Pseudorapidity dependence of the bulk properties of hadronic medium in pp collisions at 7 TeV

AbstractThe measured charged particle $$p_T$$ p T spectra in proton-proton collisions obtained by the CMS experiment at CERN is compared with the simulation results of EPOS–LHC and Pythia8.24 models at 7 TeV center-of-mass energy. The Pythia8.24 model describes the experimental data very well, particularly in the high $$p_T$$ p T region. The model also predicts the $$p_T$$ p T spectra for $$|$$ | $$\eta $$ η $$|$$ | < 2.4 at 0 $$\le $$ ≤ $$p_T$$ p T $$\le $$ ≤ 6 $$\text {GeV/}c$$ GeV/ c . The EPOS–LHC model underpredicts the $$p_T$$ p T spectra from 0.1 to 2 $$\text {GeV/}c$$ GeV/ c in all $$\eta $$ η bins for about 20% and the $$p_T$$ p T spectrum from 0.1 to 4.2 $$\text {GeV/}c$$ GeV/ c for $$|$$ | $$\eta $$ η $$|$$ | < 2.4 by about 15% while reasonably predicts well for $$p_T$$ p T > 4.2 $$\text {GeV/}c$$ GeV/ c within the experimental errors. Furthermore, to get information about collective properties of the hadronic matter, modified Hagedorn function with embedded transverse flow velocity and thermodynamically consistent Tsallis distribution functions are used to fit the experimental data and simulated results. The values of $$\chi ^2/ndf$$ χ 2 / n d f show that the functions fit the data and simulation results well. The parameter extracted by the functions: $$\beta _T$$ β T , $$T_0$$ T 0 , and $$T_{eff}$$ T eff decreases with increasing $$\eta $$ η . The decrease in $$\beta _T$$ β T with increasing $$\eta $$ η is due to the large energy deposition in lower rapidity bins producing rapid expansion due to large pressure gradient resulting quick expansion of the fireball. Similarly, large energy transfer in the lower pseudo-rapidity bin results in higher degree of excitation of the system which results larger values of $$T_0$$ T 0 and $$T_{eff}$$ T eff . The values of the fit constant $$N_0$$ N 0 increase with $$\eta $$ η where the values of $$N_0$$ N 0 extracted from Pythia8.24 are closer to the data than the EPOS–LHC model. The Pythia8.24 model has better prediction than the EPOS–LHC model which might be connected to its flow-like features and color re-connections resulting from different Parton interactions in the initial and final state.