MPI @ LHC 2013, Workshop on Multi-Parton Interactions at the LHC

2 Dec 2013, 08:00 → 6 Dec 2013, 18:00 Europe/Zurich

Nick Van Remortel (University of Antwerp), Pierre Van Mechelen (University of Antwerp (BE))

   

Welcome to the fifth International Workshop on Multiple Partonic Interactions at the LHC

The aim of this workshop on Multiple Partonic Interactions (MPI) at the LHC is to raise the profile of MPI studies, summarising the legacy from the phenomenology at previous hadron colliders, reviewing the early results from the LHC and fostering contact between the theoretical and experimental communities.

MPI are experiencing a growing popularity and are widely invoked to account for observations that cannot be explained otherwise: the activity of the Underlying Event, the rates for multiple heavy flavour production, the survival probability of large rapidity gaps in hard diffraction, etc. At the same time, the implementation of MPI effects in Monte Carlo generators is quickly proceeding at an increasing level of sophistication and complexity, which can have far reaching implications for LHC physics. The ultimate ambition of this workshop is to promote MPI as a unifying concept between apparently distinct lines of research, to profit from experimental progress in order to constrain their implementation in models, and to evaluate their impact on the LHC physics program.

Topics

• Phenomenology of MPI processes and multiparton distributions
• Considerations for the description of MPI in QCD
• Measuring multiple partonic interactions
• Experimental results on inelastic hadronic collisions: underlying event, minimum bias, forward energy flow
• Monte Carlo development and tuning
• Connections with diffraction, heavy-ion physics and cosmic rays
   

Previous meetings

MPI@LHC'12 CERN Geneva, Switserland
MPI@LHC'11 DESY Hamburg, Germany
MPI@LHC'10 Glasgow, Scotland
MPI@LHC'08 Perugia, Italy

Monday, 2 December

08:00 Registration

Opening

Conveners: Prof. Nick Van Remortel (University of Antwerp), Pierre Van Mechelen (University of Antwerp (BE))

slides MPI_intro.pdf

MPI & Monte Carlo: (chair: Albert Knutsson)

slides Info_on_photo_and_Excursion.pdf MPI_WG.pdf

1 Overview of MC models of MPI

Speaker: Mike Seymour (University of Manchester (GB))

Slides MPI2013-Seymour.pdf MPI2013-Seymour.pptx

2 MinBias in SHERPA

SHRiMPS, the new minimum bias model in SHERPA, is based on the successful Khoze-Martin-Ryskin model. The latter is a multi-channel eikonal model that by summing all multi-pomeron diagrams is capable of describing elastic and inelastic scattering, low mass and high mass diffractive dissociation and central exclusive production. The Monte Carlo realisation relies on the partonic interpretation of the model and is a considerable extension of the original KMR model. I will review the status of SHRiMPS and future plans.

Speaker: Korinna Christine Zapp (CERN)

Slides antwerpen_1312.pdf

3 DIPSY - from protons to heavy ions

We describe the status of the DIPSY program with emphasis on its application to heavy-ion collisions. The main advantage of the DIPSY model for heavy ions, is the inclusion of initial state correlations and fluctuations. We present preliminary results for pA collisions where small effects of fluctuations can be found in inclusive cross sections.

Speaker: Leif Lönnblad (Lund University (SE))

Slides MPI13.pdf

4 Constraining MPI models using sigma_effective and recent Tevatron and LHC Underlying Event data

We review the modelling of multiple interactions in the event generator Herwig++ and study implications of recent tuning efforts to Tevatron and LHC data. It is often said that measurements of the effective cross section for double-parton scattering, sigma_effective, are in contradiction with models of the final state of multi-parton interactions, but we show that the Herwig++ model is consistent with both and gives stable predictions for underlying event observables at 14 TeV.

Speaker: Andrzej Konrad Siodmok (University of Manchester (GB))

Slides siodmok_antwerp.pdf

10:55 Coffee

MPI & Monte Carlo: (chair: Andrzej Siodmok)

slides Info_on_photo_and_Excursion.pdf MPI_WG.pdf

5 Tevatron Energy Scan: Findings & Surprises

At CDF we study min-bias collisions (MB) and the “underlying event” (UE) using charged particles produced in proton-antiproton collisions at 300 GeV, 900 GeV, and 1.96 TeV. The 300 GeV and 900 GeV data are a result of the “Tevatron Energy Scan” which was performed just before the Tevatron was shut down. We use the direction of the leading charged particle in each event, PTmax, to define three regions of eta-phi space; “toward”, “away”, and “transverse”. The “transverse” region is further divided into the “transMAX” and “transMIN” contributions. The “transMIN” region is very sensitive to the MPI & BBR components of the UE, while the “transDIF” region (“transMAX” minus “transMIN”) is more sensitive to the ISR & FSR. The data are corrected to the particle level and are compared with LHC data at 900 GeV and 7 TeV. This CDF analysis together with LHC data provides a detailed information about the energy dependence of the various components of the UE.

