QCD Evolution 2016

30 May 2016, 10:00 → 3 Jun 2016, 16:10 Europe/Amsterdam

Z011 - Turing room (Nikhef)

Alexei (co-chair) Prokudin (Penn State University / Jefferson Lab), Anatoly Radyushkin (ODU/Jefferson Lab), Andrea Signori, Daniel (co-chair) Boer, Ivan Vitev (LANL), Leonard Gamberg (Penn State University), Piet (co-chair) Mulders, Tomas Kasemets, Zhongbo Kang (LANL)

The QCD Evolution 2016 workshop will be held at the National Institute for Subatomic Physics (Nikhef) in Amsterdam, May 30 – June 3, 2016. The workshop is a continuation of a series of workshops held during five consecutive years, in 2011, 2012, 2013, 2015 at Jefferson Lab, and in 2014 in Santa Fe, NM. With the rapid developments in our understanding of the evolution of parton distributions including low-x, TMDs, GPDs, higher-twist correlation functions, and the associated progress in perturbative QCD, lattice QCD and effective field theory techniques, we look forward to yet another exciting meeting in 2016. The program of QCD Evolution 2016 will pay special attention to the topics of importance for ongoing experiments, in the full range from Jefferson Lab energies to LHC energies or future experiments such as a future Electron Ion Collider, recently recommended as a highest priority in U.S. Department of Energy's 2015 Long Range Plan for Nuclear Science.

Bulletin QCD-Evolution2016-bulletin.pdf

Events in Amsterdam Events_in_Amsterdam.pdf

Group picture DSC_9637.Jjpg.JPG

Location Hermitage map_Nikhef_Casa400_Hermitage.png

Poster QCD_evolution_2016-poster_small.pdf

Monday, 30 May ¶

Welcome¶

1 Electron Ion Collider: Physics and Prospects¶

We will give an outline of the anticipated physics program for a future electron ion collider. The status and prospects for construction of such a device will be discussed.

Speaker: Hugh Montgomery

Slides Mont_qcd_evolution_may_30,_2016_v4.pdf Mont_qcd_evolution_may_30,_2016_v4.pptx

Talks¶

2 Quasi-real photon contribution to A_N in lp --> pion X in a TMD approach¶

Within a TMD approach we discuss the impact of quasi-real(Weizsacker-Williams) photon contribution to the transverse single spin asymmetry, A_N, for the inclusive process lp --> pion X. This study extends a previous one where only the leading order contribution was taken into account. The predictions are obtained adopting the Sivers and transversity distributions and the Collins fragmentation functions as extracted from fits to the azimuthal asymmetries measured in semi-inclusive deep inelastic scattering and e+e- annihilation processes. The description of the available data is very good, showing a clear general improvement with respect to the previous analysis. Predictions for future experiments are also given.

Speaker: Umberto D'Alesio

Slides Dalesio_AN_lp_ww.pdf

3 Do fragmentation functions in factorization theorems correctly treat non-perturbative effects?¶

Current all-orders proofs of factorization of hard processes are made by extracting the leading power behavior of Feynman graphs, i.e., by working strictly order-by-order in perturbation theory. The resulting parton densities and fragmentation functions include non-perturbative effects. I discuss how there could be loopholes in the proof such that other non-perturbative and semi-perturbative effects could be important. My discussion is motivated by string and cluster models of hadronization.

Speaker: John Collins

Slides collins-QCD-Evol-2016.pdf

4 Extraction of unpolarized TMDs¶

We present an extraction of unpolarized TMDs that includes the effects of TMD evolution up to next-to-leading log level. We consider semi-inclusive DIS multiplicites and Drell-Yan cross-sections. Flavor dependence of TMDs is taken into consideration.

Speaker: Alessandro Bacchetta

Slides Bacchetta_Amsterdam_2016_QCDevo.pdf

12:30 Lunch

Talks¶

5 Transverse phase space and its multipole decomposition¶

Relativistic phase space distributions are very interesting objects as they allow one to gather the information extracted from various types of experiments into a single coherent picture. Focusing on the 4-dimensional transverse phase space, we identified all the possible angular correlations providing at the same time a clear physical interpretation of all the leading-twist generalized and transverse-momentum dependent parton distributions. We also developed a convenient representation of this 4-dimensional space.

Speaker: Cedric Lorce

Slides Cedric_QCDEvolution2016.pptx

6 Quark Orbital Angular Momentum¶

Generalized Parton Distributions (GPDs) provide information on the distribution of quarks in impact parameter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. This force when acting along the whole trajectory of the active quark leads to transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark Orbital Angular Momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe-Manohar and Ji OAMs.

