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Measurement of Longitudinal Spin AsymmetriesFrom W rarr e Boson Decay in Polarized ppCollisions at radics = 500 GeV at RHIC-PHENIXTo cite this article Kensuke Okada and (forthe PHENIX collaboration) 2011 J Phys Conf Ser 295012072

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Measurement of Longitudinal Spin Asymmetries

From W rarr e Boson Decay in Polarized pp Collisions

atradic

s = 500 GeV at RHIC-PHENIX

Kensuke Okada (for the PHENIX collaboration)

BNL Bldg510A Upton NY 11973 USA

E-mail okadabnlgov

Abstract We report the measurement of the parity violating single spin asymmetries forinclusive high transverse momentum electrons and positrons in polarized p + p collisions at acenter of mass energy of

radic

s = 500 GeV with the PHENIX detector at RHIC These electronsare attributed to the decay of Wplusmn and Z0 bosons and measured production cross section isconsistent with the expectations The W production is confirmed for the first time in p + p

collisions Its spin asymmetry in the polarized p+p collisions is a important probe for the quarkflavor decomposition of the proton spin

The proton is the most basic composite particle Understanding its structure helps us gaininsight into the quark confinement Deeply inelastic scattering (DIS) experiment has been a clearand powerful approach and it is evolved to polarized DIS experiment to explore the spin structureof the proton Analyses of polarized semi-inclusive DIS experiments [1 2 3] have determined theindividual flavor separated quark and antiquark helicity distribution (∆q and ∆q) by connectingfinal state hadrons with quark flavors using fragmentation functions Colliding polarized protonsis a complementary way to approach the origin of the proton spin At the collider energy thereal W is produced via a parity violating weak process which enables to identify the quarkflavor and helicity in the proton contributed to the process by detecting decay leptons withoutthe uncertainty of fragmentation functions Another advantage is because the scale is set bythe heavy mass of the W higher order QCD corrections can be evaluated reliably

In 2009 the relativistic heavy ion collider (RHIC) succeeded to collide polarized protons ata center of mass energy of

radics = 500 GeV In this report we describe the measurement of the

production cross section of W boson and its parity violating single spin asymmetryThe PHENIX detector has been described in detail elsewhere [4] The central arm

spectrometer covers |η| lt 035 in pseudorapidity This analysis uses the electromagneticcalorimeter (EMCal) to measure the energy of electrons and tracking chambers the driftchambers (DC) and the pad chambers (PC) to determine the charge sign of the tracks fromtheir bend angle in an axial magnetic field The data set was recorded with the EMCal triggerwhich has a nominal energy threshold at 10 GeV The luminosity is monitored by beam-beamcounters those are two arrays of 64 quartz Cerenkov counters located at 31lt |η| lt39 Theircoincidence rate is connected to the luminosity from the van der Meer scan technique [5] Inthe 2009 run the integrated luminosity of 86 pbminus1 was recorded for the central arm detectoranalysis

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

Published under licence by IOP Publishing Ltd 1

This analysis searched for electrons from the W decay The data collected by the EMCaltrigger are mostly QCD events In the W event the electron has high transverse momentum(pT ) and it is isolated unlike high pT track in QCD jet events The electron energy measuredin the EMCal was used to calculate the pT Because for electrons at momentum of 40GeVc ofour interest the resolution of the energy in the EMCal is about 5 and it is better than theresolution of the momentum of about 40 Figure 1 shows the relation between the bend angleα and the transverse momentum for charged tracks after a rough electron selection Thoughthere are contaminations of hadron backgrounds in the low pT region the data points are seento be distributed around the calculation from the magnetic field strength The resolution ofthe tracking system was evaluated from the data with no magnetic field Figure 2 shows the αdistribution for 40 GeVc tracks It is capable to determine the charge sign reasonably Thedata with no magnetic field were also used to determine the center of transverse beam position

[GeVc]T

p5 10 15 20 25 30 35 40 45 50

[mra

d]α

-002

-0015

-001

-0005

0

0005

001

0015

002

Figure 1 The correlation between the bendangle (α) and the transverse momentum (pt)The lines show the expectation

[mrad]α-5 -4 -3 -2 -1 0 1 2 3 4 5

arbi

traril

y sc

ale

0

1

2

3

4

5

6

+e

-e

Figure 2 The α distribution for 40GeVtrack The ratio of two Gaussian distributionis set to the expected signal ratio of W + andWminus

