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Spin-dependent forward particle correlations in p+p collisions at = 200GeV

Nikola PoljakUniversity of Zagreb

(For the STAR Collaboration)

Forward Spin Physics at STARRHIC, BNL

STASTARR

s

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Single Spin Asymmetry

• Definition:

• dσ↑(↓) – differential cross section of when

incoming proton has spin up(down)

dd

ddAN

0, xF<0 0, xF>0

Left

Right

p p

positive AN:

more 0 going left to polarized beam

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Published measurements at STAR

PRL 97, 152302 (2006)

nucl-ex/0602011

At this energy the cross-section is consistent with NLO pQCD (run2 + run3)

STAR: arXiv:hep-ex/0801.2990

accepted for publication in PRL

RUN 6• Large transverse single-spin asymmetries at large xF

• xF dependence matches Sivers effect expectations qualitatively

• pT dependence at fixed xF not consistent with 1/pT expectation of pQCD-based calculations

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Developments in theory and experiment

• Expectations that the Collins effect is suppressed in p↑+p → +X (PRD 71 014002) were found incorrect due to a sign error (arXiv:0804.3047)

The need remains to separate Collins and Sivers effects in p↑+p → +X

• new phenomenological analyses within a generalized parton model can explain both Sivers moments in semi-inclusive deep inelastic

scattering and many features of p↑+p → +X. (PRD 77 051502(R))

• ~20% of the COMPASS transversely polarized proton data has been analyzed and reported.  COMPASS finds non-zero Collins moments and Sivers moments compatible with zero, although expected Sivers

moments are small in the x,Q2 range of their experiment. (Levorato, for COMPASS; Ferrara, 2008).

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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 jet-like events

Sensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

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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 will provide 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 acceptancethan previous detectors

uses 3 different settingsof modular detectors

uses a single monolithic detector

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New FMS CalorimeterLead Glass From FNAL E831804 cells of 5.8cm5.8cm60cmSchott F2 lead glass

Students prepare cells at test Lab at BNL

Forward Meson Spectrometer (FMS)

Cockcroft-Walton HV bases with computer control through USB. Designed/built in house for FEU-84.

Designed and built at Penn State University

Small Cell PSU Type224 of 476

Readout of 1264 channels of FMS provided by QT boards. Each board has

• 32 analog inputs

• 5-bit TDC / channel

• Five FPGA for data and trigger

• Operates at 9.38 MHz and higher harmonics

• Produces 32 bits for each RHIC crossing for trigger

• 12-bit ADC / channelDesigned and built at UC Berkeley/SSL

QT board

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Calibration

Event selection done with:

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/19.008.0

2

2121

2

EEEEz

cGeVm

N

Offline calibration done cell-by-cell

included energy corrections

the calibration methodologies employed

for the FPD have been successfully adapted to

the FMS

minbias condition

Hightower ADC threshold

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

<0.7 (small) ; <1.0 (large cells)

fiducial volume cut (0.5 cell)

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Calorimeter stable at level of ~1%.

Minimal run-by-run dependence in mass peak observed

LED system : critical calibration tool

MIT (LED optics)

UC Berkeley/SSL (flasher boards)

Texas / Protovino / BNL (assembly)

SULI program (Stony Brook students) / BNL

(control electronics)

Details of data analysis - calibration

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Association analysis – energy corrections

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

• We consider

Eliminating energy dependence in mass peak gives the correct average

neutral pion energy

UNCORRECTED CORRECTED

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Distributions comparison• Full PYTHIA/GEANT simulations have adequate statistics to reach

moderate xF at large pT

• Cell mass resolution in data is reasonable, given run-6 FPD performance

• Simulations have somewhat better resolution than dataDATA SIMULATION

Present understanding sufficient; further investigations to be done

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• intercompare reconstructed PYTHIA+GSTAR events against reconstructed data

• verify spin information from STAR local polarimeter• extract transverse single spin asymmetries from FMS from

data for p↑+ p -> 0 + X as a point of contact with previous work

• extract transverse single spin asymmetries from FMS from data for p↑+ p -> “jet-like” + X final state.

• FMS - a new device, with many more channels (1264 detectors compared to 98 for north/south FPD modules).

