Publications

Order by: publication date number of citations

• On the generality of symmetry breaking and dissipative freezing in quantum trajectories

  Joseph Tindall, Dieter Jaksch, Carlos Sánchez Muñoz

  SciPost Phys. Core 6, 004 (2023) · published 25 January 2023 | Toggle abstract · pdf

  Recently, several studies involving open quantum systems which possess a strong symmetry have observed that every individual trajectory in the Monte Carlo unravelling of the master equation will dynamically select a specific symmetry sector to 'freeze' into in the long-time limit. This phenomenon has been termed 'dissipative freezing', and in this paper we argue, by presenting several simple mathematical perspectives on the problem, that it is a general consequence of the presence of a strong symmetry in an open system with only a few exceptions. Using a number of example systems we illustrate these arguments, uncovering an explicit relationship between the spectral properties of the Liouvillian in off-diagonal symmetry sectors and the time it takes for freezing to occur. In the limiting case that eigenmodes with purely imaginary eigenvalues are manifest in these sectors, freezing fails to occur. Such modes indicate the preservation of information and coherences between symmetry sectors of the system and can lead to phenomena such as non-stationarity and synchronisation. The absence of freezing at the level of a single quantum trajectory provides a simple, computationally efficient way of identifying these traceless modes.

  2 citations

• Monte Carlo, fitting and Machine Learning for Tau leptons

  Vladimir Cherepanov, Elzbieta Richter-Was, Zbigniew Was

  SciPost Phys. Proc. 1, 018 (2019) · published 19 February 2019 | Toggle abstract · pdf

  Status of tau lepton decay Monte Carlo generator TAUOLA, and its main recent applications are reviewed. It is underlined, that in recent efforts on development of new hadronic currents, the multi-dimensional nature of distributions of the experimental data must be taken with a great care. Studies for H to tau tau; tau to hadrons indeed demonstrate that multi-dimensional nature of distributions is important and available for evaluation of observables where tau leptons are used to constrain experimental data. For that part of the presentation, use of the TAUOLA program for phenomenology of H and Z decays at LHC is discussed, in particular in the context of the Higgs boson parity measurements with the use of Machine Learning techniques. Some additions, relevant for QED lepton pair emission and electroweak corrections are mentioned as well.

  2 citations

• Functional flows for complex effective actions

  Friederike Ihssen, Jan M. Pawlowski

  SciPost Phys. 15, 074 (2023) · published 31 August 2023 | Toggle abstract · pdf

  In the present work we set up a general functional renormalisation group framework for the computation of complex effective actions. For explicit computations we consider both flows of the Wilsonian effective action and the one-particle irreducible (1PI) effective action. The latter is based on an appropriate definition of a Legendre transform for complex actions, and we show its validity by comparison to exact results in zero dimensions, as well as a comparison to results for the Wilsonian effective action. In the present implementations of the general approaches, the flow of the Wilsonian effective action has a wider range of applicability and we obtain results for the effective potential of complex fields in $\phi^4$-theories from zero up to four dimensions. These results are also compared with results from the 1PI effective action within its range of applicability. The complex effective action also allows us to determine the location of the Lee-Yang zeros for general parameter values. We also discuss the extension of the present results to general theories including QCD.

  2 citations

• Scrambling under quench

  Adith Sai Aramthottil, Diptarka Das, Suchetan Das, Bidyut Dey

  SciPost Phys. Core 6, 021 (2023) · published 28 March 2023 | Toggle abstract · pdf

  We evaluate out of time ordered correlators in certain low dimensional quantum systems at zero temperature, subjected to homogenous quantum quenches. We find that when the Lyapunov exponent exists, it can be identified with the quenched energy. We show that the exponent naturally gets related to the post-quench effective temperature. In the context of sudden quenches the exponent is determined in terms of the quench amplitude while for smooth quenches we observe scalings (both the Kibble-Zurek as well as the fast) of the exponent with the quench rate. The scalings are identical to that of the energy generated during the quench.

  2 citations

• Positivity, low twist dominance and CSDR for CFTs

  Agnese Bissi, Aninda Sinha

  SciPost Phys. 14, 083 (2023) · published 25 April 2023 | Toggle abstract · pdf

  We consider a crossing symmetric dispersion relation (CSDR) for CFT four point correlation with identical scalar operators, which is manifestly symmetric under the cross-ratios $u,v$ interchange. This representation has several features in common with the CSDR for quantum field theories. It enables a study of the expansion of the correlation function around $u=v=1/4$, which is used in the numerical conformal bootstrap program. We elucidate several remarkable features of the dispersive representation using the four point correlation function of $\Phi_{1,2}$ operators in 2d minimal models as a test-bed. When the dimension of the external scalar operator ($\Delta_\sigma$) is less than $\frac{1}{2}$, the CSDR gets contribution from only a single tower of global primary operators with the second tower being projected out. We find that there is a notion of low twist dominance (LTD) which, as a function of $\Delta_\sigma$, is maximized near the 2d Ising model as well as the non-unitary Yang-Lee model. The CSDR and LTD further explain positivity of the Taylor expansion coefficients of the correlation function around the crossing symmetric point and lead to universal predictions for specific ratios of these coefficients. These results carry over to the epsilon expansion in $4-\epsilon$ dimensions. We also conduct a preliminary investigation of geometric function theory ideas, namely the Bieberbach-Rogosinski bounds.

