-
P. Monaghan,
R. Shneor,
R. Subedi, B. D. Anderson,
K. Aniol,
J. Annand,
J. Arrington,
H. Benaoum,
F. Benmokhtar,
P. Bertin, [......],
G. M. Urciuoli,
E. Voutier,
K. Wang,
J. W. Watson,
L. B. Weinstein,
B. Wojtsekhowski,
S. Wood,
H. Yao,
X. Zheng,
L. Zhu
[show abstract]
[hide abstract]
ABSTRACT: The five-fold differential cross section for the 12C(e,e'p)11B reaction was
determined over a missing momentum range of 200-400 MeV/c, in a kinematics
regime with Bjorken x > 1 and Q2 = 2.0 (GeV/c)2. A comparison of the results
and theoretical models and previous lower missing momentum data is shown. The
theoretical calculations agree well with the data up to a missing momentum
value of 325 MeV/c and then diverge for larger missing momenta. The extracted
distorted momentum distribution is shown to be consistent with previous data
and extends the range of available data up to 400 MeV/c.
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).
-
H. Fonvieille,
G. Laveissière,
N. Degrande,
S. Jaminion,
C. Jutier,
L. Todor,
R. Di Salvo,
L. Van Hoorebeke,
L. C. Alexa, B. D. Anderson, [......],
E. Voutier,
J. W. Watson,
L. B. Weinstein,
K. Wijesooriya,
R. Wilson,
B. B. Wojtsekhowski,
D. G. Zainea,
W-M. Zhang,
J. Zhao,
Z.-L. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Virtual Compton scattering (VCS) on the proton has been studied at the Jefferson Laboratory using the exclusive photon electroproduction reaction ep→epγ. This paper gives a detailed account of the analysis which has led to the determination of the structure functions PLL−PTT/ε and PLT and the electric and magnetic generalized polarizabilities (GPs) αE(Q2) and βM(Q2) at values of the four-momentum transfer squared Q2=0.92 and 1.76 GeV2. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured Q2 range and point to their nontrivial behavior.
Phys. Rev. C. 07/2012; 86(1).
-
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).
-
H. Fonvieille,
G. Laveissiere,
N. Degrande,
S. Jaminion,
C. Jutier,
L. Todor,
R. Di Salvo,
L. Van Hoorebeke,
L. C. Alexa, B. D. Anderson, [......],
E. Voutier,
J. W. Watson,
L. B. Weinstein,
K. Wijesooriya,
R. Wilson,
B. B. Wojtsekhowski,
D. G. Zainea,
W. -M. Zhang,
J. Zhao,
Z. -L. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson
Lab using the exclusive photon electroproduction reaction (e p --> e p gamma).
This paper gives a detailed account of the analysis which has led to the
determination of the structure functions P_LL-P_TT/epsilon and P_LT, and the
electric and magnetic generalized polarizabilities (GPs) alpha_E(Q^2) and
beta_M(Q^2) at values of the four-momentum transfer squared Q^2= 0.92 and 1.76
GeV^2. These data, together with the results of VCS experiments at lower
momenta, help building a coherent picture of the electric and magnetic GPs of
the proton over the full measured Q^2-range, and point to their non-trivial
behavior.
05/2012;
-
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: 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
-
M. Mihovilovic,
K. Allada, B. D. Anderson,
J. R. M. Annand,
T. Averett,
A. Camsonne,
R. W. Chan,
J. -P. Chen,
K. Chirapatpimol,
C. W. de Jager, [......],
G. Rosner,
B. Sawatzky,
M. Shabestari,
A. Shahinyan,
R. Shneor,
S. Sirca,
R. Subedi,
V. Sulkosky,
J. W. Watson,
Y. -W. Zhang
[show abstract]
[hide abstract]
ABSTRACT: The techniques for optical calibration of Jefferson Lab's large-acceptance
magnetic hadron spectrometer, BigBite, have been examined. The most consistent
and stable results were obtained by using a method based on singular value
decomposition. In spite of the complexity of the optics, the particles'
positions and momenta at the target have been precisely reconstructed from the
coordinates measured in the detectors by means of a single back-tracing matrix.
The technique is applicable to any similar magnetic spectrometer and any
particle type. For 0.55 GeV/c protons, we have established the vertex
resolution of 1.2 cm, angular resolutions of 7 mrad and 16 mrad (in-plane and
out-of-plane, respectively), and a relative momentum resolution of 1.6%.
01/2012;
-
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: 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
[show abstract]
[hide abstract]
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;
-
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: Two-particle azimuthal ($\Delta\phi$) and pseudorapidity ($\Delta\eta$)
correlations using a trigger particle with large transverse momentum ($p_T$) in
$d$+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV and
200~GeV from the STAR experiment at RHIC are presented. The \ns correlation is
separated into a jet-like component, narrow in both $\Delta\phi$ and
$\Delta\eta$, and the ridge, narrow in $\Delta\phi$ but broad in $\Delta\eta$.
