If you’re interested in particle physics and not regularly reading Tommaso Dorigo’s blog, you should be. His latest posting reports on incendiary claims by Michael Dittmar of the CMS collaboration that recent Tevatron Higgs mass limits are wrong and not to be believed. According to Dittmar, the Tevatron is basically useless for looking for a SM Higgs, with only the future LHC experiments ever having a chance to see anything or produce real limits. You can look at the slides and the blog posting and make up your own mind. From what I can tell, Dittmar doesn’t make a strong enough case to show that the Tevatron results are wrong. It remains true of course that the statistical significance of the limits being set (“95% confidence level”), is right at the edge of what is normally taken as capable of seriously ruling something out.
In the latest New York Review of Books, Freeman Dyson, in context of a review of Frank Wilczek’s The Lightness of Being, engages in his own smackdown of particle physics at colliders. Here’s what Dyson has to say about the LHC, and colliders in general:
Wilczek’s expectation, that the advent of the LHC will bring a Golden Age of particle physics, is widely shared among physicists and widely propagated in the press and television. The public is led to believe that the LHC is the only road to glory. This belief is dangerous because it promises too much. If it should happen that the LHC fails, the public may decide that particle physics is no longer worth supporting. The public needs to hear some bad news and some good news. The bad news is that the LHC may fail. The good news is that if the LHC fails, there are other ways to explore the world of particles and arrive at a Golden Age. The failure of the LHC would be a serious setback, but it would not be the end of particle physics.
There are two reasons to be skeptical about the importance of the LHC, one technical and one historical. The technical weakness of the LHC arises from the nature of the collisions that it studies. These are collisions of protons with protons, and they have the unfortunate habit of being messy. Two protons colliding at the energy of the LHC behave rather like two sandbags, splitting open and strewing sand in all directions. A typical proton–proton collision in the LHC will produce a large spray of secondary particles, and the collisions are occurring at a rate of millions per second. The machine must automatically discard the vast majority of the collisions, so that the small minority that might be scientifically important can be precisely recorded and analyzed. The criteria for discarding events must be written into the software program that controls the handling of information. The software program tells the detectors which collisions to ignore. There is a serious danger that the LHC can discover only things that the programmers of the software expected. The most important discoveries may be things that nobody expected. The most important discoveries may be missed.
He goes on to somehow count Nobel prizes for experimental results in particle physics, with the conclusion:
The results of my survey are then as follows: four discoveries on the energy frontier, four on the rarity frontier, eight on the accuracy frontier. Only a quarter of the discoveries were made on the energy frontier, while half of them were made on the accuracy frontier. For making important discoveries, high accuracy was more useful than high energy. The historical record contradicts the prevailing view that the LHC is the indispensable tool for new discoveries because it has the highest energy.
His argument that proton collider physics is problematic because of the huge backgrounds and difficulty of designing triggers just states the reasons why these are complicated and difficult experiments. Despite the difficulties, they have produced a huge number of new physics results. He doesn’t give the details of how he is counting and categorizing Nobel Prize winning results, so that part of his argument is hard to evaluate.
In opposition to colliders, Dyson wants to make the case for passive detectors, with his main example Raymond Davis’s discovery that the neutrino flux from the sun is 1/3 what it should be. I don’t really see though why he sets up such experiments in opposition to high energy accelerator experiments. Right now many of them actually are accelerator experiments (for example MiniBoone), with an accelerator being used to produce a beam of neutrinos sent to the passive detector. Dyson’s point that if one is very smart and lucky one may get indirect evidence about physics at high energy scales from passive detectors looking at cosmic rays is valid enough, but there is no shortage of people trying to do this, and it is every bit as problematic as working with colliders. There are inherent reasons that such experiments can’t directly investigate the highest energies or shortest distance scales the way a collider experiment can. It’s extremely hard to come up with a plausible scenario in which cosmic ray experiments will give you any information about the big remaining mystery of particle physics, electroweak symmetry breaking.
While I agree with Dyson that the huge sales job to the public about a new golden age of physics coming out of the LHC is a mistake. I don’t see any reason to believe that if it fails cosmic ray experiments are going to get us to a golden age. If and when particle physics reaches a final energy frontier, with higher energies forever inaccessible to direct experiment, hopes for a golden age are going to rest on theory, not experiment, and recent experience with such hopes isn’t very promising.
Update: This Sunday the New York Times will have a profile of Dyson, see here.