At this week’s EPS conference we have seen the release of new results from both CMS and LHCb on a search for a very rare decay of the Bs meson, to a muon-antimuon pair. I’ve written about this before (yikes, two years ago!); this decay turns out to be amazingly sensitive to possible new physics processes. This is in part because the decay (which violates flavor conservation, to leading order) is highly suppressed in the standard model, and thus the presence of additional particles could have a big impact on the decay rate. Just what impact depends on the particle; depending on the model, the rate for this decay could be either increased or decreased. It’s a somewhat unusual situation — you have something interesting to say either if you see this decay when you don’t expect to (because you don’t yet expect to have sensitivity due to insufficient data), or you if you don’t see the decay when you do expect to.
But in this particular case, the standard model wins again. CMS and LHCb now have essentially identical results, both claiming observation of this process at the rate predicted by the standard model. (LHCb had shown the first clear evidence of this decay last November.) I’m not going to claim any great expertise on this topic, but this result should put stronger constraints on theories such as supersymmetry, as it will restrict the possible characteristics of possible SUSY particles. In addition, this observation is the culmination of years of searching for this decay. I reproduce the CMS plot of the history of the searches below; over the course of about 25 years, our ability to detect this decay has improved by a factor of about 10,000.
But here’s what’s really on my mind: I’m thinking about this measurement in the context of the Snowmass workshop, which begins one week from today in Minneapolis. The studies of the workshop have been divided up into categories of “frontiers”, where the physics can fall into Energy, Intensity or Cosmic Frontiers. This categorization arises from the 2008 report of a US HEP program planning committee. It is certainly a useful intellectual organization of the work that we do in particle physics that is easy to explain to people outside the field. The Department of Energy budget for particle physics is now also organized according to these frontiers.
But where exactly does this Bs measurement fit? The physics of quark flavors and the search for rare decays would be considered part of the Intensity Frontier. But the measurements are being done at the LHC, which is considered an Energy Frontier facility because it has the largest collision energy of any accelerator ever built, and the process is sensitive to the effects of putative particles of very high mass. This is just one example of physics measurements that cut across frontiers. Another that comes to mind is that the LHC experiments have sufficient sensitivity to the production of potential dark-matter particles that in some cases, they can be competitive with searches done in non-accelerator experiments that are classified as being in the Cosmic Frontier.
Heading into next week’s workshop, I am hoping that we will be cognizant of the interconnected work of all the research that we do, regardless of how they might get classified for accounting purposes. We have many ways to explore each of our physics questions, and we need to figure out how to pursue as many of them as possible within the resources that are available.
