Hanbury-Brown-Twiss Radii Research Articles

Squeezed correlations of bosons with nonzero widths for expanding sources

We explore the squeezed back-to-back correlation (SBBC) and investigate how the squeezing effect influences the Hanbury-Brown–Twiss (HBT) interferometry using an expanding Gaussian source with nonzero width. The SBBC and HBT of D0 and ϕ mesons with finite in-medium widths are studied. The expanding flow of the source may enhance the SBBC strength of D0D¯0 and ϕϕ in the low momentum region but suppress the SBBC in the larger momentum region. The squeezing effect suppresses the influence of flow on the HBT radii, which is significant for two identical bosons with large pair momentum or with large mass. Due to the squeezing effect, the relationship between the HBT radii and the pair momentum exhibits nonflow behavior for the D0D0 pair. Likewise, nonflow behavior also appears in the HBT radii of ϕϕ with large pair momentum. This phenomenon may bring new insights for studying the squeezing effect. Published by the American Physical Society 2024

Effects of a phase transition on two-pion interferometry in heavy ion collisions at $$\sqrt {{s_{{\rm{NN}}}}} = 2.4 - 7.7\,\,{\rm{GeV}}$$

Hanbury-Brown-Twiss (HBT) correlations for charged pions in central Au+Au collisions at $\sqrt{s_\mathrm{NN}}=2.4 - 7.7~\text{GeV}$ (corresponding to beam kinetic energies in the fixed target frame from $E_{\rm{lab}}=1.23~\text{to}~30~\text{GeV/nucleon}$) are calculated using the UrQMD model with different equations of state. The effects of a phase transition at high baryon densities are clearly observed in the HBT parameters that are explored. It is found that the available data on the HBT radii, $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$, in the investigated energy region favors a relatively stiff equation of state at low beam energies which then turns into a soft equation of state at high collision energies consistent with astrophysical constraints on the high density equation of state of QCD. The specific effects of two different phase transition scenarios on the $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are investigated. It is found that a phase transition with a significant softening of the equation of state below 4 times nuclear saturation density can be excluded using HBT data. Our results highlight that the pion's $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are sensitive to the stiffness of the equation of state, and can be used to constrain and understand the QCD equation of state in the high baryon density region.

Pion interferometry at 200 GeV using anisotropic hydrodynamics

In this paper, we continue our phenomenological studies of heavy-ion collisions using 3+1d anisotropic hydrodynamics (aHydro). In previous works, we compared quasiparticle aHydro (aHydroQP) with ALICE 2.76 TeV Pb-Pb and RHIC 200 GeV Au-Au collision results. At both energies, the agreement was quite good between aHydroQP and the experimental data for many observables. In this work, we present comparisons of the Hanbury Brown--Twiss (HBT) radii and their ratios determined using pi+ pi+ pairs produced in 200 GeV Au-Au collisions. We first present comparisons with STAR results for the HBT radii and their ratios. We then present comparisons with PHENIX results for the HBT radii and their ratios. In both cases, we find reasonable agreement between aHydroQP predictions and available experimental results for the ratios of HBT radii. At the level of the radii themselves, in some cases quantitative differences on the order of 10-20% remain, which deserve further study.

Hanbury-Brown–Twiss correlation functions and radii from event-by-event hydrodynamics

Hanbury-Brown--Twiss correlation functions and radii from event-by-event hydrodynamics (HoTCoffeeh) is a new computational tool which determines Hanbury-Brown--Twiss (HBT) charged-pion (${\ensuremath{\pi}}^{+}$) correlation functions and radii for event-by-event hydrodynamics with fluctuating initial conditions in terms of Cooper--Frye integrals, including resonance-decay contributions. In this paper, we review the basic formalism for computing the HBT correlation functions and radii with resonance-decay contributions included and discuss our implementation of this formalism in the form of HoTCoffeeh. This tool may easily be integrated with other numerical packages [see, e.g., [Comput. Phys. Commun. 199, 61 (2016)]] for the purpose of simulating the evolution of heavy-ion collisions and thereby extracting predictions for heavy-ion observables.

