A 'known' CP asymmetry in τ decays

We point out that dynamics known from the observed CP violation in KL→π∓l±ν coupled with CPT invariance generate a CP asymmetry of 3.3×10−3 in τ±→νKSπ±→ν[π+π−]Kπ±. The equality of the τ+ and τ− lifetimes required by CPT symmetry is restored in an intriguing way as the combined effect of long-lived K→π+π− as well as contributions from KL-KS interference. While little new can be learnt from this CP asymmetry, the latter has to be accounted for, since CP asymmetries in τ→νKπ channels are prime candidates for revealing the intervention of New Physics. This 'known' CP asymmetry provides a very useful calibration point in such searches. It also provides a test of CPT symmetry (as well as the ΔQ=ΔS selection rule).