Engineering topics

Ghimire, N.

Publications and source records attributed to Ghimire, N..

At least 19 records

DOE OSTI2025

Semi-inclusive direct photon + jet and 𝜋 0 + jet correlations measured in 𝑝 + 𝑝 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 (γ dir ) and neutral pion (π 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 −1 for Au + Au and 23pb −1 for 𝑝𝑝 collisions. Jets are reconstructed using the anti-𝑘 𝑇 algorithm with resolution parameters 𝑅 = 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 𝑝$^{ch}_{𝑇,jet}$ and large 𝑅. Recoil-jet distributions are reported in the range 𝑝$^{ch}_{𝑇,jet}$ < 25 GeV/𝑐. Comparison of the distributions measured in 𝑝𝑝 and Au + Au collisions reveals strong medium-induced jet yield suppression for 𝑅 = 0.2 with markedly less suppression for 𝑅 = 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 PHYSICS↗

DOE OSTI2025

Light nuclei femtoscopy and baryon interactions in 3 GeV Au+Au collisions at RHIC

We report the measurements of proton-deuteron (𝑝-𝑑) and deuteron-deuteron (𝑑-𝑑) 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 (𝑅 𝐺 ), scattering length (𝑓 0 ), and effective range (𝑑 0 ) are extracted from the measured correlation functions with a simultaneous fit. The spin-averaged 𝑓 0 for 𝑝-𝑑 and 𝑑-𝑑 interactions are determined to be -5.28 ± 0.11(stat.) ± 0.82(syst.) fm and -2.62 ± 0.02(stat.) ±0.24(syst.) fm, respectively. The measured 𝑝-𝑑 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 PHYSICS↗

DOE OSTI2025

Electric-charge-dependent directed flow splitting of produced quarks in Au+Au collisions

We report directed flow (v 1 ) of multistrange baryons ($Ξ$ and Ω) and improved v 1 data for K – ,$\overline{p}$, $\overline{Λ}$, and ϕ 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 (“splitting”) 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 FIELDS↗

DOE OSTI2024

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 (𝑝 𝑇 ) spectra of protons (𝑝) , deuterons (𝑑) , tritons (𝑡) , 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 (𝑑⁢𝑁/𝑑⁢𝑦), average transverse momentum (⟨𝑝 𝑇 ⟩), yield ratios (𝑑/𝑝, 𝑡/𝑝, 3 He/𝑝, 4 He/𝑝), as well as the coalescence parameters (𝐵 2 ,𝐵 3 ). The 4⁢𝜋 yields for various particles are determined by utilizing the measured rapidity distributions, 𝑑⁢𝑁/𝑑⁢𝑦. Furthermore, we present the energy, centrality, and rapidity dependence of the compound yield ratios (𝑁 𝑝 ×𝑁 𝑡 /𝑁$^{2}_{d}$) 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 PHYSICS↗

DOE OSTI2024

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 PHYSICS↗

DOE OSTI2024

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 η ϵ[–5,–3.4] in the Au-going direction and report correlations between this EA and hard- and soft-scale particle production at midrapidity (η ϵ[–1,1]). 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 PHYSICS↗

DOE OSTI2024

Measurements of charged-particle multiplicity dependence of higher-order net-proton cumulants in p + p collisions at $\sqrt{s}$ = 200 GeV from STAR at RHIC

We report on the charged-particle multiplicity dependence of net-proton cumulant ratios up to sixth order from $\sqrt{s}$ = 200 GeV p+p collisions at the Relativistic Heavy Ion Collider (RHIC). The measured ratios C 4 /C 2 , C 5 /C 1 , and C 6 /C 2 decrease with increased charged-particle multiplicity and rapidity acceptance. Neither the Skellam baselines nor PYTHIA8 calculations account for the observed multiplicity dependence. In addition, the ratios C 5 /C 1 and C 6 /C 2 approach negative values in the highest-multiplicity events, which implies that thermalized QCD matter may be formed in p+p collisions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Observation of the antimatter hypernucleus $^4_{\bar{\Lambda}}\overline{\hbox{H}}$

