114 research outputs found
Measurement of the Groomed Jet Radius and Momentum Splitting Fraction in pp and Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV
No full textThis article presents groomed jet substructure measurements in pp and Pb-Pb collisions at root s(NN )= 5.02 TeV with the ALICE detector. The soft drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, z(g), and the (scaled) groomed jet radius, theta(g). Charged-particle jets are reconstructed at midrapidity using the anti-k(T) algorithm with resolution parameters R = 0.2 and R = 0.4. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the theta(g) distribution in Pb-Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark-gluon plasma. No significant modification of the z(g) distribution in Pb-Pb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark-gluon plasma
Measurements of the groomed and ungroomed jet angularities in pp collisions at √s = 5.02 TeV
No full textThe 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 = 5.02 TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity (|η| < 0.9). The anti-kT algorithm is used with jet resolution parameters R = 0.2 and R = 0.4 for several transverse momentum pTchjet 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, λα, and groomed jet angularities, λα,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. [Figure not available: see fulltext.
Measurements of the groomed and ungroomed jet angularities in pp collisions at s = 5.02 TeV
No full textThe 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
= 5.02 TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity (|η| < 0.9). The anti-kT algorithm is used with jet resolution parameters R = 0.2 and R = 0.4 for several transverse momentum
p
T
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, λα, and groomed jet angularities, λα,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
Publisher Correction: Direct observation of the dead-cone effect in quantum chromodynamics (Nature, (2022), 605, 7910, (440-446), 10.1038/s41586-022-04572-w)
No full textIn the version of this article initially published, there was a typographical error in the first sentence following the “Exposing the dead cone” heading, now reading, “The measurements of R(θ), in the three radiator (charmquark) energy intervals 5 &lt; ERadiator &lt; 10 GeV, 10 &lt; ERadiator &lt; 20 GeV and 20 &lt; ERadiator &lt; 35 GeV…,” where “35 GeV” initially appeared as “3 GeV.” The error has been corrected in the HTML and PDF versions of the article. *A list of authors and their affiliations appears online. © The Author(s) 2022
Measurement of the groomed jet radius and momentum splitting fraction in pp and PbPb collisions at TeV
No full textThis article presents groomed jet substructure measurements in pp and PbPb collisions at TeV with the ALICE detector. The Soft Drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, , and the (scaled) groomed jet radius, . Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with resolution parameters and . In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the distribution in PbPb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quarkgluon plasma. No significant modification of the distribution in PbPb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quarkgluon plasma.This article presents groomed jet substructure measurements in pp and Pb-Pb collisions at sNN=5.02 TeV with the ALICE detector. The soft drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, zg, and the (scaled) groomed jet radius, θg. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with resolution parameters R=0.2 and R=0.4. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the θg distribution in Pb-Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark-gluon plasma. No significant modification of the zg distribution in Pb-Pb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark-gluon plasma.This article presents groomed jet substructure measurements in pp and PbPb collisions at TeV with the ALICE detector. The Soft Drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, , and the (scaled) groomed jet radius, . Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with resolution parameters and . In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the distribution in PbPb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quarkgluon plasma. No significant modification of the distribution in PbPb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quarkgluon plasma
Experimental Evidence for an Attractive p-phi Interaction
