Putting the theory of chiral perturbations to the test

Scientists put the chiral perturbation theory to the test with a set of new experiments that helped define the fundamental properties of protons.

Protons – the building blocks of ordinary matter – are composite objects made up of quarks bound together by a powerful force mediated by gluons. These elementary particles and their interactions are accurately described by a theory called quantum chromodynamics, the modern understanding of which allows scientists to perform calculations if the relative velocities of the interacting particles are large enough.

However, in most interactions between protons and other strongly interacting particles called hadrons, their motion is relatively “slow”, so other theoretical approaches must be used. Their need is even more urgent if one is interested in the details of the distribution of quarks and gluons inside hadrons.

The most developed of these approaches is chiral perturbation theory, which allows the results of hadron interaction to be quantitatively described and properties such as mass, electric charge, and an internal angular momentum called spin to be calculated.

However, there is a problem: despite the fact that this theory is inspired by quantum chromodynamics, it is not rigorously derived from it and its predictions therefore require independent empirical verification.

Putting theory into practice

To test some of them, a team of physicists from collaborating US institutions along with colleagues from other countries carried out experimental studies of the structure of the proton at the Thomas Jefferson National Accelerator Facility. The results of the studies were recently published in the journal Natural Physics.

The first proton parameter the researchers measured was spin polarizability, which describes a spin-dependent response of the proton to an electromagnetic field and has never been measured before. The second parameter was color polarizability, which contains information about how a proton’s spin affects the particle’s strong interaction features and has never been measured under current experimental conditions.

To measure these quantities, a standard method for elementary particle physics was used in which particles collide and information about their properties is extracted from observed scattering data. The protons studied were in a solid ammonia (NH3) target placed in a strong magnetic field and their structure probed with an incident electron beam, which scattered away from the target.

The measured polarizabilities were then compared to predictions made using chiral perturbation theory.

How did the theory of chiral perturbations accumulate?

Since it is not as fundamental as quantum chromodynamics, this theory is actually a set of models whose parameters differ according to assumptions about the subtle properties of the strong interaction. Currently, there are two main such models that predict different values ​​for proton polarizabilities.

The scientists’ measurements determined which of the two models better described the structure of the strongly interacting particles, and indeed, the predictions of one of them matched the experimental data very well.

Despite the importance of their results, the scientists note that they still have a lot of work to do. The quantities they studied were measured at three fixed collision energies, but chiral perturbation theory predicts these values ​​for a fairly wide energy range, for which it would be interesting to compare them with experimental data.

Despite the complexity of conducting such experiments and the time it takes to analyze their results, the researchers say they will complete the studies in the future.

Reference: D. Ruth, et al., Proton spin structure and generalized polarizabilities in the strong quantum chromodynamics regime, Physics of Nature (2022). DOI: 10.1038/s41567-022-01781-y

Image credit: Tareq Ajalyakin on Unslpash

Sharon D. Cole