To select electron track candidates we applied two levels of criteria The first one is a minimaltrack criteria which requires the projected point to the beam to be less than 30 cm loose cutson the time of flight and rough energy-momentum matching This criteria was used to calculatethe production cross section The second one requires the electron track to be isolated Thethreshold was set to the sum of energy and momentum in the cone around the electron candidatetrack at 2 GeV This cut purifies the signal fraction However it is hard to evaluate the cutefficiency so this sample was only used to calculate the spin asymmetry

Figure 3 shows the pT spectra for positron and electron candidates The bands represent ourestimated background which are dominated by charged hadrons with hadronic interactions inthe EMCal and electrons from photon conversions before the tracking system In the figure clearelectron signals from the W decay are seen by the Jacobian peak at MW 2 40 GeV It has tobe noted that our selection is limited to one hemisphere so that the signal is a combination ofW and Z bosons decay

From the yields in the signal region (30ltpT lt50 GeVc) the production cross sections werecalculated The tracks within the nominal geometric acceptance of the central spectrometerwere reconstructed with sim37 efficiency defined by the overlap of live areas in the trackingdetectors and fiducial areas on the calorimeters and drift chambers The efficiency for retainingelectron candidates after all cuts was 99 The resulting reconstruction efficiency was not pT

dependent for pT gt 30 GeVc Table 1 shows the production cross sections for electrons andpositrons from W and Z boson decays The first error is statistical the second error is systematic

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

2

Figure 3 (color online) The spec-tra of positron (upper panel) andelectron (lower panel) candidatesbefore (solid histogram) and af-ter (dashed histogram) an isolationcut The estimated backgroundbands are also shown

from the uncertainty in the background and the third error is a normalization uncertainty Thenormalization uncertainty is due to the luminosity (10) multiple collision (5) and acceptanceand efficiency uncertainties (10) The results are compared with NLO and NNLO calculationsThe difference of the two calculations indicates a level of theory uncertainties Within theuncertainty our measurements are consistent with expectations From theory calculations thefraction of Z boson contributions to our sample are 7 and 31 for positrons and electronsrespectively

Table 1 Comparison of measured cross sections for electrons and positrons with 30 lt pT lt 50GeVc from W and Z decays with NLO [6 7] and NNLO [8] calculations The first error isstatistical the second error is systematic from the uncertainty in the background and the thirderror is a normalization uncertainty

dσdy

(30ltpeT lt50GeVc)|y=0 [pb]

Lepton Data NLO NNLO

e+ 502 plusmn 72+12minus36 plusmn 75 432 468

eminus 97 plusmn 37+21minus25 plusmn 15 113 135

e+ and eminus 599 plusmn 81+31minus60 plusmn 90 545 603

To calculate the spin asymmetry the sample with the isolation cut was used to minimize thebackground contamination To reduce the ambiguity of charge misidentification to a negligiblelevel a further cut was applied to the bend angle (α) to be |α| gt 1 mrad When a polarizedbeam collides with a unpolarized beam the raw parity violating single spin asymmetry is definedby

εL =N+ minus R middot Nminus

N+ + R middot Nminus

(1)

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

3

where N+ is the number of events from collisions with the beam polarization is positive andNminus for the negative Generally the integrated luminosities are not equal in the two cases therelative luminosity R is to adjust the difference The physics asymmetry is calculated from theraw asymmetry according to

AL =εL middot D

P (2)

where P is the beam polarization and D is a dilution correction to account for the remainingbackground in the signal region

In reality RHIC has two polarized beams Since the measurement in the central armspectrometer is symmetric to the beams both beams contribute equally We built a likelihoodfunction from yields sorted by four helicity states The statistical uncertainty of the rawasymmetry is confirmed to be consistent with what we expect from Poisson distribution oftwo times of number of candidates

The second column in Table 2 shows the measured raw asymmetries For the sample inthe background region (12 lt pT lt 20 GeVc) the asymmetry is consistent with zero which isexpected from the fact that they are dominated by the QCD process For the signal region(30 lt pT lt 50 GeVc) large asymmetries were observed Especially it is significant for thepositrons

