Measurements of Transverse Spin Effects with the Forward Pion Detector of STAR

Larisa NogachInstitute of High Energy Physics, Protvino

for the STAR collaboration

Outline• Motivation and previous measurements• Detector layout• Status of the analysis of Run 6 data and results • Plans for Run 7 and beyond• Summary

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Motivation

Phenomenological models expect that 0 analyzing power decreases with increasing pT

hep-ph/0609238 AN for production at RHIC at √s=200 GeV

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Existing measurements at STARPRL 92, 171801 (2004)

√s=200 GeV, <η> = 3.8

nucl-ex/0602011

Cross-section is consistent with NLO pQCD calculations

Asymmetry revealed at lower energiespersists at √s=200 GeV

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Transverse spin runs at STAR

with forward calorimetry: 2001→2006 Run2 Run3 Run5 Run6

detector EEMC

and FPD

prototypes

6 matrices

of FPD

full FPD

(8 matrices)

East FPD

West FPD++

~15 ~30 ~45 ~60

0.15 0.25 0.1 6.8

3.8 ±3.3/±4.0 ±3.7/±4.0 -3.7/3.3

%,BEAMP

∫ −1, pbLdt

><η

sampled

FOM (P2L) in Run 6 is ~50 times larger than from all the previous STAR runs

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STAR detector layout

• TPC: -1.0 < η < 1.0• FTPC: 2.8 < η < 3.8• BBC : 2.2 < η < 5.0• EEMC: 1 < η < 2• BEMC: -1 < η < 1• FPD++/FPD:

η ~ 3.3/-3.7

FPD++: engineering test of the Forward Meson Spectrometer

Run 6 – FPD++

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Detector acceptance

• Strong correlation between xF and pT in the individual detectors

• Rapid change in number of events for either increasing pT at fixed xF or increasing xF at fixed pT

• Broader pT range in xF bins when combining data at <η>=3.3 and 3.7

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Details of data analysis

Event selection:

2

2121

/19.008.0

)5.0(cutvolumefiducial

7.0)(||

2

)/15/25(

thresholdenergysummed

conditionminbias

cGeVm

cell

EEEEz

N

FPDWestEasttheforGeV

<<

<+−=

=

γγ

γγγγγγ

γ

Calibration: on hardware level offline cell-by-cell energy and time-dependent corrections−> is known at the level of 2%

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Details of data analysis

runN

NrunN

A

AA

δ><−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅+⋅

⋅−⋅⋅=

↓↑↓↑

↓↑↓↑

LRRL

LRRL

BeamN

NNNN

NNNN

PA

1

Analyzing power is measured by “cross-ratio” method with two-arms (left-right) detector:

Run-by-run comparison with meanis consistent with statistics, exceptright near the threshold

<η>=3.7<η>=3.3

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π0 AN at √s=200 GeV – xF-dependence

• AN at positive xF grows with increasing xF

• AN at negative xF is consistent with zero

• Run 6 data at <η>=3.7 are consistent with the existing measurements

• Small errors of the data points allow quantitative comparison with theory predictions

• Theory expects the reverse dependence on η

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AN(pT) at xF > 0.4

Run3+Run5 data (hep-ex/0512013):

Run6 data:

• more precise measurements

• consistent with the previous runs in the overlapping pT region

• complicated dependence on pT

• Online calibration of CNI polarimeter• Hint of AN decrease with increasing pT at pT~1-2 GeV/c

residual xF-dependence? => AN mapping in (xF,pT) plane is required

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AN(pT) in xF-bins

• Combined data from three runs at <η>=3.3, 3.7 and 4.0

• In each xF bin, <xF> does not significantly changes with pT

• Measured AN is not a smooth decreasing function of pT

as predicted by theoretical models

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Jet-like events

• PYTHIA simulations: - trigger on small cells; sum over entire side - vector sum of photon momenta reproduces most forward hard-scattered parton

• Expect 4-5σ effect from existing Run 6 data

if observed π0 AN is due to Sivers effect

hep-ex/0602012• Is the single spin asymmetry observed for 0 also present for the jet the 0 comes from?