Speaker: Richard Dryden Field (University of Florida (US))

Slides MPI@LHC13_RickField_12-2-13.pdf MPI@LHC13_RickField_12-2-13.ppt

6 Measurement of the UE activity in pp collisions with the CMS and ATLAS detectors

A measurement of the underlying event activity is performed on proton-proton collisions, for different centre-of-mass energies, using the CMS detector. The charged particles in the azimuthal region transverse to the leading track with transverse momentum larger than 0.5 GeV/c and at central pseudorapidities are studied. The measurements are compared with various theoretical predictions. A significant increase in the average multiplicity and the scalar transverse momentum sum in the transverse region is observed up to transverse momenta of few GeV/c followed by a slower rate of increase. Both quantities are observed to increase at higher centre-of-mass energies.

Speaker: Mr Mohammed Zakaria (University of Florida (US))

Slides Untitled_1.pdf

An overview of the UE analyses done at CMS and ATLAS in the past 2 years is presented. Emphasis has been given to the physics and MC feedback involved. 

7 Jet and underlying event properties as a function of particle multiplicity in proton-proton collisions at 7 TeV

Characteristics of multiparticle production in proton-proton collisions at a centre-of-mass energy of 7 TeV are presented as a function of the charged-particle multiplicity. The produced particles are separated into those belonging to jets and those belonging to the underlying event (UE). Results are compared to the predictions of the PYTHIA and HERWIG Monte Carlo event generators. For increasing charge particle multiplicities, PYTHIA systematically predicts higher jet rates and harder transverse momenta spectra than seen in the data, whereas HERWIG shows the opposite trends. Predictions without multiparton interactions fail completely to describe the dependence observed in the data. In the lowest-multiplicity events, the data show narrower jets than predicted by both Monte Carlo event generators.

Speaker: Maxim Azarkin (Russian Academy of Sciences (RU))

Slides Jet_and_UEPropertiesMPI13.pdf

8 Using substructure techniques to probe Multiple Parton Interactions

Angular Structure Function (ASF) has been proposed as a way to study MPI contribution in large radius jets (arXiv:1201.2688v2 [hep-ph]). We extend the study to include more topologies and Monte Carlo models, and look at the traditional transverse regions used for probing the underlying event. The ASF gives a reasonable discrimination between MPI and hard scattered jets.

Speaker: Deepak Kar (University of Glasgow (GB))

Slides dkar_mpilhc13.pdf

13:00 Lunch

Ten Prinsenhove

MPI & Small-x: (chair: Krzysztof Kutak)

slides Workshop_Dinner_info.pdf

9 Introduction to small x and MPI physics

The talk is intended to be an introduction to the low x session. Overview of the problems of low x and MPI

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

Slides mpiintro4.pdf

10 Measurements probing small-x physics using the ATLAS detector

The high-energy pp collisions at the LHC provide unique opportunity to study particle flow and event shapes of the hadronic final state particles. Evolution of the event shape variables, such as the transverse thrust, thrust minor and transverse sphericity have been studied for minimum bias events. Particle distributions sensitive to the underlying event have been measured using calorimeter jet events. Separate inclusive jet and exclusive dijet event selections are included in this study for various observables. The production of jets in association with a vector boson is an important process to study the contribution of multi-particle interactions and for understanding the effect of QCD radiation on forward and central jet activity. ATLAS results on events with vector boson plus jets are compared with predictions from Monte Carlo simulations.

Speaker: Alexei Myagkov (Institute for High Energy Physics (RU))

Slides ATL-COM-PHYS-2013-1574-v3.pdf

11 Measurements probing small-x physics using the CMS detector

We present recent results sensitive to small-x physics, using data collected by the CMS detector at the LHC. Measurements of the azimuthal decorrelations of Mueller-Navalet dijets at 7 TeV are discussed, and compared to BFKL and DGLAP-based predictions, including the effects of angular ordering and multiparton interactions. Recent studies of low-pT and forward jets at 8TeV, and of the hadronic activity in electroweak Z+jets production at 7 and 8 TeV, are also discussed.

Speaker: Ivan Pozdnyakov (ITEP Institute for Theoretical and Experimental Physics (RU))

Slides small_x_cms_pozdnyakov_antwerp_v4.pdf

12 Forward jets and saturation within high energy factorization

We present three jet cross sections for proton-proton and proton-lead collisions in a situation where one or all of the jets are in the forward region. The calculation is performed using gauge invariant tree-level off-shell matrix elements and unintegrated gluon density with dynamical nonlinear effects allowing for gluon saturation. Since we deal with asymmetric collisions in the fractions of the hadrons momenta we use so called hybrid high-energy factorization, i.e. there is a single off-shell gluon in the partonic hard subprocess. We observe certain effects due to saturation e.g. in the azimuthal decorrelations.

Speaker: Piotr Kotko

Slides MPI@LHC_2013_2.pdf

13 Forward jet production and BFKL effects at the LHC

Phenomenological aspects of search for BFKL effects at LHC energies are briefly reviewed. Different observables for forward dijets with large rapidity separation are discussed. In particular, azimuthal angle decorrelations of Mueller-Navelet dijets and ratios for inclusive dijet and Mueller-Navelet dijet cross sections to two-jet cross section are considered.

