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 DØ 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 DØ 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), DØ
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 DØ 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).