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 …

OK, actually it is kind of a big deal. Discovering a new particle is not something that happens every day, and it’s a concrete result of having a well-tuned detector. Besides, it’s just cool. So congratulations to the CMS collaboration!

In case you haven’t heard the story, late last week CMS announced that they had a statistically significant observation of the \({\Xi^*}_b^0\) baryon, a particle made up of an up quark, a strange quark, and a bottom quark. In this case, “statistically significant” means that they detected this particular decay signature 21 times, of which only \(3\pm 1.4\) of them can be attributed to random coincidences in the detector. So they’re about as sure as you can be in physics that they are seeing signs of a real particle. They’ve also managed to reconstruct various properties of this particle by examining the decay products, and everything matches up with the predicted properties of the \({\Xi^*}_b^0\).

Now, why isn’t this a bigger deal, and why didn’t I write about it right away? Well, as I just mentioned, this particle was predicted to exist. Of course, the Higgs boson …

There are a few interesting experimental results and analyses from the physics world this week, mostly having to do with dark matter. Probably the biggest of these is a fairly detailed paper on the local density of dark matter by the team of Moni Bidin, Carraro, Méndez, and Smith. As you may know, dark matter is astrophysicists’ favorite method to explain how the tangential velocity of stars in large galaxies can be nearly constant all the way from the center out to the (visible) edge, despite the fact that a simple model would tell you that the velocity should be slower for stars further out. It explains a bunch of other observations too, including measurements of gravitational lensing by large galaxy clusters, so we’re pretty confident that dark matter exists.

With that in mind, it’s kind of surprising that the analysis done by Moni Bidin, Carraro, Méndez, and Smith finds no dark matter at all within a few kiloparsecs of the solar system! Basically, what they’ve done is apply Newtonian gravity (which applies fairly well on these scales), along with ten reasonable-sounding assumptions, to find a formula which relates the velocities of stars in some region of …

It hasn’t escaped my notice that Mythbusters is back with a new season! Actually, it’s not really that new, since we’re now three (well, now four) weeks in, but I missed the first two episodes since I was out of the country. But it works out because this (actually last) week’s myth is full of interesting physics to analyze!

This past Sunday, Adam and Jamie tested the myth that if you’re driving fast enough, square wheels can actually provide a surprisingly smooth ride. At first, the idea of square wheels working at all, much less actually being smooth, can seem a little wacky, but with a bit of physical intuition, it’s not hard to convince yourself that it’s actually pretty plausible. As they explained in the show, the reason a square wheel is expected to bounce you up and down is that the distance from the axle to the bottom of the wheel changes as it turns. If you’re going slowly, every time the wheel tips over another corner, it’s going to fall down until its side is resting against the ground, taking you with it. But if you speed up …

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 …

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.

• SM Higgs production in theory
• A theoretical review of the BSM Higgs
• Impact of LHC data on (NN)PDFs
• GPDs experimental
• Low x physics: a critical phenomenon?
• On the NNLO QCD corrections to deep-inelastic …

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 …

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 …