CERN Accelerating science

The LHC is the experimental setup to test the theoretical predictions made by the standard model. The interactions of the fundamental particles which form hadrons are explained by the strong interaction theory called quantum chromo-dynamics (QCD). QCD provides a good explanation of hadron collider data to conduct a more precise experiment to test the theory, a technique called perturbative QCD (pQCD) is adopted. It is a subfield of particle physics by using the fact that the strong coupling constant $~\alpha_s$ is small in high energy and short-distance interactions, thus allowing the perturbation theory techniques to be applied. ALICE (A Large Ion Collider Experiment) is one of the biggest experiments performed on the LHC at CERN. It is devoted to collecting the data from high-energy heavy ion collisions, where a new state of nuclear matter that is thought to exist in the early universe can be created in QGP (quark-gluon plasma). From the analysis of data we can investigate its formation and properties. The data that ALICE collects are obtained from different detectors which can form ALICE. Other than heavy ion collisions small collisions system based on pp and p-Pb collisions are also under investigation at LHC. In this work TPC, ITS, and V0 detectors are used, which are also considered in this research work. The main goal of this work is to study underlaying event (UE) in small collision systems. The components of particle production in hadronic interactions, which are not directly related to the hardest partonic interaction, are studied. It includes the multi-parton interaction (MPI), initial and beam remnants. The UE and the LHC simulations are critical to fully grasping the LHC physics program at CERN's Large Hadron Collider (LHC). This is due to the fact that the UE is a component of the same proton-proton collision that any signal-partonic interaction occurs in. In order to comprehend the backdrops to other observables in small collisions systems, it is necessary to have a good understanding of the UE. A hard scattering and the UE cannot be distinguished in an individual collision, however there can be observables that can be estimated that are particularly sensitive to the UE. Traditional UE analysis relies on measuring particle production in three distinct topological regions at an azumuthal angle to the event's leading particle: toward, away, and transverse. The Toward and Away regions are dominated by particle production from the hard process and are hence relatively insensitive to the UE. In contrast, transverse region has contributions from UE, ISR, and FSR but the transverse region is more sensitive to the UE, and observables defined inside it are the primary focus of UE measurements. A number of experiments have reported UE by using transverse region method at different center-of-mass energies. The goal of this work is to perform the UE analysis in pp collisions at $\sqrt{s}~=~5.02~{\rm TeV}$ it is the first time that results for this energy are presented pp collisions at $\sqrt{s}~=~5.02~{\rm TeV}$ is analysed and also the same method is used for p-Pb collisions with same $\sqrt{s_{\mathrm{NN}}}~$ energy and same thresholds of transverse momentum. We report about the properties of the underlying event measured with ALICE at the LHC in pp and p-Pb collisions at$\sqrt{s_{\mathrm{NN}}}~=~5.02~{\rm TeV}$. The event activity, quantified by charged-particle number and summed-$p_{\rm T}$ densities, is measured as a function of the leading-particle transverse momentum ($p_{\rm T}^{\rm trig}$ ). These quantities are studied toward, away, and transverse regions. Results are presented for three different $p_{\rm T}$ thresholds (0.15, 0.5 and 1 GeV/$c$) at mid-pseudorapidity ($~\left | \eta \right |~<~0.8~$). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p-Pb collisions, namely, a steep increase with $p_{\rm T}^{\rm trig}$, followed by a saturation at $p_{\rm T}^{\rm trig}$ $\approx$ 5 GeV/$c$. The results obtained in this analysis from pp collisions are compared with existing measurements at other centre-of-mass energies, e.g, 0.9, 7 and 13 TeV. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p-Pb collisions for $p_{\rm T}^{\rm trig}$ $>$ 10 GeV/$c$, whereas for lower $p_{\rm T}^{\rm trig}$ values the jet-like particles production is slightly higher in p-Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.