Matter-antimatter asymmetry is a research topic of fundamental interest, as it is the basis for the existence of the matter world, which survived annihilation with antimatter in the early Universe. High energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter. Much of the antimatter created escapes the rapidly expanding fireball without annihilation, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and study their properties. In this paper, we report the first observation of the antimatter hypernucleus $^4_{\bar{\Lambda}}\overline{\hbox{H}}$, composed of an $\bar{\Lambda}$, an antiproton and two antineutrons. The discovery was made through its two-body decay after production in ultrarelativistic heavy ion collisions by the STAR experiment at the Relativistic Heavy Ion Collider. In total, 15.6 candidate $^4_{\bar{\Lambda}}\overline{\hbox{H}}$ antimatter hypernuclei are obtained with an estimated background count of 6.4. Lifetimes of the antihypernuclei $^3_{\bar{\Lambda}}\overline{\hbox{H}}$ and $^4_{\bar{\Lambda}}\overline{\hbox{H}}$ are measured and compared with lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei and (anti)nuclei are also measured and compared with theoretical model predictions, shedding light on their production mechanism.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2024

Exclusive J / ψ , ψ ( 2 s ) , and e + e - pair production in Au + Au ultraperipheral collisions at the BNL Relativistic Heavy Ion Collider

Measurements of exclusive J/ψ, ψ(2⁢s), and electron-positron (e + ⁢e - ) pair photoproduction in Au+Au ultraperipheral collisions are reported by the STAR experiment at $\sqrt{s_{NN}}$ = 200GeV. Here, we report several first measurements at the BNL Relativistic Heavy Ion Collider, which are (i) J/ψ photoproduction with large momentum transfer up to 2.2⁢(GeV/c) 2 , (ii) coherent J/ψ photoproduction associated with neutron emissions from nuclear breakup, (iii) the rapidity dependence of incoherent J/ψ photoproduction, (iv) the ψ⁡(2⁢s) photoproduction cross section at midrapidity, and (v) e + ⁢e - pair photoproduction up to high invariant mass of 6GeV/c 2 . For measurement (ii), the coherent J/ψ total cross section of γ + Au → J/ψ + Au as a function of the center-of-mass energy W γN has been obtained without photon energy ambiguities. The data are quantitatively compared with the Monte Carlo models STARlight, Sartre, BeAGLE, and theoretical calculations of gluon saturation with color glass condensate, nuclear shadowing with leading twist approximation, quantum electrodynamics, and the next-to-leading-order perturbative QCD. At the photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent J/ψ cross sections of Au nuclei are found to be 71% ± 10% and 36% ± 7%, respectively, of that of free protons. These data provide an important experimental constraint for nuclear parton distribution functions and a unique opportunity to advance the understanding of the nuclear modification effect at the top RHIC energy.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Observation of Strong Nuclear Suppression in Exclusive J/ψ Photoproduction in Au + Au Ultraperipheral Collisions at RHIC

We report a measurement of exclusive J/ψ and ψ⁡(2⁢s) photoproduction in Au+Au ultraperipheral collisions at √s NN = 200 GeV using the STAR detector. For the first time, (i) the ψ⁡(2⁢s) photoproduction in midrapidity at the Relativistic Heavy-Ion Collider has been experimentally measured; (ii) nuclear suppression factors are measured for both the coherent and incoherent J/ψ production. At average photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent J/ψ cross sections of Au nuclei are found to be 71 ±10% and 36 ±7%, respectively, of that of free protons. The stronger suppression observed in the incoherent production provides a new experimental handle to study the initial-state parton density in heavy nuclei. As a result, data are compared with theoretical models quantitatively.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Estimate of background baseline and upper limit on the chiral magnetic effect in isobar collisions at $\sqrt{S_{NN}}$=200 GeV at the BNL Relativistic Heavy Ion Collider

