With a slightly anti-climatic timing if we consider the just ended orgy of new results presented at winter conferences in particle physics (which I touched on here), the LHCb collaboration outed today the results of a measurement of unity, drawing attention on the fact that unity was found to be not equal to 1.0.
I have been supportive, even enthusiastic at times, of the recent LHCb measurements - in general I like the business of this small collaboration (well, small, you know - I'm comparing with ATLAS and CMS) which plays the part of the David compared not with one but with two angry Goliaths. They have been doing their job quite well, measuring with precision a number of things that the giants cannot reach, or ignore because they are too busy excluding the presence of new physics anywhere they can reach.
But I am starting to be a bit disturbed by what might start to look, to the eyes of a cynical son of a bitch, like a continuous search for headlines.
Okay, any 2-sigma departure from Standard Model predictions nowadays is in principle worth our undivided attention: in the absence of any clear avenue of physics beyond the Standard Model, whatever has the potential to keep us dreaming is worth its weight in morphine. But there's a limit, and at least as far as I am personally concerned, that limit has been reached in the case at hand.
What am I talking about, exactly? I'll tell you later. Why am I ranting? Because I know that these continuous announcements of wannabe departures from Standard Model predictions cause the community of theorists to engage in a race of paper spewing, whereby whoever publishes first gets to be cited by everybody else who come later. We have seen this in action following the announcement of hints of a 750 GeV resonance in ATLAS and CMS data less than 16 months ago, and there is no reason to believe this kind of game is going to be disattended in the near future (this would require changing entirely the rules of the game for the appraisal of scientists: i.e. a total disregard of the size of their citation balls). Since I think this is a very idle game that nobody benefits from, I cannot be very happy about the news that a new wave is starting.
So what is this new 2-sigma thing? It is, as I mentioned, a measurement of 1.0 which returns 0.6 or so, give or take some 0.2 uncertainty. In physics the easiest thing is to measure 0.0, such as when you look for a non-existing particle: you measure its rate, and find a number compatible with zero. The nice thing about measuring something that you expect to be equal to zero is that you can be very sensitive in your measurement. An example is the determination of the g-2 "gyromagnetic ratio" of the muon, again something that should be very close to zero. I have written about that somewhere else, so I will not divagate here.
Measuring something that should be 1.0 is also quite frequent in physics. However, as compared with measuring "zero", measuring "one" is a bit trickier, as you have issues connected with the scale of your measurement. There are also, in truth, some nice features in constructing 1.0 as the ratio between two different particle reaction rates, as LHCb did in its latest result. The reason is that many systematic uncertainties that affect in the same way both the numerator and the denominator cancel, leaving you with a higher precision on the number you want to determine.
What LHCb did was to try and determine if B mesons (particles containing a bottom quark together with a lighter one, which are copiously produced at small angle from the beam in LHC proton-proton collisions) decay preferentially into one of the two combinations including a "K*" meson (read "kappa-star", a particle made up by a strange quark and a down antiquark) and a muon-antimuon or an electron-positron pair. By taking the ratio of the measured yield of the two combinations, LHCb tried to measure "one", in fact - as a thing called "universality of the weak currents" demands that the two lepton species appear in equal proportions in these B decays, once one accounts for small effects due to the different masses of the leptons in question (half a MeV the electron, 105 MeV the muon).
So, the money plot above shows that the experiment found numbers departing from 1.0: indeed, they found a dearth of decays to muon-antimuon pairs in two independent data samples (divided by a kinematic variable, on the x axis). The discrepancy is a two-point-something-sigma thing, i.e. "don't even bother" in my Torino scale of anomalous effects.
Indeed, there are two-sigma effects everywhere you look: a similar thing arises once every twenty measurements, if the theory you are comparing the data to is indeed correct. Of course, these things happen more often if the theory does not hold, but... Is it enough to call the Standard Model off ?
I beg to say no, it does not. Not only it does not: to me, it doesn't even qualify as something exciting. And the fact that LHCb until yesterday had been pointing out that something else was awry in the kinematics of the B decays (some of those in the know may recall the issue of the P5' parameter, which was at some point at 3-sigma discrepancy with theoretical models, but is now in much better agreement overall, once other experiments also measured it, including CMS), and now points to something unrelated (yes, yes I know there are theorists out there ready to prove that the two things are indeed related, because, you know... Z' bosons, the whatchamacallits, and so forth), to me is a clear sign of inviting a new ambulance chase. [UPDATE: the chase has indeed started already, with 6 papers in the ArXiv in the matter of a few hours after the LHCb announcements. These days theorists are fast! ;-) ]
Don't get me wrong: first of all, LHCb is a beautiful, and quite successful experiment if you ask me. Second of all, I would be the first to rejoice if the Standard Model broke down in this somewhat unexpected way. To show that I put my money where my mouth is, here is a $100 bill saying that lepton universality will not be disproven, by an established observation-level measurement, within the next 24 months. It is yours if you bet against it and indeed the Standard Model crumbles on that very issue.
Any takers ? You must be somebody whose internet footprint is not elusive - so that if you lose, you prefer to pay off rather than carry the shame of not honoring your debt. Ah, you also need to be over 18, sorry. I do not fancy stealing candy from kids' hands.
LHCb Measures Unity, Finds 0.6