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Neutrino and meson breakthroughs
July 20, 2013 12:06 AM   Subscribe

While perhaps not quite as errm climactic as yesterday's news of pitch dripping in Trinity College, physics news dripping out of Stockholm reveals that
  • The Super Kamiokande T2K group has verified with great certainty that neutrinos oscillate among 3 flavors in flight (which could help explain what happened to the antimatter in the universe), and
  • CERN has used the LHC to measure the decay time of the rare B sub s meson, ending a 25-year search.

  • posted by Twang (9 comments total) 22 users marked this as a favorite

     
    Here are more Super Kamiokande photos. It really is the world's biggest swinging sixties bachelor pad.
    posted by Joakim Ziegler at 12:18 AM on July 20, 2013


    That sounds like something one of my college professors would've said... "since I got tenure, I've spent most of my time searching for the B.S. meson."
    posted by XMLicious at 4:45 AM on July 20, 2013 [1 favorite]


    Other physics/chem related news: Discovery of highly absorptive, high surface area, "impossible" material (via PLOS One). Gizmodo summary.
    posted by JoeXIII007 at 6:56 AM on July 20, 2013 [2 favorites]


    our sense of physics is much informed by our rather accidental position wrt to energy density and time-scale (with reference to the history of matter). It seems like the world of neutrinos maybe represents a more generic physics and what we are familiar with, a set of special coincidences...
    posted by ennui.bz at 8:11 AM on July 20, 2013 [1 favorite]


    Damnit, now I have to remember flavor physics.

    First, the basics (and I really should invest in some sort of resource for Intro to Particle Physics for Metafilter so that in the future I can just jump straight to the new stuff). In the Standard Model of particle physics, matter fields are spin-1/2 fermions and come in two general types: leptons and hadrons. Leptons are the ones that don't feel the strong nuclear force, hadrons do. Most "normal" matter that we're made out of is primarily up and down quarks (hadrons). Up quarks have electric charge +2/3 and down quarks are -1/3 (so two up and 1 down makes a charge +1 proton and two downs and one up are a neutral neutron - though protons and neutrons are only one average made up of 2 ups and 1 down or vice versa). Electrons are leptons. There is also an electrically neutral lepton called a neutrino, partner of the electron. They're electrically neutral and don't feel the strong force (the "charge" of the strong force is sometimes called "color" so leptons are often said to be color-less). Due to the lack of charge and color, they only interact via the weak nuclear force (mediated by particles called the W and Z bosons, as the electromagnetic force is mediated by photons and the color force by gluons). As the weak force is weak, neutrinos interact very rarely, and so you need huge masses of target to see them; that's what Super-K is for. I will skip neutrino physics for this comment and try to come back to it later. Right now let's concentrate on the B_S meson.

    So, continuing on the tour of the Standard Model, we have 2 quarks and 2 leptons (one charged, one neutral). Then, weirdly, it turns out we have copies of these particles which have the same charge assignments but are just heavier. So the up and down quarks have two additional copies: the charm and strange quarks, and the top and bottom (or truth and beauty if you go with the older alternate names that never caught on). Charm and top are up-type (charge +2/3) and strange and bottom are down type (charge -1/3). They are identical in every way to the up and down, just heavier (which is due to larger couplings to the Higgs boson: the top as the heaviest fermion has large Higgs interactions, and I'm dropping this factoid here in the hopes that I'll have reason to link back to it someday). The electron has two heavier partners too: the muon and the tau leptons. The electron neutrino has mu-type and tau-type neutrino partners.

    So that's the particle content of the Standard Model, and it's bizarre. There's no obvious reason that we need three copies (called generations or "flavors") of everything, when one set would do (and recent evidence from Higgs physics implies that there are only three copies that work like the ones we know about. A 4th generation would have to be fundamentally different from the preceding three to avoid being seen in experiments at this stage.

    Now, it turns out that electromagnetic and strong interactions conserve "flavor." So if you start with a strange quark and only consider interactions with photons and gluons, you'll end up with a strange quark at the end of the day. "Strangeness" is conserved. This is actually the origin of the name for the strange quark. It's the lightest of the non-up or down quarks, and when bound states containing the strange quark were created in early particle colliders (basically copies of protons, neutrons, or quark-antiquark states called "mesons" with a down quark swapped out with a strange quark) physicists noted that these new particles were a) heavy, and b) lasted a strangely long time compared to the lighter ones we were used to. This is because the lighter particles could decay via the strong interaction (powerful interactions occur quickly in particle physics). The strange quark can only decay via the weak force, which results in comparatively long times for decays.

    The W bosons, which are charged, can convert a up-type quark to a down-type quark in the same generation, or an charged lepton to a neutrino of the same flavor. Most of the time, this is what happens, and the "generation number" is conserved (u to d, s to c, t to b, e to e-neutrino etc etc etc). However, there's a slight mis-match between the interactions in the weak interaction and the flavor assignment, so very rarely, strange quarks can become up quarks by radiating away a W- boson. This is called flavor changing "charged current" interactions (or FCCC).

    The other weak interaction particle, the Z boson is electrically neutral, and in the Standard Model, cannot change flavors. So flavor changing neutral currents (FCNC) are barred in the Standard Model if you just exchange one weak force boson. So, in the Standard Model, the only flavor-changing that happens is through the W, and it's fairly small.

