Stephen Pate (for the PHENIX Collaboration) New Mexico State University
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Transcript of Stephen Pate (for the PHENIX Collaboration) New Mexico State University
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PHENIX Measurement of Parity-Violating Single Spin Asymmetry
in W Production in p+p Collisions at 500 GeV
Stephen Pate(for the PHENIX Collaboration)New Mexico State University
12 September 2010
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DSSV Global Fit -- PRD80 (2009) 034030 [0904.3821]Over many years, the worldwide program of form factor measurements, inclusive deep-inelastic and semi-inclusive deep-inelastic scattering has made possible a definitive measurement of the polarized PDFs of the u and d quarks. But the contribution of the sea and gluons to the spin of the proton remains relatively unclear.
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PHENIX Central Arm Rapidity
PHENIX Muon Arm Rapidity
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ALl +
=Δd (x1)u(x2)(1+ cosθ )2 − Δu(x1)d (x2)(1 − cosθ )2
d (x1)u(x2)(1+ cosθ )2 + u(x1)d (x2)(1− cosθ )2
ALl −
=Δu (x1)d(x2)(1 − cosθ )2 − Δd(x1)u (x2)(1+ cosθ )2
u (x1)d(x2)(1 − cosθ )2 + d(x1)u (x2)(1+ cosθ )2
θ = lepton angle in partonic c.m.s.
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ALl ±
(W ± → l ±ν )
l+
PHENIX@RHIC is measuring asymmetries in W production that are sensitive to the light quarksea contribution to the proton spin.
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STAR
RHIC-PHENIX 2009 run
• Run 09: First 500 GeV Run, March 17-April 13, 2009• Machine development in parallel with physics running to
increase luminosity, polarization• Integrated luminosity (with vertex cut) is Ldt=8.6 pb-1
• Polarization is <P>=0.39±0.04 (scale uncertainty)
Spin Rotators
Siberian SnakepC and H-jet polarimeters
Siberian Snake
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Polarization
• Measured with two polarimeters– CNI polarimeter
measurements available during run
– H jet polarimeter provides absolute polarization
– Measured residual polarization in real time after rotation at PHENIX
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Polarization measured by CNIpolarimeters fill-by-fill
P
P
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• EMC 4x4 Tower Sum Trigger
• ±30 cm vertex cut• High energy EM Calorimeter
clusters matched to charged track
• Loose timing cut to reduce cosmic ray bkg
• Loose E/p cut
PHENIX Central arm: We
Central Arm Acceptance : ||<0.35 in rapidity 2 arms covering =
e
Fully efficient at 12 GeV
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Clusters
•Smooth spectrum of EMC clusters after removing bad towers
Effect of Cuts
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ClustersGood track match
•Smooth spectrum of EMC clusters after removing bad towers•Have a good track pointing to an EMC cluster
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Effect of Cuts
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ClustersGood track matchTOF cut
•Smooth spectrum of EMC clusters after removing bad towers•Have a good track pointing to an EMC cluster•Timing within start time of collision
• Reduces out of time backgrounds (cosmics) by ~80%
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Effect of Cuts
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ClustersGood track matchTOF cutE/p
•Smooth spectrum of EMC clusters after removing bad towers•Have a good track pointing to an EMC cluster•Timing within start time of collision
• Reduces out of time backgrounds (cosmics) by ~80%•E/P cut
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Effect of Cuts
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•QCD provides the most obvious background (W. Vogelsang)•Not shown here but very important
• Cosmics and photons (from meson decays and direct), which can have accidental matches to tracks or conversions
•c/b relatively small above 30 GeV, calculated at FONLL (Matteo Carciari)•Z/* background is estimated from PYTHIA (~1 count is expected in Run09).•We is also small
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Signal and Background
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•QCD provides the most obvious background (W. Vogelsang)•Not shown here but very important
• Cosmics and photons (from meson decays and direct), which can have accidental matches to tracks or conversions
•c/b relatively small above 30 GeV, calculated at FONLL (Matteo Carciari)•Z/* background is estimated from PYTHIA (~1 count is expected in Run09).•We is also small
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Hadron spectrum suppressed by poor response of EMC to hadronsSignal and Background
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Background subtracted spectra of positron and electron candidates
Gray bands = range of background estimates.
Compared to spectrum of W and Zdecays from a NLO calculation [D. de Florian and W. Vogelsang, Phys. Rev. D81, 094020 (2010).P. M. Nadolsky and C. P. Yuan, Nucl. Phys. B666, 31 (2003)]
These yields were used for cross section results. For the asymmetry measurement, additional cuts were applied to make the background contribution negligible.
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Isolation Cut – use fact that lepton from W decay is not from a jet
E < 2GeV
Sum up energy in a cone around electron/positron
>80% of signal is kept (blue dashed line)
Factor ~4 reduction in background
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Signal for Asymmetry Measurement After Isolation Cut
Positrons:42 signal events(background 1.7 1.0)
Electrons:13 signal events(background 1.6 1.0)
Signal region is30 < pT < 50 GeV/c
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Longitudinal spin asymmetry AL
Parity-violating longitudinal single spin asymmetry
• N+ = right-handed production of W• N- = left-handed production of W• P = Polarization• R = relative luminosities of the helicity states• D = dilution by background and Z0
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Parity-violating AsymmetriesIn W Production in PHENIX
Final Results for RHIC 2009 Run[arXiv:1009.0505]
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ALe +
is non - zero at 95% CL.
Direct observation of parity-violation in W production.
Likelihood function used to determine value and limits for AL (low statistics)
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ALe +
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ALe −
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Muon Trigger Upgrade
muIDnorth
muIDsouth
muTr northmuTr south
Existing Muon Arms:•muID (triggering)•muTr (tracking)•trigger rejection ~ 200 - 500
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Muon Trigger Upgrade
RPC1(a,b)RPC3
RPC3
muIDnorth
muIDsouth
muTr northmuTr south
muTr-trigger
Existing Muon Arms:•muID (triggering)•muTr (tracking)•trigger rejection ~ 200 - 500
Upgrade:1) muTr trigger electronics: muTr 1-3 send tracking info to level-1 trigger 2) RPC stations: RPC 1+3 tracking + timing info to level-1 trigger
Trigger idea: Reject low momen- tum muons Cut out-of-time beam background
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RPCs: trigger level timing• Timing used in Run-9 to
characterize background• RPC3-N installed for Run-
10• Commissioned & ready From collision or
out going beamFrom beambackground
Test assembly of RPC-3 half octant support structure at UIUC
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MUTRIG ready for physics in 2011 Run
Minimum Ionizing Particle
• Good efficiency for MIPs• MUTR.N installed for Run-9• MUTR.s installed for Run-10• ready to go
Station 1
Station 2
Station 3
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Expectation for uncertainties in parity-violating asymmetries in high-pT muon production
Assuming:50 pb-1
50% polarizationS/B = 0.3
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Expectation for uncertainties in parity-violating asymmetries in high-pT muon production
Assuming:50 pb-1
50% polarizationS/B = 3.0
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Expectation for uncertainties in parity-violating asymmetries in high-pT muon production
Assuming:150 pb-1
50% polarizationS/B = 3.0
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Powerful measurement of Δu u
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Summary
• PHENIX has observed a non-zero parity-violating asymmetry in W production in pp collisions at 500 GeV
• Future measurements with greater precision will allow a measurement of the light quark sea contribution to the spin of the proton, using a technique complementary to that used in leptonic SIDIS
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Cross Sectionsfor W Production in PHENIX
Final Results for RHIC 2009 Run[arXiv:1009.0505]