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B. Povh

Publications and source records attributed to B. Povh.

At least 19 recordsLinked to original sources
arXiv2018

Proton Spin in Deep Inelastic Scattering

So far the analyses of the polarized structure functions of the proton and neutron have been limited to the evaluation of their integrals and comparing them to the prediction of the static quark model of the nucleon. We extended our analysis to the x dependence of the polarized structure functions and observe: the measured structure function excellently agrees with the prediction of the static quark model for Bjorken $x>0.2$ and drops rapidly for $x<0.2$. It is suggested that for Bjorken $x>0.2$ electrons get scattered on the undamaged constituent quarks (alias valence quarks) denoted as quasi-elastic scattering on the constituent quarks and for $x<0.2$ the constituent quarks fragment. In the fragmentation strong interaction is involved which does not preserve the polarization.

hep-ph↗
arXiv2012

Pion structure function at small x from DIS data

Production of leading neutrons in DIS is usually considered as a tool to measure the pion structure function at small x accessible at HERA. The main obstacle is the lack of reliable evaluations of the absorption corrections, which significantly suppress the cross section. We performed a parameter free calculation within the dipole approach and found the absorption corrections to be nearly as strong, as for neutron production in pp collisions. We also included the significant contribution of the iso-vector Reggeons with natural (ρ, a_2) and unnatural (a_1, ρ-π cut) parity with parameters constrained by phenomenology. With a certain modeling for the pion-to-proton ratio of the structure functions we reached good agreement with data from the ZEUS and H1 experiments, successfully reproducing the observed dependence on the fractional neutron momentum z, the photon virtuality Q^2, and the transverse momentum transfer q_T.

hep-ph↗
arXiv2011

The qq$\bar{\mathbf{q}}$ component in the constituent quark generates the pion cloud of the nucleon

The spin properties and the flavor asymmetric sea of the nucleon can be well explained by assuming that each constituent quark is surrounded with about 30% probability by a quark-antiquark pair coupled to the pion quantum numbers. We show that about one quarter of these quark-antiquark pairs show up as the pion in $\p\to\nπ^+$ and $\p\to\pπ^0$ fluctuation in agreement with the observed value in the ($\e+\p\to\e+$ forward neutron+X) experiment.

hep-ph↗
arXiv2007

Evidences for two scales in hadrons

Some unusual features observed in hadronic collisions at high energies can be understood assuming that gluons in hadrons are located within small spots occupying only about 10% of the hadron's area. Such a conjecture about the presence of two scales in hadrons helps to explain: why diffractive gluon radiation so much suppressed; why the triple-Pomeron coupling shows no t-dependence; why total hadronic cross sections rise with energy so slowly; why diffraction cone shrinks so slowly, and why $α^\prime_P\llα^\prime_R$; why the transition from hard to soft regimes in the structure functions occurs at rather large $Q^2$; why the observed Cronin effect at collider energies is so weak; why hard reactions sensitive to primordial parton motion (direct photon, Drell-Yan dileptons, heavy flavors, back-to-back di-hadrons, seagull effect, etc.) demand such a large transverse momenta of the projectile partons, which is not explained by NLO calculations; why the onset of nuclear shadowing for gluons is so much delayed compared to quarks, and why shadowing is so weak.

hep-ph↗
arXiv2007

Observation of a resonance in the K$_s$p decay channel at a mass of 1765 MeV/c$^2$

We report on the observation of a K$_s$p resonance signal at a mass of 1765$\pm$5 MeV/c$^2$, with intrinsic width $Γ= 108\pm 22$ MeV/c$^2$, produced inclusively in $Σ^-$-nucleus interactions at 340 GeV/c in the hyperon beam experiment WA89 at CERN. The signal was observed in the kinematic region $x_F>0.7$, in this region its production cross section rises approximately linearly with $(1-x_F)$, reaching $BR(X\to K_S p)\cdot dσ/dx_F = (5.2\pm 2.3) μb $ per nucleon at $x_F=0.8$. The hard \xf spectrum suggests the presence of a strong leading particle effect in the production and hence the identification as a $Σ^{*+}$ state. No corresponding peaks were observed in the $K^- p$ and $Λπ^{\pm}$ mass spectra.

