W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ.
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Transcript of W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ.
Probing the Proton’s Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results
W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ.
STARSTAR
Introduction STAR longitudinal spin - Recent results
The polarized proton collider RHIC
The STAR detector
Topics: “Topics: “SliceSlice” of STAR Spin Physics Status & ” of STAR Spin Physics Status & OverviewOverview
STAR Transverse spin di-jets - Recent results
STAR EMC Calorimetry! Summary
Look at near future and outlook
Spin Problem & DIS Gluon Helicity Preference Spin Problem & DIS Gluon Helicity Preference ConstraintsConstraints
World data (2005) on g1p = ½
ei2 [qi (x,Q2) + qi (x,Q2)]
All fixed-target data
Only ~30% of proton spin
arises from q and q helicity preferences !
limited info on scaling
violations, on shape or integral of
gluon helicity
preference g(x,Q2).
)GeV1(@
8.1
2
dxg
)GeV5,( 2xgx4.0
g
7.1 g
Only valence
quarks are strongly polarized
gz
qz
pz LLGS
2
1
2
1
ssd
duu
RHIC p+p: Pert QCD Probe of Spin-Dep Partonic RHIC p+p: Pert QCD Probe of Spin-Dep Partonic StructureStructure
pp hX
“soft” parton distribution functions
“soft” frag. function
“hard” dQCD parton-parton
∧
Theory ingredients: pQCD factorization
LO pQCD
+ large parton-level
2-spin sensitivity
• Prefer dominant/”clean” reaction mechanism w/ large “aLL” ( -jet)
• But jet and 0 rates are sufficient to give significant G constrain in initial RHIC polarized p data
Inclusive cross-section (jets, , and ±)
Good agreement with NLO pQCD over many orders of magnitude
PRL 97, 252001 (2006)2003+2004
PLB 637, 161 (2006)
STARSTAR
Hadron inclusive
production
1st RHIC inclusive Jet x-sec
pQCD works!
also direct photon incl. @ PHENIX;
forward incl. @ STAR & incl chg.
hadrons @ BRAHMS
jets
Xpp o
2005 STAR Preliminary
±
Relativistic Heavy Ion Collider 100 GeV beam proton beams
Each bunch filled with a distinct polarization state
Spin Rotators at STAR IR allow for transverse and longitudinal spin orientation
Bunch Xings every 100-200ns
CNI polarimeters + Hydrogen Jet target provide run by run & absolute polarization
HJT calibr to ~ 5% goal in progress
…world’s 1st Collider
p p
pp Run Year FOM=P4L 2002 2003 2004 2005
2006
< Polarization> % 15 30 40-45 45-50 60
Lmax [ 1030 s-1cm-2 ] 2 6 6 16 30
Lint [pb-1 ] at STAR (L/T) 0 / 0.3 0.3 / 0.25 0.4 / 0 3.1 / 0.1 8.5 / 3.4,6.8
BRAHMS
PHENIX
AGS
BOOSTER
Spin Rotators(longitudinal polarization)
Solenoid Partial Siberian Snake
Siberian Snakes
200 MeV PolarimeterAGS Internal Polarimeter
Rf Dipole
RHIC pC PolarimetersAbsolute Polarimeter (H jet)
AGS pC Polarimeters
Strong Helical AGS Snake
Helical Partial Siberian Snake
Spin Rotators(longitudinal polarization)
Spin flipper
Siberian Snakes
STAR
PHOBOS
Pol. H- SourceLINAC
1
2
3
4
jet patch
size:1x1 x
7
8
9
10
11
12
+13,…,18
Allocated Jet Rate to tape: ~15 Hz
1x1 Jet patch ET/GeV
4 8
2006 rate ~ 2.5 Hz, sent to tape without prescaling
Level 0 High Tower & Jet Triggering in STARLevel 0 High Tower & Jet Triggering in STAR
2006 rate ~ 150 Hz, combine with L2 trigger to fit in limited bandwidth
Trigger either on
HT: 1 (of 4800 BEMC or 720 EEMC) tower ET > thresh - Or
JP: 1 (of 12 BEMC or 6 EEMC) hard-wired jet patch ET > thresh.
[note: also a minbias cond]
BARREL EMC
ENDCAP EMC
• Theoretically allowed range in Δg:
• GRSV-STDGRSV-STD: Higher order QCD analysis of polarized DIS
experiments!
