arXiv subjects

Soumya Mohapatra

Publications and source records attributed to Soumya Mohapatra.

8 recordsLinked to original sources

arXiv2025

Upgrades of the ATLAS Zero Degree Calorimeter System for Run 3 at the Large Hadron Collider

Experimental studies of ultra-relativistic heavy ion collisions at the Large Hadron Collider (LHC) depend crucially on Zero Degree Calorimeters (ZDCs) that measure neutrons produced at near-beam rapidity in nucleus-nucleus collisions. In hadronic nuclear collisions these neutrons are mainly spectator neutrons, those that do not scatter from opposing nucleons during the collision. As a result, the ZDCs provide a vital probe of heavy ion collision geometry. The ZDCs are also essential in the study of ultra-peripheral collisions that are initiated by photons associated with the electric fields of one or both nuclei. Coherent photon emission typically leaves the photon emitter intact, making the observation of no ZDC signal, on one or both sides, a tag of such processes. The ATLAS ZDCs, built prior to Run 1 were substantially upgraded for LHC Run 3. The primary upgrades included replacement of the quartz Cherenkov radiator with $\text{H}_2$-doped fused silica rods; installation of fast air-core signal cables between the ZDC and the ATLAS USA15 cavern; new LED-based calibration system; and new electronics implemented for readout and fully-digital triggering. The ZDCs were also augmented with new "Reaction Plane Detectors" (RPDs) designed to measure the transverse centroid of multi-neutron showers to allow event-by-event reconstruction of the directed-flow plane in nuclear collisions. The Run~3 ZDC detectors, including the RPDs, are described in detail with emphasis on aspects that are new for Run~3.

physics.ins-det↗

arXiv2014

Elucidating the event-by-event flow fluctuations in heavy-ion collisions via the event shape selection technique

The presence of large event-by-event flow fluctuations in heavy ion collisions at RHIC and the LHC provides an opportunity to study a broad class of flow observables. This paper explores the correlations among harmonic flow coefficients $v_n$ and their phases $Φ_n$, and the rapidity fluctuation of $v_n$. The study is carried out usin Pb+Pb events generated by the AMPT model with fixed impact parameter. The overall ellipticity/triangularity of events is varied by selecting on the eccentricities $ε_n$ or the magnitudes of the flow vector $q_n$ for n=2 and 3, respectively. The responses of the $v_n$, the event-plane correlations, and the rapidity fluctuations, to the change in $ε_n$ and $q_n$ are then systematized. Strong positive correlations are observed among all even harmonics $v_2, v_4$, and $v_6$ (all increase with $q_2$), between $v_2$ and $v_5$ (both increase with $q_2$) and between $v_3$ and $v_5$ (both increase with $q_3$), consistent with the effects of nonlinear collective response. In contrast, an anti-correlation is observed between $v_2$ and $v_3$ similar to that seen between $ε_2$ and $ε_3$. These correlation patterns are found to be independent of whether selecting on $q_n$ or $ε_n$, validating the ability of $q_n$ in selecting the initial geometry. A forward/backward asymmetry of $v_n(η)$ is observed for events selected on $q_n$ but not on $ε_n$, reflecting dynamical fluctuations exposed by the $q_n$ selection. Many event-plane correlators show good agreement between $q_n$ and $ε_n$ selections, suggesting that their variations with $q_n$ are controlled by the change of $ε_n$ in the initial geometry. Hence these correlators may serve as promising observables for disentangling the fluctuations generated in various stages of the evolution of the matter created in heavy ion collisions.

nucl-ex↗

arXiv2013

Disentangling flow and nonflow correlations via Bayesian unfolding of the event-by-event distributions of harmonic coefficients in ultrarelativistic heavy-ion collisions

The performance of the Bayesian unfolding method in extracting the event-by-event (EbyE) distributions of harmonic flow coefficients v_n is investigated using a toy model simulation, as well as simulations based on the HIJING and AMPT models. The unfolding method is shown to recover the input v_2-v_4 distributions for multiplicities similar to those observed in Pb+Pb collisions at the LHC. The effects of the nonflow are evaluated using HIJING simulation with and without a flow afterburner. The probability distribution of v_n resulting only from nonflow in HIJING is nearly a Gaussian and can be largely suppressed in the data-driven unfolding method used by the ATLAS Collaboration. The residual nonflow effects have no appreciable impact on the v_3 distributions, but are found to affect the tails of the v_2 and v_4 distributions; these effects manifest as a small simultaneous change in the mean and standard deviation of the $v_n$ distributions. For the AMPT model, which contains both flow fluctuations and nonflow effects, the reduced shape of the extracted v_n distributions is found to be independent of pT in the low pT region, similar to what is observed in the ATLAS data. The prospect of using the EbyE distribution of the harmonic spectrum aided by the unfolding technique as a general tool to study azimuthal correlations in high energy collisions is also discussed.

