Papers by Naser Ahmadiniaz
Physical Review D, 2020
Quantum electrodynamics predicts the vacuum to behave as a non-linear medium, including effects s... more Quantum electrodynamics predicts the vacuum to behave as a non-linear medium, including effects such as birefringence. However, for experimentally available field strengths, this vacuum polarizability is extremely small and thus very hard to measure. In analogy to the Heisenberg limit in quantum metrology, we study the minimum requirements for such a detection in a given strong field (the pump field). Using a laser pulse as the probe field, we find that its energy must exceed a certain threshold depending on the interaction time. However, a detection at that threshold, i.e., the Heisenberg limit, requires highly non-linear measurement schemes-while for ordinary linear-optics schemes, the required energy (Poisson or shot noise limit) is much larger. Finally, we discuss several currently considered experimental scenarios from this point of view.
Physical Review D, 2013
An interesting class of background field configurations in quantum electrodynamics (QED) are the ... more An interesting class of background field configurations in quantum electrodynamics (QED) are the O(2) × O(3) symmetric fields, originally introduced by S.L. Adler in 1972. Those backgrounds have some instanton-like properties and yield a one-loop effective action that is highly nontrivial, but amenable to numerical calculation. Here, we use the recently developed "partial-wave-cutoff method" for a full mass range numerical analysis of the effective action for the "standard" O(2) × O(3) symmetric field, modified by a radial suppression factor. At large mass, we are able to match the asymptotics of the physically renormalized effective action against the leading two mass levels of the inverse mass expansion. For small masses, with a suitable choice of the renormalization scheme we obtain stable numerical results even in the massless limit. We analyze the N-point functions in this background and show that, even in the absence of the radial suppression factor, the two-point contribution to the effective action is the only obstacle to taking its massless limit. The standard O(2) × O(3) background leads to a chiral anomaly term in the effective action, and both our perturbative and nonperturbative results strongly suggest that the small-mass asymptotic behavior of the effective action is, after the subtraction of the two-point contribution, dominated by this anomaly term as the only source of a logarithmic mass dependence. This confirms a conjecture by M. Fry.
arXiv: High Energy Physics - Theory, 2015
The first part of the lectures, given by O. Corradini, covers introductory material on quantum-me... more The first part of the lectures, given by O. Corradini, covers introductory material on quantum-mechanical Feynman path integrals, which are here derived and applied to several particle models. We start considering the nonrelativistic bosonic particle, for which we compute the exact path integrals for the case of the free particle and for the harmonic oscillator, and then describe perturbation theory for an arbitrary potential. We then move to relativistic particles, both bosonic and fermionic (spinning) particles. We first investigate them from the classical view-point, studying the symmetries of their actions, then consider their canonical quantization and path integrals, and underline the role these models have in the study of space-time quantum field theories (QFT), by introducing the "worldline" path integral representation of propagators and effective actions. We also describe a special class of spinning particles that constitute a first-quantized approach to higher-s...
Physical Review D
We present a novel procedure to construct Yang-Mills Berends-Giele currents from the Bern-Kosower... more We present a novel procedure to construct Yang-Mills Berends-Giele currents from the Bern-Kosower formalism for one-loop gluon amplitudes. The obtained currents naturally obey color-kinematics duality. As a feedback to the Bern-Kosower formalism we outline how the multiparticle fields can be used to significantly streamline the pinch procedure for the reducible one-loop amplitudes.
Proceedings of Loops and Legs in Quantum Field Theory — PoS(LL2016)
We summarize recent work on the application of the string-inspired worldline formalism to the der... more We summarize recent work on the application of the string-inspired worldline formalism to the derivation of gauge-invariant form factor decompositions of the QCD off-shell N-gluon amplitudes. This formalism allows one to achieve such decompositions by way of integration-by-parts, rather than the usual tedious analysis of the non-abelian off-shell Ward identities, and to combine the scalar, spinor and gluon loop cases. For the three-gluon case, we rederive the standard Ball-Chiu decomposition in an efficient way. For the four-gluon vertex, we obtain a new decomposition in terms of 19 tensors, of which only 14 have full four-point kinematics; the remaining five are the Ball-Chiu tensors reappearing as boundary terms. A particularly compact representation is obtained for the case of N = 4 SYM theory. We compare with the low-energy effective action.
