lundi 13 juillet 2015

Summer program 2015 : riding the W ' ave .... (LHC season 2, episode 1)

... to reach new Heig(g)hts ?
I carry on reporting some news about the hint of a signal for a new weak gauge boson W' at LHC: a potential discovery classified as "not unexpected" in a recent review by Chris Quigg. In this post I chose to follow the work from authors of a specific model reported previously in this blog just for the sake of obstinacy ;-). 
The reader will encounter a detailed discussion of the scalar sector proposed to accommodate the spontaneous breaking of a left-right symmetric extension of the standard model at the TeV scale and she will get more information about the potential falsification of such a phenomenological construction expected for the end of this year thanks to the 13 TeV available now for LHC Run2! 

Using LHC data at √s=8 TeV, the ATLAS and CMS Collaborations, have reported deviations from the Standard Model (SM) of statistical significance between 2 and 3σ in several final states, indicating mass peaks in the 1.8–2 TeV range [1]-[5]. The cross sections required for producing these mass peaks are consistent with the properties of a W' boson in an SU(2)L×SU(2)R×U(1)B-L gauge theory with right-handed neutrinos that have Dirac masses at the TeV scale [6]. 
The spontaneous breaking of SU(2)×SU(2)×U(1) gauge groups require an extended Higgs sector. For large regions of parameter space, the W' boson have large branching fractions into heavy scalars from the Higgs sector [7][8]. We show here that the W' boson hinted by the LHC data is likely to decay into H+A0 and H+H0, where H+A0 and H0 are heavy spin-0 particles present in Two-Higgs-Doublet models. We compute the branching fractions for these decays and present evidence that signals for the W'→ H+A0/H0 processes may already be visible in the 8 TeV LHC data.  
There are numerous and diverse studies of SU(2)L×SU(2)R×U(1)B-L models, spanning four decades [10]. An interesting aspect of the left-right symmetric models is that they can be embedded in the minimal SO(10) grand unified theory. This scenario must be significantly modified due to the presence of Dirac masses for right-handed neutrinos required by the CMS e+e−jj events [Unless the right-handed neutrinos have TeV-scale masses with the split between two of them at the MeV scale [9]].1 The theory introduced in [6] involves at least one vectorlike fermion transforming as a doublet under SU(2)R. This may be part of an additional SO(10) multiplet, but it may also be associated with completely different UV completions... 
The Higgs sector of the SU(2)L×SU(2)R×U(1)B-L gauge theory discussed in [6] consists of two complex scalar fields: an SU(2)R triplet T of B−L charge +2, and an SU(2)L× SU(2)R bidoublet Σ of B−L charge 0.  
The renormalizable Higgs potential is given by V(T)+V(T,Σ)+V(Σ)... The bidoublet-only potential V(Σ) is chosen such that by itself it does not generate a VEV for Σ ... the T scalar acquires a VEV ... this breaks SU(2)R×U(1)B-L down to the SM hypercharge gauge group, U(1)Y , leading to large masses for the W' and Z' bosons. The value of the T VEV is related to the parameters of the Wboson. In the next section we will show that the parameters indicated by the LHC mass peaks near 2 TeV imply uT≈ 3−4 TeV. 
The triplet field includes 6 degrees of freedom, and can be written as T = (T1, T2, T3), with Ti (i = 1, 2, 3) complex scalars... The fields of definite electric charge, which are combinations of the Ti components, include three Nambu-Goldstone bosons (GR±, GR-). These become the longitudinal degrees of freedom of the W'± and Z' bosons. The three remaining fields are a real scalar T0, a doubly-charged scalar T++ and its charge conjugate state T--... For quartic couplings in the 0.1–1 range and in the absence of fine tuning, the T0 and T++ particles have masses comparable to, or heavier than W' 
The mixed terms, which involve both the T and Σ scalars, will induce a nonzero VEV {proportional to vH} where vH=174 GeV is the electroweak scale. We are interested in the case where vH/uT∼1/20. The effect of the Σ VEV on the T0 and T++ masses and couplings is thus negligible. At energy scales below the T0 and T++ masses, the scalar sector consists only of Σ, which is the same as two Higgs doublets.
.... Besides h0 and the longitudinal W± and Z, the bidoublet field Σ includes the heavy scalars H±H0A0. The range of allowed masses for these particles spans more than an order of magnitude, from the weak scale to the SU(2)R breaking scale. If they are lighter than the W' boson, then W' decays may provide the main mechanisms for production of these scalar particles at hadron colliders. 
...For MW',≈1.9 TeV and gR≈0.45–0.6 (as determined in [6], by comparing the W' production cross section to the CMS dijet excess [4]), we find the SU(2)breaking scale uT ≈ 3–4 TeV.
...The main decay modes for the heavy Higgs bosons are A0H0t and H+→t. If their masses are below MW'/2, then the cascade decay W'H±A0/H0→3t+b→3W+4b has a branching fraction of up to 3% and provides a promising way for discovering all these particles. An excess of events, with a statistical significance of about 3σ, has been reported by the ATLAS Collaboration [17] in the final state with two leptons of same charge and two or more b jets. We have shown that this can be explained by the cascade decay of W' if the heavy Higgs bosons have masses in the 400–700 GeV range. 
The SU(2)L×SU(2)R×U(1)B-L gauge theory presented here depends on only a few parameters, whose ranges are already determined by accounting for the deviations from the SM mentioned above. The various phenomena predicted by this gauge theory can thus be confirmed or ruled out in the near future. 
In Run 2 of the LHC, the W' production cross section is large, in the 1–2 pb range at √s=13 TeV. Besides resonant production of WZ, Wh0, jj and tb, there are several W' discovery modes: W'→ τNτ → ττjj, ττtb, eτjj, eτtb and W'→eNτ→eejj, eetb would test the existence of the heavy right-handed neutrino Nτ {with benchmark mass of 1 TeV}, while W'→H±A0/H0→3t+b, W'→WA0/H0→Wt, W'→H±h0→tbh0 and others would test the existence of the heavy Higgs bosons. 
Another promising search channel for the heavy neutral Higgs bosons, independent of the W' , follows from production in association with a t pair, which has a cross section of the order of 10 fb at √s=13 TeV. With more data, the Z' boson analyzed in [6] will also be accessible in a variety of channels.
(Submitted on 7 Jul 2015)

Summertime is a good opportunity to build wonderful sandcastles but few withstand waves ... 
No offence to the hard work of phenomenologists of course but due respect to these wizards and their fascinating ingenuity as quantum "mechanics" ;-)

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