Resummation, Evolution, Factorization 2021

15 Nov 2021, 08:00 → 19 Nov 2021, 18:30 Europe/Berlin

Ami Rostomyan (DESY), Armando Bermudez Martinez (CMS (CMS Fachgruppe QCD)), Hannes Jung (CMS (CMS Fachgruppe QCD)), Markus Diehl (T (Phenomenology)), Qun Wang (DESY), Sara Taheri Monfared (DESY)

REF 2021 is the 8th edition in the series of workshops on Resummation, Evolution, Factorization. The workshop focuses on transverse momentum dependent (TMD) parton densities and their connection with Monte Carlo event generators, as well as on the experimental measurements aimed at extracting information on TMD densities at the LHC, HERA and future colliders. The interplay between the necessary factorization theorems, resummation of large logarithms and the corresponding evolution equations are crucial towards higher precision calculations, necessary not only for the understanding of the data recorded by facilities like the LHC but especially for future experiments like the EIC. The workshop is sponsored by European Union's Horizon 2020 research and innovation programme under grant agreement No. 824093 (STRONG 2020).

The workshop takes place online only.

 

The abstract submission is closed now. If you have any questions, inquiries, please contact the organizing committee.

 

Local Organizing Committee:

Armando Bermúdez Martínez (DESY)

Sara Taheri Monfared (DESY)

Qun Wang (DESY)

Hannes Jung (DESY)

Markus Diehl (DESY)

Armine Rostomyan (DESY)

 

Advisory Board:

Elke Aschenauer, Daniel Boer, Igor Cherednikov, Markus Diehl, Miguel G. Echevarria, Laurent Favart, Francesco Hautmann, Hannes Jung, Krzysztof Kutak, Piet Mulders, Gunar Schnell, Ignazio Scimemi, Andrea Signori, Pierre Van Mechelen

 

Previous meetings:

 

7-11 December 2020, Online

25-29 November 2019, Pavia (Italy)

19-23 November 2018, Krakow (Poland)

13-16 November 2017, Madrid (Spain)

7-10 November 2016, Antwerp (Belgium)

2-5 November 2015, Hamburg (Germany)

8-11 December 2014, Antwerp (Belgium)

23-24 June 2014, Antwerp (Belgium)

 

REF2021_poster_.pdf

Monday, 15 November

1 Introduction

Online

Speaker: Hannes Jung (CMS (CMS Fachgruppe QCD))

Welcome.mp4

Welcome.pdf

MC generators: Parton showers and TMDs

Overview on Parton Branching, overview on traditional Parton shower, parton shower accuracy and consistency, TMDs from PSMC

Convener: Hannes Jung (CMS (CMS Fachgruppe QCD))

2 Parton showers and PDFs ⊕ N3LO preview

Speaker: Stefan Prestel (Lund University)

Stefan.mp4

talk20211115.pdf

3 QCD/QED interference in fixed color parton showers

We assess the effect of QCD/QED interference effects in fixed color parton showers with tree level matrix element corrections.

Speaker: Leif Gellersen (Lund UNiversity)

leif_gellersen.pdf

Leif.mp4

4 Determining Transverse Momentum Dependent Parton Distribution Functions with Monte Carlo generators

Transverse Momentum Dependent (TMD) parton distribution is a very powerful concept for the description of low and high transverse momentum effects in high energy collisions. The Parton Branching (PB) method provides TMD distributions that can be used in parton shower simulations, as already implemented in CASCADE Monte Carlo event generator.
We develop a method, PS2TMD, that allows determining effective TMDs from the standard Monte Carlo parton showers. This method is validated and implemented to successfully reconstruct the PB-TMDs with different configuration settings. We also discuss kinematic shifts in longitudinal momentum distributions from initial state showering and point out the sizable influence of different reconstruction definitions on both collinear and transverse momentum PDFs.

Speaker: Sara Taheri Monfared (DESY)

MC4TMD-sara-REF2021-15-11-2021.pdf

Sara.mp4

5 Intrinsic kt and DY production at different masses and energies

We present a study of intrinsic kt effects in DY production from very low DY masses up to the Z-boson mass, covering a large range of center-of-mass energies from \sqrt{s} = 40 GeV up to LHC energies.
The description of measurements from different experiments is studied with MCatNLO supplemented with Pythia8 as well as with Parton Branching TMDs.
An energy dependent intrinsic kt width is needed to described the measurements when Pythia8 is used, while a fixed intrinsic kt width works with PB TMDs.

Speaker: Mikel Mendizabal Morentin (DESY)

Mikel.mp4

REF2021.pdf

6 Rapidity correlations in multiperipheral models and high energy QCD

Particle-particle correlations, especially rapidity-rapidity correlations was already an important tool for hadron collider physics since the early 70s. Recently, we studied one-particle rapidity distributions and two-particle rapidity-rapidity correlations at hadron colliders revisiting one of the old models, the Chew-Pigniotti multiperipheral model and we were surprised to realize that the predictions were very much in line with the predictions one gets for the minijet radiation by using perturbative high energy QCD. Here, we report on further results we obtain from studying rapidity-rapidity correlations by using the Monte Carlo code BFKLex.

Speaker: Grigorios Chachamis (IFT UAM-CSIC)

Grigorios.mp4

ref2021.pdf

Break

Multiparton interactions: Multi-parton interactions

MPI and TMDs

Convener: Markus Diehl (T (Phenomenology))

7 A Monte-Carlo Simulation of Double Parton Scattering

Double parton scattering (DPS) is the process in which one has two separate hard scatterings in an individual proton-proton collision. In recent years much progress has been made towards the proper description of this phenomenon in QCD, including a consistent description of perturbative splittings that yield the parton pair in one or both protons, and a coherent framework for combining both double and single scattering without double counting. However, phenomenological predictions for DPS are often made using Monte Carlo codes or the "pocket formula", which are based on a simple picture of two uncorrelated scatters. I describe the development of a new Monte Carlo simulation for DPS, dShower, that is based on the modern theoretical picture. The structure of the DPS shower will be discussed, as well as the procedure to combine this shower with an SPS shower such that one can simulate both processes without double counting.