hep-ex↗
arXiv2006

Glue drops inside hadrons

We present experimental evidences for the existence of a semi-hard scale in light hadrons. This includes the suppression of gluon radiation that is seen in high mass hadron diffraction; the weak energy dependence of hadronic total cross sections; the small value of the Pomeron trajectory slope measured in photoproduction of J/Psi; the weakness of gluon shadowing in nuclei; shortage of gluons in the proton revealed by an unusual behavior of the proton structure function in the soft limit, and the enhanced intrinsic transverse momentum of quarks and gluons, which considerably exceeds the inverse hadronic size. All these observations suggest that gluons in hadrons are located within spots of a small size relative to the confinement radius.

hep-ph↗
arXiv2004

Small Gluonic Spots in the Nucleon: Searching for Signatures in Data

Nuclear shadowing and color glass condensate are possible only at sufficiently small x where parton clouds of different nucleons overlap in the longitudinal direction. Another condition vital for these effect, an overlap of partons in impact parameters, is not easy to fulfill for gluons which are located within small spots, as follows from the observed weakness of diffractive gluon radiation (smallness of the triple-Pomeron coupling). The predicted weakness of the leading twist gluon shadowing has been confirmed recently by data for J/Psi production and Cronin effect in d-Au collisions at RHIC. Smallness of gluonic spots also leads to a rather low value of the slope of the Pomeron trajectory, confirmed by ZEUS data on elastic photoproduction of J/Psi. At the same time, saturation of unitarity for central pp collisions leads to a substantial increase of the Pomeron slope in good agreement with elastic pp data.

hep-ph↗
arXiv2002

Baryon Number Flow in High-Energy Collisions

It is not obvious which partons in the proton carry its baryon number (BN). We present arguments that BN is associated with a specific topology of gluonic fields, rather than with the valence quarks. The BN distribution is easily confused with the difference between the quark and antiquark distributions. We argue, however, that they have quite different x-dependences. The distribution of BN asymmetry distribution is nearly constant at small x while q(x)-\bar q(x) \propto \sqrt{x}. This constancy of BN produces energy independence of the \bar pp annihilation cross section at high energies. Recent measurement of the baryon asymmetry at small x at HERA confirms this expectation. The BN asymmetry at mid-rapidities in heavy ion collisions is substantially enhanced by multiple interactions, as has been observed in recent experiments at the SPS. The same gluonic mechanism of BN stopping increases the production rate for cascade hyperons in a good accord with data. We expect nearly the same as at SPS amount of BN stopped in higher energy collisions at RHIC and LHC, which is, however, spread ove larger rapidity intervals.

hep-ph↗
arXiv2000

Soft QCD Dynamics of Elastic Scattering in Impact Parameter Representation

The elastic hadronic amplitude is calculated using the nonperturbative light-cone dipole representation for gluon bremsstrahlung. The data for large mass diffraction demand a two-scale structure of light hadrons: the gluon clouds of the valence quarks with a size of 0.3fm and the hadronic size 1fm. The presence of the two scales unavoidably leads to a specific form for the total hadronic cross section which consists of a steeply rising \propto s^Δ (Δ=0.17 +/- 0.01) term related to gluon radiation, and a large constant term originating from soft interactions which does not induce any gluon emission. Our calculations reproduce well the total cross sections and elastic slopes \cite{k3p}. To further test the model, we analyze the elastic pp and ppbar differential cross sections and extract the partial amplitudes in the impact parameter representation. The Pomeron trajectory as a function of the impact parameter is only slightly above one for central collisions, but steeply grows towards the periphery. The model predicts correctly the shape and energy dependence of the partial amplitude at all impact parameters.

hep-ph↗
arXiv2000

Nonperturbative Gluon Radiation and Energy Dependence of Elastic Scattering

The energy dependence of the total hadronic cross sections is caused by gluon bremsstrahlung which we treat nonperturbatively. It is located at small transverse distances about 0.3 fm from the valence quarks. The cross section of gluon radiation is predicted to exponentiate and rise with energy as s^Δ with Δ=0.17 +/- 0.01. The total cross section also includes a large energy independent Born term which corresponds to no gluon radiation. The calculated total cross section and the slope of elastic scattering are in good agreement with the data.

