Some questions about particles being produced in certain interactions

[jeebs]

Hi,
I keep reading about particles being produced in certain interactions, but I'm not completely clear on what this means. The image I have in my head is this. Say two protons collide with each other and move off with less energy than they started with, and the energy deficit goes into creating a new particle. The protons would have been repelling from their electromagnetic interaction, but also attracting from the strong interaction if they were close enough.

How does this energy actually get transferred out of the protons and into this new particle, given that it doesn't exist prior to the collision? Is this mechanism even remotely known about?

Also, I keep coming across things like "such and such a particle was created via the strong interaction". In a collision such as my example, how would we know whether the new particle produced was as a result of the strong interaction or the EM one, seeing as both interactions are present?

One other thing I've been wondering about - it's said that no single quarks are ever observed because the energy required to pull them out of a particle is always enough to create a new particle, right?
In other words, work is being done against the attractive strong force to separate them, so their potential energy in each other's strong field is being increased, right?
Does that mean that mass is equivalent to potential energy (that it's not technically their kinetic energy that matters) - and if that's true, is the cause of larger particle production not actually the collision but what happens just after the collision, ie. the colliding (and attracting) particles move away from each other until their increase in potential energy exceeds the new particle's rest mass?
(Conversely, for two repelling particles that create a new one, the new particle arises from the increase in potential energy just before the collision?)

Sorry if these questions don't quite make sense to you, I'm not even sure they make sense to me...

 

[mharley]

Hi there,

It's actually a rather interesting set of questions, I'll do my best to answer as much as I can here.

How does this energy actually get transferred out of the protons and into this new particle, given that it doesn't exist prior to the collision? Is this mechanism even remotely known about?

This can only really be described using quantum field theory, which if you haven't studied before can be rather complicated. For a start it might be worth looking into Griffiths' "Introduction to Elementary Particles" or, if you have some mathematical background, I'd recommend Peskin and Schroeder's "An Introduction to Quantum Field Theory".

Anyway, in field theory we replace all of our classical ideas of particles with disturbances of fields (fermion fields, vector boson fields, etc.) and so in a collision it is possible for fields to exchange energy with each other via. an interaction.

e.g. In the case of an electron-positron collision: the electron and positron field "disturbances" meet and annihilate to a photon (an example of a particle being produced) and then this photon can decay into another pair of electron-positron or even a muon and an antimuon (or even heavier particles) if the original pair had enough energy.

Proton-proton collisions are far more complicated due to the strong force and because of the internal structure. The LHC makes use of the physics here but I don't think I'd be able to fit a useful introduction in here.

Also, I keep coming across things like "such and such a particle was created via the strong interaction". In a collision such as my example, how would we know whether the new particle produced was as a result of the strong interaction or the EM one, seeing as both interactions are present?

The strong force (mediated by the gluon rather than the photon of electromagnetism) is described by a much more complicated field theory than that of electromagnetism (Quantum Electrodynamics). This theory is called Quantum Chromodynamics if you're interested.

It can be tricky at times to tell which interaction has occurred here. It is usually done by considering the rate at which the process occurred (i.e. how probable it was to occur tells us how "strong" the force mediating it was) and using other conservation laws (parity, isospin and others). Also, the strong force only interacts with quarks not electrons (or the other leptons) so that can give you an idea.

One other thing I've been wondering about - it's said that no single quarks are ever observed because the energy required to pull them out of a particle is always enough to create a new particle, right?

This is known as "confinement" and is an open problem in high energy physics at the moment. In fact, the Clay Mathematics Institute (http://www.claymath.org/millennium/Yang-Mills_Theory/ ) will give you a $1million if you can prove it's always true in QCD.
It is thought to occur because, unlike photons, the gluons interact with each other. This reigns in the distance over which the strong force acts, but increases its potency.

The production of the new particle is again a property of quantum field theory. After work equivalent to Mc^2 (where M is the mass of the new particle) has been done on the system it becomes energetically favourable for the fields to be perturbed into the new state with more particles [remember: the laws of physics usually act to minimise the potential energy or equivalently the action].

Saying then that mass is equivalent to energy is correct since in quantum field theory energy only really appears as the momentum of disturbances of fields.

I hope this has answered some of your questions. If I can be of any further help at all with this let me know and I'd be happy to help. I've written this in a rush during a break from my research so I'd be more than willing to take my time and clarify anything that's too brief or covered in too much jargon!

 

[jeebs]

yeah cheers for the response, was starting to think I wouldn't get one. I don't know about QFT or QCD etc. beyond the most basic of levels yet, but you've at least painted a picture for me.

There is one other thing though, a bit of a departure from the initial questions but I might as well ask it. It's about the weak interaction, I'm having trouble visualizing how it does it's thing. With gravitation I can picture two masses attracting, with EM I can picture two charges attracting/repelling, and with the strong force I can picture repelling protons being held together in a nucleus. All I ever see written about the weak force is that "it is responsible for beta decay" and it's left at that.
So, negative beta decay is neutron -> proton + electron + antineutrino, right? or in quark terms it's udd -> uud + electron + antineutrino, so at it's most fundamental it's d -> u + electron + antineutrino.

where is the "force" or the "acceleration" involved here in changing this quark flavour? I can't see where the change in momentum comes in that I'd expect when dealing with a force...