9 Search Results

This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p–Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p–Pb collisions at centre-of-mass energies per nucleon pair of $$\sqrt{s_{NN}}$$ = 5.02 Te and 8.16 TeV, as well as in pp collisions at $$\sqrt{s}$$ = 5.02 TeV and 8 TeV. The cross section is obtained at midrapidity (|γ| < 0.7) using a charged-track based isolation momentum p$$^{iso,~ch}_{T}$$ < 1.5 GeV/c in a cone with radius R = 0.4. The data for both collision systems are well reproduced by perturbativemore » QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor R_pA for both collision energies is consistent with unity for p_T > 20 GeV/c. However, deviations from unity (R_pA < 1) of up to 20% are observed for p_T < 20 GeV/c with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision.« less

A signal consistent with the Ω⁡(2012) baryon has been observed with a significance of 15⁢𝜎 in pp collisions at $$\sqrt{𝑠}$$ = 13 TeV at the LHC. In this paper, the analysis technique is described and measurements of the mass and width of the Ω⁡(2012) are reported, along with the first measurement of its transverse-momentum spectrum and yield. This paper corroborates the observation by the Belle Collaboration of this excited Ω state and the observation that the Ω⁡(2012) has a rather narrow width for a strongly decaying resonance. The yield measurement is combined with a statistical thermal model calculation of strangemore » baryon yield ratios to obtain estimates of the Ω⁢(2012)⁻ → $$Ξ⁢\bar{\textrm{K}}$$ branching ratios. These results will improve our understanding of the internal structure and mass spectrum of excited baryon states and serve as a baseline for searches regarding modifications of these properties in high-temperature media.« less

We present the measurement of the 𝑝_T-differential production cross section of 𝜔 mesons in pp and 𝑝-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV at midrapidity by ALICE. In addition, the first measurement of the nuclear modification factor 𝑅_𝑝⁢Pb for 𝜔 mesons at LHC energies is presented, complementing the existing measurements of lighter neutral mesons such as the 𝜋⁰ and 𝜂. Within the measured 𝑝_T range, the 𝑅_𝑝⁢Pb of 𝜔 mesons is compatible with no cold nuclear matter effects within the uncertainties, consistent with previous measurements at lower energies. The 𝜔/𝜋⁰ ratio is presented for both collision systems, showing no collisionmore » system dependence within the uncertainties. The comparison to previously published 𝜔/𝜋⁰ ratios at lower and higher collision energies in pp collisions suggests a decreasing trend of the ratio above _𝑝T = 4 GeV/𝑐 with increasing collision energy. The data in both collision systems are compared to predictions from PYTHIA 8, EPOS LHC, and DPMJET event generators, revealing significant shortcomings in these models’ ability to describe the production of 𝜔 mesons.« less

This paper presents the primary charged-particle multiplicity distributions in proton–lead collisions at a centre-of-mass energy per nucleon–nucleon collision of $$\sqrt{s_{NN}}$$ = 5.02 TeV. The distributions are reported for non-single diffractive collisions in different pseudorapidity ranges. The measurements are performed using the combined information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE. The multiplicity distributions are parametrised with a double negative binomial distribution function which provides satisfactory descriptions of the distributions for all the studied pseudorapidity intervals. The data are compared to models and analyzed quantitatively, evaluating the first four moments (mean, standard deviation, skewness, and kurtosis).more » The shape evolution of the measured multiplicity distributions is studied in terms of KNO variables and it is found that none of the considered models reproduces the measurements. This paper also reports on the average charged-particle multiplicity, normalised by the average number of participating nucleon pairs, as a function of the collision energy. The multiplicity results are then compared to measurements made in proton–proton and nucleus–nucleus collisions across a wide range of collision energies.« 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

Correlations between event-by-event fluctuations of anisotropic flow harmonics are measured in Pb-Pb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV, as recorded by the ALICE detector at the LHC. This study presents correlations up to the hexagonal flow harmonic 𝑣₆, which was measured for the first time. The magnitudes of these higher-order correlations are found to vary as a function of collision centrality and harmonic order. These measurements are compared to viscous hydrodynamic model calculations with EKRT initial conditions and to the iEBE-VISHNU model with T_R⁢ENTo initial conditions. The observed discrepancies between the data and the modelmore » calculations vary depending on the harmonic combinations. Due to the sensitivity of model parameters estimated with Bayesian analyses to these higher-order observables, the results presented in this work provide new and independent constraints on the initial conditions and transport properties in theoretical models used to describe the system created in heavy-ion collisions.« 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 momentum-differential invariant cross sections of π⁰ and η mesons are reported for pp collisions at $$\sqrt{s}$$ = 13 TeV at midrapidity (|y| < 0.8). The measurement is performed in a broad transverse-momentum range of 0.2 < p_T < 200 GeV/c and 0.4 < p_T < 60 GeV/c for the π⁰ and η, respectively, extending the p_T coverage of previous measurements. Transverse-mass-scaling violation of up to 60% at low transverse momentum has been observed, agreeing with measurements at lower collision energies. Transverse Bjorken x (x_T) scaling of the π⁰ cross sections at LHC energies is fulfilled with a power-law exponentmore » of n = 5.01 ± 0.05, consistent with values obtained for charged pions at similar collision energies. The data are compared to predictions from next-to-leading order perturbative QCD calculations, where the π⁰ spectrum is best described using the CT18 parton distribution function and the NNFF1.0 or BDSS fragmentation function. Expectations from PYTHIA8 and EPOS LHC overestimate the spectrum for the π⁰ and are not able to describe the shape and magnitude of the η spectrum. The charged-particle multiplicity dependent π⁰ and η p_T spectra show the expected change of the spectral shape, characterized by a flatter slope with increasing multiplicity. This is demonstrated across a broad transverse-momentum range and up to events with a charged-particle multiplicity exceeding five times the mean value in minimum bias collisions. The η/π⁰ ratio depends on the charged-particle multiplicity for p_T < 4 GeV/c. PYTHIA8 and EPOS LHC qualitatively explain this behavior with an increasing contribution from the feed-down of heavier particles to the π⁰ spectrum.« less

The p_T-differential cross section of ω meson production in pp collisions at $$\sqrt{s}$$ =13 TeV at midrapidity (|y| < 0.5) was measured with the ALICE detector at the LHC, covering an unprecedented transverse-momentum range of 1.6 < p_T < 50 GeV/c. The meson is reconstructed via the ω → π⁺π⁻π⁰ decay channel. The results are compared with various theoretical calculations: PYTHIA8.2 with the Monash 2013 tune overestimates the data by up to 50%, whereas good agreement is observed with Next-to-Leading Order (NLO) calculations incorporating ω fragmentation using a broken SU(3) model. The ω/π⁰ ratio is presented and compared with theoreticalmore » calculations and the available measurements at lower collision energies. The presented data triples the p_T ranges of previously available measurements. A constant ratio of C^ω/π0 = 0.578 ± 0.006 (stat.) ± 0.013 (syst.) is found above a transverse momentum of 4 GeV/c, which is in agreement with previous findings at lower collision energies within the systematic and statistical uncertainties.« less