Speaker: Jonathan Gaunt (University of Manchester)

Jonathan.mp4

REF_2021_Gaunt.pdf

8 Sum rules for triple parton distribution functions

Recent measurements of the triple J/psi production performed by the CMS collaboration suggest non-negligible contribution from a triple parton scattering (TPS). The CMS results can be seen as a first experimental observation of a TPS process. Unfortunately, due to the absence of experimental fits, triple parton distribution functions (tPDFs), needed for the theoretical computation of TPS cross sections, remain unknown. Therefore, in phenomenological studies of the TPS phenomenon one still has to rely on certain model-dependent assumptions about the shape of tPDFs and their relation to standard single parton distribution functions.

In this talk I will discuss how one can extend the sum rules, previously proposed by Gaunt and Stirling for the double parton distribution functions, to the case of tPDFs and how one can use PYTHIA8 code to construct tPDFs which will approximately obey such sum rules. Additionally, I will demonstrate how one can proof the sum rules for ''bare'' tPDFs using the light-cone formalism.

Speaker: Oleh Fedkevych (University of Genoa and INFN - National Institute for Nuclear Physics (sezione di Genova))

Oleh.mp4

REF_2021_OFedkevych.pdf

9 DPDs in colour space: perturbative splitting and positivity

Double parton scattering (DPS) - the situation where two individual hard interactions happen during a single hadron-hadron collision - is sensitive to all sorts of correlations between two partons inside a hadron, e.g.~spin, colour, or spatial separation. Therefore DPS gives access to information about hadron structure not accessible in single parton scattering (SPS). This information is encoded in double parton distributions (DPDs) which are the DPS counterpart of PDFs in SPS.
At small inter-parton distances the leading contribution to DPDs is due to a perturbative splitting mechanism, which makes it possible to calculate DPDs in perturbation theory in this regime. We compute the unpolarized perturbative splitting DPDs at next-to-leading order (NLO) for all possible colour correlations and study their numerical importance compared to the leading order (LO) splitting DPDs.
With these NLO splitting DPDs we can show that positivity bounds for colour space DPDs can be violated. We furthermore find that even at LO positivity can be violated by evolution from lower to higher scales, in contrast to the situation for ordinary PDFs.

Speaker: Peter Josef Ploessl (T (Phenomenology))

2021_11_15_REF_Workshop_Ploessl.pdf

Peter.mp4

Break

pp processes

Convener: Qun Wang (DESY)

10 Measurements of the Z boson production at ATLAS

Precision measurements of the Drell-Yan process at LHC provide most precise tests of perturbative QCD and information about the parton distribution functions for quarks within the proton. We present the measurement of the transverse momentum and phistart_eta distributions of Drell-Yan lepton pairs at 13 TeV. The measurement is corrected for detector inefficiency and resolution and compared with state-of-the-art theoretical calculations. A summary of similar measurements at 7 and 8 TeV is also presented.

Speaker: Aleksei Ezhilov (ATLAS)

aezhilov_ref2021_talk_v2.pdf

Aleksei.mp4

11 Measurement of the Drell-Yan transverse momentum dependence over a wide mass range with CMS

The double differential cross sections of the Drell-Yan lepton pair
(electron or muon) production, as a function of its invariant mass m_ll,
transverse momentum pT, and $\phi*$ are measured. Drell-Yan masses between
50 GeV and 1 TeV are investigated. Additionally, a measurement is
performed requiring at least one jet in the final state. The collected
data correspond to an integrated luminosity of 36.3 fb-1 of proton-proton
collisions recorded with the CMS detector at the LHC at the center-of-mass
energy of 13 TeV in 2016. Measurements are compared to state-of-the-art
predictions based on perturbative quantum chromodynamics including soft
gluon resummation. Theses results correspond to the CMS preliminary
results SMP-20-003.

Speaker: Kyeongpil Lee (ULB)

20211115_REF2021_DYPt_KPLee_v2_1.pdf

Kyeongpil.mp4

12 Transverse momentum distributions in low-mass Drell-Yan lepton pair production at NNLO QCD

The production of lepton pairs at low invariant mass and finite transverse momentum resolves QCD dynamics at the boundary between the perturbative and non-perturbative domains. We investigate the impact of NNLO QCD corrections for the lepton pair transverse momentum distributions at energies corresponding to the BNL RHIC collider and to fixed-target experiments. Satisfactory perturbative convergence is observed in both cases. Only the collider data are found to be well-described by perturbative QCD.

Speaker: Adrian Rodriguez Garcia (ETH Zurich)

Adrian.mp4

DrellYan_talk_Adrian_REF.pdf

Tuesday, 16 November

TMDs

Convener: Sara Taheri Monfared (DESY)

13 Disentangling Long and Short Distances in Momentum-Space TMDs

The extraction of nonperturbative TMD physics is made challenging by prescriptions that shield the Landau pole, which entangle long- and short-distance contributions in momentum space. The use of different prescriptions then makes the comparison of fit results for underlying nonperturbative contributions meaningless on their own. We propose a model-independent method to restrict momentum-space observables to the perturbative domain. This method is based on a set of integral functionals that act linearly on terms in the conventional position-space operator product expansion (OPE). Artifacts from the truncation of the integral can be systematically pushed to higher powers in $\Lambda_{\rm QCD}/k_T$. We demonstrate that this method can be used to compute the cumulative integral of TMDPDFs over $k_T$ in terms of collinear PDFs, accounting for both radiative corrections and evolution effects. This gives a systematic way of correcting the naive picture where the TMDPDF integrates to a collinear PDF, and we find that for the unpolarized distribution the corrections are a percent-level effect. We also show that, when supplemented with experimental data and improved perturbative inputs, these functionals will enable model-independent limits to be put on the nonperturbative OPE contributions in the Collins-Soper kernel and intrinsic TMD distributions.

Speaker: Zhiquan Sun (Massachusetts Institute of Technology)

REFtalkslides_ZS.pdf

Zhiquan.mp4

14 Factorization for the Transverse Momentum Distributions in SIDIS at Subleading Power

Fully differential measurements of the Semi-Inclusive DIS process with polarized beams provide important information on the three-dimensional structure of hadrons. In particular, azimuthal asymmetries of the fragmenting hadron probe interesting new correlations between partons in the hadronic system through novel hadronic distribution functions. In the framework of factorization for transverse momentum distributions (TMDs) some key asymmetries start at subleading order in the power expansion, where the formalism needed to fully relate observables and distributions is not yet under control, being primarily at tree level in the parton model. In this talk I utilize the soft collinear effective theory formalism to answer a few open questions, including providing generalized definitions of the distribution functions that are valid beyond tree level, giving a proof of completeness of the basis of functions, and providing a description of soft dynamical effects. These results enable a QCD description of SIDIS azimuthal asymmetries beyond tree level, and hence provide a framework for systematically improvable relations to the underlying distributions.

