Mark Strikman - Academia.edu (original) (raw)
Papers by Mark Strikman
Physical Review Letters, 1993
arXiv (Cornell University), 1998
We evaluate the amplitude of Bethe-Heitler process: \gamma^* p -> \gamma \Delta^+ -electron brems... more We evaluate the amplitude of Bethe-Heitler process: \gamma^* p -> \gamma \Delta^+ -electron bremsstrahlung of photon accompanied by the excitation of \Delta$ isobar. We show that this background is suppressed at small momentum transfer squared t to a proton relative to that for \gamma^* p -> \gamma p and \gamma^* p\to -> \Delta^+ - DVCS processes. From experimental point of view, this means that N -> N and N -> \Delta skewed quark distributions (SQD's) might be measurable at small momentum transfer. Several implications and applications of the QCD factorization theorem for the processes \gamma_{L}^* + p -> h_f +h_s are discussed where h_f - the particle produced along photon momentum vecq\vec qvecq maybe either a meson or a baryon. We discuss also t dependence of DVCS and exclusive meson production as the practical criteria to distinguish between soft and hard regime. Basing on the large-N_c picture of the nucleon as a soliton of the effective chiral Lagrangian we derive relations between N -> N and N ->\Delta SQD's which can be used to estimate amplitude of N -> \Delta DVCS .
Deep Inelastic Scattering DIS 2006, 2007
We summarize the QCD expectations for hard diffraction in DGLAP and Black Disk Limits and argue t... more We summarize the QCD expectations for hard diffraction in DGLAP and Black Disk Limits and argue that LHC will provide unique new oportunities for studing of these phenomena using ultraperipheral heavy ion (proton-ion) collisions at the LHC.
The European Physical Journal C, 2014
We examine the role played in double-parton interactions (DPI) by the parton-parton correlations ... more 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 a resolution of the longstanding puzzle of an excess of multi-jet production events in the back-to-back kinematics observed at the Tevatron.
Physical Review D, 2011
We propose a new method to determine at what transverse momenta particle production in high-energ... more We propose a new method to determine at what transverse momenta particle production in high-energy pp collisions is governed by hard parton-parton processes. Using information on the transverse spatial distribution of partons obtained from hard exclusive processes in ep/γp scattering, we evaluate the impact parameter distribution of pp collisions with a hard parton-parton process as a function of pT of the produced parton (jet). We find that the average pp impact parameters in such events depend very weakly on pT in the range 2 < pT < few 100 GeV, while they are much smaller than those in minimum-bias inelastic collisions. The impact parameters in turn govern the observable transverse multiplicity in such events (in the direction perpendicular to the trigger particle or jet). Measuring the transverse multiplicity as a function of pT thus provides an effective tool for determining the minimum pT for which a given trigger particle originates from a hard parton-parton process. Additional tests of the proposed geometric correlations are possible by measuring the dependence on the trigger rapidity. Various strategies for implementing this method are outlined.
Exclusive Processes at High Momentum Transfer, 2002
Physical Review D, 2011
We analyze the structure of the high multiplicity events observed by the CMS collaboration at the... more We analyze the structure of the high multiplicity events observed by the CMS collaboration at the LHC. We argue that the bulk of the observed correlations is due to the production of a pair of jets with pt > 15 GeV/c. We also suggest that high multiplicity events are due to a combination of three effects: high underlying multiplicity for collisions at small impact parameters, upward fluctuations of the gluon density in the colliding protons, and production of hadrons in the fragmentation of dijets. The data analysis is suggested which may clarify the underlying dynamics of the high multiplicity events and probe fluctuations of the gluon field as a function of x.
15th International Workshop on Deep-Inelastic Scattering and Related Subjects DIS 2007, 2007
We summarize how the approach to the black-disk regime (BDR) of strong interactions at TeV energi... more We summarize how the approach to the black-disk regime (BDR) of strong interactions at TeV energies influences rapidity gap survival in exclusive hard diffraction ÔÔ Ô • À • Ô´À dijet ÉÉ Higgsµ. Employing a recently developed partonic description of such processes, we discuss (a) the suppression of diffraction at small impact parameters by soft spectator interactions in the BDR; (b) further suppression by inelastic interactions of hard spectator partons in the BDR; (c) effects of correlations between hard and soft interactions, as suggested by various models of proton structure (color fluctuations, spatial correlations of partons). Hard spectator interactions in the BDR substantially reduce the rapidity gap survival probability at LHC energies compared to previously reported estimates.