EPL (Europhysics Letters)
EPJ Web of Conferences
The Bern-Kosower formalism, originally developed around 1990 as a novel way of obtaining on-shell... more The Bern-Kosower formalism, originally developed around 1990 as a novel way of obtaining on-shell amplitudes in field theory as limits of string amplitudes, has recently been shown to be extremely efficient as a tool for obtaining form factor decompositions of the N-gluon vertices. Its main advantages are that gauge invariant structures can be generated by certain systematic integration-by-parts procedures, making unnecessary the usual tedious analysis of the non-abelian off-shell Ward identities, and that the scalar, spinor and gluon loop cases can be treated in a unified way. After discussing the method in general for the N-gluon case, I will show in detail how to rederive the Ball-Chiu decomposition of the three-gluon vertex, and finally present two slightly different decompositions of the four-gluon vertex, one generalizing the Ball Chiu one, the other one closely linked to the QCD effective action.
Nuclear Physics B
Tree-level scattering amplitudes for a scalar particle coupled to an arbitrary number N of photon... more Tree-level scattering amplitudes for a scalar particle coupled to an arbitrary number N of photons and a single graviton are computed. We employ the worldline formalism as the main tool to compute the irreducible part of the amplitude, where all the photons and the graviton are directly attached to the scalar line, then derive a "tree replacement" rule to construct the reducible parts of the amplitude which involve irreducible pure N-photon two-scalar amplitudes where one photon line emits the graviton. We test our construction by verifying the on-shell gauge and diffeomorphism Ward identities, at arbitrary N .
Journal of Physics: Conference Series
In this contribution we discuss off-shell gluon amplitudes from worldline formalism perspective. ... more In this contribution we discuss off-shell gluon amplitudes from worldline formalism perspective. We use the Bern-Kosower master formula and two IBP methods to extract the form factor of the off-shell three-and four-gluon vertices. For the four-gluon case we also discus its representation in N = 4 SYM.
Journal of High Energy Physics
We consider one-particle reducible (1PR) contributions to QED and scalar QED processes in externa... more We consider one-particle reducible (1PR) contributions to QED and scalar QED processes in external fields, at one-loop and two-loop order. We investigate three cases in detail: constant crossed fields, constant magnetic fields, and plane waves. We find that 1PR tadpole contributions in plane waves and constant crossed fields are non-zero, but contribute only divergences to be renormalised away. In constant magnetic fields, on the other hand, tadpole contributions give physical corrections to processes at one loop and beyond. Our calculations are exact in the external fields and we give strong and weak field expansions in the magnetic case.
Journal of High Energy Physics
We present an approach to U⋆(N) Yang-Mills theory in non-commutative space based upon a novel pha... more We present an approach to U⋆(N) Yang-Mills theory in non-commutative space based upon a novel phase-space analysis of the dynamical fields with additional auxiliary variables that generate Lorentz structure and colour degrees of freedom. To illustrate this formalism we compute the quadratic terms in the effective action focusing on the planar divergences so as to extract the β-function for the Yang-Mills coupling constant. Nonetheless the method presented is general and can be applied to calculate the effective action at arbitrary order of expansion in the coupling constant and is well suited to the computation of low energy one-loop scattering amplitudes.
Journal of High Energy Physics
In this work we have studied the Kleiss-Kuijf relations for the recently introduced Parke-Taylor ... more In this work we have studied the Kleiss-Kuijf relations for the recently introduced Parke-Taylor factors at one-loop in the CHY approach, that reproduce quadratic Feynman propagators. By doing this, we were able to identify the non-planar one-loop Parke-Taylor factors. In order to check that, in fact, these new factors can describe nonplanar amplitudes, we applied them to the bi-adjoint Φ 3 theory. As a byproduct, we found a new type of graphs that we called the non-planar CHY-graphs. These graphs encode all the information for the subleading order at one-loop, and there is not an equivalent of these in the Feynman formalism.
