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Register a new userDirect Measurement of the Forbidden $2^{3!}S_1 rightarrow 3^{3!}S_1$ Atomic Transition in Helium
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Kieran Francis Thomas
Publication date
2020
fields
Physics
and research's language is
English
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We present the detection of the highly forbidden $2^{3!}S_1 rightarrow 3^{3!}S_1$ atomic transition in helium, the weakest transition observed in any neutral atom. Our measurements of the transition frequency, upper state lifetime, and transition strength agree well with published theoretical values, and can lead to tests of both QED contributions and different QED frameworks. To measure such a weak transition, we developed two methods using ultracold metastable ($2^{3!}S_1$) helium atoms: low background direct detection of excited then decayed atoms for sensitive measurement of the transition frequency and lifetime; and a pulsed atom laser heating measurement for determining the transition strength. These methods could possibly be applied to other atoms, providing new tools in the search for ultra-weak transitions and precision metrology.
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Despite quantum electrodynamics (QED) being one of the most stringently tested theories underpinning modern physics, recent precision atomic spectroscopy measurements have uncovered several small discrepancies between experiment and theory. One particularly powerful experimental observable that tests QED independently of traditional energy level measurements is the `tune-out frequency, where the dynamic polarizability vanishes and the atom does not interact with applied laser light. In this work, we measure the `tune-out frequency for the $2^{3!}S_1$ state of helium between transitions to the $2^{3!}P$ and $3^{3!}P$ manifolds and compare it to new theoretical QED calculations. The experimentally determined value of $725,736,700,$$(40_{mathrm{stat}},260_{mathrm{syst}})$ MHz is within ${sim} 2.5sigma$ of theory ($725,736,053(9)$ MHz), and importantly resolves both the QED contributions (${sim} 30 sigma$) and novel retardation (${sim} 2 sigma$) corrections.
The workhorse of atomic physics, quantum electrodynamics, is one of the best-tested theories in physics. However recent discrepancies have shed doubt on its accuracy for complex atomic systems. To facilitate the development of the theory further we aim to measure transition dipole matrix elements of metastable helium (He*) (the ideal 3 body test-bed) to the highest accuracy thus far. We have undertaken a measurement of the `tune-out wavelength which occurs when the contributions to the dynamic polarizability from all atomic transitions sum to zero; thus illuminating an atom with this wavelength of light then produces no net energy shift. This provides a strict constraint on the transition dipole matrix elements without the complication and inaccuracy of other methods. Using a novel atom-laser based technique we have made the first measurement of the tune-out wavelength in metastable helium between the $3^{3}P_{1,2,3}$ and $2^{3}P_{1,2,3}$ states at 413.07(2) nm which compares well with the predicted valuecite{Mitroy2013} of 413.02(9) nm. We have additionally developed many of the methods necessary to improve this measurement to the 100 fm level of accuracy where it will form the most accurate determination of transition rate information ever made in He* and provide a stringent test for atomic QED simulations. We believe this measurement to be one of the most sensitive ever made of an optical dipole potential, able to detect changes in potentials of $sim$200 pK and is widely applicable to other species and areas of atom optics.
We study the mass spectra of the $NOmega$ dibaryons in the $^{3}S_1$ and $^{5}S_2$ channels with $J^{P}=1^{+}$ and $2^{+}$ respectively, by using the method of QCD sum rules. We construct two dibaryon interpolating currents in the molecular picture and calculate their correlation functions and spectral densities up to dimension-16 condensates. Our results indicate that there may exist an $NOmega$ dibaryon bound state in the $^{5}S_2$ channel with a binding energy of about $21 mathrm{MeV}$. The masses of the $^{3}S_1$ $NOmega$ dibaryons with $J^{P}=1^{+}$ are predicted to be higher than the $NOmega$ and $LambdaXi$ thresholds, and thus can decay into these final states directly in S-wave. The $NOmega (^{5}S_2)$ dibaryon bound state can decay into the octet-octet final states $LambdaXi$ and $SigmaXi$ in D-wave via the quark rearrangement mechanism. The existence of these $NOmega$ dibaryons may be identified in the relativistic heavy-ion collision experiments in the future.
We have completed a measurement of the $(6s^26p^2), ^3!P_0 rightarrow , ^3!P_2$ 939 nm electric quadrupole ($E2$) transition amplitude in atomic lead. Using a Faraday rotation spectroscopy technique and a sensitive polarimeter, we have measured this very weak $E2$ transition for the first time, and determined its amplitude to be $langle ^3!P_2 || Q || ^3!P_0 rangle$ = 8.91(9) a.u.. We also present an ab initio theoretical calculation of this matrix element, which agrees with experiment at the 0.5% level. We heat a quartz vapor cell containing $^{208}$Pb to between 800 and 940 $^{circ}$C, apply a $sim ! 10 , {rm G}$ longitudinal magnetic field, and use polarization modulation/lock-in detection to measure optical rotation amplitudes of order 1 mrad with noise near 1 $mu$rad. We compare the Faraday rotation amplitude of the $E2$ transition to that of the $^3!P_0 -, ^3!P_1$ 1279 nm magnetic dipole ($M1$) transition under identical sample conditions.
The present knowledge of Lamb shift, fine-, and hyperfine structure of the 2S and 2P states in muonic helium-3 ions is reviewed in anticipation of the results of a first measurement of several $mathrm{2Srightarrow2P}$ transition frequencies in the muonic helium-3 ion, $mathrm{mu^3He^+}$. This ion is the bound state of a single negative muon $mu^-$ and a bare helium-3 nucleus (helion), $mathrm{^3He^{++}}$. A term-by-term comparison of all available sources, including new, updated, and so far unpublished calculations, reveals reliable values and uncertainties of the QED and nuclear structure-dependent contributions to the Lamb shift and the hyperfine splitting. These values are essential for the determination of the helion rms charge radius and the nuclear structure effects to the hyperfine splitting in $mathrm{mu^3He^+}$. With this review we continue our series of theory summaries in light muonic atoms; see Antognini et al., Ann. Phys. 331, 127 (2013), Krauth et al., Ann.Phys. 366, 168 (2016), and Diepold et al., ArXiv 1606.05231 (2016).
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