1. 2013

    Have we really found a tetraquark?

    Hooray, it’s time for another blog post! What I’m writing about this time is kind of old news — and don’t worry, there’s more to come on just why I haven’t been able to write about it for so long — but very interesting nonetheless.

    A few weeks ago, two separate physics experiments announced that they had discovered a tetraquark, a composite particle made of four quarks. Or rather, that’s what all the popular news coverage said. But what really happened? The discovery of a real tetraquark would be huge news, so I’m sure not going to trust the media reports on this one. As always, I’m going straight to the source: the original papers by the BES III and Belle collaborations.

    Two or Three is Company, Four’s a Crowd

    Before delving into the discovery itself, I’m going to tackle the burning question on everybody’s mind: what’s so special about a particle made of four quarks?

    To understand that, we have to look to quantum chromodynamics (QCD), the theory of how “color-charged” particles interact. In some ways, QCD is superficially similar to quantum electrodynamics, the theory of how electrically charged …

  2. 2012

    Off to DC! Quark Matter 2012

    Updates on this blog have been kind of sporadic over the past couple months, I know. Part of the reason for that is that the posts I want to do involve some research which I haven’t entirely had time for, but besides that, I’ve also been busy preparing for my presentation at Quark Matter 2012! This is a physics conference that focuses on the study of the strong force in systems that contain lots of quarks and gluons — not just individual protons and neutrons, but entire atomic nuclei as well.

    The particular piece of research I’ll be presenting is about the particle reaction \(\mathrm{p}A\to e^- e^+ \pi^0 X\): a proton collides with an atomic nucleus (the \(A\)) and produces an electron-positron pair, a pion, and other junk which we don’t care about (the \(X\)). The electron and positron themselves are produced from a (virtual) photon, so when you get down to the core of it, the reaction is really \(qg\to q\gamma^*\): a quark from the proton and a gluon from the nucleus interact to give a quark and a photon.

    What we’re interested in is the angle between that …

  3. 2012

    Day 5: Plenary sessions (again!)

    DIS 2012 wrapped up today, and the last day of the conference was filled with another round of plenary sessions (attended by everybody). This time, though, the talks were mostly devoted to summarizing the parallel sessions which took place over the previous three days.

    The conference was divided up by topic into seven working groups: structure functions, the future of DIS, diffraction and vector mesons, electroweak and new physics searches, hadronic final states, heavy flavor, and spin physics. Each of these working groups was organized by two or three conveners, who were also responsible for putting together and presenting the summary slides. I have to recognize the impressive amount of work this must have taken: in one afternoon, the conveners went through every single presentation given in the conference, and organized and adapted the main conclusions from all of them into an experimental and a theoretical summary talk for each working group. Not to mention they had to stay awake and attentive for the entire three days of talks — much easier said than done!

    Anyway, the full summary presentations can be found on Indico, so if you’re interested, go ahead and check those out. I’ll post a more …

  4. 2012

    Midweek report: parallel sessions

    We are now in the middle of DIS 2012, the part known as the parallel sessions because, well, they are in parallel. Specifically, at any given time during the conference there will be 5-7 presentations going on in different rooms. With four sessions per day and three or four 20-minute talks per session, that means there have probably been almost 200 physics presentations given in this one building in just the past two days!

    With that breadth of material, I can’t hope to cover them all — in fact, I haven’t even been able to properly “digest” just the ones I’ve been to! Unfortunately there are no particularly attention-grabbing talks like major experimental results, so nothing necessarily stands out of the pack; instead, here’s a somewhat arbitrary selection of some of the interesting presentation titles. If you are interested in this sort of thing, feel free to follow the links and read them; if not, make it into a drinking game or something.

  5. 2012

    Day 1: Plenary sessions

    DIS 2012 kicked off today with a full day of plenary sessions, general talks that everyone in the conference attends. (Well, not everyone attends, but there’s nothing else going on at any rate.) The slides of all the talks presented today are available on the conference website, but here are some of the interesting results.

    Results from the Tevatron and LHC

    Under the principle of “save the best for last,” I am getting this out of the way first: none of the major experiments have any new results of widespread importance to present. In particular, the Higgs search stands exactly where it was two weeks ago when the Moriond results were presented. This is no surprise because, for one thing, the Higgs boson is an electroweak phenomenon whereas DIS is more about the strong force; also, any major results would be presented at a bigger conference. DIS is a fairly specialized field of study so it doesn’t attract all that many people, in the grand scheme of things.

    Of course, that’s not to say there is nothing to report at all. The Tevatron experiments are finishing up analysis of their data and they have found some interesting …

  6. 2012

    What is Deep Inelastic Scattering?

    Since I’ll be writing about the Deep Inelastic Scattering Workshop this week, I was planning to make a pre-conference introduction post explaining in some detail what DIS actually is. But as it turns out, one of the plenary talks tomorrow is devoted to exactly that subject — plus I’m really tired after traveling for about 18 hours and walking around the city for another four or so. So I’ll start with a quick introduction and update this with more information tomorrow.

    Deep Inelastic Scattering

    Deep inelastic scattering itself is a particular type of physical process that occurs when a hadron (a particle made of quarks and gluons, such as a proton) collides with a lepton (a particle that, as far as we know, has no constituents).

    • It’s “deep” because the lepton has very high momentum as measured in the proton’s reference frame, so the way it behaves in the interaction can depend on very small features of the proton’s structure.
    • It’s “inelastic” because some of the kinetic energy of the original two particles is lost. In modern DIS, that energy goes into splitting the proton into many outgoing particles.
    • It’s “scattering” because the …