Measurement of Cp Asymmetries at Belle

The Belle experiment at the KEK B factory has collected 93 fb of electron positron collisions at √ s = 10.6 GeV. This has produced a sample of 85 million BB̄ meson pairs that can be used to study CP violation in rare (and not so rare) B decay modes. Here I report on a measurement of indirect CP violation in the decay B → J/ψK S, as well as time dependent CP asymmetries in rarer modes such as B → ππ, ηK S and φK S. I summarise the prospects for improving the precision on these and related measurements. ∗With support from Princeton University and the Japanese Accelerator Laboratory, KEK. 1 CP Violation in B Decay When CP violation was first observed in neutral kaon decay, in the early 1960s, it shook the foundations of particle physics. It had previously been assumed that the combination of charge conjugation and a parity transformation left all known particle interactions invariant, despite the fact that weak interactions violated parity alone. Over the following three decades CP violation in the K system was measured with ever increasing precision in an attempt to pin down its source. In the 1970s Kobyashi and Maskawa showed that the Standard Model could accommodate CP violation in a three quark weak mixing matrix, VCKM. The single non-trivial phase in such a 3 × 3 unitary matrix could explain the small effect first seen in K meson decay, where CP violation was observed at the 10 level. It was suggested, in the early 1980s, that the comparable amplitudes for the direct decay of B mesons into CP eigenstates and the mixing of BB̄ mesons would make neutral B meson decay an ideal place to observe large indirect CP violating effects. One way to understand the magnitude of CP violation predicted by the CKM model, in neutral B meson decay, is to consider the unitarity relation between the first and third columns of VCKM: V ∗ tbVtd + V ∗ cbVcd + V ∗ ubVud = 0. While each term in this expression is relatively small (O(sin θC)), they are all the same size. When plotted in the complex plane (see fig. 1) one expects significant angles at each apex because the sides of the triangle have similar lengths. In neutral kaon decay the corresponding unitarity triangle has two sides that are much larger than the third – making the decay rates for K mesons much larger, but making the angles, and hence the observable phases small and more challenging to measure. In B meson decay CP violating phases are most readily observed through the indirect mixing of two amplitudes. Given a CP eigenstate accessible to both B and B̄ decays – such as J/ψK S – one can observe the interference between the direct decay amplitude for: B(B̄) → J/ψK S; and the amplitude for the same decay preceded by B mixing: B(B̄) → B̄(B) → J/ψK S. Figure 1: Unitarity of the CKM matrix relates its different elements. The study of CP violation in B meson decay involves the CKM elements depicted in this triangular relationship. When one includes the relative phase between these two amplitudes we get an expression for the time dependent CP asymmetry: ACP (∆t) ≡ dN dt (B̄→J/ψKξ)− dN dt (B→J/ψKξ) dN dt (B̄→J/ψKξ)+ dN dt (B→J/ψKξ , = ξK sin 2φ1 sin ∆m∆t. [1] Where the asymmetry in the decay rate between B and B̄ mesons is proportional to the B mixing rate (sin∆m∆t), the CP eigenvalue of the final state, ξK , (ξK = −1 for J/ψK S and +1 for J/ψK L) and sin 2φ1, the angle at the lower right apex of the unitarity triangle shown in fig. 1. 2 The KEK-B Collider and Belle Detector The main experimental challenge in measuring CP violation in B meson decay lies in the fact that B mesons decay much more quickly than K mesons, having proper flight distances of fractions of a millimeter. Furthermore the decay rate to experimentally accessible CP eigenstates are much smaller, on the order of O(10−4) for B mesons; while essentially 100% of K meson decays are to identifiable CP eigenstates. While the very much larger CP violation in B decay makes up for some of this it remained a significant experimental challenge to observe CP violation in B meson decay. The key ingredient is having a high luminosity source of B mesons – a B factory. Two dedicated machines were built in the late 1990s to address this. The Figure 2: The Belle experiment has been collecting data over the last two years. This plot shows the integral luminosity over that period (lower line) compared to the luminosity collected by the BaBar experiment at PEPII (upper line). The data samples are similar at this point. KEK-B ee accelerator complex collides beams of 8 GeV electrons and 3.5 GeV positrons, to produce Υ(4S) mesons at √ s = 10.6 GeV. These, in turn, decay into BB̄ meson pairs. Using an 11 mrad crossing angle KEK-B is able to collide beams with currents in excess of 1 Ampere, with tolerable backgrounds and luminosities approaching 10 per cm each second. An integrated luminosity comparison between the KEK-B factory and its competitor, PEP-II at SLAC, is shown in fig. 2. While the two machines have delivered similar integrated luminosities since they came online two years ago, the recent instantaneous luminosities (the slope of the curve) in the KEK machine bodes well for upcoming data-taking. The colliding beam energies are asymmetric so that the Υ(4S) meson produced is boosted along the beam direction. This results in the daughter B mesons also being boosted, with βγ ≈ 0.45, separating the two B meson decay vertices in the detector. The boost introduces some asymmetry in the design of the experiment. A cross-section of the cylindrical Belle detector is shown in fig. 3. The detector elements are arranged asymmetrically around the interaction point to increase the acceptance for B meson decay products. Working outwards from the collision point B meson decay products encounter 0 1 2 (m) ee+ 8.0 GeV 3.5 GeV SVD CDC CsI KLM TOF PID 150°