Latest discoveries about nucleus structure

The initial rendition of this article was published on Quanta Magazine.

A recent assessment of the strong nuclear force, responsible for binding protons and neutrons, reaffirms prior indications of an unsettling reality: our comprehension of even the most basic nuclear systems remains lacking in solid theoretical foundation.

In an endeavor to examine the strong nuclear force, scientists directed their focus towards the helium-4 nucleus, encompassing two protons and two neutrons. Upon subjecting helium nuclei to excitation, they observed an unconventional behavior—rather than inflating as anticipated, the nuclei expanded beyond projected limits before rupture. This expansion, termed the form factor, was measured to be twice the size of theoretical projections.

Sonia Bacca, a theoretical physicist from the Johannes Gutenberg University of Mainz and a contributor to the paper published in Physical Review Letters, expressed her view on the matter, stating, "The theoretical framework should be applicable in this context." However, she noted that the observed disparity has left the scientific community puzzled.

According to researchers, the engorged helium nucleus assumes the role of a miniature laboratory for the purpose of scrutinizing nuclear theories. Analogous to a microscope, it possesses the capacity to accentuate inadequacies within theoretical computations. Physicists postulate that specific anomalies within this inflationary phenomenon confer upon it an exceptional susceptibility to even the most minute constituents of the nuclear force—a scale of influence that is typically disregarded. The extent of nucleus inflation further correlates with the pliability of nuclear substance, a characteristic that yields illuminative perspectives into the enigmatic cores of neutron stars. However, prior to elucidating the compressive nature of substance within neutron stars, physicists must initially unravel the origins of the pronounced disparities in their prognostications.

Bira van Kolck, a nuclear theorist affiliated with the French National Center for Scientific Research, remarked that Bacca and her associates have brought to light a substantial quandary within the realm of nuclear physics. He noted that they have unearthed a notable case where the preeminent comprehension of nuclear interactions, encapsulated within the construct of chiral effective field theory, has encountered limitations.

According to van Kolck, this transition serves to accentuate the challenges inherent in the theory, challenges that might otherwise remain comparatively inconspicuous.

The Powerful Nuclear Cohesive Force

Atomic nucleons, namely protons and neutrons, are bound in unity through the agency of the potent strong force. However, it is noteworthy that the formulation of the strong force theory was not primarily intended for elucidating the mechanism of nucleonic cohesion. Rather, its initial application pertained to the explanation of the composite nature of protons and neutrons, which are intricately composed of fundamental entities referred to as quarks and gluons.

Over an extended duration, physicists encountered challenges in harnessing the strong force as a tool for comprehending the adhesive properties intrinsic to protons and neutrons. An impediment arose from the enigmatic characteristics of the strong force itself, characterized by an unconventional trait—the augmentation of its intensity as the separation distance increases, in contrast to the customary attenuation. This distinctive attribute hindered the application of their habitual computational strategies. In the realm of particle physics, the conventional approach involves the dissection of a force into more manageable and approximate components, ranking these components in terms of significance, and subsequently relegating the less crucial elements to neglect. Regrettably, this pragmatic approach proved unsuitable for the formidable complexities inherent in the strong force.

Subsequently, in 1990, Steven Weinberg achieved a breakthrough by establishing a bridge between the realm of quarks and gluons and the cohesive nature of atomic nuclei. The ingenious approach employed involved the application of an effective field theory—an approach that offers requisite detail only to match the specific dimensions (or energy levels) pertinent to natural phenomena. To characterize the dynamics of a nucleus, an in-depth understanding of quarks and gluons is not requisite. Instead, in these particular scales, a novel effective force comes to the fore—the potent strong nuclear force—transmitted amidst nucleons via the intermediary exchange of pions.

Weinberg's contributions were instrumental in providing physicists with insights into the genesis of the strong nuclear force arising from the fundamental strong force. Furthermore, his work facilitated the adoption of the conventional strategy of approximated contributions in theoretical computations. The theory he advanced, known as chiral effective theory, has now attained a pervasive recognition as the preeminent framework for conducting calculations pertaining to the forces dictating the behaviors inherent to atomic nuclei, as noted by Bacca.

