Workshop on Resummation, Evolution, Factorization

7 Nov 2016, 13:00 → 10 Nov 2016, 15:50 Europe/Zurich

Klooster van de Grauwzusters (University of Antwerp)

REF 2016

Understanding high-energy hadronic processes within the TMD and Small-X_B frameworks: comparison and matching

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REF 2016 is the 3^rd workshop in the series of workshops on `Resummation, Evolution, Factorization.

Previous discussion meetings and workshops were
2-5 November 2015 DESY Hamburg (Germany)
1-3 June 2015 Amsterdam (The Netherlands)
8-11 December 2014 in Antwerp (Belgium)
23-24 June 2014 Antwerp (Belgium)

The principal goal of this series of Workshops is to organize a discussion between the experts from the communities specialising, on the one hand, in the investigation of the nucleon structure based on the Transverse-Momentum Dependent factorization framework (TMD) and, on the other hand, in various methods of dealing with the unintegrated Parton Distribution Functions (uPDF) in the small-X_B regime. Given that these two big communities attempt to address quite overlapping physical phenomena by means of the different languages, the strong demand arises for mutual understanding, promoting the possible development of joint research programs and unified treatments of some of the crucial problems in the theory of strong interaction. 

The key topics of this 3^rd REF Workshop will cover the most important theoretical and phenomenological issues related to the study of the hadronic processes at high energy in which the transverse degrees of freedom are relevant. 

The theoretical part includes the fundamental problems of factorization, operator definitions and gauge invariance of parton densities, their renormalization and evolution in different X_B regimes. 

The phenomenological part addresses specific high-energy processes available at the LHC, JLab, RHIC and EIC (Drell-Yan process, Higgs production etc.) within the frameworks of the TMDs and uPDFs, emphasizing on the profound analysis of the matches and mismatches of their predictions. 

 

Organizing committee:		Scientific committee:
Igor Cherednikov		Elke Aschenauer
Didar Dobur		Daniel Boer
David Dudal		Igor Cherednikov
Laurent Favart		Markus Diehl
Francesco Hautmann		Didar Dobur
Fabio Maltoni		David Dudal
Pierre Van Mechelen		Miguel Echearría
		Laurent Favart
		Francesco Hautmann
		Hannes Jung
		Fabio Maltoni
		Piet Mulders
		Andrea Signori
		Pierre Van Mechelen

 

Monday, 7 November

13:00 REF 2016 Registration

Welcome

Convener: Hannes Jung (Deutsches Elektronen-Synchrotron (DE))

1 Introduction

Recap and aims of the REF workshops

Speaker: Hannes Jung (Deutsches Elektronen-Synchrotron (DE))

Intro.pdf

Higgs/DY/QQbar production

Convener: Francesco Hautmann (Institute of Theoretical Physics)

2 Lam-Tung relation in Z0 hadroproduction as a probe of gluon TMD

Recent ATLAS data of dilepton angular distribution from Z0 hadroproduction provide a stringent test of QCD predictions. The measured Lam-Tung combination of dilepton angular coefficients shows strong deviations from collinear QCD predictions at the NNLO accuracy. We describe this observable within the kT-factorization framework and show that it is sensitive to the shape of gluon TMD. An improved description of the data is obtained with a gluon TMD with a hard ~1/k^2 tail at moderate gluon x.

Speaker: Leszek Motyka

motykaLT_Ant.pdf

3 A study of the non-perturbative structure of TMDs

The non-perturbative partof the TMDs has been the object of many phenomenologucal studies.
We consider here a renormalon analysis of TMDs which provides the leading corrections to the perturbative estimations of TMDs. We can identify two sources of renormalons which should be taken into account separately.
We also show that theTMDs have a non-trivial dependence on Bjorken variables. All these information can be used to provide consistent models/ansätze for TMD phenomenology.

