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B. Blok

Publications and source records attributed to B. Blok.

At least 37 records · Page 2Linked to original sources
arXiv2015

Proceedings of the Sixth International Workshop on Multiple Partonic Interactions at the Large Hadron Collider

Multiple Partonic Interactions are often crucial for interpreting results obtained at the Large Hadron Collider (LHC). The quest for a sound understanding of the dynamics behind MPI - particularly at this time when the LHC is due to start its "Run II" operations - has focused the aim of this workshop. MPI@LHC2014 concentrated mainly on the phenomenology of LHC measurements whilst keeping in perspective those results obtained at previous hadron colliders. The workshop has also debated some of the state-of-the-art theoretical considerations and the modeling of MPI in Monte Carlo event generators. The topics debated in the workshop included: Phenomenology of MPI processes and multiparton distributions; Considerations for the description of MPI in Quantum Chromodynamics (QCD); Measuring multiple partonic interactions; Experimental results on inelastic hadronic collisions: underlying event, minimum bias, forward energy flow; Monte Carlo generator development and tuning; Connections with low-x phenomena, diffraction, heavy-ion physics and cosmic rays. In a total of 57 plenary talks the workshop covered a wide range of experimental results, Monte Carlo development and tuning, phenomenology and dedicated measurements of MPI which were produced with data from the LHC's Run I. Recent progress of theoretical understanding of MPI in pp, pA and AA collisions as well as the role of MPI in diffraction and small-x physics were also covered. The workshop forstered close contact between the experimental and theoretical communities. It provided a setting to discuss many of the different aspects of MPI, eventually identifying them as a unifying concept between apparently different lines of research and evaluating their impact on the LHC physics programme.

hep-ph↗
arXiv2014

Double parton interactions in $γp, γA$ collisions in the direct photon kinematics

We derive expressions for the differential distributions and the total cross section of double- parton interaction in direct photon interaction with proton and nuclei. We demonstrate that in this case the cross section is more directly related to the nucleon generalized parton distribution than in the case of double parton interactions in the proton - proton collisions. We focus on the production of two dijets each containing charm (anticharm) quarks and carrying $x_1,x_2>0.2$ fractions of the photon momentum. Numerical results are presented for the case of $γp$ collisions at LHeC, HERA and in the ultraperipheral $AA$ and $pA$ collisions at the LHC. We find that the events of this kind would be abundantly produced at the LHeC. For $\sqrt{s}=1.3$ TeV the expected rate is $2\cdot 10^8$ events for the luminosity $10^{34}$ cm$^{-2}s^{-1}$, the running time of $10^6$ s and the transverse cutoff of $p_t>5$ GeV. This would make it feasible to use these processes for the model independent determination of two parton GPDs in nucleon and in nuclei. For HERA the total accumulated number of the events is also high, but efficiency of the detection of charm seems too low to study the process. We also find that a significant number of such double parton interactions should be produced in $p - Pb$ and $Pb- Pb$ collisions at the LHC: $\sim 6\cdot 10^4$ for $Pb-Pb$, and $\sim 7 \cdot 10^3$ for $p-Pb$ collisions for the same transverse momentum cutoff.

hep-ph↗
arXiv2014

Shadowing in multiparton proton - deuteron collisions

We study the screening effect for the multiparton interactions (MPI) for proton--deuteron collisions in the kinematics where one parton belonging to the deuteron has small $x_1$ so the leading twist shadowing is present while the second parton ($x_2$) is involved in the interaction %%ms in the kinematics where shadowing effects are small. We find that the ratio of the shadowing and the impulse approximation terms is approximately factor of two larger for MPI than for the single parton distributions. We also calculate the double parton antishadowing (DPA) contribution to the cross section due to the independent interactions of the partons ofthe projectile proton with two nucleons of the deuteron and find that shadowing leads to a strong reduction ofthe DPA effect.For example, for the resolution scale $Q_1^2 \sim 4$~$\mbox{GeV}^2$ of the interaction with parton $x_1$ we find that shadowing reduces the DPA effect by $\sim$ 30%. It is argued that in the discussed kinematics the contribution of interference diagrams, which correspond to the interchange ofpartonsbetween the proton and neutron, constitutes only a small correction to the shadowing contributions.

