DIS 2014, Warsaw, Apr. 28-May 2, 2014 1

Measurement of the Transverse Single-Spin Asymmetries for π0 and Jet-like Events at Forward

Rapidities at STAR in p+p Collisions at √s = 500 GeV

Mriganka Mouli Mondal (for STAR experiment)Texas A&M University

DIS 2014, Warsaw, Apr. 28-May 2, 2014 2

Outline

Transverse Single Spin Asymmetries (TSSA)Forward Meson Spectrometer in the STAR

experiment EM-Jets in forward and central rapidity AN measurements from RHIC Run 11 at √s = 500 GeV

DIS 2014, Warsaw, Apr. 28-May 2, 2014 3

TSSA - 2 theoretical frameworks

Spin-dependent transverse momentumdependent (TMD) function ST.(PxkT) Brodsky, Hwang, Schmidt, 02Collins, 02, Ji, Belitsky, Yuan, 02

Twist-3 quark-gluon correlations Efremov & Teryaev: 1982 & 1984Qiu & Sterman: 1991 & 1999

Sivers fct.

QLQCD QT/PT <<<<QT/PT

Transversemomentumdependent

Q>>QT>=LQCDQ>>pT

Collinear/twist-3

Q,QT>>LQCDpT~Q

Efremov, Teryaev;Qiu, Sterman

Need 2 scalesQ2 and pt

Remember pp:most observables one scale

Exception:DY, W/Z-production

Need only 1 scaleQ2 or pt

But should be of reasonable

sizeshould be applicable to most pp observables

AN(p0/g/jet)

Intermediate QTQ>>QT/pT>>LQCD

pp-p

A-Lo

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, Feb

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14 E

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AN

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π0 AN Measurements at Forward Rapidity

Rising AN with XF

AN nearly independent of √s No evidence of fall in AN with

increasing PT

Transverse Single Spin Asymmetry

Inclusive π0 production

xF = 2pZ/√s

DIS 2014, Warsaw, Apr. 28-May 2, 2014 5

500 GeV Isolated π0 results are

AN is higher with incerasing isolation radius AN in increasing with xF

DIS-

2013

(Ste

ven

Hepp

elm

ann)

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200 GeV Isolated π0 results DI

S-20

13 (S

teve

n He

ppel

man

n)

Events opposite “side photons” or “no” photons have similar AN

Same side photons lead to much reduced AN

DIS 2014, Warsaw, Apr. 28-May 2, 2014 7

RHIC : the world’s first and only polarized proton collider

Beams: √s 62.4 - 500 GeV pp

AGSLINACBOOSTER

Polarized Source

200 MeV Polarimeter

Hydrogen Jet Polarimeter

PHENIXSTAR

Siberian Snakes

Siberian Snakes

Spin Flipper

Carbon Polarimeters

RF Dipole AGS Internal Polarimeter

AGS pC Polarimeter

Strong Snake

Tune Jump QuadsHelical Partial Snake

Spin Rotators

For 2011 : Average Blue Beam Polarization = 51.6% (Transverse) Luminosity =22 pb-1

DIS 2014, Warsaw, Apr. 28-May 2, 2014 8

Forward ECAL in STAR

Forward Meson Spectrometer (FMS) :-- Pb glass EM calorimeter covering 2.5< η <4.0-- Detect 𝜋0,η, direct photons and jet-like events in the kinematic region where transverse spin asymmetries are known to be large.

FMS Pb Glass EM Calorimeter pseudo-rapidity 2.7<<4.0 Small cells: Outer cells: 3.81x3.81 cm 5.81 x 5.81 cm

9

BEMC

EEMC FMSFPD TPC

BBCZDC ZDC

VPDVPD

DIS 2014, Warsaw, Apr. 28-May 2, 2014

FMS photon reconstruction : towers clusters photon

shower shape fitting

BEMC+EEMC towers : to find central electromagnetic jets

FMS photons : to find forward electromagnetic jets

STAR detector cross view

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RHIC Run 11 (2011) pp @ √s=500 GeV

Forward Electromagnetic Jets (EM-Jets) Jet algorithm : anti-kT

R-parameter : 0.7 pT

EM-Jet > 2.0 GeV/c

Leading EM-Jets : defined as EM-Jets with highest energy. 2.8<ηEM-Jet<4.0 40 GeV < EnergyEM-Jet < 100 GeV (0.16 < xF < 0.4)

# Je

ts

Structure in EM-Jet pT :-- Acceptance non uniformity in small and large tower boundary inside FMS -- Different trigger threshold influence different pT region

Forward EM-Jet characteristics

2-photon jets are mostly π0

Events with more than 2 photons show jet-like energy flow

No. of photons in leading EM-Jets

γγ invariant mass 2-photon EM-jets

dE/d(ΔR) distribution of EM-Jets

EM-Jet Energy 60-80 GeVZ ϒϒ<0.8, no. photons =2

EM-Jet Energy 60-80 GeV

# Je

ts#

Jets

dE/d

(ΔR)

(ar

bitr

ary

scal

e)

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AN from fits

EM-Jet Energy = 55-57.5 GeV

For 2-photon isolated π0For each slice of data averaged over

~18 fills. Fits are well in control.

