First Collisions in the LHC!

By John Conway | November 23, 2009 12:45 pm


The LHC circulated two counter-rotating beams today, and a few hours ago the CMS experiment recorded its first collision event, shown in the display above. This is a fantastic milestone for the LHC and the experiments! (Sorry the event display is fuzzy; I zoomed in on a portion of the larger one.)

The green lines are the tracks of charged particles from the collision, which are typically pions, which are unstable particles consisting of an up quark and an anti-down (or an anti-up and a down). Though they are unstable, they live long enough to nearly always leave tracks in the detector. The yellow rectangles indicate the position of the silicon strip detectors that recorded their passage.

The red and blue boxes indicate where energy was detected in the detectors outside the tracking detector, called the calorimeters. The inner calorimeter is sensitive to electromagnetic energy deposits coming from high energy photons – gamma rays – and from high energy electrons. Deposits in that one are in red here. Now, most of the high energy gamma rays here are coming from decays of neutral pions, which are much more unstable than their charged cousins. A neutral pion is a quark-antiquark combination, and since they are the same “flavor” of quark they can annihilate electromagnetically to two photons in a very short time; we see the two gammas in the electromagnetic calorimeter. Outside the electromagnetic calorimeter is the “hadronic” calorimeter which detects the energy left by charged pions and other hadrons, particles which contain quarks, such as protons, neutrons, kaons, and many others. But most if not all of the particles here are pions.

Where did these pions come from? The beams each had an energy of 450 GeV, the energy at which they were injected into the LHC. In fact the LHC has not accelerated particles to higher energy yet, but may do so soon. The beams were not tightly focused, and so only rarely when the beam bunches passed through each other did collisions occur. And most of the collisions are sort of “glancing blows” that disrupt the incoming protons, breaking them apart, and sending some particles sideways into the detector. This is presumably what we have in this first collision event.

As time goes on and more collisions are made, we will record events in which the constituents of the protons collide with more energy, leading to sprays of particles transverse to the beam direction which we call “jets”. For example, a quark in one proton hitting an antiquark in the other proton with, say, a couple hundred GeV can produce two jets of 100 GeV going in opposite directions (from the beam’s eye view). Such dijet events will provide a very useful sample of data for aligning and calibrating the detector.

Only after the beam intensity and the center of mass energy is a lot higher will we expect to see rarer and more interesting processes like W and Z boson production, and top quark pair production (top-antitop pairs). There are plans to collide at 2.4 TeV (higher than the 1.96 TeV at the Tevatron) before the end of the year if all goes well, and to 7 TeV early next year. And all is definitely going well so far!

Addendum: from a friend’s Facebook page I snagged this display of a collision event in the ATLAS detector! So cool!!


CATEGORIZED UNDER: Science, Technology
  • Sean


  • Jenna


  • Pieter Kok

    at last!

  • John

    Should remark that both experiments are calling these “collision candidate events” in that they may possibly be something other than beam protons hitting other beam protons. For example it may be beam protons hitting residual gas atoms. But that is very much less likely to produce such symmetric events.

  • Julie Simon Lakehomer

    Wow! Finally! Congratulations, and thanks for the very interesting picture and explanation!

  • graviton383

    I’ve only waited 25+ years for this…


    Hmm… I wonder how many little universes were destroyed today? 😉

  • Mandeep Gill

    Rock, and FREAKING ROLL.

    Strap yerselves in, folks, here we go…


  • Ismanidar



  • Jeff

    Fucking exhilarating

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  • Gordon Stangler

    Completely awesome. Even with all the delays, it was well worth it.

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  • GodZirra!


  • Bruceleeeowe

    Finally it has been started. We were waiting from a long time. Still questions… Finally could it detect so called higgs boson?

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  • John

    Bruceleeowe: for a standard model Higgs boson we will have to be patient for a few years. But for a non-standard-model Higgs boson, who knows? Maybe next year. I am on it…

  • Maledict

    Oh, I’m soooo tired of hearing about the LHC! What value does it have in the real world? Why are we funding it? We could field another division of Xe Corp. in Afghanistan for what that damn thing costs! Tens of thousands of suspected terraist waterboardings could be accomplished for the U.S. investment alone! Where is your perspective? At long last, sir, have you no decency?

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  • Gabriel

    Congratulations to everyone who is involved! You guys are making history!

  • ken mac

    Thanks so much for that great explanation! I can not get enough of this stuff, fascinating beyond belief.

  • Roman

    Congratulations to all involved.
    Could somebody explain to a lay person what the screen shot comes from – is this just control software for engineers working there or are physicists reading data from it?
    It looks so familiar – file, edit, view, window, help just like in my word processor. And I’ve notice that runs and event counts are kind of high already but we are just celebrating the first collisions.

  • John

    Roman: the event displays are made using the software that “reconstructs” the event from the raw digital information recorded for a collision. Only collision events that cause a readout “trigger” to fire are recorded. The trigger is typically based on a certain amount of energy deposited in the calorimeters, and we are presently running with a very low threshold, recording every collision we can. Later on, only one in a million or so will get recorded. It’s mostly physicists who write the software to do all this, with a certain fraction of software engineers.

    Anyway, we have made many, many runs with cosmic rays, and recorded many events. That’s why the counts are so high.

  • Vickey

    OMG…………finally, I have been following the LHC since it was only an article in Scientific American. I am so excited, as ken mac said above “I can not get enough of this stuff” Will it be the Higgs Boson are something even more fantastic?? What new technology will come of what we learn, what Medical Breakthroughs?? Exhilarating!

  • Ian Sample

    It’s wonderful to see the machine up and running at long last. Fingers crossed commissioning and ramping up goes smoothly. Blogs like this are a real service to the interested public. I’m from the UK, where we give £1.5m a week to Cern, and mostly that has been for the LHC. I know scientists want, and need, to run results through peer review channels etc, but don’t underestimate the value of having any kind of running commentary on how the machine is doing and what, if anything, looks interesting in the collisions. The LHC is funded by taxpayers from more than 20 countries – we might as well say it belongs to everyone, not just you lucky people who get to play with it! Happy hunting!

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  • James Mason

    I can’t wait for the angels and the demons to come pouring down on you.

  • attila eori

    great we are standing at the cradle of big discoveries

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