Predicted ALL sensitivity for different G scenarios
Inclusive ALL measurements (, ±, and jets)
±
jets
Neutral pions (in Jets); Inclusive 0 ALL
/ndf compared to NLO calculations(ignoring systematic errors): GRSV Std: 0.8 GRSV Max: 2.5 GRSV Min: 0.8 GRSV Zero: 0.4 GRSV max scenario disfavored
z pt 0
ptjet
use invariant mass spect. (data from HT trig)
– MC 0 line shape– low inv. mass bkgrd – comb. bkgrd & residual fit
assoc w/ full Jets if within a Jet cone (0.4 in , ) but typ. ~ 5 deg.
2005 data
“geant” jets
“pyt
hia”
jets
Jet Reconstruction in STARJet Reconstruction in STAR
part
on
part
icle
dete
ctor
etcp
e
,,
,
gq,
DataData SimulationSimulation
GEANT
pyth
ia
theo
ry
Search over all possible seeds (pTseed >
0.5 GeV) for stable groupings
• Check midpoints between jet-jet pairs for stable groupings
• Split/merge jets based on Eoverlap
• Add all track/tower 4-momenta
Use cone radius:
• = 0.4 for half-BEMC 2003-5
• = 0.6-0.7 for full B+EEMC 2006
22 coneR
Full jet reconstruction uses “midpoint-cone” algorithm (hep-ex/0005012):
Data well described by MC
Inclusive Jet ALL and x-sec Analysis Issues
Use Simulation (MC) to provide correction to RAW jet yield - trigger and jet inefficiencies - jet resolution & bin migration - undetected particles (n + ) - PYTHIA 6.205 CDF Tune A - GEANT (Geisha)
Verification of DATA/MC agreement essential
The shape of the Fraction of Neutral Energy in the Jet (EMF) is sensitive to the trigger bias as well as contributions from beam background.
C pT PYTHIA(pT )
GEANT(pT )
Minbias• JP2• HT2
STAR Preliminary
On average PARTICLE Jets are reconstructed in the DETECTOR with 20% increase in pT due
to ~25% jet resolution + steep jet pT distribution
Systematic offsets in pT cause dilution of the jet asymmetry which depend on the size of the asymmetry!
Effects of Jet Resolution
Trigger Bias The trigger biases jets toward higher neutral energy. This may change the ratios of qq+qg+gg and therefore change the asymmetries
10 20 30 pT(GeV)
Trigger Bias: JP << HTALL/ALL larger at low pT
Total Systematic:
ALL ALLPARTICLE ALL
TRIGGER
g = g (max)g = -g (min) g = 0GRSV-STD
2005 STAR preliminary
Systematic error band
Measured Jet PT (GeV/c)
Error bars are statistical Systematic band includes 25% scale error from current polarization uncertainties due to online values Results in good agreement with ’03/’04 ALL data* in region of overlap but ~ 4 times more precise and pT range nearly doubled [* PRL 97,252001 (2006)]
Systematic
(x 10 -3)
False Asymmetries
<6.5 (pT dep’t)
Reconstruction + Trigger Bias
2-12 (pT dep’t)
Non-long Polarization
3
Relative Luminosity
2
Backgrounds
<1
2005 Inclusive jet ALL at mid-rapidity
Vogelsang and Stratmann
Significant new constraints on G when compared to predictions derived from one global fit to DIS data
GRSV DIS best fit=0.241 = -0.45 to 0.7
PRD 63, 094005 (2001)
GRSV DIS
Constraint on G
Inclusive Jet (Inclusive Jet ()) Data Data from 2006 -> Greater from 2006 -> Greater Discr’m Power for Discr’m Power for gg
High-statistics (esp. at high pT) inclusive jet and 0 ALL data from 2006 will select among g models, assuming a shape of g(x,Q2).
Need global analysis including these ALL results!
Significant increase in sampled luminosity Polarization typically ~60% acceptance in BEMC
increased by a factor of 2significant increase in figure of merit!
G=G
GRSV-stdG=-G G=0
Projected statistical uncertainties for STAR 2006 inclusive jet ALL
jetjet
also analysis w/ Endcap EMC + vs. - analysis in Barrel
Di-Jet Asymmetry Measurements with Di-Jet Asymmetry Measurements with STAR; Sivers Transverse Spin AsymmetriesSTAR; Sivers Transverse Spin Asymmetries
Needs ISI and/or FSI to evade time-reversal violation
Assuming QCD factorization, subsume ISI/FSI contributions in gauge-invariant kT -dependent parton distribution fcns.
Characterize by Sivers effect:
Sensitive to parton orbital angular momentum.
0processobserved
partonTprotonproton kps
pproton
sproton
kTparton ?
kTparton ?
Do quarks and/or gluons have transverse motion preferences in a proton polarized transverse to its momentum?
Do we observe q Sivers consistent
w/ HERMES, after inclusion of proper
pQCD-calculable ISI/FSI gauge link factors
for pp jets? Tests limited TMD
“universality”.
First direct measurement of gluon Sivers effects.
Expect Collins and Boer-Mulders asy’m contributions
small: no charge-sign bias from EMC-only
L2 trigger jet reconstruction & small role of q-q scattering
at probed pT
2006 p + p run at RHIC; 3 wks transverse spin @ STAR
Motivation for pp Motivation for pp Di-Jet Di-Jet MeasurementMeasurement
zx
y
Colliding beams
proton spin
parton kTx
HERMES transverse spin SIDIS asymmetries u and d quark Sivers
functions of opposite sign, different magnitude.
Sivers effect in pp spin-dependent sideways boost to di-jets, suggested
by Boer & Vogelsang (PRD 69, 094025 (2004))
Both beams polarized, x +z x z can
distinguish high-x vs. low-
x (primarily
gluon) Sivers effects.
spin
bisector
Jet 1
Jet 2
Reco cos(bisector) measures
sign of net kTx for event
=+2
= -1
TPC
EMC Barrel
EMC Endcap
BBC East BBC West
Yellow (-z) beamBlue (+z)
STAR EMC-Based (Level 0 + 2) Di-Jet Trigger in STAR EMC-Based (Level 0 + 2) Di-Jet Trigger in 20062006
Endcapessential for hi-x vs.lo-x Siversdistinction
Signed azimuthal opening angle zz xx /ln21
2006 p+p run, 1.1 pb1
2.6M di-jet triggered events
2 localized clusters = 0.6 0.6, with ET
EMC > 3.5 GeV, | | > 60; ET wt’ed centroid
Broad 1,2 coverage Full, symmetric 1,2 coverage
Fast MC Simulations Illustrate Di-Jet Sivers Fast MC Simulations Illustrate Di-Jet Sivers EffectsEffects 2-parton events,
transverse plane
match full jet reco. pT distribution
Gaussian + exp’l tail kT distribution fits distribution
random kTx,y (rms = 1.27 GeV/c) for each parton
Sivers spin-dep. kTx offset shift, L-R di-jet
bisector asymetry
1-spin effects vary linearly with kTx offset
f = 0.85 dilution corrected in data
Error-weighted average of 16 independent AN(>) values for |cos(bisector)| slices
effective beam pol’n for each slice = Pbeam |cos(bisector)|
rotation samples kT
y, parity-violating sp•kT
correlation
STAR data - bothjets rotated by 90
Null Tests
STAR Results Integrated Over PseudorapiditySTAR Results Integrated Over Pseudorapidity
Sivers asymmetries consistent with zero with stat. unc. = 0.002
Fast MC sensitivity to Sivers kTx offset few MeV/c 0.002 (kT
x)21/2
Systematic uncertainties smaller than statistics
All null tests, including forbidden 2-spin asym. cos(bisector), consistent with zero, as are physics asymmetries for all polarization fill patterns
Note: P_beam from online CNI analysis, with 20% calibr. uncertainty
AN+z AN
-z 2-spin AN+z AN
-z 2-spin
STAR data
What Did We Expect? Constraints from SIDIS What Did We Expect? Constraints from SIDIS ResultsResultsFits to HERMES SIDIS Sivers asym constrain u and d quark Sivers functions, for
use in pp dijet + X predictions.
