<--
.. title: Your Week in Seminars Intro Edition
.. date: 2010-10-18 12:50:00
.. tags:
.. category: old
.. slug: your-week-in-seminars-intro-edition
.. author: Yariv
.. has_math: true
-->
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](http://lepp.cornell.edu/Events/ParticleTheory/WebHome.html).
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
B^0^~s~ and the B^0^ (mesons made up of an anti-b and an s or d quark,
respectively) is the same as the difference between the Ξ^0^ and the
Σ^0^" (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.