Speaker: Prof. Victor Kim (St. Petersburg Nuclear Physics Institute - PNPI, Gatchina)

Slides kim_MPI@LHC2013.pdf

16:05 Coffee

MPI & Small-x: (chair: Victor Kim)

slides Workshop_Dinner_info.pdf

14 Signal of high-energy resummation effects in Mueller-Navelet jets 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 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: Mr Bertrand Ducloue

Slides MPI-2013_Ducloue.pdf

15 Multiple Partonic Interactions in Production of Photons, Mesons, and Jets in Proton–Proton Collisions at the LHC

Production of real and virtual photons in coincidence with jets, J/psi - mesons and jets, and phi-meson pairs is studied at the LHC energies. Calculations for prompt-photon and lepton-pair production are performed at NLO accuracy with the help of aMC@NLO and MadGraph. Processes of mesons production in proton-proton scattering are simulated using Pythia 8. Computed cross sections are compared with results of the experiments at the LHC. Influence of the multiparton interactions (MPI) on the observables, obtained both in the central and forward regions, is analyzed. MPI effects are taken into account in the framework of HERWIG and Pythia event generators. Manifestation of MPI in the reactions is also considered in calculation at LO with Sherpa. Sensitivity of the cross sections on parton distribution functions (PDFs), in particular gluon PDFs, is discussed. Computations for processes that result in creation of 2 gammas + 2 jets are in progress.

Speaker: Volodymyr Kotlyar (Nat. Acad. of Sciences of Ukraine (UA))

Slides Kotlyar.MPIatLHC.2013.pdf Kotlyar.MPIatLHC.2013.ppt

16 Onia production in pA and Ap collisions at LHCb

With a unique forward acceptance, and excellent vertexing and particle identification capability, LHCb is well-placed to make important contributions to heavy ion physics through measurements performed using data taken during pA and Ap collisions. The first studies of these data, on the production of onia vector mesons will be presented. The results include comparisons of the production cross-sections between pA, Ap and pp collisions, and measurements of the nuclear modification factors.

Speaker: Dr Burkhard Schmidt (CERN)

Slides MPI@LHC2013-Onia_in_pA_final.pdf

17 Leading track and leading jet cross sections at small transverse momenta

The production yields of leading charged-particle jets and tracks in proton-proton collisions are measured at sqrt(s) = 8 TeV based on a data sample of 17.4 pb−1 collected with the CMS detector. The charged-particle jets (tracks) are measured in the pseudorapidity range |eta| < 2.0 (2.4) for transverse momenta pT > 1 (0.8) GeV. The yield distribution integrated over a given minimum transverse momentum pTmin, falls steeply with increasing pTmin, and provides information on the mechanism by which the underlying parton-parton cross sections are unitarized approaching the low-pT non-perturbative domain. Predictions obtained from various Monte Carlo event generators are compared to the measurements.

Speaker: Albert Knutsson (University of Antwerp (BE))

Slides knutsson_mpiatlhc_leadtrack.pdf

18:30 Welcome reception

Tuesday, 3 December

MPI & Double Parton Scattering: (chair: Paolo Bartalini)

18 Theory of double parton scattering: progress an open questions

I give a brief introduction to the theory of double hard scattering and then discuss recent theory developments and open questions.

Speaker: Markus Diehl (DESY)

Slides diehl_mpi.pdf

19 Measurement of double-parton interactions in W+2 jets events with the CMS detector

Double parton scattering (DPS) is studied in proton-proton collisions at a centre-of-mass energy of 7 TeV, using events with a W boson decaying into a muon and neutrino and the presence of two associated jets in the final state. The data sample corresponds to an integrated luminosity of 5 /fb, collected by the CMS detector at the Large Hadron Collider. Observables sensitive to double parton scattering are shown, corrected for detector effects and selection efficiencies. The fraction of double parton scattering and the effective cross section are also measured and compared to other DPS measurements.

Speaker: Dr Sunil Bansal (University of Antwerp (BE))

Slides dpsW2Jet_CMS_Sunil

20 First measurement of associated vector boson plus prompt J/psi production at the ATLAS experiment

We present the discovery of associated W+prompt J/psi production and subsequent measurement of the production rate, which is a key observable to furthering understanding of quarkonium production mechanisms. We estimate the relative contributions to the signal from single and double parton scattering and discuss possible implications of this novel final state for study of multiple parton interactions in the future. The single parton scattering cross-sections are compared to cutting-edge theoretical calculations in the colour singlet and colour octet formalisms. Future possibilities for measurements with Z+J/psi and Di-J/psi are discussed.

Speaker: Constantinos Melachrinos (University of Chicago (US))

Slides MelachrinosMPI.pdf

10:50 Coffee

MPI & Double Parton Scattering: (chair: Wouter Waalewijn)

21 Production of two $c \bar c$ pairs and two mesons with charm in double-parton scattering: inclusive and correlation observables.

We discuss production of two pairs of $c \bar c$ in proton-proton collisions at the LHC. Both double-parton scattering (DPS) and single-parton scattering (SPS) contributions are included in the analysis. Each step of DPS is calculated within $k_t$-factorization approach, i.e. effectively including next-to-leading order corrections. The conditions how to identify the DPS contribution are presented. The discussed mechanism unavoidably leads to the production of pairs of mesons: $D_i D_j$ (each containing $c$ quarks) or $\bar D_i \bar D_j$ (each containing $\bar c$ antiquarks). We calculate corresponding production rates for different combinations of charmed mesons as well as some differential distribution for $(D^0 D^0$ + $\bar D^0 \bar D^0)$ production. Within large theoretical uncertainties the predicted DPS cross section is fairly similar to the cross section measured recently by the LHCb collaboration. The best description is obtained with the Kimber-Martin-Ryskin (KMR) unintegrated gluon distribution, which very well simulates higher-order corrections. The contribution of SPS turned out to be rather small. Finally, we emphasize significant contribution of DPS mechanism to inclusive charmed meson spectra measured recently by ALICE, ATLAS and LHCb. We discuss also production of two $J/\Psi$ mesons.

