Explore the vast range of titles available.

In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD) 1 . These partons subsequently emit further partons in a process that can be described as a parton shower 2 , which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m Q and energy E , within a cone of angular size m Q / E around the emitter 3 . Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques 4 , 5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics. The direct measurement of the QCD dead cone in charm quark fragmentation is reported, using iterative declustering of jets tagged with a fully reconstructed charmed hadron.

This paper presents the measurements of π ± , K ± , p and p ¯ transverse momentum ( p T ) spectra as a function of charged-particle multiplicity density in proton–proton (pp) collisions at s = 13 TeV with the ALICE detector at the LHC. Such study allows us to isolate the center-of-mass energy dependence of light-flavour particle production. The measurements reported here cover a p T range from 0.1 to 20 GeV / c and are done in the rapidity interval | y | < 0.5 . The p T -differential particle ratios exhibit an evolution with multiplicity, similar to that observed in pp collisions at s = 7 TeV , which is qualitatively described by some of the hydrodynamical and pQCD-inspired models discussed in this paper. Furthermore, the p T -integrated hadron-to-pion yield ratios measured in pp collisions at two different center-of-mass energies are consistent when compared at similar multiplicities. This also extends to strange and multi-strange hadrons, suggesting that, at LHC energies, particle hadrochemistry scales with particle multiplicity the same way under different collision energies and colliding systems.

The coherent photoproduction of J/ψ and ψ′ mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sNN=5.02 TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The J/ψ is reconstructed using the dilepton (l+l-) and proton–antiproton decay channels, while for the ψ′ the dilepton and the l+l-π+π- decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 μb-1. The results are compared with theoretical models for coherent J/ψ and ψ′ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6×10-4, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent J/ψ cross section including ALICE measurements at forward rapidity. The ratio of ψ′ to J/ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.

The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ( y < 0.5 ) are measured in proton-proton collisions at s  = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of K S 0 , Λ , Ξ , and Ω increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.

A bstract The cross section for coherent photonuclear production of J/ ψ 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 s NN = 5.02 TeV. Cross sections are presented in five different J/ ψ rapidity ranges within | y | < 4, with the J/ ψ reconstructed via its dilepton decay channels. In some events the J/ ψ 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 γ Pb , n < 920 GeV, where W γ Pb , n is the centre-of-mass energy per nucleon of the γ Pb system. This range corresponds to a Bjorken- x interval spanning about three orders of magnitude: 1.1 × 10 − 5 < x < 3.3 × 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.

A bstract The p T -differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity ( | y | < 0 . 5) in proton-proton collisions at s = 5 . 02 TeV. The data sample used in the analysis corresponds to an integrated luminosity of (19 . 3 ± 0 . 4) nb − 1 . D mesons were reconstructed from their decays D 0 → K − π + , D + → K − π + π + , and D s + → ϕ π + → K − K + π + and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D + and D s + mesons have an extended p T coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on p T , allowing for a more precise determination of their p T -integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons, f s / ( f u + f d ), is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The b b ¯ production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is d σ b b ¯ / d y y < 0.5 = 34.5 ± 2.4 stat − 2.9 + 4.7 tot . syst μb. It is compatible with previous measurements at the same centre-of-mass energy and with the cross section pre- dicted by perturbative QCD calculations.

A bstract The production of prompt D 0 , D + , and D *+ mesons was measured at midrapidity (| y | < 0.5) in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair s NN = 5 . 02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0–10%) and semicentral (30–50%) collisions. The measurement was performed up to a transverse momentum ( p T ) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb–Pb collisions at the LHC, the yield of D 0 mesons was measured down to p T = 0, which allowed a model-independent determination of the p T -integrated yield per unit of rapidity (d N/ d y ). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor ( R AA ) of prompt D mesons at p T = 6–8 GeV/c for the 0–10% and 30–50% centrality classes, respectively. The D-meson R AA is compared with that of charged pions, charged hadrons, and J /ψ mesons as well as with theoretical predictions. The analysis of the agreement between the measured R AA , elliptic ( v 2 ) and triangular ( v 3 ) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient D s . Furthermore the comparison of R AA and v 2 with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms.

The interaction of K - with protons is characterised by the presence of several coupled channels, systems like K ¯ 0 n and π Σ with a similar mass and the same quantum numbers as the K - p state. The strengths of these couplings to the K - p system are of crucial importance for the understanding of the nature of the Λ ( 1405 ) resonance and of the attractive K - p strong interaction. In this article, we present measurements of the K - p correlation functions in relative momentum space obtained in pp collisions at s = 13  Te, in p–Pb collisions at s NN = 5.02  Te, and (semi)peripheral Pb–Pb collisions at s NN = 5.02  Te. The emitting source size, composed of a core radius anchored to the K + p correlation and of a resonance halo specific to each particle pair, varies between 1 and 2 fm in these collision systems. The strength and the effects of the K ¯ 0 n and π Σ inelastic channels on the measured K - p correlation function are investigated in the different colliding systems by comparing the data with state-of-the-art models of chiral potentials. A novel approach to determine the conversion weights ω , necessary to quantify the amount of produced inelastic channels in the correlation function, is presented. In this method, particle yields are estimated from thermal model predictions, and their kinematic distribution from blast-wave fits to measured data. The comparison of chiral potentials to the measured K - p interaction indicates that, while the π Σ – K - p dynamics is well reproduced by the model, the coupling to the K ¯ 0 n channel in the model is currently underestimated.

A bstract Measurements of the production cross sections of prompt D 0 , D + , D *+ , D s + , Λ c + , and Ξ c + charm hadrons at midrapidity in proton-proton collisions at s = 13 TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum ( p T ) are provided with improved precision and granularity. The ratios of p 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 Λ c + ( Ξ c + ) baryon production extend the measured p T intervals down to p T = 0(3) GeV /c . These measurements are used to determine the charm-quark fragmentation fractions and the c 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 D 0 , D + , D s + , Λ c + , Ξ c 0 and, for the first time, Ξ c + , and of the strongly-decaying J /ψ mesons. The first measurements of Ξ c + and Σ 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 c c ¯ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.

The production cross sections for prompt charmed mesons D 0 , D + , D ∗ + and D s + were measured at mid-rapidity in proton–proton collisions at a centre-of-mass energy s = 7 TeV with the ALICE detector at the Large Hadron Collider (LHC). D mesons were reconstructed from their decays D 0 → K - π + , D + → K - π + π + , D ∗ + → D 0 π + , D s + → ϕ π + → K - K + π + , and their charge conjugates.With respect to previous measurements in the same rapidity region, the coverage in transverse momentum ( p T ) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total c c ¯ production cross section is likewise improved. The measured p T -differential cross sections are compared with the results of three perturbative QCD calculations.