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
$p_T$. 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 $\sqrt{s_{{NN}}}$ = 200
GeV, is also found in Cu+Cu collisions and in collisions at $\sqrt{s_{{NN}}}$
=\xspace 62.4 GeV, but is found to be substantially smaller at
$\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV than at $\sqrt{s_{{NN}}}$ = 200 GeV for the
same average number of participants ($ \langle N_{\mathrm{part}}\rangle$).
Measurements of the ridge are compared to models.
10/2011;
-
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: This paper reports results for directed flow $v_{1}$ and elliptic flow
$v_{2}$ of charged particles in Cu+Cu collisions at $\sqrt{s_{NN}}=$ 22.4 GeV
at the Relativistic Heavy Ion Collider. The measurements are for the 0-60% most
central collisions, using charged particles observed in the STAR detector. Our
measurements extend to 22.4 GeV Cu+Cu collisions the prior observation that
$v_1$ is independent of the system size at 62.4 and 200 GeV, and also extend
the scaling of $v_1$ with $\eta/y_{\rm beam}$ to this system. The measured
$v_2(p_T)$ in Cu+Cu collisions is similar for $\sqrt{s_{NN}} = 22.4-200$ GeV.
We also report a comparison with results from transport model (UrQMD and AMPT)
calculations. The model results do not agree quantitatively with the measured
$v_1(\eta), v_2(p_T)$ and $v_2(\eta)$.
09/2011;
-
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 present two-dimensional (2D) two-particle angular correlations on relative
pseudorapidity $\eta$ and azimuth $\phi$ for charged particles from Au-Au
collisions at $\sqrt{s_{\rm NN}} = 62$ and 200 GeV with transverse momentum
$p_t \geq 0.15$ GeV/$c$, $|\eta| \leq 1$ and $2\pi$ azimuth. Observed
correlations include a {same-side} (relative azimuth $< \pi/2$) 2D peak, a
closely-related away-side azimuth dipole, and an azimuth quadrupole
conventionally associated with elliptic flow. The same-side 2D peak and
away-side dipole are explained by semihard parton scattering and fragmentation
(minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those
structures follow N-N binary-collision scaling in Au-Au collisions until
mid-centrality where a transition to a qualitatively different centrality trend
occurs within a small centrality interval. Above the transition point the
number of same-side and away-side correlated pairs increases rapidly {relative
to} binary-collision scaling, the $\eta$ width of the same-side 2D peak also
increases rapidly ($\eta$ elongation) and the $\phi$ width actually decreases
significantly. Those centrality trends are more remarkable when contrasted with
expectations of jet quenching in a dense medium. Observed centrality trends are
compared to {\sc hijing} predictions and to the expected trends for semihard
parton scattering and fragmentation in a thermalized opaque medium. We are
unable to reconcile a semihard parton scattering and fragmentation origin for
the observed correlation structure and centrality trends with heavy ion
collision scenarios which invoke rapid parton thermalization. On the other
hand, if the collision system is effectively opaque to few-GeV partons the
observations reported here would be inconsistent with a minijet picture.
09/2011;
-
STAR Collaboration,
H. 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,
J Zhao,
C. Zhong,
W Zhou,
X Zhu,
Y H Zhu,
R. Zoulkarneev,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: We report new STAR measurements of mid-rapidity yields for the $\Lambda$,
$\bar{\Lambda}$, $K^{0}_{S}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$,
$\bar{\Omega}^{+}$ particles in Cu+Cu collisions at \sNN{200}, and mid-rapidity
yields for the $\Lambda$, $\bar{\Lambda}$, $K^{0}_{S}$ particles in Au+Au at
\sNN{200}. 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 parameterization
based on the fraction of participants that undergo multiple collisions.
07/2011;
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The STAR Collaboration H. Agakishiev,
M. M. Aggarwal,
Z. Ahammed,
A. V. Alakhverdyants,
I. Alekseev,
J. Alford, B. D. Anderson,
C. D. Anson,
D. Arkhipkin,
G. S. Averichev, [......],
Y. Zhang,
Z. P. Zhang,
F. Zhao,
J. Zhao,
C. Zhong,
W. Zhou,
X. Zhu,
Y. H. Zhu,
R. Zoulkarneev,
Y. Zoulkarneeva
[show abstract]
[hide abstract]
ABSTRACT: High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang1; in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus ( ), also known as the anti-α ( ), consists of two antiprotons and two antineutrons (baryon number B = −4). It has not been observed previously, although the α-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level2. Antimatter nuclei with B < −1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1,000 with each additional antinucleon3, 4, 5. Here we report the observation of , the heaviest observed antinucleus to date. In total, 18 counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC; ref. 6) in 109 recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair. The yield is consistent with expectations from thermodynamic7 and coalescent nucleosynthesis8 models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of in cosmic radiation.
Nature 05/2011; 473(Letter-473, 353–356 (19 May 2011)):353-356. · 36.28 Impact Factor