Speaker: Ignazio Scimemi (Universidad Complutense Madrid)

Antwerp2016.pdf

15:10 Coffee/tea

Higgs/DY/QQbar production: B

Convener: Francesco Hautmann (Institute of Theoretical Physics)

4 Transverse-momentum resummation for Higgs boson pair production with top-quark mass effects

We consider Higgs boson pair production via gluon fusion in hadronic collisions. We report the calculation of the transverse-momentum ($q_T$) distribution of the Higgs boson pair with top-quark mass ($M_t$) effects fully taken into account. At small values of $q_T$ we resum the logarithmically-enhanced perturbative QCD contributions up to next-to-leading logarithmic (NLL) accuracy. At intermediate and large values of $q_T$ we consistently combine resummation with the $\mathcal{O}(\alpha_S^3)$ fixed-order results. After integration over $q_T$, we recover the next-to-leading order (NLO) result for the inclusive cross section with full dependence on $M_t$. We present illustrative numerical results at LHC energies, together with an estimate of the corresponding perturbative uncertainties, and we study the impact of the top-quark mass effects.

Speaker: Giancarlo Ferrera (University of Milan)

ref16-ferrera.pdf

5 Factorization and Resummation for Massive Quark Effects in Exclusive Drell-Yan

Transverse momentum spectra at hadron colliders are nowadays studied at an unprecedented precision in theory and experiment. Within the framework of Soft-Collinear Effective Theory we discuss how to incorporate massive bottom/charm quark effects in the resummed cross section for Drell-Yan at small $q_T$. These can e.g. play an important role in W-mass measurements at the LHC. At NNLL' order, i.e. including matrix elements at $\mathcal{O}(\alpha_s^2)$ and NNLL resummation, one has to account for both heavy quark initiated (primary) corrections to the hard scattering process as well as secondary radiation effects. The theoretical description depends on the hierarchy between the involved scales, ranging from the decoupling limit for large masses to the massless limit for small masses, and involves quark mass dependent beam functions/TMDs and soft functions in between. The rapidity divergences for $q_T \sim m_Q$ are affected by the quark mass and we also discuss the resulting evolution which resums the associated rapidity logarithms.

Speakers: Piotr Pietrulewicz (DESY), Piotr Pietrulewicz

REF2016.pdf

6 Discussion

Speaker: Francesco Hautmann (Institute of Theoretical Physics)

Tuesday, 8 November

(TMD) PDFs

Non-Perturbative TMDS, ...

Convener: Daniel Boer (University of Groningen)

7 Double parton scattering: evolution and matching

In double parton scattering (DPS), we consider the limit of perturbative transverse momenta and a large spatial separation between the two hard processes. We discuss the scale and rapidity evolution as well as the matching of transverse momentum dependent double parton distributions onto collinear double parton distributions for processes with a colorless final state. Although the evolution and matching equations (and their derivation) are more elaborate than just doubling the TMD/PDF evolution and matching, we show that the NLO coefficient functions for DPS matching can be recycled from the TMD/PDF matching.

Speaker: Maarten Buffing (DESY)

Buffing_2016_11(nov)_08_REF.pdf

8 Predictions of Drell-Yan in p+Pb and calculation of RpA using hybrid High Energy Factorization

We calculate cross sections for various observables in Drell-Yan lepton pair production in p+p and p+Pb collisions within Hybrid High Energy Factrorization. In particular we obtain reasonably good description of invariant mass distribution data using KS linear and KS-nonlinear gluon distribution function. Furthermore we provide prediction for saturation in p+Pb collisions and calculate nuclear modifications factors. We compare our results for p+p to predictions obtained within colinear based Monte Carlo generators working at LO and NLO accuracy. We see that differential cross section as a function of rapidity, delta R and azimuthal angle show significant difference between results of Madgraph NLO and HEF.

