-
STAR Collaboration,
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford, C. D. Anson, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: A study is reported of the same- and opposite-sign charge-dependent azimuthal
correlations with respect to the event plane in Au+Au collisions at 200 GeV.
The charge multiplicity asymmetries between the up/down and left/right
hemispheres relative to the event plane are utilized. The contributions from
statistical fluctuations and detector effects were subtracted from the
(co-)variance of the observed charge multiplicity asymmetries. In the mid- to
most-central collisions, the same- (opposite-) sign pairs are preferentially
emitted in back-to-back (aligned on the same-side) directions. The charge
separation across the event plane, measured by the difference, $\Delta$,
between the like- and unlike-sign up/down $-$ left/right correlations, is
largest near the event plane. The difference is found to be proportional to the
event-by-event final-state particle ellipticity (via the observed second-order
harmonic $v_2^{\rm obs}$, where $\Delta=(1.3\pm1.4({\rm
stat})^{+4.0}_{-1.0}({\rm syst}))\times10^{-5}+(3.2\pm0.2({\rm
stat})^{+0.4}_{-0.3}({\rm syst}))\times10^{-3}v_2^{\rm obs}$ for 20-40% Au+Au
collisions. The implications for the proposed chiral magnetic effect (\cme) are
discussed.
03/2013;
-
STAR Collaboration,
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: Azimuthal angular correlations of charged hadrons with respect to the axis of
a reconstructed (trigger) jet in Au+Au and p+p collisions at $\sqrt{s_{NN}} =
200$ GeV in STAR are presented. The trigger jet population in Au+Au collisions
is biased towards jets that have not interacted with the medium, allowing
easier matching of jet energies between Au+Au and p+p collisions while
enhancing medium effects on the recoil jet. The associated hadron yield of the
recoil jet is significantly suppressed at high transverse momentum
($p_{T}^{assoc}$) and enhanced at low $p_{T}^{assoc}$ in Au+Au collisions,
which is indicative of medium-induced parton energy loss in ultrarelativistic
heavy-ion collisions.
02/2013;
-
STAR Collaboration,
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: Recent experimental results from the STAR collaboration suggest
event-by-event charge separation fluctuations perpendicular to the event plane
in non-central heavy-ion collisions. Here we present the correlator previously
used split into its two component parts to reveal correlations parallel and
perpendicular to the event plane. The results are from a high statistics 200
GeV Au+Au collisions data set collected by the STAR experiment at RHIC. We
explicitly count units of charge separation from which we find clear evidence
for more charge separation fluctuations perpendicular than parallel to the
event plane. We also employ a modified correlator to study the possible P-even
background in same and opposite charge correlations.
02/2013;
-
STAR Collaboration,
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: We present measurements of three-dimensional correlation functions of
like-sign low transverse momentum kaon pairs from sqrt(sNN)=200 GeV Au+Au
collisions. A Cartesian surface-spherical harmonic decomposition technique was
used to extract the kaon source function. The latter was found to have a
three-dimensional Gaussian shape and can be adequately reproduced by
Therminator event generator simulations with resonance contributions taken into
account. Compared to the pion one, the kaon source function is generally
narrower and does not have the long tail along the pair transverse momentum
direction. The kaon Gaussian radii display a monotonic decrease with increasing
transverse mass m_T over the interval of 0.55<=m_T<=1.15 GeV/c^2. While the
kaon radii are adequately described by the m_T-scaling in the outward and
sideward directions, in the longitudinal direction the lowest m_T value exceeds
the expectations from a pure hydrodynamical model prediction.
02/2013;
-
STAR Collaboration,
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: We present a study of the average transverse momentum ($p_t$) fluctuations
and $p_t$ correlations for charged particles produced in Cu+Cu collisions at
midrapidity for $\sqrt{s_{NN}} =$ 62.4 and 200 GeV. These results are compared
with those published for Au+Au collisions at same energies, to explore the
system size dependence. In addition to the collision energy and system size
dependence, the $p_t$ correlations results have been studied as functions of
the collision centralities, the ranges in $p_t$, the pseudo-rapidity $\eta$,
and the azimuthal angle $\phi$, for which the correlations are measured. The
square root of the measured $p_t$ correlations when scaled by mean-$p_t$ are
found to be independent of both colliding beam energy and system size studied.