For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions ($^{96}_{44}$Ru + $^{96}_{44}$Ru, $^{96}_{40}$Zr + $^{96}_{40}$Zr) obtained through a blind analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the CME-sensitive charge-dependent azimuthal correlator (Δ⁢𝛾), normalized by elliptic anisotropy (𝑣2), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, 𝑌= (Δ⁢𝛾/𝑣2) Ru /(Δ⁢𝛾/𝑣2) Zr , is naively expected to be (1/𝑁) Ru /(1/𝑁) Zr ; however, genuine two- and three-particle correlations are expected to alter it. We estimate the contributions to 𝑌 from those correlations, utilizing both the isobar data and hijing simulations. After including those contributions, we arrive at a final background baseline for 𝑌, which is consistent with the isobar data. Here, we extract an upper limit for the CME fraction in the Δ⁢𝛾 measurement of approximately 10% at a 95% confidence level on in isobar collisions at $\sqrt{S_{NN}}$=200 GeV, with an expected 15% difference in their squared magnetic fields.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Upper limit on the chiral magnetic effect in isobar collisions at the Relativistic Heavy-Ion Collider

The chiral magnetic effect (CME) is a phenomenon that arises from the QCD anomaly in the presence of an external magnetic field. The experimental search for its evidence has been one of the key goals of the physics program of the Relativistic Heavy-Ion Collider. The STAR Collaboration has previously presented the results of a blind analysis of isobar collisions ( Ru 44 96 + Ru 44 96 , Zr 40 96 + Zr 40 96 ) in the search for the CME. The isobar ratio ( Y ) of CME-sensitive observable, charge separation scaled by elliptic anisotropy, is close to but systematically larger than the inverse multiplicity ratio, the naive background baseline. This indicates the potential existence of a CME signal and the presence of remaining nonflow background due to two- and three-particle correlations, which are different between the isobars. In this postblind analysis, we estimate the contributions from those nonflow correlations as a background baseline to Y , utilizing the isobar data as well as Heavy Ion Jet Interaction Generator simulations. This baseline is found consistent with the isobar ratio measurement, and an upper limit of 10% at 95% confidence level is extracted for the CME fraction in the charge separation measurement in isobar collisions at s NN = 200 GeV. Published by the American Physical Society 2024

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Collision-energy dependence of deuteron cumulants and proton-deuteron correlations in Au+Au collisions at RHIC

We report the first measurements of cumulants, up to 4th order, of deuteron number distributions and proton-deuteron correlations in Au+Au collisions recorded by the STAR experiment in phase-I of Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider. Deuteron cumulants, their ratios, and proton-deuteron mixed cumulants are presented for different collision centralities covering a range of center-of-mass energy per nucleon pair $\sqrt{s_{NN}} = 7.7$ to 200 GeV. It is found that the cumulant ratios at lower collision energies favor a canonical ensemble over a grand canonical ensemble in thermal models. An anti-correlation between proton and deuteron multiplicity is observed across all collision energies and centralities, consistent with the expectation from global baryon number conservation. The UrQMD model coupled with a phase-space coalescence mechanism qualitatively reproduces the collision-energy dependence of cumulant ratios and proton-deuteron correlations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Measurements of the $Z$ 0 /$γ$* cross section and transverse single spin asymmetry in 510 GeV $p$ + $p$ collisions

The differential cross section for Z 0 production, measured as a function of the boson’s transverse momentum (p T ), provides important constraints on the evolution of the transverse momentum dependent parton distribution functions (TMDs). The transverse single spin asymmetry (TSSA) of the Z 0 is sensitive to one of the polarized TMDs, the Sivers function, which is predicted to have the opposite sign in p + p → W/Z + X from that which enters in semi-inclusive deep inelastic scattering. In this Letter, the STAR Collaboration reports the first measurement of the Z 0 /γ* differential cross section as a function of its p T in p+p collisions at a center-of-mass energy of 510 GeV, together with the Z 0 /γ* total cross section. We also report the measurement of Z 0 /γ* TSSA in transversely polarized p+p collisions at 510 GeV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Reaction plane correlated triangular flow in Au + Au collisions at $\sqrt{s_{NN}}$ = 3 GeV

