Transcript pp Collisions at RHIC

Transverse Spin Physics in
pp Collisions at RHIC
Carl A. Gagliardi
Texas A&M University
Outline
•
•
•
•
Introduction
Forward rapidity measurements
Mid-rapidity measurements
Looking ahead
Carl Gagliardi – WWND – Trans Spin at RHIC
1
RHIC: the Relativistic Heavy Ion Collider
• Search for and study the Quark-Gluon Plasma
• Explore the partonic structure of the proton
• Determine the partonic structure of nuclei
Carl Gagliardi – WWND – Trans Spin at RHIC
2
From PDFs to polarized PDFs
Consider a proton moving toward the right
q(x)
g(x)
Δq(x)
Δg(x)

+

Proton spin 


Unpolarized PDF
“Polarized”
distribution
Proton spin 
δq(x)


“Transversity”
distribution
• There are really three different sets of PDFs for the proton
Carl Gagliardi – WWND – Trans Spin at RHIC
3
Current knowledge of the polarized distributions
• Quarks and antiquarks only carry ~30% of total proton spin
• Proton “spin crisis”
Anselmino et al,
DSSV, PRL
101, 072001
arXiv:0807.0173
1
 S     G   Lqz    Lgz 
2
p
z
• Gluon and anti-quark • Know very little about • Transversity also has
polarizations have
orbital motion
large uncertainties
large uncertainties
RHIC transverse spin program
Carl Gagliardi – WWND – Trans Spin at RHIC
4
RHIC: the world’s first polarized hadron collider
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
BRAHMS
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
-
Pol. H Source
Spin flipper
LINAC
Helical Partial Siberian Snake
BOOSTER
AGS
200 MeV Polarimeter
AGS Internal Polarimeter
AGS pC Polarimeters
Rf Dipole
•
•
•
•
Spin Rotators
(longitudinal polarization)
Solenoid Partial Siberian Snake
Strong Helical AGS Snake
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
Carl Gagliardi – WWND – Trans Spin at RHIC
5
Transverse single-spin asymmetries
• Definition:
•
d   d 
AN 
d   d 
Two methods of measurements:
• Single arm calorimeter:
dσ↑(↓) –
AN 
cross section for scattering
to the left when incoming proton
has spin up(down)
Pbeam
 N   RN  

  
 
 N  RN 
R
L
L
R – relative luminosity
Pbeam – beam polarization
Left
π0, xF<0
1
π0, xF>0
• Two arm (left-right) calorimeter:
p
p
Right
p0 going
Positive AN: more
to
left of the polarized beam
Carl Gagliardi – WWND – Trans Spin at RHIC
 N  N  N  N
L
R
R
L
AN 

