Summary of the Workshop on Multi-Parton Interactions (MPI@LHC 2012)
With short resumes and highlights the discussions in the different working groups of the workshop MPI@LHC 2012 is documented.
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arXiv subjects
E. Levin
Publications and source records attributed to E. Levin.
Diffraction Production in a Soft Interaction Model: Mass Distributions
In the framework of our model (GLM) for soft interaction with $α'_{\pom}(0)=0$, we propose a procedure based on Gribov's partonic interpretation of the Pomeron, which enables one to calculate the diffractive mass distributions in hadron-hadron scattering. Using the analogy with deep-inelastic scattering, we associate the Pomeron-quark interaction with the Good-Walker sector of the hadron-hadron scattering, and the Pomeron-gluon sector with the t-channel Pomeron interactions. We present predicted mass distributions for the LHC energies
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Description of LHC data in a soft interaction model
We show in this paper that we have found a set of parameters in our model for the soft interactions at high energy, that successfully describes all high energy experimental data, including the LHC data. This model is based on a single Pomeron with large intercept $Δ_\pom = 0.23$ and slope $α'_\pom = 0$, that describes both long and short distance processes. It also provides a natural matching with perturbative QCD. All features of our model are similar to the expectations of N=4 SYM, which at present is the only theory that is able to treat srong interactions on a theoretical basis.
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The BFKL Pomeron calculus: summing enhanced diagrams
The goal of this paper is to sum over a class of enhanced diagrams, and derive a new Pomeron Green function. It is found that this sum gives the Pomeron contribution to the scattering amplitude that decreases with energy. In other words, we found that the total cross section of two colourless dipoles of small but equal sizes, falls down at high energies.
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Soft interaction model and the LHC data
Most models for soft interactions which were proposed prior to the measurements at the LHC, are only marginally compatible with LHC data, our GLM model has the same deficiency. In this paper we investigate possible causes of the problem, by considering separate fits to the high energy ($W > 500\, GeV$), and low energy ($W < 500\, GeV$) data. Our new results are moderately higher than our previous predictions. Our results for total and elastic cross sections are systematically lower that the recent Totem and Alice published values, while our results for the inelastic and forward slope agree with the data. If with additional experimental data, the errors are reduced, while the central cross section values remain unchanged, we will need to reconsider the physics on which our model is built.
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Survival probability of large rapidity gaps in QCD and N=4 SYM motivated model
In this paper we present a self consistent theoretical approach for the calculation of the Survival Probability for central dijet production . These calculations are performed in a model of high energy soft interactions based on two ingredients:(i) the results of N=4 SYM, which at the moment is the only theory that is able to deal with a large coupling constant; and (ii) the required matching with high energy QCD. Assuming, in accordance with these prerequisites, that soft Pomeron intercept is rather large and the slope of the Pomeron trajectory is equal to zero, we derive analytical formulae that sum both enhanced and semi-enhanced diagrams for elastic and diffractive amplitudes. Using parameters obtained from a fit to the available experimental data, we calculate the Survival Probability for central dijet production at energies accessible at the LHC. The results presented here which include the contribution of semi-enhanced and net diagrams, are considerably larger than our previous estimates.
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N=4 SYM and QCD motivated approach to soft interactions at high energies
In this paper we construct a model that satisfies the theoretical requisites of high energy soft interactions, based on two ingredients:(i) the results of N=4 SYM, which at present is a unique theory that allows one to deal with a large coupling constant; and (ii) the requirement of matching with high energy QCD. In accordance with these ideas, we assume that the soft Pomeron intercept is rather large, and the slope of the Pomeron trajectory is equal to zero. We derive analytical formulae that sum both enhanced and semi-enhanced diagrams for elastic and diffractive amplitudes. We fit the available experimental data, and predict the valuefor cross sections at the energies accessible at the LHC. The main corrections to the model are studied and evaluated.
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Inelastic processes in DIS and N=4 SYM
In this paper we compare the prediction for deep inelastic scattering from N=4 SYM with the HERA experimental data. The paper conveys two results. The first is the message that N=4 SYM is able to describe the DIS data with very good accuracy ($χ^2/d.o.f. \,\leq\,1.5$) in the region of $Q^2 = 0.85 ÷60\,GeV^2$ with $2/\sqrtλ = 0.7 ÷0.8 $ . The second is that the value of string coupling constant $g_s$ turns out to be so small that none of saturation effects will be visible in the region of accessible energies, including the maximal energy of the LHC (W = 14 \,\,TeV).
