Neutrinos That Need Directions

[IanTBlack]

I was reading an article about neutrinos in a Science Illustrated magazine about a month ago. And in this article it stated that when a neutrino collides with another particle, you know the direction from which the neutrino came from and the flavor of it.

A neutrino is a non-charged, non-zero massed particle right? And objects that have mass, whether it be negative or positive, are affected by gravity.

If a neutrino is affected by gravity, and in this article it states that they are, then it should be impossible to know which direction the neutrino comes from. The neutrino could slingshot around billions of particles before it collides.

You can infer the direction of the neutrino by the initial and final velocities. Yet you can't get a truly accurate number.

Is it really possible to know the true direction a neutrino came from?


Thanks!

 

[mgb_phys]

And in this article it stated that when a neutrino collides with another particle, you know the direction from which the neutrino came from and the flavor of it.

You know the direction it was going immediately before the collision

And objects that have mass, whether it be negative or positive, are affected by gravity.

Yes it only meant it's direction before the collision

And remember that even things without mass (like photons) are affected by gravity.

 

[DJsTeLF]

I think what the article might have been referring to is the fact that in nature only left-handed chiral neutrinos are observed (and right-handed anti-neutrinos), an example of CP-violation. The is accounted for in the GSW-model, that is, the electroweak sector of the Standard Model.

Remember that particle collisions (unless in a very low energy limit) are not ballistic collisions but that the interation is mediated by the force-carrying vector bosons, aka virtual particles.

The upshot of all this is that, in the case of pion decay in the presence of an applied external electromagnetic field for example, the neutrinos are emiited preferentially in one direction, despite their being massless (as in the Standard Model) and uncharged.

I suspect however that when the article you read suggested that one can know the direction of the incoming neutrino the reason is simply a matter of relativistic kinematics. Irrespective of all the aforementioned chirality considerations, all collisions must (and do) obey the principles of energy and momentum conservation. By measuring the outgoing neutrino's energy and direction, and knowing the mass and position of the particle with which it collided, one can calculate (more appropriately we should probably say approximate) the original direction of the incoming neutrino.

In practice however neutrinos are notoriously difficult to detect since they interact so weakly with ordinary matter. Typical neutrino detectors normally consist of huge undergroud tanks of liquid surrounded by photomultipliers that detect tiny amounts of light emitted in the collision. As the whole spherical surface of the tank is surrounded by these detectors it is possible to triangulate the direction of the incoming neutrino based on where the flashes of light are detected on the outside of the sphere.