Indirect Hints and Direct Search for 4th Generation in LHC Era -- George Hou (National Taiwan University)

jeudi 14 oct. 2010, 14:00 → 15:00 Europe/Paris

salle 12-22-RC-08 (LPNHE)

The 4th generation has been deemed ruled out by neutrino counting and electroweak precision tests. However, neutrinos have mass, while S and T variables are not as unforgiving as one had thought. In 2004, indications appeared for a difference between direct CP violation in charged vs neutral B -> Kpi decay, confirmed by 2007, points to the possibility of new CPV sources in electroweak b -> s penguins. Although the effect could also arise from so-called color-suppressed tree diagram, a prediction was made in 2005 (strengthened in 2007), by correlation between Z penguin and box diagrams, for large and negative TCPV in Bs -> J/psi phi, where tantalizing hints appeared in 2008 at the Tevatron. Though the situation in 2010 appears mixed, the situation would be clarified soon by the LHCb experiment. No amount of indirect hints, however, can replace the power of direct discovery, and the CDF and Dzero experiments have both set stringent bounds on 4th generation t' (and b') quark masses, but with the LHC coming up, direct search would make great progress in the next two years by the ATLAS and CMS collaborations. The ever increasing bounds, on the other hand, rekindles the thought that perhaps electroweak symmetry breaking itself could arise from the very large (and possibly nonperturbative) Yukawa couplings. While this still needs to be clarified, even for the purpose of conducting very high mass search, perhaps the best reason why there may be a 4th generation is that the "Jarlskog"-like CPV measure could jump up 1000 trillion fold from the 3 generation case, and thereby touch the problem of baryon asymmetry of the Universe. Thus, the search for the 4th generation quarks at the LHC is not only mandatory, but carries grand goals as well.