Transcript Jet Production in Polarized pp Collisions at RHIC

Jet Production in Polarized pp
Collisions at RHIC
Carl A. Gagliardi
Texas A&M University
for the
STAR Collaboration
Outline
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Carl Gagliardi – DIS2008 – Jets in pp at RHIC
Introduction
2005 results
2006 results
Looking ahead
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Origin of the proton spin?
Polarized DIS: 0.2~0.3
Poorly Constrained
1 1
 S      G   Lqz    Lgz 
2 2
p
z
Three recent fits of equal quality:
– ΔG = 0.13 ± 0.16
– ΔG ~ 0.006
– ΔG = -0.20 ± 0.41
all at Q2 = 1 GeV2
Leader et al, PRD 75, 074027
• A primary goal of the RHIC Spin program is to
determine the gluon polarization distribution
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Polarized pp collisions at RHIC
       f a f b
ALL   

aˆ LL

 
f a fb
f: polarized parton distribution functions
Partonic fractions in jet
production at 200 GeV
qg -> qg FOM
1
0.75
0.5
0.25
0
0
0.2
0.4
0.6
0.8
1
|cos (theta*)|
cos 
0
10
20
30 pT(GeV)
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
For most RHIC kinematics, gg and qg
dominate, making ALL for jets sensitive
to gluon polarization.
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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
Spin Rotators
(longitudinal polarization)
Solenoid Partial Siberian Snake
LINAC
Helical Partial Siberian Snake
BOOSTER
AGS
200 MeV Polarimeter
AGS Internal Polarimeter
AGS pC Polarimeters
Rf Dipole
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
“Thousands of millions”
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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STAR detector
E-M
Calorimeter
Projection
Chamber
Time
of
Flight
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Jet reconstruction in STAR
MC jets
GEANT
e,   
 , p, etc
q, g
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
PYTHIA
Particle
Detector
Data jets
Midpoint cone algorithm
(Adapted from Tevatron II - hep-ex/0005012)
• Seed energy = 0.5 GeV
• Cone radius in -
• R=0.4 with 0.2 <  < 0.8 (2005)
• R=0.7 with -0.7 <  < 0.9 (2006)
• Splitting/merging fraction f=0.5
Use PYTHIA + GEANT to
quantify detector response
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Jet production at RHIC
STAR
PRL 97, 252001
• Jet structure at 200 GeV is well
understood
• Mid-rapidity jet cross section is
well described by pQCD over 7
orders of magnitude
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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2005 inclusive jets ALL systematics
ALL systematics
(x 10 -3)
Reconstruction +
Trigger Bias
2-5 (pT dep)
Non-longitudinal
Polarization
0.1-0.8 (pT dep)
Relative Luminosity
0.94
Backgrounds
0.70
pT systematic
[-5.4%,+6.7%]
Neutral Energy Fraction
• Reconstruction bias: combination of
steeply falling spectrum and ~25% jet
ET resolution
• Trigger bias: non-uniform sampling of
gg, qg, and qq processes
• Will illustrate impact on ALL with 2006
data
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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2005 inclusive jets ALL
STAR
hep-ex arXiv:0710.2048
Model calcs from:
Jager et al, PRD 70, 034010
200 GeV
0.2 < η < 0.8
• Comparison to predictions within the GRSV framework
with various input values of ΔG demonstrate sensitivity
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Limits on ΔG from 2005 jet results
STAR
hep-ex arXiv:0710.2048
GRSV DIS
• Significant new constraints on ΔG when compared to
predictions derived from one global fit to DIS data
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Other global analyses
xΔg(x) at
Q2 = 10 GeV2
• There are many other global analyses of the polarized DIS data
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Additional ALL predictions for 2005
• Many predictions for ALL vs. pT
• Is the sensitivity unique to GRSV?
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Comparison to other global analyses
STAR
• The STAR data exclude a broad range of models that have
ΔG larger than that in GRSV-std
• The counterexample is GS-C, which is negative at large x, has
a node near x ~ 0.1, and has a large first moment at small x
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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2006 data
Improved figure of merit
 Luminosity :
 Polarization:
 Barrel EMC  coverage:
2  4.7 pb-1
50%  60% (online polarization)
[0,1]  [-1,1]
In addition
 Jet cone radius: 0.4  0.7
 -0.7 < jet axis < 0.9
 Neutral energy fraction < 0.85
 Increased trigger thresholds
 Inclusion of Endcap EMC towers
 Improved tracking at large ||
 Shielding from backgrounds
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Jet reconstruction and trigger bias for 2006 data
Step 1: Use PYTHIA+GEANT to
estimate the pT shift from
detector jets to particle jets
Step 2: Simulate the difference in ALL
between particle and detector jets for
various gluon polarization scenarios
Particle jets
Shifted detector jets
Detector jets
•
•
Maximum deviation
determines ALL systematic
Confidence level calculations
take account of any residual
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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2006 inclusive jets ALL
STAR
Model curves calculated with cone
radius 0.7 and -0.7 <  < 0.9
200 GeV
-0.7 <  < 0.9
ALL systematics
(x 10 -3)
Reconstruction +
Trigger Bias
[-1,+3]
(pT dep)
Non-longitudinal
Polarization
~ 0.03
(pT dep)
Relative
Luminosity
0.94
Backgrounds
1st bin ~ 0.5
else ~ 0.1
pT systematic
6.7%
• Statistical uncertainties are 3-4 times smaller than in ’05 data
for pT > 13 GeV/c
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Limits on ΔG from 2006 jet results
STAR
GRSV DIS
• Within the GRSV framework:
– GRSV-std excluded with 99% CL
– ΔG < -0.7 excluded with 90% CL
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Looking beyond inclusive ALL measurements
• Inclusive ALL measurements at fixed pT average over a
broad x range.
• Need a global analysis to determine the implications
• Can hide considerable structure if Δg(x) has a node
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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DSSV – first global analysis with polarized jets
de Florian et al., arXiv:0804.0422 [hep-ph]
• 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
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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The next few years: di-jets and Δg(x)
STAR
Mass

