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!!
November 23rd, 2009 at 12:52 pm
Woohoo!
November 23rd, 2009 at 1:41 pm
Hooray!
November 23rd, 2009 at 2:04 pm
at last!
November 23rd, 2009 at 2:06 pm
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.
November 23rd, 2009 at 2:57 pm
Wow! Finally! Congratulations, and thanks for the very interesting picture and explanation!
November 23rd, 2009 at 3:38 pm
I’ve only waited 25+ years for this…
November 23rd, 2009 at 3:55 pm
Hmm… I wonder how many little universes were destroyed today?
November 23rd, 2009 at 5:04 pm
Rock, and FREAKING ROLL.
Strap yerselves in, folks, here we go…
:->
November 23rd, 2009 at 9:13 pm
awesome..
congratulation…!!
November 23rd, 2009 at 9:18 pm
Fucking exhilarating
November 23rd, 2009 at 10:18 pm
[...] The LHC collided particles today! [...]
November 23rd, 2009 at 10:38 pm
Completely awesome. Even with all the delays, it was well worth it.
November 24th, 2009 at 3:38 am
[...] 在大强子对撞机（LHC）首次尝试注入粒子束四天之后，欧洲粒子物理研究所（CERN）宣布它成功完成了首次质子束对撞，为了这一时刻科学家们已经等待了数年。未来几周他们将会增加质子束的能量强度和首次尝试加速。 CERN新闻稿称，他们注入了两束质子流，每束能量为450 GeV，粒子没有加速到更高能量，它们分别从两个方向同步绕行，能在环型隧道内完成多达两处交会。在注入几小时后，CMS探测器记录到了首次粒子对撞。如果一切运转正常，CERN计划在年底前完成2.4 TeV（高于费米实验室Tevatron对撞机的1.96 TeV）能量级别的对撞，明年初达到7 TeV。 [...]
November 24th, 2009 at 3:48 am
Finally.
November 24th, 2009 at 7:01 am
Finally it has been started. We were waiting from a long time. Still questions… Finally could it detect so called higgs boson?
November 24th, 2009 at 8:24 am
[...] dann sollten auch schon eine Menge Kollisionsereignisse im Kasten sein. Mehr von Symmtry Breaking, Cosmic Variance, New York [...]
November 24th, 2009 at 1:07 pm
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…
November 24th, 2009 at 1:46 pm
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?
November 24th, 2009 at 2:18 pm
[...] technical. A far more knowledgeable account of it, and the source of the quote above, can be found here (which, by the way, is a great [...]
November 24th, 2009 at 11:13 pm
Congratulations to everyone who is involved! You guys are making history!
November 25th, 2009 at 3:51 am
Thanks so much for that great explanation! I can not get enough of this stuff, fascinating beyond belief.
November 25th, 2009 at 9:05 am
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.
November 25th, 2009 at 4:13 pm
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.
November 25th, 2009 at 11:24 pm
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!
November 26th, 2009 at 12:17 am
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!
November 26th, 2009 at 9:25 am
[...] the Tevatron at Fermilab on the outskirts of Chicago [Guardian]. The LHC’s Atlas detector snapped an image of two counter-rotating proton beams that collided [...]
November 26th, 2009 at 10:11 am
I can’t wait for the angels and the demons to come pouring down on you.
November 26th, 2009 at 3:34 pm
great we are standing at the cradle of big discoveries
November 27th, 2009 at 4:38 pm
[...] the Tevatron at Fermilab on the outskirts of Chicago [Guardian]. The LHC’s Atlas detector snapped an image of two counter-rotating proton beams that collided [...]
November 30th, 2009 at 7:01 am
[...] Again; Scientists Celebrate with Caution DISCOVER: A Tumultuous Year at the LHC Cosmic Variance: First Collisions at the LHC! Cosmic Variance: Spooky Signals from the Future Telling Us to Cancel the [...]
December 4th, 2009 at 8:13 pm
[...] the Tevatron at Fermilab on the outskirts of Chicago [Guardian]. The LHC’s Atlas detector snapped an image of two counter-rotating proton beams that collided [...]