-
S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
R. Amirikas,
L. Aphecetche,
S. H. Aronson,
R. Averbeck, [......],
Y. Yang,
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Measurements of neutral pion (π0) production at midrapidity in √sNN=200 GeV Au+Au collisions as a function of transverse momentum, pT, collision centrality, and angle with respect to reaction plane are presented. The data represent the final π0 results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of 3 increase in statistics over previously published results for pT>6 GeV/c. We evaluate the suppression in the yield of high-pT π0's relative to pointlike scaling expectations using the nuclear modification factor RAA. We present the pT dependence of RAA for nine bins in collision centrality. We separately integrate RAA over larger pT bins to show more precisely the centrality dependence of the high-pT suppression. We then evaluate the dependence of the high-pT suppression on the emission angle Δϕ of the pions with respect to event reaction plane for seven bins in collision centrality. We show that the yields of high-pT π0's vary strongly with Δϕ, consistent with prior measurements [1, 2]. We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane, whereas the yield of π0's produced perpendicular to the reaction plane is suppressed by a factor of ~2. We analyze the combined centrality and Δϕ dependence of the π0 suppression in different pT bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.
Phys. Rev. C. 09/2007; 76(3).
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S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
R. Amirikas,
L. Aphecetche,
S. H. Aronson,
R. Averbeck, [......],
Y. Yang,
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Measurements of neutral pion (π0) production at midrapidity in √sNN=200 GeV Au+Au collisions as a function of transverse momentum, pT, collision centrality, and angle with respect to reaction plane are presented. The data represent the final π0 results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of 3 increase in statistics over previously published results for pT>6 GeV/c. We evaluate the suppression in the yield of high-pT π0's relative to pointlike scaling expectations using the nuclear modification factor RAA. We present the pT dependence of RAA for nine bins in collision centrality. We separately integrate RAA over larger pT bins to show more precisely the centrality dependence of the high-pT suppression. We then evaluate the dependence of the high-pT suppression on the emission angle Δϕ of the pions with respect to event reaction plane for seven bins in collision centrality. We show that the yields of high-pT π0's vary strongly with Δϕ, consistent with prior measurements [1, 2]. We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane, whereas the yield of π0's produced perpendicular to the reaction plane is suppressed by a factor of ~2. We analyze the combined centrality and Δϕ dependence of the π0 suppression in different pT bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.
Phys. Rev. C. 09/2007; 76(3).
-
S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
R. Amirikas,
L. Aphecetche,
S. H. Aronson,
R. Averbeck, [......],
Y. Yang,
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Longitudinal density correlations of produced matter in Au+Au collisions at √sNN=200 GeV have been measured from the inclusive charged particle distributions as a function of pseudorapidity window sizes. The extracted αξ parameter, related to the susceptibility of the density fluctuations in the long-wavelength limit, exhibits a nonmonotonic behavior as a function of the number of participant nucleons, Npart. A local maximum is seen at Npart~90, with corresponding energy density based on the Bjorken picture of εBjτ~2.4 GeV/(fm2c) with a transverse area size of 60 fm2. This behavior may suggest a critical phase boundary based on the Ginzburg-Landau framework.
Phys. Rev. C. 09/2007; 76(3).
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Emission source functions are extracted from correlation functions constructed from charged pions produced at midrapidity in Au+Au collisions at sqrt[s(NN)]=200 GeV. The source parameters extracted from these functions at low k(T) give first indications of a long tail for the pion emission source. The source extension cannot be explained solely by simple kinematic considerations. The possible role of a halo of secondary pions from resonance emissions is explored.
