RHIC Spin Accomplishments, Plans and Issues

Report
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 )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
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)
10
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
16
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
17
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)
18
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
19
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?
F0 as mq0 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…
30
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+p0+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:
39
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
40
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

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