The transport based model calculations are found to have a better quantitative
agreement with the measurements compared to models which incorporate only
jet-like correlations.
01/2013;
-
STAR Collaboration,
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: We report measurements of the third harmonic coefficient of the azimuthal
anisotropy, v_3, known as triangular flow. The analysis is for charged
particles in Au+Au collisions at $\sqrtsNN = 200$ GeV, based on data from the
STAR experiment at the Relativistic Heavy Ion Collider. Two-particle
correlations as a function of their pseudorapidity separation are fit with
narrow and wide Gaussians. Measurements of triangular flow are extracted from
the wide Gaussian, from two-particle cumulants with a pseudorapidity gap, and
also from event plane analysis methods with a large pseudorapidity gap between
the particles and the event plane. These results are reported as a function of
transverse momentum and centrality. A large dependence on the pseudorapidity
gap is found. Results are compared with other experiments and model
calculations.
01/2013;
-
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin,
D. Arkhipkin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: Elliptic flow ($v_{2}$) values for identified particles at mid-rapidity in
Au+Au collisions, measured by the STAR experiment in the Beam Energy Scan at
RHIC at $\sqrt{s_{NN}}=$ 7.7--62.4 GeV, are presented. A beam-energy dependent
difference of the values of $v_{2}$ between particles and corresponding
anti-particles was observed. The difference increases with decreasing beam
energy and is larger for baryons compared to mesons. This implies that, at
lower energies, particles and anti-particles are not consistent with the
universal number-of-constituent-quark (NCQ) scaling of $v_{2}$ that was
observed at $\sqrt{s_{NN}}=$ 200 GeV.
01/2013;
-
L. Adamczyk,
J. K. Adkins,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
I. Alekseev,
J. Alford, C. D. Anson,
A. Aparin,
D. Arkhipkin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the elliptic flow, $v_{2}$, of identified hadrons
($\pi^{\pm}$, $K^{\pm}$, $K_{s}^{0}$, $p$, $\bar{p}$, $\phi$, $\Lambda$,
$\bar{\Lambda}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$, $\bar{\Omega}^{+}$)
in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV
are presented. The measurements were done at mid-rapidity using the Time
Projection Chamber and the Time-of-Flight detectors of the STAR experiment
during the Beam Energy Scan program at RHIC. A significant difference in the
$v_{2}$ values for particles and the corresponding anti-particles was observed
at all transverse momenta for the first time. The difference increases with
decreasing center-of-mass energy, $\sqrt{s_{NN}}$ (or increasing baryon
chemical potential, $\mu_{B}$) and is larger for the baryons as compared to the
mesons. This implies that particles and anti-particles are no longer consistent
with the universal number-of-constituent quark (NCQ) scaling of $v_{2}$ that
was observed at $\sqrt{s_{NN}}=$ 200 GeV. However, for the group of particles
NCQ scaling at $(m_{T}-m_{0})/n_{q}>$ 0.4 GeV/$c^{2}$ is not violated within
$\pm$10%. The $v_{2}$ values for $\phi$ mesons at 7.7 and 11.5 GeV are
approximately two standard deviations from the trend defined by the other
hadrons at the highest measured $p_{T}$ values.
01/2013;
-
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson,
D. Arkhipkin, [......],
W. M. Zhang,
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report STAR measurements of the longitudinal double-spin asymmetry ALL, the transverse single-spin asymmetry AN, and the transverse double-spin asymmetries AΣ and ATT for inclusive jet production at mid-rapidity in polarized p+p collisions at a center-of-mass energy of √s=200 GeV. The data represent integrated luminosities of 7.6 pb-1 with longitudinal polarization and 1.8 pb-1 with transverse polarization, with 50%–55% beam polarization, and were recorded in 2005 and 2006. No evidence is found for the existence of statistically significant jet AN, AΣ, or ATT at mid-rapidity. Recent model calculations indicate the AN results may provide new limits on the gluon Sivers distribution in the proton. The asymmetry ALL significantly improves the knowledge of gluon polarization in the nucleon.
Phys. Rev. D. 08/2012; 86(3).
