Research Summary

  Chris Collins-Tooth

• HERA, the Transverse Polarimeter (TPOL) and ZEUS
• Charged Current Deep Inelastic Scattering cross
  sections using ZEUS data gathered in 99-00.
• Polarisation - its’ impact and measurement
• PDF analysis using ZEUS 99-00 data
    HERA, the TPOL and the ZEUS detector

                 27.5 GeV

                            • Schematic layout, showing locations
                              of ZEUS, TPOL and two new sets of
                              spin rotators - required by expts.
                              (e.g. ZEUS)
                Charged Current DIS
•   99-00 ZEUS e+p data used to extract Charged Current events.
•   CC signature: large missing Pt (ZEUS=99.7% solid angle coverage)
•   Typical backgrounds: mis-measured NC and Photoproduction.
•   CC Interaction characterised by;
    – Q2 (the negative square of the 4-momentum transfer)
    – x (fraction of incident proton momentum carried by struck quark)
    – y (fractional energy transfer to proton in its’ rest frame )
   My single differential cross sections
                                       Single differential cross section
                                       with respect to Q2, plus ratio plot
• Single differential cross sections
  in Q2, x and y compared to SM
  expectation (evaluated using
• Ratio plot shows data points as a
  fraction of SM expectation using
• Ratio plot also shows the
  uncertainty arising from the
• Data points dominated by
  statistical uncertainty.
• SM gives good description of the
Single differential cross section    Single differential cross section
with respect to x, plus ratio plot   with respect to y, plus ratio plot
 My double differential cross sections
    Reduced double differential               Reduced double differential
   cross section in bins of fixed x         cross section in bins of fixed Q2

                              99/00 e+p                               99/00 e+p
                              SM CTEQ6D
                              SM MRST2001                             SM CTEQ6D [NLO]
                              ZEUS-S                                  x(1-y2)(d+s) [LO]
                                                                      x(û+ ) [LO]
                                                                      ZEUS-S [NLO]

• At Leading Order, and at HERA energies, scc(e+p) = x[(u+c)+(1-y2).(d+s)]
• =>At high x, we are really probing the d-valence density.
Charged Current and Polarisation
              • Lepton beam polarisation upgrades
                were made to HERA - e.g. spin
                rotators, TPOL.
              • Future running will allow
                measurement of s obs(P) (i.e. see

                how cross section varies with
              • Standard Model: no right handed
                Charged Currents (WR)
              • Eventually will allow (for example)
                direct measurement of WR mass. -
                present limit is 720 GeV set in
                10/2000 by D0.
              • BUT- Must measure polarisation
                accurately (-my part in polarisation-).
                                 The TPOL                                        Silicon Detector

• Transversely polarised leptons collide with circularly polarised laser light to give angular
  asymmetry at TPOL IP.
• Angular asymmetry of Compton photons at IP becomes spatial asymmetry at TPOL.
• Calorimeter is in two halves to measure up-down energy asymmetry;  =(EU-ED) / (EU+ED)
•  is used to get photon y-position on calorimeter face - the “-y transformation”
                              -LARGE UNCERTAINTY-
• Silicon upgrade in front of calorimeter allows fast up-down calibration of calorimeter.
Silicon Upgrade
        • Two 6x6cm2 silicon detectors
          (from ATLAS collab.!!!)
          mounted together.
        • Composite silicon detector has
          horizontal and vertical strips
          (80,120mm pitch respectively).
        • 1 Xo Pb preshower to convert
          Compton photons.
        • Should improve accuracy of
          Polarisation measurement to
          under 1%.
        • 2 Testbeams performed at
               ZEUS-01-019; ZEUS-02-019
        • Polarised data now being taken
          by ZEUS.
        PDF analysis with (unpolarised)
            99-00 cross sections
• I performed a set of NLO QCD fits with new ZEUS data (incl. NC,CC).
• PDF determination (especially d-valence) should benefit from new
  ZEUS data.
• FIRST - successfully replicated the published ZEUS fits (94-98 data);
    – ZEUS-S (global fit) -Large systematic errors from heavy target data
    – ZEUS-O (only ZEUS data) -Small systematic errors, but large stat. errors
• New data added to ZEUS-O fit. (ZEUS-S fit showed only marginal
  benefit from new data, due to dominance of systematic errors).
• Effects of new data - (highlighted problems with old fitting model).
• Model dependence was then investigated to produce a final fit with
  ZEUS data.
                      ‘S’ and ‘O’ fits
 First task - replicate published ZEUS-S and -O fits published in DESY-02-105.
 -O errors mostly statistical and d-valence/sea look different in -O fit.
               1994-2000 ‘O’ fit

 Include 99-00 ZEUS data.
 Encouraging decrease in
  uncertainties, especially in d-
 PROBLEM- reduced
  uncertainties now mean
  differences in valence central
  values for ‘S’ and 94-00 ‘O’ fit
  are statistically significant.
 High-x sea still too low.
 Parameterisation dependence
  now NEEDS to be investigated -
  esp. valence & sea.
               94-00 ‘O-final’ fit
   (revised fixed and free parameters in fit)

 MANY fits performed to investigate
  effect of fixing/freeing PDF params.
 ‘O-final’ fit has:
        – low-x u,d valence =freed
          (determined by fit to data)
        – high-x sea and gluon fixed to
          sensible values obtained in global
          ZEUS-S fit.
        – mid-x sea and gluon freed.
 94-00 ‘O-final’ fit removes the
  valence and sea differences to a large
 d-valence uncertainty not reduced as
  much as first hoped, but still
  improved over published ZEUS-O.
 Extracted 99-00 charged current cross section using ZEUS.
 Contributed to enabling the measurement of polarised cross sections.
 Performed a PDF analysis using the 99-00 ZEUS data, where;
     Model changes were required- even the old ZEUS-S and -O fits showed
     Adding 99-00 data to the original ‘O’ fit made this more obvious- it
      improved the uncertainties, but central values remained far from
     ‘O-final’ fit: parameter changes made to valence, sea and gluon PDFs
     94-00 ‘O-final’ PDF central values are much more consistent with MRST
      / global ZEUS-S fit, when ZEUS-Only data is used.
     94-00 ‘O-final’ PDF Uncertainties: some d-valence improvement, but
      limited by model changes made.
        Polarisation principles
 Relativistic e+/- emit

                                     Polarisation (%)
  synchrotron radiation in curved
  portions of a storage ring.
 Emission can cause spin flip.
 UD and DU flip rates differ.
 e- become polarised antiparallel
  to the guide field, e+ become
  polarised parallel to field
  (Sokolov-Ternov effect).
 P(t) =Pst [1-exp(-t/Tst)]
        = (NU-ND)/(NU+ND)
 Pst was ~0.51 at HERA.
 Tst=time constant ~20min.
                                                        Time (min)
d-valence uncertainty smaller using
            99-00 data
Global d-valence vs. 94-00 d-valence

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