To get the physics asymmetries the dilution correction of D = 104plusmn003 and 114plusmn010 forpositive and negative charges respectively and the average beam polarization P = 039 plusmn 001were applied The longitudinal polarization fractions were monitored using very forward neutronasymmetries [9] and found to be 99 or greater The contribution to AL from the residualtransverse component of the polarization was negligible thanks to the almost left-right symmetricdetectors Table 2 shows the results of AL and its confidence intervals those contains the effect ofbroadening of the likelihood function due to the uncertainties of D and P When the confidenceinterval was calculated the physical boundary (AL = plusmn1) was applied With limited statisticsone side of 68 CL and 95 CL hits this physical boundary A non-zero parity violating singlespin asymmetry is observed in positrons

Table 2 Longitudinal single-spin asymmetries The confidence intervals are defined for AL

Sample εL AL(W+Z) 68 CL 95 CL

Background + minus0015 plusmn 004Signal + minus031 plusmn 010 minus086 [minus1minus056] [minus1minus016]

Background minus minus0025 plusmn 004Signal minus 029 plusmn 020 +088 [017 1] [minus060 1]

The results are compared with estimations of various polarized parton-distribution functions(PDFs) [6] in Fig 4 With the current limited statistics the measured asymmetries are consitentwith estimations Another message from this figure is that the admixture of W and Z bosonscan be a probe to separate models

In summary we presented the first measurement of production cross section and non-zeroparity violating asymmetry in W and Z production in polarized p + p collisions at

radics = 500

GeV A non-zero spin asymmetry in positron candidates is a direct demonstration of the prity-violating coupling of the W to the light quraks In the following years a precise measurementis the main goal in the RHIC spin program PHENIX is preparing a detector upgrade which

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

4

enables us to measure the forward and backward muons from the W decay We will get moreinsight into flavor separated quark and antiquark helicity distributions in the proton

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z+W

gt 30 GeVcTp|y|lt035

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z-W

gt 30 GeVcTp|y|lt035

DSSVDNS kkpDNS kreGRSV stdGRSV val

Figure 4 (color online)Longitudinal single-spinasymmetries for electronsand positrons from Wand Z decays The errorbars represent 68 CLThe theoretical curvesare calculated using NLOwith different polarizedPDFs

References[1] Alekseev M G 2010 Phys Lett B693 227ndash235[2] Airapetian A et al 2005 Phys Rev D71 012003[3] Adeva B et al 1998 Phys Lett B420 180ndash190[4] Adcox K et al 2003 Nucl Instrum Meth A499 469ndash479[5] Adare A et al 2009 Phys Rev D79 012003[6] de Florian D and Vogelsang W 2010 Phys Rev D81 094020[7] Nadolsky P M and Yuan C P 2003 Nucl Phys B666 31ndash55[8] Melnikov K and Petriello F 2006 Phys Rev D74 114017[9] Adare A et al 2007 Phys Rev D76 051106

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

5

This analysis searched for electrons from the W decay The data collected by the EMCaltrigger are mostly QCD events In the W event the electron has high transverse momentum(pT ) and it is isolated unlike high pT track in QCD jet events The electron energy measuredin the EMCal was used to calculate the pT Because for electrons at momentum of 40GeVc ofour interest the resolution of the energy in the EMCal is about 5 and it is better than theresolution of the momentum of about 40 Figure 1 shows the relation between the bend angleα and the transverse momentum for charged tracks after a rough electron selection Thoughthere are contaminations of hadron backgrounds in the low pT region the data points are seento be distributed around the calculation from the magnetic field strength The resolution ofthe tracking system was evaluated from the data with no magnetic field Figure 2 shows the αdistribution for 40 GeVc tracks It is capable to determine the charge sign reasonably Thedata with no magnetic field were also used to determine the center of transverse beam position

[GeVc]T

p5 10 15 20 25 30 35 40 45 50

[mra

d]α

-002

-0015

-001

-0005

0

0005

001

0015

002

Figure 1 The correlation between the bendangle (α) and the transverse momentum (pt)The lines show the expectation

[mrad]α-5 -4 -3 -2 -1 0 1 2 3 4 5

arbi

traril

y sc

ale

0

1

2

3

4

5

6

+e

-e

Figure 2 The α distribution for 40GeVtrack The ratio of two Gaussian distributionis set to the expected signal ratio of W + andWminus

To select electron track candidates we applied two levels of criteria The first one is a minimaltrack criteria which requires the projected point to the beam to be less than 30 cm loose cutson the time of flight and rough energy-momentum matching This criteria was used to calculatethe production cross section The second one requires the electron track to be isolated Thethreshold was set to the sum of energy and momentum in the cone around the electron candidatetrack at 2 GeV This cut purifies the signal fraction However it is hard to evaluate the cutefficiency so this sample was only used to calculate the spin asymmetry