HBT Radii: Comparative Studies on Collision Systems and Beam Energies

Two-particle Hanbury-Brown-Twiss (HBT) interferometry is an important probe for understanding the space-time structure of particle emission sources in high energy heavy ion collisions. We present the comparative studies of HBT radii in Pb+Pb collisions at sNN = 17.3 GeV with Au+Au collisions at sNN = 19.6 GeV. To further understand this specific energy regime, we also compare the HBT radii for Au+Au collisions at sNN = 19.6 GeV with Cu+Cu collisions at sNN = 22.4 GeV. We have found interesting similarity in the Rout/Rside ratio with mT across the collision systems while comparing the data for this specific energy zone which is interesting as it acts as a bridge from SPS energy regime to the RHIC energy domain.

Pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss interferometry in a viscous granular source model**Supported by National Natural Science Foundation of China (11675034, 11275037)

We examine the evolution of quark-gluon plasma (QGP) droplets with viscous hydrodynamics and analyze the pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in a granular source model consisting of viscous QGP droplets. The shear viscosity of the QGP droplet speeds up and slows down the droplet evolution in the central and peripheral regions of the droplet, respectively. The effect of the bulk viscosity on the evolution is negligible. Although there are viscous effects on the droplet evolution, the pion momentum spectrum and elliptic flow change little for granular sources with and without viscosity. On the other hand, the influence of viscosity on HBT radius Rout is considerable. It makes Rout decrease in the granular source model. We determine the model parameters of granular sources using the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii together, and investigate the effects of viscosity on the model parameters. The results indicate that the granular source model may reproduce the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii in heavy-ion collisions of Au-Au at and Pb-Pb at in different centrality intervals. The viscosity of the droplet leads to an increase in the initial droplet radius and a decrease of the source shell parameter in the granular source model.

Imprints of fluctuating proton shapes on flow in proton-lead collisions at the LHC

Results for particle production in s=5.02TeV p+Pb collisions at the Large Hadron Collider within a combined classical Yang–Mills and relativistic viscous hydrodynamic calculation are presented. We emphasize the importance of sub-nucleon scale fluctuations in the proton projectile to describe the experimentally observed azimuthal harmonic coefficients vn, demonstrating their sensitivity to the proton shape. We stress that the proton shape and its fluctuations are not free parameters in our calculations. Instead, they have been constrained using experimental data from HERA on exclusive vector meson production. Including temperature dependent shear and bulk viscosities, as well as UrQMD for the low temperature regime, we present results for mean transverse momenta, harmonic flow coefficients for charged hadrons and identified particles, as well as Hanbury–Brown–Twiss radii.

Observable consequences of event-by-event fluctuations of HBT radii

We explore the effects of event-by-event fluctuations of Hanbury Brown–Twiss (HBT) radii and show how they can be observed experimentally. The relation of measured HBT radii extracted from ensemble-averaged correlation functions to the mean of their event-by-event probability distribution is clarified. We propose a method to experimentally determine the mean and variance of this distribution and test it on an ensemble of fluctuating events generated with the viscous hydrodynamic code VISH2+1. Using the same code, the sensitivity of the mean and variance of the HBT radii to the specific QGP shear viscosity η/s is studied. We report sensitivity of the mean pion HBT radii and their variances to the temperature dependence of η/s near the quark-hadron transition at a level similar (10–20%) to that which was previously observed for elliptic and quadrangular flow of charged hadrons [1].

Publisher's Note: Probing the properties of event-by-event distributions in Hanbury-Brown–Twiss radii [Phys. Rev. C92, 044906 (2015)

Received 16 October 2015DOI:https://doi.org/10.1103/PhysRevC.92.049901©2015 American Physical Society

Probing the properties of event-by-event distributions in Hanbury-Brown–Twiss radii

Hanbury-Brown--Twiss interferometry is a technique which yields effective widths (i.e., "HBT radii") of homogeneity regions in the fireballs produced in heavy ion collisions. Because the initial conditions of these collisions are stochastically fluctuating, the measured HBT radii also exhibit variation on an event-by-event basis. However, HBT measurements have, to date, been performed only on an ensemble-averaged basis, due to inherent limitations of finite particle statistics. In this paper, we show that experimental measurements to date are best characterized theoretically as weighted averages of the event-by-event HBT radii, and we propose a new method for extracting experimentally both the arithmetic mean and the variance of the event-by-event distribution of HBT radii. We demonstrate the extraction of the mean and variance of this distribution for a particular ensemble of numerically generated events, and offer some ideas to extend and generalize the method to enable measurement of higher moments of the HBT distribution as well.

