Isotopic variation of parity violation in atomic ytterbium

Abstract

The weak force is the only fundamental interaction known to violate the symmetry with respect to spatial inversion (parity). This parity violation (PV) can be used to isolate the effects of the weak interaction in atomic systems, providing a unique, low-energy test of the Standard Model [see for example reviews 1, 2, 3]. These experiments are primarily sensitive to the weak force between the valence electrons and the nucleus, mediated by the neutral Z 0 boson and dependent on the weak charge of the nucleus, Q w . The Standard Model (SM) parameter Q w was most precisely determined in cesium (Cs) [4, 5] and has provided a stringent test of the SM at low energy. The SM also predicts a variation of Q w with the number of neutrons in the nucleus, an effect whose direct observation we are reporting here for the first time. Our studies, made on a chain of ytterbium (Yb) isotopes, provide a measurement of isotopic variation in atomic PV, confirm the predicted SM Q w scaling and offer information about an additional Z´ boson.The large PV observable in Yb was first predicted by DeMille [6], a prediction further supported by subsequent calculations [7,8,9] and confirmed by experiment [10,11]. The PV effect in Yb is approximately 100 times larger than that in Cs. Moreover, Yb has a chain of stable isotopes, allowing for an isotopic comparison of the effect [12]. Such a comparison has the potential to be a probe of neutron distributions in the Yb nuclei [13] and is sensitive to physics beyond the SM [14,15]. A related measurement, in which the PV effects are compared for different hyperfine components of isotopes with non-zero nuclear spin, is expected to improve the understanding of the weak interaction within the nucleus [3,16,17,18].The principle of our measurements is similar to that of the 1 st -generation experiment [10,11]. We optically excite Yb atoms in a beam, on the 6s 2 1 S 0 → 5d6s 3 D 1 transition ( fig. 1), in a region in which in addition to the applied optical field, static electric and static magnetic fields are applied to the atoms [19].The directions of the magnetic and static electric field and that of the optical-field polarization define the handedness for the experimental coordinate system. As the 1 S 0 and 3 D 1 states are of nominally same parity, an electric-dipole (E1) transition between them is forbidden by selection rules. In the presence of the weak interaction, however, mixing of the 1 P 1 state into 3 D 1 results in a E1 PV amplitude for the transition. The applied dc (or quasi-static) electric field results in additional mixing of these states, allowing for a larger and controlled Stark-induced E1 amplitude [20]. The Stark-induced and PV amplitudes will interfere with appropriate choice of field geometry. Field reversals flip the handedness of the field geometry, leading to a sign reversal of the Stark-PV interference term and a change in the transition rate. This change provides an experimental observable.We measured the PV effect in four nuclear-spin-zero isotopes ( 170 Yb, 172 Yb, 174 Yb a...

Electroweak probes of ground state densities

“…Yet, in the case of the weak form factor of 132 Xe displayed in Fig. 8(b), the weak model dependence found in 40 Ar still persists-a fact that reinforces CEνNS as a powerful tool for the search for new physics [65]. In addition, we continue to find excellent agreement between the (analytic) two-parameter symmetrized Fermi function and the numerically generated weak form factor over a significant momentum-transfer range.…”

“…These predictions suggest a fairly soft symmetry energy; indeed, the predicted value for the slope of the symmetry energy falls in the fairly narrow range of L = 37.8 -47.7 MeV [76]. The associated weak form factor of 40 Ar is displayed in Fig.7(b) as a function of momentum transfer. Recall that for spinless nuclei, the weak form factor encodes the entire nuclear-structure contribution to the coherent cross section.…”

“…6(b), a central CREX value placed arbitrarily at R 48 skin ≈ 0.2 fm-but accurately reflecting the anticipated experimental error-results in a neutron skin thickness for argon of R 40 skin = 0.097 (14) fm. The rectangular section attached to the figure reflects predictions from microscopic models of the neutron skin thickness of 48 Ca [76] and 40 Ar [77]. These predictions suggest a fairly soft symmetry energy; indeed, the predicted value for the slope of the symmetry energy falls in the fairly narrow range of L = 37.8 -47.7 MeV [76].…”

“…weak form factor of 132 Xe and 40 Ar at low momentum transfer. Both liquid noble gases, xenon and argon are currently being used as active targets for the detection of neutrinos as well as dark matter particles.…”

“…As in any weak-interaction process, the signal is inherently small and hindered by uncertainties in both atomic and nuclear-structure theory. However, measuring ratios of parity-violating observables along isotopic chains mitigates the sensitivity to atomic theory [40]. As a result, nuclear-structure uncertainties, primarily in the form of neutron radii, remain the limiting factor in the search for new physics [41][42][43][44][45].…”