arXiv subjects
Publications and source records attributed to S. Tsuno.
A new process with the lepton flavor violation (LFV) is presented in the setup for LHC. The LFV is induced by the one-loop effect through Higgs bosons in the framework of the type-III two Higgs doublet model. It is demonstrated that the vast of parameter space in the Yukawa sector could be accessed by current and future LHC experiments.
Many theoretical models, like the Standard Model or SUSY at large tan(beta), predict Higgs bosons or new particles which decay more abundantly to final states including tau leptons than to other leptons. At the energy scale of the LHC, the identification of tau leptons, in particular in the hadronic decay mode, will be a challenging task due to an overwhelming QCD background which gives rise to jets of particles that can be hard to distinguish from hadronic tau decays. Equipped with excellent tracking and calorimetry, the ATLAS experiment has developed tau identification tools capable of working at the trigger level. This contribution presents tau trigger algorithms which exploit the main features of hadronic tau decays and describes the current tau trigger commissioning activities.
The GR@PPA event generator has been updated to version 2.7. This distribution provides event generators for $V$ ($W$ or $Z$) + jets ($\leq$ 4 jets), $VV$ + jets ($\leq$ 2 jets) and QCD multi-jet ($\leq$ 4 jets) production processes at $pp$ and $p\bar{p}$ collisions, in addition to the four bottom quark productions implemented in our previous work (GR@PPA\_4b). Also included are the top-pair and top-pair + jet production processes, where the correlation between the decay products are fully reproduced at the tree level. Namely, processes up to seven-body productions can be simulated, based on ordinary Feynman diagram calculations at the tree level. In this version, the GR@PPA framework and the process dependent matrix-element routines are separately provided. This makes it easier to add further new processes, and allows users to make a choice of processes to implement. This version also has several new features to handle complicated multi-body production processes. A systematic way to combine many subprocesses to a single base-subprocess has been introduced, and a new method has been adopted to calculate the color factors of complicated QCD processes. They speed up the calculation significantly.
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 ttbar 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.
We present a quantitative study on sensitivities to the top-decay anomalous couplings, taking into account realistic experimental conditions expected at Tevatron and LHC. A double angular distribution of W and charged lepton in the top decay is analyzed, using ttbar events in the lepton+jets channel. In order to improve sensitivities to the anomalous couplings, we apply two techniques: (1) We use a likelihood fitting method for full kinematical reconstruction of each top event. (2) We develop a new effective spin reconstruction method for leptonically-decayed top quarks; this method does not require spin information of the antitop side. For simplicity, we neglect couplings of right-handed bottom quark as well as CP violating couplings. The 95% C.L. estimated bound on a ratio of anomalous couplings reads -0.81 < f_2/f_1 < -0.70, -0.12<f_2/f_1<0.14 using 1000 reconstructed top events at Tevatron, while -0.74<f_2/f_1<-0.72, -0.01<f_2/f_1<0.01 is expected with 100k reconstructed top events at LHC, where only statistical errors are taken into account. A two-fold ambiguity in the allowed range remains when the number of events exceeds a few hundred.
We have developed an event generator, named GR@PPA_4b, for the four bottom quark (b bbar b bbar) production processes at PP and PPbar collisions. The program implements all of the possible processes at the tree level within the framework of the Standard Model. Users can generate events from the Higgs boson and photon/Z mediated processes, as well as those from pure QCD interactions. The integration and the event generation are performed within the newly developed GR@PPA framework, an extension of the GRACE automatic event-generator generation system to hadron collisions. This program is so designed that it can be embedded in a general-purpose event generator PYTHIA version 6.1. PYTHIA adds the initial- and final-state parton showers and simulates the hadronization and decays to make generated events realistic. It should be emphasized that a huge number of diagrams and complicated four-body kinematics are dealt with strictly in GR@PPA_4b. This program will be useful for studies of Higgs boson productions, especially those in extended models where the Yukawa coupling to b quarks is greatly enhanced. The source code is located in http://atlas.kek.jp/physics/nlo-wg/index.html .
We have successfully developed a technique to integrate an automatic event-generator generation system GRACE and a general-purpose event generator framework PYTHIA. The codes generated by GRACE are embedded in PYTHIA in the created event generator program. The embedded codes give information on parton-level hard interactions directly to PYTHIA. The choice of PDF is controlled by the ordinary parameter setting in PYTHIA. This technique enables us to create easy-to-handle event generators for any processes in hadron collisions. Especially, in virtue of large capability of GRACE, we can easily deal with those processes containing many (four or more) partons in the final state, such as multiple heavy particle productions. This project is being carried out as a collaboration between the Japanese Atlas group and the Minami-Tateya group, aiming at developing event generators for Tevatron and LHC experiments.