[Johann Mahne]

 I remember once reading about how physicists had built big traps underground and were looking for a theoretical particle but had no success for years?
  What causes them to appear?

[Soul Surfer]

neutrinos were predicted to exist to balance out the equations in certain very common nuclear interactions.  Let me explain.

An isolated neutron is unstable and decays with a half life of about 12 minutes into a proton and an electron.  So far so good the proton has a positive charge and the electron has a negative charge so the charges cancel out.  BUT a neutron has one quantum of spin(angular momentum) and a proton and an electron each have one quantum of spin so the spins cannot balance out and this contravenes the law of the conservation of momentum.  Furthermore if you looked at the mass energy of the neutron and the mass energy and momentum of the proton and electron you would find that some of the energy was missing.  this contravenes the law of conservation of energy so something must be missing.  The simplest solution is a particle that has one quantum of spin no charge and the energy to make up the deficit.  This particle was called the neutrino and it was very many years before one was actually detected because it interacts so weakly with other particles.  It can pass straight through a star without ever bumping into anything!

[Johann Mahne]

A good explanation thank you,
  If neutrinos can pass through anything then they could not have been caught in the traps that were built for them.
When were they first detected?

[syhprum]

Although a single Neutrino can pass thru a star without hitting any thing there are really vast numbers of them and they have a tiny chance of interacting so that in a large enough detector some will register.

[damocles]

neutrinos interact very weakly with an isotope of chlorine, and the eventual result is decay to an isotope of argon. The "traps" were vats of carbon tetrachloride or similar compound put in a deep mineshaft, and they were analysed for tiny argon bubbles after a zillion or more neutrinos had passed through.

[Johann Mahne]

 Are neutrinos considered to be particles, although they have no mass?

[damocles]

Are neutrinos considered to be particles, although they have no mass?

Well, there is some argument that neutrinos may have a tiny mass. But photons certainly have no mass, yet both photons and neutrinos are considered particles.

[Soul Surfer]

Neutrinos probably do have a tiny mass because it is important that they do or they could not oscillate between the three different types electron, muon and tau.  They have spin and a property (as yet not described fully) that causes them to interact via the weak interaction like the other leptons which is much stronger than gravity.

Gravitational interactions have as yet no place in particle physics because gravitation is so indescribably weak it is extremely improbable that even in neutral interactions particles pass close enough to each other to experience a gravitational interaction.  Even the weak interactions of neutrinos that than pass through a star unscathed are far more probable.

[Johann Mahne]

Neutrinos probably do have a tiny mass because it is important that they do or they could not oscillate between the three different types electron, muon and tau

If they do have some mass, then does this not violate relativity in that they can achieve the speed of light?
 Would they have less mass than an electron?
  I don't know what muon or tau are?

[damocles]

Answering Johann's latest:

(1)

If they do have some mass, then does this not violate relativity in that they can achieve the speed of light?

not if they do NOT QUITE achieve the speed of light. But the latest finding is that they may be exceeding the speed of light, and that certainly does violate (special) relativity.

(2)

Would they have less mass than an electron?

they are much less massive than an electron

(3)

I don't know what muon or tau are?

muons and tau mesons are short-lived, high energy particles that are more massive than electrons but less massive than protons or neutrons

[imatfaal]

Answering Johann's latest:

(1)

If they do have some mass, then does this not violate relativity in that they can achieve the speed of light?

not if they do NOT QUITE achieve the speed of light. But the latest finding is that they may be exceeding the speed of light, and that certainly does violate (special) relativity.

(2)

Would they have less mass than an electron?

they are much less massive than an electron

(3)

I don't know what muon or tau are?

muons and tau mesons are short-lived, high energy particles that are more massive than electrons but less massive than protons or neutrons

Nice answers Damocles - would only quibble with one thing - tau meson are heavier than protons/neutrons

μ^- = 105 MeV/c²
p⁺ = 938 MeV/c²
n⁰ = 939 MeV/c²
τ^- = 1776 MeV/c²

For Johann's further reference the combined mass of the three variants of neutrino (tau, mu and electron) must be less than 0.27 eV/c² - ie at least 10^9 times smaller than a proton.  This is only an upper bound and the actual amount could be orders of magnitude less - I don't know of the lower bound, other than the mass is greater than zero.