Here, we measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au + Au collisions at the BNL Relativistic Heavy Ion Collider. A significant v 3 signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at midrapidity, dv 3 /dy| (y=0) , opposite in sign compared to the slope for directed flow. No significant v 3 signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Jet-hadron correlations with respect to the event plane in $\sqrt{s_{NN}}$ = 200 GeV Au + Au collisions in STAR

Angular distributions of charged particles relative to jet axes are studied in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions as a function of the jet orientation with respect to the event plane. This differential study tests the expected path-length dependence of energy loss experienced by a hard-scattered parton as it traverses the hot and dense medium formed in heavy-ion collisions. A second-order event plane is used in the analysis as an experimental estimate of the reaction plane formed by the collision impact parameter and the beam direction. Charged-particle jets with 15 < 𝑝 T,jet < 20 and 20 < 𝑝 T,jet < 40 ⁢GeV/𝑐 were reconstructed with the anti-𝑘 T algorithm with radius parameter setting of 𝑅 = 0.4 in the 20–50% centrality bin to maximize the initial-state eccentricity of the interaction region. The reaction plane fit method is implemented to remove the flow-modulated background with better precision than prior methods. Yields and widths of jet-associated charged-hadron distributions are extracted in three angular bins between the jet axis and the event plane. The event-plane (EP) dependence is further quantified by ratios of the associated yields in different EP bins. No dependence on orientation of the jet axis with respect to the event plane is seen within the uncertainties in the kinematic regime studied. This finding is consistent with a similar experimental observation by ALICE in $\sqrt{s_{NN}}$ = 2.76 TeV Pb-Pb collision data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Results on elastic cross sections in proton–proton collisions at $\sqrt{s}$ = 510 GeV with the STAR detector at RHIC

We report results on an elastic cross section measurement in proton–proton collisions at a center-of-mass energy $\sqrt{s}$ = 510 GeV, obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section is measured in the four-momentum transfer squared range 0.23 ≤ −t ≤ 0.67 GeV 2 . This is the only measurement of the proton-proton elastic cross section in this t range for collision energies above the Intersecting Storage Rings (ISR) and below the Large Hadron Collider (LHC) colliders. We find that a constant slope B does not fit the data in the aforementioned t range, and we obtain a much better fit using a second-order polynomial for B(t). This is the first measurement below the LHC energies for which the non-constant behavior B(t) is observed. The t dependence of B is also determined using six subintervals of t in the STAR measured t range, and is in good agreement with the phenomenological models. The measured elastic differential cross section dσ/dt agrees well with the results obtained at $\sqrt{s}$ = 540 GeV for proton–antiproton collisions by the UA4 experiment. We also determine that the integrated elastic cross section within the STAR t-range is σ$^{fid}_{el}$ = 462.1 ± 0.9(stat.) ± 1.1(syst.) ± 11.6(scale) μb.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Observation of the Electromagnetic Field Effect via Charge-Dependent Directed Flow in Heavy-Ion Collisions at the Relativistic Heavy Ion Collider

The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Noncentral collisions can produce strong magnetic fields on the order of 10 18 G , which offers a probe into the electrical conductivity of the QGP. In particular, quarks and antiquarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as v 1 ( y ) . Here, we present the charge-dependent measurements of d v 1 / d y near midrapidities for π ± , K ± , and p ( p ¯ ) in Au + Au and isobar ( Ru 44 96 + Ru 44 96 and Zr 40 96 + Zr 40 96 ) collisions at s NN = 200 GeV , and in Au + Au collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the v 1 signal on collision system, particle species, and collision centrality can be qualitatively and semiquantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the u and d quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations. Published by the American Physical Society 2024

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