Dorigo-Corfu by dandanhuanghuang

VIEWS: 3 PAGES: 113

									                                         Corfù, September 6th, 2005



    High Pt Physics:
from the Tevatron to LHC
                      Tommaso Dorigo
                University of Padova and INFN


• Introduction: the Tevatron, CDF and D0 in Run II
• Tools for high-Pt physics: jets, leptons, b-tags, and all that
• Higgs boson searches and prospects
• Top quark physics searches and prospects
• Electroweak physics searches and prospects
     • Not discussing today:
         • Precision QCD measurements
         • Searches for new Physics / BSM / SUSY
• Conclusions and perspectives
         What this talk is not
• Not a showroom
  – skipping / forgetting / ignoring many interesting new
    results
  – some analyses only briefly mentioned
  – not giving a complete panorama
• Not a fair balance between CDF and D0
  – actually totally unfair
  – mostly focusing on CDF
• Not a snapshot of where we stand
  – rather, a view of the issues we are facing in high-Pt
    physics in preparation for the LHC
The present trenches of
   high-Pt physics:
     the Tevatron
              The Tevatron in Run II
•   Massive upgrade with respect to Run I,
    to increase L by 1.5 orders of magnitude
     – Main injector, pbar recycler
     – crossing time from 3.5 ms to 396 ns
     – increased antiproton yield and transfer
       efficiency

•   From a endured start in 2001-2002, the
    Tevatron is now working excellently
     – So far collected more than 800 pb-1 /exp
     – Peak instantaneous luminosity by now
       regularly above 1032
     – less downtime, fewer stops for beam
       studies needed – just fine smooth
       running

• In 2005-2006 crucial upgrades are being
worked at to complete the picture
     • electron cooling
     • stacktail bandwidth upgrade
• Two foreseen plans for data accumulation
     • Base plan: the minimal objective
     • Design plan: if everything works great
    Run II: where we are right now
Have been following design curve!
Upgrades continuing – electron
cooling of antiprotons is critical.
As L increases, CDF and D0
catching up by modifying trigger
tables, improving DAQ
Design curve means 8 fb-1 by
2009!


                           Total Luminosity (fb-1)
                                                                                                 Integrated Weekly Luminosity (pb-1)
   WE ARE HERE
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    10/1/03   9/30/04   9/30/05    9/30/06     9/30/07   9/29/08    9/29/09    0
                                                                              10/1/03     9/30/04      9/30/05    9/30/06     9/30/07   9/29/08      9/29/09
CDF and D0 in Run 2
The CDF Detector
    CDF significantly upgraded from Run 1:
•   New L00+SVX+ISL silicon detector
•   New central tracker
•   Extended muon coverage to |h|<1.5
•   New end-plug calorimeters
•   SVT measures IP to 45 mm at Level 2!

    The challenge is now a smooth
    operation for many years of running…
The D0 Detector
Massively upgraded from Run 1 to
include:
• 77,000 ch scintillating fiber tracking
• 2.0 Tesla solenoid
• 800,000 channel silicon detector
  (4 barrel layers, 2-sided disks)
• Extended muon coverage (MDT)
Tracker working well despite low
volume (R=1/3 RCDF)
High performance b-tag to |h|<2.0
Tools for high-Pt
    Physics
   The most common animals: Jets
In hadronic interactions, jets of hadrons
are the most common things one can
observe

They are common, but are they obvious
to define ?

“Obvious: something you may think
about for 20 years and maybe
understand”

After 20 years of studies of pQCD, we
think we understand what is going on…

What we measure in our detectors is
the combination of a multitude of effects

Disentangling them is the key
to understanding each of them better
     Identification and measurement
              of hadronic jets
Both CDF and D0 mainly use a cone algorithm (R=0.4 or 0.5) to identify localized
depositions of energy in their calorimeters and measure hard partons
Other algorithms (midpoint, Kt) are mainly used in QCD studies

When faced with the measurement of the kinematics of hard
parton emissions, one has to deal with two distinct issues:

- SCALE: to calibrate the energy response, to minimize the average
measurement error on a sample of jets

- RESOLUTION: to improve the precision of the energy measurement,
decreasing the measurement error on an individual jet

The first issue is fundamental for precision mass measurements
of hadronically decaying objects (e.g. top quarks)
The second issue is critical for the successful identification of
low S/N signals (e.g. Higgs bosons)
              The JetClu Algorithm
• Was initially designed to meet specifications from the Snowmass
  Accord (1992)
    – A seeded, iterative cone algorithm, with R=0.7
    – custom prescription for splitting and merging
• Several drawbacks
    – not best option for QCD measurements
    – pQCD uses larger cone (Rsep=1.3) to emulate experimental procedure
    – not collinear safe, not IR safe (see next slide)
• But also strong points
    – conceptually simple
    – sensible choice for 2 TeV physics
    – makes it easier to compute corrections and systematics
• Start from Et-ordered list of seed towers (Et>1 GeV)
    – do preclustering by creating list of cones centered on seed towers,
      removing seeds as they are englobed in cones
    – then add to cones Et of towers within, recompute baricenter, move
      cone, to convergence
    – if two cones share too much energy (>75%) they are merged
Shortcomings of standard cone algorithms
Infrared safety
The jet multiplicity changes
if an arbitrarily soft emission
is detected between two
partons
 the cone algorithm does not give
a stable answer in the IR limit

Collinear safety
Replacing a massless parton
by the sum of two collinear
particles a jet may fail detection due
to lack of a seed, and the jet
multiplicity changes

