17 Search Results

The nuclear modification factor R AA of charm jets, identified by the presence of a D0 meson among the jet constituents, has been measured for the first time in Pb–Pb collisions at a centre-of-mass energy per nucleon pair s NN = 5.02 TeV with the ALICE detector at the LHC. The D0 mesons and their charge conjugates are reconstructed from the hadronic decay D 0 → K − π + . Jets are reconstructed from D0-meson candidates and charged particles using the anti-k T algorithm with jet resolution parameter R = 0.3 , in the jet transverse momentum (p T)more » range 5 < p T ch jet < 50 GeV/c and pseudorapidity |η ch jet| < 0.6. A hint of reduced suppression in the charm-jet R AA is observed in comparison to inclusive jets in central Pb–Pb collisions with a significance of about 2σ in 20 < p T ch jet < 50 GeV/c, suggesting the in-medium energy loss to depend on both the difference between quark and gluon coupling strength (Casimir colour-charge effect) and quark mass (dead-cone effect). The data are compared with model calculations that include mass effects in the in-medium energy loss. Several state-of-the-art models are consistent with the data, with the LIDO model providing the best description of the data in the common kinematic range of inclusive and D0-tagged jets, highlighting the role of mass effects in interpreting the results.« less

Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGPmore » formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions.« less

The distribution of angular correlations between prompt charm hadrons and primary charged particles in pp collisions is sensitive to the charm-quark hadronisation process. In this letter, charm-baryon correlations are measured for the first time by studying the azimuthal-angle difference between charged particles and prompt Λ c + baryons produced in pp collisions at a centre-of-mass energy of s = 13 TeV, with the ALICE detector. Λ c + baryons are reconstructed at midrapidity ( | y | < 0.5 ) in the transverse-momentum interval 3 < p T < 16 GeV/c, and correlated with charged particles with p T >more » 0.3 GeV/c and pseudorapidity | η | < 0.8 . For 3 < p T Λ c + , D < 5 GeV/c, the comparison with published measurements of D-meson and charged-particle correlations in the same collision system hints at a larger number of low-momentum particles associated with Λ c + -baryon triggers than with D-meson triggers, both in the collinear and opposite directions with respect to the trigger particle. These differences can be quantified by the comparison of the properties of the near- and away-side correlation peaks, and are not reproduced by predictions of various Monte Carlo event generators, generally underpredicting the associated particle yields at p T assoc < 1 GeV/c. This tension between Λ c + -baryon and D-meson associated peak yields could suggest a modified fragmentation of the charm quark, or a different hadronisation process, when a charm baryon is produced in the final state.« less

The interaction between pions and nucleons plays a crucial role in hadron physics. It represents a fundamental building block of the low-energy QCD dynamics and is subject to several resonance excitations. This work studies the p--π^±dynamics using femtoscopic correlations in high-multiplicity pp collisions at $$\sqrt{s}$$ = 13 TeV measured by ALICE at the LHC. As the final-state interaction between protons and pions is well constrained by scattering experiments and the study of pionic hydrogen, the results give access to information on the particle-emitting source in pp collisions using the femtoscopy methods. The scaling of the source size of primordial protonsmore » and pions against their pair transverse mass is extracted. The results are compared with the source sizes studied with p–p, p--K⁺, and π^±–π^± pairs by ALICE in the same collision system and are found to be in agreement for the different particle pairs. This reinforces recent findings by ALICE of a common emission source for all hadron-pairs in pp collisions at LHC energies. Furthermore, the p--p--π^± systems are studied using three-particle femtoscopy in pp collisions at TeV. The presence of three-body effects is analyzed utilizing the cumulant expansion method. In this formalism, the known two-body interactions are subtracted in order to isolate the three-body effects. For both, p--p--π⁺ and p--p--π^–, a non-zero cumulant is found, indicating effects beyond pairwise interactions. These results give information on the coupling of the pion to multiple nucleons.« less

Differential two-particle normalized cumulants (R₂) and transverse momentum correlations (P₂) are measured as a function of the relative pseudorapidity and azimuthal angle difference (ΔηΔφ) of charged particle pairs in minimum bias pp collisions at $$\sqrt{s}$$ = 13 TeV. The measurements use charged hadrons in the pseudorapidity region of |η| < 0.8 and the transverse momentum range 0.2 < p_T < 2.0 in order to focus on soft multiparticle interactions and to complement prior measurements of these correlation functions in p–Pb and Pb–Pb collisions. The correlation functions are reported for both unlike-sign and like-sign pairs and their charge-independent and charge-dependent combinations.more » Both the R₂ and P₂ measured in pp collisions exhibit features qualitatively similar to those observed in p–Pb and Pb–Pb collisions. The Δη and Δφ root mean square widths of the near-side peak of the correlation functions are evaluated and compared with those observed in p–Pb and Pb–Pb collisions and show smooth evolution with the multiplicity of charged particles produced in the collision. The comparison of the measured correlation functions with predictions from PYTHIA8 shows that this model qualitatively captures their basic structure and characteristics but feature important differences. In addition, the R$$^{CD}_{2}$$ is used to determine the charge balance function of hadrons produced within the detector acceptance of the measurements. The integral of the balance function is found to be compatible with those reported by a previous measurement in Pb–Pb collisions.« less

