We discussed earlier in the PiBeta collaboration the possibility and desirability of the precise measurement of radiative pion decays. For this measurements the new trigger HiLo was proposed. The HiLo trigger has high efficiency and the statistics for pion beta and radiative decays could be obtained simultaneously by measuring decays of pions stopped in the target.

The precise measurement of radiative pion decays will allow one to
find out for sure if a tensor interaction makes any contribution to
this decay. The Standard Model allows here only V-A interaction. For
instance, the ISTRA collaboration [1] claimed existence of tensor
charged-current interaction (T) contribution to the decay. Destructive
interference between the standard amplitude and a non-conventional
amplitude of the tensor type can lead to a lack of events observed in
the ISTRA experiment. The authors obtained the tensor interaction term
in the amplitude FT=-(5.6+/-1.7)x10^{-3}, and considered this
value as an upper limit on |FT|< 10 ^{-2}. It corresponds
to a coupling constant fT of about 10^{-2} whereas even in
supersymmetric extensions of the standard model the maximum value of
fT is 10^{-4} - 10^{-5} [2]. Nevertheless somewhat
exotic models [3,4] were proposed to explain the ISTRA result.

Kinematics of this process allows to separate the region where structure-dependent contribution (SD) is dominated over uninteresting inner bremsstrahlung (IB). SD is parametrized with vector (FV) and axial-vector (FA) form factors. So in the Standard Model V-A approach one can test predictions of different strong interaction models.

The main goals of the measurement are:

- to improve accuracy of pion form factors FV and FA;
- to obtain an upper limit on possible contribution of the tensor interaction.

A simulation of PiBeta calorimeter with HiLo trigger ensures a substantial suppression of the IB contribution. For ISTRA value of FT the ratio |IB|/|SD|/|T| is about 260/1/2 for all decay events whereas for triggered events this ratio is 6/1/1. The very preliminary results of radiative pion decays with and without tensor interaction one can see on Fig1:

The registration efficiency of radiative pion decay events is about 1%
for such a trigger. For events with large photon energies (x > 0.9)
the registration efficiency is about 8%, but the majority of such
events rejected by the trigger lies in the range L < 0.2. The
registration efficiency of the main physical background (pi -> e nu
decay) is about 0.2%. A calculation of the radiative pion decay rate
for photons with energy > 5 MeV gives 3.84x10^{-6}. We
expect about 1.2x10^{-7} radiative pion decay triggers per one
pion decay (without prescaling) or 5.2 of these triggers (prescaling
factor = 4) per one PiBeta decay. In the interesting region
(E_{Gamma} > 63 MeV) we expect about 0.7 radiative pion
decays (prescaling factor = 4) per one PiBeta decay. It means that in
statistics equivalent to 10^{4} PiBeta decays we will have
7000 radiative pion events. We can determine the tensor interaction on
the level of a few percents.

So this trigger is rather suitable to investigate possible tensor contribution in the most sensitive region of phase space. On the other hand, one can see that all "PiBeta" events are selected by "PiBeta Hi" trigger will be selected by "HiLo" trigger also.

[1] V.N.Bolotov et al., Phys.Lett. B243 (1990) 308.

[2] V.M.Belyaev and Ya.I.Kogan, Preprint UBCTP 91-38 (1991).

[3] P.Herczeg, Phys.Rev. D49 (1994) 247.

[4] L.V.Avdeev and M.V.Chizhov, Phys.Lett. B321 (1994) 212.

[5] A.V.Chernyshev et al., Mod.Phys.Lett.A, Vol.12, No.23 (1997) 1669.