Overview of Orbital Angular Momentum via TMD Measurements

in Hadronic Collisions

L.C. Bland

Brookhaven National Lab

ECT*, Trento 28 August 2014

OUTLINE

• General comments

• Transverse SSA for ppX

• Transverse SSA for ppjetX

• Outlook

• Conclusions

8/28/2014 2

Introductory Comments…

• This is at least the second ECT* workshop on OAM. There has been progress, but there remain many open questions for both experiment and theory

• The role of polarized proton collisions, and more generally hadronic interactions with polarization, needs to be continually emphasized [see below].

• As an experimentalist, “to measure” has special meaning… In my opinion, experiments never measure distribution functions. Instead, experiments measure [spin-dependent] cross sections that are interpreted by theory, ideally to provide universal distribution [or fragmentation] functions. Comparing measurements to theory is the way we understand hadronic structure.

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ANDY

Schematic of Measurement ApparatusRHIC for Spin

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

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Measured QuantitySTARSTAR

arXiv:1405.5134

• Helicity asymmetry for inclusive jet production is measured as a function of pT.

• Measurements are sensitive to <x>~2pT/s

8/28/2014 5

gluon

quark pion or jetquark

RHIC Spin ProbesProton collisions / collinear factorization

Describe p+p particle production at RHIC energies (s 62 GeV) using perturbative QCD at Next to Leading Order,

relying on universal parton distribution functions and fragmentation functions

cabccbb

cbaaacba dzDxfxfdzdxdxd

ˆ)()()( ,,

8/28/2014 6

Implications of Measurement

de Florian, Sassot, Stratmann, Vogelsang PRL 113 (2014) 012001 / arXiv:1404.4293

Evidence for polarization of gluons from global NLO fit to preliminary version of inclusive jet data from STAR, neutral pion data from PHENIX and polarized deep inelastic scattering

8/28/2014 7

Why Study Hadronic Collisions?

• Many examples where new particles are first observed in the interactions of hadrons: J/ (hidden charm); (hidden beauty); W,Z0 (weak bosons); H (Higgs boson), as just a few well-known examples.

• Hadro-production has provided many examples of first observation of unexpected phenomena: transverse single-spin asymmetries (SSA) - both analyzing power [AN] for production and induced polarization [P] for production; Lam-Tung violation in Drell-Yan [see backup for details/references]; …

• Hadro-production provides direct sensitivity to gluons: unpolarized gluon PDF, especially to low-x ; gluon spin contribution (G)

• Hadro-production is pursued because of emergent phenomena: quark-gluon plasma ; onset of gluon saturation?

• Hadro-production can provide an important test of universality – are the quantities measured in hard scattering processes really telling us about the structure of the proton?

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What Measurements Can Be Sensitive to OAM?

What would we see from this gedanken experiment?

F0 as mq0 in vector gauge theories, so AN ~ mq/pT

or,AN ~ 0.001 for pT ~ 2 GeV/cKane, Pumplin and Repko PRL 41 (1978) 1689

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Transverse SSA in p+p CollisionsInclusive 0 AN persists to large s

J. Adams et al. (STAR), PRL 92 (2004) 171801; and PRL 97 (2006) 152302

Even though the kinematics of the SIDIS measurement and the forward data have little overlap, it was possible to account for most of the features of the RHIC data by calculations based on phenomenological fits to the SIDIS data

Data: B.I. Abelev et al. (STAR), submitted to PRL [arXiv:hep-ex/0801.2990]

Theory (red): M. Boglione, U. D’Alesio, F. Murgia PRD 77 (2008) 051502.

Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013

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Inclusive 0 AN at s=62.4 GeV

• Muon piston calorimeter provides PHENIX access to large rapidity

• Observe large AN that is iincreasing with xF, as seen at higher and lower s

arXiv:1312.1995

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Inclusive Transverse SSA at Midrapidity

• pT range of midrapidity production comparable to pT values for large-xF production.

