We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100  MeV as an explanation of the muon g_{μ}-2 anomaly.

A search is performed for a new sub-GeV vector boson (A ) mediated production of Dark Matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A , called dark photon, can be generated in the reaction e − Z → e − ZA of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A → χχ. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to 4.3 × 10 10 electrons on target no evidence of such a process has been found. New stringent constraints on the A mixing strength with photons, 10 −5 10 −2 , for the A mass range m A 1 GeV are derived. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the 90% C.L. limits 10 −11 y 10 −6 on the dark-matter parameter y = 2 αD( mχ m A ) 4 are obtained for the dark coupling constant αD = 0.5 and dark-matter masses 0.001 mχ 0.5 GeV. The lower limits αD 10 −3 for pseudo-Dirac Dark Matter in the mass region mχ 0.05 GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using exact tree level calculations of the A production cross-sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region m A 0.1 GeV.

Heavy Neutral Leptons (HNLs) are hypothetical particles predicted by many extensions of the Standard Model. These particles can, among other things, explain the origin of neutrino masses, generate the observed matter-antimatter asymmetry in the Universe and provide a dark matter candidate. The SHiP experiment will be able to search for HNLs produced in decays of heavy mesons and travelling distances ranging between O(50 m) and tens of kilometers before decaying. We present the sensitivity of the SHiP experiment to a number of HNL's benchmark models and provide a way to calculate the SHiP's sensitivity to HNLs for arbitrary patterns of flavour mixings. The corresponding tools and data files are also made publicly available.

NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) is a multi-purpose experimental facility to study hadron production in hadron-proton, hadron-nucleus and nucleus-nucleus collisions at the CERN Super Proton Synchrotron. It recorded the first physics data with hadron beams in 2009 and with ion beams (secondary 7 Be beams) in 2011.NA61/SHINE has greatly profited from the long development of the CERN proton and ion sources and the accelerator chain as well as the H2 beamline of the CERN North Area. The latter has recently been modified to also serve as a fragment separator as needed to produce the Be beams for NA61/SHINE. Numerous components of the NA61/SHINE set-up were inherited from its predecessors, in particular, the last one, the NA49 experiment. Important new detectors and upgrades of the legacy equipment were introduced by the NA61/SHINE Collaboration.This paper describes the state of the NA61/SHINE facility -the beams and the detector system -before the CERN Long Shutdown I, which started in March 2013.

The Search for Hidden Particles (SHiP) Collaboration has shown that the CERN SPS accelerator with its 400 GeV/c proton beam offers a unique opportunity to explore the Hidden Sector [1–3]. The proposed experiment is an intensity frontier experiment which is capable of searching for hidden particles through both visible decays and through scattering signatures from recoil of electrons or nuclei. The high-intensity experimental facility developed by the SHiP Collaboration is based on a number of key features and developments which provide the possibility of probing a large part of the parameter space for a wide range of models with light long-lived super-weakly interacting particles with masses up to 𝒪(10) GeV/c2 in an environment of extremely clean background conditions. This paper describes the proposal for the experimental facility together with the most important feasibility studies. The paper focuses on the challenging new ideas behind the beam extraction and beam delivery, the proton beam dump, and the suppression of beam-induced background.