Shin-ichi Kawada

Feasibility study of the measurement of Higgs pair creation at a photon linear collider

We studied the feasibility of the measurement of Higgs pair creation at a photon linear collider. From the sensitivity to the anomalous self-coupling of the Higgs boson, the optimum gamma gamma collision energy was found to be around 270 GeV for a Higgs mass of 120 GeV/c(2). We found that large backgrounds such as gamma gamma -> W+W-, ZZ, and b (b) over barb (b) over bar can be suppressed if correct assignment of tracks to parent partons is achieved and Higgs pair events can be observed with a statistical significance of similar to 5 sigma by operating the photon linear collider for 5 years.

Development of an intense positron source using a crystal-amorphous hybrid target for linear colliders

In a conventional positron source driven by a few GeV electron beam, a high amount of heat is loaded into a positron converter target to generate intense positrons required by linear colliders, and which would eventually damage the converter target. A hybrid target, composed of a single crystal target as a radiator of intense gamma–rays, and an amorphous converter target placed downstream of the crystal, was proposed as a scheme which could overcome the problem. This paper describes the development of an intense positron source with the hybrid target. A series of experiments on positron generation with the hybrid target has been carried out with a 8–GeV electron beam at the KEKB linac. We observed that positron yield from the hybrid target increased when the incident electron beam was aligned to the crystal axis and exceeded the one from the conventional target with the converter target of the same thickness, when its thickness is less than about 2 radiation length. The measurements in the temperature rise of the amorphous converter target was successfully carried out by use of thermocouples. These results lead to establishment to the evaluation of the hybrid target as an intense positron source.

Spatial resolutions of GEM TPC. A novel theoretical formula and its comparison to latest beam test data

Unprecedented charged particle momentum resolution is required for precision Higgs studies at the International Linear Collider (ILC), which in turn demands as many as 200 sampling points with a high spatial resolution of 100 microns or better if we are to adopt a TPC for the central tracker. We discuss a novel theoretical resolution formula for a GEM-readout TPC, which is applicable to inclined tracks as opposed to the previous formula which is valid only for normal incidence. The formula identifies key factors that determine the spatial resolution and helps optimize the readout pad geometry and High Voltage settings for a given gas mixture. The formula is compared to the latest beam test results for a LC TPC Large prototype.

Blocking positive ion backflow using a GEM gate: experiment and simulations

Positive ion feedback can be problematic in a high precision Time Projection Chamber (TPC) as proposed for the International Linear Collider (ILC). Use of a traditional wire gating device would increase the dead areas in the planned module structure. F. Sauli proposed, in 2006, the use of a Gas Electron Multiplier (GEM) as a gating device. We have measured the electron transparency for a 14 μm thick GEM in a 1 T magnetic field. The transparency does not meet the requirement for a TPC at the ILC. We performed a simulation study using Garfield++ to understand the important parameters. Simulations show that a new GEM structure with wider aperture, for example a hexagonal honeycomb structure, can improve the performance as a gate. Results of measurements will be compared to the simulation and the predicted performance of the new GEM structure will be described.

A conventional positron source for international linear collider

A possible solution to realize a conventional positron source driven by a several-GeV electron beam for the International Linear Collider is proposed. A 300 Hz electron linac is employed to create positrons with stretching pulse length in order to cure target thermal load. ILC requires about 2600 bunches in a train which pulse length is 1 ms. Each pulse of the 300 Hz linac creates about 130 bunches, then 2600 bunches are created in 63 ms. Optimized parameters such as drive beam energy, beam size, and target thickness, are discussed assuming a L-band capture system to maximize the capture efficiency and to mitigate the target thermal load. A slow rotating tungsten disk is employed as positron generation target

A novel technique for the measurement of the avalanche fluctuation of gaseous detectors

We have developed a novel technique for the measurement of the avalanche fluctuation of gaseous detectors using a UV laser. The technique is simple and requires a short data-taking time of about ten minutes. Furthermore, it is applicable for relatively low gas gains. Our experimental setup as well as the measurement principle, and the results obtained with a stack of Gas Electron Multipliers (GEMs) operated in several gas mixtures are presented.

Higgs Boson Decays to Tau Pairs at the ILC with the ILD Detector

Since the discovery of a Higgs boson at the LHC, the investigation of its properties has become one of the most important themes in particle physics, particularly the verification of the mass generation mechanism. In the Standard Model (SM), the fermion mass is generated via the Yukawa coupling. The Higgs decay to tau leptons is expected to be the best probe for the precision test of the Yukawa coupling since the hadronic Higgs decays suffer from the theoretical uncertainties from QCD. Even in the absence of further new discoveries at the LHC, the effects of new physics may still manifest as few-percent deviations in the Yukawa couplings from the values predicted by the SM [1], thus motivating the precise measurement of the tau Yukawa coupling. The International Linear Collider (ILC), a next generation electron-positron collider, is the ideal place to precisely measure the Higgs properties and particularly its branching ratios. In this talk, we report on the expected precision of the branching ratio of the Higgs boson decaying to tau pairs at the ILC using the full detector simulation of the ILD detector model designed as part of the ILC Technical Design Reprot effort. We assume a center-of-mass energy of √ s = 250 GeV, an integrated luminosity of 250 fb−1, and beam polarizations of P (e−; e) = (−0.8;+0.3). The signal is the Higgs-strahlung process e+e− → ZH with H → τ+τ−. The hadronic Z decay offers the most sensitivity because of the large statistics. The main backgrounds are those including tau pairs, e.g. e+e− → ZZ → qq̄τ+τ−. The tau selection is optimized for signal reconstruction in the presence of jet backgrounds. The collinear approximation is readily available in e+e− collisions. From the statistical significance of the signal, the expected precision of the tau Yukawa coupling is extracted. The results are rescaled to the case of the 125 GeV Higgs boson.