D0 Searching for Higgs particles, in a nearly supersymmetric world...

    One of the most fundamental properties of an elementary particle (i.e. particle that has no substructure) is its mass. Different particles have different masses, from the very light electron, to the nearly million times more massive top quark. The modern theory of particle physics called the Standard Model (SM) cannot explain the origin of mass and why various particles have the masses that they do. Instead, it postulates the existance of a new particle, the Higgs boson, and  the corresponding Higgs field that fills the Universe (!). Particles become heavy because they are coupled to (interact with) the Higgs field. It also follows from this model that one can create Higgs particles for example in experiments at accelerators. Since the Higgs particle itself is predicted to be heavy, a great amount of energy must be concentrated into a small volume of space to have any hope of creating one. The only machine capable of doing this today is the Tevatron proton-antiproton collider at Fermilab, and the D0 experiment is one of the two detectors that is specifically designed to watch for traces of Higgs particles.

But why are the Higgs particles heavy? Where do they get their masses from? No one really knows... but it's worse than that! Within the SM framework the Higgs particle mass is not even finite. However, an extension of the model, called supersymmetry, helps to control the Higgs mass. The 'symmetry' of supersymmetry is that any particle with half-integer spin (like 1/2 or 3/2) has a partner, identical in all respects, but with integer spin (like 0,1,or 2). It is called 'super' because it solves many problems in particle physics! We know that the theory of supersymmetry is not completely correct since there is not an integer-spin electron observed in experiments, for instance. However, if the theory was almost correct, there might be an integer-spin electron, but it's just a bit too heavy for us to see. This 'near-supersymmetry' would be good enough to solve the mystery of the Higgs particle masses.

If we do live in a nearly supersymmetric world, the Higgs particles would be a bit different. First of all, there would be at least 5 kinds of them! This is required for the theory to be 'well behaved' so that calculated masses are finite. Theory also predicts that some of the Higgs particles might be made at the Tevatron in a very special way. Instead of being produced alone, they would often be accompanied by two bottom quarks. Furthermore, Higgs particles themselves would also tend to decay into a pair of bottom quarks. The result would then be an excess, a signal, of events with many bottom quarks, where a pair of these bottom quarks would have energies that add up to the Higgs mass.

The number of multi-bottom-quark events we would expect to see with the D0 detector can be accurately predicted using computer simulations of the physical processes we know about, and models of the detector geometry, materials, and electronics. The predictions are compared to what is observed from about 2 years of the experimental data. The result is shown in Figure 1, where the solid line is the prediction, assuming no Higgs particles, and the points with error bars are the data. The signal of Higgs events, if it were to exist, would have a size and shape like the dotted line. Since no excess in data is seen, we conclude that either

a) Higgs particles don't really exist
b) they exist but are too heavy for the Tevatron to create in large numbers
c) the world is not nearly-supersymmetric
d) the parameters of supersymmetry do not lead to a large number of Higgs particles to be created

Figure 1: A comparison of the data to predictions

The parameter of the supersymmetric theories which most directly controls the number of Higgs events we would expect to see is called tanB (pronounced 'tan', like a sun-tan, and the Greek letter: 'beta'). So we can set limits on the possible value of tanB, for each assumed Higgs mass, as shown in Figure 2. The two different blue lines correspond to two slightly different models of supersymmetry. The other shaded regions have been ruled out by other experiments in the past, like the Large Electron Positron collider at CERN. Since supersymmetry has not been discovered, we certainly don't know the values of its parameters!

D0: public mA tanB plot
Figure 2: Limits on tanB versus the Higgs mass

As we continue to take data with the D0 detector, we will become more and more sensitive to lower tanB values and heavier Higgs boson masses. Some theorists believe that the Higgs particles may be just outside our current reach. If we are lucky, therefore, we could discover Higgs particles within the next few years, marking a major milestone in the history of particle physics, and our understanding of the world around us!

An article on this analysis has been submitted to Physical Review Letters in April 2005. Please e-mail for an additional information.