hep-ph↗
arXiv1999

Hyperon Radii

A survey of the experimental data on the charge and strong hadronic radii is given. A new, however preliminary value of the Σ^- charge radius allows to include also hyperons in the systematics. The presented data on the hadronic radii give the first hint of their dependence on the strangeness content. An estimate of the constituent quark size is given.

hep-ph↗
arXiv1998

Baryon Stopping at HERA: Evidence for Gluonic Mechanism

Recent results from the H1 experiment [1] have confirmed the existence of a substantial baryon asymmetry of the proton sea at small $x$ with magnitude predicted in hep-ph/9607486. This is a strong support for the idea [3] that baryon number can be transferred by gluons through a large rapidity interval without attenuation. In this paper we calculate the dependence of baryon asymmetry on associated multiplicity of produced hadrons which turns out to be very sensitive to the underlying dynamics. Comparison with data [1] confirms the dominance of the gluonic mechanism of baryon number transfer and excludes any substantial contribution of valence quark exchange.

hep-ph↗
arXiv1998

Energy Dependence of the Pomeron Spin-Flip

There is no theoretical reason to think that the spin-flip component of the Pomeron is zero. One can measure the spin-flip part using Coulomb-nuclear interference (CNI). Perturbative QCD calculations show that the spin-flip component is sensitive to the smallest quark separation in the proton, while the non-flip part probes the largest separation. According to HERA results on the proton structure function at very low x the energy dependence of the cross-section correlates with the size of the color dipole. Analysing the data from HERA we predict that the ratio of the spin-flip to non-flip amplitude grows with energy as $r(s)\propto (1/x)^{0.1-0.2}$, violating Regge factorisation of the Pomeron.

hep-ph↗
arXiv1998

Hadron Interactions - Hadron Sizes

A geometrical model of hadron-proton interactions has been extended to describe gamma-proton total cross section measured in deep-inelastic scattering at small x (x<0.04). Experimental data are consistent with this model assuming that the hadronic fluctuation of the virtual photon has a size of ~ 1/(4m_q^2 + Q^2), where m_q is the mass of the light constituent quark.

hep-ph↗
arXiv1997

Unitarity effects in DIS

We argue that diffractive DIS, dominated by soft interactions, is probably the unique process which allows us to observe unitarity effects in DIS. Guided by a close analogy between the diffractive dissociation of a highly virtual photon and the elastic scattering of hadrons we propose a specific procedure to analyse the data in order to detect the onset of the unitarity limit. Lacking appropriate data, we use the predictions of a realistic model as an input for our analysis, to demonstrate that the output unitarity signal is sufficiently large to be detectable.

hep-ph↗
arXiv1996

Baryon Asymmetry of the Proton Sea at Low $x$

We predict a nonvanishing baryon asymmetry of the proton sea at low $x$. It is expected to be about $7\%$ and nearly $x$-independent at $x < 0.5 \times 10^{-3}$. The asymmetry arises from the baryon-antibaryon component of the Pomeron, rather than from the valence quarks of the proton, which are wide believed carriers of baryon number. Experimental study of $x$-distribution of the baryon asymmetry of the proton sea can be performed in $ep$ or $γp$ interactions at HERA, where $x\sim 10^{-5}$ are reachable, smaller than at any of existing or planned proton colliders.

hep-ph↗
arXiv1996

Interplay of Soft and Hard Interactions in Nuclear Shadowing at High $Q^2$ and Low x

Nuclear shadowing corrections are dominated by soft interaction and grow as function of $1/x$ more slowly than the single scattering term, which has an essential contribution from hard interaction. Therefore, we predict vanishing nuclear shadowing at very low $x$ provided that $Q^2$ is high and fixed. At the same time, at medium and low $Q^2$, nuclear shadowing grows with $1/x$ as is well known for soft hadronic interactions.

nucl-th↗