Monday, October 18, 2010

Your Week in Seminars Intro Edition

We've done a lot of talking over the past few months here on the Virtuosi, but one important subject has not come up so far. An issue that is central to the day to day life of the average grad student. The subject of free food.

The average graduate student in an American university shops for food 0.7 times per semester, paying a total of $13.22. He eats an average of three vegetables and one fruit, all at home during Thanksgiving. He turns his oven on once per year while trying to ascertain if the power is out or the light bulb in the kitchen needs to be replaced. The rest of his nutrition is made up entirely of free donuts, bagels and pizza.

The place to get all this free food, naturally, is various department talks and seminars. And while we're there, we may as well try to learn some physics.



With that noble goal in mind, I'd like to welcome you to the first edition of Your Week in Seminars, where I shall endeavor to relay the content of the weekly seminars I attend in Cornell. On an average week this will be one general interest colloquium and two particle theory talks. One of my colleagues may want to take up the LASSP (Condensed Matter) talk or any of the other seminars going around in the department

I'll try to relate what I got out of each talk, with more words than equations and with no figures. I'll aim for a general audience level but I think I'm likely to end up at a physics undergrad or a popular-science-savvy level, as technical terms are bound to be thrown about. If there's one you don't know, feel free to ask over in the comments or take this as an opportunity to delve into Wikipedia.

I'll also provide two handy metrics to the quality of the talk, my Interest Level, defined as the amount of time before I start playing with my phone, and my Comprehension level, defined as the amount of time where I was still following the speaker.

Last week there was no colloquium due to Fall break, so this post will cover just the Wednesday and Friday particle seminars.

On Wednesday we had David Kagan from Columbia University tell us about Conifunneling - Stringy Tunneling Between Flux Vacua.

As you may know, string theory demands that our universe have a large number of dimensions, generally 10 or 11, to avoid such nastiness mass particles. To bridge the gap between the theoretical and observed number of dimensions (four) one has to "compactify" the extra dimensions, that is, to posit that they have some shape and size and write down an effective four-dimensional theory that takes their presence into account.

This compactification creates an energy surface, or some effective potential in space. What we call "vacuum", the ground state of the universe, rests in one of the minimum points of that potential, as ground levels are wont to do. But it need not be the absolute minimum, just a local one, and where there are local minima in a quantum theory we know that there is also tunneling.

Kagan, then, talks of tunneling between these local energy minima created by compactification of the extra dimensions of string theory. This tunneling, from what I gathered, can be described as an evolution in time of the manifold, the geometric layout of spacetime.

The main conceit of the talk was that this evolution takes the manifold into the form of a "conifold", which is a manifold with a conic singularity. This conifold then nucleates a 5d-brane; branes are a objects in string theory that have some dimensionality less than that of the entire spacetime. After creating this object, the conifold transforms back into a non-singular manifold, but one where the vacuum is in another energy minimum.

We can visualize this process by thinking of spacetime as an elastic sheet of of sorts, pinched at a point and pulled. It is deformed, creating an elongated cone-like area, until finally it tears, emitting a five-dimensional brane, and reverting back to its original form.

There was some discussion at the end which mostly went over my head, but at some point Henry Tye, Liam and Maxim were trying to figure out whether the tunneling is necessarily done via a conifold or whether Kagan was just describing what happens if it does. The conclusion, I believe, was that it is the latter case, though Kagan said they have some good arguments on why the conifold tunneling had to happen.

Interest: 40 minutes.
Understanding: 20 minutes.


On Friday we had Zvi Lipkin from the Weizmann Institute tell us about Heavy quark hadrons and exotics, a challenge for QCD.

This talk revolved around the constituent quark model for QCD. Our usual picture of hadrons is one of two or three valence quarks sitting in a sea of gluons and virtual quark-antiquark pairs, due to the strong interactions of Strong Interaction. Lipkin's work focuses on trying to abstract this sea away and focus on the valence quarks as if we were discussing a hydrogen-atom-like system of two particles and a potential between them.

This kind of treatment allows us to maximize the use of flavor symmetries. Flavor is QCD-speak for "type of particle", that is, up, down, strange, charm and bottom quarks. Using the constituent quark model we may be able to say things like "the difference between the B0s and the B0 (mesons made up of an anti-b and an s or d quark, respectively) is the same as the difference between the Ξ0 and the Σ0" (baryons made up of uss and uds quarks, respectively).

(Don't take that last example too seriously - I made it up by looking at lists of baryons and mesons. But that was the gist of the talk)

Lipkin showed done by him and Marek Karliner, (who taught me differential equations in Tel Aviv) including lots of numbers nicely matching between their theory and experiment as well as a less-convincing attempt to characterize the two-body potential in this two-body problem.

At the end of the talk he also mentioned the X(3872) seen by the Belle experiment. This is a particle that does not seem to fit into our regular models as either a baryon or a meson, and Lipkin suggested that this might be a "tetraquark," a combination of two quarks and two antiquarks. This kind of exotic hadron has been talked about for a long time, and there was some excitement a few years ago with the discovery and eventual un-discovery of the Θ+ pentaquark. (made up of four quarks and an antiquark)

Interest: 60 minutes. (I was sitting in the front and could not politely take out the phone)
Understanding: 60 minutes.

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