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In high energy experiments, people smash particles into particles.

But how to make sure that they really run into each other, instead of just passing by?

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  • $\begingroup$ They, uh, aim carefully? Are you asking about the mechanisms one uses to adjust the beams? $\endgroup$ – ACuriousMind Feb 7 '15 at 19:54
  • $\begingroup$ My concern is that, we spend so much energy to accelerate the particles, if they miss each other, we waste a lot. So we have to take great efforts to make sure that they indeed collide. $\endgroup$ – kaiser Feb 7 '15 at 22:10
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    $\begingroup$ @kaiser Did you notice that most colliders are rings? Energy cost is one of several reasons for this (along with capital costs and beam cooling arrangements). $\endgroup$ – dmckee Feb 7 '15 at 22:12
  • $\begingroup$ Possible bloody obvious, but CERN and its webpages full of information. Including the beams. $\endgroup$ – Hennes Feb 7 '15 at 22:19
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Short answer: you don't.

Slightly longer answer:

You're using beams of particles, and you focus each of them as much as you (practically1) can so that the particles in each beam are reasonably close together.

The result is a wide variety of interaction distances from far apart through near misses to closer interactions still. You mentioned electrons which are treated as point particles so that every interaction is a "miss" at some level. For hadrons (protons, neutrons, mesons, etc), alphas and heavy nuclei the constituents of the beam have physical extent and can reasonably be said to collide at times.

To recap: the particles in the beams necessarily interact with each other at a variety of distance.

If we don't control how close they come, then how can we say what kind of event it is that we're measured?

Now you've asked the critical question. The answer is that we design and operate the detector package to measure enough data about scattered particle to reconstruct that information.

A simple example that was used for decades is to simply only place detectors at positions that represent large scattering angles (which implies high momentum transfer, and therefore very close approaches or actual collisions).

As data acquisition systems have become faster it has become more a more feasible to simple collect everything and sort the details out later.


I've written the above assuming a beam-beam machine, but similar remarks apply to fixed target work.


1 There are good reasons not to just get the tightest focus that can be achieved. In particular the tighter the focus you attempt to create the move transverse momentum you give the beam particles and the harder it is to contain the beam and bring it around for another go.

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  • $\begingroup$ There is extra focusing in colliders where the experiment sits "Just prior to collision, another type of magnet is used to "squeeze" the particles closer together to increase the chances of collisions." home.web.cern.ch/topics/large-hadron-collider $\endgroup$ – anna v Feb 8 '15 at 4:25
  • $\begingroup$ @annav If the beam is charged, the beam radius is constantly trying to expand due to electrostatic repulsion. An accelerator or storage ring must counteract this expansion by "squeezing" the beam with a magnetic quadrupole at regular intervals. While a collider experiment should certainly place an interaction region equally distant from two "squeezes", where the particle density will be the largest, I don't know that it's proper to call that out any more specially than the squeezing done for ordinary beam transport/preservation. $\endgroup$ – rob Feb 11 '15 at 23:28
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    $\begingroup$ @rob LHC keeps the beam in a pretty broad profile most of the time to make it easy cool then they really focus it down near the active interaction points. It does cause extra trouble keeping the beam cool in the transverse direction but it pays in terms of interaction per crossing, and they had been thinking in those terms since early in the design process so the quadrupole positioning scheme takes this into account. I'm not aware of other colliders that use that scheme, but this could be an expression of my ignorance as I'm not a collider person. $\endgroup$ – dmckee Feb 11 '15 at 23:42
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    $\begingroup$ @rob Yes, they do extra focus at the interaction regions. I gave a quote from the cern blurb. The beams are brought into collision at the interaction points. this is the design report for anybody ambitious enough ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html . "The most direct way of increasing luminosity is to focus the beam more tightly at the collision point (reduce Seff , or more especifically the so-called β* parameter)" from lhc-closer.es/1/4/9/0 $\endgroup$ – anna v Feb 12 '15 at 4:19

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