arXiv subjects

Arkady Vainshtein

Publications and source records attributed to Arkady Vainshtein.

At least 19 recordsLinked to original sources

arXiv2025

Non-perturbaitve effects in Higgs boson decays to electroweak vector bosons and photons

We estimate the magnitude of the leading non-perturbative QCD corrections to the decays of the Higgs boson to the $\gamma Z$ and $\gamma \gamma$ final states. These corrections originate from the light-quark contributions to such decays. We show that the non-perturbative effects are suppressed by the small Yukawa couplings of light quarks, but that there is no further quark-mass suppression. This is at variance with what is found in the standard perturbative calculations of the light-quark contributions. We demonstrate that the non-perturbative corrections modify the $H \to \gamma Z$ and $H \to \gamma \gamma$ decay rates by $O(10^{-5})$, well below the expected precision with which such decays can be studied both at the high-luminosity LHC and at future colliders.

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arXiv2023

Spectral Flow in Instanton Computations and the \boldmath{$\b$} functions

We discuss various differences in the instanton-based calculations of the $\beta$ functions in theories such as Yang-Mills and $\mathbb{CP}(N\!-\!1)$ on one hand, and $\lambda\phi^4$ theory with Symanzik's sign-reversed prescription for the coupling constant $\lambda$ on the other hand. Although the aforementioned theories are asymptotically free, in the first two theories, instantons are topological, whereas the Fubini-Lipatov instanton in the third theory is topologically trivial. The spectral structure in the background of the Fubini-Lipatov instanton can be continuously deformed into that in the flat background, establishing a one-to-one correspondence between the two spectra. However, when considering topologically nontrivial backgrounds for Yang-Mills and $\mathbb{CP}(N\!-\!1)$ theories, the spectrum undergoes restructuring. In these cases, a mismatch between the spectra around the instanton and the trivial vacuum occurs.

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arXiv2022

Four-fermion deformations of the massless Schwinger model and confinement

We consider the massless charge-$N$ Schwinger model and its deformation with two four-fermion operators. Without the deformations, this model exhibits chiral symmetry breaking without confinement. It is usually asserted that the massless Schwinger model is always deconfined and a string tension emerges only when a mass for the fermion field is turned on. We show that in the presence of these four-fermion operators, the massless theory can in fact confine. One of the four-fermion deformations is chirally neutral, and is a marginal deformation. The other operator can be relevant or irrelevant, and respects a $\mathbb{Z}_2$ subgroup of chiral symmetry for even $N$, hence forbidding a mass term. When it is relevant, even the exactly massless theory exhibits both confinement and spontaneous chiral symmetry breaking. The construction is analogous to QCD(adj) in 2d. While the theory without four-fermion deformations is deconfined, the theory with these deformations is generically in a confining phase. We study the model on $\mathbb{R}^2$ using bosonization, and also analyze the mechanism of confinement on $\mathbb{R}\times S^1$, where we find that confinement is driven by fractional instantons.

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arXiv2021

Magic Zeroes and Hidden Symmetries

Selection rules arising from accidental or broken symmetries may be sufficiently obscure that their agency is hidden, leading to the appearance of "magic zeroes" -- quantities that are suppressed without apparent recourse to a symmetry explanation. Magic zeroes and their corresponding hidden symmetries may shed new light on parametric hierarchies in the Standard Model and beyond. We identify the hidden symmetry responsible for a recently-discovered magic zero, the vanishing of the putative leading contribution to the anomalous dipole moments of the muon upon integrating out weak doublet and singlet vector-like fermions. Some of the tools involved -- spurion analysis leveraging discrete symmetries of the free theory, field redefinitions, spectator fields, and non-supersymmetric non-renormalization theorems -- may prove useful in the hunt for new magic zeroes and their hidden symmetries.

