Monojet search for heavy neutrinos at future $Z$-factories

Abstract

The existence of heavy neutrinos is the most prominent feature of type-I seesaw-like mechanisms, which naturally explain tiny masses of observed neutrinos. Various experiments have been conducted to search for heavy neutrinos, with meson decays sensitive to heavy neutrinos lighter than B mesons and high energy collider experiments sensitive to the ones with masses larger than O(10) GeV. In between, there is a mass gap uncovered by these two kinds of experiments, and by this work, we investigate the 5-15 GeV neutrino searches at future Z-factories via the process e − e + → Z → νN → ν qq with = e, µ. Based on the simulation of signal and background events, we find that the signals appear as monojet events owning to the large Lorentz boost of the heavy neutrinos, and that the monojet substructures are essential to reconstruct the signal and suppress the background. The upper bounds on the cross section of the signal process and the mixing parameter |V N | 2 are obtained. Compared with other experiments, the monojet method at future Z-factories will be able to fill the gap around this mass range.

The present and future status of heavy neutral leptons

“…The FCC-ee will have good sensitivity above the charm mass all the way up to the W boson mass for different values of the mixing angle with the existing neutrinos. Sensitivity down to heavy-light mixing of |V ℓN | 2 ∼ 10 −8 − 10 −12 was obtained in [505,506], covering a large parameter space for heavy neutrino masses between 5 and 80 GeV. This sensitivity includes the regime where Z → N decays result in boosted HNLs [506].…”

“…Sensitivity down to heavy-light mixing of |V ℓN | 2 ∼ 10 −8 − 10 −12 was obtained in [505,506], covering a large parameter space for heavy neutrino masses between 5 and 80 GeV. This sensitivity includes the regime where Z → N decays result in boosted HNLs [506].…”

“…Colourful lines: estimated sensitivities of the main detectors at the HL-LHC [85,451,723,724] and NA62 [139], compared to the sensitivities of selected proposed experiments (FASER2 [484], DUNE [600], MATHUSLA [482], SHiP [515,595], CODEX-b [480], see [64] for a more comprehensive list) as well as selected future colliders (ILC [523,725], FCC-ee or CEPC [506,516,726], FCC-hh [85,507,514,727], muon colliders [728], LHeC and FCC-he [519], where DV indicates displaced vertex searches). The curves from [506,723,726] have been re-scaled to an integrated luminosity consistent with [85,451,516]. The sensitivity of future colliders could be considerably improved with dedicated long-lived particle detectors [510,511,514,729].…”

“…Colourful lines: estimated sensitivities of the main detectors at the HL-LHC[85,451,723,724] and NA62[139], compared to the sensitivities of selected proposed experiments (FASER2[484], DUNE[600], MATHUSLA[482], SHiP[515,595], CODEX-b[480], see[64] for a more comprehensive list) as well as selected future colliders (ILC[523,725], FCC-ee or CEPC[506,516,726], FCC-hh[85,507,514,727], muon colliders[728], LHeC and FCC-he[519], where DV indicates displaced vertex searches). The curves from[506,723,726] have been re-scaled to an integrated luminosity consistent with[85,451,516]. The sensitivity of future colliders could be considerably improved with dedicated long-lived particle detectors[510,511,514,729].Brown band: lower bound on the total HNL mixing U U U e 2 2 2 + + m t from the requirement to explain the light neutrino properties [40].…”

Exploring the potential of FCC-hh to search for particles from $B$ mesons

“…Such searches are currently running [8,9], with being significantly improved during the High-Luminosity phase of the LHC (see, e.g., [10]). Future leptonic colliders, such as FCC-ee [11,12] and CEPC [13], that would operate in the regime of Z resonance ( √ s = m Z ), may further explore the probed parameter space [14], (see [15][16][17][18] for the discussion of other type of searches).…”