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Low-functioning pinhead...
December 22, 2005
Bye Bye, Raoul
Raoul Bott was a huge influence on my life. I learned Differential Geometry from him, in a reading course at Dunster House, where he was House Master, and I was an undergraduate. I and a couple of other Dunster House undergraduates met with him once a week, while working through the notes to his graduate course. Of course, there was that one, somewhat intimidating, week, when Sir Michael Atiyah showed up in his stead…
Raoul started out studying Engineering at McGill and, I think, that “nuts 'n bolts” approach to things mathematical never left him (to be fair, though, Marc Grisaru, who knew him from those days, reports that, “Even then, we knew he was a Mathematician.”). He was one of the people responsible for the close working relationship between Physics and Mathematics at Harvard, and was known to admonish his more skeptical colleagues, “Don’t argue with the physicists; they know how to compute!”
He was a warm and approachable House Master, greeting everyone at the Master’s Teas (back in the days when it was still legal to serve sherry to the undergraduates) with a big bear hug. He and Phyllis also had continuous stream of interesting visitors staying with them at Dunster: from the aforementioned Sir Michael to trumpeter Red Rodney to poet Allan Ginsburg.
But I think what impressed my fellow (non-Physics/Math) Dunsterites most was that, on Martha’s Vineyard, where he and Phyllis summered, Raoul was known as “King of the Nude Beach” — a claim-to-fame few other Harvard House Masters could match.
There’s more from Luboš and Sean and the Harvard Math Department.
Creeping Up on the MSSM
There’s an interesting paper by Diaconescu, Florea, Kachru and Svrček on gauge-mediated supersymmetry-breaking in String Theory. In perturbative heterotic string theory and, I think, in heterotic M-theory as well, supersymmetry-breaking is generally expected to occur in a hidden sector, associated to the second , and is communicated only indirectly to the visible sector via gravity-mediation. Gauge mediation requires a messenger field charged under both the visible and the hidden gauge group. There are such fields in the heterotic string but their masses are string-scale in the weakly-coupled heterotic string and super-Planckian in heterotic M-theory. So gravity-mediation typically dominates1.
Gauge-mediated SUSY-breaking is attractive, because it solves the “flavour problem” by making the SUSY-breaking squark masses flavour-independent. That’s because the messenger(s) couple directly only to the gauge sector of the Standard Model. Gaugino masses are a 1-loop effect. Squark and slepton masses are a 2-loop effect, and are flavour-independent. Gravity-mediation (despite the name) has no such flavour-universality (a-priori).
Diaconescu et al find some new classes of heterotic models in which gauge-mediated SUSY-breaking dominates. In the dual F-theory picture (these models all have F-theory duals), SUSY is broken on a stack of D3-branes probing a singularity corresponding to a shrunken del Pezzo. The corresponding quiver gauge theories are known to be supersymmetric to all orders in perturbation theory, but to dynamically break supersymmetry at the nonperturbative level (see also Berenstein et al and Bertolini et al).
If this stack of D-branes is located sufficiently close to the stack on which the Standard Model arises, then the ground states of the open strings between the two stacks are the desired messengers for gauge-mediated SUSY-breaking.
Most of the discussion takes place in the context of the elliptically-fibered Calabi-Yaus explored by the Penn Group (for some recent papers getting ever-closer to the precise MSSM field content and couplings, see Bouchard and Donagi and Braun, He, Ovrut, and Pantev). So there’s a good chance that one can actually build a fully-realistic compactification along these lines. (Foes of F-theory flux vacua are permitted, at this point, to go berserk.)
So much for “top-down,” what about “bottom-up”? What will we be able to learn about SUSY at the LHC? I’ve mentioned before the paper by Arkani-Hamed, Kane, Thaler and Wang, which has finally appeared. In it, they discuss the “inverse problem” of deducing a model in the MSSM parameter space from the LHC data. To study the question, they simulate a huge number of MSSM models, look at the experimental signatures they produce. When the sleptons are somewhat heavy (so that they don’t appear in the decay-chain of neutralinos which result in opposite-sign dilepton events), there’s a relatively large2 degeneracy (~10-100 models), corresponding to different orderings of the slepton masses. If the sleptons are lighter, then the degeneracies are smaller, and they present a rough taxonomy of some of the degenerate models.
- Flippers
- The spectrum of masses of the Electroweak 'inos is fixed, but the identities of which ino has which mass are exchanged.
- Sliders
- The Electroweak 'ino masses are moved up or down, keeping the mass splittings fixed.
- Squeezers
- The information of some of the Electroweak 'inos is hidden because the mass splittings are small enough that the leptons in the decay products are too soft to be seen.
With the limited available signatures, the LHC will be unable to distinguish between these radically models.
But, before losing hope, one of the key messages to take away from their paper is that, if you have some reason to choose between the degenerate models (either a theoretical prejudice, or some experimental signature that they did not consider), then the LHC data can determine the parameters of that single model to very high accuracy.
1 There are exceptions to this general rule, in certain orbifold models, as my colleague, Vadim Kaplunovsky, is fond of pointing out. Perhaps that’s true more generally.
2 On the other hand, the degeneracy isn’t , as you might have feared.
December 21, 2005
The Sorry State of Spambot Writing
While Trackback spam is a source of continuing fascination hereabouts, Comment spam is but a fading memory. Yes, we occasionally get the odd piece of hand-entered Comment spam from India or Thailand or the Former Soviet Bloc, but Comment Spambots pretty much pass us by. Which is a shame, really, because I’d like to keep abreast of developments in that field.
So you can imagine my delight in finding that this blog had been visited by a new (to me, at least) Comment Spambot the other day.
In the space of 14 minutes, it
- made 2257 requests
- from 91 distinct IP addresses (all, as far as I can tell, zombie PCs)
- of which, 467 were requests for my comment script
- among which were 151 (unsuccessful, of course) attempts to POST a comment
- which resulted in 48 new IP addresses automagically added to my IP-banlist
How do I know all these details? Because the Spambot issues a malformed HTTP REQUEST header. (Fortunately, Apache is liberal in what it accepts, and equanimously records the malformed header to the logs.) I guess the Spambot author found the HTTP 1.1 Specification too difficult to understand.
Perhaps some public-spirited person, like Sam, could put together a Spambot Validation Service, in the interest of improving the overall quality of the Web.
December 19, 2005
Shredding Party
Kieran Healy has written up an excellent list of rationales for supporting the President’s Executive Order authorizing domestic spying without Judicial oversight. I’d been trying to draft such a list myself, but Kieran’s is wittier than I could have come up with.
But the thing that I find most puzzling about the current fuss over renewing the Patriot Act has not been much discussed. It’s apparent that, under the Yoo Doctrine, the President can, by Executive Order, authorize all of the activities covered by the Patriot Act, with or without Congressional approval.
So who cares whether the Patriot Act is renewed?
I assume it’s just a matter of principle. One can’t let the namby-pamby Democratic “enemies of freedom” in Congress give aid and comfort to the terrorists by denying the President their approval for his activities.
December 18, 2005
That’s Better!
After considerable frustration with Apache 2.2.0, I was about to throw in the towel and downgrade to 2.0.55. On a lark, I decided to download the latest Development Version and try that instead.