Energy Correlators within Jets in Transversely Polarized Proton-Proton Collisions at βs =200 GeV
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73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Engineering topics
Bhasin, A.
Publications and source records attributed to Bhasin, A..
Tracking the baryon number with nuclear collisions
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73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
First Observation of Deuteron-Ξ Correlations at RHIC
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73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Measurement of inclusive $J/Ο$ production in Au+Au collisions at $\sqrt{s_{\textrm{NN}}}$ = 54.4 GeV at STAR
This article presents measurements of inclusive J/Ο production at midrapidity (|y| < 1.0) in Au+Au collisions at $\sqrt{s_{\textrm{NN}}}$ = 54.4 GeV with the STAR detector at the Relativistic Heavy Ion Collider. A suppression of the J/Ο yield, quantified using the nuclear modification factors (R AA , R CP ), is observed with respect to the scaled production p + p in collisions. The dependence of R AA on collision centrality and J/Ο transverse momentum is measured with improved precision compared to previous measurements at 39 and 62.4 GeV, while the centrality dependence of R CP is measured and compared to the same results at 39, 62.4, and 200 GeV. In central collisions, no significant collision energy dependence of R AA is found within uncertainties for collision energies between 17.3 and 200 GeV. Two transport model calculations that include dissociation and regeneration contributions are consistent with the experimental results within uncertainties. Although no significant collision energy dependence of the J/Ο suppression in high energy heavy-ion collisions up to $\sqrt{s_{\textrm{NN}}}$ = 200 GeV is observed within uncertainties, the newly measured results at 54.4 GeV Au+Au collisions provide additional constraints on theoretical calculations of the hot medium evolution and cold nuclear matter effects.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Probing late-stage hadronic interactions at high baryon density via $K^{β0}$ production in the RHIC beam energy scan program
A precision measurement of the $K^{β0}$ meson yield is reported in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.6, 19.6, and 27 GeV using the high-statistics data sample collected by the STAR experiment during the Beam Energy Scan II (BES-II) program at RHIC. The transeverse momentum (p T )-integrated yield ratios $\large{(}K^{β0} + \overline{K^{β0}}\large{)}/(K^+ + K^{-})$ in central collisions show a suppression relative to peripheral collisions at the (1.7β3.6) Ο level, while a thermal model without final-stage rescattering overpredicts this ratio with a deviation of (6.9β8.2) Ο. These results indicate a loss of the measured $K^{β0}$ signal in central collisions due to re-scattering of its hadronic decay products in the hadronic phase. The p T -integrated yield of charged kaons exhibits an approximate scaling with charged-particle multiplicity, independent of collision energy and system size. A similar trend is observed for the short-lived $K^{β0}$ resonance, although significant deviations emerge at lower energies. At BES energies, the $K^{β0}/K$ ratio shows stronger suppression than at the highest RHIC and LHC energies within a given multiplicity bin, particularly in central and mid-central collisions. This behavior is consistent with changes in the effective hadronic interaction cross section and is supported by transport model calculations, which indicate dominant mesonβbaryon interactions at lower energies and mesonβmeson interactions at higher energies.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Measurement of kaon directed flow in Au+Au collisions in the high baryon density region
Rapidity-odd directed flow v 1 measurements are presented for $K^Β±$ and $K^0_S$ in Au + Au collisions for $\sqrt{s_{NN}}$ from 3.0 to 3.9 GeV with the STAR experiment. For comparison, v 1 of Ο Β± , protons, and Ξ from the same collisions are also discussed. The mid-rapidity v 1 slope dv 1 /dy| y=0 for protons and Ξ is positive in these collisions. On the other hand, v 1 slope of kaons exhibits a strong dependence: negative at p T < 0.6 GeV/c and positive at higher p T . A similar p T dependence is also evident for the v 1 slope of charged pions. Compared to the spectator-removed calculations in Au+Au collisions at $\sqrt{s_{NN}}$ = 3.0β3.9 GeV, the JAM model demonstrates a pronounced shift of the v 1 slopes of mesons towards the negative direction. It suggests that the shadowing effect of the spectators plays an important role in the observed kaon anti-flow at low p T in the high baryon density region of non-central collisions.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Measuring spin correlation between quarks during QCD confinement