Speaker: Anjie Gao (Massachusetts Institute of Technology)

Anjie.mp4

Anjie_SIDIS.pdf

15 The Collins-Soper kernel from lattice QCD via quasi TMDPDFs

I will describe recent lattice QCD determinations of the nonperturbative Collins-Soper kernel, which describes the rapidity evolution of quark transverse-momentum-dependent parton distribution functions. It is found that different approaches to extract the Collins-Soper kernel from the same underlying lattice QCD matrix elements yield significantly different results and uncertainty estimates, revealing that power corrections, such as those associated with higher-twist effects, and perturbative matching between quasi and light-cone beam functions, cannot be neglected. I will discuss the implications of these observations for future controlled calculations of this quantity, and what will be required to achieve them.

Speaker: Phiala Shanahan (MIT)

Phiala.mp4

Shanahan.pdf

16 Determination of the Collins-Soper Kernel from Lattice QCD via moments of quasi-TMDs

We present lattice results for the non-perturbative Collins-Soper (CS) kernel, which describes the energy-dependence of transverse momentum-dependent parton distributions (TMDs). The CS kernel is extracted from the ratios of first Mellin moments of quasi-TMDs evaluated at different nucleon momenta.
The analysis is done with dynamical $N_f=2+1$ clover fermions for the CLS ensemble H101 ($a=0.0854\,\mathrm{fm}$, $m_{\pi}=m_K=422\,\mathrm{MeV}$). The computed CS kernel is in good agreement with experimental extractions and previous lattice studies.

Speaker: Maximilian Schlemmer

Maximilian.mp4

talk.pdf

Break

Parallel session: Small x

Convener: Armando Bermudez Martinez (CMS (CMS Fachgruppe QCD))

17 Curing high-energy instability of the NLO heavy-quarkonium hadroproduction cross section with High-Energy Factorization

The energy dependence of the total hadroproduction cross section for pseudoscalar quarkonia is computed via matching of the NLO Collinear-Factorisation (CF) results with Leading-Logarithmic resummation of higher-order corrections $\propto\alpha_s^{n}\ln^{n-1}(1/z)$ to the CF hard-scattering coefficient, where $z=M^2/\hat{s}$ with $\hat{s}$ being partonic center-of-mass energy squared. The resummation is performed using High-Energy Factorization(HEF). The resummation cures the collinear over-subtraction problem of the Next-to-Leading Order(NLO) CF calculation, stabilizing the high-energy behaviour of the cross section. Predictions of the NNLO $\alpha_s^2\ln(1/z)$ term of the CF hard-scattering coefficient are made. The matching is performed directly in $z$-space for the first time using the Inverse-Error Weighting (InEW) matching.

Speaker: Maxim Nefedov (Samara State University)

MaximNefedov.mp4

Nefedov_REF-2021.pdf

18 Evidence for the maximally entangled low x proton in Deep Inelastic Scattering from H1 data

Motivated by the Kharzeev-Levin entropy formula which postulates a maximally entangled proton wave function in Deep Inelastic Scattering at low x, we investigate the proposed relation between parton number and final state hadron multiplicity. Contrary to the original formulation we propose to determine partonic entropy from the sum of gluon and quark distribution functions at low x, which we obtain from an unintegrated gluon distribution subject to next-to-leading Balitsky-Fadin-Kuraev-Lipatov evolution. We find that our framework provides a very good description of recently measured H1 data on the final state hadron multiplicity, which provides experimental evidence for the proposed entropy formula. We furthermore provide a comparison based on CTEQ next-to-leading order parton distribution functions, which slightly overshoots the data, but provide in general an acceptable description.

Speaker: Dr Krzysztof Kutak (IFJ PAN)

entanglement.pdf

Krzysztof.mp4

19 Isolated photon production in the Parton Reggeization Approach with real NLO corrections

We study isolated photon production in the Parton Reggeization Approach with NLO corrections originated from subprocesses of additional parton emissions. Two matching schemes are tested. The first one is based on Kt-ordering of emitted partons during Kt-depended evolution, the second one applies rapidity-ordering for factorization of real parton emissions. We compare our predictions for transverse momentum spectra of isolated photons measured at the LHC.

Speaker: Vladimir Saleev (Samara National Research University)

talk_ref_2021.pdf

Vladimir.mp4

20 First solutions of the JIMWLK equation with the kinematical constraint

In this talk, we describe the implementation of the kinematical constraint within the Langevin formulation of the JIMWLK equation following the proposal of Hatta and Iancu. We present stability and scaling tests and show first comparisons of the solutions without and with the improvement. We also comment on the rapidity evolution rate of the saturation radius.

Speaker: Piotr Korcyl (Jagiellonian University)

Piotr.mp4

REF2021_referat_JIMWLK.pdf

21 Finite-𝑁𝑐 corrections to Wilson line correlators

In high-energy heavy-ion collisions, a quark-gluon plasma is produced. One way to study this exotic type of matter is to analyze the effect it has on high-energy partons traversing it. This leads to a rich phenomenology that commonly is referred to as jet quenching. High-energy partons propagating in the hot and dense medium mainly experience transverse momentum broadening which leads to additional radiative energy loss. The theoretical description of the main mechanisms behind these phenomena has been well known for some time.
One effect the medium has on partons propagating through it, is that their color continuously rotates, an effect that is encapsulated in a Wilson line along their trajectory. When calculating observables, one typically has to deal with traces of two or more medium-averaged Wilson lines. These are usually dealt with in the literature by invoking the large-$𝑁_𝑐$ limit, but exact calculations have been lacking in many cases. In our work [1], we show how correlators of multiple Wilson lines appear, and develop a method to calculate them to all orders in $𝑁_𝑐$. Specifically, we will focus on the trace of four Wilson lines, which we develop a differential equation for. We will then generalize this calculation to a product of an arbitrary number of Wilson lines, and show how to do the exact calculation numerically, and even analytically in the large-$𝑁_𝑐$ limit.
Even though these results are derived in the context of high-$𝑝_𝑇$ jet processes, they are quite general and can be used in other situations where Wilson line correlators appear.

