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Measurement of anti-3He nuclei absorption in matter and impact on their propagation in the Galaxy
(2022)
In our Galaxy, light antinuclei composed of antiprotons and antineutrons can be produced through high-energy cosmic-ray collisions with the interstellar medium or could also originate from the annihilation of dark-matter particles that have not yet been discovered. On Earth, the only way to produce and study antinuclei with high precision is to create them at high-energy particle accelerators. Although the properties of elementary antiparticles have been studied in detail, the knowledge of the interaction of light antinuclei with matter is limited. We determine the disappearance probability of 3He when it encounters matter particles and annihilates or disintegrates within the ALICE detector at the Large Hadron Collider. We extract the inelastic interaction cross section, which is then used as an input to the calculations of the transparency of our Galaxy to the propagation of 3He stemming from dark-matter annihilation and cosmic-ray interactions within the interstellar medium. For a specifc dark-matter profle, we estimate a transparency of about 50%, whereas it varies with increasing 3He momentum from 25% to 90% for cosmic-ray sources. The results indicate that 3He nuclei can travel long distances in the Galaxy, and can be used to study cosmic-ray interactions and dark-matter annihilation.
In our Galaxy, light antinuclei composed of antiprotons and antineutrons can be produced through high-energy cosmic-ray collisions with the interstellar medium or could also originate from the annihilation of dark-matter particles that have not yet been discovered. On Earth, the only way to produce and study antinuclei with high precision is to create them at high-energy particle accelerators. Although the properties of elementary antiparticles have been studied in detail, the knowledge of the interaction of light antinuclei with matter is limited. We determine the disappearance probability of 3He¯¯¯¯¯¯ when it encounters matter particles and annihilates or disintegrates within the ALICE detector at the Large Hadron Collider. We extract the inelastic interaction cross section, which is then used as input to calculations of the transparency of our Galaxy to the propagation of 3He¯¯¯¯¯¯ stemming from dark-matter annihilation and cosmic-ray interactions within the interstellar medium. For a specific dark-matter profile, we estimate a transparency of about 50%, whereas it varies with increasing 3He¯¯¯¯¯¯ momentum from 25% to 90% for cosmic-ray sources. The results indicate that 3He¯¯¯¯¯¯ nuclei can travel long distances in the Galaxy, and can be used to study cosmic-ray interactions and dark-matter annihilation.
Antimatter particles such as positrons and antiprotons abound in the cosmos. Much less common are light antinuclei, composed of antiprotons and antineutrons, which can be produced in our galaxy via high-energy cosmic-ray collisions with the interstellar medium or could also originate from the annihilation of the still undiscovered dark-matter particles. On Earth, the only way to produce and study antinuclei with high precision is to create them at high-energy particle accelerators like the Large Hadron Collider (LHC). Though the properties of elementary antiparticles have been studied in detail, knowledge of the interaction of light antinuclei with matter is rather limited. This work focuses on the determination of the disappearance probability of \ahe\ when it encounters matter particles and annihilates or disintegrates. The material of the ALICE detector at the LHC serves as a target to extract the inelastic cross section for \ahe\ in the momentum range of 1.17≤p<10 GeV/c. This inelastic cross section is measured for the first time and is used as an essential input to calculations of the transparency of our galaxy to the propagation of 3He¯¯¯¯¯¯ stemming from dark-matter decays and cosmic-ray interactions within the interstellar medium. A transparency of about 50% is estimated using the GALPROP program for a specific dark-matter profile and a standard set of propagation parameters. For cosmic-ray sources, the obtained transparency with the same propagation scheme varies with increasing 3He¯¯¯¯¯¯ momentum from 25% to 90%. The absolute uncertainties associated to the 3He¯¯¯¯¯¯ inelastic cross section measurements are of the order of 10%−15%. The reported results indicate that 3He¯¯¯¯¯¯ nuclei can travel long distances in the galaxy, and can be used to study cosmic-ray interactions and dark-matter decays.
Understanding the production mechanism of light (anti)nuclei is one of the key challenges of nuclear physics and has important consequences for astrophysics, since it provides an input for indirect dark-matter searches in space. In this paper, the latest results about the production of light (anti)nuclei in pp collisions at s√=13 TeV are presented, focusing on the comparison with the predictions of coalescence and thermal models. For the first time, the coalescence parameters B2 for deuterons and B3 for helions are compared with parameter-free theoretical predictions that are directly constrained by the femtoscopic measurement of the source radius in the same event class. A fair description of the data with a Gaussian wave function is observed for both deuteron and helion, supporting the coalescence mechanism for the production of light (anti)nuclei in pp collisions. This method paves the way for future investigations of the internal structure of more complex nuclear clusters, including the hypertriton.
