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  1. Free, publicly-accessible full text available March 1, 2024
  2. Abstract Three-body nuclear forces play an important role in the structure of nuclei and hypernuclei and are also incorporated in models to describe the dynamics of dense baryonic matter, such as in neutron stars. So far, only indirect measurements anchored to the binding energies of nuclei can be used to constrain the three-nucleon force, and if hyperons are considered, the scarce data on hypernuclei impose only weak constraints on the three-body forces. In this work, we present the first direct measurement of the p–p–p and p–p– $$\Lambda $$ Λ systems in terms of three-particle correlation functions carried out for pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV. Three-particle cumulants are extracted from the correlation functions by applying the Kubo formalism, where the three-particle interaction contribution to these correlations can be isolated after subtracting the known two-body interaction terms. A negative cumulant is found for the p–p–p system, hinting to the presence of a residual three-body effect while for p–p– $$\Lambda $$ Λ the cumulant is consistent with zero. This measurement demonstrates the accessibility of three-baryon correlations at the LHC. 
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    Free, publicly-accessible full text available July 1, 2024
  3. Free, publicly-accessible full text available February 1, 2024
  4. 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. 
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    Free, publicly-accessible full text available June 1, 2024
  5. A bstract The first measurement of the e + e − pair production at low lepton pair transverse momentum ( p T , ee ) and low invariant mass ( m ee ) in non-central Pb–Pb collisions at $$ {\sqrt{s}}_{\textrm{NN}} $$ s NN = 5 . 02 TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity ( |η e | < 0 . 8) as a function of invariant mass (0.4 ≤ m ee < 2 . 7 GeV/ c 2 ) in the 50–70% and 70–90% centrality classes for p T , ee < 0.1 GeV/ c , and as a function of p T , ee in three m ee intervals in the most peripheral Pb–Pb collisions. Below a p T , ee of 0.1 GeV/ c , a clear excess of e + e − pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The m ee excess spectra are reproduced, within uncertainties, by different predictions of the photon–photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the p T , ee spectra. The measured $$ \sqrt{\left\langle {p}_{\textrm{T},\textrm{ee}}^2\right\rangle } $$ p T , ee 2 of the excess p T , ee spectrum in peripheral Pb–Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region. 
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    Free, publicly-accessible full text available June 1, 2024
  6. A bstract The measurement of the production of charm jets, identified by the presence of a D 0 meson in the jet constituents, is presented in proton–proton collisions at centre-of-mass energies of $$ \sqrt{s} $$ s = 5.02 and 13 TeV with the ALICE detector at the CERN LHC. The D 0 mesons were reconstructed from their hadronic decay D 0 → K − π + and the respective charge conjugate. Jets were reconstructed from D 0 -meson candidates and charged particles using the anti- k T algorithm, in the jet transverse momentum range 5 < p T , chjet < 50 GeV/ c , pseudorapidity | η jet | < 0 . 9 − R , and with the jet resolution parameters R = 0 . 2 , 0 . 4 , 0 . 6. The distribution of the jet momentum fraction carried by a D 0 meson along the jet axis $$ \left({z}_{\Big\Vert}^{\textrm{ch}}\right) $$ z ‖ ch was measured in the range 0 . 4 < $$ {z}_{\Big\Vert}^{\textrm{ch}} $$ z ‖ ch < 1 . 0 in four ranges of the jet transverse momentum. Comparisons of results for different collision energies and jet resolution parameters are also presented. The measurements are compared to predictions from Monte Carlo event generators based on leading-order and next-to-leading-order perturbative quantum chromodynamics calculations. A generally good description of the main features of the data is obtained in spite of a few discrepancies at low p T , chjet . Measurements were also done for R = 0 . 3 at $$ \sqrt{s} $$ s = 5 . 02 and are shown along with their comparisons to theoretical predictions in an appendix to this paper. 