[1] J. H. Isaksen and K. Tywoniuk, “Wilson line correlators beyond the large-Nc,” 7 2021 https://arxiv.org/abs/2107.02542

Speaker: Johannes Hamre Isaksen (University of Bergen)

Johannes.mp4

ref.pdf

Parallel session: TMDs

Convener: Markus Diehl (T (Phenomenology))

22 Factorization for quark and gluon lattice TMDs

Recent years have seen prominent progress in the first-principles calculation of transverse-momentum-dependent parton distributions (TMDs) from the so-called lattice TMDs, which are equal time correlators that include quasi-TMDs and those from the Lorentz-invariant approach. In this talk, I will show that the lattice TMDs and TMDs are related by a factorization formula which can be rigorously derived in QCD. As a corollary, there is no mixing between quark and gluon sectors, or quark of different flavors, in the matching for lattice TMDs, which implies that their lattice calculations can be much simpler than the collinear parton distributions. We also verify these results at one-loop in perturbation theory.

Speaker: Yong Zhao (Argonne National Laboratory)

REF_2021_yzhao.pdf

Yong.mp4

23 Power corrections in TMD factorization

I present the recent studies of power corrections for the transverse momentum dependent (TMD) factorization. A particular emphasis is made on the novel method of TMD operator expansion that yields the TMD factorization theorem on the operator level. This method grants a systematic description of power corrections to TMD factorization, in particular, it naturally introduces the concept of TMD-twist and allows a certain separation of kinematic and genuine power corrections. Also, I present factorized expression at the next-to-leading power (NLP) (at NLO perturbative accuracy), and discuss the evolution properties for TMD distribution of higher TMD-twists.

Speaker: Alexey Vladimirov (Regensburg University)

Alexey.mp4

talk.pdf

24 T-odd transverse-momentum-dependent gluon distributions in a spectator model

We perform explorative analyses of the 3D gluon content of the proton via a study of polarized T-odd gluon TMDs at leading twist, calculated in a spectator model for the parent nucleon. Our approach encodes a flexible parametrization for the spectator-mass density, suited to describe both moderate and small-x effects. All these prospective developments are relevant in the investigation of the gluon dynamics inside nucleons and nuclei, which constitutes one of the major goals of new-generation colliding machines, as the Electron-Ion Collider, the High-Luminosity LHC, NICA-SPD, and the Forward Physics Facility.

Speaker: Francesco Giovanni Celiberto (ECT*/FBK Trento & INFN-TIFPA)

Francesco.mp4

talk_REF_2021_Celiberto_gluon_TMDs.pdf

25 Dynamical resolution scale fits for TMDs at Next-to-Leading-Order (NLO)

The parton-branching solutions of QCD evolution equations have been recently
studied. From this approach, a method to construct both collinear and transverse
momentum dependent (TMD) parton densities has been formulated. In this
formalism, soft-gluon color coherence effects are taken into account by
introducing the soft-gluon resolution scale. Both analytical studies and
numerical solutions of PB evolution equations in the presence of dynamical
resolution scales at Leading-Order (LO) in the strong coupling have been
presented. In this talk, we show preliminary results of fits to the high-precision
deep inelastic scattering (DIS) structure-function measurements regarding the
implications of dynamical resolution scales at Next-to-Leading-Order (NLO)
accuracy in the strong coupling. We also illustrate the difference between LO
and NLO distributions fits.

Speaker: Mrs Safura Sadeghi Barzani (University of Antwerp, Shahid Beheshti University)

REF-talk-2021-f.pdf

Safura.mp4

26 TMDPDFs and TMDFFs at N3LO for N3LL' Resummation

In this talk I discuss the calculation at N$^3$LO of the quark and gluon unpolarized $q_T$ Beam Functions and of the TMD Fragmentation Functions. I illustrate their role in the resummation of QCD observables at N3LL', such as $q_T$ distributions and Energy Energy Correlators. The matching kernels onto standard PDFs or FFs of these collinear gauge invariant objects can be obtained from the collinear expansion of differential partonic cross sections at the LHC, which we have calculate at N$^3$LO for both Drell-Yan and Higgs production in gluon fusion. I conclude by presenting the results for the N3LO EEC Jet Function and the resummation at N3LL' of the EEC in the back-to-back limit.

Speaker: Dr Gherardo Vita (SLAC)

Gherardo.mp4

REF2021_Vita.pdf

Break

pp processes

Convener: Qun Wang (DESY)

27 Azimuthal correlations in Z+jet events

We present a measurement of azimuthal correlations in Z+jet events measured by CMS at sqrt{s}=13 TeV. The measurements are compared to predictions at NLO using Pythia8 parton shower and hardonization as well as Parton Branching TMDs and TMD shower.
Predictions are given for the back-to-back configuration and a comparison of the prediction with measurements of multi-jets in the same kinematic range is shown.

Speaker: Heng Yang (CMS (CMS Fachgruppe QCD))

Azimuthal correlations in Z+jet events.pdf

Heng.mp4

28 Transverse-momentum resummation for boson plus jet production at hadron colliders

In this talk I discuss the associated production of a vector or Higgs boson with a jet in hadronic collisions. When the transverse momentum of the boson-jet system is much smaller than its invariant mass, the QCD perturbative expansion is affected by large logarithmic terms that must be resummed to all orders. I discuss the all-order resummation structure of the logarithmically enhanced contributions up to next-to-leading logarithmic accuracy. The presented resummation formalism is fully differential in the kinematic variables of the boson-jet system. Soft-parton radiation produces azimuthal correlations that are fully accounted for in this framework. I present explicit analytical results up to next-to-leading order and next-to-leading logarithmic accuracy, that include the exact dependence on the jet radius. I also elaborate on the presence of non-global logarithms for this observable as well as the absence of clustering logarithms when one uses anti-kt jet-clustering.

Speaker: Jürg Haag (UZH)

Juerg.mp4

qt_res_for_Bj.pdf

29 Investigation of the proton structure with associated Z + heavy jet production at the LHC

We consider the production of $Z$ bosons associated with a heavy (charm and beauty) jet at the LHC energies using two scenarios based on the transverse momentum dependent (TMD) parton densities in the proton. First of them employs the Catani-Ciafaloni-Fiorani-Marchesini gluon evolution and is implemented into the Monte-Carlo event generator PEGASUS. The second scheme is based on the Parton Branching approach implemented into the Monte-Carlo event generator CASCADE. We compare the predictions obtained within these two TMD-based approaches with each other, investigate their sensitivity to the TMD gluon densities in the proton and estimate the effects coming from parton showers, double parton scattering mechanism and possible intrinsic charm. Additionally, we compare our predictions with the results of traditional (collinear) pQCD calculations performed with the NLO accuracy. It is shown that the TMD-based results do not contradict LHC experimental data.

