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Forward rapidity J/ψ production as a function of charged-particle multiplicity in pp collisions at √s = 5.02 and 13 TeV
(Springer Science and Business Media Deutschland GmbH, 2022-06-01)
The production of J/ψ is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies s = 5.02 and 13 TeV. The J/ψ mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5 < y < 4.0), whereas the charged-particle multiplicity density (dNch/dη) is measured at midrapidity (|η| < 1). The production rate as a function of multiplicity is reported as the ratio of the yield in a given multiplicity interval to the multiplicity-integrated one. This observable shows a linear increase with charged-particle multiplicity normalized to the corresponding average value for inelastic events (dNch/dη/〈dNch/dη〉), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum (〈pT〉) of J/ψ in pp collisions exhibits an increasing trend as a function of dNch/dη/〈dNch/dη〉 showing a saturation towards high charged-particle multiplicities. [Figure not available: see fulltext.]
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Prompt and non-prompt J/ψ production cross sections at midrapidity in proton-proton collisions at √s = 5.02 and 13 TeV
(Springer Science and Business Media Deutschland GmbH, 2022-03-01)
The production of J/ψis measured at midrapidity (|y| <0.9) in proton-proton collisions at $$ \sqrt{s} $$ s = 5.02 and 13 TeV, through the dielectron decay channel, using the ALICE detector at the Large Hadron Collider. The data sets used for the analyses correspond to integrated luminosities of $$ \mathcal{L} $$ L int= 19.4±0.4 nb−1and $$ \mathcal{L} $$ L int= 32.2±0.5 nb−1at $$ \sqrt{s} $$ s = 5.02 and 13 TeV, respectively. The fraction of non-prompt J/ψmesons, i.e. those originating from the decay of beauty hadrons, is measured down to a transverse momentumpT= 2 GeV/c(1 GeV/c) at $$ \sqrt{s} $$ s = 5.02 TeV (13 TeV). ThepTand rapidity (y) differential cross sections, as well as the corresponding values integrated overpTandy, are carried out separately for prompt and non-prompt J/ψmesons. The results are compared with measurements from other experiments and theoretical calculations based on quantum chromodynamics (QCD). The shapes of thepTandydistributions of beauty quarks predicted by state-of-the-art perturbative QCD models are used to extrapolate an estimate of the $$ \mathrm{b}\overline{\mathrm{b}} $$ bb¯ pair cross section at midrapidity and in the total phase space. The total $$ \mathrm{b}\overline{\mathrm{b}} $$ bb¯ cross sections are found to be $$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ σbb¯ = 541±45 (stat.)±69 $$ {\left(\mathrm{syst}.\right)}_{-12}^{+10} $$ syst.−12+10 (extr.)μb and $$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ σbb¯ = 218±37 (stat.)±31 $$ {\left(\mathrm{syst}.\right)}_{-9.1}^{+8.2} $$ syst.−9.1+8.2 (extr.)μb at $$ \sqrt{s} $$ s = 13 and 5.02 TeV, respectively. The value obtained from the combination of ALICE and LHCb measurements in pp collisions at $$ \sqrt{s} $$ s = 13 TeV is also provided.
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Measurement of beauty-quark production in pp collisions at √s = 13 TeV via non-prompt D mesons
(Springer Science and Business Media Deutschland GmbH, 2024-10-01)
The pT-differential production cross sections of non-prompt D0, D+, and Ds+ mesons originating from beauty-hadron decays are measured in proton–proton collisions at a centre-of-mass energy s = 13 TeV. The measurements are performed at midrapidity, |y| < 0.5, with the data sample collected by ALICE from 2016 to 2018. The results are in agreement with predictions from several perturbative QCD calculations. The fragmentation fraction of beauty quarks to strange mesons divided by the one to non-strange mesons, fs/(fu + fd), is found to be 0.114 ± 0.016 (stat.) ± 0.006 (syst.) ± 0.003 (BR) ± 0.003 (extrap.). This value is compatible with previous measurements at lower centre-of-mass energies and in different collision systems in agreement with the assumption of universality of fragmentation functions. In addition, the dependence of the non-prompt D meson production on the centre-of-mass energy is investigated by comparing the results obtained at s = 5.02 and 13 TeV, showing a hardening of the non-prompt D-meson pT-differential production cross section at higher s. Finally, the bb¯ production cross section per unit of rapidity at midrapidity is calculated from the non-prompt D0, D+, Ds+, and Λc+ hadron measurements, obtaining dσ/dy=75.2±3.2stat.±5.2syst.−3.2+12.3extrap. μb.
