# Finding new particles

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What do ‘events’ refer to when collisions occur at the LHC.

The Higgs Boson was found from a blip in the graph of Events vs Energy at about 125Gev.

It shows an excess of events at this energy.

But what events are in excess and why does this indicate the existence of a particle?

mfb
Mentor
An event is what happens in a single collision plus whatever happened at the same time (typically some more collisions for ATLAS and CMS).

I wrote an Insights article about finding new particles.

pinball1970, berkeman, vanhees71 and 2 others
For any others with the same question, the link given above by mfb 'an Insights article' has a couple of sub links which I also found helpful.
Thanks

I read your article too, very interesting. But one thing struck me as funny. On other threads in the forum discussing QM it's all probabilities, wave functions. How does that square with shooting individual particulars in a physical object (the LHC) that actually really do collide? And how are these individual particles guided along the collider so precisely that literally two of them going in opposite directions can be aimed enough to collide? What happened to all the probability? (I know HOW they're aimed, that's not what I'm asking about.)

Great question. Sometimes it is the questions that are more enlightening than the answers. Although as in QM I think we need to apply some probabilities here. I guess that at such high energies it will be said that there is no uncertainty in the position and momentum of the Hadrons. But I really don't know.
The Heisenberg uncertainty principle has apparently been shown to have exceptions;
https://resonance.is/new-measuremen...ental-evidence-non-orthodox-quantum-theories/

mfb
Mentor
I read your article too, very interesting. But one thing struck me as funny. On other threads in the forum discussing QM it's all probabilities, wave functions. How does that square with shooting individual particulars in a physical object (the LHC) that actually really do collide? And how are these individual particles guided along the collider so precisely that literally two of them going in opposite directions can be aimed enough to collide? What happened to all the probability? (I know HOW they're aimed, that's not what I'm asking about.)
It is random chance. At the collision point the beams have a diameter of tens of micrometers - similar to the width of a human hair, and billions of times wider than a proton. Each bunch has about 100 billion protons (initially, goes down over time) - out of these, about 50 collide with one of the 100 billion protons from the opposite beam when they cross at the experiments (numbers for ATLAS and CMS. ~1-3 collisions in LHCb, 0-1 in ALICE). A tiny collision chance for any given pair of protons, but many pairs of protons.

Ok but see, how does THAT square with probability in locating particles. How do they know only two collided? I'm assuming there's a tiny collision chance for say three protons to collide. And then as far as guiding them, do experimenters just not worry about, or take into account, say the Copenhagen Interpretation that there are no actual particles, just their wave functions (well, that version of the CI). Or does it just not matter. Physicists basically spray two hoses at each other and see what comes of it.

mfb
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How do they know only two collided?
You look where the tracks come from. Here is an example from ATLAS with two collisions:

Things get more difficult if you have 50 collisions but the idea is still the same.

The uncertainty relation is irrelevant, the corresponding uncertainties are orders of magnitude smaller than the resolution of the detectors.
Physicists basically spray two hoses at each other and see what comes of it.
That.

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Here is an example from ATLAS with two collisions:

Ah...the good old days...

jtbell
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Ah...the good old days...
No, the good old days were the bubble-chamber days when you could scan ten or so pictures before finding a single useful event. With a neutrino beam, that is.

mfb
Mentor
Ah...the good old days...
You can go to LHCb, they still have just a few collisions per bunch crossing ;). Now the average is somewhere close to 1, after the upgrade they will get about 5.

Reading a great book called "The Large Hadron Collider" by Don Lincoln. Very well written and at the right level, not too watered down. It's amazing technology but I'm also struck by how, how to put it, how brute force the detectors are. All the types still seem like basically sophisticated bubble chambers, not in their technology per se, but by the means of actually detecting particles. Or should I say detecting their effects. Of course how else would you detect their effects except how they actually do now. But I feel like there should be a Star Trek movie scene where Scotty is inspecting the warp core, and he giggles, gee just think of how they used to use nuclear energy, hee hee. Kinda like that but about particle detectors...gee just think how they used to actually slam particles together like billiard balls and watch the pieces fly into slabs of metal and plastic. Hee hee if they only knew how to X. Not knocking the LHC by any means though, don't get me wrong. Utterly amazing technology and amazing people who design and build these machines.

The uncertainty relation is irrelevant, the corresponding uncertainties are orders of magnitude smaller than the resolution of the detectors.
Isn't that putting the cart before the horse? Or are you saying uncertainties might be in play, just we can't detect them so we're not going to worry about their effects?

mfb
Mentor
Isn't that putting the cart before the horse? Or are you saying uncertainties might be in play, just we can't detect them so we're not going to worry about their effects?
It is like wondering how good an atomic clock is if you don't want to be late to the next meeting.

It is like wondering how good an atomic clock is if you don't want to be late to the next meeting.
Ooh that's a good one! :) Except that if your atomic clock, because of uncertainties, is sometimes off and your boss demands that you arrive on time to the zillionth of a second. (Where 'boss demands' = 'boss demands results that reflect what's actually going on given QM uncertainties. :) )