AN is calculated from p0 + P×AN cos(ϕ) fits over each fill on p0 = relative luminosity AN = asymmetry P = polarization --- AN’s are corrected for polarization values from RHIC-fills --- AN and χ2/NDF are calculated over entire fills

DIS 2014, Warsaw, Apr. 28-May 2, 2014 13

AN vs. EM-Jet Energy

π0-Jets –2γ-EM-Jets with mγγ <0.3 Zγγ <0.8

EM-Jets – with no. photons >2

Isolated π0’s have large asymmetries consistent with previous observation (CIPANP-2012 Steven Heppelmann)

https://indico.triumf.ca/contributionDisplay.pycontribId=349&sessionId=44&confId=1383 Asymmetries for jettier events are much smaller

DIS 2014, Warsaw, Apr. 28-May 2, 2014 14

AN vs. EM-Jet Energy

Isolated π0’s have large asymmetries consistent with previous observation (CIPANP-2012 Steven Heppelmann)

https://indico.triumf.ca/contributionDisplay.pycontribId=349&sessionId=44&confId=1383 Asymmetries for jettier events are much smaller

π0-Jets – 2γ-EM-Jets with mγγ <0.3 Zγγ <0.8

2γ-EM-Jets (η + continuum) - with mγγ > 0.3

EM-Jets – with no. photons >2

DIS 2014, Warsaw, Apr. 28-May 2, 2014 15

AN for different # photons in EM-Jets

1-photon events, which include a large π0 contribution in this analysis, are similar to 2-photon events

Three-photon jet-like events have a clear non-zero asymmetry, but substantially smaller than that for isolated π0’s

AN decreases as the event complexity increases (i.e., the "jettiness”

AN for #photons >5 is similar to that for #photons = 5

Jettier events

DIS 2014, Warsaw, Apr. 28-May 2, 2014 16

AN with midrapidity activities

η = 1.09,2.0η = 2.6-4.2

BEMC EEMCFMS

η = -1.0,1.0

central EMJets forward EMJets

• Case-I : having no central jet• Case-II : having a central jet

Jet algorithm : anti-kT, R = 0.7 pT

EM-Jet > 2.0 GeV/c, -1.0<ηEM-Jet<2.0

Inputs for central EMJets : towers from BEMC and EEMCLeading central EM-Jets : Jet with highest pT

Midrapidity EM Jets

DIS 2014, Warsaw, Apr. 28-May 2, 2014 17

Characteristics of Coincident Central EM-Jets (case-II)Energy flow within central EMJets

energy sharing (asymmetric scatterings)Forward-central correlations

For Central EMJets

pT distribution

pT balance (di-jet like)

central/forward central/forward

DIS 2014, Warsaw, Apr. 28-May 2, 2014 18

Δφ correlation between forward and central EMJets

Correlation is stronger for more N_photon Jets For higher EMJets energy, correlation grows stronger

Number of photons for forward EMJets : 1,2 3 and more

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AN for with and without a central EM-Jet

An EM-jet in the central rapidity region reduces the asymmetries for the forward isolated π0

0.06

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AN for the central jet : near and away in ϕ to the forward jet

Near and away side

Correlatedcentral EM-Jet

Uncorrelated central EM-Jet

Uncorrelated central EM-Jet is separated out

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AN for correlated central jets and no central jet cases

≈√

Asymmetries for the forward isolated π0 are low when there is a correlated away-side jet.

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Conclusion

EM-jets are reconstructed from photons detected in the FMS at STAR.

Jets with isolated π0 have large asymmetry. Three-photon jet-like events have a clear non-zero asymmetry, but

substantially smaller than that for isolated π0’s. AN decreases as the event complexity increases(i.e., the "jettiness”) Isolated π0 asymmetries are smaller when there is a correlated EM-

jet at mid-rapidity.

Both of these dependences raise serious question how much of the large forward π0 AN comes from 2 2 parton scattering.

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Backup ..

Systematics arising from intermixing of event classes PRD 86 051101(R) (2012)

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No EM-jet within -1<η<2 (pT>2.0GeV/c)

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With a EM-jet with -1<η<2 (pT>2.0GeV/c)