The Standard Model: In Trouble?

     The Standard Model has been a pillar of fundamental physics for more than 30 years. It took decades to construct, and it is the crown jewel of 20th-century particle physics. In 1979, Sheldon Glashow, Abdus Salam and Steven Weinberg won the Nobel Prize for unifying the sectors of the theory comprising the weak subnuclear and electromagnetic forces in what is now known as the electroweak theory. The remaining sector, the strong interactions, is based on a so-called non-abelian gauge theory (a theoretical invention by R. L. Mills and C. N. Yang in 1954). The strong force holds the protons and neutrons together in a nucleus. The only fundamental force not included in the Standard Model of particle physics is gravity: Physicists have been unable to construct a consistent quantum theory of gravity although string theory holds some promise.

     All matter is made up of two types of generic particles: leptons and quarks. There are three types of charged leptons: electrons, muons and taus. Of these particles, only the electrons are stable; the other charged leptons, which are heavier, decay in a small fraction of a second. Electrons, which flow through a wire to make electricity, form the electronic cloud of the outer part of an atom. There also exist leptons that are electrically neutral and have very tiny masses; they are neutrinos. There are six types of quarks: down, up, strange, charm, bottom and top. Two up-quarks and one down-quark compose a proton, while two down-quarks and one up-quark compose a neutron. The neutrons and protons fuse together in a nucleus, which is the tiny, central core of an atom. Through the weak subnuclear interactions, the heavier quarks (strange, charm, bottom and top) decay into lighter quarks and other particles.

     The exchange of vector gauge bosons generates the particle forces: Passing virtual photons between charged particles yields the electromagnetic forces, exchanging virtual W's and Z's between quarks and leptons produces the weak subnuclear forces, and rapidly emission between gluons themselves and quarks creates the strong interactions.

     The masses of the quarks and leptons, W's and Z's are generated through what is known as electroweak symmetry breaking. This process leaves the photon massless thereby rending the electromagnetic forces long-ranged, macroscopic and quite different from the subnuclear weak interactions. In the Standard Model, symmetry breaking is achieved using a field called the Higgs particle. Experimentalists have searched for the Higgs but, to date, have not detected it. The Higgs sector of the electroweak theory is the least appealing part, and many theorists believe the nature might use a more elegant way to accomplish symmetry breaking and mass generation.

     For decades, the Standard Model has been subjected to thorough experimental scrutiny and has survived several attacks. For example, in the 1990's, the experimental result for the fractional production of bottom anti-bottom (denoted by Rb) through a Z gauge boson differed from the theoretical prediction. However, further experiments and analysis eventually eliminated the discrepancy. The Standard Model has overcome so many of these experimental-theoretical conflicts that particle physicists are almost complacent about deviations and are willing to assume that any problem is merely a statistical fluctuation that will eventually resolve itself.

     The CERN collider near Geneva, Switzerland, the SLAC facility in Stanford, California and the proton accelerator at Fermilab near Chicago, Illinois have been the main machines testing the Standard Model. In 1998, a comparison of dozens of quantities revealed no discrepancy between experiment and theory. The Standard Model appeared to be in perfect shape, much to the dismay of inventive theorists, who had hoped to find small imperfections that would indicate new physics such as supersymmetry, compositeness or undiscovered subnuclear forces.

     Much of the effort has focused on Z production. An electron and positron annihilate to produce a Z, which exists virtually for less than a slit second, before decaying. The decay is most often into a lepton and antilepton or into a quark and an antiquark. Here is a simulation for the case of a muon-antimuon decay:

electron-positron annihilation

     During the last few years, new data has accumulated indicating that the forward-backward asymmetry in bottom quark production, AbFB, through the decay of a Z differs from expectations with a confidence level of 99.6%, meaning that there is only four-1000th of a chance (1) that the Standard Model is correct or (2) that a statistical fluke has occurred. Since this sort of thing has happened from time to time during that past 30 years, most particle physicists are not concerned.

     Mathematically, the bottom quark forward-backward asymmetry is

AbFB = (s bF - s bB)/(s bF + s bB)

where sbF is the cross section for producing a bottom quark in the forward hemisphere: When an electron and a positron annihilate, the motion of the electron defines a direction. If the bottom quark is generated in the direction of motion of this initial electron then it contributes to sbF; if the bottom quark is generated in the direction opposite then it contributes to sbB. For example, in the above animated image, the muon is produced in the forward hemisphere so that it is included in smF, the analogous forward cross section for muons. If the muon had been a bottom quark then this event would provide a positive contribution to AbFB. A cross section, by the way, is a measure of how likely two particles will collide.

     In a paper published in Physical Review Letter on December 3, 2001, theorist Dr. Michael Chanowitz at the Lawrence Berkeley National Laboratory argues that there is currently a "lose-lose" situation for the Standard Model: the data of the electroweak theory has evolved to the point where new physics is suggested independent of whether the measurement of AbFB is correct or not. The reasoning is quite simple: If the measured value of AbFB is correct then a modification of the Standard Model is needed since the experimental result does not agree with the theoretical calculation. If the measurement of AbFB is not correct due to a statistical fluctuation or a systematic error then it should not be included in the analysis. However, when AbFB is excluded, it turns out that the predicted mass for the Higgs particle is in conflict with current experimental limits. Measurements at CERN rule out a Higgs mass below 113.5 GeV at the 95% confidence level. So even if AbFB is excluded, the Standard Model has only a few percent chance of being correct assuming that electroweak breaking is accomplished with a Higgs.

     What does this all mean? One possibility is that the mass of the Higgs is below 113.5 GeV and has somehow escaped detection. This is unlikely but possible. Usually, extraordinary evidence is needed to establish a new theory. However, since the Higgs sector has not been experimentally confirmed, perhaps the inconsistencies in the data is telling scientists that electroweak breaking does not make use of a Higgs field. This would be an exciting and intriguing result. Until additional experiments are analyzed, the resolution to the above quandary remains unknown. Stay tuned to developments by book-marking and Jupiter Scientific will keep you updated.

This report was prepared by the staff of Jupiter Scientific, an organization devoted to the promotion of science through books, the internet and other means of communication.

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