Speaker: Krzysztof Bozek (King's College London)

Bozek-ForwardDY.pdf

9 TMD fragmentation in jets

We study the transverse momentum dependence of hadrons fragmenting inside jets. By measuring the transverse momentum with respect to a judiciously chosen axis (not the standard jet axis), the measurement is insensitive to soft radiation. Furthermore, for small transverse momenta the effects of the jet boundaries can be factorized, leading to a TMD fragmentation function. This TMD fragmentation function is universal, i.e. it is independent of the number of jets and appears in pp, ep and ee collisions.

Speaker: Wouter Waalewijn (University of Amsterdam)

ref_2016_waalewijn.pdf

10:30 Coffee/tea

(TMD) PDFs

Non-Perturbative TMDS, ...

Convener: Daniel Boer (University of Groningen)

10 Reconstructing PDFs at large x

PDFs are nonperturbative objects defined by nonlocal light-cone correlations. They cannot be computed directly from QCD. Using a standard lattice QCD approach, it is possible to compute moments of PDFs, which are matrix elements of local operators. Recently, an alternative approach has been proposed, based on the introduction of quasi-PDFs, which are matrix elements of equal-time spatial correlations and hence calculable on lattice. Quasi-PDFs approach standard PDFs in the limit of very large longitudinal proton momenta $P^z$. This limit is not attainable in lattice simulations, and quasi-PDFs fail to reproduce PDFs at high x. Here, we propose a method to improve the reconstruction of PDFs by combining information from quasi-PDFs and from the Mellin moments of regular PDFs. We test our method using the diquark spectator model for up and down valence distributions of both unpolarized and helicity PDFs. In the future, the method can be used to produce PDFs entirely based on lattice QCD results.

Speaker: Marco Radici (INFN - Pavia)

radici.pdf

11 Extraction of partonic unpolarized TMD distributions from SIDIS and DY processes

We present an extraction of unpolarized partonic transverse momentum distributions (TMDs) from a simultaneous fit of available data measured in semi-inclusive deep inelastic scattering, Drell-Yan processes, and Z boson production by different experimental collaborations. To connect data at different scales, we use TMD evolution at next-to-leading logarithmic accuracy. The analysis is restricted to the low-transverse-momentum region, with no matching to fixed-order calculations at high transverse momentum. We introduce specific choices to deal with TMD evolution at low scales, of the order of 1 GeV$^2$. We show the results of our fit, discussing the characteristics of the TMD functional forms extracted and what improvements can be made to our analysis. This could be considered as a first attempt at a global fit of TMDs.

Speakers: Cristian Pisano (Vrije Universiteit Amsterdam), Dr Cristian Pisano (University of Pavia and INFN Pavia), Cristian Pisano

REF16-pisano.pdf

12 Discussion

Speaker: Daniel Boer (University of Groningen)

13:00 Lunch

(TMD) PDFs

Non-Perturbative TMDS, ...

Convener: Hannes Jung (Deutsches Elektronen-Synchrotron (DE))

13 TMD PDFs from Monte Carlo evolution

We describe a new approach to solve coupled quark and gluon DGLAP
evolution equations with a Monte Carlo method. We show that this method
is equivalent to other semi-analytical methods. We apply this method to
extract quark and gluon parton densities, both collinear and transverse
momentum dependent (TMD), using the precision HERA DIS data. We present
also first results including NLO spliting functions.

Speaker: Aleksandra Anna Lelek (Deutsches Elektronen-Synchrotron (DE))

OlaLelek.pdf

14 Monte Carlos & transverse momentum

Monte Carlos & transverse momentum

Speaker: Simon Plaetzer (ITP, Universitaet Karlsruhe)

slides.pdf

15 TMDs from Parton Shower MCs (Discussion)

We will present a method to obtain TMDs from standard parton shower Monte Carlo generators. We compare the prediction from different MC generators and argue that TMDs are a unique way to compare features of parton shower generators at a theoretical level.

