The cavernous experimental hall that houses the Compact Muon Solenoid detector at CERN lies 100 meters underground and could easily be mistaken for a Bond supervillain’s lair—perhaps the secret rocket base in You Only Live Twice. The detector’s components were assembled on the surface and then lowered down the shaft with centimeters to spare—the first time such an approach has been tried. Here the final component, the 1,430-ton YE+1 end cap, is being mated to the detector on January 9, 2007.

Beam On!

Watching the first slug of protons jog their victory lap around the Large Hadron Collider (LHC) was a bit like watching JPL land the Phoenix on Mars: would the nail-biting, brow-furrowing, breath-holding physicists in Mission Control burst into applause as the batch of high-energy particles reached its checkpoint? Or would their stunned silence indicate failure?

For 53 minutes, you couldn’t cut this kind of tension with a knife.

Fortunately for the LHC, it was all cheers at 10:30 a.m. on September 10 near Geneva, Switzerland, where CERN, the European Organization for Nuclear Research, operates the goliath physics experiment. The LHC’s beam line, a ring 8.6 kilometers in diameter straddling the Swiss-French border, will recreate the conditions seen during the Big Bang. In the head-on collisions of billions of protons at a time, physicists have their fingers crossed that they will discover new particles, including the famed but as yet undetected Higgs boson.

As the LHC’s controllers popped open champagne bottles, some 75 Techers in Pasadena celebrated the beam’s first run at a midnight pajama party with pizza and beer, communicating via video conference with a dozen Caltech physicists in Geneva.

As soon as next month, two beams of protons zipping in opposite directions at 99.999998 percent the speed of light will collide at 10 trillion electron volts (TeV), ramping up to 14 TeV in the spring—about the energy released by four tons of TNT. At seven times the energy of the Tevatron at Fermilab, now the world’s second most powerful accelerator, the LHC will push beyond the limits of what is known in physics as the Standard Model, which has so far described all the known interactions between particles with high precision.

This spray of subatomic particle debris from the proton beam hitting a tungsten block was one of the first images recorded by the Compact Muon Solenoid’s detectors.

Some 40 Caltech scientists under Professor of Physics Harvey Newman and Associate Professor of Physics Maria Spiropulu work on the LHC’s Compact Muon Solenoid (CMS), one of two experiments searching for the Higgs particle. In anticipation of the terabytes of data soon to come, Newman has also directed a project that bops bits between computers at breakneck speeds—151 gigabits per second, or some 15,000 times faster than the best cable service in California.

Physicists hope the discovery of the Higgs will fill one of the gaping holes in the Standard Model—why do most particles have mass, and why are some particles heavier than others? If the model is correct, it’s the interaction between the Higgs and other particles that creates mass. So the Higgs decays much more often into heavier particles, such as the Z or W bosons, than it does into lighter particles such as muons or electrons. If most physicists’ hunches are right, the LHC will provide the energy needed to strip a 120–160 billion-electron-volt (GeV) Higgs particle from the wreckage of a proton.

The CMS is the heaviest CERN experiment, packing the weight of the Eiffel Tower into a volume 400 times smaller. Most of the weight is due to a massive metal coil that generates a 4-tesla magnetic field—enough stored energy in principle to lift the 12,500-ton magnet by 20 meters—which imparts a curve to the path of any charged particles created in the collision. The slower, heavier ones are the most affected—the more momentum a particle has, the straighter its course—allowing the momentum of each individual particle to be calculated by its trajectory through a nested set of detectors. Inside the magnet, a layer of lead tungstate crystals measures the energies of photons and electrons with high resolution, while layers of other materials capture the signatures of hadrons, a class of particles that includes protons, neutrons, and pions. Outside the magnet, another detector tracks muons, which come not only from collisions within the beam line, but from cosmic rays hitting air molecules in the upper atmosphere. Together, the system inventories the flurry of exotic particles from which the exceedingly rare Higgs signature must be plucked.

Because the Higgs will be so quick to decay, CMS isn’t designed to capture the particle itself. Instead, the physicists will look for a decay of the Higgs into two photons, the massless carriers of light. Though this is a rare decay mode—only happening once in every ten trillion collisions—its signature should lurk in the crystals that Caltech has spent the last decade and a half perfecting.

Caltech physicists, including grad student Vladlen Timciuc, have analyzed the data from many millions of simulated proton-proton collisions, most of which result in bombardments of photons and pions well documented in other experiments. These simulations are crucial to separating false alarms from the true Higgs. “Yesterday’s signal is today’s background,” says Timciuc, who will be looking through CMS data to find new particles.

CMS, located in Cessy, France, isn’t the only station on the LHC hoping to find the Higgs. A competing experiment, ATLAS, takes place on the other side of the ring, on the Swiss side of the border. Though CMS is the better at seeing the two-photon signature of a Higgs decay, there is another possible outcome: one Higgs decaying into two Z bosons, each of which decays into two muons. Both experiments have equal sensitivity to the latter mode, so if one experiment sees it, the other should too. Though both the CMS and the ATLAS teams will race to publish first, Spiropulu says it’s a friendly competition.
“We cannot play war games,” said Spiropulu. “We need each other.”

The LHC should start up in earnest on October 21, when the two proton beams will first run into each other. If the Standard Model is right, ATLAS and CMS should see a few Higgs decays a year. But being stood up by the guest of honor would just as exciting. Stephen Hawking recently made a $100 bet that the Higgs won’t show.

“When do you give up on the Higgs?” asks Newman. “There is no other compelling explanation for how mass is generated, and people have gotten used to it.”

With no Higgs, it would be back to the chalkboard for theorists. In the quantum world, forces are carried by particles—electromagnetism by photons, the weak force by W and Z bosons, and so on. A particle that supplies mass fits snugly in this framework. But if there’s no Higgs, perhaps other particles will pop out of the collisions.

“Whenever you turn on a telescope, you see something new,” says Sean Carroll, a senior research associate in physics at Caltech. “It’s the same with particle accelerators.” —MC

Marissa Cevallos (physics ’09) is the editor of the California Tech and a member of the women’s Ultimate Frisbee team. She reported on LHC’s first beam from Geneva, Switzerland, before starting a semester exchange program in Edinburgh, Scotland.