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.

Starting from a molecular picture for the X (3872) resonance, this state and its J PC = 2 ++ heavy-quark spin symmetry partner [X 2 (4012)] are analyzed within a model which incorporates possible mixings with 2P charmonium (cc) states. Since it is reasonable to expect the bare χ c1 (2P) to be located above the DD * threshold, but relatively close to it, the presence of the charmonium state provides an effective attraction that will contribute to binding the X (3872), but it will not appear in the 2 ++ sector. Indeed in the latter sector, the χ c2 (2P) should provide an effective small repulsion, because it is placed well below the D * D * threshold. We show how the 1 ++ and 2 ++ bare charmonium poles are modified due to the D ( * )D( * ) loop effects, and the first one is moved to the complex plane. The meson loops produce, besides some shifts in the masses of the charmonia, a finite width for the 1 ++ dressed charmonium state. On the other hand, X (3872) and X 2 (4012) start developing some charmonium content, which is estimated by means of the compositeness Weinberg sum rule. It turns out that in the heavy-quark limit, there is only one coupling between the 2P charmonia and the D ( * )D( * ) pairs. We also show that, for reasonable values of this coupling, leading to X (3872) molecular probabilities of around 70-90 %, the X 2 resonance destabilizes and disappears from the spectrum, becoming either a virtual state or one being located deep into the complex plane, with decreasing influence in the D * D * scattering line. Moreover, we also discuss how around 10-30 % charmonium probability in the X (3872) might explain the ratio of radiative decays of this resonance into ψ(2S)γ and J/ψγ . Finally, we qualitatively discuss within this scheme, the hidden bottom flavor sector, paying a special attention to the implications for the X b and X b2 states, heavy-quark spin-flavor partners of the X (3872).a

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.

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The SHiP experiment is designed to search for very weakly interacting particles beyond the Standard Model which are produced in a 400 GeV/c proton beam dump at the CERN SPS. An essential task for the experiment is to keep the Standard Model background level to less than 0.1 event after 2× 1020 protons on target. In the beam dump, around 1011 muons will be produced per second. The muon rate in the spectrometer has to be reduced by at least four orders of magnitude to avoid muon-induced combinatorial background. A novel active muon shield is used to magnetically deflect the muons out of the acceptance of the spectrometer. This paper describes the basic principle of such a shield, its optimization and its performance.