Transcript The STAR Detector at RHIC

Transverse Spin Results from STAR
William Christie, BNL
Mini-symposium on Orbital Motion of Quarks in Hard Scattering I
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and
the Physical Society of Japan
September 21, 2005.
Outline
• Physics motivation
• Brief introduction to the STAR Detector
• STAR Transverse polarization Data sets
• Cross section measurements
• Forward 0 asymmetry
• Future measurements
• Summary
STAR
STAR
The STAR Collaboration
500+ collaborators
52 institutions
14 countries
Austria: Bern
Brazil:
Sao Paolo
China: IHEP-Beijing, IMP-Lanzhou, Shanghai INR, Tsinghua, USTC, IPP-Wuhan Croatia: Zagreb
Czech Republic: Nuclear Physics Institute-AS-CR
England: Birmingham
France: IReS - Strasbourg, SUBATECH-Nantes
Germany: Frankfurt, MPI-Munich
India: Bhubaneswar, Jammu, IIT, Panjab, Rajasthan, VECC-Kolkata
Netherlands: NIKHEF
Poland: Warsaw U. of Technology
Russia: JINR - Dubna, IHEP – Protvino, MEPHI - Moscow
S. Korea: Pusan
U.S.:
Argonne, Berkeley, Brookhaven National Laboratories
UC Berkeley, UC Davis, UCLA, CalTech, Creighton, Carnegie-Mellon, Indiana, Kent State,
Michigan State, CCNY, Ohio State, Penn State, Purdue, Rice, Texas, Texas A&M, Valparaiso,
Washington, Wayne State, Yale Universities
STAR
Physics Motivation
1
Proton Spin:
2

(DS)  DG + L
1
2 Quark spin
Gluon Spin Angular momentum
It has been determined, through polarized deep inelastic scattering experiments,
that the quarks alone can not account for the spin of the proton (i.e. DS ~ 0.2)
To account for the spin of the proton, either the gluons are polarized and/or
there are significant contributions to the protons spin from the orbital motion of
its constituents.
Would like to unravel the contributions to transverse spin asymmetries (an area
of intense recent theoretical development) from:
a) quark transverse spin preferences in a transversely polarized proton (p)
 “transversity”  quark property decoupled from gluons
b) quark and gluon transverse motion preferences in p
 spin-kT correlation related to quark/gluon orbital ang. mom.
Triggering
Barrel EM
Calorimeter
STAR
STAR Detector
-1<η< 1
  - ln(tan(q/2)
Lum. Monitor
Local Polarim.
Beam-Beam
Counters
Forward
TPC
2.8 <  < 3.8
Central Trigger
Barrel
Forward Pion
Detector
= -1
2<|η|< 5
=0
=2
Silicon
Vertex
Tracker
Triggering
Endcap EM
Calorimeter
1<η< 2
-4.1<η< -3.3
Time Projection
Chamber  Tracking
-1<η< 1
Solenoidal Magnetic
Field (5 kG) analyzes
transverse momentum
pT of charged particles
2005
2004
2003
STAR
Transverse Polarization Data sets and FPD Configurations
2002 Run: <Pb> ~ 15%, Lint = 0.3 pb-1
Forward 0 Detector (FPD, aka pEEMC)
- 24 layer Pb-scintillator sampling calorimeter
- 2 orthogonal planes of finely segmented
triangular scintillator strips (Shower-Maximum
Detector, or SMD)
- 2 Preshower layers
2003 Run: <Pb> ~ 30%, Lint = 0.5
Upgraded Forward 0 Detector (FPD)
pb-1
East of STAR
North
South
Top
• Pb-glass EM calorimeter
(from IHEP Protovino, used in E704)
• Shower-Maximum Detector (SMD)
• Preshower
Bottom
Spin asymmetries in proton-proton collider
Requires 3 different process/measurements
Single Spin Asymmetries (F.o.M = P2L)
A 
1
P
1
= P
(



