Hunting down the X17 boson at the CERN SPS

Indexación ScopusRecently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4He, that they previously observed in measurements with 8Be. These observations could be explained by the existence of a new vector X17 boson. So far, the search for the decay X17 → 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 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X17 decay with the proposed method. To reach this goal an optimization of the X17 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. © 2020, The Author(s).https://link-springer-com.recursosbiblioteca.unab.cl/article/10.1140%2Fepjc%2Fs10052-020-08725-

Dark matter search in missing energy events with NA64

A search for sub-GeV dark matter production mediated by a new vector boson A′, called a dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with 2.84×1011 electrons on target no evidence of such a process has been found. The most stringent constraints on the A′ mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range ≲0.2  GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.A search for sub-GeV dark matter production mediated by a new vector boson $A'$, called dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with $2.84\times10^{11}$ electrons on target no evidence of such a process has been found. The most stringent constraints on the $A'$ mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range $\lesssim 0.2$ GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search

Improved exclusion limit for light dark matter from e+e−e^+e^- annihilation in NA64

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson A′ were set by the NA64 experiment for the mass region mA′≲250  MeV, by analyzing data from the interaction of 2.84×1011 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including A′ production via secondary positron annihilation with atomic electrons, we extend these limits in the 200–300 MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated e+ beam efforts.The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^\prime$ were set by the NA64 experiment for the mass region $m_{A^\prime}\lesssim 250$ MeV, by analyzing data from the interaction of $2.84\cdot10^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^\prime$ production via secondary positron annihilation with atomic electrons, we extend these limits in the $200$-$300$ MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^+$ beam efforts

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

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 Be*8 nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, He4, seems to support this anomaly spurring the need for an independent measurement. If the X boson exists, it 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 Super Proton Synchrotron and observed through its subsequent decay into e+e- pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4×1010 electrons on target collected in 2017 and 2018. This allows one to set new limits on the X-e- coupling in the range 1.2×10-4≲εe≲6.8×10-4, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons (A′) with a mass ≲24  MeV. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling ε. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.The improved results on a direct search for a new $X$(16.7 MeV) boson which could explain the anomalous excess of $e^+e^-$ pairs observed in the excited $^8Be^*$ nucleus decays ("Berillium anomaly") are reported. Due to its coupling to electrons, the $X$ boson could be produced in the bremsstrahlung reaction $e^-Z\rightarrow e^-ZX$ by a high-energy beam of electrons incident on 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 $8.4 \times 10^{10}$ electrons on target collected in 2017 and 2018. This allows to set the new limits on the $X-e^-$ coupling in the range $1.2 \times 10^{-4}\underset{\sim}{<}\epsilon_e \underset{\sim}{<}6.8\times 10^{-4}$, excluding part of the parameter space favored by the Berillium anomaly. We also set new bounds on the mixing strength of photons with dark photons ($A'$) from non-observation of the decay $A' \to e^+e^-$ of the bremsstrahlung $A'$ with a mass below 24 MeV