Towards grand unification?

I wrote earlier in the blog "The countdown to the beginning" about the early nature of the universe. The cosmic inflation theories suggest that all energy was bound in the form of the inflation field in the beginning. The field decayed eventually to the particles presented by the standard model. The standard model is established and well proved model for elementary particles and their interactions. Making it simple, it divides particles to six quarks, six leptons, and bosons that are force carriers for the three fundamental interactions. Quarks are interconnected together by strong force, and leptons by weak and electromagnetic forces. In the high enough energy scale (over 246 GeV) leptons interact by combined electroweak force (at the first 10^-12 seconds of universe). Even the standard model has been successful and self-consistent it does not incorporate the gravity interaction described in the general relativity. Additionally, the standard model is quite complex and many parameters are given. It does not explain sufficient reasoning for charges, large hierarchy of arbitrary masses and different weak interactions, the current excess of matter over antimatter, and small Higgs boson mass. Thus it is believed that standard model is an approximation of more complete and fundamental theory. 

In 1974 Howard Georgi and Sheldon Glashow proposed the unified electroweak and strong interaction described by a single, larger simple symmetry group SU(5) containing the standard model SU(3) x SU(2) x SU(1). The SU(5) presented the start for the later labeled Grand Unification Theory (GUT) family which developed through some variations to larger symmetry groups and Supersymmetry models (SUSY). One motivation for the theory is that leptons and quarks seem to cancel each others charges exactly to extreme precision. Furthermore, mass symmetry is apparently visible between quark and lepton generations. Fluctuation between quarks and leptons also enables asymmetric generation quarks over antiquarks. Decay to The unification energy scale is extreme (10¹⁶ GeV) and will never be the tested in particle collision experiments. On the other hand, the theory has failed in its predictions in reachable energy scales. The unified electronuclear force is carried by heavy hypothetical elementary X and Y bosons that interact between quarks and leptons. The X boson would decay to two up quarks or to one positron and one down antiquark. Consequently, in an energetic collision of two up quarks an intermediate X boson would emerge and decay to positron and down antiquark. In this case a proton would decay to a positron and antimuon that is left over of annihilation of down quark and down antiquark. A proton decay could be experimented even within ordinary energy scales by increasing the likelihood of occurrence with a huge amount of protons. The results from the Super-Kamiokande water Cherenkov radiation detector (detecting 50 000 tons of water) in Japan have placed lower limits on the lifetime of a proton that now exceed 10³⁴ years. This has ruled out the simplest GUT models unless the universe is both supersymmetric and contains extra dimensions. However, experiments so far are done for protons bound to atoms and molecules, which could make them much more stable than free protons. So there's still hope. If proton is truly stable it will cripple many other theories extending the standard model, including the SUSY and string theory. 

The first generation of fermions are sufficient to describe matter under ordinary circumstances: top and down quark; electron and electron neutrino. The two other generations presented with muon and tau lepton flavors are not stable and have been observed only in laboratory and cosmic ray experiments. In last year several B meson decay measurements showed that some electroweak processes were not lepton-flavor independent, contrary to the lepton universality principle ingrained in the standard model. As unpredicted, the decay process produced more tau flavored leptons compared to other leptons. Also electrons dominated muons in earlier LHC results. This discrepancy was explained by a new interaction meditated by a proposed W prime or leptoquark boson. CERN CMS researchers continued to investigate a previously unexplored leptoquark signature involving the third generation of fermions. The analy­sis assumed that the leptoquark decays half the time to each of the possible quark–lepton flavor pairs, for example, in the case of a spin-1 leptoquark to a top quark and a tau neutrino, or to a bottom quark and a tau lepton. CMS finds a range of lower limits on the leptoquark mass between 0.98 and 1.73 TeV. This means that we can soon expect direct observations of the leptoquark from particle collider tests. The first generation leptoquark coupled to up/down quarks and electrons is expected to have much higher mass which would make the probability of proton decay very low. 

Another taking from the last year was Muon g-2 experiment done in Fermilab that didn’t perfectly match the standard model prediction of the magnetic moment of the muon particle. This finding could also point to undiscovered particles (such as Z prime boson) that interact with muons. The SUSY can also explain this kind of discrepancies. It is still too early to say but it seems that we will get soon solid evidence beyond the standard model. Some could show a way to more unified and symmetric particle physics. Currently searched leptoquarks may be a milestone in a puzzle towards grand unification theories that wait for revising.  Many questions arise like whether the hypothetical X and Y bosons are related to the three eagerly investigated leptoquark candidates, are they more unified particle presentations binding the fermion generations together, or are they needed at all.