Tuesday, August 30, 2011

Supersymmetry, the Higgs boson, the LHC, and all that

Lately there has been a big kerfluffle (technical term of art, there) in the blog-o-sphere about what the high energy physics experimentalists are finding, or not finding, at the LHC. See, for example, posts here and here, which reference newspaper articles and the like. Someone asked me what I thought about this the other day, and I thought it might be worth a post.

For non-experts (and in high energy matters, that's about the right level for me to be talking anyway), the main issues can be summarized as follows. There is a theoretical picture, the Standard Model of particle physics, that does an extremely good job (perhaps an unreasonably good job) of describing what appear to be the fundamental building blocks of matter (the quarks and leptons) and their interactions. Unfortunately, the Standard Model has several problems. First, it's not at all clear why many of the parameters in the model (e.g., the masses of the particles) have the values that they do. This may only be a problem with our world view, meaning the precise values of parameters may come essentially from random chance, in which case we'll just have to deal with it. However, it's hard to know that for sure. Moreover, there is an elegant (to some) theoretical idea called the Higgs mechanism that is thought to explain at the same time why particles have mass at all, and how the electroweak interaction has the strength and symmetry that it does. Unfortunately, that mechanism predicts at least one particle which hasn't been seen yet, the Higgs boson. Second, we know that the Standard Model is incomplete, because it doesn't cover gravitational interactions. Attempts to develop a truly complete "theory of everything" have, over the last couple of decades, become increasingly exotic, encompassing ideas like supersymmetry (which would require every particle to have a "superpartner" with the other kind of quantum statistics), extra dimensions (perhaps the universe really has more than 3 spatial dimensions), and flavors of string theory, multiverses, and whatnot. There is zero experimental evidence for any of those concepts so far, and a number of people are concerned that some of the ideas aren't even testable (or falsifiable) in the conventional science sense.

So, the LHC has been running for a while now, the detectors are working well, and data is coming in, and so far, no exotic stuff has been seen. No supersymmetric partners, no Higgs boson over the range of parameters examined, etc. Now, this is not scientifically unreasonable or worrisome. There are many possible scales for supersymmetric partners and we've only looked at a small fraction (though this verges into the issue of falsifiability - will theorists always claim that the superpartners are hiding out there just beyond the edge of what's measurable?). The experts running the LHC experiments knew ahead of time that the most likely mass range for the Higgs would require a *lot* of data before any strong statement can be made. Fine.

So what's the big deal? Why all the attention? It's partly because the LHC is expensive, but mostly it's because the hype surrounding the LHC and the proposed physics exotica has been absolutely out of control for years. If the CERN press office hadn't put out a steady stream of news releases promising that extra dimensions and superpartners and mini black holes and so forth were just around the corner, the reaction out there wouldn't be nearly so strong. The news backlash isn't rational scientifically, but it makes complete sense sociologically. In the mean time, the right thing to do is to sit back and wait patiently while the data comes in and is analyzed. The truth will out - that's the point of science. What will really be interesting from the history and philosophy of science perspective will be the reactions down the line to what is found.

Wednesday, August 24, 2011

great post by ZZ

Before I go to teach class this morning, I wanted to link to this great post by ZapperZ about the grad student/research adviser relationship.  Excellent.

Saturday, August 20, 2011

Gating and "real" metals.

Orientation week has kept me very busy - hence the paucity of posts.  I did see something intriguing on the arxiv recently (several things, actually, but time is limited at the moment), though.

Suppose I want to make a capacitor out of two metal plates separated by empty space.  If I apply a voltage, V, across the capacitor using a battery, the electrons in the two plates shift their positions slightly, producing a bit of excess charge density at the plate surfaces.  One electrode ends up with an excess of electrons at the surface, so that it has a negative surface charge density.  The other electrode ends up with a deficit of electrons at the surface, and the ion cores of the metal atoms lead to a positive surface charge density.  The net charge on one plate is Q, and the capacitance is defined as C = Q/V.

So, how deep into the metal surfaces is the charge density altered from that in the bulk metal?  The relevant distance is called the screening length, and it's set in large part by the density of mobile electrons.  In a normal metal like copper or gold, which has a high density of mobile (conduction) electrons on the order of 1022 per cm3, the screening length is comparable to an atomic diameter!  That's very short, and it tells you that it's extremely hard to alter the electronic properties of a piece of normal metal by capacitively messing about with its surface - you just don't mess with the electronic density in most of the material.  (This is in contrast to the situation in semiconductors or graphene, by the way, when a capacitive "gate" electrode can change the number of mobile electrons by orders of magnitude.)