PBeam  N   N   N   N 
L
R
R
L

1




Less sensitive to instrumental
effects
6
Transverse single-spin asymmetries at
forward rapidity
FNAL E704
• Large single-spin asymmetries at CM energy of 19.4 GeV
• Weren’t supposed to be there in naïve pQCD
AN  αS mq /pT
Carl Gagliardi – WWND – Trans Spin at RHIC
7
Forward π0 production at ISR energies
√s=23.3GeV
√s=52.8GeV
Data-pQCD
differences
at pT=1.5GeV
q5o
q10o
q15o
NLO
calculations
with different
scales:
pT and pT/2
q53o
q22o
xF
xF
Bourrely and Soffer, EPJ C36, 371:
NLO pQCD calculations underpredict the data at ISR energies
Maybe the E704 results arise from soft physics?
Carl Gagliardi – WWND – Trans Spin at RHIC
8
First AN measurement at RHIC
STAR
PRL 92, 171801 (2004)
Similar to result from E704 experiment
(√s=19.4 GeV, 0.5 < pT < 2.0 GeV/c)
Can be described by several models:
Sivers: spin and kT correlation in
parton distribution functions
(initial state)
Collins: spin and kT correlation in
fragmentation function (final
state)
Qiu and Sterman (initial state) /
Koike (final state): twist-3 pQCD
calculations, multi-parton
correlations
√s=200 GeV, <η> = 3.8
Carl Gagliardi – WWND – Trans Spin at RHIC
9
Forward pp  π0 + X cross sections at 200 GeV
STAR PRL 97, 152302
The error bars are statistical
plus point-to-point systematic
Consistent with NLO pQCD
calculations at 3.3 < η < 4.0
Data at low pT trend from KKP
fragmentation functions toward
Kretzer.
NLO pQCD calculations by Vogelsang, et al.
Carl Gagliardi – WWND – Trans Spin at RHIC
10
Sivers and Collins effects in pp collisions
Sivers mechanism: initial-state
kT dependence in the parton
distribution
SP
kT,q
p
Collins mechanism: final-state
asymmetry in the forward jet
fragmentation
SP
p
p
Sensitive to proton spin –
parton transverse motion
correlations
p
Sensitive to
transversity
Sq
kT,π
Observed transverse single-spin asymmetries could
arise from the Sivers effect or Collins effect, or
from a linear combination of the two
Carl Gagliardi – WWND – Trans Spin at RHIC
11
Sivers and Collins effects in deep-inelastic scattering
Sivers
Collins
• Semi-inclusive DIS can distinguish the Sivers and Collins effects
• HERMES finds both are non-zero
Carl Gagliardi – WWND – Trans Spin at RHIC
12
Recent π0 results at 200 GeV from STAR
STAR
PRL 101, 222001
• Sivers fit to HERMES SIDIS describes η = 3.3; overpredicts η = 3.7
• Twist-3 fit to E704 plus preliminary STAR Runs 3 and 5 data
describes η = 3.7; underpredicts η = 3.3
Carl Gagliardi – WWND – Trans Spin at RHIC
13
Charged pion measurements at 200 GeV from
BRAHMS
BRAHMS
4 deg (~3)
2.3 deg (~3.4)
• Sign dependence of charged pion asymmetries seen in FNAL E704
persists to 200 GeV
Carl Gagliardi – WWND – Trans Spin at RHIC
14
p0 results at 62.4 GeV from PHENIX
+pp00+X
+X at
at s=62.4
s=62.4 GeV/c
GeV/c22
pp+pp
3.0<<4.0
• Asymmetries are comparable or larger at 62.4 GeV than they are at
200 GeV
Carl Gagliardi – WWND – Trans Spin at RHIC
15
Charged-hadron results at 62.4 GeV from BRAHMS
BRAHMS PRL 101, 042001
• Very large asymmetries!
• K- (= su ) asymmetry is a surprise. Sea-quark Sivers
effect or disfavored fragmentation?
Limitation of the BRAHMS measurements:
Very strong correlation between xF and pT
from small acceptance
Carl Gagliardi – WWND – Trans Spin at RHIC
16
STAR
Inclusive π0 AN(pT) in xF bins
PRL 101, 222001
• Combined data from three runs
at <η>=3.3, 3.7 and 4.0
• Measured AN is not a smooth
decreasing function of pT
as predicted by theoretical
models
Carl Gagliardi – WWND – Trans Spin at RHIC
Kouvaris et al,
PRD 74, 114013
17
A potential fly in the ointment?
STAR
STAR 2006 PRELIMINARY
η ~ 3.66
• 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
Carl Gagliardi – WWND – Trans Spin at RHIC
18
Mid-rapidity inclusive pion AN
PRL 95, 202001
