61 Search Results

The PHENIX experiment measured the centrality dependence of two-pion Bose-Einstein correlation functions in $$\sqrt{^{S}NN}$$ = 200 GeV Au + Au collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The data are well represented by Lévy-stable source distributions. The extracted source parameters are the correlation-strength parameter λ, the Lévy index of stability α, and the Lévy-scale parameter R as a function of transverse mass m_T and centrality. The λ⁡(m_T) parameter is constant at larger values of m_T, but decreases as m_T decreases. The Lévy-scale parameter R⁡(m_T) decreases with m_T and exhibits proportionality to the length scale of the nuclear overlap region. The Lévy exponent α⁡(m_T) is independent of m_T within uncertainties in each investigated centrality bin, but shows a clear centrality dependence. At all centralities, the Lévy exponent α is significantly different from that of Gaussian (α = 2) or Cauchy (α = 1) source distributions. Comparisons to the predictions of Monte-Carlo simulations of resonance-decay chains show that, in all but the most peripheral centrality class (50%–60%), the obtained results are inconsistent with the measurements, unless a significant reduction of the in-medium mass of the η' meson is included. Finally, in each centrality class, the best value of the in-medium η' mass is compared to the mass of the η meson, as well as to several theoretical predictions that consider restoration of U_A(1) symmetry in hot hadronic matter.

The nearby elliptical galaxy M87 contains one of only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to γ-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high-energy (VHE) γ-rays as well as details of the individual observations and light curves. We also conducted phenomenological modelling to investigate the basic source properties. We present the first VHE γ-ray flare from M87 detected since 2010. The flux above 350 GeV more than doubled within a period of ≈36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE γ-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and it emphasises the need for combined image and spectral modelling.

High-momentum two-particle correlations are a useful tool for studying jet-quenching effects in the quark-gluon plasma. Angular correlations between neutral-pion triggers and charged hadrons with transverse momenta in the range 4–12 GeV/c and 0.5–7 GeV/c, respectively, have been measured by the PHENIX experiment in 2014 for Au + Au collisions at $$\sqrt{S_{NN}}$$ =200 GeV. Suppression is observed in the yield of high-momentum jet fragments opposite the trigger particle, which indicates jet suppression stemming from in-medium partonic energy loss, while enhancement is observed for low-momentum particles. Here, the ratio and differences between the yield in Au + Au collisions and p + p collisions, I_AA and Δ_AA, as a function of the trigger-hadron azimuthal separation, Δφ, are measured for the first time at the BNL Relativistic Heavy Ion Collider. These results better quantify how the yield of low-p_T associated hadrons is enhanced at wide angle, which is crucial for studying energy loss as well as medium-response effects.

The PHENIX experiment has performed a systematic study of identified charged-hadron ($π^±$, $K^±$, $$p$$, $$\overline{p}$$) production at midrapidity in $$p$$ + Al, ³He +Au, and Cu + Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV and U + U collisions at $$\sqrt{s_{NN}}$$ = 193 GeV. Identified charged-hadron invariant transverse-momentum ($$p_T$$) and transverse-mass ($$m_T$$) spectra are presented and interpreted in terms of radially expanding thermalized systems. The particle ratios of $K/π$ and $p/π$ have been measured in different centrality ranges of large (Cu + Au and U + U) and small ($$p$$ + Al and ³He +Au) collision systems. The values of $K/π$ ratios measured in all considered collision systems were found to be consistent with those measured in $p + p$ collisions. However, the values of $p/π$ ratios measured in large collision systems reach the values of ≈0.6, which is a factor of ≈2 larger than in $p + p$ collisions. These results can be qualitatively understood in terms of the baryon enhancement expected from hadronization by recombination. Identified charged-hadron nuclear-modification factors ($$R_{AB}$$) are also presented. Enhancement of proton $$R_{AB}$$ values over meson $$R_{AB}$$ values was observed in central ³He +Au, Cu + Au, and U + U collisions. Here, the proton $$R_{AB}$$ values measured in the $$p$$ + Al collision system were found to be consistent with $$R_{AB}$$ values of $$Φ$$, $π^±$, $K^±$, and $π^0$ mesons, which may indicate that the size of the system produced in $$p$$ + Al collisions is too small for recombination to cause a noticeable increase in proton production.

Here, the measurement of the direct-photon spectrum from Au+Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV is presented by the PHENIX collaboration using the external-photon-conversion technique for 0%– 93% central collisions in a transverse-momentum (p_T) range of 0.8–10 GeV/c. An excess of direct photons, above prompt-photon production from hard-scattering processes, is observed for p_T < 6 GeV/c. Nonprompt direct photons are measured by subtracting the prompt component, which is estimated as N_coll-scaled direct photons from p+p collisions at 200 GeV, from the direct-photon spectrum. Results are obtained for 0.8 < p_T < 6.0 GeV/c and suggest that the spectrum has an increasing inverse slope from ≈0.2 to 0.4 GeV/c with increasing p_T, which indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct-photon production, the p_T-integrated nonprompt direct-photon yields also follow a power-law scaling behavior as a function of collision-system size. The exponent, α, for the nonprompt component is found to be consistent with 1.1 with no apparent p_T dependence.

