Archive for May 30, 2006

ATLAS observes cosmic-ray events underground!

Very good news – according to Travis Stewart the ATLAS tile calorimeter has recorded clean cosmic-ray events in the underground cavern. Here is the picture he posted:

Please visit his blog in order to see more information about this.While people are not interested in using ATLAS to do comic-ray physics, this accomplishment is important because it shows an expected signal successfully recorded and understood – the hits are in the right place, there are no garbage hits, the event has the right features, etc. Progress over the next year or two will consist of moving from one modest success (like this one) to the next one, which will be slightly less trivial. So, after a year of comic rays, we will hope to count the number of tracks in minimum-bias events…

(A disclosure: while I am happy about this success, it also makes me slightly nervous and envious, since I am a member of the competing CMS collaboration…)

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May 30, 2006 at 6:40 pm 7 comments

predictions from Lattice QCD

Georg from Life on the Lattice recommended a couple of papers which show the success of LQCD.

He is right! The post- and pre-dictions of real experimentally accessible quantities are confirmed now at the couple percent level – something which is not generally appreciated in the HEP community.

For example, in hep-lat/0304004 show that computations with staggered quarks correctly reproduce the known values for nine diverse and independent non-perturbative quantities, after the quarks masses and lattice spacing (equivalent to the strong coupling constant) have been tuned. As pointed out in the paper, this result is important not only for the impressive level of agreement for those nine quantities, but also for the way LQCD physics is inextricably linked to B-physics. To put it bluntly, we have a confirmed theory here, not just a clevel phenomenological model. (Not that clever phenomenology is bad – it does help us to think about the physics, at least at an early stage of the game.)

Even more impressive are the successes discussed briefly in hep-lat/0509169. These are true predictions – LQCD was used to obtain precise values for three quantities which were first published and then subsequently confirmed by actual measurements in real experiments. These quantities are, specifically:

  1. the q2-dependence of the form-factor in semileptonic D-decays
  2. the decay constants for D+ and Ds mesons
  3. the mass of the Bc-meson

I consider Fig.1 in Kronfeld et al. (hep-lat/0509169) to be especially striking.This is a big deal for particle physics. There are many places in which fundamental quantities such as elements of the CKM matrix cannot be extracted reliably from measurements due to poorly known “phenomenological” constants. For example, we would like to extract precise values for third-generation CKM matrix elements from the recent measurements of Bs-mixing (D0: hep-ex/0603029 and CDF: FERMILAB-CONF/06-076-e) and lattice will play a key role. It’s time to start paying closer attention to LQCD, at least for me!

Thanks, Georg, for the info!!

May 30, 2006 at 1:23 pm 8 comments


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