Speaker: Antoni Szczurek (Institute of Nuclear Physics)

Slides szczurek.pdf

22 Open charm and J/ψ production as a function of the charged particle multiplicity density in pp collisions at √s = 7 TeV with ALICE at the LHC

The study of heavy-flavour hadron production as a function of the charged particle multiplicity in pp collisions helps one to understand the interplay between hard and soft QCD processes, in particular it allows one to study the role of Multi-Parton Interactions (MPI). At the LHC energies the MPI might affect not only processes involving light quarks and gluons, but also the hard momentum scales relevant for open heavy-flavour and J/ψ production. This could induce a correlation between the yield of heavy-flavour hadrons and the total charged particle multiplicity. In ALICE, the production of prompt D mesons, reconstructed at central rapidity (|y| < 0.9) via hadronic decay channels, and inclusive J/ψ, measured at both central and forward (2.5 < y < 4) rapidity via di-electron and di-muon decay channels respectively, has been studied in pp collisions at √s = 7 TeV as a function of the charged particle multiplicity density. Furthermore, the fraction of J/ψ coming from beauty hadron decays has been measured at central rapidity, providing the multiplicity dependence of beauty hadron production. In this talk results on open charm and J/ψ multiplicity dependence in pp collisions at √s = 7 TeV will be discussed. Predictions from selected Pythia tunes will also be shown.

Speaker: Fiorella Fionda (Universita e INFN (IT))

Slides MPISlides.pdf

23 Extracting sigma_effective from the LHCb double-charm measurement

We discuss various issues related to the definition of single and double open charm cross sections. We conclude that LHCb's extraction of sigma_effective, the effective cross section for double-parton scattering, is too large by a factor of two. This correction brings the data from open-charm pairs closer to that from J/psi plus open charm and jet production.

Speaker: Andrzej Konrad Siodmok (University of Manchester (GB))

Slides Siodmok_LHCb.pdf

24 Production of vector bosons in association with heavy flavor jets at the LHC

The production of vector bosons in association with jets from heavy flavor quarks is possibly sensitive to contributions from MPI. This presentation summarizes the status of studies of the production of W+bb and Z+bb at the LHC.

Speaker: Tristan Arnoldus Du Pree (Universite Catholique de Louvain (BE))

Slides 20131203_VHF_MPILHC_TristanduPree.pdf

13:00 Lunch

MPI & Double Parton Scattering

25 DPS in p-A and A-A collisions

The DPS cross sections in p-A and A-A collisions are compared to those in p-p collisions. Such DPS formalism is applied to two different final-states at CERN LHC energies: (i) same-sign W-boson pair production in p-Pb, and (ii) double-J/psi production in Pb-Pb.

Speakers: Dr Alexander Snigirev (SINP MSU), Prof. David d'Enterria (CERN)

Slides mpi13-snig.pdf

26 DPS in High-Energy p-A Collisions and Partonic Correlations

The joint study of Double Parton Scatterings, in high energy proton-proton and proton-nucleus collisions, can provide a lot of information on multi-parton correlations. The multi-parton structure is in fact probed in a different way by DPS, in p-p and in p-A collisions. In p-A collisions the interpretation of the experimental results may be however complicated by the presence of interference terms, which are missing in p-p. A suitable reaction channel, where interference terms are absent, is WJJ production. By studying WJJ production in p-Pb collisions, we estimate that the fraction of events due to DPS may be larger by a factor 3 or 4, as compared to p-p, while the amount of the increased fraction can give a direct indication on the importance of different correlation terms.

Speaker: Daniele Treleani (University of Trieste)

Slides MPI_2013.pdf

27 Factorization of double Drell-Yan at low transverse momentum

Multi-Parton Interactions become ever more important with increasing hadron collider energies and are highly relevant for the LHC. Therefore, they have to be understood based on first principles within QCD. Unfortunately this is much easier said than done as it calls for a substantial extension of present day collinear perturbative QCD techniques. The most crucial and perhaps most difficult step is the proof of factorization for double parton distributions. As one step in this direction we study factorization for one specific process (double Drell-Yan) at one-loop level and with hypothetical spinless quarks.

Speaker: Daniel Ostermeier (University of Regensburg)

Slides MPI13_Ostermeier.pdf

15:30 Coffee

MPI & Diffraction

28 Overivew of results on diffraction from the ATLAS Experiment

Speaker: Hardeep Singh Bansil (University of Birmingham (GB))

Slides ATLAS-Diffraction-MPI.pdf

29 Recent results on diffraction from CMS

We present recent CMS results on diffraction in pp collisions at 7 TeV. Measurements of soft diffractive cross sections, as well as hard diffractive production of dijets and vector bosons are discussed. The results are compared to predictions of various Monte Carlo models.

