Search for Resonant Second Generation Slepton Production at the Tevatron

What are Sleptons ?

An important goal of particle physics is to describe the elementary constituents of matter, and the forces which govern them, with a simple Unified Theory . Supersymmetry, often called SUSY, is a promising theory that may offer this unification.

SUSY predicts a new partner for each known particle with similar properties but different spin. For example, the smuon would be the partner of the muon μ, the charged lepton of the second generation (or familiy) of matter particles. While muons have a spin of 1/2, smuons carry no angular momentum. Similarly, sneutrinos are the predicted partners of the neutrinos , electrically neutral particles with a small mass. The generic name 'slepton' is used to denote the spinless partners of the known leptons, namely electrons, muons, tauons and the corresponding three neutrinos. So far no SUSY-particle has been discovered. Therefore we do not know yet if SUSY is a good description of nature. Thus the search for SUSY-particles is of high relevance, and many experiments all over the world try to find them.

Slepton search at the Tevatron

Different SUSY models have been proposed, some of them allow a quark and an antiquark to annihilate such that their total energy is transformed into a single SUSY-particle, for example a smuon or a sneutrino . The production probability is highest when the (anti)quark energies match the mass of the SUSY-particle, therefore particle physicists call this process 'resonant'. Since quarks (antiquarks) are the constituents of protons (antiprotons), SUSY particles can be produced in proton-antiproton collisions at the Tevatron - if they are not too heavy. Smuons and Sneutrinos are unstable and decay after a fraction of a microsecond, for example the smuon could disintegrate into a muon and a neutralino , see graph below.

Neutralinos would be supersymmetric partners to the W and Z bosons , the photon and the Higgs boson . In the SUSY model considered here, also the neutralino is unstable and could decay into a muon μ, a quark and an antiquark. A quark is not observable directly, but it forms several hadrons flying in about the same direction as the quark would, thus they are detectable as hadron 'jets'. In case of smuon production and decay the detector will find in total two muons and two jets. (Sneutrinos do not decay in exactly the same way, see publication; in this summary we focus on smuon production and its decay into muon and neutralino.)

Challenges

The D0 detector is well suited to measure the direction and energy of muons and jets. Since the Tevatron collider provides a very high collision energy of nearly 2000 GeV, this experiment has a good chance to detect sleptons - if they exist and if they are not too heavy. However, not all proton-antiproton interactions with a final state consisting of two muons and two jets can be attributed to smuon production. 'Background' processes can fake the production of SUSY particles, for example the reaction proton+antiproton → Z plus two jets with the Z decaying into two muons.

The figure above shows for all the collision events, for which the D0 detector has reconstructed 2 muons, the mass of the parent particle of the two muons, which can be calculated from the measured muon momenta. As can be seen, several thousand events of this type were recorded, the bulk of them is compatible with the principal background source, Z production (green), giving a clear peak in the vicinity of 91 GeV, the mass of the Z boson. Also displayed in the figure is the expected mass distribution for smuon and slepton production (red). Since in this case the muons do not stem from the same parent particle, there is no peak in the mass distribution. Note that only very few SUSY-particles are expected (that's why the red curves are scaled up by a factor of 100), which makes the search very difficult.

Search Results

Fortunately we can calculate and measure the contribution of the background processes and subtract it from the data sample measured. We find, that in particular for the sample with two muons and two jets the difference between measurement and background vanishes, that is, there is no indication for SUSY. Note that the difference is not exactly zero, since the number of collision events, which are due to quantum processes, obtained in a certain measuring period (in this case about 2 years) fluctuates. These statistical effects have to be taken into account in quantifying the non-observation of sleptons. The figure below displays the exclusion limits for smuons, that is an upper bound on the cross section (and subsequent decay) for the slepton production.

The cross section is a measure of the production rate and given in the unit pb = picobarn. The limit depends on both the assumed smuon mass and the neutralino mass. The lower right half of the graph is empty since we must assume the smuon to be heavier than the neutralino, else the former couldn't decay into the latter. The experimental cross section limit can be compared to the cross section predicted by our SUSY model, as a function of the masses and other model parameters. If the predicted cross section exceeds the experimental upper bound, the corresponding smuon mass and neutralino mass values are excluded, such SUSY particles dont exist! Depending on the detailed model assumptions smuons with masses below 210 GeV (363 GeV) can be excluded. These bounds are more stringent than those obtained in other experiments - thus an important step forward has been made in testing supersymmetry.

D0 will continue to search for evidence of SUSY in the new data coming in, and will be able to probe new SUSY territory.

If you have any questions about this analysis, please contact the primary authors.