Kent State University
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Longitudinal Double-Spin Asymmetry for
Inclusive Jet Production in (P)Over-Right-Arrow +
(P)Over-Right-Arrow Collisions at Root S=200
Gev
B. I. Abelev
Quan Li
Kent State University - Kent Campus4/-.)17)(8
STAR Collaboration
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Longitudinal Double-Spin Asymmetry for Inclusive Jet Production in
~
p
~
p Collisions
at
s
p
200 GeV
B. I. Abelev,
9
M. M. Aggarwal,
31
Z. Ahammed,
46
B. D. Anderson,
20
D. Arkhipkin,
13
G. S. Averichev,
12
Y. Bai,
29
J. Balewski,
17
O. Barannikova,
9
L. S. Barnby,
2
J. Baudot,
18
S. Baumgart,
51
V. V. Belaga,
12
A. Bellingeri-Laurikainen,
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R. Bellwied,
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F. Benedosso,
29
R. R. Betts,
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S. Bhardwaj,
36
A. Bhasin,
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A. K. Bhati,
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H. Bichsel,
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J. Bielcik,
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J. Bielcikova,
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L. C. Bland,
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M. Bombara,
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´
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PRL 100, 232003 (2008)
PHYSICAL REVIEW LETTERS
week ending
13 JUNE 2008
0031-9007=08=100(23)=232003(7) 232003-1 © 2008 The American Physical Society
(STAR Collaboration)
1
Argonne National Laboratory, Argonne, Illinois 60439, USA
2
University of Birmingham, Birmingham, United Kingdom
3
Brookhaven National Laboratory, Upton, New York 11973, USA
4
California Institute of Technology, Pasadena, California 91125, USA
5
University of California, Berkeley, California 94720, USA
6
University of California, Davis, California 95616, USA
7
University of California, Los Angeles, California 90095, USA
8
Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
9
University of Illinois at Chicago, Chicago, Illinois 60607, USA
10
Creighton University, Omaha, Nebraska 68178, USA
11
Nuclear Physics Institute AS CR, 250 68 R
ˇ
ez
ˇ
/Prague, Czech Republic
12
Laboratory for High Energy (JINR), Dubna, Russia
13
Particle Physics Laboratory (JINR), Dubna, Russia
14
University of Frankfurt, Frankfurt, Germany
15
Institute of Physics, Bhubaneswar 751005, India
16
Indian Institute of Technology, Mumbai, India
17
Indiana University, Bloomington, Indiana 47408, USA
18
Institut de Recherches Subatomiques, Strasbourg, France
19
University of Jammu, Jammu 180001, India
20
Kent State University, Kent, Ohio 44242, USA
21
University of Kentucky, Lexington, Kentucky, 40506-0055, USA
22
Institute of Modern Physics, Lanzhou, China
23
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
24
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA
25
Max-Planck-Institut fu
¨
r Physik, Munich, Germany
26
Michigan State University, East Lansing, Michigan 48824, USA
27
Moscow Engineering Physics Institute, Moscow Russia
28
City College of New York, New York City, New York 10031, USA
29
NIKHEF and Utrecht University, Amsterdam, The Netherlands
30
Ohio State University, Columbus, Ohio 43210, USA
31
Panjab University, Chandigarh 160014, India
32
Pennsylvania State University, University Park, Pennsylvania 16802, USA
33
Institute of High Energy Physics, Protvino, Russia
34
Purdue University, West Lafayette, Indiana 47907, USA
35
Pusan National University, Pusan, Republic of Korea
36
University of Rajasthan, Jaipur 302004, India
37
Rice University, Houston, Texas 77251, USA
38
Universidade de Sao Paulo, Sao Paulo, Brazil
39
University of Science & Technology of China, Hefei 230026, China
40
Shanghai Institute of Applied Physics, Shanghai 201800, China
41
SUBATECH, Nantes, France
42
Texas A&M University, College Station, Texas 77843, USA
43
University of Texas, Austin, Texas 78712, USA
44
Tsinghua University, Beijing 100084, China
45
Valparaiso University, Valparaiso, Indiana 46383, USA
46
Variable Energy Cyclotron Centre, Kolkata 700064, India
47
Warsaw University of Technology, Warsaw, Poland
48
University of Washington, Seattle, Washington 98195, USA
49
Wayne State University, Detroit, Michigan 48201, USA
50
Institute of Particle Physics, CCNU (HZNU), Wuhan 430079, China
51
Yale University, New Haven, Connecticut 06520, USA
52
University of Zagreb, Zagreb, HR-10002, Croatia
(Received 12 October 2007; published 13 June 2008)
We report a new STAR measurement of the longitudinal double-spin asymmetry A
LL
for inclusive jet
production at midrapidity in polarized p p collisions at a center-of-mass energy of
s
p
200 GeV. The
data, which cover jet transverse momenta 5 <p
T
< 30 GeV=c, are substantially more precise than
previous measurements. They provide significant new constraints on the gluon spin contribution to the
PRL 100, 232003 (2008)
PHYSICAL REVIEW LETTERS
week ending
13 JUNE 2008
232003-2
nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic
scattering measurements.