Journal of High Energy Physics, 1999
Predictions for the diffractive photoproduction of the Υ-family at HERA energies, within the fram... more Predictions for the diffractive photoproduction of the Υ-family at HERA energies, within the framework of the analysis by Frankfurt, Koepf and Strikman, are presented. Two novel effects lead to a significant enhancement of the original calculation: the non-diagonal (or skewed) kinematics, calculated to leading-log(Q 2) accuracy, and the large magnitude of the real part of the amplitude. The resultant cross sections are found to agree fairly well with recent preliminary data from ZEUS and H1. A strong correlation between the mass of the diffractively produced state and the energy dependence of the cross section is found. In particular, a considerably stronger rise in energy is predicted than that found in J/ψ-production.
Physics Letters B, 2007
We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton ... more We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton propagating through the nuclear matter should loose a significant and increasing with energy and atomic number fraction of its initial energy as a result of dominance of inelastic interactions, causality and energy-momentum conservation. We evaluate these energy losses and find them to lead to the significant suppression of the forward jet production in the central NA collisions at collider energies with a moderate suppression of recoiling jet at central rapidities. We confront our expectations with the recent RHIC data of the STAR collaboration on the probability, P , for emission of at least one fast hadron at a central rapidity in association with production of a very forward high p t neutral pion in pp and dAu collisions. We calculate the A-dependence of P , and find that the data imply a strong suppression of leading pion production at central impact parameters. We also conclude that production of recoil jets in the hard subprocess is not suppressed providing further evidence for the dominance of peripheral collisions. Both features of the data are consistent with the onset of BDR. We suggest new phenomena and new observables to investigate BDR at RHIC and LHC.
Physics Letters B, 2005
We discuss the production of charmonium states in antiproton-nucleus collisions at the ψ ′ thresh... more We discuss the production of charmonium states in antiproton-nucleus collisions at the ψ ′ threshold. It is explained that measurements inpA collisions will allow to get new information about the strengths of the inelastic J/ψN and ψ ′ N interaction, on the production of Λc andD in charmonium-nucleon interactions and for the first time about the nondiagonal transitions ψ ′ N → J/ψN. The inelastic J/ψ-nucleon cross section is extracted from the comparison of hadronnucleus collisions with hadron-nucleon collisions. We extract the total J/ψ-nucleon cross section from photon-nucleon collisions by accounting for the color transparency phenomenon within the frame of the GVDM (Generalized Vector meson Dominance Model). We evaluate within the GVDM the inelastic ψ ′-nucleon cross section as well as the cross section for the nondiagonal transitions. Predictions for the ratio of J/ψ to ψ ′ yields in antiproton-nucleus scatterings close to the threshold of ψ ′ production for different nuclear targets are presented.
Physics Letters B, 1997
We argue that as the atomic number of the target nucleus A → ∞, the multiplicity of leading parti... more We argue that as the atomic number of the target nucleus A → ∞, the multiplicity of leading particles in hadron-nucleus collisions tends to a finite limit. The limiting multiplicities for various particle production are computed for both proton and pion projectiles. Signatures at finite A are discussed. Data from 100 GeV/c central hadron-nucleus collisions are analyzed and found to be in qualitative agreement with this picture.
Physics Letters B, 1997
We discuss baryon production in the nucleon fragmentation region in deep inelastic scattering. Th... more We discuss baryon production in the nucleon fragmentation region in deep inelastic scattering. The dependence of the nucleon spectra on Bjorken-x is evaluated within the parton model and in QCD, and ways to look for the breakdown of the DGLAP approximation via baryon production are suggested. We argue also that the leading neutron production in these small x processes is rather insensitive to the pion parton densities of the nucleon.
