arXiv subjects

P. Dixit

Publications and source records attributed to P. Dixit.

61 records · Page 4Linked to original sources

arXiv2022

Disappearance of partonic collectivity in $\sqrt{s_{NN}}$ = 3 GeV Au+Au collisions at RHIC

We report on the measurements of directed flow $v_1$ and elliptic flow $v_2$ for hadrons ($π^{\pm}$, $K^{\pm}$, $K_{S}^0$, $p$, $ϕ$, $Λ$ and $Ξ^{-}$) from Au+Au collisions at $\sqrt{s_{NN}}$ = 3\,GeV and $v_{2}$ for ($π^{\pm}$, $K^{\pm}$, $p$ and $\overline{p}$) at 27 and 54.4\,GeV with the STAR experiment. While at the two higher energy midcentral collisions the number-of-constituent-quark (NCQ) scaling holds, at 3\,GeV the $v_{2}$ at midrapidity is negative for all hadrons and the NCQ scaling is absent. In addition, the $v_1$ slopes at midrapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative $v_2$ and positive $v_1$ slope at 3\,GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that the medium in such collisions is likely characterized by baryonic interactions.

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arXiv2022

Measurement of inclusive electrons from open heavy-flavor hadron decays in $p$+$p$ collisions at $\sqrt{s} = 200$ GeV with the STAR detector

We report a new measurement of the production cross section for inclusive electrons from open heavy-flavor hadron decays as a function of transverse momentum ($p_{\rm T}$) at mid-rapidity ($|y|<$ 0.7) in $p$+$p$ collisions at $\sqrt{s} = 200$ GeV. The result is presented for 2.5 $<p_{\rm T}<$ 10 GeV/$c$ with an improved precision above 6 GeV/$c$ with respect to the previous measurements, providing more constraints on perturbative QCD calculations. Moreover, this measurement also provides a high-precision reference for measurements of nuclear modification factors for inclusive electrons from open-charm and -bottom hadron decays in heavy-ion collisions.

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arXiv2022

Light Nuclei Collectivity from $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au Collisions at RHIC

In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, $v_1$ and $v_2$, of light nuclei ($d$, $t$, $^{3}$He, $^{4}$He) produced in $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured $v_1$ slopes of light nuclei at mid-rapidity. For the measured $v_2$ magnitude, a strong rapidity dependence is observed. Unlike $v_2$ at higher collision energies, the $v_2$ values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.

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arXiv2022

Global $Λ$-hyperon polarization in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV

Global hyperon polarization, $\overline{P}_\mathrm{H}$, in Au+Au collisions over a large range of collision energy, $\sqrt{s_\mathrm{NN}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While naïve extrapolation of data trends suggests a large $\overline{P}_\mathrm{H}$ as the collision energy is reduced, the behavior of $\overline{P}_\mathrm{H}$ at small $\sqrt{s_\mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $Λ$ hyperons along the direction of global angular momentum in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91\pm0.81(\rm stat.)\pm0.15(\rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.

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arXiv2022

Measurement of cold nuclear matter effects for inclusive $J/ψ$ in $p$+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 200 GeV

Measurement by the STAR experiment at RHIC of the cold nuclear matter (CNM) effects experienced by inclusive $J/ψ$ at mid-rapidity in 0-100\% $p$+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 200 GeV is presented. Such effects are quantified utilizing the nuclear modification factor, $R_{p\mathrm{Au}}$, obtained by taking a ratio of $J/ψ$ yield in $p$+Au collisions to that in $p$+$p$ collisions scaled by the number of binary nucleon-nucleon collisions. The differential $J/ψ$ yield in both $p$+$p$ and $p$+Au collisions is measured through the dimuon decay channel, taking advantage of the trigger capability provided by the Muon Telescope Detector in the RHIC 2015 run. Consequently, the $J/ψ$ $R_{p\mathrm{Au}}$ is derived within the transverse momentum ($p_{\mathrm{T}}$) range of 0 to 10 GeV/$c$. A suppression of approximately 30% is observed for $p_{\mathrm{T}}<2$ GeV/$c$, while $J/ψ$ $R_{p\mathrm{Au}}$ becomes compatible with unity for $p_{\mathrm{T}}$ greater than 3 GeV/$c$, indicating the $J/ψ$ yield is minimally affected by the CNM effects at high $p_{\mathrm{T}}$. Comparison to a similar measurement from 0-20% central Au+Au collisions reveals that the observed strong $J/ψ$ suppression above 3 Gev/$c$ is mostly due to the hot medium effects, providing strong evidence for the formation of the quark-gluon plasma in these collisions. Several model calculations show qualitative agreement with the measured $J/ψ$ $R_{p\mathrm{Au}}$, while their agreement with the $J/ψ$ yields in $p$+$p$ and $p$+Au collisions is worse.

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arXiv2021

Search for the Chiral Magnetic Effect with Isobar Collisions at $\sqrt{s_{NN}}$ = 200 GeV by the STAR Collaboration at RHIC

The chiral magnetic effect (CME) is predicted to occur as a consequence of a local violation of $\cal P$ and $\cal CP$ symmetries of the strong interaction amidst a strong electro-magnetic field generated in relativistic heavy-ion collisions. Experimental manifestation of the CME involves a separation of positively and negatively charged hadrons along the direction of the magnetic field. Previous measurements of the CME-sensitive charge-separation observables remain inconclusive because of large background contributions. In order to better control the influence of signal and backgrounds, the STAR Collaboration performed a blind analysis of a large data sample of approximately 3.8 billion isobar collisions of $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr at $\sqrt{s_{\rm NN}}=200$ GeV. Prior to the blind analysis, the CME signatures are predefined as a significant excess of the CME-sensitive observables in Ru+Ru collisions over those in Zr+Zr collisions, owing to a larger magnetic field in the former. A precision down to 0.4% is achieved, as anticipated, in the relative magnitudes of the pertinent observables between the two isobar systems. Observed differences in the multiplicity and flow harmonics at the matching centrality indicate that the magnitude of the CME background is different between the two species. No CME signature that satisfies the predefined criteria has been observed in isobar collisions in this blind analysis.

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arXiv2021

Measurement of $e^+e^-$ Momentum and Angular Distributions from Linearly Polarized Photon Collisions

The Breit-Wheeler process which produces matter and anti-matter from photon collisions is investigated experimentally through the observation of 6085 exclusive electron-positron pairs in ultra-peripheral Au+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The measurements reveal a large fourth-order angular modulation of $\cos{4Δϕ}=(16.8\pm2.5)\%$ and smooth invariant mass distribution absent of vector mesons ($ϕ$, $ω$ and $ρ$) at the experimental limit of $\le 0.2\%$ of the observed yields. The differential cross section as a function of $e^+e^-$ pair transverse momentum $P_\perp$ peaks at low value with $\sqrt{ \langle P_\perp^2 \rangle } = 38.1\pm0.9$ MeV and displays a significant centrality dependence. These features are consistent with QED calculations for the collision of linearly polarized photons quantized from the extremely strong electromagnetic fields generated by the highly charged Au nuclei at ultra-relativistic speed. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.

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