Measurement of the WW production cross section in proton anti-proton collisions

The question of the structure of the universe and the existence of humanity has kept physicists and philosophers of many generations busy. In the last hundred years, the modern science has succeeded in pushing the knowledge farther and farther in these fields. Current and future experiments in particle and astro physics will enlarge our knowledge and advance in new spheres to address the fundamental questions.

According to our present knowledge the matter is composed of fermions, the leptons and quarks. Four fundamental forces act on these particles: the strong, weak, electromagnetic, and gravitational force. The force carriers are called bosons (more) and are responsible for the intermediation of the forces. The knowledge of the constitution of matter and the forces between the components is summarized in the Standard Model of particle physics. The Standard Model is in very good agreement with the experimental results. One of the big successes of the Standard Model is the unification of the electromagnetic and the weak force.

The pair production of W bosons is one of the interesting processes that can occur in nature. At the Tevatron the W boson pairs can be produced in two different processes, via the exchange of a quark or in resonant production via a photon (γ) or Z boson, which are illustrated on the right.

The Standard Model with its gauge symmetry in the electroweak sector makes precise predictions for the WWγ and WWZ couplings. Thus a measurement of the W boson pair cross section offers a good test of the Standard Model. In addition the pair production of W bosons is one of the most important background processes for the production of the Higgs boson or supersymmetric particles. Thus a good understanding of this process is also desirable for many other analyses.

The W bosons cannot be observed directly with the Dě detector, because after production they decay immediately into other particles. In 2/3 of the cases the decay products are two quarks that appear as jets of particles in our detector. In 10% of the time the W bosons decay into an electron (e) and a neutrino (ν) or a muon (μ) and a neutrino. Other particles produced in proton anti-proton collisions can lead to similar final states as the W boson pair production. In most of the collisons the final state consists of quarks and since the direct production of quarks happens more than one million times more often than the pair production of W bosons, it is impossible to distinguish the W bosons that decay into quarks from direct quark production. In addition the Dě detector is optimized to detect events involving electrons and/or muons. Thus final states including either two electrons, two muons or one electron and one muon, are the best to reliably detect the pair production of W bosons.

However there are still other processes that can mimic the pair production of W bosons. For example two electrons or muons can also be produced via a photon or Z boson which is called the Drell Yan process (just replace the two W bosons in the right diagram above by electrons or muons). For every W boson pair production approximately 2000 Drell Yan events are observed in our detector. But our situation is not hopeless since we can exploit different features of WW production to suppress other backgrounds. The most important one is the fact that in the leptonic decay of every W boson one neutrino is produced. This is not the case for other processes such as Drell Yan. The neutrinos cannot be detected with our detector but we can use the energy that is carried away by the neutrinos to observe them indirectly via an imbalance of transverse energy. Requiring a large amount of this missing energy rejects a large number of the Drell Yan events but keeps events from WW production.

After scanning our data which was taken between April 2002 and March 2004 for events with two electrons, two muons, or one electron and one muon, we count approximately 40000 events. Only a small fraction (less than 0.1%) are expected to be WW events. Applying different selection criteria we observe 25 events at the end, fifteen of which have one electron and muon, six have two electrons and four have two muons. To measure the cross section of the WW pair production we have to carefully estimate the background contributions from all possible processes. For our data sample we expect 8.1 events from non WW production with an uncertainty of 0.6 events. Taking into account the efficiency for detecting a WW event in our detector and also the probability for W bosons to decay into electrons and muons, we find a cross section for the WW production of 13.8 picobarn (pb) with an uncertainty of +4.5 pb and -4.0 pb. The unit of a cross section is the barn (b): 1 b = 10-28 m2. Thus 13.8 pb are 13.8*10-40m2 which is a very small area. This area can be interpreted as area of overlap of the interacting fields of the particles in proximity of each other.

We cannot say exactly which of the 25 events are from WW pair production and which are from background processes. We also have to test if we observed the pair production of W bosons or if our observation is only a fluctuation of the other possible processes. Thus a large number of pseudo experiments is performed on a computer. With our knowledge of the expected and observed events, we can test how often an expectation of 8.1 events would result in an observation of 25 or more events. We find that only in 2.3*10-7 of the times this would be the case. The conclusion is that the outcome of our experiment is not compatible with a hypothesis that only background processes produced the number of observed events. The significance of this measurement is 5.2 σ.

After performing the measurement we can now compare our result to the theoretical prediction. The theoretical calculations assume the Standard Model thus a comparison of experiment and theory can show wether the Standard Model is in agreement with the experimental results. The graph on the right shows our result in blue, the lines correspond to the experimental uncertainty. The theortical calculation for the WW cross section is shown in red as function of center of mass energy of the colliding protons and anti-protons. We performed our measurement at a center of mass energy of 1.96 TeV. So far our experiment is in good agreement with the Standard Model predictions. In the graph the result of the CDF experiment, which operated at a center of mass energy of 1.8 TeV, is shown in green.

In the next years we will record 20-40 times more data that will allows to study the WW production in more detail and with greater accuracy. With this increased data set it will be possible to make more stringent test of the Standard Model and test the WWγ and WWZ couplings in more detail. With a more detailed analysis and the large data set it will also be possible to search for the Higgs boson which is believed to give masses to all the other particles.

The full article can be found here.

For more information on this analysis please contact Johannes Elmsheuser (Ludwig Maximilians University, Munich, Germany) or Marc Hohlfeld (Johannes Gutenberg University, Mainz, Germany)

by MH and JE