Resummation, Evolution, Factorization

8 Dec 2014, 08:00 → 11 Dec 2014, 18:00 Europe/Zurich

Hof Van Liere, Prentenkabinet (University of Antwerp)

Andrea Signori (V), Hannes Jung (Deutsches Elektronen-Synchrotron (DE)), Igor Cherednikov (University of Antwerp)

REF 2014

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

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The first Workshop `Resummation, Evolution, Factorization' to be held on 8 - 11 December 2014 in Antwerp (Belgium) follows up the previous TMD/uPDF Workshop and is aimed to continue, extend and develop the discussion initiated on the latter.

The principal goal of this series of Workshops is to organise 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 the first 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. 

You are welcome to register by 1 Dec 2014 at https://indico.cern.ch/event/330428/registration/register#/register

More information on the scientific program, working schedule and local organisation will follow in the second bulletin. We are looking forward to seeing you in Antwerp on 8 - 11 Dec 2014.

Monday, 8 December

1 Welcome and opening remarks

Opening address and introduction to philosophy of the meeting

Speaker: Igor Cherednikov (University of Antwerp)

Slides

• REF_2014_opening.pdf
• REF_timetable.pdf
• Screen_Shot_2014-12-07_at_15.23.26.png

2 Presentation of the TMDlib project

Speaker: Andrea Signori (VU University Amsterdam - Nikhef)

Slides REF_TMDlib_Signori.pdf

3 Results on TMD evolution

We solve recently proposed TMD evolution equations for unpolarised quark in a particular limit which is suitable for comparison with other results in the literature.

Speaker: Federico Alberto Ceccopieri (Universita & INFN, Milano-Bicocca (IT))

Slides REF14_ceccopieri.pdf

4 Evolution and extraction of TMDs

In this talk I would like to point out the importance of evolution and resummations in the extraction of TMDs. I will make the case of unpolarized TMDPDF and discuss recent results. I will point out how the knowledge of TMDs can affect the interpretation of the LHC outcomes is some relevant cases.

Speaker: Ignazio Scimemi (Universidad Complutense (ES))

Slides Scimemi_Antwerp2014.pdf

5 Introduction to QCD evolution of parton distributions: Comparative view

Speaker: Igor Cherednikov (University of Antwerp)

Slides cherednikov_REF_2014.pdf

6 Overview of TMD evolution

Speaker: Daniel Boer (University of Groningen)

Slides Boer_Antwerp-Dec-2014-2.pdf

7 Effect of TMD evolution and partonic flavor on e+e- annihilation into hadrons

We calculate the transverse momentum dependence in the production of two back-to-back hadrons in electron-positron annihilations at 100 GeV^2. We use the parameters of the transverse-momentum-dependent (TMD) fragmentation functions that we recently extracted from the HERMES multiplicity data at 2.4 GeV^2. We apply TMD evolution according to two different approaches and using different parameters for the socalled nonperturbative part of TMD evolution. We explore the sensitivity of our results to these different choices and to the flavor dependence of parton fragmentation functions. We discuss how experimental measurements could discriminate among various scenarios.

Speaker: Marco Radici (urn:Google)

Slides radici.pdf

8 Discussion

Tuesday, 9 December

9 Collinear QCD, TMD resummation and non-perturbative aspects in SIDIS processes

TMD resummation for SIDIS processes is studied in the framework of the original Collins-Soper-Sterman formalism, with special attention to the interplay between perturbative QCD and non-perturbative contributions. Phenomenological implementations of the TMD formalism to SIDIS processes will be discussed with practical examples, exploring different kinematical configurations of SIDIS experiments.

Speaker: Mariaelena Boglione (University of Turin)

Slides elena_REF2014.pdf

10 Non-linear evolution equations

Speaker: Ian Balitsky (ODU/JLab)

Slides Balitsky_REF_2014.pdf

11 Production of forward jets within high-energy factorization

We propose a method to introduce Sudakov effects to unintegrated gluon density promoting it to be hard scale dependent. The advantage of the approach is that it guarantees that the gluon density is positive definite and that on integrated level the Sudakov effects cancel. Besides that the method to introduce the Sudakov effects is convenient since it does not need evaluation of cross section in the process of imposing the effects. As a case study we apply the method to calculate angular correlations in production of forward-forward dijet and RpA ratio for p+p vs. p+Pb collision.

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

Slides kutak_jets.pdf

12 Discussion

13 Applications of the high-energy QCD effective action

We introduce Lipatov's effective action, which displays high-energy factorization in a gauge invariant way through the inclusion of reggeon fields, and show several computational applications of this approach, including the gluon Regge trajectory at two loops and the NLL effective jet vertices for semi-inclusive forward jet production and jet-gap-jet configurations.

