The Pittsburgh Particle physics, Astrophysics & Cosmology Center (PITT PACC) welcomes everyone to LoopFest XX conference, which will be held on May 12-14 at the University of Pittsburgh.
LoopFest provides a forum for discussing the latest results in precision quantum field theory and their applications to understanding experimental data at current and future colliders.
The workshop will consist of three days of plenary talks.
Topics include:
- The potential of the LHC and future colliders for precision measurements
- Progress in multi-loop and multi-leg calculations
- Interfacing fixed-order higher-order calculations with multi-purpose event generators
- Application of effective field theory techniques to precision calculations
- Prospects for improving PDFs for precision measurements
NOTE: Proper vaccination for Covid-19 is required for all in-person participants. Participants are expected to wear face masks during the conference, except when delivering a presentation and for eating/drinking during breaks. |
Local organizers: Arnab Dasgupta, Ayres Freitas (chair), Joni George, Tao Han, Keping Xie
Advisory Committee: Radja Boughezal (ANL), Fernando Febres Cordero (FSU), Bernhard Mistlberger (SLAC), Doreen Wackeroth, and Ciaran Williams (UB)
Conference banquet: May 13 (Friday), 19:00-21:00
Sponsored by the Pittsburgh Particle physics, Astrophysics & Cosmology Center (PITT PACC)
PHENO 2022 will be also held at the University of Pittsburgh right before LoopFest 2022, during 9-11 May.
Drell-Yan process plays an important role in the phenomenology of
the LHC era. This process is used extensively in parton distribution
function extractions. Predictions with high theoretical accuracy and
good numerical stability are key for high precision PDF extractions.
This process is calculated in the literature at the NNLO QCD accuracy
by multiple groups. So far no thorough comparison was done between
these calculations. In my talk I would like to present the first such
comparison of available computer programs with an emphasis on the differences
found in numerical results with discussion on their possible sources.
In this talk, I will present the recent calculation of the NNLO mixed QCD-electroweak corrections to the neutral-current Drell-Yan production of a pair of massless leptons within the nested soft-collinear subtraction framework. Thus, our computation is fully differential with respect to the final state particles.
Interestingly, the mixed corrections corrections are larger than what one would expect based on the magnitude of the coupling constants, and they
can exceed the pure NNLO QCD contribution in a large portion of the phase space.
At relatively low values of the dilepton invariant mass, around 200 GeV, we find unexpectedly large mixed QCD-electroweak corrections at the level of -1%.
At higher invariant masses, in the TeV region, we observe that these corrections can be well approximated by the product of QCD and electroweak corrections.
The plan of the talk is to first cover some technical aspects of the calculation, and then devote a discussion to results for fiducial cross sections and a selection of kinematic distributions.
We compute the fragmentation functions for the production of a Higgs boson at order $\mathcal{O}(y_t^2 \alpha_s)$. Real and virtual corrections have been computed by using modern loop computation techniques. In particular, we combine the unitary cut method with the differential equation approach. We find the criteria to bring the master integrals in canonical form, so that their computation is analytically performed, order by order in the dimensional regulator $\epsilon$. Our results can be used to compute differential cross sections with arbitrary top-quark and Higgs-boson masses from massless calculations. They can also be used to resum logarithms of the form $\ln(p_T/m)$ at large transverse momentum $p_T$ to next-to-leading-logarithmic accuracy by solving the DGLAP equations.
Since the discovery of the Higgs boson at the Large Hadron Collider, many of its properties such as the mass and the couplings to Standard Model gauge bosons have been studied extensively and determined to a good level of accuracy. The direct measurement of the Higgs width $\Gamma_H$, however, remains elusive due to limited detector resolution. In the past it has been observed that $\Gamma_H$ can be constrained by studying the interference between the $gg \to H \to \gamma \gamma$ signal and the continuum $gg \to \gamma \gamma$ background. More specifically, one can extract information on $\Gamma_H$ by investigating the diphoton invariant mass distribution. So far, this study has been performed up to NLO QCD.
Very recently, three-loop amplitudes for the background process $gg \to \gamma\gamma$ have been calculated, thus making possible to extend this analysis up to NNLO QCD. In this talk I will present a first step towards this goal. More precisely, I will discuss an analysis of the diphoton invariant mass distribution at the LHC in an "improved" soft-virtual approximation up to NNLO QCD. I will consider the interplay of signal, background and interference and study the impact of QCD radiative corrections on $\Gamma_H$ determinations.
I present new results for fully differential next-to-next-to-leading-order QCD corrections to Higgs boson production in vector-boson fusion in the factorizing limit. In contrast to earlier computations of this process, decays of the Higgs boson at leading order are included.
We present a novel formalism to calculate beam and jet functions automatically at next-to-next-to-leading
order in perturbation theory. By employing suitable phase-space parameterisations in combination with
sector-decomposition steps and selector functions, we managed to factorise all divergences in the
phase-space integrations, and we implemented our framework in the publicly available code pySecDec.
Our approach covers a wide class of SCET-1 and SCET-2 observables, and we present results for
several event-shape observables for both quark and gluon jet functions, as well as for $p_T$-resummation,
jet vetoes and hadronic event shapes for quark beam functions.
I will discuss the application of Soft Collinear Effective Theory (SCET) to the extraction of the strong coupling constant from e+e- event shape distributions, where state-of-the-art results exhibit a few sigma discrepancy with respect to the PDG world average. After briefly introducing event shape distributions and the SCET resummation formalism we use to study them, I will then focus on the canonical 'Thrust' variable, and on the phenomenological treatment of non-perturbative effects stemming from the soft sector. In particular, I will show that equivalently well-defined schemes for combining perturbative resummed and fixed-order contributions together with non-perturbative effects (notably renormalon cancellations) can lead to significant shifts in the extracted values of the strong coupling, when studying two-parameter fits in the dijet region. I also hope to briefly discuss novel (non-)perturbative extraction opportunities using the 'Angularities' class of observables, which generalizes the Thrust variable.
The QCD energy-momentum tensor exhibits the well-known property of trace anomaly. The anomalous contribution can be distributed among the quark and gluon parts. Although the total energy-momentum tensor remains unrenormalized owing to the conservation of energy and momentum, the individual components do go through ultraviolet renormalization. We perform this renormalization at four-loop level. As a spin-off, the phenomenological consequences of our result concerning the anomaly induced mass structure of hadrons are discussed.
We present an implementation of the nested soft-collinear subtraction scheme for color singlet decay processes at NNLO. The scheme is of particular utility in the typically poorly convergent subtracted double-real emission corrections. We demonstrate the cancellation of soft and collinear singularities through scaling behaviour analysis. We also discuss possible avenues to parton shower matching and the general implementation into the MCFM Monte Carlo program.
I discuss the recent advances in the analytic computation of two-loop scattering amplitudes for five-particle processes with one external massive leg. The latter are crucial ingredients to obtain NNLO QCD predictions for many interesting LHC processes. I present a basis of transcendental functions which enables a fast and stable evaluation of all the required planar Feynman integrals, and a workflow based on finite field arithmetic which allows us to compute the amplitudes efficiently. Finally, I present analytic results for several amplitudes of this kind in the leading colour approximation.
Single top quark is mainly produced through the t-channel W boson exchange q + b -> q + t at LHC. This process probes Wtb vertex directly and can be used to measure the CKM matrix element Vtb or constrain the bottom quark PDF. The non-factorisable contributions are the last missing piece of the NNLO QCD correction. In this talk, I will first motivate the computation of these corrections, then I will discuss the calculation procedure and techniques we applied in this work. Finally I will present some results.
The new precision frontier laid out by future e+e- colliders requires higher-order electroweak and mixed EW-QCD radiative corrections of electroweak precision observables(EWPOs) defined at Z-resonance. It has also long been considered that a manifestly gauge invariant theoretical set-up of the scattering in the vicinity of the resonance is a must to incorporate higher-order corrections from any model. We thus introduce a new C++ library GRIFFIN (Gauge-invariant Resonance In Four-Fermion Interactions) out of such necessity. In this talk, we will show the framework set up, class structure, and sample results of this library in comparison with predecessors such as ZFITTER.
We present state-of-the-art SCETlib predictions for the $W$ and $Z/\gamma^\ast$ transverse-momentum ($q_T$) distributions at the LHC at complete three-loop order in resummed perturbation theory (N$^3$LL$'$) and matched to available fixed order. We pay particular attention to the estimation of theory uncertainties via profile scale variations in such a way that perturbative uncertainties due to PDF evolution, perturbative resummation uncertainties, and nonperturbative uncertainties for $q_T \to 0$ are cleanly disentangled, and compare our predictions to high-precision measurements by the ATLAS and CMS experiments. The speed and versatility of our resummed calculation also allow us to study the dependence on the strong coupling, the PDFs, and their parametric uncertainties at this order. We find intriguing early evidence that the normalized ATLAS and CMS $Z$ $q_T$ spectra may prefer a lower strong coupling than the PDG value.
Constantly increasing accuracy of the experimental data and high-order calculations
require rethinking the theoretical uncertainties due to the missing higher orders.
The traditionally used simple but ad hoc scale variation prescription has no
probabilistic interpretation. The Bayesian approach to theoretical uncertainties
introduced by Cacciari and Houdeau offers an alternative. I will discuss the pros
and cons of the Bayesian approach, present recent developments, and illustrate
them with some practical examples.
Up to now, NNLO QCD calculations of photon production cross sections applied an idealised photon isolation procedure, which differs from the isolation used in experiments. We present first numerical results for NNLO QCD predictions of isolated photon cross sections at the LHC with a realistic cone-based isolation. Photon fragmentation processes are included for the first time at NNLO, by extending the antenna subtraction method to handle infrared-singular parton-photon configurations while retaining the information on the photonic energy inside the collinear parton-photon cluster. We describe how these singularities are subtracted in antenna subtraction using new fragmentation antenna functions and outline their integration.
Recent developments have shown that the numerical solution of loop integrals
using generalized series expansions of associated differential equation (DE)
systems allows for the calculation of state-of-the-art multi-loop problems with
many scales. In principle the bottleneck of efficient numerical evaluations is
then reduced to the IBP reduction to master integrals. We present a library to
solve such DE systems in the canonical and non-canonical sectors with an
application to the massive two-loop integrals in H+jet production. Through its
implementation in the free open-source computer algebra system FriCAS, the
library is compiled to efficient machine code and can be interfaced to other
codes, as well as deployed in massively parallel cluster setups for precision
phenomenology.
The appearance of large logarithmic corrections is a well-known phenomenon in the presence of widely separated mass scales. In this talk, we point out the existence of large Sudakov-like logarithmic contributions related to external-leg corrections of heavy scalar particles which cannot be resummed straightforwardly using renormalisation group equations. Based on a toy model, we discuss in detail how these corrections appear in theories containing at least one light and one heavy particle that couple to each other with a potentially large trilinear coupling. We show how the occurrence of the large logarithms is related to infrared singularities. In addition to a discussion at the one-loop level, we also explicitly derive the two-loop corrections containing the large logarithms. We point out in this context the importance of choosing an on-shell-like renormalisation scheme. As exemplary applications, we present results for the two-loop external-leg corrections for the decay of a gluino into a scalar top quark and a top quark in the Minimal Supersymmetric extension of the Standard Model as well as for a heavy Higgs boson decay into two tau leptons in the singlet-extended Two-Higgs-Doublet Model.
We present our recent computation of classical gravitational potentials for spinning black holes.
The work is based on the modern scattering amplitude approach.
A wide range of techniques ranging from numerical unitarity, loop integration,Integration-by-parts,
expansion-by-regions and effective field theories are used in order to extract the classical information
from quantum scattering amplitudes.
In this talk, I discuss the calculation of all helicity amplitudes for four-parton scattering in three-loop massless QCD. Our results allow us, for the first time, to verify completely the structure of quadrupole IR divergences at this perturbative order in QCD. From the high-energy limit of the amplitudes, we have extracted the three-loop gluon Regge trajectory in full QCD. Our findings provide a highly non-trivial test of the universality of high-energy factorization in QCD.
In this talk, I will present a calculation of the three-loop massless QCD helicity amplitudes for diphoton production in gluon fusion, based on arXiv:2111.13595. I will argue that employing a recently proposed projector method reduces the amount of independent Lorentz tensor structures. I will also discuss the derivation of a complete set of Master Integrals in all the relevant physical regions. It is based on regularity properties of the underlying Feynman integrals, which were observed before. Finally, I will mention potential applications to LHC phenomenology.
I will present our calculation of massive quark form factors at three loops in QCD.
After reducing the Feynman integrals in the amplitudes to master integrals, these were computed by solving differential equations. By constructing expansions around regular as well as singular points and numerical matching, we obtain sufficient precision over the whole kinematic range.
In this talk I discuss peculiarities that arise in the computation of real-emission contributions to observables that contain Heaviside functions. Specifically I will discuss the calculation of the zero-jettiness soft function in SCET at next-to-next-to-next-to-leading order in perturbative QCD. The Heaviside functions prevent a direct use of multi-loop methods based on reverse unitarity. I will present a way to bypass this problem and illustrate key aspects of the calculation. Finally I will present some results for various non-trivial contributions to the zero-jettiness soft function.
I present new analytic results for Feynman integrals that contribute to mixed QCD-EW corrections to partial decay width for 𝐻→𝑍𝑍*. These corrections include the massive top-quark contributions. The analytic computation of these integrals is challenging due to the presence of many massive scales. These two-loop integrals are solved using the method of differential equations by bringing them to a canonical form, keeping full dependence on masses of the internal propagators.
I explain the construction of a dlog-form for the differential equation, which is obtained despite the presence of (non-) rationalizable square roots.
It is well-known that perturbative expansions of QFT observable suffer from infrared (IR) divergences both in the phase-space of real-emission contributions and in the loop amplitudes of virtual contributions.
Traditionally, the two are handled separately through a combination of local subtraction counterterm and dimensional regulation.
Local Unitarity is an alternative formulation using the Loop-Tree Duality (LTD) theorem and where the Kinoshita–Lee–Nauenberg (KLN) cancellation pattern is leveraged to achieve a direct cancellation of real-emission and loop IR divergences, independently for each forward scattering graph.
Together with an automated local renormalization procedure based on the R-operation, the resulting expression is locally finite and thus amenable to numerical integration at arbitrary perturbative orders and for processes with final-state singularities only.
I will showcase our first computations within this new paradigm, for physical cross-sections up to N3LO.
In this talk, we will first present the renormalization of twist-two operators in covariant gauge. And then we apply it to the computation of splitting functions in three-loop order.
Numerical tools, such as OpenLoops, compute NLO scattering amplitudes in a fully automated way. In order to meet the precision requirements of the LHC era and future experiments, however, NNLO calculations are crucial, and their automation in a similar tool highly desirable.
In the OpenLoops framework, D-dimensional two-loop amplitudes are decomposed into loop momentum tensor integrals and corresponding tensor coefficients constructed in 4 dimensions, as well as rational terms.
In this talk we present a new and fully general algorithm for the construction of two-loop tensor coefficients, which exploits the factorization of Feynman diagrams into universal building blocks derived from the Feynman Rules of the model at hand.
This algorithm has been implemented in a fully automated way for two-loop QED and QCD corrections to the Standard Model. We will discuss the general structure of this algorithm and its implementation in OpenLoops, and present detailed studies on its numerical stability and efficiency.