hep-ph↗
arXiv2014

Perturbative QCD correlations in multi-parton collisions

We examine the role played in double parton interactions (DPI) by the parton--parton correlations originating from perturbative QCD parton splittings. Also presented are the results of the numerical analysis of the integrated DPI cross sections at Tevatron and LHC energies. To obtain the numerical results the knowledge of the single-parton GPDs gained by the HERA experiments was used to construct the non-perturbative input for generalized double parton distributions. The perturbative two-parton correlations induced by three-parton interactions contribute significantly to resolution of the longstanding puzzle of an excess of multi-jet production events in the back-to-back kinematics observed at the Tevatron.

hep-ph↗
arXiv2012

Origins of parton correlations in nucleon and multi-parton collisions

We demonstrate that perturbative QCD leads to positive 3D parton--parton correlations inside nucleon explaining a factor two enhancement of the cross section of multi-parton interactions observed at Tevatron at $x_i\ge 0.01$ as compared to the predictions of the independent parton approximation. We also find that though perturbative correlations decrease with $x$ decreasing, the nonperturbative mechanism kicks in and should generate correlation which, at $x$ below $10^{-3}$, is comparable in magnitude with the perturbative one for $x\sim 0.01$.

hep-ph↗
arXiv2012

pQCD Physics of multiparton interactions

We study production of two pairs of jets in %hard hadron--hadron collisions in view of extracting contribution of {\em double hard interactions} of three and four partons ($3\to4$, $4\to4$). Such interactions, in spite of being power suppressed at the level of the total cross section, become comparable with the standard hard collisions of two partons, $2\to4$, in the {\em back-to-back kinematics} when the transverse momentum imbalances of two pairing jets are relatively small. We express differential and total cross sections for two-dijet production in double parton collisions through the generalized two-parton distributions, $_2$GPDs \cite{BDFS1}, that contain large-distance two-parton correlations of non-perturbative origin as well as small-distance correlations due to parton evolution. We find that these large- and small-distance correlations participate in different manner in 4-jet production, and treat them in the leading logarithmic approximation of pQCD that resums collinear logarithms in all orders. A special emphasis is given to $3\to4$ double hard interaction processes that occur as an interplay between large- and short-distance parton correlations and were not taken into consideration by approaches inspired by the parton model picture. We demonstrate that the $3\to4$ mechanism, being of the same order in $\as$ as the $4\to4$ process, turns out to be {\em geometrically enhanced} compared to the latter and should contribute significantly to 4-jet production. The framework developed here takes into systematic consideration perturbative $Q^2$ evolution of $_2$GPDs. It can be used as a basis for future analysis of NLO corrections to multi-parton interactions (MPI) at LHC and Tevatron colliders, in particular for improving evaluation of QCD backgrounds to new physics searches.

hep-ph↗
arXiv2011

Multi-Parton Interactions at the LHC

We review the recent progress in the theoretical description and experimental observation of multiple parton interactions. Subjects covered include experimental measurements of minimum bias interactions and of the underlying event, models of soft physics implemented in Monte Carlo generators, developments in the theoretical description of multiple parton interactions and phenomenological studies of double parton scattering. This article stems from contributions presented at the Helmholtz Alliance workshop on "Multi-Parton Interactions at the LHC", DESY Hamburg, 13-15 September 2010.