Speaker: Matthias Burkardt

Slides Burkardt_evolution16.pdf

7 Structure of the Energy-Momentum Tensor and Applications¶

The probably most fundamental information about a particle is contained in the matrix elements of its energy momentum tensor which are accessible via GPDs. The spin decomposition of the nucleon and Ji sum rule is just one example. Less prominent but equally important information is encoded in the stress tensor related to the spatial components of the energy-momentum tensor which shows in detail how the internal forces balance to form a stable bound state. This provides not only unique insights on hadronic structure. It also leads to unexpected fascinating new applications to hadron spectroscopy which allow us to formulate new interpretations of the charmonium-nucleon recently discovered by LHCb. Recent progress is reviewed.

Speaker: Peter Schweitzer

Slides Talk-Schweitzer-QCD-evol-2016.pdf

15:30 Coffee break

Talks¶

8 Helicity Evolution at Small-x¶

We construct small-x evolution equations which can be used to calculate quark and anti-quark helicity TMDs and PDFs, along with the g1 structure function. These evolution equations resum powers of αs ln2(1/x) in the polarization-dependent evolution: hence our equations collect powers of the parameter excluded in the DGLAP and CSS evolutions. The full non-linear version of our equations also resums the powers of αs ln(1/x) in the unpolarized evolution which includes saturation effects. The equations are written in an operator form in terms of polarization-dependent Wilson line-like operators. While the equations do not close in general, they become closed and self-contained systems of non-linear equations in the large-Nc and large-Nc and Nf limits. The resulting x-dependence of quark helicity TMD and PDF can be obtained by solving these equations numerically.

Speaker: Yuri Kovchegov

Slides Kovchegov_Amsterdam2016.pdf

9 Resumming large collinear logarithms in the non-linear QCD evolution at high energy¶

The Balitsky-JIMWLK equations which govern the QCD evolution at high energy and in the presence of unitarity corrections are presently known to next-to-leading order accuracy. However, the NLO approximation turns out to be unstable, due to large radiative corrections enhanced by double, or single, collinear logarithms, i.e. logarithms generated by the transverse phase-space. Via an explicit diagrammatic analysis, we clarify the origin of these logarithmic corrections and perform their resummation at the level of the BK equation (the large Nc limit of the Balitsky-JIMWLK hierarchy). Numerical studies demonstrate that, due to this resummation, the evolution becomes stable and it is also considerably slowed down (as it should, to be consistent with the phenomenology). Using the solution to the collinearly-improved BK equation, we obtain excellent fits to the HERA data for electron-proton deep inelastic scattering at high energy.

Speaker: Edmond Iancu

Slides Iancu_Amsterdam2016.pdf

10 Initial state correlations¶

I discuss several mechanisms that result in correlations between gluons in the initial state wave functions. These correlations exhibit themselves as correlated particle production in certain kinematical regime. I also discuss the fate of these correlations under high energy evolution.

Speaker: Alex Kovner

Slides Kovner-evolution_2016.pdf

Reception¶

Tuesday, 31 May ¶

Talks¶

11 The RHIC transverse spin physics program: a crucial step to EIC¶

The most recent results and future possibilities to measure observables to constrain quark and gluon TMDs and their evolution through transverse polarised pp collisions at a center of mass energy of 200 GeV and 500 GeV will be reviewed. Further selective measurements for TMDs at an eRHIC will be discussed

Speaker: Elke-Caroline Aschenauer

Slides eca.RHIC.transverse.pptx

12 Spin asymmetries for vector boson production in polarized p+p collisions¶

We investigated the cross section and the associated spin asymmetries for vector boson (W±/Z0/γ∗) production in polarized proton-proton collisions at tree level within the TMD factorization formalism. Besides the well-known Sivers function f1T , the single transverse asymmetry could also probe the transversal helicity distribution g1T via the parity-violating nature of W/Z0 production. Contrary to Sivers function, which is expect to change sign from SIDIS to DY-type of processes, transversal helicity is universal between SIDIS and DY. To assess the feasibility of experimental measurements, we estimate the spin asymmetries for W±/Z0 boson production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) by using current knowledge of the relevant TMDs. We find that both the parity-conserving and parity-violating single transverse asymmetries can be sizable, if the suppression effect from TMD evolution is not too strong.

