Net-proton number fluctuations and the Quantum Chromodynamics critical point

Abstract

Observations from collisions of heavy-ion at relativistic energies have established the formation of a new phase of matter, Quark Gluon Plasma (QGP), a deconfined state of quarks and gluons in a specific region of the temperature versus baryonic chemical potential phase diagram of strong interactions. A program to study the features of the phase diagram, such as a possible critical point, by varying the collision energy (<math xmlns="http://www.w3.org/1998/Math/MathML"> <msqrt> <msub> <mi>s</mi> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">N</mi> </mrow> </msub> </msqrt> </math>), is performed at the Relativistic Heavy-Ion Collider (RHIC) facility. Non-monotonic variation with <math xmlns="http://www.w3.org/1998/Math/MathML"> <msqrt> <msub> <mi>s</mi> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">N</mi> </mrow> </msub> </msqrt> </math> of moments of the net-baryon number distribution, related to the correlation length and the susceptibilities of the system, is suggested as a signature for a critical point. We report the first evidence of a non-monotonic variation in kurtosis × variance of the net-proton number (proxy for net-baryon number) distribution as a function of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msqrt> <msub> <mi>s</mi> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">N</mi> </mrow> </msub> </msqrt> </math> with 3.1 σ significance, for head-on (central) gold-on-gold (Au+Au) collisions measured using the STAR detector at RHIC. Non-central Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msqrt> <msub> <mi>s</mi> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">N</mi> </mrow> </msub> </msqrt> </math>.