Speaker: Maxim Malyshev (CMS (CMS Fachgruppe QCD))

Maksim.mp4

malyshev_Zcb1.pdf

Wednesday, 17 November

Small x

Convener: Hannes Jung (CMS (CMS Fachgruppe QCD))

30 Sudakov resummation from the BFKL evolution

Based on: Phys. Rev. D 104, 054039 (2021) [https://arxiv.org/abs/2105.13915]

The Leding-Logarithmic (LL) gluon Sudakov formfactor is derived from rapidity-ordered BFKL evolution with longitudinal-momentum conservation. This derivation further clarifies the relation between High-Energy and TMD-factorizations and can be extended beyond LL-approximation as well.

Speaker: Maxim Nefedov (Samara State University)

Maxim.mp4

Nefedov_BFKL-Sudakov_REF-21.pdf

31 Hunting stabilization effects of the high-energy resummation via heavy flavor production at the LHC

With the aim of targeting for clear signals of the stabilization of the high-energy resummation under higher-order corrections and energy-scale variation, we investigate the inclusive semi-hard production of two bottom-flavored hadrons, as well as a single bottom-flavored hadron accompanied by a light jet. Working within the hybrid high-energy and collinear factorization formalism, which encodes the BFKL resummation of energy logarithms up to NLL accuracy, we present predictions for cross sections differential in rapidity and transverse momentum, and for azimuthal-angle correlations. The fair stability the emergers from the analysis of our distributions motivates our interest toward future studies in wider kinematic ranges, as the ones reachable at new-generation colliding facilities (HL-LHC, the FPF, the EIC, and NICA). Here, our hybrid factorization could serve as an additional tool to improve the standard fixed-order description of heavy-flavored emissions and to possibly gauge the feasibility of precision studies of these channels in the high-energy regime.

Speaker: Mohammed Maher Abdelrahim Mohammed (Università della Calabria and INFN - Gruppo collegato di Cosenza)

Mohammed.mp4

REF_2021_Mohammed.pdf

32 Gauge-invariant TMD factorization for Drell-Yan hadronic tensor at small $x$

The Drell-Yan process is studied in the framework of TMD factorization in the Sudakov region $s\gg Q^2\gg q_\perp^2$ corresponding to recent LHC experiments with $Q^2$ of order of mass of Z-boson and transverse momentum of DY pair $\sim$ few tens GeV. The Drell-Yan hadronic tensors are calculated with ${1\over Q^2}$ accuracy, first at the tree level and then with the double-log accuracy. It is demonstrated that in the leading order in $N_c$ the higher-twist quark-quark-gluon TMDs reduce to leading-twist TMDs due to QCD equation of motion. The resulting tensor for unpolarized hadrons is EM gauge-invariant and depends on two leading-twist TMDs: $f_1$ responsible for total DY cross section, and Boer-Mulders function $h_1^\perp$. The corresponding qualitative and semi-quantitative predictions for angular coefficients of Z-boson production seem to agree with LHC data at ​corresponding kinematics.

Speaker: Ian Balitsky (JLab/ODU)

Ian.mp4

33 Helicity Evolution at Small x: the Single-Logarithmic Contribution

We calculate single-logarithmic corrections to the small-$x$ flavor-singlet helicity evolution equations derived previously at the double-logarithmic approximation. The new single-logarithmic part of the evolution kernel sums up powers of $\alpha_s\ln(1/x)$, which are an important correction to the dominant powers of $\alpha_s\ln^2(1/x)$ summed up previously by the double-logarithmic kernel at small values of Bjorken $x$ and with $\alpha_s$ the strong coupling constant. The single-logarithmic terms arise separately from either the longitudinal or transverse momentum integrals. Consequently, the evolution equations we derive not only include the small-$x$ evolution kernel, but they also coincide in assumptions with the leading-order polarized DGLAP splitting functions. We further enhance the equations by calculating the running coupling corrections to the kernel.

Speaker: Yossathorn Tawabutr (The Ohio State University)

Yossathorn.mp4

34 Transverse Momentum Dependent Gluon Distribution within High Energy Factorization at Next-to-Leading Order

We discuss Transverse Momentum Dependent (TMD) gluon distributions within high energy factorization at next-to-leading order in the strong coupling within the framework of Lipatov's high energy effective action. Unlike previous results obtained within high energy factorization, our framework allows to recover the complete anomalous dimension associated with Collins-Soper-Sterman (CSS) evolution of the TMD distribution, including also single-logarithmic terms in the CSS evolution. Another interesting feature concerns the relation between unpolarized and linearly polarized gluon TMD in the BFKL limit. While our calculation confirms that both distributions agree in the BFKL limit at leading order, we find that they differ, once next-to-leading order corrections are being included. We furthermore establish a link between the QCD operator definition of the TMD gluon distribution and a previously derived off-shell TMD gluon-to-gluon splitting function, which is within the present framework obtained as the real 1-loop correction.

Speaker: Martin Hentschinski (Universidad de las Americas Puebla)

Martin.mp4

REF21onlineDESY.pdf

Break

ep processes

Convener: Armando Bermudez Martinez (CMS (CMS Fachgruppe QCD))

35 HERAPDF2.0Jets and prospects for parton-branching PDFs with HERA jets data.

HERA inclusive and jet data has been used in collinear HERAPDF2.0Jets fits at NLO and NNLO. The addition of the jet data allows a simultaneous fit of parton distribution functions of the proton and determination of the strong coupling alpha_s(M2_Z). In the parton branching (PB) approach, Collinear and TMD parton densities have been determined by fits to inclusive deep inelastic scattering HERA data. This method allows one to simultaneously take into account soft-gluon emission and the transverse momentum recoils in the parton branchings along the QCD cascade. The latter leads to a natural determination of the TMD PDFs in a proton. A crucial development will be to include data from other measurements in a wider kinematic range in order to constrain the TMD PDFs and gain sensitivity to intrinsic transverse momentum contributions. We present very preliminary results for PB TMD fits using the same HERA inclusive and jet data at NLO. The results are compared to the HERAPDF2.0Jets predictions and a prospect of including LHC data sets is discussed.