The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quarkβantiquark pairs. The spontaneous breaking of the approximate chiral symmetry, signalled by the nonvanishing quark condensate $\langle$$q\bar{q}$$\rangle$, is dynamically generated through topologically nontrivial gauge configurations such as instantons. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement remains a profound open question in quantum chromodynamics (QCD)βthe fundamental theory of strong interaction. High-energy protonβproton collisions could liberate virtual quarkβantiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in $Ξ\bar{Ξ}$ hyperon pairs inherited from spin-correlated strange quarkβantiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of (18 Β± 4)% that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Temperature measurement of Quark-Gluon plasma at different stages
In a Quark-Gluon Plasma (QGP), the fundamental building blocks of matter, quarks and gluons, are under extreme conditions of temperature and density. A QGP could exist in the early stages of the Universe, and in various objects and events in the cosmos. The thermodynamic and hydrodynamic properties of the QGP are described by Quantum Chromodynamics (QCD) and can be studied in heavy-ion collisions. Despite being a key thermodynamic parameter, the QGP temperature is still poorly known. Thermal lepton pairs (e + e β and ΞΌ + ΞΌ β ) are ideal penetrating probes of the true temperature of the emitting source, since their invariant-mass spectra suffer neither from strong final-state interactions nor from blue-shift effects due to rapid expansion. Here we measure the QGP temperature using thermal e+eβ production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-medium Ο 0 vector-meson dominant) is (2.01 Β± 0.23) Γ 10 12 K, consistent with the chemical freeze-out temperature from statistical models and the phase transition temperature from Lattice QCD. The average temperature from the intermediate mass region (above the Ο0 mass, QGP dominant) is significantly higher at (3.25 Β± 0.60) Γ 10 12 K. This work provides essential experimental thermodynamic measurements to map out the QCD phase diagram and understand the properties of matter under extreme conditions.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Hyperon global polarization in isobar Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200Ge V
The polarization of Ξ, $\overline{Ξ}$, $Ξ$ β , and $\overline{Ξ}$ + hyperons along the angular momentum of the system has been measured in isobar collisions of Ru+Ru and Zr+Zr at $\sqrt{s_{NN}}$ = 200 GeV with the STAR detector at RHIC. The polarization dependence on collision centrality exhibits an increasing trend in more peripheral collisions. Ξ and $\overline{Ξ}$ polarization dependence on the transverse momentum and pseudorapidity have been investigated, but no significant dependence was observed. The polarizations of Ξ and $\overline{Ξ}$ are found to be consistent with each other, indicating little contribution of the spin-magnetic coupling to the measured polarization. Comparison to previously measured polarization in Au+Au collisions show no obvious system size dependence. The results are qualitatively consistent with hydrodynamic calculations including contributions from shear-induced polarization and thermal vorticity. For the first time in heavy-ion collisions, the dependence of the global polarization on the hyperonβs emission azimuthal angle relative to the second-order event plane has been measured, indicating stronger polarization for the in-plane emitted hyperons at the level of 2.4 Ο significance in 20β50 % centrality. The $Ξ$ hyperon polarization measurements via polarization transfer analysis yield finite positive values with 2.9 Ο significance in 20β50 % centrality, slightly larger compared to the inclusive Ξ polarization.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Semi-inclusive direct photon + jet and π 0 + jet correlations measured in π + π and central Au + Au collisions at $\sqrt{s_{NN}}$ = 200GeV
The STAR experiment at RHIC reports new measurements of jet quenching based on the semi- inclusive distribution of charged-particle jets recoiling from direct photon (Ξ³ dir ) and neutral pion (Ο 0 ) triggers in pp and central Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, for triggers in the range 9 < $E$$^{trig}_{T}$ < 20 GeV. The datasets have integrated luminosities of 3.9nb β1 for Au + Au and 23pb β1 for ππ collisions. Jets are reconstructed using the anti-π π algorithm with resolution parameters π = 0.2 and 0.5. The large uncorrelated jet background in central Au + Au collisions is corrected using a mixed-event approach, which enables precise charged-particle jet measurements at low transverse momentum π$^{ch}_{π,jet}$ and large π . Recoil-jet distributions are reported in the range π$^{ch}_{π,jet}$ < 25 GeV/π. Comparison of the distributions measured in ππ and Au + Au collisions reveals strong medium-induced jet yield suppression for π = 0.2 with markedly less suppression for π = 0.5. Comparison is also made to theoretical models incorporating jet quenching. Furthermore, these data provide new insight into the mechanisms underlying jet quenching and the angular dependence of medium-induced jet-energy transport and provide new constraints on modeling such effects.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Light nuclei femtoscopy and baryon interactions in 3 GeV Au+Au collisions at RHIC
