What could a hint of non-supersymmetric physics beyond the standard model look like at the LHC?

A putative fluke in an experimental exclusion limit study looking for right-handed bosons arising in the minimal left-right symmetric extension of the Standard Model



 A search for right-handed bosons (WR) and heavy right-handed neutrinos (Nl) in the left-right symmetric extension of the standard model has been presented. The data sample is in agreement with expectations from standard model processes in the µµjj final state. An excess is observed in the electron channel with a local significance of 2.8σ at Meejj ≈ 2.1 TeV. The excess does not appear to be consistent with expectations from left-right symmetric theory. Considering WR → e Ne and WR → µNµ searches separately, regions in the (MWR, MNl) mass space are excluded at 95% confidence level that extend up to MWR < 3.0 TeV for both channels. Assuming Wregion in the two-dimensional (MWR, MNl) mass plane compared to previous searches, and for the first time this search has excluded MWvalues beyond the theoretical lower mass limit of MWR > 2.5 TeV.

... turning into a speculative smoking-gun for a left-right symmetry model with a spontaneous D-parity breaking motivated by a non-SUSY SO(10) with a Pati-Salam subgroup

The CMS analysis compares the experimental result with the theoretically predicted cross section in the minimal left right symmetric model (LRSM) using  gL=gR. In this case, the ee excess cannot be explained by the theoretical prediction since the predicted cross section is too large by a factor of ≈3−4. This discrepancy could be reconciled in our model with a smaller gR. In the following we assume that the excess is due to the production of a WR which decays to a heavy neutrino N that dominantly couples to electrons with a large right-handed current mixing matrix element VNe≤1. We also assume that there is a negligible mixing between the heavy and light neutrinos as well as the left and right W bosons. In this case, both WR and N couple only through right-handed currents and the total cross section of the process under consider where σCMS(pp→eejj) corresponds to the scenario with gL=gR and  VNe=1 as used in the CMS analysis. Instead, using the value derived in the LRSM with spontaneous symmetry breaking and SO(10) unification, the predicted cross section is suppressed by a factor of ≈0.4. This is already sufficient to allow the excess to be interpreted as a signal. In addition, even a small deviation in VNe will lead to a sizable further suppression. This is shown in Fig. 2 where we compare our calculated process cross section with the CMS result.

We have shown that a TeV scale left-right symmetric model can naturally arise via spontaneous D-parity breaking. The asymmetry between the gauge couplings near the electroweak symmetry breaking scale is then a consequence of gauge coupling unification. Assuming that the Pati-Salam symmetry SU(2)L×SU(2)R×SU(4)C is the largest sub-group of a non-supersymmetric SO(10) grand unified theory we obtain gR/gL≈0.6. This gives rise to an extra suppression in the production of WR in proton-proton collisions. As a result we could reconcile our prediction for WR → eejj events at the LHC with the recent 2.8σ excess within the mass range 1.9 TeV < MWR < 2.4 TeV, reported recently by the CMS collaboration. If this result is confirmed by future data, it would be the first direct evidence for physics beyond the standard model from LHC, which will rule out the SU(5) GUT. Moreover, a TeV scale WR would imply B − L violation at the TeV scale (which will also be the first evidence for baryon or lepton number violation), which has strong implication on the generation of baryon asymmetry of the Universe as well as the mechanism of neutrino mass generation. For example, if the excess were to be confirmed for the same sign lepton events, sizable contributions to neutrinoless double beta decay are possible and high scale models of Leptogenesis would be strongly disfavoured [13]. While the excess cannot be considered a significant deviation from the SM as of now, the model we discussed here demonstrates that the excess can be explained in well-motivated extensions of the minimal left-right symmetric model.
Frank F. Deppisch et al, (Submitted on 21 Jul 2014)


... en route to a long march from TeV scale partial unification to YeV scale grand unification?