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QCD motivated approach to soft interactions at high energy: inclusive production
We extend our two component Pomeron model (GLMM) for soft high energy scattering to single inclusive cross sections. We present a suitable formulation which also includes the semi enhanced Pomeron-particle vertex corrections. The available data on single inclusive density $(1/σ_{in})dσ/dy$ in the c.m. energy range of 200-1800 GeV, including the just published Alice Collaboration point at 900 GeV,and the CMS collaboration measurements at 900 and 2360 GeV are well reproduced by our model. We also present predictions covering the complete LHC energy range which can be readily tested in the early low luminosity LHC runs. The results presented in this communication provide additional support to our Pomeron model approach.
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QCD motivated approach to soft interactions at high energies: nucleus-nucleus and hadron-nucleus collisions
In this paper we consider nucleus-nucleus and hadron-nucleus reactions in the kinematic region: $g A^{1/3} G_{3\pom} \exp\Lb ΔY\Rb \approx 1 G^2_{3\pom} \exp\Lb ΔY\Rb \approx 1 $, where $G_{3\pom}$ is the triple Pomeron coupling, $g$ is the vertex of Pomeron nucleon interaction, and 1 + $Δ_{\pom}$ denotes the Pomeron intercept. We find that in this kinematic region the traditional Glauber-Gribov eikonal approach is inadequate. We show that it is necesssary to take into account inelastic Glauber corrections, which can not be expressed in terms of the nucleon-nucleon scattering amplitudes. In the wide range of energies where $α'_\pom Y \ll R^2_A$,the scattering amplitude for the nucleus-nucleus interaction, does not depend on the details of the nucleon-nucleon interaction at high energy. In the formalism we present, the only (correlated) parameters that are required to describe the data are $Δ_{\pom}$, $G_{3\pom}$ and $g$. These parameters were taken from our description of the nucleon-nucleon data at high energies \cite{GLMM}.The predicted nucleus modification factor is compared with RHIC Au-Au data at $W = 200 GeV.$ Estimates for LHC energies are presented and discusssed.
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Soft interaction at high energy and N=4 SYM
In this paper we show that the N=4 SYM total cross section violates the Froissart theorem, and in the huge range of energy this cross section is proportional to $s^{1/3}$. The graviton reggeization will change this increase to the normal logarithmic behaviour $σ\propto \ln^2 s$. However, we demonstrated that this happens at ultra high energy, much higher than the LHC energy. In the region of accessible energy we need to assume that there is a different source for the total cross section, with the value of the cross section about 40 mb. With this assumption we successfully describe $σ_{tot}, σ_{el}$ and $σ_{diff}$ for the accessible range of energy from the fixed target Fermilab to the Tevatron energies. It turns out that the N=4 SYM mechanism can be responsible only for a small part of the inelastic cross section for this energy region (about $2 mb$). However, at the LHC energy the N=4 SYM theory can describe the multipartticle production with $σ_{in} \approx 28 mb$. The second surprise is the fact that the total cross section and the diffraction cross section can increase considerably from the Tevatron to the LHC energy. The bad description of $B_{el}$ gives the strong argument that the non N=4 SYM background should depend on energy. We believe that we have a dilemma: to find a new mechanism for the inelastic production in the framework of N=4 SYM other than the reggeized graviton interaction, or to accept that N=4 SYM is irrelevant to any experimental data that has been measured before the LHC era.
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Direct photons at forward rapidities in high-energy pp collisions
We investigate direct photon production in pp collisions at the energies of RHIC, CDF and LHC, at different rapidities employing various color-dipole models. The cross section peaks at forward rapidities due to the abelian dynamics of photon radiation. This opens new opportunities for measurement of direct photons at forward rapidities, where the background from radiative hadronic decays is strongly suppressed. Our model calculations show that photon production is sensitive to the gluon saturation effects, and strongly depends on the value of the anomalous dimension.
