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Register a new userTopological studies of light-flavor hadron production in high multiplicity pp collisions with ALICE at the LHC
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Sushanta Tripathy
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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.
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Recent results for high multiplicity pp and p-Pb collisions have revealed that they exhibit heavy-ion-like behaviors. To understand the origin(s) of these unexpected phenomena, event shape observables such as transverse spherocity ($S_{rm 0}^{p_{rm T} = 1}$) and the relative transverse activity classifier ($R_{rm{T}}$) can be exploited as a powerful tools to disentangle soft (non-perturbative) and hard (perturbative) particle production. Here, the production of light-flavor hadrons is shown for various $S_{rm 0}^{p_{rm T} = 1}$ classes in pp collisions at $sqrt{s}$ = 13 $textrm{TeV}$ measured with the ALICE detector at the LHC are presented. The evolution of average transverse momentum ($langle p_{rm T}rangle$) with charged-particle multiplicity, and identified particle ratios as a function of $p_{rm T}$ for different $S_{rm 0}^{p_{rm T} = 1}$ are also presented. 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. The evolution of $langle p_{rm T}rangle$ in different topological regions as a function of $R_{rm{T}}$ are presented. Finally, using the same approach, we present a search for jet quenching behavior in small collision systems.
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.
Next-to-leading order (NLO) QCD predictions for the production of heavy quarks in proton-proton collisions are presented within three different approaches to quark mass, resummation and fragmentation effects. In particular, new NLO and parton shower simulations with POWHEG are performed in the ALICE kinematic regime at three different centre-of-mass energies, including scale and parton density variations, in order to establish a reliable baseline for future detailed studies of heavy-quark suppression in heavy-ion collisions. Very good agreement of POWHEG is found with FONLL, in particular for centrally produced D^0, D^+ and D^*+ mesons and electrons from charm and bottom quark decays, but also with the generally somewhat higher GM-VFNS predictions within the theoretical uncertainties. The latter are dominated by scale rather than quark mass variations. Parton density uncertainties for charm and bottom quark production are computed here with POWHEG for the first time and shown to be dominant in the forward regime, e.g. for muons coming from heavy-flavour decays. The fragmentation into D_s^+ mesons seems to require further tuning within the NLO Monte Carlo approach.
Proton-proton (pp) collisions have been used extensively as a reference for the study of interactions of larger colliding systems at the LHC. Recent measurements performed in high-multiplicity pp and proton-lead (p-Pb) collisions have shown features that are reminiscent of those observed in lead-lead (Pb-Pb) collisions. In this context, the study of identified particle spectra and yields as a function of multiplicity is a key tool for the understanding of similarities and differences between small and large systems. We report on the production of pions, kaons, protons, $K^{0}_{rm S}$, $Lambda$, $Xi$, $Omega$ and $K^{*0}$ as a function of multiplicity in pp collisions at $sqrt{s}=$ 7 TeV measured with the ALICE experiment. The work presented here represents the most comprehensive set of results on identified particle production in pp collisions at the LHC. Spectral shapes, studied both for individual particles and via particle ratios as a function of $p_{rm T}$, exhibit an evolution with charged particle multiplicity that is similar to the one observed in larger systems. In addition, results on the production of light flavour hadrons in pp collisions at $sqrt{s}=$ 13 TeV, the highest centre-of-mass energy ever reached in the laboratory, are also presented and compared with previous, lower energy results.
Recent measurements of charm-baryon production in proton--proton (pp) collisions at midrapidity by the ALICE collaboration showed that baryon-to-meson yield ratios are significantly higher than those measured in $rm e^+e^-$ collisions. The charm baryon-to-meson and charm baryon-to-baryon yield ratios provide unique information on hadronization mechanisms since the contributions from parton distribution function and parton--parton scattering terms cancel in the ratios. In this contribution, the first measurement of $rm Omega_{c}^{0}$ production via the hadronic decay channel $rm Omega_{c}^{0} rightarrow Omega^{-}pi^{+}$ (and its charge conjugate) in $2<p_{rm T}<12$ GeV/$c$ performed with the ALICE detector at midrapidity in pp collisions at $sqrt{s}=$ 13 TeV is presented. The $rm Omega_c^0/D^0$ and $rm Omega_c^0/Xi_c^0$ ratios multiplied by the decay branching ratio $rm BR(Omega_{c}^{0} rightarrow Omega^{-}pi^+)$, which is not experimentally measured, are compared to MC generators with fragmentation fractions based on $rm e^+e^-$ measurements and models including hadronization of charm quark via coalescence.
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