Nuclear spin-dependent parity violation arises from weak interactions between electrons and nucleons, and from nuclear anapole moments. We outline a method to measure such effects, using a Stark-interference technique to determine the mixing between opposite-parity rotational/hyperfine levels of ground-state molecules. The technique is applicable to nuclei over a wide range of atomic number, in diatomic species that are theoretically tractable for interpretation. This should provide data on anapole moments of many nuclei, and on previously unmeasured neutral weak couplings.PACS numbers: 32.80. Ys, 12.15.Mm, 21.10.Ky Up to now, atomic parity violation (PV) experiments have primarily focused on the PV effect arising from the weak charge of the nucleus Q W [1], a nuclear spinindependent quantity that parameterizes the electroweak neutral coupling between electron axial-and nucleon vector-currents (A e V n ). Here we propose a highly sensitive and widely applicable technique to measure nuclear spin-dependent (NSD) PV effects. Such effects arise primarily from two underlying causes. The first is the nuclear anapole moment, a P-odd magnetic moment induced by weak interactions within the nucleus, which couples to the spin of a penetrating electron [2]. Measurements of anapole moments can provide useful data on purely hadronic PV interactions [3,4]. So far, only one nuclear anapole moment has been measured, in 133 Cs [5]. The second source of NSD-PV is the electroweak neutral coupling between electron vector-and nucleon axialcurrents (V e A n ). This can be parameterized by two constants, C 2u,d , which describe the V e A n couplings to up and down quarks. These are suppressed in the Standard Model (SM), making C 2u,d difficult to measure and at present perhaps the most poorly characterized parameters in the SM [6]. However, because of this suppression, even moderately precise measurements of C 2u,d could be sensitive to new physics at TeV energy scales [7].Our method to measure NSD-PV exploits the properties of diatomic molecules [8,9,10] to amplify the observable signals. Rotational/hyperfine (HF) levels of opposite parity can be mixed by NSD-PV interactions, and are inherently close in energy. Accessible laboratory magnetic fields can Zeeman-shift these levels to degeneracy, dramatically enhancing the state mixing. The matrix element (m.e.) of the NSD-PV interaction can be measured with a Stark-interference technique of demonstrated sensitivity [11]. Use of ground-state molecules leads to enhanced resolution because of their long lifetimes [11,12]. (Two recent papers also proposed measuring NSD-PV in the ground state HF levels of heavy alkali atoms [13].) The technique is applicable to a wide class of molecules and hence to NSD-PV couplings to the variety of nuclei within them. We specifically consider diatomic molecules with a single valence electron in a 2 Σ electronic state. These are the molecular equivalent of alkali atoms, with a simple, regular structure of rotational/HF levels. This makes it possible to re...

Rotational levels of molecular free radicals can be tuned to degeneracy using laboratory-scale magnetic fields. Because of their intrinsically narrow width, these level crossings of opposite-parity states have been proposed for use in the study of parity-violating interactions and other applications. We experimentally study a typical manifestation of this system using 138 BaF. Using a Stark-mixing method for detection, we demonstrate level-crossing signals with spectral width as small as 6 kHz. We use our data to verify the predicted lineshapes, transition dipole moments, and Stark shifts, and to precisely determine molecular magnetic g-factors. Our results constitute an initial proof-ofconcept for use of this system to study nuclear spin-dependent parity violating effects.PACS numbers: 32.80. Ys, 12.15.Mm, 21.10.Ky It has been suggested that diatomic molecules could be used as a system to measure classes of parity-violating (PV) electroweak interactions that are difficult to access through other means [1][2][3]. The level structure of diatomic free radicals systematically makes it possible to tune states of opposite parity to near degeneracy, using a magnetic field such that the Zeeman shift of the electron spin matches the rotational splitting. Near such a level crossing, the mixing of these long-lived states due to nuclear spin-dependent (NSD) PV interactions is greatly enhanced [4]. This should make it feasible to measure small, poorly understood effects such as those due to nuclear anapole moments and axial hadronic-vector electronic electroweak couplings [3,5,6]. This type of level crossing has also been identified as an attractive system for quantum simulations of conical intersections [7] or magnetic excitons [8], and for sensitive detection of electric fields [9].Here we report an experimental study of Zeeman-tuned rotational level crossings in 138 BaF. Using an electric field pulse to induce transitions between the near-degenerate levels, we demonstrate the ability to understand and control the system with energy resolution at the kHz scale, as desired for the measurement of nuclear spin-dependent PV effects in similar systems. By measuring the magnetic field at several crossings, we extract precise values for poorly known magnetic g-factors; also, by studying transfer efficiency vs. electric field, we deduce values for electric dipole matrix elements between the crossing levels, and for off-resonant Stark shifts not previously con- * e-mail: sidney.cahn@yale. 138 Ba is spinless. In the absence of external fields, the lowest energy levels are described by the Hamiltonianwhere N is the rotational angular momentum, S = 1/2 is the electron spin, and n is a unit vector along the internuclear axis ( = 1 throughout) [11,12]. All parameters of H 0 have been precisely measured [13][14][15]. The rotational constant B is much larger than the spin-rotation (SR) constant γ, the hyperfine (HF) constants b and c, and the centrifugal correction constant D; thus N is a good quantum number, with eigenstates of energ...

No abstract