Does An Anomaly In A Proton's Reaction To Electric Fields Reveal BSM Physics?

Overview

When a new experimental anomaly that seems to contradict the Standard Model of Particle Physics fails to secure even ambulance chasing paper writers to propose new physics to explain it, it probably isn't really new physics. 

This is the situation in the case of an anomaly described in a new article in the journal Nature regarding how quarks within protons act in electromagnetic fields of particular strengths in an overall low energy system (the anomalous effect peaks at a momentum exchange scale of about 18.7 MeV, which is about 350 MeV squared).

The money chart of the paper which shows the anomaly is Figure 4 below:

But, for a variety of reasons, there is good reason to think that the anomaly observed, whatever its source, isn't beyond the Standard Model (BSM) physics.

The New Anomaly

New experiments seem to show that the quarks respond more than expected to an electric field pulling on them, physicist Nikolaos Sparveris and colleagues report October 19 in Nature. The result suggests that the strong force isn’t quite as strong as theory predicts.

It’s a finding at odds with the standard model of particle physics, which describes the particles and forces that combine to make up us and everything around us. The result has some physicists stumped about how to explain it — or whether to even try.

At the Thomas Jefferson National Accelerator Facility in Newport News, Va., the team probed protons by firing electrons at a target of ultracold liquid hydrogen. Electrons scattering off protons in the hydrogen revealed how the protons’ quarks respond to electric fields (SN: 9/13/22). The higher the electron energy, the deeper the researchers could see into the protons, and the more the electrons revealed about how the strong force works inside protons.

For the most part, the quarks moved as expected when electric interactions pulled the particles in opposite directions. But at one point, as the electron energy was ramped up, the quarks appeared to respond more strongly to an electric field than theory predicted they would.

But it only happened for a small range of electron energies, leading to a bump in a plot of the proton’s stretch.

“Usually, behaviors of these things are quite, let’s say, smooth and there are no bumps,” says physicist Vladimir Pascalutsa of the Johannes Gutenberg University Mainz in Germany.

Pascalutsa says he’s often eager to dive into puzzling problems, but the odd stretchiness of protons is too sketchy for him to put pencil to paper at this time. “You need to be very, very inventive to come up with a whole framework which somehow finds you a new effect” to explain the bump, he says. “I don’t want to kill the buzz, but yeah, I’m quite skeptical as a theorist that this thing is going to stay.”

It will take more experiments to get theorists like him excited about unusually stretchy protons, Pascalutsa says. He could get his wish if Sparveris’ hopes are fulfilled to try the experiment again with positrons, the antimatter version of electrons, scattered from protons instead.

From Science News discussing primarily the following paper whose abstract and citations are set forth below. 

The Paper and Its Abstract

The abstract of the new article in Nature state:

The visible world is founded on the proton, the only composite building block of matter that is stable in nature. Consequently, understanding the formation of matter relies on explaining the dynamics and the properties of the proton’s bound state. A fundamental property of the proton involves the response of the system to an external electromagnetic field. It is characterized by the electromagnetic polarizabilities that describe how easily the charge and magnetization distributions inside the system are distorted by the electromagnetic field. Moreover, the generalized polarizabilities map out the resulting deformation of the densities in a proton subject to an electromagnetic field. They disclose essential information about the underlying system dynamics and provide a key for decoding the proton structure in terms of the theory of the strong interaction that binds its elementary quark and gluon constituents. Of particular interest is a puzzle in the electric generalized polarizability of the proton that remains unresolved for two decades. Here we report measurements of the proton’s electromagnetic generalized polarizabilities at low four-momentum transfer squared. We show evidence of an anomaly to the behaviour of the proton’s electric generalized polarizability that contradicts the predictions of nuclear theory and derive its signature in the spatial distribution of the induced polarization in the proton. The reported measurements suggest the presence of a new, not-yet-understood dynamical mechanism in the proton and present notable challenges to the nuclear theory.

R. Li et al. Measured proton electromagnetic structure deviates from theoretical predictions. Nature (October 19, 2022). doi: 10.1038/s41586-022-05248-1.

Background Regarding QCD

The paper also provides a general background introduction to Quantum Chromodynamics (QCD) to give the paper context before it launches into its body text. 

Explaining how the nucleons—protons and neutrons—emerge from the dynamics of their quark and gluon constituents is a central goal of modern nuclear physics. The importance of the question arises from the fact that the nucleons account for 99% of the visible matter in the universe. Moreover, the proton holds a unique role of being nature’s only stable composite building block. 

