I haven’t said much since this year’s start-up of the LHC, but there have been some interesting developments, so I’ll add one last update. If you haven’t been following the LHC status, it has been exponentially increasing in collision rate while maintaining a fixed collision energy (about 3 pb-1 of 7 TeV collisions have been collected by the LHC experiments, which is a few thousand times less than the Tevatron’s 9 fb-1 of 1.96 TeV collisions, collected since 2001). My “particle body counts” are now completely obsolete: nearly all known particles have been re-discovered in the LHC experiments. And today, the first unexpected effect has been presented by an LHC experiment: “Long-range, near-side angular correlations,” which is presented in detail on the CMS public page. Below the cut here, I’ll explain what this means.
Posts Tagged ‘CMS’
At least it looks like it from this CMS event display:
See the CMS e-commentary for hourly updates and more information. (That’s how I know which results are public. :))
The yellow boxes are silicon strips that detected the passage of particles (most likely pions in this case) and the green lines radiating from the center are tracks reconstructed from those hits. They’re not constrained to meet at the center: that’s an indication that these particles actually originated where the beams collide. Beyond that, the red and blue bars show how much energy was collected in the electromagnetic calorimeter (electrons, photons, and hadrons) and the hadronic calorimeter (hadrons only), respectively. No activity can be seen in the muon detector (red boxes).
This is all consistent with what one should expect from the collision of two protons— a strong (QCD) interaction between the quarks and gluons producing a handful of strongly-interacting hadrons, rather than photons, electrons, muons, or taus, which are insensitive to the strong force. An electroweak interaction between the quarks and gluons, producing possible Higgs bosons or any of a number of other exciting possibilities * * * * * * * *…, are more rare, and will require collecting and sifting through huge numbers of collisions.
The CERN twitter site says that all four experiments saw collision-like events. It’s finally happening!
Protons have not only entered the LHC, but they have travelled through one of the experiments, ALICE, to a point part-way between ALICE and CMS. On the first try, the protons threaded through all the dipole magnets and appeared on target at the IP3 beam-dump.
Here’s the spot of protons, as seen by the beam-TV at IP3:
Full details are available on the LHC Injection Test webpage.
Meanwhile, CMS has entered more or less full-time data-taking mode. Until recently, we had been mostly installing detectors and taking cosmic ray data intermittantly, but now we are primarily taking data and closing up the detector. This week, we started our fourth major cosmic ray run without a magnetic field, in which charged cosmic ray muons appear as straight tracks through the detector, and soon we will turn on the magnetic field so that the tracks are curved with a radius inversely proportional to their momenta. In addition to bending muon tracks, it will tug our five-story iron endcaps so strongly that they will bend a few centimeters in the middle, with a force about 5 times their weight. Therefore, all components must be completely closed and bolted into position, closing off access to the inner detectors. Half of the detector is already closed.
This is it! It’s finally happening! More updates later.
Things are happening pretty quickly, after all! Six hours ago, protons were injected into the main LHC ring and they travelled several meters before (presumably) crashing into a deliberate beam dump. I had heard about an upcoming test in which the beam would be injected into one sector only (Sector 1-2, near ATLAS— the ring is divided into eight sectors). This must be the beginning of that test.
Just after I finished saying that there aren’t any clear “start” dates, CERN defined one. In this press release, the Conseil announced that on September 10, a 450 GeV beam will circulate the LHC storage ring. Note that “beam” is singular: colliding beams are not expected until 1 or 2 months later (that part was not in the press release).
I’d like to try something unusually blog-like on The Everything Seminar. As some of you know, the Large Hadron Collider (LHC) will come online sometime this fall to answer basic questions in particle physics, and some of these questions have been waiting for 20, 30, or even 40 years (the Higgs mechanism was proposed in 1964). My day-job is to commission the Compact Muon Solenoid (CMS) experiment, one of the two general-purpose particle detectors on the ring (the other is ATLAS), “general-purpose” meaning that it is designed for surprises, a discovery machine. Since it seems that I won’t be interrupting any math conversations, I’d like to give short updates on what’s going on and when things will be working, and maybe correct some newspapers. I’ll be spending some of the first data runs in the CMS Centre at CERN, and can relay a little bit of what it’s like in the first days of a new particle physics experiment.