Two-loop Computation Research Articles

Two-loop amplitude for mixed QCD-EW corrections to gg → Hg

We report on the two-loop amplitude computation for the mixed QCD-electroweak corrections to the process gg → Hg, with exact dependence on the electroweak boson masses. This amplitude has been employed in the computation of next-to-leading order (NLO) mixed QCD-electroweak corrections to the Higgs-boson production rate in [47]. The master integrals that appear in the amplitude are evaluated by means of generalized power series expansions, which allows for fast and high-precision numerical evaluation of the amplitude in the physical phase-space, proving to be a powerful tool for phenomenological applications.

Next-to-Next-to-Leading-Order QCD Prediction for the Photon-Pion Form Factor.

We accomplish the complete two-loop computation of the leading-twist contribution to the photon-pion transition form factor γγ^{*}→π^{0} by applying the hard-collinear factorization theorem together with modern multiloop techniques. The resulting predictions for the form factor indicate that the two-loop perturbative correction is numerically important. We also demonstrate that our results will play a key role in disentangling various models of the twist-two pion distribution amplitude thanks to the envisaged precision at Belle II.

Soft Interactions in Cold Quark Matter.

Accurate knowledge of the thermodynamic properties of zero-temperature, high-density quark matter plays an integral role in attempts to constrain the behavior of the dense QCD matter found inside neutron-star cores, irrespective of the phase realized inside the stars. In this Letter, we consider the weak-coupling expansion of the dense QCD equation of state and compute the next-to-next-to-next-to-leading-order contribution arising from the non-Abelian interactions among long-wavelength, dynamically screened gluonic fields. Accounting for these interactions requires an all-loop resummation, which can be performed using hard-thermal-loop (HTL) kinematic approximations. Concretely, we perform a full two-loop computation using the HTL effective theory, valid for the long-wavelength, or soft, modes. We find that the soft sector is well behaved within cold quark matter, contrary to the case encountered at high temperatures, and find that the new contribution decreases the renormalization-scale dependence of the equation of state at high density.

Wrinkling transition in quenched disordered membranes at two loops.

We investigate the flat phase of quenched disordered polymerized membranes by means of a two-loop, weak-coupling computation performed near their upper critical dimension D_{uc}=4, generalizing the one-loop computation of Morse etal. [D. C. Morse etal., Phys. Rev. A 45, R2151 (1992)PLRAAN1050-294710.1103/PhysRevA.45.R2151; D. C. Morse and T. C. Lubensky, Phys. Rev. A 46, 1751 (1992)PLRAAN1050-294710.1103/PhysRevA.46.1751]. Our work confirms the existence of the finite-temperature, finite-disorder wrinkling transition, which has been recently identified by Coquand etal. [O. Coquand etal., Phys. Rev. E 97, 030102(R) (2018)2470-004510.1103/PhysRevE.97.030102] using a nonperturbative renormalization group approach. We also point out ambiguities in the two-loop computation that prevent the exact identification of the properties of the novel fixed point associated with the wrinkling transition, which very likely requires a three-loop order approach.

Numerical stochastic perturbation theory estimate of the improvement coefficient cA to two loops

By using Numerical Stochastic Perturbation Theory (NSPT), we carry out a quenched two-loop computation of the improvement coefficient $c_A$ associated to the isovector axial current. Within the Schr\"odinger Functional formalism, we compute the bare quark mass $m$ and fix $c_A$ by requiring discretization corrections on $m$ to be of order $O(a^2)$ in the lattice spacing $a$.

Muon g−2 in the 2HDM: Maximum results and detailed phenomenology

We present a comprehensive analysis of the muon magnetic moment $a_\mu$ in the flavour-aligned two-Higgs doublet model (2HDM) and parameter constraints relevant for $a_\mu$. We employ a recent full two-loop computation of $a_\mu$ and take into account experimental constraints from Higgs and flavour physics on the parameter space. Large $a_\mu$ is possible for light pseudoscalar Higgs A with large Yukawa couplings to leptons, and it can be further increased by large A coupling to top quarks. We investigate in detail the maximum possible Yukawa couplings to leptons and quarks of a light A, finding values of around 50...100 (leptons) and O(0.5) (quarks). As a result we find that an overall maximum of $a_\mu$ in the 2HDM of more than $45\times10^{-10}$ is possible in a very small parameter region around M_A=20 GeV. The parameter regions in which the currently observed deviation can be explained are characterized.

