1. 2012

    B meson oscillations and the CPT theorem

    As if last week’s announcements of new Higgs results, B-dimuon decay, the rediscovered Y(4140), and all sorts of other goodies at HCP 2012 weren’t enough, there’s more big news from the world of experimental particle physics this week. A paper published just a few days ago in Physical Review Letters (here’s the PDF, and the arXiv page) describes the first observation ever of actual time reversal asymmetry: a difference between the behavior of a particular physical process and the time-reversed version of the same process.

    Lest you get too excited, though: this has nothing to do with actual reversing of time, so it doesn’t mean time travel is possible or anything like that. And in fact, nobody in physics is the least bit surprised that it worked out the way it did. There is a theorem in physics called the CPT theorem (or sometimes PCT, or TCP, but not that TCP) which basically guaranteed that time reversal asymmetry had to show up somewhere. The theorem is suddenly getting a lot of attention in the news coverage of the discovery, but it’s technical enough that most people aren’t bothering to explain it. I …

  2. 2012

    Why, oh Y(4140) are you so tantalizing?

    Note: I’m posting this from the road, so it will be somewhat lacking in pictures and details. Stay tuned for an update that fills all that in!

    I’ve spent a lot of time poring over the results coming out of the Hadron Collider Physics conference this week, and I’ve noticed a trend. Higgs candidate cross sections are consistent with the standard model. B meson branching ratios are consistent with the standard model. Multijet event counts are consistent with the standard model. Maybe you can see where this is going. Vector boson production rates are consistent with the standard model. CMS rediscovered an unknown particle. Meson masses are consistent with—

    Wait, what?

    Yeah, that happened. CMS announced the second observation ever [PDF] of a mysterious new particle which defies classification.

    This mystery object, called the Y(4140) or sometimes X(4140), was first seen in 2009 by CDF, one of the experiments at the Tevatron. It wasn’t just a slight fluctuation, either; the CDF data excluded the background-only hypothesis at more than a \(5\sigma\) level, which is the threshold physics uses to define a proper discovery. Certainly much ado has been made about less strong statistical …

  3. 2012

    Extra! Extra! Higgs results from HCP 2012

    The Hadron Collider Physics conference is nearing its end now, and that means one thing: Higgs results! The LHC collaborations have just presented their updated measurements of the Higgs boson candidate whose discovery was announced in July, based on the \(\SI{7}{fb^{-1}}\) of new data that was collected in late summer.

    I’ll jump straight to the punch line: what they see is mostly consistent with the new particle actually being a plain old standard model Higgs boson (although it’s not absolutely confirmed yet). Which is kind of disappointing, because it indicates a lack of exciting new stuff to discover. Back in July when the discovery of the particle was originally announced, there were some slight discrepancies between the results obtained by the experiments and the predictions, and a lot of physicists were hoping that was a hint at something new and unexpected, but it’s looking more and more as though that is not the case.

    Anyway. On to the results. As in the original announcement, ATLAS and CMS are searching for the Higgs in five different channels: they’re trying to detect five different sets of particles that the standard model Higgs boson can decay …

  4. 2012

    B meson decay and the calculation of cross sections

    Now that I’ve finished my quest to identify the misbehaving sinc function, I can bring you the latest news from the Hadron Collider Physics conference in Kyoto, Japan. This is a major conference at which several new results from the LHC experiments are being announced — an exciting time for the physics world indeed!

    B meson-muon decay

    The most interesting result to come out of the conference so far is an observation by the LHCb experiment of a \(\mathrm{B}^0_s\) meson, made of bottom and antistrange quarks, decaying to a muon-antimuon pair. This is a reaction that physicists have been searching for since 1987. This year, the LHCb collaboration has actually seen it happen — or at least, they’ve collected enough statistical evidence to be fairly confident (\(3.5\sigma\)) that it does happen — for the first time.

    It’s not quite as dramatic a discovery as that makes it sound, though; the reason \(\mathrm{B}^0_s\to\mu^+\mu^-\) has never been seen before is that it’s incredibly rare. B mesons are not exactly easy to produce, and then once you’ve got one, the standard model predicts that only one out of every 300 million will …

  5. 2012

    Results from HPT 2012

    One unexpected perk of being in China: I woke up before 7:30 this morning. That would never happen without jet lag.

    Unfortunately, even waking up at 7:30 every day hasn’t given me any time to write up a mid-conference blog post. Talks have been running from 8:30-6:30, with the rest of the time mostly taken up by meals and discussions. So I’ll just post this “teaser” of some of the more interesting results that were presented.

    Of the presentations that gave new results, most of them are based the September proton-lead run at the LHC. This was just a pilot run, meant to ensure that there wouldn’t be any unexpected problems with colliding two different types of particles, so there wasn’t a lot of data collected — only 2 million collisions — but it was already enough to start shedding some light on the underlying physics.

