In my last post, I promised you some comments on a couple of other news stories you may have seen. Promise kept! see below.
But before I go there, I should mention (after questions from readers) an important distinction. Wednesday’s post was about the simple process by which a Bs meson (a hadron containing a bottom quark and a
down[typo] strange anti-quark, or vice versa, along with the usual crowd of gluons and quark/antiquark pairs) decays to a muon and an anti-muon. The data currently shows nothing out of the ordinary there. This is not to be confused with another story, loosely related but with crucially different details. There are some apparent discrepancies (as much as 3.7 standard deviations, but only 2.8 after accounting for the look-elsewhere effect) cropping up in details of the intricate process by which a Bd meson (a hadron containing a bottom quark and a down antiquark, or vice versa, plus the usual crowd) decays to a muon, an anti-muon, and a spin-one Kaon (a hadron containing a strange quark and a down anti-quark, or vice versa, plus the usual crowd). The measurements made by the LHCb experiment at the Large Hadron Collider disagree, in some but not all features, with the (technically difficult) predictions made using the Standard Model (the equations used to describe the known particles and forces.)
A few theorists have even gone so far as to claim this discrepancy is clearly a new phenomenon — the end of the Standard Model’s hegemony — and have gotten some press people to write (very poorly and inaccurately) about their claim. Well, aside from the fact that every year we see several 3 standard deviation discrepancies turn out to be nothing, let’s remember to be cautious when a few scientists try to convince journalists before they’ve convinced their colleagues… (remember this example that went nowhere? …) And in this case we have them serving as judge and jury as well as press office: these same theorists did the calculation which disagrees with the data. So maybe the Standard Model is wrong, or maybe their calculation is wrong. In any case, you certainly musn’t believe the news article as currently written, because it has so many misleading statements and overstatements as to be completely beyond repair. [For one thing, it’s a case study in how to misuse the word “prove”.] I’ll try to get you the real story, but I have to study the data and the various Standard Model predictions more carefully first before I can do that with complete confidence.
Ok, back to the promised comments: on twists and turns for neutrinos and for muons…
Another measurement which has gone from strong evidence to an official discovery is the process by which muon-type neutrinos, traveling about 300 hundred kilometers (180 miles) through the Earth, turn into [“oscillate into”] electron-type neutrinos. [Click here for a discussion of neutrino oscillations, and a discussion of how beams of muon neutrinos are made, and a discussion of how neutrinos can [with difficulty] be measured.] Actually, the oscillation of electron-type neutrinos into something had already been discovered last year, as I described here. But now the T2K experiment, which also provided the initial evidence for this type of oscillation, has confirmed (by observing the reverse process) that electron neutrinos are oscillating into muon neutrinos (and probably tau neutrinos too); they aren’t turning into some other, previously unknown type of neutrino-like particle. This is as one would expect if there are only the three known neutrinos in nature.
In T2K, a beam of mostly muon-type neutrinos is made at Tokai (T), at the J-PARC laboratory; the beam is measured at Tokai to check its composition, travels to (2) the laboratory in the Kamioka mine (K), and is measured in the SuperKamiokande experiment. Comparing the measurements at Tokai and Kamioka, the composition of the beam is determined to have changed; a larger fraction of the neutrinos that collide with an atomic nucleus turn into electrons at Kamioka than do at Tokai, indicating some of the muon neutrinos have changed into electron neutrinos.
The fact that this conversion occurs with a substantial rate is interesting on its own, and has implications for those of us trying to understand why the masses of quarks, charged leptons, and neutrinos form such a strange and difficult-to-understand pattern. Equally important, it will make it easier for experiments to study another property of neutrinos in the future: whether their behavior violates the symmetry CP (which you can read about here.) In the long-term, discovery of CP violation in neutrinos could perhaps have implications for understanding how the universe ended up with so much more matter (i.e., in this context, electrons, protons, and neutrons) than anti-matter (i.e., positrons [anti-electrons], anti-protons and anti-neutrons.) Don’t hold your breath though; this will be a story lasting decades or more.
By the way, as I recently learned, the lab at Tokai is temporarily closed down, along with all of its science, due to a fortunately very small but nevertheless badly-handled radiation accident. This is a case where apparently the lab’s safety systems were sufficient to contain the problem, but either bad training or bad judgment caused humans to override the system. Tokai was also hit hard during the 2011 earthquake, so it’s been a rough period for the lab, to say the least.
The spectacular journey via water and road by a giant electromagnet, 50 feet across, from New York’s Brookhaven Laboratory, where it was built, to the Fermi Laboratory outside Chicago, where it will be reused, has been widely reported. Of course, almost nobody has reported what the magnet is going to be used for, because, hey, it’s a bit more complicated and the pictures aren’t quite as good. I promise an article on this, but the short story is this:
The precise way that a particles of a certain type (electrons, neutrons, muons, etc) respond to electric and to magnetic fields turns out, in some cases, to be a remarkably powerful tool for testing the Standard Model (the equations used to describe all of the known particles and forces.) If there are additional particles or forces in nature that we haven’t discovered yet, it is possible that they will affect these responses in a subtle way, via effects known as “virtual particles” (i.e., via non-particle-like disturbances in the corresponding as-yet undiscovered fields). These effects, though very small, may sometimes be detectable, if sufficient precision can be obtained both in the experiment that does the measurement and in the theoretical calculation of what is expected within the Standard Model.
The response of electrons to magnetic fields (the electron’s “magnetic moment”) has been measured to better than one part in a million million (1,000,000,000,000) and it agrees with the Standard Model prediction, meaning that nothing unexpected showed up in that measurement. [The magnetic moment is proportional to a quantity called “g”, which in the absence of virtual particles would equal 2; the interesting part of the measurement is therefore often called “g-2”, pronounced “gee-minus-2”.] But the muon, with a mass about 200 times larger than the electron, could in many cases react to new particles or forces with a strength (200)² times larger, or even more. So measuring the muon’s magnetic moment to better than one part in a thousand million (1,000,000,000) — which is what this giant electromagnet was previously and will in future be used for — could actually be a more efficient way to discover something new.
These measurements, which don’t require much energy per muon but do require a huge number of muons, are complementary to the types of measurements done at the LHC, where the colliding protons have as much energy as we can give them. Some types of as yet unknown particles and forces would show up first in the direct searches performed at the LHC, but others would show up first in a precise, indirect measurement, such as “muon g-2”. It is this complementarity that explains why particle physicists pursue both types of approaches: extremely high-energy collisions with good precision (the Tevatron and the LHC) and extremely high-precision measurements with only moderate energy (muon g-2, and many others). [Interestingly, the bottom meson measurements mentioned at the top of this post are a blend of both; they need the large energy of the LHC to produce the moderate-weight bottom mesons — which have a mass about five times larger than that of a proton — in great abundance.]
A challenge for this measurement, which will begin in 2016, is that it isn’t so easy for theorists to calculate the Standard Model prediction at high enough precision to match what this experiment can do; an apparent discrepancy between the last g-2 measurement and theorists’ calculations was a source of widespread, but unresolved, controversy in the previous decade. We’ll see if the theorists can improve their predictions by as much as the experimentalists will be improving their results.