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MSSM Higgs at 160 GeV: one more piece of non-evidence August 27, 2007

Posted by dorigo in news, physics, science.

2007 has been a rather bumpy year for blogging about Higgs searches, and in particular those for a MSSM Higgs decay to third-generation fermions. It all started when John Conway discussed at Cosmic Variance his brand new result on the decay to tau lepton pairs, which showed an intriguing 2-\sigma excess attributable to a 160 GeV boson. In a series of posts here and there, the discussion on whether a corresponding signal was observable in data containing two b-quark jets in CDF and D0 samples used to extract the Z \rightarrow b \bar b decay led the press to become intrigued. In March, New Scientist produced a slightly incorrect account of the whole picture, and the Economist pitched in, adding more inaccuracies. Many in the CDF and D0 experiments resented that.

Just as the buzz level on the internet was getting back down to a physiological level, rumors started again about a possible large signal found by D0 in a search for MSSM H->bb  decays in events with three or four b-quark jets. Since that is another good sample where to search for that particle, people were again very interested in the issue, and a new wave of interest arose everywhere, and got picked up by slate and other high-traffic web sites, and still more newspapers.

The rumor was not based on a public result, so there was not much to do but sit and wait for the moment when D0 would be ready to discuss their achievements. But D0 did not produce anything, only making the curiosity grow.

I am too lazy to replicate all the links here, so if you want to dig in the issue please find all the required addresses in this more comprehensive summary .

Anyway, that is the past.  The news now are that, for the Lepton-Photon 2007 conference, CDF has produced a public result on the same search. Since CDF and D0 have similarly sized datasets, and similar sensitivities, it is quite interesting to see what they find! So let us look at the new result, whose main authors are Thomas Wright and Dante Amidei – two knowledgeable and skilled physicists who usually (and this is no exception) produce very accurate results. 

The search is based on a trigger collecting multi-jet events enriched with b-quarks. The trigger is made possible in CDF by the Silicon Vertex Tracker, a wonderfully complex set of custom-designed hardware boards organized in a highly parallel architecture. The parallelism allows speed of execution: in less than 20 microseconds, the hardware collects information on hits in the silicon layers, compares hit patterns to those stored in associative memory boards, and performs a linearized fit, achieving a measurement of charged track transverse momentum, azimuthal angle, and -crucially- impact parameter with respect to the beam position with a precision quite similar to the best achievable offline with a glass of brandy in one’s hands. A sketch of the operation flow of SVT is shown below, where you also get to see the actual boards performing the operation.

The multijet trigger collects events with at least three jets, two of E_T > 15 GeV and a third with looser E_T>10 GeV. Each of the two leading jets is required to be matched in azimuth to a track with P_T > 2 GeV, \ |d_0| > 120 \mu m. The latter is the impact parameter (I.P.) cut, which allows to spot tracks which are likely originated in the decay point of a B hadron. The drawing on the left shows what happens to tracks originated from a decay in flight (of the B particle): they produce a I.P. (blue segment) with respect to the interaction point (in red). Forget the yellow arrows please.

The sample of data collected by the SVT multijet trigger is enriched in b-quark jets, but a more stringent selection is needed. Such is provided by the SecVtX algorithm, which explicitly searches inside jet cones for sets of charged tracks which can be fit to a common origin displaced from the interaction point: a secondary vertex. CDF selects events with all three jets tagged by SecVtX, to select a sample enriched with the process bg \rightarrow bH \rightarrow bb \bar b.

Tom and Dan found that to search for MSSM production of Higgs and one b-jet, the “smoking gun” distribution was the invariant mass of the two highest-E_T b-jets: they are most likely to be those coming from Higgs decay. However, even after triple SecVtX b-tagging, CDF data contains a mixture of several different QCD processes, not just a single background of “multiple b-jet production”: events with three b-jets, but also events with two b-jets and a light-quark jet, or a charm jet.  These backgrounds have to be modeled with percent accuracy in order to achieve a reasonable sensitivity on Higgs decay.

Monte Carlo generators do not predict with the necessary accuracy the correct mixture of processes with b-jets and c-jets in multijet events.  So one has to rely on the data to model them. Events with only two b-tagged jets provide a good handle, but the key to understanding how the different backgrounds contribute to the 3-tag sample is given by a variable called M_{diff}, constructed with the invariant mass of tracks fitted to the secondary vertex in each jet:

M_{diff} = M_1 + M_2 - M_3

where M_i is the “vertex mass” of tracks in jet i. By fitting simultaneously the invariant mass distribution of the leading jets and M_{diff}, backgrounds get constrained much better, allowing a much more precise modeling and a better result for the Higgs search.

So here is the result of the fit: The dijet mass distribution with the best fit overlaid is shown in the plot on the left, for a hypothesized MSSM Higgs of 150 GeV. The black points are CDF data with three b-tags, the four main backgrounds are shown in light blue (bbb= three b-jets), dark blue (bb+light-quark jet), violet (bcb= events where a charm jet produced along with two b-jets is one of the two leading jets), and yellow (bqb=events where a mistagged light-quark jet is one of the leading jets). The best fit allows some small fraction of signal (in red), but it is also compatible with the no signal hypothesis, such that a limit can be set to the cross section of Hb production, and an exclusion obtained in the M_A – tan(beta)  plane. The limit is shown below.