               Fixed-order pQCD calculations contain
              uncanceled divergencies…
        The Midpoint Algorithm
• Conceived to remove
  some of the problems of
  JetClu when compared to
  theoretical calculations
• IR safety mended by
  introducing imaginary
  seeds at midpoint of
  each pair of jets close in
  angle, and iterating to
  convergence
              The Kt algorithm
QCD appears to separate
partons into different jets
according to their relative
transverse momentum
The Kt algorithm is therefore
preferred by theory, and
comparisons between
experimental
measurements and theoretical
calculations are more
straightforward
One event, three algorithms
           Calibration of Jet Energy
•   To calibrate the energy measurement in CDF we use a detector-dependent
    correction, a scale correction, and a treatment of additional small physical
    effects
     – eta-dependent correction  dijet balancing
     – multiple interaction correction  f(Nvtx)
     – absolute scale correction: E/p of single tracks are used to tune the MC, which is
       then used to derive “calhad” corrections.
     – last, out of cone and underlying event corrections are made
     – Systematic errors reduced to 3% (data/MC comparisons, g-jet balancing)
          • Calorimeter stability, MC (fragmentation, simulation of single particle resp.)
          • Understanding of out-of-cone radiation and UE
          • Simulation of response function versus jet rapidity

•   D0 has an almost-compensated calorimeter (e/p <1.05, linear with energy );
    disuniformities and gaps among cryostats need to be corrected
     – EM part is calibrated with Zee decays
     – U noise measured in situ; other offset corrections address pile-up (energy from
       previous interactions) and underlying event
     – Response is measured as a function of rapidity and Et with gamma-jet events
     – showering correction: Et flux vs DR off jet cones
CDF Jet Energy Corrections
  Ptcorr = (Ptraw frel – MI) fabs – UE + OOC
         The b-Jet Energy Scale Issue
•   b-jets are different from generic jets
     – large mass of leading hadron
     – semileptonic decay
     – hard fragmentation
•   Originally thought the most pressing issue
    for precision Mtop measurements
     – after demonstration of auto-calibration with Wjj
       the picture is brighter
     – residual systematics of b-JES to top mass estimated
       at less than percent level
     – but that is MC extrapolation… Need to measure b-JES anyhow!

•   To calibrate b-jets, CDF exploits the SVT
    to trigger on Zbb events in Run II
     – extract signal, fit, get scale from Z mass (more on that later)
     – But this technique is unfeasible at the LHC
          • background cross section is huge
          • rate of any b-jet trigger impossible to handle
•   Another possibility is searching for gamma-b events
     – balancing the photon in the transverse plane with the jet, one obtains
       a calibration
     – but b-fraction of jets is typically 40-50% even after a tight b-tagging
       by secondary vertex identification
     – D0 and CDF currently studying this technique – expect results soon
     b-jet calibration with gB events
• Use the MPF method:
   – select back-to-back gj events
   – determine Rhad from missing Et
     projection
   – apply b-tagging, separate into different
     samples for more handles
       • resulting sample is a mixture of b and
         charm, light quarks
       • use also tighter b-tag by exploiting mass
         of tracks in secondary vertex
   – can fit for Rb
        What to do at the LHC ?
• Zbb signal extraction is unfeasible
• gamma-B balancing techniques might work – studies are ongoing
• Calibrations using top quark decays are possible, but one would
  prefer an independent determination
• I have a suggestion: use Zgbbg events
Advantages:
   – Automatically selects qq initial state, boosting the S/N by an order of
     magnitude at typical TeVatron energy, surely more at LHC, with respect
     to inclusive Zbb vs gluonbb ratio
       • Typical initial state of gluonbb does not produce photons!
   – Can fully exploit dedicated detectors for Hgg
   – Resolution on Eg is so good, one can determine b-jet scale by just
     looking at jet-jet ANGLE!
Disadvantages:
   – Statistical power is limited by small cross section
Improving the jet energy resolution

• Calibrating the calorimeter response to streams of
  hadrons is one of the foundations of mass
  measurements
   – It is a correction to the average systematic offset to the
     measurement
• But the precision of an individual jet’s energy
  determination is no less a foundation
   – Separation of reconstructed hadronic resonances
     (W,Z,top,Higgs, other fancier animals) critically depends on it
   – Even continuous-Q2 distributions benefit from a more precise
     measurement
   – Less known is that top mass measurements do benefit greatly
     from improved resolutions even in high S/N samples
               Tools for the improvement
                  of the Et resolution
    • CDF has taken seriously the challenge to improve the jet Et resolution
         – Triggered by HSWG studies (more later)
         – Issue is complex: resolution can be improved in different ways depending
           on event characteristics, jet rapidity, flavor of parton…
         – focus is improvement of dijet mass resolution through more precise jet Et
           measurement
         – also focusing on b-jets

Three candidate algorithms identified and
studied:
     - H1 algorithm: use tracker for central
     charged hadrons
     - Track+Cal algorithm: categorize cal
     towers, disentangle photon response, use
     tracker for charged tracks
     - Hyperball algorithm: olistic approach to
     the problem. Use ALL information on jet
     measurement, exploit intercorrelation
     between jet observables and Et
              The Hyperball algorithm:
              statement of the problem
•   From an idea developed for the HSWG, 2003
•   Imagine one measures a scalar quantity (say the Et of a jet), which is
    subject to all sorts of biases
•   Alongside with Et, one measures heaps of other characteristics of the jet
     –   several quantities in the calorimeter
     –   track Pt information
     –   photon clusters in Strip Chambers
     –   b-tagging information
     –   etcetera
•   Many of the latter carry information about the biases of the Et measurement
     – for instance, a charged fraction larger than one speaks of a undermeasurement
       of calorimeter Et
•   Simple minded approaches to remove biases neglect cross-correlations
     – First I correct for the charged fraction, then the presence of muons, then the
       missing Et along the jet direction… In the end the computed biases wash out
       each other to some extent
•   How to correct for these biases all at once ?
             Basics of the algorithm
•   Main hypothesis: A scalar field DEt:RNR exists and is continuous
•   Its value is the average error (pos or neg) in the jet Et measurement
    performed in the calorimeter, as a function of all thinkable jet observables:
    DEt = Etmeas- Ettrue
•   Cannot determine DEt with infinite precision
•   How to best measure it ?
•   Hyperball method:
     – Fill RN with MC b-jets (we know DEt for them!)
     – Need to average locally the value of DEt
     – What does locality mean ?
         • close to point to be estimated
         • similar value of most important variables
         • smaller correlation variables are less important for averaging
     – Generalized distance in RN: D2(x,y) = Siwi(xi-yi)2
     – Use D to find MC points closest to point where scalar field is needed
     – Need to determine W vector such that closest MC points provide best estimate of
       <DEt>
     – Geometrically it means determining shape of hyperellipsoids
Promising results… Work in progress!
Applied the algorithm to
B-jets (QCD direct prod.)
Resolution improves
by about 30% throughout
the Et spectrum studied