During 2013, Bacca leveraged the framework of effective field theory to forecast the degree of enlargement that an energized helium nucleus would experience. However, upon juxtaposing her computational predictions with data gleaned from experiments conducted during the 1970s and 1980s, Bacca identified a substantial discordance. Her projections indicated a level of swelling that was comparatively lower than the quantified magnitudes, yet the extensive range of uncertainty inherent in the experimental data hindered a definitive conclusion.

Expanding Nuclei

Following the initial indication of an issue, Bacca initiated a process of prompting her associates at the Mainz facility to replicate the experiments conducted over decades past. Given their access to more refined instruments and enhanced measurement capabilities, these endeavors were poised to deliver heightened precision. These deliberations subsequently laid the foundation for a novel collaborative effort: Simon Kegel and his team would embark upon a modernization of the experimental protocols, while Bacca and her associates would concurrently undertake the task of comprehending any potential divergence, should it become apparent.

Within their experimental undertaking, Kegel and his team activated the nuclei by subjecting a stream of electrons to a reservoir of chilled helium gas. In the event that an electron traversed the vicinity of a helium nucleus, it imparted a portion of its superfluous energy to the constituents—protons and neutrons—prompting an expansion of the nucleus. This augmented configuration proved evanescent, as the nucleus swiftly relinquished the binding of one of its protons, culminating in its transformation into a hydrogen nucleus comprising two neutrons, alongside a liberated proton.

Similar to other nuclear transitions, the nucleus can undergo an expansion solely with a specific quantum of injected energy. By systematically altering the momentum of electrons and meticulously observing the ensuing response of helium, scientists were able to ascertain the extent of this expansion. The research team subsequently embarked on a comparative investigation, contrasting this shift in the nucleus's configuration—referred to as the form factor—with a spectrum of theoretical calculations. Surprisingly, none of these theoretical frameworks concurred with the empirical observations. Intriguingly, the calculation that exhibited the closest conformity employed a rudimentary representation of the nuclear force, a deviation from the principles of chiral effective field theory.

"The outcomes were entirely unforeseen," Bacca commented.

Other scholars share the same sense of bewilderment. "The experiment has been meticulously executed and is characterized by its clarity," remarked Laura Elisa Marcucci, a physicist associated with the University of Pisa in Italy. However, she stipulated that an incongruity exists between the experiment and theoretical projections, necessitating the recognition that an error may exist in either of the two.

Harmonizing the Force

Upon retrospection, physicists possessed multiple indications to anticipate that this uncomplicated measurement would delve into the boundaries of our comprehension pertaining to nuclear forces.

To start, this system showcases a specific intricacy. The energy needed to induce the momentarily expanded state of the helium nucleus—precisely the state researchers wish to examine—exists in a precise position slightly above the energy threshold for proton emission and just beneath the same threshold for neutron release. This intricate energy configuration introduces challenges into the arena of accurate calculation.

The second factor is tied to Weinberg's effective field theory. Its efficacy stemmed from the ability it conferred upon physicists to neglect elements of lesser import within the equations. Van Kolck contends that certain facets, conventionally relegated as secondary and habitually overlooked, bear substantive importance. The magnification granted by this specific helium measurement, he asserted, is elucidating this fundamental omission.

"I must exercise restraint in criticism, given the inherent complexity of these calculations," he appended. "Their endeavor represents a commendable effort to navigate challenging terrain."

Several research cohorts, van Kolck's contingent included, are in the process of duplicating Bacca's calculations with the aim of identifying the sources of error. The potential avenue for resolution might entail an expansion of terms within the nuclear force approximation. Conversely, it remains plausible that the phenomenon of helium nuclei expanding in size has laid bare a fundamental vulnerability in our comprehension of the nuclear force.

"We have illuminated the enigma, yet regrettably, the enigma remains unsolved," remarked Bacca. "Not as of now."

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