DOI: 10.1103/PhysRevLett.100.232003 PACS numbers: 14.20.Dh, 13.87.Ce, 13.88.+e, 14.70.Dj
Understanding how the spin of the nucleon is con-
structed from the spin and angular momentum of the
constituent quarks and gluons is a fundamental and unre-
solved question in quantum chromodynamics (QCD). The
gluon spin contribution to the nucleon spin G has been
the focus of theoretical and experimental efforts since
polarized deep-inelastic scattering (DIS) experiments
found the quark spin contribution to be unexpectedly small
[1]. Unlike quarks, gluons do not couple directly to the
virtual photon emitted by the lepton in DIS. Information
about G must be extracted from the next-to-leading-order
(NLO) perturbative QCD (pQCD) analysis of the momen-
tum transfer dependence of the same inclusive spin struc-
ture functions used in determining the quark spin
contribution [2–7]. Measurements of high-p
T
hadron pairs
and charm mesons resulting from the photon-gluon fusion
process in DIS have provided additional but limited con-
straints [8]. In contrast to DIS, hadronic interactions pro-
vide direct, leading order access to both the quark and
gluon polarized parton distribution functions (PDFs) via
detection of the jets of particles fragmenting from the
scattered partons. Early hadroproduction measurements
utilized fixed targets [9], but more recently results from
the first ever polarized collider RHIC [10,11] are now
being incorporated into NLO pQCD fits [12] with the
ultimate goal of extracting their constraint on G.
Despite recent progress, significant uncertainty remains
regarding the magnitude and sign of G [13]. The inclu-
sive measurements presented here span more than an order
of magnitude in partonic momentum fraction (x) and are
expected to sample a sizable piece of the total integral.
Comparisons with predictions derived from one global fit
[4,14] to deep-inelastic scattering measurements are used
to demonstrate the substantial new constraints these results
place on G.
In this Letter, we report a new measurement of the
longitudinal double-spin asymmetry A
LL
for midrapidity
inclusive jet production in polarized p p collisions at
s
p
200 GeV center-of-mass energy,
A
LL
; (1)
where
is the differential cross section when the
beam protons have equal (opposite) helicities. We have
previously measured the helicity-averaged cross section
[10] for transverse momenta (p
T
)upto50 GeV=c and
it is well described by NLO pQCD evaluations. Inclusive
jet production in the kinematic regime studied here is
dominated by gluon-gluon (gg) and quark-gluon (qg) scat-
tering. Therefore, A
LL
provides direct sensitivity to gluon
polarization [14] and the cross-section result motivates the
use of NLO pQCD to interpret our measurements.
The data presented here are extracted from an integrated
luminosity of 2pb
1
recorded in the year 2005 with the
STAR detector [15] at RHIC. The polarization was mea-
sured independently for each of the two counter-rotating
proton beams and for each fill using Coulomb-Nuclear
Interference (CNI) proton-Carbon polarimeters [16],
which were calibrated via a polarized atomic hydrogen
gas-jet target [17]. Averaged over RHIC fills, the luminos-
ity weighted polarizations for the two beams were 52
3% and 48 3%. The proton helicities were alternated
between successive bunches in one beam and between
bunch pairs in the other beam. Additionally, the helicity
configurations of the colliding beam bunches were
changed between beam fills to minimize systematic un-
certainties in the A
LL
measurement. Segmented beam-
beam counters (BBC) [18] located up and downstream of
the STAR interaction region (IR) measured the helicity
dependent relative luminosities, identified minimum bias
(MB) collisions, and served as local polarimeters.
The STAR subsystems used to measure jets are the time
projection chamber (TPC) and the barrel electromagnetic
calorimeter (BEMC) [15]. The TPC provides tracking for
charged particles in the 0.5 T solenoidal magnetic field for
pseudorapidities 1:3 & & 1:3 and 2 in the azimuthal
angle . In 2005 the BEMC, covering a fiducial area of
2 and 0 <<1, provided triggering and detection
of photons and electrons.
Events were recorded if they satisfied both the MB
condition, defined as a coincidence between east and
west BBCs, and either a jet patch (JP) or high tower
(HT) trigger. The HT condition required the energy of a
single calorimeter tower to be at least 2.6 (HT1) or 3.6
(HT2) GeV. The JP trigger fired if the sum of a
1 1 patch of towers, the typical size of a jet,
exceeded 4.5 (JP1) or 6.5 (JP2) GeV. Approximately half
of the 2:38 10
6
jets extracted from the 12 10
6
event set
originated from the JP2 trigger sample.
Jets were reconstructed using a midpoint cone algorithm
[19] with the same parameters as described in Ref. [10].
