CERN Accelerating science

In heavy-ion collider experiments, a hot and dense, strongly-interacting nuclear matter, the so-called quark-gluon plasma (QGP) is formed in central collisions (collisions with small impact parameter). For many years, pp collisions were considered as a control experiment to extract the genuine QGP effects from heavy-ion data. However, recent studies on high-multiplicity proton-proton (pp) and proton-lead (p-Pb) collisions, the so called small-collision systems, unveiled QGP-like effects such as collectivity and strangeness enhancement. The origin of the new effects is still unknown. Two approaches based on either final-state effects (QGP formation) or initial-state effects are currently under investigation. In particular, the Monte Carlo (MC) models widely used in pp collisions have been successful at describing some aspects of data. For example, multi-parton interactions (MPI) and colour reconnection (CR) can produce collective-like effects. The effects increase with the increase of the number of MPI (decrease of the impact parameter) involved in the collision. This is not surprising because in these collisions, several scattered patrons are produced and they are allowed to interact before the hadronization. This sort of collective hadronization mimics radial flow effects. The study of pp collisions as a function of MPI is therefore interesting. One issue with the study of high multiplicity pp and p-Pb collisions are the selection biases, which are basically biases towards local multiplicity fluctuations. For example, if the multiplicity is measured in a very narrow pseudorapidity interval then requiring a high multiplicity could act as a trigger on jet multiplicity, which is not what we want to study. This is, the correlation between the impact parameter and the charged particle multiplicity is very weak in small-collision systems. In this thesis we investigate two event activity estimators aimed at reducing the unwanted selection biases. One of them is the multiplicity in the transverse region of the di-hadron correlation, and the second one is a proposal to further improve the sensitivity to the impact parameter of the collision using the so-called flattenicity estimator. The first estimator aims at triggering on the underlying event. A typical pp collision can be viewed as a hard parton-parton scattering accompanied by UE which consists of particles from beam-beam remnants (BBR) and MPI. MPI refers to two or more semi-hard parton-parton scatterings occurring within the same pp collision. Experimentally, it is impossible to uniquely separate the UE from the event-by-event hard scattering process. In order to enhance the sensitivity to UE, we followed the technique introduced by the CDF Collaboration, which is based on the definition of three distinct topological regions with different sensitivities to the UE. The three topological regions are defined from the angular difference between the trigger and associated particles, $|\Delta \varphi| = |\varphi^{\rm assoc} - \varphi^{\rm trig}|$, where $\varphi^{\rm trig}$ and $\varphi^{\rm assoc}$ refer to the value of the azimuthal angle for the trigger particle and for associated particles in the event, respectively. The trigger particle is the one with the largest transverse momentum ($p_{\rm T}^{\rm trig}$) in the event, and the rest are termed as associated particles. The toward ($|\Delta \varphi|<\pi/3$) and away ($|\Delta \varphi|>2\pi/3$) regions are dominated by the fragments of jets. Although UE appears everywhere, in general, these two regions are less sensitive to the UE. In contrast, the transverse region ($\pi/3$ $<|\Delta \varphi|<2\pi/3$) is the most sensitive to UE since it is less affected by contributions from the hard scattering. Measurements of UE in pp collisions at different energies showed that in the transverse region the mean charged-particle multiplicity as a function of $p_{\rm T}^{\rm trig}$ (``number density'') increased with $p_{\rm T}^{\rm trig}$ up to about 5 GeV/$c$ (plateau) where it saturated. Such saturation effect observed in the number density is commonly interpreted as a bias towards collisions with small impact parameter. In other words, selecting pp collisions with $p_{\rm T}^{\rm trig}$ above 5 GeV/$c$ is an experimental approach to select central pp collisions. Among the results presented in this thesis, we report the CDF analysis implemented in pp and p-Pb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV (published in JHEP 2023). The results exhibit a very similar saturation of the number density in p-Pb collisions, suggesting a bias in the impact parameter of the nucleon-nucleon collision, and to some extend in the p-Pb impact parameter. This finding suggests that the event activity in the transverse region of the di-hadron correlations seems as a good candidate to select pp and p-Pb collision with small impact parameter. As explained above, the selection on $p_{\rm T}^{\rm trig}$ allows to select pp collisions with average impact parameter near to zero. Therefore, the fluctuations on impact parameter diminish, and a Koba-Nielsen-Olesen (KNO) scaling of the multiplicity distributions in the transverse region is expected. This scaling was observed in MC simulation for pp collisions at the LHC energies for $0.5 < R_{\rm T} < 2.5$, where $R_{\rm T}=N_{\rm ch}^{\rm T}/ \langle N_{\rm ch}^{\rm T}\rangle$ is the relative transverse activity classifier, $N_{\rm ch}^{\rm T}$ being the charged-particle multiplicity and $\langle N_{\rm ch}^{\rm T} \rangle$ being the mean charged-particle multiplicity in the transverse region that is calculated considering all the events. The KNO-like scaling is expected in models which assume that a single pp collision is merely a superposition of a given number of elementary partonic collisions emitting particles independently. Therefore, MPI is expected to produce such an effect. One has to keep in mind that the transverse region does not only include contributions from UE, but also from initial- and final-state radiation (ISR-FSR). Therefore, it raises a question whether the violation of the KNO-like scaling at lower or higher $R_{\rm T}$ is attributed to ISR-FSR. To understand this violation, in this work the transverse region is further subdivided in two regions, defined according to their relative multiplicities: ``trans-max'' (the sub-transverse region with the larger multiplicity) and ``trans-min'' (the sub-transverse region with the smaller multiplicity) regions which have an enhanced sensitivity to ISR-FSR and UE, respectively. Using $N_{\rm ch}^{\rm Tmax}$ (the multiplicity in the trans-max region) and $N_{\rm ch}^{\rm Tmin}$ (the multiplicity in the trans-min region), instead of $N_{\rm ch}^{\rm T}$, we can also define the quantities $R_{\rm T}^{\rm max}$ and $R_{\rm T}^{\rm min}$, respectively. In this thesis, we report for the first time the measurement of the multiplicity distributions in KNO variables for the transverse, trans-max and trans-min regions using pp data at $\sqrt{s}=$2.76, 5.02, 7, and 13 TeV reconstructed with the ALICE detector (published in JHEP 2024). Our results show that, in the transverse region, within 20%, a KNO-like scaling holds for $0 < R_{\rm T} < 3$, whereas for higher $R_{\rm T}$ values ($R_{\rm T}>3$), the KNO-like scaling is broken which might be attributed to jet fragmentation bias. The results for trans-max are qualitatively similar to those reported for the transverse region. On the other hand, for the trans-min region, the KNO-like scaling holds within a wider $R_{\rm T}^{\rm min}$ interval ($0< R_{\rm T}^{\rm min} <4$), whereas for $R_{\rm T}^{\rm min} >4$ the KNO-like scaling is still broken which might be also a result of jet fragments bias. To further understand the production mechanism of particles in the three regions, single negative binomial distributions (NBD) were fitted to data. The parameterisation suggests that a single NBD is enough to describe $N_{\rm ch}^{\rm T}$ and $N_{\rm ch}^{\rm Tmin}$ distributions, and the best result is observed for $N_{\rm ch}^{\rm Tmin}$ distributions. This result is consistent with the reduction in the fluctuations of impact parameter. In contrast, the quality of the fit gets worst for the trans-max region because there two components are present, the UE as well as jet fragments from ISR and FSR. We also report the centre-of-mass energy dependence of the average multiplicity density in the transverse, trans-max, and trans-min regions. Our results in pp collisions at $\sqrt{s}=$2.76, 5.02, 7, and 13 TeV follow the trend of existing data results. The average multiplicity density as a function of centre-of-mass energy can be described by the parameterisation of the form $s^{0.27} + 0.14{\rm log}(s)$. The power-law term and the logarithmic term describe the MPI- and ISR-FSR-sensitive topological region of the collision, respectively. The results reported here support the interpretation that the multiplicity in transverse region and trans-min are good candidates to select pp collisions with small impact parameter. However, the multiplicity reach is limited to around $2-3$ times the average multiplicity in minimum-bias pp collisions. As already stated, the studies as a function of the charged-particle are affected by selection biases. This bias is also observed in $R_{\rm T}$, as mentioned above, we could only explore multiplicities up to $R_{\rm T}\approx2-3$. In order to have a better control on the biases, this thesis explores the use of the new event classifier, flattenicity, which is calculated in the forward pseudorapidity region, and which could be more powerful than $R_{\rm T}$. A comprehensive study of flattenicity in pp collisions $\sqrt{s}=$13.6 TeV is performed using PYTHIA 8 simulations (published in PRD 2023). A comparison between the widely used multiplicity estimator and flattenicity shows that although both of them show the same level of correlation with the average MPI activity, flattenicity is less affected by the bias due to local multiplicity fluctuations. The implementation of flattenicity in ALICE during RUN 3 is currently ongoing by other groups. This thesis is organised as follows. In chapter 1 the main aspects of strong interactions are briefly introduced, as well as the particle production mechanisms in pp collisions, the UE and KNO scaling, and the MC models used in this theses. Chapter 2 gives a brief review of the ALICE detectors, especially focusing on the main detectors used in this thesis. Chapter 3 presents the data sample and analysis strategy. In chapter 4 the energy dependence of UE are discussed. In chapter 5 UE properties in pp and p-Pb collisions is discussed. In chapter 6 a MC study of flattenicity is reported. Chapter 7 summarises the main results with outlook.