Speaker: Benoit Roland (University of Antwerp (BE))

Slides mpilhc-2013-roland.pdf

30 TOTEM Physics Results

Speaker: Dr Mario Deile (CERN)

Slides MPI_Antwerpen2013.pdf MPI_Antwerpen2013.ppt

31 ALICE Results on Ultra-Peripheral and Diffractive Production

Speaker: Orlando Villalobos Baillie (University of Birmingham (GB))

Slides Villalobos-ALICE-UPC-Diffractive-Tuesday.V3.pptx

32 Exclusive dimuon production with LHCb

Exclusive vector meson production is studied in the dimuon channel using the LHCb detector. The production cross-sections are measured in the kinematic region corresponding to the LHCb acceptance. The J/ψ photoproduction cross-section is reported as a function of the photon-proton centre-of-mass energy, and compared to previous measurements and theoretical predictions.

Speaker: Victor Coco (CERN)

Slides MPI_Coco.pdf

Wednesday, 4 December

MPI & Monte Carlo

slides Info_on_photo_and_Excursion.pdf MPI_WG.pdf

33 The fused string model for Hadronisation

The Lund string model for hadronisation provides an intuitive and strong physical picture of the hadronisation process by realising a linear confinement potential as a flux tube stretched between colour-singlet pairs. This picture was developed in the context of the very clean LEP environment, but faces some difficulties describing the hadro-chemistry in the busy environment of LHC, where flux tubes exhibits a spatial overlap. The fused string model allows such overlapping flux tubes to fuse with one another, and thus provides corrections to ordinary string hadronisation in busy events. The effect of allowing string fusion greatly affects the parameters controlling strange particle production and baryon to meson ratios in pp collisions, and thus provides a scheme for improving predictions of such observables.

Speaker: Mr Christian Bierlich (Lund University (SE))

Slides christian_bierlich.pdf

34 Color reconnection and flow-like patterns in pp collisions

Recently we have been studying the predictions of Pythia 8 with respect to several observables: the ratios of identified particles in the 2-6 GeV/c range the production of jets in Pythia in function of multiplicity and transverse sphericity and momentum correlations in the φ-η plane in function of multiplicity and transverse sphericity. In some cases the results of varying the number of MPI will be shown. The results point to the fact that the use of event structure type variables may be very important in the understanding of the mechanisms of pp interactions.

Speaker: Guy Paic (Universidad Nacional Autonoma (MX))

Slides antwerpen.pdf

35 PHOJET - status and recent developments

PHOJET is a well established multi-purpose Monte-Carlo generator, focused towards the simulation of minimum bias, diffractive and forward phenomena. It is relying on the ideas of Regge-Theory and the topological expansion of QCD. Multi parton interactions evolve naturally from the Eikonal structure of scattering amplitudes. Although the program is still widely applied in high-energy physics and various technical contexts, it has not received updates or tuning since 2001. Latest measurements at high energies show that simulated multiplicity distributions do not properly describe experimental data and that adjustments to the model parameters have become necessary. In this talk, we will present the recent development status. We hope to shed light on the limitations of the involved Eikonal picture and we will present ideas for the upcoming version.

Speaker: Anatoli Fedynitch (KIT - Karlsruhe Institute of Technology (DE))

Slides PHOJET-MPI-Antwerpen-2013.pdf

36 Mini-Jet Absorption and its Consequences in Color String Models

It is well known that jets or mini-jets in heavy ion and to a degree in dense proton reactions are suppressed as a consequence of re-scattering. Here we consider indirect consequences of such re-scattering. It is done in a toy model with a fixed survival length and a uniform emission density, adjusted to fit observed suppression of jets. In transverse direction the simple picture provides a novel component of a central aspect of azimuthal correlations. The angular flow v_{2} is obtained from the directional dependence of geometrically relevant surface size without added dynamical assumptions. In longitudinal direction the rather long range ridge structure of particles accompanying jets reflects the string structure. It relies on a usually assumed tight relation between space and momentum coordinates within one string and considers the spatial configuration of a hard scattered parton reaching a distant string. The resulting offset between the initial hard scattering and the re-scattering introduces a rapidity shift of the right magnitude for the debris visible as ridge.

Speaker: Prof. Fritz, W. Bopp (University of Siegen)

Slides fixedInteractionLength_v8.pdf

37 Markov Chain Monte Carlo solution of BK equation in integral form

I would like to present a Monte Carlo based method that solves the non-linear Balitsky–Kovchegov (BK) equation in the integral form. The approach is based on reformulation of the BK equation as a system of linear integral equations through the Newton–Kantorovich transformation. The system is then solved through a Markov Chain Monte Carlo (MCMC) algorithm. Our results agree at the level of 0.1% with the solution obtained with the publicly available program BKSolver. Our approach can be generalized to more dimensions therefore offers possibility to solve large class of non-linear integral equations via Monte Carlo method. Furthermore it opens the possibility to construct a MC generator for the BK equation. I will discuss efficiency of our method and possible applications.

Speaker: Krzysztof Bozek (K)

Slides MCMC_Bozek.pdf

11:00 Coffee

MPI & Monte Carlo

slides Info_on_photo_and_Excursion.pdf MPI_WG.pdf

38 HERAPDF1.5LO PDF Set with Experimental Uncertainties

This note presents the HERAPDF1.5 PDF set evolved to leading order (LO) s using DGLAP evolution equations. This LO PDF is particularly useful for Monte Carlo event generators, based on LO matrix elements plus parton showers. The PDF fit is based on DIS data alone, using the H1 and ZEUS experiments at HERA.

