RHIC Spin Accomplishments, Plans and Issues
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Transcript RHIC Spin Accomplishments, Plans and Issues
Spin at RHIC
Present Status and Future Plans
OUTLINE
• Spin at RHIC: how its done, why its done, and who does it
• Status of ALL measurements at RHIC: sensitivity to DG
• Status of transverse single-spin asymmetries at RHIC
• Future plans
L.C. Bland, BNL
IWHSS08, Torino
2 April 2008
RHIC Spin Collaboration
Confederation of Groups Involved in the Effort
• Accelerator Physicists
Primarily BNL Collider-Accelerator Department
RHIC spin is a successful accelerator physics experiment
Essential contributions from RIKEN for spin at RHIC
• RHIC Experiments (PHENIX,STAR,BRAHMS)
RIKEN / RBRC, University and National Laboratory groups
RHIC spin planning fully integrated in experiment collaborations
• Polarimetry
Primarily BNL effort, with “detailees” from RHIC experiments
Essential measurements required for RHIC spin results
• Theory
RIKEN / RBRC, University and National Laboratory groups
RHIC spin results require QCD global analyses to extract physics
2
RHIC Spin Goals - I
How is the proton built from its known quark and gluon constituents?
As with atomic and nuclear structure, this is an evolving understanding
In QCD: proton is not
just 3 quarks !
Recall:
simple quark model
Rich structure of quarks
anti-quarks, gluons
3
RHIC Spin Goals - II
Understanding the Origin of Proton Spin
Spin Sum Rules
Longitudinal Spin
Transverse Spin
PRD 70 (2004) 114001
Understanding the origin of proton spin helps to understand its structure
4
RHIC Spin Goals - III
Milestones and Other Objectives
•
Direct measurement of polarized gluon distribution (DG)
using multiple probes
•
Direct measurement of flavor identified anti-quark
polarization using parity violating production of W
•
Transverse spin: connections to partonic orbital angular
momentum (Ly) and transversity (dS)
5
RHIC Spin Probes - I
Polarized proton collisions / hard scattering probes of DG
quark
pion or jet
quark
gluon
c
d dxa dxb dzc f a ( xa ) fb ( xb ) Dc ( zc )dˆ 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
RHIC Spin Probes - II
Unpolarized cross sections as benchmarks and heavy-ion references
0 + X, s = 200 GeV
p + pp
+ p, s = 200 GeV
arXiv:0704.3599 [hep-ex]
PRL 97 (2006) 252001
jets
PRL 92 (2004) 171801
direct g
PRL 98 (2007) 012002
Good agreement between experiment and theory
calibrated hard scattering probes of proton spin
7
RHIC
As a Polarized Collider
RHIC pC Polarimeters
BRAHMS & PP2PP
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
2005: Pblue = 49.3% +/- 1.5% +/- 1.4%
Pyellow= 44.3% +/- 1.3% +/- 1.3%
DP/P = 4.2% (goal=5%)
2006: 1 MHz collision rate;
Polarization=0.6 (online)
8
RHIC Run-6
-1
)
(pb
Integrated Luminosity
Plot by Phil Pile
100 x 100 Gev pp RUN05-06,
PHENIX Integrated Luminosity
Final delivered
An extraordinary Run-6!
45
40
35
30
25
20
15
10
5
0
RUN05
RUN06
Average Polarization 60%!
0
7
14 21 28 35 42 49 56 63 70 77 84 91
PHENIX Days in Physics mode
Outstanding luminosity and polarization performance from RHIC
for polarized proton collisions at s = 200 GeV
9
PHENIX Detector
EMCal
0/g/h detection
• Electromagnetic Calorimeter (PbSc/PbGl):
• High pT photon trigger to collect 0's,
h’s, g’s
• Acceptance: |h|<0.35, f 2 x /2
• High granularity (~10*10mrad2)
+/ • Drift Chamber (DC) for Charged Tracks
• Ring Imaging Cherenkov Detector (RICH)
• High pT charged pions (pT>4.7 GeV).
