All of our previous Zoom seminars

19 April 2024

via Zoom

Speaker: Maxim Nefedov, IJCLab Orsay

Title: Heavy quarkonium production at high energy via matching of Collinear and High-Energy factorisations

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22 March, 14:00

via Zoom

Speaker: Yizhuang Liu, Jagiellonian University, Krakow

Title: Renormalon for threshold-like factorization of space-like parton correlators

Abstract: In this talk , I present our recent work on renormalon and power correction for threshold-like factorizations of space-like parton distributions (such as pPDF&&qPDF ). I show that the common Sudakov hard kernel at LP in this case has a linear IR renormalon and show how it cancels with UV renormalons for NLP operators. I will also discuss some other features of threshold expansion in this case.

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06 October 2023

via Zoom

Speaker: Long Chen, Shandong University in Jinan, China

Title: A unique γ5 prescription for the Standard Model in Dimensional Regularization

Abstract: Calculating correctly the high-order perturbative corrections to quantities involving an axial-current operator in dimensional regularization (DR) is non-trivial due to the well-known γ5 issue in D dimensions. In this talk we will review a few practical mainstream prescriptions that have been successfully applied in many high-order perturbative calculations, and discuss their advantages and disadvantages both from the technical side and the conceptual side. In the end, we propose a unique prescription for treating γ5 in the Standard Model in DR.

19 May 2023

via Zoom

Speaker: Qin-Tao Song

Title: Kinematical higher-twist corrections in γ* γ → M1 M2 and γ* → M1 M2 γ

Abstract: We carry out the calculation of kinematical higher-twist corrections to the cross sections ofγ* γ → M1 M2 and γ* → M1 M2 γ up to twist 4, where Mi is a scalar or pseudoscalar neutral meson. The three independent helicity amplitudes are presented in terms of the twist-2 generalized distribution amplitudes (GDAs), which are important non-perturbative quantities for understanding the 3D structure of hadrons. Since those processes can be measured in e+e- collisions, we also perform the numerical estimate of the kinematical corrections for γ* γ → M1 M2 and γ* → M1 M2 γ by using the kinematics of Belle and BESIII, respectively. We adopt the π π GDA extracted from Belle measurements and the asymptotic π π GDA to study the size of the kinematical corrections in the case of pion meson pair, and a model η η GDA is used to see the impact of target mass corrections O(m2/s) for γ* γ → η η and γ* → η η γ. Our results show that the kinematical higher-twist corrections account for ~ 20% of the cross sections on the average, and it is necessary to include them if one tries to extract GDAs from experimental measurements precisely.

17 March 2023

via Zoom

Speaker: Felix Hekhorn

Title: The new theory prediction pipeline and towards N3LO PDF

Abstract: In this talk I review the recent progress on providing theory predictions inside a PDF fit and show the progress towards N3LO PDF determination. In the first part of the talk, I introduce the new framework in NNPDF for providing theory predictions, dubbed pineline. I list and comment on all the necessary ingredients and their programs developed as Open Source codes. In the second part of the talk I report on the progress of the extension of PDF fits to next-to-next-to-next-to-leading order (N3LO) using the new theory pipeline.

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08 Dezember 2022

via Zoom

Speaker: Dr. Lisa K. Horvath

Title: "Gender Biases in der Wissenschaft - Überblick und Gegenmaßnahmen"

english: "Gender Biases in science - overview and countermeasures "

Abstract: Der Vortrag präsentiert verschiedene Biases in der Wissenschaft und zeigt Bias-Management-Strategien auf, mit denen ihnen entgegengewirkt werden kann. Dr. L. Horvath zählt zu den renommiertesten Beraterinnen im Bereich Universitäten und Organisationen. Es ergeht herzliche Einladung an alle Mitglieder der Universität seitens der Koordinationsstelle Chancengleichheit.

The Talk presents different biases in science and shows bias management strategies, with which they can be antagonized. Dr. L. Horvath is one of the most renommated consultans regarding universities and oranizations. All members of the University (Regensburg) are warmly invited by the department for equal opportunity.

