Re: What Happens to an Atom When it Dies?

I did an extensive search for the concept of "dead atom" in the Physical Review site, which has the only version of that prestigious physics journal, and the arXiv.org e-Print archive, which stores millions of articles on physics, math and astronomy. There are six articles on dead atoms in the Physical Review and one in the arXiv, referring to atoms on a surface that become chemically inactive. None seems to fit the idea of an atom that dies and turns into subatomic particles. As far as we know, atoms are stable as long as they are not smashed or destroyed by other particles or interactions. The main constituents of an atom are protons, neutrons and electrons. Isolated neutrons decay into a proton and an electron after about 10 minutes. That time is called the mean life-time of the neutron. Thus in a sense, neutrons die after 10 minutes, but when they form part of an atom, neutrons can be perfectly stable in many cases. Isolated protons and electrons are stable as far as we know, and thus are eternal. Protons and neutrons form the nucleus (plural: nuclei) of most atoms. The only exception is the hydrogen atom, which has no neutrons and a single proton. There has been much speculation on the issue of proton stability. We know that isolated protons live a long time because hydrogen atoms, which have a single proton as a nucleus and an electron around it, have lived in space for billions of years. Thus there is no evidence that atoms die or decay spontaneously.

The electrons in an atom are located around the nucleus, and are more loosely bound that the protons and neutrons in the nucleus. If an atom collides with another particle, or if the atom absorbs energetic light, it can loose some or all of its electrons. If the collision is strong enough, the nucleus can be destroyed into electrons, protons, neutrons and a zoo of other particles, like gluons, bosons, Z particles, neutrinos and the like. Those other particles are understood to be the remains of the energy that bounds the protons and neutrons in a nucleus. The inventory of particles that result of such collisions is quite long. It is, however, quite difficult to destroy completely an atomic nucleus. In fact, if two nuclei collide, they can form larger nuclei not found in nature.

The best way to destroy an atomic nucleus with a collision is by using another atomic nucleus. The problem is that atomic nuclei are positively charged, and thus repeal each other. The only way to make two nuclei collide is by smashing them at very high speed in order to overcome their electric repulsion. That is achieved in particle accelerators on Earth. In the Universe atom collisions can happen in certain energetic phenomena like supernova explosions, but the details are still uncertain. We do observe the arrival of all kinds of subatomic particles on Earth, but cannot pinpoint their origin. The possibility that you suggest, that dark matter is formed by subatomic particles of known of unknown nature, has been considered by many scientists, and this is a field full of speculation.

The idea of subatomic particles as dark matter is to find some hitherto undetected subatomic particle that is abundant enough to account for the observed dark matter, and that produces some kind of observable signature. Recently Michael Loewenstein and Alexander Kusenko reported the detection of X-rays at an energy that would correspond to a new kind of neutrino in The Astrophysical Journal, Vol. 714, pp. 652-662 (2010). Neutrinos are a kind of particles that result from nuclear reactions. There are plenty of neutrinos in the Universe, and they almost don't interact with matter. Neutrinos traverse Earth all the time. If the new kind of neutrino exists, it can explain all of the dark matter according to Lowenstein and Kusenko. It is going to be some time before scientists are convinced of this new theory. There are three known types of neutrinos. They interact with matter through the so called weak nuclear force. Some theories of the Big Bang and experiments in particle accelerators suggest that there must be a fourth type of neutrino. This neutrino has been called "sterile" because it is not affected by the weak nuclear force. That means that it cannot be detected directly by their interaction with matter. The theory predicts that sterile neutrinos must emit X-rays at the energy observed by Lowenstein and Kusenko. In conclusion, yes, subatomic particles can be the answer to the problem of dark matter. How and where those particles are formed is another problem. Supernovae are the most likely candidates. Interestingly the neutrinos are supposed to be produced when iron atoms are smashed during the supernova explosion. In this case the protons and electrons of the atom combine to form neutrons and neutrinos. You can find more information and references on the search of the sterile neutrino in "The hunt for the sterile neutrino heats up", Nature, Vol. 464, p. 334 (2010) by Eric Hand.

Greetings from,

Vladimir Escalante Ram�rez
Center for Radio Astronomy and Astrophysics
National University of Mexico, Morelia, Mexico