1. Confronting the Unknown: The Large Hadron Collider The Detectors Just as telescopes gaze outwards toward the most massive objects and the most immense distances in the universe, particle detectors peer inwards at the smallest constituents of matter. Four detectors are poised to record the products of the proton collisions. ATLAS, CMS, ALICE and LHCb are each the size of a cathedral and are housed in subterranean chambers at collision points on the LHC ring. Andrea Bangert, Santa Cruz Institute for Particle Physics CERN Messier 81, NASA Above: Assembly of an endcap of the ATLAS silicon strip detector. Technicians are mounting power distribution cables. Photograph by Peter Ginter. Known Unknowns The Standard Model describes the matter particles and the forces which communicate between them. The quarks and the leptons are the fundamental constituents of matter which bind together to form protons, neutrons, and atoms. The W and Z bosons and the familiar photon transmit the forces responsible for binding matter particles into atoms. The Standard Model describes what we see very, very well. However, scientists have recently discovered that the matter familiar to human experience forms only a very small part of the universe. Rotation rates of spiral galaxies demonstrate that luminescent stars do not contain the total mass of the galaxies. Most of the mass is attributed to matter which does not glow: we call this dark matter. Dark matter is unlike any form of matter observed on earth, so in order to study it we must create it using accelerators such as the Large Hadron Collider. Undiscovered Symmetries The laws of physics are derived from symmetries of nature. Laws which have not yet been formulated may reflect undiscovered symmetries. One promising candidate is called Supersymmetry, which predicts that every known particle is mirrored by a supersymmetric partner, doubling the ranks of the matter particles. The least massive supersymmetric particle provides an attractive candidate for dark matter. However, no experiment has yet detected such a particle. The discovery of supersymmetry, if it is realized in nature, is one of the most pressing challenges which the LHC will face. Arpeggios on a Single Superstring String theory unifies the fundamental forces by postulating that all known particles and forces represent different modes of vibration of objects called superstrings. String theory predicts Supersymmetry. It also predicts seven yet undiscovered spatial dimensions. The observation of currently unknown dimensions would represent a Copernican shift in our understanding of the universe. Above: An engineer installs a magnet in the LHC tunnel. The magnets guide the protons in their trajectory around the LHC ring. Right: A technician connects cables to an endcap of the ATLAS silicon strip detector. Photographs by Peter Ginter. The Machine The Large Hadron Collider (LHC) is situated at the European Organization for Nuclear Research (CERN) in Switzerland. The accelerator ring is 17 miles in circumference and is housed in a tunnel 300 feet below the surface of the earth. The machine is designed to accelerate protons to near light speeds, then guide clockwise and counter- clockwise proton beams to produce highly energetic collisions. The LHC will begin operation in December 2009. Left: the LHC lies between the Swiss alps and the Jura mountains, close to Lake Geneva. The circumference of the subterranean accelerator ring is shown in red. Above: The world’s largest solenoid magnet sits at the heart of the CMS detector. Photograph: Peter Ginter.