Indication of New Physics in B→ΦKs Decays at the BELLE Experiment

The BELLE experiment at the asymmetric e+e− (3.5 GeV and 8 GeV) accelerator KEK-B at Tsukuba, Japan, has recorded 152 Million BB meson pairs in 4 years of data taking. In the standard model, the size of CP violation in the B meson system is characterized by a quantity called sin(2φ1), in which φ1 denotes one of the angles in the unitarity triangle. This quantity was determined precisely to be sin(2φ1)=0.733±0.059±0.028, based upon N=1997 events with the decay B→J /ΨKs and N=914 other B→charmonium+Kaon decays [1] [2] [3]. The measurement is consistent with the measurement of the BABAR detector at SLAC [4]. In the standard model, the decay B→ΦKs is expected to show identical CP violation. However, a measurement of N=68 events revealed −0.96±0.50+0.09−0.11 (opposite sign from expectation), which is a 3.5σ deviation from the expected standard model value [1] [5]. The probability of obtaining the observed result for B→ΦKs as a statistical fluctuation of the B→J /ΨKs result is less than 0.1%. In the standard model, B→ΦKs is dominated by a penguin diagram with a W boson and a top quark in a loop. The top quark itself radiates a gluon which creates an ss pair. Two additional decay channels, namely B→η′Ks and B→K+K−Ks are dominated by the same diagram. However, their CP violation seems to be consistent (within still large statistical errors) with the sin(2φ1) value from B→J /ΨKs [6] [7]. The difference between the three different decay channels is given by the spin of the Φ meson J =1− (vector meson), while all Kaons and the η′ are J=0− (pseudoscalar mesons). In an additional analysis, angular distributions have been investigated. In the case of B→V V 1 a helicity analysis can be performed. For this purpose, the B→J /ΨK∗ [8] (pseudoscalar meson Ks replaced by vector meson K∗) and B→ΦKs [9] were compared. As pointed out recently by Grossman [10], differences between the two decay channels seem visible, and are difficult to be explained within the standard model. BELLE will resume data taking in 10/2003, with an improved Silicon Vertex Detector (4 layers, larger angular coverage). An update of the analysis results is envisaged for summer 2004.