arXiv subjects

Mohamed M. Anber

Publications and source records attributed to Mohamed M. Anber.

At least 37 records · Page 2Linked to original sources

arXiv2021

Condensates and anomaly cascade in vector-like theories

We study the bilinear and higher-order fermion condensates in $4$-dimensional $SU(N)$ gauge theories with a single Dirac fermion in a general representation. Augmented with a mixed anomaly between the $0$-form discrete chiral, $1$-form center, and $0$-form baryon number symmetries (BC anomaly), we sort out theories that admit higher-order condensates and vanishing fermion bilinears. Then, the BC anomaly is utilized to prove, in the absence of a topological quantum field theory, that nonvanishing fermion bilinears are inevitable in infrared-gapped theories with $2$-index (anti)symmetric fermions. We also contrast the BC anomaly with the $0$-form anomalies and show that it is the former anomaly that determines the infrared physics; we argue that the BC anomaly lurks deep to the infrared while the $0$-form anomalies are just variations of local terms. We provide evidence of this assertion by studying the BC anomaly in vector-like theories compactified on a small spacial circle. These theories are weakly-coupled, under analytical control, and they admit a dual description in terms of abelian photons that determine the deep infrared dynamics. We show that the dual photons talk directly to the $1$-form center symmetry in order to match the BC anomaly, while the $0$-form anomalies are variations of local terms and are matched by fiat. Finally, we study the fate of the BC anomaly in the compactified theories when they are held at a finite temperature. The effective field theory that describes the low-energy physics is $2$-dimensional. We show that the BC anomaly cascades from $4$ to $2$ dimensions.

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arXiv2021

Natural inflation, strong dynamics, and the role of generalized anomalies

We revisit models of natural inflation and show that the single-field effective theory described by the potential $V(a)\sim \cos\frac{a}{f}$ breaks down as the inflaton $a$ makes large-field excursions, even for values of $f$ smaller than the Planck scale. In order to remedy the problem, we modify the potential in order to account for the heavy degrees of freedom (hadrons) that become intertwined with the light inflaton as the latter rolls down its potential. By embedding the low energy degrees of freedom into an ultraviolet complete gauge theory, we argue that the intertwining between the two scales can be explained as the result of a generalized mixed 't Hooft anomaly between the discrete chiral symmetry and background fractional fluxes in the baryon number, color, and flavor directions. Further, we study the multi-field inflation and show that it entertains rich dynamics. Inflating near the hilltop excites the hadrons and spoils the slow-roll parameters, in contradistinction with the expectations in the single-field inflation. Nevertheless, we identify a safe zone where inflation can proceed successfully. We determine the conditions under which the Universe inflates by at least $60$ e-foldings and inflation leads to a power spectrum and tensor to scalar ratio that are consistent with the Cosmic Microwave Background data.

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arXiv2020

Generalized 't Hooft anomalies on non-spin manifolds

We study the mixed anomaly between the discrete chiral symmetry and general baryon-color-flavor (BCF) backgrounds in $SU(N_c)$ gauge theories with $N_f$ flavors of Dirac fermions in representations ${\cal R}_c$ of $N$-ality $n_c$, formulated on non-spin manifolds. We show how to study these theories on $\mathbb{CP}^2$ by turning on general BCF fluxes consistent with the fermion transition functions. We consider several examples in detail and argue that matching the anomaly on non-spin manifolds places stronger constraints on the infrared physics, compared to the ones on spin manifolds (e.g.~$\mathbb{T}^4$). We also show how to consistently formulate various chiral gauge theories on non-spin manifolds.

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arXiv2020

Deconfinement on axion domain walls

We study $SU(N_c)$ gauge theories with Dirac fermions in representations ${\cal{R}}$ of nonzero $N$-ality, coupled to axions. These theories have an exact discrete chiral symmetry, which has a mixed 't Hooft anomaly with general baryon-color-flavor backgrounds, called the "BCF anomaly" in arXiv:1909.09027. The infrared theory also has an emergent $\mathbb Z_{N_c}^{(1)}$ $1$-form center symmetry. We show that the BCF anomaly is matched in the infrared by axion domain walls. We argue that $\mathbb Z_{N_c}^{(1)}$ is spontaneously broken on axion domain walls, so that nonzero $N$-ality Wilson loops obey the perimeter law and probe quarks are deconfined on the walls. We give further support to our conclusion by using a calculable small-circle compactification to study the multi-scale structure of the axion domain walls and the microscopic physics of deconfinement on their worldvolume.

