[July 6 UPDATE: very busy with work here, but I have finished a short article briefly describing the data, and why it is convincing that a new particle has been found.]
July 5: A wild day at the CERN laboratory yesterday, continuing into today and surely the rest of the week. [If you’re a lay-person wondering what all the fuss is about, you might want to read my article “Why The Higgs Particle Matters“, or watch my year-old video clips that explain how one searches for the Higgs particle, or read the Higgs FAQ.] [Very well-written and almost entirely accurate articles at both the New York Times and the BBC.]
The contrast from last December is profound. After last December’s presentation on the search for the Higgs particle at the Large Hadron Collider [LHC], whose results I called “inconclusive” and other bloggers called “firm”, most ATLAS and CMS experimentalists I talked to were expressing, in public and in private, something between extreme caution and cautious optimism. I can’t count the number of times that senior experimentalists told me a story about a 3 sigma result that they’d seen disappear because of a subtle mistake. Though many theorists were convinced, many other theorists (despite what you hear from some other bloggers) shook their heads and expressed deep caution. Discussion at the CERN cafeteria and in the hallways focused on which parts of the data were most trustworthy and whether the discrepancies between what ATLAS and CMS had observed made the result unstable to small changes. All in all, despite what all the other bloggers said, I personally did not talk to a single experimentalist who felt the result was secure — which is not to say (this is an important distinction, please read carefully and please do not misquote) that anyone (including me) thought the hints were clearly a mirage. The problem, as I explained at the time, was that you could run an argument that made you confident in the hints, and you could also run an argument that made you lack confidence. Well, evidence isn’t firm until the argument in favor is a lot stronger than the argument against, and that wasn’t true in December. The evidence did improve somewhat by March, though the improvements were kind of up and down.
By enormous contrast, after yesterday’s presentation I did not speak to a single person who expressed serious doubts that ATLAS and CMS had made a discovery. (The most negative comment was from an experimentalist who wished ATLAS and CMS hadn’t been forced, by the early-July scheduling of the ICHEP conference, to do their data analysis so soon; he would have preferred to have twice as much data and a result that left him with absolute confidence. Well, if that’s the worst thing anyone can say…) What’s changed from December is that every argument you can make comes out in favor of the result being real; I’ve been unable to think of any argument against it, nor have I spoken to anyone who has proposed one. All the conversation now is on what the discovery means and on what to do in coming months.
How did a result that was inconclusive in December become so convincing in July, with just a doubling of the amount of data? The answer, as far as I can tell, is that a number of things that could have gone wrong all went right. I’ll go through this in more detail later, but suffice it to say for now that the experts who operate the accelerator and the two detectors all proved, yet again, that they are extremely smart and capable, and on top of that, they had some good luck that brought them to results almost as strong as anyone could hope for. Only time will tell whether the two experiments both got a bit lucky with a statistical fluke or whether the Higgs is actually a bit easier to find than theorists expected — but on top of this, the LHC produced somewhat more collisions than estimated in advance, the experimenters dealt admirably with the extreme collision rates, many new techniques for improving the measurements were developed, and these numerous small bits of good news combined for a nearly best-case scenario. Yesterday’s results were close to the optimal that could have been possible, given the amount of data; to have both experiments see clear signs of a new particle with a mass of about 125 GeV/c2 in both of their “easy” searches (for Higgs particles decaying to two photons, and for Higgs particles decaying to two lepton/anti-lepton pairs) was a lot to ask for, but it’s what we got!
My goal for today and tomorrow, if I can squeeze it in — obviously my highest priority right now is conversations with my professional colleagues about interpreting the result and discussing what needs to be on the agenda for the rest of 2012 and 2013 — is to explain to you:
- What does the current data actually show?
- What in the data makes the evidence in favor of a new particle so convincing?
- What elements of first-rate experimental technique and plain-old luck combined to make the result so strong?
- Why I am personally so convinced (more than the experimentalists seem to be right now) that the new particle is a Higgs particle, as opposed to something else?
- To what extent is this particle consistent with the simplest Higgs (a “Standard Model Higgs”) as opposed to a more complicated type?
Maybe getting through this list will take me into next week. At some point I’ll write more about the implications of this being a simplest Higgs or not, but probably not before the middle of next week. And hey, it’s also time to revise the Higgs FAQ!!!!
So stay tuned to this channel! I’ll alert you when I finish writing the answer to each of these questions.