That means we can really
get back the 10% relative
resolution we promised for
Hbb decay searches

Still lots to improve:
-refine list of variables used
-use more MC for DEt estimates
-optimize everything
 Identification of High Pt Leptons
Most high Pt final states studied at the Tevatron involve the
detection of leptons
      - easy to trigger on
      - high signal purity
      - easy to calibrate using standard candles (W,Z bosons)

Tevatron experiments are exploiting to the fullest these
signatures, producing lots of precision Electroweak physics
                                                                 CDF
measurements with them

Tau leptons are also beginning to contribute appreciably,
especially to new physics searches which may be generation-
dependent



                         CDF

                                                                D0
        Tagging b-jets
 Identifying b-jets is of paramount importance
 for low-mass Higgs boson searches.
 Three methods are well-tested and used:
  – Soft lepton tagging
                                                        D0
  – Secondary vertex tagging
  – Jet Probability tagging
 For double tag searches, efficiency factors
 get squared! To retain signal, both CDF/D0
 have loose and tight tagging options             Tight/loose SV tag eff.
 Efficiency drops at low jet Et and high
 rapidity but is 45-50% for central b-jets from
 Higgs decay
 Mistag rates are kept typically at 0.5%

SV tagging: tracks with
significant IP are used in a                           CDF
iterative fit to identify the   I.P.
secondary vertex inside                B
the jet
          Secondary vertex tagging

This event display shows how
charged tracks are used to fit
for secondary vertices in jets
from a ttbar candidate (single
lepton decay)

Decay lengths for 50 GeV b-
jets are typically of the order of
a few millimeters and they can
be easily reconstructed with
tracks having at least 3
associated hits in the silicon
detectors
(sd is around 20 microns)
    The last resort – or the main one?
      The Monte Carlo Simulation
•   The technology of reproducing the known behavior of high energy
    interactions has reached exquisite heights
     – Now available several choices which model QCD (let alone EW interactions) very
       successfully
          • full matrix element computations and parton shower modeling agree better by the day
     – But tuned with the data… Will they stand the test of a x7 jump in CM energy ?
     – Unfortunately we are still critically dependent on PDF fits
          • even larger extrapolation in the unknown at LHC (more on that later)

•   Almost every analysis of high Pt processes now relies heavily on Monte
    Carlo simulations

•   Let’s not forget that gross mistakes are brought by relying too much on MC
    to extrapolate into the unknown

•   Need to keep a cool head
     – Lesson for the future from the past: use of data is fundamental at the start of a
       new endeavour, as was in CDF and D0 in the early days of top searches
          • method 1 vs method 2
          • likelihood methods vs pure counting experiments
               “Simulation”
From the Latin “simulacrum”…

         My Webster’s offers the following:
1) The act of simulating; pretense; feigning.
2) A simulated resemblance
3) An imitation or counterfeit
4) The use of a computer to calculate, by means of
  extrapolation, the effect of a given physical
  process
Higgs Boson Searches
     SM Higgs: Production and Decay
At the Tevatron, about five 120 GeV Higgs
bosons are produced in a typical day of
running (will be 15/day in two years).

Direct production occurs mostly via gluon-




                                                 Excluded
                                                 Excluded
gluon fusion diagrams.
Associated production through a virtual W or Z
boson provides sensitivity in the region where
LHC will have more trouble. At higher mass,
the WW(*) final state becomes dominant.                              mH (GeV/c2)


Even the WHWWW(*)
process is promising
despite the low yield, due
to the striking signature of                                     e       l
missing Et plus three                                                        n
                                                     q           W
leptons, two of which may                                   W*
be of the same charge                                                H
but different flavor.                                q                       b
                                                                         b
     What we know
     about the Higgs
• Although they did not directly observe it,
  the LEP experiments have collected a
  wealth of information on the Higgs boson
  through comparisons of EW observables
  to EW theory + radiative corrections

• From theory we know its couplings, its
  decay modes, and how its mass impacts
  the W and top masses.
• If it exists, then we know its mass with
  about 60 GeV accuracy, and the direct
  search limit already cuts away a large part
  of the allowed mass region

• Latest LEP results: MH=126+73-48 GeV,
  MH<280 GeV @ 95% CL (Winter ‘05) 
  now being updated for new Mtop…
             Higgs Sensitivity WG Predictions
             In 2003 the Tevatron chances for
             Higgs discovery were re-evaluated

             Idea: with available data and operating
             detectors, can better assess Tevatron
                                                                 CDF
             reach
             Surprisingly, the new results meet or
             exceed 1998 Susy/Higgs WG ones.
Lum (fb-1)




                                DESIGN                 Keys to success:
                                           BASE           -mass resolution improvements;
                                                          - optimized b-tagging;
                                                          - shape information vs counting.
           Can we see dijet resonances
                if they are there?
   A low mass Higgs search entails believing that we can:
          - appropriately reconstruct hadronically-decaying objects
          - accurately understand our background shapes
   All of that can be proven if we see the Zbb decay in our data.

The S/N is not higher than 1/5 at the most
   in the signal region

    – good testing ground for H!
    – can use to test/improve dijet mass
      resolution with advanced
      algorithms


   We barely saw it in Run 1…

   Can we use it in Run 2 ??
   CDF sees Zbb decays in Run 2!

Double b-tagged events with no extra
jets and a back-to-back topology are
the signal-enriched sample:
Et3<10 GeV, DF12>3

Among 85,784 selected events CDF finds
3400±500 Zbb decays
     - signal size ok
     - resolution as expected
     - jet energy scale ok!

This is a proof that we are in business
with small S/N jet resonances!

