Of tiny distances (microns) and the
structure of matter
know that the world is made of particles: leptons and quarks. Leptons
electron, muon, tau and their neutrinos; quarks are six and in
increasing order of mass are: up, down, strange, charm, bottom and top.
We also know
that the particles interact exchanging other particles, called gauge
The Standard Model is the theory that describes the interactions among
particles. An interesting feature of the quarks is that they are never
observed isolated, they make hadrons, there are groups of two and three
quarks, called mesons and baryons respectively. If a quark is created
collision of protons and antiproton then this quark picks up other
form a hadron, this process is called hadronization.
collisions of protons and antiprotons in the Tevatron, hadrons
the quark bottom are produced, but those hadrons live less than 1/1012
seconds. They become other particles due to the weak interaction. There
various models that try to explain how these hadrons decay into other
particles. The most simple model predicts the same lifetime for all
containing the bottom quark, however there are experiments that
different lifetimes for the different B hadrons. Newer and more complex
models have predicted different lifetimes of B hadrons very much in
agreement with the measurements. One piece of this picture however is
not right, the measured lifetime of the Λb baryon does not
the theoretical prediction. Therefore a new measurement in a different
environment can shed some light in this problem.
lifetime of the Λb was previously
measured in cases where the Λb became a set of
particles that included a muon. In D0 we can
measure that lifetime when the Λb becomes a J/psi and
a Λ0. The
advantage of this measurement over the previous ones is that all
that come from the Λb are observed while
in those previous
measurements one of the decay products was missing.
measure of the lifetime of a particle means to measure the distance
between two points, the point where the particle is created and the
where the particle ceases to exist and becomes other particles. In the
of the Λb this distance is of
the order of hundreds of microns. To be
able to measure these distances one has to have tools as precise as
microns, which we have. The D0 detector consists among other things of
component that is made of silicon that is capable of measuring
the order of hundreds of microns and therefore ideal to measure
we had a set of Λb baryons we would be
ready to make the
measurement, however, every time a proton and antiproton collide in the
Tevatron many things can happen, rarely a Λb will be produced.
why we need to find a way to distinguish events that look like a Λb.
We collect only those events that have the signature of a Λb, i.e., Λb becomes a J/psi and
a Λ0, but the J/psi
becomes two muons and
the Λ0 becomes a proton
and a pion. These particles are stable and can
be seen in our detector.
the Λb there are other
processes that have the same signature
and are impossible to separate. They are present in the data analyzed
measure the lifetime. Nevertheless they can be taken into account in
model used to measure the lifetime and in the end are not a problem.
the Λb, the B0 meson can decay
into a J/psi and a K0s, having
a very similar topology to the Λb decay used in the
lifetime of the B0 was also measured
using the previous decay. Theory
predicts the ratio of the Λb and B0
lifetimes and therefore both
measurements, the Λb and B0 lifetimes, can be
used to estimate the
ratio and compare with the prediction. Predictions of recent models
to describe the decay of these B hadrons agree with the result of our
measurements, validating their model.
description of how we measured the Λb and B0 lifetimes is being published in the
Scientific Journal Physical Review Letters.
or questions to About the Λb lifetime analysis