Antihydrogen and Fundamental Physics

Abstract

The recent advent of high precision antihydrogen (H) spectroscopy opens the way to stringent experimental tests of the fundamental principles underlying particle physics and general relativity (GR), such as Lorentz and CPT invariance and the Einstein Equivalence Principle (EEP), on pure antimatter systems. In this paper, the nature and implications of these tests is investigated, with special reference to the ALPHA antihydrogen programme at CERN. This is underpinned by a theoretical review of the role of antiparticles, causality and fundamental symmetries in relativistic quantum field theory (QFT) and the theory of time measurement in GR. Low-energy effective theories which break Lorentz and CPT symmetry, or the Strong Equivalence Principle (SEP), are then introduced, together with a review of several 'fifth force' scenarios involving new long-range forces which would effectively violate the universality of free-fall, or Weak Equivalence Principle (WEPff). The possible role of CPT violation in determining the matter-antimatter asymmetry of the Universe is discussed. Explicit calculations are given for the dependence on possible Lorentz and CPT violating couplings of the transition frequencies amongst the 1S, 2S and 2P hyperfine states measured in the magnetic field of the ALPHA trap and the resulting bounds are compared with existing limits. An analysis of the implications for the EEP of current free-fall and spectroscopic measurements with antihydrogen is presented and existing and potential bounds on WEPff and the universality of clocks (WEPc) are derived, together with constraints on fifth forces. Future prospects for high-precision antihydrogen spectroscopy, free-fall and gravitational redshift experiments, and antiatom matter-wave interferometry are described and experimental possibilities involving other antimatter species are briefly outlined.

Snowmass White Paper: Precision Studies of Spacetime Symmetries and Gravitational Physics

“…The availability of cold antiprotons at CERN's Antiproton Decelerator has paved the way for unprecedented studies of antihydrogen. One class of these is concerned with antihydrogen precision spectroscopy: the ALPHA and ASACUSA experiments are designed for such antihydrogen measurements, including 1S-2S, 1S-2P, and hyperfine spectroscopy, and compare these to the corresponding frequencies in ordinary hydrogen for a direct CPT test [48]. These efforts are well underway with the completion of various extraordinary milestones, such as a 1S-2S measurement just three orders of magnitude shy of the corresponding accuracy in hydrogen.…”

“…Another class of antihydrogen experiments seeks to study the interaction of antimatter with gravity. For example, AEgIS, ALPHA-g, and GBAR at CERN will be employing complementary methods to measure the rate of free fall of antihydrogen in the gravitational field [48], and a proposal for a further antimatter gravity experiment at Fermilab exists [56]. Both spectroscopic and free-fall efforts are currently straining at the leash to resume antihydrogen studies as the current Long Shutdown 2 at the LHC draws to a close and the new Extra-Low Energy Antiproton Ring ELENA goes into full operation.…”

Fundamental Physics in Small Experiments

“…Cold antiprotons at CERN's Antiproton Decelerator have been essential for precision measurements with antihydrogen and pave the way for various CPT tests. The ALPHA and ASACUSA experiments will perform antihydrogen hyperfine spectroscopy for direct CPT tests through comparison with the same measurements in hydrogen [152]. The anticipated precision in terms of the mass-antimass difference will even exceed neutral-kaon interferometry, the particle-physics standard for CPT tests.…”

“…The anticipated precision in terms of the mass-antimass difference will even exceed neutral-kaon interferometry, the particle-physics standard for CPT tests. Other antihydrogen efforts like AEgIS, ALPHA-g, or GBAR at CERN [152] or a proposed future experiment at Fermilab [153], will study CPT symmetry via the interaction of antimatter with gravity by measuring the free-fall of antihydrogen. Both spectroscopic and gravitational methods will be able to provide qualitatively new Lorentz and CPT tests.…”

An ion trap source of cold atomic hydrogen via photodissociation of the BaH+ molecular ion

“…By comparing it to antihydrogen, hydrogen can be used to test matter-antimatter symmetries with high precision [8]. In the last 10 years, antihydrogen physics has advanced to the point where more than 1000 atoms can be simultaneously trapped [9], cooled [10], and probed with lasers [11,12] and microwaves [13].…”