Gradient flows without blow-up for Lefschetz thimbles

We propose new gradient flows that define Lefschetz thimbles and do not blow up in a finite flow time. We study analytic properties of these gradient flows, and confirm them by numerical tests in simple examples.Comment: 31 pages, 11 figures, (v2) conclusion part is expande

Random Matrix Models for Dirac Operators at finite Lattice Spacing

We study discretization effects of the Wilson and staggered Dirac operator with $N_{\rm c}>2$ using chiral random matrix theory (chRMT). We obtain analytical results for the joint probability density of Wilson-chRMT in terms of a determinantal expression over complex pairs of eigenvalues, and real eigenvalues corresponding to eigenvectors of positive or negative chirality as well as for the eigenvalue densities. The explicit dependence on the lattice spacing can be readily read off from our results which are compared to numerical simulations of Wilson-chRMT. For the staggered Dirac operator we have studied random matrices modeling the transition from non-degenerate eigenvalues at non-zero lattice spacing to degenerate ones in the continuum limit.Comment: 7 pages, 6 figures, Proceedings for the XXIX International Symposium on Lattice Field Theory, July 10 -- 16 2011, Squaw Valley, Lake Tahoe, California, PACS: 12.38.Gc, 05.50.+q, 02.10.Yn, 11.15.H

Large NN expansion of the moments and free energy of Sachdev-Ye-Kitaev model, and the enumeration of intersection graphs

In this paper we explain the relation between the free energy of the SYK model for $N$ Majorana fermions with a random $q$-body interaction and the moments of its spectral density. The high temperature expansion of the free energy gives the cumulants of the spectral density. Using that the cumulants are extensive we find the $p$ dependence of the $1/N^2$ correction of the $2p$-th moments obtained in 1801.02696. Conversely, the $1/N^2$ corrections to the moments give the correction (even $q$) to the $\beta^6$ coefficient of the high temperature expansion of the free energy for arbitrary $q$. Our result agrees with the $1/q^3$ correction obtained by Tarnopolsky using a mean field expansion. These considerations also lead to a more powerful method for solving the moment problem and intersection-graph enumeration problems. We take advantage of this and push the moment calculation to $1/N^3$ order and find surprisingly simple enumeration identities for intersection graphs. The $1/N^3$ corrections to the moments, give corrections to the $\beta^8$ coefficient (for even $q$) of the high temperature expansion of the free energy which have not been calculated before. Results for odd $q$, where the SYK `Hamiltonian' is the supercharge of a supersymmetric theory are discussed as well

The Factorization Method for Simulating Systems With a Complex Action

We propose a method for Monte Carlo simulations of systems with a complex action. The method has the advantages of being in principle applicable to any such system and provides a solution to the overlap problem. We apply it in random matrix theory of finite density QCD where we compare with analytic results. In this model we find non--commutativity of the limits $\mu\to 0$ and $N\to\infty$ which could be of relevance in QCD at finite density.Comment: Talk by K.N.A. at Confinement 2003, Tokyo, July 2003, 5 pages, 4 figures, ws-procs9x6.cl

Exact moments of the Sachdev-Ye-Kitaev model up to order 1/N21/N^2

We analytically evaluate the moments of the spectral density of the $q$-body Sachdev-Ye-Kitaev (SYK) model, and obtain order $1/N^2$ corrections for all moments, where $N$ is the total number of Majorana fermions. To order $1/N$, moments are given by those of the weight function of the Q-Hermite polynomials. Representing Wick contractions by rooted chord diagrams, we show that the $1/N^2$ correction for each chord diagram is proportional to the number of triangular loops of the corresponding intersection graph, with an extra grading factor when $q$ is odd. Therefore the problem of finding $1/N^2$ corrections is mapped to a triangle counting problem. Since the total number of triangles is a purely graph-theoretic property, we can compute them for the $q=1$ and $q=2$ SYK models, where the exact moments can be obtained analytically using other methods, and therefore we have solved the moment problem for any $q$ to $1/N^2$ accuracy. The moments are then used to obtain the spectral density of the SYK model to order $1/N^2$. We also obtain an exact analytical result for all contraction diagrams contributing to the moments, which can be evaluated up to eighth order. This shows that the Q-Hermite approximation is accurate even for small values of $N$.Comment: 49 pages, 16 figure

Universality and its limits in non-Hermitian many-body quantum chaos using the Sachdev-Ye-Kitaev model

Spectral rigidity in Hermitian quantum chaotic systems signals the presence of dynamical universal features at time scales that can be much shorter than the Heisenberg time. We study the analogue of this time scale in many-body non-Hermitian quantum chaos by a detailed analysis of long-range spectral correlators. For that purpose, we investigate the number variance and the spectral form factor of a non-Hermitian $q$-body Sachdev-Ye-Kitaev (nHSYK) model, which describes $N$ fermions in zero spatial dimensions. After an analytical and numerical analysis of these spectral observables for non-Hermitian random matrices, and a careful unfolding, we find good agreement with the nHSYK model for $q > 2$ starting at a time scale that decreases sharply with $q$. The source of deviation from universality, identified analytically, is ensemble fluctuations not related to the quantum dynamics. For fixed $q$ and large enough $N$, these fluctuations become dominant up until after the Heisenberg time, so that the spectral form factor is no longer useful for the study of quantum chaos. In all cases, our results point to a weakened or vanishing spectral rigidity that effectively delays the observation of full quantum ergodicity. We also show that the number variance displays non-stationary spectral correlations for both the nHSYK model and random matrices. This non-stationarity, also not related to the quantum dynamics, points to intrinsic limitations of these observables to describe the quantum chaotic motion. On the other hand, we introduce the local spectral form factor, which is shown to be stationary and not affected by collective fluctuations, and propose it as an effective diagnostic of non-Hermitian quantum chaos. For $q = 2$, we find saturation to Poisson statistics at a time scale of $\log D$, compared to a scale of $\sqrt D$ for $q>2$, with $D$ the total number of states.Comment: 47 pages, 19 figure