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

Abstract

Motivated by an anomaly observed in the decay of an excited state of Beryllium ( 8 Be) by the Atomki collaboration, we study an extension of the Standard Model with a gauged U (1) ′ symmetry in presence of a 2-Higgs Doublet Model structure of the Higgs sector. We show that this scenario complies with a variety of experimental results and is able to explain the potential presence of a resonant spin-1 gauge boson, Z ′ , with a mass of 17 MeV in the Atomki experimental data, for appropriate choices of U (1) ′ charges and Yukawa interactions.The Atomki pair spectrometer experiment [1] was set up for searching e + e − internal pair creation in the decay of excited 8 Be nuclei (henceforth, 8 Be * ), the latter being produced with the help of a beam of protons directed on a Lithium ( 7 Li) target. The proton beam was tuned in such a way that the different 8 Be excitations could be separated with high accuracy.In the data collection stage, a clear anomaly was observed in the decay of 8 Be * with spin-parity J P = 1 + into the ground state 8 Be with spin-parity 0 + (both with isospin T = 0), where 8 Be * had an excitation energy of 18.15 MeV. 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]. Subsequently, further studies of such models have been performed in [5-12] 1 .1 An alternative explanation was given in [13], wherein the X was In the footsteps of this literature, we consider here an extension of the SM described by a generic U (1) ′ group. Due to the presence of two such Abelian symmetries, U (1) Y × U (1) ′ , the most general kinetic Lagrangian of the corresponding fields,B µ andB ′ µ , respectively, allows for a gauge invariant mixing of the two field-strengthswhere κ is the kinetic mixing parameter between U (1) Y and U (1) ′ . A diagonal form for this Lagrangian can be obtained by transformation of the Abelian fields such that the gauge covariant derivative becomeswhere Y and z are the hypercharge and U (1) ′ charge, respectively, andg the gauge coupling mixing between the two Abelian groups. ...

TeV‐ and MeV‐Physics Out of an Model

“…GeV, and considering that Weinberg's angle has the experimental value sin 2 θ W 0.23, the parametrization (15) gives us the masses of W ± ,Z andZ in terms of the fundamental constants. Thus, the W-mass is given like in the electroweak model…”

“…Therefore, an Extended SM with the presence of the X-boson could be described by the unification of the gauge group SU c (3) × SU L (2) × U R (1) Y × U (1) X . Nowadays, there is a huge number of alternative models that describe this unification [11][12][13][14][15][16].…”

“…Nowadays, there is a huge number of alternative models that describe this unification. [11][12][13][14][15][16] The appearance of such a new boson mediating an interaction with typical range of 12 fm may put into evidence a fifth fundamental interaction in Nature. On the other hand, there has been in the literature a great deal of interest on the activity related to the phenomenology of the so-called hidden sector para-photons and milli-charged particles.…”

Chiral Froggatt-Nielsen models, gauge anomalies and flavourful axions

“…For instance, it has been shown [19][20][21][22] that in minimal supersymmetric models, the MSSM spectrum induces gauge anomalies when charged under a FN symmetry, such that one must design an extra fermionic spectrum or a Green-Schwarz (GS) mechanism in order to make the model consistent (see e.g. [23][24][25][26][27][28][29][30][31][32][33][34]). One way to do this is to add chiral spectator fermions at the scale where the FN symmetry is broken.…”

Neutrino masses in a two Higgs doublet model with a U(1) gauge symmetry

“…Nevertheless, some proposals to extend the 2HDM via the presence of gauge symmetries have been put forth [43][44][45]. Some were triggered by anomalies in flavor and collider physics [46][47][48] and others devoted to explain neutrino masses via type I seesaw mechanism and absence of flavor changing interactions [49][50][51][52].…”