• FMS has 20x more acceptance than the previous modular detectors• the FMS involves the large cells, not used in the FPD

• methodologies used in FPD successfully adapted to FMS

Summary & goals

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BBC polarization time dependence samples

Star preliminary

Flat line fits shown

Correlation of multiplicity topology in beam-beam counter (BBC) with polarization direction turns out to be good polarimeter for s = 200 GeV

see J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1

For run-8 data, analysis of BBC asymmetries, using effective analyzing powers from run-6, is effective quality assurance for the FMS analysis

Every run for which there is FMS data, also has BBC data.

Po

lari

zati

on

Relative polarimetry consistent with CNI

14First look at analysis results

Octant subdivision of FMS for inclusive spin sorting.

• AN comparable to prior measurements

• Azimuthal variation appears to be as expected

• Systematic errors being evaluated

• First estimate tot. 1.2 stat.

STAR preliminary

stat.errors only75% of run-8 data

15First look at “jet-like” eventsEvent selection done with:

• >15 cells with energy > 0.4GeV in the event (no single pions in the event sample)

• cone radius = 0.5 (eta-phi space)

• “Jet-like” pT > 1 GeV/c ; xF > 0.2

• 2 perimeter fiducial volume cut (small/large cells)

The agreement between data and simulation looks

convincing

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Conclusions

• FMS is complete and in place. Commissioned and

operated in run-8. It has 20x the acceptance of FPD

• Reconstruction and calibration procedures successfully

ported from FPD to FMS

• Calibration is mostly complete and data shows good

agreement with the simulated sample of events

• Inclusive 0 AN(xF) from FMS is comparable to FPD

precision measurements

• analysis of jet-like events is under way

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Outlook

• Complete analysis of “jet-like” events

• Determine AN(pT) for p↑+ p -> 0 + X

• Determine AN for final state that contains 0 pairs

• Determine AN for final states with heavier mesons

• Run-9 - Go beyond detection to direct photons + jet final

state AN

THANK YOU

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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:

How can the cross section depend on the proton transversity?

1. Proton quark scattering is insensitive to transverse spin. However, the quark retains its initial spin after a hard scattering, and the quark π0 fragmentation can have azimuthal dependence on the transverse spin of the quark. This process is referred to as the Collins Effect. [Nucl. Phys. B396, 161 (1993)]

2. A quark inside a proton may have orbital angular momentum that is correlated to the spin of the proton. If two quarks with opposite transverse momentum contribute different scattering amplitudes to the same final state, a case can be made where the proton quark scattering is sensitive to the transverse spin of the proton. This process is referred to as the Sivers Effect. [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]

Collins Effect

SP

p

π0

π0

pq

sq

kTπ

P (any polarization)

sq = Spin of the struck quark

pq = Momentum of the struck quark

kTπ = Transverse momentum of the neutral pion

p + p + X

The spin of the scattered quark is correlated with the spin of the proton

The fragmentation of the quark to 0 has sq dependence

Spin of the proton affects the scattering angle through the spin of the large x quark

xy

z

Sivers Effect

SP

Pp

π0

π0

pq

kTq

P (any polarization)

Sp = Spin of the proton

Pp = Momentum of the proton

kTq = Transverse momentum of the quark inside the proton

Quark Parton Distribution Function has kT

q dependence

p + p + X

Spin of the proton affects the scattering angle through the quark transverse momentum

Quark transverse momentum is correlated

with the spin of the proton

xy

z

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Background fitting

df/dx = S / [σ(2π)½] exp[-(x-μ)2/2σ2] + B[β2 / (t1 - t2)](x - x0)exp[-β(x - x0)];

S: Gaussian peak integral,μ: Gaussian peak centroid,σ: Gaussian width,B:  integral of background function for |xi - μ| < 3σ,xP:  background peak position,β:  background exponential falloff parameter.

S and B are spin dependent

Tuned 2-γ fit, especially for Large cells.  Reduced from 6 to 5 parameters by fixing Eγγ

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Energy-dependent corrections

0 peak position depends on the energy

• Linear correction extracted from 0 peak position and being applied to photon energies

• works for both 0s and ηs, and significantly decreases shift from zero in E = Esimu - Ereco.

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Resolution smearing

• A data-driven model is applied to

introduce irresolution to the

simulation

• This smearing is taken from the

individual detector performance, as

measured from high-tower

associated invariant mass

• Applying this to the full

PYTHIA/GSTAR simulations of the

small cells results in a better match

between simulation and data

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Simulation and search algorithm for 00 in FMS and its engineering protoype

• Without Z vertex information in the calculation above, it is possible to find events where the 0 pair originated at a significant distance from the origin

• One source of such events are decays KS→00 (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