The meaning behind observed $p_T$ regions at the LHC energies

Abstract

We argue that $p_T$ distribution data from the LHC on the invariant differential yield of the charged primary particles in $pp$ collisions at $\sqrt{s}=0.9 TeV, 2.76 TeV,7TeV$ and in $Pb-Pb$ collisions at $\sqrt{s_{NN}}=2.76 TeV$ with 6 centrality bins contains several $p_T$ regions with special properties. These distributions were analysed by fitting the data with exponential functions. We conclude that the regions reflect features of fragmentation and hadronization of partons through the string dynamics. The nuclear transparency results in negligible influence of the medium in the III region ($p_T >17-20 GeV/c$), which has highest $p_T$ values. The effects and changes by the medium start to appear weakly in the II region ($4-6 GeV/c < p_T< 17-20 GeV/c$) and become stronger in the I region ($p_T <4-6 GeV/c$) . It seems that the II region has highest number of strings. The increase in string density in this region could lead to fusion of strings, appearance of a new string and collective behaviour of the partons in the most central collisions. These phenomena can explain anomalous behaviour of the Nuclear Modification Factor in the II region. We propose the II region as a possible area of Quark Gluon Plasma formation through string fusion. The first $p_T$ regions are the ones with the maximum number of hadrons and minimum number of strings due to direct hadronization of the low energy strings into two quark systems - mesons.