I’m a few days behind (thanks to an NSF grant proposal that had to be finished last week) but I wanted to write a bit more about my visit to CERN, which concluded Nov. 21st in a whirlwind of activity. I was working full tilt on timely issues related to Run 2 of the Large Hadron Collider [LHC], currently scheduled to start early next May. (You may recall the LHC has been shut down for repairs and upgrades since the end of 2012.)
A certain fraction of my time for the last decade has been taken up by concerns about the LHC experiments’ ability to observe new long-lived particles, specifically ones that aren’t affected by the electromagnetic or strong nuclear forces. (Long-lived particles that are affected by those forces are easier to search for, and are much more constrained by the LHC experiments. More about them some other time.)
This subject is important to me because it is a classic example of how the trigger systems at LHC experiments could fail us — whereby a spectacular signal of a new phenomena could be discarded and lost in the very process of taking and storing the data! If no one thinks carefully about the challenges of finding long-lived particles in advance of running the LHC, we can end up losing a huge opportunity, unnecessarily. Fortunately some of us are thinking about it, but we are small in number. It is an uphill battle for those experimenters within ATLAS and CMS [the two general purpose experiments at the LHC] who are working hard to make sure they have the required triggers available. I can’t tell you how many times people within the experiments — even at the Naturalness conference I wrote about recently — have told me “such efforts are hopeless”… despite the fact that their own experiments have actually shown, already in public and in some cases published measurements (including this, this, this, this, this, and this), that it is not. Conversely, many completely practical searches for long-lived particles have not been carried out, often because there was no trigger strategy able to capture them, or because, despite the events having been recorded, no one at ATLAS or CMS has had time or energy to actually search through their data for this signal.
Now what is meant by “long-lived particles”?
Most types of particles decay (i.e., disintegrate into lower-mass particles). Any particular type of particle has an average lifetime, though an individual one may live a longer or shorter life than the average. A neutron, if found outside a stable atomic nucleus, decays into a proton, and electron and an anti-neutrino after about 15 minutes… sometimes shorter, sometimes longer, but 15 minutes on average.
In the context of the LHC, the term “long-lived particle” doesn’t mean a particle will outlast you and me! or even a neutron. It merely that its average lifetime is long enough that, if traveling at a good fraction of the speed of light, it is likely to move a measurable distance before it decays… i.e. that it lives a trillionth of a second or longer. The ATLAS and CMS experiments, despite being the size of small office buildings, are able to detect when a particle’s decay occurs a millimeter (about 1/25 of an inch) away, or even less, from the location of the proton-proton collision where it was created. (Pretty amazing, huh? My experimental colleagues are awesome.) This astounding ability is critical in identifying bottom and charm quarks, which often decay at a location separated from the collision point by as little as a millimeter.
In contrast to a neutron, the Higgs particle which was discovered at ATLAS and CMS in 2012 is very short-lived; a typical Higgs particle, from its birth to its death at the LHC, won’t even travel an atom’s width from the location at which it was created, living about a billionth of a trillionth of a second. But there are other known particles that are much longer lived, such as the muon. A muon, once produced, will survive for two millionths of a second on average — during which, if it is traveling at a good fraction of the speed of light*, it can travel a good fraction of a mile.
*Due to time dilation, a consequence of relativity à la Einstein, if the muon is traveling very close to the speed of light relative to us, it will actually, from our point of view, live longer and travel further than average. This is why very high energy muons created in cosmic ray collisions high in the atmosphere often are able to travel hundreds of miles, often reaching the Earth’s surface.
But the really important question for the LHC is whether it might be able to produce as-yet unknown particles that have long lifetimes. We haven’t found a long-lived particle for a while — the bottom quark was the most recently discovered long-lived elementary particle, and that was about 40 years ago — but we found many before, and why shouldn’t there be others? Indeed such particles are predicted, with small to moderate certainty, in various speculations that have been considered by theorists over the years, including some creative ideas about the nature of dark matter, certain forms of supersymmetry, various attempts to understand the masses of the known particles, etc. What got me interested in the problem, back in 2001 or so, is that I noticed that if one takes a broad view, and considers a wider set of variants of these speculative ideas, one quickly sees that long-lived particles are far more common than they are in the most popular variants of these ideas. (After some intermittent research, this observation was finally published in 2006 work with Kathryn Zurek.)
In other words, due to cultural biases among scientists, but not due to anything about nature itself, the possibility of long-lived particles was viewed as far more remote than it really is. (Though the biases have improved since 2006, to a certain extent they still remain.) And for this reason, the LHC experiments, as of 2006, had prepared for only a fraction of the possibilities, and were even lacking trigger strategies that could be sensitive to certain types of long-lived particles. The worry was especially acute if the long-lived particles were created mainly in the decays of a lightweight Higgs particle. A Higgs particle of mass 125 GeV/c², such as the one we have in nature, has such low mass-energy (125 GeV) compared to the energy of LHC proton-on-proton collisions (7000 GeV in 2011, 8000 GeV in 2012, and 13,000 GeV starting in 2015) that its decays are generally unspectacular and are difficult to observe; the 2012 discovery of the Higgs required careful trigger design, and this will become even more important for Higgs studies in Run 2. So I have spent a lot of time since 2007 trying to help change the situation, so that we can be sure that, if such particles exist, we will discover them, and if we don’t find them, we’ll be confident we did a thorough search and didn’t just miss one for lack of trying. To this end, I spent much of my time at CERN giving talks and having extended conversations, both planned and spontaneous, with experimental colleagues at ATLAS and CMS, and discussing the issues with theorist colleagues as well.
I realize this is a bit abstract, so in a day or two I’ll give you a more detailed example of how a new trigger strategy can be crucial in allowing for the possibility of a discovery of long-lived particles.