Article 39 Why an electron does not fall into the nucleus in terms of the strong and weak nuclear forces.

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It can be shown one may able to derive the strong and weak nuclear forces and the internal geometry of protons and neutrons in terms of the orientation of…

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Electrons in the atom do enter the nucleus. In fact, electrons in the s states tend to peak at the nucleus. Electrons are not little balls that can fall into the nucleus under electrostatic attraction. Rather, electrons are quantized wavefunctions that spread out in space and can sometimes act like particles in limited ways. An electron in an atom spreads out according to its energy. The states with more energy are more spread out. All electron states overlap with the nucleus, so the concept of an electron “falling into” or “entering” the nucleus does not really make sense. Electrons are always partially in the nucleus.

If the question was supposed to ask, “Why don’t electrons in the atom get localized in the nucleus?” then the answer is still “they do”. Electrons can get localized in the nucleus, but it takes an interaction to make it happen. The process is known as “electron capture” and it is an important mode of radioactive decay. In electron capture, an atomic electron is absorbed by a proton in the nucleus, turning the proton into a neutron. The electron starts as a regular atomic electron, with its wavefunction spreading through the atom and overlapping with the nucleus. In time, the electron reacts with the proton via its overlapping portion, collapses to a point in the nucleus, and disappears as it becomes part of the new neutron. Because the atom now has one less proton, electron capture is a type of radioactive decay that turns one element into another element.

If the question was supposed to ask, “Why is it rare for electrons to get localized in the nucleus?” then the answer is: it takes an interaction in the nucleus to completely localize an electron there, and there is often nothing for the electron to interact with. An electron will only react with a proton in the nucleus via electron capture if there are too many protons in the nucleus. When there are too many protons, some of the outer protons are loosely bound and more free to react with the electron. But most atoms do not have too many protons, so there is nothing for the electron to interact with. As a result, each electron in a stable atom remains in its spread-out wavefunction shape. Each electron continues to flow in, out, and around the nucleus without finding anything in the nucleus to interact with that would collapse it down inside the nucleus.

Why are neutrinos so light?

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Did you know that every second more than 100 trillion tiny particles called neutrinos pass through your body without causing any harm? These mysterious particles are produced abundantly throughout the universe in events like nuclear reactions in the sun, radioactive decays in the Earth’s crust, and in high-energy collisions in space. In particular, these subatomic particles play a crucial role in the explosive deaths of stars known as supernovae, where they act as the driving force behind the explosion. Despite their abundance in the universe, they are incredibly difficult to detect directly in experiments since they pass right through any matter and only interact extremely rarely. At the LHC, their existence can only be inferred indirectly by summing up the energy of all other particles produced from the proton collisions and looking for missing energy that has been carried away by the neutrino, which escaped the experiment undetected.

Neutrinos are a type of fundamental particle known as a lepton and they are electrically neutral. They stand out among fundamental particles because of their peculiar characteristics. Not only do they interact exceptionally rarely, but they also possess a minuscule mass, approximately 500,000 times lighter than that of an electron. One possible explanation for the smallness of their mass is given by the “seesaw” mechanism. According to this theory, there exist additional new fundamental particles that are electrically neutral. The mechanism postulates that the masses of these new particles, known as “heavy neutral leptons” (HNLs), are mathematically linked to those of the normal neutrinos, like two sides of a seesaw. The theory also predicts that the HNLs will “mix” with their known cousins, neutrinos. This means that a neutrino, produced in an LHC collision, can change into an HNL, and the HNL can then decay back into known particles that the LHC experiments can detect!

The seesaw explanation for the neutrino mass is particularly attractive and various searches for HNLs have been performed at the LHC and by other experiments in the past (see an example where CMS muon detectors are exploited in such a search). The CMS Collaboration has recently published a new search that makes the assumption that the mixing between the HNLs and neutrinos is very small. In this special case, the HNL can be “long lived” and travel macroscopic distances away from the collision point before decaying. Experiments can then take advantage of the unusual signatures from these “displaced” particle decays when trying to find evidence for the existence of HNLs.