MUON POLARIZATION MEASUREMENTS AT THE AGS

Muon polarization is a sensitive probe of CP and T symmetries in many decay modes. Following table lists a few such processes.

1. K+ -> Pi Mu+ Nu

2. K+ -> Mu+ Gamma nu

3. K_l -> Mu+ Mu-

4. K+ -> Pi+ Mu+ Mu-

5. Eta -> Mu+ Mu-

The BNL-AGS is particularly suited for these types of experiments because it can provide high intensity kaon beams at low energies so that the muons from the final states can be stopped and analysed with relative ease. We briefly comment on the first decay mode in the paragraph below. A more complete discussion of these opportunities will be made during the AGS-2000 workshop.

T VIOLATING MUON POLARIZATION IN DECAYS

Draft Proposal: Postcript File

A new search is proposed for the time reversal violating polarization (P_T) of the muon normal to the plane of the KMU3 (K+ -> Pi0 + muon + neutrino) decay. The projection of the muon spin out of the plane defined by the muon and pion momenta changes sign upon time reversal; therefore a finite expectation value for this quantity indicates a violation of time reversal invariance.

In the Standard Model with only one Higgs doublet KMU3 decay occurs at first order through the weak charged current or W exchange. In such an interaction P_T=0. For a non-zero transverse polarization an effective scalar interaction with a different phase is needed. Thus the study of this polarization naturally leads to the physics of charged Higgs particles or other non-standard scalar interactions such as lepto-quarks. It should be noted that CP-violation is required to generate the observed baryon asymmetry of the Universe, and it is unclear whether the CP-violation embodied in the CKM matrix has sufficient strength for this purpose. Several recent theoretical attempts [1] postulate multiple-Higgs-doublet models with CP violation to explain the baryon asymmetry. These models can also generate transverse muon polarization [2]; and so T-violation in KMU3 decays could be related to the baryon number asymmetry of the Universe.

The best previous experimental limits were obtained almost 15 years ago with both neutral [3] and charged kaons [4] at the BNL-AGS. The experiment with K+ decays produced a measurement of the transverse polarization, P_T=0.0031+-0.0053. This 15 year old measurement was based on 21 Million KMU3 events and was statistically limited. The AGS can now produce low energy kaon beams with almost ten times the intensity; this large flux makes possible high intensity separated kaon beams with low background from pions and low singles rates per kaon. An experiment designed for higher acceptance than References [1] and [2] combined with the high kaon flux should make it possible to improve the limit on the polarization or , the T-violating amplitude, by more than an order of magnitude.

REFERENCES

1.   L. Mclerran, M. Shaposhnikov, N. Turok, and M. Voloshin, Phys. Lett. B 256, 451 (1991); N. Turok and M. Voloshin, Phys. Lett. B 256, 451 (1991).
2.   R. Garisto and G. Lane, Phys. Rev. D 44, 2038 (1991).
3.   M. Schmidt, et al., Phys. Rev. Lett. 43, 556 (1979). W. Morse, et al., Phys. Rev. D 21, 1750 (1980).
4.   M. Campbell, et al., Phys. Rev. Lett. 47, 1032 (1981). S. Blatt, et al., Phys. Rev. D 27, 1056 (1983).