International Journal of Modern Physics E
The Bern–Kosower formalism, developed around 1990 as a novel way of obtaining QCD amplitudes as t... more The Bern–Kosower formalism, developed around 1990 as a novel way of obtaining QCD amplitudes as the limit of infinite string tension of the corresponding string amplitudes, was originally designed as an on-shell formalism. Building on early work by Strassler, the authors have recently shown that this “string-inspired formalism” is extremely efficient also as a tool for the study of off-shell amplitudes in QCD, and in particular for achieving compact form factor decompositions of the [Formula: see text]-gluon vertices. Among other things, this formalism allows one to achieve a manifestly gauge invariant decomposition of these vertices by way of integration-by-parts, rather than the usual tedious analysis of the non-abelian off-shell Ward identities, and to combine the spin zero, half and one cases. Here, we will provide a summary of the method, as well as its application to the three- and four-gluon vertices.
International Journal of Modern Physics: Conference Series, 2016
We report on an ongoing study of photon amplitudes, graviton amplitudes and mixed photon-graviton... more We report on an ongoing study of photon amplitudes, graviton amplitudes and mixed photon-graviton amplitudes at tree-level using the worldline formalism. We explicitly recalculate the amplitude with one photon and one graviton coupled to a scalar propagator, relevant for graviton photoproduction. We comment on the factorization properties of this amplitude, and outline a generalization to similar processes involving more gravitons.
Physical Review D, 2016
We apply the worldline formalism to amplitudes in scalar quantum electrodynamics (QED) involving ... more We apply the worldline formalism to amplitudes in scalar quantum electrodynamics (QED) involving open scalar lines, with an emphasis on their non-perturbative gauge dependence. At the tree-level, we study the scalar propagator interacting with any number of photons in configuration space as well as in momentum space. At one-loop we rederive, in an efficient way, the off-shell vertex in an arbitrary dimension and any covariant gauge. Generalizing the Landau-Khalatnikov-Fradkin transformation (LKFT) for the non-perturbative propagator, we find simple non-perturbative transformation rules for arbitrary x-space amplitudes under a change of the covariant gauge parameter in terms of conformal cross ratios.
Physical Review D, 2016
We study the propagator of a colored scalar particle in the background of a non-Abelian gauge fie... more We study the propagator of a colored scalar particle in the background of a non-Abelian gauge field using the worldline formalism. It is obtained by considering the open worldline of a scalar particle with extra degrees of freedom needed to take into account the color charge of the particle, which we choose to be in the fundamental representation of the gauge group. Specializing the external gauge field to be given by a sum of plane waves, i.e. a sum of external gluons, we produce a master formula for the scalar propagator with an arbitrary number of gluons directly attached to the scalar line, akin to similar formulas derived in the literature for the case of the scalar particle performing a loop. Our worldline description produces at the same time the situation in which the particle has a color charge given by an arbitrarily chosen symmetric or antisymmetric tensor product of the fundamental.
Journal of High Energy Physics, 2015
We study pure noncommutative U(1) gauge theory representing its one-loop effective action in term... more We study pure noncommutative U(1) gauge theory representing its one-loop effective action in terms of a phase space worldline path integral. We write the quadratic action using the background field method to keep explicit gauge invariance, and then employ the worldline formalism to write the one-loop effective action, singling out UV-divergent parts and finite (planar and non-planar) parts, and study renormalization properties of the theory. This amounts to employ worldline Feynman rules for the phase space path integral, that nicely incorporate the Fadeev-Popov ghost contribution and efficiently separate planar and non-planar contributions. We also show that the effective action calculation is independent of the choice of the worldline Green's function, that corresponds to a particular way of factoring out a particle zero-mode. This allows to employ homogeneous string-inspired Feynman rules that greatly simplify the computation.
We show how to use the Bern-Kosower master formula, originally a generating functional for on-she... more We show how to use the Bern-Kosower master formula, originally a generating functional for on-shell gluon matrix elements, to derive well-organized form factor decompositions of the off-shell one-particle-irreducible N - gluon vertices. Two such algorithms are presented which can be used for any N, the first one optimized with respect to the nonabelian gauge invariance, the second one with respect to transversality. We give explicit results for the three- and four-gluon cases. The second algorithm in the three-point case reproduces precisely the well-known Ball-Chiu decomposition, and in the four-point case a natural generalization thereof. A particularly simple structure emerges in the N=4 SYM case.
Uploads
Papers by Naser Ahmadiniaz