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    Free, publicly-accessible full text available June 1, 2024
  7. Abstract The transverse-momentum $$(p_{\textrm{T}})$$ ( p T ) spectra of K $$^{*}(892)^{0}~$$ ∗ ( 892 ) 0 and $$\mathrm {\phi (1020)}~$$ ϕ ( 1020 ) measured with the ALICE detector up to $$p_{\textrm{T}} $$ p T  = 16 GeV/ c in the rapidity range $$-1.2< y < 0.3,$$ - 1.2 < y < 0.3 , in p–Pb collisions at the center-of-mass energy per nucleon–nucleon collision $$\sqrt{s_{\textrm{NN}}} = 5.02$$ s NN = 5.02  TeV are presented as a function of charged particle multiplicity and rapidity. The measured $$p_{\textrm{T}} $$ p T distributions show a dependence on both multiplicity and rapidity at low $$p_{\textrm{T}} $$ p T whereas no significant dependence is observed at high $$p_{\textrm{T}} $$ p T . A rapidity dependence is observed in the $$p_{\textrm{T}} $$ p T -integrated yield (d N /d y ), whereas the mean transverse momentum $$\left( \langle p_{\textrm{T}} \rangle \right) $$ ⟨ p T ⟩ shows a flat behavior as a function of rapidity. The rapidity asymmetry ( $$Y_{\textrm{asym}}$$ Y asym ) at low $$p_{\textrm{T}} $$ p T (< 5 GeV/ c ) is more significant for higher multiplicity classes. At high $$p_{\textrm{T}} $$ p T , no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K $$^{*}(892)^{0}~$$ ∗ ( 892 ) 0 and $$\mathrm {\phi (1020)}~$$ ϕ ( 1020 ) show similar $$Y_{\textrm{asym}}$$ Y asym . The nuclear modification factor $$(Q_{\textrm{CP}})$$ ( Q CP ) as a function of $$p_{\textrm{T}} $$ p T shows a Cronin-like enhancement at intermediate $$p_{\textrm{T}} $$ p T , which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high $$p_{\textrm{T}}$$ p T (> 5 GeV/ $$c$$ c ), the $$Q_{\textrm{CP}}$$ Q CP values are greater than unity and no significant rapidity dependence is observed. 
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    Free, publicly-accessible full text available June 1, 2024
  8. A bstract Production of inclusive charmonia in pp collisions at center-of-mass energy of $$ \sqrt{s} $$ s = 13 TeV and p–Pb collisions at center-of-mass energy per nucleon pair of $$ \sqrt{s_{\textrm{NN}}} $$ s NN = 8 . 16 TeV is studied as a function of charged-particle pseudorapidity density with ALICE. Ground and excited charmonium states ( J/ψ , ψ (2S)) are measured from their dimuon decays in the interval of rapidity in the center-of-mass frame 2 . 5 < y cms < 4 . 0 for pp collisions, and 2 . 03 < y cms < 3 . 53 and −4 . 46 < y cms < −2 . 96 for p–Pb collisions. The charged-particle pseudorapidity density is measured around midrapidity (| η | < 1 . 0). In pp collisions, the measured charged-particle multiplicity extends to about six times the average value, while in p-Pb collisions at forward (backward) rapidity a multiplicity corresponding to about three (four) times the average is reached. The ψ (2S) yield increases with the charged-particle pseudorapidity density. The ratio of ψ (2S) over J/ψ yield does not show a significant multiplicity dependence in either colliding system, suggesting a similar behavior of J/ψ and ψ (2S) yields with respect to charged-particle pseudorapidity density. Results for the ψ (2S) yield and its ratio with respect to J/ψ agree with available model calculations. 
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    Free, publicly-accessible full text available June 1, 2024
  9. A bstract We report about the properties of the underlying event measured with ALICE at the LHC in pp and p–Pb collisions at $$ \sqrt{s_{\textrm{NN}}} $$ s 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 $$ \left({p}_{\textrm{T}}^{\textrm{trig}}\right) $$ p T trig . 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 $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ p T trig for low $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ p T trig , followed by a saturation at $$ {p}_{\textrm{T}}^{\textrm{trig}}\approx 5 $$ p T trig ≈ 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 $$ {p}_{\textrm{T}}^{\textrm{trig}}>10 $$ p T trig > 10 GeV/ c , whereas for lower $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ p T 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. 
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    Free, publicly-accessible full text available June 1, 2024
  10. Free, publicly-accessible full text available February 1, 2024