Speaker: amanda sarkar

Slides amanda_herapdf15lo_2013.pdf

39 Measurement of Charged Particle Spectra in Deep-Inelastic ep Scattering at HERA

Charged particle production in deep-inelastic ep scattering is measured with the H1 detector at HERA. The kinematic range of the analysis covers low photon virtualities, 5 < Q^2 < 100 GeV^2, and small values of Bjorken-x, 10^{-4} < x < 10^{-2}. The analysis is performed in the hadronic centre-of-mass system. The charged particle densities are measured as a function of pseudorapidity (eta^*) and transverse momentum (p_T^*) in the range 0< \eta^* < 5 and 0< p_T^* < 10 GeV differentially in x and Q^2. The data are compared to predictions from different Monte Carlo generators implementing various options for hadronisation and parton evolutions.

Speaker: Anastasia Grebenyuk (Deutsches Elektronen-Synchrotron (DE))

Slides MPIatLHC_H1_4.12.13.pdf

40 Pseudorapidity distributions of charged particles in proton-proton collisions by the CMS and TOTEM experiments

The pseudorapidity distributions of charged particles produced in proton-proton collisions at a centre-of-mass energy of 8 TeV are measured in the ranges |eta| < 2.2 and 5.3 < |eta| < 6.4, by the CMS and TOTEM detectors, respectively. The measurement is performed with a one-side TOTEM trigger, which is sensitive to 99% of non-diffractive interactions and all diffractive interactions with masses above 3.6 GeV, for three different event selections: an inclusive sample with the least selection bias, a sample enhanced in non-single diffractive events, and a sample enhanced in singlediffractive events. The data are compared to models used to describe high-energy hadronic interactions which are found not to consistently describe the measured distributions.

Speaker: Vardan Khachatryan (ANSL (Yerevan Physics Institute) (AM))

Slides MPI_v4.pdf

41 Determination of Sigma_eff (Discussion)

Speaker: Paolo Gunnellini (Deutsches Elektronen-Synchrotron (DE))

Slides DiscussSigmaEff.pdf

14:00 Harbour tour with lunch

Thursday, 5 December

MPI & Small-x

slides Workshop_Dinner_info.pdf

42 LHC soft physics and TMD gluon density at small x

We study the unintegrated TMD (transverse momentum dependent) gluon density obtained from the best description of the LHC data on the inclusive spectra of charged hadrons produced in the mid-rapidity region and low transverse momenta at starting scale Q_0^2 = 1 (GeV/c)^2. To extend this unintegrated gluon density (u.g.d) at higher Q^2 we use the Catani-Ciafoloni-Fiorani-Marchesini (CCFM) evolution equation. The influence of the initial (starting) non-perturbative gluon density is studied. It is shown that the evoluted u.g.d. is very sensitive to the starting u.g.d. especially at low intrinsic transverse momenta |k_T|< 1. GeV/c. The inclusion of the CCFM evolution results in a large increase of the u.g.d. magnitude at small x and large |k_T| above a few GeV/c. The application of the obtained gluon distribution to the analysis of the e-p 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). So, the connection between the soft processes at LHC and small x physics at HERA has been confirmed and extended to a wide kinematical region. This work is the continuation of our previous study of the unintegrated gluon densities at small x.

Speaker: Prof. Gennady Lykasov (Joint Institute for Nuclear Research , Dubna 141980, Moscow region, Russia)

Slides LYKASOV_MPI13_30.11.13.pdf LYKASOV_MPI13_30.11.13.ppt

43 Drell-Yan at small x via kT_cut; treatment of the infrared region.

We show that the observation of the Drell-Yan production of low-mass lepton-pairs (M < 20 GeV) at high rapidities (Y > 3) at the LHC can make a direct measurement of parton distribution functions (PDFs) in the low x region, x < 10^{-4}. We describe a procedure that greatly reduces the sensitivity of the predictions to the choice of the factorization scale and, in particular, show how, by imposing a cutoff on the transverse momentum of the lepton-pair, the data are able to probe PDFs in the important low scale, low x domain. We also show that the conventional prescription used for DGLAP parton evolution at NLO has an inconsistent treatment of the contribution from the infrared (IR) region. The deficiency is not present in a physical approach which removes the IR divergency and allows calculation in the normal 4-dimensional space.

Speaker: Emmanuel de Oliveira (IF - USP, Brazil)

Slides egdeoliveira_2013MPI.pdf

44 Minijets analysis in p-Pb collisions with ALICE at LHC

At LHC energies, several pairs of partons can collide in each pp, pA or AA collision. Multiple Parton Interactions (MPIs) can affect many physical observables, such as the charged particle multiplicity and the average transverse momentum per event. In order to include jets down to the lowest energies ("minijets"), a two-particle correlation analysis is performed with the ALICE Detector. The correlation is expressed as associated yield per trigger particle and allows one to extract the number of uncorrelated seeds which in MC simulations is proportional to the number of MPIs. In this talk, the latest results for nearside and awayside per trigger yields and uncorrelated seeds as a function of VZERO multiplicity class are reported in p-Pb collisions recorded by the ALICE Detector. The talk will discuss how the number of MPIs scale with the number of nucleon-nucleon collisions which are typically calculated in Glauber MCs.

Speaker: Emilia Leogrande (University of Utrecht (NL))

Slides EL_MPI@LHC_2013.pdf

45 MPI in gamma A & pA collions

I will review the recent theoretical studies of the MPI in collisions with nuclei including effects of leading twist nuclear shadowing as well as the recent tests of the dynamics of leading twist shadowing in coherent and incoherent J/psi photoproduction studied experimentally by ALICE.

