**CDF: B-physics from 10 meters to 10 microns**

In high energy physics the constituents of matter are studied with
particle beams and detectors analogous to studying small objects with a
microscope: the accelerator provides the \"light\" in the form of
particle beam radiation with the smallest possible wavelength, and the
detectors provide the optics to collect the scattered \"light\" into a
visual image of the object at a human scale. Recent events at the
Collider Detector at Fermilab (CDF) illustrate just how apt this
microscope analogy has become.

CDF is a complex instrument which records 115,000 pieces of information
about each selected event. With the 1992 installation of a silicon
microstrip vertex detector (SVX), CDF became capable of observing the
character of proton-antiproton
(![](media/image1.pct){width="0.1388888888888889in"
height="0.19444444444444445in"} p) collisions at the scale of 10 microns
(about 0.0004 inch ). Event details inside the Tevatron vacuum beam pipe
can now be studied even though no detector resides inside that beampipe.
In 1992 the original 4,500 ton detector was also upgraded by the
addition of 2500 new muon wire chambers and 630 tons of steel, all
required to make some of these special event observations possible.

The detector is an intelligent microscope, capable of selecting only the
events of interest out of the 500,000
![](media/image1.pct){width="0.1388888888888889in"
height="0.19444444444444445in"} p collisions occurring each second
during the 1992 collider run. Approximately 28 events of the type
described below have been seen so far out of a sample of 9 pb-1 (
approximately 5 x 1011 collisions ). Compared to the classical
microscope, this is like having an instrument which can be programmed to
select the one exciting specimen out of a collection of 20 billion
slides.

Events containing quarks are the specimens of interest described here.
CDF looks for collisions resulting in a B meson ( containing a b quark )
plus anything else; the B mesons are unstable and decay after living
only 1.3 x 10-13 seconds. Some of the time the B\'s decay into a J-Psi (
J/ψ ) particle and a K-star particle ( K\* ); subsequently the
J/ψ decays into two muons ( µ ) of opposite charge and the K\* decays
into a K meson ( K ) and a pi meson ( π ) of opposite charges. We look
for the process ![](media/image2.pct){width="1.9583333333333333in"
height="0.2222222222222222in"}. The \"X\" is shorthand for all other
particles which may be produced in the event.

The figure shows a computer generated graphic of an event recorded on
magnetic tape. In this view the proton and antiproton beams head into
and out of the page at the center of the diagram. The detector forms a
[10 meter]{.underline} square box around the beam collision point. The
red circle in the lower right hand corner is an expanded view of the
innermost 1 centimeter of the 10 meter box.

In the full size figure, the red and blue shaded areas indicate detector
components containing many layers of lead and steel; the yellow shading
indicates solid steel. Particles that come from the interaction point
and penetrate all this material are, typically, charged muons. Such
particles deposit energy in the muon wire chambers behind the lead and
steel, and that energy is shown in the graphic as +\'s. CDF also has a
region of magnetic field generated by a superconducting solenoid inside
the lead and steel components. Charged particles in this region are
detected in another wire chamber and the detected particle path is shown
in the center of the graphic as a curved line. This particular event was
selected by the detector triggering system because it had two muon
candidates and two matching curved tracks. The momentum information from
these two muons can be combined to measure the mass of the parent J/ψ
particle.

Next we zoom in to see what the SVX tells us about the event details
right at the interaction point ( look at the inset graphic in the lower
right hand corner of the picture ). The SVX is a 20-centimeter diameter
device with 46,000 strips etched in silicon wafers at separations of 60
microns between the strips. There are four layers of silicon and these
four position measurements can be used to extrapolate the particle paths
deep inside the Tevatron vacuum beampipe. The inset figure shows these
extrapolated tracks inside a circle with [1 centimeter]{.underline}
diameter. The tiny circle in the center of the expanded picture
represents the Tevatron beam \-- 90% of the Tevatron beam is inside a
circle about 70 microns in radius. Using the tracks from the primary
vertex in a given event, the SVX can determine the actual position of a
particular ![](media/image1.pct){width="0.1388888888888889in"
height="0.19444444444444445in"} p interaction to about 10 microns.

If a B meson moves at nearly the speed of light, then it travels a
finite distance before decaying. This distance can be simply calculated
by multiplying speed by its short lifetime of 1.3 x 10-13 seconds to
give 390 microns. This is the average lifetime; some B\'s live a longer
time and some a shorter time. One can see that the muons in this event
do not appear to come from the beam spot but instead come from a
secondary position seven hundred microns away from the beam. This is
just what we expect for a B-meson. Looking closely you will see

that two other tracks also come from this secondary vertex. These are
the K meson and the π meson; the reconstructed mass from these two
particles is consistent with the mass of the K\* . The information from
the µ+, µ-, K and π tracks can be combined to measure the mass of this
meson as 5.3 GeV. This is almost exactly the known value of the B-meson
mass and confirms that we are probably looking at a B-meson.

Finally, in the inset figure notice there is [another]{.underline}
secondary vertex located [below]{.underline} the Tevatron beam. The
complete process is ![](media/image3.pct){width="1.1388888888888888in"
height="0.2222222222222222in"}; we now see evidence of the other B meson
! The decay of the second B is unconstrained \-- we did not demand that
it undergo the decay into µ µ K π and so now we can begin to study the
properties of B-mesons in general by looking at the second after
triggering on the first of the pair. While the combined branching ratio
for B-mesons into J/ψ followed by J/ψ into µ+µ- is only about one in
every 1500 decays, other decay modes have higher probabilities. For
example, the most obvious B characteristic is its short lifetime and
subsequent decay leading to a potentially visible secondary vertex.
[Every]{.underline} B-meson has this property.

Once the collaboration devises a fast trigger to select these events
with secondary vertices, CDF will open a new window for the detailed
study of one of the six fundamental quarks.