TeV scale resonant leptogenesis with \nLμ−Lτ\n gauge symmetry in light of the muon \ng−2

Abstract

Motivated by the growing evidence for the possible 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, to find a common explanation for muon (g − 2) as well as baryon asymmetry of the universe via leptogenesis. The minimal setup allows TeV scale resonant leptogenesis satisfying light neutrino data while the existence of light Lμ − Lτ gauge boson affects the scale of leptogenesis as the right handed neutrinos are charged under it. For Lμ − Lτ gauge boson mass at GeV scale or above, the muon (g−2) favoured parameter space is already ruled out by other experimental data while bringing down its mass to sub-GeV regime leads to vanishing lepton asymmetry due to highly restrictive structures of lepton mass matrices at the scale of leptogenesis. Extending the minimal model with two additional Higgs doublets can lead to a scenario consistent with successful resonant leptogenesis and muon (g− 2) while satisfying all relevant experimental data. Introduction: The recent measurement of the muon anomalous magnetic moment, aμ = (g − 2)μ/2, by the E989 experiment at Fermilab for the first time 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 1. The status of the SM calculation of muon magnetic moment has been updated recently in [4]. For more details, one may refer to [5]. The latest Fermilab measurements have also led to several recent works on updating possible theoretical models with new data, a comprehensive review of which may be found in [6]. Gauged lepton flavour models like U(1)Lμ−Lτ provide a natural origin of muon (g−2) in a very minimal setup while also addressing the question of light neutrino mass simultaneously. Recent studies on this model related to muon (g−2) may be found in [7–11]. While this could be due to lepton flavour universality (LFU) violation, similar anomalies, 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 [12] has led to the most precise measurement ever ∗Electronic address: [email protected] †Electronic address: [email protected] ‡Electronic address: [email protected] 1 The latest lattice results [2] however, predict a larger value of muon (g−2) bringing it closer to experimental value. Tension of measured muon (g − 2) with global electroweak fits from e+e− to hadron data has also been reported in [3]. 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. Here we consider the popular and minimal model based on the gauged Lμ − Lτ symmetry which is anomaly free [13, 14]. Apart from the SM fermion content, the minimal version of this model has three heavy right handed neutrinos (RHN) leading to to type I seesaw origin of light neutrino masses [15–20]. The same RHNs can lead to leptogenesis [21, 22] via out-of-equilibrium decay into SM leptons. However, with hierarchical RHN spectrum, there exists a lower bound on the scale of leptogenesis, known as the Davidson-Ibarra boundM1 > 10 GeV [23]. Several earlier works [24–28] considered different scenarios like supersymmetry, high scale leptogenesis within type I seesaw framework of this model. However, it is also possible to have TeV scale leptogenesis via resonant enhancement due to tiny mass splitting between RHNs, known as the resonant leptogenesis [29–32]. It should be noted that earlier works on leptogenesis in gauged Lμ−Lτ model considered high scale breaking of such gauge symmetry and compatibility with muon (g − 2) explanation from a low scale vector boson was missing. We intend to perform a general analysis as well as to bridge this gap showing the possibility of TeV scale leptogenesis along with muon (g − 2) from a light Lμ − Lτ vector boson. Motivated by the recent measurements of the muon (g−2), in this work we consider the possibility of low scale leptogenesis and constrain the Lμ−Lτ gauge sector from the requirements of successful leptogenesis and (g − 2)μ. Since the decaying RHNs can have this new gauge interaction which can keep them in equilibrium for a longer epochs and can also initiate some washout processes, the requirement of successful leptogenesis for a fixed scale of leptogenesis around a TeV can lead to constraints on the gauge sector parameter space. Since leptogenesis is a high scale phenomena and the requirement of (g − 2)μ needs a low scale breaking Lμ−Lτ gauge symmetry, one can not realise both in the minimal version of the model. 1 ar X iv :2 10 6. 14 41 0v 2 [ he pph ] 1 5 Se p 20 21 We first discuss the minimal model from the requirement of satisfying neutrino data and baryon asymmetry from leptogenesis and then consider an extended model which can accommodate muon (g − 2) as well. While we do not pursue the study of RK anomalies in this model, one may refer to [33] for common origin of muon (g− 2) and RK anomalies along with dark matter in extensions of minimal Lμ − Lτ model. Explanation of similar flavour anomalies along with muon (g − 2) in this model have also been studied [34, 35]. Recently, a dark matter extension of the Lμ−Lτ model was also found to provide a common origin of muon (g−2) and electron recoil excess reported by the XENON1T collaboration [7, 11].