Speaker: Jin Huang

Slides Huang_W_spin.pdf

13 Exploring universality of transversity in p-p collisions¶

We consider azimuthal asymmetries of charged pion pairs produced inside the same jet from proton-proton collisions with one transversely polarized proton. A specific asymmetry directly probes the chiral-odd transversity parton distribution in connection with the chiral-odd interference fragmentation function. We present predictions for this observable based on previous extractions of transversity (from charged pion pair production in SIDIS off transversely polarized targets) and of the interference fragmentation function (from the semi-inclusive production of charged pion pairs in back-to-back jets from e+e-). All analyses are performed in the framework of collinear factorization. We compare our predictions with recently released data by the STAR collaboration, and we find them compatible. It is the first attempt to explore the universality of transversity.

Speaker: Marco Radici

Slides radici.pdf

10:30 Coffee break

Talks¶

14 Double parton scattering in the ultraviolet: addressing the double counting problem¶

At short distance between partons, double parton scattering overlaps with loop contributions to standard single hard scattering. Responsible for this is the splitting of one parton into two, and different choices for the partitioning of these contributions between single and double scattering give rise to different cross section formulae and to different evolution equations for the double parton distributions. We present a consistent scheme to separate single from double hard scattering and illustrate numerically the relative importance of the respective contributions.

Speaker: Jonathan Gaunt

Slides Gaunt_QCDev16.odp_1.pdf

15 Double parton scattering for perturbative transverse momenta¶

The cross section for transverse momentum dependent double parton scattering involves transverse momentum dependent double parton distributions (DTMDs). In the region of perturbative transverse momentum the DTMDs can be matched onto collinear double parton distributions. In this talk, we present the framework and results for this matching, as well as the evolution equations for DTMDs in the region of large distance between the two partons. We discuss NLO matching coefficients and LO evolution kernels.

Speaker: Maarten Buffing

Slides Buffing_2016_QCD_Evolution.pdf

16 Double parton scattering in a multi-scale regime¶

At perturbative transverse momenta, double parton scattering receives contributions from several different regimes, which need to be merged in a consistent way. I show how this can be done by adapting Collins’ subtraction formalism to position space.

Speaker: Markus Diehl

Slides Diehl_evolution.pdf

12:30 Lunch

Talks¶

17 Generalized Parton Distributions in Deeply Virtual Lepton Scattering Processes¶

Spin and transverse momentum dependent Generalized Parton Distributions (GPDs) exist at the interface between the non-perturbative regime of QCD hadron structure and observable quantities. The distributions appear as linear superpositions and convolutions within helicity amplitudes for parton-nucleon scattering processes, which, in turn, occur in amplitudes for leptoproduction processes. We have developed a ”flexible model” of quark and gluon GPDs that incorporates diquark and other spectators, Regge behavior and evolution. Chiral even GPDs determine deeply virtual Compton scattering amplitudes and are compared with cross section and polarization data. The chiral odd GPDs can be generated from these via parity relations. Those chiral odd GPDs, including ”transversity”, lead to predictions for pseudoscalar leptoproduction. We will present relations between crucial quark-nucleon or gluon-nucleon GPDs and the rich array of angular distributions in Deeply Virtual Scattering processes.

Speaker: Gary Goldstein

Slides QCD_Evoln2016_goldstein_v4.pdf

18 COMPASS results on hard exclusive muoproduction¶

The high energy polarised muon beam available at CERN, with positive or negative charge, makes COMPASS a unique place for studies of General Parton Distributions (GPDs). The first GPD related COMPASS results were obtained for exclusive vector meson production on transversely polarised protons and deuterons. The data were taken in 2003-2010 with solid-state polarised target, although without recoil detector. Results on various transverse target spin dependent azimuthal asymmetries will be presented and their relation to GPDs will be discussed. The dedicated COMPASS GPD program started in 2012 with commissioning of a new long liquid hydrogen target and new detectors, such as the large recoil proton detector CAMERA and the large-angle electromagnetic calorimeter. It was followed by a short pilot DVCS run. The performance of the setup and first results on DVCS and exclusive π0 channels will be demonstrated. The full data taking for the GPD program is planned in 2016 and 2017.

Speaker: Andrzej Sandacz

Slides sandacz-qcdevol16.pdf

19 Towards three loop evolution equation for GPDs¶

QCD evolution equations in minimal subtraction schemes have a hidden symmetry: One can construct three operators that commute with the evolution kernel and form an SL(2) algebra. We found explicit expressions for these operators to two-loop accuracy going over to QCD in non-integer d = 4 − 2ǫ space-time dimensions at the intermediate stage. In this way conformal symmetry of QCD is restored on quantum level at the specially chosen (critical) value of the coupling. Quantum corrections to conformal generators in d = 4−2ǫ effectively correspond to the conformal symmetry breaking in the physical theory in four dimensions. Our result allows one to derive three-loop evolution equations for generalized parton distributions and light-cone meson distribution amplitudes.