Figure 3 shows the pT spectra for positron and electron candidates The bands represent ourestimated background which are dominated by charged hadrons with hadronic interactions inthe EMCal and electrons from photon conversions before the tracking system In the figure clearelectron signals from the W decay are seen by the Jacobian peak at MW 2 40 GeV It has tobe noted that our selection is limited to one hemisphere so that the signal is a combination ofW and Z bosons decay

From the yields in the signal region (30ltpT lt50 GeVc) the production cross sections werecalculated The tracks within the nominal geometric acceptance of the central spectrometerwere reconstructed with sim37 efficiency defined by the overlap of live areas in the trackingdetectors and fiducial areas on the calorimeters and drift chambers The efficiency for retainingelectron candidates after all cuts was 99 The resulting reconstruction efficiency was not pT

dependent for pT gt 30 GeVc Table 1 shows the production cross sections for electrons andpositrons from W and Z boson decays The first error is statistical the second error is systematic

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

2

Figure 3 (color online) The spec-tra of positron (upper panel) andelectron (lower panel) candidatesbefore (solid histogram) and af-ter (dashed histogram) an isolationcut The estimated backgroundbands are also shown

from the uncertainty in the background and the third error is a normalization uncertainty Thenormalization uncertainty is due to the luminosity (10) multiple collision (5) and acceptanceand efficiency uncertainties (10) The results are compared with NLO and NNLO calculationsThe difference of the two calculations indicates a level of theory uncertainties Within theuncertainty our measurements are consistent with expectations From theory calculations thefraction of Z boson contributions to our sample are 7 and 31 for positrons and electronsrespectively

Table 1 Comparison of measured cross sections for electrons and positrons with 30 lt pT lt 50GeVc from W and Z decays with NLO [6 7] and NNLO [8] calculations The first error isstatistical the second error is systematic from the uncertainty in the background and the thirderror is a normalization uncertainty

dσdy

(30ltpeT lt50GeVc)|y=0 [pb]

Lepton Data NLO NNLO

e+ 502 plusmn 72+12minus36 plusmn 75 432 468

eminus 97 plusmn 37+21minus25 plusmn 15 113 135

e+ and eminus 599 plusmn 81+31minus60 plusmn 90 545 603

To calculate the spin asymmetry the sample with the isolation cut was used to minimize thebackground contamination To reduce the ambiguity of charge misidentification to a negligiblelevel a further cut was applied to the bend angle (α) to be |α| gt 1 mrad When a polarizedbeam collides with a unpolarized beam the raw parity violating single spin asymmetry is definedby

εL =N+ minus R middot Nminus

N+ + R middot Nminus

(1)

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

3

where N+ is the number of events from collisions with the beam polarization is positive andNminus for the negative Generally the integrated luminosities are not equal in the two cases therelative luminosity R is to adjust the difference The physics asymmetry is calculated from theraw asymmetry according to

AL =εL middot D

P (2)

where P is the beam polarization and D is a dilution correction to account for the remainingbackground in the signal region

In reality RHIC has two polarized beams Since the measurement in the central armspectrometer is symmetric to the beams both beams contribute equally We built a likelihoodfunction from yields sorted by four helicity states The statistical uncertainty of the rawasymmetry is confirmed to be consistent with what we expect from Poisson distribution oftwo times of number of candidates

The second column in Table 2 shows the measured raw asymmetries For the sample inthe background region (12 lt pT lt 20 GeVc) the asymmetry is consistent with zero which isexpected from the fact that they are dominated by the QCD process For the signal region(30 lt pT lt 50 GeVc) large asymmetries were observed Especially it is significant for thepositrons

To get the physics asymmetries the dilution correction of D = 104plusmn003 and 114plusmn010 forpositive and negative charges respectively and the average beam polarization P = 039 plusmn 001were applied The longitudinal polarization fractions were monitored using very forward neutronasymmetries [9] and found to be 99 or greater The contribution to AL from the residualtransverse component of the polarization was negligible thanks to the almost left-right symmetricdetectors Table 2 shows the results of AL and its confidence intervals those contains the effect ofbroadening of the likelihood function due to the uncertainties of D and P When the confidenceinterval was calculated the physical boundary (AL = plusmn1) was applied With limited statisticsone side of 68 CL and 95 CL hits this physical boundary A non-zero parity violating singlespin asymmetry is observed in positrons