The algorithm clusters TPC charged track momenta and
BEMC tower energies within a cone radius of R
2
2
p
0:4. Jets were required to have p
T
>
5 GeV=c and point between 0:2–0:8 in order to mini-
mize the effects of the BEMC acceptance on the jet energy
scale. BBC timing information was used to select events
with reconstructed vertex positions within 60 cm of the
center of the detector, ensuring uniform tracking efficiency
and matching the conditions used in determining the rela-
PRL 100, 232003 (2008)
PHYSICAL REVIEW LETTERS
week ending
13 JUNE 2008
232003-3
tive luminosity measurements. Beam background from
upstream sources observed as neutral energy deposits in
the BEMC were minimized by requiring the neutral energy
fraction of the jet energy (NEF) to be less than 0.8. A
minimum NEF of 0.1 was also imposed in order to reduce
pileup effects. Finally, only jets which contained a trigger
tower or pointed to a triggered jet patch were considered
for analysis.
Figure 1 compares the NEF spectra for MB, HT, and JP
triggered jets from data and simulations. Monte Carlo
events were generated using
PYTHIA 6.205 [20] with pa-
rameters adjusted to CDF ‘‘Tune A’’ settings [21] and
processed through the STAR detector response package
based on
GEANT 3[22]. The shapes of the data distributions
are sufficiently reproduced by the simulations for the pur-
pose of estimating systematic errors. In contrast to the
calorimeter triggers, the mean and shape of the MB distri-
bution is relatively stable as a function of jet p
T
. The HT
jets, and to a lesser extent the JP jets, show a strong bias
towards higher NEF at low p
T
which diminishes for higher
jet p
T
. The enhancement of jets near NEF 1 in the data
compared to simulation is consistent with contributions
from beam background, as discussed above.
We present the inclusive jet A
LL
measurement, not as a
function of the measured transverse momentum
(
DETECTOR jet p
T
), but instead corrected to reflect the jet
energy scale before interaction with the STAR detector
(
PARTICLE jet p
T
). This correction was carried out by
applying the same jet reconstruction algorithm to the si-
mulated event samples at both the
PARTICLE and DETECTOR
levels. PARTICLE jets are composed of stable, final-state
particles which result from the fragmentation and hadro-
nization of the scattered partons and remnant protons.
DETECTOR jets consist of the reconstructed TPC tracks
and BEMC tower energies in simulated events that pass
the same trigger conditions that were placed on the data. As
shown in Fig. 2(a) the jet yield is a rapidly falling function
of jet p
T
. This effect combined with the jet p
T
resolution of
(GeV/c)
T
DETECTOR Jet p
5 101520253035
-4
10
-3
10
-2
10
-1
10
1
Jet Yield
a)
2005 STAR
JP2 Data
STAR JP2
Simulations
(GeV/c)
T
DETECTOR Jet p
0 5 10 15 20 25 30 35
0
5
10
15
20
25
30
35
b)
(GeV/c)
T
PARTICLE Jet
p
FIG. 2. (a) The raw detected jet yield in data (points) compared with the STAR Monte Carlo simulations. (b) Correlation between the
reconstructed jet transverse momenta at the
PARTICLE and DETECTOR levels. The points indicate the means and the vertical error bars
show the rms widths of the associated
PARTICLE jet distributions within the DETECTOR jet bins. The dashed line represents the condition
when the
PARTICLE and DETECTOR jet p
T
values are equal.
NEF
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Normalized Jet Yield
-5
10
-4
10
-3
10
-2
10
-1
10
1
a)
< 11.4 GeV/c
T
6.2 < p
HT1 data
JP1 data
MB data
NEF
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-5
10
-4
10
-3
10
-2
10
-1
10
1
b)
< 32.2 GeV/c
T
17.3 < p
HT2 data
JP2 data
FIG. 1 (color online). Neutral energy fraction of the jet energy for MB (crosses), HT (circles), and JP (squares) data compared with
STAR simulations for two jet p
T
bins, (a) 6:2 <p
T
< 11:4 GeV=c and (b) 17:3 <p
T
< 32:2 GeV=c. The statistical uncertainties are
represented as error bars for the data points and bands for the simulations.
PRL 100, 232003 (2008)
PHYSICAL REVIEW LETTERS
week ending
13 JUNE 2008
232003-4
![FIG. 3 (color online). Longitudinal double-spin asymmetry ALL for inclusive jet production at s p 200 GeV versus jet pT . The points show results for PARTICLE jets with statistical error bars, while the curves show predictions for NLO parton jets [14] from the GRSV [4] and GS-C [23] global analysis. The gray boxes indicate the systematic uncertainties on the measured ALL values (vertical) and in the corrections to the measured jet pT and the conversion between PARTICLE jet and NLO parton jet pT (horizontal).](/figures/fig-3-color-online-longitudinal-double-spin-asymmetry-all-dsxbv7ns.webp)
![FIG. 4 (color online). (a) The solid curve represents the fraction of G for GRSV-std that has x > xmin for scale Q2 100 GeV2=c2. The histograms show the xgluon sampled in the lowest and highest jet pT bins. (b) Confidence levels for several gluon polarization distributions, characterized by their G at an input scale of 0:4 GeV2=c2 [4,14,24].](/figures/fig-4-color-online-a-the-solid-curve-represents-the-fraction-1btgrp1y.webp)