Is there a sign of new physics in beryllium transitions?

Abstract

We discuss the current status of the anomaly in beryllium-8 nuclear transitions recently reported in the angular distribution of internal conversion electron-positron pairs. We present a phenomenological analysis of the signal and review the models proposed to explain it, focusing on those involving a new light protophobic vector gauge boson. We also elaborate on the prospects of verifying the anomaly in present and future experiments. * Invited talk given at the American Physical Society April Meeting 2017, Washington, DC, January 28, 2017; based on the work done in collaboration with Jonathan Feng, Iftah Galon, Susan Gardner, Jordan Smolinsky, Tim Tait and Philip Tanedo [1,2].arXiv:1707.09749v1 [hep-ph] Our current understanding of Nature at the fundamental level is successfully and elegantly captured by the Standard Model of elementary particles. Sadly, this theory describes only five percent of the entire content of the Universe. The overwhelming unknown constituents are dark matter and dark energy. Although not much is known about the origin of dark energy, we have quite compelling reasons to believe that dark matter is made up of new kind of particles. The experimental limits on their non-gravitational interactions with the particles of the Standard Model are severe, forcing those interactions to be very weak, if at all nonzero. The sole existence of dark matter is the key motivator for new particles searches.The Standard Model is based on the local symmetry SU(3) c × SU(2) L × U(1) Y [3,4,5,6,7]. All fundamental particles discovered so far come in representations of this gauge group. It seems reasonable to expect that dark matter also fits into this general picture, carrying charges under a new gauge group, with a very small mixing with the Standard Model. The simplest realization of this idea is to postulate a new "dark" U(1) local symmetry, forming a hidden sector in which the dark matter resides. In such a scenario the dark matter can communicate with the Standard Model only through a new gauge boson mediating interactions between the two sectors. If such a gauge boson has small couplings to the Standard Model particles, it could have escaped experimental detection even if it is light, on the scale of a few MeV [8,9].It is quite difficult to look for light bosons at particle colliders. Thankfully, as pointed out already a long time ago [10,11], nuclear transitions provide powerful probes of MeV-scale new physics, especially in systems with a large splitting between nuclear energy levels connected via electromagnetic transitions. This has been used over the years to look for light bosons [12,13,14,15,16,17,18,19,20,21,22], though without any strong evidence. However, recently an experiment looking at electron-positron internal conversion pairs in beryllium-8 ( 8 Be) claimed a 6.8 sigma evidence for an anomaly [23]. Before describing those results, let us first discuss why the 8 Be nucleus offers one of the most appealing environments for this type of search. New Physics Search in BerylliumTh...

Searching for new light gauge bosons at e+e− colliders

“…The latter will give a ∼10% contribution to the cross sections. In the situation when the coupling is unknown, the observables will be described with a reasonable accuracy even when this term is omitted in (50). This makes all the results of Secs.…”

“…It is important and relevant to compare (50) with (3). The dependences of both amplitudes on the Mandelstam variables are identical except the last term in (50). The latter will give a ∼10% contribution to the cross sections.…”

“…To describe the experimental distributions, the new boson should have mass m Z ¼ 16.70 AE 0.35ðstatÞ AE 0.50ðsysÞ MeV [34]. In the last year, there have appeared many articles devoted to this hypothetical 17 MeV boson [39][40][41][42][43][44][45][46][47][48][49][50]. First limits on its coupling to electrons have been set in the NA64 experiment at the Super Proton Synchrotron (SPS) of the European Organization for Nuclear Research (CERN) [51].…”

X(16.7) production in electron–positron collision

“…Other possible explanations on this new boson, light pseudoscalar boson, protophobic vector boson, axialvector particle, etc., are also widely studied [28][29][30][31][32][33][34][35]. Among them, the axial-vector particle proposal is suggested in Ref.s [26,29] but not pursued systematically.…”

Explanation of the 511 keV line: cascade annihilating dark matter with the $$^8$$ 8 Be anomaly

“…[23] (for more discussions, see e.g., Refs. [24][25][26][27][28][29]). For the possible X portal DM investigated in Refs.…”