Transcript Probing Spin Structure Proton the

Probing the Spin Structure of
the Proton at STAR
J. Sowinski
Indiana University
For the STAR Collaboration
1
STAR Collaboration
• RHIC and STAR
• STAR results both transverse and longitudinal
• Future measurements
Many special
devices in
RHIC to
generate,
preserve and
measure
polarization
Snakes
Development
runs
First devoted
physics run
Long prod run 9
RHIC
RUN
s [GeV]
LRec [pb-1]
Long. *
LRec [pb-1]
Tran. *
Pol. (%)
2002
200
0.3
0.15
15
2003
200
0.3
0.25
30
2004
200
0.4
0
40-45
2005
200
3.1
0.1
45-50
2006
200
8.5
3.4/6.8
60
2008-9
200, 200 + 500
25 + 10
7.8
45, 55 + 35
* Lum.
recorded
.at STAR
Barrel EM Calorimeter
-1<h< 1
Detector
STAR
Special interest for spin
Triggering by all calorimeters & BBC
h = - ln(tan(q/2)
h= -1
h=0
h=2
Forward Pion
Detector
FMS EM
Calorimeter
h=2.5
h=4
Endcap EM
Calorimeter
1<h< 2
-4.1<h< -3.3
Time Projection
Chamber
-2<h< 2
Lum. Monitor
Local Polarim.
Beam-Beam
Counters
2<|h|< 5
Tracking
Solenoidal Magnetic
Field 5kG
2005
2004
2003
2008
Summary:
• STAR’s large solid angle coverage allows
detection of correlations between particles,
jets, two jets etc
• Significant new data sets at 200 GeV taken
in Run 9
• First 500 GeV collisions in Run 9
Next: STAR Transverse Results and
Prospects for the Future
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The First Spin Results from
Transverse Program
Confirmed in following
measurements
PRL 92,
171801
PRL
101, (2004)
222001
• Runs 2-3 with Forward Pion
Detector (FPD)
• Transverse SSA are Large!
• Run 6
• Sivers effect, Collins and
Twist 3 all nominally
describe data
• SSA for h even larger
• Run 8
• First early data with FMS
confirms previous results
STAR
6
STAR p0 data pT dependence
does not fall as pQCD calcs
predict
STAR + BRAHMS data are very
similar to E704 at 1/10 cm Energy
pT ~ 1GeV / c
PRL 101, 222001
s  19.4GeV
What is connection to low
energy physics?
E704 Nucl. Phys. B 510 (1998) 3
7
Both Sivers and Collins effects promise access to
interesting physics
Sivers mechanism: Transverse
motion (orbital angular momentum)
wrt proton spin + ISI and FSI
“deflect” jets
SP
Collins mechanism: Polarization of
incident quark (transversity)
transferred to scattered quark
which self analyzes in spin
dependent fragmentation
SP
kT,q
p
p
Reconstruct
jets and
compare left
to right wrt p
pol. vector
p
p
Analyze
correlations
within jet on
one side
Sq
kT,π
Of interest in both forward and mid-rapidity regions!
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Spin Transfer to Determine Transversity dq
pQCD predicts the transverse polarization of quarks is preserved in
forward scattering - spin transfer parameter dTT
dTT
Collins, Heppelman et al.
How might we measure dq
the polarization of
scattered q?
Fragmentation can be spin
dependent!
• Collins functions
• Interference functions
• Leading hadron preference
to one side of jet wrt s x pjet
• Preference for orientation
of two leading hadrons wrt
s x pjet
Measured jet asymmetry = dq x dTT x e(z)
Determined in e+e-.
experiment from Belle
~10% effects seen
Sivers with di-Jets at Mid-rapidity
Initially thought effect
could be large and give
sensitivity to q and g
orbital angular
momentum
proton
spin
parton kTx
x
y
z
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Sivers with di-Jets at Mid-rapidity
Initially thought could
be large and give
sensitivity to q and g
orbital angular
momentum
proton
spin
parton kTx
x
y
z
PRL 99 (2007) 142003
Observe small SSA
Much smaller than Sivers
effects observed in SIDIS
STAR Analysis Mtg – July 7-11 2009
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Non-Universality in Sivers Functions
W. Vogelsang and F. Yuan,
PRD 72, 054028 (2005).
ISI only
ISI+FSI
• SSA requires interference between
amplitudes
• FSI have one sign (SIDIS)
• ISI have opposite sign (Drell Yan)
• Di-jets have both - cancellation
Forward g-jet also has only ISI
Important upcoming FMS meas.
u-quark
u+d
Bomhof et al.,
hep-ph/0701277 d-quark
31 Jan 2007
STAR Analysis Mtg – July 7-11 2009
12
Forward Correlation Studies Have
Begun with 2008 Data
STAR Preliminary
First large xF J/Ψ
measurement at
a collider
Correlation of particles in
jet cone
Triple clusters to construct
the w
13
Including Correlations within
and between “jets”
p+p-> π0+h±+X
Forward – mid rapidity correlations
FMS p0 – TPC h+-
Forward correlations
Two FMS p0‘s
14
Transverse Summary:
• Large inclusive p0 asymmetries at forward rapidity
observed in first runs and now reproduced with higher
precision and in h meson
• A Sivers effect would indicate orbital angular momentum