Physical Review Letters 04/2007; 98(13):132301. · 7.37 Impact Factor
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S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
A. Al-Jamel,
R. Amirikas,
K. Aoki,
L. Aphecetche, [......],
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
J. Zimányi,
L. Zolin,
X. Zong
[show abstract]
[hide abstract]
ABSTRACT: Inclusive transverse momentum spectra of η mesons in the range pT≈2–12 GeV/c have been measured at midrapidity (|η|<0.35) by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at √sNN=200 GeV. The η mesons are reconstructed through their η→γ γ channel for the three colliding systems as well as through the η→π0π+π- decay mode in p+p and d+Au collisions. The nuclear modification factor in d+Au collisions, RdAu(pT)≈1.0–1.1, suggests at most only modest pT broadening (“Cronin enhancement”). In central Au+Au reactions, the η yields are significantly suppressed, with RAuAu(pT)≈0.2. The ratio of η to π0 yields is approximately constant as a function of pT for the three colliding systems in agreement with the high-pT world average of Rη/π0≈0.5 in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies (√sNN≈3–1800 GeV) as well as, for high scaled momentum xp, in e+e- annihilations at √s=91.2 GeV. These results are consistent with a scenario where high-pT η production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (π0,η) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.
Phys. Rev. C. 02/2007; 75(2).
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Azimuthal correlations of jet-induced high-p(T) charged hadron pairs are studied at midrapidity in Au+Au collisions at sqrt[s(NN)]=200 GeV. The distribution of jet-associated partner hadrons (1.0<p(T)<2.5 GeV/c) per trigger hadron (2.5<p(T)<4.0 GeV/c) is found to vary with collision centrality, in both shape and yield, indicating a significant effect of the nuclear collision medium on the jet fragmentation process.
Physical Review Letters 08/2006; 97(5):052301. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Inclusive transverse momentum spectra of eta mesons have been measured within p(T) = 2-10 GeV/c at midrapidity by the PHENIX experiment in Au + Au collisions at square root S(NN) = 200 GeV. In central Au+Au the eta yields are significantly suppressed compared to peripheral Au + Au, d + Au, and p + p yields scaled by the corresponding number of nucleon-nucleon collisions. The magnitude, centrality, and p(T) dependence of the suppression is common, within errors, for eta and pi0. The ratio of eta to pi0 spectra at high p(T) amounts to 0.40 < R(eta/pi)0 < 0.48 for the three systems, in agreement with the world average measured in hadronic and nuclear reactions and, at large scaled momentum, in e+e- collisions.
Physical Review Letters 06/2006; 96(20):202301. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: The azimuthal distribution of identified pi0 and inclusive photons has been measured in [FORMULA: SEE TEXT] Au+Au collisions with the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The second-harmonic parameter (nu2) was measured to describe the observed anisotropy of the azimuthal distribution. The measured inclusive photon is consistent with the value expected for the photons from hadron decay and is also consistent with the lack of direct photon signal over the measured pT range 1-6 GeV/c. An attempt is made to extract nu2 of direct photons.
Physical Review Letters 02/2006; 96(3):032302. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured midrapidity ([FORMULA: SEE TEXT]) transverse momentum spectra ([FORMULA: SEE TEXT]) of electrons as a function of centrality in Au+Au collisions at [FORMULA: SEE TEXT]. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of pi0 and eta mesons, were removed. The resulting nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to nonphotonic electrons in p+p collisions. A significant suppression of electrons at high pT is observed in central Au+Au collisions, indicating substantial energy loss of heavy quarks.
Physical Review Letters 02/2006; 96(3):032301. · 7.37 Impact Factor
-
S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: The invariant differential cross section for inclusive electron production in p+p collisions at [FORMULA: SEE TEXT] has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over the transverse momentum range 0.4<or=pT<OR=5.0 GeV/c in the central rapidity region ([FORMULA: SEE TEXT]). The contribution to the inclusive electron spectrum from semileptonic decays of hadrons carrying heavy flavor, i.e., charm quarks or, at high , bottom quarks, is determined via three independent methods. The resulting electron spectrum from heavy-flavor decays is compared to recent leading and next-to-leading order perturbative QCD calculations. The total cross section of charm quark-antiquark pair production is determined to be [FORMULA: SEE TEXT].
