Measurement of theΛc+decay-asymmetry parameter

Abstract

We report a measurement of A polarization in the two-body decay A/-* A/r + in nonresonant e *e ~ interactions from data taken with the CLEO detector. Using these data we have determined the parityviolating asymmetry decay parameter a Af to be -1.0-8o-We see no evidence for significant A r + polarization.PACS numbers: 14.20. Kp, ll.30.Er, 13.30.Eg, 13.88.+eThe A f + (udc) is the lowest-mass baryon containing a charmed quark. Its spin is y, and its decays proceed via the weak interaction, usually resulting in strange-particle final states. As in hyperon decays, the weak decay of charmed baryons is expected to violate parity conservation. The decay A r + -• Ax + is analogous to the hyperon decays H~-•A/r~ and A-+pn~, where the parityviolating asymmetry decay parameters have been measured to be a E --0.456 ±0.014 and a A =0.642 ±0.013.' We present a measurement of the parityviolating asymmetry decay parameter « Af for the decay A r + -»A/r + . A nonzero value of a Af indicates parity violation; in this decay, the parity violation takes the form of an up-down asymmetry in the decay A's direction relative to the A r + spin. A nonzero a Af also gives rise to polarization of the daughter A, as we discuss below. Throughout this paper, unless noted otherwise, use of a particle or decay implies the use of its charge conjugate.In a recent paper, Bjorken 2 argues that a Ac --1 for several A f + decay modes including A/-* A;r + . Bjorken, making a factorization ansatz, writes the decay amplitude as T=x(p,s)(gvY M +g A Y 5 r,h(P,S)J\where T is the decay amplitude, j£, s, and p are the Dirac wave function, the spin, and momentum of the A, respectively; #, P, and S are the Dirac wave function, momentum, and spin of the A f + , and J^ is the pion weak current. The pion weak current is proportional to its momentum q^\ 2842

Subleading corrections to semileptonic Λc decays

“…The first indirect measurement of this parameter has been performed in a nonleptonic decay, where the polarization variable α 1 in the nonleptonic decay Λ c → Λπ was measured. Both ARGUS [7] and CLEO [8] observe polarizations which are consistent with α = −1. If factorization is assumed to relate the decay Λ c → Λπ to semileptonic Λ c → Λ transitions the observed value exactly corresponds to G A /G V = −1 predicted by heavy quark symmetry.…”

Infinite momentum frame calculation of semileptonic heavyΛb→Λctransitions including HQET improvements

“…The structure of the lepton-side decay W − → l − +ν l is determined by the Standard Model (V − A) coupling and has 100% analyzing power. For the hadron side the two-body decay Λ + c → Λ+π + is best suited for this analysis since the decay structure has recently been determined in two experiments [23,24] which 5 Judging from the numbers in Table 1 the exclusive semileptonic decay rate Λ b → Λ + c + l − +ν l would be predicted to amount to 57% -87% (IMF quark model) and 48% -72% (dipole model) of the total inclusive semileptonic rate…”

“…IMF quark model predictions for the O(1/m Q ) reduced form factor η(ω) as functions of ω = v 1 · v 2 (m = n = 1/2).characterized by comparing the charge radii ρ 2 of the form factors defined in Eq (23)…”

Second-order power corrections in the heavy-quark effective theory. II. Baryon form factors

“…where α is the asymmetry parameter of the Λ b decay, and α Λc is the measured asymmetry parameter in the decay Λ c → Λ X. For the nonleptonic decay Λ + c → Λ π + , a particularly useful mode, there are recent measurements α Λc = −1.0 +0.4 −0.0 [23] and α Λc = −0.96 ± 0.42 [24].…”