Searching for the Wild Higgs
If found, it will be the capstone of the 'standard model' of particles and fields
31 December 2003--A machine designed to uncover the ultimate nature of matter is beginning to take shape beneath a Geneva suburb, straddling the border of Switzerland and France. Two weeks ago, shortly before the holiday shut-down of the European Center for Nuclear Research in Geneva, better known as CERN, the first magnets were installed in a tunnel that will connect an existing accelerator with a particle collider being built from scratch. The first dipole magnets for the collider will be installed in February. When the whole machine is completed and operational in about three years, the Large Hadron Collider (LHC)named for the class of relatively large particles it will smash togetherwill be poised to answer truly fundamental questions, some of which have troubled humanity in one form or another since the days of the pre-Socratic philosophers.
Why does a material universe exist? Why, in particular, does matter hang together the way it does? How is it that matter predominates over antimatter in the universe, so that it is matter we are made of? What is the mysterious ”dark matter” that eludes detection, and what about the dark energy that seems to be accelerating the expansion of the universe?
During the past generation, physicists have constructed a theory of energy and matter called the ”standard model.” The basic divisions are quarks, the constituents of protons, neutrons, and mesons; leptons, which include the trusty electron; and force-carrying particles (”gauge bosons”), notably the photon, the mediator of electromagnetism.
Putting the finishing touches on the standard model has been just as much a feat of engineering as of science. In the early 1980s, the Italian physicist Carlo Rubbia sold CERN on the idea of using magnets to propel protons and antiprotons around a ring in opposite directions, in order to pin down the W and Z gauge bosons, which carry the short-range ”weak” force associated with certain atomic decay processes. Simon van der Meer, a CERN scientist and engineer, came up with a crucial technique called stochastic cooling, in which computerized control systems nudge particles as they come around one side of the ring at near the speed of light so that they are perfectly aligned to hit each other when they arrive at the opposite side. Rubbia and van der Meer built a proton-antiproton collider at CERN based on those principles, got their quarry, and were promptly rewarded with a Nobel prize.
The next step was to build a machine that would be capable of finding the Higgs particle (named for the physicist Peter Higgs), a postulated force carrier that gives the W and Z gauge bosons their mass and probably gives quarks and leptons their mass as well. After the U.S. Superconducting Supercollider (SSC) project fell afoul of congressional budget cutters in the early 1990s, the honor of searching for the Higgs fell to the envisioned LHC, which would have the advantage of being built in a tunnel already created to accommodate a collider finishing its run.
In the Higgs field, which is believed to permeate all of space-time almost as if it were the ether of old, particles coupling to the field acquire effective masses, explains Chris Quigg, a particle theorist at Fermilab in Illinois who was deputy director of the design team for the SSC. Those that don't couplelike the photon or muonremain massless.
Everything about the LHC, scheduled to host its first proton-proton collisions in 2007, is superlative: the highest collision energy ever achieved, on the order of 1 TeV; the most powerful and reliable superconducting magnets ever mass-produced for an accelerator, which will be cooled with superfluid helium to generate fields of 9 Tesla; the largest and most complex detectors, whose installation is a feat of civil engineering in its own right and which require the hardest of radiation hardening to keep from self-destructing; control electronics, data acquisition and processing technologies, and digital communications that all push the state of the art much further than ever before; and automated data analysis procedures requiring batteries of the world's best software engineers.
Not least, anticipation of gigantically high data rates from the LHC's detectors has driven the concept of distributed supercomputing, dubbed ”grid computing,” on a global scale, notes LHC technical coordinator Paul Proudlock.
How matter matters
Considered purely as a technical challenge, the LHC is not to be underestimated. When the project was first conceived, recalls Roger Cashmore, the deputy director general of CERN and director of research for collider programs, engineers didn't actually know how to build the accelerator. Then, when they finally figured that out around 1994, it still took another five years to discover the key to constructing its detectors.
If the Higgs particle is discovered by means of the LHC, electrical and electronic engineers may take a large part of the credit. But the much more important point is that when the Higgs is discovered, it will be one of humankind's epochal achievements. The Higgs will tell us why the electron has the mass it has; that in turn will explain the shape and size of the atom, Cashmore says. In that sense, the mystery of matter will be solved.
What if the universe didn't include something like the Higgs? Protons and neutrons, bound by the strong force, would still exist, Quigg observes. But radioactive decay processes would be much faster in the absence of the weak force, and electrons would be absent. Atoms could not exist; nor could composite matter or, of course, life. The universe would consist predominantly of electromagnetic radiation.