  2 citations

• SINDRUM II

  A. van der Schaaf

  SciPost Phys. Proc. 5, 008 (2021) · published 6 September 2021 | Toggle abstract · pdf

  In 1987 a collaboration including ETHZ - UZH - PSI - RWTH Aachen - Univ. Tbilisi proposed a new search for μe conversion in muonic atoms. The SINDRUM II spectrometer came into operation in the PSI μE1 area in 1989, but a dedicated beam line was delayed until 1998 by technical setbacks.

  2 citations

• Bootstrapping traceless symmetric $O(N)$ scalars

  Marten Reehorst, Maria Refinetti, Alessandro Vichi

  SciPost Phys. 14, 068 (2023) · published 12 April 2023 | Toggle abstract · pdf

  We use numerical bootstrap techniques to study correlation functions of traceless symmetric tensors of $O(N)$ with two indices $t_{ij}$. We obtain upper bounds on operator dimensions for all the relevant representations and several values of $N$. We discover several families of kinks, which do not correspond to any known model and we discuss possible candidates. We then specialize to the case $N=4$, which has been conjectured to describe a phase transition in the antiferromagnetic real projective model $ARP^{3}$. Lattice simulations provide strong evidence for the existence of a second order phase transition, while an effective field theory approach does not predict any fixed point. We identify a set of assumptions that constrain operator dimensions to a closed region overlapping with the lattice prediction. The region is still present after pushing the numerics in the single correlator case or when considering a mixed system involving $t$ and the lowest dimension scalar singlet.

  2 citations

• Born again

  Jorge G. Russo, Paul K. Townsend

  SciPost Phys. 16, 124 (2024) · published 13 May 2024 | Toggle abstract · pdf

  Born's original 1933 theory of nonlinear electrodynamics (in contrast to the later Born-Infeld theory) is acausal for strong fields. We explore the issue of strong-field causality violation in families of theories containing Born and/or Born-Infeld, and many variants that have been previously proposed in contexts that include cosmology and black hole physics. Many of these variants are acausal and hence unphysical. A notable exception is the modified Born-Infeld theory with ModMax as its conformal weak-field limit.

  2 citations

• On the quantum simulation of complex networks

  Duarte Magano, João Moutinho, Bruno Coutinho

  SciPost Phys. Core 6, 058 (2023) · published 24 August 2023 | Toggle abstract · pdf

  Quantum walks provide a natural framework to approach graph problems with quantum computers, exhibiting speedups over their classical counterparts for tasks such as the search for marked nodes or the prediction of missing links. Continuous-time quantum walk algorithms assume that we can simulate the dynamics of quantum systems where the Hamiltonian is given by the adjacency matrix of the graph. It is known that such can be simulated efficiently if the underlying graph is row-sparse and efficiently row-computable. While this is sufficient for many applications, it limits the applicability for this class of algorithms to study real world complex networks, which, among other properties, are characterized by the existence of a few densely connected nodes, called hubs. In other words, complex networks are typically not row-sparse, even though the average connectivity over all nodes can be very small. In this work, we extend the state-of-the-art results on quantum simulation to graphs that contain a small number of hubs, but that are otherwise sparse. Hopefully, our results may lead to new applications of quantum computing to network science.

  2 citations

• Confined vs. extended Dirac surface states in topological crystalline insulator nanowires

  Roni Majlin Skiff, Fernando de Juan, Raquel Queiroz, Subramanian Mathimalar, Haim Beidenkopf, Roni Ilan

  SciPost Phys. Core 6, 011 (2023) · published 13 March 2023 | Toggle abstract · pdf

  Confining two dimensional Dirac fermions on the surface of topological insulators has remained an outstanding conceptual challenge. Here we show that Dirac fermion confinement is achievable in topological crystalline insulators (TCI), which host multiple surface Dirac cones depending on the surface termination and the symmetries it preserves. This confinement is most dramatically reflected in the flux dependence of these Dirac states in the nanowire geometry, where different facets connect to form a closed surface. Using SnTe as a case study, we show how wires with all four facets of the $\langle 100 \rangle$ type display novel Aharonov-Bohm oscillations, while nanowires with the four facets of the $\langle 110 \rangle$ type such oscillations are absent due to strong confinement of the Dirac states to each facet separately. Our results place TCI nanowires as a versatile platform for confining and manipulating Dirac surface states.

  2 citations