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Probing the Hbb vertex at the LHC February 21, 2007

Posted by dorigo in Blogroll, internet, news, physics, politics, science.

In a comment to a recent post about non-mainstream searches for the Higgs boson at the LHC, Jester asked me with what precision the CMS and ATLAS experiments are expected to measure the Higgs-bb coupling.

That made me think… In fact, the LHC experiments are not putting great trust in the H->bb decay to spot that elusive particle. The reason is that the signature of two b-quark jets is simply not distinctive enough to allow a reasonable signal-to-noise ratio -indeed, the  observation of  Z to bb decays  is already quite tough, and its rate is a thousand times larger than the H->bb process. Therefore, to study the Hbb vertex one has to rely on the production of a Higgs boson together with other bodies, whose characteristics allow a significant background reduction. The candidates are:

1) associated production with a vector boson:

  • WH -> l nu b anti-b;
  • ZH -> l l b anti-b.

These processes are not so favorable at the LHC as they are at the Tevatron, and indeed backgrounds from QCD production of b-jets associated with the vector boson prevent a significant measurement of the rate of these processes, from which the Hbb vertex coupling could in principle be extracted.

2) associated production with a top-antitop pair: in this case the signature is quite distinctive, but the rate is small enough to make the measurement of the Hbb vertex quite hard. Moreover, what one would be probing in a ttbb final state would be the combination of top and b-quark vertices, and thus the Hbb coupling would require a separate measurement of Htt coupling to be extracted.

A study of the chances of measuring the various couplings of the Higgs boson at the LHC has been done three years ago by M.Duhrssen, S.Heinemeyer, H.Logan, D.Rainwater, G.Weiglein and D.Zeppenfeld. They put together a global likelihood fit that included information from all the observable Higgs decay channels by the CMS and ATLAS experiments, in two scenarios: 30/fb (which the LHC should deliver in the first few years of running), and 300/fb (which represents a much longer time scale).

The fit considers the size of the expected signals obtained by the experiments as a function of a large parameter space of the different couplings, to extract the sensitivity of a global analysis. Some of the results are summarized in the plot below.

One sees that for 30/fb of data (a similar plot is available from page 11 of their preprint, available  here  ) the Hbb coupling can be constrained to about 40% if the Higgs boson mass is smaller than 130 GeV. For larger values of the mass, the decay cannot be probed meaningfully, due to its rapidly falling probability in favor of decays to vector boson pairs (WW, ZZ).

It is not too interesting to ask what could be done at the Tevatron, if the Higgs were found there. For  even in the rosiest of scenarios -say, 8/fb of collected data per experiment, a Higgs mass of 125 GeV, and all the suggested analysis improvements working like a charm- CDF and D0 would be unable to see much more than a faint evidence of only a couple of final states: the associated production of WH and ZH yielding leptons plus a bb pair (at a 3-sigma level, once every subprocess were thrown in the mix), and maybe the direct production of H decays to W boson pairs (at maybe 1.5-sigma of significance). A fit of these two “excesses” could prove far-fetched.

Instead, the question of what could be done at the ILC (if, and that is a big if, it were constructed) might be more interesting to answer. A  recent paper addresses that issue: a linear collider producing 500/fb of electron-positron collisions with a reasonably achievable polarization of the beams could study all Higgs boson couplings quite well, and in particular the Hbb vertex could be probed to 1% accuracy by determining the heavy flavor composition of jet pairs produced in association with two leptons from Z decay in the e+e- –> ZH production process. Interestingly, even the Hcc vertex could be probed with that technique, to about 10% accuracy. 

It remains to be seen if the ILC will be constructed after all. Recently, the nobel prize winner Burton Richter has joined the group of those of us who gloomily believe the LHC has to find something really new in order to motivate the large investment for the linear collider – and gloomy is the feeling, since chances that no new physics is behind the next door are fat.


1. Jester - February 22, 2007

Thanks a lot for this one. The reason i’m interested is precisely the nightmare scenario of finding just a higgs at the LHC. What we can do then, apart from sitting down and crying, is to measure the higgs couplings to see if this is really the standard model higgs. For example, there exist models where the higgs is a pseudo-goldstone and it couples to fermions via \cos(v/f) \lambda_b h \bar b b. If the LHC could see that this \cos(v/f) is not 1, that would justify a need for more precise studies at the ILC. Your anwers seems to indicate that there is not much chance in this direction. Remains some hope in WW scattering…

2. dorigo - February 22, 2007

Hi Jester,

you are welcome… It is always nice to be “forced” to learn something. The two papers cited above contain interesting stuff of which I was unaware.

Indeed, the bare SM scenario is scaring the hell out of all of us, the closer we get to start running those darn protons down the LHC pipe.


3. Andrea Giammanco - February 28, 2007

What about pp->bbH, with H decaying in something distinctive (like ZZ, WW, gammagamma)?

4. dorigo - March 1, 2007

Hi Andrea,

I guess it would be hard although probably feasible with enough statistics. The higgsstrahlung off a bb-pair is a small xs process… But I have not read the full paper by Heinemeyer et al., they might be mentioning it somewhere.


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