arXiv subjects

Andreas Wipf

Publications and source records attributed to Andreas Wipf.

At least 55 records · Page 3Linked to original sources

arXiv2012

Phases of supersymmetric O(N) theories

We perform a global renormalization group study of O(N) symmetric Wess-Zumino theories and their phases in three euclidean dimensions. At infinite N the theory is solved exactly. The phases and phase transitions are worked out for finite and infinite short-distance cutoffs. A distinctive new feature arises at strong coupling, where the effective superfield potential becomes multi-valued, signalled by divergences in the fermion-boson interaction. Our findings resolve the long-standing puzzle about the occurrence of degenerate O(N) symmetric phases. At finite N, we find a strongly-coupled fixed point in the local potential approximation and explain its impact on the phase transition. We also examine the possibility for a supersymmetric Bardeen-Moshe-Bander phenomenon, and relate our findings with the spontaneous breaking of supersymmetry in other models.

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arXiv2011

From confinement to new states of dense QCD matter

Transitions between centre sectors are related to confinement in pure Yang-Mills theories. We study the impact of these transitions in QCD-like theories for which centre symmetry is explicitly broken by the presence of matter. For low temperatures, we provide numerical evidence that centre transitions do occur with matter merely providing a bias towards the trivial centre sector until centre symmetry is spontaneously broken at high temperatures. The phenomenological consequences of these transitions for dense hadron matter are illustrated in an SU(3) effective quark theory: centre dressed quarks undergo condensation due to Bose-type statistics forming a hitherto unknown state of dense but cold quark matter.

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arXiv2011

Fermi-Einstein condensation in dense QCD-like theories

While pure Yang-Mills theory feature the centre symmetry, this symmetry is explicitly broken by the presence of dynamical matter. We study the impact of the centre symmetry in such QCD-like theories. In the analytically solvable Schwinger model, centre transitions take place even under extreme conditions, temperature and/or density, and we show that they are key to the solution of the Silver-Blaze problem. We then develop an effective SU(3) quark model which confines quarks by virtue of centre sector transitions. The phase diagram by confinement is obtained as a function of the temperature and the chemical potential. We show that at low temperatures and intermediate values for the chemical potential the centre dressed quarks undergo condensation due to Bose like statistics. This is the Fermi Einstein condensation. To corroborate the existence of centre sector transitions in gauge theories with matter, we study (at vanishing chemical potential) the interface tension in the three-dimensional Z2 gauge theory with Ising matter, the distribution of the Polyakov line in the four-dimensional SU(2)-Higgs model and devise a new type of order parameter which is designed to detect centre sector transitions. Our analytical and numerical findings lead us to conjecture a new state of cold, but dense matter in the hadronic phase for which Fermi Einstein condensation is realised.

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arXiv2011

Supersymmetry Breaking in Low Dimensional Models

We analyse supersymmetric models that show supersymmetry breaking in one and two dimensions using lattice methods. Starting from supersymmetric quantum mechanics we explain the fundamental principles and problems that arise in putting supersymmetric models onto the lattice. We compare our lattice results (built upon the non-local SLAC derivative) with numerically exact results obtained within the Hamiltonian approach. A particular emphasis is put on the discussion of boundary conditions. We investigate the ground state structure, mass spectrum, effective potential and Ward identities and conclude that lattice methods are suitable to derive the physical properties of supersymmetric quantum mechanics, even with broken supersymmetry. Based on this result we analyse the two dimensional N=1 Wess-Zumino model with spontaneous supersymmetry breaking. First we show that (in agreement with earlier analytical and numerical studies) the SLAC derivative is a sensible choice in the quenched model, which is nothing but the two dimensional phi^4 model. Then, we present the very first computation of a renormalised critical coupling for the complete supersymmetric model. This calculation makes use of Binder cumulants and is supported by a direct comparison to Ward identity results, both in the continuum and infinite volume limit. The physical picture is completed by masses at two selected couplings, one in the supersymmetric phase and one in the supersymmetry broken phase. Signatures of the Goldstino in the fermionic correlator are clearly visible in the broken case.

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arXiv2011

Critical behavior of supersymmetric O(N) models in the large-N limit

We derive a supersymmetric renormalization group (RG) equation for the scale-dependent superpotential of the supersymmetric O(N) model in three dimensions. For a supersymmetric optimized regulator function we solve the RG equation for the superpotential exactly in the large-N limit. The fixed-point solutions are classified by an exactly marginal coupling. In the weakly coupled regime there exists a unique fixed point solution, for intermediate couplings we find two separate fixed point solutions and in the strong coupling regime no globally defined fixed-point potentials exist. We determine the exact critical exponents both for the superpotential and the associated scalar potential. Finally we relate the high-temperature limit of the four-dimensional theory to the Wilson-Fisher fixed point of the purely scalar theory.

