Adam Bzdak

Initial-state geometry and the role of hydrodynamics in proton-proton, proton-nucleus, and deuteron-nucleus collisions

We apply the successful Monte Carlo Glauber and IP-Glasma initial state models of heavy ion collisions to the much smaller size systems produced in proton-proton, proton-nucleus and deuteron- nucleus collisions. We observe a significantly greater sensitivity of the initial state geometry to details of multi-particle production in these models compared to nucleus-nucleus collisions. In particular, we find that the size of the system produced in p+A collisions is very similar to the one produced in p+p collisions, and predict comparable Hanbury-Brown-Twiss radii in the absence of flow in both systems. Differences in the eccentricities computed in the models are large, while differences amongst the generated flow coefficients v_2 and v_3 are smaller. For a large number of participants in proton-lead collisions, the v_2 generated in the IP-Glasma model is comparable to the value obtained in proton-proton collisions. Viscous corrections to flow are large over characteristic lifetimes in the smaller size systems. In contrast, viscous contributions are significantly diminished over the longer space-time evolution of a heavy ion collision.

Event-by-event fluctuations of magnetic and electric fields in heavy ion collisions

Abstract We show that fluctuating proton positions in the colliding nuclei generate, on the event-by-event basis, very strong magnetic and electric fields in the direction both parallel and perpendicular to the reaction plane. The magnitude of E and B fields in each event is of the order of m π 2 ≈ 10 18 Gauss. Implications on the observation of electric dipole in heavy ion collisions is discussed, and the possibility of measuring the electric conductivity of the hot medium is pointed out.

Elliptic and Triangular Flow in p-Pb and Peripheral Pb-Pb Collisions from Parton Scatterings

Using a multiphase transport model (AMPT) we calculate the elliptic v_{2} and triangular v_{3} Fourier coefficients of the two-particle azimuthal correlation function in proton-nucleus (p-Pb) and peripheral nucleus-nucleus (Pb-Pb) collisions. Our results for v_{3} are in a good agreement with the CMS data collected at the Large Hadron Collider. The v_{2} coefficient is very well described in p-Pb collisions and is underestimated for higher transverse momenta in Pb-Pb interactions. The characteristic mass ordering of v_{2} in p-Pb is reproduced, whereas for v_{3}, this effect is not observed. We further predict the pseudorapidity dependence of v_{2} and v_{3} in p-Pb and observe that both are increasing when going from a proton side to a Pb-nucleus side. Predictions for the higher-order Fourier coefficients, v_{4} and v_{5}, in p-Pb are also presented.

Charge-Dependent Correlations in Relativistic Heavy Ion Collisions and the Chiral Magnetic Effect

We provide a phenomenological analysis of present experimental searches for local parity violation manifested through the Chiral Magnetic Effect. We introduce and discuss the relevant correlation functions used for the measurements. Our analysis of the available data from both RHIC and LHC shows that the present experimental evidence for the Chiral Magnetic Effect is rather ambiguous. We further discuss in some detail various background contributions due to conventional physics, which need to be understood quantitatively in order to draw a definitive conclusion about the existence of local parity violation in heavy ion collisions.

Baryon number conservation and the cumulants of the net proton distribution

We discuss the modification of the cumulants of the net baryon and net proton distributions due to the global conservation of baryon number in heavy-ion collisions. Corresponding probability distributions and their cumulants are derived analytically. We show that the conservation of baryon number results in a substantial decrease of higher order cumulants. Based on our studies, we propose an observable that is insensitive to the modifications due to baryon number conservation.

Acceptance corrections to net baryon and net charge cumulants

We show that the effect of finite acceptance drastically influences the net-baryon and net-charge cumulants, which are believed to be sensitive probes of the QCD phase diagram. We derive the general formulae that relate the true cumulants Kn which reflect the full dynamics of the system with the actually measured cumulants cn for a given acceptance, modeled by a binomial probability parameter p. We find that this relation involves additional moments which cannot be expressed by cumulants and should be measured in order to extract any potential information about the QCD critical point. We demonstrate that for a wide range of the true cumulant ratios Kn/Km the measured ratios cn/cm quickly converge if p < 1/2, which makes the interpretation of the data very challenging, especially in case of the net-proton cumulants. Our study further suggests that the measurement of net-charge cumulants may be more advantageous for the investigation of the QCD phase diagram.

On the Charge Separation Effect in Relativistic Heavy Ion Collisions

In this paper, we discuss alternative means of measuring the possible presence of local parity violation in relativistic heavy ion collisions. We focus on the phenomenon of charge separation and introduce the charged dipole vector Q{sub 1}{sup c}, which will measure the charge separation on an event-by-event basis. Using Monte Carlo events, we demonstrate the method and its discriminating power. In particular we show that such an analysis will reveal the strength of the charge separation effect and its azimuthal correlation with the reaction plane. We further show that our proposed method may be able to distinguish between the actual charge separation effect and effects due to certain two-particle correlations. The connection to present measurements based on particle correlations is discussed.

Long-range azimuthal correlations in proton–proton and proton–nucleus collisions from the incoherent scattering of partons

We show that the incoherent elastic scattering of partons, as present in a multi-phase transport model (AMPT), with a modest parton-parton cross-section of sigma = 1.5-3 mb, naturally explains the long-range two-particle azimuthal correlation as observed in proton-proton and proton-nucleus collisions at the Large Hadron Collider

Anisotropy of Photon Production: Initial Eccentricity or Magnetic Field

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of Relativistic Heavy Ion Collider show that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this Letter, we propose a method to experimentally measure the degree to which a magnetic field in heavy-ion collisions is responsible for the observed anisotropy of photon production. The experimental test proposed in this Letter may potentially change our understanding of the nonequilibrium stage and possible thermalization in heavy-ion collisions.

Longitudinal fluctuations of the fireball density in heavy-ion collisions

We show that fluctuations of the fireball shape in the longitudinal direction generate nontrivial rapidity correlations that depend not only on the rapidity difference, y_{1} - y_{2}, but also on the rapidity sum, y_{1} + y_{2}. This is explicitly demonstrated in a simple wounded nucleon model, and the general case is also discussed. We show how to extract different components of the fluctuating fireball shape from the measured two-particle rapidity correlation function. The experimental possibility of studying the longitudinal initial conditions in heavy-ion and proton-proton collisions is emphasized.