Table 2 Longitudinal single-spin asymmetries The confidence intervals are defined for AL

Sample εL AL(W+Z) 68 CL 95 CL

Background + minus0015 plusmn 004Signal + minus031 plusmn 010 minus086 [minus1minus056] [minus1minus016]

Background minus minus0025 plusmn 004Signal minus 029 plusmn 020 +088 [017 1] [minus060 1]

The results are compared with estimations of various polarized parton-distribution functions(PDFs) [6] in Fig 4 With the current limited statistics the measured asymmetries are consitentwith estimations Another message from this figure is that the admixture of W and Z bosonscan be a probe to separate models

In summary we presented the first measurement of production cross section and non-zeroparity violating asymmetry in W and Z production in polarized p + p collisions at

radics = 500

GeV A non-zero spin asymmetry in positron candidates is a direct demonstration of the prity-violating coupling of the W to the light quraks In the following years a precise measurementis the main goal in the RHIC spin program PHENIX is preparing a detector upgrade which

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

4

enables us to measure the forward and backward muons from the W decay We will get moreinsight into flavor separated quark and antiquark helicity distributions in the proton

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z+W

gt 30 GeVcTp|y|lt035

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z-W

gt 30 GeVcTp|y|lt035

DSSVDNS kkpDNS kreGRSV stdGRSV val

Figure 4 (color online)Longitudinal single-spinasymmetries for electronsand positrons from Wand Z decays The errorbars represent 68 CLThe theoretical curvesare calculated using NLOwith different polarizedPDFs

References[1] Alekseev M G 2010 Phys Lett B693 227ndash235[2] Airapetian A et al 2005 Phys Rev D71 012003[3] Adeva B et al 1998 Phys Lett B420 180ndash190[4] Adcox K et al 2003 Nucl Instrum Meth A499 469ndash479[5] Adare A et al 2009 Phys Rev D79 012003[6] de Florian D and Vogelsang W 2010 Phys Rev D81 094020[7] Nadolsky P M and Yuan C P 2003 Nucl Phys B666 31ndash55[8] Melnikov K and Petriello F 2006 Phys Rev D74 114017[9] Adare A et al 2007 Phys Rev D76 051106

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

5

Figure 3 (color online) The spec-tra of positron (upper panel) andelectron (lower panel) candidatesbefore (solid histogram) and af-ter (dashed histogram) an isolationcut The estimated backgroundbands are also shown

from the uncertainty in the background and the third error is a normalization uncertainty Thenormalization uncertainty is due to the luminosity (10) multiple collision (5) and acceptanceand efficiency uncertainties (10) The results are compared with NLO and NNLO calculationsThe difference of the two calculations indicates a level of theory uncertainties Within theuncertainty our measurements are consistent with expectations From theory calculations thefraction of Z boson contributions to our sample are 7 and 31 for positrons and electronsrespectively

Table 1 Comparison of measured cross sections for electrons and positrons with 30 lt pT lt 50GeVc from W and Z decays with NLO [6 7] and NNLO [8] calculations The first error isstatistical the second error is systematic from the uncertainty in the background and the thirderror is a normalization uncertainty

dσdy

(30ltpeT lt50GeVc)|y=0 [pb]

Lepton Data NLO NNLO

e+ 502 plusmn 72+12minus36 plusmn 75 432 468

eminus 97 plusmn 37+21minus25 plusmn 15 113 135

e+ and eminus 599 plusmn 81+31minus60 plusmn 90 545 603

To calculate the spin asymmetry the sample with the isolation cut was used to minimize thebackground contamination To reduce the ambiguity of charge misidentification to a negligiblelevel a further cut was applied to the bend angle (α) to be |α| gt 1 mrad When a polarizedbeam collides with a unpolarized beam the raw parity violating single spin asymmetry is definedby

εL =N+ minus R middot Nminus

N+ + R middot Nminus

(1)

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

3

where N+ is the number of events from collisions with the beam polarization is positive andNminus for the negative Generally the integrated luminosities are not equal in the two cases therelative luminosity R is to adjust the difference The physics asymmetry is calculated from theraw asymmetry according to

AL =εL middot D

P (2)

where P is the beam polarization and D is a dilution correction to account for the remainingbackground in the signal region

