Atomic precision tests and light scalar couplings

Physical Review D

In a large class of scalar-tensor theories that are potential candidates for dark energy, a nonminimal coupling between the scalar and the photon is possible. The presence of such an interaction grants us the exciting prospect of directly observing dark sector phenomenology in the electromagnetic spectrum. This paper investigates the behavior of one-electron atoms in this class of modified gravity models, exploring their viability as probes of deviations from general relativity in both laboratory and astrophysical settings. Building heavily on earlier studies, our main contribution is threefold: A thorough analysis finds additional finestructure corrections previously unaccounted for, which now predict a contribution to the Lamb shift that is larger by nearly 4 orders of magnitude. In addition, they also predict a scalar-mediated photon-photon interaction, which now constrains the scalar's coupling to the photon independently of the matter coupling. This was not previously possible with atomic precision tests. Our updated constraints are log 10 β m ≲ 13.4 and log 10 β γ ≲ 19.0 for the matter and photon coupling, respectively, although these remain uncompetitive with bounds from other experiments. Second, we include the effects of the nuclear magnetic moment, allowing for the study of hyperfine structure and the 21 cm line, which hitherto have been unexplored in this context. Finally, we also examine how a background scalar leads to equivalence principle violations.

Physical Review D

Physical Review D

Most theories that predict time and/or space variation of fundamental constants also predict violations of the Weak Equivalence Principle (WEP). Khoury and Weltmann proposed the chameleon model in 2004 and claimed that this model avoids experimental bounds on WEP. We present a contrasting view based on an approximate calculation of the two body problem for the chameleon field and show that the force depends on the test body composition. Furthermore, we compare the prediction of the force on a test body with Eötvös type experiments and find that the chameleon field effect cannot account for current bounds.

Physical Review D

In a previous paper [1] we pointed out some shortcomings of the standard approach to chameleon theories consisting in treating the small bodies used to test the Weak Equivalence Principle (WEP) as test particles, whose presence do not modify the chameleon field configuration. In that paper we developed an alternative method to determine the relevant field configuration which takes into account the influence of both test and source bodies, and computed the chamaleon mediated force. Relying on that analysis we showed that the effective acceleration of test bodies is composition dependent even when the model is based on universal couplings. In this paper, we improve our method by using a more suitable approximation for the effective chameleon potential in situations where the bodies are in the so called "thick shell regime". We then find new and more restrictive bounds on the model's parametres by confronting the new theoretical predictions with the empirical bounds on Eötvös parameter comming from the Lunar Laser Ranging experiments.

Physical Review Letters, 2011

Physical Review Research, 2020

We consider the role of high-lying Rydberg states of simple atomic systems such as 1 H in setting constraints on physics beyond the Standard Model. We obtain highly accurate bound states energies for a hydrogen atom in the presence of an additional force carrier (the energy levels of the Hellmann potential). These results show that varying the size and shape of the Rydberg state by varying the quantum numbers provides a way to probe the range of new forces. By combining these results with the current state-of-the-art QED corrections, we determine a robust global constraint on new physics that includes all current spectroscopic data in hydrogen. Lastly we show that improved measurements that fully exploit modern cooling and trapping methods as well as higher-lying states could lead to a strong, statistically robust global constraint on new physics based on laboratory measurements only.

Physical Review D, 2015

The chameleon mechanism enables a long range fifth force to be screened in dense environments when non-trivial self interactions of the field cause its mass to increase with the local density. To date, chameleon fifth forces have mainly been studied for spherically symmetric sources, however the non-linear self interactions mean that the chameleon responds to changes in the shape of the source differently to gravity. In this work we focus on ellipsoidal departures from spherical symmetry and compute the full form of the chameleon force, comparing it's shape dependence to that of gravity. Enhancement of the chameleon force by up to 40% is possible when deforming a sphere to an ellipsoid of the same mass, with an ellipticity 0.99.

