The LHC at CERN, the European Organization for Nuclear Research, is the largest, most complex and most powerful particle accelerator ever built. It operates in a circular tunnel almost 17 miles in circumference about 330 feet underground, between Switzerland's Lake Geneva and France's Jura mountains. The LHC can create almost a billion proton-proton collisions per second. In March 2010, it collided protons at a center-of-mass energy of 7 trillion electron volts, 3.5 TeV per beam. It is eventually expected to reach a center-of-mass energy of 14 TeV, seven times higher than Fermilab’s Tevatron. Physicists conduct experiments at the energy frontier with two general purpose detectors, ATLAS and CMS, and two specialized detectors, ALICE and LHCb.
Scientists predict that its very-high-energy proton collisions will yield extraordinary discoveries about the nature of the physical universe. Beyond revealing a new world of unknown particles, the LHC experiments could explain why those particles exist and behave as they do. The LHC experiments could reveal the origins of mass, shed light on dark matter, uncover hidden symmetries of the universe, and possibly find extra dimensions of space.
The Large Hadron Collider guides beams of protons in opposite directions through two vacuum pipes around the accelerator ring using 1,232 superconducting dipole magnets.
At the heart of the LHC are superconducting magnets made of niobium-titanium. When cooled with liquid helium to negative 514 degrees Fahrenheit, the cable inside the superconducting magnets can conduct electric current without resistance. The lack of resistance permits the magnets to operate at higher currents, which gives operators the ability to steer particles accelerated to higher energies.
Because the LHC is a proton-proton collider, it must have two separate magnet rings to guide the proton in opposite directions. Several thousand additional magnets fine-tune the beams’ orbits, and some 400 quadrupole magnets keep the beams in focus as they travel millions of miles around the ring. An acceleration section gives additional energy to the particles as they circle around the tunnel about 11,000 times per second.
Additional special magnets near the interaction regions focus the protons into hair-thin beams that collide within the LHC experiments.
Once they have reached their maximum energy, the beams of protons collide at each of two eight-story general purpose detectors, CMS and ATLAS, which are positioned at two different points along the LHC. The beams cross paths an average of 3.1 million times per second at each detector. Each time presents an opportunity for one or more collisions between circulating protons.
Billions of protons in the LHC’s two counter-rotating particle beams will collide at an energy of 14 trillion electron volts. The LHC physics program mainly uses proton-proton collisions, but the LHC will also be able to accelerate lead ions to create collisions with an energy of 1,150 TeV during shorter running periods, typically one month per year.
Collisions create showers of new particles. Physicists take interest in collisions that stand out due to the force of their impact or for the types of particles they produce. For interesting events, about 100 of which occur each second, CMS and ATLAS record each particle’s flight path, energy, momentum and electric charge. Physicists use this information to identify the types of particles created by the collisions and to determine if they have discovered something new.
CMS and ATLAS, two detectors positioned on opposite sides of the LHC beam pipe, use different technologies to capture and analyze similar information about the particles proton-proton collisions produce.
Like CDF and DZero, CMS and ATLAS are built differently, but the particles that spray out from collisions travel through three similar basic layers in both. The inner layer of each detector is comprised of tracking detectors, which record the flight paths of passing electrically charged particles.
Because the layers of tracking detector are located within magnetic fields, charged particles such as electrons, muons and charged hadrons follow curved paths through them. The slower or less massive the particles, the greater the magnet’s effect on them, and the more they curve. Scientists therefore use the amount which a particle’s track curves to determine its momentum. This information helps them determine what kinds of particles were produced immediately after the proton and antiproton collided.
Particles curve differently in the tracking detectors of CMS and ATLAS because the two detectors use different types of magnets. CMS uses a high-strength solenoidal field, while ATLAS uses a toroidal field.
The next layers are made up of calorimeters, which measure the energy of particles. These are massive detectors that absorb the energy of charged particles. When a high-energy particle collides with a layer of lead, steel or uranium in the calorimeter, it showers, creating a cascade of lower-energy, charged particles. These in turn hit other metal atoms and create their own showers of even lower-energy particles. In this way, one 100-GeV electron could become 100 1-GeV electrons by the end of the ride.
Only muons, which are like electrons but much heavier, can pass through the inner layers of the detectors and still leave their traces in the outer layers of the detector. Normally only muons and neutrinos pass all the way through to the muon chambers. But neutrinos hardly interact with matter at all; practically all of them course through the detector without leaving any signal. By recording how many particles the outermost detector finds, physicists can determine which particles that came out of the collision were high-energy muons.
Physicists also gather information about particles that slip through detectors undetected. Physicists know that according to the law of conservation of energy, what they put into the system should equal what they get out of it. Missing energy means the collision created some particles that the detector cannot see. These could be particles such as neutrinos that don’t interact with matter very often. Or they could be particles that have not been discovered yet. Many searches for new physics postulate the existence of particles that the detector would not be able to see directly. Looking for missing energy is the only way to validate the existence of these particles using the detectors.
The two collaborations verify each other’s results, which ensures that they do not misinterpret data anomalies as discoveries. Working in tandem, the detector teams can search mass and energy ranges more efficiently than they could alone.