In reality RHIC has two polarized beams Since the measurement in the central armspectrometer is symmetric to the beams both beams contribute equally We built a likelihoodfunction from yields sorted by four helicity states The statistical uncertainty of the rawasymmetry is confirmed to be consistent with what we expect from Poisson distribution oftwo times of number of candidates

The second column in Table 2 shows the measured raw asymmetries For the sample inthe background region (12 lt pT lt 20 GeVc) the asymmetry is consistent with zero which isexpected from the fact that they are dominated by the QCD process For the signal region(30 lt pT lt 50 GeVc) large asymmetries were observed Especially it is significant for thepositrons

To get the physics asymmetries the dilution correction of D = 104plusmn003 and 114plusmn010 forpositive and negative charges respectively and the average beam polarization P = 039 plusmn 001were applied The longitudinal polarization fractions were monitored using very forward neutronasymmetries [9] and found to be 99 or greater The contribution to AL from the residualtransverse component of the polarization was negligible thanks to the almost left-right symmetricdetectors Table 2 shows the results of AL and its confidence intervals those contains the effect ofbroadening of the likelihood function due to the uncertainties of D and P When the confidenceinterval was calculated the physical boundary (AL = plusmn1) was applied With limited statisticsone side of 68 CL and 95 CL hits this physical boundary A non-zero parity violating singlespin asymmetry is observed in positrons

Table 2 Longitudinal single-spin asymmetries The confidence intervals are defined for AL

Sample εL AL(W+Z) 68 CL 95 CL

Background + minus0015 plusmn 004Signal + minus031 plusmn 010 minus086 [minus1minus056] [minus1minus016]

Background minus minus0025 plusmn 004Signal minus 029 plusmn 020 +088 [017 1] [minus060 1]

The results are compared with estimations of various polarized parton-distribution functions(PDFs) [6] in Fig 4 With the current limited statistics the measured asymmetries are consitentwith estimations Another message from this figure is that the admixture of W and Z bosonscan be a probe to separate models

In summary we presented the first measurement of production cross section and non-zeroparity violating asymmetry in W and Z production in polarized p + p collisions at

radics = 500

GeV A non-zero spin asymmetry in positron candidates is a direct demonstration of the prity-violating coupling of the W to the light quraks In the following years a precise measurementis the main goal in the RHIC spin program PHENIX is preparing a detector upgrade which

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

4

enables us to measure the forward and backward muons from the W decay We will get moreinsight into flavor separated quark and antiquark helicity distributions in the proton

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z+W

gt 30 GeVcTp|y|lt035

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z-W

gt 30 GeVcTp|y|lt035

DSSVDNS kkpDNS kreGRSV stdGRSV val

Figure 4 (color online)Longitudinal single-spinasymmetries for electronsand positrons from Wand Z decays The errorbars represent 68 CLThe theoretical curvesare calculated using NLOwith different polarizedPDFs

References[1] Alekseev M G 2010 Phys Lett B693 227ndash235[2] Airapetian A et al 2005 Phys Rev D71 012003[3] Adeva B et al 1998 Phys Lett B420 180ndash190[4] Adcox K et al 2003 Nucl Instrum Meth A499 469ndash479[5] Adare A et al 2009 Phys Rev D79 012003[6] de Florian D and Vogelsang W 2010 Phys Rev D81 094020[7] Nadolsky P M and Yuan C P 2003 Nucl Phys B666 31ndash55[8] Melnikov K and Petriello F 2006 Phys Rev D74 114017[9] Adare A et al 2007 Phys Rev D76 051106

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

5

where N+ is the number of events from collisions with the beam polarization is positive andNminus for the negative Generally the integrated luminosities are not equal in the two cases therelative luminosity R is to adjust the difference The physics asymmetry is calculated from theraw asymmetry according to

AL =εL middot D

P (2)

where P is the beam polarization and D is a dilution correction to account for the remainingbackground in the signal region

In reality RHIC has two polarized beams Since the measurement in the central armspectrometer is symmetric to the beams both beams contribute equally We built a likelihoodfunction from yields sorted by four helicity states The statistical uncertainty of the rawasymmetry is confirmed to be consistent with what we expect from Poisson distribution oftwo times of number of candidates

The second column in Table 2 shows the measured raw asymmetries For the sample inthe background region (12 lt pT lt 20 GeVc) the asymmetry is consistent with zero which isexpected from the fact that they are dominated by the QCD process For the signal region(30 lt pT lt 50 GeVc) large asymmetries were observed Especially it is significant for thepositrons