Speaker: Mark Strikman (Pennsylvania State University (US))

Slides MPI13strikman.pdf

46 Measurement of the H-->gg differential distributions with the ATLAS detector

A review of fiducial and differential cross section measurements of the recently discovered Higgs boson production in the diphoton channel. These results were obtained with the pp collision dataset corresponding to am integrated luminosity of 20.3 fb-1, for several diphoton and jet distributions.

Speaker: Estelle Scifo (Universite de Paris-Sud 11 (FR))

Slides ATLAS_differentialXS_Hgamgam.pdf

10:45 Coffee

MPI & Small-x

slides Workshop_Dinner_info.pdf

47 Higgs as a gluon trigger

In the forthcoming high-luminosity phase at the LHC many of the most interesting QCD measurements so far become prohibitively difficult due to the high pile-up. We suggest a program of QCD measurements based on the observed Higgs boson which can be started now and can be carried through also in the large pile-up environment at high luminosity. It focuses on gluonic processes at high mass scales, and tier distinctive QCD features compared to classic probes such as Drell-Yan. It explores the strong-interaction sector of the Standard Model both at high transverse momenta and at low transverse momenta, by investigating issues on gluon fusion processes which have never been addressed experimentally before. We discuss a few specific examples and present results of Monte Carlo simulations.

Speakers: F Hautmann (Oxford / Sussex), H Jung (Desy / Antwerp)

Slides HiggsForGluons.pdf

48 Jet quenching in a Wilson lines formalism

We investigate the use of the Wilson lines approach in the analysis of the transverse-momentum broadening probability function, the first moment of which is associated with the jet quenching parameter. Starting from the Euclidean space formulation and then transforming the result to the Minkowski light-cone geometry, we discuss the issues of the UV, rapidity and IR singularities, and possible applications of the proposed approach in lattice simulations.

Speaker: Dr Igor Cherednikov (Universiteit Antwerpen)

Slides cherednikov_MPI_2013.pdf

49 Measurements with electroweak gauge bosons at LHCb

We report measurements of electroweak boson production in the forward region, using data collected at the LHCb experiment with a centre of mass energy of √s=7 TeV with an integrated luminosity of up to 1.0 fb–1. W and Z bosons are reconstructed in leptonic decay channels, and their cross-sections determined using data-driven techniques. The production of gauge bosons in association with jets and with other particles is studied. The results are compared to various theoretical predictions.

Speaker: Albert Bursche (Universitaet Zuerich (CH))

Slides Zs.pdf

Discussion on MPI and related subjects

slides Leeg.pdf

Slides: Diffraction MPILHC-discussion.pdf

Slides: Monte Carlo MPI_WG.pdf

Small-x Small-x & MPI WG

13:00 Lunch

MPI & Double Parton Scattering

50 Correlations in Double Parton Scattering

I will discuss the framework for double parton scattering that I helped develop, which is based on QCD factorization. The initial state is described by double parton distribution functions, and offers a window into diparton correlations in the proton. I will discuss its renormalization group evolution, which implies that color correlations and parton-exchange interference contributions are Sudakov suppressed at high energies. In addition, I will provide an estimate of the diparton correlations using a bag model of the proton.

Speaker: Wouter Waalewijn (UCSD)

Slides mpi_waalewijn.pdf

51 Double parton correlations and constituent quark models

In a recent paper, double parton correlations have been studied in the valence quark region, by means of constituent quark models.In this framework, two particle correlations are present without any additional prescription, at variance with what happens, for example, in independent particle models, such as the MIT bag model in its simplest version. The conclusions are similar to the ones obtained in a modified version of the bag model: correlations in the longitudinal momenta of the active quarks are found to be sizable, while those in transverse momentum are much smaller. The used framework allows to understand clearly the dynamical origin of the correlations and to establish which, among the features of the results, are model independent. Recent relevant preliminary results obtained in a relativistic light front scheme, able to overcome some drawbacks of the previous calculation, such as the so called poor support problem, will be also presented. The possibility to test experimentally the studied effects will be discussed.

Speaker: Prof. Sergio Scopetta (Perugia University and INFN)

Slides mpi13_scopetta.pdf

52 Double parton distributions under evolution

We examine the effect of double DGLAP evolution on the double parton distributions. Through a series of numerical investigations with focus on two types of parton correlations, we quantify the degree to which evolution affects the dependence on the transverse distance between partons as well as the importance of the different polarized distributions.

Speaker: Tomas Kasemets (DESY)

Slides KasemetsMPI2013.pdf

53 Double-parton scattering studies in 4-jet events with the CMS detector

We present measurements of exclusive 4-jet production cross sections as a function of the transverse momentum, pseudorapidity, as well as of correlations in azimuthal angle and transverse momentum balance among the jets. The data sample was collected in proton-proton collisions at a centre-of-mass energy of 7 TeV with the CMS detector at the LHC, corresponding to an integrated luminosity of 36 /pb. The measured cross sections agree within uncertainties with the predictions of parton shower Monte Carlo event generators and with NLO calculations matched to parton showers. The study of the angular correlations between the four jets provide a useful baseline for future studies to investigate possible contributions from double parton scattering.