Speaker: Hannes Jung (Deutsches Elektronen-Synchrotron (DE))

TMDfromPS.pdf

16:30 Coffee/tea

Advisory Committee meeting: A

Wednesday, 9 November

Tools, Monte Carlo generators: Q

16 Underlying event sensitive observables in Drell-Yan production using GENEVA

I will discuss the GENEVA Monte Carlo framework, that allows to obtain predictions for underlying-event sensitive measurements in Drell-Yan production using PYTHIA8's MPI model, in conjunction with fully-differential NNLO calculation, NNLL' resummation for the 0-jet resolution variable (beam thrust), and NLL resummation for the $1$-jet resolution variable. I will present results for several underlying-event sensitive observables and compare to data from ATLAS and CMS as well as to standalone PYTHIA8 predictions, including a comparison with the recent ATLAS measurement of the beam thrust spectrum, which provides a potential avenue to fully disentangle the physical effects from the primary hard interaction, primary soft radiation, multiple parton interactions, and nonperturbative hadronization.

Speaker: Simone Alioli (CERN)

REF_2016_Alioli.pdf

17 APFEL: A PDF Evolution Library

APFEL is a numerical code specialized for PDF fits that provides a
fast and accurate solution of the DGLAP equations. In addition to PDF
evolution, APFEL also provides a module that computes DIS cross
sections in several mass schemes. In this contribution I will present
the most recent and relevant developments carried out in APFEL. They
include: the computation of the time-like evolution, the
implementation of the SIA cross sections needed for the determination
of FFs, the small-x resummed evolution, and the implementation of the
NLO QED corrections both to the DGLAP evolution and the DIS structure
functions. I will then mention some of the most recent results
obtained using APFEL and finally I will conclude discussing some
relevant future plans related to the determation of FFs and TMD

Speaker: Valerio Bertone (NIKHEF)

bertone_REF_09_11_2016.pdf

18 TMDlib and TMDplotter

We present the latest versions of TMDlib and
TMDplotter. Basic features of TMDplotter include plotting as a function of the
momentum fraction, the transverse momentum, or the evolution scale, as well as
integration of TMDs and comparison to sets from LHAPDF. Latest improvements
include a upload of new sets from the web interface and the plotting of the
uncertainties. Luminosity for collinear PDFs is also described.

Speaker: Patrick Connor (Deutsches Elektronen-Synchrotron (DE))

talk.pdf

10:20 Coffee/tea

Tools, Monte Carlo generators: B

19 xFitter project

We present the xFitter project (former HERAFitter) which provides a unique open-source software framework for the determination of the proton's PDFs and for the interpretation of the physics analyses in the context of Quantum Chromodynamics.
We highlight the new xFitter software release which includes many new
features and additions, e.g. the possibility of the inclusion of photon PDF,
updated variable and fixed-flavour schemes for heavy quarks, interface to
the APFEL library and n-space evolution program MELA, updates to the
latest theory calculations and many more.
We will also report on the latest xFitter results on the new determination of
the mass of the charm quark extracted by analysing the statistical quality of
fits to inclusive and exclusive charm deep-inelastic scattering data from
the HERA collider.

Speaker: Ringaile Placakyte

placakyte_xfitter_ref16.pdf

20 TMDs in xFitter

The xFitter (former HERAFitter) is an open-source package that provides
a framework for the determination of the proton PDFs and covers a large
number of the existing methods and schemes for PDF determination.
xFitter also provides facilities to use diverse methodologies alternative to the
standard collinear formalism, as provided by DGLAP evolution equations.
The emphasis of this presentation is on the recent developments related to
these different approaches ranging from simple dipole models to address
the saturation phenomenology, to the advances in using resummed PDFs
and transverse momentum dependent PDFs (TMDs), as implemented in the xFitter package.

Speaker: Voica Ana Maria Radescu (University of Oxford (GB))

REF2016_VR.pdf

21 Discussion

13:00 Lunch

Jet Final States

Convener: Krzysztof Kutak (Instytut Fizyki Jadrowej Polskiej Akademii Nauk)

22 Matrix elements and Monte Carlo calculations with k_T-factorization

Besides parton distribution functions with an explicit dependence on the transverse momentum, k_T-type factorizations require matrix elements with off-shell initial-state partons. We discuss issues regarding their definition, and how they can be calculated efficiently. We also present a complete Monte Carlo program for the calculation of cross sections and event generation for arbitrary processes within k_T-type factorization formulas.