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Studying strangeness and baryon production mechanisms through angular correlations between charged Ξ baryons and identified hadrons in pp collisions at √s = 13 TeV
(Springer Science and Business Media Deutschland GmbH, 2024-09-01)
The angular correlations between charged Ξ baryons and associated identified hadrons (pions, kaons, protons, Λ baryons, and Ξ baryons) are measured in pp collisions at s = 13 TeV with the ALICE detector to give insight into the particle production mechanisms and balancing of quantum numbers on the microscopic level. In particular, the distribution of strangeness is investigated in the correlations between the doubly-strange Ξ baryon and mesons and baryons that contain a single strange quark, K and Λ. As a reference, the results are compared to Ξπ and Ξp correlations, where the associated mesons and baryons do not contain a strange valence quark. These measurements are expected to be sensitive to whether strangeness is produced through string breaking or in a thermal production scenario. Furthermore, the multiplicity dependence of the correlation functions is measured to look for the turn-on of additional particle production mechanisms with event activity. The results are compared to predictions from the string-breaking model Pythia 8, including tunes with baryon junctions and rope hadronisation enabled, the cluster hadronisation model Herwig 7, and the core-corona model Epos-lhc. While some aspects of the experimental data are described quantitatively or qualitatively by the Monte Carlo models, no model can match all features of the data. These results provide stringent constraints on the strangeness and baryon number production mechanisms in pp collisions.
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Investigating strangeness enhancement with multiplicity in pp collisions using angular correlations
(Springer Science and Business Media Deutschland GmbH, 2024-09-01)
A study of strange hadron production associated with hard scattering processes and with the underlying event is conducted to investigate the origin of the enhanced production of strange hadrons in small collision systems characterised by large charged-particle multiplicities. For this purpose, the production of the single-strange meson KS0 and the double-strange baryon Ξ± is measured, in each event, in the azimuthal direction of the highest-pT particle (“trigger” particle), related to hard scattering processes, and in the direction transverse to it in azimuth, associated with the underlying event, in pp collisions at s = 5.02 TeV and s = 13 TeV using the ALICE detector at the LHC. The per-trigger yields of KS0 and Ξ± are dominated by the transverse-to-leading production (i.e., in the direction transverse to the trigger particle), whose contribution relative to the toward-leading production is observed to increase with the event charged-particle multiplicity. The transverse-to-leading and the toward-leading Ξ±/KS0 yield ratios increase with the multiplicity of charged particles, suggesting that strangeness enhancement with multiplicity is associated with both hard scattering processes and the underlying event. The relative production of Ξ± with respect to KS0 is higher in transverse-to-leading processes over the whole multiplicity interval covered by the measurement. The KS0 and Ξ± per-trigger yields and yield ratios are compared with predictions of three different phenomenological models, namely Pythia8.2 with the Monash tune, Pythia8.2 with ropes and EPOS LHC. The comparison shows that none of them can quantitatively describe either the transverse-to-leading or the toward-leading yields of KS0 and Ξ±.