Relative Luminosity
• Beam Beam Counter (BBC)
ZDC
• Acceptance: 3.0< h<3.9
BBC
ZDC
• Zero Degree Calorimeter (ZDC)
• Acceptance: ±2 mrad
Local Polarimetry
• ZDC
• Shower Maximum Detector (SMD)
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STAR
Run-6 STAR detector layout
Detectors used for jets:
Time Projection Chamber,
|h|<1.4 Tracking
Barrel EM Calorimeter, |h|<1
Triggering & Calorimetry
Endcap EM Calorimeter,
1.09<h<2 Triggering &
Calorimetry
Beam-Beam Counters,
3.4<|h|<5 Triggering, 11
Luminosity, local Polarimetry
Longitudinal Two-Spin (ALL)
Status of probing for gluon polarization via
measurements of ALLfor midrapidity jet,0 production
12
ALL: 0
PHENIX Preliminary Run6 (s=200 GeV)
5
10
pT(GeV)
GRSV model:
“DG = 0”: DG(Q2=1GeV2)=0.1
“DG = std”: DG(Q2=1GeV2)=0.4
Statistical uncertainties now to
the point of distinguishing “std”
and “0” scenarios?
13
Run3,4,5: PRL 93, 202002; PRD 73, 091102;
hep-ex-0704.3599
From pT to xgluon
midrapidity neutral pion ALL
Each pT bin corresponds to a
wide range in xgluon, heavily
overlapping with other pT bins
• Data are not sensitive to variation
of DG(xgluon) within our x range
• Quantitative analysis needs to
assume some DG(xgluon) shape
log10(xgluon)
NLO pQCD: for 0, 2< pT < 9 GeV/c 0.02 < xgluon < 0.3
GRSV model: DG(0.02 < xgluon< 0.3) ~ 0.6DG(0 < xgluon <141 )
Constraining DG from 0 ALL
Similar approach as used for jets
Calculations by W.Vogelsang and M.Stratmann
“GRSV-std” scenario,
DG(Q2=1GeV2)=0.4, is excluded
by data on >3 level
• Only exp. stat. uncertainties are included
(the effect of syst. uncertainties is expected
to be small in the final results)
• Theoretical uncertainties are not included
15
Jet reconstruction in STAR
Data jets
MC jets
Midpoint cone algorithm
Geant
Detector
(Adapted from Tevatron II - hep-ex/0005012 )
•Seed energy = 0.5 GeV
•Cone radius in h-f
• R=0.4 (2005)
e, , g ,
, p, etc
Pythia
Particle
• R=0.7 (2006)
•Splitting/merging fraction f=0.5
Use Pythia+GEANT to quantify
detector response
q, g
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Final 2005 Inclusive ALLjets Results
STAR hep-ex arXiv:0710.2048
0.2 < h < 0.8
Data are compared to
predictions within the
GRSV framework with
several input values of DG.