Speaker: Andreas van Hameren (IFJ PAN)

vanHameren.pdf

23 4-jet production with kt-factorization plus parton showers

In this talk, I give a review of our work on the exploration of optimal conditions to pin down double parton scattering
in four jet production and the LHC in the 7 and 13 TeV data sets, working in the theoretical framework of high energy (kt-) factorisation.
Beside, I elaborate on the first stages of the update of the project, consisting in repeating the investigation by augmenting
the theoretical predictions with final state parton showers.

Speaker: Mirko Serino

REF_2016_Serino.pdf

15:20 Coffee/tea

Jet Final States

Convener: Krzysztof Kutak (Instytut Fizyki Jadrowej Polskiej Akademii Nauk)

24 TeV jets at the LHC Run II and future colliders - CANCELLED

We present Monte Carlo simulations of di-jet production at very
high transverse momenta and invariant masses, discussing effects of
NLO QCD contributions, initial-state and final-state multi-parton
radiation, and their impact on measurements at the LHC Run II and
future high-energy collider experiments.

Speaker: Maria Vittoria Garzelli (University of Hamburg)

25 High pt jets in the back-to-back region

Scenarios where at least two high pt jets in the almost back to back configuration ($175^{\circ} <\Delta \phi<180$) are produced, are sensitive to soft gluon resummation effects. We studied these scenarios looking at different observables of the leading system ($\Delta \phi$, $\phi^\ast$, $\Delta pt$ and $\Delta \eta$) as well as other observables as the extra jets multiplicity.

Speaker: Daniela Dominguez Damiani (DESY)

REF_Presentation.pdf

26 NLO predictions for dijet azimuthal correlation

We present a phenomenological study of the inclusive dijet cross section as a function of the azimuthal separation between the leading jets. We focus on the Sudakov region for high pt leading jets scenarios. We performed the studies using 3 final partons @ NLO matrix elements as well as POWHEG three jets NLO matched to pt ordered parton showers.

Speaker: Armando Bermudez Martinez (CMS-DESY)

talk_Armando.pdf

27 Discussion

Speaker: Krzysztof Kutak (Instytut Fizyki Jadrowej Polskiej Akademii Nauk)

19:30 Workshop dinner

Thursday, 10 November

Small-x

Convener: Piet Mulders (VU/Nikhef)

28 Gluon TMD's at low x and diffractive processes

Gluon TMD's at low x and diffractive processes

2016-REF-Antwerp-Mulders.pdf

2016-REF-Antwerp-Mulders.pdf

2016-REF-Antwerp-Mulders.pptx

29 Transverse momentum dependent splitting functions from kT factorization

The formulation of kT factorization is build on factorization of scattering amplitudes in the high energy limit and the resummation of corresponding logarithms. As a consequence its applicability is at first limited to the so-called low x region. Moreover the resulting LO evolution is purely gluonic. Building on the lessons learned from (NLO) BFKL evolution, a possibility to extend the formalism to the region of medium and maybe even large Bjorken x might be to include quarks into the evolution, mimicking in this way the matrix structure of conventional DGLAP evolution. In this contribution we report on recent attempts to calculate TMD quark splitting functions within kT factorization and to construct corresponding extended evolution equations.