No full textThis Letter presents the first experimental evidence of the attractive strong interaction between a proton and a phi meson. The result is obtained from two-particle correlations of combined p-phi circle plus (p) over bar-phi pairs measured in high-multiplicity pp collisions at root s = 13 TeV by the ALICE Collaboration. The spin-averaged scattering length and effective range of the p-phi interaction are extracted from the fully corrected correlation function employing the Lednicky-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the p-phi interaction. These data demonstrate that the interaction is dominated by elastic p-phi scattering. Furthermore, an analysis employing phenomenological Gaussian-and Yukawa-type potentials is conducted. Under the assumption of the latter, the N-phi coupling constant is found to be g(N-phi) = 0.14 +/- 0.03(stat) +/- 0.02(syst). This work provides valuable experimental input to accomplish a self-consistent description of the N-phi interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium
Polarization of Λ and Λ¯ Hyperons along the Beam Direction in Pb-Pb Collisions at sNN=5.02 TeV
No full textThe polarization of the Λ and Λ[over ¯] hyperons along the beam (z) direction, P_{z}, has been measured in Pb-Pb collisions at sqrt[s_{NN}]=5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to P_{z} comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient P_{z,s2}=⟨P_{z}sin(2φ-2Ψ_{2})⟩, where φ is the hyperon azimuthal emission angle and Ψ_{2} is the elliptic flow plane angle. We report the measurement of P_{z,s2} for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P_{z,s2} is positive similarly as measured by the STAR Collaboration in Au-Au collisions at sqrt[s_{NN}]=200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon P_{z} in Pb-Pb collisions at the LHC. The comparison of the measured P_{z,s2} with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase
Experimental Evidence for an Attractive p−φ Interaction
No full textThis Letter presents the first experimental evidence of the attractive strong interaction between a proton and a φ meson. The result is obtained from two-particle correlations of combined p−φ⊕ ̄p−φ pairs measured in high-multiplicity pp collisions at √s=13 TeV by the ALICE Collaboration. The spin-averaged scattering length and effective range of the p−φ interaction are extracted from the fully corrected correlation function employing the Lednický-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the p−φ interaction. These data demonstrate that the interaction is dominated by elastic p−φ scattering. Furthermore, an analysis employing phenomenological Gaussian- and Yukawa-type potentials is conducted. Under the assumption of the latter, the N−φ coupling constant is found to be gN−φ=0.14±0.03(stat)±0.02(syst). This work provides valuable experimental input to accomplish a self-consistent description of the N−φ interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium
Hypertriton Production in p-Pb Collisions at √sNN = 5.02 TeV
No full textThe study of nuclei and antinuclei production has proven to be a powerful
tool to investigate the formation mechanism of loosely bound states in
high-energy hadronic collisions. The first measurement of the production of
in p-Pb collisions at = 5.02
TeV is presented in this Letter. Its production yield measured in the rapidity
interval -1 < y < 0 for the 40% highest multiplicity p-Pb collisions is . The measurement is compared with the expectations of statistical
hadronisation and coalescence models, which describe the nucleosynthesis in
hadronic collisions. These two models predict very different yields of the
hypertriton in small collision systems such as p-Pb and therefore the
measurement of is crucial to distinguish between them.
The precision of this measurement leads to the exclusion with a significance
larger than 6 of some configurations of the statistical hadronisation,
thus constraining the production mechanism of loosely bound states
Extending the ALICE strong-interaction studies to nuclei: measurement of proton-deuteron correlations in pp collisions at √s = 13 TeV
No full textThe large data sample of high-multiplicity pp collisions collected by ALICE allows for the precise measurement of the size of source producing primary hadrons, opening the doors to a study of the interaction of different hadron species using femtoscopy techniques. The momentum correlation between (anti)protons and (anti)deuterons measured in pp collisions at √s = 13 TeV with ALICE is studied here for the first time. The measured correlation function for ((equation presented))p-((equation presented))d pairs is compared with theoretical predictions obtained considering Coulomb and Coulomb plus strong interactions and employing the Lednický-Lyuboshitz model with scattering parameters extracted from traditional scattering experiments for the p-d system. The measured correlation function can not be reproduced by any of the obtained predictions. This deviation can to large extent be interpreted as a demonstration of the late formation time of (anti)deuterons in hadron-hadron collisions. This observation is key for the understanding of the production mechanism of light (anti)nuclei, which is an open issue in high-energy physics and has also important consequences for the study of antinuclei formation in the interstellar medium either from collisions triggered by high-energy cosmic rays or by dark matter decays
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