Hanbury-Brown-Twiss Radii Research Articles

Correspondence between Hanbury-Brown–Twiss radii and the emission zone in noncentral heavy ion collisions

In noncentral collisions between ultrarelativistic heavy ions, the freeze-out distribution is anisotropic, and its major longitudinal axis may be tilted away from the beam direction. The shape and orientation of this distribution are particularly interesting, as they provide a snapshot of the evolving source and reflect the space-time aspect of anisotropic flow. Experimentally, this information is extracted by measuring pion Hanbury-Brown--Twiss (HBT) radii as a function of angle with respect to the reaction plane. The connection between measured radius oscillations and the underlying geometry is necessarily model-dependent; many existing formulas are strictly valid only for Gaussian sources with no collective flow. With a realistic transport model of the collision, which generates flow and non-Gaussian sources, we find that these formulas approximately reflect the anisotropy of the freeze-out distribution.

Characterizing quark gluon plasma by dilepton interferometry

The Hanbury-Brown-Twiss (HBT) radii have been calculated from the two particle correlation functions with virtual photons produced in the collisions of two nuclei at ultra-relativistic energies. We show that the variation of the HBT radii with the invariant mass of the virtual photon can be used to characterize and distinguish the hadronic as well as the partonic phase that might have produced initially in the collisions. It has been illustrated that the non-monotonic variation of the HBT radii with invariant mass provides an access to the development of collective flow in the system.

EFFECTS OF BULK VISCOSITY ON THE EVOLUTION OF RELATIVISTIC CAUSAL VISCOUS HYDRODYNAMICS

The Hanbury-Brown Twiss (HBT) radii of Au + Au collisions at RHIC energy are investigated by a hydrodynamical expanding source with both shear (η) and bulk viscosities (ζ). With different height of the ratio of ζ to entropy density s, the ratio Rout/Rside of HBT radii can not describe the experimental data. But with large enough peak of ζ/s, the instability suggests that the source may clusterize which gives a hint to resolve the HBT puzzle.

Effects of bulk viscosity on hadron spectra and the Hanbury–Brown Twiss radius by causal viscous hydrodynamics

The particle spectra and Hanbury–Brown Twiss (HBT) radius of Au+Au collisions at RHIC energy are investigated by a hydrodynamical expanding source with both shear and bulk viscosities (ζ). With a large width of the ratio of ζ to entropy density s, both the particle transverse momentum spectra and the ratio Rout/Rside of HBT radii in the direction of the total transverse momentum of detected two particles (Rout) and perpendicular to both this direction and the beam direction (Rside) become a little steeper.

Photon Hanbury-Brown–Twiss interferometry for noncentral heavy-ion collisions

Currently, the only known way to obtain experimental information about the space-time structure of a heavy-ion collision is through 2-particle momentum correlations. Azimuthally sensitive HBT interferometry (Hanbury Brown-Twiss intensity interferometry) can complement elliptic flow measurements by constraining the spatial deformation of the source and its time evolution. Performing these measurements on photons allows us to access the fireball evolution at earlier times than with hadrons. Using ideal hydrodynamics to model the space-time evolution of the collision fireball, we explore theoretically various aspects of 2-photon intensity interferometry with transverse momenta up to 2 GeV, in particular the azimuthal angle dependence of the HBT radii in non-central collisions. We highlight the dual nature of thermal photon emission, in both central and non-central collisions, resulting from the superposition of QGP and hadron resonance gas photon production. This signature is present in both the thermal photon source function and the HBT radii extracted from Gaussian fits of the 2-photon correlation function.

A model comparison of resonance lifetime modifications, a soft equation of state and non-Gaussian effects on π–π correlations at FAIR/AGS energies

Hanbury-Brown–Twiss (HBT) correlations of π−–π− pairs at FAIR/AGS energies are investigated by using the UrQMD transport model and the CRAB analyzing program. Three different possible sources (treatment of resonance lifetimes, a soft equation of state and non-Gaussian effects) to understand the HBT RO/RS puzzle are investigated. First, we find that different treatments of the resonance decay time cannot resolve the HBT time-related puzzle, however it can modify the HBT radii at low transverse momenta to some extent to explain the data slightly. Second, with a soft equation of state with momentum dependence, the measured transverse momentum-dependent HBT radii and the RO/RS ratio can be described fairly well. Third, non-Gaussian effects are visible in the calculated correlation function. Using the Edgeworth expansion, one finds that the non-Gaussian effect is strongest in the longitudinal direction and weakest in the sideward direction.