Speaker: Alexander Manashov

Slides manashov.pdf

15:30 Coffee break

Talks¶

20 An EFT for Both Hard and Forward Scattering¶

I will discuss an EFT framework that is valid for both hard scattering and forward scattering processes by including Glauber interactions in the Soft-Collinear Effective Theory. A Lagrangian is constructed that is valid to all orders in alphas. The role of rapidity renormalization and subtractions is highlighted. Applications include understanding factorization violation in hard scattering, as well as M¯S style derivations of rapidity renormalization and evolution equations (such as BFKL, small-x resummation in DIS).

Speaker: Iain Stewart

Slides Stewart_QCDEvolution.pdf

21 Multi-differential resummation with SCET+¶

I will discuss how to perform the simultaneous resummation of various series of logarithms using an extension of Soft Collinear Effective Theory, called SCET+. I will discuss this for the simultaneous measurement of the pT of the Z boson and the hadronic beam thrust event shape [arXiv:1410.6483], which was recently measured at the LHC. However, I will also consider other types of logarithms, including kinematic logarithms [arXiv:1601.05088], jet radius logarithms and threshold logarithms.

Speaker: Wouter Waalewijn

Slides qcd_evolution_waalewijn.pdf

22 Jet fragmentation functions in proton-proton collisions¶

The jet fragmentation function describes the longitudinal momentum distribution of hadrons inside a reconstructed jet. We study the jet fragmentation function in proton-proton collisions in the framework of soft-collinear effective theory (SCET). We find that, up to power corrections, the jet fragmentation function can be expressed as the ratio of the fragmenting jet function and the unmeasured jet function. We use our theoretical formalism to describe the jet fragmentation functions for light hadron and heavy meson production measured at the Large Hadron Collider (LHC). Our calculations agree very well with the experimental data for the light hadron production. On the other hand, although our calculations for the heavy meson production inside jets are consistent with the PYTHIA simulation, they fail to describe the LHC data. We find that the jet fragmentation function for heavy meson production is very sensitive to the gluon-to-heavy-meson fragmentation function.

Speaker: Felix Ringer

Slides Ringer_QCDEVOL16.pdf

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

Within the framework of Soft-Collinear Effective Theory we discuss how to incorporate massive bottom quark effects in the resummed cross section for exclusive 0-jet Drell-Yan, where as a specific example for a measured exclusive observables we consider the transverse momentum of the electroweak boson. At NNLL’ order, i.e. including matrix elements at O(α2 s) 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 TMDPDFs and soft functions in between. The emerging rapidity divergences can be different with respect to the massless quark case and we also discuss the resulting evolution which resums the associated rapidity logarithms.

Speaker: Daniel Samitz

Slides Samitz_QCDEvlution2016.pdf

Wednesday, 1 June ¶

Talks¶

24 Gluon TMD in the Loop Space Formalism¶

We discuss a method to derive the gauge-invariant gluon transverse-momentum dependent distribution function (gTMD) within the Wilson loop space framework. The basic ingredient of this approach is a set of hadronic matrix elements of arbitrary Wilson loops. We compare this approach with other definitions of the gTMD at small- and large-x existing in the literature.

Speaker: Igor Cherednikov

Slides cherednikov_QCDEvolution_2016.pdf

25 TMD factorization from small to large transverse momentum¶

I will discuss issues that arise when TMD factorization theorems are combined with descriptions of large transverse momentum. The importance of accurate treatments of large transverse momentum in processes with lower hard scales, like semi-inclusive deep inelastic scattering, will be emphasized. I will also discuss the relationship between TMD factorization and collinear factorization.

Speaker: Ted Rogers

Slides Evo2016Rogers.pdf

26 SIDIS cross sections: perturbative and non-perturbative aspects¶

I will address the study of semi-inclusive deep inelastic scattering cross sections over a wide range of transverse momenta, and in particular in the region of low to moderate $q_T$, when the interplay between perturbative and non-perturbative effects becomes more relevant. Some realistic SIDIS kinematical configuration will be considered where the commonly used prescription of “matching” through the so-called Y-factor cannot be applied. Moreover, I will discuss the theoretical uncertainties of the transverse momentum resummed cross sections in the CSS formalism related to the scale parameters C_1, C_2 and C_3, and their interplay with the uncertainties of the non perturbative parameters g_1, g_2 and g_3.