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arXiv2019

On the baryon-color-flavor (BCF) anomaly in vector-like theories

We consider the most general fractional background fluxes in the color, flavor, and baryon number directions, compatible with the faithful action of the global symmetry of a given theory. We call the obstruction to gauging symmetries revealed by such backgrounds the baryon-color-flavor (BCF) 't Hooft anomaly. We apply the BCF anomaly to vector-like theories, with fermions in higher-dimensional representations of arbitrary N-ality, and derive non-trivial constraints on their IR dynamics. In particular, this class of theories enjoys an independent discrete chiral symmetry and one may ask about the fate of this symmetry in the background of BCF fluxes. We show that, under certain conditions, an anomaly between the chiral symmetry and the BCF background rules out massless composite fermions as the sole player in the IR: either the composites do not form or additional contributions to the matching of the BCF anomaly are required. We can also give a flavor-symmetric mass to the fermions, smaller than or of order the strong scale of the theory, and examine the $θ$-angle periodicity of the theory in the BCF background. Interestingly, we find that the conditions that rule out the composites are the exact same conditions that lead to an anomaly of the $θ$ periodicity: the massive theory will experience a phase transition as we vary $θ$ from $0$ to $2π$.

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arXiv2019

Self-conjugate QCD

We carry out a systematic study of $SU(6)$ Yang-Mills theory endowed with fermions in the adjoint and $3$-index antisymmetric mixed-representation. The fermion bilinear in the $3$-index antisymmetric representation vanishes identically, which leads to interesting new phenomena. We first study the theory on a small circle, i.e., on $\mathbb R^3\times \mathbb S^1_L$, employing symmetry-twisted boundary conditions and semi-classical techniques. We find that the ground state is $3$-fold degenerate, which can be explained as a consequence of a $1$-form/$0$-form mixed 't Hooft anomaly. In addition, the theory may admit massless bosonic and fermionic degrees of freedom, depending on the number of flavors, and confines the electric probes in the infrared. Empowered by 't Hooft anomaly matching conditions along with the $2$-loop $β$-function, we further examine the possible infrared symmetry realizations on $\mathbb R^4$ for various number of adjoint and $3$-index antisymmetric fermions. The infrared theory is either a conformal field theory, which is expected for a large number of flavors, or it is confining with or without chiral symmetry breaking. In a few cases, we are able to give enough evidence for adiabatic continuity between the small- and large-circle limits.

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arXiv2019

Domain walls in high-T SU(N) super Yang-Mills theory and QCD(adj)

We study the domain walls in hot $4$-D $SU(N)$ super Yang-Mills theory and QCD(adj), with $n_f$ Weyl flavors. We find that the $k$-wall worldvolume theory is $2$-D QCD with gauge group $SU(N-k)\times SU(k) \times U(1)$ and Dirac fermions charged under $U(1)$ and transforming in the bi-fundamental representation of the nonabelian factors. We show that the DW theory has a $1$-form $\mathbb Z_{N}^{(1)}$ center symmetry and a $0$-form $\mathbb Z_{2Nn_f}^{dχ}$ discrete chiral symmetry, with a mixed 't Hooft anomaly consistent with bulk/wall anomaly inflow. We argue that $\mathbb Z_{N}^{(1)}$ is broken on the wall, and hence, Wilson loops obey the perimeter law. The breaking of the worldvolume center symmetry implies that bulk $p$-strings can end on the wall, a phenomenon first discovered using string-theoretic constructions. We invoke $2$-D bosonization and gauged Wess-Zumino-Witten models to suggest that $\mathbb Z_{2Nn_f}^{dχ}$ is also broken in the IR, which implies that the $0$-form/$1$-form mixed 't Hooft anomaly in the gapped $k$-wall theory is saturated by a topological quantum field theory. We also find interesting parallels between the physics of high-temperature domain walls studied here and domain walls between chiral symmetry breaking vacua in the zero temperature phase of the theory (studied earlier in the semiclassically calculable small spatial circle regime), arising from the similar mode of saturation of the relevant 't Hooft anomalies.