We report the measurements of proton-deuteron (π-π) and deuteron-deuteron (π-π) correlation functions in Au + Au collisions at $\sqrt{s_{NN}}$ = 3 GeV using fixed-target mode with the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). For the first time, the source size (π πΊ ), scattering length (π 0 ), and effective range (π 0 ) are extracted from the measured correlation functions with a simultaneous fit. The spin-averaged π 0 for π-π and π-π interactions are determined to be -5.28 Β± 0.11(stat.) Β± 0.82(syst.) fm and -2.62 Β± 0.02(stat.) Β±0.24(syst.) fm, respectively. The measured π-π interaction is consistent with theoretical calculations and low energy scattering experiment results, demonstrating the feasibility of extracting interaction parameters using the femtoscopy technique. The reasonable agreement between the experimental data and the calculations from the transport model indicates that deuteron production in these collisions is primarily governed by nucleon coalescence.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Electric-charge-dependent directed flow splitting of produced quarks in Au+Au collisions
We report directed flow (v 1 ) of multistrange baryons ($Ξ$ and Ξ©) and improved v 1 data for K β ,$\overline{p}$, $\overline{Ξ}$, and Ο in Au+Au collisions at $\sqrt{s_{NN}}$ = 27 and 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). We focus on particles whose constituent quarks are not transported from the incoming nuclei but instead are produced in the collisions. At intermediate impact parameters, we examine quark coalescence behavior for particle combinations with identical quark content, and search for any departure from this behavior (βsplittingβ) for combinations having non-identical quark content. Under the assumption of quark coalescence for produced quarks, the splitting strength appears to increase with the electric charge difference of the constituent quarks in the combinations, consistent with electromagnetic effect expectations.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSβ
Production of protons and light nuclei in Au +Au collisions at $\sqrt{s_{NN}}$ = 3 GeV with the STAR detector
We report the systematic measurement of protons and light nuclei production in Au +Au collisions at $\sqrt{s_{NN}}$ = 3 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The transverse momentum (π π ) spectra of protons (π) , deuterons (π) , tritons (π‘) , 3 He , and 4 He have been measured from midrapidity to target rapidity for different collision centralities. We present the rapidity and centrality dependence of particle yields (πβ’π/πβ’π¦), average transverse momentum (β¨π π β©), yield ratios (π/π, π‘/π, 3 He/π, 4 He/π), as well as the coalescence parameters (π΅ 2 ,π΅ 3 ). The 4β’π yields for various particles are determined by utilizing the measured rapidity distributions, πβ’π/πβ’π¦. Furthermore, we present the energy, centrality, and rapidity dependence of the compound yield ratios (π π Γπ π‘ /π$^{2}_{d}$) and compare them with various model calculations. Furthermore, the physics implications of these results on the production mechanism of light nuclei and the QCD phase structure are discussed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Imaging shapes of atomic nuclei in high-energy nuclear collisions
Atomic nuclei are self-organized, many-body quantum systems bound by strong nuclear forces within femtometre-scale space. These complex systems manifest a variety of shapes, traditionally explored using non-invasive spectroscopic techniques at low energies. However, at these energies, their instantaneous shapes are obscured by long-timescale quantum fluctuations, making direct observation challenging. Here we introduce the collective-flow-assisted nuclear shape-imaging method, which images the nuclear global shape by colliding them at ultrarelativistic speeds and analysing the collective response of outgoing debris. This technique captures a collision-specific snapshot of the spatial matter distribution within the nuclei, which, through the hydrodynamic expansion, imprints patterns on the particle momentum distribution observed in detectors. We benchmark this method in collisions of ground-state uranium-238 nuclei, known for their elongated, axial-symmetric shape. Our findings show a large deformation with a slight deviation from axial symmetry in the nuclear ground state, aligning broadly with previous low-energy experiments. This approach offers a new method for imaging nuclear shapes, enhances our understanding of the initial conditions in high-energy collisions and addresses the important issue of nuclear structure evolution across energy scales.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Correlations of event activity with hard and soft processes in p + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at the RHIC STAR experiment
