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V. R. Zoller

Publications and source records attributed to V. R. Zoller.

At least 55 records · Page 3Linked to original sources
arXiv2002

Diffractive Hard Dijets and Nuclear Parton Distributions

Diffraction plays an exceptional role in DIS off heavy nuclei. First, diffraction into hard dijets is an unique probe of the unintegrated glue in the target. Second, because diffraction makes 50 per cent of total DIS off a heavy target, understanding diffraction in a saturation regime is crucial for a definition of saturated nuclear parton densities. After brief comments on the Nikolaev-Zakharov (NZ) pomeron-splitting mechanism for diffractive hard dijet production, I review an extension of the Nikolaev-Schafer-Schwiete (NSS) analysis of diffractive dijet production off nuclei to the definition of nuclear partons in the saturation regime. I emphasize the importance of intranuclear final state interactions for the parton momentum distributions.

hep-ph↗
arXiv2002

Coherent Coulomb excitation of relativistic nuclei in aligned crystal targets

We study coherent Coulomb excitation of ultrarelativistic nuclei passing through the aligned crystal target. We develop multiple scattering theory description of this process which consistently incorporates both the specific resonant properties of particle-crystal interactions and the shadowing effect typical of the diffractive scattering. We emphasise that the effect of quantum mechanical diffraction makes the physics of ultrarelativistic nuclear excitations entirely different from the physics of non-relativistic atomic excitations experimentally studied so far. It is found that at small transverse momenta $q_{\perp}$ the shadowing effect drastically changes the dependence of coherent amplitudes on the crystal thickness $L$, from the widely discussed growth $\propto L$ typical of the Born approximation to the inverse thickness attenuation law. At relatively large $q_{\perp}$ no attenuation effect is found but the coherency condition is shown to put stringent constrain on the growth of the transition rate with growing $L$.

hep-ph↗
arXiv2001

Predicitions for high-energy real and virtual photon-photon scattering from color dipole BFKL-Regge factorization

High-energy virtual photon-virtual photon scattering can be viewed as interaction of small size color dipoles from the beam and target photons, which makes $γ^{*}γ^{*}, γ^{*}γ$ scattering at high energies (LEP, LEP200 & NLC) an indispensable probe of short distance properties of the QCD pomeron exchange. Based on the color dipole representation, we investigate consequences for the $γ^{*}γ^{*},γ^{*}γ$ scattering of the incorporation of asymptotic freedom into the BFKL equation which makes the QCD pomeron a series of isolated poles in the angular momentum plane. The emerging color dipole BFKL-Regge factorization allows us to relate in a model-independent way the contributions of each BFKL pole to $γ^{*}γ^{*},γ^{*} γ$ scattering and DIS off protons. Numerical predictions based on our early works on color dipole BFKL phenomenology of DIS on protons are in a good agreement with the experimental data on the photon structure function $F_{2γ}$ and most recent data on the $γ^*γ^*$ cross section $σ^{γ^*γ^*}(Y)$ from OPAL and L3 experiments at LEP200. We discuss the role of non-perturbative dynamics and predict pronounced effect of the Regge-factorization breaking due to large unfactorizable non-perturbative corrections to the perturbative vacuum exchange. We comment on the salient features of the BFKL-Regge expansion for $γ^{*}γ^{*},γ^{*}γ$ scattering including the issue of decoupling of subleading BFKL poles and the soft plus rightmost hard BFKL pole dominance .

hep-ph↗
arXiv2001

Diffractive beauty photoproduction as a short distance probe of QCD pomeron

High-energy open beauty photoproduction probes the vacuum exchange at distances $\sim 1/m_b$ and detects significant corrections to the BFKL asymptotics coming from the subleading vacuum poles. We show that the interplay of leading and subleading vacuum exchanges gives rise to the cross section $σ^{b\bar b}(W)$ growing much faster than it is prescribed by the exchange of the leading pomeron trajectory with intercept $α_{\Pom}(0)-1=Δ_{\Pom}=0.4$. Our calculations within the color dipole BFKL model are in agreement with the recent determination of $σ^{b\bar b}(W)$ by the H1 collaboration. The comparative analysis of diffractive photoproduction of beauty, charm and light quarks exhibits the hierarchy of pre-asymptotic pomeron intercepts which follows the hierarchy of corresponding hardness scales. We comment on the phenomenon of decoupling of soft and subleading BFKL singularities at the scale of elastic $Υ(1S)$-photoproduction which results in precocious color dipole BFKL asymptotics of the process $γp \to Υp$.

hep-ph↗
arXiv2001

How open charm production and scaling violations probe the rightmost hard BFKL pole exchange

