Numerical simulation of heavy fermions in an SU(2)_L x SU(2)_R symmetric Yukawa model

An exploratory numerical study of the influence of heavy fermion doublets on the mass of the Higgs boson is performed in the decoupling limit of a chiral $\rm SU(2)_L \otimes SU(2)_R$ symmetric Yukawa model with mirror fermions. The behaviour of fermion and boson masses is investigated at infinite bare quartic coupling on $4^3 \cdot 8$, $6^3 \cdot 12$ and $8^3 \cdot 16$ lattices. A first estimate of the upper bound on the renormalized quartic coupling as a function of the renormalized Yukawa-coupling is given.Comment: 15 pp + 11 Figures appended as Postscript file

Characterization of new hybrid pixel module concepts for the ATLAS Insertable B-Layer upgrade

The ATLAS Insertable B-Layer (IBL) collaboration plans to insert a fourth pixel layer inside the present Pixel Detector to recover from eventual failures in the current pixel system, especially the b-layer. Additionally the IBL will ensure excellent tracking, vertexing and b-tagging performance during the LHC phase I and add robustness in tracking with high luminosity pile-up. The expected peak luminosity for IBL is 2 to 3centerdot1034 cm-2s-1 and IBL is designed for an integrated luminosity of 700 fb-1. This corresponds to an expected fluence of 5centerdot1015 1 MeV neqcm-2 and a total ionizing dose of 250 MRad. In order to cope with these requirements, two new module concepts are under investigation, both based on a new front end IC, called FE-I4. This IC was designed as readout chip for future ATLAS Pixel Detectors and its first application will be the IBL. The planar pixel sensor (PPS) based module concept benefits from its well understood design, which is kept as similar as possible to the design of the current ATLAS Pixel Detector sensor. The second approach of the new three dimensional (3D) silicon sensor technology benefits from the shorter charge carrier drift distance to the electrodes, which completely penetrate the sensor bulk. Prototype modules of both sensor concepts have been build and tested in laboratory and test beam environment before and after irradiation. Both concepts show very high performance even after irradiation to 5centerdot1015 1 MeV neqcm-2 and meet the IBL specifications in terms of hit efficiency being larger than 97%. Lowest operational threshold studies have been effected and prove independent of the used sensor concept the excellent performance of FE-I4 based module concepts in terms of noise hit occupancy at low thresholds.Comment: Part of 9th International Conference on Position Sensitive Detectors (PSD9

Quenching Effects in the Hadron Spectrum

Lattice QCD has generated a wealth of data in hadronic physics over the last two decades. Until relatively recently, most of this information has been within the "quenched approximation" where virtual quark--anti-quark pairs are neglected. This review presents a descriptive discussion of the effects of removing this approximation in the calculation of hadronic masses.Comment: To appear in "Lattice Hadron Physics", ed. A.C. Kalloniatis, D.B. Leinweber and A.G. William

Mass Spectrum and Bounds on the Couplings in Yukawa Models With Mirror-Fermions

The $\rm SU(2)_L\otimes SU(2)_R$ symmetric Yukawa model with mirror-fermions in the limit where the mirror-fermion is decoupled is studied both analytically and numerically. The bare scalar self-coupling $\lambda$ is fixed at zero and infinity. The phase structure is explored and the relevant phase transition is found to be consistent with a second order one. The fermionic mass spectrum close to that transition is discussed and a first non-perturbative estimate of the influence of fermions on the upper and lower bounds on the renormalized scalar self-coupling is given. Numerical results are confronted with perturbative predictions.Comment: 7 (Latex) page

Nucleon axial form factors from two-flavour Lattice QCD

We present preliminary results on the axial form factor $G_A(Q^2)$ and the induced pseudoscalar form factor $G_P(Q^2)$ of the nucleon. A systematic analysis of the excited-state contributions to form factors is performed on the CLS ensemble `N6' with $m_\pi = 340 \ \text{MeV}$ and lattice spacing $a \sim 0.05 \ \text{fm}$. The relevant three-point functions were computed with source-sink separations ranging from $t_s \sim 0.6 \ \text{fm}$ to $t_s \sim \ 1.4 \ \text{fm}$. We observe that the form factors suffer from non-trivial excited-state contributions at the source-sink separations available to us. It is noted that naive plateau fits underestimate the excited-state contributions and that the method of summed operator insertions correctly accounts for these effects.Comment: 7 pages, 12 figures; talk presented at Lattice 2014 -- 32nd International Symposium on Lattice Field Theory, 23-28 June, 2014, Columbia University New York, N

Nucleon electromagnetic form factors in two-flavour QCD

We present results for the nucleon electromagnetic form factors, including the momentum transfer dependence and derived quantities (charge radii and magnetic moment). The analysis is performed using O(a) improved Wilson fermions in Nf=2 QCD measured on the CLS ensembles. Particular focus is placed on a systematic evaluation of the influence of excited states in three-point correlation functions, which lead to a biased evaluation, if not accounted for correctly. We argue that the use of summed operator insertions and fit ans\"atze including excited states allow us to suppress and control this effect. We employ a novel method to perform joint chiral and continuum extrapolations, by fitting the form factors directly to the expressions of covariant baryonic chiral effective field theory. The final results for the charge radii and magnetic moment from our lattice calculations include, for the first time, a full error budget. We find that our estimates are compatible with experimental results within their overall uncertainties.Comment: 22 pages, 10 figures, citations modifie