-
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;
-
A Adare,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
R Akimoto,
H Al-Ta'ani,
J Alexander,
K R Andrews,
A Angerami, [......],
J S Yoo,
Z You,
G R Young,
I Younus,
I E Yushmanov,
W A Zajc,
A Zelenski,
S Zhou,
L Zolin,
PHENIX Collaboration
[show abstract]
[hide abstract]
ABSTRACT: We report on the first measurement of the double-spin asymmetry, A(LL), of electrons from the decays of hadrons containing heavy flavor in longitudinally polarized p + p collisions at root s = 200 GeV for p(T) = 0.5 to 3.0 GeV/c. The asymmetry was measured at midrapidity (vertical bar eta vertical bar < 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 vertical bar Delta g/g(log(10)(x) = -1.6(-0.4)(+0.5), mu = m(T)(c)vertical bar(2) < 0.030 (1 sigma) based on a leading-order perturbative quantum chromodynamics model, using the measured asymmetry. DOI: 10.1103/PhysRevD.87.012011
Physical Review D 01/2013; 87(1). · 4.56 Impact Factor
-
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;
-
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
-
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;
-
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
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
Y. Aramaki,
E. T. Atomssa, [......],
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured the production of neutral pions in Au+Au
collisions at sqrt(s_NN)=200 GeV. The new data offer a fourfold increase in
recorded luminosity, providing higher precision and a larger reach in
transverse momentum, p_T, to 20 GeV/c. The production ratio of eta/pi^0 is
0.46+/-0.01(stat)+/-0.05(syst), constant with p_T and collision centrality. The
observed ratio is consistent with earlier measurements, as well as with the p+p
and d+Au values. The production of pi^0 is suppressed by a factor of 5, as in
earlier findings. However, with the improved statistical precision a small but
significant rise of the nuclear modification factor, R_AA, vs p_T, with a slope
of 0.0106+/-^(0.0034)_(0.0029)[GeV/c]^-1, is discernible in central collisions.
A phenomenological extraction of the average fractional parton energy loss
shows a decrease with increasing p_T. To study the path length dependence of
suppression, the pi^0 yield was measured at different angles with respect to
the event plane; a strong azimuthal dependence of the pi^0 R_AA is observed.
The data are compared to theoretical models of parton energy loss as a function
of the path length, L, in the medium. Models based on pQCD are insufficient to
describe the data, while a hybrid model utilizing pQCD for the hard
interactions and AdS/CFT for the soft interactions is consistent with the data.
08/2012;
-
A. Adare,
S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
A. Al-Jamel,
J. Alexander,
A. Angerami, [......],
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
J. Zimányi,
L. Zolin,
X. Zong
[show abstract]
[hide abstract]
ABSTRACT: Direct photons have been measured in sqrt(s_NN)=200 GeV d+Au collisions at
midrapidity. A wide p_T range is covered by measurements of nearly-real virtual
photons (1<p_T<6 GeV/c) and real photons (5<p_T<16 GeV/c). The invariant yield
of the direct photons in d+Au collisions over the scaled p+p cross section is
consistent with unity. Theoretical calculations assuming standard cold nuclear
matter effects describe the data well for the entire p_T range. This indicates
that the large enhancement of direct photons observed in Au+Au collisions for
1.0<p_T<2.5 GeV/c is due to a source other than the initial-state nuclear
effects.
08/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
A. Angerami,
K. Aoki,
N. Apadula, [......],
A. Yanovich,
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
S. Zhou,
L. Zolin
[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 at the Relativistic Heavy Ion
Collider, in the transverse-momentum range 0.85 < pT < 8.5 GeV/c. In central
d+Au collisions, the nuclear modification factor R_dA at 1.5 < pT < 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 RHIC 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 pi0 and heavy-flavor-electron nuclear modification factor R_AA.
08/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
Y. Aramaki,
E. T. Atomssa, [......],
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the anisotropy parameter v2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum pT, and transverse kinetic energy KET at midrapidity (|η|<0.35) in Au + Au collisions at √sNN=200 GeV are presented. Pions and protons are identified up to pT= 6 GeV/c, and kaons up to pT= 4 GeV/c, by combining information from time-of-flight and aerogel Čerenkov detectors in the PHENIX Experiment. The scaling of v2 with the number of valence quarks (nq) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KET/nq in noncentral Au + Au collisions (20–60%), but this scaling remains valid in central collisions (0–10%).
Physical Review C 06/2012; 85:064914. · 3.31 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
Y. Aramaki,
E. T. Atomssa, [......],
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the anisotropy parameter v2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum pT, and transverse kinetic energy KET at midrapidity (|η|<0.35) in Au + Au collisions at √sNN=200 GeV are presented. Pions and protons are identified up to pT= 6 GeV/c, and kaons up to pT= 4 GeV/c, by combining information from time-of-flight and aerogel Čerenkov detectors in the PHENIX Experiment. The scaling of v2 with the number of valence quarks (nq) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KET/nq in noncentral Au + Au collisions (20–60%), but this scaling remains valid in central collisions (0–10%).
Phys. Rev. C. 06/2012; 85(6).
-
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 differential cross section for the production of direct photons in p+p collisions at √s = 200
GeV at midrapidity was measured in the PHENIX detector at the Relativistic Heavy Ion Collider.
Inclusive-direct photons were measured in the transverse momentum range from 5.5–25 GeV/c,
extending the range beyond previous measurements. Event structure was studied with an isolation
criterion. Next-to-leading-order perturbative-quantum-chromodynamics calculations give a good
description of the spectrum. When the cross section is expressed versus xT , the PHENIX data
are seen to be in agreement with measurements from other experiments at different center-of-mass
energies.
Physical Review C 06/2012; · 3.31 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 and 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.
05/2012;
-
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 differential cross section for the production of direct photons in p+p
collisions at sqrt(s)=200 GeV at midrapidity was measured in the PHENIX
detector at the Relativistic Heavy Ion Collider. Inclusive-direct photons were
measured in the transverse-momentum range from 5.5--25 GeV/c, extending the
range beyond previous measurements. Event structure was studied with an
isolation criterion. Next-to-leading-order perturbative-quantum-chromodynamics
calculations give a good description of the spectrum. When the cross section is
expressed versus x_T, the PHENIX data are seen to be in agreement with
measurements from other experiments at different center-of-mass energies.
05/2012;
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A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
A. Angerami,
K. Aoki,
N. Apadula, [......],
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: We report on charmonium measurements [J/ψ (1S), ψ′ (2S), and χc (1P)] in p+p collisions at √s=200 GeV. We find that the fraction of J/ψ coming from the feed-down decay of ψ′ and χc in the midrapidity region (|y|<0.35) is 9.6±2.4% and 32±9%, respectively. We also present the pT and rapidity dependencies of the J/ψ yield measured via dielectron decay at midrapidity (|y|<0.35) and via dimuon decay at forward rapidity (1.2<|y|<2.2). The statistical precision greatly exceeds that reported in our previous publication [ Phys. Rev. Lett. 98 232002 (2007)]. The new results are compared with other experiments and discussed in the context of current charmonium production models.
Phys. Rev. D. 05/2012; 85(9).