Speaker: José Daniel Madrigal Martínez

Slides Applications_Lipatov.pdf

14 High-energy resummation effects in Mueller-Navelet jet production at the LHC

The study of the production of two forward jets with a large interval of rapidity at hadron colliders was proposed by Mueller and Navelet as a possible test of the high energy dynamics of QCD. We analyze this process within a complete next-to-leading logarithm framework, supplemented by the use of the Brodsky-Lepage-Mackenzie procedure extended to the perturbative Regge dynamics, to find the optimal renormalization scale. This leads to a very good description of the recent CMS data at LHC for the azimuthal correlations of the jets.

Speaker: Samuel Wallon

Slides Wallon-Anvers.pdf

15 Impact factor for quark-antiquark-gluon jet production in diffractive DIS

We present the calculation of the impact factor for the photon to quark, antiquark and gluon transition within Balitsky's high energy OPE. We also rederive the impact factor for photon to quark and antiquark transition within the same framework. These results provide the necessary building blocks for further phenomenological studies of inclusive diffractive DIS as well as for two and three jets diffractive production which go beyond approximations discussed in the litterature.

Speaker: Renaud Boussarie

Slides REF2014Boussarie.pdf

16 Discussion

17 Workshop Dinner

Slides Da_Giovanni.png

Wednesday, 10 December

18 Overview on transverse momentum resummation

We consider the transverse-momentum distribution of generic high-mass systems of non-strongly interacting particles (lepton pairs, vector bosons, Higgs particles, ...) produced in hadronic collisions. The logarithmically-enhanced contributions at small transverse momentum are treated to all perturbative orders by a universal resummation formula that depends on a single process-dependent hard factor. The formalism is applied to Drell-Yan lepton pairs and Higgs boson production at Tevatron and LHC energies. We combine the most advanced perturbative information available at present for these processes: resummation up to next-to-next-to-leading logarithmic accuracy and fixed-order perturbation theory up to next-to-next-to-leading order. We show and discuss the reduction in the scale dependence of the results with respect to lower-order calculations, estimating the corresponding perturbative uncertainty.

Speaker: Giancarlo Ferrera (University of Milan)

Slides ferrera-ref.pdf

19 Soft gluons and the ordering problem

Recent developments (JHEP07 (2012) 026) have shown that is not always possible to factorize all collinear singularities into process independent and universal functions. This breakdown of collinear factorization was anticipated using an algorithm to compute the leading soft gluon corrections to a hard process (JHEP08(2006)059). Such algorithm is base on the assumption that the successive emissions can be ordered in transverse momentum. In this work we show that this assumption is correct at the first two non-trivial orders. We do this by studying the leading behavior of the soft corrections to a hard process due to one virtual exchange and one and two real emissions.

Speaker: Rene angeles martinez (urn:Google)

Slides REF14_RAM.pdf

20 Discussion

21 Gluon-gluon to Higgs in TMD and kT-factorization: SCET approach

Speaker: Miguel Echevarría (VU/Nikhef)

Slides Echevarria_REF2014.pdf

22 Gluon-gluon to Higgs in TMD and kT-factorization: Small-x framework

Speaker: Francesco Hautmann (Institute of Theoretical Physics)

23 Transverse momentum gluon density at low-x

We present new results on the unintegrated TMD (transverse momentum dependent) gluon density (u.g.d.) at low $x$, which based on our previous study [1]. We match this u.g.d. at low transverse momenta $\mid k_T\mid$ and starting scale $Q_0^2 = 1-3$~GeV$^2$ to the exact solution of the BFKL equation outside of the saturation region at large $\mid k_T\mid$ obtained in [2], which includes all multiple Pomeron exchanges. Then, to extend this u.g.d at higher $Q^2$ we use the Catani-Ciafoloni-Fiorani-Marchesini (CCFM) evolution equation. The inclusion of the CCFM evolution results in a large increase of the u.g.d. magnitude at low $x$ and large $\mid k_T\mid$ above a few GeV$/$c. The application of the obtained gluon distribution to the analysis of the $ep$ deep inelastic scattering allows us to get the results, which describe reasonably well the H1 and ZEUS data on the longitudinal proton structure function $F_L(x,Q^2)$, $F_{2c}(x,Q^2$ and $F_{2b}(x,Q^2$. In addition to this the use of new u.g.d. allows us to describe satisfactorily the LHC data on heavy meson production and especially the correlation between two B-mesons produced in $pp$ collisions. The comparison of our new TMD unintegrated gluon density to the another ones is presented. We present new results on the unintegrated TMD (transverse momentum dependent) gluon density (u.g.d.) at low $x$, which based on our previous study [1]. We match this u.g.d. at low transverse momenta $\mid k_T\mid$ and starting scale $Q_0^2 = 1-3$~GeV$^2$ to the exact solution of the BFKL equation outside of the saturation region at large $\mid k_T\mid$ obtained in [2], which includes all multiple Pomeron exchanges. Then, to extend this u.g.d at higher $Q^2$ we use the Catani-Ciafoloni-Fiorani-Marchesini (CCFM) evolution equation. The inclusion of the CCFM evolution results in a large increase of the u.g.d. magnitude at low $x$ and large $\mid k_T\mid$ above a few GeV$/$c. The application of the obtained gluon distribution to the analysis of the $ep$ deep inelastic scattering allows us to get the results, which describe reasonably well the H1 and ZEUS data on the longitudinal proton structure function $F_L(x,Q^2)$, $F_{2c}(x,Q^2$ and $F_{2b}(x,Q^2$. In addition to this the use of new u.g.d. allows us to describe satisfactorily the LHC data on heavy meson production and especially the correlation between two B-mesons produced in $pp$ collisions. The comparison of our new TMD unintegrated gluon density to the another ones is presented. References [1] A.V.Lipatov, G.I.Lykasov, N.P.Zotov, Phys.Rev. D89 (2014) 014001; arXiv:1310.7893. [2] Yuri V. Kovchegov, Phys.Rev. D 61 (2000) 074018.