The limit above, shown with a black line, is compared -as has become customary lately- with expectations from pseudo-experiments (dashed black line, and red 1-sigma and purple 2-sigma bands). You can see that between 140 and 160 GeV CDF obtained a worse limit than the one they expected to set, mainly because of the small fluctuation which is shown in the dijet mass fit above. In all cases, the fluctuation is at most a 1.5-sigma excess, which again, is one more piece of non-evidence for MSSM Higgs at the Tevatron. Nonetheless, let me say that this analysis is a very beautiful attempt at something which I know out of experience to be a quite difficult task: understanding the composition of multijet datasets in terms of their flavor composition, and modeling correctly the different nuisances of the mass distribution of the leading jet pairs, is no small feat.

For the more technically inclined, I should also mention that the plot above was derived by assuming no width effects in the Higgs mass templates. That is to say, the increased bbH coupling due to a large value of tan(beta) will increase the width of the breit-wigner shape. The cross section also gets modified by the change, and so one has better compute a limit taking those effects into account. Of course, both including and neglecting the effect is useful. You can find the width-included limit in  the public note describing the result.


1. anomalous cowherd - August 27, 2007

Very nice result! Congratulations to all concerned.

2. Jester - August 27, 2007

Thanks a lot, especially for the link to the public note. Do you know when to expect a similar analysis from D0?

3. dorigo - August 27, 2007

Hi Jester,

unfortunately, no. I was betting on a result for LP07 from them too, but if two (early unconfirmed rumors of excesses) and two (no news) is four, I bet they found that their own background model required more work. Another interpretation could be that they are trying to include all the data they can in a more solid new result. In both cases, I am quite interested in seeing what they will publish.


4. Internet, I wish I knew how to quit you « An American Physics Student in England - August 28, 2007

[…] sifting through the daily arXiv delivery, only to be distracted by a new informative blog post on Higgs detection, and then you’re off clicking on suggested review papers, online lectures, and the whole kit […]

5. Jon Lester - August 28, 2007

Is it possible that Higgs is strongly self-interacting so to escape current detection methods?


6. hep skeptic - August 28, 2007

Dear T.,

I agree with your hunch that D0 found that their background model
required more work is the most probable explanation for the delay.

Any comment about what conclusions should be drawn when a published result cannot be updated without having to change background rate/shape predictions beyond the systematic range listed in the PRL? One might conjecture that in the current climate at the tevatron the quickest path to publication (a.k.a. job advancement) is to have excellent agreement with SM predictions.

hep skeptic

7. Guess Who - August 28, 2007

Lester, a strongly self-interacting Higgs would also be a very massive Higgs, at least if we are talking about an SM-style one, and we know it can’t be more massive than 800 GeV or so without violating unitarity. I think the LHC should be able to see it all the way up there.

8. Jon Lester - August 28, 2007

The point is that the Higgs part of the Lagrangian can become non-perturbative at far lower values of the Higgs mass. You can see this from \sqrt{\lambda}=M/\sqrt{2}v with v=246 GeV. At this limit the asymptotic states of the Higgs boson are no more free particle ones and, at best, the estimation of the production rates could be not correct. This could make difficult a possible identification of the particle.


9. anomalous cowherd - August 28, 2007

John Lester writes:

“the Lagrangian can become non-perturbative at far lower values of the Higgs mass. You can see this from \sqrt{\lambda}=M/\sqrt{2}v with v=246 GeV.”

You have to be careful; the coupling constant $\lambda$ is not the real effective expansion parameter in your Feynman diagram expansion. The parameter that effectively governs succeeding terms in the expansion of a $\lambda \phi^4$ theory is $\lambda / 16 \pi^{2}$. See section 4.4 thru 4.7 of the book by Ramond: “Field Theory: A Modern Primer” for a detailed study of the perturbative expansion of $\lambda \phi^4$ and its renormalization.

This is actually a common property of field theories, that the effective expansion parameter is not just the Lagrangian coupling but includes inverse powers of factors of $4\pi$. Take Quantum Electrodynamics: the Lagrangian coupling at the vertices is “e”, but the expansion parameter in the Feynman diagram expansion for QED is “$\alpha = e^{2}/(4\pi)$”. Incidentally, the Lagrangian coupling parameter for QED is not that small. For $\alpha$ = 1/137 you can see that “e” is about 1/3, or in other words about the same value as the parameter in the 1/N expansion of QCD [this observation is known as “Coleman’s joke” among field theorists].

You should not worry. The unitarity bounds that Guess Who quotes above are based on calculations that got all their factors of $4\pi$ right, and give about the number he quotes. For example see:
-Weak Interactions at Very High-Energies: The Role of the Higgs -Boson Mass.
-Benjamin W. Lee, C. Quigg, H.B. Thacker (Fermilab) . -FERMILAB-PUB-77-030-THY, FERMILAB-PUB-77-030-T, Mar -1977. 50pp.
-Published in Phys.Rev.D16:1519,1977.
-TOPCITE = 1000+

And yes, a lot of LHC studies have been done to convince the experimentalists that they can see a Higgs up to that mass!

10. Jon Lester - August 29, 2007

Dear anomalous cowherd,

Thanks a lot for your helpful post. Of course, you are right. So, let us wait and see.


11. dorigo - August 29, 2007

Hi all,

I’ve been sort of slow in replying to this thread – and I think it was a lucky idea, because an interesting discussion has grown by itself and answers (with even links to papers, thanks Anomalous!) have been provided. I could do no better, so thanks to all involved.


12. dorigo - August 29, 2007

Dear Hep skeptic,

of course publishing agreement with the SM is easier than forcing an anomaly through a collaboration. Most researches in science are plagued by the decision process of the people doing the analysis, in a number of ways: choice of what to work on, for instance, is a usually overlooked source of bias. I think Physics is not immune, and we can only rely to the fair dealing of the scientists involved.


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