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Л.Ногач, ИФВЭ, Протвиноthe STAR Collaboration

Односпиновая асимметрия в образовании π0-мезонов в области фрагментации

поляризованного протонного пучка на установке

СТАР в Брукхейвене

STASTARR

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Motivation

• Contrary to simple pQCD predictions,Contrary to simple pQCD predictions, f first measurements of production in production in pp↑p↑p collisions found collisions found large transverse single-spin asymmetries. Similar large large transverse single-spin asymmetries. Similar large effects have recently been found in effects have recently been found in ee++ee-- and semi- and semi-inclusive deep-inelastic scattering.inclusive deep-inelastic scattering.

• Significant developments in theory in the past few years Significant developments in theory in the past few years suggest common origins for these effects, but suggest common origins for these effects, but large large transverse spin asymmetries intransverse spin asymmetries in pp↑+p↑+p → → + X+ X productionproduction are not yet fully understood.are not yet fully understood.

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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:

According to the latest theoretical developments, both mechanisms contribute to AN

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RHIC Polarized Collider

BRAHMS & PP2PP

STAR

PHENIX

AGS

LINACBOOSTER

Pol. H- Source

Spin Rotators(longitudinal polarization)

Siberian Snakes

200 MeV Polarimeter

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC PolarimeterStrong AGS Snake

Helical Partial Siberian Snake

PHOBOS

Spin Rotators(longitudinal polarization)

Siberian Snakes

1 MHz collision ratePolarization ~ 0.5-0.6

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STAR detector layout

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

Forward Meson Spectrometer

FPD/FPD++: ~ -3.7/3.3

FPD/FMS: ~ -4.1/2.5-4.0

1/2 of the FMS

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run/year Run2

2001-2002

Run3 Run5 Run6 Run8

2008

detector EEMC

and FPD

prototypes

6 matrices

of FPD

full FPD

(8 matrices)

East FPD

West

FPD++

East FPD

FMS

Pbeam, % ~15 ~30 ~45 ~55 ~45

0.15 0.25 0.1 6.8 ~7.8

3.8 ±3.3/±4.0 ±3.7/±4.0 -3.7/3.3 -4.1/

2.5-4.0

1, pbLdtsampled

η

Transverse spin runs at STAR with forward calorimetry: 2001→2008

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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

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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Runs 3-6: FPD, FPD++

• Inclusive 0 cross sections

• AN for inclusive 0 production

Detectors – from FPD to FMS

Run8 and beyond: FMS • FMS provides full azimuthal coverage

for range 2.5 4.0

• broad acceptance in xF-p

T plane for

inclusive ,,,K,… production in p+p and d(p)+Au

• broad acceptance for and from forward jet pairs

20x more acceptancethen previous detectors

three different positionsof modular detectors

single monolithic detector

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Published measurements with FPD

PRL 101, 222001 (2008)

• Large transverse single-spin asymmetries at large xF

• xF dependence matches Sivers effect expectations qualitatively

• pT dependence at fixed xF is not consistent with 1/pT expectation of theoretical model calculations

PRL 92, 171801 (2004)

√s=200 GeV, <η> = 3.8

Asymmetry revealed at lower energiespersists at √s=200 GeV

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Event selection done with:

• the calibration methodologies employed for the FPD have been successfully adapted to the FMS• offline cell-by-cell calibration using π0 peak position in the sorted by high tower mass distributions

minbias condition

hightower ADC threshold

(400/200 cts. for small/large cells)

(for small cells only)

fiducial volume cut (0.5 cell)

Run8 data analysis -calibration

7.0)(||

/2.007.0

2

2121

2

EEEEz

cGeVm

N

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Calorimeter stable at the level of a few percents

Minimal time dependence in π0 peak position is observed

LED system: critical calibration tool

LED monitoring system

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Data to simulations comparison

• Full PYTHIA/GEANT simulations have adequate statistics to reach moderate xF • Mass resolution in data is reasonable, given Run6 FPD performance• Simulations have somewhat better resolution than data

DATA SIMULATION

Present understanding sufficient; further investigations to be done

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First look at π0 AN from Run8 data

Octant subdivision of FMS for inclusive spin sorting

• AN comparable to prior measurements

• Azimuthal variation appears to be as expected

STAR preliminary

stat.errors only75% of Run8 data

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Summary• High precision AN measurements with the FPD allows for

a quantitative comparison with theoretical models.

• FMS was commissioned and fully operated in Run8. It

has 20x the acceptance of FPD (1264 channels

compared to 98 for the pair of FPD modules).

• Reconstruction and calibration procedures successfully

ported from FPD to FMS.

• Calibration is completed and data show good agreement

with the simulated sample of events.

• Inclusive 0 AN(xF) from the FMS is in agreement with the

FPD measurements.

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Outlook

Complete analysis of π0 asymmetry

Going beyond π0 detection:

- AN for a state that contains 0 pairs

- AN for states with heavier mesons

(η, ω, J/ψ…)

- direct photons, “jet-like” events – can help

to discriminate between Sivers and

Collins effects

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BACKUP

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π0 cross section measurementsPRL 97, 152302 (2006)

Cross-section is consistent with NLO pQCD calculations;

it was used in a recent global analysis of e+e-, SIDIS and pp data to determine fragmentation functions [PR D76, 074033 (2007)]

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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

To discriminate between the two effects we need to go beyond π0 detection to direct photons or jet-like events

Sensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

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Energy dependent corrections

• π0 peak position depends on the energy

• linear correction extracted from π0

peak position and being applied to photon energies works for both π0s and ηs, and significantly decreases shift from zero in δEγγ = Esimu -

Ereco.

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Association analysis

gensim EEE ,, • comparison of generated quantities to GEANT simulations

• we consider

Eliminating energy dependence in mass peak gives the correct average

neutral pion energy

UNCORRECTED CORRECTED

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Simulation and search algorithm for π0π0 in FMS and its engineering prototype

• it is possible to find events where the π0 pair originated at a significant distance from the vertex

• one source of such events are decays KS→π0π0 (31% branching fraction)

• plot shows the mass distribution for displaced vertices above 100 cm from the BBC vertex; a pronounced KS mass bump is visible