Physical Review Letters 01/2006; 96(3):032001. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Transverse single-spin asymmetries to probe the transverse-spin structure of the proton have been measured for neutral pions and nonidentified charged hadrons from polarized proton-proton collisions at midrapidity and square root(s) = 200 GeV. The data cover a transverse momentum (pT) range 1.0-5.0 GeV/c for neutral pions and 0.5-5.0 GeV/c for charged hadrons, at a Feynman-x value of approximately zero. The asymmetries seen in this previously unexplored kinematic region are consistent with zero within errors of a few percent. In addition, the inclusive charged hadron cross section at midrapidity from 0.5 < pT < 7.0 GeV/c is presented and compared to next-to-leading order perturbative QCD (pQCD) calculations. Successful description of the unpolarized cross section above approximately 2 GeV/c suggests that pQCD is applicable in the interpretation of the asymmetry results in the relevant kinematic range.
Physical Review Letters 11/2005; 95(20):202001. · 7.37 Impact Factor
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S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
R. Amirikas,
L. Aphecetche,
S. H. Aronson,
R. Averbeck, [......],
Y. Yang,
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The transverse momentum dependence of the azimuthal anisotropy parameter v2, the second harmonic of the azimuthal distribution, for electrons at midrapidity (|η|<0.35) has been measured with the PHENIX detector in Au+Au collisions at √sNN=200 GeV. The measurement was made with respect to the reaction plane defined at high rapidities (|η|=3.1-3.9). From the result we have measured the v2 of electrons from heavy flavor decay after subtraction of the v2 of electrons from other sources such as photon conversions and Dalitz decay from light neutral mesons. We observe a nonzero single electron v2 with a 90% confidence level in the intermediate-pT region. The precision of the present data set does not permit us to conclude definitively that heavy quarks exhibit thermalization with the transverse flow of the bulk matter.
Phys. Rev. C. 08/2005; 72(2).
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S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
R. Amirikas,
L. Aphecetche,
S. H. Aronson,
R. Averbeck, [......],
Y. Yang,
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: We present the results of ϕ meson production in the K+K- decay channel from Au+Au collisions at √sNN=200 GeV as measured at midrapidity by the PHENIX detector at Brookhaven National Laboratory's Relativistic Heavy Ion Collider. Precision resonance centroid and width values are extracted as a function of collision centrality. No significant variation from the Particle Data Group accepted values is observed, contrary to some model predictions. The ϕ transverse mass spectra are fitted with a linear exponential function for which the derived inverse slope parameter is seen to be constant as a function of centrality. However, when these data are fitted by a hydrodynamic model the result is that the centrality-dependent freeze-out temperature and the expansion velocity values are consistent with the values previously derived from fitting identified charged hadron data. As a function of transverse momentum the collisions scaled peripheral-to-central yield ratio RCP for the ϕ is comparable to that of pions rather than that of protons. This result lends support to theoretical models that distinguish between baryons and mesons instead of particle mass for explaining the anomalous (anti) proton yield.
Phys. Rev. C. 07/2005; 72(1).
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: The first measurement of direct photons in Au + Au collisions at (square root)S(NN) = 200 GeV is presented. The direct photon signal is extracted as a function of the Au + Au collision centrality and compared to next-to-leading order perturbative quantum chromodynamics calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.
Physical Review Letters 07/2005; 94(23):232301. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
A Al-Jamel,
J Alexander,
R Amirikas,
K Aoki,
L Aphecetche, [......],
G R Young,
I Younus,
I E Yushmanov,
W A Zajc,
O Zaudkte,
C Zhang,
S Zhou, S J Zhou,
J Zimányi,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: New measurements are presented for charged hadron azimuthal correlations at midrapidity in Au+Au collisions at (square root)s(NN) = 62.4 and 200 GeV. They are compared to earlier measurements obtained at (square root)s(NN) = 130 GeV and in Pb + Pb collisions at (square root)s(NN) = 17.2 GeV. Sizeable anisotropies are observed with centrality and transverse momentum (pT) dependence characteristic of elliptic flow (upsilon2). For a broad range of centralities, the observed magnitudes and trends of the differential anisotropy, upsilon2(pT), change very little over the collision energy range (square root)s(NN) = 62-200 GeV, indicating saturation of the excitation function for upsilon2 at these energies. Such a saturation may be indicative of the dominance of a very soft equation of state for (square root)s(NN) approximately 60-200 GeV.