Effect of the shear viscosity to entropy density ratio on Hanbury-Brown–Twiss radii in a multiphase transport model

Using a multiphase transport (AMPT) model, we investigate the effect of shear viscosity to entropy density ratio on the Hanbury-Brown--Twiss (HBT) parameters and freeze-out eccentricity from the azimuthal angle dependence of the HBT radii with a constant specific shear viscous approach. The specific shear viscosity gives smaller values of ${R}_{\mathrm{o}},\phantom{\rule{0.16em}{0ex}}{R}_{\mathrm{l}}$ and ${R}_{\mathrm{o}}/{R}_{\mathrm{s}}$, but increases value of ${R}_{\mathrm{s}}$. The freeze-out eccentricity increases with decreasing $\ensuremath{\eta}/s$ which suggests a possible slow drop of ${\ensuremath{\varepsilon}}_{\mathrm{f}}$ in the excitation function with increasing energy in relativistic heavy-ion collisions.

Beam-energy-dependent two-pion interferometry and the freeze-out eccentricity of pions measured in heavy ion collisions at the STAR detector

We present results of analyses of two-pion interferometry in Au+Au collisions at sNN=7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV measured in the STAR detector as part of the BNL Relativistic Heavy Ion Collider Beam Energy Scan program. The extracted correlation lengths (Hanbury-Brown–Twiss radii) are studied as a function of beam energy, azimuthal angle relative to the reaction plane, centrality, and transverse mass (mT) of the particles. The azimuthal analysis allows extraction of the eccentricity of the entire fireball at kinetic freeze-out. The energy dependence of this observable is expected to be sensitive to changes in the equation of state. A new global fit method is studied as an alternate method to directly measure the parameters in the azimuthal analysis. The eccentricity shows a monotonic decrease with beam energy that is qualitatively consistent with the trend from all model predictions and quantitatively consistent with a hadronic transport model.21 MoreReceived 30 January 2015DOI:https://doi.org/10.1103/PhysRevC.92.014904©2015 American Physical Society

Kinetic freeze-out from an anisotropic fluid in high-energy heavy-ion collisions: particle spectra, Hanbury Brown–Twiss radii, and anisotropic flow

Dissipative relativistic fluid-dynamical descriptions of the extended fireball formed in high-energy heavy-ion collisions are quite successful, yet require a prescription for converting the fluid into particles. We present arguments in favour of using a locally anisotropic momentum distribution for the particles emitted from the fluid, so as to smooth out discontinuities introduced by the usual conversion prescriptions. Building on this ansatz, we investigate the effect of the asymmetry on several observables of heavy ion physics.

Pion Transverse Momentum Spectrum, Elliptic Flow, and Interferometry in the Granular Source Model for RHIC and LHC Heavy Ion Collisions

We systematically investigate the pion transverse momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in the granular source model for the heavy ion collisions of Au-Au atsNN=200 GeV and Pb-Pb atsNN=2.76 TeV with different centralities. The granular source model can well reproduce the experimental results of the heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). We examine the parameters involved in the granular source model. The experimental data of the momentum spectrum, elliptic flow, and HBT radii for the two collision energies and different centralities impose very strict constraints on the model parameters. They exhibit certain regularities for collision centrality and energy. The space-time structure and expansion velocities of the granular sources for the heavy ion collisions at the RHIC and LHC energies with different centralities are investigated.

Azimuthally sensitive femtoscopy in event-by-event hydrodynamics

I analyze the pion femtoscopy correlations in noncentral Au-Au and Pb-Pb collisions. The azimuthally sensitive Hanbury-Brown-Twiss (HBT) method is used to extract the interferometry radii depending on the azimuthal angle with respect to the second- and third-order event plane. The results are in semiquantitative agreement with the STAR collaboration data on the HBT radii with respect to the second-order reaction plane, with the preliminary PHENIX collaboration data on the HBT radii with respect to the third-order reaction plane in Au-Au collisions at 200 GeV, and with the preliminary ALICE collaboration data for the HBT radii with respect to the second-order event plane for Pb-Pb collisions at $2.76$ TeV.

