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.

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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

Diffraction 2010 212 September 2010

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.


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

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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%

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

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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

12 September 2010

Hadron spectrum suppressed by poor response of EMC to hadronsSignal and Background


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.


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


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


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


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 northm

uTr southExisting 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 northm

uTr 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



Assuming:50 pb-1




Assuming:150 pb-1


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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

12 September 2010



Cross Sectionsfor W Production in PHENIX

Final Results for RHIC 2009 Run[arXiv:1009.0505]

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.

Documents

trigger trigger idea

lefthanded production

asymmetriesin w production

mutr trigger electronics

muon trigger upgrade

asymmetry measurement

righthanded production

p p collisions