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 $0.3\le p<4\mathrm{GeV}/c$. The measurement is carried out using $p$-Pb collisions at a center-of-mass energy per nucleon–nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, 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 $0.3\le p<0.47\mathrm{GeV}/c$ 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.

Figure 1

Raw primary $\overline{p}/p$ (left) and $\overline{d}/d$ (right) ratios as a function of the momentum $p_{\text{primary}}$. Experimental data are shown in blue, and the statistical and systematic uncertainties are shown as vertical bars and boxes. The results from ALICE MC simulations based on geant4 using the FTFP_INCLXX_EMV physics list are shown in black. The width of the MC band represents the statistical uncertainty of the simulation. The global uncertainty due to the primordial ratio (1.5% for $\overline{p}/p$ and 3% for $\overline{d}/d$) is not shown in the top panels. The bottom panels display the ratios of experimental data to MC simulations with statistical, systematic and global uncertainties added in quadrature.

Figure 2

Raw primary $\overline{p}/p$ ratio as a function of momentum. Blue boxes indicate $\pm 1\sigma$ experimental limits. The results from MC simulations with varied ${\sigma }_{\mathrm{inel}}(\overline{p})$ are shown as green and magenta bands, and the gray band corresponds to the results with default ${\sigma }_{\mathrm{inel}}(\overline{p})$. The uncertainties on MC results include the variations of elastic cross sections and the variation of ${\sigma }_{\mathrm{inel}}(p)$.

Figure 3

Inelastic interaction cross section for antiprotons and antideuterons on an average material element of the ALICE detector as a function of the momentum $p$, at which the interaction occurs. The top row shows the results for antiprotons, the bottom row for antideuterons, and the results from the $\mathrm{ITS}+\mathrm{TPC}$ ($\mathrm{ITS}+\mathrm{TPC}+\mathrm{TOF}$) analysis are shown on the left (right). Dashed black lines represent the geant4 parametrizations for antinuclei, and full gray lines show the parametrizations for protons and deuterons. The experimental data points are shown connected by solid black lines, with green and orange bands corresponding to $\pm 1$ and $\pm 2\sigma$ constraints from the raw primary ratios.