The goal of collider programs of high energy heavy-ion collisions is to produce a form of matter that can test non-perturbative aspects of Quantum Chromodynamics (QCD), the theory of strong interactions. One of the primary predictions from a computational approach to this quantum field theory is the existence of distinct thermodynamic states of QCD, particularly a state of deconfined quarks and gluons. An important step in establishing changes of state of QCD matter is to demonstrate that femto-scale system created in the laboratory can attain thermodynamic equilibrium. We present such a demonstration by checking that event-to-event fluctuations of several different conserved quantities have a consistent treatment in a grand canonical ensemble through a common temperature and chemical potentials. A key component of this work is that we used cumulants of up to third order in the net proton number, charge, and strangeness. We found a clear indication that the femto-scale fireball is thermalized when the collision energy in the centre of mass, <math><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></mrow></msqrt></math>, is high enough. Also, the use of higher order cumulants, within the framework of grand canonical ensemble, reveals for the first time that the fireballs created at lower <math><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></mrow></msqrt></math> are not in thermodynamic equilibrium. This opens the door to further studies of the approach to equilibrium, and the effects that a critical point in the phase diagram of QCD could have on such phenomena.

India

Sourendu Gupta

Debasish Mallick

Dipak Kumar Mishra

Bedangadas Mohanty

United States

Nu Xu

Limits of thermalization in relativistic heavy ion collisions

We study the fluctuations of net charge, net pion, net kaon, and net proton using the <math><msub><mi>ν</mi><mtext>dyn</mtext></msub></math> variable in the heavy-ion jet interaction generator (HIJING), ultrarelativistic quantum molecular dynamics (UrQMD), and hadron resonance gas (HRG) model at different collision energies <math><msqrt><msub><mi>s</mi><msub><mrow></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></msub></msqrt></math>. It has been observed that the values of <math><msub><mi>ν</mi><mtext>dyn</mtext></msub></math> strongly depend on <math><mrow><mi>Δ</mi><mi>η</mi></mrow></math> in the HIJING and UrQMD models and are independent in the HRG model. The present work emphasizes the particle species dependence of net charge fluctuation strength and provides a baseline for comparison with the experimental data.

Vivek Kumar Singh

Zubayer Ahammed

Fluctuations of identified particle yields using the <math><msub><mi>ν</mi><mi>dyn</mi></msub></math> variable at energies available at the BNL Relativistic Heavy Ion Collider