Speaker: Gennady Lykasov (Joint Inst. for Nuclear Research (RU))

Slides LYKASOV_REF_2014.pdf

24 Discussion

Thursday, 11 December

25 Impact of gluon polarization on Higgs production at the LHC

Linearly polarized gluons inside an unpolarized proton contribute to the transverse momentum distributions of (pseudo)scalar particles produced inclusively in hadronic collisions, such as Higgs bosons and quarkonia with even charge conjugation ($\eta_c$, $\eta_b$, $\chi_{c0}$, $\chi_{b0}$). Moreover, they can produce azimuthal asymmetries in the associated production of a Higgs boson and a jet, in a kinematic region in which they are almost back to back, and modify the transverse spectrum of the pair. We show how these effects could be measured in the running experiments at the LHC.

Speaker: Cristian Pisano

Slides cristian_pisano.pdf

26 Gluon TMDs and quarkonium production in (un)polarized proton-proton collisions

In this talk, I discuss how the study of quarkonium production in unpolarised and polarised proton-proton collisions can provide important insights on the gluon TMDs. The study of back-to-back production of quarkonium + isolated photon provides a unique way to extract the gluon TMDs f1^g and h1^perp,g at the LHC [1] and at a proposed Fixed-Target ExpeRiment at the LHC (AFTER@LHC). Although with smaller rates [2], Psi+Z and Upsilon+Z also offer interesting prospects at LHC energies. In addition, Quarkonium + isolated photon can also be used to extract the gluon Sivers function via Transverse Single Spin Asymmetries (TSSA), complementing the possible study of low-pT C-even quarkonium TSSA. Finally, I will comment on the additional information which can be obtained through J/psi-pair production in both unpolarised and polarised proton-proton collisions, extending our previous work [3].

Speaker: Jean-Philippe Lansberg (IPN Orsay, Paris Sud U. / IN2P3-CNRS)

Slides REF-2014-111214.pdf

27 Higgs and Drell-Yan production in high luminosity proton-proton collisions

In the forthcoming high luminosity phase of the LHC many of the most interesting measurements for precision QCD studies are affected by large pile-up conditions. However, with the recently discovered Higgs boson, which couples in the heavy top limit directly to gluons, we have access to a novel production process to probe QCD by a colour-singlet current. In this study we compare observables in Higgs boson and Drell-Yan production and evaluate their stability with respect to pile-up effects. We present first attempts to apply various pile-up correction methods both to underlying event and to boson + jet event topologies, and compare Pythia8, Powheg and CASCADE event generator predictions to demonstrate the sensitivity to different multiple-parton radiation mechanisms.

Speaker: Hans Van Haevermaet (University of Antwerp (BE))

Slides DYHiggs_Antwerp_111214_v2.pdf

28 Generating function for web diagrams

We present the description of the exponentiated diagrams in terms of generating function within the universal diagrammatic technique. In particular, we show the exponentiation of the gauge theory amplitudes involving products of an arbitrary number of Wilson lines of arbitrary shapes, which generalizes the concept of web diagrams. The presented method gives a new viewpoint on the web diagrams and proves the non-Abelian exponentiation theorem.

Speaker: Alexey Vladimirov (urn:Google)

Slides Vladimirov_REF_2014.pdf

29 Discussion

30 Concluding remarks and future plans