Tuesday, 23 October 2007

Higgs and Beyond

Every year in autumn, physicists follow atavistic instincts and spam the arXive with conference proceedings. While book proceedings may be of some use as a paper weight, the ones posted on the arXive are typically cut&paste from existing papers awkwardly clipped to fit the page limit. From time to time, however, one may stumble upon a nice, concise review. I recommend the recent short article by Gian Giudice about theories of electroweak symmetry breaking. The article gives a pretty accurate picture of the current state-of-art in the field; it covers everything that deserves being mentioned plus a few things that does not.

As an appetizer, I review here one interesting and not so well known point raised by Gian. Everybody knows that the LEP and SLD precision measurements hint towards a light higgs boson and constrain the standard model higgs boson mass to a quite narrow range. The best fit value, $76_{-24}^{+33}$ GeV, is a little disturbing, given the direct search limit 115 GeV, but still it lies comfortably within the 2-sigma limits. The situation is however more involved, as explained below.

The plot shows the higgs boson mass as inferred from individual measurements . The two most sensitive probes: the leptonic left-right asymmetry and the bottom forward-backward asymmetry do not really agree. The former points to a very light higgs boson, $31_{-19}^{+33}$ GeV, already excluded by LEP, while the latter suggests a heavy higgs $420_{-190}^{+420}$ GeV. Only when we combine these two, partially incompatible measurements in an overall fit of the standard model parameters, we get an estimate that is roughly compatible with the direct search limit. The bottom asymmetry is often considered a mote in the eye, as this is the only LEP measurement that is more than 3-sigma away from the standard model predictions. However, if we decided that this measurement suffers from some systematic errors and removed it from the fit, we would conclude that the standard model is almost excluded by direct higgs searches. That's irony.

This tension raises some hopes that the standard model higss boson is not the whole story and some new physics will emerge at the LHC. For a critical review of the alternatives, read the article.

8 comments:

a quantum diaries survivor said...

Hi Jester,

thank you for pointing to the paper by Gian Giudice. Gian is from my university, and I followed a course in group theory he held ten years ago during my PhD studies...

About the A_fb(b) being the one result that keeps the SM together in the Higgs hunt, I did mention that point in a post last May... See http://dorigo.wordpress.com/2007/05/31/higgs-bosons-global-fits-mssm-and-dark-matter/ . However, I do not think I was the first to point that out either ...

Cheers,
T.

a quantum diaries survivor said...

Hmmm I should learn to put tags in the right place. Let me do this again for who wants to read it: post on Higgs ...

Cheers,
T.

Jester said...

Désolée Tommaso, to have forgotten a citation ;-) I agree that this thing isn't new; since a year i've seen it emerging in seminars and private conservations. I guess the problem was sharpened by you guys, who constantly lower the top mass :-) I think it's important to spread the news, as it is one of the very few sources of optimism for the LHC...

Matti Pitkanen said...

Hi Jester,


a couple of questions, which I already made for Tommaso.

It seems that high precision electroweak parameters containing radiative corrections involving Higgs exhange give widely varying Higgs masses. The variation range is one order of magnitude.

This might be due to the logarithmic dependence on Higgs mass. On the other hand, TGD allows the possibility that the couplings of Higgs to fermions are much weaker than in standard model since vacuum expectation does not determine fermion mass. This would effectively drop loops in which fermion couples to Higgs and modify profoundly the formulas from which one can estimate Higgs mass.


The high precision parameters deduced from Z^0 resonance certainly depend on Higgs exchange between fermion in loop. Is it possible to make rough estimates what happens if these contributions are simply dropped but boson-Higgs couplings kept by using existing programs? What mass estimate would result for Higgs mass if only bosonic couplings to Higgs are present?

Best Regards,

Matti Pitkanen

Jester said...

I'm not 100% sure, but here is what i imagine. The LEP and SLD precision tests are sensitive to the higgs boson only through its couplings to W and Z. The higgs mass dependence of various observables enters because loops with the higgs boson affect the gauge boson propagators. The couplings to the light fermions, including the bottom quark, is too small to influence the LEP observables (if we had top asymmetries at hand, that would be different).

In summary, i think that the LEP observables tell you nothing about couplings of the higgs boson to the fermions. Wait till the LHC measures the higgs decay branching ratios...

Matti Pitkanen said...

Thank you for your kind answer.

Matti

a quantum diaries survivor said...

True Jester, the top mass has been getting leaner by the day last year. But this year we have a new very precise measurement placing it at 172.7 with 2.1 GeV total error, and that is a single measurement! So I think the Higgs will get a bit heavier too...

And don't worry about missed citations :) Indeed it was a bit inelegant of me to point that out - but I thought the post contained some additional interesting points made by Sven Heinemeyer.

Cheers,
T.

Anonymous said...

The A_FB^b feature and its relation to the Higgs mass has been (under-)appreciated for some time. I would say the person who has really taken it the most seriously is Mike Chanowitz:

http://arxiv.org/abs/hep-ph/0104024

Though Gian himself and collaborators proposed an explanation which discounts the asymmetry and then fixes the Higgs mass using light supersymmetry:

http://arxiv.org/abs/hep-ph/0106029

There was also work by Carlos Wagner et al, who took the result seriously, and modified the fit directly by proposing the bottom has some small mixing with vector-like quarks:

http://arxiv.org/abs/hep-ph/0109097

It's an interesting feature in the data, and one which is not likely to go away any time soon. I think it is great you (re-)called our attention to it, Jester.

It's also interesting to see how this "anomaly" responds to changes in the top mass. As the top wanders within error bars, it doesn't change things by a huge amount. I seem to remember hearing that a lighter top makes the discrepancy a little worse, so that is another interesting result as the Tevatron shrinks its error bars.