Fri·Apr 22, 2022Friday·April 22, 2022

1. 01*

   ALICE luminosity determination for Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV

   ALICE Collaboration

   Luminosity determination within the ALICE experiment is based on the measurement, in van der Meer scans, of the cross sections for visible processes involving one or more detectors (visible cross sections). In 2015 and 2018, the Large Hadron Collider provided Pb$-$Pb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. Two visible cross sections, associated with particle detection in the Zero Degree Calorimeter (ZDC) and in the V0 detector, were measured in a van der Meer scan. This article describes the experimental set-up and the analysis procedure, and presents the measurement results. The analysis involves a comprehensive study of beam-related effects and an improved fitting procedure, compared to previous ALICE studies, for the extraction of the visible cross section. The resulting uncertainty of both the ZDC-based and the V0-based luminosity measurement for the full sample is 2.5%. The inelastic cross section for hadronic interactions in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV, obtained by efficiency correction of the V0-based visible cross section, was measured to be $7.67\pm 0.25$ b, in agreement with predictions using the Glauber model.

   nucl-exhep-exJINST(2024)·52 citations
2. 02*

   Study of charged particle production at high $p_{\mathrm{T}}$ using event topology in pp, p$-$Pb and Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV

   ALICE Collaboration

   This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity ($|\eta |<0.8$) in pp, p$-$Pb and Pb$-$Pb collisions at centre-of-mass energy per nucleon pair, $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The measurements are performed with ALICE at the LHC. Different multiplicity classes are defined based on the event activity measured at forward rapidities. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum ($p_{\mathrm{T}}$) in the collision and having $8<p_{\mathrm{T}}<15$ GeV/$c$. The $p_{\mathrm{T}}$ spectra of associated particles ($0.5\le p_{\mathrm{T}}<6$ GeV/$c$) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-$p_{\mathrm{T}}$ associated particles in the toward region in central Pb$-$Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb$-$Pb collisions and are not observed in either high-multiplicity pp or p$-$Pb collisions.

   nucl-exhep-exPLB(2023)·22 citations
3. 03*

   First measurement of antideuteron number fluctuations at energies available at the Large Hadron Collider

   ALICE Collaboration

   The first measurement of event-by-event antideuteron number fluctuations in high energy heavy-ion collisions is presented. The measurements are carried out at midrapidity ($|\eta |<0.8$) as a function of collision centrality in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV using the ALICE detector. A significant negative correlation between the produced antiprotons and antideuterons is observed in all collision centralities. The results are compared with a state-of-the-art coalescence calculation. While it describes the ratio of higher order cumulants of the antideuteron multiplicity distribution, it fails to describe quantitatively the magnitude of the correlation between antiproton and antideuteron production. On the other hand, thermal-statistical model calculations describe all the measured observables within uncertainties only for correlation volumes that are different with respect to those describing proton yields and a similar measurement of net-proton number fluctuations.

   nucl-exhep-exPRL(2023)·42 citations
4. 04*

   Measurement of the production of charm jets tagged with ${\mathrm{D}}^0$ mesons in pp collisions at $\sqrt{s}$ = 5.02 and 13 TeV

   ALICE Collaboration

   The measurement of the production of charm jets, identified by the presence of a ${\mathrm{D}}^0$ meson in the jet constituents, is presented in proton-proton collisions at centre-of-mass energies of $\sqrt{s}$ = 5.02 and 13 TeV with the ALICE detector at the CERN LHC. The ${\mathrm{D}}^0$ mesons were reconstructed from their hadronic decay ${\mathrm{D}}^0\to {\mathrm{K}}^{-}{\pi }^{+}$ and the respective charge conjugate. Jets were reconstructed from ${\mathrm{D}}^0$-meson candidates and charged particles using the anti-$k_{\mathrm{T}}$ algorithm, in the jet transverse momentum range $5<p_{\mathrm{T};\mathrm{chjet}}<50$ GeV/$c$, pseudorapidity $|{\eta }_{\mathrm{jet}}|<0.9-R$, and with the jet resolution parameters $R$ = 0.2, 0.4, 0.6. The distribution of the jet momentum fraction carried by a ${\mathrm{D}}^0$ meson along the jet axis ($z_{||}^{\mathrm{ch}}$) was measured in the range $0.4<z_{||}^{\mathrm{ch}}<1.0$ in four ranges of the jet transverse momentum. Comparisons of results for different collision energies and jet resolution parameters are also presented. The measurements are compared to predictions from Monte Carlo event generators based on leading-order and next-to-leading-order perturbative quantum chromodynamics calculations. A generally good description of the main features of the data is obtained in spite of a few discrepancies at low $p_{\mathrm{T};\mathrm{chjet}}$. Measurements were also done for $R=0.3$ at $\sqrt{s}$ = 5.02 TeV and are shown along with their comparisons to theoretical predictions in an appendix to this paper.

