Accurate simulation of the finite density lattice Thirring model

We present a study of the finite density lattice Thirring model in 1+1 dimensions using the world-line/fermion-bag algorithm. The model has features similar to QCD and provides a test case for exploring the accuracy of various methods of solving sign problems. In the massless limit and with open boundary conditions we show that the sign problem is an artifact of the auxiliary field approach and is completely eliminated in the fermion bag approach. With periodic boundary conditions the sign problem is mild in the fermion bag method. We present accurate results for various quantities in the model that can be used as a benchmark for comparison with other methods of solving sign problems.Comment: 8 pages, 7 figures. Proceedings of the 35th International Symposium on Lattice Field Theor

Ideal Walking Dynamics via a Gauged NJL Model

According to the Ideal Walking Technicolor paradigm large mass anomalous dimensions arise in gauged Nambu--Jona-Lasinio (NJL) models when the four-fermion coupling is sufficiently strong to induce spontaneous symmetry breaking in an otherwise conformal gauge theory. We therefore study the $SU(2)$ gauged NJL model with two adjoint fermions using lattice simulations. The model is in an infrared conformal phase at small NJL coupling while it displays a chirally broken phase at large NJL couplings. In the infrared conformal phase we find that the mass anomalous dimension varies with the NJL coupling reaching $\gamma_m \sim 1$ close to the chiral symmetry breaking transition, de facto making the present model the first explicit realization of the Ideal Walking scenario.Comment: 10 pages, 4 tables and 7 figure

Running coupling in SU(2) with adjoint fermions

We present a measurement of the Schr\"odinger Functional running coupling in SU(2) lattice gauge theory with adjoint fermions. We use HEX smearing and clover improvement to reduce the discretization effects. We obtain a robust continuum limit for the step scaling, which confirms the existence of a non-trivial fixed point.Comment: Contribution to SCGT12 "KMI-GCOE Workshop on Strong Coupling Gauge Theories in the LHC Perspective", 4-7 Dec. 2012, Nagoya University, 4 pages, 2 figure

Konformaalisten kenttäteorioiden kartoittamista hilalla

The Standard Model of particle physics and the modern understanding of physics at small length scales rests on the foundation of non-Abelian gauge theories. Mapping their behavior at large energies is important in understanding the Standard Model and in extending it to include a wider range of physical phenomena. There is a region in the parameter space of these models where the interaction strength runs to a constant at high energies. These conformal models are useful for explaining the masses of Standard Model particles trough the extended technicolor mechanism. In this thesis I present studies of the conformal window using lattice simulations. This turns out to be a significant challenge in the study non-perturbative quantum physics. We have studied the phase structure of the SU(2) gauge theory coupled to fermions that transform in the fundamental or the adjoint representation of the symmetry group. We have found evidence of large discretization errors and studied them by constructing a Symanzik improved model. The running of the renormalized coupling is greatly affected by improvements in the model. Using the Symanzik improved model we have studied the running of the coupling in the SU(2) gauge theory with fundamental fermions. The models behave as expected with 4 and 10 flavors of fermions. We were unable to distinguish between chirally broken and conformal behavior in the model with 6 fermions. In order to push further into large energy scales we have studied actions with NHYP and HEX smearing. This helps reduce the discretization effects and reduce the corrections in the Symanzik improvement. Using the improved HEX smeared action we have been able to run simulations at larger energy scales than before.Hiukkasfysiikan Standardimalli kuvaa fysikaalisia ilmiöitä erittäin hyvin pienessä mittakaavassa. Malli perustuu kvanttikenttäteorioille, jotka kuvaavat ainetta keskenään vuorovaikuttavien hiukkaskenttien avulla. Standardimallin paremman ymmärtämisen ja laajentamisen vuoksi on hyvä ymmärtää näiden teorioiden käyttäytymistä. Näillä teorioilla on parametrialue, jossa hiukkasten välisen vuorovaikutuksen voimakkuus ei riipu etäisyydestä. Tällaisia malleja kutsutaan konformaalisiksi. Ne voisivat olla hyödyllisiä hiukkasten massojen selittämiseen niin kutsutun technicolor-mekanismin kautta. Tässä tutkimuksessa selvitimme sitä parametrialuetta, jolla kvanttikenttäteoriat ovat konformaalisia. Tämä on osoittautunut merkittäväksi haasteeksi laskennallisessa kvanttikenttäteoriassa. Olemme tutkineet SU(2)-mittakenttäteoriaa johon kytkeytyy fermioneja. Löysimme viitteitä suurista systemaattisista, simulaatiomallista riippuvista, virheistä ja tutkimme niitä määrittämällä Symanzikin menetelmällä parannetun simulaatiomallin. Tutkimuksessa kävi ilmi että mallin parantaminen muuttaa huomattavasti teorian mitattua käyttäytymistä. Käyttäen parannettua mallia olemme tutkineet vuorovaikutuksen voimakkuutta SU(2)-mittakenttäteoriassa johon kytkeytyy 4, 6 tai 10 fermionia. Näistä mallit, jossa on 4 tai 10 fermionia käyttäytyvät odotetulla tavalla. Kiinnostavimmassa tapauksessa, jossa fermioneja on 6, mallin konformaalisuus jää epävarmaksi. Tulosten tarkentamiseksi olemme tutkineet NHYP- ja HEX-parannettuja malleja. Nämä vähentävät systemaattisia virheitä ja pienentävät Symanzikin menetelmän korjauksia

Running coupling in SU(2) gauge theory with two adjoint fermions

We study SU(2) gauge theory with two Dirac fermions in the adjoint representation of the gauge group on the lattice. Using clover improved Wilson fermion action with hypercubic truncated stout smearing we perform simulations at larger coupling than before. We measure the evolution of the coupling constant using the step scaling method with the Schrodinger functional and study the remaining discretization effects. At weak coupling we observe significant discretization effects, which make it difficult to obtain a fully controlled continuum limit. Nevertheless, the data remains consistent with the existence of a fixed point in the interval 2.2 less than or similar to g(*2) less than or similar to 3. We also measure the anomalous dimension and find that its value at the fixed point is gamma(*) similar or equal to 0.2 +/- 0.03.Peer reviewe