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In this study, we demonstrate the potential of the NA64 experiment at CERN SPS to search for New Physics processes involving e → μ transitions after the collision of 100 GeV electrons with target nuclei. A new Dark Sector leptonic portal in which a scalar boson φ could be produced in the lepton-flavor-changing bremsstrahlung-like reaction, e N → μ N φ , is used as benchmark process. In this work, we develop a realistic Monte Carlo simulation of the NA64 experimental setup implementing the differential and total production cross-section computed at exact tree-level and applying the Weiszäcker–Williams phase space approximation. Using this framework, we investigate the main background sources and calculate the expected sensitivity of the experiment. The results indicate that with minor setup optimization, NA64 can probe a large fraction of the available parameter space compatible with the muon g - 2 anomaly and the Dark Matter relic predictions in the context of a new Dark Sector leptonic portal with 10 11 EOT. This result paves the way to the exploration of lepton-flavour-changing transitions in NA64.

Standard Model fails to explain neutrino oscillations, dark matter, and baryon asymmetry of the Universe. All these problems can be solved with three sterile neutrinos added to SM. Quite remarkably, if sterile neutrino masses are well below the electroweak scale, this modification—Neutrino Minimal Standard Model (νMSM)—can be tested experimentally. We discuss a new experiment on search for decays of GeV-scale sterile neutrinos, which are responsible for the matter-antimatter asymmetry generation and for the active neutrino masses. If lighter than 2 GeV, these particles can be produced in decays of charm mesons generated by high energy protons in a target, and subsequently decay into SM particles. To fully explore this sector of νMSM, the new experiment requires data obtained with at least 1020 incident protons on target (achievable at CERN SPS in future) and a big volume detector constructed from a large amount of identical single modules, with a total sterile neutrino decay length of few kilometers. The preliminary feasibility study for the proposed experiment shows that it has sensitivity which may either lead to the discovery of new particles below the Fermi scale—right-handed partners of neutrinos—or rule out seesaw sterile neutrinos with masses below 2 GeV.

Decays of light pseudoscalar mesons play an important role in the search for dark matter signals in fixed-target experiments. The intermediate vector meson state is shown to be of importance in the analysis of dark photon physics in the decays of neutral mesons. Constraints on the dark photon model for the optimistic statistics of the NA64 experiment with a beam of negative pions are presented.

We consider the SM extension with additional light real singlet scalar, right-handed neutrino and nonrenormalizable Yukawa interaction for the first two generations. We show that the proposed model can explain the observed (g – 2) muon anomaly. Phenomenological consequenses as flavour violating decays τ → μμμ, μμe, μee are briefly discussed. We also propose the UR(1) gauge generalization of the SM with complex scalar singlet and nonzero right-handed charges for the first two generations.

Mirror matter from the dark hidden sector with the same particle content and gauge interactions as in the standard model is still an interesting candidate for dark matter. Several experiments on search for positronium and neutron oscillations into their mirror partner have been conducted recently. In this work we consider the transitions \\(K^0-K^0_m\\) of a neutral kaon into a hidden mirror kaon. It is shown that their probability can be probed with the sensitivity of \\(P(K^0-K^0_m ) 10^-7\\) from the search for missing-energy events in the NA64 experiment at the CERN SPS with \\( 10^11\\) \\(K^+\\) on target.This would result in a limit on the \\(K^0-K^0_m\\) mixing parameter much stronger than the similar bound estimated from the cosmological considerations.

A bstract The inclusion of an additional U(1) gauge L μ − L τ symmetry would release the tension between the measured and the predicted value of the anomalous muon magnetic moment: this paradigm assumes the existence of a new, light Z ′ vector boson, with dominant coupling to μ and τ leptons and interacting with electrons via a loop mechanism. The L μ − L τ model can also explain the Dark Matter relic abundance, by assuming that the Z ′ boson acts as a “portal” to a new Dark Sector of particles in Nature, not charged under known interactions. In this work we present the results of the Z ′ search performed by the NA64- e experiment at CERN SPS, that collected ~ 9 × 10 11 100 GeV electrons impinging on an active thick target. Despite the suppressed Z ′ production yield with an electron beam, NA64- e provides the first accelerator-based results excluding the g − 2 preferred band of the Z ′ parameter space in the 1 keV < m Z ′ ≲ 2 MeV range, in complementarity with the limits recently obtained by the NA64- μ experiment with a muon beam.