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arXiv2018

Entanglement entropy, dualities, and deconfinement in gauge theories

Computing the entanglement entropy in confining gauge theories is often accompanied by puzzles and ambiguities. In this work we show that compactifying the theory on a small circle $\mathbb S^1_L$ evades these difficulties. In particular, we study Yang-Mills theory on $\mathbb R^3\times \mathbb S^1_L$ with double-trace deformations or adjoint fermions and hold it at temperatures near the deconfinement transition. This theory is dual to a multi-component (electric-magnetic) Coulomb gas that can be mapped either to an XY-spin model with $\mathbb Z_p$-preserving perturbations or dual Sine-Gordon model. The entanglement entropy of the dual Sine-Gordon model exhibits an extremum at the critical temperature/crossover. We also compute Renyi mutual information (RMI) of the XY-spin model by means of the replica trick and Monte Carlo simulations. These are expensive calculations, since one in general needs to suppress lower winding vortices that do not correspond to physical excitations of the system. We use a T-duality that maps the original XY model to its mirror image, making the extraction of RMI a much efficient process. Our simulations indicate that RMI follows the area law scaling, with subleading corrections, and this quantity can be used as a genuine probe to detect deconfinement transitions. We also discuss the effect of fundamental matter on RMI and the implications of our findings in gauge theories and beyond.

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arXiv2018

Anomaly matching, (axial) Schwinger models, and high-T super Yang-Mills domain walls

We study the discrete chiral- and center-symmetry 't Hooft anomaly matching in the charge-$q$ two-dimensional Schwinger model. We show that the algebra of the discrete symmetry operators involves a central extension, implying the existence of $q$ vacua, and that the chiral and center symmetries are spontaneously broken. We then argue that an axial version of the $q$$=$$2$ model appears in the worldvolume theory on domain walls between center-symmetry breaking vacua in the high-temperature $SU(2)$ ${\cal N}$$=$$1$ super-Yang-Mills theory and that it inherits the discrete 't Hooft anomalies of the four-dimensional bulk. The Schwinger model results suggest that the high-temperature domain wall exhibits a surprisingly rich structure: it supports a non-vanishing fermion condensate and perimeter law for spacelike Wilson loops, thus mirroring many properties of the strongly coupled four-dimensional low-temperature theory. We also discuss generalizations to theories with multiple adjoint fermions and possible lattice tests.

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arXiv2018

Two-flavor adjoint QCD

We study four dimensional $SU(2)$ Yang-Mills theory with two massless adjoint Weyl fermions. When compactified on a spatial circle of size $L$ much smaller than the strong-coupling scale, this theory can be solved by weak-coupling nonperturbative semiclassical methods. We study the possible realizations of symmetries in the $\mathbb R^4$ limit and find that all continuous and discrete $0$-form and $1$-form 't Hooft anomaly matching conditions are saturated by a symmetry realization and massless spectrum identical to that found in the small-$L$ limit, with only a single massless flavor-doublet fermion in the infrared. This observation raises the possibility that the class of theories which undergo no phase transition between the analytically-solvable small-size circle and strongly-coupled infinite-size circle is larger than previously thought, and offers new challenges for lattice studies.