Here, the invariant yield of electrons from open-heavy-flavor decays for 1 < p_T < 8 GeV/c at midrapidity |y| < 0.35 in Au+Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV has been measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. A displaced-vertex analysis with the PHENIX silicon-vertex detector enables extraction of the fraction of charm and bottom hadron decays and unfolding of the invariant yield of parent charm and bottom hadrons. The nuclear-modification factors RAA for electrons from charm and bottom hadron decays and heavy-flavor hadrons show both a centrality and a quark-mass dependence, indicating suppression in the quark-gluon plasma produced in these collisions that is medium sized and quark-mass dependent.

We previously published in 2018 a detailed measurement recorded in 2010 for Au+Au collisions at $$\sqrt{^SNN}$$ = 200 GeV of charged two-pion correlation functions in 0%-30% centrality. We found the data to be well described by Bose-Einstein correlation functions stemming from L$$\acute{e}$$vy-stable source distributions. Using a fine transverse momentum binning, we extracted the correlation strength parameter λ, the L$$\acute{e}$$vy index of stability α and the L$$\acute{e}$$vy length scale parameter R as a function of average transverse mass of the pair m_T.

We present measurements of the cross section and double-helicity asymmetry A_LL of directphoton production in $$\vec{p}$$ + $$\vec{p}$$ collisions at $$\sqrt {s}$$ = 510 GeV. The measurements have been performed at midrapidity (|η| < 0.25) with the PHENIX detector at the Relativistic Heavy Ion Collider. At relativistic energies, direct photons are dominantly produced from the initial quark-gluon hard scattering and do not interact via the strong force at leading order. Therefore, at $$\sqrt {s}$$ = 510 GeV, where leading-order-effects dominate, these measurements provide clean and direct access to the gluon helicity in the polarized proton in the gluon-momentum-fraction range 0.02 < x < 0.08, with direct sensitivity to the sign of the gluon contribution.

Here, the measurement of direct photons from Au + Au collisions at $$\sqrt{s_{NN}}$$ = 39 and 62.4 GeV in the transverse-momentum range 0.4 < $$p_T$$ < 3 Gev/c is presented by the PHENIX collaboration at the BNLRelativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $$p_T$$ spectra for different center-of-mass energies and for different centrality selections at $$\sqrt{s_{NN}}$$ = 62.4 GeV is scaled with $$(dN_{\text{ch}}/dη)^α$$ for α = 1.21 ± 0.04. This scaling also holds true for direct-photon spectra from Au + Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV measured earlier by PHENIX, as well as the spectra from Pb + Pb at $$\sqrt{s_{NN}}$$ = 2760 GeV published by ALICE. The scaling power α seems to be independent of $$p_T$$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $$p_T$$ of 2 Gev/c. The spectra have a local inverse slope $$T_{\text{eff}}$$ increasing with $$p_T$$ of 0.174 ± 0.018 Gev/c in the range 0.4 < $$p_T$$ < 1.3 Gev/c and increasing to 0.289 ± 0.024 Gev/c for 0.9 < $$p_T$$ < 2.1 Gev/c. The observed similarity of low-$$p_T$$ direct-photon production from $$\sqrt{s_{NN}}$$ = 39 to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.

Recently, the PHENIX Collaboration has published second- and third-harmonic Fourier coefficients v₂ and v₃ for midrapidity (|η|< 0.35 ) charged hadrons in 0%–5% central p+Au, d+Au, and ³He+Au collisions at $$\sqrt{s_{NN}}$$= 200 GeV, utilizing three sets of two-particle correlations for two detector combinations with different pseudorapidity acceptance [Acharya et al., Phys. Rev. C 105, 024901 (2022)]. Here, this paper extends these measurements of v₂ to all centralities in p+Au, d+Au, and ³He+Au collisions, as well as p+p collisions, as a function of transverse momentum (p_T) and event multiplicity. The kinematic dependence of v₂ is quantified as the ratio R of v₂ between the two detector combinations as a function of event multiplicity for 0.5 < p_T <1 and 2 _T < 2.5 GeV/c. A multiphase-transport (AMPT) model can reproduce the observed v₂ in most-central to midcentral d Au and ³He+Au collisions. However, the AMPT model systematically overestimates the measurements in p+p, p+Au, and peripheral d+Au and ³He+Au collisions, indicating a higher nonflow contribution in the AMPT model than in the experimental data. The AMPT model fails to describe the observed R for 0.5 < p_T < 1 GeV/c , but there is qualitative agreement with the measurements for 2 < p_T < 2.5 GeV/c.