• Why is large xF so important for transverse SSA?

arXiv:1312.1995

PRL 91 (2003) 241803

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Inclusive Transverse SSA at Midcentral Rapidity

• Mid-central cross section consistent with NLO pQCD within scale uncertainties

• Mid-central transverse SSA are consistent with zero, as expected by twist-3 model.

STARSTARarXiv:1309.1800

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xF and pT dependence of AN for p+p±+X, s=62 GeV

• AN(+) ~ -AN(-), consistent with results at lower s and u,d valence differences

• At fixed xF, evidence that AN grows with pT

I. Arsene, et al. PRL101 (2008) 042001

8/28/2014 14

Forward Pion Transverse SSA Versus s

RHIC s=62.4 GeV

FNAL s=19.4 GeV

BNL s=6.6 GeV

ANL s=4.9 GeV

Forward pion analyzing power in p+p collisions exhibits similar xF dependence over a broad range of s

Aidala, Bass, Hasch, Mallot RMP 85 (2013) 655 / arXiv:1209.2803

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What are the issues?

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Issue 1 – Inclusive production does not distinguish initial-state versus final-state kT

Other mechanisms have been suggested in recent years

Collins mechanism requires transverse quark polarization and spin-dependent

fragmentation

Sivers mechanism requires spin-correlated transverse

momentum in the proton (orbital motion) and color-charge interaction. SSA is

present for jet or

final state

initial state

8/28/2014 17 PRL 99 (2007) 142003

Emphasizes (50%+ ) quark Sivers

AN consistent with zero

~order of magnitude smaller in pp di-jets than in semi-inclusive DIS quark Sivers asymmetry!

VY 1, VY 2 are calculations by Vogelsang & Yuan, PRD 72 (2005) 054028

Idea: directly measure kT by observing momentum imbalance of a pair of jets produced in p+p collision and attempt to measure if kT is correlated with incoming proton spin

Boer & Vogelsang, PRD 69 (2004) 094025

jet

jet

AN pbeam (kT ST)

pbeam into page

Midrapidity Di-Jet ProductionMidrapidity Di-Jet Production

STARSTAR

8/28/2014 18

Issue 2 - Factorization

• Factorization is used for inclusive particle production for collinear distribution and fragmentation functions, and in general, works well for RHIC energies

• TMD factorization is not proven for inclusive (or, more complicated) hadro-production, although has been proven for Drell-Yan production.

• Twist-3 collinear calculations are based on factorization. Moments of qg correlators from twist-3 analyses are related to the Sivers function

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Issue 3 – Initial-State versus Final-State Interactions

DIS: attractive Drell-Yan: repulsive

Simple QEDexample:

Same in QCD:

In general, particle production in p+p collisions will mix initial-state (DY-like) and final-state (SIDIS-like) interactions. Present understanding is that p+pX is DY-like “sign-mismatch” between SIDIS and p+pX transverse SSA [arXiv:1103.1591]

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SummaryTransverse SSA for Inclusive Pion Production

• Large s pion production cross sections are consistent with NLO pQCD

• Large AN is found to increase with xF over a very broad range of s

• xF and pT dependences are now disentangled, with AN increasing with pT to a plateau

• Pion asymmetries are consistent with zero at central and mid-central rapidities

Go beyond inclusive pion production to test present understanding

jets, Drell-Yan production, direct photons

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From Pions to Jets

21

Left/right symmetric HCal

Left/right symmetric ECal

Left/right symmetric preshower

Trigger/DAQ electronics

Blue-facing BBC

Beryllium vacuum pipe

Forward JetsANDY at IP2

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Data/Simulation Comparison

Jet pT and xF are calculated ignoring mass good agreement between data and simulations above trigger threshold

Jet data is well described by simulation

Tower multiplicity Jet pT

30-50GeV

50-70GeV

70-90GeV

Jet shape

8/28/2014 23

Forward JetsarXiv:1304.1454

• Forward jet cross section is consistent with NLO pQCD [Mukerjee & Vogelsang, PRD 86 (2012) 094009 / arXiv:1209.1775]

