Asymmetry in the Nonmesonic Weak Decay of PolarizedΛ5HeHypernuclei

Abstract

We have measured the asymmetric emission of protons from the nonmesonic decay of polarized (5)(Lambda)He produced by the (pi(+), K+) reaction. (5)(Lambda)He is an s-shell hypernucleus and its polarization is due to the Lambda. One expects to obtain direct information on the elementary weak Lambda-->p-->np process. The asymmetry parameter has been determined to be 0.24+/-0.22. The implication of the result is discussed.

Asymmetry parameter for the nonmesonic decay of 5<FONT FACE=Symbol>L</font>He

“…It is experimentally extracted from measurements of the angular distribution of the emitted protons in the nonmesonic decay of polarized hypernuclei. There are large discrepancies, both experimentally and theoretically, in the determination of a Λ [1], specially after the newest experimental results for the decay of 5 Λ He [3] giving a positive value for this observable, differently from previous measurements based on the decay of 12 Λ C [4], which would be expected to yield approximately the same result (see below). In strong disagreement with the new measurement, all calculations so far fi nd a negative value for a Λ , which makes the investigation of this observable particularly relevant.…”

Intrinsic asymmetry parameter in nonmesonic hypernuclear decay: analytical proof of the independence on hypernucleus within an independent particle shell model

“…Similarly, the results for even-even core hypernucleus are reproduced with j p c = j n c = 0 (which means J C = 0 and J F = j p ) in odd-odd core formulas. Thus, we have arrived to the general expression (12) for the f J (j p , J 1 J F ) coefficients of an odd-odd hypernuclei, which includes the even-odd and even-even hypernuclei as particular cases.…”

“…[14,37] 0.11 ± 0.08 ± 0.04 [15,38] 0.24 ± 0.22 [39] 11 Λ B −0.16 ± 0.28 +0.18 −0.00 [37] −0.20 ± 0.26 ± 0.04 [38] 0.28 ± 0.14 [40] 12 Λ C −0.24 ± 0.26 +0.08 −0.00 [14] −0.16 ± 0.28 +0. 18 −0.00 [37] −0.20 ± 0.26 ± 0.04 [15,38] 0.02 ± 0.20 [40] 16 …”

“…One expects, therefore, that the asymmetry parameter will have more discriminating power to constrain the proposed mechanisms for NMHD. As known, the measurements of a Λ favor a negative value for 12 Λ C and a positive value for 5 Λ He [11]- [15]. On the other hand, majority of the existing calculations based on strict one-meson exchange models (OMEM) [16]- [26] find values between −0.73 and −0.19 for 5 Λ He [27,28] and, when results are available in the same model, very similar values for 12 Λ C. Thus, there is now a renewed interest to solve this NMHD puzzle.…”

“…As known, the measurements of a Λ favor a negative value for 12 Λ C and a positive value for 5 Λ He [11]- [15]. On the other hand, majority of the existing calculations based on strict one-meson exchange models (OMEM) [16]- [26] find values between −0.73 and −0.19 for 5 Λ He [27,28] and, when results are available in the same model, very similar values for 12 Λ C. Thus, there is now a renewed interest to solve this NMHD puzzle. A first application of effective field theory (EFT) to nonmesonic decay [29] and the inclusion of σ meson exchange plus direct-quark interaction, which strongly violates the ∆T = 1/2 rule [30], have stressed that the scalar-isoscalar type of interactions are specially important in explaining the asymmetry.…”