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Soft interactions at high energies: QCD motivated approach
We propose a QCD motivated theoretical approach to high energy soft interactions, which successfully describes the experimental data on total, elastic and diffraction cross sections. We predict that the survival probability for the diffractive Higgs production at the LHC energy is small (less than 1%), and investigate the influence of suggested corrections e.g. threshhold effects and semi-enhanced diagrams, on this value.
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The Phenomenology of Pomeron Enhancement
Multi Pomeron interactions are the main source of high mass diffraction. Their role in high energy dynamics greatly influences the predictions for high energy cross sections and survival probabilities of hard diffraction channels, notably, diffractive Higgs production at the LHC. Our approach, is motivated by the fact that we obtain a very small value for the fitted slope of the Pomeron trajectory, which justifies the use of perturbative QCD for soft scattering. Our suggested model differs from the proposal of the Durham KMR group which is based on a parton model interpretation of the Reggeon calculus in the complex J-plane in which multi Pomeron vertices are arbitrarily defined. The theoretical input and predictions of the two groups, as well as their data analysis and procedures are compared and evaluated.
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Soft processes at high energy without soft Pomeron: a QCD motivated model
In this paper we develop a QCD motivated model for both hard and soft interactions at high energies. In this model the long distance behavior of the scattering amplitude is determined by the approximate solution to the non-linear evolution equation for parton system in the saturation domain. All phenomenological parameters for dipole-proton interaction were fitted from the deep inelastic scattering data and the soft processes are described with only one new parameter, related to the wave function of hadron. It turns out that we do not need to introduce the so called soft Pomeron that has been used in high energy phenomenology for four decades. The model described all data on soft interactions: the values of total, elastic and diffractive cross sections as well as their $s$ and $t$ behavior. The value for the survival probability of the diffractive Higgs production is calculated being less 1% for the LHC energy range.
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Glauber - Gribov approach for DIS on nuclei in N=4 SYM
In this paper the Glauber-Gribov approach for deep-inelastic scattering (DIS) with nuclei is developed in N=4 SYM. It is shown that the amplitude displays the same general properties, such as geometrical scaling, as is the case in the high density QCD approach. We found that the quantum effects leading to the graviton reggeization, give rise to an imaginary part of the nucleon amplitude, which makes the DIS in N=4 SYM almost identical to the one expected in high density QCD. We concluded that the impact parameter dependence of the nucleon amplitude is very essential for N=4 SYM, and the entire kinematic region can be divided into three regions which are discussed in the paper. We revisited the dipole description for DIS and proposed a new renormalized Lagrangian for the shock wave formalism which reproduces the Glauber-Gribov approach in a certain kinematic region. However the saturation momentum turns out to be independent of energy, as it has been discussed by Albacete, Kovchegov and Taliotis. We discuss the physical meaning of such a saturation momentum $Q_s(A)$ and argue that one can consider only $Q>Q_s(A)$ within the shock wave approximation.
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Unitarity bound for gluon shadowing
Although at small Bjorken x gluons originated from different nucleons in a nucleus overlap in the longitudinal direction, most of them are still well separated in the transverse plane, therefore cannot fuse. For this reason the gluon density in nuclei cannot drop at small x below a certain bottom bound, which we evaluated in a model independent manner assuming the maximal strength of gluon fusion. We also calculated gluon shadowing in the saturated regime using on the Balitsky-Kovchegov equation, and found the nuclear ratio to be well above the unitarity bound. The recently updated analysis of parton distributions in nuclei \cite{eps08} including RHIC data on high-pT hadron production at forward rapidities, led to astonishingly strong gluon shadowing, which is far beyond the unitarity bound. This indicates a misconception in the interpretation of the nuclear suppression observed at HRIC.
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A QCD motivated model for soft processes
In this talk we give a brief description of a QCD motivated model for both hard and soft interactions at high energies. In this model the long distance behaviour of the scattering amplitude is determined by the dipole scattering amplitude in the saturation domain. All phenomenological parameters for dipole-proton interaction were fitted from the deep inelastic scattering data and the soft processes are described with only one new parameter, related to the wave function of hadron. It turns out that we do not need to introduce the so called soft Pomeron that has been used in high energy phenomenology for four decades.
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