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Standard Model In Trouble Again: Berkeley Lab Theory

What we know about the fundamental nature of matter is not enough to explain what has been observed in high energy physics experiments during the past decade, according to the "Lose-Lose Theorem" proposed by a physicist with Lawrence Berkeley National Laboratory.

Michael Chanowitz, a theoretician with Berkeley Lab's Physics Division, says that a measurement of the breakdown or decay of Z particles, carriers of the weak nuclear force, shows that the theory that has successfully explained fundamental physics since the 1970s, the Standard Model of Particles and Fields, is no longer quite so successful.

In a paper published in Physical Review Letters, Chanowitz argues that whether scientists accept the measurement as valid or dismiss it as an anomaly, the Standard Model loses.

"An analysis of all relevant data, including searches for the Higgs particle, favors a breakdown of the Standard Model," Chanowitz says. "This implies with a high probability that there is new physics beyond the Standard Model waiting to be discovered."

The Standard Model provides a theoretical framework for describing the fundamental particles of matter and all of the forces that interact with them except gravity.

It holds that there are two kinds of fermions, or matter particles, called quarks and leptons (leptons include electrons and neutrinos), which are grouped into three distinct "generations" of increasing mass.

Ordinary matter is composed of the lightest generation of fermions: up and down quarks, which combine to form the protons and neutrons of atomic nuclei; electrons, which bind atoms together into molecules; and electron-neutrinos, which influence the stability of this matter.

Through the years, the Standard Model has been used to predict particle properties even before the particles were experimentally found. For example, it accurately predicted the mass of the top quark, which was found in 1994.

The final particle needed to complete the Standard Model's predictions is the Higgs particle, named after Peter Higgs of the University of Edinburgh, who first proposed it.

A boson, or force-carrier, the Higgs particle is thought to give mass to the elementary particles through its interactions with them. Among the measurements used to predict the mass of the Higgs particle is the direction of the decay of Z particles into bottom quarks and antibottom quarks, the bottom quark's antimatter counterparts.

Says Chanowitz, "The result of this measurement disagrees significantly with the Standard Model's predicted value. If genuine, the discrepancy implies a breakdown of the Standard Model."

Because this measurement of Z decay into bottom and antibottom quarks is extremely difficult, Chanowitz says the possibility that the discrepancy is the result of "subtle experimental error" cannot be excluded. However, under the terms of his Lose-Lose Theorem, the possibility of experimental error cannot save the Standard Model because "the predicted value of the Higgs particle mass would then be so low that it should have already been observed at existing experiments."

Chanowitz bases his conclusion on the fact that experiments at CERN (European Organization for Nuclear Research) on the Large Electron Positron collider (LEP) have already set the lower limit for the mass of the Higgs particle at 114.1 GeV (billion electron volts).

If the questionnable Z-decay measurement is discarded, the Standard Model becomes an excellent fit with other important measurements. However, its predicted value for the Higgs mass then falls far below the established minimum.

"The Standard Model is caught in a Catch-22 dilemma," Chanowitz says. "Whether the questionnable Z-decay measurement is right or wrong, new physics beyond the Standard Model is required, either to explain the discrepant measurement or to explain the failure to have already observed the Higgs particle."

Some physicists have already speculated about new kinds of quarks, or a symmetry of fermions and bosons known as "supersymmetry," as possible explanations. But Chanowitz says there is not enough evidence at this time to know what the new physics beyond the Standard Model might be.

"We need new particles or new forces at higher scales that can interact with the particles we already know about, in order to explain those effects we've been seeing that cannot be explained by the Standard Model," he says. "Until this new physics is known, we cannot predict the Higgs particle mass except for the general statement that it is at or below the trillion electron volt (TeV) scale."

Chanowitz says that the Large Hadron Collider, which is scheduled to begin operations at CERN in 2005 and will smash together protons in the multi-TeV energy range, is likely to provide answers to the questions posed by his Lose-Lose Theorem. - By Lynn Yarris

[Contact: Lynn Yarris]






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