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In this work we explore the possibility to perform ``effective energy studies in very high energy collisions at the CERN Large Hadron Collider (LHC). In particular, we focus on the possibility to measure in $pp$ collisions the average charged multiplicity as a function of the effective energy with the ALICE experiment, using its capability to measure the energy of the leading baryons with the Zero Degree Calorimeters. Analyses of this kind have been done at lower centre--of--mass energies and have shown that, once the appropriate kinematic variables are chosen, particle production is characterized by universal properties: no matter the nature of the interacting particles, the final states have identical features. Assuming that this universality picture can be extended to {it ion--ion} collisions, as suggested by recent results from RHIC experiments, a novel approach based on the scaling hypothesis for limiting fragmentation has been used to derive the expected charged event multiplicity in $AA$ interactions at LHC. This leads to scenarios where the multiplicity is significantly lower compared to most of the predictions from the models currently used to describe high energy $AA$ collisions. A mean charged multiplicity of about 1000-2000 per rapidity unit (at $eta sim 0$) is expected for the most central $Pb-Pb$ collisions at $sqrt{s_{NN}} = 5.5 TeV$.
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Studies of the production of heavy-flavour baryons are of prominent importance to investigate hadronization mechanisms at the LHC, in particular through the study of the evolution of the baryon-over-meson production ratio. Measurements performed in pp and p--Pb collisions at the LHC have revealed unexpected features, qualitatively similar to what was observed in heavy-ion collisions and, in the charm sector, not in line with the expectations based on previous measurements from $rm e^+e^-$ colliders and in ep collisions. These results suggest that charmed baryon formation might not be universal and that the baryon-over-meson ratio depends on the collision system or multiplicity. A review of ALICE measurements of charmed baryons, including $rm Lambda_c^+/D^0$ as a function of charged-particle multiplicity in pp, p--Pb and Pb--Pb collisions, $rm Sigma_c^{0, +, ++}/D^0$ and $rm Xi_c^{0, +}/D^0$ as a function of $p_{rm T}$ in pp collisions and $rm Gamma(Xi_c^0rightarrowXi^-e^+ u_e)/Gamma(Xi_c^0rightarrowXi^-pi^+)$, will be presented. Comparison to phenomenological models will be also discussed. Emphasis will be given to the discussion of the impact of these studies on the understanding of hadronization processes.
Recent measurements in high-multiplicity pp and p-A collisions have revealed that these small collision systems exhibit collective-like behaviour, formerly thought to be achievable only in heavy-ion collisions. To understand the origins of these unexpected phenomena, event shape observables can be exploited, as they serve as a powerful tool to disentangle soft and hard contributions to particle production. Here, results on the production of light flavor hadrons for different classes of unweighted transverse spherocity ($S_{rm 0}^{p_{rm T}= 1}$) and relative transverse activity ($R_{rm{T}}$) in high multiplicity pp collisions at $sqrt{s}$ = 13 $textrm{TeV}$ measured with the ALICE detector are presented. Hadron-to-pion ratios in different $S_{rm 0}^{p_{rm T}= 1}$ and $R_{rm{T}}$ classes are also presented and compared with state-of-the-art QCD-inspired Monte Carlo event generators. The evolution of charged particle average transverse momentum ($langle p_{rm T}rangle$) with multiplicity and $S_{rm 0}^{p_{rm T}= 1}$ is also discussed. In addition, the system size dependence of charged particle production in pp, p-Pb, and Pb-Pb collisions at $sqrt{s_{rm NN}}$= 5.02 TeV is presented. Finally, within the same approach, we present a search for jet quenching behavior in small collision systems.
Jet tomography probes provide a means to explore the properties of highly compressed and excited nuclear matter created in heavy ion collisions. The capabilities of the ALICE experiment, with its electromagnetic calorimeter (EMCal) upgrade, to trigger on and reconstruct jets in p+p and Pb+Pb collisions at $sqrt{s_{NN}}$=5.5 TeV are presented.
At LHC energies, the charged-particle multiplicity dependence of particle production is a topic of considerable interest in $pp$ collisions. It has been argued that multiple partonic interactions play an important role in particle production mechanisms, not only affecting the soft processes but also the hard processes. Recently, ALICE has measured $J/psi$ production as a function of charged-particle multiplicity to study the correlation between soft and hard processes. In this contribution, we present the $J/psi$ production versus multiplicity for $pp$ and $p-Pb$ collisions measured by ALICE. We compare the results with different theoretical models.
One of the key results of the LHC Run 1 was the observation of an enhanced production of strange particles in high multiplicity pp and p--Pb collisions at $sqrt{s_mathrm{NN}}$ = 7 and 5.02 TeV, respectively. The strangeness enhancement is investigated by measuring the evolution with multiplicity of single-strange and multi-strange baryon production relative to non-strange particles. A smooth increase of strange particle yields relative to the non-strange ones with event multiplicity has been observed in such systems. We report the latest results on multiplicity dependence of strange and multi-strange hadron production in pp collisions at $sqrt{s} = $ 13 TeV with ALICE. We also presented recent measurements of mesonic and baryonic resonances in small collision systems like pp and p--Pb at $sqrt{s_mathrm{NN}}$ = 13 and 8.16 TeV, respectively. The system size dependent studies in pp and p-Pb collisions have been used to investigate how the hadronic scattering processes affect measured resonance yields and to better understand the interplay between canonical suppression and strangeness enhancement. The measurement of the $phi(1020)$ meson as a function of multiplicity provides crucial constraints in this context.
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