Saturday, 22 May 2010

Meanwhile at the LHC

For the time being the most interesting physics results arrive from the Tevatron, as we were reminded this week by D0's announcement. The LHC cannot compete yet, but it's steadily working its way to becoming the leader sometime next year. According to the latest report, things are going pretty smoothly. So far the peak luminosity is $6x10^{28}/cm^2/s$ (corresponding to roughly an inverse picobarn per year), and the goal for the present run is to increase it by a factor of a thousand. Currently the machine people are working on increasing the numbers of protons in the bunches up to the nominal value of $\sim 10^{11}$. This step alone should allow them to reach $2x10^{29}/cm^2/s$ assuming just 2 bunches circulating in the LHC ring. After that, they will progressively add more and more bunches to the beam.
For the moment, the acquired luminosity is around 10 inverse nanobarns per experiment. This means that CMS and ATLAS have already collected almost 1000 W bosons (85 nanobarn cross section), hundreds of Z bosons (25 nanobarn cross section), and a few top quark pairs Poisson permitting (0.2 nanobarn cross section). ATLAS now shows on its public pages the first event displays with leptonically decaying Z bosons. The one reproduced above features a beautiful Z decaying into electrons (the two blobs in the electromagnetic calorimeter). Meanwhile, CMS has no new events on its public pages since the first collisions on March 30. The only logical explanation is that a giant octopus has eaten the detector together with the entire collaboration. As otherwise, if they had anything to share they would share it... or wouldn't they ;-)


billk said...

Jester, Comments from CMS have been appearing regularly on Twitter and Facebook.

Jester said...

still it might have been an octopus who tweets...they're damn clever

Mitchell said...

Off-topic... I have a question about the calculation of particle masses in GUT models. I get the impression that almost universally, the way it works is, you have one or more Higgs bosons, they acquire VEVs thanks to mutual interaction, and then the other particles get their masses by interacting with the Higgses.

That's how it is in principle, but in practice I can't seem to find any complete calculations of the masses in such models. When I look at GUT papers, mostly they seem busy with getting the SM gauge group and representations. Occasionally there will be a significant lone fact, like the Georgi-Jarlskog relation, or we will have potentials with a lot of free parameters (MSSM) and there will be a few remarks about how experiment constrains the possible values of those parameters.

So in short I'm trying to figure out how predictive these models are with respect to the known masses and mixing parameters. Do people construct Higgs potentials but not solve them, because they're busy trying to improve the qualitative features? Or does everything outside of string theory have so many free parameters that it inherently *can't* explain many of the SM parameters, except a few relationships here and there?