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Transcript of Carl Gagliardi – STAR Spin: Recent Results, Future Directions 1 Spin Recent Results, Future...
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 1
Spin
Recent Results, Future Directions
Carl A. GagliardiTexas A&M University
for the CollaborationSTARSTAR
STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 2
What contributes to the proton spin?Consider a proton moving
toward the right
Δq(x)Δg(x)
δq(x)
Proton spin
Proton spin
Spin sum rule:
Polarized DIS: ~ 0.3 Both are poorly constrained
Transversity – very little data
STAR spin program:
Exploring poorly determined components of the proton
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 3
RHIC: the world’s first polarized hadron collider
• Spin varies from rf bucket to rf bucket (9.4 MHz)• Spin pattern changes from fill to fill• Spin rotators provide choice of spin orientation• “Billions” of spin reversals during a fill with little if any depolarization
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
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 4
STAR detector in cross section
BEMC
EEMC FMSFPD TPC
BBC
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 5
Essential benchmark – unpolarized cross sections
• Mid-rapidity jet cross section is consistent with NLO pQCD over 7 orders of magnitude
• Forward rapidity π0 cross section also consistent with NLO pQCD• Many other examples• pQCD works over a very broad kinematic range at RHIC energies
STARSTAR PRL 97, 252001 STARSTAR PRL 97, 152302
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 6
STAR longitudinal spin program
• What is the polarization of the gluons in the proton?
Three recent fits of equal quality:– ΔG = 0.13 ± 0.16– ΔG ~ 0.006– ΔG = -0.20 ± 0.41all at Q2 = 1 GeV2
Leader et al, PRD 75, 074027
• Additional question for future 500 GeV runs:
What is the polarization of the anti-quarks?
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 7
LLba
baLL a
ff
ffA ˆ
f: polarized parton distribution functions
cos
For most RHIC kinematics, gg and qg dominate, making ALL for inclusive jets and hadrons sensitive to gluon polarization. 10 20 30 pT(GeV)
Partonic fractions in jet production at 200 GeV
0
Longitudinally polarized pp collisions at RHIC
0
0.25
0.5
0.75
1
0 0.2 0.4 0.6 0.8 1
|cos (theta*)|
qg
->
qg
FO
M
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 8
STAR inclusive π0 ALL at various rapidities
• During Run 6, STAR measured ALL for inclusive π0 for three different rapidity regions
• Mid-rapidity result excludes large gluon polarization scenarios• Larger rapidity correlates to stronger dominance of qg scattering
with larger x quarks and smaller x gluons
• Expect ALL to decrease as increases
• Forward rapidity: baseline for future γ and γ-jet measurements
|| < 0.95 1 < < 2 = 3.2, 3.7
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 9
STAR inclusive charged pions
• STAR measured ALL for inclusive charged pions during Run 5
• ALL(π+) – ALL(π-) is sensitive to the sign of G
• Difficult to trigger on charged pions• Used the EMCal jet patch trigger as a surrogate, which introduces
significant trigger bias (dominates syst. error band)
STARSTAR
π+ π-
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 10
Lambda longitudinal polarization transfer
• STAR can measure the longitudinal polarization transfer from one of the proton beams to outgoing Λ and anti-Λ
• Outgoing anti-Λ polarization is particularly sensitive to s• Has some of the same trigger difficulties as charged pions
STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 11
STAR inclusive jet ALL from Run 6
200 GeV-0.7 < < 0.9
STARSTAR
• Confidence level calculations from comparison to the GRSV polarized parton distributions
• Also compare to other polarized parton fits
• Large gluon polarization scenarios excluded, except for GS-C
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 12
DSSV – first global analysis with polarized jets
• The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing
• Finds a node in the gluon distribution near x ~ 0.1, but with the opposite phase from GS-C
de Florian et al., PRL 101, 072001
STAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 13
Future inclusive jet ALL sensitivity
• Goal for the current 200 GeV run:– 50 pb-1 @ 60% pol – reduce ALL uncertainties a factor of ~4
– Will provide much stronger constraints on gluon polarization
• Goal for future 500 GeV running:– 300 pb-1 @ 70% pol
– Extend precision determination to lower xg
Projected improvement in xg from Run 9
Projected sensitivities:Run 9 & future 500 GeV running
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 14
Looking beyond inclusive ALL measurements
• Inclusive ALL measurements at fixed pT average over a broad x range.
