Category Archives: Dave

Mazel Tov!

Let’s not let this blog die, posters!

It’s my pleasure to congratulate co-blogger, Dr. Bonna Newman, who successfully defended last week. Congrats Bonna! 

I think one of the Homer’s is up next.

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Podcast Self Promotion!

Sorry for the long time since last post. Today I have a shortish lunch talk to all types of physics graduate students. The talk was titled “Physics at Strong Coupling: Why String Theory and Supersymmetry are Ridiculously Cool, Part I,” with future parts to follow in the coming years. The talk went ok, I hope I convinced some graduate students that string theorists are not completely crazy.

But maybe now I can convince you! A friend helped me make the talk a podcast, with audio track and all. You can subscribe to it (and future, podcasts, I guess) here. Hitting “subscribe” on that page imports it into your i-tunes. Let me know if you enjoyed it! I guess, also let me know if you did not enjoy it…

Quantum Mechanics in Your Face

If, today, you feel as if you should be doing work, but don’t really want to do any, may I suggest a video of a Sidney Coleman Lecture: Quantum Mechanics in Your Face? You’ll learn and be entertained.

The lecture is fantastic and consists of the late, great Coleman discussing a version of Bell’s theorem (which is much easier to understand than the standard treatment), and then going on to discuss the “mysterious” “collapse of the wavefunction”. It’s great stuff. All that’s required for enjoyment is a basic undergrad QM course…

False Vacua I: The End of the Universe As We Know it

A few weeks ago, I wrote the first script of what I hoped would become a pop-sci podcast. Since this podcast, if it ever happens, will not happen for a rather long time, I figured it’d be fun to make it a series of blog posts. Many apologies if the post seems too simplistic or condescending….

The subject of false vacua is fascinating in its own right, but also nice because it brings together many different areas of theoretical physics. The discussion encapsulates many current areas of beautiful physical research such as quantum field theory, gravity, cosmology, supersymmetry and string theory. There is also a science-fiction like quality about the subject; After all, we’ll be talking about the possibility that at any second a bubble might form out of nothing and eat you. Hopefully, after these podcasts/blogposts the viewer/reader will appreciate that nowadays science fiction is somewhat superfluous; some of the craziest ideas are found in modern physics.

The fusion of Einstein’s special relativity and quantum mechanics under the heading of “Quantum Field Theories,” which describes things that are both very small and very fast. Continue reading

SUSY QCD/Seiberg Duality Reading List

The supersymmetry club is hitting Seiberg Duality this week! In preparation, I’ve been rereading the very good Argyres notes (here linked is strictly the ’96 Weyl spinor version; If you want the Majorana version, I might ask you, why do you like unhappiness and pain, sir (or madam)?), and also the book by Terning (which I find less helpful on first reading, but very beautiful once I sort of know the stuff already).

 I also tonight started reading these notes by Strassler, which (though I’ve only read the first fifteen pages or so) are freakin’ incredible!  Why did nobody tell me these notes were so good?! (Answer: Tom and Qudsia have been telling me to read these notes for weeks now). For those who mournfully don’t know about the beauty of Seiberg Duality, there will surely be an explanatory post to follow. I seem to use lots of parentheses in my posts…. 

A Non-Technical Explanation of Flavo(u)r Physics

The extra “u” is for Helen. 

 I figured that with all of Homer’s excited postings about the new physics CKM fit, the blog could benefit with a too simple introduction to why flavor physics is interesting and important. The standard model of particle physics has 6 known types, called flavors, of quarks with somewhat fanciful names: up, down, charm, strange, top and bottom. Now, the structure of the standard model is that these quarks come in what’s known as “generations,” eg. up/down, charm/strange, top/bottom are the three generations of quarks. Because of this generational structure, an up quark really “wants to” interact with a down quark (and a W boson), the charm quark really wants to interact with a strange, etc. However, there’s also a small chance that an up quark will interact with a strange quark or a bottom quark (and a W boson). In fact any of the up type quarks (up, charm, top) can interact with any of the down type quarks (down, strange, bottom), although the dominant interaction is within generations (it is still a mystery why this is, however). The relative strengths of these interactions is parametrized, in the standard model, by what’s known as the CKM (Cabibbo-Kobayashi-Maskawa) matrix.   Now, it turns out that you can specify the CKM matrix with three real angles and one complex phase. The fact that part of the matrix is complex is extremely important (and wouldn’t happen, for example, if there were only two generations). What it means in nature is that certain processes happen at different rates than their “mirror” images, where the definition of “mirror” is slightly technical. For example, a positively charged pion decaying to a positively charged muon and a muon neutrino is the mirror process of a negatively charged pion decaying to a negatively charged muon and a muon antineutrino. This complex phase in the CKM matrix predicts that the rate for these two mirror decay processes is slightly different! This phenomenon, of nature being non-mirror symmetric, is known as CP violation, and rocked the physics world when it was discovered experimentally in the 60’s. In the standard model, all measured CP violation is thought to come from the complex phase in the CKM matrix.  

