arXiv subjects

Michael Annan Lisa

Publications and source records attributed to Michael Annan Lisa.

11 recordsLinked to original sources

arXiv2026

When positive and negative pairs differ in femtoscopy: residual Coulomb and isospin effects

We study charge-dependent modifications of identical-pion and identical-kaon femtoscopic correlation functions from two sources: the residual Coulomb field of the charged source and isospin-related hadronic dynamics. The residual Coulomb effect is modeled with a modified Retiere--Lisa blast-wave source, where the same emitted particles are propagated with positive and negative charge signs through an effective residual field. The residual Coulomb field produces a small but systematic positive-to-negative splitting of the correlation functions, strongest at low $k_T$ and sensitive to the effective charge, spatial width, and expansion velocity of the residual source. It also modifies the height and shape of the correlation function, complicating interpretation of the fitted radii. UrQMD 3.4 cascade calculations for Au+Au collisions at $\sqrt{s_{NN}}=7.7~\mathrm{GeV}$ show that charge-dependent splittings can also appear even without a residual Coulomb field. For pions, this difference is mainly driven by the initial isospin composition, while for kaons it is strongly affected by different $K^+$ and $K^-$ production mechanisms and subsequent hadronic evolution. These results show that residual Coulomb and isospin-related effects can compete, and neither can be interpreted reliably without constraining the other within the same model framework.

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arXiv2024

Pair momentum dependence of tilted source in heavy ion collisions

In non-central heavy-ion collisions, the particle-emitting source can be tilted away from the beam direction, an effect that becomes particularly significant at collision energies of a few GeV and lower. This phenomenon, manifest itself in many observables such as directed flow, polarization, and vorticity, is therefore important to investigate. In this paper, we study the consistency between the tilt extracted directly from the freeze-out distribution of pions and the tilt parameter obtained using the azimuthally sensitive femtoscopy (asHBT) method. Using the UrQMD model, we demonstrate a strong dependence of the tilt parameter extracted with asHBT on the momentum of the particle pair. Considering the experimental challenges in accessing low particle momenta - where the tilt parameter extracted with asHBT closely matches the tilt of the freeze-out distribution of pions - we propose an exponential extrapolation method to obtain the tilt of the entire freeze-out distribution. This approach aims to enhance the accuracy of experimental measurements of tilt in non-central heavy-ion collisions.

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arXiv2024

Vortex Rings in Event-by-Event Relativistic Heavy-Ion Collisions

We present event-by-event simulations for central asymmetric light+heavy and Au+Au collisions to investigate the formation and evolution of vortex-ring structures in the longitudinal flow velocity profile. The production-plane polarization of $Λ$ hyperons, defined w.r.t. the $Λ$ momentum and the beam, can track the "vortex-ring" feature in the event, a characteristic vortical structure generated by longitudinal flow gradients. We make comprehensive model predictions for the rapidity-dependent vortex-ring observables for different collision system sizes at $\sqrt{s_\mathrm{NN}} = 200$ and 72 GeV. Our predictions at the latter energy can be explored in the future LHCb fixed-target experiment at the Large Hadron Collider.

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arXiv2023

$Λ$ polarization from vortex ring as medium response for jet thermalization

We performed a systematic study on the formation of vorticity rings as the process for jet thermalization in the medium created in high-energy nuclear collisions. In this work, we expanded our previous analysis to a more realistic framework by considering non-central events and fluctuations in the initial condition. We simulate the formation and evolution of the flow vortex structure in a relativistic viscous hydrodynamic model and study the sensitivity of the proposed ``ring observable'' ($\mathcal{R}^{t}_Λ$) that can be measured experimentally through the polarization of $Λ$ hyperons. We show that this observable is robust with respect to fluctuating initial conditions to capture the jet-induce vortex flow signal and further study its dependence on different model parameters, such as the jet's velocity, position, the fluid's shear viscosity, and the collision's centrality. The proposed observable is associated to the formation of vorticity in a quark-gluon plasma, showing that the measurement of particle polarization can be a powerful tool to probe different properties of jet-medium interactions and to understand better the polarization induced by the transverse and longitudinal expansions of the medium.

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arXiv2021

Vortex rings from high energy central p+A collisions

Relativistic p+A collisions may produce droplets of quark gluon plasma (QGP) that quickly develop a toroidal vortex structure similar to that of an expanding smoke ring. We present viscous relativistic hydrodynamic calculations of ultra-central p+A collisions and develop an experimental observable to probe the structure, correlating the polarization and momentum of hyperons emitted from the collision. The effect is robust against changes in the definition of vorticity used to calculate the polarization. Experiments at RHIC and LHC may test the existence and strength of the vortex toroids, bringing new evidence to bear on the question of collectivity in the smallest QGP droplets.

