February 19, 2004

Superstuff, and not-so-super stuff.

Apologies for my lack of posts lately. I don't intend to let this turn into an Ed-only website. I've also been somewhat sick, and before that I've had a huge number of things going on. But, let's not waste time giving excuses. I'll talk about some physics. (The background required is not as minimal as I would like; hopefully I can fill in more prerequisites in future posts.)

A new paper here, by Buchmueller et. al., addresses issues involved in looking for gravitinos at future colliders. This is an interesting topic, as it's a way of trying to show the existence of supergravity. Supergravity is a combination of supersymmetry and gravity, as the name suggests. So first, what's supersymmetry?

Supersymmetry is believed by many (occasionally by me, when I'm in an optimistic mood) to be an approximate symmetry of nature. It posits that for every particle we know of with half-integer spin (i.e., a fermion), there is a "superpartner" of integer spin (i.e., a boson), and vice versa. This may sound like a ridiculous idea; Occam's razor dictates that, when we haven't seen a single one of these superpartners, we had better have a good reason for believing they exist. And there is a good reason, namely that supersymmetry protects against all sorts of nonsense that appears in the Standard Model. Not the least of this nonsense is that the (also hypothetical) Higgs boson has a mass that tends to blow up to unphysically large values, unless nature has fine-tuned certain parameters to a high degree of accuracy. This is a bit hard to swallow, but supersymmetry causes this fine-tuning to become natural. It makes a very precarious theory into a robust one.

(There are other ways of addressing this Higgs issue, though, some very recently thought up and very exciting, and I'd like to talk about them in the future.)

Supergravity, roughly speaking, comes from taking the extra symmetry of supersymmetry and making it vary a bit from place to place. Those of you who know about gauge theories can probably see why this is nice. The really amazing thing about having this symmetry become a local one is that gravity automatically appears in the theory. This is really remarkable. You have this funny problem with the Higgs that gives things mass, so you add in a somewhat ad hoc symmetry, and suddenly your theory includes gravity. This magic is probably a large part of the reason people persist in believing in supersymmetry, despite the lack of experimental evidence for it so far. I should note that, compelling as supergravity is, it does not provide a feasible theory of quantum gravity. Otherwise people wouldn't be doing stringy things all the time.

[Those of you who know some particle physics, but are not familiar with SUSY, might want to take a look at John Ellis's review here.]

Supergravity, then, has a particle called the "graviton," representing a weak fluctuation of the gravitational field. (These are the quanta of gravitational waves, which the LIGO collaboration searches for.) The graviton, like any force-carrying particle, is a boson, so supersymmetry requires it to have a partner. This partner is given the name "gravitino." It's rather exotic; unlike every other fermion ever discovered, it has spin 3/2 instead of 1/2.

The paper I linked to above is one of many papers on looking for supersymmetry at colliders, but it caught my attention for a few reasons. One is that, when the Large Hadron Collider turns on in a few years (and the Linear Collider after that), we're going to have huge amounts of data that will give us hints about new physics. It seems likely to me that we will quickly get data we do not understand, but we will have a very hard time deciding which of many theoretical scenarios accounts for these data. In particular, supersymmetry is such a popular idea that many people will start trying to interpret whatever is found in terms of SUSY. This paper, then, considers a scenario that is a "smoking gun" for supergravity: finding the spin 3/2 gravitino. It's such an exotic particle that few other theories could look like it. The interesting thing is that there are plausible models in which it is the lightest supersymmetric particle, making it easier to find. Another thing that caught my eye about this paper is that it proposes searches involving tau leptons, which I work on identifying. If the superpartner of the tau is the second lightest supersymmetric particle, it could decay to a tau lepton and a gravitino.

If you've followed this discussion, I think a few things about the way high-energy physics works currently should stand out. The first is that there are often complex chains of reasoning leading people to be very convinced of things for which there is little direct evidence. This may seem risky, but it's proven suprisingly effective in the past. Eugene Wigner once posed the question of why mathematics is so effective in describing the natural world. I'm not sure that anyone can satisfactorily answer that question, but so far it hasn't failed us. Another thing I would like to point out is the interplay of theory and experiment. There are a number of ideas out there now about what physics looks like at higher energy scales than we have probed so far. Some of them sound very wacky. But ultimately, experiment is the arbiter of what becomes accepted in the next "Standard Model." To this end it is important to try to understand what tests can be most effective at culling the huge data samples we will have over the next decade, and what signals might be strong clues pointing us in a given direction.

I'm not sure how comprehensible this was; probably it seems simplistic and odd to those of you who know much physics, and boring to those who don't. But I hope that now and then I can try to get across some of the things I find so exciting about high-energy physics.

Posted by Matt at February 19, 2004 12:53 AM

Matt -

Thanks for this. I know very little about physics and I found it interesting, so I think you found a good balance between simplicity and technicality.

I'd really enjoy more entries like this in the future.


Posted by: Ray at February 19, 2004 04:43 PM
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