-
A. Adare,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
A. Angerami,
K. Aoki,
N. Apadula,
Y. Aramaki, [......],
R. Yang,
A. Yanovich,
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
S. Zhou
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC)
reports measurements of azimuthal dihadron correlations near midrapidity in
d+Au collisions at sqrt(s_NN)=200 GeV. These measurements complement recent
analyses by experiments at the Large Hadron Collider (LHC) involving central
p+Pb collisions at sqrt(s_NN)=5.02 TeV, which have indicated strong anisotropic
long-range correlations in angular distributions of hadron pairs. The origin of
these anisotropies is currently unknown. Various competing explanations include
parton saturation and hydrodynamic flow. We observe qualitatively similar
anisotropies at RHIC to those seen at the LHC, and when both are divided by an
estimate of the initial-state eccentricity, the anisotropies follow a common
multiplicity scaling. This scaling is also found to extend to heavy ion data at
RHIC and the LHC, where the anisotropies are widely thought to be due to
hydrodynamic flow. The results presented here, at much lower collision energy
and with a deuteron projectile (instead of a proton), provide important new
information for understanding the origin of these new long-range correlations.
03/2013;
-
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;
-
A Adare,
C Aidala,
N N Ajitanand,
Y Akiba,
H Al-Bataineh,
J Alexander,
A Angerami,
K Aoki,
N Apadula,
Y Aramaki, [......],
R Yang,
A Yanovich,
J Ying,
S Yokkaichi,
Z You,
G R Young,
I Younus,
I E Yushmanov,
W A Zajc,
S Zhou
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt[s_{NN}]=200 GeV in the transverse-momentum range 0.85≤p_{T}^{e}≤8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R_{dA} at 1.5<p_{T}<5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at the Relativistic Heavy Ion Collider extends to the heavy D meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the π^{0} and heavy-flavor-electron nuclear modification factors R_{AA}.
Physical Review Letters 12/2012; 109(24):242301. · 7.37 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
R. Akimoto,
H. Al-Bataineh,
H. Al-Ta'ani,
J. Alexander,
A. Angerami, [......],
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
A. Zelenski,
C. Zhang,
S. Zhou,
J. Zimányi,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The jet fragmentation function is measured with direct photon-hadron
correlations in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The p_T of the
photon is an excellent approximation to the initial p_T of the jet and the
ratio z_T=p_T^h/p_T^\gamma is used as a proxy for the jet fragmentation
function. A statistical subtraction is used to extract the direct photon-hadron
yields in Au+Au collisions while a photon isolation cut is applied in p+p. I_
AA, the ratio of jet fragment yield in Au+Au to that in p+p, indicates
modification of the jet fragmentation function. Suppression, most likely due to
energy loss in the medium, is seen at high z_T. The fragment yield at low z_T
is enhanced at large angles. Such a trend is expected from redistribution of
the lost energy into increased production of low-momentum particles.
12/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
R. Akimoto,
H. Al-Bataineh,
H. Al-Ta'ani,
J. Alexander,
K. R. Andrews, [......],
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
A. Zelenski,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The three Upsilon states, Upsilon(1S+2S+3S), are measured in d+Au and p+p
collisions at sqrt(s_NN)=200 GeV and rapidities 1.2<|y|<2.2 by the PHENIX
experiment at the Relativistic Heavy-Ion Collider. Cross sections for the
inclusive Upsilon(1S+2S+3S) production are obtained. The inclusive yields per
binary collision for d+Au collisions relative to those in p+p collisions
(R_dAu) are found to be 0.62 +/- 0.26 (stat) +/- 0.13 (syst) in the gold-going
direction and 0.91 +/- 0.33 (stat) +/- 0.16 (syst) in the deuteron-going
direction. The measured results are compared to a nuclear-shadowing model,
EPS09 [JHEP 04, 065 (2009)], combined with a final-state breakup cross section,
sigma_br, and compared to lower energy p+A results. We also compare the results
to the PHENIX J/psi results [Phys. Rev. Lett. 107, 142301 (2011)]. The rapidity
dependence of the observed Upsilon suppression is consistent with lower energy
p+A measurements.
11/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
H. Al-Ta'ani,
A. Angerami,
K. Aoki, [......],
S. Yokkaichi,
G. R. Young,
I. Younus,
Z. You,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: We previously reported [Phys. Rev. D 82, 112008 (2010)] measurements of
transverse single-spin asymmetries, A_N, in J/psi production from transversely
polarized p+p collisions at sqrt(s)=200 GeV with data taken by the PHENIX
experiment at the Relativistic Heavy Ion Collider in 2006 and 2008.
Subsequently, we have found errors in the analysis procedures for the 2008
data, which resulted in an erroneous value for the extracted A_N. The errors
affected the sorting of events into the correct left/right and forward/backward
bins. This produced an incorrect value for the 2008 result, but the 2006 result
is unaffected. We have conducted two independent reanalyses with these errors
corrected, and we present here the corrected values for the 2008 data and the
combined results for 2006 and 2008. The new combined spin asymmetry in the
forward region is A_N = -0.026+/-0.026(stat)+/-0.003(sys). Since this asymmetry
is consistent with zero, we no longer claim that our results suggest a possible
non-zero trigluon correlation function in transversely polarized protons.
