Multiplicity dependence of freezeout scenarios in pp collisions at 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 ${\ensuremath{\gamma}}_{S}$ which accounts for possible out-of-equilibrium production of strange hadrons ($1\mathrm{CFO}+{\ensuremath{\gamma}}_{S}$), and (iii) separate freeze-out for hadrons with and without strangeness content (2CFO). It has been observed that the $1\mathrm{CFO}+{\ensuremath{\gamma}}_{S}$ scheme gives the best description of the hadronic yields at midrapidity when multiplicity ($\ensuremath{\langle}d{N}_{ch}/d\ensuremath{\eta}\ensuremath{\rangle}$) 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 three parameters of this CFO scheme, temperature $T$, radius $R$ of the fireball, and strangeness suppression factor ${\ensuremath{\gamma}}_{S}$ increase with the increase of $\ensuremath{\langle}d{N}_{ch}/d\ensuremath{\eta}\ensuremath{\rangle}$. Furthermore, we have compared applicability of different CFO schemes considering two more colliding systems $p+\text{Pb}$ at $\sqrt{{s}_{\mathrm{NN}}}=5.02$ and $\text{Pb}+\text{Pb}$ at $\sqrt{{s}_{\mathrm{NN}}}=2.76$ 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 $1\text{CFO}+{\ensuremath{\gamma}}_{S}$, 1CFO, and 2CFO schemes provide the best description of the data when the dimensionless 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 $\ensuremath{\langle}d{N}_{ch}/d\ensuremath{\eta}\ensuremath{\rangle}<30, 30<d{N}_{ch}/d\ensuremath{\eta}<60$, and $\ensuremath{\langle}d{N}_{ch}/d\ensuremath{\eta}\ensuremath{\rangle}>100$, respectively.