nucl-ex↗

arXiv2013

A study of the anisotropy associated with dipole asymmetry in heavy ion collisions

The anisotropy associated with the initial dipole asymmetry in heavy ion collisions is studied via the two-particle relative azimuthal azimuthal angle (Δϕ=ϕ^a-ϕ^b) correlations, within a multi-phase transport model. For a broad selection of centrality, transverse momenta (pT^{a,b}) and pseudorapidity (η^{a,b}), a fitting method is used to decompose the anisotropy into a rapidity-even component, characterized by the Fourier coefficient v1, and a global momentum conservation component. The extracted v1 values are negative for pT<=0.7-0.9 GeV, reach a maximum at 2-3 GeV, and decreases at higher pT. The v1 values vary weakly with ηand centrality, but increases with collision energy and parton cross-section. The extracted global momentum conservation component is found to depend on Δη= η^a-η^b for Δη<3.

nucl-th↗

arXiv2013

A method for studying initial geometry fluctuations via event plane correlations in heavy ion collisions

A method is proposed to measure the relative azimuthal angle distributions involving two or more event planes of different order in heavy ion collisions using a Fourier analysis technique. The analysis procedure is demonstrated for correlations involving two and three event planes (Phi_n, Phi_m and Phi_h). The Fourier coefficients of these distributions are found to coincide with previously proposed correlators, such as cos(6Phi_2-6Phi_3) and cos(Phi_1+2Phi_2-3Phi_3) etc, hence the method provides a natural framework for studying these correlators at the same time. Using a Monte Carlo Glauber model to simulate Au+Au collisions with fluctuating initial geometry, we are able to identify several new two- or three-plane correlators that have sizable magnitudes and should be measured experimentally.

nucl-th↗

arXiv2012

Measurement of $v_{n}$ coefficients and event plane correlations in 2.76 TeV Pb-Pb collisions with ATLAS

Measurements of flow harmonics $v_1-v_6$ are presented in a broad range of transverse momentum and centrality for $\sqrt{s_{NN}} = 2.76$ TeV Pb-Pb collisions with the ATLAS experiment. The fourier coefficients of two-particle correlations are shown to factorize as products of the single particle $v_n$ for $n=2-6$. The factorization breaks for $n=1$ due to effects of global momentum conservation. The dipolar $v_1$ associated with the initial dipole asymmetry is extracted from the two-particle correlations via a two component fit that accounts for momentum conservation. The magnitude of the extracted dioplar $v_1$ is comparable to $v_3$. Measurements of correlations between harmonic planes of different orders, which give additional constraints on the initial geometry and expansion mechanism of the medium produced in the heavy ion collisions, are also presented.

nucl-ex↗

arXiv2011

Measurement of elliptic and higher order flow harmonics at $\sqrt{s_{NN}}=2.76$ TeV Pb+Pb collisions with the ATLAS Detector

The measurements of flow harmonics $v_2$-$v_6$ using the event plane and two particle correlations methods in broad $p_T$, $η$ and centrality ranges using the ATLAS detector at LHC are presented. ATLAS recorded about 9 $μ\textrm{b}^{-1}$ of lead-lead collision data in the 2010 heavy ion run. The full azimuthal acceptance of the ATLAS detector in $\pm2.5$ units of pseudorapidity for charged hadrons and the large amount of data allows for a detailed study of the flow harmonics. The $p_T$, centrality and $η$ ranges where the two methods give consistent $v_n$ and where they disagree are discussed. It is shown that the ridge as well as the so called "mach-cone" seen in two particle correlations are largely accounted for by the collective flow. Some scaling relations in the $p_T$ dependence of the $v_n$ are also discussed.

nucl-ex↗

arXiv2009

Identifying Galactic PeVatrons with Neutrinos

We perform a realistic evaluation of the potential of IceCube, a kilometer-scale neutrino detector under construction at the South Pole, to detect neutrinos in the direction of the potential accelerators of the Galactic cosmic rays. We take fully account of the fact that the measurement of the energy of the secondary muons can be used to further discriminate between the signal and the background of atmospheric neutrinos. We conclude that IceCube could identify the sources in the Milagro sky map as the sources of the Galactic cosmic rays at the 3 sigma level in one year and at the 5 sigma level in three years. We discuss the dependence of these expectations on ambiguities, mostly associated with our incomplete knowledge of the astrophysics of the sources.

astro-ph.HE↗