The effect of threshold singularities induced by unstable particles on two-loop observables is investigated and it is shown how to cure them working in the complex-mass scheme. The impact on radiative corrections around thresholds is thoroughly analyzed and shown to be relevant for two selected LHC and ILC applications: Higgs production via gluon fusion and decay into two photons at two loops in the Standard Model. Concerning Higgs production, it is essential to understand possible sources of large corrections in addition to the well-known QCD effects. It is shown that NLO electroweak corrections can incongruently reach a 10 % level around the WW vector-boson threshold without a complete implementation of the complex-mass scheme in the two-loop calculation.
We present the first comprehensive analysis of the unitarity thresholds and anomalous thresholds of scattering amplitudes at two loops and beyond based on the loop-tree duality, and show how non-causal unphysical thresholds are locally cancelled in an efficient way when the forest of all the dual on-shell cuts is considered as one. We also prove that soft and collinear singularities at two loops and beyond are restricted to a compact region of the loop three-momenta, which is a necessary condition for implementing a local cancellation of loop infrared singularities with the ones appearing in r…
We propose a new approach that allows for the separate numerical calculation of the real and imaginary parts of finite loop integrals. We find that at one-loop the real part is given by the Loop-Tree Duality integral supplemented with suitable counterterms and the imaginary part is a sum of two-body phase space integrals, constituting a locally finite representation of the generalised optical theorem. These expressions are integrals in momentum space, whose integrands were specially designed to feature local cancellations of threshold singularities. Such a representation is well suited for Mon…
On the analytic continuation and matching of threshold singularities from Vacuum Amplitudes
Universidad Autónoma de Sinaloa（西诺阿自治大学）；Universität Zürich（苏黎世大学）；Universidad de Salamanca（萨拉曼卡大学）；Universitat de València – Consejo Superior de Investigaciones Científicas（瓦伦西亚大学–西班牙国家研究委员会）
In perturbation theory, the spectral densities of two-point functions develop non-integrable threshold singularities at higher orders. In QCD, such singularities emerge when calculating the diagrams in terms of the pole quark mass, and they become stronger when one rearranges the perturbative expansion in terms of the running quark mass. In this paper we discuss the proper way to handle such singularities.
In this work we explore general leading singularities of one-loop amplitudes in higher-derivative Yang-Mills and quadratic gravity. These theories are known to possess propagators which contain quadratic and quartic momentum dependence, which leads to the presence of an unstable ghostlike resonance. However, unitarity cuts are not to be taken through unstable particles and therefore unitarity is still satisfied. On the other hand, this could engender issues when calculating leading singularities which are generalizations of unitarity cuts. Nevertheless, we will show with explicit examples how …
Threshold and infrared divergences are studied as possible mechanisms of particle production and compared to the usual decay process in a model quantum field theory from which generalizations are obtained. A spectral representation of the propagator of the decaying particle suggests that decay, threshold and infrared singularities while seemingly different phenomena are qualitatively related. We implement a non-perturbative dynamical resummation method to study the time evolution of an initial state. It is manifestly unitary and yields the asymptotic state and the distribution function of prod…
The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle…
In this work, we introduce a modification of the Dyson series based on the perturbative unitarity as a starting point. The presented approach systematically avoids singularities and double-counting related to the presence of unstable particles as intermediate states and, at the same time, it does not rely on a specific choice of the renormalization scheme.
We demonstrate that triangle singularity (TS) and box singularity (BS) mechanisms can produce unique narrow enhancements at the $Λ_c\bar{D}$ and $Λ_c\bar{D}^*$ thresholds in the invariant mass spectra of $J/ψp$ and $J/ψpπ$, respectively. Taking into account that such mechanisms only depend on the initial $Σ_c^{(*)}\bar{D}^{(*)}$ interactions near threshold within the TS or BS kinematic regimes, the $Λ_c\bar{D}$ and $Λ_c\bar{D}^*$ threshold enhancements can be regarded as a feed-down phenomenon originated from both the heavier pentaquark decays and the $Σ_c^{(*)}\bar{D}^{(*)}$ scatterings from …
In the past years, we have been developing a novel technique, called Four-Dimensional Unsubtraction (FDU) which aims to obtain purely four-dimensional representations of the matrix elements contributing to physical observables. In this talk, we describe the application of the loop-tree duality (LTD) theorem to represent loop amplitudes in terms of tree-level like objects, focusing on the origin of possible singularities of scattering amplitudes. In particular, we analyze the regions responsible of infrared and threshold singularities. With this information, we aim to extend the FDU formalism t…
A Study of Degenerate Two-Body and Three-Body Coupled-Channel Systems -Renormalized Effective AGS Equations and Near-Threshold Resonances-
由于存在质量接近两体和三体强子阈值的奇异强子候选者，我们研究了简并的两体和三体耦合通道系统.本文首先将非简并二体和三体耦合道的散射问题表示为一个有效的三体问题，即有效Alt-Grassberger-Sandhas（AGS）方程。接下来，我们研究的行为$S$矩阵极点附近的阈值时，两体和三体阈值退化。通过求解AGS方程核的本征值方程而不是AGS方程本身来获得$S$ -矩阵极点能量。然后，我们面临一个非物理奇异性问题：虽然物理跃迁振幅只有物理奇异性，但AGS方程的核具有非物理奇异性。然而，我们表明，这些非物理的奇异性可以通过适当的重组的散射方程和质量重整化。退化阈值附近的$S$矩阵极点的行为被发现是普遍的意义上的复极点能量，$E$，是由一个真正的参数，$c$，作为$c - E \log{\left( - E \right)} = 0$，或等价地，${\rm Im} E = - π{\rm Re} E / \log{\mid {\rm Re} E \mid}$。这种行为不同于二体或三体系统，是简并二体和三体耦合通道系统的特征。我们期望这类新的普遍行为可能在理解奇异强子方面发挥关键作用。
