<--
.. title: Your Week in Seminars Fermionic Edition
.. date: 2010-10-25 11:56:00
.. tags:
.. category: old
.. slug: your-week-in-seminars-fermionic-edition
.. author: Yariv
-->
Good evening, and welcome to the second edition of YWiS. Last week I
took in the full range of seminars, from colloquium to Friday lunch. I
don't know if I can say I took in the full content of these talks as
well, but let's see what I learned On Monday we had Andrew Millis from
Columbia University talk about Materials with Strong Electronic
Correlations: The (Theoretical) End of the Beginning? (I think the
subtitle wasn't actually there in the talk itself). This was a condensed
matter theory talk, and like all condensed matter talks it started off
with the phase diagram for cuprates and a mention of the illustrious
pseudogap, the Dark Energy of condensed matter. The pseudogap is a phase
of cuprates - the materials that make high-temperature superconductors -
that occurs at about the same concentration of defects as
superconductivity but at a higher temperature. It is a little-understood
phase that sits between two well-understood phases (antiferromagnetism
and Fermi liquid) and perhaps holds some answers to the nature of
high-temperature superconductivity. Millis started with the pseudogap
picture and a short overview of the current state of condensed matter
theory. He claimed that perhaps some of the phases of matter in question
have local, short-range ordering, but no overarching long-range order in
the system, and that the investigation of these phases should take this
into account. At the end of this introduction he asked why we cannot
easily solve the problems of condensed matter. The basic equations that
govern the interactions in the field are known - the electromagnetic
potential and SchrÃ¶dinger's equation - and we should be able to just
plug them into a computer and calculate away. The trouble, as Millis
presented it, comes from the fermionic nature of the problem. What we're
trying to calculate, in metals, is the behavior of the electrons running
through the bulk of the metal. Electrons are fermions, which means that
no two can have the same quantum numbers, that is no two can be in the
same place with the same momentum and spin. It turns out that the
configurations with lowest energies tend to be symmetric, with many
particles in the same position. Finding low-energy configurations that
put every particle in a different place is much harder. I didn't get a
lot more from this talk. Millis went on to suggest a method that avoids
tackling the problem directly, but rather solves an analogous one that
we can translate to into a solution. I believe that there was some talk
of a local, rather than global, solution, and of the Hubbard model,
which is a popular approximation used in modeling electrons in a solid.
I phased in and out of this talk, but I'd peg my Understanding at 25
minutes, and my Interest at about 35 minutes. The Wedenesday particle
talk was by Jesse Thaler from MIT. He talked about Aspects of Goldstini.
Goldstini is the Italian plural form of goldstino, which is the
fermionic version - we put "ino" at the end of fermionic particles,
influenced by the neutrino - of the Goldstone boson. A Goldstone boson
is a massless particle that we find in theories of spontaneous symmetry
breaking. Spontaneous symmetry breaking is a popular concept in particle
physics, which springs from the concept of an unstable energy maximum.
Imagine a pencil standing on its tip, a system which is symmetric in
every direction. The pencil is unstable, though, and left by itself it
would fall down in any one of the equivalent directions around it. Once
it has fallen, it's broken the symmetry and created one preferred
direction. Thus the symmetry of the system is broken when one direction
is chosen spontaneously. This sort of thing is at the bottom of our
understanding the electroweak force, and pops up quite a bit in particle
physics. When it does, we expect a Goldstone boson, a massless particle
that roughly corresponds to spinning the fallen-down pencil around its
tip. The goldstini is the fermionic version of that particle which
springs from the breaking of supersymmetry - the symmetry that relates
fermions and bosons. The goldstino, then, is well known and accepted in
common theories of supersymmetry. It breaks supersymmetry, and then
interacts with the gravitino - another fermion, which mediates the force
of gravity - to become massless. Thaler's work posits more than one
goldstino, hence, goldstini. How can we have more than one goldstino? By
breaking supersymmetry more than once. We do this by imagining several
"sectors" in our theory, different sets of fields (particles) that break
supersymmetry but don't interact with each other significantly. When you
work through this model it turns out that you can have several
goldstini. Also, as the original goldstini lost its mass by giving it to
the gravitino, and the gravitino is now satisfied, the new goldstini get
to keep their mass, which turns out to be exactly twice that of the
(satisfied) gravitino. Thaler then discussed three possible scenarios
for this mass, and what we would expect to see at the LHC in each case.
The important thing, it turns out, is how this mass compares with that
of the lightest ordinary superpartner, the first supersymmetry-related
particle we expect to see in the LHC. If the mass of the goldstini is
very small, they will not come into play as the LOSP will decay into
particles we already know. If the mass of the goldstini is too large,
then the LOSP cannot decay into it. But if the mass is in some
goldstinilocks region in-between, things become interesting and we can
expect to see evidence of the gravitino and the goldstini, and
distinctly see one having double the mass of the other. I followed a
good portion of this talk, with Understanding of 30 minutes all in all,
and perhaps 45 minutes of interest. Finally, the Friday particle theory
lunch had a talk by our own David Curtin, one of Csaba's grad students.
He talked about Solving the gaugino mass problem in Direct Gauge
Mediation. I came into this one to follow more of it, on account of the
speaker being a student, but ended up following very little as it was
technical and above my level. It revolved, again, around supersymmetry
breaking. David does model building, which means he starts out with some
acceptable results, i.e. the universe as we know it, and tries to tinker
up a combination of particles and interactions that would reproduce it,
one portion at a time. What he was trying to build this time was a
metastable level in the broken supersymmetric potential. If we think
back to our pencil, we had an unstable maximum, the pencil standing on
its tip, and a minimum point, the pencil laying on the table, from which
it cannot fall. But we can also imagine a midpoint - perhaps resting one
side of the pencil on a book. It can't fall any further right away, but
there is another, preferred position lying flat on the table. That's
what we call a metastable energy level. As it turns out, the metastable
level has some desirable outcomes within the context of supersymmetry,
and the talk revolved around the ways we have of getting the right
energy structure to our system while avoiding things we don't want in
our models - arbitrary particle masses, a large number of new particles,
or anything blatantly unphysical. My Understanding here was quite close
to 0, as the technicalities were beyond me. (in fact, the pre-seminar
discussion was about soccer, so one might say my understanding was
negative). I probably kept trying to follow for about half the talk, or
30 minutes. That's it for last week. This week we can expect gravity,
(heavy!) Conformality Lost (literary!) and CFT/AdS (buzzwordy!). And
hopefully less headscratching and more nodding in a agreement.