Exploring the non-universality of charm hadronisation through the measurement of the fraction of jet longitudinal momentum carried by $\Lambda_{\rm c}^+$ baryons in pp collisions

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Adler, Alexander ; et al.
Phys.Rev.D 109 (2024) 072005, 2024.
Inspire Record 2628372 DOI 10.17182/hepdata.151164

Recent measurements of charm-baryon production in hadronic collisions have questioned the universality of charm-quark fragmentation across different collision systems. In this work the fragmentation of charm quarks into charm baryons is probed, by presenting the first measurement of the longitudinal jet momentum fraction carried by $\Lambda_{\rm c}^{+}$ baryons, $z^{\mathrm{ch}}_\mathrm{||}$, in hadronic collisions. The results are obtained in proton-proton (pp) collisions at $\sqrt{s}=13$ TeV at the LHC, with $\Lambda_{\rm c}^{+}$ baryons and charged (track-based) jets reconstructed in the transverse momentum intervals of $3 \leq p_{\rm T}^{\Lambda_{\rm c}^{+}} < 15$ GeV/$c$ and $7 \leq p_{\rm T}^{\rm jet\;ch} < 15$ GeV/$c$, respectively. The $z^{\mathrm{ch}}_\mathrm{||}$ distribution is compared to a measurement of ${\rm D}^0$-tagged charged jets in pp collisions as well as to PYTHIA 8 simulations. The data hints that the fragmentation of charm quarks into charm baryons is softer with respect to charm mesons, in the measured kinematic interval, as predicted by hadronisation models which include colour correlations beyond leading-colour in the string formation.

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Prompt and non-prompt J$/\psi$ production at midrapidity in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Aglieri Rinella, Gianluca ; et al.
JHEP 02 (2024) 066, 2024.
Inspire Record 2692201 DOI 10.17182/hepdata.146723

The transverse momentum ($p_{\rm T}$) and centrality dependence of the nuclear modification factor $R_{\rm AA}$ of prompt and non-prompt J$/\psi$, the latter originating from the weak decays of beauty hadrons, have been measured by the ALICE collaboration in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. The measurements are carried out through the ${\rm e}^{+}{\rm e}^{-}$ decay channel at midrapidity ($|y| < 0.9$) in the transverse momentum region $1.5 < p_{\rm T} < 10$ GeV/$c$. Both prompt and non-prompt J$/\psi$ measurements indicate a significant suppression for $p_{\rm T} >$ 5 GeV/$c$, which becomes stronger with increasing collision centrality. The results are consistent with similar LHC measurements in the overlapping $p_{\rm T}$ intervals, and cover the kinematic region down to $p_{\rm T}$ = 1.5 GeV/$c$ at midrapidity, not accessible by other LHC experiments. The suppression of prompt J$/\psi$ in central and semicentral collisions exhibits a decreasing trend towards lower transverse momentum, described within uncertainties by models implementing J$/\psi$ production from recombination of c and $\overline{\rm c}$ quarks produced independently in different partonic scatterings. At high transverse momentum, transport models including quarkonium dissociation are able to describe the suppression for prompt J$/\psi$. For non-prompt J$/\psi$, the suppression predicted by models including both collisional and radiative processes for the computation of the beauty-quark energy loss inside the quark$-$gluon plasma is consistent with measurements within uncertainties.

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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

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Aglieri Rinella, Gianluca ; et al.
JHEP 01 (2024) 199, 2024.
Inspire Record 2709103 DOI 10.17182/hepdata.146104

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 ($p_{\mathrm{T}}^{\rm trig}$) in the range $8<p_{\mathrm{T}}^{\rm trig}<15$ 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, $R_{\mathrm{T}}=N_{\mathrm{ch}}^{\mathrm{T}}/\langle N_{\mathrm{ch}}^{\mathrm{T}}\rangle$, is used to group events according to their UE activity, where $N_{\mathrm{ch}}^{\mathrm{T}}$ is the charged-particle multiplicity per event in the transverse region and $\langle N_{\mathrm{ch}}^{\mathrm{T}}\rangle$ is the mean value over the whole analysed sample. The energy dependence of the $R_{\mathrm{T}}$ distributions in pp collisions at $\sqrt{s}=2.76$, 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 $p_{\rm T}$ spectra as a function of $R_{\mathrm{T}}$ in the three azimuthal regions in pp, p$-$Pb, and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ 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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Probing the Chiral Magnetic Wave with charge-dependent flow measurements in Pb-Pb collisions at the LHC

