Archive | 2021
Muon $(g-2)$ and XENON1T Excess with Boosted Dark Matter in $L_{\\mu}-L_{\\tau}$ Model
Abstract
Motivated by the growing evidence for lepton flavour universality violation after the first results from Fermilab’s muon (g−2) measurement, we revisit one of the most widely studied anomaly free extensions of the standard model namely, gauged Lμ−Lτ model, known to be providing a natural explanation for muon (g − 2). We also incorporate the presence of dark matter (DM) in this model in order to explain the recently reported electron recoil excess by the XENON1T collaboration. We show that the same neutral gauge boson responsible for generating the required muon (g − 2) can also mediate interactions between electron and dark fermions boosted by dark matter annihilation. The required DM annihilation rate into dark fermion require a hybrid setup of thermal and non-thermal mechanisms to generate DM relic density. The tightly constrained parameter space from all requirements remain sensitive to ongoing and near future experiments, keeping the scenario very predictive. Introduction: The recent measurement of the muon anomalous magnetic moment, aμ = (g − 2)μ/2, by the E989 experiment at Fermilab shows a discrepancy with respect to the theoretical prediction of the Standard Model (SM) [1] a μ = 116592040(54)× 10−11 (1) a μ = 116591810(43)× 10−11 (2) which when combined with the previous Brookhaven determination of a μ = 116592089(63)× 10−11 (3) leads to a 4.2 σ observed excess of ∆aμ = 251(59)×10−11. The status of the SM calculation of muon magnetic moment has been updated recently in [2]1. For more details, one may refer to [4, 5]. The latest Fermilab measurements have also led to several recent works on updating possible theoretical models with new data. For example, see [6–12] for minimal dark matter (DM) motivated scenarios, [13–15] for axion like particle (ALP) motivated scenarios, [16, 17] for gauged lepton flavour models like U(1)Lμ−Lτ and [18–41] for other phenomenological scenarios like supersymmetry, multi-Higgs doublet models etc. and other implications of this new measurement. For a comprehensive review on new physics explanations of muon (g− 2) anomaly, please see [42]. Another evidence of such lepton flavour universality (LFU) violation, that too in the context of muon, comes from the measurement of RK = BR(B → Kμ+μ−)/BR(B → Ke+e−). While the hint for this anomaly, like muon (g − 2) was there for several years, recent update from the LHCb collaboration [43] has led to the most precise measurement ever ∗Electronic address: [email protected] †Electronic address: [email protected] ‡Electronic address: [email protected] §Electronic address: [email protected] 1 The latest lattice results [3] however, predict a larger value of muon (g − 2) bringing it closer to experimental value. with more than 3σ deviation from the SM predictions. In the light of growing evidences for such LFU violations, need for beyond standard model physics around the TeV corner has become very prominent. Another recent anomaly is the one reported by the XENON1T collaboration in 2020 related to their observation of an excess of electron recoil events over the background in the recoil energy Er in a range 1-7 keV, peaked around 2.4 keV[44]. Although solar axions and neutrinos with magnetic moment can explain the excess at 3.5σ and 3.2σ significance respectively, they are severely plagued by stellar cooling bounds. This has led to several interesting new physics explanations, see [45–65] and references therein. The DM interpretations out of these examples, typically have a light mediator via which DM interacts with electrons. The recoil can occur either due to light boosted DM or inelastic up or down-scattering [52– 61, 65–75, 75–77]. Here we consider the popular and minimal model based on the gauged Lμ−Lτ symmetry which is anomaly free [78, 79]. In earlier attempts to explain XENON1T excess with inelastic DM in gauged Lμ − Lτ model [53] which can also explain (g − 2)μ, only a tiny parameter space was allowed from all requirements even while considering a much larger error bars in (g − 2)μ namely ∆aμ = (27.9 ± 22.8) × 10−10, consistent with the 3.7σ discrepancy prior to the Fermilab measurement. As can be seen from [53], the main obstacle in satisfying both the excess is the constraint on heavier DM lifetime. To be more specific, in such scenarios, the heavier DM must be present in the universe at current epoch so that it can give rise to inelastic down-scattering at XENON1T detector. However, the same process responsible for such scattering also leads to heavier DM decay into lighter DM and SM particles leading to stringent constraints. Therefore, in this work, we consider a single component DM scenario which can annihilate into boosted lighter particles so that the latter can scatter off electron elastically, giving rise to the required excess. Boosted DM interpretation of XENON1T excess in the context of different models 1 ar X iv :2 10 4. 05 65 6v 2 [ he pph ] 7 A ug 2 02 1 have been discussed in [47–50, 74, 75, 75–77] 2. We study this possibility within the framework of gauged Lμ − Lτ model along with the possibility of explaining the muon (g−2) data. While we do not pursue the study of RK anomalies in this model, one may refer to [82] for common origin of muon (g− 2) and RK anomalies along with dark matter in extensions of minimal Lμ−Lτ model. Gauged Lμ − Lτ Symmetry: The SM fermion content with their gauge charges under SU(3)c × SU(2)L × U(1)Y × U(1)Lμ−Lτ gauge symmetry are denoted as follows.