New Method for Reconstructing Effective Top Quark Spin

We propose a new method for reconstructing an effective spin direction of a semi-leptonically decayed top quark. The method is simple: for instance, it does not require the spin information of the antitop quark in a tt̄ event. The reconstructed effective spin is expected to be useful in hadron collider experiments. We demonstrate its usefulness in an analysis of the top decay vertex. After ten years since the discovery of the top quark [1], as yet only limited experimental data on its properties are available, and details of the nature of the top quark are still to be unveiled. So far, there exists no evidence for significant deviations from the Standard Model (SM) predictions concerning top quark properties. On the other hand, since the mass of the top quark approximates the electroweak symmetry-breaking scale, there are hopes that symmetrybreaking physics may manifest itself through non-standard interactions of the top quark. For detailed examinations of top quark interactions, it is expected that the top quark spin can be used as a powerful analysis tool. This is because, in the SM (and in many of its extensions), the top quark decays before it hadronizes, and the spin information of the top quark is directly reflected to the distributions of its decay products. Hence, we may utilize the top quark spin for disentangling different top quark interactions efficiently. This is in contrast to the other lighter quarks, for which hadronization effects dilute the spin information at the quark level severely. In a future ee linear collider experiment, we will be able to utilize the spin of the top quark quite efficiently. This is because produced top quarks will naturally be polarized, due to parity-violating nature of the interactions in the top quark production process. The top quark polarization can even be controlled or raised to high values by tuning the polarization of the initial electron beam. By contrast, spin reconstruction of top quarks in hadron collider experiments is a nontrivial task. At Tevatron and LHC, top quarks are produced mainly through tt̄ production processes, and these top quarks are known to be hardly polarized [2]. Two types of methods have been proposed and studied for utilizing the top quark spin at these colliders. One is to take advantage of the correlation between the top quark spin and antitop quark spin in the tt̄ events. The other method is to use polarized top quarks produced through the single top production process. Unfortunately, so far, not much information has been obtained by applying these methods in analyzing real top quark data in the Tevatron experiments, due to intrinsic disadvantages of the methods. The only analysis that has been performed is a spin-correlation measurement by D0, which put a fairly loose bound on a correlation coefficient [3]. Disadvantages in using the top-antitop spin correlation in the tt̄ events are as follows. If we analyze the tt̄ events which decay in the dilepton channel (both t and t̄ decay semi-leptonically), data statistics is low due to the small branching fraction. Furthermore, there are two missing neutrino momenta in each event, which make reconstruction of the event topology non-trivial. Instead, if we analyze the tt̄ events which decay in the lepton + jets channel (one of the top quarks decays semi-leptonically and the other decays hadronically), we have more statistics, but reconstruction of the spin of the hadronically-decayed top quark needs to go through complicated procedures. The reconstruction process is affected significantly by kinematical cuts, and often important information is lost by the cuts. Accurate estimation of the effects of kinematical cuts and event reconstruction is crucial as well. The complex procedures bring in sizable systematic uncertainties in the top spin reconstruction. Disadvantages in using the single top production process are that there are huge background cross sections for Wbb̄ and Wbb̄+jets processes, and that these background cross sections have not been estimated accurately. In fact, up to now the single top production process has not been observed at Tevatron, as opposed to original expectations, due to lack of data statistics and difficulty in the background estimation. We would expect that, when a huge top quark sample is available at LHC, these difficulties