Of Particular Significance

Some Weird Twists and Turns

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 08/02/2013

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.)

Don't confuse these two processes!  (Top) The process B_s --> muon + anti-muon, covered in Wednesday's post, agrees with Standard Model predictions.   (Bottom) The process B_d --> muon + anti-muon + K* is claimed to deviate by nearly 3 standard deviations from the Standard Model, but (as far as I am aware) the prediction and associated claim has not yet been verified by multiple groups of people, nor has the measurement been repeated.
Don’t confuse these two processes! (Top) The process B_s –> muon + anti-muon, covered in Wednesday’s post, agrees with Standard Model predictions. (Bottom) The process B_d –> muon + anti-muon + K* is claimed to deviate by nearly 3 standard deviations from the Standard Model, but (as far as I am aware) the prediction and associated claim has not yet been verified by multiple groups of people, nor has the measurement been repeated.

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.

Schematic drawing how a neutrino beam, made at J-PARC in Tokai, is measured both in Tokai and again, nearly 300 kilometers away, in the SuperKamiokande detector.
Schematic drawing (taken from the T2K website) showing how a muon-neutrino beam, made at J-PARC in Tokai, is measured both in Tokai’s “Near Detector”, and again, nearly 300 kilometers away, in the SuperKamiokande detector, at which it is measured to see whether it has any electron neutrinos.  And it does!  Note only a tiny, tiny fraction of the neutrinos will hit something, so despite the huge number of neutrinos in the beam, this measurement takes many months.

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.

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17 Responses

  1. Matt, I followed your link to matter and energy and wasn’t keen. Take a fresh-eyed look at Compton scattering where some of the photon E=hf energy is converted into the motion of an electron. The thing is, you could do another Compton scatter on the residual photon, and another and another ad infinitum. In the limit there’s no photon left. Take all the energy out of a wave, and it just isn’t there any more. It has been converted entirely into the motion of electrons. It has been converted entirely into motion. And yet in pair production, you can make an electron (and a positron) out of a photon. So in a way, matter is made of motion. Or kinetic energy if you prefer. Or just energy. Hence E=mc². It isn’t something it has, it’s what it is. I really wish I could engage guys like you with stuff like this. I don’t know if you’ve read Unzicker’s book, but it is scarey, and makes me feel that time is running out.

  2. There is a mystery about neutrinos that I discussed on a different blog but I’m still having trouble wrapping my head around.

    Neutrinos all seem to have a -1/2. Anti-neutrinos all have a +1/2 spin. If neutrinos are massless this isn’t a problem. But since they have mass they are going slower than the speed of light and you should be able to choose a frame of reference where a -1/2 spin looks like a +1/2 spin. This would seem to convert matter to antimatter simply by a change of frame of reference.

    How to deal with this seems to be what makes neutrinos so interesting.

  3. Is the present precision in g-2 consistent with QED? I would guess there are lots of presently known virtual particles which would contribute to g-2 even before we talk about virtual exotic particles. What do you think?

  4. There’s a “down” that should be “strange” at the beginning, talking about Wednesday’s post.

  5. If we were able to solve the problems of generating and detecting neutrinos are there any technical reasons that would prevent neutrino based data links?

    1. Those two problems are very profound, but they are the only two problems you’d need to solve to have data links. But there are other issues. One problem you could not solve is that the neutrinos cannot be shielded or stopped, so your data could be detected by anyone sitting behind or in front of your data link. Another potentially insoluble problem is that by the time the neutrinos could be detected efficiently, you might have such a powerful beam that it would be dangerous to any humans in the way of the beam. Finally, the expense of such a data link may make it completely impractical, even if it becomes possible.

      I do suspect that someday neutrinos will be used by geologists, and perhaps by industry, to probe the earth’s interior. It’s hard to imagine right now exactly how that will be done… but it is easier than a data link.

      1. Is it possible to accelerate electron/muon beam into W-resonance with incoming neutrino beam? Or the required energy will be too high for any practical use?

      2. Geologists have already used neutrinos, not for image probing, but to probe the radiogenic heat flux out of Earth:

        ” But it was not until 2005 that the KamLAND collaboration reported the first detection of geoneutrinos.1 Now, with five times as much data in hand, the international collaboration has reported its first substantive geophysics result.

        The principal finding is that present-day radioactivity accounts for about half of the total heat flux from Earth’s interior. That’s no great surprise to geologists. But it does supply the first explicit measurement of the radiogenic heat flux—albeit still with large error bars—for comparison with the broad range of detailed models of Earth’s formation and the energy sources of plate tectonics and the geodynamo.”

        [ Physics Today, 2011 (the system doesn’t let me link) ]

  6. It is common nowadays for people to want to be in the news quickly rather than correctly. This apparently also applies to scientists.

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