hep-ph↗
arXiv2011

The four jet production at LHC and Tevatron in QCD

We demonstrate that in the back-to-back kinematics the production of four jets in the collision of two partons is suppressed in the leading log approximation of pQCD, compared to the hard processes involving the collision of four partons. We derive the basic equation for four-jet production in QCD in terms of the convolution of generalized two-parton distributions of colliding hadrons in the momentum space representation. Our derivation leads to geometrical approach in the impact parameter space close to that suggested within the parton model and used before to describe the four-jet production. We develop the independent parton approximation to the light-cone wave function of the proton. Comparison with the CDF and D0 data shows that the independent parton approximation to the light-cone wave function of the proton is insufficient to explain the data. We argue that the data indicate the presence of significant multiparton correlations in the light-cone wave functions of colliding protons.

hep-ph↗
arXiv2010

Proceedings of the First International Workshop on Multiple Partonic Interactions at the LHC (MPI08)

The objective of this first workshop on Multiple Partonic Interactions (MPI) at the LHC is to raise the profile of MPI studies, summarizing the legacy from the older phenomenology at hadronic colliders and favouring further specific contacts between the theory and experimental communities. The MPI are experiencing a growing popularity and are currently widely invoked to account for observations that would not be explained otherwise: the activity of the Underlying Event, the cross sections for multiple heavy flavour production, the survival probability of large rapidity gaps in hard diffraction, etc. At the same time, the implementation of the MPI effects in the Monte Carlo models is quickly proceeding through an increasing level of sophistication and complexity that in perspective achieves deep general implications for the LHC physics. The ultimate ambition of this workshop is to promote the MPI as unification concept between seemingly heterogeneous research lines and to profit of the complete experimental picture in order to constrain their implementation in the models, evaluating the spin offs on the LHC physics program.

hep-ex↗
arXiv2010

DGLAP versus perturbative Pomeron in large momentum transfer hard diffractive processes at HERA and LHC

We evaluate within the LO DGLAP approximation the dependence on energy of the cross section of the photo(electro)production of vector meson V ($V=J/ψ,...$) in the hard elastic processes off a parton $γ^*+g\to V=J/ψ+g$ as the function of momentum transfer $t=(q_γ-p_V)^2$. We demonstrate that in the limit $-t \ge m_V^2 +Q^2$ the cross section does not contain double logarithmic terms in any order of the DGLAP approximation leading to the energy independent cross section. Thus the energy dependence of cross section $γ^*+p\to J/ψ+{\rm rapidity ~gap} +X$ is governed at large $t$ by the gluon distribution within a proton, i.e. it is unambiguously predicted within the DGLAP approximation including the stronger $W_{γN}$ dependence at larger $-t$ . This prediction explains recent HERA data. The calculations which follow perturbative Pomeron logic predict opposite trend of a weaker $W_{γN}$ dependence at larger $t$. We explain that at the HERA energies double logarithmic terms characteristic for DGLAP approximation dominate in the hard processes as the consequence of constraints due to energy-momentum conservation. We give predictions for ultraperipheral hard diffractive processes at the LHC and show that these processes are well suited for looking for the contribution of the single logarithmic terms due to the gluon emission in the multiRegge kinematics. We also comment on the interrelation between energy and $t$ dependence of the cross sections of the hard exclusive processes.

hep-ph↗
arXiv2010

The energy dependence of the hard exclusive diffractive processes in pQCD as the function of momentum transfer

We predict the dependence on energy of photo(electro) production processes: $γ(γ^*)+p\to V+ X$ with large rapidity gap at small x and large momentum $-t$ transferred to $V$ in pQCD. Here V is a heavy quarkonium ($J/ψ, Υ$) or longitudinally polarized light vector meson (in the electroproduction processes), etc. In the kinematics of HERA we calculate the dependence on energy of cross sections of these processes as the function of momentum transfer $t$, photon virtuality $Q^2$ and/or quarkonium mass. In the kinematical region $Q_0^2\le -t\ll Q^2+M^2_V$ the nontrivial energy dependence of the cross section for the vector meson production due to the photon scattering off a parton follows within QCD from the summing of the double logarithmic terms. In the second regime $-t\ge Q^2+M^2_V$ within DGLAP approximation in all orders of perturbation theory the $q\bar q - {\rm parton}$ elastic cross section is energy independent. We show that the correct account of the double logarithmic terms and of the gluon radiation including kinematical constraints removes the disagreement between pQCD calculations and recent HERA experimental data. The explicit formula for the dependence of the differential cross section$\displaystyle{\frac{d^2σ}{dtdx_J}}$ of these processes on $s_{γ^*N}$ is obtained. We show that perturbative Pomeron type behavior may reveal itself only at energies significantly larger than those available at HERA. In addition we evaluate the energy dependence of DCVS processes.

