Probing scalar couplings through tests of the equivalence principle
Jennifer Hechao Chen · CERN Document Server (European Organization for Nuclear Research) · 2005
It could be that our universe contains one or more nearly-massless neutral scalars, either as low energy relics of the UV complete theory, or as dynamical dark energy as called upon by observations. Here we discuss phenomenological ramifications of the coupling of a light scalar to the Standard Model. More precisely, we argue that low energy effects of this scalar are dominated by its linear couplings to gauge field kinetic terms and to fermion mass terms, which could then source fifth forces and induce variations in the 'constants.' We determine the limits on each of these couplings, first by determining the strength of the source from each sector. We find that couplings to the gluon kinetic term and to the strange quark mass term are most constrained by current null results for long range composition dependent fifth forces. Should such as detection occur, it would most likely arise from couplings to these sectors. If we are fortunate enough to make multiple measurements of scalar forces with test body pairs of different compositions, it would be possible to determine each of these couplings. Since any high energy theory containing a light scalar coupled to matter and gauge fields has a four dimensional effective field theory of this form, knowing these coupling would provide an indirect test of theories beyond the Standard Model. In addition to fifth forces arising from spatial variations from the scalar, the time variation of the scalar can drive a time variation of the constants. There has been a claimed detection of a lower value of the fine structure constant at a redshift z ∼ 1.5. We find that this detection of a 0.001% change in the fine structure constant is not possible to explain with a simple quintessence model while remaining consistent with other constraints on the time variation of the fine structure constant. On the other end of the spectrum, a different group has obtained results with tight error bars consistent with no variation. From this, we place even better bounds on scalar couplings to the photon kinetic term.