Wednesday, September 15, 2021

Experimental Bounds On A Fifth Force

An ultra-precise measurement of neutrons traveling through silicon crystals has rivaled prior record precision measurement of the neutron charge radius, revealed unexpected things about how atoms in a silicon crystal behave, and established upper and lower bounds on the range and mass of a hypothetical fifth force in addition to the electromagnetic force, the strong force, the weak force, and gravity.

To be clear, there is no positive evidence that the fifth force considered in this study exists at all, it only addresses what kinds of fifth forces can be ruled out experimentally.

"Generally, if there's a force carrier, the length scale over which it acts is inversely proportional to its mass," meaning it can only influence other particles over a limited range, Heacock said. But the photon, which has no mass, can act over an unlimited range. "So, if we can bracket the range over which it might act, we can limit its strength." The scientists' results improve constraints on the strength of a potential fifth force by tenfold over a length scale between 0.02 nanometers (nm, billionths of a meter) and 10 nm, giving fifth-force hunters a narrowed range over which to look.

From Science Daily reporting on the following NIST study: 

Benjamin Heacock, et al., "Pendellösung interferometry probes the neutron charge radius, lattice dynamics, and fifth forces." 373 (6560) Science 1239 (2021) DOI: 10.1126/science.abc2794.

The constrained length scale would correspond to a similar order of magnitude to the distance between atoms in molecules and crystals.

A force carrier with a mass of a pion (about 139 MeV) has a range of about 1 fm (10^-15 meters). The new experimental result implies that the carrier for a fifth force, if there is one, must be on the order of 10^4 to 10^7 times less massive than a pion to have the proportionately longer range that is not ruled out. Thus, a force carrier (presumably a boson) for a fifth force must have a mass on the order of 6 eV to 3 keV, if it exists at all. 

A hypothetical fifth force carrier boson would have to be much less massive than any fundamental massive boson in the Standard Model the lightest of which is about 80.4 GeV, much less massive than any composite particle bound by gluons (i.e. any hadron) the lightest of which is about 139 MeV, much less massive than any quark the lightest of which is about 2 MeV, and much less massive than an electron (or other charged leptons which are more massive than an electron) which is about 0.511 MeV.

But, this is significantly more massive than any neutrino the most massive of which appears to have a mass of less than 0.1 eV, and of course, much more massive than the zero rest mass of an isolated gluon (something not observed in Nature outside of hadrons that actually has an effective range in most circumstances on the order of 1 fm or less), a photon (with infinite range), or a hypothetical graviton (with infinite range).

At the low end towards 6 eV, this is a mass range that has been considered for hypothetical sterile neutrinos that oscillate with the three active neutrinos with low probabilities (an observationally disfavored possibility). 

At the high end towards 3 keV, this is in the same ballpark as hypothetical "warm dark matter" particles (a possibility that also has observational evidence that disfavors it).

This experiment rules out (within the assumptions it makes for setting these bounds), for example, a fifth force carried by a 17 MeV mass boson that was hypothesized most recently in November of 2019 (which would have an effective range on the order of 8 fm, which would reach from one proton or neutron to most other ones in most atomic nuclei, but not much further).

As an aside, the range of the weak force, by this (oversimplified) analysis, is roughly 0.0017 fm, which makes it very nearly a contact force (which is how it was originally modeled by Fermi) and is smaller than the size of a proton or neutron.

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