Speaker: Katarzyna Wichmann (DESY)

Katarzyna.mp4

kwichmann-ref21.pdf

36 Transverse momentum dependent distributions in dijet and heavy hadron pair production at EIC

We discuss the measurement of gluon transverse momentum distribution (TMD) in dijet and heavy hadron pair (HHP) production in semi-inclusive deep inelastic scattering. The factorization of these processes in impact parameter space shows the appearance of a specific new soft factor matrix element on top of angular a complex valued anomalous dimensions. We show in detail how these features can be treated consistently and we discuss a scale prescription for the evolution kernel of the dijet soft function. As a result, we obtain phenomenological predictions for unpolarized and angular modulated cross sections for the electron-ion collider (EIC) using current available information on unpolarized TMD.

Speaker: Rafael F. del Castillo (Universidad Complutense de Madrid)

Rafael.mp4

REF2021.pdf

37 Probing gluon number density with electron-dijet correlations at EIC

We propose a novel way of studying the gluon number density (the so-called Weizsäcker-Williams gluon distribution) using the planned Electron Ion Collider. Namely, with the help of the azimuthal correlations between the total transverse momentum of the dijet system and the scattered electron, we examine an interplay between the effect of the soft gluon emissions (the Sudakov form factor) and the gluon saturation effects. The kinematic cuts are chosen such that the dijet system is produced in the forward direction in the laboratory frame, which provides an upper bound on the probed longitudinal fractions of the hadron momentum carried by scattered gluons. Further cuts enable us to use the factorization formalism that directly involves the unpolarized Weizsäcker-Williams gluon distribution. We find this observable to be very sensitive to the soft gluon emission and moderately sensitive to the gluon saturation.

Speaker: Krzysztof Kutak (Institute of Nuclear Physics Polish Academy of Sciences)

dijetsEIC.pdf

Krzysztof.mp4

38 Forward dijet production at the EIC: beyond the TMD factorization

Measurements of azimuthal correlations in the production of forward dijets in deep inelastic scattering provide the opportunity to probe the Weizsäcker-Williams gluon transverse momentum dependent (TMD) parton distribution, and the possibility for the discovery of gluon saturation at the future Electron-Ion Collider (EIC). While the TMD factorization is expected to hold near back-to-back production, there are important kinematic (perturbative power) and genuine saturation contributions that must be resummed for more controlled phenomenological predictions.
In this talk, I will compare the results of the TMD and the improved TMD factorization framework to those in the Color Glass Condensate (CGC), and report on the expected size of kinematic and genuine saturation corrections at different kinematics accessible at the EIC [1]. I will discuss recent progress towards the computation of dijet production at the next-to-leading order in the CGC [2], and the possibility of extending the TMD-CGC correspondence to higher orders in perturbation theory.

References:

[1] The importance of kinematic twists and genuine saturation effects in dijet production at the Electron-Ion Collider. JHEP 09 (2021) 178. [arXiv: 2106.11301 hep-ph]
R. Boussarie, F. Salazar, H. Mäntysaari, and B. Schenke.
[2] Dijet impact factor in DIS at next-to-leading order in the Color Glass Condensate. Submitted to JHEP. [arXiv: 2108.06347 hep-ph]
P. Caucal, F. Salazar, and R. Venugopalan.

Speaker: Farid Salazar (UCLA)

Farid.mp4

REF_dijets_CGC_Salazar.pdf

39 Jet-based TMD measurements with H1 data and machine-learning unfolding

Recently, jet measurements in deep-inelastic scattering (DIS) events close to Born kinematics have been proposed as a new probe to study transverse-momentum-dependent (TMD) PDFs, TMD fragmentation functions, and TMD evolution. In this talk, I will report measurements of lepton-jet momentum imbalance in high-Q2 DIS events collected with the H1 detector at HERA.  These data bridge DIS measurements from fixed target experiments and Drell-Yan measurements at colliders, thus providing a stringent test of TMD factorization, evolution and universality. This measurement also represents the first example of unfolding assisted with machine learning. These results serve as a pathfinder for the Electron-Ion Collider jet-based 3D imaging program, which I will describe briefly.

Speaker: Miguel Arratia (University of California, Riverside)

Miguel.mp4

Resumation2021(3).pdf

Break

Discussion session

Convener: Krzysztof Kutak (Institute of Nuclear Physics Polish Academy of Sciences)

Discussion.mp4

Intellectual Excursion

40 A Philosophical Perspective on Problems in Physics

The everyday practice of physicists is to a large extent determined by the scientific problems they are confronted with. The conceptual analysis of scientific problems and how they change, therefore, may allow for a fine-grained investigation of the development of a scientific discipline. In this talk I discuss what constitutes a scientific problem, what its elements are and how they change. I will illustrate the advantages of a more problem-focused approach in understanding the development of modern particle physics and provide a perspective that may shed some light in the assessment of open problems and whether they constitute “genuine” problems.

Speaker: Radin Dardashti (U. Wuppertal)

Radin.mp4

REF2021.pdf

Thursday, 18 November

TMDs

Convener: Sara Taheri Monfared (DESY)

41 Track functions and TMD physics

I will talk about track functions and TMD physics.

Speaker: Wouter Waalewijn (University of Amsterdam / Nikhef)

TMD_tracks.pdf

Wouter.mp4

42 The k_t- and (z,k_t)-factorizations and their applications to the e-p and p-p collisions

In the frameworks of $k_t$- and $(z, k_t)$-factorizations we discuss various TMDs (UPDFs) such as KMR, MRW, and PB which are based on the Sudakov form factor and the different angular ordering. The comparison is also made with the TMDlib and its possible extensions. The dependence of the above TMDs on input PDFs i.e. MSHT20, MMHT2014, CT14, etc are investigated. Finally, different processes such as e.g. $p-p$ Drell-Yan and $e-p$ inclusive jets productions in the above frameworks and KATIE parton-level event generator are discussed and the comparison is also made with the available experimental data.