Physics Letters B, 2005
We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisio... more We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisions in kinematics of RHIC. Our improved estimate of the total coherent cross section is a factor of two smaller than the earlier ones. We find that the counting rate of quasielastic J/ψ photoproduction tagged by neutrons emitted due to cascading of the recoiled nucleon within the residual nucleus exceeds the rate of the coherent events. We argue that measurements of this process can be used to learn about the dynamics of color dipole-nucleon interactions in nuclei in the wide effective range of energies of γA interactions.
Physics Letters B, 2012
We present predictions for the J/ψ and ρ meson production in the heavy ion ultraperipheral collis... more We present predictions for the J/ψ and ρ meson production in the heavy ion ultraperipheral collisions (UPC) for the current energy 2.76 TeV at the LHC. Both total cross sections and cross sections with the neutron emission from one or both nuclei are presented. We also perform analysis of the RHIC ρ-meson photoproduction data and emphasize importance of these data to test the current model for nucleus break up in UPC.
Physics Letters B, 2004
We point out that the suppression in the ratio R dAu recently observed by the BRAHMS Collaboratio... more We point out that the suppression in the ratio R dAu recently observed by the BRAHMS Collaboration in forward scattering is stronger than usually appreciated. This is related to the fact that at forward rapidities BRAHMS measures negatively charged hadrons and that R dAu is defined from the ratio of dA and pp scattering cross sections. We also investigate the influence of standard shadowing on R dAu , and the typical values of partonic momentum fractions relevant in forward scattering. We find that x Au 0.02 dominate in the cross section.
Physical Review Letters, 1990
Current data on deep-inelastic processes oA' nuclei are analyzed by using exact QCD sum rules for... more Current data on deep-inelastic processes oA' nuclei are analyzed by using exact QCD sum rules for the total momentum and baryon charge of nuclei. Evidence for an overall enhancement of the gluon field in heavy nuclei (~4%) is found which, combined with the calculation of nuclear shadowing, yields a substantial enhancement of valence-quark and gluon distributions at x =0.1, as well as a suppression of the sea for x~O. l. It is also shown that scaling violations lead to a strong decrease of shadowing for the sea at x = 0.05. Implications for disentangling the origin of internuclear forces are further discussed.
Physical Review Letters, 1993
The scattering of hadrons at high energies can be described by a distribution function of cross s... more The scattering of hadrons at high energies can be described by a distribution function of cross sections, which incorporates the phenomena of color transparency and color opacity. We infer here a cross-section distribution for pionic projectiles on nucleons consistent with the data on the moments of the distribution. Using this distributi. on we calculate the probability for a pion to be in a pointlike configuration and compare with predictions based on a combination of perturbative @CD and @CD sum rules. Comparison with the corresponding distribution for nucleon-nucleon scattering shows that color transparency efT'ects should be more pronounced for a meson beam.
Physical Review Letters, 2000
The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of dec... more The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large Nc limit and found to be comparable to that of octet baryons. Large Nc selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.
Physical Review D, 2004
We study the distribution of gluons in transverse space in the nucleon at moderately small x (∼ 1... more We study the distribution of gluons in transverse space in the nucleon at moderately small x (∼ 10 −2). At large transverse distances (impact parameters) the gluon density is generated by the "pion cloud" of the nucleon, and can be calculated in terms of the gluon density in the pion. We investigate the large-distance behavior in two different approaches to chiral dynamics: i) phenomenological softpion exchange, ii) the large-Nc picture of the nucleon as a classical soliton of the pion field, which corresponds to degenerate N and ∆ states. The large-distance contributions from the "pion cloud" cause a ∼ 20% increase in the overall transverse size of the nucleon if x drops significantly below Mπ/MN. This is in qualitative agreement with the observed increase of the slope of the t-dependence of the J/ψ photoproduction cross section at HERA compared to fixed-target energies. We argue that the glue in the pion cloud could be probed directly in hard electroproduction processes accompanied by "pion knockout", γ * + N → γ (or ρ, J/ψ) + π + N ′ , where the transverse momentum of the emitted pion is large while that of the outgoing nucleon is restricted to values of order Mπ.
