Search for a light dark photon in muonium decay

Dubinina, V. V.; Egorenkova, N. P.; Pozharova, E. A.; Polukhina, N.; Smirnitsky, V. A.; Старков, Н. И.

doi:10.1134/s106377881703005x

“…A possibility is that this new force is carried by a vector boson A , called dark photon. Stringent limits on the coupling strength and mass m A of such dark photons, excluding the parameter space region favored by the anomalous magnetic moment of the muon, the so called (g − 2) μ anomaly, have already been placed by beam dump [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], fixed target [21][22][23], collider [24][25][26], rare particle decay searches [27][28][29][30][31][32][33][34][35][36][37] and the new determination of the fine structure constant α combined with the measurement of (g − 2) e [38,39].…”

Section: Introductionmentioning

confidence: 99%

Hunting down the X17 boson at the CERN SPS

Depero¹,

Andreev²,

Banerjee³

et al. 2020

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Recently, the ATOMKI experiment has reported new evidence for the excess of $$e^+e^-$$ e + e - events with a mass $$\sim $$ ∼ 17 MeV in the nuclear transitions of $$^4$$ 4 He, that they previously observed in measurements with $$^8$$ 8 Be. These observations could be explained by the existence of a new vector $$X17$$ X 17 boson. So far, the search for the decay $$X17 \rightarrow e^+ e^-$$ X 17 → e + e - with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining $$X17$$ X 17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the $$X17$$ X 17 decay with the proposed method. To reach this goal an optimization of the $$X17$$ X 17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.

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“…[5,23,24]. A number of previous experiments, such as beam dump [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], fixed target [40][41][42], collider [43][44][45] and rare particle decay searches [46][47][48][49][50][51][52][53][54][55][56][57], put stringent constraints on the and mass m A of such dark photons, excluding, in particular, the parameter space region favored by the g µ − 2 anomaly. However, a large range of mixing strengths 10 −4 10 −3 corresponding to short-lived A remains unexplored.…”

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confidence: 99%

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

Banerjee¹,

Bernhard²,

Burtsev³

et al. 2020

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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

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“…[4,18,19]. A number of previous beam dump [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], fixed target [35][36][37], collider [38][39][40] and rare particle decay [41][42][43][44][45][46][47][48][49][50][51][52][53] tons excluding, in particular, the parameter space region favored by the g µ − 2 anomaly. However, the range of mixing strengths 10 −4 10 −3 corresponding to a short-lived A still remains unexplored.…”

mentioning

confidence: 99%

Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN

Banerjee¹,

Burtsev²,

Chumakov³

et al. 2018

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We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e^{+}e^{-} pairs observed in the excited ^{8}Be^{*} nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e^{-}Z→e^{-}ZX by a 100 GeV e^{-} beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a e^{+}e^{-} pair. With 5.4×10^{10} electrons on target, no evidence for such decays was found, allowing us to set first limits on the X-e^{-} coupling in the range 1.3×10^{-4}≲ε_{e}≲4.2×10^{-4} excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A^{'}) from nonobservation of the decay A^{'}→e^{+}e^{-} of the bremsstrahlung A^{'} with a mass ≲23 MeV.

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Search for a light dark photon in muonium decay

Cited by 3 publications

References 2 publications

Hunting down the X17 boson at the CERN SPS

Hunting down the X17 boson at the CERN SPS

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

Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN

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