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arXiv2020

On Dimensional Transmutation in 1+1D Quantum Hydrodynamics

Recently a detailed correspondence was established between, on one side, four and five-dimensional large-N supersymmetric gauge theories with $\mathcal{N}=2$ supersymmetry and adjoint matter, and, on the other side, integrable 1+1-dimensional quantum hydrodynamics. Under this correspondence the phenomenon of dimensional transmutation, familiar in asymptotically free QFTs, gets mapped to the transition from the elliptic Calogero-Moser many-body system to the closed Toda chain. In this paper we attempt to formulate the hydrodynamical counterpart of the dimensional transmutation phenomenon inspired by the identification of the periodic Intermediate Long Wave (ILW) equation as the hydrodynamical limit of the elliptic Calogero-Moser/Ruijsenaars-Schneider system. We also conjecture that the chiral flow in the vortex fluid provides the proper framework for the microscopic description of such dimensional transmutation in the 1+1d hydrodynamics. We provide a geometric description of this phenomenon in terms of the ADHM moduli space.

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arXiv2019

On dispersion relations and hadronic light-by-light scattering contribution to the muon anomalous magnetic moment

We discuss the use of dispersion relations for the evaluation of the pseudoscalar contributions to the muon anomalous magnetic moment. We point out that, in the absence of experimental data, reconstruction of light-by-light scattering amplitudes from their absorptive parts is ambiguous and requires additional theoretical input. The need for an additional input makes dispersive computations of the hadronic light-by-light scattering contribution to g-2 akin to phenomenological models, in spite of pretense to the contrary. In particular, we argue that the recent proposal [1], based on the dispersive approach, satisfies short distance constraints at the expense of unjustifiably large deviations from the chiral limit.

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arXiv2018

Neutron--Antineutron Oscillations: Discrete Symmetries and Quark Operators

We analyze status of ${\bf C}$, ${\bf P}$ and ${\bf T}$ discrete symmetries in application to neutron-antineutron transitions breaking conservation of baryon charge ${\cal B}$ by two units. At the level of free particles all these symmetries are preserved. This includes ${\bf P}$ reflection in spite of the opposite internal parities usually ascribed to neutron and antineutron. Explanation, which goes back to the 1937 papers by E. Majorana and by G. Racah, is based on a definition of parity satisfying ${\bf P}^{2}=-1$, instead of ${\bf P}^{2}=1$, and ascribing $ {\bf P}=i$ to both, neutron and antineutron. We apply this to ${\bf C}$, ${\bf P}$ and ${\bf T}$ classification of six-quark operators with $|Δ{\cal B} |=2$. It allows to specify operators contributing to neutron-antineutron oscillations. Remaining operators contribute to other $|Δ{\cal B} |=2$ processes and, in particular, to nuclei instability. We also show that presence of external magnetic field does not induce any new operator mixing the neutron and antineutron provided that rotational invariance is not broken.

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arXiv2017

(In)dependence of Theta in the Higgs Regime without Axions

We revisit the issue of the vacuum angle theta dependence in weakly coupled (Higgsed) Yang-Mills theories. Two most popular mechanisms for eliminating physical theta dependence are massless quarks and axions. Anselm and Johansen noted that the vacuum angle theta(EW), associated with the electroweak SU(2) in the Glashow-Weinberg-Salam model, is unobservable although all fermion fields obtain masses through Higgsing and there is no axion. We generalize this idea to a broad class of Higgsed Yang-Mills theories. In the second part we consider consequences of Grand Unification. We start from a unifying group, e.g. SU(5), at a high ultraviolet scale and evolve the theory down within the Wilson procedure. If on the way to infrared the unifying group is broken down into a few factors, all factor groups inherit one and the same theta angle -- that of the unifying group. We show that embedding the SM in SU(5) drastically changes the Anselm-Johansen conclusion: the electroweak vacuum angle theta(EW), equal to theta(QCD) becomes in principle observable in ΔB=ΔL =\pm 1 processes. We also note in passing that if the axion mechanism is set up above the unification scale, we have one and the same axion in the electroweak theory and QCD, and their impacts are interdependent.