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Aglieri Rinella, Gianluca ; et al.
JHEP 12 (2023) 067, 2023.
Inspire Record 2692198 DOI 10.17182/hepdata.145839

The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic ($v_{2}$) and triangular ($v_{3}$) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope $r_{3}^{\rm Norm}$ is found to be larger than zero and to have a magnitude similar to $r_{2}^{\rm Norm}$, thus pointing to a large background contribution for these measurements. Furthermore, $r_{2}^{\rm Norm}$ can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW ($f_{\rm CMW}$) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for $f_{\rm CMW}$, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level.

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Measurement of Non-prompt $\rm D^0$-meson Elliptic Flow in Pb-Pb Collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Aglieri Rinella, Gianluca ; et al.
Eur.Phys.J.C 83 (2023) 1123, 2023.
Inspire Record 2681666 DOI 10.17182/hepdata.145800

The elliptic flow ($v_2$) of $\rm D^0$ mesons from beauty-hadron decays (non-prompt $\rm D^0$) was measured in midcentral (30-50%) Pb-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The $\rm D^0$ mesons were reconstructed at midrapidity ($|y|<0.8$) from their hadronic decay $\mathrm{D^0 \to K^-\pi^+}$, in the transverse momentum interval $2 < p_{\rm T} < 12$ GeV/$c$. The result indicates a positive $v_2$ for non-prompt $\rm D^0$ mesons with a significance of 2.7$\sigma$. The non-prompt $\rm D^0$-meson $v_2$ is lower than that of prompt non-strange D mesons with 3.2$\sigma$ significance in $2 < p_{\rm T} < 8$ GeV/$c$, and compatible with the $v_2$ of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.

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Charm production and fragmentation fractions at midrapidity in pp collisions at $\sqrt{s} = 13$ TeV

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Aglieri Rinella, Gianluca ; et al.
JHEP 12 (2023) 086, 2023.
Inspire Record 2697877 DOI 10.17182/hepdata.145759

Measurements of the production cross sections of prompt ${\rm D^0}$, ${\rm D^+}$, ${\rm D^{\ast +}}$, ${\rm D_s^+}$, ${\rm \Lambda_{c}^{+}}$, and ${\rm \Xi_{c}^{+}}$ charm hadrons at midrapidity in proton$-$proton collisions at $\sqrt{s}=13$ TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum ($p_{\rm T}$) are provided with improved precision and granularity. The ratios of $p_{\rm T}$-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-$x$ ($10^{-5}-10^{-4}$). The measurements of ${\rm \Lambda_{c}^{+}}$ (${\rm \Xi_{c}^{+}}$) baryon production extend the measured $p_{\rm T}$ intervals down to $p_{\rm T}=0(3)$~GeV$/c$. These measurements are used to determine the charm-quark fragmentation fractions and the ${\rm c\overline{c}}$ production cross section at midrapidity ($|y|<0.5$) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons ${\rm D^0}$, ${\rm D^+}$, ${\rm D_s^+}$, ${\rm \Lambda_{c}^{+}}$, ${\rm \Xi_{c}^{0}}$ and, for the first time, ${\rm \Xi_{c}^{+}}$, and of the strongly-decaying J/$psi$ mesons. The first measurements of ${\rm \Xi_{c}^{+}}$ and ${\rm \Sigma_{c}^{0,++}}$ fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e$^+$e$^-$ and ep collisions. The ${\rm c\overline{c}}$ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.

