Over the top
Jun 9, 2004
Physicists in the DØ collaboration at Fermilab have made the most accurate ever measurement of the mass of the top quark. The result has important implications in the search for the Higgs boson -- the particle that is thought to explain where mass comes from -- and the search for "new physics" beyond the Standard Model of particle physics (Nature 429 638).
The top quark was discovered at the Tevatron proton-antiproton collider at Fermilab in 1995 and is the heaviest elementary particle detected to date. The latest DØ result is based on data taken before the Tevatron was shutdown in 1999 for an upgrade. A new analysis of this so-called Run I data gives a new "world average" for the mass of the top quark mass of 178.0 GeV plus or minus 4.3 GeV. Strictly speaking the mass is 178.0 GeV/c2, where c is the speed of light, but particle physicists often drop the "c squared" term.
In the Standard Model the masses of particles are generated as a result of their interactions with a field called the Higgs field. It should also be possible to detect excitations of this field in the form of a particle known as the Higgs boson. Detecting the Higgs -- the only particle in the Standard Model that has not been detected experimentally -- is therefore one of the outstanding challenges in particle physics. However, careful measurements of the masses of the top quark and the W+ and W- bosons -- two of the particles responsible for transmitting the electroweak force -- allow particle physicists to place upper and lower limits on the likely mass of the Higgs.
The new value for the mass of the top quark is some 5.3 GeV higher than the previous value and causes the best-fit value for the expected mass of the Higgs boson to increase from 96 to 117 GeV. Moreover, the upper limit on the expected Higgs mass increases from 219 to 251 GeV, possibly placing the Higgs beyond the reach of the current generation of accelerators.
Detecting the Higgs would allow particle physicists to search for new physics beyond the Standard Model, such as supersymmetry. Supersymmetric extensions of the Standard Model predict that all fundamental particles -- such as quarks, photons and electrons -- have so-called superpartners.
"Not only has the most probable value of the Higgs mass moved up by a good 25%, but certain previous limits on supersymmetry parameters no longer apply with the new value of the top quark mass," DØ group member Greg Landsberg of Brown University told PhysicsWeb. According to Landsberg the new analysis technique is equivalent to running the Tevatron for another three years. The new technique is now being applied by the CDF collaboration, the other detector team at the Tevatron, and on Run II data from the upgraded Tevatron.
About the author
Peter Rodgers is Editor of Physics World