B. Jager et.al, Phys.Rev.D70, 034010
GRSV-std
The inclusive measurements give
sensitivity to gluon polarization over a
broad momentum range
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Constraining DG from jet production ALL
Before global analyses…
• Compute ALL in NLO pQCD varying integral DG, but maintaining GRSV shape
• Perform 2 analysis between calculations and measured jet ALL
Vogelsang and Stratmann
GRSV DIS
GRSV DIS best fit=0.24
1 DG varies from -0.45 to 0.7
PRD 63, 094005 (2001)
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STAR
Run 2006 Data
Improved FOM
Luminosity :
Polarization:
Barrel EMC h coverage:
2 4.7 pb-1
50% 60% (online polarization)
[0,1] [-1,1]
In addition
Jet Cone Radius: 0.4 0.7
-0.7 < hjet axis < 0.9
Neutral Energy Fraction < 0.85
Increased trigger thresholds
Inclusion of Endcap EMC towers
Improved tracking at large |h|
STAR Preliminary
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STAR
2006 Inclusive ALLjets
-0.7 < h < 0.9
GRSV curves with
cone radius 0.7
and -0.7 < h < 0.9
Using jet patch trigger (DhxDf = 1x1 patch of towers) only
Statistical uncertainties are 3-4 times smaller than
2005 in high pT region (pT>13 GeV/c)
ALL systematics
(x 10 -3)
Reconstruction +
Trigger Bias
[-1,+3]
(pT dep)
Non-longitudinal
Polarization
~ 0.03
(pT dep)
Relative
Luminosity
0.94
Backgrounds
1st bin ~ 0.5
Else ~ 0.1
pT systematic
6.7%
20
STAR
2006 Inclusive ALLjets
*Theoretical uncertainties not included
GRSV DIS best fit=0.24
1 = -0.45 to 0.7
PRD 63, 094005 (2001)
GRSV DIS
Within GRSV framework:
Dg_std excluded with 99% CL
Dg<-0.7 excluded with 90% CL
21
Summary of ALL Measurements
•
Data accumulated through RHIC run 6 for inclusive 0 and jet ALL has reached
high statistical significance to constrain DG in the limited x range (~0.02-0.3)
DG is found to be consistent with zero, to date
Theoretical uncertainties (x dependence) are significant
Future Plans for Probing DG
• Improve statistical precision of midrapidity 0,jet ALL measurements and extend
measurements to higher pT
• Determine x-dependence of DG via correlation measurements: jet1+jet2 and g+jet
• Extend gluon-x range probed to lower x via measurements of ALL at larger rapidity
and higher s
• Global analysis in NLO pQCD of RHIC data and HERMES,COMPASS data
22
Transverse Single-Spin Asymmetries (AN)
Probing for orbital motion within transversely polarized protons
23
Expectations from Theory
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/c
Kane, Pumplin and Repko PRL 41 (1978) 1689
24
A Brief History…
p + p + X
s=20 GeV, pT=0.5-2.0 GeV/c
• QCD theory expects very small
(AN~10-3) transverse SSA for particles
produced by hard scattering.
• The FermiLab E-704 experiment
found strikingly large transverse singlespin effects in p+p fixed-target
collisions with 200 GeV polarized
proton beam (s = 20 GeV).
•
•
0 – E704, PLB261 (1991) 201.
+/- - E704, PLB264 (1991) 462.
25
Two of the Explanations for Large Transverse SSA
Spin-correlated kT
Collins/Hepplemann mechanism
requires transverse quark polarization
and spin-dependent fragmentation
Sivers mechanism
requires spin-correlated transverse
momentum in the proton (orbital motion).
SSA is present for jet or g
initial
state
final
state
26
Require experimental separation of Collins and Sivers contributions
Transverse Single-Spin Asymmetries
World-wide experimental and theoretical efforts
• Transverse single-spin asymmetries are observed in semi-inclusive
deep inelastic scattering with transversely polarized proton targets
HERMES (e); COMPASS (m); and planned at JLab
• Collins fragmentation function is observed in hadron-pair production
in e+e- collisions (BELLE)
• Intense theory activity underway
SPIRES-HEP: search title including:
“Transverse spin, Transversity, single spin”
Total number: 625
(1968~2006)
Experimental results ~14%
27
Spin Effects in the Forward Direction
Status prior to RHIC run 6
J. Adams et al. (STAR), PRL 92 (2004)
171801; and PRL 97 (2006) 152302
D. Morozov, for STAR [hep-ex/0512013]
Transverse SSA persist at large xF at RHIC energies
where unpolarized cross sections are calculable 28
STAR Results vs. Di-Jet Pseudorapidity Sum
Run-6 Result
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!