CDF expects to stringently constrain the
b-jet energy scale with this dataset
                  A few additional notes
b-jet Et scale = dominant systematics in Run I top mass measurements
     • top decay is a two-body one
     • very nearly linear relationship between Eb and Mt
     • At Tevatron, b Jet Energy Scale syst. is approx. s(Mt) (GeV) ~ s(Eb) (%)
     • At LHC, typical top quark boost softens the dependence: s(Mt) (GeV) ~ 0.7
     s(Eb) (%)
          - for light-quark jets s(Mt) (GeV) ~ 0.3 s(Eb) (%)
•   In Run II we are demonstrating that
    by measuring with precision the JES
    of light-quark jets using Wjj, the
    part of s(Mt) due to modeling of b
    jets (decays, fragmentation, color
    connection) can be reduced to below
    1% (more later).
•   The Zbb signal becomes
    important mainly as a testing ground
    of algorithms targeting the jet
    resolution improvements
•   Anyway Zbb decays may
    contribute appreciably to b-JES
    determinations: already with 300 pb-1
    one gets a statistical error well below
    2%
        Search for WH in Run 2
To search for WHlnbb events
a detailed understanding of the
composition of the W+jets sample is mandatory.

In the 2-jet bin CDF finds 187 events with a b-tag,
where 175±26 are expected, mostly from
Wbb production and mistags.



                                A fit to the dijet mass
                                distribution allows to
                                extract a 95% CL
                                limit of 5 pb to SM
                                WH production.
                                The obtained limit
                                is consistent both
                                with a priori
                                predictions and with
                                expectations based
                                on HSWG results.
 Results with double tagged events
When two jets are required to be b-tagged, backgrounds are
strongly reduced and mostly Wbb, ttbar remain
The data is still in good agreement with expectations
The extracted limit of WH production is 3-10 pb for MH=110-150 GeV
   WH Search in D0
D0 also study their W+2jet bin with b-
tagging in 384 pb-1 of high-Pt leptons from
Run 2 data.
The dijet mass distribution shows no
anomaly with 1 b-tag. The 2-tag distribution
is divided in search windows to set limits to
Higgs production.
They find 4 events with two b-tags in the
mass window centered on 115 GeV (exp.
2.4±0.6)
                         95%CL limits on
                         sWH*B(Hbb) are set
                         at 7 to 9 pb for
                         MH=105-135 GeV

                         By-product: a 95% CL
                         limit is set to Wbb
                         production (DR>0.75,
                         Pt>20 GeV) at 4.6 pb.
   High Mass Searches: HWW(*)
The SM production of WW pairs has been
measured by CDF in Run 1 and by both
CDF and D0 in Run 2: excellent
agreement with NLO.




To search for Higgs boson decays, events
with two high-Pt leptons (e,m) and large
missing Et are selected; the tt background
is rejected with a jet veto.
                                             n   W+   e+
Then both experiments use the
helicity-preferred alignment of
charged leptons in F to
discriminate known backgrounds.              n   W-   e-
CDF results on HWW
CDF searches for HWW events by selecting two
tight leptons (ee,em,mm) with Ete(Ptm)>20 GeV and
missing Et>25 GeV (50 GeV if DFll<20°).
A strict jet veto (Et<15 GeV if |h|<2.5) rejects top
candidates.
Finally, a small dilepton mass is required (Mll<55-
80 GeV for MH=140-180 GeV).

                               8 events are observed in 184 pb-1 of Run 2 data with
                               the Mll <80 GeV cut, with an expected background of
                               8.9±1.0.

                               A likelihood fit to the DFll distribution is performed to
                               extract a limit on the HWW cross section as a
                               function of its mass.

                               The result is sHWW*B(WWllnn)>5.6 pb for MH=160
                               GeV.
Putting it all together…
 Higgs Physics: perspectives
The Higgs boson is being hunted at the Tevatron in all
advantageous search channels. D0 and CDF are competing –
that’s good! – but will soon start to also combine their results.

No surprises with the analyzed 200 pb-1 samples, but we have
already three times more data on tape to look at!

We are on track to supersede the LEP2 lower limit on MH by 2007

By the end of 2009, the Tevatron might be able to see a MH=115
GeV Higgs at 5s, or exclude it all the way to 180 GeV.

…but that will require both cunning and the Tevatron delivering
according to the design plan!

What I feel I can promise at 95% CL: exclusion up to 135 GeV,
3s evidence at 115 GeV.
                    Implications for LHC
    LHC starts collecting physics data in April 2008  if everything works as it
    should, the Higgs is discovered by CMS and ATLAS in 2009 (a few fb-1 should
    suffice)
    However, fits prefer a Higgs mass in the region favoring Tevatron and hampering
    LHC…. Let’s hypothesize MH=115 GeV. Three possible scenarios:


•   Scenario A: Tevatron design, LHC
    delays  firs hints from CDF e D0
    (3s, early 2008) allow LHC to put
    their chips in the right place 
    confirmation, common discovery (as
    did Adone for J/y? Seems
    improbable…

•   Scenario B: Tevatron design, LHC
    in time  Tevatron “confirms” the
    first signal from LHC

•   Scenario C: Tevatron base plan (or
    killed), LHC whatever  you know
    the story.
Top Quark Physics
   The Top Quark at the Tevatron
• The top quark just turned 10! Run I results:
    – s(tt) =5.7±1.6 pb (D0), 6.5±1.4 pb (CDF) (@1.8TeV)
    – Mt = 178.0±2.7±3.3 GeV (D0+CDF)
    – many other measurements – but still imprecise – of Vtb, BR, spin; limits to
      single production, non-SM production and decays.

