Search for Axionlike and Scalar Particles with the NA64 Experiment

We carried out a model-independent search for light scalar (s) and pseudoscalar axionlike (a) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The a(s) would penetrate the downstream HCAL module, serving as shielding, and would be observed either through their $a(s)\to\gamma \gamma$ decay in the rest of the HCAL detector or as events with large missing energy if the a(s) decays downstream of the HCAL. This method allows for the probing the a(s) parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to $2.84\times10^{11}$ electrons on target allowing to set new limits on the $a(s)\gamma\gamma$-coupling strength for a(s) masses below 55 MeV.Comment: This publication is dedicated to the memory of our colleague Danila Tlisov. 7 pages, 5 figures, revised version accepted for publication in Phys. Rev. Let

Search for invisible decays of sub-GeV dark photons in missing-energy events at the CERN SPS

We report on a direct search for sub-GeV dark photons (A') which might be produced in the reaction e^- Z \to e^- Z A' via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The A's would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75\cdot 10^{9} electrons on target. We set new limits on the \gamma-A' mixing strength and exclude the invisible A' with a mass < 100 MeV as an explanation of the muon g_\mu-2 anomaly.Comment: 6 pages, 3 figures; Typos corrected, references adde

Search for Axionlike and Scalar Particles with the NA64 Experiment

We carried out a model-independent search for light scalar (s) and pseudoscalar axionlike (a) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The a(s) would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their a(s)→γγ decay in the rest of the HCAL detector, or as events with a large missing energy if the a(s) decays downstream of the HCAL. This method allows for the probing of the a(s) parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to 2.84×10^{11} electrons on target, allowing us to set new limits on the a(s)γγ-coupling strength for a(s) masses below 55 MeV

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-

NA64 status report 2023

In this report, we summarise the 2016-2022 combined analysis on A$'\rightarrow$invisible searches with 9.37$\times10^{11}$ electrons on target. With such statistics, NA64 started probing for the first time the well-motivated region of parameter space suggested by benchmark light Dark Matter models, thus making further searches extremely exciting and important. In addition, the experiment has significantly extended its physics potential exploiting the use of positrons and muons beams. This year, we present the first results obtained at H4 using 100 GeV positrons and at M2 using 160 GeV muons. Both techniques would allow to improve the LDM sensitivity at higher dark photon masses. Finally, we will also report on the two runs carried out in 2023 at H4 and M2 beam lines and conclude with an overview of our plans for the next year

Light dark matter search with positron beams at NA64

We propose to extend the current NA64 scientific program to include a positron-beam, missing-energy search for light dark matter (LDM) in the mass region above 100 MeV, exploiting the high-quality CERN $e^+$ beam at the SPS H4 beamline. After an initial R$\&$D phase, we plan to run a multi-stage measurement, varying the energy of the beam in order to fully exploit the resonant signature of the LDM production via $e^+e^-$ annihilation. Specifically, we propose to run a first phase at 60 GeV beam energy, accumulating up to $10^{11}$ positrons-on-target, followed by a 40 GeV measurement with the same accumulated statistics. This experimental program will allow NA64 to explore the LDM parameter space in the mass range between 135~MeV and 250 MeV, probing for the first time the so-called ``thermal target lines'', i.e the value of the dark sector coupling predicted by astrophysical and cosmological arguments, for selected values of the model parameters, in the aforementioned mass range. Based on the obtained results, we also envisage to perform subsequent measurements to accumulate further statistics and explore further regions of the LDM parameter space. We plan to run the experiment exploiting the existing NA64 setup, with specific upgrades necessary to match the detector performances to the lower-energy kinematic regime. Thanks to the versatility of the H4 beamline, we will be able to perform each positron measurement at the end of the main electron-beam runs, thus maximizing the efficiency of use of the H4 beam

Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into 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 8Be∗ nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, 4He, 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.ISSN:1550-7998ISSN:0556-2821ISSN:1550-236

Hunting down the X17 boson at the CERN SPS

Recently, 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.ISSN:1434-6044ISSN:1434-605

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

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 8Be∗ 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 5.4×1010 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≲εe≲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.ISSN:0031-9007ISSN:1079-711