Here, we have the events display of an event which is a candidate of a pair of Top quarks decaying into a pair of W Bosons(one of the 3 Intermediate Vector boson which carries the force of the weak interaction, which causes such phenomenom as radiative decays) + 2 B jets. One of the W which subsequently decays into an electron and a neutrino (which escapes our detector without interaction, and thus is observed by as MET (Missing E Transverse) in the event--while the other W decays into 2 jets (either u + d-bar or c + s-bar), leading to a total of 4 jets (2 of which are b-jets) + electron + neutrino.
The lower left
figure shows the end-view of the tracking system along with the
calorimetry, indicating a stiff track at 8 o'clock
(one that shows almost no bending,
and thus is measured to have a very high Momentum of about 40 GeV).
That track, which does not have other high energy tracks nearby (we call
this "isolated" track) also points directly to an energy deposit in
the EM ealorimetry consistant with the track momentum measured-- thus
confirming that this is a good electron candidate.
The same events also have 4 jets--2 of these jets (jet # 1 at 2:30 and
jet # 4 at 4:30 directions) each have a secondary vertex
unpon close examination of the tracks near the collision vertex
(which we can do only because of the superb accuracy of track determination
by the SVX track detectors)--see the close up on the lower right.
These secondary vertices--with liftime consistant with a B meson decay--
indicate strongly that these 2 jets are good candidates for b jets.
Thus, this event is a good candidate for the physics process described in
the picture at right (except that we have an electron and an electron
neutrino instead).
3 sets of event displays provides a graphic image of the event and is
useful to "see" what is happening in the event. We use it as a tool to
understand the event, but people outside of CDF would also get a better
feeling of what a CDF event and detector looks like.
Let me list the 3 sets of CDF event displays :
Now for some details
For a introduction to the CDF event displays, see the event display below
and the explanation at the bottom of this web page
EXPLANATION OF THE EVENT DISPLAYS
Isolated stiff track (very little bending indicated very high momentem)
oftens indicates an electron candidate (if accompanied by an EM
calorimeter energy deposit consistant with the track momentum, as
is the case of the electron candidate at 8 o'clock above).
Other isolated still tracks with very little energy deposite but with
hits in the outer muon chamber (outside of the picture above)
are muon candidate (see W --> Muon + neutrino
for an example of the expanded Beam view (diameter roughly 10 meters)
Groups of high and low energy tracks indicates a jet--see the 4 jets
displayed above--these usually points to energy deposite in both
the EM and HAD calorimetries.
Note that the beam view only displays the Central calorimetry--which
goes from roughly 30 degrees to 90 degrees from the beam. Energy
deposited in the Plug (roughly 10 to 30 degrees) are not shown--
as indicated by jet 4 above, which pointes to a track--you do see
the energy deposited in the logo plot described below.
Note also that occasionally, 2 jets/isolated-track could overlap in
Phi (the axumuthal angles, which goes from 0 degrees at 3 o'clock,
to 90/180/270 degrees at (6/9/12 o'clock))--so occasionally
it may be difficult to separate the 2 items in this view--the lego
plot is better.
displaying this "cylinder" of energy flow in 2 dimension is difficult--
so, we "unroll" the cylinder, and make it into a flat 2-d plot, with the
magnitude of the towers being the total energy deposited. The figure at
the right shows such a process of un-folding.
The scale of this plot require some explanation--one of the dimension
is the Phi, or Azumuthal angle. In the CDF convention, 3 o'clock is 0 degrees
(or 0 radians), and 6/9/12 o'clock are 90/180/270 degrees (or pi/2, pi, and
1-1/2 pi radians).
The other dimension we jus is call Eta (or Psuedo-rapidity). Eta of
0 is 90 degrees to the beam (perpendicular to the entering beam), and Eta of
+1/-1, +2/-2, +3/-3 corresponds to 140/40 degrees, 165/15 degrees, and
174/6 degrees to the beam.
The secondary vertices indicate that something produced at the interaction
vertex has travalled several mm before decaying (note that this decay region
is inside the vacuum pipe, so interaction of particles are unlikely--this
is of cource not the case for the beam pipe--several centimeters from the
interaction point, and probability of particles interacting in the beam
pipe causing a secondary vertex is of order few tenths of a percent).
It turns out that a B meson, as well as a Charmed D meson, would fit
as the source of this decaying particle. The lifetime of the B meson
is about .0017 of a nanosecond.
Thus, if we select a sample of jets with secondary vertices inside the
beam pipe, there would be strong candidates for B or D mesons.
This is important because many new physics processes preferentially
decay into B mesons--one example is the Top quark, which, as far as we
know, always decay into a b quark + a W boson.
The study of B mesons are also inherently intersting, such as lifetime,
decay properties, etc. Thus, the ability to measure such short secondary
vertices is an important feature of our detector, and we have spend a huge
amount of effort to make this ability present--the SVX detector is one
of the most challenging part of the CDF detector !!!