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The Higgs almost excluded at 160 GeV!! February 2, 2008

Posted by dorigo in news, physics, science.
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A new, hot-of-the-press, combination of the Higgs boson search results by the CDF and D0 collaborations at Fermilab is out. The combination is based on a score of analyses based on one to two inverse femtobarns of integrated luminosity collected by the two experiments at the Tevatron collider since the start of Run II. Both CDF and D0 already have collected almost twice that amount of data, but they are still analyzing the remainder. So let us look at the current status of Higgs boson searches with the very informative summary plot shown below.

It has become customary, in this search, to show as a function of the unknown Higgs boson mass the 95% confidence level limit on Higgs production as a limit on the ratio between excluded cross section and standard model prediction. A ratio smaller than unity means that the Higgs boson should not be there if we believe the standard model to hold; a ratio larger than unity still allows the existence of the particle at that mass value. As years go by, the exclusion curve has gone down in small steps as a result of the inclusion of more data and, crucially, continuous improvements in the analysis techniques.

In the plot, the black line shows the actual limit found at the Tevatron. You can see by yourself that the limit is at 1.1 times the standard model for Higgs mass of 160 GeV – the most sensitive point in the whole mass region. What that means is that CDF and D0 are not quite able to exclude the existence of the Higgs boson in any mass point, but they have gotten very, very close to achieve that. But the line also tells more. Indeed, the fact that the limit obtained is lower, at 160 GeV, than that expected from pseudoexperiments (the band called “expected limit”), shows that there is less data at 160 GeV than what was expected by backgrounds alone. A normal downward fluctuation, and in fact the black line is only one standard deviation below the average expectation. But that also gives one the scale of what the limit really means.

In fact, a 95% confidence level limit says very, very little. It is like describing a probability distribution by saying, “oh, well, on this side there is only 5% of it”. I explained elsewhere what is hidden behind a claim of a 95% cl exclusion, and will not repeat the whole issue here. Suffices to say that if the limit at the Tevatron is 1.1 x SM at 160 GeV, this could mean that the real SM cross section has indeed been “excluded” at xx% (with xx<95) confidence level, since the most probable value of the Higgs cross section implied by Tevatron data must be below the SM prediction to have produced a limit at the quoted value.

In other words, if the probability distribution for the Higgs cross section were all lying above 1.0xSM, and only 5% of it leaked above 1.1xSM, one would conclude it is a very, very narrow distribution – and indeed, it would be a measurement!, since such a narrow distribution would definitely not be compatible with zero cross section: the Higgs boson having a non-null cross section would mean it exists. But the above is of course not the case, and the probability distribution is very well compatible with zero – so it extends down and it is wide. How wide ? Well, if one examines the expected limit, one gets the scale of how wide the observed limit might be by examining the +- 1-sigma countours of the expectation. So, xx% is not small… Eighty percent ? I guess maybe 85. That would mean that the likelihood that the Higgs boson mass is at 160 GeV has already shrunk considerably if we consider all the available Tevatron data… Bad news for LHC experiments, whose sensitivity is highest if the Higgs has a mass of 160 GeV (but the LHC will find the Higgs anywhere, given enough statistics).

Finally, let me add to this note a word of self-praise. Two months ago I discussed the D0 limit on Higgs cross sections in the WW production mode, and I ventured to make the following prediction:

After seeing this plot, which reaches a x2.4 SM value at 160 GeV, I am starting to be very curious to see the combination with CDF results (which stood at x1.9 SM at 160 GeV already last August). I think we will have to wait for winter conferences to get that plot, but I smell a x1.1 limit at 160 GeV: not yet any mass exclusion for winter 2008 (for the latest combination, yielding x1.4 SM, see here).

My prediction was based on incomplete results from D0, and I did not have insider information on the CDF limit in the WW final state in my hands yet… A good call.

Comments

1. Kea - February 2, 2008

Congratulations! Now that’s what I call teamwork. Don’t worry – the LHC will probably be useful for looking at other stuff, which actually exists.