• From the “discovery” mode the Tevatron soon adapted to using top
  quarks as a perfect pQCD laboratory
• As new data pours in, the plan is the same: first, cross section
  measurements are performed; then the mass, then the kinematics and
  the search of anomalies, and lastly, the measurement of intrinsic
  phhysical properties

• That modus operandi allows to optimize the output of physics results as
  analysis tools get perfected and more sophisticated:
    – high- Pt lepton identification
    – b-tagging
    – precise measurement of jet energy scale
A bit of history of Top Quark Quest
        Production of top at Tevatron
• At Tevatron production of ttbar pairs occurs by qq annihilation (85%) or
  gluon fusion (15%)  proportions inverted WRT LHC!
• Theoretical cross section (NNLO) is 6.1 pb 1/1010 collisions  2 events
  per hour
• Single top production is not irrelevant (3 pb), but its signature is way less
  characteristic  so far obtained only upper limits to single top production
                   Top Quark Decays
  Since Vtb=1 and Mt>Mb+MW, the decay tWb is dominant. Final states of tt
  pairs are classified according to the decay of the two W bosons:

Measurements exploit mainly three decay
channels
• Dileptonic: B=4/81, S/N ~ 3-10
• Single lepton: B=8/27, S/N ~2-5
• All hadronic: B=4/9, S/N ~1/6

Top quark decays also constitute an
excellent laboratory to study weak
interactions of quarks freed from any QCD
effects:
   Mt large  Gt =f(Mt3) ~ 1.5 GeV >> LQCD
   ~ 0.2 GeV and thus:
   • t is produced and decays free;
   • polarization can be studied in the
     decay, since the depolarization time
    td ~ Mt/L2 is much longer
   Cross section measurements
• The most precise stt measurements come from the analysis of
  single lepton decays  compromise between S/N and yield
• “standard” recipe:
   –   trigger on electrons or m with Et (Pt)>15 GeV
   –   offline selection: Et (Pt)>20 GeV
   –   Missing Et>25 GeV
   –   3 or 4 jets with Et>15 GeV
   –   B tagging and/or cut on Ht (sum of transverse energy or all objects,
       including missing Et)
• Both CDF and D0 use MC to estimate physical backgrounds and
  data to handle false b-tags
• “W+1 jet” and “W+2 jet” events are used to verify sample
  composition and yield
Example: D0 lepton+jets with b-tag
The measurement from D0 uses events with a lepton (e, m), missing Et>30
GeV, and 3 or 4 jets with Et>20 GeV, and one or two b-tags. The eight
determinations of the signal (e or mu, 3 or 4 jets, 1 or 2 b-tags) are used in a
likelihood fit to obtain stt = 8.6 ±1.6 ±0.6 (lum.) pb


     Single tags                                        Double tags
        stt in the all-hadronic Channel
• The “least fortunate” of measured
  channels
• First measurement in Run I, now
  repeated by CDF

• Selection: multi-jet trigger, and
    – >=6 jets (Et>20 GeV)
    – >=1 b-tag
    – kinematical selection (centrality, SEt,
      Aplanarity, subleading SEt)

• Background (essentially QCD HF prod.
  with radiation, and fake HF+rad.) estimate
  by parametrizing the probability
  P(Et,Ntrk,…) to get a b-tag
    – works well pre-kinematic selection
• With 311 pb-1:
  stt= 7.5 ± 1.7 (stat.) +3.3-2.2 (syst.) pb
    Summary of measurements of stt
The ttbar production is studied at the Tevatron in all significant final states and
with different methodologies. The experimental error is still larger than the theoretical
one (NNLO, Cacciari, Kidonakis – 15%) but is reaching it quickly


                   CDF
       Top mass measurements
The final Run I results, recently updated with the latest
measurement by D0, reach a combined precision of 2.5%
In Run II the goal is to reach a precision close to 1% per
experiment  That would mean getting very close to LHC
promises and not far to the physical limit of precision with
direct reconstruction techniques


                                                 73
                                        M H  126 GeV / c
                                                  48
                                                            2




                               M H  280GeV / c 2 @ 95%C.L.
     CDF Template Analysis Overview
 Datasets
                      c2 mass fitter:
 Data                 Finds best top mass and jet-parton assignment
         Wbb MC
                      One number per event
             Mass     Additional selection cut on resulting c2
 tt MC       fitter
                                Templates



                                            Likelihoo
                                                d         Result
                                               fit
Likelihood fit:
Best signal + bkgd templates to fit data
Compare to paramiz’n, not directly
Constraint on background normalization
         Result with 318 pb-1
Mtop = 173.2   +2.9
                      -2.8   (stat) ± 3.4 (syst) GeV/c2

                                              Green histos:
                                               data
                                               distributions
                                              Curves:
                                               expected
                                               signal and
                                               background
                                               from global
                                               best fit
                      Systematics Summary
                                         Was 6.8 !!
                                           - Reduced double counting
                                           - Tuned simulation to data
Systematic Source         Uncertainty
                          (GeV/c2)
                                           Systematics dominated
Jet Energy Scale              3.1
                                           by jet energy scale.
B-jet energy                  0.6
Initial State Radiation       0.4
Final State Radiation         0.4
Parton Distribution           0.4
Functions                                    Jet Systematic Source         Uncertainty
                                                                            (GeV/c2)
Generators                    0.3
                                        Relative to Central                    0.6
Background Shape              1.0
                                        Hadronic energy (Absolute Scale)       2.2
MC statistics                 0.4
                                        Parton energy (Out-of-Cone)            2.1
B-tagging                     0.2
                                                       Total                   3.1
Total                         3.4
   W mass resonance in tt events!

• Can we use Wjj mass
  resonance to constrain JES?
• Mtop measurement sensitive
  primarily to energy scale of b
  jets. (W mass constraint in
  c2.)
   – But studies show most
                                    So use Wjj to improve
      uncertainty is shared by       understanding of q jets,
      light quark, b jets.           therefore b jets, therefore
   – Only 0.6 GeV/c2 additional      Mtop.
      uncertainty on Mtop due to    This constraint will only
      b-jet-specific systematics.    improve with statistics!
   Measure JES using dijet mass
Build templates using
invariant mass mjj of all
non-tagged jet pairs.




                            • Rather than assuming JES and
                              measuring MW...
                            • Assume MW and measure JES
                            • Parameterize P(mjj;JES) same as
                              P(mtreco;Mtop)
            The “2D” measurement

• Too many correlations to treat this as an independent
  measurement of JES.
• Take the plunge and fit for Mtop and JES simultaneously…
   – Need “2D” templates: P(mtreco;Mtop,JES) and P(mjj;Mtop,JES).
   – More complex, but still tractable.
   – Constrain to prior knowledge: JES = 0 ± 1.
• Advantages:
   – Improve uncertainty on JES (dominant systematic)improve
     uncertainty on Mtop.
   – With this method, JES uncertainty begins to scale directly
     with statistics!
              Apply 2D fit to the data
                    JES = -0.10 +0.78-0.80 (stat only) s
                    3.7
 M top  173.5      3.6 (stat. JES)  1.7 (syst.)GeV/c            2


• Reported error includes
  both “pure statistics” and
  (reduced) JES
  systematic.
• Breaks down to
       -2.6 (stat) ± 2.5 (JES)
  +2.7

• 20% improvement in
  uncertainty due to JES!

