Abstract We report about the properties of the underlying event measured with ALICE at the LHC in pp and p–Pb collisions at s NN $$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV. The event activity, ...quantified by charged-particle number and summed-p T densities, is measured as a function of the leading-particle transverse momentum p T trig $$ \left({p}_{\textrm{T}}^{\textrm{trig}}\right) $$ . These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different p T thresholds (0.15, 0.5 and 1 GeV/c) at mid-pseudorapidity (|η| < 0.8). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p–Pb collisions, namely, a steep increase with p T trig $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ for low p T trig $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ , followed by a saturation at p T trig ≈ 5 $$ {p}_{\textrm{T}}^{\textrm{trig}}\approx 5 $$ GeV/c. The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p–Pb collisions for p T trig > 10 $$ {p}_{\textrm{T}}^{\textrm{trig}}>10 $$ GeV/c, whereas for lower p T trig $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ values the event activity is slightly higher in p–Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.
Abstract 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 Large Hadron Collider ...(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 T algorithm with jet radius R = 0.4, and subjets are reconstructed by reclustering the jet constituents using the anti-k T algorithm with radii r = 0.1 and r = 0.2. In proton–proton 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 frag- mentation 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 → 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.
Abstract The transverse momentum ($$p_\mathrm{T} $$ pT ) spectra and elliptic flow coefficient ($$v_{2}$$ v2 ) of deuterons and anti-deuterons at mid-rapidity ($$|y|<0.5$$ |y|<0.5 ) are measured with ...the ALICE detector at the LHC in Pb–Pb collisions at $$\sqrt{s_{\mathrm {NN}}}$$ sNN = 2.76 TeV. The measurement of the $$p_\mathrm{T} $$ pT spectra of (anti-)deuterons is done up to 8 GeV$$/c$$ /c in 0–10% centrality class and up to 6 GeV$$/c$$ /c in 10–20% and 20–40% centrality classes. The $$v_{2}$$ v2 is measured in the 0.8 < $$p_\mathrm{T} $$ pT $$<~$$ < 5 GeV$$/c$$ /c interval and in six different centrality intervals (0–5, 5–10, 10–20, 20–30, 30–40 and 40–50%) using the scalar product technique. Measured $$\pi $$ π $$^{\pm }$$ ± , K$$^{\pm }$$ ± and p+$$\overline{\mathrm {p}}$$ p¯ transverse-momentum spectra and $$v_{2}$$ v2 are used to predict the deuteron $$p_\mathrm{T} $$ pT spectra and $$v_{2}$$ v2 within the Blast-Wave model. The predictions are able to reproduce the $$v_{2}$$ v2 coefficient in the measured $$p_\mathrm{T} $$ pT range and the transverse-momentum spectra for $$p_\mathrm{T} $$ pT > 1.8 GeV$$/c$$ /c within the experimental uncertainties. The measurement of the coalescence parameter $$B_2$$ B2 is performed, showing a $$p_\mathrm{T} $$ pT dependence in contrast with the simplest coalescence model, which fails to reproduce also the measured $$v_{2}$$ v2 coefficient. In addition, the coalescence parameter $$B_2$$ B2 and the elliptic flow coefficient in the 20–40% centrality interval are compared with the AMPT model which is able, in its version without string melting, to reproduce the measured $$v_{2}$$ v2 ($$p_\mathrm{T} $$ pT ) and the $$B_2$$ B2 ($$p_\mathrm{T} $$ pT ) trend.
