First measurement of the absorption of anti-3He and anti-3H nuclei in matter and its impact on anti-3He propagation in the galaxy

Antinuclei in cosmic rays have long been considered a golden channel for indirect WIMP dark matter searches, since WIMPs are predicted to be able to annihilate to create antinuclei. They are considered such a promising probe because the expected antinuclei signal from dark matter at low kinetic energies exceeds the background ex- pected from other astrophysical sources by sever orders of magnitude. Indeed, only a single relevant background source is considered: the collision of high energy cosmic rays with the interstellar medium. Current generation experiments are reaching sensitivities which can probe optimistic models, and next generation experiments will be able to fully resolve any such signal, if it exists. In order to decode any information from such a signal, all effects acting on it must be understood, and the uncertainties on each of these effects must me known. The relevant processes are the production, propagation, and finally annihilation of these antinuclei. On earth, antinuclei are produced in high energy particle collisions at particle colliders. Due to their rarity, traditional fixed target experiments employed to measure the annihilation probabilities (called the inelastic cross section) of particles cannot be used for low energy antinuclei. The work presented in this thesis used a recently developed new experimental method to measure the inelastic cross sections of anti-3He and anti-3H for the first time, and used these measurements in order to infer the effect of annihilation on the expected antinuclei flux in cosmic rays. Furthermore, the same procedure for evaluating the effect of antinuclei inelastic cross sections on their propagation has been applied to antideuterons. In the course of this work, the uncertainties concerning the propagation and production of antinuclei have also been re-evaluated. The work carried out as part of my PhD has thus involved measuring the measure- ment of the inelastic cross sections of the A=3 antinuclei anti-3He and anti-3H , as well as using them in order to determine the experimental uncertainties on anti-3He and antideuteron fluxes due to annihilation, both for the first time

Measurement of the low-energy antideuteron inelastic cross section

In this Letter, we report the first measurement of the inelastic cross section for antideuteron-nucleus interactions at low particle momenta, covering a range of 0.3 ≤ p < 4 GeV/c. The measurement is carried out using p-Pb collisions at a center-of-mass energy per nucleon–nucleon pair of $\sqrt{s_{\rm NN}}$ = 5.02 TeV, recorded with the ALICE detector at the CERN LHC and utilizing the detector material as an absorber for antideuterons and antiprotons. The extracted raw primary antiparticle-to-particle ratios are compared to the results from detailed ALICE simulations based on the geant4 toolkit for the propagation of (anti)particles through the detector material. The analysis of the raw primary (anti)proton spectra serves as a benchmark for this study, since their hadronic interaction cross sections are well constrained experimentally. The first measurement of the inelastic cross section for antideuteron-nucleus interactions averaged over the ALICE detector material with atomic mass numbers ⟨A⟩ = 17.4 and 31.8 is obtained. The measured inelastic cross section points to a possible excess with respect to the Glauber model parametrization used in geant4 in the lowest momentum interval of 0.3 ≤ p < 0.47 GeV/c up to a factor 2.1. This result is relevant for the understanding of antimatter propagation and the contributions to antinuclei production from cosmic ray interactions within the interstellar medium. In addition, the momentum range covered by this measurement is of particular importance to evaluate signal predictions for indirect dark-matter searches

Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at sNN\sqrt {s_{NN}} = 5.02 TeV

The elliptic flow of electrons from beauty hadron decays at midrapidity (|y|<0.8) is measured in Pb-Pb collisions at $\sqrt {s_{NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, $v_2$. The $v_2$ coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum ($p_T$) range 1.3–6 GeV/c in the centrality class 30%–50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length cτ ≈ 500 μm compared to that of charm hadrons and most of the other background sources. The $v_2$ of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 σ. The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high $p_T$, but is in agreement with the results at low $p_T$. Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties

J/ψ\psi elliptic and triangular flow in Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 5.02 TeV

The inclusive J/ψ elliptic (v$_{2}$) and triangular (v$_{3}$) flow coefficients measured at forward rapidity (2.5 < y < 4) and the v$_{2}$ measured at midrapidity (|y| < 0.9) in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750 μb$^{−1}$ at forward rapidity and 93 μb$^{−1}$ at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum p$_{T}$ and collision centrality. At midrapidity, the J/ψ v$_{2}$ is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/ψ v$_{3}$ with a significance of more than 5σ at forward rapidity in the p$_{T}$ range 2 < p$_{T}$< 5 GeV/c. The forward rapidity v$_{2}$, v$_{3}$, and v$_{3}$/v$_{2}$ results at low and intermediate p$_{T}$ (p$_{T}$ ≲ 8 GeV/c) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher p$_{T}$. At low and intermediate p$_{T}$, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high p$_{T}$, path-length dependent effects seem to dominate. The J/ψ v$_{2}$ measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson v$_{n}$ measurements can be described based on those for charged pions and J/ψ flow