 1D result
             M top  173.22..9 (stat.) 3.1 (JES)  1.5 (syst.)GeV/c2
                          2 8
  was:
   Current Top mass Measurements
CDF took full advantage of the new jet energy corrections to surpass D0’s
precision recenlty; D0 still has a systematic uncertainty close to 6 GeV on jet
energy scale, but will soon improve both statistics and systematics of its
measurements
        Other top physics results
 Besides cross section
 and mass, CDF and D0
 have begun to study in
 detail production and
 decay characteristics of
 top quarks


See talk by Gervasio G.
this afternoon
Electroweak Physics
  Production of W and Z bosons
At the Tevatron W and Z bosons are
fundamental tools for calibrations and
checks
- EM energy scale
- Momentum scale in tracker
- Studies of the resolution in missing
transverse energy
- Studies of the calorimeter response to
low Et hadronic activity (boson recoil)
- input to PDF at low x from W charge
asymmetry measurements
Their cross section, known at NNLO (2%
precision), may provide an important
normalization point to sidestep the
luminosity error
Assorted signals
W bosons collected by CDF and D0
are now close to a million, and 105
the Z bosons
(but analyzed signals are still catching
up WRT these numbers)
    sW with high rapidity electrons
• CDF measured the Wen process
  with forward electrons (1.1<h<2.8)
  using the ISL and the new plug
  calorimeter
• Important measurement for the
  determination of the PDF’s (d/u a x
  piccolo) thanks to the asymmetry in
  production and decay (PDF e V-A)
• Important also to detect effect from
  soft gluon resummation in low-x
  production  impact in all LHC
  cross section measurements

 sW = 2.874 ± 0.034(stat) ±
  0.167(syst) ± 0.172 (lum.) nb.

These new CDF W asymmetry data points will
    be included in the MRST04 fits soon
Zooming out: 22 years of EW
      measurements
     Wln as a luminosity monitor
All cross section
measurements refer to the
inelastic cross section, which
is known with a 4% precision
Given the high precision
reached by NNLO calculations
for s(W), one may think of
using the latter as a
normalization point of
integrated luminosity of a     However, it is necessary to known with precision the stability
given dataset                  of data taking and the collection efficiencies…
HEP-PH/0405130 (Frixione,
Mangano) investigates the
uncertainties from Tevatron
and LHC acceptance and
other effects
It appears possible to measure
s(W) to within1-2% both at
Tevatron and LHC – it is
critical however to study the
PDF’s at high rapidity
 A possible important contribution of
    Tevatron to LHC: the PDFs ?
At LHC all cross section measurements will strongly depend on the knowledge of
PDF at low x
    - would benefit from high-rapidity W/Z measurements at Tevatron
    - Heavily relying on HERA data
LHC goal on W mass: 15 MeV  requires an accurate knowledge of production
mechanisms at low x (5x10-4 : 10-2)

- Top mass measurement systematics
-the search of new physics signals
with low cross section is often based
on an accurate knowledge of the rate
of production of physics backgrounds
                    How to measure PDFs at low x ?
                                                                                                    • In order to determine the PDFs
                                                                                                      at low x, one needs to study
            9
                            Tevatron parton kinematics                                                light things going forward …
         10

                   x1,2 = (M/1.96 TeV) exp(y)                                                      • An attempt: quarkonia in D0…
         10
            8
                   Q=M
                                                                                                      Still lacking enough statistics.
            7
         10


            6
         10                                                     M = 1 TeV



                                                                                                x=10-3, Q2=100 GeV2                                             x=10-4, Q2=10 GeV2
            5
         10
(GeV )
2




            4
         10                         M = 100 GeV                                                    1000                                                        10000
2




                                                                                                              data: D0
Q




                                                                                                                                                                           Tevatron
                                                                                                   900        theory: gg  normalised to data                  9000
                                                                                                                                                                           theory: gg y arbitrary normalisation
            3
         10
                                                                                                   800
                                                                                         (pb)




                                                                                                                                                               8000
                                    y=         4            2    0     2        4
            2                                                                                      700                                                         7000
         10          M = 10 GeV
                                                                                                                                                                                                    CTEQ6




                                                                                                                                                    Bmmds/dy
                                                                                        Bmmds/dy




                                                                                                   600                                                         6000
            1
                                                                      fixed
         10                                        HERA                                            500                                                         5000
                                                                      target                                                         MRST2002                                           MRST2002
                                                                                                   400                                                         4000
            0
         10
              -7       -6      -5         -4            -3       -2        -1       0
           10        10       10         10            10       10    10        10                 300                       CTEQ6                             3000

                                                   x                                               200                                                         2000
                                                                                                          0          1         2        3       4                      0          1         2         3           4

