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I want to know about the multi-b excess too! May 28, 2007

Posted by dorigo in news, physics, science.

A anonymous physicist left a comment to a recent post here mentioning that the Dzero experiment is allegedly observing an excess of multi-b-quark events, asking me whether I have more details on the matter. Since I am not a member of the Dzero collaboration, it is too bad that the simple answer is NO. Too bad since I would not fear of being crucified if I did leak restricted information, as it would happen if it came from the CDF experiment.

However, I have friends. And they have friends too… You certainly know that the internet is less than a couple dozen links “thick”. Well, by the same token the high energy particle physics community is certainly less than five “friendship links” thick in turn: so it should be easy to gather information by milking your closest contacts – a fact that underlines how silly it is to pretend that individual experiments keep their result secret for half-year-long periods of time, waiting for all sorts of checks before diffusing preliminary results (which still have a non-zero chance of being wrong anyways).  I have been asking friends about the Dzero signal, and hopefully I will get some hints from them…

In the meantime, being in the dark is nice in a sense. I have zero information on what is the exact channel where Dzero has allegedly seen an excess of events. I only know it seems significant (4-sigma ? 5-sigma ?), and that it implies some narrow new state at about 180 GeV of mass. So I am free to speculate wildly: let me do it.

A supersymmetric Higgs boson ? That is certainly possible, although such a state would not be narrow if it had to show a production rate large enough to allow significant detection with a few inverse femtobarns of data. A supersymmetric b-quark ? Hmm, that hypothesis does not seem to fly too much either. A fourth generation b-quark with that mass would decay to a top quark – unless flavor-changing neutral currents were also playing a trick there. What else ? A gluino ? Also a possibility – it may decay to b-quarks.

I would favor the hypothesis of the SUSY Higgs boson: it would be great if Dzero were seeing a signal at 160-170 GeV – it could turn out to be compatible with the excess seen by the Higgs to tau-pairs search performed by CDF recently: the plot would thicken considerably if a tentative signal of a b-quark pair resonance were seen at that mass. John Conway, who is working on the tau-pairs search, tells me CDF will have a new 2-inverse-femtobarn result out for summer conferences. So the scenario of a joint SUSY Higgs discovery by CDF and Dzero in a few months is pretty much open!

In any case, with the low level of information we have available this far, practically anything is possible. We need more leaks. Dzero folks, please do not leave us in the dark! Anonymity for commenters here will be for once respected…

UPDATE: I think the analysis is an update of the one reported  here, with twice as much luminosity. I might be wrong though… Still gathering information. That work was a search for SUSY Higgs in events with three b-jets in the final state, selected by multiple secondary vertex tagging. In the paper, there is a cross section exclusion plot which shows they did obtain an observed limit worse than the expected one, for 170 GeV…

UPDATE: Ok, below you can see the plot I am referring to above. You see that with the multijet search D0 excluded MSSM Susy models implying a light higgs boson of 170 GeV for tan(beta) values above 121, when they expected to be able to exclude down to tan(beta)=104. The discrepancy is due to the small excess seen, 6748 events observed with three b-tagged jets versus 6687 predicted, in 900 inverse picobarns of data: in particular, those 6748 events had more of them with a mass of the two leading jets around 170 GeV than their model predicted. Now they are analyzing twice as much data, but I do not know whether they have improved their technique.


In the plot above, (let me stress it, it is the published old result by D0 in the same channel discussed in this post) the red line marks the lower limit of the region in the M(A):tan(beta) plane which is excluded by the multijet Susy Higgs search by D0 with 0.9/fb of data. It means that all pairs of values corresponding to coordinates above the red line are excluded at 95% confidence level or more. The black line, instead, shows what limit D0 expected to set. The expected and observed limits roughly coincide for M(A) lower than 150 GeV, and then divide, due to the slight excess observed in their analysis.


1. chinese whisperer - May 28, 2007

i heard (at about fifth hand) that it involved a 5-sigma 4 b-jet resonance at 180GeV, which could be something decaying to h h* -> 4b.

2. Kea - May 29, 2007

Ah, gossip, gossip. Well, it does sound intriguing, I must admit! Of course, if the bump is real it CAN’T be a Higgs boson, because they don’t exist….

3. Rumors! « Charm &c. - May 29, 2007

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4. Tony Smith - May 29, 2007

Experimental observations such as “… a 5-sigma 4 b-jet event resonance at 180 GeV …” are very interesting to me, because in my physics model the Higgs, T-quark, and vacuum form a system with three states, roughly related to three types of Nambu-Jona-Lasinio Higgs-as-T-quark-condensate models, described as:

Low Energy: 130 to 160 GeV –
Tquark_low (130 GeV),
Higgs_low (146 GeV),
W+W- pair (160 GeV)

Middle Energy: 170 to 190 GeV –
Tquark_middle (175 GeV),
Higgs_middle (180 GeV),
Z0 pair (180 GeV)

High Energy: 220 to 260 GeV –
Tquark_high (225 GeV),
Higgs_high (240 GeV),
W+Z0W- triple (252 GeV),
Higgs VEV (252 GeV)

The T-quark and Higgs masses are only tree-level estimates, and so could vary from experimental results by 10 per cent or so.

It seems interesting to me that those things fit together, and that preliminary indications from experiments seem to see things that are consistent with the overall scheme of the model.

