Stormbringer note: This wikipedia article outlines the more well known problems with the standard model. there are more recent findings which call the standard model into question. remember the scientific maxim; it takes but one contradiction to false a scientific theory.
Challenges to the Standard Model
Although the Standard Model has had great success in explaining experimental results, it has never been accepted as a complete theory of fundamental physics. This is because it has two important defects:
The model contains 19 free parameters, such as particle masses, which must be determined experimentally (plus another 10 for neutrino masses). These parameters cannot be independently calculated.
The model does not describe the gravitational interaction.
Since the completion of the Standard Model, many efforts have been made to address these problems.
One attempt to address the first defect is known as grand unification. The so-called grand unified theories (GUTs) hypothesized that the SU(3), SU(2), and U(1) groups are actually subgroups of a single large symmetry group. At high energies (far beyond the reach of current experiments), the symmetry of the unifying group is preserved; at low energies, it reduces to SU(3)×SU(2)×U(1) by a process known as spontaneous symmetry breaking. The first theory of this kind was proposed in 1974 by Georgi and Glashow, using SU(5) as the unifying group. A distinguishing characteristic of these GUTs is that, unlike the Standard model, they predict the existence of proton decay. In 1999, the Super-Kamiokande neutrino observatory reported that it had not detected proton decay, establishing a lower limit on the proton half-life of 6.7× 1032 years. This and other experiments have falsified numerous GUTs, including SU(5). Another effort to address the first defect has been to develop Preon models which attempt to set forth a substructure of more fundamental particles than those set forth in the Standard Model.
In addition, there are cosmological reasons why the standard model is believed to be incomplete. Within it, matter and antimatter are symmetric. While the preponderance of matter in the universe can be explained by saying that the universe just started out this way, this explanation strikes most physicists as inelegant. Furthermore, the Standard Model provides no mechanism to generate the cosmic inflation that is believed to have occurred at the beginning of the universe, a consequence of its omission of gravity.
The Higgs boson, which is predicted by the Standard Model, has not been observed as of 2005 (though some phenomena were observed in the last days of the LEP collider that could be related to the Higgs; one of the reasons to build the LHC is that the increase in energy is expected to make the Higgs observable).
The first experimental deviation from the Standard Model came in 1998, when Super-Kamiokande published results indicating neutrino oscillation. This implied the existence of non-zero neutrino masses since massless particles travel at the speed of light and so do not experience the passage of time. The Standard Model did not accommodate massive neutrinos, because it assumed the existence of only "left-handed" neutrinos, which have spin aligned counter-clockwise to their axis of motion. If neutrinos have non-zero mass, they necessarily travel slower than the speed of light. Therefore, it would be possible to "overtake" a neutrino, choosing a reference frame in which its direction of motion is reversed without affecting its spin (making it right-handed). Since then, physicists have revised the Standard Model to allow neutrinos to have mass, which make up additional free parameters beyond the initial 19.
A further extension of the Standard Model can be found in the theory of supersymmetry, which proposes a massive supersymmetric "partner" for every particle in the conventional Standard Model. Supersymmetric particles have been suggested as a candidate for explaining dark matter. Although supersymmetric particles have not been observed experimentally to date, the theory is one of the most popular avenues of research in theoretical particle physics.
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See also
The theoretical formulation of the standard model
Weak interactions, Fermi theory of beta decay and electroweak theory
Strong interactions, flavour, quark model and quantum chromodynamics
For open questions, see quark matter, CP violation and neutrino masses
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References
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Textbooks
Griffiths, David J. (1987). Introduction to Elementary Particles, Wiley, John & Sons, Inc. ISBN 0471603864
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Journal Articles
Y. Hayato et al., Search for Proton Decay through p → νK+ in a Large Water Cherenkov Detector. Phys. Rev. Lett. 83, 1529 (1999).
S.F. Novaes, Standard Model: An Introduction, hep-ph:0001283
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External links
New Scientist story: Standard Model may be found incomplete
The Universe Is A Strange Place, a lecture by Frank Wilczek
Observation of the Top Quark at Fermilab
MISN-0-305 The Standard Model of Fundamental Particles and Their Interactions (PDF file) by Mesgun Sebhatu for Project PHYSNET.
PostScript version of the Standard Model Lagrangian
The particle adventure.
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