10/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
R. Akimoto,
H. Al-Taani,
J. Alexander,
A. Angerami,
K. Aoki, [......],
R. Yang,
A. Yanovich,
J. Ying,
S. Yokkaichi,
Z. You,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
A. Zelenski,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Neutral-pion �pi^0 spectra were measured at midrapidity (jyj < 0:35) in Au + Au collisions at sqrt(s_NN) = 39
and 62.4 GeV and compared with earlier measurements at 200 GeV in a transverse-momentum range of
1<pT < 10 GeV/c. The high - pT tail is well described by a power law in all cases, and the powers
decrease significantly with decreasing center-of-mass energy. The change of powers is very similar to that
observed in the corresponding spectra for p + p collisions. The nuclear modification factors (RAA) show
significant suppression, with a distinct energy, centrality, and pT dependence. Above pT =7 GeVc, RAA
is similar for sqrt(s_NN) = 62.4 and 200 GeV at all centralities. Perturbative-quantum-chromodynamics
calculations that describe RAA well at 200 GeV fail to describe the 39 GeV data, raising the possibility
that, for the same pT region, the relative importance of initial-state effects and soft processes increases at
lower energies. The pT range where pi^�0 spectra in central Au + Au collisions have the same power as in
p + p collisions is � 5 and 7 GeV/c for sqrt(s_NN) = 200 and 62.4 GeV, respectively. For the sqrt(s_NN) = 39 GeV data, it is not clear whether such a region is reached, and the xT dependence of the xT-scaling
power-law exponent is very different from that observed in the sqrt(s_NN) = 62 and 200 GeV data, providing
further evidence that initial-state effects and soft processes mask the in-medium suppression of hard scattered partons to higher pT as the collision energy decreases.
Physical Review Letters 10/2012; 109:152301. · 7.37 Impact Factor
-
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
A Al-Jamel,
J Alexander,
K Aoki,
L Aphecetche,
R Armendariz,
S H Aronson, [......],
F Staley,
P W Stankus,
E Stenlund,
M Stepanov,
A Ster,
S P Stoll,
T Sugitate,
C Suire,
J P Sullivan,
J Sziklai
[show abstract]
[hide abstract]
ABSTRACT: We report the measurement of direct photons at midrapidity in Au þ Au collisions at sqrt(s_NN) = 200 GeV. The direct photon signal was extracted for the transverse momentum range of 4 GeV=
c < pT < 22 GeV/c, using a statistical method to subtract decay photons from the inclusive photon
sample. The direct photon nuclear modification factor RAA was calculated as a function of pT for different
Au þ Au collision centralities using the measured p þ p direct photon spectrum and compared to
theoretical predictions. RAA was found to be consistent with unity for all centralities over the entire
measured pT range. Theoretical models that account for modifications of initial direct photon production due to modified parton distribution functions in Au and the different isospin composition of the nuclei
predict a modest change of RAA from unity. They are consistent with the data. Models with compensating
effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation
photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.
Physical Review Letters 10/2012; 443737374637382038(109):152302. · 7.37 Impact Factor
-
A Adare,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
R Akimoto,
H Al-Ta'ani,
J Alexander,
A Angerami,
K Aoki, [......],
R Yang,
A Yanovich,
J Ying,
S Yokkaichi,
Z You,
I Younus,
I E Yushmanov,
W A Zajc,
A Zelenski,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Neutral-pion π^{0} spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at sqrt[s_{NN}]=39 and 62.4 GeV and compared with earlier measurements at 200 GeV in a transverse-momentum range of 1<p_{T}<10 GeV/c. The high-p_{T} tail is well described by a power law in all cases, and the powers decrease significantly with decreasing center-of-mass energy. The change of powers is very similar to that observed in the corresponding spectra for p+p collisions. The nuclear modification factors (R_{AA}) show significant suppression, with a distinct energy, centrality, and p_{T} dependence. Above p_{T}=7 GeV/c, R_{AA} is similar for sqrt[s_{NN}]=62.4 and 200 GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe R_{AA} well at 200 GeV fail to describe the 39 GeV data, raising the possibility that, for the same p_{T} region, the relative importance of initial-state effects and soft processes increases at lower energies. The p_{T} range where π^{0} spectra in central Au+Au collisions have the same power as in p+p collisions is ≈5 and 7 GeV/c for sqrt[s_{NN}]=200 and 62.4 GeV, respectively. For the sqrt[s_{NN}]=39 GeV data, it is not clear whether such a region is reached, and the x_{T} dependence of the x_{T}-scaling power-law exponent is very different from that observed in the sqrt[s_{NN}]=62 and 200 GeV data, providing further evidence that initial-state effects and soft processes mask the in-medium suppression of hard-scattered partons to higher p_{T} as the collision energy decreases.
