Constraints from the CHARM experiment on heavy neutral leptons with tau mixing

Searches for new physics at SND@LHC

“…The actual sensitivity of SND@LHC can be derived taking into account the signal identification efficiency and background level, which are subjects of detailed studies that go beyond the scope of this paper, see section 3.2. Sensitivities of previous experiments and of the FASER/FASER2 experiment are reproduced from [14,68].…”

The Present and Future Status of Heavy Neutral Leptons

“…Recent peak search experiments have been conducted by the E949 experiment at BNL [23], the PIENU experiment at TRIUMF [24,25], and the NA62 experiment at CERN [26][27][28][29]. Other accelerator experiments searching for HNLs include PS-191 [30,31], CHARM [32,33], NuTEV [34], DELPHI [35], T2K [36], and a recent reinterpretation of BOREXINO data [37]. The peak search experiments looking for anomalous monochromatic peaks in the charged lepton spectra of decaying π + and K + appear to place more stringent upper bounds on mixing angles of HNLs than any other terrestrial experiments over a large range of HNL masses from ∼ 40 MeV to ∼ 2 GeV.…”

Constraints on Heavy Neutral Leptons interacting with a singlet scalar

“…7 of Ref. [48]). We then fix log 10 f = −15.6 to match the original limit, giving the number of events N E = N D × f = 2.24 < 2.3 at 90% C.L..…”

“…Although the original CHARM analysis did not include U τ constraints, Ref. [48] extended their results to do so by including neutral current contributions to the HNL decays, and we have done likewise.…”

“…The CHARM Collaboration searched for HNLs in the mass range 0.5 − 1.8 GeV [45]. In subsequent analyses [11,48], this range was expanded up to 2 GeV by considering also the production of D s , which we emulate here.…”