Physical Review Letters 07/2005; 94(23):232302. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: The production of deuterons and antideuterons in the transverse momentum range 1.1<p(T)<4.3 GeV/c at midrapidity in Au+Au collisions at square root of s(NN)=200 GeV has been studied by the PHENIX experiment at RHIC. A coalescence analysis, comparing the deuteron and antideuteron spectra with that of proton and antiproton, has been performed. The coalescence probability is equal for both deuterons and antideuterons and it increases as a function of p(T), which is consistent with an expanding collision zone. Comparing (anti)proton yields, p /p=0.73+/-0.01, with (anti)deuteron yields, d /d=0.47+/-0.03, we estimate that n /n=0.64+/-0.04. The nucleon phase space density is estimated from the coalescence measurement.
Physical Review Letters 04/2005; 94(12):122302. · 7.37 Impact Factor
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S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
J. Alexander,
R. Amirikas,
L. Aphecetche,
S. H. Aronson,
R. Averbeck, [......],
Y. Yang,
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou, S. J. Zhou,
L. Zolin
Physical Review C 03/2005; 71(4):49901. · 3.31 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured midrapidity transverse momentum spectra (0.4<p(T)<4.0 GeV/c) of single electrons as a function of centrality in Au+Au collisions at sqrt[s(NN)]=200 GeV. Contributions from photon conversions and Dalitz decays of light neutral mesons are measured by introducing a thin (1.7% X0) converter into the PHENIX acceptance and are statistically removed. The subtracted nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons containing heavy quarks, mainly charm at lower p(T). For all centralities, the charm production cross section is found to scale with the nuclear overlap function, T(AA). For minimum-bias collisions the charm cross section per binary collision is N(cc )/T(AA)=622+/-57(stat)+/-160(syst) microb.
Physical Review Letters 03/2005; 94(8):082301. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Bose-Einstein correlations of identically charged pion pairs were measured by the PHENIX experiment at midrapidity in Au + Au collisions at square root s(NN)=200 GeV. The Bertsch-Pratt radius parameters were determined as a function of the transverse momentum of the pair and as a function of the centrality of the collision. Using the standard core-halo partial Coulomb fits, and a new parametrization which constrains the Coulomb fraction as determined from the unlike-sign pion correlation, the ratio R(out)/R(side) is within 0.8-1.1 for 0.25< <k(T)> <1.2 GeV/c. The centrality dependence of all radii is well described by a linear scaling in N(1/3)(part), and R(out)/R(side) for <k(T)> approximately 0.45 GeV/c is approximately constant at unity as a function of centrality.
Physical Review Letters 10/2004; 93(15):152302. · 7.37 Impact Factor
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S S Adler,
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
J Alexander,
R Amirikas,
L Aphecetche,
S H Aronson,
R Averbeck, [......],
Y Yang,
A Yanovich,
S Yokkaichi,
G R Young,
I E Yushmanov,
W A Zajc,
C Zhang,
S Zhou, S J Zhou,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: Event-by-event fluctuations of the average transverse momentum of produced particles near midrapidity have been measured by the PHENIX Collaboration in square root of (sNN)=200 GeV Au+Au, and p+p collisions at the Relativistic Heavy Ion Collider. The fluctuations are observed to be in excess of the expectation for statistically independent particle emission for all centralities. The excess fluctuations exhibit a dependence on both the centrality of the collision and on the pT range over which the average is calculated. Both the centrality and pT dependence can be well reproduced by a simulation of random particle production with the addition of contributions from hard-scattering processes.
Physical Review Letters 09/2004; 93(9):092301. · 7.37 Impact Factor