-
STAR Collaboration,
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford, C. D. Anson,
D. Arkhipkin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: We report a high precision measurement of the transverse single spin
asymmetry $A_N$ at the center of mass energy $\sqrt{s}=200$ GeV in elastic
proton-proton scattering by the STAR experiment at RHIC. The $A_N$ was measured
in the four-momentum transfer squared $t$ range $0.003 \leqslant |t| \leqslant
0.035$ $\GeVcSq$, the region of a significant interference between the
electromagnetic and hadronic scattering amplitudes. The measured values of
$A_N$ and its $t$-dependence are consistent with a vanishing hadronic spin-flip
amplitude, thus providing strong constraints on the ratio of the single
spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated
by the Pomeron amplitude at this $\sqrt{s}$, we conclude that this measurement
addresses the question about the presence of a hadronic spin flip due to the
Pomeron exchange in polarized proton-proton elastic scattering.
06/2012;
-
The STAR Collaboration,
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson, [......],
W. M. Zhang,
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the differential cross-section and the transverse single-spin
asymmetry, A_N, vs. x_F for pi0 and eta mesons are reported for 0.4 < x_F <
0.75 at an average pseudorapidity of 3.68. A data sample of approximately 6.3
pb^{-1} was analyzed, which was recorded during p+p collisions at sqrt{s} = 200
GeV by the STAR experiment at RHIC. The average transverse beam polarization
was 56%. The cross-section for pi0 is consistent with a perturbative QCD
prediction, and the eta/pi0 cross-section ratio agrees with previous
mid-rapidity measurements. For 0.55 < x_F < 0.75, A_N for eta (0.210 +- 0.056)
is 2.2 standard deviations larger than A_N for pi0 (0.081 +- 0.016).
05/2012;
-
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson,
D. Arkhipkin, [......],
W. M. Zhang,
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report measurements of the charge-separated W+(-)→e+(-)+νe(ν̅ e) and Z/γ*→e+e- production cross sections at mid-rapidity in proton-proton collisions at √s=500 GeV. These results are based on 13.2 pb-1 of data recorded in 2009 by the STAR detector at RHIC. Production cross sections for W bosons that decay via the eν channel were measured to be σ(pp→W+X)·BR(W+→e+νe)=117.3±5.9(stat)±6.2(syst)±15.2(lumi) pb, and σ(pp→W-X)·BR(W-→e-ν̅ e)=43.3±4.6(stat)± 3.4(syst)±5.6(lumi) pb. For Z/γ* production, σ(pp→Z/γ*X)·BR(Z/γ*→e+e-)=7.7±2.1(stat)-0.9+0.5(syst)±1.0(lumi) pb was measured for di-lepton invariant masses me+e- between 70 and 110 GeV/c2. First measurements of the W cross section ratio, σ(pp→W+X)/σ(pp→W-X), at √s=500 GeV are also reported. Theoretical predictions, calculated using recent parton distribution functions, are found to agree with the measured cross sections.
Phys. Rev. D. 05/2012; 85(9).
-
The STAR Collaboration,
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson, [......],
W. M. Zhang,
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report STAR measurements of the longitudinal double-spin asymmetry A_LL,
the transverse single-spin asymmetry A_N, and the transverse double-spin
asymmetries A_Sigma and A_TT for inclusive jet production at mid-rapidity in
polarized p+p collisions at a center-of-mass energy of sqrt{s} = 200 GeV. The
data represent integrated luminosities of 7.6 /pb with longitudinal
polarization and 1.8 /pb with transverse polarization, with 50-55% beam
polarization, and were recorded in 2005 and 2006. No evidence is found for the
existence of statistically significant jet A_N, A_Sigma, or A_TT at
mid-rapidity. Recent model calculations indicate the A_N results may provide
new limits on the gluon Sivers distribution in the proton. The asymmetry A_LL
significantly improves the knowledge of gluon polarization in the nucleon.