Bomhof, Mulders, Vogelsang & Yuan, hep-ph/0701277
5 < pTparton < 10 GeV/cInitial State Inter’ns only (à la Drell-Yan)Trento sign conv. (opposite Madison)
ISI only
FSI only
ISI+FSI
VY 2 SIDIS Sivers fit
Theory of Transverse SSA Developing Very Theory of Transverse SSA Developing Very Rapidly!Rapidly!Bacchetta et al. [PRD 72, 034030 (2005)] deduce gauge
link struct for pp jets, hadrons:
AN (ISI+FSI) 0.5 AN (ISI)
Gauge links more robust for SSA wt’ed by pT or |sin |, due to kT - factorization breakdown (Collins & Qiu, arXiv:0705.2141)
models (2) of Sivers fcn. x-dep
W. Vogelsang and F. Yuan, PRD 72, 054028 (’05)
Jet 1 rapidity Jet pT (GeV/c)
no hadronization no gluon Sivers functions
xxxud
xxxuuVY
T
T
186.1)(/
181.0)(/:1)2/1(
)2/1(
xxxdd
xxxuuVY
T
T
176.2)(/
175.0)(/:2)2/1(
)2/1(
STAR Di-Jet Sivers Results vs. Jet Pseudorapidity STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum ISum I
Tra
ns
Sp
in A
sym
Emphasizes (50%+ ) quark Sivers
Spin
Polquark
Unpol
qluon+z
+z
y
x
Blue beam Yellow beam
.,),(
),(
),(
),()(
1)(/1)(
)(|cos|
etcN
N
N
Nr
withrr
APf
bj
bj
bj
bjbz
bzbz
zNbz
Extract analyzing powers averaged over and b w/ fit to asym’s in |cos b| using cross ratio:
Typical xT ~ 0.05 - 0.10;
1+2 range 0.01 < xBj < 0.4
STAR Di-Jet Sivers Results vs. Jet Pseudorapidity STAR Di-Jet Sivers Results vs. Jet Pseudorapidity SumSum
STAR AN all consistent with zero both net high-x parton and low-x gluon Sivers effects ~10x smaller in pp di-jets than SIDIS quark Sivers asym.!
Blue beam Yellow beam All calcs. for STAR acceptance
Reverse calc. AN signs for Madison convention
Scale Bomhof calcs by 1/|sin | 3.0 to get AN of unit max. magnitude
u vs d and FSI vs ISI cancellations sizable SSA in inclusive fwd. h prod’n and SIDIS (weighted SSA) compatible with small weighted di-jet SSA -- test via LCP flavor select
Near Term future, and RHIC run 8 & 9Near Term future, and RHIC run 8 & 9
Inclusive channels for longitudinal spin prog suffer from broad integration over x model-dep. G extraction as well as other systematic issues
With improved beam & detector performance, focus will now shift to jet-jet and -jet coincidences for event-by-event constraints on colliding parton x1,2 .
0 10 3020pT (GeV)
xgluon
500200
GeV101
102
Inclusive 0
N.B. x-range sampled depends on g(x,Q2) ! -- M. Stratmann
.2
;2
1tanh*cos
;2
;2
21
212121
pp
TT
TT
s
pxwith
eex
xeex
x
Sivers -> Ongoing analysis incorporates TPC tracks for full jet reconstruction allowance for cuts on jet pT , u vs. d filtering via leading hadron charge sign, etc. (w/o cuts consistent w/ EMC only)
Presently in detailed planning stages for RHIC run 8 (accelerator cool-down ~1 Nov. 2007) … as per the Beam Use Request (BUR) there will be a significant amount of polarized pp running (divided among longitudinal and transverse spin orientations) … as we also work on run 6 analysis!
Similarly we expect a signifcant amount of polarized pp beam in run 9
2008-12: Coincidence Measurements to Map 2008-12: Coincidence Measurements to Map g(x) Fullyg(x) Fully
For example, simulations (L. Bland) of STAR capabilities for - jet coincidences give rough indication of g discriminating power for various models of input gluon polarization.
Simplified LO analysis used for simulations here to illustrate sensitivity
- jet and di-jet measurements @ s = 200 & 500 GeV, will map g(x 0.01-0.3,high Q2), when included in NLO treatments of entire spin structure database.
SummarySummary NLO pQCD describes hadron x-sections at RHIC for inclusive jets,and ±) allowing spin program to access G directly
Longitudinal spin: 2005 inclusive jet data provide significant new constraint on G when compared to predictions derived from oneglobal fit to DIS data (GRSV-max scenario ruled out w/ ‘03/’04 data);2006 data should provide sizable add’l constraint (as will global fits!) STAR longitudinal spin program entering phase of correlation anddirect measurements, while continuing to expand the pT reach of the incl. channels; 2008-09, focus on ALL for di-jet and +jet production Transverse spin: spin asym’s for pp di-jet production Sivers
asym’s consistent w/ zero, whether dominated by valence or sea partons … data will constrain unified theoretical accounts of SSA in hard trans spin pQCD, and connection to parton orbital momentum.
Present pQCD calcs. reconcile small observed asym’s with larger effects seen in SIDIS (& pp forward hadron), via cancelling ISI vs. FSI and u vs. d contr.
p+p in pQCD regime viable complement to DIS more data coming!
