Probing sub-GeV leptophilic dark matter at Belle II and NA64

Abstract

An analysis is given of the Belle II sensitivities and NA64 constraints on the sub-GeV Dirac dark matter that interacts with charged leptons. We consider two different types of interactions between sub-GeV Dirac dark matter and the charged leptons: the EFT operators and the light vector mediators. We compute the Belle II mono-photon sensitivities on sub-GeV dark matter with 50 ab−1 data which are expected to be accumulated in the full Belle II runs. Although the Belle II mono-photon sensitivities on the EFT operators are of similar size as the LEP constraints, Belle II can probe new parameter space of the light vector mediator models that are unexplored by LEP. For both the EFT operators and the light vector mediator models, the Belle II mono-photon sensitivities can be several orders of magnitude stronger than the current dark matter direct detection limits, as well as the white dwarf limits. The light vector mediator can also be directly searched for by reconstructing the invariant mass of its di-lepton decay final states at Belle II, which is found to be complementary to the mono-photon channel. We compute the NA64 constraints on the sub-GeV Dirac dark matter and provide analytic expressions of the dark matter cross section in the Weizsäcker-Williams approximation, for the EFT operators, and for the light vector mediator models. We find that the current NA64 data (with 2.84 × 1011 electron-on-target events) provide strong constraints on sub-GeV dark matter. Although the NA64 constraints are found to be about one order of magnitude smaller than the Belle II sensitivities for the EFT operators, NA64 can probe some regions of the parameter space in the light vector mediator models that are beyond the reach of Belle II. We also find that Belle II and NA64 can probe the canonical dark matter annihilation cross section in thermal freeze-out in a significant portion of the parameter space of the models considered.

Forecasting dark showers at Belle II

“…For comparison, when we simply calculate the perturbative production cross section for a pair of dark quarks following ref. [60] and assume that the subsequent dark shower always remains invisible, we obtain the almost identical sensitivity estimate Λ −2 < 8 • 10 −5 GeV −2 . 11 For m ρ d 500 MeV the sensitivity decreases because there is…”

“…11. Note that this bound is significantly weaker than the one in ref [60],. because we consider an integrated luminosity of 500 fb −1 rather than 50 ab −1 and do not attempt background subtraction.…”

“…7 We remind the reader that Pythia does not distinguish between the dark pion mass and the ρ 0 d meson mass. 8 It is tempting to propose even more stringent cuts, for which even the remaining simulated backgrounds would be removed [60]. However, it seems unlikely that this search can ever be completely background-free, which is why we prefer to keep a non-negligible background in order to obtain more realistic sensitivity estimates.…”

Associated production of neutrino and dark fermion at future lepton colliders

“…particularly interesting possibility is the sterile neutrino DM that can produce astrophysical X-ray [22][23][24][25][26][27]. The leptophilic DM models have been discussed extensively in literature [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] and have the potential of explaining various experimental anomalies [47][48][49].…”

Factorizing hidden particle production rates

“…In recent years, a significant program of searches for such feebly interacting new particles has begun to emerge [1][2][3][4]. Relevant constraints can be obtained from high intensity data sets of CMS [5][6][7] and ATLAS [8], from flavour physics experiments such as LHCb [9][10][11][12][13][14][15] or Belle-II [16][17][18], and from high luminosity runs of the Large Hadron Collider [19]. There is also a number of ongoing and proposed searches at low-energy fixed target experiments such NA62 [20][21][22][23][24][25][26], KOTO [27,28], SeaQuest [29,30], or SHiP [1], and further complimentary constraints can be obtained from searches at dedicated long-lived particle experiments [31] such as MATHUSLA [32,33], FASER [34,35] and CODEX-b [36].…”