ICHEP 2012 – Higgs Session

As you may have seen from our live-blogging of the CERN seminars on Wednesday morning, after having told Sean and John I would be asleep, I woke up anyway and watched the announcement live at 3am my time. I don’t regret it for a moment – you don’t get to watch historic events like that every day! But the reason I’d originally intended to stay asleep was that I had a very long day ahead of me, since Wednesday evening I flew out to Melbourne to take part in the 36th International Conference on High Energy Physics (ICHEP). This is the major high-energy physics conference every two years; and this year, given the Higgs announcement, it is particularly exciting.

I landed in Melbourne on Friday morning (local time), but couldn’t attend a lot of the conference that day because I had to deal with a lost bag (thanks United!) and then shop for a shirt, underwear, etc. Enough about that though. For the last year or so the University of Melbourne has been the primary institution in a new Australian Center of Excellence in Particle Physics (CoEPP) at the Terascale, and I am fortunate to be one of their international partner investigators and on the International Advisory Committee. Friday evening there was a small reception and dinner for members of the Center and the IAC. Geoff Taylor (head of CoEpp), my friend Ray Volkas, Rolf Heuer (Director General of CERN and Chair of the IAC), and others were all there, dressed nicely in their suits. And I was there in the jeans I’d worn continually (and slept in on the plane) for two and a half days (thanks United! OK, I’ll stop mentioning it now). In any case, this was a lovely start to what I”m hoping will be one of the more exciting conferences of my career.

Saturday was a day filled with parallel sessions. I gave a short talk on Galileons in the Particle Astrophysics and Cosmology session that afternoon, but due to jet lag I can’t recall in any detail the other talks that I went to, except for a nice one by Stefan Antusch on Matter Inflation. Sunday was a day off for most of us, which allowed me to work on the plenary talk on Dark Energy and Cosmology that I will deliver on Wednesday, and spend some time enjoying some of the wonderful attractions of Melbourne that I know from my previous visits.

Then, on Monday, the plenary talks began. I was up early to sit on a press panel at 8am, but afterwards went straight to the auditorium, and after a nice introduction from Geoff Taylor and a number of others, the Higgs session started up. Although Fabiola Gianotti, who you may have seen in the Wednesday presentations, is here, this time the talk on ATLAS Higgs Searches and Experiment was given by Richard Hawkings from CERN. He covered the preliminary results that generated excitement at the end of last year, but quickly moved on to the 2012 results. One thing he emphasized is that all their decisions about how to select events and what analysis techniques to use were fixed before looking at the 2012 data. This is something I think is often missed in causal explanations of how these things work. You can’t come along after the fact and design analyses to optimize the chance of claiming a discovery – that could introduce bias too easily.

Hawkings discussed the two most important channels at this point separately. In the first, in which the Higgs decays to two photons, ATLAS finds a new particle at a mass of 126.5GeV with a significance of 4.5 sigma after allowing for systematic uncertainties in the photon energy. He mentioned that the signal strength is somewhat higher than expected from the Standard Model, but still compatible with it. In the second channel, in which the Higgs decays to four leptons via two Z bosons, a similar set of results yield a new particle at mass 125GeV.

Finally for ATLAS, the combined 2011 and 2012 results from both these channels yield “Evidence for a new, narrow resonance at a mass near 126.5GeV” at a significance, after taking account energy systematics, of 5.0 sigma. It was wonderful (and I felt extremely privileged) to be in a room full of other physicists watching this. This is the first conference at which we know that (to all intents and purposes) we have a Higgs. It must have been even more fun at CERN on Wednesday Sean!

Next up was Joe Incandela from UCSB, of Internet fame from Wednesday’s CERN seminar, to report on “Observation of a narrow resonance near 125GeV in CMS”. This talk was a little slower, with more focus on the challenges that pile-up presented. He pointed out that they have learned a lot and are now processing events in under 20 seconds. Incandela presented results for five decay modes for this conference, but again, the two I mentioned above for ATLAS are the most sensitive right now to the Higgs. As with ATLAS , the final result was evidence for a new particle at 125GeV at 5.0 sigma significance. The combined results of all channels show a signal only a little more than 1 sigma away from the Standard Model prediction, and so, so far, one can’t see other new physics.

The final Higgs talk was about the Tevatron results by Shalhout Shalhout, who is a postdoc at UC Davis. One big difference here is that the most important process is Higgs decaying to two b-quarks. In their new analysis for ICHEP their combined results show an excess with a significance of 2.5 sigma, compatible with Standard Model Higgs production in the range 115GeV – 135GeV.

There is a lot more going on at this conference of course, and I’ll write a few more times, but I really wanted this to be just about the Higgs. Being here was a lot different from watching alone in bed at 3am. It has been a great week for particle physics!

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