Boosting through the Darkness : Bounds on boosted dark matter from direct detection Debjyoti Bardhan, 1, ∗ Supritha Bhowmick, 1, † Diptimoy Ghosh, 1, ‡ Atanu Guha, 1, 2 and Divya Sachdeva 3, § 1 Department of Physics, Indian Institute of Science Education and Research Pune, India 2 Department of Physics, Chungnam National University, South Korea ¶ 3 Laboratoire de Physique Th´ eorique et Hautes ´ Energies (LPTHE), UMR 7589 CNRS and Sorbonne Universit´ e, 4 Place Jussieu, F-75252, Paris, France The recoil threshold of Direct Detection (DD) experiments limits the mass range of Dark Matter (DM) particles that can be detected, with most DD experiments being blind to sub-MeV DM particles. However, these light DM particles can be boosted to very high energies via collisions with energetic Cosmic Ray electrons. This allows Dark Matter particles to induce detectable recoil in the target of Direct Detection experiments. We derive constraints on scattering cross section of DM and electron, using XenonnT and Super-Kamiokande data. Vector and scalar mediators are considered, in the heavy and light regimes. We discuss the importance of including energy dependent cross sections (due to specific Lorentz structure of the vertex) in our analysis, and show that the bounds can be significantly different than the results obtained assuming constant energy-independent cross-section, often assumed in the literature for simplicity. Our bounds are also compared with other astrophysical and cosmological constraints. I. INTRODUCTION One of the strongest indicators of Physics Beyond the Standard Model (BSM) is Dark Matter (DM). Its ex- istence can be inferred from diverse observations like galaxy rotation curves, cosmic microwave background radiation (CMBR) and gravitational lensing [1–3]. Ex- pectedly, massive experimental and observational efforts have been undertaken to understand its composition and interactions. Moreover, details of structure forma- tion constrain the type of DM and we know that it is only cold dark matter (CDM) which fits all the evidence. However, these observations remain mute about the ex- act composition of DM and the interactions it has with itself and SM particles besides gravitation. Experiments aimed at investigating particle nature of DM are divided into two categories - indirect detection and direct detection (DD) experiments. Indirect detec- tion experiments [4] focus on the study of signatures of the creation or annihilation of DM. Annihilation or de- cay of DM might produce excess photons in a certain mass window over the SM background, from which the mass of the DM can be inferred. The obvious challenge in this methodology is the very low signal production, which can be difficult to distinguish over the SM back- ground, not to mention the difficulty in modelling the SM photon background in the first place. The basic idea of DD experiments is that DM particles impinge on a detector and transfer a part of their kinetic energy to the target. The rate of such scattering events in a cer- tain recoil energy bin yields DM interaction cross section * debjyoti.bardhan@acads.iiserpune.ac.in † supritha.bhowmick@students.iiserpune.ac.in ‡ diptimoy.ghosh@iiserpune.ac.in § dsachdeva@lpthe.jussieu.fr ¶ atanu@cnu.ac.kr bounds. Despite intense efforts on DD experiments all around the globe, the search for DM has been fruitless. Some experiments have seen tantalising hints [5, 6], but nothing definitive has come of those [7]. The average velocity of DM particles in the solar neighbourhood is v/c ∼ 10 −3 which limits the energy to be deposited in a detector. Therefore, scattering in the direct detection is assumed to be non-relativistic (NR). With detectors like Xenon1T that has a minimum electronic recoil energy threshold of ∼O(1 keV), the smallest accessible DM mass (m χ ) is m χ ∼O(1 MeV). For Super-Kamiokande (Super-K), which has a min- imum recoil energy threshold of ∼O(1 MeV), the small- est accessible DM mass is ∼O(1 GeV) 1 . These detec- tors cannot access lighter DM particles in this scenario. However, as DM particles interact with cosmic rays (CR), it is inevitable that some DM particles will be boosted due to scattering by energetic CR particles [11– 23]. In this study we focus on boosting of DM parti- cles by CR electrons only; for boosting of DM by CR nucleons and neutrinos, refer to [11, 13, 23–26]. Since boosted particles can carry large amounts of kinetic en- ergy, even very light DM particles can deposit a recoil energy E R >E c in a detector, where E c is the lower detector threshold. Thus, direct detection detectors as well as neutrino experiments can become sensitive to very low mass DM. However, the sensitivity at lower DM masses is achieved at larger cross sections because the upscattered subcomponent flux is substantially lower than the galactic DM population. Note that CRe is one of sources of boosting DM particles among others, such as blazars [27, 28], helium nuclei [11], Diffuse Supernova 1 An exception to this occurs in fermionic DM absorption mod- els, for which Xenon-1T can probe DM masses down to ∼ O(10 keV) and Super-K can probe masses of ∼O(1 MeV) [8– 10] arXiv:2208.09405v3 [hep-ph] 14 Nov 2022