LHC's record intensity speeds Higgs search

Squeezing more bunches of protons onto the particle accelerator's beams increases the data-gathering rate--and the odds the Higgs boson will turn up at CERN.

LHS experiments are assembled underground. Here, a silicon tracking detector--a cousin to an ordinary digital camera sensor--is inserted into one of the LHC's two general-purpose experiments, the CMS. Michael Hoch/CERN

The Large Hadron Collider has surpassed a record set by Fermilab's rival particle accelerator for what's called luminosity, a milestone that improves the odds that the gargantuan scientific experiment will produce new physics discoveries.

The LHC operators yesterday packed more bunches of protons into the beam, increasing the likelihood of collisions and therefore of the detection of very rare outcomes from those collisions.

"Beam intensity is key to the success of the LHC, so this is a very important step," said General Rolf Heuer, director of the CERN facility that operates the LHC, in a statement. "Higher intensity means more data, and more data means greater discovery potential."

It also means a better chance that the LHC will be the center of excitement in scientific circles, not the older, less powerful, but still significant Tevatron accelerator at Fermilab in Illinois. Last month, Fermilab reported enough progress in tracking down physicists' favorite unknown fundamental particle, the Higgs boson, to declare the Tevatron the "frontrunner" in the search. The Higgs boson is thought to be instrumental in endowing particles with mass.

The LHC particle accelerator, an underground ring with a 27-kilometer circumference beneath Switzerland and France near Geneva, actually has two beams of protons traveling in opposite directions. The collective energy in each direction is 3.5 tera-electron-volts, or TeV, but because the beams travel in opposite directions, the energy adds up to 7 TeV. That's only half of the LHC's ultimate planned energy level of 14TeV , but it's enough that researchers think they'll be able to find new physical phenomena.

And finding new physics is important for the facility. It postponed a shutdown to upgrade the facility to the higher energy level, betting that there were good odds that the lower level would produce fruitful results. The Higgs boson--or rather a possible collection of them--is just one focus of the facility; the thousands of physicists at the LHC also hope to investigate subjects including the quark-gluon plasma, supersymmetry in among fundamental particles, and dark matter. To perform the research, the LHC accelerates protons (and sometimes lead ions) so they're tremendously energetic, then smashes them into each other to reproduce, for a fleeting moment, a little bit of what the universe was like in its earliest fractions of a second after the Big Bang.

A proton must travel very close to the speed of light to attain energy of 3.5TeV. But even then, it's only got the energy of a flying mosquito. The LHC makes up for this apparent feebleness with numbers, though: It's designed to have enough protons that the total energy in the collider will be that of a 20,000-ton aircraft carrier traveling at 12 knots, according to James Gillies and Mike Lamont, writing on a CERN blog today.

Reaching 3.5TeV is only one challenge. Squeezing more protons into each beam is also crucial, and LHC operators have been gradually increasing that number. Each beam consists not of a continuous stream of protons, but rather a series of bunches with about 100 billion protons each. Yesterday, the LHC got the number of bunches up to 480, which is what led to the record. Ultimately, the LHC is designed to accommodate 2,808 bunches.

Each bunch is only a few centimeters long and--to maximize the odds that protons will actually collide--is compressed to the width of a human hair at the zones in the ring where collisions take place. With 100 billion protons in that volume, it may sound pretty crowded, but it's not. If protons were the size of marbles, a bunch would be as long as the distance from Earth to Uranus and the width as long a the distance from Earth to the Moon. Each marble would be more than 500 miles apart.

Thus, LHC operators want to squeeze more bunches into each beam. They also want to run the beam for as long as possible to collect data from as many collisions as possible. Much of the existing LHC work has alternated between operational priorities to get the machine working properly and research priorities to actually investigate physics, but the LHC is about to embark on months-long run solely devoted to physics.

"There's a great deal of excitement at CERN today and a tangible feeling that we're on the threshold of new discovery," said Sergio Bertolucci, CERN's director for research and scientific computing.



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