Bs Lifetime

Just as the lifetime of a spinning top depends on its rate of spin and the gravitational and frictional forces acting on it, the lifetimes of subatomic particles provide physicists with key information about fundamental particle constituents and the different forces acting between these constituents. Lifetimes can range from greater than 1032 years for the proton (we haven't seen one decay yet), to 886 seconds for the neutron (it does not decay at all when bound to protons in atomic nuclei), all the way down to about 10-24 seconds for "resonances" like the rho meson. The latter lifetime is too short to measure directly and can only be inferred using Heisenberg's uncertainty principle.

When a quark (from a proton) and an anti-quark (from an antiproton) collide at the Tevatron, currently the accelerator with the highest energy collisions in the world, they can produce among other things bottom (or "b") and anti-bottom (b(bar)) quarks. These quarks combine the remaining "fragments" of the proton and the antiproton or from heavier quarks produced in the collision to form observable particles called "b hadrons". The lifetimes of heavy particles called "b hadrons" are particularly interesting since they have the shortest lifetimes that can be measured by reconstructing their decays into more stable particles in our D detector. Traveling at near the speed of light, they decay on average a distance of about 1 mm from the proton-antiproton collision that produced them. This corresponds to an average lifetime of about 10-12 seconds (one picosecond, or one trillionth of a second).

By repeatedly measuring this decay length between the point of production and the spray of decay products, the D experiment has measured the lifetimes of the Bs and Bd mesons, which contain an anti-b quark and a "strange" (s) or "down" (d) quark, respectively. Currently the Tevatron is the only collider in the world capable of producing this heavier Bs meson and we therefore have an incredible opportunity to measure its properties.

A very simple and naive model, called the "spectator model", predicts that the lifetime of the B meson is determined solely by the decay of the b quark (the other anti-quark in the meson is a mere "spectator") and that the lifetimes of all b hadrons should be equal. However, the B meson lifetime is long on the minute scale of particle physics; during this time the constituents of the B mesons, a b quark and a lighter down or strange anti-quark can interact with each other in a variety of ways that reveal the nature of elementary forces. In other words, the other anti-quarks are not "passive spectators", but rather affect the lifetime of the b quark in different ways and we can learn a great deal by measuring these effects on lifetimes. The theory of these decays predicts that the lifetimes of the Bs and Bd mesons should differ by about one percent.

With approximately 337 Bs candidates, identified through their decays into J/y (a particle made up of a charm and anti-charm quark, subsequently quickly decaying into an observable muon and anti-muon) and a phi (made up of strange and anti-strange quarks, decaying into an observable kaon and anti-kaon), D experimentalists have measured the average lifetime of the Bs meson to be 1.444 +0.098-0.090 (stat) 0.020 (syst) picoseconds. This constitutes the best measurement of the lifetime of the Bs particle. Similarly, the D Collaboration has also measured the lifetime of the Bd meson: 1.473 + 0.052-0.050 (stat) 0.023 (syst) picoseconds. When combining both numbers.. the ratio of lifetimes of Bs to Bd was found to be 0.980 +0.075-0.070 (stat) 0.003 (syst), in very good agreement with theoretical predictions. This gives us confidence that we can rely on the theory as a guide to understand the forces that drive the decays of B mesons. The information will ultimately lead to a better understanding of the behavior of all quarks bound inside the matter of our everyday lives.

The full article can be found here. For more information on this analysis, please contact P.L.M. Podesta-Lerma, A. Sanchez-Hernandez, H. Castilla-Valdez (CINVESTAV, Mexico).