The dynamics of quarks and gluons is governed by quantum chromodynamics (QCD), the theory of the strong interaction. The application of perturbation methods renders aspects of QCD calculable at large energies and momenta— namely at high four-momentum transfer squared (Q2)—and offers a reasonable understanding of the nucleon structure at that scale. Nevertheless, to explain the emergence of the fundamental properties of nucleons from the interactions of its constituents, the dynamics of the system have to be understood at long distances (or low Q2), where the QCD coupling constant αs becomes large and the application of perturbative QCD is not possible. The challenge arises from the fact that QCD is a highly nonlinear theory, because the gluons—the carriers of the strong force—couple directly to other gluons. Here theoretical calculations can rely on lattice QCD, a space-time discretization of the theory based on the fundamental quark and gluon degrees of freedom, starting from the original QCD Lagrangian. 

An alternative path is offered by effective field theories, such as the chiral effective field theory, which use hadronic degrees of freedom and are based on the approximate and spontaneously broken chiral symmetry of QCD. Although steady progress has been made in recent years, we have yet to achieve a good understanding of how the nucleon properties emerge from the underlying dynamics of the strong interaction. To do this, the theoretical calculations require experimental guidance and confrontation with precise measurements of the system’s fundamental properties.

The conclusion states in the body text:

In conclusion, we have studied the proton’s response to an external electromagnetic field and its dependence on the distance scale within the system. We show evidence of a local enhancement in the proton’s electric generalized polarizability that the nuclear theory cannot explain. We provide a definitive answer to the existence of an anomaly in this fundamental property and we have measured with high precision the magnitude and the dynamical signature of this effect. The reported data suggest the presence of a dynamical mechanism in the system that is currently not accounted for in the theory. They pose a challenge to the chiral effective field theory, the prevalent effective theory for the strong interaction, and they serve as high-precision benchmark data for the upcoming lattice quantum chromodynamics calculations. 

The measurements of the proton’s electromagnetic generalized polarizabilities complement the counterpart of the spin-dependent generalized polarizabilities of the nucleon. Together, the two components of the generalized polarizabilities provide a puzzling picture of the nucleon’s dynamics that emerge at long-distance scales. 

Proton has the unique role of being nature’s only stable composite building block. Consequently, the observed anomaly in a fundamental system property comes with a unique scientific interest. It calls for further measurements so that the underlying dynamics can be mapped with precision and highlights the need for an improved theory so that a fundamental property of the proton can be reliably described.

Why Aren't New Physics Explanations Popular For This Kind Of Effect

Neither plain vanilla quantum electrodynamics (QED), which is the Standard Model theory of electromagnetism, nor quantum chromodynamics, which is the Standard Model theory of the strong force, which are the two principle Standard Model theories implicated in this experiment, are popular parts of fundamental physics in which to suggest beyond the Standard Model modifications. 

This is for opposite reasons. 

QED is validated at such extreme precision (often at parts per billion levels or more) with reasonably moderate amounts of calculations, which have been exhaustively tested, that there is no wiggle room for significant new physics in the context of something as ordinary as a proton in an electric field.

QCD, meanwhile, is so hard to calculate with that one of perhaps half a dozen main workable operationalizations of it must be used in practice, and the precision of those calculations is so low (general at the 10% to parts per thousand level) and is so inconsistent between methods, that it is almost more art than science to do QCD calculations that reproduce real world observations reasonably well. As a result theoretical calculation uncertainties frequently swamp any proposed new physics effects in a theory with few moving parts that can be easily manipulated by a physicist to explain observational anomalies.

So, even if there is an anomalous effect that is real and not just due to systemic error or statistical variation, it is hard to say that this is because of new physics as opposed to an oversimplification of true QCD dynamics made in the interests of actually being able to do QCD calculations.

Instead, theorists proposing new theories are far more fond of introducing new particles and forces, outside the three Standard Model forces with its dozen kinds of fundamental fermions, three massive weak force bosons, Higgs boson, photons, and gluons. 

But introducing a new particle into the dynamics of the proton, the most carefully studied composite particle made of quarks that exists, that has never been observed in any other context over many decades of ultra-precise experimental measurements of its properties is very hard to do without contradicting other experiments that should also be affected by the same new particle that weren't observed in the other experiments.

Also, if they tweak any Standard Model force, the weak force whose interactions require ten of the Standard Model's parameters to describe, is more attractive that the electromagnetic or strong forces, which have fundamentally very simple structures even when the application of those forces to physical situations is complicated. 

But this experiment is a poor candidate to demonstrate a weak force effect since it doesn't involve any kind of particle decay and because it isn't small enough for the weak force to be a good candidate to explain it.

Tweaks to General Relativity which is the state of the art theory of gravity are also popular with theorists, but this isn't part of the Standard Model, and gravitational effects are negligible at the scale of a single proton.