Preliminary Observation of Parity Nonconservation in Atomic Thallium

Abstract

Parity nonconservation is observed in the 6 2 P t /2~7 2 Pt/2 transition in thallium. Absorption of circularly polarized 293-nm photons by 6 jp t^ atoms in an E field results in polarization of the 7 2 P\/2 state through interference of Stark El amplitudes with Ml and paritynonconserving El amplitudes 3fR and S p . Detection of this polarization yields the circular dichroism 6^ + (5.2±2.4)xl0" 3 , which agrees in sign and magnitude with theoretical estimates based on the Weinberg-Salam model.We report preliminary observations of parity nonconservation (PNC) in the &P l / 2 -r l 2 P 1 / 2 transition (292.7 nm) in atomic thallium (see Fig. 1). The transition is forbidden Ml with measured amplitude m = (-2.1 ±0.3)xlO" 5 \eti/2m e c\. 1 If parity is not conserved, the 6 2 Py 2 an( * ^2P\ji states are admixed with 2 Sj 2 states. The transition amplitude then contains an additional El component S p , and circular dichroism exists, defined bywhere a± are the cross sections for absorption of 293-nm photons with ± helicity, respectively,, Theoretical estimates of 6 based on the Weinberg-Salem (W-S) model 2 yield 3 ' 4 ^theor =2lm(5 /)f theor)/^expt = (+2.3±0.9)xl0" 3 (2)for sin 2 0w = O.25, where 0^ is the Weinberg angle. The uncertainty in 6 theor arises from the uncertainties in Sftexpt (~ 15%) and S Pttheot (~ 25%). The aim of this experiment is to measure 6. Investigations of this type were first suggested by Bouchiat and Bouchiat, 5 and their experiment on Cs is in progress. 6 Also, optical-rotation experiments on bismuth have been reported (but with contradictory results), 7 " 9 while PNC in highenergy electron scattering, consistent with the W-S model, has been observed. 10 The simplest way to measure 6* would be to illuminate Tl vapor in a field-free region with circularly polarized 293-nm light and observe the helicity dependence of the decay fluorescence (e.g., at 535 nm; see Fig. 1). Unfortunately this is impractical because of background effects. Instead, using a technique first suggested by Bouchiat and Bouchiat, 5 we apply an external field E which Stark mixes 2 P x / 2 states with 2 S x / 2 and 2 D 3 / 2 stateSo The transition intensity, proportional to E 2 , is thereby increased above the background; moreover interference between the Stark transition amplitudes and the 3ft and S p amplitudes polarizes the 7 2 P 1 / 2 state, permitting measurement of 9H and 6 0 Let the 293-nm photon beam be alongx, and choose E =Ey [see Fig. 1(b)]. Ignoring terms of order [STC* 8 P ] 2 , we find the r l 2 P l / 2 polarization 8*S FIG. 1. (a) Low-lying energy levels of Tl (not to scale), (b) Coordinate system, orientation of photon beams, and electric field direction, (c) Schematic diagram indicating production and analysis of 7 2 P 1//S polarization in the 0-1 transition. The rates at which the F=l 9 m F = -l, 0, and +1 levels are populated in the 7 2 P i/2 states are proportional to i(p+WL*S p ) 2 , JOKT^) 2 , and J(0 -9R±^) 2 , respectively. The polarization is analyzed by circularly polarized 2.18-/im radiation (7 2 P 1 ...

The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules

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“…Extension to hydrogen-like and heliumlike atoms may result in a test of QED calculations, and is presently under active investigation [5]. Second-ly, the M 1 E 1 nd interference occurs in parity violation experiments, like those being performed at present on cesium [6] and thallium [7] in zero magnetic field and also those under investigation on the same elements with the use of a high magnetic field [8,9].…”

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