Two-pion interferometry in heavy ion collisions at HIRFL-CSR energy**Supported by National Natural Science Foundation of China (10575024, 10775024)

We examine the two-pion Hanbury-Brown-Twiss (HBT) interferometry for the particle-emitting source produced in heavy ion collisions at HIRFL-CSR energy. The source evolution is described by relativistic hydrodynamics with three kinds of equations of state for chemical equilibrium (CE), chemical freeze-out (CFO), and partial chemical equilibrium (PCE) models, respectively. We investigate the effects of particle decay, multiple scattering, and source collective expansion on the two-pion interferometry results. We find that the HBT radii of the evolution source for the CFO and PCE models are smaller than that for the CE model. The HBT lifetime for the CFO model is smaller than those for the PCE and CE models. The particle decay increases the HBT radius and lifetime while the source expansion decreases the HBT radius. The multiple scattering effect on the HBT results can be neglected based on our model calculations.

Hanbury-Brown–Twiss interferometry with quantum transport of the interfering pair

In the late stage of the evolution of a pion system in high-energy heavy-ion collisions when pions undergo multiple scatterings, the quantum transport of the interfering pair of identical pions plays an important role in determining the characteristics of the Hanbury-Brown-Twiss (HBT) interference. We study the quantum transport of the interfering pair using the path-integral method, in which the evolution of the bulk matter is described by relativistic hydrodynamics while the paths of the two interfering pions by test particles following the fluid positions and velocity fields. We investigate in addition the effects of secondary pion sources from particle decays, for nuclear collisions at AGS and RHIC energies. We find that quantum transport of the interfering pair leads to HBT radii close to those for the chemical freeze-out configuration. Particle decays however lead to HBT radii greater than those for the chemical freeze-out configuration. As a consequence, the combined effects give rise to HBT radii between those extracted from the chemical freeze-out configuration and the thermal freeze-out configuration. Proper quantum treatments of the interfering pairs in HBT calculations at the pion multiple scattering stage are important for our understanding of the characteristics of HBT interferometry in heavy-ion collisions.

Fitted Hanbury-Brown–Twiss radii versus space-time variances in flow-dominated models

The inability of otherwise successful dynamical models to reproduce the Hanbury-Brown--Twiss (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 that 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 behavior. By Fourier transforming the emission function, we compute the two-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 the 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.

Effect of shear viscosity on spectra, elliptic flow, and Hanbury Brown–Twiss radii

I calculate the first correction to the thermal distribution function of an expanding gas due to shear viscosity. With this modified distribution function I estimate viscous corrections to spectra, elliptic flow, and HBT radii in hydrodynamic simulations of heavy ion collisions using the blast wave model. For reasonable values of the shear viscosity, viscous corrections become of order one when the transverse momentum of the particle is larger than 1.7 GeV. This places a bound on the $p_{T}$ range accessible to hydrodynamics for this observable. Shear corrections to elliptic flow cause $v_{2}(p_{T})$ to veer below the ideal results for $p_{T} \approx 0.9$ GeV. Shear corrections to the longitudinal HBT radius $R^{2}_{L}$ are large and negative. The reduction of $R_{L}^2$ can be traced to the reduction of the longitudinal pressure. Viscous corrections cause the longitudinal radius to deviate from the $\frac{1}{\sqrt{m_T}}$ scaling which is observed in the data and which is predicted by ideal hydrodynamics. The correction to the sideward radius $R^{2}_{S}$ is small. The correction to the outward radius $R^{2}_{O}$ is also negative and tends to make $R_{O}/R_{S} \approx 1$.

Source Function Determined from Hanbury-Brown/Twiss (HBT)Correlations by the Maximum Entropy Principle

We study the reconstruction of the source function in space-timedirectly from the measured Hanbury-Brown/Twiss (HBT) correlation function using themaximum entropy principle. We find that the problem is ill-definedwithout at least one additional theoretical constraint as input.Using the requirement of a finite source lifetime for theproblem we find a new Gaussian parametrization of the source functiondirectly in terms of the measured HBT radius parameters and itslifetime, where the latter is a free parameter which is not directlymeasurable by HBT. We discuss the implications of our results for theremaining freedom in building source models consistent with a givenset of measured HBT radius parameters.

Relationship between particle freeze-out distributions and Hanbury Brown–Twiss radius parameters

The relationship between pion and kaon space-time freeze-out distributions and the Hanbury Brown--Twiss (HBT) radius parameters in high-energy nucleus-nucleus collisions is investigated. We show that the HBT radius parameters in general do not reflect the rms deviations of the single particle production points. Instead, the HBT radius parameters are most closely related to the curvature of the two-particle space-time relative position distribution at the origin. We support our arguments by studies with a dynamical model (RQMD 2.4).