Multiplicity dependence freeze-out scenarios in pp collisions at $\sqrt{s}$ = 7 TeV

Abstract

The data on transverse momentum integrated hadron yields in different multiplicity classes of p+p collisions at $\sqrt{s}=7$ TeV have been analyzed to extract the chemical freeze-out parameters using a thermal model. The chemical freeze-out parameters have been extracted for three different freeze-out schemes: i. unified freeze-out for all hadrons in complete thermal equilibrium (1CFO), ii. unified freeze-out for all hadrons with an additional parameter ${\gamma }_S$ which accounts for possible out-of-equilibrium production of strange hadrons (1CFO$+{\gamma }_S$), and iii. separate freeze-out for hadrons with and without strangeness content (2CFO). It has been observed that 1CFO$+{\gamma }_S$ scheme gives the best description of the hadronic yields at midrapidity when multiplicity ($⟨d{N}_{ch}/d\eta ⟩$) of the collision is less than 10. This indicates that the strangeness is out of equilibrium in most of the multiplicity classes of p+p collisions. All the three parameters of this CFO scheme, temperature ($T$), radius of the fireball ($R$) and strangeness suppression factor (${\gamma }_S$) increase with the increase of $⟨d{N}_{ch}/d\eta ⟩$. Further, we have compared applicability of different CFO schemes considering two more colliding system p+Pb at $\text{sNN}$ = 5.02 and Pb+Pb at $\text{sNN}$ = 2.76 TeV along with p+p collisions at $\sqrt{s}=7$ TeV. We observe a freeze-out volume (or multiplicity) dependence of CFO schemes regardless of colliding ions. The 1CFO+${\gamma }_S$, 1CFO and 2CFO schemes provide best description of the data when the dimension less quantity $V{T}^3$ approximately satisfies the conditions $V{T}^3<50$, $50<V{T}^3<100$ and $V{T}^3>100$ respectively or the corresponding multiplicity satisfies the conditions $⟨d{N}_{ch}/d\eta ⟩<30$, $30<d{N}_{ch}/d\eta <60$ and $⟨d{N}_{ch}/d\eta ⟩>100$ respectively.

Comments: 9 Pages, 6 figures