• Can hide considerable structure if Δg(x) has a node• Correlation measurements can constrain the shape of
Δg(x)
STARSTAR PRL 100, 232003
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 15
Beyond inclusives: charged pions opposite jets
STARSTAR
• Making lemons into lemonade
• Beat the trigger bias by using it
• Trigger and reconstruct a jet, then look for a charged pion on the opposite side
• Correlation measurement significantly increases the sensitivity of ALL(π+)
ALL ALL
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 16
Jet+hadron correlations at NLOfrom de Florian, arXiv:0904.4402
• NLO calculations show strong correlation between the real x and z values and LO estimates
• NLO calculations verify the increased sensitivity of ALL(π+)
• Typical scale dependence ±20% of calculated ALL value
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 17
Remaining trigger bias in charged pions + jets
• Trigger bias will be reduced with the Run 9 data:– Larger acceptance and lower thresholds increase efficiency, reduce bias
– Higher statistics will permit finer binning in jet pT
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 18
Di-jets and Δg(x)
• Di-jets provide direct access to parton kinematics at LO
Mass Rapidity |cos(*)|
Ratio
2005 preliminary di-jet distributions
2tanhcos
2ln
2
1
|ηη||θ|
ηη
x
xy
sxxM
epeps
1x
epeps
1x
43*
43
2
1
21
η4
η32
η4
η31
43
43
STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 19
Di-jets and g(x): projected sensitivity for Run 9
2exp 43
21
ηη
s
Mx,x
• Di-jets provide direct access to initial parton kinematics at LO
• Di-jets in different regions of the STAR detector– Sample different
mixtures of qq, qg, gg– Sample different
ranges of xg
• Goal for the current run: 50 pb-1 @ 60% pol
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 20
Projected di-jet sensitivity for √s = 500 GeV
• Higher energy accesses lower xg
• Expect smaller ALL
• Assumes 300 pb-1 at 70% polarization
2exp 43
21
ηη
s
Mx,x
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 21
Gamma + jet
• 90% of gamma+jet yield from quark-gluon Compton scattering• Forward photons + mid-rapidity jets provide clean access to g(x)
via scattering off highly polarized valence quarks
STAR sensitivity for: pT,γ > 10 GeV/c 1.09 < ηγ < 2 (EEMC) √s = 200 GeV 50 pb-1 @ 60% pol.
NOT INCLUDING: Efficiency and background effects
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 22
Gamma+jet experimental issues
• Signal yield is very small compared to di-jet backgrounds• Background suppression is VERY CHALLENGING• Efforts underway for photons in the STAR EEMC
– Current status from Run 6: 25-40% purity at 70% efficiency– Have less material in STAR to produce backgrounds in Run 9– FGT will improve photon isolation beginning in Run 12
• Gamma+jet with photons in the STAR FMS investigates even lower xg
– To date, only PYTHIA+GEANT background studies have been performed
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 23
500 GeV pp: quark and anti-quark polarizations
• Projected sensitivity for parity-violating W asymmetries
• With two polarized beams and W+ and W-, can separate u, d, u, d polarizations
• Explores the origin of the sea quarks
Will constrain u
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 24
Mid-rapidity W asymmetries
• W cross section is much larger at mid-rapidity than at forward and backward rapidity
• Mid-rapidity W asymmetries mix quark and anti-quark sensitivities• Provide high precision information about anti-quarks when quark
polarizations are well known
Will constrain d
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 25
Extending the Q2 reach
• W asymmetry measurements will extend the reach of polarized data to Q2 ~ 6500 GeV2
• Significant test of the evolution when combined with our g(x) measurements
World DIS
database with
DGLAP fits
All fixed-target data
World Data on g1p as of 2005
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 26
STAR transverse spin program
• Large rapidity transverse single-spin asymmetries at 200 GeV are BIG
• May arise from the Sivers effect, Collins effect, or a combination
PRL 92, 171801STARSTAR
Parton orbital motion
Transversity
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 27
Run 6 inclusive π0 AN at forward rapidity
• Large transverse single-spin asymmetries at large xF
• xF dependence matches Sivers effect expectations qualitatively (but not quantitatively)
• pT dependence at fixed xF does not follow ~1/pT expectation of pQCD-based calculations
PRL 101, 222001STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 28
• To date, the η meson has looked like a “high-mass, low-yield π0” in all measurements at RHIC
• AN for the η mass region is much larger at high xF
STAR 2006 PRELIMINARY
η ~ 3.66
STARSTARRun 6 inclusive AN at large xF
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 29
Separating Sivers and Collins effects in pp collisions
Collins mechanism: asymmetry in the forward jet fragmentation
Sivers mechanism: asymmetry in the forward jet or γ production
SPkT,q
p
p
SP
p
p
Sq kT,πSensitive to proton spin – parton transverse motion correlations
Sensitive to transversity
• Need to go beyond inclusive hadrons to measurements of jets or direct γ
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 30
Sivers di-jet measurementPRL 99, 142003
• Observed asymmetries are an order of magnitude smaller than seen in semi-inclusive DIS by HERMES
• Detailed cancellations of initial vs. final state effects and u vs. d quark effects?
STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 31
FMS: expanding STAR’s forward acceptance
• Expanded pT range for inclusive π0 AN during Run 8
STAR Forward Meson Spectrometer2.5 < η < 4.0 STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 32
More than just π0 out there
• FMS acceptance is large enough to provide good efficiency for higher mass hadrons over a broad kinematic range
Pythia+GEANT(3γ background only)
STAR Preliminary
η γγω π0γ γγγ
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 33
Run 8 forward π0 + hadron correlation measurements
• Trigger on a π0 in the FMS with pT > 2.5 GeV/c• Correlate with hadrons with pT > 1.5 GeV/c• FMS-TPC and FMS-FMS back-to-back correlations enable di-hadron / di-jet Sivers effect measurements• FMS-FMS near-side correlations sensitive to transversity• FMS-TPC: key step toward future transverse spin γ + jet study
Un
corr
ect
ed C
oin
cid
ence
P
rob
abil
ity
(rad
ian
-1)
pp π0 (FMS) + h± (TPC) + XSTARSTAR pp π0 (FMS) + π0 (FMS) + X
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 34
First look at jet-like events with the STAR FMS
• Comparisons of the jet profile and effective mass in data vs. PYTHIA + GEANT simulations
• If we find:– AN for jets must arise from Sivers effect
– Azimuthal variation in jet shape must arise from Collins effect
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 35
Proposed addition: Forward Hadron Calorimeter
• Install hadronic calorimetry behind the FMS left and right of the beam– Forward Lambdas
– Improved FMS direct photon isolation
– Forward jets
Estimated sensitivity for FMS+FHC
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 36
Future: transverse spin forward + mid-rapidity jet
Bacchetta et al., PRL 99, 212002
• Conventional calculations predict the asymmetry to have the same sign in SIDIS and +jet
• Calculations that account for the repulsive interactions between like color charges predict opposite sign
• Critical test of our basic theoretical understanding
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 37
Transverse spin possibilities with mid-rapidity jets
• Run 6 transverse spin measurements– Measured AN, AΣ while investigating systematics in ALL
– Study of Collins effect in jet fragmentation underway (please calculate)
• Will measure ATT for inclusive jets during next extended transverse spin run (please calculate)
STARPreliminary
STAR Preliminary
PT (GeV/c)
PT (GeV/c) PT (GeV/c)
PT (GeV/c)
-0.9 < < -0.5
STAR Preliminary
0 < < 0.5STAR Preliminary
0.5 < < 0.9
STAR Preliminary
-0.5 < < 0
AΣ
AN
STARSTAR
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 38
Conclusions
• STAR is making significant contributions to three poorly constrained pieces of the spin puzzle– Gluon polarization– Flavor-separated quark and anti-quark polarizations– Parton orbital motion and transversity
• STAR in embarking on precision explorations of these phenomena through correlation measurements
• STAR is generating a wealth of new data regarding the spin structure of the proton
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 39
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 40
E-M Calorimeter
Time of Flight
Projection Chamber
STAR detector
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 41
Other global analyses
• There have been many global analyses of the polarized DIS data
xΔg(x) atQ2 = 10 GeV2
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 42
Full set of DSSV polarized distributionsde Florian et al, PRL 101, 072001 and arXiv:0904.3821
Carl Gagliardi – STAR Spin: Recent Results, Future Directions 43
Current status on transversity
• Global analysis combining Collins effect measurements in SIDIS from HERMES and COMPASS with measurements of the Collins fragmentation function by BELLE
Anselmino et al, arXiv:0812.4366