BACKUPS
2005 Inclusive Jet ALL
GRSV curves*G = GG = -GG = 0G = STD
1
PY PB
N parallel RN antiparallel
N parallel RN antiparallel
Phys.Rev.Lett 97 252001 (2006)
2005 ALL is consistent
with previous
2003/2004 results.
PYTHIA+GEANT full jet reconstruction vs. parton-level resolution:
EMC-Only Information OK For 1EMC-Only Information OK For 1stst Dijet Sivers Dijet Sivers AsymmetryAsymmetry
Jet finder
•TPC+EMC
• jet cone radius 0.6
Full offline di-jet reconstruction for ~2% of all runs shows triggered jet pT spectrum:
Typical xT ~ 0.05 - 0.10;
1+2 range 0.01 < xBj < 0.4
and angle resolution loss @ L2 OK:
[()=5.0, ()=0.10] full reco. jet vs. parton angles
Net L2-to-parton (jet) = 6.3, (di-jet) = 9.0
(full reco) – (L2) [deg]
[()=3.9, ()=5.8] L2 vs. full jet << observed() 20, mostly from kT
Distinguishing Sivers from Collins AsymmetriesDistinguishing Sivers from Collins AsymmetriesIn SIDIS, can distinguish transverse motion preferences in PDF’s (Sivers) vs. in fragmentation fcns. (Collins) via asym. dependence on 2 azimuthal angles:
HERMES results both non-zero, but + vs. – difference suggests Sivers functions opposite for u and d quarks.
Collins Sivers
Theory of Transverse SSA Developing Very Theory of Transverse SSA Developing Very Rapidly!Rapidly!
Bacchetta, Bomhof, Mulders & Pijlman [PRD 72, 034030 (2005)] deduce gauge link structure for pp jets, hadrons:
AN (ISI+FSI) 0.5 AN (ISI)
Gauge links more robust for SSA weighted by pT or |sin |, due to kT - factorization breakdown (Collins & Qiu, arXiv:0705.2141)
ISI only
FSI only
ISI+FSI
Bomhof, Mulders, Vogelsang & Yuan, hep-ph/0701277
VY 2 SIDIS Sivers fit
Bomhof, Mulders, Vogelsang & Yuan, hep-ph/0701277
u quark
d quark
u+d
Sivers fcns. from twist-3 qg correl’n fits to pp forward hadron
Ji, Qiu, Vogelsang & Yuan [PRL 97, 082002 (2006)] show strong overlap between Sivers effects & twist-3 quark-gluon (Qiu-Sterman) correlations:
twist-3 fits to AN(p+p fwd. h) can constrain Sivers fcn. moment relevant to weighted di-jet SSA
Kouvaris et al. [PRD 74, 114013 (2006)] fits give nearly complete u vs. d cancellation in weighted di-jet SSA
Future: W Production @ 500 GeV Future: W Production @ 500 GeV u/ u and u/ u and d/d to Illuminate Origin of the Nucleon’s qq d/d to Illuminate Origin of the Nucleon’s qq
SeaSea
¯̄ ¯̄¯̄ ¯̄¯̄
2 asyms. 2 charges pol’n of valence q, sea q separately for u,d.
Detect W± via isolated high-pT daughter e± or ± , no away-side jet
¯
Projected uncertainties for quark and antiquark polarizations
Many non-perturbative models of nucleon structure predict sign & magni-tude difference between u and d polar-izations in nucleon sea, not yet seen.
Probe via single-spin parity-violating asym. AL for p + p W + X with respect to helicity flip of each beam.
Detector at RHICSTARSTAR
= -ln[tan(/2)]
TPC
||<1.4
Charged particle momentum
BEMC
||<1.0
Neutral Energy
High pT Trigger
EEMC
1<<2
Neutral Energy
High pT Trigger
BBC
3.4<<5
MinBias Trigger
Relative Lumi
(also ZDC)
EndCapEMC
BBCEast
Barrel EMC
BBC West
TPC
Yellow beamBlue beam
STASTARR
FMS, EEMC and BEMC provides nearly complete EM coverage from -1 η +4
STAR Calorimeter Coverage
proton spin
proton momentum
proton spin
proton momentum +
Blue beam asymmetry Yellow beam asymmetry2006 p+p run STAR measurement of Sivers transverse single-spin asymmetry for di-jets -- shows smaller effects than predicted for observable sensitive to orbital components of parton motion in proton.