Did you know that another name for Minneapolis, Minnesota is “Snowmass”? Just ask a large number of my colleagues, who are in the midst of a once-every-few-years exercise aimed at figuring out what should be the direction of the U.S. particle physics program. I quote:
- The American Physical Society’s Division of Particles and Fields is pursuing a long-term planning exercise for the high-energy physics community. Its goal is to develop the community’s long-term physics aspirations. Its narrative will communicate the opportunities for discovery in high-energy physics to the broader scientific community and to the government.
They are doing so in perhaps the worst of times, when political attacks on science are growing, government cuts to science research are severe, budgets to fund the research programs of particle physicists like me have been chopped by jaw-dropping amounts (think 25% or worse, from last year’s budget to this year’s — you can thank the sequester).. and all this at a moment when the data from the Large Hadron Collider and other experiments are not yet able to point us in an obvious direction for our future research program. Intelligent particle physicists disagree on what to do next, there’s no easy way to come to consensus, and in any case Congress is likely to ignore anything we suggest. But at least I hear Minneapolis is lovely in July and August! This is the first Snowmass workshop that I have missed in a very long time, especially embarrassing since my Ph.D. thesis advisor is one of the conveners. What can I say? I wish my colleagues well…!
Meanwhile, I’d like to comment briefly on a few particle physics stories that you’ve perhaps seen in the press over recent days. I’ll cover one of them today — a measurement of a rare process which has now been officially “discovered”, though evidence for it was quite strong already last fall — and address a couple of others later in the week. After that I’ll tell you about a couple of other stories that haven’t made the popular press…
First, an update to an ongoing measurement has allowed the LHCb and CMS experiments at the Large Hadron Collider [LHC] to claim an official discovery, where previously they had only strong evidence. In question is the long-sought decay of Bs mesons (hadrons containing a bottom quark and strange anti-quark, or vice versa) to a muon and an anti-muon. [I briefly described this process when an unconfirmed claim of evidence was made some time ago.] By combining their results and using the full 2011-2012 LHC data, they have officially “discovered” it … i.e., together they have been able to exclude the absence of this decay, at the officially sanctioned level of five standard deviations.
According to the measurement, about 3 per billion (1,000,000,000) Bs mesons decay in this fashion. But note the measurement is still not very precise, as you can see in the figure below — so let’s not jump to conclusions. That “3” could still be 2 or 4 per billion, or perhaps even 1 or 5 per billion, without surprising anyone. Now, calculations using the Standard Model (the set of equations used to predict the behavior of all known particles and non-gravitational forces) predict that this number would be about 3.5 per billion, give or take. So there is agreement — rough agreement — between data and the Standard Model’s prediction.
In the press, however, you’ll see statements that this new measurement has huge implications for supersymmetry (a popular but certainly not unique speculative idea for what might resolve the hierarchy puzzle of the Standard Model), and for other speculative ideas too. [From the BBC, you’d often get the impression that particle physics is “A Battle Between The Standard Model and Supersymmetry”, as though supersymmetry is the only other game in town and no one has ever had any other interesting speculative ideas about nature. This is, of course, a tad overstated.]
Well, what lies behind these press articles is apparently a powerful hype machine operating within LHCb. It is true that this measurement is very important, and has been since it was first studied seriously at the LHC, as presented in late 2011 and early 2012. But since that time, each step forward has been relatively small; this is no surprise, since to get a major advance you generally need about 10 times as much data as you had before, and that won’t happen til 2015 or later. Yet somehow, despite small-to-medium steps forward in November 2012 and now in July 2013, the hype machine has managed to convince the BBC and other press organizations that there have been multiple major scientific advances while actually there’s been only one. You have to admire them (and to wonder why the BBC isn’t catching on.)
Let me quote some of the purple prose from the LHCb press release: “The result is a stunning success for the Standard Model of particle physics and yet another blow for those hoping for signs of new physics from CERN’s Large Hadron Collider (LHC).” WOW! Whoosh!!
Hee-hee… gosh, if every measurement with a precision of about 30% that agreed with a theoretical prediction had been hailed as a triumph for the theory, there would be a lot of embarrassed scientists around… a lot of predictions have failed only after much better precision was available. Let’s see the precision reach 3%; then we can talk about “stunning”. And a “blow” to supersymmetry and to other speculative ideas beyond the Standard Model? Didn’t we already go through this exact same story last November? While this measurement rules out a large number of variants of supersymmetry, there are many other variants which it doesn’t yet touch. And most other speculative ideas in the scientific literature survive this measurement with an even larger fraction of their variants intact.
[Many of the speculative ideas that have been discussed by particle physicists predict multiple Higgs particles. Typically the variants ruled out by this measurement are those in which at least one type of Higgs particle interacts much more strongly with bottom quarks than does the single Higgs particle present in the Standard Model.]
In short — the recent news is a step forward, but not at a level that would justify the hype. There isn’t that much of a change from November, and in any case, you typically can’t rule out any one speculative idea with a single measurement. Constraints on speculative ideas come from combining many different measurements being made at the LHC, and elsewhere. So — let us express our congratulations to LHCb and CMS for their joint discovery, and also, in a way, to LHCb’s press office for its … skills … regarding journalists and other non-experts.
[By the way, the BBC article contains an error. “Scientists have confirmed one of the rarest phenomena of decay in particle physics, found about three times in every billion collisions at the LHCb.” No, it’s not 3 times in every billion proton-proton collisions at LHCb; it’s 3 out of every billion Bs mesons, which are certainly not produced in every proton-proton collision.]