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arXiv2021

Λ polarization from thermalized jet energy

We examine the formation of vortical "smoke rings" as a result of thermalization of energy lost by a jet. We simulate the formation and evolution of these rings using hydrodynamics and define an observable that allows to probe this phenomenon experimentally. We argue that observation of vorticity associated with jets would be an experimental confirmation of the thermalization of the energy lost by quenched jets, and also a probe of shear viscosity.

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arXiv2019

Global Polarization Effect in the Extremely Rapidly Rotating QGP in HIC

This is prepared for a featured article in Nuclear Physics News. Recently, the global polarization of Λand \barΛ hyperons in heavy-ion collisions (HIC) has been observed by the STAR Collaboration at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory. The discovery confirms the theoretical prediction made more than ten years ago and indicates that the quark gluon plasma (QGP) produced in HIC possesses a vorticity as high as 10^21s^-1, much higher than any other fluid observed in nature. This opens a new window to study properties of QGP and a new direction in high energy heavy ion physics. This featured article is aimed to report the basic idea, current status and outlook.

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arXiv2018

Alternative methods to measure global polarization of $Λ$ hyperons

We propose alternative methods to measure the global polarization of $Λ$ hyperons. These methods involve event averages of proton's and $Λ$'s momenta in the lab frame. We carry out simulations using these methods and show that all of them work equivalently well in obtaining the global polarization of $Λ$ hyperons.

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arXiv2009

Femtoscopically Probing the Freeze-out Configuration in Heavy Ion Collisions

Two-particle femtoscopy reveals the space-time substructure of the freeze-out configuration from heavy ion collisions. Detailed fingerprints of bulk collectivity are evident in space-momentum correlations, which have been systematically measured as a function of particle type, three-momentum, and collision conditions. A clear scenario, dominated by hydrodynamic-type flow emerges. Reproducing the strength and features of the femtoscopic signals in models involves important physical quantities like the Equation of State, as well as less fundamental technical details. An interesting approximate "factorization" in the measured systematics suggests that the overall physical freeze-out scale is set by final state chemistry, but the kinematic substructure is largely universal. Referring to previous results from hadron and lepton collisions, we point to the importance of determining whether these "universal" trends persist from the largest to the smallest systems. We review theoretical expectations for heavy ion femtoscopy at the LHC, and point to directions needing further theory and experimental work at RHIC and the LHC.

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arXiv2006

Fitted HBT radii versus space-time variances in flow-dominated models

The inability of otherwise successful dynamical models to reproduce the ``HBT radii'' extracted from two-particle correlations measured at the Relativistic Heavy Ion Collider (RHIC) is known as the ``RHIC HBT Puzzle.'' Most comparisons between models and experiment exploit the fact that for Gaussian sources the HBT radii agree with certain combinations of the space-time widths of the source which can be directly computed from the emission function, without having to evaluate, at significant expense, the two-particle correlation function. We here study the validity of this approach for realistic emission function models some of which exhibit significant deviations from simple Gaussian behaviour. By Fourier transforming the emission function we compute the 2-particle correlation function and fit it with a Gaussian to partially mimic the procedure used for measured correlation functions. We describe a novel algorithm to perform this Gaussian fit analytically. We find that for realistic hydrodynamic models the HBT radii extracted from this procedure agree better with the data than the values previously extracted from the space-time widths of the emission function. Although serious discrepancies between the calculated and measured HBT radii remain, we show that a more ``apples-to-apples'' comparison of models with data can play an important role in any eventually successful theoretical description of RHIC HBT data.

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arXiv2000

Tilted Pion Sources from Azimuthally Sensitive HBT Interferometry

Intensity interferometry in noncentral heavy ion collisions provides access to novel information on the geometry of the effective pion-emitting source. We demonstrate analytically that, even for vanishing pair momentum, the cross terms $R_{ol}^2$ and $R_{sl}^2$ of the HBT correlation function in general show a strong first harmonic in their azimuthal dependence. The strength of this oscillation characterizes the tilt of the major axis of the spatial emission ellipsoid away from the direction of the beam. Event generator studies indicate that this tilt can be large (>20 degrees) at AGS energies which makes it by far the most significant azimuthally sensitive HBT signal at these energies. Moreover, transport models suggest that for pions this spatial tilt is directed opposite to the tilt of the directed flow ellipsoid in momentum space. A measurement of the azimuthal dependence of the HBT cross terms $R_{ol}^2$ and $R_{sl}^2$ thus probes directly the physical origin of directed pion flow.

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