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Lambda b lifetime checks Ok! August 8, 2008

Posted by dorigo in news, physics, science.
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One year ago I reported here about the measurements of the lifetime of the Lambda b baryon, a very heavy neutron-like particle which contains one bottom quark in place of one of the two down quarks: a (udb) composition instead of the neutron’s (udd). The lifetime of the lambda b baryon is accurately predicted by theory, but a nagging disagreement had been observed in the past, and last year’s CDF measurement had made matters worse. Was maybe something new or unexpected hiding behind the too long lifetime measured for these heavy neutrons ?

The lifetime of a particle can be predicted using several methods which allow to estimate the interplay of strong and weak interactions. One such framework is called “heavy quark effective theory” (HQET), and it basically pictures a heavy hadron (hadrons are particles composed of quarks) as planetary system with the heavy quark sitting at the center, and the other(s) jiggling around it, bound by gluons to their own “sun”. The picture works well, and it allows some precise estimates. However, it turns out that the lifetime ratios between similar particles can be estimated much better than lifetimes themselves. Take a Lamdba b and a B° meson: the former is a bottom quark with a down and an up running around it, the latter is a bottom quark with just a down quark orbiting it. Since the lifetime is, to very good approximation, driven by the disintegration time of the heavy quark, the two hadrons should have a comparable lifetime, right ?

Correct. The theory can predict very accurately the ratio: R = 0.88+-0.05, indeed not too far from unity. Instead, measurements have been showing this ratio to be smaller. In 2006 the world average for experimental determinations of R was listed at 0.804+-0.049. Not much of a disagreement, but the latest CDF result last year had turned the tables, with a result in the opposite direction.

The new result by CDF, recently presented at ICHEP 2008, instead settles the issue. It uses the reconstructed \Lambda_b \to \Lambda_c ^+\pi^- decay, and it is based on a larger statistics. It is the most precise measurement of this quantity to date, and the ratio turns out to be R=0.922+-0.039, less than one standard deviation away from theory.

To measure the lifetime, CDF determines the path length of the particle from the place where it is created by a proton-antiproton interaction, to the place where it decays. This entails measuring the secondary vertex reconstructed with charged tracks by the very precise silicon vertex detector. Below, you can see the distribution of the decay length measured in the sample of events. The black points are experimental data, and the curves represent the various contributions. The blue line is the fit to the data points. The lower blue-bar plot shows the residuals of the fit from the data, indicating a good agreement throughout.

The lifetime of the lambda b turns out to be of 1.410 picoseconds, give or take 0.054: if you compare this measurement with the current world average, which includes it (1.383+-0.049) you realize that the CDF determination dominates all others. A great new results from CDF. Below you can compare the recent determinations and the world average in a simple bar graph. The next-to-last line shows last year’s result, which was higher than all previous ones. The new one is more precise, and it agrees with the former at the level of about 1.5 standard deviations.


1. Luboš Motl - August 17, 2008

Isn’t it cool that Lambda b baryons are still the old-fashioned science that no one gives a damn and that doesn’t attract a single comment after one week? 😉

Imagine that Lambda b baryons would be criticized on a daily basis for not being able to improve the life of the lumpenproletariat or for being not even wrong.

OK, let me play the devil’s advocate, anyway. Are you going to disprove QCD or what’s exactly the motivation of these measurements? You will fail and you know it very well. 😉

2. dorigo - August 17, 2008

Hi Lubos,

no, I think it’s not cool that so little interest is paid to these useful checks of QCD. You should know that supersymmetry may be found earlier in B physics phenomenology than in high-energy phenomena…. You trust SUSY more than I do, so you should salute these precision tests.


3. Luboš Motl - August 18, 2008

Dear Tommaso,

I am happy to hear about your qualified opinion that B-physics “may” uncover SUSY first. It just seems to me that such evidence would be so indirect that it couldn’t win any bets. 😉

The information is just too low. A test either holds accurately or not. When it doesn’t, something may be going on but we don’t know for sure what it is. Sometimes the reason for this “SUSY” is a sign-error in someone’s two-loop calculations, if you know what story I am talking about.


4. dorigo - August 18, 2008

Sure… I’ve written about it here
two months ago.

Yes, discovery of susy requires more evidence. I will point back to this comment in a month, when an interesting discussion of that very topic is bound to arise.


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