Features
Current Feature
The data from the CDF detector for a top-quark event produced in the proton-antiproton interactions in the Fermilab Tevatron1. We are looking at a representation of the detector from a vantage point on the beam, and along the proton direction. The axis of the colliding beams is at the center of the blue circle, which represents the cylindrical coil of the CDF solenoidal magnet, which produces an axial (parallel to the beam direction) field of 1.4 Tesla. Positive charged particles (e.g. a proton, positron, or pi-plus meson), are bent in the field so that their trajectories form a helix bending in the clock-wise direction, which projects as a circle in the r- plane represented in the picture.
The event is reconstructed a top-antitop pair. Both the top quark and the antitop decay to a W-boson and a b-quark ‘jet’ (being fractionally charged, quarks are not stable observable particles, and materialize as concentrated sprays of integrally-charged (i.e. neutral or plus or minus) particles. In this even the top quark decayed to a W+ boson that decayed to an electron and neutrino (at 8 o’clock and 6:30, respectively), and a b-quark (the jet at 4:30). The top anti-quark decayed to a W−-boson that decayed to a pair of quarks, either an up and a down pair, or a charm and strange pair, and a b antiquark2.
The electron is identified by its interaction in the ‘calorimeter’ surrounding the magnet coil, represented by the magenta and cyan colored blocks. The calorimeter around the magnet is divided into 24 cells in the azimuthal (φ) direction; in the radial direction there is one compartment, made of lead and scintillator, that is sensitive to photons and electrons, followed by a compartment to detect ‘hadrons’- strongly interacting particles, which is made of steel and scintillator. The amount of energy in the electromagnetic and hadronic compartments for each azimuthal area are represented by the size of the magenta and cyan blocks, respectively. In this case there is a large electromagnetic energy deposit at 8 o’clock, matched by a very ‘stiff’ (straight) track, a signature of an electron. The neutrino is identified by the seeming non-conservation of momentum in the plane; the red arrow gives the direction of the missing momentum, which is (presumably) due to the neutrino.
The b-quark jets are identified by locating the vertex at which the tracks in the jet intersect. The b-quark has a lifetime of 400 microns (we measure time in units of length, using the speed of light, c, as the conversion factor); the distance from the beamline is magnified by time-dilation, as the b-quark is relativistic. The jets from the other W-boson are not identifiable in the present detector.
The mass of the top quark can be reconstructed by adding up the 4-vectors component-by-component for the two tops, and taking the norm, using E2 = p2 + m2 to find m (note c = 1, the most natural and convenient set of units for length and time). In this event it is very close to the world average, around 174 GeV.
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1More information on CDF can be found at http://www-cdf.fnal.gov/
2We can't tell them apart - that's the focus of a new major effort we've started here at UC and at Argonne - see http://hep.uchicago.edu/psec/
