Azimuthal single-spin asymmetries of leptoproduced pions and charged kaons were measured on a transversely polarized hydrogen target. Evidence for a naive-T-odd, transverse-momentum-dependent parton ...distribution function is deduced from nonvanishing Sivers effects for pi(+), pi(0), and K(+/-), as well as in the difference of the pi(+) and pi(-) cross sections.
The PHENIX experiment at the BNL Relativistic Heavy Ion Collider has measured second- and third-order Fourier coefficients of the azimuthal distributions of direct photons emitted at midrapidity in ...Au + Au collisions at √ sNN = 200 GeV for various collision centralities. Combining two different analysis techniques, results were obtained in the transverse momentum range of 0.4 < pT < 4.0 GeV/c. At lowpT the second-order coefficients, v2, are similar to the ones observed in hadrons. Third-order coefficients, v3, are nonzero and almost independent of centrality. These new results on v2 and v3, combined with previously published results on yields, are compared to model calculations that provide yields and asymmetries in the same framework. Those models are challenged to explain simultaneously the observed large yield and large azimuthal anisotropies.
We present azimuthal angular correlations between charged hadrons and energy deposited in calorimeter towers in central d+Au and minimum bias p+p collisions at sNN=200 GeV. The charged hadron is ...measured at midrapidity |eta|<0.35, and the energy is measured at large rapidity (-3.7<eta< -3.1, Au-going direction). An enhanced near-side angular correlation across | Delta eta|>2.75 is observed in d+Au collisions. Using the event plane method applied to the Au-going energy distribution, we extract the anisotropy strength v2 for inclusive charged hadrons at midrapidity up to pT=4.5 GeV/c. We also present the measurement of v2 for identified pi+ or - and (anti)protons in central d+Au collisions, and observe a mass-ordering pattern similar to that seen in heavy-ion collisions. These results are compared with viscous hydrodynamic calculations and measurements from p+Pb at sNN=5.02 TeV. The magnitude of the mass ordering in d+Au is found to be smaller than that in p+Pb collisions, which may indicate smaller radial flow in lower energy d+Au collisions.
Here, we present a detailed measurement of charged two-pion correlation functions in 0–30% centrality √sNN = 200 GeV Au + Au collisions by the PHENIX experiment at the Relativistic Heavy Ion ...Collider. The data are well described by Bose-Einstein correlation functions stemming from Lévy-stable source distributions. Using a fine transverse momentum binning, we extract the correlation strength parameter λ, the Lévy index of stability α, and the Lévy length scale parameter R as a function of average transverse mass of the pair mT. We find that the positively and the negatively charged pion pairs yield consistent results, and their correlation functions are represented, within uncertainties, by the same Lévy-stable source functions. The λ(mT) measurements indicate a decrease of the strength of the correlations at low mT. The Lévy length scale parameter R(mT) decreases with increasing mT, following a hydrodynamically predicted type of scaling behavior. The values of the Lévy index of stability α are found to be significantly lower than the Gaussian case of α = 2, but also significantly larger than the conjectured value that may characterize the critical point of a second-order quark-hadron phase transition.
Charmonium suppression in hot and dense nuclear matter has been argued to be a signature for the production of the quark gluon plasma (QGP). In order to search for this effect in heavy ion collisions ...one must have a clear understanding of all the factors that can contribute to such a suppression. These may include shadowing of the partons in a nuclear environment, breakup of a correlated
c
−
c
¯
pair as it traverses the nuclear fragment, suppression of feed-down from higher mass states as well as other initial state interactions. In order to disentangle these effects one must measure charmonium production rates in both proton+proton (p+p) and proton+nucleus (p+A) collisions. The p+p collisions serve as a baseline for searching for suppression compared to binary scaling predictions, allow one to quantify the amount of feed-down from higher states as well as serve as a tool to distinguish between different theoretical calculations for charmonium production mechanisms. In order to quantify nuclear effects it is also necessary to study charmonium production in p+A collisions where the temperature and density of the system are low compared to a heavy ion collision. These measurements allow one to determine the influence of nuclear shadowing and breakup in “cold” nuclear matter which can be extrapolated to heavy ion collisions in order to determine the amount anomalous suppression. Of course, extrapolations that rely on a model based technique depend heavily on the assumption of a production mechanism, a fact that reinforces the importance of the p+p measurements. The PHENIX and STAR experiments at Brookhaven National Laboratory have measured charmonium production in p+p, d+Au, Au+Au and Cu+Cu collisions at
s
N
N
=
200
GeV
for both forward and mid rapidities. I will present a review of the latest measurements from both experiments with an emphasis on what we have and can still learned from them about charmonium production and suppression with these experimental apparatuses.
Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The ...asymmetry depends on the azimuthal angles of both the pion (phi) and the target spin axis (phi(S)) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component sin((phi+phi(S))(pi)(UT) is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component sin((phi-phi(S)(pi)(UT) arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.
Here, we report the measurement of cumulants (Cn,n = 1, ..., 4) of the net-charge distributions measured within pseudorapidity (vertical bar eta vertical bar < 0.35) in Au + Au collisions at √sNN = ...7.7-200 GeV with the PHENIX experiment at the Relativistic Heavy Ion Collider. The ratios of cumulants (e.g., C1/C2, C3/C1) of the net-charge distributions, which can be related to volume independent susceptibility ratios, are studied as a function of centrality and energy. These quantities are important to understand the quantum-chromodynamics phase diagram and possible existence of a critical end point. The measured values are very well described by expectation from negative binomial distributions. We do not observe any nonmonotonic behavior in the ratios of the cumulants as a function of collision energy. Moreover, the measured values of C1/C2 and C3/C1 can be directly compared to lattice quantum-chromodynamics calculations and thus allow extraction of both the chemical freeze-out temperature and the baryon chemical potential at each center-of-mass energy. Finally, the extracted baryon chemical potentials are in excellent agreement with a thermal-statistical analysis model.
Jet production rates are measured in p+p and d+Au collisions at $\sqrt{sNN}$=200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Jets are reconstructed using the ...R = 0.3 anti-kt algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multi-wire proportional chambers, and the jet transverse momentum (pT ) spectra are corrected for the detector response. Spectra are reported for jets with 12 < pT < 50 GeV/c, within a pseudorapidity acceptance of |η|< 0.3. The nuclear-modification factor (RdAu) values for 0%-100% d+Au events are found to be consistent with unity, constraining the role of initial state effects on jet production. However, the centrality-selected RdAu values and central-to-peripheral ratios (RCP) show large, pT -dependent deviations from unity, challenging the conventional models that relate hard-process rates and soft-particle production in collisions involving nuclei.
Here, we present measurements of e(+)e-production at midrapidity in Au + Au collisions at √sNN = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of ...mass (mee < 5 GeV/c2) and pair transverse momentum (pT < 5 GeV/c) for minimum bias and for five centrality classes. We compare the e+e- yield to the expectations from known sources. In the low-mass region (mee = 0.30-0.76 GeV/c2) there is an enhancement that increases with centrality and is distributed over the entire pair pT range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to 2.3 ± 0.4(stat) ± 0.4(syst) ± 0.2(model) or to 1.7 ± 0.3(stat) ± 0.3(syst) ± 0.2(model) for minimum bias collisions when the open heavy-flavor contribution is calculated with PYTHIA or MC@NLO, respectively. The inclusive mass and pT distributions, as well as the centrality dependence, are well reproduced by model calculations where the enhancement mainly originates from the melting of the ρ meson resonance as the system approaches chiral symmetry restoration. Finally, in the intermediate-mass region (mee = 1.2-2.8 GeV/c2), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in minimum bias Au+Au collisions at sNN−−−−√=200 GeV via the yields of electrons from ...semileptonic decays of charm and bottom hadrons. Previous heavy flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks owing to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in Au+Au collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in p+p collisions at sNN−−−−√=200 GeV and find the fractions to be similar within the large uncertainties on both measurements for pT>4GeV/c. We use the bottom electron fractions in Au+Au and p+p along with the previously measured heavy flavor electron RAA to calculate the RAA for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region 3<pT<4GeV/c.
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