Transcript FROM ZEUS TO ATLAS

Research Summary
Chris Collins-Tooth
20-2-2004
Introduction
• 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
N
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 sections
in Q2, x and y compared to SM
expectation (evaluated using
CTEQ6D & ZEUS-S PDFs).
• Ratio plot shows data points as a
fraction of SM expectation using
ZEUS-S PDFs.
• Ratio plot also shows the
uncertainty arising from the
PDFs.
• Data points dominated by
statistical uncertainty.
• SM gives good description of the
data.
Single differential cross section
with respect to Q2, plus ratio plot
Single differential cross section
with respect to x, plus ratio plot
Single differential cross section
with respect to y, plus ratio plot
My double differential cross sections
Reduced double differential
cross section in bins of fixed x
99/00 e+p
SM CTEQ6D
SM MRST2001
ZEUS-S
Reduced double differential
cross section in bins of fixed Q2
99/00 e+p
SM CTEQ6D [NLO]
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
polarisation)
• 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-).
CC
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
DESY/CERN-SPS.
publications:
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 dvalence.
 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
extent.
 d-valence uncertainty not reduced as
much as first hoped, but still
improved over published ZEUS-O.
Summary
 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
discrepancies.
 Adding 99-00 data to the original ‘O’ fit made this more obvious- it
improved the uncertainties, but central values remained far from
MRST/ZEUS-S.
 ‘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.
 Relativistic e+/- emit
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.
Polarisation (%)
Polarisation principles
Time (min)
d-valence uncertainty smaller using
99-00 data
Global d-valence vs. 94-00 d-valence