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$\psi(2S)$ suppression in Pb-Pb collisions at the LHC

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Adler, Alexander ; et al.
Phys.Rev.Lett. 132 (2024) 042301, 2024.
Inspire Record 2165947 DOI 10.17182/hepdata.145654

The production of the $\psi(2S)$ charmonium state was measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity ($2.5<y<4$). The measurement of the ratio of the inclusive production cross sections of the $\psi(2S)$ and J/$\psi$ resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region $p_{\rm T}<12$ GeV/$c$. The results are compared with the corresponding measurements in pp collisions, by forming the double ratio $[\sigma^{\psi(2S)}/\sigma^{J/\psi}]_{\rm{Pb-Pb}}/[\sigma^{\psi(2S)}/\sigma^{J/\psi}]_{\rm{pp}}$. It is found that in Pb-Pb collisions the $\psi(2S)$ is suppressed by a factor of $\sim 2$ with respect to the J/$\psi$. The $\psi(2S)$ nuclear modification factor $R_{\rm AA}$ was also obtained as a function of both centrality and $p_{\rm T}$. The results show that the $\psi(2S)$ resonance yield is strongly suppressed in Pb-Pb collisions, by a factor up to $\sim 3$ with respect to pp. Comparisons of cross section ratios with previous SPS findings by the NA50 experiment and of $R_{\rm AA}$ with higher-$p_{\rm T}$ results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC.

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Soft-dielectron excess in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Adler, Alexander ; et al.
Phys.Rev.Lett. 127 (2021) 042302, 2021.
Inspire Record 1798514 DOI 10.17182/hepdata.111331

A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass $m_{\rm ee}$ and pair transverse momentum $p_{\rm T,ee}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and event multiplicity ${\rm d} N_{\rm ch}/{\rm d} \eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta/\pi^0$ ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<m_{\rm ee}<0.6$ GeV/$c^2$ and for $p_{\rm T,ee} < 0.4$ GeV/$c$ indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13\,(\rm{stat.})\pm 0.17\,(\rm{syst., data}) \pm 0.34\,(\rm{syst., cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\rm ee}$ and $p_{\rm T,ee}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.

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Energy dependence of coherent photonuclear production of J/$\psi$ mesons in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=5.02 TeV

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Adler, Alexander ; et al.
JHEP 10 (2023) 119, 2023.
Inspire Record 2666011 DOI 10.17182/hepdata.144758

The cross section for coherent photonuclear production of J/$\psi$ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. Cross sections are presented in five different J/$\psi$ rapidity ranges within $|y|<4$, with the J/$\psi$ reconstructed via its dilepton decay channels. In some events the J/$\psi$ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range $17 < W_{\gamma\, \mathrm{Pb, n}} <920$ GeV, where $W_{\gamma\, \mathrm{Pb, n}}$ is the centre-of-mass energy per nucleon of the $\gamma\,\mathrm{Pb}$ system. This range corresponds to a Bjorken-$x$ interval spanning about three orders of magnitude: $ 1.1\times10^{-5}<x<3.3\times 10^{-2}$. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.

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Measurement of inclusive J/$\psi$ pair production cross section in pp collisions at $\sqrt{s} = 13$ TeV

The ALICE collaboration Acharya, Shreyasi ; Adamova, Dagmar ; Adler, Alexander ; et al.
Phys.Rev.C 108 (2023) 045203, 2023.
Inspire Record 2648593 DOI 10.17182/hepdata.144368

The production cross section of inclusive J/$\psi$ pairs in pp collisions at a centre-of-mass energy $\sqrt{s} = 13$ TeV is measured with ALICE. The measurement is performed for J/$\psi$ in the rapidity interval $2.5 < y < 4.0$ and for transverse momentum $p_{\rm T} > 0$. The production cross section of inclusive J/$\psi$ pairs is reported to be $10.3 \pm 2.3 {\rm (stat.)} \pm 1.3 {\rm (syst.)}$ nb in this kinematic interval. The contribution from non-prompt J/$\psi$ (i.e. originated from beauty-hadron decays) to the inclusive sample is evaluated. The results are discussed and compared with data.

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