2. dorigo - February 3, 2008

Hi Kea,
I am writing from the lobby of a hotel in Lisbon, to say thank you and …. I do believe in the Higgs, as I think you know. So it will indeed be interesting to see what is going to happen in the next few years. And well, if the LHC discovers nothing at all, it will become useful in other ways, such as sterilizing biological fertilizers or doing radiation therapy. And the caverns will be great for long-term storage.

Cheers,
T.

3. Jim Graber - February 5, 2008

Higgs bosons for football fans: I want to cheer for my favorite Higgs mass prediction team!
As I understand it, the standard model team wants a Higgs near 170 GeV.
The MSSM team prefers 130 GeV or thereabouts.
Looking at your graph two posts back from Altarelli’s talk, it looks like the Lepton asymmetry team wants an already excluded Higgs near 50 Gev, but the Hadron asymmetry data favors a heavy Higgs centered on 400 GeV, but stretching from 200 to 800 GeV. The world average seems happy up to about 150 GeV.
Are those vertical error bars one sigma?
And what is the relation to the previous Higgs mass charts you have posted?
Actually, I cheer most for the LHC team to get that monster up and working.
Best,
Jim Graber

4. dorigo - February 6, 2008

Hello Jim,

let me try to shed some light. The SM prefers a Higgs boson as light as it can be, since global fits to electroweak observables indicate a mass lower than 80 GeV; albeit with a large error bar. The direct lower limit stands at 114.4 (LEP II), so 120 GeV are distinctly better than 170 for SM.

The same thing basically goes with MSSM theories: although there are many different possibilities, a Higgs above 135 GeV is even less wanted there. Again, the lighter, the better.

As for Altarelli’s plot, the two asymmetries point to different values of the Higgs mass, but these determinations are imprecise enough that one should not give them too much weight (there are in fact a dozen more – and it is the combination of all that indicates a low Higgs mass). The error bars in the plot, as far as I understand it, represent one-sigma coverages, but no guarantee exists that they are gaussian in nature.

Cheers,
Jim

5. island - February 6, 2008

I am writing from the lobby of a hotel in Lisbon, to say thank you and …. I do believe in ghosts, I do believe in ghosts, I do, I do, I do, I do, I DO believe in… ghosts….😉

6. Jim Graber - February 7, 2008

Hi Tommaso,
Thanks for your illuminating response. I will take your word that both SM and MSSM want the lightest possible Higgs, contrary to my previous understanding. But I am still a very confused football fan. First of all, what does it mean if we find a heavy Higgs instead of a light one?
Another thing I have heard is that if low energy supersymmetry (LES) exists, LHC will see it faster than the Higgs, because the signatures are so much more obvious. Is that true?
Maybe I will root for LES and a nice neutralino to match the dark matter. That used to be the mainstream favorite, did it not? But now I hear that the precision measurements disfavor LES. Is that true?
Or maybe I will root for axions. I’d like to see them, not only for PQ, but also because it would be much easier to see them than to prove they don’t exist. Of course, this result comes not from LHC, if it comes at all.
Who do you bet on in the dark matter game?
Thanks again for your time and all the fun.
Jim Graber

7. dorigo - February 7, 2008

Island, sticks and stones…😉

Jim, if we found the Higgs at, say, 300 GeV, that would really be a fly in the face of Supersymmetry. Instead, the SM could accommodate it with some stretch of one or two electroweak measurements, but it would also imply that the SM must break down at a relatively light energy scale (above a TeV), lest we conclude that we understand nothing at all about the theory.

About light susy, yes, the LHC can in principle see it in a few days of running, for a particular combination of SUSY parameters. Not true for all cases though. Nature may have made it hard for us to find SUSY even if it is “light”.

As for dark matter, I think we understand too little about our cosmos yet. I do not believe in SUSY however, so whatever else fits the bill is ok for me🙂

Cheers,
T.

8. island - February 7, 2008

Island, sticks and stones… might break your bones, but a null result is only a good thing…🙂

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