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 $$ \mathcal{O} $$ 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.

We present a search for the e+e- decay of a hypothetical dark photon, also names U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared epsilon^{2} at 90% CL has been obtained for the mass range M(U) = 0.02 - 0.55 GeV/c2 and is compared with the present world data set. For masses 0.03 - 0.1 GeV/c^2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favoured by the muon g-2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 * 10^{-6} has been set on the helicity-suppressed direct decay of the eta meson, eta-> e+e-, at 90% CL

SND@LHC is a compact and stand-alone experiment designed to perform measurements with neutrinos produced at the LHC in the pseudo-rapidity region of 7.2 < η < 8.4. The experiment is located 480 m downstream of the ATLAS interaction point, in the TI18 tunnel. The detector is composed of a hybrid system based on an 830 kg target made of tungsten plates, interleaved with emulsion and electronic trackers, also acting as an electromagnetic calorimeter, and followed by a hadronic calorimeter and a muon identification system. The detector is able to distinguish interactions of all three neutrino flavours, which allows probing the physics of heavy flavour production at the LHC in the very forward region. This region is of particular interest for future circular colliders and for very high energy astrophysical neutrino experiments. The detector is also able to search for the scattering of Feebly Interacting Particles. In its first phase, the detector is ready to operate throughout LHC Run 3 and collect a total of 250 fb^-1.

In the framework of the ALICE collaboration we have recently studied the performance of multigap Resistive Plate Chambers operated in avalanche mode and at atmospheric pressure for time-of-flight measurements. The detector provided an overall (detector plus electronics) timing accuracy of 120 ps sigma at an efficiency of 98% for MIPs. The chambers had four gas gaps of 0.3 mm, each limited by a metallised ceramic plate and a glass plate, with an active dimension of 4×4cm 2 . The gas mixture contained C 2 H 2 F 4 +5% isobutane+10% SF 6 . A few percent of streamer discharges, each releasing about 20 pC, was tolerated without any noticeable inconvenience. This detector opens perspectives of affordable and reliable high granularity large area TOF detectors, with an efficiency and a time resolution comparable to existing scintillator-based TOF technology but with significantly, up to an order of magnitude, lower price per channel. PACS 29.40. Cs Keywords ALICE TOF RPC Glass The ALICE experiment at the Large Hadron Collider heavy ion program at CERN will include a large area Time-of-Flight (TOF) system, spanning over a 100 m 2 with around 100 000 individual read-out channels. Up to 10 000 charged particles are detected per unit of rapidity per event at midrapidity by this detector; particle identification of most of these particles is covered by the TOF technique. Several types of TOF detectors, including gaseous, have been considered for this application, aiming to provide the required timing accuracy at an affordable cost [1] . One new possibility is the Resistive Plate Chamber (RPC) described in this letter, that was recently developed and tested. Our approach contrasts with a previous application of RPCs to TOF (the Pestov counter [2] ) in that we operate a multigap [3] detector in avalanche mode with a non-flammable gas mixture at atmospheric pressure, although at some sacrifice of timing of the performance. The detector is formed by two identical square-shaped RPC cells electrically connected in parallel. Each cell has an active area of 4×4 cm 2 and is constituted by two metallic electrodes placed on each side of a central glass plate, defining two gas gaps (see Fig. 1 ). No electrical connection is made to the central glass plates, which are made of a commercial variety of dark glass having a resistivity around 10 12 Ω cm. The metallic electrodes were deposited on profiled ceramic plates [4] and the electrical contact made through a small perforation in the plate, placed outside the active gap volume. The width of each gas gap was defined by a set of four spacers placed in the corners of the ceramic plates, also lying outside the active gap volume. A schematic description of the detector is shown in Fig. 1 . The chamber signals were amplified by a 2 ns rise-time current amplifier developed specially for this application [5,6] , followed by a fixed threshold discriminator connected to a TDC with a 50 ps bin width. The output of the amplifier was also sent to a charge integrating ADC that measured the fast signal charge. The charge information was used to correct offline for the finite rise time of the amplifier (slewing correction). The contribution of the electronics noise to the timing jitter was measured to be 50 ps sigma for a signal level of 1.5 pC, with a discriminator level of 20 fC. The detector was filled with a non-flammable gas mixture consisting of C 2 H 2 F 4 +10% SF 6 +5% isobutane [7] and tested at the CERN PS using a secondary beam of 7 GeV pions. Two identical scintillation counters provided a time reference with a resolution of 120 ps sigma and a scintillator-based telescope selected those particles that crossed the detector in a central area of 1 cm 2 . The fast charge spectrum for minimum ionizing particles is shown in Fig. 2 , corresponding to an average fast signal of 1.5 pC in avalanche mode. The sharp left cutoff corresponds to the ADC pedestal and the narrow peak close to 6 pC corresponds to the saturation of the ADC. The overall shape of the spectrum above 2 pC is influenced by the non-linearity of the electronics. About 2% of the particles triggered the formation of streamers, each developing about 20 pC, in qualitative agreement with [8] . A typical streamer pulse is shown in Fig. 3 . In Fig. 4 we show a typical time difference spectrum between the start counters and our detector, with a resolution of 107 ps sigma, after quadratic subtraction of the start counters contribution (120 ps). At counting rates below 800 Hz/cm 2 the timing resolution for MIPs was between 100 and 120 ps with a detection efficiency above 98% ( Fig. 5 ). The performance was degraded for higher counting rates, presumably due to time-dependent voltage drops across the resistive plate. Other similar detectors with gas gaps of 0.4 and 0.6 mm were also tested and the available data ( Fig. 6 ) suggest that the timing resolution of thin gap parallel geometry detectors depends mainly on the width of the gas gap. One can approximately describe this dependency by a straight line with a slope of 40 ps/0.1 mm. This observation suggests that in principle further gains in timing accuracy may be achieved by reducing the gap width. For very small gas gaps one expects a reduction in efficiency, but this can be compensated for by increasing the number of intermediate resistive plates. However, in order to reach timing resolutions comparable to those featured by the Pestov Counter [2] (30 ps sigma), but working in proportional mode and at atmospheric pressure, further progress in electronics must also be made. The following colleagues, from our home institutes, contributed to this work: A. Akindinov, E. Cerron-Zeballos, R. Ferreira-Marques, V. Golovine, D. Hatzifotiadou, J. Lamas-Valverde, A. Martemianov, V. Petrov, F. Piuz, A. Policarpo, K. Voloshin. References [1] ALICE - Technical Proposal for A Large Ion Collider Experiment at the CERN LHC, CERN/LHCC/95-71, December 1995. [2]...

We present results on dielectron production in ${}^{40}\mathrm{Ar}+\mathrm{KCl}$ collisions at 1.76$A$ GeV. For the first time $\omega$ mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The $\omega$ multiplicity has been extracted and compared to the yields of other particles, in particular of the $\phi$ meson. At intermediate $e^{+}{e}^{-}$ invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions, suggesting the onset of nontrivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for a pair cocktail dominated at intermediate masses by $\Delta$ Dalitz decays.

We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e- > 0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.