In 1994 Zakharov and the present authors argued that in color dipole (CD) BFKL approach to DIS excitation of open charm at moderately large Q2 is dominated by hard BFKL exchange. In view of the rapid accumulation of the experimental data on small-x charm structure function of the proton F2cc from HERA, we subject the issue of dominance of the rightmost hard BFKL pole exchange to further scrutiny. Based on CD BFKL-Regge factorization we report parameter-free predictions for the charm structure function F2cc and show that the background to the dominant rightmost hard BFKL exchange from subleading hard BFKL and soft-pomeron exchanges is negligible small from real photo-production to DIS at Q2 lsim 50-100 GeV2. The agreement with the experiment is good and lends strong support for the intercept of the rightmost hard BFKL pole DeltaPom=alphaPom-1=0.4 as found in 1994 in the color dipole approach. We comment on the related determination of DeltaPom from the x-dependence of the longitudinal structure function FL(x,Q2) and of the scaling violation dF2/dlog Q2 taken at a suitable value of Q2. \

hep-ph↗
arXiv2000

Probing the QCD pomeron in high-energy $γ^*γ^*$ collisions

Based on the color dipole representation, we investigate consequences for the $γ^{*}γ^{*},γ^{*}γ$ scattering of the finding by Fadin, Kuraev and Lipatov that incorporation of asymptotic freedom into the BFKL equation makes the QCD pomeron a series of isolated poles in the angular momentum plane. We present parameter-free predictions for the vacuum exchange contribution to the photon structure function which agree well with OPAL and L3 determinations. A good agreement is found between our predictions for the energy and photon virtuality dependence of the photon-photon cross section $σ^{γ^*γ^*}(W,Q^2,P^2)$ and the recent data taken by the L3 Collaboration.

hep-ph↗
arXiv1999

The color dipole BFKL-Regge expansion: from DIS on protons to pions to rise of hadronic cross sections

As noticed by Fadin, Kuraev and Lipatov in 1975 incorporation of asymptotic freedom into the BFKL equation splits the QCD pomeron into a series of isolated poles in complex angular momentum plane. Following our earlier work on the proton structure function we explore the phenomenological consequences of the emerging BFKL-Regge factorized expansion for the small-$x$ structure function of the pion $F_{2π}(x,Q^2)$. We calculate $F_{2π}$ in a small-$x$ region and find good agreement with the recent H1 determination of $F_{2π}(x,Q^2)$. We also present the BFKL-Regge factorization based evaluation of the contribution from hard scattering to the observed rise of the $NN$, $πN$ and real photo-absorption $γN$ and $γπ$ total cross sections.

hep-ph↗
arXiv1998

Precocious asymptopia for charm from the running BFKL

The running BFKL equation gives rise to a series of moving poles in the complex j-plane. Corresponding eigenfunctions (color dipole cross sections) are the oscillating functions of the color dipole size $r$. The first nodes for all sub-leading solutions (color dipole cross sections) accumulate at $r_1\sim 0.1 fm$. Therefore the processes dominated by the dipole sizes $r\sim r_1$ are free of sub-leading BFKL corrections. A practically important example - the leptoproduction of charm. In a wide range of $Q^2$ the calculated $F_2^{cc}(x,Q^2)$ is exhausted by the leading BFKL pole and gives a perfect description of the experimental data.

hep-ph↗
arXiv1998

The Running BFKL: Resolution of Caldwell's Puzzle

The HERA data on the proton structure function, $F_2(x,Q^2)$, at very small $x$ and $Q^2$ show the dramatic departure of the logarithmic slope, $\partial F_2/\partial\log Q^2$, from theoretical predictions based on the DGLAP evolution. We show that the running BFKL approach provides the quantitative explanation for the observed $x$ and/or $Q^2$ -dependence of $\partial F_2/\partial\log Q^2$.

hep-ph↗
arXiv1998

The Running BFKL: Precocious Asymptopia for Charm and the $dF_{2}/d\log Q^2$ - Puzzle

The running BFKL equation gives rise to a series of moving poles in the complex $j$-plane. The first nodes for all subleading solutions (color dipole cross sections) accumulate at $r_1\sim 0.1 fm$.Therefore the processes dominated by the dipole sizes $r\sim r_1 $ are free of subleading BFKL corrections. An example - the leptoproduction of charm. The calculated $F_2^{cc}$ is exhausted by the leading BFKL pole and gives a perfect description of the experimental data. The logarithmic slope of the subleading structure functions, $dF_2^{(n)}/d\log Q^2$, at small $Q^2$ is small compared to $dF_2^{(0)}/d\log Q^2$ due to the presence of nodes. This observation provides an explanation for the observed $x$/$Q^2$ dependence of the derivative of the proton structure function $dF_2/d\log Q^2$.

