Friday, 17 August 2007

Restoration of the Fourth Generation

I'm not as successful as the others in spreading rumours. So i'm sadly returning to writing about things i've seen with my very eyes. Last week there has been several talks that would deserve a post. I pick up perhaps not the most interesting but the most comprehensible one: Graham Kribs talking about Four Generations and Higgs Physics.
There are three generations of quarks and leptons. Everyone knows that the fourth one does not exist. The Bible says it is excluded at 99.999% confidence level. Graham is spreading the heresy of claiming otherwise. He adds the 4th generation of chiral matter to the Standard Model and reanalyses the constraints on the parameters of such a model.

First of all, there are limits from direct searches. The LEPII experiment set the lower limit of roughly 100 GeV on the masses of the 4th electron and neutrino. The bounds on the 4th generation quarks from the Tevatron are more stringent. CDF excludes the 4th up-type quark lighter than 256 GeV, and Graham argues that a similar bound should hold for a down-type quark.

Indirect limits from flavour physics can be taken care of by asuming that the mixing between the three and the fourth generations is small enough. More troubling are the contraints from the electroweak precision tests. For example, the new quark doublet contributes to the S paramater:
$\Delta S = \frac{N_c}{6 \pi} \left ( 1 - 2 Y \log \frac{m_u^2}{m_d^2} \right )$
If the 4th generation quarks are degenarate in mass, the S parameter comes out too large. Splitting the masses could help keeping the S parameter under control, though it generates new contributions to the T parameter. Nevertheless, Graham argues that there is a large enough window, with the up-type quark some 50 GeV heavier than the down-type quark, where all the precision constraints are satisfied.

The fourth generation may be discovered by direct searches at the Tevatron or at the LHC. But its most dramatic effect would be a profound modification of the higgs physics. Within the Standard Model, the higgs is produced in a hadron collider dominantly via the gluon fusion:
The particle in the loop is the top quark - the only coloured particle in the Standard Model with a sizable coupling to the higgs boson. With the 4th generation at hand, we have two more quarks strongly coupled to the higgs boson. As a result, the higgs production cross section dramatically increases, roughly by the factor of 9. The first hint of the 4th generation could come from the Tevatron who would see the higgs with an abnormally large cross section. In fact, the Tevatron has already excluded the 4th generation scenario for a range of the higgs boson mass (see the latest higgs exclusion plot here).

The slides from this and other talks can be found here. If interested in more details, look at the recent paper of Graham. While reading, try not to forget that the 4th generation does not exist.

1 comment:

Anonymous said...

Hi Jester,

I was looking for a discussion of the cross section for higgs production in light of more than 3 generations, and ended up re-reading this post... Very nice.
Can I borrow your feynman graph ? ... Already done :)