Why measure the top quark mass ? June 6, 2006Posted by dorigo in news, physics, science.
This morning Sven Heinemeyer gave a nice talk on the importance of a precise determination of the top quark mass. Of course, he spent lots of time discussing the plot of top mass versus W boson mass. Here is the plot:
The plot shows graphically the inner consistency with measured data of two particle theories: the Standard Model, and an extension called "Minimal SuperSymmetric Model". Most everybody who does Higgs Hunting these days has this plot stuck to their office wall. So let me explain a little bit what is in there.
The top quark mass and the W boson mass are inextricably linked to the unknown mass of the Higgs boson; however, while in the Standard Model (red band) there is a direct connection between these three parameters – and the arrow near the lower left shows what the Higgs boson mass has to be if you pick one particular value for Mt and Mw – in the MSSM (green band) you can accommodate different values of top and W mass by varying other parameters: the mass of the supersymmetric particles (arrow near the right boundary). But in that case the lightest Higgs boson has to be lighter than 135 GeV.
The red band is constrained from above by the direct limit, MH>114 GeV, obtained by the LEP II experiments at CERN; the green band is only constrained by the inner consistency of the theory. You could extend it a bit upwards if you assumed extreme values for some free parameters of MSSM theory; and downwards, if you added Higgs bosons – but in that case you would be into real SUSY theories, and no longer into the "minimal" model.
We do not know what the Higgs boson mass is, nor do we know if there are supersymmetric particles, let alone their mass. So, this plot is of some guidance to us: the better we measure what we observe – top and W boson masses, x and y axis – and the more we can discriminate the two theories, SM and MSSM.
Right now, the two measurements constrain physics to within the blue ellipse: there is a 68% probability that the true value of Mt and Mw is withing the ellipse, given what we observed experimentally. We cannot yet say whether the pure SM is favored, or whether MSSM is the better candidate as a particle theory. We can say, however, that the Higgs boson is light: detailed computations find it consistent with measurements only if the Higgs boson mass is lower than about 175 GeV. This bodes well for the near future of the Tevatron, which has a better chance to discover the Higgs if the mass of that particle is near to the experimental limit.