In the last decades particle physicists have developed a very successful model for describing the known forces between particles, the 'Standard Model' . So far, the biggest success of the Standard Model is the unification of the electromagnetic and the weak force into the so-called electroweak force. Each force is carried by characteristic particles called bosons . The photon carries the electromagnetic force; it also transmits light. The W and Z bosons represent the weak force. The electroweak unification leads to a theoretical problem: it requires all particles which carry forces to be massless like the photon. But experiments show that the W and Z bosons are not massless at all; their mass is comparable to the mass of 80 or 90 protons.
Introducing a new particle, the Higgs boson, solves this problem. Indeed in quantum mechanics, to every particle is associated a quantum field. This Higgs field interacts with the W and Z bosons to give them their heavy masses. It also interacts with the other particles, like the electron or the quarks, and the strength of the interaction determines the mass of the particle which interacts with the Higgs field. The Higgs boson is the last undiscovered particle of the Standard Model, and could reveal further informations on the electroweak unification, so finding the Higgs boson is one of the primary goals of the Dě Experiment.
The Standard Model predicts exactly one Higgs boson. This model tells us that the Higgs boson carries no electric charge, but it does not predict its mass. However, other precision measurements in particle physics teach us that it could be light enough to be observed at the Tevatron, for instance if its mass would be about 120 times the mass of the proton. If this would be true, the Tevatron would probably need to accumulate data for another 2 or 3 years, in order to produce enough Higgs bosons to be observed in an unambiguous way. But the Higgs boson is so particular that it deserves large efforts since day one, that's why we performed the search for the production of a Higgs boson produced together with a W boson. The search for a light Higgs boson produced alone cannot be performed easily, since there are not enough characteristic particles in the final state. The best way to search for it at the Tevatron is to to detect the production of a Higgs boson in association with a W (called in the following W+Higgs) via the annihilation of quarks and antiquarks as described by the following diagram:
On the left, the incoming elementary particles (a quark inside the incoming proton and an antiquark inside the incoming antiproton) collide and annihilate into a "virtual" W boson, which decays almost immediately into a real W and a Higgs boson. The difference between virtual and real particles, is that virtual particles cannot be observed, although they are needed by the theory. This process is very rare. We expect that one Higgs boson is produced in about one Trillion scatterings of protons and antiprotons ! But this search also allows us to test our detection techniques and to study the main physical processes which might fake a Higgs boson (so-called "backgrounds"), in particular the production of a W boson in association with b quark and an anti-b quark (called in the following W+b+anti-b). The diagram which describes this background process is shown in figure 2.
The Dě Experiment is designed to measure particles produced in the collision of protons and antiprotons at very high energy. At these high energies, protons and antiprotons break up and we can observe the interactions of their building blocks, quarks and gluons. The best way to observe the production of a light Higgs boson produced in association with a W boson, is to select events which have an energetic lepton, like an electron, which would come from the decay of the W boson, and an apparent imbalance of energy in the plane transverse to the beam axis. This apparent imbalance would be the signature of the neutrino also produced by the W decay: indeed, by the law of energy-momentum conservation, the energy in the transverse plane is conserved, but since the neutrino has very weak interaction with matter, it escapes from the detector without leaving any trace, so there is no measured energy to balance the energy deposited by the electron emitted in the opposite direction. Once these events are selected, the next step is to look if 2 jets of particles are also produced in these events, and if these jets are compatible with being produced by b quarks. To find out, we need to look from where the particles creating these jets are originating. If they originate from a common point (vertex) displaced from the beam axis, there is a good chance that they come from a b quark, since the b quark decays after it had time to fly several millimeters from the place where it has been produced. If the two jets have their vertex displaced from the beam axis then we have selected exactly the events that we want: a W and 2 b-jets. Indeed, since we are not able to distinguish a jet originating from a b quark, and another jet coming from an anti-b, we simply call them b-jets
The question then arises: are these events W + Higgs events or just the W+b+anti-b background? This question can be answered only by collecting several of these events, and using the kinematic properties of the jets to derive the mass of an hypothetic particle which decays in those 2 b-jets. If the masses that we derive from these events are always almost the same, then we know that we are producing a new particle having this mass, which could be the Higgs boson. If the mass found is different for every event, then most probably these jets are not originating from the decay of the Higgs boson produced in the W+Higgs process, but could simply be due to the W+b+anti-b background, or to other backgrounds, like the production of top quarks which can also lead to similar events.
So our search was to look for these events producing one energetic electron, 2 b-jets and a large imbalance of energy in the transverse plane. After going through all the data accumulated by the D0 experiment during 2002 and 2003, we found 6 events of this kind. The mass reconstructed from the characteristics of the 2b-jets in each of the 6 events are represented as the points with error bars in figure 3 (one point per mass).
Three of them are more or less at the same mass, but at a low value (about 40 GeV, i.e. 40 times the mass of the proton) which, for a Higgs boson, is already excluded by other experiments. It is also a mass region where the expected backgrounds which are represented by the histograms are important. So we concluded that this accumulation is coincidental. The other 3 events are at larger masses, but they are at 3 different masses, so there are no hint of the production of a Higgs boson. However we were not so disappointed since we knew that the quantity of data required to observe the Higgs boson was so far too small, according to the theoretical predictions.
What could we conclude from our search? Since the standard model is a well established theory, we know how many events to expect (between 4 and 5) in our dataset, from the well known background processes. Since we observed 6 events, we could determine, using statistical methods, what is the maximum rate (cross section in our technical terminology) allowed for the production of W+b+anti-b, or for the production of W+Higgs. These limits are currently the best available in the world for W+b+anti-b, and for W+Higgs, when the Higgs mass is greater than 115 GeV. However we plan to improve them soon by increasing the data sample and using also muons as the lepton originating from the W decay, and not only the electron as we did so far. Concerning the Higgs boson, the limits are about 2 times better than the previous results from the CDF experiment achieved during the Run I of the Fermilab Tevatron, which was concluded in 1996. Fifty times more proton-antiproton collision will be recorded in the next years by the Dě Experiment. This is therefore just the beginning in our quest to find the elusive Higgs bosons.
If you have any questions about this research, please contact Gregorio Bernardi (University of Paris, France)