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At CERN - What do you call the moment (event) particles crash together in the particle accelerator?

At CERN they crash different particles together and measure what comes out. What is the name of the event (moment) the particles collide in the tunnel? Is there a name for the event itself or the crash itself?

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closed as off-topic by Dilaton, Manishearth Jul 22 '13 at 16:56

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"collision". This isn't really a physics question, and I think it's even a bit of a stretch to call it a physics terminology question... – Kyle Oman Jul 22 '13 at 14:42
This question appears to be off-topic because it is not really about physics concepts – Dilaton Jul 22 '13 at 14:58

1 Answer 1

up vote 17 down vote accepted

Actually there are terminological subtleties when you are talking about that.

Particles in the accelerator's tube are gathered in a sequence of little "bunches". For the proton-proton mode there was roughly ~3000 bunches per beam. And each bunch contained roughly $\simeq 10^{11}$ protons.

So, at the largest level what you actually have during the experiment is a...

Bunch crossing: two proton bunches passing through each other. Happening every 25ns. For each bunch crossing there is a probability of an inelastic interaction of a pair of protons (one from each bunch). Which is called a...

Collision: inelastic $pp$ scattering. In the 2012, on the average there was 20 collisions per bunch-crossing. Each collision creates a number of new particles -- products of the interaction. This products fly away from the collision point, hopefully in the direction, transverse to the beam. So physicists build complex detectors around the points where particle beams cross. So for every bunch-crossing you can obtain an...

Event: the detector response for each bunch crossing. From that data one can try to reconstruct the number of collisions and point where they did occur. Try to identify the particle tracks. Then find their energies, momenta, charges and associate them to each collision. Then one can continue and try to infer the identities of these particles and other properties of the interaction.

That's it, but I feel like I should mention an extra detail. If you do the math, you'll see that in that case one expects 800 000 000 collisions per second. Which is an enormous amount of data to record and/or process. Fortunately, most of the events are not very interesting and can be discarded on-the-fly in the real time. This kind of filtering is preformed by complex electronic systems called triggers.

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I see, i was looking for one word... maybe a word more science-fiction "like". Something that names both the event and the particles taking part... as some special something. Maybe you have a special word for the entire experiment "run", especially if it's a successful run. – unmircea Jul 23 '13 at 8:12
800e6 collisions per bunch crossing is way to high and even in conflict with with what you said previously (20). Maybe you meant collisions per second? In that case the number looks still off: $20 * 4 * 11$ kHz $= 880$ kHz. – DarioP Sep 15 at 16:29
@DarioP Thank you for noticing that. I, indeed, meant "per second". As for number I think that I'm right $20 \cdot 1/25ns = 20 \cdot 40 MHz = 800 MHz$ – Kostya Sep 16 at 23:47
My number is computed for only one bunch, your number is computed for only one IP. You are more right than me! – DarioP Sep 17 at 7:04

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