   nucl-exhep-exJHEP(2023)·37 citations
5. 05*

   Measurement of the J/$\psi$ polarization with respect to the event plane in Pb-Pb collisions at the LHC

   ALICE Collaboration

   We study the polarization of inclusive J/$\psi$ produced in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV at the LHC in the dimuon channel, via the measurement of the angular distribution of its decay products. We perform the study in the rapidity region $2.5<y<4$, for three transverse momentum intervals ($2<p_{\mathrm{T}}<4$, $4<p_{\mathrm{T}}<6$, $6<p_{\mathrm{T}}<10$ GeV/$c$) and as a function of the centrality of the collision for $2<p_{\mathrm{T}}<6$ GeV/c. For the first time, the polarization is measured with respect to the event plane of the collision, by considering the angle between the positive-charge decay muon in the J/$\psi$ rest frame and the axis perpendicular to the event-plane vector in the laboratory system. A small transverse polarization is measured, with a significance reaching 3.9$\sigma$ at low $p_{\mathrm{T}}$ and for intermediate centrality values. The polarization could be connected with the behaviour of the quark$-$gluon plasma, formed in Pb$-$Pb collisions, as a rotating fluid with large vorticity, as well as with the existence of a strong magnetic field in the early stage of its formation.

   nucl-exhep-exPRL(2023)·113 citations
6. 06*

   System-size dependence of the charged-particle pseudorapidity density at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV for pp, p-Pb, and Pb-Pb collisions

   ALICE Collaboration

   We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, p-Pb, and Pb-Pb, at the top energy of the Large Hadron Collider ($\sqrt{s_{\mathrm{NN}}}=5.02$ TeV) measured over a wide pseudorapidity range ($-3.5<\eta <5$), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for p-Pb and Pb-Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter.

   nucl-exhep-exPLB(2023)·37 citations
7. 07*

   The Proton Spin Structure Function $g_2$ and Generalized Polarizabilities in the Strong QCD Regime

   D. Ruth · R. Zielinski · C. Gu · M. Allada (Cummings) · T. Badman · M. Huang · J. Liu · P. Zhu · K. Allada · J. Zhang · A. Camsonne · J.P. Chen and 85 other authors

   The strong interaction is not well understood at low energy, or for interactions with low momentum transfer $Q^2$, but one of the clearest insights we have comes from Chiral Perturbation Theory ($\chi$PT). This effective treatment gives testable predictions for the nucleonic generalized polarizabilities -- fundamental quantities describing the nucleon's response to an external field. We have measured the proton's generalized spin polarizabilities in the region where $\chi$PT is expected to be valid. Our results include the first ever data for the transverse-longitudinal spin polarizability ${\delta }_{LT}$, and also extend the coverage of the polarizability $\overline{d_2}$ to very low $Q^2$ for the first time. These results were extracted from moments of the structure function $g_2$, a quantity which characterizes the internal spin structure of the proton. Our experiment ran at Jefferson Lab using a polarized electron beam and a polarized solid ammonia (NH${}_3$) target. The ${\delta }_{LT}$ polarizability has remained a challenging quantity for $\chi$PT to reproduce, despite its reduced sensitivity to higher resonance contributions; recent competing calculations still disagree with each other and also diverge from the measured neutron data at very low $Q^2$. Our proton results provide discriminating power between existing calculations, and will help provide a better understanding of this strong QCD regime.

   nucl-exhep-exNat.Phys.(2022)·17 citations
8. 08*

   Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb-Pb and Xe-Xe collisions

   ALICE Collaboration

   Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons $v_2${${\Psi }_{\mathrm{SP}}$} in collisions of Pb ions at center-of-mass energy per nucleon-nucleon pair $\sqrt{s_{\mathrm{NN}}}$=2.76 TeV and Xe ions at $\sqrt{s_{\mathrm{NN}}}$=5.44 TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum. The ratio between $v_2${${\Psi }_{\mathrm{SP}}$} and the elliptic flow coefficient relative to the participant plane $v_2${4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models of initial state fluctuations. A significant transverse momentum dependence of the ratio $v_2${${\Psi }_{\mathrm{SP}}$}/$v_2${4} is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state fluctuations. The ratios of $v_2${${\Psi }_{\mathrm{SP}}$} and $v_2${4} to the corresponding initial state eccentricities for Xe-Xe and Pb-Pb collisions at similar initial entropy density show a difference of $(7.0\pm 0.9)$% with an additional variation of +1.8% when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark-gluon plasma.