Now, there are many good reasons to think that at energies that will soon be obtained by the new LHC (Large Hadron Collider), we will see new physics and particles beyond the standard model. In most extensions of the standard model, the CKM matrix has to be made a bit larger to incorporate the new particles. In general, though, when we do this, we also must add in new complex phases to the extended CKM matrix. For example, if we do the stupidest possible extension–just adding another, more massive, generation of quarks (and leptons), the extended CKM matrix now has 3 complex phases. These new complex phases will contribute to CP violating observables; the CP violation in nature will be more than is predicted by just the standard model. In basically any viable extension of the standard model, say supersymmetry, there are new complex phases running around everywhere, contributing to CP violation. In fact, many models can be ruled out experimentally precisely because we have not seen the amount of CP violation predicted by their orgy of complex phases.  

The name of the experimental game is this: you measure a bunch of CP violating observables, as many as you can get your hands on. You take your experimental CP violating data and assume that all CP violation came from the standard model only.  Using this, you can translate your experimental result into a statement about some part of the CKM matrix. You can translate this result, your measurement of some part of the CKM matrix, into a nice picture called the unitarity triangle. If you do an experiment that measures some decay, you can maybe translate that result into a statement of where the apex of the triangle sits in the picture. If you do a measurement of some other decay, and translate it into a result about the apex of the triangle using the standard model, well then you should get the same place that you got with the first experiment. If you do forty different experiments that measure the apex, if the standard model is correct, they should all tell you that the apex is in the same place. However, if you a forty first experiment, and it tells you that the apex is somewhere wildly different in the picture, it would have told you that either you made some mistake, or that your assumption, that the standard model is the only source of CP violation, was incorrect. It has long been the hope that by making various measurements on the triangle in this way, some inconsistent result would be a signal of physics beyond the standard model. Until now, unitarity triangle measurements have been maddeningly consistent with the amount of CP violation predicted by the standard model alone.  

 Until now? The paper that Homer keeps talking about claims that there is a likelyhood of new physics based on averaging various data sets of triangle measurements. I haven’t talked to anybody in the field who knows if this result is really exciting, but I think I’ve heard before that there have been whispers of new physics from the flavor community before, only to have some mistakes realized and the standard model endure. So I’m taking a cautiously skeptical attitude until the numbers get a bit better than three sigma. Could be exciting though… 

A Busy Day

Today was a bit of a busy day…

 

First at bat was my Astrophysics midterm. Now, I took this whole business of a midterm as rather silly so I did not study very much. I was very much ready to be embarrassed by a test that would take advantage of this. Luckily, I think I managed to do alright. The one question I didn’t know too much about asked me to list “four reasons that globular clusters are important for stellar astrophysics.” And God help me if I didn’t actually write: “they’re pretty.” Well they are pretty.

 

Then it was off to a talk for the LHC club that I gave on “Little Higgs” theories. Now, I spent the whole time in preparation hoping there weren’t going to be any questions. However, I think my talk put everybody to sleep, and so there actually were very few questions; I was quite disappointed. I can understand though, there was nothing particularly beautiful about the model I was presenting. It was just another in a long list of interesting, possibly relevant (and phenomenologically indistinguishable) solutions to the hierarchy problem. For those in the know, I can now bore you with my blog post!

 

The hierarchy problem is basically this: the standard model “predicts” a higgs boson which is too massive. It takes a bit of “fine tuning” (basically uglifying the theory) to get the mass down to a few hundred GeV, which is where it needs to be for proper electroweak symmetry breaking. The basic reason the higgs is too massive theoretically has to do with its interaction with other particles; for example, it’s interaction with the top quark provides a very large contribution to its mass. Supersymmetry solves this problem by providing a “superpartner” (the “stop quark”) which cancels the interaction with the top and keeps the higgs light. 

 

The basic lesson of little higgs, in my view, is that you can have theories with partners that cancel the top quark that are not “super”. Indeed, given that these theories were only discovered relatively recently (early 90’s or so), I think its important to realize that solutions to the hierarchy problem come in many more flavors than supersymmetry (discovered in the 70’s). In fact, the model I was talking about, the “simplest Little Higgs,” basically just enlarges the SU(2) weak gauge group to SU(3) in a special way that turns out to solve the hierarchy problem for all LHC relevant energies. In many ways this is much simpler than supersymmetry, but it is also, sadly for my audience, much less beautiful. 

 

Anyway, from my talk, it was off to the extremely boring Graduate Student Council  Meeting. These useless meetings will be the subject of another post. Then it was off to the annually cute “Latke-Hammentashen debate,” where my loyalties remained firmly on the Latke side.

 

Then I prepared with Tom and Eric for our supersymmetry talk on holomorphy and moduli spaces for friday. The chat was long and illuminating and reminded me of why I like physics so much, and why, despite the usual woes of a graduate student, having long chats about very esoteric, beautiful (even possibly true) things is one of the most gratifying things in the world. 

 

Of course, no research got done!