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arXiv2011

Phase diagram of the lattice G(2) Higgs Model

We study the phases and phase transition lines of the finite temperature G(2) Higgs model. Our work is based on an efficient local hybrid Monte-Carlo algorithm which allows for accurate measurements of expectation values, histograms and susceptibilities. On smaller lattices we calculate the phase diagram in terms of the inverse gauge coupling $β$ and the hopping parameter $κ$. For $κ\to 0$ the model reduces to G(2) gluodynamics and for $κ\to\infty$ to SU(3) gluodynamics. In both limits the system shows a first order confinement-deconfinement transition. We show that the first order transitions at asymptotic values of the hopping parameter are almost joined by a line of first order transitions. A careful analysis reveals that there exists a small gap in the line where the first order transitions turn into continuous transitions or a cross-over region. For $β\to\infty$ the gauge degrees of freedom are frozen and one finds a nonlinear O(7) sigma model which exhibits a second order transition from a massive O(7)-symmetric to a massless O(6)-symmetric phase. The corresponding second order line for large $β$ remains second order for intermediate $β$ until it comes close to the gap between the two first order lines. Besides this second order line and the first order confinement-deconfinement transitions we find a line of monopole-driven bulk transitions which do not interfer with the confinement-deconfinment transitions.

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arXiv2010

Confinement and the quark Fermi-surface in SU(2N) QCD-like theories

Yang-Mills theories with a gauge group SU(N_c\=3)and quark matter in the fundamental representation share many properties with the theory of strong interactions, QCD with N_c=3. We show that, for N_c even and in the confinement phase, the gluonic average of the quark determinant is independent of the boundary conditions, periodic or anti-periodic ones. We then argue that a Fermi sphere of quarks can only exist under extreme conditions when the centre symmetry is spontaneously broken and colour is liberated. Our findings are supported by lattice gauge simulations for N_c=2...5 and are illustrated by means of a simple quark model.

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arXiv2010

Casimir Scaling and String Breaking in G(2) Gluodynamics

We study the potential energy between static charges in G(2) gluodynamics in three and four dimensions. Our work is based on an efficient local hybrid Monte-Carlo algorithm and a multi-level Lüscher-Weisz algorithm with exponential error reduction to accurately measure expectation values of Wilson- and Polyakov loops. Both in three and four dimensions we show that at intermediate scales the string tensions for charges in various G(2)-representations scale with the second order Casimir. In three dimensions Casimir scaling is confirmed within one percent for charges in representations of dimensions 7, 14, 27, 64, 77, 77', 182 and 189 and in 4 dimensions within 5 percent for charges in representions of dimensions 7, 14, 27 and 64. In three dimensions we detect string breaking for charges in the two fundamental representations. The scale for string breaking agrees very well with the mass of the created pair of glue-lumps.

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arXiv2010

N=1 Wess Zumino Model in d=3 at zero and finite temperature

Supersymmetric renormalization group (RG) flow equations for the effective superpotential of the three-dimensional Wess-Zumino model are derived at zero and non-zero temperature. This model with fermions and bosons interacting via a Yukawa term possesses a supersymmetric analogue of the Wilson-Fisher fixed-point. At zero temperature we determine the phase-transition line in coupling-constant space separating the supersymmetric from the non-supersymmetric phase. At finite temperature we encounter dimensional reduction from 3 to 2 dimensions in the infrared regime. We determine the finite-temperature phase diagram for the restoration of the global $Z_2$-symmetry and show that for temperatures above the $Z_2$ phase transition the pressure obeys the Stefan-Boltzmann law of a gas of massless bosons in 2+1 dimensions.

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arXiv2010

Finite Temperature Schwinger Model

The temperature dependence of the order parameter of the Schwinger model is calculated in the euclidean functional integral approach. For that we solve the model on a finite torus and let the spatial extension tend to infinity at the end of the computations. The induced actions, fermionic zero-modes, relevant Green functions and Wilson loop correlators on the torus are derived. We find the analytic form of the chiral condensate for any temperature and in particular show that it behaves like \< \barΨΨ\> \sim -2 T\exp(-π\sqrtπT/e) for temperatures large compared to the induced photon mass.

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arXiv2009

Supersymmetry breaking as a quantum phase transition

We explore supersymmetry breaking in the light of a rich fixed-point structure of two-dimensional supersymmetric Wess-Zumino models with one supercharge using the functional renormalization group (RG). We relate the dynamical breaking of supersymmetry to an RG relevant control parameter of the superpotential which is a common relevant direction of all fixed points of the system. Supersymmetry breaking can thus be understood as a quantum phase transition analogously to similar transitions in correlated fermion systems. Supersymmetry gives rise to a new superscaling relation between the critical exponent associated with the control parameter and the anomalous dimension of the field -- a scaling relation which is not known in standard spin systems.