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arXiv2016

Higgs boson decay to two photons and the dispersion relations

We discuss the computation of the Higgs boson decay amplitude to two photons through the W-loop using dispersion relations. The imaginary part of the form factor F_W(s) that parametrizes this decay is unambiguous in four dimensions. When it is used to calculate the unsubtracted dispersion integral, the finite result for the form factor F_W(s) is obtained. However, the F_W(s) obtained in this way differs by a constant term from the result of a diagrammatic computation, based on dimensional regularization. It is easy to accommodate the missing constant by writing a once-subtracted dispersion relation for F_W(s) but it is unclear why the subtraction needs to be done. The goal of this paper is to investigate this question in detail. We show that the correct constant can be recovered within a dispersive approach in a number of ways that, however, either require an introduction of an ultraviolet regulator or unphysical degrees of freedom; unregulated and unsubtracted computations in the unitary gauge are insufficient, in spite of the fact that such computations give a finite result.

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arXiv2015

Anomalies of Minimal N=(0, 1) and N=(0, 2) Sigma Models on Homogeneous Spaces

We study chiral anomalies in $\mathcal N=(0, 1)$ and $(0, 2)$ two-dimensional minimal sigma models defined on generic homogeneous spaces $G/H$. Such minimal theories contain only (left) chiral fermions and in certain cases are inconsistent because of "incurable" anomalies. We explicitly calculate the anomalous fermionic effective action and show how to remedy it by adding a series of local counter-terms. In this procedure, we derive a local anomaly matching condition, which is demonstrated to be equivalent to the well-known global topological constraint on $p_1(G/H)$. More importantly, we show that these local counter-terms further modify and constrain "curable" chiral models, some of which, for example, flow to nontrivial infrared superconformal fixed point. Finally, we also observe an interesting relation between $\mathcal N=(0, 1)$ and $(0, 2)$ two-dimensional minimal sigma models and supersymmetric gauge theories. This paper generalizes and extends the results of our previous publication arXiv:1510.04324.

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arXiv2015

On Isometry Anomalies in Minimal N=(0,1) and N=(0,2) Sigma Models

The two-dimensional minimal supersymmetric sigma models with homogeneous target spaces $G/H$ and chiral fermions of the same chirality are revisited. We demonstrate that the Moore-Nelson consistency condition revealing a global anomaly in CP(N-1) (with N>2 and ${\mathcal N}=(0,2)$ supersymmetry) due to a nontrivial first Pontryagin class is in one-to-one correspondence with the local anomalies of isometries in these models. These latter anomalies are generated by fermion loop diagrams which we explicitly calculate. In the case of O}(N) sigma models the first Pontryagin class vanishes, so there is no global obstruction for the minimal ${\mathcal N}=(0,1)$ supersymmetrization of these models. We show that at the local level isometries in these models are anomaly free. Thus, there are no obstructions to quantizing the minimal ${\mathcal N}=(0,1)$ models with the $S^{N-1}= SO(N)/SO(N-1)$ target space. This also includes CP(1) (equivalent to $S^{2}$) which is an exceptional case from the CP(N-1) series. We also discuss a relation between the geometric and gauged formulations of the CP}(N-1) models.

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arXiv2015

Neutron-Antineutron Oscillation as a Signal of CP Violation

Assuming the Lorentz and CPT invariances we show that neutron-antineutron oscillation implies breaking of CP along with baryon number violation -- i.e. two of Sakharov conditions for baryogenesis. The oscillation is produced by the unique operator in the effective Hamiltonian. This operator mixing neutron and antineutron preserves charge conjugation C and breaks P and T. External magnetic field always leads to suppression of oscillations. Its presence does not lead to any new operator mixing neutron and antineutron.