Speaker: Martin Hentschinski (Benemérita Universidad Autónoma de Puebla)

antwerp_2016_MartinH.pdf

30 On the diffractive jet production in NLO

We present the calculation of the impact factor for the γ (∗) → qq¯ transition with one loop accuracy in arbitrary kinematics. The calculation was done within Balitsky’s high energy operator expansion. Together with our previous result for the γ (∗) → qqg¯ Born impact factor it allows one to derive cross-sections for 2- (one loop) and 3-jet (Born) difractive electroproduction. We write such cross sections for the 2 and 3 jet exclusive diffractive electroproduction off a proton in terms of hadronic matrix elements of Wilson lines. For the 2-jet cross section we demonstrate the cancellation of IR, collinear and rapidity singularities. Our result can be directly exploited to describe the recently analyzed data on exclusive dijet production at HERA and used for the study of jet photoproduction in ultraperipheral proton or nuclear scattering.

Speaker: Andrey Grabovskiy (Budker Institute of Nuclear Physics (RU))

REFgrabovsky.pdf

10:20 Coffee/tea

Small-x

Convener: Piet Mulders (VU/Nikhef)

31 Inclusive forward jet and di-jet production at the LHC at 7 and 13 TeV

A description of the transverse momentum spectrum of single inclusive forward
jets produced at the LHC, at the center-of-mass energies of 7 and 13 TeV, using the high
energy factorization (HEF) framework will be presented. Subsequently, a study of double inclusive forward jet production and, in particular, calculation of contributions to azimuthal angle distributions coming from double parton scattering will be shown. Our results for double inclusive jet production are compared to those obtained with the Pythia Monte Carlo generator. This comparison confirms that the HEF resummation acts like an initial state parton shower. It also points towards the need to include final state radiation effects in the HEF formalism.

Speakers: Michal Deak (CERN), Michal Deak (Universidade de Santiago de Compostela), Michal Deak (Conseil Europeen Recherche Nucl. (CERN)-Unknown-Unknown), Michal Deák (IFJ, Krakow, Poland)

REF2016-Deak.pdf

32 Small-x behavior of TMD gluon distributions in the Color Glass Condensate

We study the operator structure of the unpolarized transverse-momentum-dependent (TMD) gluon distributions in the cross section for forward dijet production in dilute-dense collisions. At finite $N_c$, there are total of eight TMD gluon distributions for this process, including the dipole distribution and the Weizsacker-Williams gluon distribution. For each of these functions we derive their operator definitions at small-x as correlators of Wilson lines. We demonstrate their equivalence to the operator structure of the Color Glass Condensate (CGC) cross section in the nearly back-to-back limit, and we show that the correlation limit of the CGC cross section is equivalent to the small-$x$ limit of the TMD factorization formula, at finite $N_c$. We obtain an analytical result for the gluon distributions in the Golec-Biernat-Wusthoff model, their perturbative behavior at large transverse momentum in the McLerran-Venugopalan model, and, numerically, their JIMWLK evolution towards small $x$. We observe geometric scaling regime for all the TMDs after some evolution.

Speaker: Elena Petreska (Nikhef and VU Amsterdam)

petreska.pdf

33 Heavy quark production in unpolarized eA and pA scattering: the role of gluon polarization

Expanding the work done in Marquet, Petreska and Roiesnel (2016), we use the Color Glass Condensate (CGC) to study the production of forward heavy di-jets both in DIS on a nucleus and in pA collisions in the small-x regime. In the correlation limit, in which the jets are nearly back-to-back in the transverse plane, the CGC result coincides with the one in the transverse-momentum dependent (TMD) factorization approach. This allows us to extract the small-x limit of the well-known Weizsäcker-Williams gluon distribution (in both the eA and the pA case) as well as two extra gluon TMDs (for pA collisions). Interestingly, in the case of heavy quarks, each of these three different gluon TMDs is accompanied by a partner that describes the linearly polarized gluon content of the hadron. We end up with six different gluon TMDs, that we can calculate in the McLerran-Venugopalan model and evolve towards smaller Bjorken-x with a numerical implementation of the JIMWLK evolution equations.

Speaker: Pieter Taels (U)

REF 091116 - Pieter Taels.pdf

34 Discussion

Speakers: Piet Mulders (VU University), Piet Mulders (VU/Nikhef)

13:00 Lunch