Speaker: Mr Ramin Kord Valeshabadi (University of Tehran)

Ramin.mp4

ref2021.pdf

43 In-medium jet evolution via coherent medium induced radiation and scatterings

We propose a system of evolution equations that describe in-medium time-evolution of transverse-momentumdependent quark and gluon fragmentation functions.
Furthermore, we solve this system of equations using Monte Carlo methods. We use the obtained solutions to construct observables that allow us to see
different behaviour of quark and gluon initiated final-state radiation, i.e. the average transverse momentum <|k|> and energy contained
in a cone. In particular, the later allows us to conclude that in the gluon-initiated processes there is less energyin a cone, so that the quark jet
is more collimated.

Speaker: Martin Rohrmoser (Institute of Nuclear Physics, Polish Academy of Sciences)

Martin.mp4

ref2021_rohrmoser.pdf

Break

TMDs

Convener: Markus Diehl (T (Phenomenology))

44 The differential cross section of the single inclusive e+ e- annihilation to the hadrons via gamma-production, in the different k_t-factorization frameworks

For the first time, we study the differential cross section of the single inclusive ${e^ + }{e^ - }$ annihilation to the hadrons via $\gamma$-production, in the different ${k_{t}}$-factorization frameworks, using
the transverse momenta of the incoming partons, by apply the KMR and method to calculate the unintegrated parton fragmentation functions (UFFs) from the ordinary integrated one, i.e., the parton fragmentation functions (FFs). The LO sets of DSS library for the input FFs is used. The numerical results are compared with the experimental data in the different energies which are reported by the different collaborations, such as TASSO, AMY, MARK II, CELLO and Belle with the other QCD+fragmentation models. The behaviors of the normalized differential cross sections and the multiplicity versus the "transverse momentum" ($p_\bot$) are discussed. The final results demonstrate that the KMR and MRW UFFs give a good description of data and there is not much significant difference between the above three schemes. On the other hand, our results become closer to the data for the lower values of $p_\bot$ and the higher values of center of mass energies.

Speaker: Dr Roghieh Taghavi (University of Tehran)

2021 workshop (1).pptx

Roghieh.mp4

45 Heavy-flavor production and the kt-factorization approach in a variable-flavor-number scheme

We present our last work (2108.06419) on D and B-meson production in the kt-factorization approach. One of our goals is to emphasize the importance of using the same scheme for UPDFs and matrix elements. Our calculations, based on 2->2 processes computed in a general-mass variable-flavor-number scheme, are compared to ALICE and LHCb data. Finally, we discuss how the 2->1 process could be consistently included in our calculations and the expected consequences of this implementation.

Speaker: Benjamin Guiot (Universidad Tecnica Federico Santa Maria)

Benjamin.mp4

ref2021.pdf

46 TMD extraction and PDF bias problem

The extraction of TMD PDF includes PDF as input. In some recent extractions we have observed how the quality of the extraction depends on the selected PDF set. We show here how this problem, the PDF bias, is solved with a flavor dependent model for the non-perturbative TMD input.

Speaker: Ignazio Scimemi

Iganzio.mp4

Scimemi-REF2021.pdf

47 Recent results on extractions of quark TMDs

We review the recent progress on the extraction of TMD PDFs and TMD FFs from global data of Semi-Inclusive Deep-Inelastic Scattering, Drell-Yan and Z boson production. Particular attention will be devoted to the effects of the increase in theoretical accuracy.

Speaker: Alessandro Bacchetta (University of Pavia and INFN)

Alessandro.mp4

REF_Hamburg_2021.pdf

Break

pp processes

Convener: Hannes Jung (DESY)

48 Jet production and TMD evolution

We investigate the TMD evolution contribution to jet production. Calculations of jet production at the LHC using the TMD merging method are presented. The predictions show very good agreement with the data over a wide kinematic range. Lower matching cone radii are preferred by jet events than Z + jets events.

Speaker: Armando Bermudez Martinez (CMS (CMS Fachgruppe QCD))

Armando.mp4

REF2021_Armando.pdf

49 Azimuthal correlations in high pt dijet events with PB TMDs

We present a comparison of predictions obtained with PB TMDs (MG5-aMC+CA3(jj)NLO) with measurements in the back-to-back region.

The presentation is a result of the PB-TMD Monte Carlo school.

The speaker will be selected from the participants of the PB-TMD school.

Speaker: Qun Wang (DESY)

MCschool.pdf

Qun.mp4

50 Soft Gluon Resummation for Dijet Process

In this talk, we will present our results on the soft gluon resummation in the dijet production processes where the total transverse momentum of the dijet is much smaller than the jet momentum.

Speaker: Feng Yuan (Lawrence Berkeley National Laboratory)

Feng.mp4

REF21_Dijet.pdf

51 The role of pT resummation in fiducial cross sections at the LHC

I discuss the importance of pT resummation in fiducial cross sections at the LHC, using gluon-fusion Higgs production in the diphoton decay channel with fiducial cuts as an example. In particular, I discuss the treatment of fiducial power corrections that make the total cross section sensitive to small-pT resummation effects, and thereby enhance the N3LO correction. Integrating the resummed pT spectrum at N3LO+N3LL' accuracy thus allows us to predict for the first time the total fiducial cross section N3LO and improved by resummation.

Speaker: Markus Ebert (Max-Planck-Institut für Physik)

Ebert_REF2021.pdf

Markus.mp4

52 Diphoton production at small transverse momentum: resummation and challenges

At the LHC, diphoton production constitutes the main background of Higgs production in the diphoton decay and of many new physics models predicting large-mass diphoton resonances. Underestimated theoretical uncertainties and large perturbative corrections have been a long-standing issue over the years with various suggestions. Especially the transverse momentum spectrum has lead to various prescriptions. We present transverse momentum resummed diphoton predictions at the level of N$^3$LL$'$+NNLO which include the recently published 3-loop hard and beam functions. We discuss the residual uncertainties and challenges for the transverse momentum distribution and address the effect of hybrid-cone photon isolation.

Speaker: Tobias Neumann (Brookhaven National Laboratory)

Neumann_Tobias_REF2021.pdf

Tobias.mp4

Advisory Board: meeting

Ref2021Report.pdf

Friday, 19 November

pp processes

Convener: Ami Rostomyan (DESY)

53 Precision measurements using soft drop jet mass

Soft drop jet mass is an attractive candidate for precision measurements such as the top mass $m_t$ and the strong coupling constant $\alpha_s$, as it can be perturbatively calculable to high accuracy besides being more robust against nonperturbative and underlying event corrections. In this talk I will describe the prospects for the top mass and $\alpha_s$-determination at the LHC using soft drop jet mass by combining state-of-the-art resummed predictions with a field theoretic treatment of nonperturbative power corrections. I will show that the observable is promising for $m_t$ measurement in a definite top mass scheme with ${\cal O}(1)$ GeV uncertainty. On the other hand, the dominant uncertainty in the $\alpha_s$-determination is limited by nonperturbative corrections, and for measurements on $p_T \geq 1$ TeV, is saturated to about 2%.