                                                                                                                               y                                                            y
W mass measurements
Pt scale Calibrations
Calibration of EM Et scale
Measurement of hadronic recoil
Hadronic recoil, cont’d
Systematic uncertainties on MW
  Summary of MW measurement
• One of the most complex measurements at the Tevatron
  – requires a perfect understanding of tracker and
  calorimeter, and an optimization of calibrations
• Analyses are reaching conclusion
• The CDF systematic uncertainty is already defined
  and slightly better than Run I one (76 vs 79 MeV)
• D0 is finalizing calorimeter calibrations… Results are
  due soon
• The total MW error per experiment from Run II is
  expected to be about 40 MeV (compare to best single
  measurement to date: Aleph, DMW=58 MeV)
   – LHC will be able to improve these measurements by a factor 2,
     but accurate PDF measurements are required at low x
Conclusions and
 Perspectives
  Conclusions and perspectives
• Run II at the Tevatron has entered a mature stage, and precision
  measurements are coming out
   –   Mt= 173.5+3.7-3.6 ±1.7 (syst) GeV(CDF)
   –   Mt= 172.7±2.9 GeV (CDF+D0)
   –   First improvements in MW determinations expected soon
   –   Improved Higgs mass limits foreseen for 2007
• Many analysis tools are being perfected even in view of a possible
  use in LHC
   –   tools for energy calibration
   –   b-jet energy scale may not be necessary
   –   Luminosity monitoring with W production
   –   PDF at low x  important both for measurements and searches!
• The Higgs boson might be at reach of CDF and D0 before LHC
  starts playing the game
• Many a precision QCD measurement and new limits to
  supersymmetric processes and other new physics are public . but
  did not fit here…
   The inclusive jet cross section
Nel Run 1 si osserva un disagreement con NLO QCD a CDF
D0 non conferma né smentisce
Una modifica delle PDF (CTEQ4HJ) con tweaking di g(x) ad alto
x permette di riaggiustare le cose
Nel Run II, misura ripetuta con sezione d’urto x1.6 nei bins di
maggior Et  sempre un eccesso, ma minore di prima…
Single top production
                               400
                                          Design

                               350        Base
                                          Actual
Integrated Luminosity (pb-1)




                               300


                               250


                               200

                               150


                               100

                                50


                                0
                               10/01/03       12/31/03   03/31/04   06/30/04   09/29/04
                                                                                           Design
                                                          Date
                                                                                           for 2005
                                                                                           Base
                                     2005 : so far                                         for 2005
                                     along Design                                         2004

                                                                                           2003


                                                                                             2002
Jetclu e Midpoint
L’algoritmo Kt a CDF
CDF: a
WWeenn
candidate




            CDF: a
            WWemnn
            candidate
                   Method checks
• Prove to ourselves that
  parameterizations and
  likelihood machinery
  work: measure the top
  quark mass in MC
  samples.
• Mtop fit unbiased across
  input top mass and jet
  energy scales.
• Reported uncertainty
  scaled by ~1.03 as
  shown (effect of non-
  Gaussian likelihood).
  Misura di stt nel canale n+jets
• Il decadimento tttnbjjb’ non è mai stato messo in
  evidenza finora
   – mancanza di un trigger per selezionare leptoni t ad alta
     efficienza
   – difficoltà di identificare i t
   – Tuttavia, il BR è del 15%...
• E’ in realtà possibile selezionare eventi di top con la sola
  richiesta di significativa Et mancante nell’evento:
   – si selezionano eventi con soli jets adronici (veto e, m) con un
     trigger multijet
   – si usa la s(Et(miss))=Et(miss)/sqrt(SEt)
   – si eliminano eventi in cui la missing Et è prossima in f a un jet
     adronico
   – in questo modo si raccolgono eventi di tipo tttnbjjb’ ma anche
     eventi in cui un elettrone o muone fallisce i tagli di identificazione
• Si usano insomma gli scarti delle altre analisi!
    Selezione eventi e stima fondi
• Eventi con >=4 jets, e missing Et
  significance>4, DF(j,Et(miss))>0.4
• >=1 b-tag
• Il fondo viene stimato con eventi
  senza b-tags usando una
  parametrizzazione della probabilità
  di fake
   – tiene conto delle caratteristiche dei
     jets e della contaminazione da veri
     b-quarks di eventi con Et mancante
     puntante lungo il jet taggato
   – verificata su diversi campioni di
     controllo
   – buon accordo prima della selezione
     cinematica
   – sistematica conservativamente
     stimata al 10%
                                    Risultati
La procedura permette di isolare un campione di 108
eventi, contenente un segnale di oltre 60 eventi di top.
La presenza del segnale è confermata dalla distribuzione
di variabili cinematiche discriminanti.

Si trova stt=5.8±1.1(stat) +1.7 -1.1(syst) pb
                       Diboson production
•   Diverse misure di produzione associata: WW,
    Wg, Zg, WZ, ZZ… studiate da CDF e D0 con
    leptoni di alto Pt e fotoni
•   Tests stringenti del MS: limiti a accoppiamenti
    anomali, e finestra su possibile nuova fisica

•   CDF: s(WW)=14.6+5.8-5.1+1.8-3.0±0.9 pb

•
    (th: 12.4 +-0.8)
    D0: s(WZ)=4.5+3.8-2.6 pb
                                                      CDF
    (th:3.7±0.1pb)

•   Con Et(g)>7 GeV, DR>0.7(CDF):
    s(Wg)B(Wln)=18.1±3.1 pb
    (th:19.3±1.4 pb);
   s(Zg)B(Zll)=4.6±0.6 pb                     CDF
    (th:4.5±0.3 pb).

•   Con Et(g)>8 GeV, DR>0.7 (D0):
    s(Wg)B(Wln)= 14.8±2.3 pb
    (th:16.0±0.4 pb);
    s(Zg)B(Zll)= 4.7±0.5 pb
    (th: 3.9±0.2 pb).
                                                      D0
    Improving the dijet mass resolution
•   One of the keys to a successful
    extraction of the H signal is to
    increase the dijet mass
    resolution for pairs of b-jets
•   Standard CDF/D0 jet correction
    algorithms tuned for best scale
    determination, not for best
    resolution
•   H1 algorithm: use tracker to
    measure Pt of charged
    component
•   in HSWG studied prototype of b-
    specific correction using
    identified muons, Et
    dependence of had response on
    top of H1
•   Also developed advanced
    algorithm to account for subtle
    correlation among satellite
    observables and jet Et
    measurement
•   Global result is that
    sM/M=10% is achievable in
    central calorimeter
Study of V-A in the decay of quarks
   free from strong interactions
W decays and lepton Pt
Results compared
D0: upper limits to f+
               D0 Results on HWW
D0 also searches for HWW decays by selecting events with two
oppositely charged leptons (ee,em: Pt>12,8 GeV; mm: Pt>20,10 GeV),
missing Et>20 GeV (30 for mm), and imposing a loose jet veto
 (Et<90 GeV, or Et1,Et2 <50,30 GeV).

