Friday, 2 January 2015

particle physics - What came first, neutrons or electrons?



I wonder if electrons were first (at an early stage of the cosmos) embedded into neutrons, making it easier to understand why they would fit so well with protons later, or if the genesis of electrons and protons are seen as two parallel but independent evolutions.


Spinoff: Why saying that during electron capture the electron is converted to a neutrino?


Spinoff: Are fundamental particles more than their properties?



Answer



Electrons came first. In the early universe, during the so-called electroweak epoch, the temperature was so high that large numbers of heavy particles (W, Z, and Higgs bosons, for instance) were created. Once the temperature cooled down enough, all of these bosons decayed into quarks and leptons (electrons, muons, and tau particles).


At this point, during the so-called quark epoch, the universe was still too hot for protons and neutrons to form- instead the universe was filled with a quark-gluon plasma, which consisted of quarks, gluons, and leptons. It wasn't until later that the universe cooled down enough to allow for protons and neutrons to form.



Neutrons do not contain electrons. Electrons aren't really "captured" by protons in the usual sense of the word. The electron is converted to a neutrino via the weak process, while the proton is converted to a neutron. The same interaction occurs between quarks and electrons, such as in the process $\rm eu\to\nu_ed$.


This doesn't mean that the neutron contains an electron. Indeed, the neutron is perfectly happy to capture a positron and turn back into a proton, and this doesn't mean the proton contains a positron.


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