• Mid-rapidity pion yields are gluon-dominated at
these pT
• No Collins effect for gluons
• May help to constrain the gluon Sivers function
Carl Gagliardi – WWND – Trans Spin at RHIC
19
STAR
Mid-rapidity inclusive jet AN
-0.5< <0.0
-0.9< <-0.5
STAR Preliminary
0.0< <0.5
PT (GeV/c)
STAR Preliminary
PT (GeV/c)
STAR Preliminary
0.5< <0.9
PT (GeV/c)
STAR Preliminary
PT (GeV/c)
• Gluon-dominated at low pT
• Dominated by qg scattering at higher pT
• Within uncertainties, AN is consistent with zero
Carl Gagliardi – WWND – Trans Spin at RHIC
20
Sivers effect in di-jet production
• Left/right asymmetry in the kT of the
partons in a polarized proton
• Spin dependent sideways boost to
di-jets
• Measure the di-jet opening angle as
a function of proton spin
• Requires parton orbital angular
momentum
Carl Gagliardi – WWND – Trans Spin at RHIC
spin
Sivers effect:
1
open 180
kTx
di-jet
bisector
2
21
Mid-rapidity di-jet Sivers effect measurement
STAR
PRL 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, coupled with very small gluon Sivers effect?
Carl Gagliardi – WWND – Trans Spin at RHIC
22
Mid-rapidity di-hadron Sivers effect measurement
• PHENIX is performing a similar measurement
• Back-to-back correlations between a trigger π0 and an away-side
charged hadron
Carl Gagliardi – WWND – Trans Spin at RHIC
23
Separating Sivers and Collins effects in pp collisions
Sivers mechanism: asymmetry
in the forward jet or γ production
SP
Collins mechanism: asymmetry
in the forward jet fragmentation
SP
kT,q
p
p
p
Sensitive to proton spin –
parton transverse motion
correlations
p
Sensitive to
transversity
Sq
kT,π
• Need to go beyond inclusive hadrons to measurements of
jets or direct γ
Carl Gagliardi – WWND – Trans Spin at RHIC
24
Large-acceptance forward detectors
PHENIX Muon Piston Calorimeters
3.1 < |η| < 3.7
STAR Forward Meson Spectrometer
2.5 < η < 4.0
• Both PHENIX and STAR have
installed large-acceptance
electromagnetic calorimeters in the
forward direction
Carl Gagliardi – WWND – Trans Spin at RHIC
25
First look at jet-like events with the STAR FMS
• Comparisons of the jet profile and effective mass in
data vs. PYTHIA + GEANT simulations
Carl Gagliardi – WWND – Trans Spin at RHIC
26
Transverse spin forward g + mid-rapidity jet
Bacchetta et al.,
PRL 99, 212002
• Conventional calculations predict the asymmetry to have the same
sign in SIDIS and g+jet
• Calculations that account for the repulsive interactions between
like color charges predict opposite sign
• Critical test of our basic theoretical understanding
Carl Gagliardi – WWND – Trans Spin at RHIC
27
In the further future: Drell-Yan
•
•
•
In SIDIS, final-state interaction of outgoing quark with proton remnant
involves opposite color charges – attractive
In Drell-Yan, initial-state interaction of the incoming quark with the spectator
components of the proton involves like color charges – repulsive
Sign of AN should reverse
Carl Gagliardi – WWND – Trans Spin at RHIC
28
Conclusions
• RHIC has observed large transverse single-spin asymmetries for
forward particle production
• These asymmetries may provide evidence for parton orbital
angular momentum and/or quark transversity
• Measurements to identify the underlying cause(s) are underway
• Future measurements will provide a direct illustration of attractive
vs. repulsive color-charge interactions
• RHIC, the world’s first polarized hadron collider, is generating a
wealth of new data regarding the spin structure of the proton
• Stay tuned!
Carl Gagliardi – WWND – Trans Spin at RHIC
29
Carl Gagliardi – WWND – Trans Spin at RHIC
30
Low-pT forward transverse single-spin asymmetries
STAR BBCs
PHENIX ZDCs
• Rotator tuning for longitudinal polarization requires local polarimetry
Carl Gagliardi – WWND – Trans Spin at RHIC
31