University of Colorado physicists are among the several thousand from around the world who participate in research with the new Large Hadron Collider (LHC) now coming into operation at CERN, the European particle physics laboratory in Geneva, Switzerland. When fully operational the LHC will supply a handful of detector facilities with data that may answer questions such as how does matter assume mass, and what is the nature of the dark matter in the universe. Professor Bill Ford and graduate student Bernadette Heyburn are two of the Colorado group in the CMS collaboration, whose name is derived from the shorthand description of their detector, "Compact Muon Solenoid." Here they provide a first-hand account of their impressions upon jumping into the thick of the action as the LHC collider delivered its first proton collisions to CMS in December.
Graduate students Bernadette Heyburn (left) Brian Drell and in the CMS detector hall in 2007, during the assembly of CMS.
(Bernadette) Sunday Dec 6, I get a call from my brother in Portland, OR at 4:30 AM to make sure I'm awake. I take the tram and bus to the main CERN site, so I can catch the 6:15 shuttle to Point 5 (CMS, five miles away on the far side of the LHC ring) for my 7:00 shift. The control room is especially crowded - incoming and outgoing shifts are both present, plus several experts for each subdetector. My particular responsibility as a "shifter" is for the pixel subdetector, a very fine "camera" that sits closest to the collision point. We want to be sure that if any issues arise during this exhilarating new period of collisions, we can fix them right away. I spend my time checking DQM (data quality monitoring) plots, monitoring voltages, currents, and temperatures, checking the data files, etc. There are at least four other pixel people here, all experts.
(Bill) At 9:00 AM I'm just off the plane at Geneva airport. In the rental car shuttle I hear second-hand from one of the Fermilab postdocs that we've had collisions since 5:00 AM. An hour later I'm sitting in the CERN cafeteria with my laptop. I see that the run in progress has been going since 6:00 AM.
(Bernadette) At 10:00 I receive an email from an old friend who is watching the LHC Portal Page from Los Angeles. Even non-physicists are anxious! At 10:40 the run stops as we have lost Beam 2. Forty minutes later, we notice that the temperatures of the pixel detector are rising and that we have lost one of the cooling plants. We power off the detector completely as a safety measure. The cooling issue is soon fixed and we return low-voltage power to the pixel detector and wait for more stable beams.
The example posted by Kevin, Brian, and Keith of an event with a K0 (K-short) reconstructed from the pair of pions into which it decayed. The detector elements with signals are shown in yellow, the particle trajectories reconstructed from these in green, and that of the candidate K0 in red. The outer green circle bounding the tracking detector has a radius of about 1 meter.
(Bill) The K0 e-mail reaches me at the CERN cafeteria, where I'm still waiting for my room in the hostel to be ready. Jim H., also just flown in from the U.S., is frantically searching for 2-jet events. The balance of the jets' energies will give a measure of how well the calorimeters are working.
(Bernadette) After shift at 3:00 PM, the shuttle back to CERN is full, and although we are tired, we are all chatting away at what an interesting day it had been. Back at the main CERN site, I go to the cafeteria for an early dinner and to see all the people in town for CMS Week!
(Bill) Monday, Dec. 7, start of the CMS week collaboration meeting. The first talk describing the experience in operating the LHC collider exudes admiration at how well the instrumentation describes the properties of the beams, and how well these agree with the calculations from the model. Then come the reports from collaboration management, subdetector conveners, the computing guy, the safety guy. I keep seeing those pictures of K0 mesons, tracker efficiency, a new one of lambda baryons, and many others produced by our collaborators. A nice 2-jet event is shown (not Jim's actually, but someone else's).
Postdocs Mauro Dinardo (left) and Keith Ulmer in the CMS detector hall during the assembly of CMS. The central part of CMS is in the background.
Tuesday, Dec. 8. At breakfast Sridhara reminds us that we're getting data at a rate one billionth of what it will be when the machine works at its design energy and intensity. The energy of the collisions we're studying is a sixteenth of the collider's design energy, and half that of the long-running Fermilab Tevatron collider in Illinois. So let's not get carried away; we still have a lot of work to do.
(Bernadette) Friday Dec 11. My last shift and it's a good one! In the morning we run with 5x5 bunches at a relatively high intensity, but unfortunately the CMS solenoid is only at partial strength. We have a small problem with the cooling, but it is fixed within minutes and the pixel detector remains in a stable, cooled state. There's a small break for a couple of hours, and then the LHC starts to inject again (4x4 bunches). I won a bottle of champagne for having the most shifts for the pixels, which I plan to share with my colleagues later!
(Bill) Wednesday, Dec. 16 - back in Colorado. Over the past few days the LHC has finished its work for 2009, before taking a breather for the holidays, with some data at energy a bit higher than Fermilab's. We can now claim to hold the energy record for a man-made machine.
The distribution made by Kevin, Brian, and Keith of the reconstructed mass of pion pairs. The appearance of the sharp peak just below 0.5 GeV corresponds to the decay of a K0 (K-short), whose mass is known to be 0.497 Ge.
