Detection of heavy flavours

Several techniques have been devised to identify charm and bottom hadrons, such as (a) invariant mass peaks, (b) direct analysis of the decay vertex, (c) detection of leptons from semi-leptonic decays of heavy quarks. The key ingredients in these analyses are (i) the relatively long lifetime and (m) the lepton signature of heavy hadron decays, 

As we have already seen, the first evidence for charm D meson production) came from analysing the Ktttt invariant mass produced in e e collisions above 4 GeV. The sharp narrow enhancement visible in the A 7r+7r+ exotic channel is absent in the non-exotic AT+tt+tt channel (Fig. 13.9) and is convincing evidence that what is seen is indeed the decay of a i meson. 

By far the best way to study the D meson properties, however, is to analyse the decay of the l i state of charmonium, the so-called (3772) (see Chapter 11) whose mass lies less than 50 MeV/c above the DD threshold (so that the Ds move slowly) and is below the threshold for D production. This situation allows a very precise measurement of the mass from m = (E —p ) where E M n/2 is the energy of the beam which is known with only 1 MeV/c uncertainty. The momentum is very small, being so close to threshold (p ci 0.08(GeV/c) ) that any uncertainty in its value is irrelevant in the mass determination, whose resolution is then 3 MeV/c. The inveiriaiit mass spectra for various Kn combinations are shown in Fig. 13.10. 

An important point that has already been mentioned is that charm conservation in strong and electromagnetic reactions requires charm particles to be produced in associated production only. The present situation is particularly favourable since we can look at the recoil mass to verify that a has been produced in association with the D . The result shown 

Particle I(J ) Mass (MeV/c ) Main decay modes Branching fraction (%) Mean life or full width 

---