25 November 2022

Meeting ID: 949 1231 4457

Speaker: Florian Fabry, U. Hamburg

Title: NLO DGLAP splitting kernels for color non-singlet DPDs

Abstract: The two partons inside a double parton distributions (DPDs) are correlated in many of their degrees of freedom. In color space, the DPD can be projected onto color singlet and color non-singlet components, where the latter describe color correlations. Color singlet DGLAP evolution of collinear DPDs (DPDFs) uses the exact same splitting kernels as the ones for ordinary parton distribution functions (PDFs) known from single parton scattering. For color non-singlet representations, however, all splitting graphs contain additional projectors on the respective representation. For this reason, their form was unknown beyond LO until now. In this talk, I will present for the first time the full splitting kernels for unpolarized, longitudinal and quark transversity DPDFs at NLO. They were obtained using two different and independent methods. The first one is based on graph-by-graph results for PDF splitting kernels, the second one utilizes the matching formula for transverse momentum dependent matrix elements. As a by-product, the NLO anomalous dimension for rapidity evolution of DPDFs in the gluon decuplet and 27-multiplet was also obtained for the first time.

21 Oktober 2022

Meeting ID: 949 1231 4457

Speaker: Sergey Kulagin, INR RAS, Moscow

Title: What can we say on the modification of the bound nucleon on the parton level from a global QCD analysis?

Abstract: We briefly review available experimental observations on nuclear effects in deep-inelastic scattering (DIS). We then briefly discuss a few basic mechanisms responsible for nuclear corrections and review progress in understanding the observed phenomenon focusing on the valence quark region. We report the results of our global QCD analysis which includes a "standard" set of high-energy data for the proton target (DIS, DY production of lepton pair as well as W+- / Z boson production) and also nuclear 2H, 3H, and 3He DIS data. In this analysis we treat the nuclear corrections in DIS in terms of a nuclear convolution approach with off-shell bound nucleons. The off-shell correction to the nucleon structure functions is responsible for the modification of the bound nucleons on the parton level. This correction is determined from a global QCD fit along with the parton distribution functions of the proton (PDFs). A study has been performed aiming to estimate the uncertainties arising from the model of higher-twist contributions to the structure functions, from the treatment of target mass corrections, and the use of various deuterium data sets. We compare our predictions for the ratio F2n/F2p and the d/u proton PDFs ratio with the results of other analyses as well as with the recent data from the MARATHON experiment.

1 July 2022

Meeting ID: 949 1231 4457

Speaker: Jakob Schoenleber, U. Regensburg

Title: Two-loop corrections to the flavor-singlet coefficient function in DVCS

Abstract: Motivated by the research program of the Electron Ion Collider, we have calculated the two-loop (NNLO) flavor-singlet (vector) coefficient function in deeply virtual compton scattering (DVCS). Apart from presenting the result and some technical aspects of the calculation, I will discuss numerical estimates of the NNLO corrections to the corresponding Compton form factors.

27 May 2022

Meeting ID: 949 1231 4457

Speaker: Astrid Hiller Blin, U. Regensburg

Title: Proton resonances in meson production

25 Mar 2022

Meeting ID: 949 1231 4457

Speaker: Ignacio Borsa, U. Buenos Aires

Title: Global analyses of fragmentation functions: from NLO to NNLO accuracy

21 Jan 2022

Speaker: Elke Aschenauer, BNL

Title: Update on the Electron Ion Collider project

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03 Dec 2021

Speaker: Gunnar Bali

Title: Synergies between pQCD and LQCD

Abstract: We give selected examples where lattice QCD simulations require dedicated perturbative QCD calculations. We also discuss situations in which experimental and lattice QCD input ideally should be analysed simultaneously, using pQCD methods. The very subjective choice of topics is meant to initiate discussion regarding future projects.

11 Nov 2021

Speaker: Muriel Aichberger

Title: Unconscious Bias


In the framework of the Sonderforschungsbereich (SFB) 1277 a training for our members was organized by our collegues from the condensed matter group. The aim was to inform about the issue of unconcous bias and raise awareness. The trainer Mr Muriel Aichberger enjoys an excellent reputation and has, in particular, experience with training of academic staff and faculty.