arXiv:0705.4629
STAR
29
High Precision Analyzing Powers
(2003 - 2006)
STAR
B.I. Abelev, et al,
hep-ex/0801.2990
Precision measurements at s = 200 GeV
provide stringent contraints on the models…
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High Precision Analyzing Powers
(2003 - 2006)
STAR
B.I. Abelev, et al,
hep-ex/0801.2990
No model fully describes the precision data
More experimental (and theoretical) work needed…
31
32
PHENIX Goes Forward
First results with muon piston calorimeter from run 6
p+p0+X, s = 62 GeV
Transverse SSA persists with similar characteristics over
a broad range of collision energy (20 < s < 200 GeV)
33
Summary
Transverse Single Spin Asymmetry (SSA) Measurements
• Feynman-x dependence of large-rapidity pion production shows large transverse
SSA at RHIC energies, where cross sections are described by NLO pQCD
• Feynman-x dependence of large-rapidity transverse SSA are consistent with
theoretical models (Sivers effect orbital motion / twist-3 calculations)
• The pT dependence of large-rapidity 0 transverse SSA does not follow theoretical
expectations
• Direct measurement of spin-correlated kT (Sivers effect) via midrapidity di-jet spin
asymmetries completed in RHIC run 6 and found consistent with zero.
• Cancellations found in theory calculations subsequent to measurements also
expect small di-jet spin asymmetries at midrapidity.
34
Future RHIC Spin Plans
•
Measure parity violation for W
production to determine flavor
dependence of quark and antiquark
polarization.
•
Requires completion of PHENIX
muon trigger upgrade and STAR
forward tracking for charge sign
discrimination.
35
Plans for Transverse Polarization Measurements
RHIC run 8 and beyond
•
Experimental separation of Collins and Sivers effects via transverse single-spin
asymmetry measurements for large rapidity - production
•
Extend measurements of transverse single spin asymmetries from hadron
production to prompt photon production, including away-side correlations
•
Develop RHIC experiments for a future measurement of transverse single spin
asymmetries for Drell-Yan production of dilepton pairs
Transverse-Spin Drell-Yan Physics at RHIC
L. Bland, S.J. Brodsky, G. Bunce, M. Liu, M. Grosse-Perdekamp, A.
Ogawa, W. Vogelsang, F. Yuan
http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf
36
STAR
Forward Meson Spectrometer
Quantifying possible color glass condensate via
forward 0 and correlations in d+Au versus p+p at sNN=200 GeV
North FMS half
before sealing
Recycled FNAL E831 lead-glass for FMS
p+p data at s = 200 GeV
also acquired
Recorded 8 pb-1 in run 8
37
Sivers in SIDIS vs Drell Yan
Transverse-Spin Drell-Yan Physics at RHIC
L. Bland, S.J. Brodsky, G. Bunce, M. Liu, M. Grosse-Perdekamp, A.
Ogawa, W. Vogelsang, F. Yuan
http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf
• Important test at RHIC of the fundamental QCD
prediction of the non-universality of the Sivers
effect!
• requires very high luminosity (~ 250pb-1)
38
Non-universality of Sivers Asymmetries:
Unique Prediction of Gauge Theory !
Simple QED
example:
DIS: attractive
Drell-Yan: repulsive
Same in QCD:
As a result:
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Experiment SIDIS vs Drell Yan: Sivers|DIS= − Sivers|DY
*** Test QCD Prediction of Non-Universality ***
Sivers Amplitude
HERMES Sivers Results
RHIC Drell Yan Projections
0
0
Markus Diefenthaler
DIS Workshop
Műnchen, April 2007
0.1
0.2
0.3 x
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Rapidity and Collision Energy
Large rapidity acceptance required to probe valence quark Sivers function
41
Backup
42
Benchmarking Simulations
p+p J/+X l+l-+X, s=200 GeV
PHENIX,
hep-ex/0611020
e+e|h|<0.35
m+m-
1.2<|h|<2.2
J/ is a critical benchmark that must be understood before Drell-Yan
43
Dilepton Backgrounds
Drell-Yan
J/
’
cc bb
Isolation needed to discriminate open heavy flavor from DY
44