Search for a heavy resonance decaying into a Z+jet final state
in p-pbar collisions at sqrt(s) = 1.96 TeV using the D0 detector
The full paper can be found here.

All known observations in subnuclear physics are described with an astonishing precision by the Standard Model. In spite of this success the Standard Model has theoretical shortcomings, to cite a few of them: it does not include the gravity, it does not unify the four known interactions and it has too many parameters whose value is not predicted. All these problems indicate that phenomena should exist which the Standard Model does not deal with.

figure1 figure2
Figure 1 : D0 Liquid Argon Calorimeter     (eps)

Figure 2 : Particle generations     (eps)

One of the ways these phenomena may be discovered is to search for heavy new particles/resonances, with mass in the range of several hundreds of GeV. Being the highest energy accelerator in the world until the startup of the LHC, the Tevatron is the best machine today to carry out such searches. Besides the collision energy and luminosity, mass resolution and low background level are of crucial importance in this matter. The liquid argon calorimeter (figure 1.) of the D0 detector has an excellent energy resolution for electrons and jets providing almost a background free Z signal (figure 3.). Combining the reconstructed Z's in the two-electron decay channel with the jet of the highest transverse energy in the event a mass resolution of ~9% is achieved. This, together with the high luminosity delivered in Run II, offers unprecedented good conditions to observe high mass resonances decaying into a Z and a quark (figure 4.).

figure2 figure4
Figure 3 : e+ e- invariant Mass     (eps)

Figure 4 : Feynman graph of the q* production     (eps)

An example of such resonances could be an excited quark if quarks would have internal structure. As of today the ultimate building blocks of the matter is thought to be the leptons and the quarks (figure 2.). If these blocks, e.g. the quarks would have constituents, called "preons", there would exist excited states of the quarks, which would be heavy, and which would de-excite, i.e. return to the quark ground state, emitting an intermediate boson: a gluon, a photon, a W or a Z boson. The invariant mass of the particle jet produced by the quark and the emitted boson would then exhibit a resonance shape.
In the present analysis we have searched for such an enhancement in the invariant mass distribution of the leading particle jet and a Z boson in the collision of protons with antiprotons. The Standard Model predicts an exponentially falling background. The data (once again!) confirm the prediction of the Standard Model and do not show any enhancement at masses in the 100-700 GeV energy range as shown in Figure 5. One can also see in that Figure the expected shape of a 500 GeV excited quark.
figure5 figure6
Figure 5 : Z + jet invariant Mass     (eps)

Figure 6 : Upper limits on resonance cross section times branching fraction at 95% CL     (eps)

We have therefore determined upper limits of the cross section times the branching fraction into the Z+q (Z->e+e-) final state of such a hypothetical resonance as functions of the resonance mass and width (figure 6.). Excited quarks, if they exist, must have smaller production cross section in the Z+q (Z->e+e-) final state at a 95% confidence level.

Comments or questions about this analysis?
Please send email to the primary authors: Lisa Berntzon, Smaïn kermiche, Elemér Nagy and Eric Thomas