N  RN
N  RN
)
R 
L
L
N L  N R 
N R  N L
N L  N R 
N R  N L
 AN with left-right

 symmetric detectors
Double Spin Asymmetries (F.o.M = P4L)
1
A PP
1 2
(
N RN
N RN
)
R=
L
L
Polarization Pattern at STAR:
“Bunch/Spin sorting”
Up to 120 bunches in RHIC
Bunch Spacing 107nsec (9MHz)
Alternating spin pattern
Bunch/Spin sorted scaler system
interactions (kHz)/crossing
N = spin dependent yields of process interest
L = yield of luminosity monitoring process
R = relative luminosity between different spin configuration
P = beam polarization(s) from polarimeter at RHIC
Also need direction of polarization vector at IR
STAR
bunch crossing number at STAR IR
Spin Up
Spin Down
Unpolarized
STAR
BBC gives triggering, (Rel.) Luminosity, and local polarimetry.
BBC’s register hits for ~50% of tot (pp); EW
coinc. discriminates against beam-gas bkgd. for
good L monitoring; segmentation  local
polarimeter with ANobs.~0.006.
Interaction
Vertex
Left
*
Top
Right
Bottom
2.1 <||< 5.0
• Statistical uncertainty: dRstat ~10-4 -10-3
• Systematic uncertainty ( beam-gas
background ) < 10-3
L
R
1

L
Example of R
BBC East
3.3<||< 5.0 (inner tiles)
R  1 and time dependent!
BBC West
05/16/03
05/30/03
Time [Run Number]
Positive xF
Negative xF
• = BBC L/R asym.
BBC
 = BBC T/B asym.
YCNI
BCNI
Forward 0 production in a hadron collider
p
E
d N
xqp
qg
q
xgp
p
Au
EN
2E 
s
s  2E N
E
z

q
Eq
  ln(t an( ))
2
p 
xg  T e g
xq  xF / z
s
(collinear approx.)
Q 2 ~ pT2
0
E
qq
STAR
xF 
• Large rapidity  production (~4) probes asymmetric partonic collisions


p  p   ,  3.8, s  200GeV
0
• Mostly high-x valence quark + low-x gluon
• 0.3 < xq< 0.7
<z>