• Forward jet AN is consistent with Sivers effect from SIDIS [twist-3 (Gamberg, Kang, Prokudin, PRL 110, 232301(2013) / arXiv:1302.3218) and generalized parton model (M. Anselmino, et al PRD 88.054023 / arXiv:1304.7691)]

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24

Jet-like versus JetCaveat emptor

ECal triggered jet biases towards EM rich jets • more like AN

• bias extends well beyond the trigger threshold

Run11 data onlycone-jet algorithm

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Regarding Bias…(and calibrations)

• Pure EM response of HCal has nearly all aspects accounted for in full simulation relative to data []

• Pure hadronic response of HCal has most aspects accounted for in full simulation relative to data [p and conjugate]

• Pure hadronic response (that is sensitive to hadronic energy calibration) has most aspects accounted for in full simulation relative to data [K*Kp and conjugate]

Jets include both electromagnetic and hadronic components

There are many canceling effects for jet-related observables

Control of detector biases are critical

8/28/2014 26

Outlook-I

• RHIC remains the only accelerator with polarized proton beams. The large s of the collider has established via cross section measurements that particle production can be explained by NLO pQCD.

• Large acceptance RHIC experiments (STAR and PHENIX) are well instrumented at midrapidity. Spin asymmetries, in general, increase in the forward direction (valence quark phenomena?)

• Efforts are underway at both STAR and PHENIX to improve forward detector capabilities, because of the important path to a future electron-ion collider

• Economic realities may require clever reuse of existing equipment to start on the path…

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Attractive vs Repulsive Sivers EffectsUnique Prediction of Gauge Theory !

DIS: attractive Drell-Yan: repulsiveSame in QCD:

As a result:

Transverse Spin Drell-Yan Physics at RHIC (2007)

http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf

• For now, RHIC is the only accelerator with polarized beams

• RHIC should pursue polarized DY, in kinematics that match as closely as possible those from SIDIS forward

• Although STAR-forward is similar to ANDY, detailed simulation studies are still required

8/28/2014 28

Forward Instrumentation for 2016?“STAR-Forward”

Some caveats:

• Suitability of E864 calorimeter was investigated in the 2014 RHIC run

• Radiation resistance of silicon photomultipliers was investigated in the 2014 RHIC run

• Suitability of GEM tracker to be demonstrated by efficiency measurements

• Future efforts are pending review

eEMC

Existing E864 calorimeter

from IP2

FPS (preshower)TPC

bEMC

Existing forward GEM tracker

(redistributed in z)

0.5 T solenoid

What are prospects for polarized Drell-Yan with such a forward detector?

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Status of Run-14 Test

6x6 stack of calorimeter cells mounted at 730 cm from interaction point and 35 cm from beam

Pair-mass from forward calorimeter for 3He+Au collisions [online reconstructions / offline calibration in progress]

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Prospects for Polarized DY in 2016When STAR-Forward is Realized…

• Accounts for acceptance of STAR-forward calorimeter

• Assumes data sample of 400 pb-1 in 2016

• Background evaluations are in progress…

• QCD backgrounds to DY production are constrained from existing di-jet measurements [arXiv:1308.4705]

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Conclusions

• Polarized proton collisions have provided important information regarding proton structure

• Transverse SSA asymmetries for p+pX are large in the forward direction over a broad range of s. Spin-averaged cross sections can be described by NLO pQCD at RHIC energies where transverse SSA are large.

• There are many issues of interpreting p+pX transverse SSA, particularly in regard to the role of the Sivers function which is related to partonic OAM

• Forward p+pjet+X have small, positive analyzing powers. Cross sections are in agreement with NLO pQCD. This addresses some, but not all, issues regarding OAM

• Forward p+p*+X may still be possible at RHIC on a time scale relevant to address the predicted sign change