    No initial state effects

    Ion-ion collisions have already been extensively studied at both RHIC and the LHC, and as you might imagine, when you smash a blob of a hundred blobs of particles into another blob of a hundred blobs of particles, what you get is a mess …

  6. 2012

    Check-in from China

    WELCOME READERS OF THE FUTURE!! (with ominous echo) In all seriousness, I wrote most of this from the Beijing airport yesterday, but without internet access I couldn’t post it until I got to the conference.

    First impression of China: it doesn’t seem that different from the US. Classical music plays over the speakers in the airport. Signs are pretty much bilingual everywhere, or even English-only (although to be fair, a lot of those are actually made-up Western name brands that don’t mean anything anyway). And seeing a bunch of Chinese people walking around speaking Chinese doesn’t feel even a little bit strange. This is a testament to just how many Chinese students have swarmed into American higher education. It’s only when I look around for the corresponding groups of white people and Indians — and don’t find them — that it really starts to become evident that I’m in a foreign country.

    Oh, and another thing: unlike what you might hear (in the US) about China being a “police state,” the police and security officers in the airport really don’t seem to be taking their jobs more seriously than they have to. It’s …

  7. 2012

    Balloons as air bags: is there any hope?

    It’s time to write about Mythbusters again! Last night on the show, the team tested a myth that balloons can take the place of an air bag in the front seat of your car, and if you have the right configuration of balloons, you’ll be protected from the damaging effects of a crash.

    If you watched the show, you’ll know that this myth was thoroughly busted. The maximum acceleration a human body can take and still survive is \(100g = \SI{980}{m/s^2}\), and no matter what configuration of balloons they tried, they were unable to reduce the acceleration to any less than about \(120g\). This got me thinking about why that might be the case. Physically, keeping a body’s acceleration below a certain threshold is just a matter of spreading that acceleration out over a long enough distance. It’s easy to see that by looking at the relevant equation:

    $$v^2 = v_0^2 + 2a\Delta x$$

    For a car crash, you’d be given \(v_0\), the initial speed, and \(v = 0\), the final speed, and so if you want \(a\) to be small, you have to make \(\Delta x\) large. Specifically,

    $$\abs{\Delta …
  8. 2012

    Nobel Prize for the Higgs? Meh, maybe

    The physics community online is abuzz with speculation about who will be awarded this year’s Nobel Prize in Physics. And in a lot of people’s minds, the announcement by ATLAS and CMS in July of a new particle, widely expected to be the Higgs boson, is the leading candidate.

    Certainly nobody doubts that between the theoretical discovery of the Higgs mechanism and the experimental discovery of the presumed Higgs boson, there’s more than enough to deserve a Nobel Prize. And I hope and expect that some subset of the scientists involved will get it eventually. But I think making that announcement this year would be a little premature. For starters, we don’t actually know that it is the Higgs boson that was discovered at the LHC. Sure, it has a mass in the right range, and it decays to the right particles, but possibly in the wrong amounts. That could indicate that there is some extra effect that modifies the properties of the Higgs boson from what was predicted, or that it’s not the Higgs at all. The LHC will shed more light on that over the coming years, so it seems sensible to wait …

  9. 2012

    Higgs papers pass peer review! So what?

    Today the internet is abuzz with the news that the papers from ATLAS and CMS announcing their discovery of the Higgs boson have passed peer review and are officially published in Physics Letters B. That means now they’re actual science, right?

    Not really. (I’m assuming the ExtremeTech headline was a bit of a joke.) Peer review is really not as big of a deal as people outside the scientific community are often led to think. In particular, “peer-reviewed” does not mean “correct.” Peer review is just a high-level check to make sure that the paper isn’t complete nonsense and that the problem it’s addressing is relevant and interesting. Journals have limited space to publish these things, and they have to determine which of the many submissions they get are the most worthy of being put in that space. That’s what the peer review process is for.

    When something comes out of a big experimental collaboration like ATLAS or CMS, though, it has already gone through a rigorous vetting process. Doubly so for a high-profile result like this one — in fact, I’m sure the results had been double- and triple-checked by dozens of people even …

  10. 2012

    A smorgasboard of major experiment results

    We’re now halfway into Quark Matter 2012, and many of the presentation slots (at least the ones I’ve looked at) have been devoted to the experimental groups presenting their new results. In heavy ion physics, the major groups are the STAR and PHENIX collaborations at RHIC, and ATLAS, CMS, and ALICE (which I’ve learned is pronounced “ah-LEES,” not “AL-iss”) at the LHC.

    Naturally, the experts who are interested in these things will just go straight to the conference page and look at the presentations — heck, they’re pretty much all here in Washington anyway. So I’m going to try to explain some of these results in a way that makes them comprehensible for non-experts (although, apparently unlike some of my fellow conferencegoers, I’ll give you enough credit to assume you know what atoms are).

    What Quark Matter is all about

    The main focus of the Quark Matter conference is, of course, quarks, and also gluons: the most fundamental particles that make up atomic nuclei. How do they organize themselves into protons and neutrons and then into nuclei? How do the properties of those atomic nuclei emerge from their internal structure? And what really happens when …