Okay, so after the wondeful announcement of two days ago, we have the rest of our life to take stock. But we need not wait until we get old, so let me begin today by listing some "ex post" thoughts related to the Higgs hunt and discovery. I imagine I will have more thoughts as this thing cools off, but here is something to start with.
- ATLAS and CMS jointly discover the Higgs boson. They get to exactly the same level of significance (5.0 sigma) using the same final states (photon pairs and Z boson pairs decaying in turn into pairs of electrons or muons; CMS also analyzed other final states, but those do not add significance to the overall combination). This is amazing in itself: we knew all along (since 2010, in fact, as I have mentioned in this post) that 10 inverse femtobarns of data would give to each detector, roughly, the sensitivity to have 50% odds of claiming a 5-sigma discovery; but those 5 sigma could have been 6, or 4, quite easily.
- What is even more amazing is that CMS actually had a much higher expected sensitivity, by having put together more advanced multivariate search techniques and having analyzed in time for the announcement not just the two main channels but all the five important final states (W boson pairs, b-quark pairs, and tau-lepton pairs in addition to the two already mentioned). For a 125 GeV Higgs, the median sensitivity in term of Z-value (significance) was about 5.8 for CMS. ATLAS, on the other hand, expected to "only" get to 4.6 standard deviations with its data, search methods, and analyzed final states.
- The above means that a scenario just as probable (but I am eyeballing here) as the one which actually occurred was that CMS obtained over six-sigma evidence for the Higgs - a one-in-a-billion chance of their data being due to background, and a undisputable discovery by itself- and ATLAS less than four standard deviations. This would have placed the CERN laboratories in a very awkward situation: they would have had to applaud to the CMS observation of a new particle, while at the same time to say that ATLAS had "confirming evidence" at the same level of the recent Tevatron results or little more. CMS alone could have easily been seen from the outsiders to have discovered the Higgs single-handedly!!
- Please note that I am a member of CMS, but I am actually quite happy that the above scenario (an asymmetric finding in CMS and ATLAS) did not materialize, and that instead both experiments can jointly be seen as the discoverers of the Higgs boson. Any other outcome would have been unfair and would have created a totally uncalled for wave of bad feelings across the lab. So, is this dreaded scenario what the CERN management feared, when the July 4th seminar was scheduled with a very low-lying announcement ("Hggs update", "not a discovery announcement", "Opening act for ICHEP 2012") ? I guess we will never know, but at this point, ex-post, I believe it is a legitimate suspect.
- I somewhat criticized the CERN choice of reserving over 50% of the seats in the main auditorium for yesterday's event. However, having seen the outcome, I must retract a bit that criticism. After all, we all enjoyed seeing the theorists who predicted the Higgs 50 years ago sitting in the first row, and it was touching to hear them speak at the end of the seminars. Maybe the ambassadors could have been left out of the door, but otherwise most of the people sitting in the front row had the right for a reserved seat. And 20% or 50% of reserved seats would not have made the hell of a difference for those who camped outside of the auditorium door since 10PM on the previous night: the auditorium only seats about 400 people, while of the order of 2000 wanted to attend the event; so seating a hundred more or a hundred less in there does not really change matters. Instead, I did appreciate the setting up of a direct connection with the auditorium from many other conference rooms around the lab: the audio and video were flawless, and the experience was not perceptibly different.
Front page image: FABRICE COFFRINI/AFP/GettyImages
