Wednesday, May 25, 2016

Higgs to mu-tau decays would encourage \(B-L\) SSM

While Egyptian airplanes keep on collapsing (President el-Sisi's foes have decorated the same aircraft with graffiti "we will bring this aircraft down" some two years ago!) and Erdogan keeps on blackmailing Europe (and proving the cluelessness of the politicians who wanted or want to promote Turkey into a key solver of Europe's problems), a new Turkish-Egyptian hep-ph paper looks unexpectedly sexy:
Large \(BR(h \to \tau \mu)\) in Supersymmetric Models
Hammad, Khalil, and Un analyze how compatible the simple enough supersymmetric models are with a possibly emerging result of the LHC analyses – namely the decay of the Higgs to a flavor-violating pair\[

h\to\mu^\pm \tau^\mp

\] which seems to appear in roughly a 2-sigma excessive number of events both at ATLAS and CMS and the suggested branching ratio is around 1%.

The trio says that it's very unlikely that the normal minimal supersymmetric standard model may generate this flavor-violating decay. The supersymmetric seesaw model generates the the flavor mixing radiatively and the predicted branching ratio is much smaller than 1%.

On the other hand, they find out that the supersymmetric models with the \(B-L\) gauge symmetry predict the large branching ratio rather generically and naturally. That's cool because I have had other reasons to love the \(B-L\) models.

Note that \(B-L\) naturally arises as a gauge symmetry in the left-right-symmetric models that have been discussed (also on this blog) because of the (now largely abandoned) hints of new gauge bosons around \(2,3\TeV\). In these models, the left-right-asymmetric Standard Model formula for the electric charge \(Q=Y/2+T_3\) is replaced with \(Q=(B-L)/2+T_{3L}+T_{3R}\), a combination of generators of gauge groups \(U(1)_{B-L}\times SU(2)_L\times SU(2)_R\). With the extra \(SU(2)_R\), the left-handed and right-handed 2-component spinor parts of the quark and lepton fields may be reunified into a single doublet again while the \(125\GeV\) Higgs is supposed to be a state in the \((2,2)\) representation.

Such left-right-symmetric models may naturally be embedded in grand unified theories but \(SU(5)\) isn't enough. You need \(SO(10)\) or \(E_6\) – which I have always preferred, anyway, not only because they unify the representations of quarks and leptons into one (per generation; the three reps may be unified into one if one uses a flavor symmetry). One could argue that there exist several other hints, including the \(750\GeV\) cernette, which are more likely to be explained within left-right-symmetric or \(SO(10)\) or \(E_6\) models than asymmetric or \(SU(5)\)-based models.

The LHC collisions will be restarted tomorrow, on Thursday. Each major detector has collected about 0.8 inverse femtobarns in 2016. Within a month, the integrated luminosity for 2016 could surpass the luminosity for the whole year 2015, which was below 4/fb, and the LHC could accelerate its data collection campaign soon afterwards.

Many things are possible and perhaps likely: the \(750\GeV\) cernette, gluino near \(1.5\TeV\) and various other, less visible superpartners around \(0.5\TeV\), the flavor-violating decay of the Higgs, and others. 2016 could be the year when the experiments will jump ahead of the particle physics phenomenology – in the sense that the phenomenologists will have to work hard to catch up with the experiments. At the same moment, I tend to feel that the explanation of those new discoveries, if any, will be rather conservative – compatible with the scenarios that I have considered the prettiest ones for decades.

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