8 Search Results

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

Event-by-event fluctuations of the event-wise mean transverse momentum, $$\langle$$p_T$$\rangle$$, of charged particles produced in proton–proton (pp) collisions at $$\sqrt{s}$$ = 5.02 TeV, Xe–Xe collisions at $$\sqrt{s_{NN}}$$ = 5.44 TeV, and Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV are studied using the ALICE detector based on the integral correlator $$\langle$$$$\langle$$Δp_TΔp_T$$\rangle$$$$\rangle$$. The correlator strength is found to decrease monotonically with increasing produced charged-particle multiplicity measured at midrapidity in all three systems. In Xe–Xe and Pb–Pb collisions, the multiplicity dependence of the correlator deviates significantly from a simple power-law scaling as well as from the predictions of the HIJING and AMPT models. Themore » observed deviation from power-law scaling is expected from transverse radial flow in semicentral to central Xe–Xe and Pb–Pb collisions. In pp collisions, the correlation strength is also studied by classifying the events based on the transverse spherocity, S₀, of the particle production at midrapidity, used as a proxy for the presence of a pronounced back-to-back jet topology. Low-spherocity (jetty) events feature a larger correlation strength than those with high spherocity (isotropic). The strength and multiplicity dependence of jetty and isotropic events are well reproduced by calculations with the PYTHIA 8 and EPOS LHC models.« 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 transverse momentum (p_T) differential production cross section of the promptly produced charm-strange baryon $$\Xi ^0_{\textrm{c}}$$ (and its charge conjugate $$\Xi ^0_{\textrm{c}}$$) is measured at midrapidity via its hadronic decay into π⁺$$\Xi$$^– in p–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision $$\sqrt{s_{{\textrm{NN}}}}=5.02$$ TeV with the ALICE detector at the LHC. The $$\Xi ^0_{\textrm{c}}$$ nuclear modification factor (R_pPb), calculated from the cross sections in pp and p–Pb collisions, is presented and compared with the R_pPb of Λ$$^{+}_{c}$$ baryons. The ratios between the p_T-differential production cross section of $$\Xi ^0_{\textrm{c}}$$ baryons and those of D⁰ mesons and Λ$$^{+}_{c}$$ baryons are alsomore » reported and compared with results at forward and backward rapidity from the LHCb Collaboration. The measurements of the production cross section of prompt $$\Xi ^0_{\textrm{c}}$$ baryons are compared with a model based on perturbative QCD calculations of charm-quark production cross sections, which includes only cold nuclear matter effects in p–Pb collisions, and underestimates the measurement by a factor of about 50. This discrepancy is reduced when the data is compared with a model that includes string formation beyond leading-colour approximation or in which hadronisation is implemented via quark coalescence. The p_T-integrated cross section of prompt $$\Xi ^0_{\textrm{c}}$$-baryon production at midrapidity extrapolated down to p_T = 0 is also reported. These measurements offer insights and constraints for theoretical calculations of the hadronisation process. Additionally, they provide inputs for the calculation of the charm production cross section in p–Pb collisions at midrapidity.« 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