Project title: Atom interferometry to investigate screened fifth force

Superviors: Ben Sauer and Mike Tarbutt

Project description:
There are many alternatives to explain the observed accelerating universe without involving vacuum energy that needs fine-tuning to make the cosmological constant as small as observed.  [1]Most of those models can behave as a scalar field that interacts with standard model particles [2].  As a result, the coupling between the unknown scalar field and matter manifests as a fifth force, but this undetected force could have been hidden by some screening mechanism. [3] According to a hypothesised screening channel called chameleon mechanism, the effective mass of the scalar field would depend on the ambient matter density. [4, 5] Consequently, this fifth force would be extremely short-range in an environment of high mass density such that it has no effect between two macroscopic objects in this environment. However, its range might not be so short in an ultra-vacuum system where the background matter density is low. [6, 7] In this project, I am aiming to improve an experiment that has measured the acceleration of a cloud of cold atoms near a small aluminium ball in a vacuum chamber. The acceleration of atoms is obtained with light-pulse atom interferometry, which can be used to deduce whether the force between atoms (microscopic test mass) and a ball (macroscopic source mass) violates the Newtonian predication in this ultra-vacuum condition. The preliminary result was limited its signal to noise ratio [8], where the dominate noise sources were the background scattering of light on atoms, which limited the contrast of interference, and the number of atoms to perform atom interferometry, which limited the statistical error of each measurement. I am aiming to reduce these noise sources and increase the sensitivity of the previous interferometer by a factor of 10-100 to further investigate the parameter space where a fifth force hidden by chameleon mechanism might still lurk.

[1] Copeland, Edmund J., Mohammad Sami, and Shinji Tsujikawa. "Dynamics of dark energy." International Journal of Modern Physics D 15.11 (2006): 1753-1935.

[2] Upadhye, Amol. "Dark energy fifth forces in torsion pendulum experiments." Physical Review D 86.10 (2012): 102003.

[3] Burrage, Clare, and Jeremy Sakstein. "Tests of chameleon gravity." Living reviews in relativity 21.1 (2018): 1.

[4] Khoury, Justin, and Amanda Weltman. "Chameleon fields: Awaiting surprises for tests of gravity in space." Physical Review Letters 93.17 (2004): 171104.

[5] Khoury, Justin. "Chameleon field theories." Classical and Quantum Gravity 30.21 (2013): 214004.

[6]Burrage, Clare, Edmund J. Copeland, and E. A. Hinds. "Probing dark energy with atom interferometry." Journal of Cosmology and Astroparticle Physics 2015.03 (2015): 042.

 [7]Elder, Benjamin, et al. "Chameleon dark energy and atom interferometry." Physical Review D 94.4 (2016): 044051.

 [8] Sabulsky, D. O., et al. "Experiment to detect dark energy forces using atom interferometry." Physical review letters 123.6 (2019): 061102.