Transverse Single-Spin Asymmetries for Jet-like events at Forward Rapidities in p+p Collisions at √s = 500 GeV

with the STAR ExperimentMriganka Mouli Mondal (for STAR Collaboration)

Texas A&M University

APS April 5-8, 2014. Savannah, GA 1

Forward Meson Spectrometer in the STAR experiment Transverse Single Spin Asymmetries (TSSA) EM-Jets measured from FMS photons AN measurements from run-11 at √s = 500 GeV

Outline :

RHIC : the world’s first polarized proton collider

APS April 5-8, 2014. Savannah, GA 2

AGSAGSLINACLINACBOOSTERBOOSTER

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

Beams: √s 200 - 500 GeV pp; 50-60% polarization for year 2011

Forward ECALs in STAR

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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 asymmetries are known to be large.

FMS

TSSA - 2 theoretical frameworks

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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>=LQCD

Q>>pT

Collinear/twist-3

Q,QT>>LQCD

pT~QEfremov, 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 QT

Q>>QT/pT>>LQCD

π0 AN Measurements at Forward Rapidity

APS April 5-8, 2014. Savannah, GA 5

Rising AN with XF

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

increasing PT

CIPANP 2012,Steve Heppelmann

Transverse Single Spin Asymmetry

Inclusive π0 production

xF = 2pZ/√s

Run-2011 data

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p+p √s = 500 GeV transverse datasets Jet algorithm : anti-kt R-parameter : 0.7 pT

EM-Jet > 2.0 GeV/c

FMS photons with pT > 0.001 GeV fed into anti-kt

Leading EM-Jets : Multi-photon Jets with highest energy

EM-Jets used to find asymmetry within 2.8<ηEM-Jet<4.0 40 GeV < EnergyEM-Jet < 100 GeV

# Je

ts

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

EM-Jet characteristics

APS April 5-8, 2014. Savannah, GA 7

2-photon jets are mostly π0

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

No. of photons in leading EM-Jets

ϒϒ 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

) (

arbi

trar

y sc

ale)

APS April 5-8, 2014. Savannah, GA 8

EM-Jet Energy = 55-57.5 GeV

For 2-photon isolated π0

AN from fits

Individual point and χ2/NDF from averages over ~18 fills

AN is calculated from p0 + p1 cos(ϕ) fits over each fill p0 = relative luminosity, p1 = asymmetry AN’s are corrected for polarization values from fill to fill Weighted AN and χ2/NDF are calculated over entire fills

AN vs. EM-Jet Energy

APS April 5-8, 2014. Savannah, GA 9

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

AN vs. EM-Jet Energy

APS April 5-8, 2014. Savannah, GA 10

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

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

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

AN for different # photons in EM-Jets

APS April 5-8, 2014. Savannah, GA 11

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 #photons = 5

Summary

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

Jets with isolated π0 have large asymmetry as seen before.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”).

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Backup slides

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PRD 86-051101

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Systematics arising from intermixing of event classes

PYTHIA simulations

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Lavg: +15%Pavg: +8%

2013 P~55%

>=