We embed [our] model in a SO(10) grand unified theory, in which the symmetry breaking pattern goes through the Pati-Salam group G224≡SU(2)L×SU(2)R×SU(4)C  as 
SO(10) → G224D→ G224→ G2213→ G2113→ GSM→ G13 
The symmetry breaking of SO(10) to the SM is achieved by the Higgs multiplets 10H126H, 54H and 210H. However, we have introduced two extra Higgs multiplets 16H and 210H in the renormalization group evolution to achieve the unification of gauge couplings. This is shown in Fig. 1. From the gauge coupling unification, the intermediate mass scales are found to be MB-L=(3−6)TeV, M=10TeV, MC=105-6GeV, MD=109.6GeV and MU=1015.89GeV. The most desirable prediction of the model is that the values of gL and gR at TeV scale, consistent with gauge coupling unification, are given by gL ≈0.632 and gR≈0.367. As a result the ratio of right- and left-handed SU(2) gauge couplings around the TeV scale is found to be gR/gL ≈0.58...  
It is interesting that this particular ratio of the gauge coupling strengths allows us to interpret the excess of events at CMS [3] as the signature of right handed charged gauge boson.
 Id.

Some reminders...


We cannot exclude the presence of the SM Higgs boson below 127GeV because of a modest excess of events in the region between 115 and 127GeV.
Speaker: Guido Tonelli (13 december 2011)


As one can see in december 2011 the excess of events compatible with a Higgs boson reported by CMS had the same 2.8σ siginificance  as the excess of events compatible with a TeV scale right handed gauge boson reported nowadays! A big difference nevertheless is that ATLAS had seen a similarly significant excess of events in the same mass range for the Higgs boson at the time while - as far as the blogger knows - ATLAS has not (pre)published anything similar to the CMS observation for the right handed boson and its last report in the search for this kind of particle beyond the Standard Model is two-year-old!
A dedicated search for hypothetical heavy Majorana and Dirac neutrinos N, and Wbosons in final states with two high pT same-sign or opposite-sign leptons and hadronic jets has been presented. In a data sample corresponding to an integrated pp luminosity of 2.1 fb-1 at √s = 7 TeV, no signifcant deviations from the SM expectations are observed, and 95% C.L. limits are set on the contributions of new physics. Excluded mass regions for Majorana and Dirac neutrinos are presented for various operators of an effective lagrangian framework and for the LRSM. The latter interpretation was used to extract a lower limit on the mass of the gauge boson WR. For both no-mixing and maximal-mixing scenarios, WR bosons with masses below ≈1.8 TeV (≈ 2.3 TeV) are excluded for mass differences between the WR and N masses larger than 0.3 TeV (0.9 TeV).
ATLAS Collaboration (last revised 20 Jun 2012 (this version, v2))


The blogger thanks Lubos Motl for his report on the subject!

Update 30/07/2014
The search gets more momentum! Check this up:
Recent searches for a first-generation leptoquark by the CMS collaboration have shown around 2.5σ deviations from Standard Model predictions in both the eejj and eνjj channels. Furthermore, the eejj invariant mass distribution has another 2.8σ excess from the CMS right-handed W plus heavy neutrino search. We point out that additional leptoquark production from a heavy coloron decay can provide a good explanation for all three excesses. The coloron has a mass around 2.1 TeV and the leptoquark mass can vary from 550 GeV to 650 GeV. A key prediction of this model is an edge in the total mT distribution of eνjj events at around 2.1 TeV.
Yang Bai, Joshua Berger (Submitted on 16 Jul 2014)

The CMS Collaboration has published two different searches for new physics that contain possible hints for excesses in eejj and eνjj final states. Interpreting those hints as a possible signal of a right handed gauge boson WR with mass 2-2.5 TeV may have profound implications for our understanding of the gauge structure of nature and Grand Unification, the scalar sector accessible at the LHC, neutrino physics, and the baryon asymmetry of the Universe. We show that this interpretation is, indeed, consistent with all existing constraints. However, before making premature claims we propose a number of cross-checks at the LHC14 that could confirm or falsify this scenario. Those include searches for a ZR resonance and the related new scalar sector around 6-7 TeV. Additionally, large effects in top-quark spin-asymmetries in single top production are possible.
Matti Heikinheimo, Martti Raidal, Christian Spethmann (Submitted on 25 Jul 2014)





 

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