Transcript Overview of Orbital Angular Momentum via TMD Measurements

Overview of Orbital Angular
Momentum via TMD Measurements
in Hadronic Collisions
OUTLINE
• General comments
• Transverse SSA for ppX
• Transverse SSA for ppjetX
• Outlook
• Conclusions
L.C. Bland
Brookhaven National Lab
ECT*, Trento 28 August 2014
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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Schematic of Measurement Apparatus
RHIC for Spin
RHIC pC Polarimeters
BRAHMS
& PP2PP
ANDY
Absolute Polarimeter (H jet)
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
Spin Rotators
(longitudinal polarization)
Pol. H Source
LINAC
BOOSTER
Helical Partial Siberian Snake
200 MeV Polarimeter
AGS
AGS pC Polarimeter
Strong AGS Snake
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STAR
Measured Quantity
• Helicity asymmetry for inclusive jet
production is measured as a
function of pT.
• Measurements are sensitive to
<x>~2pT/s
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arXiv:1405.5134
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RHIC Spin Probes
Proton collisions / collinear factorization
quark
quark
pion or jet
gluon
c
d     dxa  dxb  dzc f a ( xa ) fb ( xb ) Dc ( zc )dˆ ab
a ,b,c
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
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Implications of Measurement
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
de Florian, Sassot, Stratmann, Vogelsang
PRL 113 (2014) 012001 / arXiv:1404.4293
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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 L
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?
F0 as mq0 in vector gauge theories, so AN ~ mq/pT
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or,AN ~ 0.001 for pT ~ 2 GeV/c
Kane, Pumplin and Repko PRL 41 (1978) 1689
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Transverse SSA in p +p Collisions
Inclusive 0 AN persists to large s
J. Adams et al. (STAR), PRL 92 (2004) 171801; and
PRL 97 (2006) 152302
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
Even though the kinematics of the SIDIS measurement and the forward 0 data have
little8/28/2014
overlap, it was possible to account for most of the features of the RHIC data 9by
calculations based on phenomenological fits to the SIDIS data
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 0 Transverse SSA at Midrapidity
PRL 91 (2003) 241803
arXiv:1312.1995
• pT range of midrapidity 0 production comparable to pT values for large-xF 0 production.
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• Why is large xF so important for transverse SSA?
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Inclusive 0 Transverse SSA at Midcentral Rapidity
arXiv:1309.1800
STAR
• Mid-central 0 cross section consistent with NLO pQCD within scale
uncertainties
• Mid-central 0 transverse SSA are consistent with zero, as expected by twist-3
model.
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xF and pT dependence of AN for p+p±+X,
s=62 GeV
I. Arsene, et al. PRL101 (2008) 042001
• AN(+) ~ -AN(-), consistent with results at lower s and u,d valence differences
• At fixed xF, evidence that AN grows with pT
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Forward Pion Transverse SSA Versus s
ANL
s=4.9 GeV
BNL
s=6.6 GeV
FNAL
s=19.4 GeV
RHIC
s=62.4 GeV
Aidala, Bass, Hasch, Mallot RMP 85 (2013) 655 / arXiv:1209.2803
Forward pion analyzing power in p+p collisions exhibits similar xF dependence
over a broad range of s
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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
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 g
initial
state
final
state
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mechanisms have been suggested in recent years 16
Midrapidity Di-Jet Production
VY 1, VY 2 are calculations by
Vogelsang & Yuan, PRD 72 (2005) 054028
AN pbeam
 (kT(50%+
 S)T)
Emphasizes
jet
quark Sivers
Boer & Vogelsang, PRD 69
(2004) 094025
pbeam
into page
jet
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
AN consistent with zero
~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS
quark Sivers asymmetry!
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STAR
PRL 99 (2007) 142003
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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) hadroproduction, 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 FinalState Interactions
Simple QED
example:
DIS: attractive
Drell-Yan: repulsive
Same in QCD:
In general, particle production in p+p collisions will mix initial-state (DY-like) and finalstate (SIDIS-like) interactions. Present understanding is that p+pX is DY-like 
“sign-mismatch” between SIDIS and p+pX transverse SSA [arXiv:1103.1591]
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Summary
Transverse 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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Forward Jets
ANDY at IP2
Left/right symmetric HCal
Left/right symmetric
ECal
Trigger/DAQ
electronics
From Pions to Jets
Blue-facing
BBC
Left/right symmetric
preshower
Beryllium vacuum
pipe
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Data/Simulation Comparison
30-50GeV
50-70GeV
70-90GeV
Tower multiplicity
Jet pT
Jet shape
Jet pT and xF are calculated ignoring mass
good agreement between data and simulations above trigger threshold
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Jet data is well described by simulation
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Forward Jets
arXiv: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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Jet-like versus Jet
Caveat emptor
Run11 data only
cone-jet algorithm
ECal triggered jet biases towards EM rich jets
• more 0 like AN
• bias extends well beyond the trigger threshold
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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 [0gg]
• Pure hadronic response of HCal has
most aspects accounted for in full
simulation relative to data [L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
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Control of detector biases are critical
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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 Effects
Unique Prediction of Gauge Theory !
Same in QCD:
DIS: attractive
Drell-Yan: repulsive
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
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• Although STAR-forward is similar to ANDY,
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detailed simulation studies are still required
Forward Instrumentation for 2016?
“STAR-Forward”
Some caveats:
0.5 T solenoid
Existing forward
GEM tracker
(redistributed in z)
• Suitability of E864
calorimeter was
investigated in the 2014
RHIC run
eEMC
• 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
TPC
Existing E864
calorimeter
from IP2
FPS
(preshower)
bEMC
What are prospects for polarized Drell-Yan with
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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
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Pair-mass from forward calorimeter for
3He+Au collisions [online reconstructions
/ offline calibration in progress]
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Prospects for Polarized DY in 2016
When 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+pX 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+pX transverse SSA, particularly in
regard to the role of the Sivers function which is related to partonic OAM
• Forward p+pjet+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g*+X may still be possible at RHIC on a time scale relevant to
address the predicted sign change
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