B hadron lifetimes: a big CDF success (Part I) May 11, 2007Posted by dorigo in physics, science.
I feel no shame in admitting that, were it not for the obligation of preparing a seminar about “recent results by CDF”, I would have remained ignorant for quite a while longer about some of the details of several excellent new results that have been recently produced by my outstanding collaborators.
Preparing a well cooked-up seminar about an experiment like CDF is no mean feat. Regardless of the time you are allotted, you will never have enough of it to do justice to all what deserves to be mentioned. So you have to summarize. And, to summarize, you need to know everything somehow – especially when talking to smart physicists who may have a question in store about something you did not plan to discuss!
So here I am, trying to digest in a week some fifty papers I should have read in the course of the last yeas or so. But that is good! It is like getting yourself to clean the garage after one year of neglect. And as you see you are making progress, and as the garage starts looking tidier, you get this nice feeling that you are doing your duty at last.
Among the things I had neglected to study in some detail I am here today to talk about the recent measurements of B-hadron lifetimes. An exciting new measurement by CDF turns the tables a bit here, as I am going to try and explain.
Hadrons are particles composed of quarks, and B-hadrons contain at least one b-quark. The presence of the b-quark makes them heavy, since that component alone weighs about 4.5 GeV, little less than five protons. And it makes many of them long-lived as far as subatomic particles go, since the only decays allowed by energy conservation are ones that change the b-quark in a lighter sibling: something only the weak interactions can do, weakly – at a leisurely rate, that is.
Imagine a proton-antiproton collision at the Tevatron, creating a jet of particles wherein lies a B-hadron. The B-hadron sits there, in the middle of the jet. It is willing to disintegrate into lighter bodies, and it is thus waiting for a W boson -the carrier of the weak force- to materialize and do the trick. It looks around and sees other hadrons, produced in the same proton-antiproton collision, being torn apart by massless gluons. Gluons are the carriers of the strong force, which acts so fast it makes you dizzy. It is easy to be so fast! Gluons are massless… But the W weighs 80.4 GeV, and at the particle energy Bank they take forever to lend you that much, even if you are going to give everything back ridiculously fast.
So the B-hadron waits. And waits. It looks like forever: if the strong force has disposed of lighter hadrons in the matter of a second, for the B-hadron a hundred years are not enough. In those 100 years it travels the immense distance of a few millimeters within the jet. And then, finally, the b-quark contained within the B-hadron manages to put together the energy to materialize a virtual W boson, thereby turning itself into a lighter quark. The W disintegrates almost instantaneously, and what remains of the original B-hadron is a lighter hadron containing the sibling of the b-quark -most often a c quark-, plus the residue of the virtual W– a lepton-neutrino pair, or a few additional pions. A time of about a picosecond has passed, for human clocks.
The picture sketched above is a simplified view of a B-hadron decay. If that were all what happens, all B-hadrons would live an equally long life. But there are complications depending on the particle mass – which is not simply the sum of the masses of its components. And then, there are even subtler effects due to the fact that hadrons are bound states – the quarks contained in them are constantly exchanging gluons to keep tied to each other. Sure, it works wonders to ignore whatever else is in the hadron: it is the b-quark the one which has the mass, and therefore the energy, to create other particles and is thus able to stage a disintegration. But for a precise understanding of the dynamics of the process, one must account for the strong interactions among the hadron components.
Enter mesons and baryons. Mesons are composed of a quark and an antiquark, baryons contain three quarks. So for instance a B° meson contains a b anti-d quark pair, while a baryon called Lambda_b contains a b, a u, and a d quark – we write the latter as (udb) for simplicity.
Should we expect those two particles -the B° and the Lambda_b– to have the same lifetime ? At first sight, yes: they both contain a b-quark, which is the one that will make you wait forever, until it transforms into a c quark, emitting a virtual W. In the case of the B° this yields a D+ meson (one made by a c-dbar quark pair), in the case of the Lambda_b the result is a Lambda_c, a particle with (udc) content. But really, does the company of two quarks or one antiquark make no difference ?
It does, but it is not easy to compute what is the effect of that difference.
[To be continued]