B mesons

Experimental Clarification in the Neutral K Meson and B Meson Systems By K. Kleinknecht 2003. 67 figs., XII, 144 pages... 

Whereas both P and C symmetries are individually violated by nature, the combination CP invariance—the exchange of particles with their corresponding antiparticles followed by a reflection in a mirror— was thought to hold absolutely until 1964, when J. W. Cronin, Val L. Fitch, and colleagues showed that CP symmetry was violated in the decay of neutral kaons. This was the one and only violation of CP symmetry that had been observed until March 2001, when the decay of neutral B mesons also violated this symmetry. But these two violations mean that CP invariance is not absolute. With this violation of CP symmetry and with the possibility of baryon nonconservation, it is possible to explain why matter dominates the universe. The explanation, however, is speculative and not very satisfying. 

The weak interactions that cause atomic PNC violate not only the symmetry of parity, P, but also the symmetry of charge conjugation, C. However, the product of these, CP, is conserved. Because any quantum field theory conserves CPT, where T is time reversal this is equivalent to saying that T is conserved. However, even this symmetry is known to be violated. To date, this incompletely understood phenomenon has been seen in only two systems, the neutral kaon system, and, quite recently, the neutral B meson system. However, as noted already in the 1950 s by Ramsey and Purcell [62], an elementary particle possessing an intrinsic electric dipole moment also violates T invariance, so that detection of such a moment would be a third way of seeing T noninvariance. 

The branching fraction measurements are for an admixture of B mesons and baryons at energies above the T(4S). Only the highest energy results (LEP, Tevatron, SppS) are used in the branching fraction averages. The production fractions give our best current estimate of the admixture at LEP. 

B-meson Symbol B . A meson that consists of a down quark and an anti-bottom quark. 

It is hoped that study of the decays of B-mesons will shed light on the problem of CP violation (see CP invariance). 

CP invariance The symmetry generated by the combined operation of changing charge conjugation (Q and parity (P). CP violation occurs in weak interactions in kaon decay and in B-mesons. See also CPT theorem time reversal. 

CP violation has been experimentally established also in B meson decays. However, the observed matter-antimatter asymmetry of the Universe requires additional sources of CP violation. 

In such B-factories, the separation of pions from kaons is vital for the identification of B- or anti-B-meson and the selection of rare decays. It is important to measure the decay time from the appearance of B-mesons to the final state. 

Figure 28.11. Decay pattern of moving B-meson pairs.

Figure 28.15. Violation of CP symmetry measured by BELLE in 2006 difference of the decay time distribution between B-mesons and anti-B-mesons.

The universality of the weak decay is reflected in the unitarity of the CKM matrix. Hence, the CKM matrix can be parametrized by three mixing angles and one irreducible phase which accounts for the Cf -violation intrinsic to the weak decay in the Standard Model. The decay width is proportional to the squared CKM matrix element. Measurements of semileptonic decays of B mesons have shown that the matrix elements relevant for the weak decay of the b quark are very small compared to other elements Vcb = 0.0412 0.0011 and Vub = 0.00393 0.00036 [30]. Consequently, the b quark decay is highly suppressed and the b quark has a relatively large lifetime of r 10 s. Since Vcb is about an order of magnitude larger than I Vub the preferred decay is ft cW with a branching ratio of almost 100%. 

On the experimental side it may come as a surprise to learn that despite 20 million J/ decays it is believed that only some 25% of all hadronic decay channels have been identified. Similarly, for the radiative decays, some 50% of the modes may still be unidentified. The situation should improve dramatically with the high luminosity (5 x 10 cm s ) and small energy spread HE 0.6 MeV) e" "e colhder BEPC in Beijing. A similar situation is envisaged for the T when the B-meson factories now under discussion are built. 

Fig. 13.6 shows how the value of R, in crossing the T(4s) peak at y s 10.55 GeV, rises by about 0.33 signalling the production of a new flavour with charge The detailed reconstruction of B meson decays is extremely difficult and was first achieved by the Argus and the CESR groups in 1987 (ARGUS, 1987b CLEO, 1987). 

Figure 1.3 are LEGO plots showing the correlation between laboratory momentum and polar angle for a variety of particles produced in B meson decays at the T(4S) at p y = 0.56. 

The boost has a substantial effect on solid angle coverage. Figure 1.4 shows the single track efficiency as a function of maximum polar angle 9 in the forward and backward directions for B meson decays at the T(4S) with a boost of 7 = 0.56. Since it is necessary to reconstruct a minimum of three tracks for a measurement of sin 2a using + 7r "7r, and at least five tracks for a measurement of sin 2/ using B there... 

Multiple Coulomb scattering is, in fact, a serious issue for the main tracking drift chamber as Well as for the vertex detector. Figure 1.10 shows the momentum distribution of pions, kaons, electrons and photons produced by B mesons at the T(4S). The particle spectra peak at low momenta, although the high momentum tails are not unimportant, as we shall see. 

The heart of any asymmetric B Factory detector aimed at the study of CP violation is the vertex detector. Primary emphasis is on resolution along the flight path of the B mesons, that is, the beam (z) direction, and not on resolution in the transverse plane, as in current silicon vertex systems. 

A Fast RICH is the reference particle identification system. The R D on this technology has been pursued primarily, but not exclusively, by the European Fast RICH Collaboration. A Fast RICH with a liquid radiator and pixel readout can provide 4cr separation for particles from two-body B meson decays over most of the solid angle. An important concern here is whether a sufficient number of Cerenkov photons can be detected in a ring with adequate angular resolution, with a device having a 20 cm radial extent and 15% of a radiation length at 90°. 

Feasibility Study for a B Meson Factory in the CERN ISR Tunnel, T. Nakada, ed., CERN 90-02 (1990). 

Figure 4.1. Distributions in momentum and angle for all pious and kaons produced in B meson decays for beam energies of 9 on 3.1 GeV a) momentum distribution for pions b) momentum distribution for kaons c) angular distribution for pions d) distribution of the average momentum versus the cosine of the polar angle 6 for pions e) distribution of the average momentum versus the cosine of the polar angle 9 for kaons.

Figure 6.2. 7t° (dashed line) and B meson reconstruction efficiency (solid line) vs. photon energy threshold. (Taken from Ref. [6].)... 

Other Physics Using Tagged B Meson Samples... 

P.R. Burchat Physics at an Asymmetric-Energy B Meson Factory Jun 28.1991... 

Other Physics using Tagged B Meson Samples.190... 

---