[damocles]

Careless of me Imatfaal; I stand corrected on the tau meson. I will add to your answer though that the electron mass in your units must be about 0.5 MeV/c^2 , which is at least 2 million times that of a neutrino

[Johann Mahne]

Thanks for all the really good replies.

Would the momentum of a neutrino simply be mv, as it's a particle?
  Would this momentum have any effect if it collided with say a neutron, or is a neutron simply to massive?

Also from Imatfaal :
How does volts/c²  relate to mass?
If e=mc²; or e/c²=m then volts would equate directly to energy, which is not true.
Or is an electron volt actually energy?

 
 

[damocles]

electron volt is an energy unit: E = Q*V; it is an electron charge = 1.6E-19 coulomb times 1 volt potential difference. So an electron volt is
1.6E-19 Joule,
or 1.6E-19 * 6.022E23 = approx 100 kJ per mol
(I leave it to you to look up exact values if you want a more exact conversion).

And the momentum of a neutrino would be simply mv, BUT the mass quoted is a rest mass m0; mass increases at near light speed according to m=m0/√(1-v^2/c^2)

When the speed v gets very close to the speed of light c, the mass can become very many times the rest mass, because the correction is very close to a division by zero.

[Johann Mahne]

m=m0/√(1-v^2/c^2)

Thanks Damocles,
I forgot about the relativity effects.
  So with this massive amount of momentum how do neutrinos go through stars unimpeded, or do they simply miss particles because of their small size?
 
 

[Johann Mahne]

BUT a neutron has one quantum of spin(angular momentum) and a proton and an electron each have one quantum of spin so the spins cannot balance out and this contravenes the law of the conservation of momentum

I don't understand what a quantum of spin actually is, and how this relates to angular momentum.
If a proton and an electron can each have one quanta of spin, does this mean that the proton will spin much slower than an electron due to it's much larger mass, and that a neutrino will spin even faster than an electron?

[Soul Surfer]

Spin in subatomic particles is a rather difficult concept.  It is not strictly like electrons and protons are little globes spinning away but is more related to underlying symmetries in the particles.  However the dimensions of Planck's constant are the same as the dimensions of angular momentum. Also an electron and a proton (and many other subatomic particles) as well as having an electric charge has a magnetic field and this implies that the electric charge must in effect be moving in circles in some way.

As for the speed with which this is happening it is not known or measurable so you just have to consider it as a quantum number

[imatfaal]

How does volts/c²  relate to mass?
If e=mc²; or e/c²=m then volts would equate directly to energy, which is not true.
Or is an electron volt actually energy?

Just to be explicit - Damocles cleared up why an electron volt is a unit of energy - in most forms of particle physics c - the speed of light - is set to one.  in these cases the mass (e/c^2) is equal to the energy - thus the masses of the subatomic particles are usually given in units of eV or MeV or even GeV.  If you are a stickler and/or a struggler (like me) you need to realise and make clear that the units you are using are (energy)/[(speed of light) squared] - and thus the mass is given in eV/c^2  ie per the mass energy equivalence equation

[imatfaal]

I hate to agree with everybody above but SSurfers comments on spin are spot on.  This is a cut-off point where you can no longer think classically - the electron cannot be seen as a little globe spinning on its axis; if it was the surface would be travelling at a velocity above c.  Spin is an intrinsic property - which is testable,predictable, and closely connect to angular momentum ; but it is not a little particle spinning like a top

[Johann Mahne]

neutrinos interact very weakly with an isotope of chlorine, and the eventual result is decay to an isotope of argon. The "traps" were vats of carbon tetrachloride or similar compound put in a deep mineshaft, and they were analysed for tiny argon bubbles after a zillion or more neutrinos had passed through.

  Did this method work?
  How does CERN detect neutrinos nowadays?
  I remember seeing pics of tracks of particle collisions. I'm not sure if they were in some type of metal plate?
The paths were all looked very strange.Were the properties of neutrinos discovered in this way as well?