Speaker: Mariaelena Boglione

Slides Elena-QCDevo2016.pdf

10:30 Coffee break

Talks¶

27 PDF from Hadronic Tensor on the Lattice and Connected Sea Evolution¶

A path-integral formulation of the hadronic tensor is reviewed where the connected and disconnected sea parsons are separated. I shall discuss how to calculate them on the lattice and will discuss how the evolution equations should be modified to accommodate both the connected and disconnected sea partons.

Speaker: Keh-Fei Liu

Slides Liu_PDF_Evolution_2106.pptx

28 Probing Gluon TMDs at a future EIC¶

We show how the study of azimuthal asymmetries for heavy quark pair and dijet production in electron-proton collisions could provide detailed information on gluon distributions, including their transverse momentum dependence. In particular, in the case of transversely polarized protons, the gluon Sivers effect is examined. Moreover, we discuss the strong analogies between the gluon single spin asymmetries for these processes and the quark asymmetries in SIDIS.

Speaker: Cristian Pisano

Slides qcde2016-c-pisano.pdf

29 Twist-3 asymmetries in proton-proton collisions¶

In this talk, I will discuss twist-3 spin asymmetries in proton-proton collisions based on the collinear twist-3 factorization. We discuss the characteristic features of the relevant twist-3 cross sections for the double spin asymmetries ALT for the hadron production in the collision between longitudinally and transversely polarized protons, and the transverse spin polarization for the produced hyperons in the unpolarized proton-proton collisions.

Speaker: Yuji Koike

Slides QCDevo2016.Koike.ppt

30 Structure of transverse momentum dependent (TMD) distributions at NNLO¶

In this work we perform the detailed analyses of TMD operators and provide a complete calculation at next-to-next-to leading order of unpolarized transverse momentum dependent parton distribution functions (TMDPDF) and fragmentation functions (TMDFF) for all flavor combinations. Our analysis is made in general framework without any reference to a specific process. We recover the known results for parton distribution functions and provide new results for the fragmentation functions. We obtain for the first time results also for the transverse momentum dependent gluon fragmentation functions at one and two loops. The analytical properties of the distribution functions, the cancellation of rapidity divergences and the details of the calculation are also discussed.

Speaker: Alexey Vladimirov

Slides Vladimirov.pdf

13:00 Lunch

Dinner¶

Thursday, 2 June ¶

Talks¶

31 Improved theoretical description of Mueller-Navelet jets at LHC¶

Mueller-Navelet jets (MNJ) are one of the most sensitive observables to QCD dynamics in the high-energy (low-x) regime. In this talk I describe two aspects that are important in order to provide accurate predictions for MNJ: 1) the matching of BFKL NLL dynamics with fixed NLO calculations; 2) a modified selection of the final state particles in order to be consistent with experimental measurements by CMS. I will show that both improvements lead to sizeable corrections with respect to the pure NLL BFKL analises carried out so far.

Speaker: Dimitri Colferai

Slides colferai.pdf

32 Evolution of sum rules and positivity constraints¶

The stability of sum rules and preservation of positivity constraints under the different versions of QCD evolution are considered. It is shown that the crucial role belongs to the kinetic interpretation of evolution equations as a gain-loss ones. The cases of DGLAP, BFKL and ERBL evolutions are considered. The various approaches to TMD evolution are compared and the role of transverse degrees of freedom in possible violation of positivity is pointed out. The relation of Burkardt sum rule to energy-momentum conservation and the outcomes to twist-3 evolution are discussed.

Speaker: Oleg Teryaev

Slides teryaevevol16.pdf

33 Selected Results and Future Perspectives for TMD Observables at CERN with COMPASS¶

Speaker: Matthias Grosse Perdekamp

Slides COMPASS-TMD-Matthias-Perdekamp-E.ppt

10:30 Coffee break

Talks¶

34 Hadronic matrix elements in three dimensions¶

I will review some basic features of hadronic matrix elements describing the three dimensional hadronic structure. I will focus on generalized transverse momentum dependent parton distributions (GTMDs) which, in many ways, are considered as the most fundamental objects in hadronic physics, and how they should be properly defined. I will also discuss some open questions regarding another three dimensional object, namely the jet quenching parameter which is of high importance in heavy-ion collisions.

Speaker: Ahmad Idilbi

Slides Idilbi-Evolution_2016.pdf

35 Definition and evolution of GTMDs¶

Three-dimensional hadronic matrix elements are fundamental objects needed to explore hadronic structure. Among them, the most basic quantities are the “generalized transverse momentum dependent parton distribution functions” (GTMDs). In my talk I will argue that, currently, they are improperly defined and will propose a new definition which is in accordance with QCD fundamentals. Using it, I will derive the (universal) evolution kernel for all (un)polarized quark/gluon GTMDs at NNLL, and obtain their matching onto the corresponding collinear GPDs at NLO.