Physical Review Letters, 1993
arXiv (Cornell University), 1998
We evaluate the amplitude of Bethe-Heitler process: \gamma^* p -> \gamma \Delta^+ -electron brems... more We evaluate the amplitude of Bethe-Heitler process: \gamma^* p -> \gamma \Delta^+ -electron bremsstrahlung of photon accompanied by the excitation of \Delta$ isobar. We show that this background is suppressed at small momentum transfer squared t to a proton relative to that for \gamma^* p -> \gamma p and \gamma^* p\to -> \Delta^+ - DVCS processes. From experimental point of view, this means that N -> N and N -> \Delta skewed quark distributions (SQD's) might be measurable at small momentum transfer. Several implications and applications of the QCD factorization theorem for the processes \gamma_{L}^* + p -> h_f +h_s are discussed where h_f - the particle produced along photon momentum vecq\vec qvecq maybe either a meson or a baryon. We discuss also t dependence of DVCS and exclusive meson production as the practical criteria to distinguish between soft and hard regime. Basing on the large-N_c picture of the nucleon as a soliton of the effective chiral Lagrangian we derive relations between N -> N and N ->\Delta SQD's which can be used to estimate amplitude of N -> \Delta DVCS .
Deep Inelastic Scattering DIS 2006, 2007
We summarize the QCD expectations for hard diffraction in DGLAP and Black Disk Limits and argue t... more We summarize the QCD expectations for hard diffraction in DGLAP and Black Disk Limits and argue that LHC will provide unique new oportunities for studing of these phenomena using ultraperipheral heavy ion (proton-ion) collisions at the LHC.
The European Physical Journal C, 2014
We examine the role played in double-parton interactions (DPI) by the parton-parton correlations ... more 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 a resolution of the longstanding puzzle of an excess of multi-jet production events in the back-to-back kinematics observed at the Tevatron.
Physical Review D, 2011
We propose a new method to determine at what transverse momenta particle production in high-energ... more We propose a new method to determine at what transverse momenta particle production in high-energy pp collisions is governed by hard parton-parton processes. Using information on the transverse spatial distribution of partons obtained from hard exclusive processes in ep/γp scattering, we evaluate the impact parameter distribution of pp collisions with a hard parton-parton process as a function of pT of the produced parton (jet). We find that the average pp impact parameters in such events depend very weakly on pT in the range 2 < pT < few 100 GeV, while they are much smaller than those in minimum-bias inelastic collisions. The impact parameters in turn govern the observable transverse multiplicity in such events (in the direction perpendicular to the trigger particle or jet). Measuring the transverse multiplicity as a function of pT thus provides an effective tool for determining the minimum pT for which a given trigger particle originates from a hard parton-parton process. Additional tests of the proposed geometric correlations are possible by measuring the dependence on the trigger rapidity. Various strategies for implementing this method are outlined.
Exclusive Processes at High Momentum Transfer, 2002
Physical Review D, 2011
We analyze the structure of the high multiplicity events observed by the CMS collaboration at the... more We analyze the structure of the high multiplicity events observed by the CMS collaboration at the LHC. We argue that the bulk of the observed correlations is due to the production of a pair of jets with pt > 15 GeV/c. We also suggest that high multiplicity events are due to a combination of three effects: high underlying multiplicity for collisions at small impact parameters, upward fluctuations of the gluon density in the colliding protons, and production of hadrons in the fragmentation of dijets. The data analysis is suggested which may clarify the underlying dynamics of the high multiplicity events and probe fluctuations of the gluon field as a function of x.
15th International Workshop on Deep-Inelastic Scattering and Related Subjects DIS 2007, 2007
We summarize how the approach to the black-disk regime (BDR) of strong interactions at TeV energi... more We summarize how the approach to the black-disk regime (BDR) of strong interactions at TeV energies influences rapidity gap survival in exclusive hard diffraction ÔÔ Ô • À • Ô´À dijet ÉÉ Higgsµ. Employing a recently developed partonic description of such processes, we discuss (a) the suppression of diffraction at small impact parameters by soft spectator interactions in the BDR; (b) further suppression by inelastic interactions of hard spectator partons in the BDR; (c) effects of correlations between hard and soft interactions, as suggested by various models of proton structure (color fluctuations, spatial correlations of partons). Hard spectator interactions in the BDR substantially reduce the rapidity gap survival probability at LHC energies compared to previously reported estimates.