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arXiv2015

Making supersymmetric connected N =(0,2) Sigma Models

We construct "connected" (0,2) sigma models starting from n copies of (2,2) CP(N-1) models. General aspects of models of this type (known as T+O deformations) had been previously studied in the context of heterotic string theories. Our construction presents a natural generalization of the nonminimally deformed (2,2) model with an extra (0,2) fermion superfield on tangent bundle T CP(N-1) x C^1. We had thoroughly analyzed the latter model previously, found the exact beta function and a spontaneous breaking of supersymmetry. In contrast, in certain connected sigma models the spontaneous breaking of supersymmetry disappears. We study the connected sigma models in the large-N limit finding supersymmetric vacua and determining the particle spectrum. While the Witten index vanishes in all the models under consideration, in these special cases of connected models one can use a permutation symmetry to define a modification of the Witten index which does not vanish. This eliminates the spontaneous breaking of supersymmetry. We then examine the exact beta functions of our connected (0,2) sigma models.

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arXiv2014

N=(0, 2) Deformation of (2, 2) Sigma Models: Geometric Structure, Holomorphic Anomaly and Exact Beta Functions

We study N=(0,2) deformed (2,2) two-dimensional sigma models. Such heterotic models were discovered previously on the world sheet of non-Abelian strings supported by certain four-dimensional N=1 theories. We study geometric aspects and holomorphic properties of these models, and derive a number of exact expressions for the beta functions in terms of the anomalous dimensions analogous to the NSVZ beta function in four-dimensional Yang-Mills. Instanton calculus provides a straightforward method for the derivation. The anomalous dimensions are calculated up to two loops implying that one of the beta functions is explicitly known up to three loops. The fixed point in the ratio of the couplings found previously at one loop is not shifted at two loops. We also consider the N=(0,2) supercurrent supermultiplet (the so-called hypercurrent) and its anomalies, as well as the "Konishi anomaly." This gives us another method for finding exact $β$ functions. We prove that despite the chiral nature of the models under consideration quantum loops preserve isometries of the target space.

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arXiv2014

Remarks on the effect of bound states and threshold in g-2

Recently, the contribution of positronium bound states to the electron anomalous magnetic moment was computed in Refs.[1,2]. It was argued there that this O(alpha^5) contribution is missed if electron g-2 is calculated within conventional perturbative QED and, as such, it must be added to the perturbative five-loop result. We show that this conclusion is flawed and that no additional contributions to g-2 are generated in QED beyond the perturbation theory.

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arXiv2009

Hadronic Light-by-Light Scattering Contribution to the Muon Anomalous Magnetic Moment

We review the current status of theoretical calculations of the hadronic light-by-light scattering contribution to the muon anomalous magnetic moment. Different approaches and related issues such as OPE constraints and large breaking of chiral symmetry are discussed. Combining results of different models with educated guesses on the errors we come to the estimate $$a^{\rm HLbL}=(10.5\pm 2.6)\times 10^{-10}.$$ The text is prepared as a contribution to the {\it Glasgow White Paper on the present status of the Muon Anomalous Magnetic Moment}.

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arXiv2008

Gravity Cutoff in Theories with Large Discrete Symmetries

We set an upper bound on the gravitational cutoff in theories with exact quantum numbers of large N periodicity, such as Z_N discrete symmetries. The bound stems from black hole physics. It is similar to the bound appearing in theories with N particle species, though a priori, a large discrete symmetry does not imply a large number of species. Thus, there emerges a potentially wide class of new theories, that address the hierarchy problem by lowering the gravitational cutoff due to existence of large Z_{10^32}-type symmetries.

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arXiv2008

Dyon dynamics near marginal stability and non-BPS states

We derive the general form of the moduli-space effective action for the long-range interaction of two BPS dyons in N=2 gauge theories. This action determines the bound state structure of various BPS and non-BPS states near marginal stability curves, and we utilise it to compute the leading correction to the BPS-mass of zero-torsion non-BPS bound states close to marginal stability.

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