landmark (n): 1. An object or feature of a landscape or town that enables someone to establish their location. 2. An event, discovery, or change marking an important stage or turning point in something.
Several times in his ICHEP 2012 closing presentation today, CERN Director General Rolf Heuer referred to ICHEP 2012 as a “landmark” conference. This inspired me to take a look at the dictionary definition of “landmark.”
Indeed, the conference seems to fulfill both definitions of the word. First, we have very much defined the location of the field of particle physics. The new particle observed this week, which at the moment appears to have the properties of the Higgs boson, now establishes that the standard model of electroweak symmetry breaking, the main working theory of particle physics for forty years, is correct. Many speakers at the conference who were not explicitly talking about the Higgs still needed to talk about the model, and they all noted with some relief that what they wrote down about the Higgs potential and so forth is now known to be true. (Implicit in all this is the usual scientific caveat that “true” only means “not yet shown to be false,” but please bear with me.) Of course we have long assumed and hoped that it was true, given how much supporting evidence there was for the model all along. But now we know our location: we have a theory that works.
(I should note that while all the attention lately has been on the Higgs, there has been another major advance this year, and that is the measurement of the neutrino mixing angle θ13 at the Daya Bay nuclear reactor experiment in China, with confirmation in other experiments. This establishes that all elements of the neutrino-mixing matrix are non-zero, allowing for the possibility of CP violation in that system. So our colleagues in the neutrino sector also have a landmark measurement to show for themselves.)
And most definitely this observation, and the conference where we first learned about it, is also a landmark in the second sense. We now turn our attention to what this discovery means. What are the next set of questions to ask, and how do we go about asking them? Our first job is to do our very best to characterize the “Higgs boson.” Once its mass is established, the standard model fully specifies all of its other properties. Does it decay to the right final states as often as it is supposed to? Various extensions to the standard model predict different values for these “branching ratios.” Does it have the right spin? We know that the spin must be an even integer, but we don’t know if it is zero as predicted. The LHC will continue taking data into February of next year, before a two-year long shutdown. We’re going to need every ounce of data we can get, and all the cleverness we can muster, to answer these questions as accurately as we can. It will keep us busy during the data drought that is about to come.
I have been spending a lot of time this week thinking about what comes next, especially in terms of future facilities. The LHC is going to be the workhorse of Higgs physics for some time, but it will have its limits. Alas, there are some bad-news landmarks from this conference too: so far, there is no evidence at all for any other new particles beyond the Higgs at this mass scale — no supersymmetry, no exotic fermions or bosons, nothing. We can’t exclude the possibility that even the 13 TeV LHC that will start to run at the end of 2014 might not have enough energy to produce any other new particles. Heuer talked a little about the “high-energy LHC,” which he also noted is really a completely new machine in the same LHC tunnel, but it’s hard to imagine that this happens before 2030. Can we do something else in the meantime? Are there revolutionary, game-changing ideas that we can bring to the table? And, as a writer from the United States, I have to ask — can we build this machine at home?
Big questions for what we hope is a big new era of particle physics! This ICHEP, the first in the Southern Hemisphere, will truly be remembered as a landmark. I would like to take this opportunity to thank our hosts for all of their work in hosting this remarkable conference in an equally remarkable location.