24 Sep 2021

Speaker: Pia Zurita

Title: Incorporating fragmentation functions in xFitter

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Abstract: Collinear PDFs are the best known partonic densities due to the amount of data available. Nevertheless, the separation of the light quark flavours in the sea sector remains challenging and different flavour sensitive observables are proposed to improve the situation. Among them, promising results have been found by the JAM collaboration, using semi-inclusive DIS. As SIDIS depends on the less constrained fragmentation functions (FFs), the use of a particular set would only create a bias into the PDF determination. Therefore, the proposal is to do a simultaneous fit of PDFs and FFs, including all relevant observables. One popular open source tool to fit PDFs is the xFitter package. In this talk I will show the changes incorporated into it to determine FFs using semi-inclusive annihilation and SIDIS, which can eventually be used to perform a simultaneous fit of initial and final state distributions. The modifications were tested by fitting SIDIS data off nuclei, is included in the EIC program.

16 July 2021

Speaker: Xiangdong Ji

Title: Scale symmetry breaking, quantum anomalous energy and proton mass decomposition

Abstract: Anomalous scale symmetry breaking effects on the proton mass in QCD arise from quantum fluctuations at ultraviolet scales. A novel contribution naturally arises as a part of the proton mass, which is called the quantum anomalous energy (QAE). I will explain the QAE origins in both lattice and dimensional regularizations and demonstrate its role as a scheme-and-scale independent component in the mass decomposition. I will further argue that QAE role in the proton mass resembles a dynamical Higgs mechanism, in which the anomalous scale symmetry breaking field generates mass scales through its vacuum condensate, as well as its static and dynamical responses to the valence quarks.

18 June 2021

Speaker: Peter Plossl

Title: Two-loop splitting in double parton distributions: the colour non-singlet case

Abstract: Double parton scattering (DPS) provides access to information about nucleon structure not accessible in single parton scattering: spatial, spin, and colour correlations between the partons inside the nucleon. At small distance between the two probed partons the double parton distributions (DPDs) containing this information can be calculated in perturbation theory. I will discuss this calculation at next-to-leading order for unpolarized DPDs with colour correlations, with a focus on the treatment of rapidity divergences. Identical results are obtained using different regulator schemes, the delta-regulator and Collins' regulator using space-like Wilson lines, providing a strong cross check. To our knowledge this is the first application of the Collins regulator in a two-loop calculation. Our results exhibit a varied pattern of enhancement at small x.

21 May 2021

Speaker: Melissa van Beekveld

Title: Next-to-leading power threshold corrections

Abstract: Cross-sections in perturbative QCD are plagued by large corrections from soft and collinear radiation. The most singular terms are known to be universal, which allows their resummation to all orders in the coupling, which cures the perturbative expansion of the cross-section. In this talk, I will give an introduction on how these large corrections arise, how resummation solves the issue, and whether we can expect the same universal structure for a class of next-to-singular contributions.

23 April 2021

Speaker: Daniel Boer

Title: Gluon TMD studies using heavy quark production processes

Abstract: Heavy quark production processes, for both open and bound heavy quark pair production, provide promising probes of transverse momentum dependent gluon distributions (gluon TMDs). In this talk the prospects for gluon TMD extractions using heavy quark production processes will be discussed, focussing on the distributions of unpolarized and linearly polarized gluons inside unpolarized protons, and on gluon distributions inside transversely polarized protons, such as the gluon Sivers TMD. Besides allowing extraction of gluon TMDs, quarkonium production TMD processes present opportunities to learn more about the quarkonium production process itself, by offering new probes of certain poorly known NRQCD long-distance matrix elements and their transverse momentum dependent generalization in the form of shape functions.

19 March 2021

Speaker: Giovanni Chirilli

Title: High-energy OPE for polarized DIS

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Abstract: High energy Operator Product Expansion is a formalism to study scattering amplitudes at high-energy (Regge limit) in perturbation theory. When it is applied to the product of two electromagnetic currents, we may write the unpolarized DIS amplitude as a convolution of coefficient functions and matrix elements of Wilson lines. The energy dependence of the cross-section is encoded in the rapidity evolution equation, the Balitsky-Kovchegov equation, of the Wilson-line operators. To study polarized scattering processes one has to include sub-eikonal corrections into the OPE formalism. I will discuss the OPE of two electromagnetic currents with sub-eikonal terms: I will present the resulting new impact factors; the matrix elements of new operators which are parametrized by new quark and gluon distributions; present new evolution equations.

19 February 2021

Speaker: Vladimir Braun

Title: Resummation of kinematic power corrections in DVCS: Operator Product Expansion.

