Monday, November 22, 2010

Your Week in Seminars: One for Two Edition

Hello everyone and welcome to another week of talks here at the physics department. I was out of Ithaca for a bit this past week, so in this very special edition I'm going to present a full week's worth of seminars (one from last week and two from the previous week) in one post covering two weeks.

The Colloquium two weeks ago was given by our own Csaba Csaki, who talked about Electroweak Symmetry Breaking and the Physics of the TeV Scale.

This was essentially an overview of beyond-the-standard-model physics and the kind of things we expect out of the LHC. Csaba started off by reminding us of the Standard Model, our very successful model of particle physics that's withstood nearly every test over the last thirty or forty years. The Standard Model is a set of three gauge theories - three forces that a relayed by massless particles - along with the theory of electroweak symmetry breaking that explains why one of these powers, the Weak one, is relayed by massive particles. This theory is well-backed by experiment, with the exception of the crucial Higgs boson, the one that gives mass to those previously massless W and Z, which we hope to see soon in the LHC.

There are a few problems with the Standard Model, and the big one is the Hierarchy problem. Given what we know of symmetry breaking and how the W and Z get their masses, we expect elementary particles to have masses that correlate with the energy scale of the interaction that gives them this mass. Since that interaction is not one we see at low energies, we expect the elementary particles to be very massive. Since they are not, we conclude that there must be some symmetry that keeps them massless or nearly so.

Some solutions to this was mentioned, beginning with current-favorite supersymmetry. This extra symmetry relation bosons to fermions and vice versa works well to solve the original problem, but creates a few of its own, like the Little Hierarchy Problem - if there's all this new physics at energies just a little higher than we've been exploring, why don't we see its effects on the low energy physics? In other words, why does the non-sypersymmetry Standard Model work so well?

Csaba went on to mention some ways of solving these problems, such as burying the Higgs by allowing it to decay only in very specific ways. He also talked about a few more, like no-Higgs theories that accomplish electroweak symmetry breaking by different means, and extra-dimensional theories that allow us to give different energy scales to different forces.

And the exciting thing about all of this is that we are likely to know a great deal of the answers soon, within the next few years, once the LHC starts giving data.

On Wednesday after it we had Sven Krippendorf from Cambridge talk about Particle Physics from local D-brane models at toric singularities.

This was a heavy string theory talk and I couldn't follow much of it. The question at hand was how to get the Standard Model, or parts of it, out of string theory models, and the gist of the talk revolved around toric singularities in the spacetime that the string theory lives in. No, I'm not entirely sure what makes a singularity toric.

There were a lot of colorful graphs and some explanations. At the end there seemed to be some analogy made between different types of singularities on the manifold in string theory language and different gauge theories in the quantum field theory language, with a way to map them to each other. Possibly exciting, but you'd have to ask a proper string theorist about it.

Then just this last Friday, we had Rachel Rosen from Stockholm University talk about Phase Transitions of Charged Scalars and White Dwarf Stars.

This was a blackboard talk, which is always exciting and is usually more illustrative than Power Point ones. The subject was the thermodynamics of white dwarf stars - stars that are very dense and old, where fusion has mostly stopped and the only thing preventing the collapse of the star upon itself is the fermionic pressure of the electrons, that cannot fall into the same quantum states.

The physical description of these stars is one of relatively free positive ions, specifically helium ions in this case, floating through a background of fermions. They are described by a quantum condensate, which has a good theory explaining it, but with the addition of Coulombic interaction between the ions.

This state applies, specifically, to a subgroup of these stars that are mostly made of helium. This kind of ion condensate tends to crystallize depending on the ratio between the kinetic and potential Coulombic energy. Quantum effects, on the other hand, depend on some ratios of mass and charge between the ions. The only material where this applies turns to be helium, but luckily there are plenty of these helium-made white dwarfs.

The derivation, as Rosen showed it, starts with a neutral Bose-Einstein condensate, which has a simple phase diagram - uncondensed above some critical temperature, and increasing condensation as the temperature is lowered to zero. The charged condensate introduces photons as it is usually done in field theory and follows the consequences.

The result is a more complicated phase diagram. Under the old Tc, the ions still condense, but things change above it. There is now some higher temperature above which there is no condensation, but in between there are two solutions to the equations of motions, a condensate and a non-condensed state, and both a local energy minima. This means that the transition into a condensate is not continuous, and this is a first order phase transition.

The nice thing here is that we have such white dwarf stars to observe and we can compare this theory to observations.

That's it for these last two weeks. All you Americans out there have a good Thanksgiving, and I'll see you next week with two new seminars.

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