Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into e+e− pairs

Abstract

The improved results on a direct search for a new X(16.7 MeV) boson that could explain the anomalous excess of e + e − pairs observed in the decays of the excited 8 Be * nucleus ("Berillium anomaly") are reported. The X boson could be produced in the bremsstrahlung reaction e − Z → e − ZX by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN SPS and observed through its subsequent decay into e + e − pair. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4 × 10 10 electrons on target collected in 2017 and 2018. This allows to set the new limits on the X − e − coupling in the range 1.2 × 10 −4 e 6.8 × 10 −4 , excluding part of the parameter space favored by the Berillium anomaly. The non-observation of the decay A → e + e − allows also to set the new bounds on the mixing strength of photons with dark photons (A ) with a mass 24 MeV.Recently, the search for new light bosons weakly coupled to SM particles was additionally inspired by the observation in the ATOMKI experiment by Krasznahorkay et al. [1,2] of a ∼7σ excess of events in the invariant mass distribution of e + e − pairs produced in the nuclear transitions of the excited 8 Be * to its ground state via internal pair creation. It was shown that this anomaly can be interpreted as the emission of a protophobic gauge boson X with a mass of 16.7 MeV decaying into e + e − pair [3,4]. This explanation of the anomaly was found to be consistent with the existing constraints assuming that the X has non-universal coupling to quarks, coupling to electrons in the range 2 × 10 −4 e 1.4 × 10 −3 and lifetime 10 −14 τ X 10 −12 s. It is interesting that a new boson with such relatively large couplings to charged leptons could also resolve the so-called (g µ − 2 ) anomaly, a discrepancy between measured and predicted values of the muon anomalous magnetic moment. This has motivated worldwide efforts towards the experimental searches, see, e.g., Refs. [5,6], and studies of the phenomenological aspects of light vector bosons weakly coupled to quarks and leptons, see, e.g., and also earlier works of Refs. [13][14][15][16]. The latest experimental results from the ATOMKI group show a similar excess of events at approximately the same invariant mass in the nuclear transitions of another nucleus, 4 He [17]. This further increases the importance of independent searches for a new particle X.Another strong motivation to search for new light bosons decaying into e + e − pair comes from the dark matter puzzle. An interesting possibility is that in addition to gravity a new force between the dark sector and visible matter, carried by a new vector boson A , called dark photon, might exist [18,19]. Such A could have a mass

Open string QED meson description of the X17 particle and dark matter

“…The recent inclusive experiment of the NA64 Collaboration [12][13][14] of finding no "dark" soft photon excesses needs to reconcile with the earlier finding of anomalous soft photon excesses in high energy hadron-p, pp, and e + e − collisions [23][24][25][26][27][28][29][30][31]. In the detection of anomalous soft photons in the DELPHI Collaboration [31], the photon detection is carried out by studying the electron and positron tracks in a TPC, while in the NA64 experiment, the produced soft photon needs to penetrate a calorimeter and is detected downstream in a separate calorimeter.…”

“…Recent observation [1] of a light, neutral boson decaying into an e + e − pair with a mass of about 17 MeV, in the decay of the I(J π ) = 0(1 + ) state of 8 Be at Atomki, has generated a great deal of interest [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Supporting evidence for this hypothetical X17 particle has been reported recently in the decay of the excited I(J π ) = 0(0 − ) state of 4 He [16].…”

“…Earlier observations of similar e + e − pairs with invariant masses between 3 to 20 MeV in the collision of nuclei with emulsion detectors have been reported [17][18][19][20][21][22]. Different theoretical interpretations, astrophysical implications, and experimental searches have been presented [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. However, a definitive description of the X17 particle has not yet emerged.…”

Dynamical evidence for a fifth force explanation of the ATOMKI nuclear anomalies

“…At the nucleus level, the decay is mediated by the operator O ð1Þ 3S in Eq. (29), which leads to the S-wave decay amplitude…”

“…A follow-up study from nuclear theory was also shown to disfavor a nuclear form factor as an explanation [27]. Follow-up measurements of the nuclear transitions have not been reported by other groups, but other experimental collaborations have provided new constraints on X boson couplings to electrons that exclude part of the viable parameter space in some models [28,29].…”