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arXiv2018

New nonperturbative scales and glueballs in confining supersymmetric gauge theories

We show that new nonperturbative scales exist in four-dimensional ${\cal{N}}$$=$$1$ super-Yang-Mills theory compactified on a circle, with an iterated-exponential dependence on the inverse gauge coupling. The lightest states with the quantum numbers of four-dimensional glueballs are nonrelativistic bound states of dual Cartan gluons and superpartners, with binding energy equal to $e^{- e^{1/g^2}}$ in units of the confining mass gap. Focusing on $SU(2)$ gauge group, we construct the nonrelativistic effective theory, show that the lightest glueball/glueballino states fill a chiral supermultiplet, and determine their "doubly-nonperturbative" binding energy. The iterated-exponential dependence on the gauge coupling is due to nonperturbative couplings in the long distance theory, $λ\sim e^{-{1 \over g^2}}$, which are responsible for attractive interactions, in turn producing exponentially small, $\sim e^{-{1\over λ}}$, effects.

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arXiv2018

Large-scale messengers from arbitrary spin fields

We show that nonperturbative production of arbitrary spin fields from vacuum will accompany the generation of non-vanishing macroscopic energy-momentum tensor correlators. This argument is based on the general causal field formalism, which gives a manifestly covariant description of higher spin particles without any reference to gauge redundancy. Our findings are direct consequence of the Poincaré covariance and anlayticity of the Green's functions and independent of any detailed particle physics model. Further, we discuss the idea that any mechanism causing imbalance between the on-shell production of left- and right-handed fields leads to a helical structure in the energy momentum correlators and violation of the macroscopic parity symmetry. We check our method for fields with spin $\frac{1}{2}$ and show that it correctly reproduces previous results. However, the formalism suffers from pathologies related to non-localities that appear for massless particles with spin $\geq 1$ in flat space. We discuss the origin of these pathologies and the relevance of our findings to cosmology.

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arXiv2017

Classification of compactified $su(N_c)$ gauge theories with fermions in all representations

We classify $su(N_c)$ gauge theories on $\mathbb R^3\times \mathbb S^1$ with massless fermions in higher representations obeying periodic boundary conditions along $\mathbb S^1$. In particular, we single out the class of theories that is asymptotically free and weakly coupled in the infrared, and therefore, is amenable to semi-classical treatment. Our study is conducted by carefully identifying the vacua inside the affine Weyl chamber using Verma bases and Frobenius formula techniques. Theories with fermions in pure representations are generally strongly coupled. The only exceptions are the four-index symmetric representation of $su(2)$ and adjoint representation of $su(N_c)$. However, we find a plethora of admissible theories with fermions in mixed representations. A sub-class of these theories have degenerate perturbative vacua separated by domain walls. In particular, $su(N_c)$ theories with fermions in the mixed representations adjoint$\oplus$fundamental and adjoint$\oplus$two-index symmetric admit degenerate vacua that spontaneously break the parity ${\cal P}$, charge conjugation ${\cal C}$, and time reversal ${\cal T}$ symmetries. These are the first examples of strictly weakly coupled gauge theories on $\mathbb R^3 \times \mathbb S^1$ with spontaneously broken ${\cal C}$, ${\cal P}$, and ${\cal T}$ symmetries. We also compute the fermion zero modes in the background of monopole-instantons. The monopoles and their composites (topological molecules) proliferate in the vacuum leading to the confinement of electric charges. Interestingly enough, some theories have also accidental degenerate vacua, which are not related by any symmetry. These vacua admit different numbers of fermionic zero modes, and hence, different kinds of topological molecules. The lack of symmetry, however, indicates that such degeneracy might be lifted by higher order corrections.

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arXiv2017

On the representation (in)dependence of $k$-strings in pure Yang-Mills theory via supersymmetry

We exploit a conjectured continuity between super Yang-Mills on $\mathbb R^3\times \mathbb S^1$ and pure Yang-Mills to study $k$-strings in the latter theory. As expected, we find that Wilson-loop correlation functions depend on the N-ality of a representation ${\cal R}$ to the leading order. However, the next-to-leading order correction is not universal and is given by the group characters, in the representation ${\cal R}$, of the permutation group. We also study W-bosons in super Yang-Mills and show that they are deconfined on the string worldsheet, and therefore, can change neither the string N-ality nor its tension. This phenomenon mirrors the fact that soft gluons do not screen probe charges with non-zero N-ality in pure Yang-Mills. Finally, we comment on the scaling law of $k$-strings in super Yang-Mills and compare our findings with strings in Seiberg-Witten theory, deformed Yang-Mills theory, and holographic studies that were performed in the 't Hooft large-$N$ limit.