With the STAR experiment at the BNL Relativistic Heavy Ion Collider, we characterize $\sqrt{s_{NN}}$ = 200GeV p + Au collisions by event activity (EA) measured within the pseudorapidity range Ξ· Ο΅[β5,β3.4] in the Au-going direction and report correlations between this EA and hard- and soft-scale particle production at midrapidity (Ξ· Ο΅[β1,1]). At the soft scale, charged particle production in low-EA p + Au collisions is comparable to that in p + p collisions and increases monotonically with increasing EA. At the hard scale, we report measurements of high transverse momentum (p T ) jets in events of different EAs. In contrast with the soft particle production, high-p T particle production and EA are found to be inversely related. Furthermore, to investigate whether this is a signal of jet quenching in high-EA events, we also report ratios of p T imbalance and azimuthal separation of dijets in high- and low-EA events. Within our measurement precision, no significant differences are observed, disfavoring the presence of jet quenching in the highest 30% EA p + Au collisions at $\sqrt{s_{NN}}$ = 200GeV.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Exploring the Strong Interaction of Three-Body Systems at the LHC
Deuterons are atomic nuclei composed of a neutron and a proton held together by the strong interaction. Unbound ensembles composed of a deuteron and a third nucleon have been investigated in the past using scattering experiments, and they constitute a fundamental reference in nuclear physics to constrain nuclear interactions and the properties of nuclei. In this work, K + -d and p-d femtoscopic correlations measured by the ALICE Collaboration in proton-proton (pp) collisions at $\sqrt{s}$ = 13 TeV at the Large Hadron Collider (LHC) are presented. It is demonstrated that correlations in momentum space between deuterons and kaons or protons allow us to study three-hadron systems at distances comparable with the proton radius. The analysis of the K + -d correlation shows that the relative distances at which deuterons and protons or kaons are produced are around 2 fm. The analysis of the p-d correlation shows that only a full three-body calculation that accounts for the internal structure of the deuteron can explain the data. In particular, the sensitivity of the observable to the short-range part of the interaction is demonstrated. These results indicate that correlations involving light nuclei in pp collisions at the LHC will also provide access to any three-body system in the strange and charm sectors.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSβ
Studying strangeness and baryon production mechanisms through angular correlations between charged Ξ baryons and identified hadrons in pp collisions at $\sqrt{s}$ = 13 TeV
The angular correlations between charged Ξ baryons and associated identified hadrons (pions, kaons, protons, Ξ baryons, and Ξ baryons) are measured in pp collisions at $\sqrt{s}$ = 13 TeV with the ALICE detector to give insight into the particle production mechanisms and balancing of quantum numbers on the microscopic level. In particular, the distribution of strangeness is investigated in the correlations between the doubly-strange Ξ baryon and mesons and baryons that contain a single strange quark, K and Ξ. As a reference, the results are compared to ΞΟ and Ξp correlations, where the associated mesons and baryons do not contain a strange valence quark. These measurements are expected to be sensitive to whether strangeness is produced through string breaking or in a thermal production scenario. Furthermore, the multiplicity dependence of the correlation functions is measured to look for the turn-on of additional particle production mechanisms with event activity. The results are compared to predictions from the string-breaking model P YTHIA 8, including tunes with baryon junctions and rope hadronisation enabled, the cluster hadronisation model H ERWIG 7, and the core-corona model E POS-LHC . While some aspects of the experimental data are described quantitatively or qualitatively by the Monte Carlo models, no model can match all features of the data. These results provide stringent constraints on the strangeness and baryon number production mechanisms in pp collisions.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSβ
Azimuthal anisotropy of jet particles in p-Pb and Pb-Pb collisions at $\sqrt{{\textrm{s}}_{\textrm{NN}}}$ = 5.02 TeV
The azimuthal anisotropy of particles associated with jets (jet particles) at midrapidity is measured for the first time in p-Pb and Pb-Pb collisions at $\sqrt{{\textrm{s}}_{\textrm{NN}}}$ = 5.02 TeV down to transverse momentum (p T ) of 0.5 GeV/c and 2 GeV/c, respectively, with ALICE. The results obtained in p-Pb collisions are based on a novel three-particle correlation technique. The azimuthal anisotropy coefficient v 2 in high-multiplicity p-Pb collisions is positive, with a significance reaching 6.8Ο at low p T , and its magnitude is smaller than in semicentral Pb-Pb collisions. In contrast to the measurements in Pb-Pb collisions, the v 2 coefficient is also found independent of p T within uncertainties. Comparisons with the inclusive charged-particle v 2 and with AMPT calculations are discussed. The predictions suggest that parton interactions play an important role in generating a non-zero jet-particle v 2 in p-Pb collisions, even though they overestimate the reported measurement. These observations shed new insights on the understanding of the origin of the collective behaviour of jet particles in small systems such as p-Pb collisions, and provide significant stringent new constraints to models.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSβ