Envisioning particle collision.

[dbertels]

I constantly hear the term 'bits flying off' in a particle collision'. Isn't this somewhat misleading? Wouldn't it be more accurate to say that, in a collision, a 'fountain' of new particles are created and that our main aim is to use the energy resulting from the collision to create new, heavier particles?

 

[BvU]

What actually happens at such small scales eludes our imagination; we should be content that we can describe the events with ever improving accuracy. Personally I like the 'bits flying off' analogon in hadron collisions. And for me the 'fountain' idea evokes a totally different association, one that to me doesn't seem to 'fit'. But it's a personal thing.

And yes, energy is partially used to create new particles. Heavier than before, but not heavier than the good old Heisenberg limit allows.

 

[Orodruin]

that our main aim is to use the energy resulting from the collision to create new, heavier particles?

Energy does not result from the collisions. It is just redistributed.

 

[dbertels]

@BvU Granted I shouldn't have used the 'fountain' image, it just introduced another analogy that can be misconstrued.
@Orodruin Does this mean that the energy/mass required to create the Higgs boson was present in the colliding hadrons?

 

[Orodruin]

Does this mean that the energy/mass required to create the Higgs boson was present in the colliding hadrons?

Energy is a frame dependent quantity. The energy needed was mainly kinetic energy in the colliding protons. Still, if you would go to the rest frame of one of the protons its kinetic energy would be zero (although the other would have an enormous kinetic energy).

 

[dbertels]

Energy is a frame dependent quantity

Is the term 'frame' here used in the sense of 'state'?
Can you point me to any articles that can help me understand this?

 

[Orodruin]

Is the term 'frame' here used in the sense of 'state'?

Definitely not. It refers to inertial frames. Energy is observer dependent.

 

[Dirk Pons]

'bits flying off' in a particle collision

The 'bits flying off' perspective is a disassembly one. It tends to imply that the particle already had all those 'bits' partially pre-formed and assembled inside itself, ready to break off.The problem with that approach is that particles in different situations show different output products, hence implying different input structures.

I find it easier to envisage this process as remanufacture of particle identities. This handily also covers the decay processes where there is only one input particle. By 'remanufacture' I mean that the input particle(s) are induced to change by some initiation event (impact, decay, photon absorption, etc) followed by a process where the energy is partitioned up differently (and excess energy is liberated as a photon). Other quantum numbers (charge, matter-antimatter attributes) are also re-distributed in the process, and usually conserved. These attributes and energies determine the identities of the output particles. Consequently small changes in the process settings can result in different output particles.

 

[dbertels]

Energy is observer dependent.

Very interesting statements - unfortunately, I'm not in a position to comprehend this. Maybe you can point me to material that may throw light on these statements?

remanufacture of particle identities

Thanks for taking the time answering this - it gives me material to research.
Interesting how all the 'realities' of the classical world lose their meaning in particle physics, hence any description will fall short of this - and maybe trying to understand the collision using classical world analogies is futile, that envisioning 'bits breaking off' is as meaningless as 'creating new particles' ..

 

[Orodruin]

Very interesting statements - unfortunately, I'm not in a position to comprehend this. Maybe you can point me to material that may throw light on these statements?

This is not anything strange or new in relativity. It is true also in classical mechanics. Obviously, if you go to an object's rest frame its kinetic energy is going to br smaller than in a frame where it is moving.

 

[jtbell]

Energy is observer dependent.

Very interesting statements - unfortunately, I'm not in a position to comprehend this.

Step away from particle physics for a moment, and into (hopefully) more familiar terrritory.

Suppose you're standing by a road, and a car goes past you at speed v = 60 m/s. It has kinetic energy ##\frac{1}{2}mv^2 = \frac{1}{2}m(60^2) ## joules, provided m is in kilograms.

Now suppose you're in a car that is going 50 m/s, and the car described above passes you in the same direction. In your new reference frame ("point of view"), that car is passing you at a rate of 10 m/s, and its kinetic energy in this reference frame is ##\frac{1}{2}m(10^2)## joules.

 

[dbertels]

Step away from particle physics for a moment, and into (hopefully) more familiar terrritory.

Great, I understand what you're saying here and I think I can see where my confusion is coming from - I was thinking particle collision involved quantum physics because of the scale of the particles involved, hence my 'creation of new particles' analogy (that possibly clears up the fact I'm no physicist).
But all your explanations seem to point to relativity (I assume because of the relativistic velocities involved?).

 

[Orodruin]

It involves both relativity and quantum physics.

 

[ChrisVer]

But all your explanations seem to point to relativity (I assume because of the relativistic velocities involved?).

It involves both relativity and quantum physics.

That's what they call "particle physics"... or "high energy physics"... the combination of both special relativity and quantum physics.

Very interesting statements - unfortunately, I'm not in a position to comprehend this. Maybe you can point me to material that may throw light on these statements?

That's relativity alone. Jtbell gave a nice way to see how this can work. For special relativity search for 4momentum.
The relation: [itex]E^2 = p^2 c^2 + m^2 c^4[/itex] where E is the energy, p is the momentum and m is the mass of the body (c=speed of light in vacuum), shows the same thing... If you change your momentum, the energy must change so that the mass of the body will be constant [ a proton's mass is always equal to a proton's mass]... the momentum obviously depends on the observer.

 

[Orodruin]

That's special relativity alone.

No it is not. It is true in classical mechanics as well. What is important here is the relativity principle, which holds just as true in classical mechanics as it does in relativity.

This is a crucial point which many seem to miss and think there is something strange going on in relativity.

 

[ChrisVer]

thanks for the point, I corrected it.

 

[dbertels]

Still, I'm interested to know what the best way would be to describe what happens in a particle collision, if you had to explain it to non-physicists in the most meaningful way possible. Any takers on this?

 

[BvU]

Does this mean that the energy/mass required to create the Higgs boson was present in the colliding hadrons?

Yes, the total energy in the center of mass of the colliding hadrons was more than the energy/mass required to create the Higgs boson.

Still, I'm interested to know what the best way would be to describe what happens in a particle collision, if you had to explain it to non-physicists in the most meaningful way possible. Any takers on this?

The concentration of energy in a very small space allows the formation of particles.

The famous Carreras would make a tralaTeV accelerator in the Cern lecture room by shooting off a small scrap of paper with a rubber band. That kinetic energy was also more than needed for a Higgs boson, but (un)fortunately not concentrrated in a small enough space.

 

[Dirk Pons]

relativistic velocities involved?

Not all particle processes involve high velocity impacts. Decay processes would be an example: they happen for static particles.

The reason for high velocities is to artificially add extra energy to the particles. The reason this is useful is that this energy is then converted (think E=mc^2 at this point) into new mass, which is then available to make new particles that otherwise would not have appeared at a slow collision.

 

[dbertels]

The concentration of energy in a very small space allows the formation of particles

I'm liking that - So the kinetic energy created by the collision allows this to happen..

 

[Orodruin]

I'm liking that - So the kinetic energy created by the collision allows this to happen..

Again, there is no energy created in the collision. There is just the energy which goes in, which for a collider is kinetic energy to a large extent.

 

[dbertels]

Ok, so is it right to say that colliding the protons causes those protons to absorb energy, resulting in the energy concentrating in a very small space which allows the formation of particles?

 

[Orodruin]

No, there is no absorbtion going on anywhere. The protons already had their kinetic energy.

 

[ChrisVer]

Ok, so is it right to say that colliding the protons causes those protons to absorb energy, resulting in the energy concentrating in a very small space which allows the formation of particles?

When you are in a car and drive, and you hit the wall, your ALREADY HAD kinetic energy is converted into other forms...
the protons already have the kinetic energy since they go round and round within the accelerator, each beam in opposite direction, at high speed/with high energy.
They are only brought together in certain points [where we have placed our detectors].
Once the beams meet, some protons from the one beam collide with protons from other. At the point of collision you have a proton with energy E1 and another with energy E2, so the total energy involved is E1+E2 [still colliding protons is just a simplification, because at those energies it's protons constituents that collide] . This energy involved can be converted into other particles that fly away from the point of collision.

 

[dbertels]

Oh - of course, the energy is already present because the protons are undergoing the acceleration - that makes perfect sense now (don't know why it didn't before - ideas get stuck sometimes) - I very much appreciate you guys explaining this with such patience.