   nucl-exhep-exPLB(2023)·13 citations
9. 09*

   Measurements of the groomed jet radius and momentum splitting fraction with the soft drop and dynamical grooming algorithms in pp collisions at $\sqrt{s}=5.02$ TeV

   ALICE Collaboration

   This article presents measurements of the groomed jet radius and momentum splitting fraction in pp collisions at $\sqrt{s}=5.02$ TeV with the ALICE detector at the Large Hadron Collider. Inclusive charged-particle jets are reconstructed at midrapidity using the anti-$k_{\mathrm{T}}$ algorithm for transverse momentum $60<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<80$ GeV/$c$. We report results using two different grooming algorithms: soft drop and, for the first time, dynamical grooming. For each grooming algorithm, a variety of grooming settings are used in order to explore the impact of collinear radiation on these jet substructure observables. These results are compared to perturbative calculations that include resummation of large logarithms at all orders in the strong coupling constant. We find good agreement of the theoretical predictions with the data for all grooming settings considered.

   nucl-exhep-exJHEP(2023)·26 citations
10. 10*

    Measurement of $\psi$(2S) production as a function of charged-particle pseudorapidity density in pp collisions at $\sqrt{s}$ = 13 TeV and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV with ALICE at the LHC

    ALICE Collaboration

    Production of inclusive charmonia in pp collisions at center-of-mass energy of $\sqrt{s}$ = 13 TeV and p-Pb collisions at center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV is studied as a function of charged-particle pseudorapidity density with ALICE. Ground and excited charmonium states (J/$\psi$, $\psi$(2S)) are measured from their dimuon decays in the interval of rapidity in the center-of-mass frame $2.5<y_{\mathrm{cms}}<4.0$ for pp collisions, and $2.03<y_{\mathrm{cms}}<3.53$ and $-4.46<y_{\mathrm{cms}}<-2.96$ for p-Pb collisions. The charged-particle pseudorapidity density is measured around midrapidity ($|\eta |<1.0$). In pp collisions, the measured charged-particle multiplicity extends to about six times the average value, while in p-Pb collisions at forward (backward) rapidity a multiplicity corresponding to about three (four) times the average is reached. The $\psi$(2S) yield increases with the charged-particle pseudorapidity density. The ratio of $\psi$(2S) over J/$\psi$ yield does not show a significant multiplicity dependence in either colliding system, suggesting a similar behavior of J/$\psi$ and $\psi$(2S) yields with respect to charged-particle pseudorapidity density. Results for the $\psi$(2S) yield and its ratio with respect to J/$\psi$ agree with available model calculations.

    nucl-exhep-exJHEP(2023)·28 citations
11. 11*

    First measurement of the $\Lambda$-$\Xi$ interaction in proton-proton collisions at the LHC

    ALICE Collaboration

    The first experimental information on the strong interaction between $\Lambda$ and ${\Xi }^{-}$ strange baryons is presented in this Letter. The correlation function of $\Lambda -{\Xi }^{-}$ and $\overline{\Lambda }-{\overline{\Xi }}^{+}$ pairs produced in high-multiplicity proton-proton (pp) collisions at $\sqrt{s}$ = 13 TeV at the LHC is measured as a function of the relative momentum of the pair. The femtoscopy method is used to calculate the correlation function, which is then compared with theoretical expectations obtained using a meson exchange model, chiral effective field theory, and Lattice QCD calculations close to the physical point. Data support predictions of small scattering parameters while discarding versions with large ones, thus suggesting a weak $\Lambda -{\Xi }^{-}$ interaction. The limited statistical significance of the data does not yet allow one to constrain the effects of coupled channels like $\Sigma -\Xi$ and N$-\Omega$.

    nucl-exhep-exPLB(2023)·42 citations
12. 12*

    Multiplicity and rapidity dependence of ${\mathrm{K}}^{\ast }(892{)}^0$ and $\varphi (1020)$ production in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

