Search for vector mediator of dark matter production in invisible decay mode

Abstract

A search is performed for a new sub-GeV vector boson (A ) mediated production of Dark Matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A , called dark photon, can be generated in the reaction e − Z → e − ZA of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A → χχ. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to 4.3 × 10 10 electrons on target no evidence of such a process has been found. New stringent constraints on the A mixing strength with photons, 10 −5 10 −2 , for the A mass range m A 1 GeV are derived. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the 90% C.L. limits 10 −11 y 10 −6 on the dark-matter parameter y = 2 αD( mχ m A ) 4 are obtained for the dark coupling constant αD = 0.5 and dark-matter masses 0.001 mχ 0.5 GeV. The lower limits αD 10 −3 for pseudo-Dirac Dark Matter in the mass region mχ 0.05 GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using exact tree level calculations of the A production cross-sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region m A 0.1 GeV.

Light resonances and the low-q2 bin of $$ {R}_{K^{*}} $$

“…In the central panel of figure 7 we show both constraints. If the invisible decays dominate, the BaBar mono-photon search [57] and the NA64 search for dark photons [55,56] lead to strong constraints on g e as summarized in the right panel of figure 7. Finally, if V also has couplings to quarks (as in the dark-photon case) additional constraints become relevant that restrict the parameter space further [29].…”

“…Additional strong constraints on the coupling to electrons can be obtained from the fixed target experiment NA64 [55,56] and from BaBar searches [31,57]. In the relevant range of V masses and couplings, there are only two possible decay modes of V : (i) the coupling g e allows the V to decay promptly to electrons; (ii) V can decay invisibly into neutrinos or a light dark sector.…”

“…For a given V mass we take the maximally allowed g e coupling from the combination of the bounds in the central panel of figure 7. In the right panel, we consider the case in of the muon and the electron are indirect constraints from having V in the loop, while the constraints from BaBar [31] and NA64 [55,56] come from direct decays to electrons. On the left panel, the constraint from ν-trident production based on measurements by the CCFR collaboration [52] assumes that the V couples with equal strength to the left-handed muon and muon neutrino [51], i.e., to the SU(2) doublet L µ .…”

Light gauge boson interpretation for (g − 2)μ and the KL→ π0 + (invisible) anomaly at the J-PARC KOTO experiment

“…Avoiding constraints from the upper limit of Br(K + → π + X), which corresponds to γX ∼ < 10 −2 , is not possible in the presence of charm quark contribution. Furthermore, invisible dark photon searches from BaBar [40] and NA64 [41] exclude large kinetic mixing down to γX ∼ < 10 −3 . For short summary, the KOTO event excess cannot be explained by a (invisibly decaying) light gauge boson, kinematically mixed with the SM photon.…”

“…is the third generation left-handed quark doublet in the SM. Similar to single X gauge boson cases, one can impose the general kinetic mixing between SM gauge bosons (γ and Z) and new gauge bosonsX 1,2 with dimension-four operators γX iB µνX µν i (i = 1, 2) although sizeable values of γX i are constrained by dark photon searches [40,41]. See Appendix A for the detailed formulation in the presence of generic kinetic and mass mixing.…”