Abstract This article reports on the inclusive production cross section of several quarkonium states, $$\textrm{J}/\psi $$ J / ψ , $$\psi \mathrm{(2S)}$$ ψ ( 2 S ) , $$\Upsilon \mathrm (1S)$$ Υ ( 1 S ...) , $$\Upsilon \mathrm{(2S)}$$ Υ ( 2 S ) , and $$\Upsilon \mathrm{(3S)}$$ Υ ( 3 S ) , measured with the ALICE detector at the LHC, in pp collisions at $$\sqrt{s} = 5.02$$ s = 5.02 TeV. The analysis is performed in the dimuon decay channel at forward rapidity ( $$2.5< y < 4$$ 2.5 < y < 4 ). The integrated cross sections and transverse-momentum ( $$p_{\textrm{T}}$$ p T ) and rapidity ( $$y$$ y ) differential cross sections for $$\textrm{J}/\psi $$ J / ψ , $$\psi \mathrm{(2S)}$$ ψ ( 2 S ) , $$\Upsilon \mathrm (1S)$$ Υ ( 1 S ) , and the $$\psi \mathrm{(2S)}$$ ψ ( 2 S ) -to- $$\textrm{J}/\psi $$ J / ψ cross section ratios are presented. The integrated cross sections, assuming unpolarized quarkonia, are: $$\sigma _{\textrm{J}/\psi }$$ σ J / ψ ( $$p_{\textrm{T}} <20$$ p T < 20 GeV/c) = 5.88 ± 0.03 ± 0.34 $$ ~\mu $$ μ b, $$\sigma _{\psi \mathrm{(2S)}}$$ σ ψ ( 2 S ) ( $$p_{\textrm{T}} <12$$ p T < 12 GeV/c) = 0.87 ± 0.06 ± 0.10 $$~\mu $$ μ b, $$\sigma _{\Upsilon \mathrm (1S)}$$ σ Υ ( 1 S ) ( $$p_{\textrm{T}} <15$$ p T < 15 GeV/c) = 45.5 ± 3.9 ± 3.5 nb, $$\sigma _{\Upsilon \mathrm{(2S)}}$$ σ Υ ( 2 S ) ( $$p_{\textrm{T}} <15$$ p T < 15 GeV/c) = 22.4 ± 3.2 ± 2.7 nb, and $$\sigma _{\Upsilon \mathrm{(3S)}}$$ σ Υ ( 3 S ) ( $$p_{\textrm{T}} <15$$ p T < 15 GeV/c) = 4.9 ± 2.2 ± 1.0 nb, where the first (second) uncertainty is the statistical (systematic) one. For the first time, the cross sections of the three $$\Upsilon $$ Υ states, as well as the $$\psi \mathrm{(2S)}$$ ψ ( 2 S ) one as a function of $$p_{\textrm{T}}$$ p T and $$y$$ y , are measured at $$\sqrt{s} = 5.02$$ s = 5.02 TeV at forward rapidity. These measurements also significantly extend the $$\textrm{J}/\psi $$ J / ψ $$p_{\textrm{T}}$$ p T reach and supersede previously published results. A comparison with ALICE measurements in pp collisions at $$\sqrt{s} = 2.76$$ s = 2.76 , 7, 8, and 13 TeV is presented and the energy dependence of quarkonium production cross sections is discussed. Finally, the results are compared with the predictions from several production models.
Abstract The production of non-prompt D0 mesons from beauty-hadron decays was measured at midrapidity (|y| < 0.5) in Pb-Pb collisions at a nucleon-nucleon center-of-mass energy of s NN $$ ...\sqrt{{\textrm{s}}_{\textrm{NN}}} $$ = 5.02 TeV with the ALICE experiment at the LHC. Their nuclear modification factor (R AA), measured for the first time down to p T = 1 GeV/c in the 0–10% and 30–50% centrality classes, indicates a significant suppression, up to a factor of about three, for p T > 5 GeV/c in the 0–10% central Pb-Pb collisions. The data are described by models that include both collisional and radiative processes in the calculation of beauty-quark energy loss in the quark-gluon plasma, and quark recombination in addition to fragmentation as a hadronisation mechanism. The ratio of the non-prompt to prompt D0-meson R AA is larger than unity for p T > 4 GeV/c in the 0–10% central Pb-Pb collisions, as predicted by models in which beauty quarks lose less energy than charm quarks in the quark-gluon plasma because of their larger mass.
Abstract We present results on transverse momentum ( $$p_{\mathrm {\textsc {t}}}$$ p T ) and rapidity ( $$y$$ y ) differential production cross sections, mean transverse momentum and mean transverse ...momentum square of inclusive $$\mathrm {J}/\psi $$ J / ψ and $$\psi \mathrm {(2S)}$$ ψ ( 2 S ) at forward rapidity ( $$2.5<y<4$$ 2.5 < y < 4 ) as well as $$\psi \mathrm {(2S)}$$ ψ ( 2 S ) -to- $$\mathrm {J}/\psi $$ J / ψ cross section ratios. These quantities are measured in pp collisions at center of mass energies $$\sqrt{s}\,=5.02$$ s = 5.02 and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at $$\sqrt{s}\,=2.76$$ s = 2.76 , 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full $$p_{\mathrm {\textsc {t}}}$$ p T range, provided that both contributions are summed. In particular, it is found that for $$p_{\mathrm {\textsc {t}}}>15$$ p T > 15 GeV/c the non-prompt contribution reaches up to 50% of the total charmonium yield.
