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

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