• A Collins effect would provide sensitivity to transversity
• Origin of large SSA to be illuminated via future correlation
measurements with FMS and STAR’s large W coverage
• Investigate extension to 500 GeV (some run 9 data)
• Sivers in g-jet and Drell-Yan should be opposite in sign to
SIDIS – future measurement with FMS
Next: Results on DG
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STAR inclusive π0 ALL at various rapidities
|h| < 0.95
•
•
•
•
•
1<h<2
h = 3.2, 3.7
Run 6 we measured ALL for inclusive π0 for three different rapidity regions
Mid-rapidity result excludes large gluon polarization scenarios
While statistics similar, signal in calculations decreases with h
Forward rapidity: p0 prod. baseline for future γ and γ-jet measurements
It remains important to confirm results in multiple channels
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Correlations at mid-rapidity used in DG program
Trigger on jet and analyze awayside
charged pions to avoid trigger bias
NLO calculations indicate that LO
reconstructed z here, and x1, x2 in di-jets,
are good approximations to NLO quantities
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To date inclusive jets have been the work horse at
STAR
2003-4 Disfavor extreme PDFs offered as fix to proton spin puzzle PRL 97, 252001
2005 data add strong contraints on large positive DG for 0.02<x<0.3 PRL 100, 232003
2006 GRSV-std (DIS data best fit) now upper limit. Constrains large neg. values
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STAR
2006 Preliminary Results
STAR incl. jets
Existing data, STAR and
others, have placed
strong constraints on DG
19
Additional ALL predictions for 2005
ALL
Implications
for many
previous
PDFs
Small range is allowed by
current measurements
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Global fit D. de Florian, R. Sassot, M.
Stratmann and W. Vogelsang
arXiv:0804.0422 [hep-ph]
Significant constraints in RHIC range 0.05<x<0.2
Uncertainty at low x prevents a constraint on the full
integral DG
Shape Dg(x) and low x behavior clearly important
21
Message for new measurements
xΔg(x) at
Q2 = 10 GeV2
Remaining PDFs either
have small DG or
nodes, large contributions
at low x are not ruled out
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DG Results Summary:
• Large contributions to DG for x>0.05 unlikely
• Full integral not constrained due to uncertain low x
contribution
• Shape Dg(x) important
Next: Future Directions to
constrain Dg(x)
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fraction
PRL 100, 232003
Increased statistics at high
pT just begin to separate
large x behavior from wide
integration range at low pT,
but also complicated with
subprocess mix
Inclusive Jets: LO
(W. Vogelsang)
10
20
pT/GeV
30
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Inclusive Jet Projections
• Accumulated FoM at 200 GeV from run 9 was ~1/3 that assumed in
fig. below
• Substantial future running at 500 GeV gives sensitivity to lower x
• Higher √s at same pT gives lower xg, lower xq and hence less q
polarization, but better statistical precision.
Projected statistics
based on measured
yields and sys. error
limits
xT = 2 pT / √s
Spin measurements, ALL ~ Dg/g
Negative polarizations evolve toward 0 with incr. pT
Need to measure small gluon polarization at low x
100
40
104
2 (GeV/c)
(GeV/c)22
Gehrmann and
Stirling - C
Small contribution
at large x
Large contribution
at low x
Dg/g(x=0.02) ~
0.04 for high
scales
Di –Jets analyzed in 2005 data
M
Pythia based
full detector
MC vs. 2005
data
• Di-jets require large coincident solid angle
• High yields allow near triple differential distributions
dM, dh, dcos(θ*)
• Select kinematics for xg dependence
• Select kinematics for valence quarks and favorable aLL
200 GeV projections 50 pb-1 P=60%
Run 9 ~ 1/3rd FoM: expect 2x error bars shown
Two body
Same
•kinematics
NLO M
and LO
X=0.06
reasonable
X=0.16
agreement
Left
two panels
•same
NLO scale
η1+η2 (x1/x2)
variations
but
differentsmall,
η1-η2
pT>7,10soGeV
(cosθ*)
smaller
GeV
AM>20
in
upper
LL
• Detector
regions
select x1/x2
• Also select âLL
Each panel gives
M / s  x1 x2
•an
x dependence
•eg.
Strong sensitivity
in range0.10,
of curr.
X=0.08,
0.13
allowed PDFs
dDg/g~0.02-0.03
500 GeV projections 300 pb-1 P=70%
• Access to lower x
• Asymmetries smaller
than at 200 GeV
• But uncertainties
smaller as well
• No significant data
so far
De Florian, Frixione,
Signer and Vogelsang
NPB 539 (1999) 455 and
PC for present calc.
Direct photon – jet from qg Compton
90% from qg process