That's why this paper was surprising.  The authors use ionic liquids (essentially a kind of salt that's molten at room temperature) to modulate the surface charge density of gold films by something like 1015 electrons per cm2.  The surprising thing is that they claim to see large (e.g., 10%) changes in the conductance of quite thick (40 nm) gold films as a result of this.  This is weird.  For example, the total number of electrons per cm2 already in such a film is something like (6 x 1022/cm3) x (4 x 10-5 cm) = 2.4 x 1018 per cm2.  That means that the gating should only be changing the 2d electron density by something like a tenth of a percent.  Moreover, only the top 0.1 nm of the Au should really be affected.  The data are what they are, but boy this is odd.  There's no doubt that these ionic liquids are an amazing enabling tool for pushing the frontiers of high charge densities in CM physics....

Sunday, August 14, 2011

Topological insulator question

I have a question, and I'm hoping one of my reader experts might be able to answer it for me.  Let me set the stage.  One reason 3d topological insulators are a hot topic these days is the idea that they have special 2d states that live at their surfaces.  These surface states are supposed to be "topologically protected" - in lay terms, this means that they are very robust; something deep about their character means that true back-scattering is forbidden.  What this means is, if an electron is in such a state traveling to the right, it is forbidden by symmetry for simple disorder (like a missing atom in the lattice) to scatter the electron into a state traveling to the left.  Now, these surface states are also supposed to have some unusual properties when particle positions are swapped around.  These unconventional statistics are supposed to be of great potential use for quantum computation.  Of course, to do any experiments that are sensitive to these statistics, one needs to do quantum interference measurements using these states.   The lore goes that since the states are topologically protected and therefore robust, this should be not too bad.

Here's my question.  While topological protection suppresses 180 degree backscattering, it does not suppress (as far as I can tell) small angle scattering, and in the case of quantum decoherence, it's the small angle scattering that actually dominates.  It looks to me like the coherence of these surface states shouldn't necessarily be any better than that in conventional materials.  Am I wrong about this?  If so, how?  I've now seen multiple papers in the literature (here, here, and here, for example) that show weak antilocalization physics at work in such materials.  In the last one in particular, it looks like the coherence lengths in these systems (a few hundred nanometers at 1 K) are not even as good as what one would see in a conventional metal film (e.g., high purity Ag or Au) at the same temperatures.  That doesn't seem too protected or robust to me....  I know that the situation is likely to be much more exciting if superconductivity is induced in these systems.  Are the normal state coherence properties just not that important?

Tuesday, August 09, 2011


Went for the cryptic headline.  I'm off for a Department of Energy Basic Energy Sciences Condensed Matter Experiment principal investigator meeting (the first of its kind, I believe) in the DC area.  This should be really interesting, getting a chance to get a perspective on the variety of condensed matter and materials physics being done out there.  This looks like it will be much more useful than a dog-and-pony show that I went to for one part of another agency a few years ago....

Monday, August 08, 2011

Evolution of blogger spam

Over the last couple of weeks, new forms of spam comments have been appearing on blogger. One type takes a sentence or two from the post itself, and feeds them through a parser reminiscent of ELIZA, to produce a vaguely coherent statement in a comment. Another type that I've noticed grabs a sentence or two from an article that was linked in the original post. A third type combines these two, taking a sentence from a linked article, and chewing on it with the ELIZA-like parser. A few more years of this, and we'll have the spontaneous evolutionary development of generalized natural-language artificial intelligence from blogger spam....

Friday, August 05, 2011

Summer colloquium

Every year at Rice in early August, the Rice Quantum Institute (old website) (shorthand: people who care about interdisciplinary science and engineering involving hbar) has its annual Summer Colloquium. Today is the twenty-fifth such event. It's a day-long miniconference, featuring oral presentations by grad students and posters, by both grad students and undergrad researchers from a couple of REU programs (this year, the RQI REU and the NanoJapan REU). It's a full day, with many talks. It's a friendly way for students to get more presentation experience, and a good way for faculty to learn what their colleagues are doing. I'd be curious to know if other institutions have similar things - my impression has been that this is comparatively unique, particularly its very broad interdisciplinary nature (e.g., talks on spectroscopy for pollution monitoring, topological insulators, plasmons, carbon nanotube composites, batteries) and combination of undergrads and grad students.