Wednesday, March 9, 2011

Losing the last motivation I had for an extra fermion family!

As we all know, the Standard Model(SM) of particle physics is a gauge theory of three families of quarks and leptons. Recently there were lots of discussions in the community regarding one more family of quarks and leptons, bounds on their masses and mixing with the observed fermions etc etc. Although the precision electroweak data still allows the inclusion of one more chiral family into the SM (not more than one), it hardly provides a solution to any of the problems SM suffers from. But "anything which is not forbidden must happen" attitude forces one to study the implications of four generation SM in colliders as well as cosmology. LEP II results constrains the mass of the fourth generation neutrino to lie above the Z-boson mass threshold i.e. around 45 GeV. Similarly there are various lower bounds on the charged fermions coming from precision data at LEP as well as null searches at Tevatron. From cosmology also, there are restrictions on number of light (~eV) neutrinos which prevents us from incorporating one more light neutrino into the model. One additional problem arises with the perturbativity. Since the fourth generation fermions get masses have to lie above the experimental lower bounds, their Yukawa couplings are quite large. And they become non-perturbative around few TeV scale when evolved under the Renormalization Group Equations (RGE). This demands new physics around the TeV scale which can keep the couplings perturbative. However, I have not seen any good candidate so far which can achieve this purpose. Enlarging the gauge symmetry or incorporating Supersymmetry do not help. TeV scale Extra dimensions might help although. Adding a large number of vector like particles in an ad-hoc fashion seems to work. This serious issue has to be addressed in any models involving four generations and no good models have come up so far. One motivation for fourth generation seems to come from the like sign dimuon charge asymmetry seen in Tevatron last year which was around 3 sigma deviation from SM predictions. Four generation models seems to have an explanation for this. But there are many other new physics explanation for this anomaly and there is no good reason to prefer four generation models over others. One more motivation could be related to Dark Matter. However it turns out that such a heavy neutrino will have very little abundance in the present Universe due to too much self annihilation. However if it's mass is as heavy as few TeVs then it can account for the correct abundance of Dark Matter in the present Universe. But to explain the origin of mass of such a heavy neutrino, we need non-perturbative Yukawa couplings. Also, such a heavy neutrino with SM couplings to the Z boson will give too high cross-section with a nuclei in direct detection experiments and hence will be ruled out from CDMS and XENON upper limits. Thus four generation scenario do not seem to be very motivating like the other beyond standard model frameworks. I would not be surprised if LHC rules out its existence :-)

No comments: