LoopFest XXI

26 Jun 2023, 09:00 → 28 Jun 2023, 18:10 US/Pacific

Kavli Auditorium (SLAC National Accelerator Laboratory)

Bernhard Mistlberger (SLAC National Accelerator Laboratory (US)), Gherardo Vita (SLAC), Glenna Paige (Stanford University), LANCE Dixon

The SLAC Theory Group welcomes everyone to LoopFest XXI conference, which will be held on June 26-28 2023 at SLAC National Accelerator Laboratory.

 

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.

 

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: 

• A limited number of rooms is available at the SLAC guest house until April 25th - see Accommodation.
• To access SLAC you have to fill out an access form - see Registration.
• If you require an invitation letter to attend LoopFest, please send an e-mail to Bernhard Mistlberger. 
• Scam artists have targeted a participant of this conference by pretending to be in charge of their travel. Be aware when engaging in conversation with third parties and when in doubt verify their identity. Please feel free to contact the organizers.

 

Local organizers: Lance Dixon, Bernhard Mistlberger, Gherardo Vita

Advisory Committee: Radja Boughezal (ANL), Fernando Febres Cordero (FSU), Bernhard Mistlberger (SLAC) and Ciaran Williams (UB)

Monday 26 June

Session 1: Morning Session 1

Convener: LANCE Dixon

1 Welcome

Speaker: Bernhard Mistlberger (SLAC National Accelerator Laboratory (US))

RestaurantInfo.pdf

ThanksAndAnnouncements.pdf

Welcome.pdf

2 Di-Higgs Searches at the LHC

Speaker: Caterina Vernieri (SLAC National Accelerator Laboratory (US))

higgs-2023-loopfest.pdf

3 Uncertainties in SMEFT Predictions: The Role of Flavor

Speakers: Sally Dawson, Sally Dawson (BNL), Sally Dawson (BNL)

dawson_loop23.pdf

4 Exploring the impact of AFB on SMEFT fits with high invariant-mass Drell-Yan data

Speaker: Yingsheng Huang (Northwestern University & Argonne National Laboratory)

DY_AFB-LoopFest.pdf

5 Azimuthal asymmetries in Drell-Yan and semi-inclusive DIS beyond leading order

Since shortly after the discovery of partons, people have thought about probing their transverse momentum inside hadrons. For example, in 1978 it was shown to give rise to an azimuthal cos(phi) asymmetry of the outgoing hadrons in the process of semi-inclusive DIS (SIDIS), known as the Cahn effect. The cos(phi) distribution, as well as a number of other asymmetries in both SIDIS and Drell-Yan, are difficult to study since in QCD they first appear at subleading order in the power expansion of small transverse momentum. These observables have traditionally been studied at tree level using the parton model. In this talk I show that the use of effective field theory makes it possible to treat these observables systematically. Utilizing the soft-collinear effective theory formalism we completely determine the structure of contributions to all orders in perturbation theory. Interestingly, we find that dynamical soft gluon contributions remains simple at this power. We show that the only new ingredients are a set of the quark-gluon-quark (qgq) correlators, which come along with only one new Wilson coefficient. Perturbative matching calculations for the qgq correlators reveal novel additive rapidity divergences as well as endpoint divergences in the convolution of energy fractions, thus making the renormalization and factorization nontrivial and interesting. I discuss our solution to removing these divergences to define renormalized qgq correlators. Our results for the subleading power azimuthal asymmetries, establish them as useful observables within QCD, and enable higher precision predictions.

Speaker: Anjie Gao

Anjie_Loopfest.pdf

Coffee Break: Morning Coffee Break

Session 2: Morning Session 2

Convener: Radja Boughezal

6 Epsilon factorised differential equations beyond polylogs

I will introduce an algorithmic procedure to build epsilon-factorised bases of differential equations, when the corresponding master integrals cannot be expressed in terms of multiple polylogarithms.

Speaker: Lorenzo Tancredi (Technische Universitat Munchen (DE))

LoopFest_2023_Tancredi.pdf

7 p-adic reconstruction of rational functions in loop calculations

The calculation and manipulation of large multi-variable rational functions is a key bottleneck in multi-loop calculations. In this talk I will present work using p-adic numbers to reconstruct rational functions in a compact form. I will apply this to examples such as rational functions appearing in non-planar 2-loop 5-point amplitudes.

Speaker: Herschel Chawdhry (University of Oxford)

Chawdhry_Loopfest_2023_06_26_padic_reconstruction.pdf

8 Beauty and Charm Energy Correlators

Speaker: Bianka Meçaj (Yale University)

Loopfest23_Bianka Mecaj.pdf

Session 3: Afternoon Session 1

Convener: Alessandro Vicini (Università degli Studi e INFN Milano (IT))

9 Three-Loop Scattering Amplitudes in full color QCD: results in the IR and Regge limit.

I will describe the methods used for the first computation of three-loop QCD scattering amplitudes involving four external massless partons in full color complexity. These calculations require careful management of color structures and, more importantly, of the large numbers of Feynman integrals induced by the Feynman rules of the theory. The resulting amplitudes result allowed a direct confirmation of the quadrupole radiation contribution to the infrared structure of QCD as well as the first computation of the three-loop QCD gluon Regge trajectory.

Speaker: Giulio Gambuti

LoopFest.pdf

10 On the evaluation of three-loop non-planar Feynman diagrams for Higgs plus jet production

Speaker: Dr William J. Torres Bobadilla (Max-Planck-Institute for Physics)

Torres.pdf

11 Master Integrals for Electroweak corrections to gg → γγ

We present a calculation of the master integrals (MI's) required for the calculation of the Electroweak corrections to $gg\rightarrow \gamma\gamma$ production in which the process contains a light quark loop.
The integrals can be broken down into four categories based on the flow of the heavy vector bosons throughout the loop. Two of the families are planar, and two are non-planar. We determine a canonical basis for each family which allows an efficient solution of the resulting differential equations via iterated integrals. We compute the families in relevant physical kinematics and obtain an efficient numerical evaluation based on an implementation of Chen-iterated integrals.

Speaker: Gabriele Fiore (SUNY Buffalo)

Loopfest 2023_Final.pdf

Coffee Break: Afternoon Coffee Break

Session 4: Afternoon Session 2

Convener: Davide Napoletano (Universita & INFN, Milano-Bicocca (IT))

12 Alaric: A NLL accurate Parton Shower algorithm

Speaker: Florian Herren (Fermilab)

LoopFest_2023_Herren.pdf

13 A novel approach to QCD evolution in processes with massive partons

We present a simple model for massive parton evolution that replaces the explicit angular ordering of the coherent branching formalism with a differentially accurate simulation of soft-gluon radiation by means of a non-trivial dependence on azimuthal angles. In the new algorithm, initial and final state evolution are treated on the same footing. We provide an implementation for final-state evolution and present all formula required for fully differential matching at next-to-leading order in QCD.

Speaker: Benoit Assi (Fermilab)

LoopFest 2023.pdf

14 NNDL event shapes with the PanScales showers

Speaker: Dr Alexander Karlberg (CERN)

260623LOOPFEST.pdf

15 Current status of the GENEVA event generator and recent improvements

The GENEVA Monte Carlo event generator interfaces fully-differential fixed order calculations up to NNLO with high-order resummation in a 0-jet resolution variable and with parton showers, by combining them at the appropriate scales. The resulting simulated events have therefore NNLO accuracy in the fixed-order region and NNLL' accuracy in the resummation region. The parton shower matching is performed such that the nominal accuracy is not spoiled, and incorporates the hadronization and MPI effects.
In the last few years, many colour singlet production processes have been implemented in GENEVA at a theoretical accuracy of NNLO+NNLL' in the 0-jettiness or transverse momentum resolution variables. Some steps have been undertaken towards the implementation of processes with coloured final state. Parallel to these developments, a number of general technical improvements have been integrated into this framework. In this talk we introduce the GENEVA method, make an overview of the currently available processes, and demonstrate the main improvements that have been implemented so far.

Speaker: Mr Riccardo Nagar (University of Milan Bicocca)

Nagar_Loopfest.pdf

Tuesday 27 June

Session 1: Morning Session 3

Convener: George Sterman (Stony Brook University)

16 Machine Learning and the LHC

Speaker: Ben Nachman (Lawrence Berkeley National Lab. (US))

LoopFest2023.pdf

17 IR-safe flavoured jet algorithms for the precision era

Speaker: Fabrizio Arturo Caola (University of Oxford (GB))

LoopFest23_FlavourV2.pdf

18 MW determination at hadron colliders: QCD uncertainties

Speakers: Alessandro Vicini (University of Milano), Alessandro Vicini (Università degli Studi e INFN Milano (IT))

Vicini-MWdetermination-LoopFest2023.pdf

Coffee Break: Morning Coffee Break

Session 2: Morning Session 4

Convener: Lorenzo Tancredi (Technische Universitat Munchen (DE))

19 Feynman integrals from positivity constraints

We describe a new method for computing Feynman integrals based on solving inequality constraints. The starting point is the simple observation that a convergent Euclidean integral is non-negative if its integrand is non-negative. Combined with integration-by-parts reduction, this places powerful constraints on the values of master integrals, which can be solved efficiently using the numerical technique of semidefinite programming. We also find hidden consistency relations between terms at different orders in $\epsilon$ in dimensional regularization. We present examples with up to three loops.

Speaker: Mao Zeng (University of Edinburgh)

talkLoopFest_short.pdf

20 On phase-space integrals with Heaviside functions

Subtraction and slicing schemes allow for systematic isolation and cancellation of infrared singularities in fully-differential QCD calculations. They usually require integrals of approximations of matrix elements over the phase-space of unresolved real emission. In his talk, I will focus on soft contributions to the zero- and one-jettiness observable, which is defined through Heaviside functions. I will discuss an extended notion of reverse unitarity and integration-by-parts (IBP) relations for corresponding phase-space integrals, as well as their computation through multi-loop methods, such as IBP reduction to master integrals and the method of differential equations.

Speaker: Mr Maximilian Delto

loopfest2023_delto.pdf

21 Hyperfunction method for two loop calculations for PRadII experiment

In this talk we will discuss hyperfunctions associated with the two loop integrals relevant for the calculation of QED radiative corrections for the PRadII experiment. We will start with a description of the relevant cohomology groups and methods for their computation. We will discuss the vanishing cycles in PRadII kinematics and the corresponding Landau varieties. Then we will proceed with generalization of the method of master integrals for diagrams with cut lines ( on shell particles). We will describe the generalization of Griffiths' method to this case. We will discuss dimensional regularization of the relevant integrals and how IR poles are absorbed in Gamma factors. We will discuss the flat bundles associated with the double Bethe-Heitler process and ways to numerically compute the IR finite part of these functions.

Speaker: Stan Srednyak

stan7.pdf

Session 3: Afternoon Session 3

Convener: Fernando Febres Cordero

22 Interference studies in Higgs to gamma gamma

In this talk I will present results for the interference between higgs-mediated diphoton production and its continuum background at the LHC. We compute the total cross section at NNLO QCD in the soft-virtual approximation and in particular we focus on the diphoton invariant mass distribution. Following up on previous literature, we exploit the shift in the diphoton invariant mass induced by interference effects to provide updated bound on the Higgs width at the LHC.

Speaker: Federica Devoto

LoopFestXXI.pdf

23 Non-planar two-loop QCD corrections to $q\bar q \rightarrow \gamma\gamma\gamma$: finite remainders in the spinor-helicity formalism

I will present the computation of the NNLO double-virtual subleading-color corrections to the production of three photons at hadron colliders. I will briefly review the numerical generalized unitarity computation over finite fields performed with the C++ code Caravel, including updates that made this computation possible. I will then formulate the rational reconstruction problem for the finite remainders in spinor variables and obtain compact analytic expressions. Finally, I will show the effect of the subleading-color corrections on the hard functions.

Speaker: Dr Giuseppe DeLaurentis (Paul Scherrer Institut)

HTML Slides

LoopFestXXIGDL.pdf

24 Five-point QCD amplitudes in full colour

In this talk I will present recent developments on the calculation of five-point scattering amplitudes in massless QCD beyond the leading-colour approximation.
I will discuss the methodology that we pursued to compute these highly non-trivial amplitudes. In this respect, I will argue that it is possible to tackle and tame the seemingly intractable algebraic complexity at each step of the calculation.
I will then illustrate the salient features of the final results and discuss their relevance in view of current and future phenomenological studies at hadron colliders.

Speakers: Federico Buccioni, Federico Buccioni

buccioni_LFXXI.pdf

25 Towards NNLO QCD corrections for the production of a heavy-quark pair in association with a massive boson

In the past few years, remarkable progresses in multi-loop calculations have opened the doors to the computation of massless two-loop five-point scattering amplitudes, allowing to complete NNLO QCD predictions for complicated $2 \rightarrow 3$ processes like tri-photon, tri-jet and di-photon plus jet production. Very recently, also the two-loop five-point scattering amplitudes with one external massive leg have been made available in literature.
Due to these successful progresses in multi-loop computations, it is reasonable thinking that two-loop five-point amplitudes with more than one massive leg will also be accessible in the next future, allowing to complete NNLO QCD computations for higher-multiplicity processes with massive final-state particles.
In this talk we will present recent progresses in NNLO QCD computations for processes where a heavy-quark pair is produced in association with a massive boson.
We will discuss about $t \bar{t} H$ and $Wb\bar b$ production, mainly focusing on the techniques we adopted to circumvent the bottleneck of the missing two-loop amplitudes, namely a soft Higgs-boson approximation and the massification of massless $Wb\bar b$ amplitudes.

Speaker: Chiara Savoini

LoopFest_talk_Savoini.pdf

Coffee Break: Afternoon Coffee Break

Session 4: Afternoon Session 4

Convener: Alexander Penin

26 High-Energy Jets (HEJ) Resummation at the LHC

LHC data is forcing the theory community to push their calculations to an unprecedented level of precision in multiple phase-space regions. In the High-Energy region, the perturbative convergence of the hard scattering cross-sections can be impeded by large logarithms in $s/t$ which can be numerically significant at LHC energies. These logarithms are especially important in key regions of phase-space, e.g. in the VBF region in Higgs boson analyses.

The High-Energy Jets (HEJ) framework is a perturbative framework which resums these logarithms to Leading Logarithmic accuracy in $s/t$ for a large range of processes. In this talk, I will show how these logarithms appear, how they can be resummed in HEJ and present some of the studies which make their phenomenological relevance manifest, including recent new results for Higgs-plus-one-jet analyses (arXiv:2210.10671).

Speaker: Mr Jérémy Paltrinieri (University of Edinburgh)

HEJ_talk_LoopFest.pdf

27 3 loop anomalous dimension & the EFT for long-distance radiative corrections in beta decay

Superallowed ($0^+\rightarrow 0^+$) beta decays provide the most precise extraction of $|V_{ud}|$ and require radiative corrections at a precision of roughly 100 ppm. In regions where photon wavelengths are long compared to the radius of the nucleus these corrections are coherently enhanced by the nuclear charge, $Z\in [6,37]$. In this talk I will describe how effective field theory can be used to isolate coherently enhanced regions, and will discuss new results for the anomalous dimension that governs logarithmically enhanced contributions at high orders in perturbation theory.

Speaker: Ryan Plestid

LOOPFEST-XXI.pdf

28 Renormalization of abelian chiral Gauge Theories with non-anticommuting $\gamma_5$ at the Multi-Loop Level

The dimensional renormalization of chiral gauge theories such as the electroweak Standard Model inevitably leads to the problem of accommodating the manifestly 4-dimensional nature of $\gamma_{5}$ in $D$ dimensions. In order to avoid inconsistencies at the multi-loop level, $\gamma_{5}$ can be treated rigorously as a non-anticommuting object using the Breitenlohner-Maison/'t Hooft-Veltman (BMHV) scheme within dimensional regularization (DReg). Employing the BMHV scheme, however, violates gauge invariance, which subsequently needs to be restored using symmetry-restoring counterterms guaranteed to exist by the methods of algebraic renormalization. These counterterms may be calculated via special Feynman diagrams with an insertion of the $\widehat{\Delta}$-operator, which reflects the breaking of chiral gauge invariance, using the regularized quantum action principle of DReg. In the case of an abelian chiral gauge theory this is consistently done at the multi-loop level, showing that the counterterm structure in the BMHV scheme may be written in a very compact form, suitable for computer implementations. While results up to the 2-loop level have already been published, the renormalization procedure has now been performed at the 3-loop level. The UV-divergences are extracted utilizing an infrared rearrangement via the so called all massive tadpoles method, where all occurring Feynman diagrams are mapped to fully massive single-scale vacuum bubbles. Ultimately, this renormalization procedure will be needed for high-precision calculations of e.g. electroweak observables.

Speaker: Matthias Weißwange (TU Dresden)

LoopFest_Matthias_Weisswange.pdf

Conference Dinner: Bus to Conference Dinner at Building 53

Conference Dinner: Conference Dinner at Trellis

Wednesday 28 June

Session 1: Morning Session 5

Convener: Dr Alexander Karlberg (CERN)

29 Two excursions in four dimensions

Speaker: George Sterman (Stony Brook University)

sterman-loopfest-2023.pdf

30 A gauge choice for IR singularities

For constructing the subtraction terms for the calculation of infrared safe observables or for constructing the splitting functions for a parton shower beyond leading order, one needs to understand the infrared behavior of loop diagrams. For this purpose, the choice of gauge matters. I will describe the interpolating gauge of Doust (1987) and Baulieu and Zwanziger (1999). This gauge interpolates between Feynman gauge and Coulomb gauge. I will argue that it may be useful for algorithms for perturbative subtractions or parton showers because of the way that it organizes collinear singularities.

Speaker: Prof. Davison Soper (University of Oregon)

Soper-LoopFest2023.pdf

31 Numerical integration of multi-loop integrals in momentum space using threshold subtraction

I will present a technique that enables the direct numerical integration of finite multi-loop integrals in momentum space without the need for contour deformation.
After first analytically integrating out the energy degrees of freedom of the loop momenta (yielding the Loop-Tree Duality), I show how the threshold singularities can be locally removed from the integration domain by subtracting suitable counterterms. I will argue that the subtracted integral evaluates to the real part and that the integrated counterterms simplify to phase space integrals, which in virtue of the optical theorem sum up to the imaginary part of the loop integral.
The resulting representation features intricate local cancellations of remaining threshold singularities among the phase space integrals. These cancellations remain realised in the limit of soft and collinear momenta and thereby imply an alternative formulation of the Local Unitarity representation, where the infrared singularities of real emission and virtual loop contributions locally cancel at integrand level.
I will also show preliminary results for the application of the threshold subtraction method to the numerical evaluation of two-loop amplitudes.

Speaker: Dario Kermanschah

kermanschah_loopfest_2023

32 Higgs boson production at the next-to-next-to-leading power

Speaker: Alexander Penin

loopfest23.pdf

Coffee Break: Morning Coffee Break

Session 2: Morning Session 6

Convener: Fabrizio Arturo Caola (University of Oxford (GB))

33 Analytic properties of the S-matrix

Speakers: Hofie Hannesdottir, Hofie Hannesdottir

Hannesdottir_AnalyticSmatrix.pdf

34 Precise Predictions for Gravitational Wave Experiments

Speaker: MICHAEL RUF

Loopfest_Ruf.pdf

35 Full NNLO QCD corrections to diphoton production

We compute the NNLO massive corrections for diphoton production in quantum chromodynamics (QCD). We compute semi-analitically the master integrals via power series expansion, classifying Feynman diagrams in different topologies and finding the canonical basis for non elliptic integrals. We present a study of the maximal cut for the non-planar topology showing the elliptic curve defining the integral. We then present the matrix element computed for the first time, in terms of form factors. Finally, we study the impact of our novel massive corrections on the phenomenology of the process, for different observables.

Speaker: Federico Coro

Coro_Federico_LoopFest.pdf

Session 3: Afternoon Session 5

Convener: Sally Dawson (BNL)

36 Two-Loop Electroweak Corrections with Fermion Loops to e^+e^- to ZH

Motivated by the anticipated high precision for the measurement of the cross section of e^+e^- to ZH process, we present a complete calculation of the next-to-next-to-leading (NNLO) electroweak corrections involving closed fermion loops. This has been achieved by using a semi-numerical technique for the two-loop vertex and box diagrams, which is based on Feynman parameters and dispersion relations for one of the two sub-loops. UV divergences are treated with suitable subtraction terms. Virtual IR divergence is regulated by introducing a small fictitous photon mass. This calculation method could also be applied to NNLO EW corrections for other 2 to 2 scattering process.

Speaker: Qian Song

qian_song_loopfest23.pdf

37 Three-point energy correlator in N=4 and QCD

We present the analytic result of three-point energy correlator in N=4 SYM and QCD. This is the first analytic computation of trijet event shape observables, which provides precise data for both theoretical analysis and phenomenological studies in jet substructure. The beauty of energy correlator allows us to rationalize all square roots and finish the full four-particle phase space integral. Compared with the standard energy-energy correlation, three-point energy correlator depends on the three angles among the detectors, containing the nontrivial shape information about the scattering process. The full shape dependence enables us to explore the kinematic space from various kinematic limits and their overlaps. This talk is based on 2203.04349, 2208.01051 and ongoing work.

Speaker: Xiaoyuan Zhang (Harvard University)

EEEC_XYZ.pdf

38 Two-loop mixed QCD-electroweak amplitudes for Z+jet production

In this talk, I will present a calculation of the two-loop mixed QCD-electroweak amplitudes for Z+jet production in proton colliders. I will argue that employing a recently proposed projector method reduces the amount of independent Lorentz tensor structures. I will discuss the numerical evaluation of multi-scale Feynman integrals with the Auxiliary Mass Flow method. Finally, I will mention the Sudakov behaviour at large transverse momentum, as well as potential applications to Dark Matter searches at the LHC.

Speaker: Piotr Bargiela

slides.pdf

Coffee Break: Afternoon Coffee Break

Session 4: Afternoon Session 6

Convener: Davison Soper

39 RI-(S)MOM to $\overline{\mbox{MS}}$ conversion for $B_K$ at two-loop order

The Kaon bag parameter, $B_K$, is a key non-perturbative ingredient in the search for new physics through CP-violation. It parameterizes the QCD hadronic matrix element of the effective weak $\Delta S=2$ four-quark operator which can be computed non-perturbatively on the lattice. The perturbative matching of $B_K$ between the lattice renormalization schemes and $\overline{\mbox{MS}}$ scheme has been done before at one-loop order. In this talk, I will present the conversion factors for $B_K$ between the RI-(S)MOM schemes and the $\overline{\mbox{MS}}$ scheme computed at two-loop order in perturbation theory.

Speaker: Dr Sandra Kvedaraite

LoopFest talk.pdf

40 Conformal Window for Asymptotic Safety up to Four Loops

In this talk I present our high-precision estimate for the conformal window of the interacting UV fixed point in the Litim-Sannino model.
Four loop gauge, three loop Yukawa and three loop quartic beta functions are determined both by a direct loop calculation and by generalizing existing literature results.
The UV fixed point and corresponding critical exponents are determined at N$^3$LO in the conformal expansion. Several approaches to estimate the conformal window are applied, and the phenomena behind the disappearance of the fixed point are discussed.

Speaker: Tom Steudtner

presentation_loopfest.pdf

41 Outlook

Speaker: LANCE Dixon

DixonLoopFest23Outlook.pdf