Physical Review Letters 10/2012; 109(15):152301. · 7.37 Impact Factor
-
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
A Al-Jamel,
J Alexander,
K Aoki,
L Aphecetche,
R Armendariz,
S H Aronson, [......],
A Yanovich,
S Yokkaichi,
G R Young,
I Younus,
I E Yushmanov,
W A Zajc,
O Zaudtke,
C Zhang,
J Zimányi,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: We report the measurement of direct photons at midrapidity in Au+Au collisions at sqrt[s_{NN}]=200 GeV. The direct photon signal was extracted for the transverse momentum range of 4 GeV/c<p_{T}<22 GeV/c, using a statistical method to subtract decay photons from the inclusive photon sample. The direct photon nuclear modification factor R_{AA} was calculated as a function of p_{T} for different Au+Au collision centralities using the measured p+p direct photon spectrum and compared to theoretical predictions. R_{AA} was found to be consistent with unity for all centralities over the entire measured p_{T} range. Theoretical models that account for modifications of initial direct photon production due to modified parton distribution functions in Au and the different isospin composition of the nuclei predict a modest change of R_{AA} from unity. They are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.
Physical Review Letters 10/2012; 109(15):152302. · 7.37 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
L. Aphecetche,
J. Asai, [......],
J. Ying,
S. Yokkaichi,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The energy dependence of the single-transverse-spin asymmetry, A_N, and the
cross section for neutron production at very forward angles were measured in
the PHENIX experiment at RHIC for polarized p+p collisions at sqrt(s)=200 GeV.
The neutrons were observed in forward detectors covering an angular range of up
to 2.2 mrad. We report results for neutrons with momentum fraction of x_F=0.45
to 1.0. The energy dependence of the measured cross sections were consistent
with x_F scaling, compared to measurements by an ISR experiment which measured
neutron production in unpolarized p+p collisions at sqrt(s)=30.6--62.7 GeV. The
cross sections for large x_F neutron production for p+p collisions, as well as
those in e+p collisions measured at HERA, are described by a pion exchange
mechanism. The observed forward neutron asymmetries were large, reaching
A_N=-0.08+/-0.02 for x_F=0.8; the measured backward asymmetries, for negative
x_F, were consistent with zero. The observed asymmetry for forward neutron
production is discussed within the pion exchange framework, with interference
between the spin-flip amplitude due to the pion exchange and nonflip amplitudes
from all Reggeon exchanges. Within the pion exchange description, the measured
neutron asymmetry is sensitive to the contribution of other Reggeon exchanges
even for small amplitudes.
09/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
R. Akimoto,
H. Al-Ta'ani,
J. Alexander,
K. R. Andrews,
A. Angerami, [......],
S. Yokkaichi,
J. S. Yoo,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
A. Zelenski,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: We report on the first measurement of double-spin asymmetry, A_LL, of
electrons from the decays of hadrons containing heavy flavor in longitudinally
polarized p+p collisions at sqrt(s)=200 GeV for p_T= 0.5 to 3.0 GeV/c. The
asymmetry was measured at mid-rapidity (|eta|<0.35) with the PHENIX detector at
the Relativistic Heavy Ion Collider. The measured asymmetries are consistent
with zero within the statistical errors. We obtained a constraint for the
polarized gluon distribution in the proton of |Delta g/g(log{_10}x=
-1.6^+0.5_-0.4, {mu}=m_T^c)|^2 < 0.033 (1 sigma), based on a leading-order
perturbative-quantum-chromodynamics model, using the measured asymmetry.
09/2012;
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A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
L. Aphecetche,
R. Armendariz, [......],
S. Yokkaichi,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
J. Zimanyi,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Heavy-flavor production in p + p collisions is a good test of perturbative-quantumchromodynamics
(pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative
to binary-collision scaling from p + p results, quantified with the nuclear-modification factor (RAA), provides
information on both cold- and hot-nuclear-matter effects. Midrapidity heavy-flavor RAA measurements at the Relativistic Heavy Ion Collider have challenged parton-energy-loss models and resulted in upper limits on the viscosity-entropy ratio that are near the quantum lower bound. Such measurements have not been made in the forward-rapidity region. The charm-production cross section in p + p collisions at √s = 200 GeV, integrated over pT and in the rapidity range 1.4 < y < 1.9, is found to be dσc ¯ c/dy = 0.139 ± 0.029 (stat) +0.051 −0.058 (syst) mb. This result is consistent with a perturbative fixed-order-plus-next-to-leading-log calculation within scale uncertainties and is also consistent with expectations based on the corresponding midrapidity charm-production cross section measured by PHENIX. The RAA for heavy-flavor muons in Cu + Cu collisions is measured in three centrality bins for 1 < pT < 4 GeV/c. Suppression relative to binary-collision scaling (RAA < 1) increases with centrality. Within experimental and theoretical uncertainties, the measured charm yield in p + p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu + Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss.
Physical Review C 08/2012; 86:024909. · 3.31 Impact Factor