05/2012;
-
STAR Collaboration,
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: We report measurements of charmed-hadron ($D^{0}$, $D^{*}$) production cross
sections at mid-rapidity in $p$ + $p$ collisions at a center-of-mass energy of
200 GeV by the STAR experiment. Charmed hadrons were reconstructed via the
hadronic decays $D^{0}\rightarrow K^{-}\pi^{+}$, $D^{*+}\rightarrow
D^{0}\pi^{+}\rightarrow K^{-}\pi^{+}\pi^{+}$ and their charge conjugates,
covering the $p_T$ range of 0.6$-$2.0 GeV/$c$ and 2.0$-$6.0 GeV/$c$ for $D^{0}$
and $D^{*+}$, respectively. From this analysis, the charm-pair production cross
section at mid-rapidity is $d\sigma/dy|_{y=0}^{c\bar{c}}$ = 170 $\pm$ 45
(stat.) $^{+38}_{-59}$ (sys.) $\mu$b. The extracted charm-pair cross section is
compared to perturbative QCD calculations. The transverse momentum differential
cross section is found to be consistent with the upper bound of a Fixed-Order
Next-to-Leading Logarithm calculation.
04/2012;
-
L. Adamczyk,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson,
D. Arkhipkin, [......],
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva,
M. Zyzak
[show abstract]
[hide abstract]
ABSTRACT: We report on mid-rapidity mass spectrum of di-electrons and cross sections of
pseudoscalar and vector mesons via $e^{+}e^{-}$ decays, from $\sqrt{s} = 200$
GeV $p+p$ collisions, measured by the large acceptance experiment STAR at RHIC.
The ratio of the di-electron continuum to the combinatorial background is
larger than 10% over the entire mass range. Simulations of di-electrons from
light-meson decays and heavy-flavor decays (charmonium and open charm
correlation) are found to describe the data. The extracted $\omega\rightarrow
e^{+}e^{-}$ invariant yields are consistent with previous measurements. The
mid-rapidity yields ($dN/dy$) of $\phi$ and $J/\psi$ are extracted through
their di-electron decay channels and are consistent with the previous
measurements of $\phi\rightarrow K^{+}K^{-}$ and $J/\psi\rightarrow
e^{+}e^{-}$. Our results suggest a new upper limit of the branching ratio of
the $\eta \rightarrow e^{+}e^{-}$ of $1.7\times10^{-5}$ at 90% confidence
level.
04/2012;
-
G Agakishiev,
M M Aggarwal,
Z Ahammed,
A V Alakhverdyants,
I Alekseev,
J Alford,
B D Anderson, C D Anson,
D Arkhipkin,
G S Averichev, [......],
W M Zhang,
X P Zhang,
Y Zhang,
Z P Zhang,
F Zhao,
J Zhao,
C Zhong,
X Zhu,
Y H Zhu,
Y Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report new STAR measurements of midrapidity yields for the Λ, Λ[over ¯], K(S)(0), Ξ(-), Ξ[over ¯](+), Ω(-), Ω[over ¯](+) particles in Cu+Cu collisions at √S(NN)==200 GeV, and midrapidity yields for the Λ, Λ[over ¯], K(S)(0) particles in Au+Au at √S(NN)==200 GeV. We show that, at a given number of participating nucleons, the production of strange hadrons is higher in Cu+Cu collisions than in Au+Au collisions at the same center-of-mass energy. We find that aspects of the enhancement factors for all particles can be described by a parametrization based on the fraction of participants that undergo multiple collisions.
Physical Review Letters 02/2012; 108(7):072301. · 7.37 Impact Factor
-
G Agakishiev,
M M Aggarwal,
Z Ahammed,
A V Alakhverdyants,
I Alekseev,
J Alford,
B D Anderson, C D Anson,
D Arkhipkin,
G S Averichev, [......],
W M Zhang,
X P Zhang,
Y Zhang,
Z P Zhang,
F Zhao,
J Zhao,
C Zhong,
X Zhu,
Y H Zhu,
Y Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report transverse momentum (p(T)≤15 GeV/c) spectra of π(±), K(±), p, p[over ¯], K(S)(0), and ρ(0) at midrapidity in p+p and Au+Au collisions at √S(NN)=200 GeV. Perturbative QCD calculations are consistent with π(±) spectra in p+p collisions but do not reproduce K and p(p[over ¯]) spectra. The observed decreasing antiparticle-to-particle ratios with increasing p(T) provide experimental evidence for varying quark and gluon jet contributions to high-p(T) hadron yields. The relative hadron abundances in Au+Au at p(T)≳8 GeV/c are measured to be similar to the p+p results, despite the expected Casimir effect for parton energy loss.