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arXiv2009

The Phase Diagram for Wess-Zumino Models

Dynamical supersymmetry breaking is an important issue for applications of supersymmetry in particle physics. The functional renormalization group equations allow for a nonperturbative approach that leaves supersymmetry intact. Therefore they offer a promising tool to investigate dynamical supersymmetry breaking. Here we will employ this method to derive the phase diagram and a surprisingly rich RG fixed-point structure with corresponding critical exponents for the $\mathcal N=1$ Wess-Zumino model in two dimensions.

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arXiv2009

Phase Diagram and Fixed-Point Structure of two dimensional N=1 Wess-Zumino Models

We study the phases and fixed-point structure of two-dimensional supersymmetric Wess-Zumino models with one supercharge. Our work is based on the functional renormalization group formulated in terms of a manifestly off-shell supersymmetric flow equation for the effective action. Within the derivative expansion, we solve the flow of the superpotential also including the anomalous dimension of the superfield. The models exhibit a surprisingly rich fixed-point structure with a discrete number of fixed-point superpotentials. Each fixed-point superpotential is characterized by its number of nodes and by the number of RG relevant directions. In limiting cases, we find periodic superpotentials and potentials which confine the fields to a compact target space. The maximally IR-attractive fixed point has one relevant direction, the tuning of which distinguishes between supersymmetric and broken phases. For the Wess-Zumino model defined near the Gaussian fixed point, we determine the phase diagram and compute the corresponding ground-state masses.

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arXiv2009

Effective Polyakov Loop Dynamics for Finite Temperature G(2) Gluodynamics

Based on the strong coupling expansion we obtain effective 3-dimensional models for the Polyakov loop in finite-temperature G_2 gluodynamics. The Svetitsky-Jaffe conjecture relates the resulting continuous spin models with G_2 gluodynamics near phase transition points. In the present work we analyse the effective theory in leading order with the help of a generalised mean field approximation and with detailed Monte-Carlo simulations. In addition we derive a Potts-type discrete spin model by restricting the characters of the Polyakov loops to the three extremal points of the fundamental domain of G_2. Both the continuous and discrete effective models show a rich phase structure with a ferromagnetic, symmetric and several anti-ferromagnetic phases. The phase diagram contains first and second order transition lines and tricritical points. The modified mean field predictions compare very well with the results of our simulations.

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arXiv2009

Analytic results for the effective action

Motivated by the seminal work of Schwinger, we obtain explicit closed form expressions for the one-loop effective action in a constant electromagnetic field. We discuss both massive and massless charged scalars and spinors in two, three, and four dimensions. Both strong field and weak field limits are calculable. The latter limit results in an asymptotic expansion whose first term reproduces the Euler-Heisenberg effective Lagrangian. We use the zeta function renormalization prescription, and indicate its relationship to Schwinger's renormalized effective action.

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arXiv2009

Zeta Functions and the Casimir Energy

We use zeta function techniques to give a finite definition for the Casimir energy of an arbitrary ultrastatic spacetime with or without boundaries. We find that the Casimir energy is intimately related to, but not identical to, the one-loop effective energy. We show that in general the Casimir energy depends on a normalization scale. This phenomenon has relevance to applications of the Casimir energy in bag models of QCD. Within the framework of Kaluza-Klein theories we discuss the one-loop corrections to the induced cosmological and Newton constants in terms of a Casimir like effect. We can calculate the dependence of these constants on the radius of the compact dimensions, without having to resort to detailed calculations.

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arXiv2009

Flow Equation for Supersymmetric Quantum Mechanics

We study supersymmetric quantum mechanics with the functional RG formulated in terms of an exact and manifestly off-shell supersymmetric flow equation for the effective action. We solve the flow equation nonperturbatively in a systematic super-covariant derivative expansion and concentrate on systems with unbroken supersymmetry. Already at next-to-leading order, the energy of the first excited state for convex potentials is accurately determined within a 1% error for a wide range of couplings including deeply nonperturbative regimes.

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arXiv2009

Instanton constituents and fermionic zero modes in twisted CP(n) models

We construct twisted instanton solutions of CP(n) models. Generically a charge-k instanton splits into k(n+1) well-separated and almost static constituents carrying fractional topological charges and being ordered along the noncompact direction. The locations, sizes and charges of the constituents are related to the moduli parameters of the instantons. We sketch how solutions with fractional total charge can be obtained. We also calculate the fermionic zero modes with quasi-periodic boundary conditions in the background of twisted instantons for minimally and supersymmetrically coupled fermions. The zero modes are tracers for the constituents and show a characteristic hopping. The analytical findings are compared to results extracted from Monte-Carlo generated and cooled configurations of the corresponding lattice models. Analytical and numerical results are in full agreement and it is demonstrated that the fermionic zero modes are excellent filters for constituents hidden in fluctuating lattice configurations.

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