To get the physics asymmetries the dilution correction of D = 104plusmn003 and 114plusmn010 forpositive and negative charges respectively and the average beam polarization P = 039 plusmn 001were applied The longitudinal polarization fractions were monitored using very forward neutronasymmetries [9] and found to be 99 or greater The contribution to AL from the residualtransverse component of the polarization was negligible thanks to the almost left-right symmetricdetectors Table 2 shows the results of AL and its confidence intervals those contains the effect ofbroadening of the likelihood function due to the uncertainties of D and P When the confidenceinterval was calculated the physical boundary (AL = plusmn1) was applied With limited statisticsone side of 68 CL and 95 CL hits this physical boundary A non-zero parity violating singlespin asymmetry is observed in positrons

Table 2 Longitudinal single-spin asymmetries The confidence intervals are defined for AL

Sample εL AL(W+Z) 68 CL 95 CL

Background + minus0015 plusmn 004Signal + minus031 plusmn 010 minus086 [minus1minus056] [minus1minus016]

Background minus minus0025 plusmn 004Signal minus 029 plusmn 020 +088 [017 1] [minus060 1]

The results are compared with estimations of various polarized parton-distribution functions(PDFs) [6] in Fig 4 With the current limited statistics the measured asymmetries are consitentwith estimations Another message from this figure is that the admixture of W and Z bosonscan be a probe to separate models

In summary we presented the first measurement of production cross section and non-zeroparity violating asymmetry in W and Z production in polarized p + p collisions at

radics = 500

GeV A non-zero spin asymmetry in positron candidates is a direct demonstration of the prity-violating coupling of the W to the light quraks In the following years a precise measurementis the main goal in the RHIC spin program PHENIX is preparing a detector upgrade which

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

4

enables us to measure the forward and backward muons from the W decay We will get moreinsight into flavor separated quark and antiquark helicity distributions in the proton

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z+W

gt 30 GeVcTp|y|lt035

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z-W

gt 30 GeVcTp|y|lt035

DSSVDNS kkpDNS kreGRSV stdGRSV val

Figure 4 (color online)Longitudinal single-spinasymmetries for electronsand positrons from Wand Z decays The errorbars represent 68 CLThe theoretical curvesare calculated using NLOwith different polarizedPDFs

References[1] Alekseev M G 2010 Phys Lett B693 227ndash235[2] Airapetian A et al 2005 Phys Rev D71 012003[3] Adeva B et al 1998 Phys Lett B420 180ndash190[4] Adcox K et al 2003 Nucl Instrum Meth A499 469ndash479[5] Adare A et al 2009 Phys Rev D79 012003[6] de Florian D and Vogelsang W 2010 Phys Rev D81 094020[7] Nadolsky P M and Yuan C P 2003 Nucl Phys B666 31ndash55[8] Melnikov K and Petriello F 2006 Phys Rev D74 114017[9] Adare A et al 2007 Phys Rev D76 051106

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

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enables us to measure the forward and backward muons from the W decay We will get moreinsight into flavor separated quark and antiquark helicity distributions in the proton

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z+W

gt 30 GeVcTp|y|lt035

y-2 -15 -1 -05 0 05 1 15 2

LA

-1

-05

0

05

10+Z-W

gt 30 GeVcTp|y|lt035

DSSVDNS kkpDNS kreGRSV stdGRSV val

Figure 4 (color online)Longitudinal single-spinasymmetries for electronsand positrons from Wand Z decays The errorbars represent 68 CLThe theoretical curvesare calculated using NLOwith different polarizedPDFs

References[1] Alekseev M G 2010 Phys Lett B693 227ndash235[2] Airapetian A et al 2005 Phys Rev D71 012003[3] Adeva B et al 1998 Phys Lett B420 180ndash190[4] Adcox K et al 2003 Nucl Instrum Meth A499 469ndash479[5] Adare A et al 2009 Phys Rev D79 012003[6] de Florian D and Vogelsang W 2010 Phys Rev D81 094020[7] Nadolsky P M and Yuan C P 2003 Nucl Phys B666 31ndash55[8] Melnikov K and Petriello F 2006 Phys Rev D74 114017[9] Adare A et al 2007 Phys Rev D76 051106

19th International Spin Physics Symposium (SPIN2010) IOP PublishingJournal of Physics Conference Series 295 (2011) 012072 doi1010881742-65962951012072

5