hep-ph↗
arXiv1998

Forward Cone in Exclusive Heavy Vector Meson Production

The color dipole gBFKL phenomenology of a diffraction cone for photo- and electroproduction $γ^{*}N \to VN$ of heavy vector mesons (charmonium & bottonium) at HERA and in fixed target experiments is presented. A substantial shrinkage of the diffraction cone from the CERN/FNAL to the HERA range of c.m.s. energy $W$ is predicted. The $Q^{2}$-controlled selectivity to the color dipole size (scanning phenomenon) is shown to lead to a decrease of the diffraction slope with $Q^{2}$ (which is supported by the available experimental data). An approximate flavor independence of the diffraction slope in the scaling variable $Q^{2}+m_{V}^{2}$ is shown to hold.

hep-ph↗
arXiv1997

The diffraction cone for exclusive vector meson production in deep inelastic scattering

We develop the color dipole gBFKL phenomenology of a diffraction cone for photo- and electroproduction $γ^{*}N \to VN$ of heavy vector mesons (charmonium & bottonium) at HERA and in fixed target experiments. We predict a substantial shrinkage of the diffraction cone from the CERN/FNAL to the HERA range of c.m.s. energy $W$. The $Q^{2}$-controlled selectivity to the color dipole size (scanning phenomenon) is shown to lead to a decrease of the diffraction slope with $Q^{2}$ (which is supported by the available experimental data). We predict an approximate flavor independence of the diffraction slope in the scaling variable $Q^{2}+m_{V}^{2}$. For diffractive production of the radially excited $2S$ states ($Ψ',Υ'$) the counterintuitive inequality of diffraction slopes $B(2S) \lsim B(1S)$ is predicted, which defies the common wisdom that diffraction slopes are larger for reactions with larger size particles.

hep-ph↗
arXiv1997

The BFKL-Regge Phenomenology of Deep Inelastic Scattering

We calculate the Regge trajectories of the subleading BFKL singularities and eigenfunctions for the running BFKL pomeron in the color dipole representation. We obtain a viable BFKL-Regge expansion of the proton structure function $F_{2p}(x,Q^2)$ in terms of several rightmost BFKL singularities. We find large subleading contributions to $F_{2p}(x,Q^2)$ in the HERA kinematical region which explains the lack of a predictive power of GLDAP-extrapolations of $F_{2p}(x,Q^2)$ to a domain of small $x$. We point out the relation of our early finding of the precocious BFKL asymptotics to the nodal structure of subleading BFKL eigenfunctions.

hep-ph↗
arXiv1997

Coherent neutrino magnetic conversion in crystals

We study coherent enhancement of neutrino electro-magnetic conversion in crystals. Large coherency length which grows with the neutrino energy makes the coherent enhancement particularly effective for very small masses. We derive constraints on the conversion rate which follow from the Fresnel effects in scattering of neutrinos on atomic chains. We comment on possible applications of the crystal converter to searches for non-diagonal neutrino magnetic transitions in the CERN neutrino beam.

hep-ph↗
arXiv1996

Rare processes and coherent phenomena in crystals

We study coherent enhancement of Coulomb excitation of high energy particles in crystals. We develop multiple scattering theory description of coherent excitation which consistently incorporates both the specific resonant properties of particle-crystal interactions and the final/initial state interaction effects typical of the diffractive scattering. Possible applications to observation of induced radiative neutrino transitions are discussed.

hep-ph↗
arXiv1996

Diffraction in DIS on nuclear targets

We discuss the implications of a strong enhancement of diffraction for multiproduction in DIS off nuclear targets. The predicted effects are large and observable at HERA. We present the prediction of a large tensor structure function b_2(x) of the deuteron which does not vanish at small x.

hep-ph↗
arXiv1995

Exploratory study of shrinkage of the diffraction cone for the generalized BFKL pomeron

In color dipole gBFKL dynamics, we describe the emerging gBFKL phenomenology of a subasymptotic energy dependence of the diffraction slope and discuss possibilities of testing the gBFKL predictions in exclusive photo- and electroproduction of vector mesons $V$ at HERA. A substantial shrinkage of the diffraction cone $γ^{*}p\rightarrow Vp$ processes from the CERN/FNAL to HERA range of energy $W$ is predicted. This subasymptotic shrinkage is faster than expected from the small slope of pomeron's Regge trajectory $α_{\Pom}'$. We point out that the diffraction slope is a scaling function of $(m_{V}^{2}+Q^{2})$, what relates production of different vector mesons.

hep-ph↗