The CERN laboratory is the European center for research in elementary particles based on the study of reactions of protons or electrons from high-energy accelerators. The Large Hadron Collider (LHC) is the laboratory's latest project, under design and construction since about 1990. When fully operational it will store protons in counter-circulating beams with energy of 7 TeV (7 trillion electron volts). The beams are confined by superconducting magnets to the inside of a vacuum pipe of a few centimeters cross section traversing a 27-kilometer diameter circle, deep underground straddling the Swiss-French border near Geneva. The two beams collide at several locations around the ring, at which experiments are set up.
The colliding protons react to produce elementary particles such as pions, kaons, protons, neutrons, electrons, muons, etc. The experiments are performed with detectors that can sample the trajectories of some of these particles, measuring their speeds (or more precisely, their momenta) by how much they bend in the magnetic field of a solenoid coil; others are detected more crudely by the splash of photons, electrons, and other debris produced when they react violently with matter (or for muons, by their lack of strong interaction with matter). Six detectors at LHC are in place, each designed with unique features and goals.
High on the list of potential discoveries at the LHC is the Higgs boson. The theory ("standard model") of particle physics states that all matter is built of fundamental "fermion" particles (quarks and leptons), which come in a dozen flavors. These include electrons, and the "up"- and "down"-quarks of which protons and neutrons are made. The interactions of these particles are mediated by half a dozen "boson" particles, such as the photon (quantum of light). But in this theory the matter particles can only attain mass through the action of a hypothetical boson, Higgs, which has yet to be seen experimentally. The Higgs is predicted to appear within our reach by most any reasonable extrapolation of current knowledge.
Extensions of the standard model to higher energies lead to theoretical difficulties that are solved in some theories by the introduction of a family of hypothetical objects known as SUperSYmetry (SUSY) particles. The LHC experiments will also search for these. Among them would be a candidate for the dark matter in the universe.
The Compact Muon Solenoid (CMS) detector is one of the experimental facilities within CERN. Currently fourteen members of the physics department at the University of Colorado are among the collaborators that contribute to its research program. The CU participants are
faculty: John Cumalat, Bill Ford, Uriel Nauenberg, Jim Smith, Kevin Stenson, Steve Wagner
Also mentioned in the journal:
CMS has a diameter of 15 meters, length of 21 meters, and has around 100 million electronics readout channels. Its main subdetectors are (1) the "tracker", silicon pixel and silicon strip cylinders and disks nearest the beam line, which are used to measure where the various charged particles originated, and by their curvature in the magnetic field, their momentum; (2) the superconducting coil and iron return yoke (12,000 tonnes worth) that provide the magnetic field; (3) "ECAL", sampling calorimeter optimized for electron and photon location and energy measurement; (4) "HCAL", sampling calorimeter optimized for hadron location and energy measurement; (5) ionization detectors embedded in the return yoke to measure the trajectories of muons; (6) electronics, over and above the readout circuits of each subdetector, to collect all the data, and to decide which are valuable enough to keep.
Perhaps it's not too surprising that such a device would require approximately 2000 physicists, plus nearly as many engineers and support staff, to assemble, operate, calibrate, and maintain it. These come from 180 institutions in 40 countries. The Colorado group is particularly concerned with the pixel tracking detector.
The LHC started its maiden run in September 2008. Beams were made to circulate in the machine with remarkable ease. But very soon thereafter, during tests to bring a section of the magnet ring toward its design energy, an electrical connection in the superconducting circuit failed, creating thermal runaway that boiled a large volume of liquid helium coolant and caused extensive mechanical damage. About 14 months elapsed before repairs, and the installation of new protection systems, were completed. The 2009-10 run plan calls for a much more gradual approach to full operation of the LHC. The first circulating beams were achieved on November 20, 2009.
Some of the pictures described in the journal appear in one of the semi-monthly CMS Times newsletters from the December run period.
Point 5, the site on the LHC ring at which the CMS detector sits. It's about 5 miles away from the main CERN site, across the ring.
CMS center: A satellite control room for CMS, at the main CERN site.
event: A pattern of signals from the detector within a narrow time window that meets pre established criteria for recording, normally corresponding to the reaction of a particular proton in one LHC beam with one in the opposing beam.
hadron: a particle that is affected by strong (nuclear) forces, or interactions. Among these are protons and neutrons.
meson: a hadron that can be created or destroyed (may act as a quantum of force).
baryon: a hadron that cannot be created or destroyed, except as one of a pair with an anti-baryon; proton, neutron, and quark are examples.
pion, or pi meson, is the lightest hadron. kaon is a bit heavier (about half the mass of a proton).
K0 (neutral kaon) is a meson that decays into two pions (also called a K-short). It has no electric charge, so leaves no track in the detector. But the two (charged) pions from the decay do leave tracks.
pi0 (neutral pi meson) is so short-lived that it travels no measurable distance before decaying into a pair of photons (quanta of light).
Lambda hyperon: a baryon, a bit heavier than a neutron.
muon: a lepton, a particle much like an electron but with mass 200 times greater.
Fermilab: The Fermi National Accelerator Laboratory in Batavia, Illinois; central facility for accelerator-based particle physics in the United States.
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