Polarization of Λ and ¯Λ hyperons along the beam direction in Pb-Pb collisions at √sNN=5.02 TeV
(2022)
The polarization of the Λ and ¯Λ hyperons along the beam (z) direction, Pz, has been measured in Pb-Pb collisions at √sNN=5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to Pz comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient Pz,s2=⟨Pzsin(2φ−2Ψ2)⟩, where φ is thhyperon azimuthal emission angle and Ψ2 is the elliptic flow plane angle. We report the measurement of Pz,s2 for different collision centralities and in the 30%–50% centrality interval as a function of the hyperon transverse momentum and rapidity. The Pz,s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at √sNN=200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon Pz in Pb-Pb collisions at the LHC. The comparison of the measured Pz,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.
This article presents the first measurement of the interaction between charm hadrons and nucleons. The two-particle momentum correlations of pD− and ¯pD+ pairs are measured by the ALICE Collaboration in high-multiplicity pp collisions at √s=13 TeV. The data are compatible with the Coulomb-only interaction hypothesis within (1.1–1.5)σ. The level of agreement slightly improves if an attractive nucleon (N)¯D strong interaction is considered, in contrast to most model predictions which suggest an overall repulsive interaction. This measurement allows for the first time an estimation of the 68% confidence level interval for the isospin I=0 inverse scattering length of the N¯D state f−10, I=0∈[−0.4,0.9] fm−1, assuming negligible interaction for the isospin I=1 channel.
We report about the properties of the underlying event measured with ALICE at the LHC in pp and p−Pb collisions at sNN−−−√=5.02 TeV. The event activity, quantified by charged-particle number and summed-pT densities, is measured as a function of the leading-particle transverse momentum (ptrigT). 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 pT 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 ptrigT for low ptrigT, followed by a saturation at ptrigT≈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 ptrigT>10 GeV/c, whereas for lower ptrigT 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.
We report about the properties of the underlying event measured with ALICE at the LHC in pp and p−Pb collisions at sNN−−−√=5.02 TeV. The event activity, quantified by charged-particle number and summed-pT densities, is measured as a function of the leading-particle transverse momentum (ptrigT). 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 pT 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 ptrigT for low ptrigT, followed by a saturation at ptrigT≈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 ptrigT>10 GeV/c, whereas for lower ptrigT 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.
The production of π±, K±, and (p¯¯¯)p is measured in pp collisions at s√=13 TeV in different topological regions. Particle transverse momentum (pT) spectra are measured in the ``toward'', ``transverse'', and ``away'' angular regions defined with respect to the direction of the leading particle in the event. While the toward and away regions contain the fragmentation products of the near-side and away-side jets, respectively, the transverse region is dominated by particles from the Underlying Event (UE). The relative transverse activity classifier, RT=NT/⟨NT⟩, is used to group events according to their UE activity, where NT is the measured charged-particle multiplicity per event in the transverse region and ⟨NT⟩ is the mean value over all the analysed events. The first measurements of identified particle pT spectra as a function of RT in the three topological regions are reported. The yield of high transverse momentum particles relative to the RT-integrated measurement decreases with increasing RT in both the toward and away regions, indicating that the softer UE dominates particle production as RT increases and validating that RT can be used to control the magnitude of the UE. Conversely, the spectral shapes in the transverse region harden significantly with increasing RT. This hardening follows a mass ordering, being more significant for heavier particles. The pT-differential particle ratios (p+p¯¯¯)/(π++π−) and (K++K−)/(π++π−) in the low UE limit (RT→0) approach expectations from Monte Carlo generators such as PYTHIA 8 with Monash 2013 tune and EPOS LHC, where the jet-fragmentation models have been tuned to reproduce e+e− results.
The transverse-momentum (pT) spectra and coalescence parameters B2 of (anti)deuterons are measured in p-p collisions at √s=13 TeV for the first time in and out of jets. In this measurement, the direction of the leading particle with the highest pT in the event (p lead T>5 GeV/c) is used as an approximation for the jet axis. The event is consequently divided into three azimuthal regions, and the jet signal is obtained as the difference between the toward region, that contains jet fragmentation products in addition to the underlying event (UE), and the transverse region, which is dominated by the UE. The coalescence parameter in the jet is found to be approximately a factor of 10 larger than that in the underlying event. This experimental observation is consistent with the coalescence picture and can be attributed to the smaller average phase-space distance between nucleons in the jet cone as compared with the underlying event. The results presented in this Letter are compared to predictions from a simple nucleon coalescence model, where the phase-space distributions of nucleons are generated using pythia8 with the Monash 2013 tuning, and to predictions from a deuteron production model based on ordinary nuclear reactions with parametrized energy-dependent cross sections tuned on data. The latter model is implemented in pythia8.3. Both models reproduce the observed large difference between in-jet and out-of-jet coalescence parameters, although the almost flat trend of the BJet2 is not reproduced by the models, which instead give a decreasing trend.