First Measurement of the Z production cross section in proton anti-proton collisions using Z decays into tau leptons

The Z boson is a heavy partner of the photon and one of the fundamental carriers of the electroweak force. Its properties are predicted in exquisite detail by the standard model of particle physics. The standard model is a theoretical framework formulated in the 1970's that describes the basic constituents of matter as fermions ( leptons and quarks) and all their interactions as mediated by force carriers called bosons. This model has proven to be extraordinarily successful. The matter constituents are grouped into three families. For every constituent there is an anti-matter partner. Each family has one charged lepton, so there are three different types of charged lepton: electron, muon and tau. The only basic difference between them is that each one is heavier then the one before.

The fundamental fermions interact with each other by exchanging bosons, the Z boson being the most massive (~100 times the mass of the proton). Z bosons decay almost as soon as they are produced into a matter constituent and its anti-matter partner. The standard model makes precise predictions for the probability (cross section) of producing a Z boson in a proton anti-proton collision and the probability (branching ratio) that it will decay into a particular matter anti-matter pair. The cross section multiplied by the branching ratio for producing Z bosons decaying to electron or muon pairs has been measured before with fairly high precision. It was found to be in very good agreement with the standard model. However, observing Z bosons produced in proton anti-proton collisions decaying to tau pairs is much more difficult. The standard model predicts that we should observe 2.5 Z bosons decaying into a tau and anti-tau in 10 billion individual proton anti-proton collisions with 2 TeV center-of-mass energy (~2000 times the mass of the proton). The reason for such a small probability is that the Z boson is massive , produced by electroweak interactions, while most proton anti-proton collisions involve strong interactions. The strong interactions produce prodigious numbers of quarks, anti-quarks, and gluons (the carriers of the strong interaction) that cannot be observed directly. They get bound into strong interacting particles called hadrons by combining with other quarks and anti-quarks pulled out of the vacuum. When an energetic quark (or gluon) is produced in a collision, what is observed in a detector is a jet of hadrons carrying the energy and direction of the original quark. Tau leptons are short-lived. They decay 35% of the time into a lighter lepton plus two neutrinos and 65% of the time into lighter hadrons plus one neutrino before they can be observed directly in the DZero detector; therefore, they can only be identified by their decay products. Because the neutrinos cannot be detected, only a portion of the original tau energy is measured. That complicates tau identification as one cannot compute its mass from its decay products. An energetic tau decaying to hadrons will look like an exceptionally narrow jet with a low hadron multiplicity.

The DZero detector has been designed to cleanly identify electrons, muons, and jets. The strategy for finding Z->tau anti-tau events is to start by looking for a muon that could come from a tau or anti-tau decay. Identifying such a muon removes most of the events produced by strong interactions, but still leaves a background that is 30 times larger than the signal. To reduce the background further one makes use of the fact that the other tau is expected to be almost 180 degrees away from the muon in the plane perpendicular to the proton anti-proton beams. Most of the time what one finds there is a jet rather than a tau. In order to distinguish between them, detailed information from the energy deposition in the detector is used to calculate a probability that one is observing a tau rather than a jet. The probability is calculated by means of neural network techniques. Such techniques are based on parallel processing of non-linear signals (much like patterns in the human brain) and have proven to be very powerful for distinguishing between processes that generate subtly different information. About 2000 events were selected for having a muon and another object with a high probability of being a tau. From studies of the expected background distributions, one can deduce that about 900 of those events have a Z boson decaying to tau anti-tau and the other 1100 are background . The number of expected events is in very good agreement with the standard model prediction and so are the distributions of those events in any measured parameter. The figure shows the transverse energy distribution in the 2000 event sample expected for jets (red triangles), the distribution subtracting the contribution from jets (black points), and the predicted distribution for taus from Z decays (green histograms). It illustrates the good agreement within statistics between the standard model predictions and our measurement.

In addition to testing the standard model, our measurement demonstrates that the DZero detector can be used to identify taus in the presence of very large backgrounds. This could play an important role in the search for supersymmetric particles. Supersymmetry (SUSY) is an extension of the standard model that postulates that every fundamental fermion (boson) should have a corresponding boson (fermion) superpartner. SUSY solves some intrinsic problems in the standard model and seems to be required to unify gravitation with the electroweak and strong interactions. The known fermions and bosons do not have the right properties to be superpartners; thus their corresponding superpartners remain to be found. In some formulations of SUSY many superpartners will preferentially decay to final states with taus. A strong indication of superpartner production in proton anti-proton collisions would be observing events with multiple taus. That would be a major discovery with profound implications.

The full article can be found here.

For more information on this analysis contact Serban Protopopescu (Brookhaven National Laboratory), Abid Patwa (Brookhaven National Laboratory), Silke Nelson (Florida State University) or Cristina Galea (University of Nijmegen)