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