# Forschungsgruppe FOR 2926/2

Next Generation Perturbative QCD for Hadron Structure:

Preparing for the Electron-Ion Collider

# Seminars

## 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

#### 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.

Download presentation (Powerpoint)

# ----- 2023 -----

#### 06 October 2023

**via Zoom**

**Speaker:** Long Chen, Shandong University in Jinan, China

**Title:** A unique γ_{5} 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
γ_{5} 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 γ_{5} in the
Standard Model in DR.

#### 19 May 2023

**via Zoom**

**Speaker:** Qin-Tao Song

**Title:** Kinematical higher-twist corrections in γ* γ → M_{1} M_{2} and
γ* → M_{1} M_{2} γ

**Abstract:** We carry out the calculation of kinematical higher-twist corrections to the
cross sections ofγ* γ → M_{1} M_{2} and γ* → M_{1} M_{2} γ
up to twist 4, where M_{i} 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 → M_{1} M_{2} and
γ* → M_{1} M_{2} 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(m^{2}/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.

# ----- 2022 -----

#### 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 zĂ¤hlt 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

# ----- 2021 -----

#### 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

**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

**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.

**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.

# ----- 2020 -----

#### 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.