The experiments CDF (Collider Detector Facility) andD0 (named after its location on the accelerator ring) at Fermilab have searched for the top quark by looking for evidence of its decay to the next heaviest quark, the bottomquark. This decay bears a direct relationship to the more familiar beta-decayof the neutron, in the following way.
The neutron decays into a proton, an electron and an electron-antineutrino. Interms of quarks, a neutron consists of one up quark and two down quarks(udd), while the proton consists of two up quarks and one downquark (uud). So at the quark level, a d changes into a u. This decay occurs through the weak force, which in this case is mediated by thenegative carrier particle of weak force, the W-. Thus the dchanges into a u by emitting a W- which almostimmediately materialises as an electron (e-) and anelectron-antineutrino.
The top quark, t, decays similarly via the weak force, but to the bottomquark, b. In this case a positive carrier particle, W+, isemitted, which can materialise in various ways since the mass difference betweenthe t and the b quarks is sufficient to create a variety ofparticles. However, the easiest decay modes to detect are those when theW+ decays into a positron (e+) and an electron-neutrino, or apositive muon and a muon-neutrino.
In the collisions at Fermilab we expect the top quark to be createdsimultaneously with its antiquark. The antitop will decay in similar ways tothe top, but with particles replaced by antiparticles and vice versa. So theantitop can decay to a bottom antiquark, together with an electron and anelectron-antineutrino, or a negative muon and a muon-antineutrino.
The bottom or anti-bottom cannot emerge on their own into the detectors, butinstead create a characteristic "jet" of particles. The electron or muon (ortheir antiparticles) do emerge, and can be identified through theirinteractions in the detector. The neutrinos (or antineutrinos) likewise emergebut leave the detectors unseen as they are so weakly interacting; they can beidentified only by the "missing" energy with which they escape.
The picture circulated by Fermilab shows an identified muon, an identifiedelectron and two jets of particles - in other words, one of the "signatures"expected for the decay of a top-antitop pair.
The neutron decays into a proton, an electron and an electron-antineutrino. Interms of quarks, a neutron consists of one up quark and two down quarks(udd), while the proton consists of two up quarks and one downquark (uud). So at the quark level, a d changes into a u. This decay occurs through the weak force, which in this case is mediated by thenegative carrier particle of weak force, the W-. Thus the dchanges into a u by emitting a W- which almostimmediately materialises as an electron (e-) and anelectron-antineutrino.
The top quark, t, decays similarly via the weak force, but to the bottomquark, b. In this case a positive carrier particle, W+, isemitted, which can materialise in various ways since the mass difference betweenthe t and the b quarks is sufficient to create a variety ofparticles. However, the easiest decay modes to detect are those when theW+ decays into a positron (e+) and an electron-neutrino, or apositive muon and a muon-neutrino.
In the collisions at Fermilab we expect the top quark to be createdsimultaneously with its antiquark. The antitop will decay in similar ways tothe top, but with particles replaced by antiparticles and vice versa. So theantitop can decay to a bottom antiquark, together with an electron and anelectron-antineutrino, or a negative muon and a muon-antineutrino.
The bottom or anti-bottom cannot emerge on their own into the detectors, butinstead create a characteristic "jet" of particles. The electron or muon (ortheir antiparticles) do emerge, and can be identified through theirinteractions in the detector. The neutrinos (or antineutrinos) likewise emergebut leave the detectors unseen as they are so weakly interacting; they can beidentified only by the "missing" energy with which they escape.
The picture circulated by Fermilab shows an identified muon, an identifiedelectron and two jets of particles - in other words, one of the "signatures"expected for the decay of a top-antitop pair.
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