hep-ph↗
arXiv2009

Increase with energy of parton transverse momenta in the fragmentation region in DIS and related phenomena

The dipole and the DGLAP approximations are combined with the $k_t$ factorization theorem to demonstrate the fundamental property of pQCD: smaller is the size of the colorless quark-gluon configurations in the fragmentation region, more rapid is the increase of its interaction with the target as a function of energy. First, we show that the transverse momenta of the quark(antiquark) within the $q\bar{q}$ pair, produced in the fragmentation region by the strongly virtual photon, increase with the decrease of x for fixed $Q^2$. As practical consequence of these effects we show that the cross sections of DIS and DVCS. We predict that the ratio of DVCS to DIS amplitudes should very slowly approach one from above at very large collision energies. Second, we study a closely related phenomenon of the increase of the transverse momenta with the energy of the characteristic transverse momenta of the gluon/quark configurations responsible for the transition to the black disk regime. We discuss the impact of this phenomenona on the slowing of the dependence on the initial energy of the coherence length. We demonstrate that a rapid projectile has the biconcave shape, which is different from the expectations of the preQCD parton model where a fast hadron has a pancake shape. We show that the increase of the transverse momenta leads to a new expression for the total cross section of a DIS scattering at very large energies, relevant to LHeC and LHC. We discuss the impact of the discovered phenomena on the hard processes in pp collisions, and on the dominance ofdifferent phases of chiral and conformal symmetries in the central and peripheral pp, pA, and AA collisions.

hep-ph↗
arXiv2008

On the shape of a rapid hadron in QCD

We visualize the fundamental property of pQCD: the smaller size of the colorless quark-gluon configurations leads to a more rapid increase of its interaction with energy. Within the frame of dipole model we use the $k_t$ factorization theorem to generalize the DGLAP approximation and/or leading $\ln(x_0/x)$ approximation and evaluate the interaction of quark dipole with a target. In the limit of fixed $Q^2$ and $x\to 0$ we found the increase with energy of transverse momenta of quark(antiquark) within q$\bar q$ pair produced by strongly virtual photon. The average $p^2_t$ is evaluated analytically within the double logarithmic approximation. We demonstrate that the invariant mass$^2$ of the q$\bar q$ pair increases with the energy as $M^2_0(x_0/x)^λ$, where $λ\sim 0.4α_sN_c/π$ for transverse photons, and as $\sim M^2_0 \exp{0.17[(4α_sN_c/π)\log(x_0/x)]^{1/2}}$ for longitudinal photons, where $M^2_0 \approx 0.7Q^2$ at the energies of the order $s_0\sim 10^4$ GeV$^2$ ($x_0\sim 10^{-2}$). The magnitude of the effect depends strongly on the small $x$ behavior of the gluon distribution. Similar pattern of the energy dependence of $M^2$ is found in the LO DGLAP approximation generalized to account for $k_t$ factorization. We discuss the impact of the found phenomenon on the dependence of the coherence length on the initial energy and demonstrate that the shape of final hadron state in DIS has biconcave form instead of pancake. Some implications of the found phenomena for the hard processes in pp collisions are discussed.

hep-ph↗
arXiv2008

The increase with energy of the parton transverse momenta in the current fragmentation region and related pQCD phenomena in DIS at very high energies