<xq>
• 0.001< xg < 0.1
• <z> nearly constant and high 0.7 ~ 0.8
NLO pQCD
Jaeger,Stratmann,Vogelsang,Kretzer
• Large-x quark polarization is known to be large from DIS
• Directly couple to gluons = A probe of low x gluons
<xg>
2002 STAR Forward 0 Detector (aka pEEMC)
STAR
0 reconstruction at
E=20~80GeV, 1 <pT < 4 GeV 3<<4
Event
Display
SMD
EMC
o Cluster separation in shower
maximum detector and measured
calorimeter energy serves as input to the
0 mass determination.
M
Run 2 Results: Forward 0 Inclusive Cross Section
STAR
• STAR data at
•= 3.8 (hep-ex/0310058,
Phys. Rev. Lett. 92 (2004) 171801)
• = 3.3 (hep-ex/0403012,
Preliminary)
• NLO pQCD calculations at fixed
 with equal factorization and
renormalization scales = pT
• Solid and dashed curves differ
primarily in the g  
fragmentation function
STAR data consistent with Next-to-Leading Order pQCD calculations
in contrast to data at lower s (Bourrely and Soffer, Eur.Phys.J. C36 (2004) 371-374)
Run 2 Results: Large Analyzing Powers at RHIC
STAR
First measurement of AN for forward π0 production at s=200GeV
STAR collaboration, hep-ex/0310058,
Phys. Rev. Lett. 92 (2004) 171801
STAR
Similar to FNAL E704 result at s = 20 GeV
In agreement with several models including
different dynamics:
 Sivers: spin and k correlation in initial
state (related to orbital angular
momentum?)
 Collins: Transversity distribution
function & spin-dependent
fragmentation function
 suppressed? (hep-ph/0408356)
First shown at spin2002
 Qiu and Sterman (initial-state) / Koike
(final-state) twist-3 pQCD calculations
• pT dependence?
• Spin dependence in jet?
• xF<0?
•AN with mid-rapidity correlation?
Run 3 Results: AN for forward & backward 0 production at s=200GeV
STAR
Statistical error only for <>=4.1
Positive AN at large positive xF has
been confirmed
 Larger significance to be non-zero &
positive than published data
The first measurement of negative xF
AN has been done, and is consistent
with zero
 Sensitive to twist-3 gluon-gluon
correlation
Run 3 Results: Add 0 cross sections at  = 4.0
STAR
xF and pT range of the data
Different position for FPD relative to the
beam, relative to already accumulated 2002
data, allows mapping of AN in xF and pT plane
to begin
Outlook
• Disentangling the dynamics of AN via
• Higher precision AN measurement vs xF and pT
• AN with mid rapidity correlation
• Forward jet
• Proposal for forward calorimeter upgrade
• Heavy mesons and direct photons
• Low x gluons in nuclei
• Mid rapidity jets
• Di-jet kT balance  gluon Sivers function
STAR
Current FPD
=4.2
=3.2
~2.4m square
~1500 cells
• Inside jet particle correlation  Collins function * Transversity
=2.5
Summary
STAR
• Forward hadron production at hadron-hadron collider selects
high-x (thus high polarization) quark + low-x gluon scatterings
• Inclusive cross section is consistent with NLO pQCD
calculations and PYTHIA(LO pQCD + parton showers)
• Analyzing power for forward 0 mesons at large positive xF was
found to be large and positive
• The first measurement of negative xF AN has been done, and is
consistent with zero
• Accumulation of significant (O 10 pb-1, P  50%) transverse
polarization data set expected in upcoming FY06 RHIC run.
Expect to start extracting information on dynamics responsible
for transverse spin asymmetries.
STAR
Backup
Coincidence Transverse Spin Measurements Should Unravel
STAR
Transversity, Collins, Sivers Effects
 Study transversity by exploiting chiral-odd fragment’n “analyzing
powers” (Collins or interference frag. fcns.) calibrated at BELLE
 Search for spin-dependent transverse motion preferences inside
proton (related to parton Lorbit ) via predicted leading-twist spindependent deviation from back-to-back alignment of di-jet axes 
study unique to RHIC spin

p
AN
q
g
+
pp  dijet + X
s = 200 GeV
8  pT(1,2)  12 GeV
|(1,2)|  1.0
p
spin
p
Jets with 2
hadrons detected

p
q
q
p
+…
D. Boer & W.
Vogelsang
predictions
STAR projections for 30 pb1,
Pbeam=70%
D
parton
kT
STAR
Analyzing Powers at Mid-Rapidity
Do processes invoked in forward
scattering show up at large angles?
STAR Collab. Phys. Rev. Lett. 92 (2004) 171801
Measure
Jet


1 YDijet  YDijet
AN 


Pol YDijet
 YDijet
For given
parton at
some x
kTL=kTR
XF 
2Eπ0
D. Boer and W. Vogelsang,
Phys.Rev. D 69 (2004) 094025
s
Sivers Function – correlation between kT and spin
S P  (Pp  k  )
1 N
ƒ q (x, k , s P )  ƒ q (x, k )  Δ q ƒ q (x, k  )
2
S P PP k 


Jet
Partonic kT from Dijet Analysis
4.1 x 10 -4
STAR
Sivers Effect Predic
T. Henry
σ = 0.23 ± 0.02 ±
0.03
0.05
D. Boer and W. Vogelsang,
Phys.Rev. D 69 (2004) 094025
AN
8 < pT1,2 < 12 GeV
|η1,2 | < 1
kT =  <kT>2 = ET sin (σ)
ET = 13.0 ± 0.7sys → Trigger Jet
STAR agrees
well with World
Data on
Partonic kT
STAR
kT distribution
d
• Curves are for various gluoni
Sivers functions
• Connection to partonic orbita
angular momentum
• Suppressed by Sudakov effe