# Why are neutrinos more weakly interacting than light?

When people describe neutrino interactions they describe them as rare/infrequent due to the fact that the neutrinos are electrically neutral and have little mass, if any. Well why then is the photon much more strongly interactive considering that it is neutral and massless? What is the discrepancy between the two? Why is the neutrino so elusive, whereas light is so prevalent? Does this perhaps have anything to do with the fact that a neutrino is a fermion whereas a photon is a boson?

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The interaction of various particles depends on their coupling constant, for instance light and electrons have a coupling constant which is very low compared to the coupling constant of strong interactions. In particular Neutrinos belong to a group of particles which are weakly interacting i.e they have a really small coupling constant. –  user2900110 Sep 4 at 8:33

The photon does couple directly to charged stuff, e.g. via Compton scattering. This is indirectly related to the spin, as direct interactions between fermions are hard to construct.

The neutrino on the other hand does not couple immediately to any other matter particle. It requires a force-carrier. Now as it turns out the only force carriers that care about the neutrino are the $W^\pm$ (and to lesser extent the $Z^0$ boson). These are really really massive, however, making the interaction very rare.

This is a different situation than for an electron, for example. It also does not couple directly to other matter particles. Still, it can interact via the photon, which is massless and can therefore easily transmit energy and momentum.

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Neutrinos having no charge means they don't participate in electromagnetic interactions, which are the strongest (at least long range). Them being leptons means they don't interact with the strong force (which is, as the name says, strong), hence they can only interact via the weak force, which is, as the name says, weak.

Photons on the other hand do not carry charge, but they transmit electromagnetic energy, hence they in fact DO participate in electromagnetic interactions (oscillating electric circuits radiate). Why is that? The photon is in fact (in a sense) the agent that transmits electromagnetic force between other particles; it is the gauge boson of the electromagnetic field. Almost all elementary bosons in the standard model (except the Higgs) are in fact gauge bosons, i.e. they are the force carriers of some of the elementary forces (strong, weak, electromagnetic). Therefore, they all have to interact quite strongly in some sense.

All in all, this means that yes, the two are very different and it has something to do with the fact that the photon is a boson and the neutrino is a fermion, but on a different level than you might think.

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Nice answer. You should rewrite one sentence though: The Higgs boson is not a gauge boson!! –  Neuneck Sep 4 at 8:35
Right, thanks a lot! I had this originally, I don't know why I changed it - probably didn't want to make it too complicated, but one shouldn't propagate wrong things. –  Martin Sep 4 at 11:48

We know four fundamental forces, three of them being included in the Standard Model. At low energies (compared to the mass of $W^\pm$) the forces have strengths as follows:

strong force > EM force > weak force > gravity

Also, photon and neutrino are really quite different. Photon is a force carrier, while neutrino is a matter particle.

Now, matter particles only interact if they carry the relevant charge. The neutrino only carries weak charge, and can therefore only interact via the weak force. The photon is the carrier of the EM force.

Now, the above inequality says that the EM force is stronger , which means that the photon interacts more often than a neutrino.

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Martin wrote:

", hence they can only interact via the weak force, which is, as the name says, weak."

Makes me wonder, is "weak" a good name? Or is this interaction better described as short, and still full of surprises? I'm thinking of {Dirac, Weyl, Majorana} spinors, rates of chiral oscillation, inertial-mass...

... which makes me wonder about the real playground for weak interactions -- that condensate exploited by the Higgs mechanism: is the presence/absence of galaxies an indicator of a variable distribution of what Leonard Susskind likes to call zilch?

... which makes me wonder, now that we think of "electroweak" in a unified way, should we expect some subtle effect leaking from the electro- onto the -weak domain?

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