Speaker: Paolo Gunnellini (Deutsches Elektronen-Synchrotron (DE))

Slides GunnelliniAntwerp.pdf

16:10 Coffee

MPI & Double Parton Scattering

54 Perturbative QCD correlations in multi-parton collisions at LHC and Tevatron.

Tevatron studies of multi-jet production in the back-to-back kinematics corresponding to parton momenta x of order 0.01 have confirmed the presence of double hard interactions in a single hadron hadron collision. The independent parton approximation underestimates the observed cross section by a factor of two. In this paper we point at previously overlooked three-parton Interactions that introduce perturbative correlation between partons and resolve this longstanding puzzle. We give prediction for the MPI rate for different processes at LHC and Tevatron in different kinematics (including 4 jets, W+ 2 jets, WW,ZZ, 3 jets +photon) and show that the MPI rate enhancement is dynamic depending on both on process and on kinematics. We present both basic formalism and numerical data.

Speaker: Dr Boris Blok (Technion)

Slides belgium4.pdf

55 Single Perturbative Splitting Diagrams in Double Parton Scattering

I will discuss the role of a particular class of graph in the double parton scattering total cross section. These are the '2v1' or `single perturbative splitting' graphs, in which two 'nonperturbatively generated' (NP) ladders interact with two ladders that have been generated via a perturbative 1->2 branching process. I will argue that these graphs do contribute to the LO DPS cross section, albeit with a different geometrical prefactor to the one that applies to the '2v2'/'zero perturbative splitting' graphs. I will also show that in these graphs there can be 'crosstalk' between the NP ladders, and discuss the numerical significance of this crosstalk.

Speaker: Jonathan Gaunt (DESY)

Slides GauntMPI13.pdf

56 Central diffractive production processes at the LHC energies

Speaker: Valery Khoze (University of Durham (GB))

Slides vak-Antwerp13.pdf vak-Antwerp13_-_.ppt

57 Double J/psi production with CMS

Speaker: Diego Ciangottini (Universita e INFN (IT))

Slides DoubleJJ.pdf

Organisation committee

slides

• LOC_report.pdf
• mpi@lhc14_PUproposal.pdf
• proposal_MPI_2014.pdf

20:00 Workshop dinner

Friday, 6 December

MPI & Diffraction

58 Soft Scattering at LHC Energies, Amplitudes and Cross Sections

We briefly describe the elements of the GLM model, that successfully describes both elastic and diffractive hadronic interactions at energies from ISR to LHC. The model is based on a single Pomeron with a large intercept \Delta_{Pom} = 0.23 and slope \alpha'_{Pom} ~ 0, and so provides a natural matching with perturbative QCD. We analyze the energy behaviour of the elastic, single diffractive and double diffractive amplitudes, and compare their behaviour with competing parametrizations and available data. We present results of the application of the GLM model in calculating long range rapidity correlations in soft interactions, and compare with LHC data.

Speaker: Errol (Asher) Gotsman (Tel Aviv University)

Slides gotsmpi.pdf

59 Recent Results on Diffraction at HERA

Speaker: Laurent Favart (Universite Libre de Bruxelles (BE))

Slides LFavart-MPI13v2.pdf

60 Central Exclusive Production in CDF

Speaker: Michael Albrow (Fermi National Accelerator Lab. (US))

Slides MPI-CDFexcl.pdf MPI-CDFexcl.pptx

61 Future Diffraction at the LHC

Speaker: Christophe Royon (CEA/IRFU,Centre d'etude de Saclay Gif-sur-Yvette (FR))

Slides anvers2013.pdf

10:40 Coffee

MPI & Diffraction

62 Measurement of the inelastic pPb cross section at 5.02 TeV with the CMS experiment

The measurement of the inelastic cross section for proton-lead (pPb) collisions at a centre-of-mass energy of 5.02 TeV using data from the CMS detector at the LHC will be presented. The analysis is based on an event counting method that takes into account corrections for pileup, acceptance, beam related and electromagnetic background as well as selection inefficiencies due to diffractive processes. Finally the accurate measurement of the beam density profiles by Van-der-Meer scans performed by CMS during the pPb run are used to obtain the integrated luminosity. The results will be compared to theory predictions to test the performance of specific models in the multi-TeV range - in particular also the Glauber model. The relefvance of nuclear shadowing or nucleon correlation effects can be estimated from such comparisons.

Speaker: Colin Baus (KIT - Karlsruhe Institute of Technology (DE))

Slides mpi_ppb_sigma_inel_colin_baus.pdf

63 Studies of soft QCD in the forward region at LHCb

Due to its unique pseudorapidity coverage and the possibility of extending measurements to low transverse momenta, LHCb provides important input to the understanding of particle production in a kinematical range where QCD models have large uncertainties. Measurements of particle production are performed in the approximate pseudorapidity range 2<η<5, which corresponds to the acceptance of the LHCb spectrometer. The results are compared to predictions given by several Monte Carlo event generators.

Speaker: Johan Blouw (Max-Planck-Gesellschaft (DE))

Slides blouw.pdf

Discussion on MPI and related subjects

slides Leeg.pdf

Slides: Diffraction MPILHC-discussion.pdf

Slides: Monte Carlo MPI_WG.pdf

Small-x Small-x & MPI WG

Closing

Conveners: Prof. Nick Van Remortel (University of Antwerp), Pierre Van Mechelen (University of Antwerp (BE))