Multiple parton interactions and production of charged particles up to the intermediate-$p_T$ range in high-multiplicity $pp$ events at the LHC

Satisfactory description of data by hydrodynamics-motivated models, as has been reported recently by experimental collaborations at the LHC, confirm “collectivity” in high-multiplicity proton-proton $(pp)$ collisions. Notwithstanding this, a detailed study of high-multiplicity $pp$ data in other approaches or models is essential for better understanding of the specific phenomenon. In this study, the focus is on a pQCD-inspired multiparton interaction (MPI) model, including a color reconnection (CR) scheme as implemented in the Monte Carlo code, PYTHIA8 tune 4C. The MPI with the color reconnection reproduces the dependence of the mean transverse momentum $⟨p_T⟩$ on the charged particle multiplicity $N_{\mathrm{ch}}$ in $pp$ collisions at the LHC, providing an alternate explanation to the signature of “hydrodynamic collectivity” in $pp$ data. It is, therefore, worth exploring how this model responds to other related features of high-multiplicity $pp$ events. This comparative study with recent experimental results demonstrates the limitations of the model in explaining some of the prominent features of the final-state charged particles up to the intermediate-$p_T$ ($p_T<2.0\mathrm{GeV}/c$) range in high-multiplicity $pp$ events.

Figure 1

The average transverse momentum, $⟨p_T⟩$, as a function of charged particle multiplicity, $N_{\mathrm{ch}}$, as measured [25] by ALICE is compared with the simulated events of the PYTHIA8 tune 4C event generator with and without CR.

Figure 2

Average transverse momentum, $⟨p_T⟩$, for the identified charged particles in the $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$, as a function of mean charged particle multiplicity for several event classes. The CMS data [26, 27] have been compared with simulated events using the PYTHIA8 tune 4C event generator with and without CR.

Figure 3

The inverse slope parameter $T_{\text{effective}}$ as a function of the masses of identified charged particles ($m_{{\pi }^{\pm }}=0.14$, $m_{K^{\pm }}=0.495$, $m_{p(\overline{p})}=0.938\mathrm{GeV}/c^2$) produced in $pp$ collisions [26, 27] at $\sqrt{s}=7\mathrm{TeV}$. $⟨N_{\mathrm{ch}}⟩$ is the mean multiplicity of the charged particles representing event classes.

Figure 4

$m_T$ spectra for identified charged particles as obtained from the CMS data [26, 27] of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$. The data are compared with the simulated events using the PYTHIA8 tune 4C event generator with and without CR.

Figure 5

One-dimensional $\mathrm{\Delta }\phi$ projection for the region of ridge-like correlations from the data [15, 34] and for the simulated events by PYTHIA.

Figure 6

Comparison of multiplicity-dependent near-side ridge-like associated yields for the data [15, 34] and simulated PYTHIA events.