Speaker: Miguel Echevarria

Slides Echevarria_QCD-Evolution16.pdf

36 Evolution equations for light-cone distribution amplitudes of heavy-light hadrons¶

QCD factorization approach for heavy hadron decays introduces light-cone distribution amplitudes for heavy mesons and baryons as the main nonperturbative input. The scale dependence of these distributions is nontrivial and was understood only recently. I review these results and their relation to more familiar DGLAP-and ERBL-type evolution for light hadrons.

Speaker: Vladimir Braun

Slides Braun-Amsterdam.pdf

12:30 Lunch

Talks¶

37 Gluon TMD in particle production from low to moderate x¶

I discuss the rapidity evolution of gluon transverse momentum dependent distributions appearing in processes of particle production and show how this evolution changes from small to moderate Bjorken $x$.

Speaker: Ian Balitsky

Slides Balitsky_gTMD.pdf

38 Sub-gauge Conditions for the Gluon Propagator Singularities in the Light-Cone Gauge and Temporal Gauge¶

Perturbative QCD calculations in the light-cone gauge have long suffered from the ambiguity associated with the regularization of the poles in the gluon propagator. Using the functional integral method, we re-derive the known sub-gauge conditions for the θ-function gauges and identify the sub-gauge condition for the principal value regularization of the gluon propagator’s light-cone poles. Within the same method we obtain the sub-gauge condition for the gluon propagator in the temporal gauge as well.

Speaker: Giovanni Antonio Chirilli

Slides Chirilli-QCD-evolution-2016-Amsterdam.pdf

39 Selected topics on QCD evolution¶

In this talk, I am going to disscuss various topics on QCD evolution, including the scale evolution of the collinear twist-3 correlation functions relevant for the single spin asymmtries phenomenology and the evolution of the small x gluon TMD. It is shown that the Burkardt sum rule can be used to check the consistency of the derived evolution equations for the twist-3 functions. In addition, we show that the computed gluon TMD at NLO satisifies both the BFKL and Collins-Soper evolution equations, thus leading an unified description of the BFKL and CS dynamics.

Speaker: Jian Zhou

Slides Zhou_Selected_topics_on_QCD_evolution_(1).pdf

15:30 Coffee break

Talks¶

40 Pion structure function and SU(2) flavor asymmetry¶

We refine the computation of the ¯ d − ¯u flavor asymmetry in the proton sea with a complementary effort to reveal the dynamics of pion exchange in high-energy processes. In particular, we discuss the efficacy of pion exchange models to simultaneously describe leading neutron electroproduction at HERA along with the ¯ d−¯u flavor asymmetry in the proton.The analysis disfavors several models of the pion flux used in the literature, and yields an improved extraction of the pion structure function and its uncertainties at parton momentum fractions in the pion of 4 × 10−4 <_ x_ <_ 0.05 at a scale of Q2 = 10 GeV2. Based on the fit results, we also address a possible estimate for leading proton structure functions in upcoming tagged deep-inelastic scattering experiments at Jefferson Lab on the deuteron with forward protons.

Speaker: Chueng-Ryong Ji

Slides QCD-Evolution-2016-CJi.pdf

41 Tagged spectator DIS off a polarized spin-1 target¶

We introduce the theoretical framework for deep-inelastic electron scattering (DIS) on the polarized deuteron with detection of a nucleon in the nuclear fragmentation region (“spectator tagging”). Such experiments enable measurements of neutron spin structure and nuclear modifications with full control of the nuclear configuration. We discuss the general expressions of the cross section of electron scattering from a polarized spin-1 target with an identified hadron in the final state, including the dependence on the transverse momentum and azimuthal angle. The target is described by a spin-1 density matrix with vector and tensor polarization. We calculate the DIS cross section on the deuteron in a factorized approximation suitable for high-energy scattering (virtual nucleon scheme). Deuteron structure is described by the NN light-front wave function, which can be related to the non-relativistic wave function in a well-defined scheme. We evaluate the tagged DIS cross section in the impulse approximation, discuss the analytic properties in the nucleon recoil momentum (free nucleon pole at $t=M_N^2$, “pole extrapolation”), and comment on the effects of final-state interactions (FSI). The pole extrapolation has method has the advantage that nuclear effects such as FSI and Fermi motion are suppressed in a model-independent way at the free nucleon pole. We discuss the application of the method to measured semi-inclusive DIS data (BONuS). The neutron structure functions obtained in this way demonstrate a surprising x dependence at x> 0.6, indicating a possible rise of the neutron to proton structure function ratio. As a second application we consider the extraction of the free neutron spin structure function $g_{1n}$ through DIS on the longitudinally polarized deuteron with proton tagging and on-shell extrapolation in the recoil momentum. Tagged DIS measurements on the polarized deuteron will become feasible at a future Electron-Ion Collider (EIC) with forward proton and neutron detectors.

