Lecture 35

Last time we discussed the Feynman diagrams for W and Z interactions, and the types of charged current interactions.

Weak Interactions

Classification of weak interactions

Neutral current interactions

Neutral current processes are harder to detect experimentally than charged current processes because neutral current exchange between charged particles competes with the standard electromagnetic process, and the EM process is normally orders of magnitude larger. The existence of neutral current interactions was established only when it was observed in neutrino reactions where the photon doesn't participate.

W and Z production and discovery

Direct production of W and Z bosons (real, "on the mass shell" particles and not virtual states) were first produced in ppbar collisions:

ppbar -> Z⁰ X⁰
Z⁰ -> l⁺ l^-

ppbar -> W^± X^-/+
W^± -> l^± ν

The idea behind missing energy is that the colliding proton and anti-proton have equal and opposite momenta, so the initial state has a fixed total energy and zero net momentum. Calorimeters are used to measure the flow of energy out of the collision. If a neutrino carries off a substantial amount of energy/momentum (in this case, half the W mass was substantial) then this missing energy/momentum can be measured, at least approximately. Care must be taken, since other particles, notably neutrons, K⁰_L, and neutrinos from weakly decaying particles, can also carry away energy and escape detection. Luckily these generally account for much smaller amounts of energy, so that a neutrino with M_W/2 of energy can be detected.

We can potentially detect Z0's and W's decaying hadronically. The idea is that a single, high energy quark (or gluon) produces a jet of particles in the detector. A jet is a group of particles with similar momenta, producing what looks like a cone of tracks and a cluster of calorimeter energy in the detector (see the cover of Martin&Shaw for a good example of a four jet event where the tracks are colored red, green, yellow, and blue to indicate their association with a particular jet. This concept is summarized by the term quark-hadron duality. It hasn't worked exceptionally well yet for reconstructing W's and Z's, but is important in the discover of the top quark.

Discovery of the top quark

Production

The top quark was discovered in 1600 GeV ppbar interactions at the Tevatron. At these energies the dominant production mechanism is top--anti-top pair production via quark-quark interaction.

At the higher energies of the LHC, single top production via virtual W decay will become important.

Decay

The identification of characteristic decay topologies was crucial to observe top. The top quark nearly always decays to a W and b quark:

t -> W⁺ bbar
tbar -> W^- b

ttbar -> W⁺W^- b bbar

This results in several observable decay topologies:

When one W decays leptonically (lν) and the other decays hadronically (U_iD_j), the event has a high momentum lepton, missing energy, and 4 jets. Additional discrimination comes by looking for evidence of "long-lived" b-particles in one or two of the jets. This is called b-tagging.
If both W's decay leptonically (l⁺νl^-ν), then the signature is a pair of opposite-sign high-momentum leptons, missing energy, and 2 jets, one or both of which can be b-tagged. This is the cleanest decay signature, but has the smallest probability to occur.
If both W's decay hadronically then the event has 6 jets, at least two of which come from b quarks. This is the most probable topology, but is hardest to correctly reconstruct.

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