Physical Review Letters, 2012

The possible involvement of weakly bound three-body systems in the muonic hydrogen spectroscopy experiment [1], which could resolve the current discrepancy between determinations of the proton radius, is investigated. Using variational calculations with complex coordinate rotation, it is shown that the pµe ion, which was recently proposed as a possible candidate [2], has no resonant states in the energy region of interest. QED level shifts are included phenomenologically by including a Yukawa potential in the three-body Coulomb Hamiltonian before diagonalization. It is also shown that the ppµ molecular ion cannot play any role in the observed line.

Nature Physics, 2018

The standard model of cosmology provides a robust description of the evolution of the universe. Nevertheless, the small magnitude of the vacuum energy is troubling from a theoretical point of view 9. An appealing resolution to this problem is to introduce additional scalar fields. However, these have so far escaped experimental detection, suggesting some kind of screening mechanism may be at play. Although extensive exclusion regions in parameter space have been established for one screening candidate-chameleon fields 10,17-another natural screening mechanism based on spontaneous symmetry breaking has also been proposed, in the form of symmetrons 11. Such fields would change the energy of quantum states of ultra-cold neutrons in the gravitational potential of the earth. Here we demonstrate a spectroscopic approach based on the Rabi resonance method that probes these quantum states with a resolution of Δ = 2 × 10 −15 eV. This allows us to exclude the symmetron as the origin of Dark Energy for a large volume of the three-dimensional parameter space. Resonance spectroscopy-originally introduced by I. Rabi 1 as a "molecular beam resonance method"-has evolved into an indispensable method for precision experiments with two-levelsystems. Here, the energy difference between two quantum states translates via the Planck-Einstein relation = ℎ into a frequency, which can be measured with unprecedented accuracy. Rabi's experiment has led to new insights into physics, chemistry and biology providing detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. This work, realized by the qBounce collaboration, extends the quantum technique of Rabi spectroscopy to the gravitational sector. The experiment measures transition frequencies of bound quantum states of neutrons in the gravitational field of the earth. These discrete quantum states occur when very slow ultra-cold neutrons (UCN) totally reflect on perfectly polished horizontal surfaces. The typical spatial extent of the corresponding wave functions is of order of ten microns. The eigenenergies are in the pico-eV range and depend on the local acceleration g, the neutron mass m, and the reduced Planck constant ℏ. Furthermore, any two states can be treated as an effective two-level system. This offers the possibility of applying resonance spectroscopy techniques to test gravity at short distances.

Physical Review A

When collecting spectroscopic data on at least four isotopes, nonlinearities in the King plot are a possible sign of physics beyond the standard model. In this work, an improved approach to the search for hypothetical new interactions with isotope-shift spectroscopy of few-electron ions is presented. Careful account is taken of the small nuclear corrections to the energy levels and the gyromagnetic factors, which cause deviations from King linearity within the standard model and are hence a possible source of confusion. In this approach, the experimental King nonlinearity is not compared to the vanishing prediction of the standard model at the leading order, but to the calculated full standard model contribution to King nonlinearity. This makes searching for beyond-the-standard-model physics with King linearity analysis possible in a high-precision experimental regime, avoiding confusion. The bounds which can be set on beyond-the-standard-model parameters remain limited by the uncertainties on the small standard model nuclear corrections which cause King nonlinearity. Direct comparison between theory and experiment on a single pair of isotopes is advocated as a more suitable approach for few-electron ions.

Physical Review D, 2014

We combine constraints from analyticity with experimental electron-proton scattering data to determine the proton magnetic radius without model-dependent assumptions on the shape of the form factor. We also study the impact of including electron-neutron scattering data, and ππ → NN data. Using representative datasets we find for a cut of Q 2 ≤ 0.5 GeV 2 , r p M = 0.91 +0.03 −0.06 ± 0.02 fm using just proton scattering data; r p M = 0.87 +0.04 −0.05 ± 0.01 fm adding neutron data; and r p M = 0.87 +0.02 −0.02 fm adding ππ data. We also extract the neutron magnetic radius from these data sets obtaining r n M = 0.89 +0.03 −0.03 fm from the combined proton, neutron, and ππ data.