Speaker: Wim Cosyn

Slides Cosyn_QCDevo_polspin1.pdf

42 Cos (2φ) azimuthal asymmetry in virtual photon-jet production in pA collisions¶

We numerically investigate the cos(2phi) azimuthal asymmetry in virtual photon-jet production in pA collisions, which provides the direct access to the linearly polarized gluon distribution. Our analysis incorporates both the saturation effect and the Sudakov suppression effect. The preliminary numerical result suggests that the asymmetry could be sizeable in the kinematical region accessible at RHIC . It is thus promising to probe the linearly polarized gluon distribution through this channel at RHIC.

Speaker: Yajin Zhou

Slides Yajin_Zhou_cos2phiQCD_.pdf

43 Gluon TMDs in the small-x limit¶

We investigate the gluon transverse momentum dependent (TMD) correlators as Fourier transforms of matrix elements of nonlocal operator combinations. At the operator level these correlators include both field strength operators and gauge links bridging the nonlocality. In contrast to the collinear PDFs, the gauge links are no longer unique for TMD PDFs (TMDs) and also Wilson loops lead to nontrivial effects. Single Wilson loop operators become important when one considers the small-x limit for gluon TMDs. We look at gluon TMDs for unpolarized, vector polarized, and tensor polarized targets. We also propose to standardize the parametrizations for gluon TMD correlators using completely symmetric, traceless tensors in kT , which is useful for applications related to TMD evolution.

Speaker: Tom van Daal

Slides Tom_van_Daal_-_QCD_Evolution_2016.pdf

Friday, 3 June ¶

Talks¶

44 Interpretation of Angular Distributions of Z-boson Production¶

High precision data of dilepton angular distributions in γ∗/Z production were reported recently by the CMS Collaboration covering a broad range of the dilepton transverse momentum, qT , up to _ 300 GeV. Pronounced qT dependencies of the λ and ν parameters, characterizing the cos2 θ and cos 2φ angular distributions, were found. Violation of the Lam-Tung relation was also clearly observed. We show that the qT dependence of λ allows a determination of the relative contributions of the q¯q annihilation versus the qG Compton process. The violation of the Lam-Tung relation is attributed to the presence of a non-zero component of the q − ¯q axis in the direction normal to the “hadron plane” formed by the colliding hadrons. The observed qT dependencies of λ and ν from the CMS and the earlier CDF data can be well described using this approach.

Speaker: Jen-Chieh Peng

Slides QCD_2016_Peng.pdf

45 Drell-Yan lepton angular distributions in perturbative QCD¶

We present a comprehensive comparison of the available experimental data for the Drell-Yan lepton angular coefficients $\lambda$ and $\nu$ to calculations at leading and next-to-leading order of perturbative QCD. Our comparisons reveal that perturbative QCD is able to describe the experimental data overall rather well, especially at colliders, but also in the fixed-target regime.

Speaker: Werner Vogelsang

Slides Vogelsang.pdf

46 Accessing gluon TMDs with quarkonium production¶

I will discuss the opportunities offered by the study of single, double and associated quarkonium production in hadronic collisions in order to access gluon TMDs. I will argue that, at the LHC, with unpolarised nucleons, the distributions of linearly polarised gluons, h_1^perp,g, is already accessible with various data on tape. I will the discuss the case of low pT C=+1 quarkonia which can be studied with LHCb or AFTER@LHC. Finally, I will present the prospects to access the gluon Sivers effect via single transverse spin asymmetries in quarkonium production.

Speaker: Jean-Philippe Lansberg

Slides Lansberg_QCD-Evolution-2016-030616.pdf

10:30 Coffee break

Talks¶

47 Probing TMDs in heavy quarkonium production¶

We investigate the effect of unpolarized transverse momentum dependent gluon parton distributions as well as the effect of linearly polarized gluons on transverse momentum and rapidity distributions of J/ψ and _ production within the framework of transverse momentum dependent factorization employing color evaporation model (CEM) in unpolarized proton-proton collision. We estimate the transverse momentum and rapidity distributions of J/ψ and _ at LHCb, RHIC and AFTER energies using TMD evolution formalism.