In this note we estimate the sensitivity of the NA64 experiment to millicharged particles (\\(\\chi\\)). That experimental facility is dedicated to the searching for dark sector particles in missing energy events at the CERN SPS. We consider missing momentum signatures in the \\(\\simeq\\) 100 GeV electron and muon beams and show that the later one allows to obtain more stringent bounds on the millicharge \\(Q_{\\chi}\\), which for the \\(\\chi\\) masses \\(100\\)~MeV \\(\\leq m_{\\chi} \\leq 500\\)~MeV at the level \\( Q_{\\chi}/e\\lesssim O(10^{-3}) - O(10^{-2})\\).

The NA64 experiment consists of two detectors which are planned to be located at the electron (NA64e) and muon (NA64\\(\\mu\\)) beams of the CERN SPS and start operation after the LHC long-stop 2 in 2021. Its main goals include searches for dark sector physics - particularly light dark matter (LDM), visible and invisible decays of dark photons (\\(A'\\)), and new light particles that could explain the \\(^8\\)Be and \\(g_{\\mu}-2\\) anomalies. Here we review these physics goals, the current status of NA64 including recent results and perspectives of further searches, as well as other ongoing or planned experiments in this field. The main theoretical results on LDM, the problem of the origin of the \\(\\gamma-A'\\) mixing term and its connection to loop corrections, possible existence of a new light \\(Z'\\) coupled to \\(L_\\mu-L_\\tau\\) current are also discussed.

In addition to gravity, there might be another very weak interaction between the ordinary and dark matter transmitted by U'(1) gauge bosons A' (dark photons) mixing with our photons. If such A's exist, they could be searched for in a light-shining-through-a-wall experiment with a high energy electron beam. The electron energy absorption in a calorimeter (CAL1) is accompanied by the emission of bremsstrahlung A's in the reaction eZ -> eZA' of electrons scattering on nuclei due to the \\gamma - A' mixing. A part of the primary beam energy is deposited in the CAL1, while the rest of the energy is transmitted by the A' through the \"CAL1 wall\" and deposited in another downstream calorimeter CAL2 by the e+e- pair from the A'->e+e- decay in flight. Thus, the A's could be observed by looking for an excess of events with the two-shower signature generated by a single high energy electron in the CAL1 and CAL2. A proposal to perform such an experiment to probe the still unexplored area of the mixing strength 10^{-5} < \\epsilon < 10^{-3} and masses M_{A'} < 100 MeV by using 10-300 GeV electron beams from the CERN SPS is presented. The experiment can provide complementary coverage of the parameter space, which is intended to be probed by other searches. It has also a capability for a sensitive search for A's decaying invisibly to dark-sector particles, such as dark matter, which could cover a significant part of the still allowed parameter space.

We discuss prospects of searching for a dark photon (\\(A'\\)) which serves as mediator between Standard model (SM) particles and light dark matter (LDM) by using the combined results from the NA64 experiment at the CERN SPS running in high-energy electron (NA64e) and muon (NA64\\(\\mu\\)) modes. We discuss the most natural values and upper bounds on the \\(A'\\) coupling constant to LDM and show they are lying in the range accessible at NA64. While for the projected \\( 5\\times10^{12}\\) electrons on target (EOT) NA64e is able to probe the scalar and Majorana LDM scenarios, the combined NA64e and NA64\\(\\mu\\) results with \\(\\simeq 10^{13}\\) EOT and a few \\(10^{13}\\) MOT, respectively, will allow covering significant region in the parameter space of the most interesting LDM models. This makes NA64e and NA64\\(\\mu\\) extremely complementary to each other and increases significantly the discovery potential of sub-GeV DM.