    However, if you throw in new physics at some higher energy than we're currently probing, it is actually very difficult to avoid large new flavor-violating interactions. This is mostly because we have no idea where flavor "comes from," and so when writing down possible new physics, there's no fundamental principle we can rely on that says "oh, flavor has to act like this..." Supersymmetry, for example, would generically lead to many new interactions that, like the honey badger, don't give a fuck about flavor. While until recently we wouldn't expect to see that new physics directly, we'd expect it to have large effects in low energy flavor physics: large FCNC, for example. Why those effects are small is called "the flavor problem."

    Now: B_S to mu mu. The B_S (pronounced Bee-sub-Ess) meson is a combination of a strange quark and a bottom anti-quark (a s anti-quark and a b quark is a B_S-bar, the antiparticle). So it has both strangeness and "bottomness." It's also charge-neutral. Now, to decay into two muons (really a muon and an anti-muon) in the Standard Model, it can't go through a Z-boson: both s and b number would change, and the Z can't change either. So what has to happen is that it goes through a "loop process." You can think of this as: on some rare occasion the anti-b quarks emits a W+ boson, and changes into an anti-top. The anti-top and the strange quark combine and turn into a W- boson. Now we have two W's and no meson. The W+ boson emits an anti-muon and a mu-neutrino. The mu-neutrino combines with the W- boson and emits a muon. Ta-da: a s and anti-b quark turned into a muon and anti-muon, which fly off into the sunset. (If I had the ability to draw pictures here, this would be a lot easier I'll try it in ASCII: =|=|=. The horizontal lines on the far left are the incoming b and s, the vertical line is the top, the middle horizontal lines are the W- and W+, the 2nd vertical line is the mu-neutrino, and the last two horizontal lines are the muons. I'm sure you're ALL following that, because it's so clear.) There's also another diagram that's a bit harder to describe called a "penguin" diagram, which is a hilarious name involving John Ellis losing a bar bet and getting stoned. Ah, physics is so funny.

    These processes sound really complicated I'm sure. And they are. They're also really really rare. My particle data book (don't leave home without it) tells me that the B_S decays 93% of the time into other mesons with strangeness and/or charmness, and most of the rest of the time into a smattering of other final states that don't require this double emission and absorption of W's. It also says that the decay into mu mu final states occurs less than a few times 10^-7% of the time, but that is hasn't been seen. We'll have to update that now (it decays to this state 3x10^-7% of the time, it turns out)

    So why do we care. Well, remember that new physics can do this sort of thing "easily." So, if this process occurred more often than the Standard Model loop process predicted, that would be evidence that new physics existed, had large flavor-violating interactions, and would give a hint as to the energy scale of new physics. All very good things. Before we measured it, we knew it was small, which was consistent with both Standard Model predictions AND the possibility of new physics (it could have been small but larger or smaller than the Standard Model predicted). Now, we measure it to be small but consistent with the Standard Model, reducing the possibilities.

    Does this mean there is no new physics hanging around? No. It means that either the new physics is at higher energies (reducing the impact it would have at the low energies relevant to B_S decays), or that it is at low energies, but for some reason respects flavor and so cannot contribute to this sort of process. Why this could be is a mystery, but everything about flavor is a complete mystery to me and the rest of the field.

    I'll get around to the neutrino thing in a bit.
    posted by physicsmatt at 8:59 AM on July 20, 2013 [24 favorites]


    Thank you, matt, for your explanation. It took two or three reads for me to get it, but I think I did, in the end.
    posted by SPrintF at 9:06 PM on July 20, 2013


    It dropped? IT DROPPED? Damn it! I have been watching the pitch drop feed for YEARS! I have been literally sitting at my computer watching it. Actually, several computers. When I was upgrading my desktop I had a laptop watching... Hoping... Dreaming of the time that I would watch that black lump drop off of the main mass!

    Sure, it wasn't easy. I stopped eating sold food five years ago. I had the wife bring me bags of nutrient rich fluid that I would suck on all day. But I had to have a catheter. And just the other day I had to change the damn catheter. And BANG! The damn thing drops when I'm out of the room. Well. There is nothing to do but sit down and wait again.

    Honey? Bring me a bag of food. I'm in it for the long run.

    But seriously. I think this is very cool.
    posted by Splunge at 10:01 PM on July 20, 2013


    In the Standard Model of particle physics, matter fields are spin-1/2 fermions and come in two general types: leptons and hadrons. Leptons are the ones that don't feel the strong nuclear force, hadrons do. Most "normal" matter that we're made out of is primarily up and down quarks (hadrons). Up quarks have electric charge +2/3 and down quarks are -1/3 (so two up and 1 down makes a charge +1 proton and two downs and one up are a neutral neutron - though protons and neutrons are only one average made up of 2 ups and 1 down or vice versa). Electrons are leptons. There is also an electrically neutral lepton called a neutrino, partner of the electron. They're electrically neutral and don't feel the strong force (the "charge" of the strong force is sometimes called "color" so leptons are often said to be color-less). Due to the lack of charge and color, they only interact via the weak nuclear force (mediated by particles called the W and Z bosons, as the electromagnetic force is mediated by photons and the color force by gluons).

    Well, you know, that's all very interesting, but I'm afraid I'm not into Pokémon.
    posted by kcds at 7:15 AM on July 21, 2013


    I really should invest in some sort of resource for Intro to Particle Physics for Metafilter

    Why does Metafilter need its own Physics Department when there's already so many good educational resources on the web (some of which even include diagrams....)?
    posted by crazy_yeti at 7:15 AM on July 22, 2013


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