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Abstract: Separation of longitudinal and transverse directions in scattering processes with nonvanishing momentum transfer $t$ is not uniquely defined. As a consequence, the leading twist appriximation is inherently ambiguous. This is formally a higher-twist effect but, in particular in DVCS, it can be 100% for the cross section in certain kinematic regions. In addition, electromagnetic Ward identities for the Compton amplitude in the leading twist approximation are not satisfied.
The problem (both problems) are due to the contributions of "invisible" operators in the OPE, the operators with zero matrix elements over free quarks (in leading order). I will explain how the contributions of such operators can be resummed to all orders in the power series in $\sqrt{-t}/Q$ using conformal symmetry.
The talk is based on 2011.04533

22 January 2021

Speaker: Kresimir Kumericki

Title: Separation of Quark Flavors using DVCS Data and other results.

Abstract: Using the available data on deeply virtual Compton scattering (DVCS) off protons and utilizing neural networks enhanced by the dispersion relation constraint, we determine six out of eight leading Compton form factors in the valence quark kinematic region. Furthermore, adding recent data on DVCS off neutrons, we separate contributions of up and down quarks to the dominant form factor.

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18 December 2020

Speaker: Sam Van Thurenhout

Title: Renormalization of quark operator matrix elements for deep inelastic scattering in the chiral limit.

Abstract: We calculate non-singlet quark operator matrix elements of deep inelastic scattering in the chiral limit including operators with total derivatives. This extends previous calculations with zero-momentum transfer through the operator vertex which provides the well-known anomalous dimensions for the evolution of parton distributions. Non-vanishing momentum-flow through the operator vertex leads to mixing with total derivative operators under renormalization. We determine the complete anomalous dimension matrix to second order in the MS-bar scheme by direct calculation of the relevant diagrams and by exploiting consistency relations for the anomalous dimension matrix which follows from the renormalization structure of the operators.

20 November 2020

Speaker: A.H. Ajjath

Title: Next to soft corrections to Inclusive cross sections at the colliders.

Abstract: We present a framework that resums the threshold enhanced large logarithms to all orders in perturbative theory for inclusive processes such as the production of a pair of leptons in Drell-Yan and of Higgs boson in gluon fusion as well as in bottom quark annihilation at the hadron colliders. These large logarithms are comprises of distributions log^i(1-z)/(1-z) resulting from soft plus virtual (SV) and logarithms of the kind log^i(1-z) from next to SV (NSV) contributions. We use collinear factorisation and renormalisation group invariance as the building blocks to achieve this. Using the integral representation of our all order z space result, we show how the NSV contributions of the form log^i(N)/N can be resummed in Mellin space.

23 October 2020

Speaker: Ignazio Scimeni

Title: Dijet and Heavy meson production in DIS: TMD factorization and challenges

Abstract: I will consider the factorization of dijet production in DIS measuring the measurement of the transverse momentum imbalance of the two hard probes in the Breit frame. These processes are interesting because of their sensitivity to gluon polarized and linearly polarized distributions but one has to acknowledge also the appearance of new elements like a new soft function whose properties will be discussed.

17 July 2020

Speaker: Alexander Manashov

Title: Two-loop coefficient function for DVCS

Abstract: Using the approach based on conformal symmetry we calculate the two-loop coefficient function for the vector flavor-nonsinglet contribution to deeply-virtual Compton scattering (DVCS). Analytic expression is derived for the coefficient function in momentum fraction space in the $\overline{\text{MS}}$ scheme. The corresponding (NNLO) correction to the Compton form factor H appears to be rather large: a factor two smaller than the NLO correction, or $\sim 10$\% of the tree level result in the bulk of the interesting kinematic range.

19 June 2020

Speaker: Alexey Vladimirov

Title: Self-contained definition of Collins-Soper kernel

Abstract: Rapidity anomalous dimension (RAD, or Collins-Soper kernel) defines the scaling properties of transverse momentum dependent distributions and can be extracted from the experimental data. I derive a self-contained non-perturbative definition that represents RAD without reference to a particular process. This definition makes possible exploration of the properties of RAD by theoretical methods on one side, and the properties of QCD vacuum with collider measurements on another side. To demonstrate these possibilities, I compute the power correction to RAD, it's large-b asymptotic and compare these estimations with recent phenomenological extractions.