Journal of High Energy Physics, 1999
Predictions for the diffractive photoproduction of the Υ-family at HERA energies, within the fram... more Predictions for the diffractive photoproduction of the Υ-family at HERA energies, within the framework of the analysis by Frankfurt, Koepf and Strikman, are presented. Two novel effects lead to a significant enhancement of the original calculation: the non-diagonal (or skewed) kinematics, calculated to leading-log(Q 2) accuracy, and the large magnitude of the real part of the amplitude. The resultant cross sections are found to agree fairly well with recent preliminary data from ZEUS and H1. A strong correlation between the mass of the diffractively produced state and the energy dependence of the cross section is found. In particular, a considerably stronger rise in energy is predicted than that found in J/ψ-production.
Physics Letters B, 2007
We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton ... more We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton propagating through the nuclear matter should loose a significant and increasing with energy and atomic number fraction of its initial energy as a result of dominance of inelastic interactions, causality and energy-momentum conservation. We evaluate these energy losses and find them to lead to the significant suppression of the forward jet production in the central NA collisions at collider energies with a moderate suppression of recoiling jet at central rapidities. We confront our expectations with the recent RHIC data of the STAR collaboration on the probability, P , for emission of at least one fast hadron at a central rapidity in association with production of a very forward high p t neutral pion in pp and dAu collisions. We calculate the A-dependence of P , and find that the data imply a strong suppression of leading pion production at central impact parameters. We also conclude that production of recoil jets in the hard subprocess is not suppressed providing further evidence for the dominance of peripheral collisions. Both features of the data are consistent with the onset of BDR. We suggest new phenomena and new observables to investigate BDR at RHIC and LHC.
Physics Letters B, 2005
We discuss the production of charmonium states in antiproton-nucleus collisions at the ψ ′ thresh... more We discuss the production of charmonium states in antiproton-nucleus collisions at the ψ ′ threshold. It is explained that measurements inpA collisions will allow to get new information about the strengths of the inelastic J/ψN and ψ ′ N interaction, on the production of Λc andD in charmonium-nucleon interactions and for the first time about the nondiagonal transitions ψ ′ N → J/ψN. The inelastic J/ψ-nucleon cross section is extracted from the comparison of hadronnucleus collisions with hadron-nucleon collisions. We extract the total J/ψ-nucleon cross section from photon-nucleon collisions by accounting for the color transparency phenomenon within the frame of the GVDM (Generalized Vector meson Dominance Model). We evaluate within the GVDM the inelastic ψ ′-nucleon cross section as well as the cross section for the nondiagonal transitions. Predictions for the ratio of J/ψ to ψ ′ yields in antiproton-nucleus scatterings close to the threshold of ψ ′ production for different nuclear targets are presented.
Physics Letters B, 1997
We argue that as the atomic number of the target nucleus A → ∞, the multiplicity of leading parti... more We argue that as the atomic number of the target nucleus A → ∞, the multiplicity of leading particles in hadron-nucleus collisions tends to a finite limit. The limiting multiplicities for various particle production are computed for both proton and pion projectiles. Signatures at finite A are discussed. Data from 100 GeV/c central hadron-nucleus collisions are analyzed and found to be in qualitative agreement with this picture.
Physics Letters B, 1997
We discuss baryon production in the nucleon fragmentation region in deep inelastic scattering. Th... more We discuss baryon production in the nucleon fragmentation region in deep inelastic scattering. The dependence of the nucleon spectra on Bjorken-x is evaluated within the parton model and in QCD, and ways to look for the breakdown of the DGLAP approximation via baryon production are suggested. We argue also that the leading neutron production in these small x processes is rather insensitive to the pion parton densities of the nucleon.