Free streaming and sudden equilibration approximation for the early stages of relativistic heavy-ion collisions

Using a multi-phase transport (AMPT) model, we study the equilibration process of the early collision stages with free streaming and sudden equilibration approximation. In addition, the results of the transverse momentum spectra, the elliptic flow, the Hanbury-Brown twiss (HBT) parameters and the freeze-out eccentricity from the azimuthal angle dependence of the HBT radii are described within our approach. The results of the transverse momentum spectra and the elliptic flow are close to the experimental data. The HBT radii result is similar to that from the AMPT model. The eccentricity results indicate that the approximation leads to a slow drop of the eccentricity with increasing time. The slow decrease can be reflected in the freeze-out eccentricity obtained by HBT radii.

Two-pion interferometry for viscous hydrodynamic sources

The space-time evolution of the (1+1)-dimensional viscous hydrodynamics with an initial quark-gluon plasma (QGP) produced in ultrarelativistic heavy ion collisions is studied numerically. The particle-emitting sources undergo a crossover transition from the QGP to hadronic gas. We take into account a usual shear viscosity for the strongly coupled QGP as well as the bulk viscosity which increases significantly in the crossover region. The two-pion Hanbury-Brown-Twiss (HBT) interferometry for the viscous hydrodynamic sources is performed. The HBT analyses indicate that the viscosity effect on the two-pion HBT results is small if only the shear viscosity is taken into consideration in the calculations. The bulk viscosity leads to a larger transverse freeze-out configuration of the pion-emitting sources, and thus increases the transverse HBT radii. The results of the longitudinal HBT radius for the source with Bjorken longitudinal scaling are consistent with the experimental data.

Predictions from a simple hadron rescattering model for pp collisions at the LHC

With studies of heavy ion and pp physics already under way at the LHC, it is necessary to consider how hadronic rescattering will affect the observed results from experiments. Through the use of a simple relativistic kinematics-based hadron rescattering model we show that the hadron rescattering phase can obscure some signals for radial flow in pp collisions at LHC energies. It is shown that the hadron rescattering model does not distort pseudorapidity distributions for pp collisions at , 7, 10 and 14 TeV, but does effect two-boson Hanbury–Brown Twiss (HBT) observables. We also compare HBT results from the model with recent measurements of ππ HBT in 0.9 TeV pp collisions. We show that there is very good agreement between the rescattering model and experiment in terms of multiplicity dependence of 1D HBT radii. There is also considerable agreement between the two on the kT dependence of 1D HBT radii. Predictions are also made for 1D and 3D identical pion and kaon correlation functions from pp collisions using the rescattering model for , 7, 10 and 14 TeV. These correlation functions are binned by multiplicity and kT, both separately and together. Fits to these correlation functions indicate that any kT signal of radial flow in pp collisions would be largely obscured by rescattering in the post-hadronization phase.

Two-pion interferometry for granular sources

A review on the two-pion Hanbury-Brown—Twiss (HBT) interferometry in the granular source model of quark-gluon plasma droplets is presented. The characteristic quantities of the granular source extracted by imaging analysis are presented and compared with the HBT radii obtained by the usual Gaussian formula fit. The signals of granular sources are presented.

Energy dependence of the freeze-out eccentricity from the azimuthal dependence of HBT at STAR

Non-central heavy-ion collisions create an out-of-plane-extended participant zone that expands toward a more round state as the system evolves. The recent RHIC beam energy scan at of 7.7, 11.5 and 39 GeV provides an opportunity to explore the energy dependence of the freeze-out eccentricity. The new low-energy data from STAR complement high statistics data sets at of 62.4 and 200 GeV. Hanbury Brown–Twiss (HBT) interferometry allows us to determine the size of pion-emitting source regions. The dependence of the HBT radius parameters on the azimuthal angle relative to the reaction plane has been extracted. These dependences can be related to the freeze-out eccentricity. The new results from STAR are consistent with a monotonically decreasing freeze-out eccentricity and constrain any minimum, suggested by previously available data, to lie in the range between 11.5 and 39 GeV. Of several models, UrQMD appears to best predict the STAR and AGS data.