Rapidity
|cos(*)|

1
p3 e η3  p4 e η4
s
1
x2 
p3 e η3  p4 e η4
s
x1 
2005 preliminary di-jet distributions


M  x1 x2 s
y
1 x1 η3  η4
ln 
2 x2
2
| cosθ* | tanh
| η3  η4 |
2
Ratio
• Di-jets provide direct access to parton kinematics at LO
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Projected di-jet sensitivity for the 2009 run
Rel FOM
~ 70%
x1 ~ 0.33
x2 ~ 0.044
GS-C
x1 ~ 0.45
x2 ~ 0.16
x1 ~ 0.27
x2 ~ 0.27
• Assumes 50 pb-1 of 200 GeV pp collisions with 60% polarization
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Conclusions
• STAR inclusive jet ALL measurements have excluded
extreme gluon polarization scenarios
• Over the next few years, STAR di-jet measurements
will provide direct information about Δg(x)
• STAR will also obtain complementary information about
Δg(x) from γ + jet
• Stay tuned!
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Double longitudinal spin asymmetry, ALL
     1 N   RN
ALL 


     P1P2 N   RN
N : Spin dependent yields (# of reconstructed jets)

P : Beam polarization (measured by RHIC Polarimeter)
R : Relative luminosities between different spin states
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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STAR detectors for jet measurements

Time Projection Chamber
||<1.4 tracking

Barrel EM Calorimeter
||<1 triggering & calorimetry

Endcap EM Calorimeter
1.09<<2 triggering & calorimetry

Beam-Beam Counters
3.4<||<5 triggering, luminosity,
local polarimetry
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Other global analyses
xΔg(x) at
Q2 = 10 GeV2
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Additional ALL predictions for 2005
ALL
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Estimating the non-jet background
Data
Monte Carlo
Charged Energy Fraction
Data
Carl Gagliardi – DIS2008 – JetsCharged
in pp atEnergy
RHICFraction
Charged Energy Fraction
•
Non-jet backgrounds stand out in the
data as single-jet events that have a
small charged energy fraction
•
Isolate the background in the ratio of
mono-jet to di-jet events vs. charged
energy fraction
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ALL vs. run index for 2006 data
STAR Preliminary
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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False asymmetries in 2006 data
STAR Preliminary
STAR Preliminary
• Longitudinal single-spin (parity-violating) asymmetries are
consistent with zero, as expected at this precision
Carl Gagliardi – DIS2008 – Jets in pp at RHIC
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Inclusive jet ALL for fixed values of ΔG
• Calculations by Stratmann and Vogelsang
within the GRSV framework
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