Physical Review Letters 02/2012; 108(7):072302. · 7.37 Impact Factor
-
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson,
D. Arkhipkin,
G. S. Averichev, [......],
W. M. Zhang,
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: Two-particle azimuthal (Δφ) and pseudorapidity (Δη) correlations using a trigger particle with large transverse momentum (pT) in d+Au, Cu+Cu, and Au+Au collisions at √sNN = 62.4 GeV and 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider are presented. The near-side correlation is separated into a jet-like component, narrow in both Δφ and Δη, and the ridge, narrow in Δφ but broad in Δη. Both components are studied as a function of collision centrality, and the jet-like correlation is studied as a function of the trigger and associated pT. The behavior of the jet-like component is remarkably consistent for different collision systems, suggesting it is produced by fragmentation. The width of the jet-like correlation is found to increase with the system size. The ridge, previously observed in Au+Au collisions at √sNN = 200 GeV, is also found in Cu+Cu collisions and in collisions at √sNN = 62.4 GeV, but is found to be substantially smaller at √sNN = 62.4 GeV than at √sNN = 200 GeV for the same average number of participants (〈Npart〉). Measurements of the ridge are compared to models.
Phys. Rev. C. 01/2012; 85(1).
-
STAR Collaboration,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B D Anderson, C. D. Anson,
D. Arkhipkin, [......],
W M Zhang,
X P Zhang,
Y Zhang,
Z P Zhang,
F Zhao,
J Zhao,
C. Zhong,
X Zhu,
Y H Zhu,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report measurements of the charge-separated $W^{+(-)} \to e^{+(-)} +
\nu_e(\bar{\nu}_e)$ and $Z/\gamma^* \to e^+e^-$ production cross sections at
mid-rapidity in proton-proton collisions at $\sqrt{s}$ = 500 GeV. These results
are based on 13.2 pb$^{-1}$ of data recorded in 2009 by the STAR detector at
RHIC. Production cross sections for W bosons that decay via the $e \nu$ channel
were measured to be $\sigma(pp \to W^+ X) \cdot BR(W^+ \to e^+ \nu_e)$ = 117.3
\pm 5.9(stat) \pm 6.2(syst) \pm 15.2(lumi) pb, and $\sigma(pp \to W^- X) \cdot
BR(W^- \to e^- \bar{\nu}_e)$ = 43.3 \pm 4.6(stat) \pm 3.4(syst) \pm 5.6(lumi)
pb. For $Z/\gamma^*$ production, $\sigma(pp \to Z/\gamma^* X) \cdot
BR(Z/\gamma^* \to e^+ e^-)$ = 7.7 \pm 2.1(stat) $^{+0.5}_{-0.9}$(syst) \pm
1.0(lumi) pb for di-lepton invariant masses $m_{e^+e^-}$ between 70 and 110
GeV/$c^2$. First measurements of the W cross section ratio, $\sigma(pp \to W^+
X) / \sigma(pp \to W^- X)$, at $\sqrt{s}$ = 500 GeV are also reported.
Theoretical predictions, calculated using recent parton distribution functions,
are found to agree with the measured cross sections.
12/2011;
-
The STAR Collaboration,
G. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford,
B. D. Anderson, C. D. Anson,
D. Arkhipkin, [......],
W. M. Zhang,
X. P. Zhang,
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
X. Zhu,
Y. H. Zhu,
Y. Zoulkarneeva
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ABSTRACT: We present STAR measurements of azimuthal anisotropy by means of the two- and
four-particle cumulants $v_2$ ($v_2\{2\}$ and $v_2\{4\}$) for Au+Au and Cu+Cu
collisions at center of mass energies $\sqrt{s_{_{\mathrm{NN}}}} = 62.4$ and
200 GeV. The difference between $v_2\{2\}^2$ and $v_2\{4\}^2$ is related to
$v_{2}$ fluctuations ($\sigma_{v_2}$) and nonflow $(\delta_{2})$. We present an
upper limit to $\sigma_{v_2}/v_{2}$. Following the assumption that eccentricity
fluctuations $\sigma_{\epsilon}$ dominate $v_2$ fluctuations
$\frac{\sigma_{v_2}}{v_2} \approx \frac{\sigma_{\epsilon}}{\epsilon}$ we deduce
the nonflow implied for several models of eccentricity fluctuations that would
be required for consistency with $v_2\{2\}$ and $v_2\{4\}$. We also present
results on the ratio of $v_2$ to eccentricity.
11/2011;