We find, within the pQCD dipole model for DIS processes, a rapid increase with energy of the scale of parton momenta in the current fragmentation region in the limit $s\to \infty$ and fixed $Q^2$ . We explain the equivalence between the dispersion representation over $Q^2$ for LT pQCD zero angle amplitude of $γ^*+T\to γ^*+T$ scattering at large energies and the $k_t$ factorization, and use it to evaluate the scales of the hard processes in the current fragmentation region. We derive within the black disc (BD) regime the modified Gribov formula for the total cross-section of the DIS. We evaluate the coherence length of the processes relevant for the BD regime and find that it increases with energy as $\sim s^{0.6}$ i.e. significantly slower than in the parton model ($1/2m_Nx$ - the Ioffe length) as well as in pQCD. In the BD regime we estimate the gluon densities, local in the coordinate space, and find that they do not decrease with the energy. We discuss briefly how the new pQCD phenomena may reveal itself in the proton-proton collisions at LHC

hep-ph↗
arXiv2006

On the behaviour of single scale hard small $x$ processes in QCD near the black disc limit

We argue that at sufficiently small Bjorken $x$ where pQCD amplitude rapidly increases with energy and violates probability conservation the shadowing effects in the single-scale small $x$ hard QCD processes can be described by an effective quantum field theory of interacting quasiparticles. The quasiparticles are the perturbative QCD ladders. We find, within the WKB approximation, that the smallness of the QCD coupling constant ensures the hierarchy among many-quasiparticle interactions evaluated within physical vacuum and in particular, the dominance in the Lagrangian of the triple quasiparticle interaction. It is explained that the effective field theory considered near the perturbative QCD vacuum contains a tachyon relevant for the divergency of the perturbative QCD series at sufficiently small $x$. We solve the equations of motion of the effective field theory within the WKB approximation and find the physical vacuum and the transitions between the false (perturbative) and physical vacua. Classical solutions which dominate transitions between the false and physical vacua are kinks that cannot be decomposed into perturbative series over the powers of $α_s$. These kinks lead to color inflation and the Bose-Einstein condensation of quasiparticles. The account of the quantum fluctuations around the WKB solution reveals the appearance of the "massless" particles-- "phonons". It is explained that "phonons" are relevant for the black disc behaviour of small $x$ processes, leading to a Froissart rise of the cross-section. The condensation of the ladders produces a color network occupying a "macroscopic" longitudinal volume. We discuss briefly the possible detection of new QCD effects.

hep-ph↗
arXiv2005

Tachyon dynamics of the black disc limit in the hard small $x$ processes in QCD

We investigate the effective field theory (EFT) which gives the approximate description of the scattering of the two hard small dipoles in the small $x$ processes in QCD near the black disc limit(BDL). We argue that the perturbative QCD approaches predict the existence of tachyon and visualize it in the approximation where $α'_P=0$ . We demonstrate that the high energy behavior of the cross-section depends strongly on the diffusion law in the impact parameter plane. On the other hand, almost threshold behavior of the cross section of the hard processes and multiplicities, i.e. fast increase of cross sections (color inflation), melting of ladders into color network and softening of the longitudinal distributions of hadrons are qualitatively insensitive to the value of diffusion in the impact parameter space. We evaluate $α'_P$ near the black disk limit and find significant $α'_P$ as the consequence of the probability conservation.

hep-ph↗
arXiv2005

Spectator Effects in the Heavy Quark Effective Theory

We present a complete analysis of the Heavy Quark Effective Theory Lagrangian at order $1/m^2$ in the leading logarithmic approximation, including effects induced by spectator quarks. At this order new correction terms appear in the effective Lagrangian, as four-quark operators containing both heavy and light quark fields. We compute the coefficients of these operators to one-loop order and in the leading-logarithmic approximation. Two of them break the heavy quark spin symmetry and we estimate their contribution to the hyperfine splitting of the heavy mesons in the factorization approximation. We find that they make a positive contribution to the hyperfine splitting of about 10% of the measured splitting in the charm case and of up to 5% in the bottom case.

hep-ph↗