• Answer discriminates between Sivers and Collins contributions

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Run-7 FMS as seen from STAR interaction point

Forward Meson Spectrometer for Run 7• FMS will provide full azimuthal coverage for

range 2.5 η 4.0

• broad acceptance in xF-p

T plane for inclusive

γ,,,K,… production in p+p and d(p)+Au

• broad acceptance for γ and from forward jet pairs to probe low-x gluon density in p+p and d(p)+Au collisions

spacers

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Summary

In Run 6, 6.8 pb-1 of data with average beam polarization ~60% has been acquired in p+p collisions at √s=200 GeV

(xF,pT)-mapping of the transverse single spin asymmetry of inclusive 0 production at forward rapidity has been done • AN at positive xF grows with increasing xF • AN at negative xF is consistent with zero • Measured AN at “fixed” xF up to xF~0.5 does not decrease with increasing pT as expected by the theory • High precision of the measurements allows for a quantitative comparison with theoretical models and distinguishing between different dynamics

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Outlook

Analysis of the data at √s=62 GeV is underway

Jet-like events: • detector response simulation has been started • calibration of the FPD++ outer matrices needs to be completed

Construction of the FMS is ongoing − expected to be completed by Run 7

Backup

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Possible mechanisms

• Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]: Flavor dependent correlation between the proton spin (Sp), proton momentum

(Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to:

• Collins effect [Nucl. Phys. B396, 161 (1993)]: Correlation between the quark spin (sq), quark momentum (pq) and transverse

momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form:

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Separating Sivers and Collins effects

Collins mechanism: asymmetry in the forward jet fragmentation

Sivers mechanism: asymmetry in the forward jet or γ production

SPkT,q

p

p

SP

p

p

Sq kT,π

Need to go beyond π0 detection tojets and direct photons

Sensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

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Single Spin Asymmetry

• Definition:

• dσ↑(↓) – differential cross section of when incoming proton has spin

up(down)

Two methods of measurements: • Single arm calorimeter:

R – relative luminosity (by BBC)

Pbeam – beam polarization

• Two arm (left-right) calorimeter:

No relative luminosity needed

↓↑

↓↑

+−=

σσσσ

dd

ddAN

↓

↑

↓↑

↓↑

=⎟⎟⎠

⎞⎜⎜⎝

⎛

+−

⋅=L

LR

RNN

RNN

PA

beamN

1

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅+⋅

⋅−⋅⋅=

↓↑↓↑

↓↑↓↑

LRRL

LRRL

BeamN

NNNN

NNNN

PA

1

π0, xF<0

π0, xF>0

Left

Right

p p

positive AN: more 0 going

left to polarized beam

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Run6 − FPD++

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Cell-by-cell calibration

gains are determined from π0 peak position in 2γ invariant mass distributions sorted by high towers accuracy of the calibration is at the level of ~2%

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Energy-dependent corrections

π0 peak position depends on the energy – due to energy leakages and ADC granularity (dedicated MC study has been done)

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Run-dependent corrections

The FPD response vary with time/beam conditions:

used for spin sorting

Correlation between FPD gains and BBC rates

(<xF>,<pT>)

pT bin <pT>, GeV/c <xF>

0.5-1.1 0.95 0.273

1.1-1.4 1.26 0.282

1.4-1.9 1.61 0.278

1.9-2.4 2.10 0.273

2.4-5.0 2.53 0.284

pT bin <pT>, GeV/c <xF>

0.5-1.2 1.06 0.321

1.2-1.7 1.47 0.322

1.7-2.2 1.87 0.330

2.2-2.7 2.41 0.323

2.7-5.0 2.85 0.330

0.25<xF<0.30 0.30<xF<0.35

0.5-1.5 1.29 0.368

1.5-1.9 1.70 0.370

1.9-2.4 2.08 0.374

2.4-3.0 2.64 0.373

3.0-5.0 3.18 0.377

0.5-1.7 1.45 0.423

1.7-2.2 1.94 0.426

2.2-2.7 2.38 0.433

2.7-3.3 2.94 0.430

3.3-5.0 3.54 0.435

0.35<xF<0.40 0.40<xF<0.47

0.5-2.0 1.70 0.496

2.0-2.3 2.16 0.496

2.3-2.9 2.52 0.507

2.9-3.4 3.10 0.508

3.4-5.5 3.79 0.506

0.47<xF<0.56

distributions in pT binsirunN

NirunN

A

AA

δ><−

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Cross section − separated xF and pT dependence

( )

6

5

13

3

≈

≈

−∝ −

B

N

pxdp

dE B

TN

F

σ

Similar to ISR analysis:J. Singh et al., Nucl. Phys. B140, 189 (1978)

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Jet-like events

Expect that jet-like events Expect that jet-like events

are are ~~15% of 15% of events events