The MSSM Higgs signal buried in my plot March 1, 2007Posted by dorigo in internet, news, personal, physics, science.
Maybe a few readers of this blog will remember my comment of the recent 2.1-sigma excess of H->tau tau candidates found by a CDF analysis looking for MSSM Higgs bosons (also see the followup with the D0 search of the same signal ). In it, I hinted at the intriguing coincidence of the CDF excess of tau-pairs – whose combined mass pointed at a possible MSSM Higgs boson of 160 GeV – with some disturbance in a D0 plot of their Z->bb excess (see graph on the left, where the points represent a deviation from the best fit) which could again be attributed to H->bb decays, and added half-jokingly that the much larger-statistics Z->bb signal about to be approved by the CDF collaboration was going to show the same trend.
Now, after the blessing of the Z signal by CDF , I can discuss openly the matter. Indeed, I at least owe it to Anil Ananthaswamy, a reporter from New Scientist, who contacted me to discuss my post on the 160 GeV excess and to get more information from me. Although I discussed the matter in detail with him, I did not give him any insider information on the CDF analysis beforehand, because I am determined to follow the rather strict rules that my experiment has set to its collaborators. But I also promised I would answer all his questions in due time.
Here are some of the questions that were raised, with my answers [Between square brackets some of my additional editing].
Q: What is the difference between your search and the search that was carried out by John Conway’s team (Higgs->tau pairs)? Were you also searching for a MSSM Higgs?
A: No. We have been searching for a Standard Model signal, the Z->bb decay.
The difference with the MSSM search of H->tau tau is that the final
state is different, and that we have tried to see a very well known signal
to extract a calibration of jet energy, while John’s group has been
searching for physics outside the Standard Model (MSSM) from the outset.
Q: What is the nature of the deviation that you have found? And at what mass scale have you found it?
A: We have not found any significant deviation, in the sense that physicists attribute to the word “significant”. What the mass spectrum we are publishing today shows [see picture on the left, better seen in another post ] is that we understand the data well, and any discrepancy from our model is understandable as a statistical fluke. However, if there were a SUSY Higgs boson at 160 GeV, then our plot should contain maybe a hundred of those events. A hundred events at 160 GeV of mass would be impossible to see with certainty, but our plot does not rule out that possibility either – and actually there is an upward fluctuation of the data right at about that mass value, of the size one would expect from MSSM [see blowup of region around 160 GeV, where the “excess” of data – the blue points – with respect to the fit is shown].
Q: How significant is this deviation?
A: Not tested yet, but probably less than one standard deviation. [One should remember that plots such as the one shown here do not include systematics on the background shape parametrization, which would easily eat up the upward wave at 160 GeV.]
Q: What did you think when you first saw this deviation? Knowing that Conway’s team has also seen a 2-sigma deviation, what do you think is going on when you look at both results?
A: I have always considered the possibility that a new resonance could
show up in a b-bbar mass distribution which also contains a clear signal of
Z->bb decays. But I have also known that it would be quite hard to see it first in this final state, due to the large backgrounds. When I saw the fluctuating points around 160 GeV I did not think much, nor do I now. But I know several colleagues who could take the three pseudo-signals, D0’s one, Conway’s 2-sigma one and my fluke, and take that quite seriously. However, I am not as easily excited, and I still believe it is a fluctuation in Conway’s plot and another fluctuation in my plot.
Q: You had mentioned [in the past] that you think the standard model is right. Given the latest data from your work, do you still think the same?
A: Yes. And extraordinary claims need extraordinary evidence, so – after
thirty years of incredibly precise confirmations of the Standard Model –
we need a HUGE signal of new physics before I get convinced there is
something beyond. Make Conway’s signal a 5-sigma one, and I will still
believe the SM is right and there is something wrong with that analysis.
Only after a very careful scrutiny and independent checks could I get