Speaker: Asmita Mukherjee

Slides Asmita.pdf

48 Differential cross section for Higgs boson production in 4 lepton channel at LHC and kT factorisation approach¶

We study the inclusive Higgs boson production cross section within the QCD kT factorization approach where the intrinsic transverse momentum effect of gluons is taken in to account. Differential cross section for the process pp ! H ! ZZ∗ ! ¯ll′¯l is estimated considering gluon to be off-shell and compared with the usual QCD results obtained within collinear factorization approach. Unintegrated parton distribution functions are used to give an estimates. The results are compared with the recent experimental data from the ATLAS collaboration. These results are useful in constraining and exploring the unintegrated parton distribution of gluon in the proton.

Speaker: Vaibhav Rawoot

Slides Vaibhav_Rawoot_qcdevolution2016_nikhef.pdf

49 Off-shell amplitudes and 4-jet production in kt-factorization¶

We present a framework for the evaluation of tree-level amplitudes with off-shell legs for kt-factorization, based on the BCFW recursion relation, and discuss its recent application to 4-jet production. We evaluate both single and double parton scattering contributions to the cross section and discuss the differences between the results of collinear and kt-factorization, comparing them to the data reported by the ATLAS and CMS collaboration and concluding that an analysis with asymmetric and soft cuts on transverse momenta would be helpful in order to better pin down the double parton scattering contribution.

Speaker: Mirko Serino

Slides QCDEvolution2016_Serino.pdf

12:30 Lunch

Talks¶

50 Azimuthal asymmetries as the probe of nuclear matter at EIC¶

Nuclear dependence of transverse momentum dependent parton distributions and azimuthal asymmetries in SIDIS off polarized nuclear targets are studied. Multiple gluon interaction, which is responsible for gauge link, contain information of nuclear dependence of parton distributions. We show the suppression of azimuthal asymmetries for eA SIDIS relative to that of eN case, and the suppression factor is related to the jet quenching parameter ˆq. With ˆq extracted from other experiments, we present a numeric estimate of the size of the suppression of azimuthal asymmetries which will be helpful for the future experimental study of SIDIS at EIC.

Speaker: Yu-kun Song

Slides songyk-qcd-evolution-2016.pdf

51 On the high-energy evolution of the jet quenching parameter¶

We present ongoing work concerning the high-energy evolution of the jet quenching parameter (the transport coefficient which characterizes the transverse momentum broadening of an energetic jet propagating through a dense partonic medium, such as the quark-gluon plasma created in the intermediate stages of nucleus-nucleus collisions at the LHC). Within the framework developed in Iancu (2014) [1], which represents a generalization of the JIMWLK evolution equation beyond the eikonal approximation, we calculate the high-energy evolution of the jet quenching parameter in the double logarithmic approximation and thus recover previous results in the literature (notably by Mueller et al (2013) [2]). We furthermore present preliminary results for the respective evolution in a more accurate, single logarithmic, approximation. [1] Iancu, E., JHEP 1410 (2014) 95 [2] Liou, T., Mueller, A.H. and Wu, B., Nucl.Phys. A916 (2013) 102-125

Speaker: Pieter Taels

Slides QCD_Evolution_2016_Pieter_Taels.pdf

52 Heavy flavour corrections to polarised and unpolarised deep-inelastic scattering at 3-loop order¶

We report on progress in the calculation of 3-loop corrections to the deep-inelastic structure functions from massive quarks in the asymptotic region of large momentum transfer Q2. Recently completed results allow us to obtain the O(a3 s) contributions to several heavy flavour Wilson coefficients which enter both polarised and unpolarised structure functions for lepton-nucleon scattering. In particular, we obtain the non-singlet contributions to the unpolarised structure functions F2(x,Q2) and xF3(x,Q2) and the polarised structure function g1(x,Q2). From these results we also obtain the heavy flavour contributions to the Gross-Llewellyn-Smith and the Bjorken sum rules. While the size of the corrections in the polarised and charged current cases are below the current experimental uncertainty, they may become relevant at a future Electron-Ion Collider.

Speaker: Arnd Behring

Slides behring-qcdevo16.pdf

53 Transverse Single Spin Asymmetries in Hard Processes¶

The results of a combined TMD and twist-3 formalism analysis of single spin asymmetries in SIDIS, e+e- annihilation into hadron pairs, and proton-proton scattering are presented.

Speaker: Leonard Gamberg

Slides QCD_EV16_Gamberg-reduce.pdf

Closing¶

54 Closing¶

Speaker: Alexei Prokudin

Slides Prokudin.pdf