The 2TeVbump@LHC chronicle : turning the degree of lepton number violation knob
Recently, a number of resonance searches at the √s=8 TeV LHC have reported a handful of excess events around invariant mass of 1.8 – 2 TeV. The most significant ones are: (i) a 3.4σ local (2.5σ global) excess in the ATLAS search  for a heavy resonance decaying into a pair of Standard Model (SM) gauge bosons V V (with V = W, Z); (ii) a 2.8σ excess in the CMS search  for a heavy right-handed (RH) gauge boson WR decaying into an electron and RH neutrino NR, whose further decay gives an eejj final state; (iii) a 2.2σ excess in the CMS search  for W' → WH, where the SM Higgs boson H decays into bb̅ and W→ℓν (with ℓ=e,µ); (iv) a 2.1σ excess in the CMS dijet search . These excesses of course need to be confirmed with more statistics at the LHC run II before any firm conclusion about their origin can be derived. Nevertheless, taking them as possible indications of new physics beyond the SM, it is worthwhile examining whether all of them could be simultaneously explained within a self-consistent, ultra-violet (UV) complete theory that could be tested in foreseeable future.
One class of models that seems to broadly fit the observed features in all the above-mentioned channels is the Left-Right Symmetric Model (LRSM) of weak interactions based on the gauge group SU(2)L×SU(2)R×U(1)B-L , with the RH charged gauge boson mass MWR∼2 TeV and with gR<gL at the TeV-scale , gL(R) being the SU(2)L(R) gauge coupling strength. Within this framework, the eejj excess  can be understood [6,7,8,9] as pp → WR → eNR → eejj  (for a review, see e.g. ) and is related to the type-I seesaw mechanism  for neutrino masses. The W Z excess  and WH excess  can be understood [6, 13, 14] in terms of WR → WZ, WH, since these couplings naturally arise in these models from the vacuum expectation values (VEVs) of the bidoublet field used to generate quark and lepton masses . Finally, the dijet excess  can simply be understood in terms of WR → jj.
However, a particular aspect of the observations in the eejj channel, namely, a suppression of same-sign electron pairs with respect to opposite-sign pairs , cannot be explained within the minimal LRSM with type-I seesaw mechanism. This is because of the fact that for a type I seesaw interpretation of the eejj excess, one expects equal number of same and opposite-sign dileptons due to the purely Majorana nature of the RH neutrinos [an exception is when the interference of two non-degenerate RH Majorana neutrinos with mixed flavor content and opposite CP parities can partially suppress the same-sign dilepton signal ]. Thus, a heavy pseudo-Dirac neutrino, as naturally occurs in the inverse seesaw mechanism , seems to be the simplest possibility to explain the suppression of same-sign dilepton events in both CMS [2, 16] and ATLAS  searches.
The main result of this paper is that if the difference between same and opposite-sign dilepton signal becomes statistically significant, it more likely suggests an inverse seesaw interpretation rather than a type-I seesaw. Note that in the original inverse seesaw proposal , the lepton number violation is small, being directly proportional to the light neutrino masses, and hence, it is rather unlikely to observe any same-sign dilepton events in this scenario. In this paper, we show that there exists a class of inverse seesaw models where the heavy neutrinos are still Majorana fermions with non-negligible lepton number violation, without affecting the inverse seesaw neutrino mass formula. In this class of models, it is possible to accommodate a non-zero same-sign dilepton signal, while being consistent with the suppression with respect to the opposite-sign signal. In particular, a statistically significant non-zero ratio of same and opposite-sign dilepton signal events could be used to test the relative strength between the Dirac and Majorana nature of the heavy neutrinos at the LHC.
Another important result of this paper is that our TeV scale LRSM with inverse seesaw unifies to an SO(10) Grand Unified Theory (GUT) at a high scale MU∼1017GeV without introducing any other intermediate scales, which is remarkable for a non-supersymmetric theory. This is achieved with a minimal TeV-scale particle content, which predicts the value of gR≃0.51 at the TeV scale, thus naturally satisfying the requirement gR<gL to explain the excess events mentioned above. Moreover, such a single-step unification without introducing supersymmetry (SUSY) also requires the existence of SU(2)- triplet fermions [at TeV scale which can play the role of Dark Matter  and TeV-scale color-octet, SU(2)L-singlet scalar fields which can lead to new signals  at the LHC and future colliders]. Finally, this model could also explain the observed baryon asymmetry of the Universe via leptogenesis through the out-of-equilibrium decay of the heavy Majorana neutrinos, while avoiding the stringent leptogenesis bounds on MWR  due to suppressed WR -induced washout in this case .
(Submitted on 10 Aug 2015)