Signatures for Majorana Neutrinos at Hadron Colliders

Abstract

The Majorana nature of neutrinos may only be experimentally verified via lepton-number violating processes involving charged leptons. We explore the ∆L = 2 like-sign dilepton production at hadron colliders to search for signals of Majorana neutrinos. We find significant sensitivity for resonant production of a Majorana neutrino in the mass range of 10 − 80 GeV at the current run of the Tevatron with 2 fb −1 integrated luminosity, and in the range of 10 − 400 GeV at the LHC with 100 fb −1 .Neutrinos are arguably the most elusive particles in the standard model (SM) spectrum. The evidence is strong that neutrinos are massive and their flavors defined with respect to the charged leptons oscillate [1], indicating the need of extension beyond the SM. We do not know the nature of the mass generation and flavor mixing. In particular, we are clueless if neutrinos are of Dirac or Majorana type -the former preserves the lepton number (L), and the latter violates it by two units. Thus, the unambiguous proof of the existence of a Majorana neutrino is the observation of a lepton-number violation process, which would have profound implications in particle physics, nuclear physics, and cosmology. Since neutrinos are so weakly interacting and leave no trace in ordinary detectors, the only appropriate signatures must involve charged leptons via the charge-current interactions for a ∆L = 2 process.The simplest extension of the Standard Model to include Majorana neutrinos is to introduce n right-handed SM singlet neutrinos N aR (a = 1, 2, · · · , n), and n ≥ 2 in order to generate at least two massive neutrinos. Besides the Dirac masses m D from the Yukawa interactions, there is also a possible heavy Majorana mass term relations between the flavors defined with respect to the charged leptons ℓ and mass eigenstatesIn the simplest incarnation without further flavor structure or new states, the light neutrino masses m ν m are of the order of magnitude m 2 D /B, while the heavy neutrino masses are m N m ′ ≃ B. The corresponding mixing angles are of V V * ∼ m ν m /m N m ′ , and thus U U † ≈ I. However, we will take a phenomenological approach toward the mass and mixing parameters without assuming any relationship a priori.In terms of the mass eigenstates, the gauge interaction Lagrangian can be written aswhere P L is the left-handed chirality projection operator. There exist constraints on the heavy neutrino mass and the mixing elements V ℓm ′ . Since we are interested in the collider searches, we consider m N ≫ 1 GeV. By far, the strongest bound is from the non-observation of the neutrinoless double-β decay (0νββ) [2]. It translates

Broadening the Higgs boson with right-handed neutrinos and a higher dimension operator at the electroweak scale

“…I note that these formulae for the masses and mixing angles are exact in the limit of universal Majorana masses and no CP violation in the Dirac masses [11]. For these fiducial values of the parameters no limits exist from the neutrinoless double β decay experiments or collider searches [5] because the mixing angles are too tiny. No limits from cosmology exist either since the right-handed neutrinos decay before big bang nucleosynthesis if M I ∼ > O(GeV), which will be assumed throughout (see Section 5 for the decay length of the right-handed neutrinos).…”

“…it will be proportional to at least two powers of the neutrino couplings. Likewise, in generating O (5) 3 from such loops its coefficient is always proportional to at least one power of the neutrino coupling. In particular, O (5) 2 is generated directly at two-loops, with c…”

LHC bounds on lepton number violation mediated by doubly and singly-charged scalars

“…The consideration of doubly-charged scalar decays into dileptons and dibosons on 6 We assume that the new fields do not carry color because we search for LNV particles which mainly manifest as dileptonic resonances. 7 Even if vector boson fusion contributions are large [63], EWPD including LHC data on the SM Higgs further reduce the limits on lepton mixing [16] and hence, the LHC potential for heavy neutrino detection [54][55][56]. In any case, LHC direct limits on heavy neutrino production provide independent evidence and restrict the allowed range of heavy neutrino masses [64,65], which are indirectly not accessible to lowest order in the expansion in the small lepton mixing.…”

“…6 (Obviously, singlets can be produced through mixing with non-singlet states but this mechanism is in general suppressed by the corresponding mixing angles. An example is heavy neutrino production through mixing with SM leptons [54][55][56], which is suppressed because the corresponding mixing angles are bounded to be small by EW precision data (EWPD) [62]. 7 )…”

“…What in practice means observing events with an excess of leptons, or anti-leptons. Among the three see-saw mechanisms, the see-saw of type II gives the cleanest signal because doubly-charged scalars can decay into pairs of same-sign charged leptons, ∆ ±± → l ± l ± , which accumulate around the doubly-charged scalar mass and allow for a very efficient search [52,53]; which is not the case for heavy fermions because they decay into an odd number of light fermions (≥ 3, if we exclude decays into Higgs bosons decaying in turn into two photons) [54][55][56][57][58][59], as required by rotational invariance. As a matter of fact, CMS [60] and ATLAS [61] have already set stringent limits on this process, excluding doubly-charged scalar masses m ∆ ±± ranging from 200 to 400 GeV, depending on the assumptions on the branching ratios into same-sign dileptons; although they have not reported on the corresponding limits for LNV.…”