Luminosity Spectrum _amp; Top Quark Threshold Studies at the ILC by yurtgc548


									Luminosity Spectrum &
Top Quark Threshold Studies
at the ILC

Filimon Gournaris
4C00 Project

              Filimon Gournaris - 15/4/05   1
Particle Physics needs… The ILC
   Precision physics needs e+e- machine with CoM energies of
    0.5 – 1 TeV to complement the LHC…
   Cannot build synchrotron, LEPII was the limit that e+e-
    circular machines can reach, at higher energies energy loss
    due to synchrotron radiation is prohibiting. Thus need
    something LINEAR!

   The ILC will be a high energy high luminosity linear collider,
    and is expected to do the precision physics of all the
    discoveries of the LHC, and new discoveries of its own..
   Based on superconducting (“cold”) technology (TESLA) for
    the accelerating cavities.
   ~35 km long linear accelerator.
   Due to the high energy and high luminosity however,
    complications are introduced to the Luminosity Spectrum of
    the machine.
   And to do precision physics, we need to understand it well…

                    Filimon Gournaris - 15/4/05                      2
    Luminosity Spectrum
   Three main energy loses:
       Initial state radiation
       Beamspread
       Beamstrahlung
   ISR calculable in QED
   Beamspread assumed
    Gaussian for TESLA design
   Beamstrahlung is the
       Can be modelled microscopically
        with Guinea-Pig
       Parameterized into a function

                      Filimon Gournaris - 15/4/05   3
    Guinea-Pig Microscopic Simulation
   550 bunch-bunch collisions including full accelerator
    effects (ground motion etc) using FONT @ 0.5 TeV

   This includes beamspread

   Cannot be used for
    fitting/reading spectra
   Provides the basis for
    testing parameterizations

                      Filimon Gournaris - 15/4/05           4
Beamstrahlung Parameterization
   Κιρκη Function:

                      With parameters

   However, this is not flexible enough…+ we want to
    parameterize beamstrahlung together with beamspread
   Naturally, this means convoluting κιρκη with a Gaussian
    (assuming beamspread is Gaussian), but the β-function
    in κιρκη does not allow for this to be done analytically…
            Thus, we need a new parameterization

                  Filimon Gournaris - 15/4/05               5
New Parameterization
   The proposed parameterization has the form:

   The sum with n = 4 (4 exponential terms) is used in
    this study.

   10 parameters describing the spectrum, 8 are used
    for the exponentials (describing different regions of
    the spectrum), and σ is the spread of the Gaussian

   Leads to a 10-11 parameter fit (using MINUIT) ! ! !

                  Filimon Gournaris - 15/4/05               6
Fitting the Guinea-Pig Spectrum

         Filimon Gournaris - 15/4/05   7
Numerical Convolution
   Can use numerical methods
    to convolute κιρκη with
   Two techniques available:
        Standard Trapezium/Simpson
        Crude Monte-Carlo
   Only managed to do MC:
        Results look OK…
        However, did not manage to
         fit with this algorithm…
   Still useful! Provides a simple
    data set to test the new

                     Filimon Gournaris - 15/4/05   8
    Fitting the Monte Carlo LumiSpectrum

    Standard beamspread 0.1% used in convolution…
    The Fit Looks GOOD!!
    Next step is to make the algorithm work for fitting..
    And Develop a proper numerical technique as well..

                     Filimon Gournaris - 15/4/05             9
Top Quark @ the ILC
   Top quark measurement is one of the first tasks of the ILC..
   Current measurement  5GeV, we think ILC can deliver 100MeV
   Top quarks are produced @ s = 2Mt (~350GeV)via channel:

       Clear detector signatures
       However, precision depends on knowledge of the luminosity
   With luminosity spectrum formalism developed, we can simulate
    the effects of the lumiSpectrum at the top threshold…

                     Filimon Gournaris - 15/4/05                    10
Effects of LumiSpectrum at
the Top Threshold
   Top cross-section does not have clear toponium
   Vulnerable to LumiSpectrum uncertainties
   Simulation based on applying luminosity spectra with
    different parameters at cross-section and comparing the

                 Filimon Gournaris - 15/4/05             11
Fitting Method
   Top Cross-section calculated to
    NNLO through TOPPIK
        Interpolated
        Parameter space f(Mt,Γτ,αs;   s)

   Then smear cross-section with
   Smear again another dataset
    with different lumiSpectrum.
   Treat one as data, other as
   Fit theory to data, minimize the
    Chi2 and see shift introduced in
    the top parameters by difference
    in the lumiSpectrum
   Can check how uncertainty in
    lumiSpectrum parameters
    influences top measurement.
                      Filimon Gournaris - 15/4/05   12
Varying Beamspread
  Data 0.3%
  Fit 0.1%

  Large effect -> Keep it small, or measure it well !!

               Filimon Gournaris - 15/4/05               13
Varying Beamstrahlung
This is done in two ways:
 Can use uncertainties introduced (K.Monig) by
   measuring the lumiSpectrum using the Bhabha
   acolinearity method (D.Miller):

Κιρκη parameters:

Does NOT include detector effects, just acolinearity
o Or can introduce larger random uncertainties
   o   ->Wait for two slides

                   Filimon Gournaris - 15/4/05         14
Bhabha Acolinearity Uncertainties

                                       Acolinearity method OK.
                                       N.B. Does not include
                                       detector effects ! ! !
         Filimon Gournaris - 15/4/05                            15
More Variation: Random Uncertainties…
   Since Acolinearity uncertainties have no effect, we can
    introduce larger uncertainties to see an effect.
   We choose  5% and  10%.
   The cross-section becomes :

                    Filimon Gournaris - 15/4/05               16
              More Variation: Random Uncertainties…

Data: Circe+10%
 Fit: Circe

                        Filimon Gournaris - 15/4/05   17
 Effective LumiSpectrum for Top production

   What is the fraction of the lumiSpectrum effective for Top production?
   Can check by truncating the spectrum and using our fitting strategy to
    see how it influences the top measurement.
   LumiSpectrum still described using standard κιρκη…

20% used                                     5% used

                     Filimon Gournaris - 15/4/05                        18
   Effective LumiSpectrum for Top production II

~3-4% @ peak produces t-tbar

Does not mean that all previous
analysis is useless..
Due to detector efficiencies etc.
need a good description of ~20%

                         Filimon Gournaris - 15/4/05   19
Outlook and the Future…
   Examined luminosity spectrum and devised a more flexible
    parameterization of it.
   Developed MC numerical convolution method
        Needs to be tested by fitting..
        Also need to do a proper numerical convolution
   Examined behaviour of top threshold under variations of the
    luminosity spectrum:
        Beamspread effect determined.
        Bhabha acolinearity method OK!
        Beamstrahlung (κιρκη) behaviour determined.
   Determined Percentage of Lumi Spectrum effective for top
    production at threshold.
   NEED to develop a scan strategy…
   Also should redo the Bhabha acolinearity simulation based on
    the new parameterization, and examine the effects to the top.
   Include/Determine detector efficiencies for both beam
    spectrometry and top reconstruction ->Have a conclusive (??)
    study for top precision..!!!

                      Filimon Gournaris - 15/4/05                 20

               THE END!

             Filimon Gournaris - 15/4/05   21
Back-Up: The Function.

         Filimon Gournaris - 15/4/05   22

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