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This travel stackexchange answer has kinda got me wondering... how long do experiments involving the large hadron collider usually take? I'd expect you run it for a few seconds and bam - higgs boson detected or whatever. Maybe it'd take a few months to set the experiment up but once it's setup it doesn't seem like it'd take that much time at all to run the experiment?

I mean, maybe you'd want to run it a few times to verify your results but if each run takes just a few seconds it seems like you could still be done with your multiple runs even in a single day.

Any ideas?

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4 Answers 4

up vote 8 down vote accepted

This document (NB it's a pdf) contains details of the beam operation. Here's a key graph nabbed from the presentation:

LHC

At the end of an experimental run the beam is dumped, and it takes about an hour and a half to get the beam back up to full energy and intensity. Once the beam is at full strength the LHC generates data continuously for somewhere between 10 and 20 hours before the beam intensity is too low and the beam needs to be dumped again.

Note that the LHC isn't an experiment that runs once and generates one result, then repeated to generate a second result and so on. Once the beam is live it generates data continuously and this data builds up for days and months. Because signals like the Higgs are so weak you need months and months worth of data, i.e. months and months of beam time, to get enough data to see these small signals. CERN have made an animation showing how the Higgs signal built up over time, which you can see here.

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What madness is this? There seems to be a noticeable lack of pink on your profile :P –  Manishearth Nov 20 '13 at 17:59
    
@Manishearth :-) Given we're about to get hats I thought I should have a head to put it on. That really is me, though it's an old photo and I'm older and more grizzled these days. –  John Rennie Nov 20 '13 at 19:06
    
Ah, makes sense. I know it really is you, I recall it from Facebook. :) –  Manishearth Nov 20 '13 at 19:10

I'd expect you run it for a few seconds and bam - higgs boson detected or whatever.

How does one detect the Higgs Boson? That's the question you must consider.

Since a real Higgs almost immediately decays upon being produced, all we can detect are the stable decay products, those particles that last long enough to leave the vicinity of the collision and interact with the detectors that surround the beam pipe.

But, remember that there are many collisions and particles created that decay into stable particles that register with the detectors.

So, all we can really do is calculate what we would statistically expect from all these particles (besides the Higgs) being created and decaying and then, after running the LHC for a long period of time, look at the results statistically and compare with the calculations.

The "signature" of a new particle or particles would then be a deviation in the experimental statistics from the calculated statistics. In order to be confident that any detected deviation is statistically significant requires an enormous amount of data since, for one thing, the production rate of the Higgs is so low; about 1 in 10 billion at the LHC.

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The experiments take many years. Using the Higgs boson search as an example, the reason it takes so long is because there is no "smoking gun" signature of a Higgs boson. You don't search for the Higgs boson directly, but for the Standard Model particles to which the Higgs decays. Since other Standard Model processes (called "backgrounds") can produce these particles, for any given collision at the LHC you cannot be sure that you have seen the Higgs boson. You have to produce thousands of Higgs bosons before you have a large enough statistical sample from which to draw any conclusions. And even then it can be a difficult process of comparing your observed rate of Higgs-like events to the predicted background rate. So finding the Higgs boson is not trivial; it corresponds to the experimental observation of a Higgs-like rate that is slightly larger than the predicted background rate. Since the background rate prediction has its own set of errors and assumptions, and statistical fluctuations are possible, it takes a long time to create enough Higgs bosons, and understand the detector well enough, to be confident that an observed excess of Higgs-like events is real.

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There's actually a FAQ for the LHC (CERN approved too). In that link , you'll see your exact question with the answer (all emphasis mine),

When a proton leaves the source, it crosses the linac and reaches the PSB in a few microseconds. In the PSB it is accelerated from 50 MeV to 1.4 GeV in 530 ms, then after less than a microsecond it is injected in the PS where it can either:

  • be accelerated/manipulated/extracted in 1025 ms
  • or wait for 1.2 more seconds before being accelerated, if it's part of the first PSB batch to the PS.

Then it is sent to the SPS where it waits for 10.8, 7.2, 3.6, or zero seconds whether it's part of the first, second, third, or fourth PS batch to the SPS. The SPS accelerates it to 450 GeV in 4.3 seconds, and sends it to the LHC.

So the time it takes from the source to the exit of the SPS is between

0.53 + 1.025 + 4.3 = 5.86 seconds and

0.53 + 1.2 + 1.025 + 10.8 + 4.3 = 17.86 seconds

Then our proton has to wait up to 20 minutes on the LHC 450 GeV injection plateau before the 25 minutes ramp to high energy, and these 45 minutes dominates the transit time.

The current number of experiments per day at the LHC is zero, as it's been temporarily shut down for some revamping. I think, though, that when it was running earlier in the year there were only a few experiments done per day (on the order of 5-10) but don't quote me on this, as I'm not 100% sure--if someone has the right answer, I'll amend this statement.

EDIT From the comments from user1247, the above quoted "experiments per day" is incorrect. The particle beam is stored in the rings for 10-20 hours providing several hundred million collisions per second before the supply of protons needs to be refilled.

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It doesn't make sense to say that "X number of experiments were done per day," because when the LHC is in operation the experiments run more or less continuously. The LHC produces millions of proton-proton collisions per second, more or less continuously, for months (taking short breaks to refill the proton ring every day or so), and the experiments continuously take data during this time. –  user1247 Nov 20 '13 at 14:54
    
Like I said, I'm not 100% sure. The downvote from you seems unnecessary given the fact that I pretty much asked for a correction to that aspect while the "how long to set up the experiment" aspect of the question (which your answer ignores) is factual. –  Kyle Kanos Nov 20 '13 at 15:09
    
Nothing personal, I just don't think your answer really addresses the question. I don't think the question is "how long does it take to get the proton beam to the experiments?" As such the answer is just not all that relevant. –  user1247 Nov 20 '13 at 15:29

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