The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum (p$$^{γ}_{T}$$), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of $$\sqrt{s_{NN}}$$ = 5.02 TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/c, depending on the collision system and on the Pb–Pb centrality class. The result extends to lower p$$^{γ}_{T}$$ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is |η^γ| <more » 0.67. The isolation selection is based on a charged particle isolation momentum threshold p$$^{iso, ch}_{T}$$ GeV/c within a cone of radii R = 0.2 and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z⁰ bosons from the CMS experiment, which are all found to be in agreement.« less

The femtoscopic study of pairs of identical pions is particularly suited to investigate the effective source function of particle emission, due to the resulting Bose–Einstein correlation signal. In small collision systems at the LHC, pp in particular, the majority of the pions are produced in resonance decays, which significantly affect the profile and size of the source. In this work, we explicitly model this effect in order to extract the primordial source in pp collisions at $$\sqrt{s}$$ = 13 TeV from charged π–π correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible withmore » the data and that the effective source, resulting from modifications due to resonances, is approximately exponential, as found in previous measurements at the LHC. The universality of hadron emission in pp collisions is further investigated by applying the same methodology to characterize the primordial source of K–p pairs. The size of the primordial source is evaluated as a function of the transverse mass (m_T) of the pairs, leading to the observation of a common scaling for both π–π and K–p, suggesting a collective effect. Further, the present results are compatible with the m_T scaling of the p–p and p–Λ primordial source measured by ALICE in high multiplicity pp collisions, providing additional evidence for the presence of a common emission source for all hadrons in small collision systems at the LHC. This will allow the determination of the source function for any hadron–hadron pairs with high precision, granting access to the properties of the possible final-state interaction among pairs of less abundantly produced hadrons, such as strange or charmed particles.« less

The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at centre-of-momentum energy of $$\mathbf {\sqrt{\textit{s}}=13}$$ TeV collected during the LHC Run 2 data-taking period. The measurement is performed by combining the measurements of the electromagnetic calorimeter EMCal and the central tracking detectors ITS and TPC, covering a pseudorapidity range of and a transverse momentum range of |η^γ| < 0.67 and a transverse momentum range 7 < p$$^{γ}_{T}$$ < 200 GeV/c. The result extends to lower p$$^{γ}_{T}$$ and x$$^{γ}_{T}$$ = 2p$$^{γ}_{T}$$/$$\sqrt{s}$$ ranges, the lowest x$$^{γ}_{T}$$ of any isolatedmore » photon measurements to date, extending significantly those measured by the ATLAS and CMS experiments towards lower p$$^{γ}_{T}$$ at the same collision energy with a small overlap between the measurements. The measurement is compared with next-to-leading order perturbative QCD calculations and the results from the ATLAS and CMS experiments as well as with measurements at other collision energies. The measurement and theory prediction are in agreement with each other within the experimental and theoretical uncertainties.« less

The total charm-quark production cross section per unit of rapidity dσ($$\mathbf {c\overline{c}}$$)/dy, and the fragmentation fractions of charm quarks to different charm-hadron species f(c → h_c), are measured for the first time in p–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV at midrapidity (–0.96 < y < 0.04 in the centre-of-mass frame) using data collected by ALICE at the CERN LHC. The results are obtained based on all the available measurements of prompt production of ground-state charm-hadron species: D⁰, D⁺, D$$^{+}_{s}$$, and J/ψ mesons, and Λ$$^{+}_{c}$$ and $$Ξ$$$^{0}_{c}$$ baryons. The resulting cross section is dσ($$\mathbf {c\overline{c}}$$)/dy = 219.6 ± 6.3 (stat.)more » $$^{+10.5}_{–11.8}$$ (syst.) $$^{+8.3}_{–2.9}$$ (extr.) ± 5.4 (BR) ± 4.6 (lumi.) ± 19.5 (rapidity shape) + 15.0 (Ω$$^{0}_{c}$$) mb, which is consistent with a binary scaling of pQCD calculations from pp collisions. The measured fragmentation fractions are compatible with those measured in pp collisions at $$\sqrt{s_{NN}}$$ = 5.02 and 13 TeV, showing an increase in the relative production rates of charm baryons with respect to charm mesons in pp and p–Pb collisions compared with e⁺e^– and e^–p collisions. The p_T-integrated nuclear modification factor of charm quarks, R_pPb ($$\mathbf {c\overline{c}}$$) = 0.91 ± 0.04 (stat.) $$^{+0.08}_{–0.09}$$ (syst.) $$^{+0.05}_{–0.03}$$ (extr.) ± 0.03 (lumi.), is found to be consistent with unity and with theoretical predictions including nuclear modifications of the parton distribution functions.« less

The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy $$\sqrt{s}$$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η| < 0.9) using the sequential recombination anti-k_T algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the trans verse momentum (p_T) interval 5–110 GeV/c. The high multiplicity events are selected by the forward V0 scintilla tor detectors.more » The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet p_T in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables z^ch and ξ^ch are measured for different jet-p_T intervals. Jet-p_T independent fragmentation of leading jets is observed for wider jets except at high- and low-z^ch values. The observed “hump-backed plateau” structure in the ξ^ch distribution indicates suppression of low-p_T particles. In high-multiplicity events, an enhancement of the fragmen tation probability of low-z^ch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more promi nent for low-p_T jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet p_T. These measurements provide important constraints to models of jet fragmentation.« less