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arXiv2017

Chiral Gravitational Waves from Chiral Fermions

We report on a new mechanism that leads to the generation of primordial chiral gravitational waves, and hence, the violation of the parity symmetry in the Universe. We show that nonperturbative production of fermions with a definite helicity is accompanied by the generation of chiral gravitational waves. This is a generic and model-independent phenomenon that can occur during inflation, reheating and radiation eras, and can leave imprints in the cosmic microwave background polarization and may be observed in future ground- and space-based interferometers. We also discuss a specific model where chiral gravitational waves are generated via the production of light chiral fermions during pseudoscalar inflation.

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arXiv2017

Oblique Confinement at $θ\neq 0$ in weakly coupled gauge theories with deformations

The main focus of this work is to test the ideas related to the oblique confinement in a theoretically controllable manner using the "deformed QCD" as a toy model. We explicitly show that the oblique confinement in the weakly coupled gauge theories emerges as a result of condensation of $N$ types of monopoles shifted by the phase $\exp(i\frac{θ+2πm}{N})$ in Bloch type construction. It should be contrasted with conventional and commonly accepted viewpoint that the confinement at $θ\neq 0$ is due to the condensation of the electrically charged dyons which indeed normally emerge in the systems with $θ\neq 0$ as a result of Witten's effect. We explain the basic reason why the "dyon" mechanism does not materialize-- it is because the Witten's effect holds for a static magnetic monopole treated as an external source. It should be contrasted with our case when $N$- types of monopoles are not static, but rather the dynamical degrees of freedom which fluctuate and themselves determine the ground state of the system.

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arXiv2015

On the Global Structure of Deformed Yang-Mills Theory and QCD(adj) on $\mathbb R^3 \times \mathbb S^1$

Spatial compactification on $\mathbb R^{3} \times \mathbb S^1_L$ at small $\mathbb S^1$-size $L$ often leads to a calculable vacuum structure, where various "topological molecules" are responsible for confinement and the realization of the center and discrete chiral symmetries. Within this semiclassically calculable framework, we study how distinct theories with the same $SU(N_c)/\mathbb Z_k$ gauge group (labeled by "discrete $θ$-angles") arise upon gauging of appropriate $\mathbb Z_k$ subgroups of the one-form global center symmetry of an $SU(N_c)$ gauge theory. We determine the possible $\mathbb Z_k$ actions on the local electric and magnetic effective degrees of freedom, find the ground states, and use domain walls and confining strings to give a physical picture of the vacuum structure of the different $SU(N_c)/\mathbb Z_k$ theories. Some of our results reproduce ones from earlier supersymmetric studies, but most are new and do not invoke supersymmetry. We also study a further finite-temperature compactification to $\mathbb R^{2}\times \mathbb S^1_β\times\mathbb S^1_L$. We argue that, in deformed Yang-Mills theory, the effective theory near the deconfinement temperature $β_c \gg L$ exhibits an emergent Kramers-Wannier duality and that it exchanges high- and low-temperature theories with different global structure, sharing features with both the Ising model and $S$-duality in ${\cal N}$$=$$4$ supersymmetric Yang-Mills theory.

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arXiv2015

Diamagnetic Vortices in Chern Simons Theory

We find a new type of topological vortex solution in the $U(1)_Z \times U(1)_A$ Chern Simons gauge theory in the presence of a $U(1)_A$ magnetic field background. In this theory $U(1)_Z$ is broken spontaneously by the $U(1)_A$ magnetic field. These vortices exhibit long range interactions as they are charged under the unbroken $U(1)_A$. They deplete the $U(1)_A$ magnetic field near their core and also break both $C$ and $P$ symmetries. Understanding the nature of these vortices sheds light on the ground state structure of the superconductivity studied in [1]. We also study the Berezinsky-Kosterlitz-Thouless phase transition in this class of theories and point out that superconductivity can be achieved at high temperatures by increasing the $U(1)_A$ magnetic field.

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