Abstract The multiplicity dependence of jet production in pp collisions at the centre-of-mass energy of $$\sqrt{s} = 13\ {\mathrm {TeV}}$$ s = 13 TeV is studied for the first time. Jets are ...reconstructed from charged particles using the anti- $$k_\mathrm {T}$$ k T algorithm with resolution parameters R varying from 0.2 to 0.7. The jets are measured in the pseudorapidity range $$|\eta _{\mathrm{jet}}|< 0.9-R$$ | η jet | < 0.9 - R and in the transverse momentum range $$5<p_\mathrm {T,jet}^{\mathrm{ch}}<140\ {\mathrm {GeV}}/c$$ 5 < p T , jet ch < 140 GeV / c . The multiplicity intervals are categorised by the ALICE forward detector V0. The $$p_{\mathrm T}$$ p T differential cross section of charged-particle jets are compared to leading order (LO) and next-to-leading order (NLO) perturbative quantum chromodynamics (pQCD) calculations. It is found that the data are better described by the NLO calculation, although the NLO prediction overestimates the jet cross section below $$20\ {\mathrm {GeV}}/c$$ 20 GeV / c . The cross section ratios for different R are also measured and compared to model calculations. These measurements provide insights into the angular dependence of jet fragmentation. The jet yield increases with increasing self-normalised charged-particle multiplicity. This increase shows only a weak dependence on jet transverse momentum and resolution parameter at the highest multiplicity. While such behaviour is qualitatively described by the present version of PYTHIA, quantitative description may require implementing new mechanisms for multi-particle production in hadronic collisions.
Abstract A measurement of inclusive, prompt, and non-prompt J/ψ production in p-Pb collisions at a nucleon-nucleon centre-of-mass energy s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV is presented. The ...inclusive J/ψ mesons are reconstructed in the dielectron decay channel at midrapidity down to a transverse momentum p T = 0. The inclusive J/ψ nuclear modification factor R pPb is calculated by comparing the new results in p-Pb collisions to a recently measured proton-proton reference at the same centre-of-mass energy. Non-prompt J/ψ mesons, which originate from the decay of beauty hadrons, are separated from promptly produced J/ψ on a statistical basis for p T larger than 1.0 GeV/c. These results are based on the data sample collected by the ALICE detector during the 2016 LHC p-Pb run, corresponding to an integrated luminosity L $$ \mathcal{L} $$ int = 292 ± 11 μb −1, which is six times larger than the previous publications. The total uncertainty on the p T-integrated inclusive J/ψ and non-prompt J/ψ cross section are reduced by a factor 1.7 and 2.2, respectively. The measured cross sections and R pPb are compared with theoretical models that include various combinations of cold nuclear matter effects. From the non-prompt J/ψ production cross section, the b b ¯ $$ \mathrm{b}\overline{\mathrm{b}} $$ production cross section at midrapidity, d σ b b ¯ $$ {\mathrm{d}\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ /dy, and the total cross section extrapolated over full phase space, σ b b ¯ $$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ , are derived.
Abstract The production of J/ψ is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies s $$ \sqrt{s} $$ = 5.02 and ...13 TeV. The J/ψ mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5 < y < 4.0), whereas the charged-particle multiplicity density (dN ch /dη) is measured at midrapidity (|η| < 1). The production rate as a function of multiplicity is reported as the ratio of the yield in a given multiplicity interval to the multiplicity-integrated one. This observable shows a linear increase with charged-particle multiplicity normalized to the corresponding average value for inelastic events (dN ch/dη/〈dN ch/dη〉), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum (〈p T〉) of J/ψ in pp collisions exhibits an increasing trend as a function of dN ch/dη/〈dN ch/dη〉 showing a saturation towards high charged-particle multiplicities.
Abstract The jet angularities are a class of jet substructure observables which characterize the angular and momentum distribution of particles within jets. These observables are sensitive to ...momentum scales ranging from perturbative hard scatterings to nonperturbative fragmentation into final-state hadrons. We report measurements of several groomed and ungroomed jet angularities in pp collisions at s $$ \sqrt{s} $$ = 5.02 TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity (|η| < 0.9). The anti-k T algorithm is used with jet resolution parameters R = 0.2 and R = 0.4 for several transverse momentum p T ch $$ {p}_{\mathrm{T}}^{\mathrm{ch}} $$ jet intervals in the 20–100 GeV/c range. Using the jet grooming algorithm Soft Drop, the sensitivity to softer, wide-angle processes, as well as the underlying event, can be reduced in a way which is well-controlled in theoretical calculations. We report the ungroomed jet angularities, λ α , and groomed jet angularities, λ α,g, to investigate the interplay between perturbative and nonperturbative effects at low jet momenta. Various angular exponent parameters α = 1, 1.5, 2, and 3 are used to systematically vary the sensitivity of the observable to collinear and soft radiation. Results are compared to analytical predictions at next-to-leading-logarithmic accuracy, which provide a generally good description of the data in the perturbative regime but exhibit discrepancies in the nonperturbative regime. Moreover, these measurements serve as a baseline for future ones in heavy-ion collisions by providing new insight into the interplay between perturbative and nonperturbative effects in the angular and momentum substructure of jets. They supply crucial guidance on the selection of jet resolution parameter, jet transverse momentum, and angular scaling variable for jet quenching studies.
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