    ALICE Collaboration

    The transverse-momentum ($p_{\mathrm{T}}$) spectra of ${\mathrm{K}}^{\ast }(892{)}^0$ and $\varphi (1020)$ measured with the ALICE detector up to $p_{\mathrm{T}}$ = 16 GeV/$c$ in the rapidity range $-1.2<y<0.3$, in p-Pb collisions at the center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV are presented as a function of charged particle multiplicity and rapidity. The measured $p_{\mathrm{T}}$ distributions show a dependence on both multiplicity and rapidity at low $p_{\mathrm{T}}$ whereas no significant dependence is observed at high $p_{\mathrm{T}}$. A rapidity dependence is observed in the $p_{\mathrm{T}}$-integrated yield (d$N$/d$y$), whereas the mean transverse momentum ($⟨p_{\mathrm{T}}⟩$) shows a flat behavior as a function of rapidity. The rapidity asymmetry ($Y_{\mathrm{asym}}$) at low $p_{\mathrm{T}}$ ( < 5 GeV/$c$) is more significant for higher multiplicity classes. At high $p_{\mathrm{T}}$, no significant rapidity asymmetry is observed in any of the multiplicity classes. Both ${\mathrm{K}}^{\ast }(892{)}^0$ and $\varphi (1020)$ show similar $Y_{\mathrm{asym}}$. The nuclear modification factor ($Q_{\mathrm{CP}}$) as a function of $p_{\mathrm{T}}$ shows a Cronin-like enhancement at intermediate $p_{\mathrm{T}}$, which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high $p_{\mathrm{T}}$ (> 5 GeV/$c$), the $Q_{\mathrm{CP}}$ values are greater than unity and no significant rapidity dependence is observed.

    nucl-exhep-exEPJC(2023)·19 citations
13. 13*

    Measurement of inclusive and leading subjet fragmentation in pp and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

    ALICE Collaboration

    This article presents new measurements of the fragmentation properties of jets in both proton-proton (pp) and heavy-ion collisions with the ALICE experiment at the LHC. We report distributions of the fraction $z_r$ of transverse momentum carried by subjets of radius $r$ within jets of radius $R$. Charged-particle jets are reconstructed at midrapidity using the anti-$k_{\mathrm{T}}$ algorithm with jet radius $R=0.4$, and subjets are reconstructed by reclustering the jet constituents using the anti-$k_{\mathrm{T}}$ algorithm with radii $r=0.1$ and $r=0.2$. In pp collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calculations at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the $z_r$ distribution. In heavy-ion collisions, we measure the leading subjet distributions, which allow access to a region of harder jet fragmentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The $z_r$ distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark-gluon plasma (QGP). We find no significant modification of $z_r$ distributions in Pb-Pb compared to pp collisions. However, the distributions are also consistent with a hardening trend for $z_r<0.95$, as predicted by several jet quenching models. As $z_r\to 1$ our results indicate that any such hardening effects cease, exposing qualitatively new possibilities to disentangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP.

    nucl-exhep-exJHEP(2023)·24 citations
14. 14*

    Medium modification of the nucleon mechanical properties: Abel tomography case

    June-Young Kim🇩🇪 · Ulugbek Yakhshiev🇰🇷 · Hyun-Chul Kim🇰🇷

    We investigate how the gravitational form factors of the nucleon undergo changes in nuclear matter, emphasizing the Abel transformation from the three-dimensional (3D) Breit frame to the two-dimensional (2D) light-front frame. Since the gravitational form factors reveal the mechanical structure of the nucleon, we examine also the medium modifications of the energy-momentum, pressure, and shear-force distributions. We scrutinize the stabilities of the nucleon in nuclear matter. For this purpose, we employ the in-medium modified SU(2) Skyrme model to study these mechanical quantities of the nucleon, since it provides a simple but clear framework. In this in-medium modified SU(2) Skyrme model, the modification of pionic properties is performed by using low-energy pion-nucleus scattering data and the saturation properties of nuclear matter near the normal nuclear matter density, ${\rho }_0=0.5m_{\pi }^3$. The results reveal how the nucleon swells in nuclear matter as the mass distribution of the nucleon is broadened in medium. We also show that the mean square radii corresponding to the mass and angular momentum distributions increase in nuclear medium. This feature is kept both in 3D and 2D cases. We visualize how the strong force fields inside the nucleon in the 2D plane are distributed and illustrate how these forces undergo change in nuclear matter.

    ↳ hep-phhep-exnucl-exnucl-thEPJC(2022)·8 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.