Physics Letters B, 2005
We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisio... more We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisions in kinematics of RHIC. Our improved estimate of the total coherent cross section is a factor of two smaller than the earlier ones. We find that the counting rate of quasielastic J/ψ photoproduction tagged by neutrons emitted due to cascading of the recoiled nucleon within the residual nucleus exceeds the rate of the coherent events. We argue that measurements of this process can be used to learn about the dynamics of color dipole-nucleon interactions in nuclei in the wide effective range of energies of γA interactions.
Physics Letters B, 2012
We present predictions for the J/ψ and ρ meson production in the heavy ion ultraperipheral collis... more We present predictions for the J/ψ and ρ meson production in the heavy ion ultraperipheral collisions (UPC) for the current energy 2.76 TeV at the LHC. Both total cross sections and cross sections with the neutron emission from one or both nuclei are presented. We also perform analysis of the RHIC ρ-meson photoproduction data and emphasize importance of these data to test the current model for nucleus break up in UPC.
Physics Letters B, 2004
We point out that the suppression in the ratio R dAu recently observed by the BRAHMS Collaboratio... more We point out that the suppression in the ratio R dAu recently observed by the BRAHMS Collaboration in forward scattering is stronger than usually appreciated. This is related to the fact that at forward rapidities BRAHMS measures negatively charged hadrons and that R dAu is defined from the ratio of dA and pp scattering cross sections. We also investigate the influence of standard shadowing on R dAu , and the typical values of partonic momentum fractions relevant in forward scattering. We find that x Au 0.02 dominate in the cross section.
Physical Review Letters, 1990
Current data on deep-inelastic processes oA' nuclei are analyzed by using exact QCD sum rules for... more Current data on deep-inelastic processes oA' nuclei are analyzed by using exact QCD sum rules for the total momentum and baryon charge of nuclei. Evidence for an overall enhancement of the gluon field in heavy nuclei (~4%) is found which, combined with the calculation of nuclear shadowing, yields a substantial enhancement of valence-quark and gluon distributions at x =0.1, as well as a suppression of the sea for x~O. l. It is also shown that scaling violations lead to a strong decrease of shadowing for the sea at x = 0.05. Implications for disentangling the origin of internuclear forces are further discussed.
Physical Review Letters, 1993
The scattering of hadrons at high energies can be described by a distribution function of cross s... more The scattering of hadrons at high energies can be described by a distribution function of cross sections, which incorporates the phenomena of color transparency and color opacity. We infer here a cross-section distribution for pionic projectiles on nucleons consistent with the data on the moments of the distribution. Using this distributi. on we calculate the probability for a pion to be in a pointlike configuration and compare with predictions based on a combination of perturbative @CD and @CD sum rules. Comparison with the corresponding distribution for nucleon-nucleon scattering shows that color transparency efT'ects should be more pronounced for a meson beam.
Physical Review Letters, 2000
The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of dec... more The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large Nc limit and found to be comparable to that of octet baryons. Large Nc selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.
Physical Review D, 2004
We study the distribution of gluons in transverse space in the nucleon at moderately small x (∼ 1... more We study the distribution of gluons in transverse space in the nucleon at moderately small x (∼ 10 −2). At large transverse distances (impact parameters) the gluon density is generated by the "pion cloud" of the nucleon, and can be calculated in terms of the gluon density in the pion. We investigate the large-distance behavior in two different approaches to chiral dynamics: i) phenomenological softpion exchange, ii) the large-Nc picture of the nucleon as a classical soliton of the pion field, which corresponds to degenerate N and ∆ states. The large-distance contributions from the "pion cloud" cause a ∼ 20% increase in the overall transverse size of the nucleon if x drops significantly below Mπ/MN. This is in qualitative agreement with the observed increase of the slope of the t-dependence of the J/ψ photoproduction cross section at HERA compared to fixed-target energies. We argue that the glue in the pion cloud could be probed directly in hard electroproduction processes accompanied by "pion knockout", γ * + N → γ (or ρ, J/ψ) + π + N ′ , where the transverse momentum of the emitted pion is large while that of the outgoing nucleon is restricted to values of order Mπ.