Speaker: Aditya Pathak (University of Manchester)

Aditya.mp4

Pathak_REF_21.pdf

54 Role of parton fragmentation for associated J/psi production at high energies

We investigate the role of parton fragmentation processes for associated gauge boson (Z or W) and J/psi production at the LHC conditions. We focused on new partonic subprocesses, namely, the charmed or strange quark excitation followed by the charmed quark fragmentation c -> J/psi + c. Additionaly we take into account the effects of multiple quark and gluon radiation in the initial and final states. We find that these contributions are important and reduce the gap between the theoretical predictions and experimental results.

Speaker: Andrei Prokhorov (Lomonosov Moscow State University (RU), Joint Institute for Nuclear Research (RU))

Andrei.mp4

Prokhorov_REF2021.pdf

55 Zero-jettiness resummation for top-quark pair production at the LHC

The N-jettiness is a useful resolution variable to distinguish between events
with a different number of jets in the final state. It has
been successfully employed in slicing calculations for colour singlet processes at
NNLO and its resummation forms the basis for the Geneva approach to matching
NNLO calculations to parton showers. I will discuss the extension of the zero-jettiness
resummation for colour-singlet production to include the production of heavy
coloured particles at the LHC. Having derived a factorisation theorem for the observable
in this process, we have resummed large logarithms up to approximate NNLL' accuracy.
I will describe the resummation framework we have developed and outline
future applications to slicing calculations and NNLO+PS event generation.

Speaker: Matthew Lim (DESY)

MALim_REF2021.pdf

Matthew.mp4

56 Next-to SV resummed Drell-Yan cross section beyond leading-logarithm

We present the resummed predictions for inclusive cross section for Drell-Yan (DY) production up to next-to-next-to leading logarithmic ($\rm \overline{ NNLL}$) accuracy taking into account both soft virtual (SV) and next-to SV (NSV) threshold logarithms. We restrict ourselves to resummed contributions only from quark anti-quark ($q \bar q$) initiated channels. The resummation is performed in Mellin-$N$ space. We derive the $N$-dependent coefficients and the $N$-independent constants to desired accuracy for our study. The resummed results are matched through the minimal prescription procedure with the fixed order results. We find that the resummation, taking into account the NSV terms, appreciably increases the cross section while decreasing the sensitivity to renormalisation scale. We observe that, at 13 TeV LHC energies, the SV+NSV resummation at $\rm \overline{ NLL} (\rm \overline{NNLL})$ gives about 8\% (2\%) corrections respectively to the NLO (NNLO) results for the considered $Q$ range: 150-3500 GeV. In addition, the absence of quark gluon initiated contributions to NSV part in the resummed terms leaves large factorisation scale dependence indicating their importance at NSV level. We also study the numerical impact of $N$-independent constants and explore the ambiguity involved in exponentiating them.Finally we present our predictions for the neutral Drell-Yan process at various center of mass of energies.

Speaker: aparna sankar (IMSc, India)

Aparna.mp4

REF_Aparna.pdf

Photo session

Break

Parallel session: Small x

Convener: Ami Rostomyan (DESY)

57 Twist decomposition of the proton structure from BK equation

We perform a twist decomposition of the Balitsky-Kovchegov (BK) amplitude for the proton structure functions in the leading logarithmic limit. Assuming a single iteration of the triple pomeron vertex and using the Mellin space technique we discuss the BK corrections for the leading and subleading twist contributions.

Speaker: Mariusz Sadzikowski (Jagiellonian University)

lecture_REF_2021.pdf

Mariusz.mp4

58 Universality of quantum corrections to transverse momentum broadening

We show that the resummation of double logarithmic corrections to transverse momentum broadening of fast partons in large QCD media in the vicinity of the saturation boundary yields a universal distribution and exhibits anomalous scaling akin to super diffusive processes such as Lévy random walks. This asymptotic, all order result may have implications in the phenomenology of dijet production in pA and AA collisions and provides a new initial condition for small-x evolution that accounts for gluon fluctuations that live within the target.

Speaker: Yacine Mehtar-Tani (BNL)

REF-11:2021.pdf

Yacine.mp4

Parallel session: pp processes

Convener: Qun Wang (DESY)

59 Renormalization of the radiative jet function and its zero-bin subtraction

We show how to compute directly the renormalization/evolution of the
radiative jet function that appears in the factorization theorems for
$B\to \gamma\ell\nu$ and $H\to \gamma\gamma$ through a $b$-quark
loop. We point out that, in order to avoid double counting of soft
contributions, one should use in the factorization theorems a
subtracted radiative jet function, from which soft contributions
have been removed. The soft-contribution subtractions are zero-bin
subtractions in the terminology of soft-collinear effective theory. We
show that they can be factored from the radiative jet function and that
the resulting soft-subtraction function gives rise to a nonlocal
renormalization of the subtracted radiative jet function. This is a
novel instance in which zero-bin subtractions lead to a nonlocality in
the renormalization of a subtracted quantity that is not present in
the renormalization of the unsubtracted quantity. We demonstrate the
use of our formalism by computing the order-$\alpha_s$ evolution kernel
for the subtracted radiative jet function.

Speaker: June-Haak Ee (Fudan university)

EE-REF2021.pdf

June.mp4

60 T-odd effects in QCD and beyond

In this talk we will describe a parallel between phenomenological and symmetry-based descriptions of the process dependence of the Time-reversal-odd phenomena in QCD, such as the Sivers effect, with the goal of defining the essential elements that lead to such process dependence in QCD. With the necessary elements clarified across these two complementary descriptions, this will then serve as a starting point to explore possible generalizations in a different gauge theory.

Speaker: Dylan Manna (University of Michigan)

Dylan.mp4

T-odd effects in QCD and beyond REF 2021.pdf

Break

Closing talk

Convener: Pierre Van Mechelen (U Antwerp)

61 Closing talk

Speaker: Pierre Van Mechelen (U Antwerp)

Pierre.mp4

REF closing talk.pdf