The azimuthal angle DFll between the
two leptons is then required to be less
than 1.5 for electron pairs (2.0 for the
em,mm combinations).


Combining the three channels they
find 9 events, when 11.2±3.2 are
expected from background sources
in 177 pb-1 of Run 2 data.

They can thus exclude s*B>5.7 pb
at 95% C.L. for MH=160 GeV.
          WHWWW(*) Search
CDF also searches for the striking signature
of three W bosons in 193.5 pb-1 of Run 2 data.
First, the dataset with a lepton with Pt>20 GeV
and a second with Pt>6 GeV of same charge
is analyzed and found in agreement with
expectations.




                                      Then, optimized cuts are applied to the
                                      second lepton (e.g. Pt>18 GeV for MH>160
                                      GeV) and on the vector sum of leptons
                                      transverse momenta (Ptll>35 GeV).
                                      Zero events are observed, when 0.95±0.61±
                                      0.18 are expected from known sources.
                                      95% CL limits are thus set at 12 (8) pb for
                                      MH=110 (160) GeV.
Low Mass H Searches
The only chance to see Hbb at the Tevatron
is through associated production with bosons

ZHllbb is the cleanest signature, but it yields
too few events
W/ZHjjbb has the lowest S/N but the high BR
helps at larger Higgs mass
WHlnbb is next-to-best, but CDF was
“unlucky” in Run I
The best channel is ZHnnbb

CDF has a new combination of Run 1 results
with ZHllbb, nnbb channels.
They search events with two jets with DF<2.6,
missing Et>40 GeV, no isolated track with Pt>10 GeV.
The limit is obtained by a fit to the
mass distribution of b-tagged events.

The Run 1 CDF limit is now at 7.2 to 6.6 pb for
MH=110 to 130 GeV.
      Reconstructed masses

                 Green histos: data distributions
                 Curves: expected signal and
                  background from global best fit




mjj     mtreco
 If this were the only Mtop result…
Electroweak fit using
 only this result as top   Most direct
 quark mass.               implication: the
                           Higgs boson mass
A full combination of      moves back down
 CDF/D0, Run I/II is       into the excluded
                           region, and away
 months away…              from the
                           “theoretically
                           loathed” M>135
                           GeV region…
                           …And back well
                           into the LHC-
                           problematic
                           region!
                           Also note:
                           restored hope for
                           MSSM-
                           aficionados
                Summary of Results
Nice measurements, although I can’t help feeling they sound more and more
as “cross-checks”…
              Event-by-event Mass Fitter
          l                                                                 • Distill all event information
                     W+                            b-jet
      ν                                                                       into one number (called
Constraints                          t    X
                                                                              reconstructed mass).
 PT balance                                                                 • Select most probable jet-
 mlν=mW
                                 t
                                                                              parton assgnmt based on
 mjj=mW                                                                       c2, after requiring b-tagged
 mt1=mt2                                 W-                                   jets assigned to b partons.
              b-jet                                                                                Reconstructed
                                                                                                   top mass is
                                                                                                   free parameter

                                ( pT, fit  pT,meas ) 2
                                   i         i
                                                                          ( pUE , fit  pUE ,meas ) 2
     c2           
               i   , 4 jets             s    2
                                                              
                                                               j  x, y
                                                                             j

                                                                                     s2
                                                                                         j

                                              i                                       j

                  ( M jj  M W ) 2              ( M n  M W ) 2 ( M bjj  M t ) ( M bn  M t ) 2
                                                                                2

                                                                               
                            GW
                             2
                                                       GW
                                                        2
                                                                        Gt2
                                                                                        Gt2
             Signal templates
Selected templates (GeV)


140      150      160       Parameterization:
                            Build signal p.d.f. as a function
                            of generated mass.

170      180          190



200      210          220

 Reconstructed Mass
            Background template   CDF Run II Preliminary (162 pb-1)

Mass Template         Background Source                # of
   Source                                             events          Constraint used
W+jets (mistags)           Mistags, QCD              4.4 ± 1.0        in likelihood fit.
     Wbb                   Wbb, Wcc, Wc,             2.1 ± 0.7
                             WW/WZ
   Single Top                Single Top               0.33 ±
                                                       0.04
                   Total                             6.8 ± 1.2



  Major contributions:
        W+heavy flavor
        W+jets (mistag)
                       QCD
            Unbinned likelihood fit
• Free parameters are Mtop, ns, and nb.
    – Profile likelihood: minimize w.r.t. ns,nb, no integration
• Fluctuations of nb are a systematic effect. Allowing nb to float in the
  fit means information in data is used to reduce the systematic
  uncertainty.
                                                                    Interesting
      L  Lshape  Lbg                                              Parameter!
                                                N      ns Psig (mi ; M top )  nb Pbg (mi )
  Lshape  e    ( ns  nb )
                               (ns  nb )   N
                                                
                                                i 1                ns  nb
                   ( nbfit  nb ) 2
                              exp
               
                       2s nb
                          2                     bkgd (mean)
    Lbg  e                                     constraint
          Templates: subdivide sample
• Use 4 categories of
  events with different
  background content
  and reconstructed
  mass shape.
• More b tags are better
      – Increases S:B
      – More “golden” events,
        where correct jet-parton
        assignment is found.


 Category         2-tag    1-tag(T)   1-tag(L)   0-tag
 j1-j3            ET>15    ET>15      ET>15      ET>21
 j4               ET>8     ET>15      15>ET>8    ET>21
 S:B              18:1     4.2:1      1.2:1      0.9:1
              Result with 318 pb-1
• Subdivision improves
  statistical uncertainty.
   – Pure and well                       2-tag          1-tag(T)
     reconstructed events
     contribute more to
     result.
   – Adds 0-tag events.
                                     1-tag(L)
• Subdivision does not                                   0-tag
  improve systematic
  uncertainty.
   – Most systematics,
     including jet energy
     scale, are highly        Expected Fraction of Sensitivity
     correlated among the    2-tag 1-tag(T) 1-tag(L) 0-tag
     samples.
                             35%      45%        9%      11%

								
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