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We report the results of a search for a new vector boson (A′) decaying into two dark matter particles χ1χ2 of different mass. The heavier χ2 particle subsequently decays to χ1 and an off-shell Dark Photon A′∗→e+e-. For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay A′→χχ¯ and axion-like or pseudo-scalar particles a→γγ. With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for A′ masses from 2me up to 390 MeV and mixing parameter ε between 3×10-5 and 2×10-2.

A bstract Thermal light dark matter (LDM) with particle masses in the 1 MeV–1 GeV range could successfully explain the observed dark matter abundance as a relic from the primordial Universe. In this picture, a new feeble interaction acts as a “portal” between the Standard Model and LDM particles, allowing for the exploration of this paradigm at accelerator experiments. In the last years, the “missing energy” experiment NA64 e at CERN SPS (Super Proton Synchrotron) has set world-leading constraints in the vector-mediated LDM parameter space, by exploiting a 100 GeV electron beam impinging on an electromagnetic calorimeter, acting as an active target. In this paper, we report a detailed description of the analysis of a preliminary measurement with a 70 GeV/c positron beam at NA64 e , performed during summer 2023 with an accumulated statistics of 1 . 596 × 10 10 positrons on target (hereafter referred to as e + OT). This data set was analyzed with the primary aim of evaluating the performance of the NA64 e detector with a lower energy positron beam, towards the realization of the post-LS3 program. The analysis results, other than additionally probing unexplored regions in the LDM parameter space, provide valuable information towards the future NA64 e positron campaign.

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 4 He, that they previously observed in measurements with 8 Be. These observations could be explained by the existence of a new vector X 17 boson. So far, the search for the decay 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 X 17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X 17 decay with the proposed method. To reach this goal an optimization of the 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.

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.

Hadrons and multiquark states are discussed within the context of holographic quantum chromodynamics. This approach is based on an action that describes the hadron structure with breaking of conformal and chiral symmetry and includes confinement through the presence of a background dilaton field. According to gauge/gravity duality, five-dimensional boson and fermion fields, moving in AdS space, are dual to the four-dimensional fields on the surface of the AdS sphere, which correspond to hadrons. In this framework, the hadron wave functions – the building blocks of the hadron properties – are dual to the profiles of the AdS fields in the fifth (holographic) dimension, which is identified with a scale. As applications, we consider the properties of hadrons and multiquark states.

The basic shape of di-pion mass spectra in the two-pion transitions of both charmonia and bottomonia states is explained by a unified mechanism based on contributions of the ππ, KK̅, and ηη coupled channels including their interference. The role of the individual f0 resonances in shaping the di-pion mass distributions in the charmonia and bottomonia decays is considered.

It is shown that the basic shape of dipion mass distributions in the twopion transitions of both charmonia and bottomonia states are explained by an unified mechanism based on the contribution of the ππ, KK̄ and ηη coupled channels including their interference.

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.

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive$$\\rho ^0$$ρ 0 meson muoproduction at COMPASS using 160 GeV/ c polarised$$ \\mu ^{+}$$μ + and$$ \\mu ^{-}$$μ - beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/$$c^2$$c 2$$< W<$$< W < 17.0 GeV/$$c^2$$c 2 , 1.0 (GeV/ c )$$^2$$2$$< Q^2<$$< Q 2 < 10.0 (GeV/ c )$$^2$$2 and 0.01 (GeV/ c )$$^2$$2$$< p_{\\textrm{T}}^2<$$< p T 2 < 0.5 (GeV/ c )$$^2$$2 . Here, W denotes the mass of the final hadronic system,$$Q^2$$Q 2 the virtuality of the exchanged photon, and$$p_{\\textrm{T}}$$p T the transverse momentum of the$$\\rho ^0$$ρ 0 meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ($$\\gamma ^*_T \\rightarrow V^{ }_L$$γ T ∗ → V L ) indicate a violation of s -channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive$$\\rho ^0$$ρ 0 production.

We propose an extension of the Standard Model (SM) with a \\(U_{A'}(1)\\) gauge invariant dark dector connected to the SM via a new portal -- the Stueckelberg portal, arising in the framework of dark photon \\(A'\\) mass generation via Stueckelberg mechanism. This portal opens through the effective dim=5 operators constructed from the covariant term of the auxiliary Stueckelberg scalar field \\(\\sigma\\) providing flavor non-diagonal renormalizable couplings of both \\(\\sigma\\) and \\(A'\\) to the SM fermions \\(\\psi\\). The Stueckelberg scalar plays a role of Goldstone boson in the generation of mass of the Dark Photon. Contrary to the conventional kinetic mixing portal, in our scenario flavor diagonal \\(A'\\)-\\(\\psi\\) couplings are not proportional to the fermion charges and are, in general, flavor nondiagonal. These features drastically change the phenomenology of dark photon \\(A'\\) relaxing or avoiding some previously established experimental constraints. We focus on the phenomenology of the described scenario of the Stueckelberg portal in the lepton sector and analyze the contribution of the dark sector fields \\(A'\\) to the anomalous magnetic moment of muon \\((g-2)_{\\mu}\\), lepton flavor violating decays \\(l_{i}\\to l_{k}\\gamma\\) and \\(\\mu-e\\) conversion in nuclei. We obtain limits on the model parameters from the existing data on the corresponding observables.