First measurement of quarkonium polarization in nuclear collisions at the LHC

International audienceThe polarization of inclusive J/ψ and ϒ(1S) produced in Pb–Pb collisions at sNN=5.02 TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region 2.5<y<4 and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters λθ , λϕ and λθϕ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval 2<pT<10 GeV/ c and pT<15 GeV/ c for the J/ψ and ϒ(1S) , respectively. The polarization parameters for the J/ψ are found to be compatible with zero, within a maximum of about two standard deviations at low pT , for both reference frames and over the whole pT range. The values are compared with the corresponding results obtained for pp collisions at s=7 and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for ϒ(1S) production in Pb–Pb collisions are also consistent with zero

Elliptic and triangular flow of (anti)deuterons in Pb-Pb collisions at sNN\sqrt{s_{\mathrm{NN}}} = 5.02 TeV

International audienceThe measurements of the (anti)deuteron elliptic flow (v2) and the first measurements of triangular flow (v3) in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon collision sNN = 5.02 TeV are presented. A mass ordering at low transverse momentum (pT) is observed when comparing these measurements with those of other identified hadrons, as expected from relativistic hydrodynamics. The measured (anti)deuteron v2 lies between the predictions from the simple coalescence and blast-wave models, which provide a good description of the data only for more peripheral and for more central collisions, respectively. The mass number scaling, which is violated for v2, is approximately valid for the (anti)deuterons v3. The measured v2 and v3 are also compared with the predictions from a coalescence approach with phase-space distributions of nucleons generated by iebe-vishnu with ampt initial conditions coupled with urqmd, and from a dynamical model based on relativistic hydrodynamics coupled to the hadronic afterburner smash. The model predictions are consistent with the data within the uncertainties in midcentral collisions, while a deviation is observed in the most central collisions

ΛK\Lambda\rm{K} femtoscopy in Pb-Pb collisions at sNN\sqrt{s_{\rm{NN}}} = 2.76 TeV

The first measurements of the scattering parameters of ΛK pairs in all three charge combinations (ΛK+, ΛK−, and ΛKS0) are presented. The results are achieved through a femtoscopic analysis of ΛK correlations in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV recorded by ALICE at the Large Hadron Collider. The femtoscopic correlations result from strong final-state interactions and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies with the THERMINATOR 2 event generator provide a good description of the nonfemtoscopic background, which results mainly from collective effects, with unprecedented precision. Furthermore, together with HIJING simulations, this model is used to account for contributions from residual correlations induced by feed-down from particle decays. The extracted scattering parameters indicate that the strong force is repulsive in the ΛK$^+$ interaction and attractive in the ΛK$^−$ interaction. The data hint that the ΛK$_S^0$ interaction is attractive; however, the uncertainty of the result does not permit such a decisive conclusion. The results suggest an effect arising either from different quark-antiquark interactions between the pairs (s$\bar{\rm s}$ in ΛK$^+$ and u$\bar{\rm u}$ in ΛK$^−$) or from different net strangeness for each system (S=0 for ΛK$^+$, and S=−2 for Λ$K^−$). Finally, the ΛK systems exhibit source radii larger than expected from extrapolation from identical particle femtoscopic studies. This effect is interpreted as resulting from the separation in space-time of the single-particle Λ and K source distributions

A new laboratory to study hadron-hadron interactions

One of the big challenges for nuclear physics today is to understand, starting from first principles, the effective interaction between hadrons with different quark content. First successes have been achieved utilizing techniques to solve the dynamics of quarks and gluons on discrete space-time lattices. Experimentally, the dynamics of the strong interaction have been studied by scattering hadrons off each other. Such scattering experiments are difficult or impossible for unstable hadrons and hence, high quality measurements exist only for hadrons containing up and down quarks. In this work, we demonstrate that measuring correlations in the momentum space between hadron pairs produced in ultrarelativistic proton-proton collisions at the CERN LHC provides a precise method to obtain the missing information on the interaction dynamics between any pair of unstable hadrons. Specifically, we discuss the case of the interaction of baryons containing strange quarks (hyperons). We demonstrate for the first time how, using precision measurements of p-$\Omega^{-}$ correlations, the effect of the strong interaction for this hadron-hadron pair can be studied and compared with predictions from lattice calculations

Measurement of isolated photon-hadron correlations in sNN\sqrt{s_{\rm{NN}}} = 5.02 TeV pppp and pp-Pb collisions

This paper presents isolated photon-hadron correlations using pp and p-Pb data collected by the ALICE detector at the LHC. For photons with |η| < 0.67 and 12 < $p_{\rm T}$ < 40 GeV/c, the associated yield of charged particles in the range |η| < 0.80 and 0.5 < $p_{\rm T}$ <10 GeV/c is presented. These momenta are much lower than previous measurements at the LHC. No significant difference between pp and p-Pb is observed, with Pythia 8.2 describing both data sets within uncertainties. This measurement constrains nuclear effects on the parton fragmentation in p-Pb collisions, and provides a benchmark for future studies of Pb-Pb collisions