Protophobic Fifth-Force Interpretation of the Observed Anomaly inBe8Nuclear Transitions

Abstract

Recently a 6.8σ anomaly has been reported in the opening angle and invariant mass distributions of e + e − pairs produced in 8 Be nuclear transitions. The data are explained by a 17 MeV vector gauge boson X that is produced in the decay of an excited state to the ground state,8 Be * → 8 Be X, and then decays through X → e + e − . The X boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The protophobic X boson may also alleviate the current 3.6σ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

Detecting the Lμ−Lτ gauge boson at Belle II

“…In fact, extensions of the SM with a new boson possessing a mass around the MeV scale and only feebly interacting with our visible sector have been recently discussed in much literature, which is motivated by phenomenological observations. Those include the discrepancy between the SM predictions and the experimental measurements of the muon anomalous magnetic moment [8,9], inconsistency in the measurement of the e + e − pair produced in the transition between an excited state of 8 Be and its ground state [10][11][12][13][14][15], the tension between the sterile neutrino suggested by the short-baseline neutrino experiments and cosmological observations [16][17][18], the deficit of high-energy cosmic neutrino events, which is reported by the IceCube experiment [19][20][21][22][23][24][25][26][27], and the disagreement in the measurement of the proton radius [28][29][30][31]. It is also known that a light force carrier that intermediates between the DM promotes the annihilation of the DM in the early Universe and helps to reproduce the correct relic density [32,33].…”

Explanation of the 17 MeV Atomki anomaly in a U(1)′ -extended two Higgs doublet model

“…An explanation of the X nature was attempted by [2,3], in the form of models featuring a new vector boson Z ′ with a mass M Z ′ of about 17 MeV, with vector-like couplings to quarks and leptons. Constraints on such a new state, notably from searches for π 0 → Z ′ + γ by the NA48/2 experiment [4], require the couplings of the Z ′ to up and down quarks to be 'protophobic', i.e., that the charges eǫ u and eǫ d of up and down quarks -written as multiples of the positron charge e -satisfy the relation 2ǫ u + ǫ d < ∼ 10 −3 [2,3].…”

“…Constraints on such a new state, notably from searches for π 0 → Z ′ + γ by the NA48/2 experiment [4], require the couplings of the Z ′ to up and down quarks to be 'protophobic', i.e., that the charges eǫ u and eǫ d of up and down quarks -written as multiples of the positron charge e -satisfy the relation 2ǫ u + ǫ d < ∼ 10 −3 [2,3]. Subsequently, further studies of such models have been performed in [5][6][7][8][9][10][11][12] 1 .…”

“…Upon analysis of the electron-positron properties, the spectra of both their opening angle θ and invariant mass M presented the characteristics of an excess consistent with an intermediate boson X being produced on-shell in the decay of the 8 Be * state, with the X object subsequently decaying into e + e − pairs. The best fit to the mass M X of X was given as [1] M X = 16.7 ± 0.35 (stat) ± 0.5 (sys) MeV, in correspondence of a ratio of Branching Ratios (BRs) obtained asThis combination yields a statistical significance of the excess of about 6.8 σ [1].An explanation of the X nature was attempted by [2,3], in the form of models featuring a new vector boson Z ′ with a mass M Z ′ of about 17 MeV, with vector-like couplings to quarks and leptons. Constraints on such a new state, notably from searches for π 0 → Z ′ + γ by the NA48/2 experiment [4], require the couplings of the Z ′ to up and down quarks to be 'protophobic', i.e., that the charges eǫ u and eǫ d of up and down quarks -written as multiples of the positron charge e -satisfy the relation 2ǫ u + ǫ d < ∼ 10 −3 [2,3].…”

Surprises from complete vector portal theories: New insights into the dark sector and its interplay with Higgs physics

“…This motivates a systematic study of the realizations of these possibilities, with extra SM charged exotics and/or flavor-dependent Z couplings, along the lines of the explicit solutions found in Refs. [63][64][65][66][67][68][69][70][71][72][73][74][75][76][77].…”