Large Hadron Collider: The Discovery Machine

 

1. ______ The mammoth machine, Large Hadron Collider, after a nine-year construction period, is scheduled (touch wood) to begin producing its beams of particles later this year. The commissioning process is planned to proceed from one beam to two beams etc; from lower energies to the terascale; from weaker test intensities to stronger ones suitable for producing data at useful rates. Each step along the way will produce challenges to be overcome by the more than 5,000 scientists, engineers and students collaborating on the gargantuan effort. To break into the new territory that is the terascale, the LHC’s basic parameters outdo those of previous colliders in almost every respect. Its nearly 7,000 magnets, chilled by liquid helium to less than two kelvins to make them superconducting, will steer and focus two beams of protons traveling within a millionth of a percent of the speed of light. Each proton will have about 7 TeV of energy − 7,000 times as much energy as a proton at rest has embodied in its mass.

2._____ That is about seven times the energy of the reigning record holder, the Tevatron collider at Fermi National Accelerator Laboratory in Batavia, Ill.. The protons will travel in nearly 3,000 bunches, spaced all around the 27-kilometer circumference of the collider. Each bunch of up to 100 billion protons will be the size of a needle, just a few centimeters long and squeezed down to 16 microns in diameter (about the same as the thinnest of human hairs) at the collision points. Four giant detectors − the largest would roughly half-fill the Notre Dame cathedral in Paris, and the heaviest contains more iron than the Eiffel Tower − will track and measure the thousands of particles spewed out by each collision occurring at their centers. Despite the detectors’ vast size, some elements of them must be positioned with a precision of 50 microns. A “farm” of a few thousand computers at CERN will turn the filtered raw data into more compact data sets organized for physicists to comb through.

3._____ As the full commissioning of the accelerator proceeds in measured step-by-step fashion, problems are sure to come up. The big unknown is how long the engineers and scientists will take to overcome each challenge. If a sector has to be brought back to room temperature for repairs, it will add months. When everything is working together, the task faced by the detectors and the data-processing systems will be Herculean. At the design luminosity, as many as 20 events will occur, with each crossing of the needlelike bunches of protons. The millions of channels of data streaming away from the detector produce about a megabyte of data from each event: a petabyte, or a billion megabytes, of it every two seconds.

The trigger system that will reduce this flood of data to manageable proportions has multiple levels. The data passed to Tier 0 by the four LHC experiments’ data-acquisition systems will be archived on magnetic tape. That may sound old-fashioned and low-tech in this age of DVD-RAM disks and flash drives, but the experts of the CERN Computing Center think it turns out to be the most cost-effective and secure approach.

4. _______ With all the novel technologies being prepared to come online, it is not surprising that the LHC has experienced some hiccups − and some more serious setbacks − along the way. Last March a magnet suffered a “serious failure” during a test of its ability to stand up against the kind of significant forces that could occur if, for instance, the magnet’s coils lost their superconductivity during operation of the beam (a mishap called quenching). Part of the supports of the magnet had collapsed under the pressure of the test, producing a loud bang like an explosion and releasing helium gas.  The problem was a design flaw: the magnet designers (researchers at Fermilab) had failed to take account of all the kinds of forces the magnets had to withstand. CERN and Fermilab researchers worked feverishly, identifying the problem and coming up with a strategy to fix the undamaged magnets in the accelerator tunnel.

5._______    The seemingly ever receding start-up date is a concern because the longer the LHC takes to begin producing sizable quantities of data, the more opportunity the Tevatron has − it is still running − to scoop it. The Tevatron could find evidence of the Higgs boson or something equally exciting if nature has played a cruel trick and given it just enough mass for it to show up only now in Fermilab’s growing mountain of data. Holdups also can cause personal woes through the price individual students and scientists pay as they fall behind with stages of their careers waiting for data. However, for a machine of this complexity, things are going remarkably smoothly, and everyone is looking forward to doing physics with the LHC very soon.

 

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