Higher spin currents in the critical O(N) vector model at 1/N2

We calculate the anomalous dimensions of higher spin singlet currents in the critical O(N ) vector model at order 1/N2. The results are shown to be in agreement with the four-loop perturbative computation in ϕ4 theory in 4 − 2ϵ dimensions. It is known that the order 1/N anomalous dimensions of higher-spin currents happen to be the same in the Gross-Neveu and the critical vector model. On the contrary, the order 1/N2 corrections are different. The results can also be interpreted as a prediction for the two-loop computation in the dual higher-spin gravity.

Reformulating the TBA equations for the quark anti-quark potential and their two loop expansion

The boundary thermodynamic Bethe Ansatz (BTBA) equations introduced in arXiv:1203.1913 and arXiv:1203.1617 to describe the cusp anomalous dimension contain imaginary chemical potentials and singular boundary fugacities, which make its systematic expansion problematic. We propose an alternative formulation based on real chemical potentials and additional source terms. We expand our equations to double wrapping order and find complete agreement with the direct two-loop gauge theory computation of the cusp anomalous dimension.

NNLO soft function for electroweak boson production at large transverse momentum

The soft function relevant for the production of an electroweak boson (photon, W, Z or H) with large transverse momentum at a hadron collider is computed at next-to-next-to-leading order. This is the first two-loop computation of a soft function involving three light-cone directions. With the result, the threshold resummation for these processes can now be performed at next-to-next-to-next-to-leading logarithmic accuracy.

Renormalization-group computation of the critical exponents of hierarchical spin glasses: Large-scale behavior and divergence of the correlation length

In a recent work [M. Castellana and G. Parisi, Phys. Rev. E 82, 040105(R) (2010)], the large-scale behavior of the simplest non-mean-field spin-glass system has been analyzed, and the critical exponent related to the divergence of the correlation length has been computed at two loops within the ε-expansion technique by two independent methods. By performing the explicit calculation of the critical exponents at two loops, one obtains that the two methods yield the same result. This shows that the underlying renormalization group ideas apply consistently in this disordered model, in such a way that an ε-expansion can be set up. The question of the extension to high orders of this ε-expansion is particularly interesting from the physical point of view. Indeed, once high orders of the series in ε for the critical exponents are known, one could check the convergence properties of the series, and find out if the ordinary series resummation techniques, yielding very accurate predictions for the Ising model, work also for this model. If this is the case, a consistent and predictive non-mean-field theory for such a disordered system could be established. In that regard, in this work we expose the underlying techniques of such a two-loop computation. We show with an explicit example that such a computation could be quite easily automatized, i.e., performed by a computer program, in order to compute high orders of the ε-expansion, and so eventually make this theory physically predictive. Moreover, all the underlying renormalization group ideas implemented in such a computation are widely discussed and exposed.

On SQCD with massive and massless flavors

We consider supersymmetric QCD in the free magnetic phase with massless and massive flavors. The theory has a supersymmetry breaking pseudo-moduli space of vacua and a runaway behavior far away from the origin. A two-loop computation reveals that the origin is destabilized and there is no meta-stable SUSY breaking solution. We also study the embedding of this model in type IIA string theory and find evidence for similar behavior. The perturbative brane dynamics involves simple interactions between branes, correctly predicting the two-loop result in the gauge theory. Our results also apply to the case when all the flavors are massive but have hierarchy among them, leading to possible instability which is manifest both in field theory and the brane description.

Classical simulation of quantum fields II

We consider the classical time evolution of a real scalar field in two-dimensional Minkowski space with a [Formula: see text] interaction. We compute the spatial and temporal two-point correlation functions and extract the renormalized mass of the interacting theory. We find our results are consistent with the one- and two-loop quantum computation. We also perform Monte Carlo simulations of the quantum theory and conclude that the classical scheme is able to produce more accurate results with a fraction of the CPU time. PACS Nos.: 03.70.+k, 03.50.–z, 11.15.Tk

Two-loop renormalization of the magnetic coupling and non-perturbative sector in hot QCD

The goal of this paper is two-fold. The first aim is to present a detailed version of the computation of the two-loop renormalization of the magnetic coupling in hot QCD. The second is to compare with lattice simulations the string tension of a spatial Wilson-loop using the result of our two-loop computation.

B and B s decay constants from moments of finite energy sum rules in QCD

We use an appropriate combination of moments of finite energy sum rules in QCD in order to compute the B_q-meson decays constants f_B and f_{B_s}.We perform the calculation using a two-loop computation of the imaginary part of the pseudoscalar two point function in terms of the running bottom quark mass. The results are stable with the so called QCD duality threshold and they are in agreement with the estimates obtained from Borel transform QCD sum rules and lattice computations.

Renormalization of quantum gravity coupled with matter in three dimensions

In three spacetime dimensions, where no graviton propagates, pure gravity is known to be finite. It is natural to inquire whether finiteness survives the coupling with matter. Standard arguments ensure that there exists a subtraction scheme where no Lorentz–Chern–Simons term is generated by radiative corrections, but are not sufficiently powerful to ensure finiteness. Therefore, it is necessary to perform an explicit (two-loop) computation in a specific model. I consider quantum gravity coupled with Chern–Simons U(1) gauge theory and massless fermions and show that renormalization originates four-fermion divergent vertices at the second loop order. I conclude that quantum gravity coupled with matter, as it stands, is not finite in three spacetime dimensions.

Two-loop critical mass for Wilson fermions

We have redone a recent two-loop computation of the critical mass for Wilson fermions in lattice QCD by evaluating Feynman integrals with the coordinate-space method. We present the results for different types of infrared regularization. We confirm both the previous numerical estimates and the power of the coordinate-space method whenever high accuracy is needed.

Two-loop Higgs mass in supersymmetric Kaluza–Klein theories

We perform an explicit two-loop computation of the Higgs mass in a five-dimensional supersymmetric theory compactified on the orbifold S 1/ Z 2 , with superpotential localized on a fixed-point brane and supersymmetry broken in the bulk of the extra dimension by Scherk–Schwarz boundary conditions. At one-loop, the Higgs mass is finite and proportional to 1/R 2 , R being the radius of the extra dimension. The two-loop corrections contain finite ( ∝1/R 2 ) and linearly divergent (∝ Λ/ R) terms, where Λ is the cutoff of the five-dimensional theory. The divergent terms are cancelled by diagrams containing one-loop wave-function renormalization counterterms, i.e., by linear threshold effects of Yukawa (and gauge) couplings, at the scale 1/ R. As a result there is no counterterm for the Higgs mass, which is finite and calculable to the considered order.

Noncommutative gauge theories and the cosmological constant

We discuss the issue of the cosmological constant in noncommutative nonsupersymmetric gauge theories. In particular, in orbifold field theories noncommutativity acts as a UV cutoff. We suggest that in these theories quantum corrections give rise to a vacuum energy \ensuremath{\rho} that is controlled by the noncommutativity parameter \ensuremath{\theta}, $\ensuremath{\rho}\ensuremath{\sim}1/{\ensuremath{\theta}}^{2}$ (only a soft logarithmic dependence on the Planck scale survives). We demonstrate our claim in a two-loop computation in field theory and by certain higher loop examples. Based on general expressions from string theory, we suggest that the vacuum energy is controlled by noncommutativity to all orders in perturbation theory.

Two-loop computation of the Schrödinger functional in lattice QCD

We compute the Schrödinger functional (SF) for the case of lattice QCD with Wilson fermions (with and without SW improvement) at two-loop order in lattice perturbation theory. This allows us to extract the three-loop β-function in the SF-scheme. These results are required to compute the running coupling, the Λ-parameter and quark masses by finite size techniques with negligible systematic errors. In addition our results enable the implementation of two-loop O( a) improvement in SF-simulations.

Remarks on two-loop free energy inN=4supersymmetric Yang-Mills theory at finite temperature

The strong coupling behavior of finite temperature free energy in N=4 supersymmetric SU(N) Yang-Mills theory has been recently discussed by Gubser, Klebanov and Tseytlin in the context of AdS-SYM correspondence. In this note, we focus on the weak coupling behavior. As a result of a two-loop computation we obtain, in the large N 't Hooft limit, $F(g^2N\to 0)\approx -\frac{\pi^2}{6}N^2V_3T^4(1-\frac{3}{2\pi^2}g^2N)$. Comparison with the strong coupling expansion provides further indication that free energy is a smooth monotonic function of the coupling constant.