Charm decay

In addition, about 10% of the subsequent charm decays also have a muon and a neutrino in the final state. The Feynman diagrams of the semileptonic decay of a Z -hadron with a muon in the final state are illustrated in Fig. 3.10. 

A harder distribution is observed in the re-weighted data with respect to simulation. In order to validate the weighting procedure, it was applied to the simulated track spectrum. The results from simulation agree within the statistical error. For comparison, the measured distribution calculated from muons which were most likely generated in light quark or charm decays is also shown in Fig. 5.8. 

After some claim of baryon charm decay seen at Fermilab the first actual complete reconstruction of a charm baryon event has come from leptonic (neutrino induced) reactions (Angelini et al, 1979) at CERN where all the momenta have been measured and the particles identified, allowing the determination of both the proper decay time and mass. The event corresponds to the decay of the charm baryon A+ ucd)... 

The relative strengths of the various possible hadronic and semi-leptonic charm decays can be estimated from the couplings to the Cabibbo allowed and forbidden channels and can be read off from Fig. 13.13. 

Let us assume that leptonic charm decays proceed via (13.1.1), i.e. neglecting the other contributions listed in Section 13.2.1. We show in Fig. 13.14 examples of Cabibbo allowed and suppressed decays. 

We expect PEP-II to produce high statistics samples of meson decays, thus presenting a wealth of new physics opportunities, beyond the principal objective of probing ti e origin of CP violation. These opj>ortunities in other aspects of 6-quark weak and electromagnetic decay, in charm decay, in r decay, and in two-photon physics have been discussed in detail in [1]. This section l)iiefly summarize and updates tliat discussion. 

The extension of the periodic system into the sectors hypermatter (strangeness) and antimatter is of general and astrophysical importance. Indeed, microseconds after the big bang the new dimensions of the periodic system, we have touched upon, certainly have been populated in the course of the baryo- and nucleo-genesis. Of course, for the creation of the universe, even higher dimensional extensions (charm, bottom, top) come into play, which we did not pursue here. It is an open question, how the depopulation (the decay) of these sectors influences the distribution of elements of our world today. Our conception of the world will certainly gain a lot through the clarification of these questions. 

In this zoo of particles, only the electron, which was discovered even before the atomic theory was proven and the atomic structure was known, is really unseeable, stable, and isolatable. The proton also is stable and isolatable, but it is made up of two quarks up (with charge -1-2/3) and one quark down (with charge —1/3). As for the quarks, while expected to be stable, they have not been isolated. The other particle constitutive of the atomic nucleus, the neutron, is also made up of three quarks, one up and two down, but it is not stable when isolated, decaying into a proton, an electron, and an antineutrino (with a 15-min lifetime). The fermions in each of the higher two classes of the electron family (muon and tau) and of the two quark families (strange charmed and bottom/top) are unstable (and not isolatable for the quarks). Only the elusive neutrinos in the three classes, which were postulated to ensure conservation laws in weak interaction processes, are also considered as being unseeable, stable, and isolatable. 

ZEUS CoUaboration, Measurement of charm and beauty production in deep in elastic ep scattering from decays into muons at HERA. Eur. Phys. J. C 65,65—79 (2010)... 

Prom the CERN SppS colliders we have a limit on a possible second generation of gauge vector bosons (CHARM, 1989) Mz > 280 GeV. No squark or gaugino (i.e. the supersymmetric partners of quarks and gauge vectors) below 100 GeV have been found nor have any heavy stable charged particles been seen at the 1.8 Tevatron by CDF (CDF, 1989). No fourth quark generation (i.e. no 6 ) is found by DELPHI at LEP 1 below 44 GeV in the decay of the Z (DELPHI, 1990). A summary of the situation can be found in Dydak (1991). 

Next, we discuss in detail the properties of the members of the J/ family (bound states of a charm-anticharm pair), their quantum numbers, decay patterns and the dynamical and kinematical reasons for their narrow widths. We then tturn to the most recently discovered family (T) whose members are remarkably heavy 10 GeV/c ) and which are supposed to be bound states of the heaviest quark-antiquark pair so far identified ( bottom or beauty ). 

The very small width of this resonance, F Ci 70 keV (about a factor 1000 smaller than a typical hadronic width), indicates that its decay mode into ordinary hadrons is highly suppressed. This discovery triggered a vast experimental search and stimulated much theoretical work. The present interpretation of the J/ is that it is the first manifestation of a cc bound state ( hidden charm ) occurring below the threshold for charm particle production. 

It turns out, however, that the lightest charm meson has a mass of 1863 MeV/c so that J/4 (3097) [as well as (3684)] is below the threshold for decay into in contradistinction to which was just... 

To the extent that strong interactions are expected to conserve charm, the decays that cc bound states will vmdergo are ... 

The subject of heavy flavours has e3q>anded tremendously in recent years stretching from the static properties (mainly spectroscopy, i.e. energy levels, lifetimes, branching ratios, decays, mixing etc.) of hadrons with one or more heavy quarks, e.g. bottom or charm, to more dynamical properties (like fragmentation, structure functions, jets etc.) and on to more exotic topics, e.g. production and decay of as yet undiscovered flavours like top, or speculations on a fourth generation or imphcations on Higgs or on non-standard effects and so on. 

We note that if the electromagnetic and strong interactions conserve flavour, then we should expect associated production of heavy flavours, i.e. that production always occurs with pairs of particles of opposite charm or bottom (this is, of course, not the case for production in neutrino interactions via weak forces). Further, the decay of a heavy particle should be generated by the weak interactions, implying very narrow widths and effects of parity non conservation. 

Chapter 19 and is in agreement with the non-existence of charm-changing neutrcd currents (as expected from the GIM mechanism, see Chapter 9). The decays, therefore, are due to the charged weak current. Nonetheless mixing is not negligible, as we will see, and this has important consequences for CP violation (see Chapter 19). 

In terms of quark diagrams (Fig. 13.1), a charm particle will decay into ordinary hadrons, according to the standard model, via the dominant quark transitions (see Section 9.2)... 

As an example Fig. 13.7 shows the mass distribution in channels occurring in charged B decays. The clear B-peak at 5275.8 1.3 3.0 MeV/c is evident. It is found that Bs decay predominantly into charm mesons, in agreement with the decay mechanism illustrated in Fig. 13.2. 

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. 

If we assume charm to be a good quantum number under strong and electromagnetic interaction (see Section 13.3), the pseudo-scalar chmm mesons D, D and Df can only decay weakly into old mesons. By contrast, D s can decay both strongly and electromagnetically whereas only the latter mode is available to T> due to its mass threshold, as can be seen from Table 13.1. The naive expectation that heavy flavoured hadron decays are entirely determined by the so-called spectator diagram (Fig. 13.13) leads to immediate predictions for the lifetimes of these... 

All Cabibbo allowed decays of non-strange charm particles involve a single strange particle which therefore provides a prominent signal for charm. As already mentioned, the same signal is expected for bottom particles given that i> —> c + W is the main decay mode of b quarks. 

Aside from the spectator contribution to all charm particle decays (this is sometimes called W-radiation ), several approaches have been suggested in order to estimate possible corrections. Among these, we have ... 

As we shall soon see, the total decay rate of D and (into hadrons) can be estimated to be of the order of 10 — 10 s so that the purely leptonic decay rate of charm mesons leads to branching ratios which are, at most, of order 10 — 10 and can, therefore, be neglected. Indeed up to the present, none of the leptonic decays has been observed (on this question, the Particle Data Group (1992) reports BR( > — ... 

Semi-leptonic and hadronic decays of charm mesons... 

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