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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

12 September 2010

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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

12 September 2010

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Clusters

•Smooth spectrum of EMC clusters after removing bad towers

Effect of Cuts

12 September 2010

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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

12 September 2010

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%

12 September 2010

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

12 September 2010

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

12 September 2010

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

12 September 2010

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

12 September 2010

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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

12 September 2010

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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

12 September 2010

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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]