Helium is the simplest element in the periodic table with more than one particle in its nucleus, yet state of the art theory and experiments on it don't add up.

Figure 1 | Measuring the size of the helium nucleus. a, The nucleus of the helium atom, also known as the α-particle, comprises two protons and two neutrons, surrounded by two electrons.

There Aldo Antognini's team generates so-called muonic helium-3, in which the two electrons of the helium atom are replaced by an elementary particle called a muon. This allows the nuclear radius ...

A small research team managed to put an antiproton in orbit around the nucleus of a helium atom that was part of some liquid helium chilled down to where it acted as a superfluid.

Based on calculations using the laser frequency, the researchers found that the helium nucleus' charge radius is 1.6782 femtometers. Measurements made by bouncing electrons off the nucleus ...

An international team of researchers has established that the radius of the helium nucleus is 1.67824 femtometers – or 1.67824×10-15 meters (5.506×10-15 feet). ... The helium atom, ...

Within a helium-4 nucleus, both protons and neutrons exist in closed shells of two nucleons each – resulting in a highly stable nucleus. Helium-8 has four extra neutrons that form a “skin” around a ...

A helium atom consists of a nucleus containing two positively charged protons and two neutrons, encircled by two orbiting electrons which carry a negative charge. A hydrogen atom has just one ...

Pionic helium In this experiment, a pion – shown here with one orange and one blue particle representing its quark and anti-quark – replaces one of the two electrons in the helium atom. This new ...