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Measurement of inclusive charged-particle jet production in pp and p–Pb collisions
(Springer Science and Business Media Deutschland GmbH, 2024-05-01)
Measurements of inclusive charged-particle jet production in pp and p-Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN = 5.02 TeV and the corresponding nuclear modification factor RpPbchjet are presented, using data collected with the ALICE detector at the LHC. Jets are reconstructed in the central rapidity region |ηjet| < 0.5 from charged particles using the anti-kT algorithm with resolution parameters R = 0.2, 0.3, and 0.4. The pT-differential inclusive production cross section of charged-particle jets, as well as the corresponding cross section ratios, are reported for pp and p-Pb collisions in the transverse momentum range 10 <pT,jetch< 140 GeV/c and 10 <pT,jetch< 160 GeV/c, respectively, together with the nuclear modification factor RpPbchjet in the range 10 <pT,jetch< 140 GeV/c. The analysis extends the pT range of the previously-reported charged-particle jet measurements by the ALICE Collaboration. The nuclear modification factor is found to be consistent with one and independent of the jet resolution parameter with the improved precision of this study, indicating that the possible influence of cold nuclear matter effects on the production cross section of charged-particle jets in p-Pb collisions at sNN = 5.02 TeV is smaller than the current precision. The obtained results are in agreement with other minimum bias jet measurements available for RHIC and LHC energies, and are well reproduced by the NLO perturbative QCD Powheg calculations with parton shower provided by Pythia8 as well as by Jetscape simulations.
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Charged-particle production as a function of the relative transverse activity classifier in pp, p–Pb, and Pb–Pb collisions at the LHC
(Springer Science and Business Media Deutschland GmbH, 2024-01-01)
Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum pTtrig in the range 8 <pTtrig< 15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, RT=NchT/NchT, is used to group events according to their UE activity, where NchT is the charged-particle multiplicity per event in the transverse region and NchT is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at s = 2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at sNN = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p–Pb).
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Multiplicity-dependent production of Σ(1385)± and Ξ(1530)0 in pp collisions at √s = 13 TeV
(Springer Science and Business Media Deutschland GmbH, 2024-05-01)
The production yields of the Σ(1385)± and Ξ(1530)0 resonances are measured in pp collisions at s = 13 TeV with ALICE. The measurements are performed as a function of the charged-particle multiplicity ⟨dNch/dη⟩, which is related to the energy density produced in the collision. The results include transverse momentum (pT) distributions, pT-integrated yields, mean transverse momenta of Σ(1385)± and Ξ(1530)0, as well as ratios of the pT-integrated resonance yields relative to yields of other hadron species. The Σ(1385)±/π± and Ξ(1530)0/π± yield ratios are consistent with the trend of the enhancement of strangeness production from low to high multiplicity pp collisions, which was previously observed for strange and multi-strange baryons. The yield ratio between the measured resonances and the long-lived baryons with the same strangeness content exhibits a hint of a mild increasing trend at low multiplicity, despite too large uncertainties to exclude the flat behaviour. The results are compared with predictions from models such as EPOS-LHC and PYTHIA 8 with Rope shoving. The latter provides the best description of the multiplicity dependence of the Σ(1385)± and Ξ(1530)0 production in pp collisions at s = 13 TeV.
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Light-flavor particle production in high-multiplicity pp collisions at √s = 13 TeV as a function of transverse spherocity
(Springer Science and Business Media Deutschland GmbH, 2024-05-01)
Results on the transverse spherocity dependence of light-flavor particle production (π, K, p, ϕ, K*0, KS0, Λ, Ξ) at midrapidity in high-multiplicity pp collisions at s = 13 TeV were obtained with the ALICE apparatus. The transverse spherocity estimator SOpT=1 categorizes events by their azimuthal topology. Utilizing narrow selections on SOpT=1, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The SOpT=1 estimator is found to effectively constrain the hardness of the events when the midrapidity (|η| < 0.8) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of SOpT=1.
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Multiplicity- and event-scale-dependent flow and jet fragmentation in pp collisions at √s = 13 TeV and in p–Pb collisions at √sNN = 5.02 TeV
(Springer Science and Business Media Deutschland GmbH, 2024-03-01)
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s = 13 TeV and p–Pb collisions at sNN = 5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆φ and pseudorapidity separation ∆η for pairs of primary charged particles within the pseudorapidity interval |η| < 0.9 and the transverse-momentum interval 1 < pT< 4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6 < |∆η| < 1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.

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