Ghostly Visits from the Higgs?
The noose is tightening about the neck of particle physics' most sought-after quarry - the Higgs boson. Precise measurements made by the SLD experiment at the SLAC Linear Collider (SLC) in California provide the tightest constraints on the mass of the Higgs boson, and serve notice that physicists are hot on the trail of this beast. When the SLC measurements are combined with results from the Fermi National Accelerator Laboratory (Fermilab) near Chicago and the European laboratory, CERN, the snare appears to be nearly closed.
Quantum mechanics provides the means for constraining an as-yet-unseen particle. Its famous "Uncertainty Principle" allows that very energetic subatomic particles spontaneously appear, and just as suddenly disappear, rapidly popping in and out of existence. Empty space is fairly quivering with these ghostly apparitions, known as "virtual effects" or "quantum fluctuations", and their existence has profound physical consequences. Their importance was first demonstrated by the exquisite correspondence between experimental results and the theoretical calculations of quantum electrodynamics (QED), the modern quantum theory of electromagnetism culminated in the late 1940s.
Over the years, "quantum field theories" such as QED have become the bread-and-butter of modern high-energy particle physics. The Standard Model, a direct descendent of QED, describes a vastly more general arena than its predecessor, incorporating not only electromagnetism, but both the weak and strong nuclear forces that govern physics at distance scales 100,000 times smaller than the size of an atomic nucleus, and energies not seen in nature since a tiny fraction of a second after the Big Bang. Since the late 1970s, physicists at numerous accelerator laboratories, such as Fermilab, SLAC and CERN, have been subjecting the Standard Model to every conceivable experimental test, and so far, the predictions of the theory have been confirmed at every turn.
There remains, however, one missing piece of the puzzle - To date, no experiment has detected an essential participant in the Standard Model known as the Higgs boson. This elementary particle plays a critical role in the unification of the weak and electromagnetic interactions that is the centerpiece of the Standard Model, and is also responsible for imbuing all the particles in this model with their distinctive masses.
The race to directly produce the Higgs boson has been led by four experiments at the Large Electron Project (LEP) machine at CERN, an enormous ring-shaped machine 27 kilometers in circumference that accelerates electrons and their positively charged antiparticle, positrons, into head-on collisions at energies of over 200 billion electron volts (200 GeV). (The electrons in your television tube reach energies about 100 million times smaller than this, about 20,000 electron volts.) The LEP machine is at the limit of its capability in this difficult search. A higher energy accelerator may be needed to produce this elusive particle, and efforts are already underway to do just that. At Fermilab, the recently upgraded Tevatron proton/antiproton accelerator will begin operation this spring with the hope of extending the Higgs search beyond the LEP range. And a proton/proton machine at CERN, the Large Hadron Collider (LHC), with peak energies more than 7 times higher than the Tevatron, is due to come on line in 2005.
But thanks to the quantum fluctuations that are part-and-parcel of the Standard Model's repertoire, there is already some very intriguing information provided by present experiments unable to directly produce the Higgs boson. Virtual effects of a Higgs boson should occur, with a magnitude that depends upon its mass. As these effects are small, high-precision measurements are needed to detect them. Certain high-precision measurements have exceptional sensitivity to virtual effects; prominent among these are measurements of the properties of the W and Z bosons (which act as force carriers responsible for transmitting the unified "electroweak" interaction) performed at CERN, Fermilab, and SLAC. These precision measurements have already demonstrated their power by exploiting virtual effects of the top quark to predict its mass ; it was discovered at the Tevatron in 1995 just where expected. Fermilabs top quark discovery was an essential turning point; the top mass is now well measured, and virtual effects due to virtual Higgs bosons can be cleanly isolated.
The first hints that the Higgs boson mass might be rather small, and in the region accessible to the LEP or upcoming Tevatron experiments, came in 1995 from precision data obtained at the SLC, a 3-kilometer long electron/positron colliding-beam accelerator, the first of its kind ever constructed. By using the SLC's polarized electron beam (a property similar to the polarization of light exploited by sunglass manufacturers) to produce Z bosons, experimenters from the SLD collaboration at SLAC were able to make a uniquely precise measurement of the weak mixing angle, a key parameter of the Standard Model. Their updated analysis includes SLDs complete body of data collected through 1998 and appears in two papers, both recently published in the Physical Review Letters. SLDs results presently provide the best limit on the Higgs mass.
Other measurements providing good estimates of the Higgs mass include measurements of the W boson mass from the LEP and Tevatron experiments and many measurements of the properties of the Z boson at LEP. In particular, the recent updates of the W boson mass measurements are in excellent agreement with the SLAC prediction that the Higgs boson may be light. Interpretation of the world-wide precision data for effects due to virtual Higgs bosons leads to the expectation that the Higgs mass should be less than 249 GeV. The result from SLAC by itself is even more restrictive, suggesting that the Higgs boson mass should be less than 147 GeV. The allowed window for this holy grail of particle physics is closing: the Higgs boson mass is constrained by the direct searches at LEP to be greater than 113 GeV.
The summer and fall of 2000 were exciting times at CERN as intriguing hints of direct production of an 115 GeV Higgs particle showed up in 3 events observed by the ALEPH detector and one event observed by the L3 detector. Despite these intiguing hints and pleas for more running in 2001, the LEP search was concluded to clear the way for the constuction of the Large Hadron Collider, which will use the same underground tunnel. Meanwhile, physicists at Fermilab have just resumed operation of the upgraded Tevatron proton collider and are early awaiting its results . In addition, the successes at SLAC have prompted physicists from the United States, Japan and Europe to propose the construction of a Linear Collider of up to ten times the energy of the SLC, a machine ideally suited to the study of a Higgs boson, particularly with a mass in the range preferred by present data.
The Higgs boson may finally emerge in the near future, the capstone of the Standard Model. Or it may not make its expected appearance, igniting an experimental crisis that could point the way to a new theory, which most physicists would regard as the more exciting prospect.
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Last updated 04-09-2001, by P. Rowson