Particle physics

From the top...

The top quark is by far the heaviest elementary particle known. A measurement of its mass with higher precision has bearing on our understanding of the fundamental interactions of nature.

The basic building-blocks of matter, as far as we know, are quarks and leptons, together with the force-carrying particles that mediate their interactions. Quarks and leptons (the latter group including the electron) are grouped in three generations; the particles in the second and third generations seem a perfect copy of those of the first generation, except that their masses are much larger. The top quark is the heaviest of all quarks and leptons, and is central to some of the most pressing questions in particle physics. For instance, why is the third-generation top quark more than 300,000 times heavier than the first-generation electron? Why are there two other quarks with precisely the same properties as the top quark but with very different masses? And what is the origin of mass itself?

Precise knowledge of the mass of the top quark and its interactions is a key ingredient in testing theory against experimental data. On page 638 of this issue1, the DØ Collaboration report an improved measurement of the top-quark mass, using data taken at the Tevatron proton–antiproton collider at Fermilab, near Chicago. Combining this with previous measurements from DØ and its sister experiment CDF, the new world average2 for the mass of the top quark is 178.0 ± 4.3 GeV/c2, where c is the speed of light (the mass of the proton expressed in these units is about 1 GeV/c2). Compared with the previous world average3, the central value of the mass has shifted upwards by about 4 GeV/c2. The experimental error has been reduced by about 15%, sharpening our view of the underlying physics.

The role of the top quark in disentangling the fundamental principles of nature is twofold. On the one hand, its large mass makes the top quark a prime target in the search for new physics that might so far be unaccounted for. For instance, the long-hypothesized Higgs boson, which is the last missing ingredient of the standard model of particle physics, is predicted to interact with other particles with a strength that is proportional to their masses. So the physics of the heavy top quark would be significantly influenced by its interaction with the Higgs boson. On the other hand, the mass of the top quark is a key parameter in the predictions for many observable quantities. Small deviations between measurement and prediction could be a signal of new physics, so the uncertainty in the predictions that arises from the experimental error on the top-quark mass limits the sensitivity of experiment to new physics.

The values of several precisely measured quantities, as predicted by the standard model, depend on the square of the top-quark mass, Mt; their dependence is much weaker on the as yet unknown mass of the Higgs boson (so far, experiment has excluded 4 any mass value below 114.4 GeV/c2). Therefore, in using a so-called global fit of the model predictions to all available data, an improved knowledge of Mt better constrains the likely value of the Higgs-boson mass. In fact, the 4 GeV/c2 shift in the central value of Mt has shifted the upper limit on the Higgs-boson mass by more than 30 GeV/c2 , to 251 GeV/c2 (at 95% confidence level)1. This upper limit has an important impact on the experimental strategies used to search for the Higgs boson at present and future colliders.

Finding the Higgs boson in the predicted range would be another triumph for the standard model — as, of course, was the discovery of the top quark itself. Historically, the mass of the top quark had been predicted from a global fit to a wealth of precise measurements made at the LEP and SLC electron–positron colliders (at CERN in Geneva and at SLAC in Stanford, respectively). The top quark was discovered at the Tevatron in 1995, with a mass value in perfect agreement with the predicted range.

Although the standard model has passed many experimental tests with great success, it cannot be the ultimate theory of the fundamental interactions. This is evident from the fact that it describes only three of the four known interactions — namely, the electromagnetic, weak and strong interactions, but not gravity. It also has several theoretical shortcomings and leaves many questions unanswered. Perhaps the most attractive framework for extending the standard model is supersymmetry. A supersymmetric extension of the standard model could be the low-energy limit of a more fundamental high-energy theory that would consistently include gravity and would describe all the fundamental forces in a unified way. Supersymmetric theories predict that there are partners for all the known particles. The minimal supersymmetric extension of the standard model — the ‘MSSM’ — comprises one pair of superpartners for each quark and lepton, superpartners for the force carriers, and five Higgs bosons.

In supersymmetric models, as a consequence of the higher degree of symmetry, the mass of the lightest Higgs boson can be predicted directly (in contrast to the standard model, in which the Higgs mass is a free parameter, allowing only an indirect determination via a global fit). The predicted mass is very sensitive to the mass of the top quark, scaling as Mt4 — an even more pronounced dependence than in the standard-model case. Figure 1 shows the prediction5,6 for the lightest Higgs-boson mass in the MSSM: the effect of the change in the top-quark mass, to 178.0 ± 4.3 GeV/c2, is clearly seen. The direct experimental detection of the Higgs boson would enable its mass to be measured with an accuracy below the 1% level. Thus, a precise knowledge of Mt with an accuracy even better than presently available will be crucial for Higgs physics in supersymmetric extensions of the standard model7.

Figure 1: The mass of the lightest Higgs boson in the minimal supersymmetric standard model (MSSM).
figure1

The predicted value5,6 is shown as a function of the parameter tanβ, which relates the properties of the different Higgs bosons of the MSSM to each other (the other MSSM parameters are chosen such that they maximize the resulting value of the Higgs mass). The predicted Higgs mass is sensitive to the value of the top-quark mass used in the calculation. The solid line indicates the prediction using the new measurement of the top-quark mass from the DØ Collaboration1; the white band indicates the uncertainty of the prediction that results from the error on the top-quark mass. The dashed line shows the situation before the new measurement (the previous experimental error of ±5.1 GeV/c2 is not shown). Based on the new value of the top-quark mass, an upper bound on the mass of the lightest MSSM Higgs boson of about 140 GeV/c2 is established.

Besides having an important impact on Higgs physics, the top-quark mass influences many other predictions of the MSSM — for instance, the masses of the superpartners of the top quark and the strengths of their interactions. The ultimate goal is to connect the predictions of the MSSM, or other extensions of the standard model, with a more fundamental theory at a higher energy scale. This may provide evidence for the unification of all of the forces of nature into a single fundamental interaction. Measurements made at the energy scales directly accessible to us in collider experiments can be extrapolated to very high energy scales, but for this to be reliable a precise knowledge of Mt is crucial7. If the extrapolation is sufficiently precise, it may even give us clues about the structure of the unified force itself.

Further progress will require new experimental data — both the discovery of new particles, such as the Higgs boson or supersymmetric partners, and more precise measurements of observable quantities that allow a sensitive test of the underlying theory. Among these, improving the accuracy of the measurement of the top-quark mass will continue to be of the utmost importance. From data taken during the present phase of operation at the Tevatron (known as ‘Run II’), the experimental error on Mt will be reduced to 2–3 GeV/c2; at the Large Hadron Collider8, currently under construction at CERN, this accuracy will be improved further to 1–2 GeV/c2.

The ultimate precision on Mt, however, will be achieved at a linear electron–positron collider. Such a machine is currently in the planning phase and could go into operation around the middle of the next decade. Data from the linear collider could improve the accuracy on the top-quark mass by about a factor of ten9,10,11. Only then will the uncertainty due to the experimental error of the top-quark mass be well enough under control for the information gleaned from the LHC in the next decade — on the Higgs boson (or bosons), supersymmetric partners or other new physics — to be fully exploited.

References

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Weiglein, G. From the top.... Nature 429, 613–614 (2004). https://doi.org/10.1038/429613a

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