I wish that I were not blacklisted, and that there were substantive discussion about the model beyond my own web site and a few blogs such as this one, but that may be about as likely to be successful as wishing for world peace. Oh, well … I am thankful for this blog and for the glimpses of real experimental results that it provides. It is more fun to compare a model with real observations than to do fancy math unconnected to such experimental details.

Tony Smith

5. dorigo - May 29, 2007

Hi Tony,

experimental 5-sigma observations would be interesting to _anybody_! Of course if they fit your models that’s very good for you, but I have to say I am personally very skeptical about this particular rumor… I think what Dzero is seeing is “some” excess, which can be new physics but can also be a incorrect modeling of the heavy flavor content of their multijet data. Of course, I have no details so mine are just wild guesses…
I wish you were not blacklisted too – I wish there was no such thing.

6. Tripitaka - May 31, 2007

“So I am free to speculate wildly: let me do it”

Yes, thats the spirit!

7. Not Even Wrong » Blog Archive » Even More Stuff Than Usual - June 1, 2007

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8. Professor - June 1, 2007

Let’s assume that this signal is really observed.
1) Why it couldn’t be a standard Higgs? You wouldn’t see its top quark decays because top is very heavy, so why do you need SUSY?
2) What about QCD production of 4b’s? It’s very sensitive to structure functions, so how would you rule it out as a source of this signal?

9. anonymous - June 1, 2007

#8 — SM Higgs at that mass would decay to W and Z. And also have a lower cross-section.

10. Professor - June 1, 2007

#9 — The rates for W and Z decays should be the same for SUSY and non-SUSY because they are determined by the identical SU(2) reps of the two Higgs doublets. So where the enhancement of 4b production would be coming from? I guess that you have to create some asymmetry between the two VEVS, parameterized by tan\beta? In other words, what is the parameter, in addition to b mass that controls H-b couplings? How would it affect other rates?

11. anonymous - June 1, 2007

Yes, you need large tan beta; the heavy Higgs would couple to b’s and tau’s, mostly.

12. dorigo - June 1, 2007

Hi professor and anonymous,

the answers of anonymous are roughly ok but let me put things more clearly down here if I manage to:

1) a SM Higgs cannot be seen at 180 GeV, for a number of reasons:
1a) the Higgs decay to bb is quite suppressed for this mass, where the decays allowed are to WW and ZZ (close to 100%).
1b) the cross section of H production in the SM at 180 GeV is very small, so at the Tevatron we would be unable to see it even with 10x more statistics, even combining information from different search channels. As for bb decays, we would not even look for those at that mass, due to the tiny BR
2) the cross section in the MSSM receives a boost by a factor tan(beta)^2, the parameter given by the ratio of v.e.v. of the higgs v1 and v2. That is if MA>>MZ, a limit one usually assumes in calculations.
2a) if tan(beta) is 40-45 or so, the cross section would agree with a significant excess observable even with 1-2 inverse femtobarns for the quoted mass
2b) the decay to bb and tau tau is the one boosted by tan beta, so you would not see a MSSM higgs in WW before you saw it in these final states – the coupling of h to bosons is not enhanced, and is exactly like the SM one, as stated by professor. To be clear: some couplings stay the same, others get boosted, and those boosted become dominant decay modes.
2c) the 4-b final state is one by which a qcd-produced bb pair radiates a h which again goes into bb.

Hope the above helps…

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19. max - July 13, 2007

All the *weaker* collider labs had enough of time to search for The Higgs in various ways during last few years.
Were any of them identified something serious they would certainly announced it.
But they didn’t didn’t and this way they(?) are just spreading incorrect rumors.
Well,it’s time to fire up our big machine in CERN isn’t it guys?


20. dorigo - July 13, 2007

Hi Max,

no, the rumor was spread by somebody who is not in D0 and who left a comment in a post here. D0 is conducting an analysis silently, as large collaborations always do, and I am sure they will let us know the results once they are done.

LHC will start colliding at the end of July 2008, but for Higgs searches the first meaningful amount of data will be available in 2009.


21. E - February 18, 2008

Higgs has indeed been observed at 5 sigma, along with evidence of 3 additional dimentions. This will be made official in March.

22. Michael J. Nemec - August 20, 2008

Would the discovery of the higgs particle possibly lead to any new type of technology. Would the discovery of the higgs particle lead to the discovery of other types of subatomic particles, or will the higgs particle be the last particle to be discovered.

23. dorigo - August 22, 2008

Hi Michael,

no, I do not imagine that the discovery of the Higgs might lead directly to something usable.
However, if a supersymmetric Higgs boson is found (and we might know instantly if it is electrically charged, for instance, or not know for a while if we mistake it for a regular one), it implies the existence of 24 new particles. A whole mirror-world of new particles.


24. Daniel de França MTd2 - November 5, 2008

“Dzero experiment is allegedly observing an excess of *multi-b-quark* events, asking me whether I have more details on the matter. Since I am not a member of the Dzero collaboration, **it is too bad that the simple answer is NO.**”

Like rummors in the computer market, which I usualy follow, they turn out to be right .

25. dorigo - November 5, 2008

Hi Daniel,

wellll… I think the signal D0 saw did peter out after all. I do not think it has anything to do with the CDF anomaly…


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