g/p0 ratio at mid-rapidity LO estimate of statistical errors for 50 challenging backgrounds
pb-1 at 200 GeV with 100% eff. and
from inclusive hadronic
no background and pT > 10 GeV.
channels
More stats at lower pT but more BG
DG Future measurements:
• Significant step in statistics at 200 GeV with run 9
• Enhanced statistics for inclusives
• Shape Dg(x) to be constrained by di-jet and g-jet
correlations
• Lower x from 500 GeV and forward detectors
Next: The Polarized Sea
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Flavor Asymmetry of the Sea
NMC and
Phys.Rev.Lett. 80 (1998) 3715
• Gottfried Sum Rule in DIS
• DIS on nuclei
• SIDIS
• Drell-Yan
• Quantitative calculations
of Pauli blocking not
conclusive
• Non-perturbative
processes seem to be
needed in generating the
sea
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E866 Results
_
Polarized q Flavor Asymmetry
¯ better for Q2 evolution
¯
• ¯d(x)-u(x)
and Du(x)-Dd(x)
¯
• E866 Results are qualitatively consistent with
pion cloud models, instanton models, chiral
quark soliton models, etc.
• pQCD motivated models predict
1
1
0
0
¯ ¯
∫ [Du(x)¯ D¯d(x)]dx [ ∫ [d(x)-u(x)]dx
• Chiral motivated models tend to disagree
m2 = (5 GeV)2
¯
x(Du¯ Dd)
¯ ¯
x(d-u)
B. Dressler et al.,
Chiral Quark
Soliton Model
Predictions
Q2 = 10 GeV2
D. De Florian et al.
Phys.Rev.D80:
034030,2009
Global fit
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W+(-) Production in p-p at  s = 500 GeV/c2
Use STAR’s EM calorimetry
for electron detection
_
u+d
_→W →e +
u + d → W + → e+ + 
•
V-A coupling
_
+
– only LH u and RH d couple
to
W
_
– Likewise LH d and RH u to W – Only LH W’s produced
WA
• Neutrino helicity gives preferential
L
directionality in decay
Parity violating single spin
asymmetry AL
(Helicity flip in one beam while
averaging over other)
_
_
~ -u(x1)Dd(x2)-d(x1)Du(x2)
Allows kinematic separation
especially for W- in EEMC
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Ws at STAR (mid-rapidity)
In preparation for analysis of
the 500 GeV data from run
9, STAR has been studying
the reconstruction of the W.
Run9 W Algorithm Simulation Results
Pythia Study
The simulations use full
detector response and
realistic QCD background.
The main source of background is hadrons so good e/h separation is necessary.
The current analysis uses a combination of tracking, shower shape, near side
isolation cuts, far side isolation cuts, and event shape cuts.
A goal of run 9 is to observe this Jacobian peak.
Analysis is ongoing.
5/23/2016
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W Detection at STAR
• Need to detect e+ e- and measure pT
• Need to find in huge hadronic background
• Need to separate e+ and e- charge
Large solid angle of STAR
h
EM
Calorimetry
e
Forward GEM Tracker
J. Sowinski Spin 2008
Charlottesville VA
36
Projections Sensitivity 500 GeV 300 pb-1
• Realistic BG subtraction
• Recent PDFs ~represent current allowed Du/Dd range
• Dd and Du (W -) isolated in forward region
• Dd and Du (W+)sensitivity
spread over h
J. Sowinski Spin 2008
Charlottesville VA
37
Conclusions
• RHIC run 9
• New level of statistics at 200 GeV
• First 500 GeV collisions
• STAR transverse results
• Surprisingly large SSA for forward incl. p0 and h
• Led to predictions for new measurements based on QCD
color interactions
• Sivers and Collins effects provide windows to orbital
angular momentum and transversity (respectively)
• STAR has made important constraints on the gluon spin
contribution to that of the proton
• Importance of x dependence shape and low x for DG
• 2009 and future Di-jets and g-jets to address these
• W production offers opportunities to constrain the flavor
asymmetry of the anti-quarks
38
Backup slides
39
Forward g – jet at mid-rapidity gives access to
Benefits from
few 10-3 < x < few 10-2
• back angle cross section rise
Narrow fiducial volume and
Eg > 35 GeV gives sufficient rates
^ ~1
• a
LL
• Large x valence quarks
Rest of FMS as isol. veto
gives S/B ~2:1
Model for high pT scattering in pp
collisions
p
p
Our tool for determining the spin of
partons in the proton
Jets
and
p0s
Assumptions:
Asymptotic freedom
Partonic Process
But
a
possibly
bigger
question: Fragmentation
Factorization
Spin Dependence
Function - measured
Calc.
NLO
pQCD
Universality
Evolution
How
well and when does this
all
work
as
f a fb  fc X
f a fb  fc X
h
ˆ
ˆ
D
f

D
f

d


a

D
 a tool?
b
LL
fc
a precise
   quantitative
   a ,b , c
ALL   

f a fb  fc X

h
ˆ
 
f

f

d


D
 a b
fc
a ,b , c
Parton Distribution Functions
g(x,Q2),q(x,Q2) - measured
Partonic Process Cross Section
Calc. NLO pQCD
41