Cp Violation in Charm

Recent results on searches for CP and CPT violation in the charm sector are presented. These results include limits on direct CP violation in several channels from the FOCUS and CLEO experiments. The first reported search for CPT violation in charm, a preliminary result by the FOCUS collaboration, is also presented. 1 Charm CP Violation Introduction CP violation is generally divided into three types: CP violation in mixing (indirect), CP violation in decay (direct), and CP violation in the interference between decay and mixing (indirect or direct). In all cases, CP violation occurs when the decay rate of a particle differs from that of its CP conjugate. This requires contributions from two different CP violating terms with different phases. In addition, two CP conserving terms must also have different phases. The CP conserving phase shift is usually generated by QCD final state interactions. In the Standard Model (SM), two CP violating terms often come from tree level and penguin decays. Extensions to the Standard Model can introduce other CP violating terms which can interfere with the SM weak decays to generate CP violation. In charm, mixing is very suppressed so at current experimental sensitivities, CP violation searches are generally searching for direct CP violation. One measures the CP violation rate by looking at the asymmetry: ACP ≡ Γ(D → f) − Γ(D → f) Γ(D → f) + Γ(D → f) (1) In the fixed-target experiments E791 and FOCUS, the production mechanism gives rise to different numbers of produced particles and antiparticles. Therefore, these experiments normalize to another (copious) decay mode which is unlikely to exhibit CP violation. 2 Overview of Experiments The most precise charm CP violation results come from the Fermilab fixedtarget experiments E791 and FOCUS and the ee central detector, CLEO. 2.1 E791 and FOCUS experiments E791 (FOCUS) took data at Fermilab during the fixed-target running of 1991– 2 (1996–7). These experiments, like all modern fixed-target charm experiments are quite similar. Both sport silicon strip detectors in the vertex region to separate the charm production and decay vertices, a key requirement in separating signal from background. Following the silicon detectors are wire chambers and magnets which track and momentum analyze the decay products. Particle identification of charged hadrons is accomplished by the use of 2 (E791) or 3 (FOCUS) multi-cell threshold Čerenkov counters. Electromagnetic calorimeters identify electrons and photons while scintillation counters downstream of absorbing steel walls are used to identify muons. Both experiments used a hadron calorimeter to trigger on interesting events with high efficiency. The targets in both experiments were segmented to allow charm decays in air. E791 used a 500 GeV/c π beam while FOCUS used a photon beam with an average energy of 180 GeV (for events with a reconstructed charm particle). The average charm momentum was around 60 GeV/c for both experiments. From a collection of 20 billion (6 billion) triggered events, E791 (FOCUS) fully reconstructed more than 200,000 (1,000,000) charm particles. 2.2 CLEO experiment The CLEO experiment utilizes the CESR storage ring at Cornell which is a symmetric ee collider. The CLEO results presented here come from data taken at and near the Υ(4S), mostly from CLEO II.V (1996–9). Both CLEO II 1) and CLEO II.V 2) detectors use wire chambers for particle tracking and an excellent electromagnetic CsI calorimeter providing good reconstruction of photons, electron, and π’s. These detectors are inside a 1.5 T axial magnetic field and surrounded by muon chambers. In CLEO II.V a silicon strip system near the beam was also present. The data presented here utilize 4.7–13.7 pb of luminosity. Charm particles produced at CLEO generally have a momentum of a few GeV/c. 3 Direct CP Violation Results 3.1 Two-body decays E791 3), FOCUS 4), and CLEO 5) have all looked for CP violating behavior in the Cabibbo suppressed decays D → KK and D → ππ. These measurements, shown in Fig. 1 and tabulated in Table 1 are approaching the 1% level where non-Standard Model effects might show up. Table 1: Measurements of the CP asymmetry from D→KK, ππ decays. Expt ACP (KK) (%) ACP (ππ) (%) E791(98) 3) −1.0± 4.9± 1.2 −4.9± 7.8± 3.0 FOCUS(00) 4) −0.1± 2.2± 1.5 4.8± 3.9± 2.5 CLEO(02) 5) 0.0± 2.2± 0.8 1.9± 3.2± 0.8 FOCUS has recently published 6) results using the two-body decay modes D →K Sπ , where Cabibbo favored and doubly Cabibbo suppressed amplitudes can interfere and D → K SK + which is a singly Cabibbo suppressed -15 -10 -5 0 5 10 15 Average 〈ACP(KK)〉 = −0.1% ± 1.7% A C P (% )