arXiv subjects

M. Strikman

Publications and source records attributed to M. Strikman.

At least 181 records · Page 10Linked to original sources

arXiv2002

Nuclear shadowing in deep inelastic scattering on nuclei: leading twist versus eikonal approaches

We use several diverse parameterizations of diffractive parton distributions, extracted in leading twist QCD analyses of the HERA diffractive deep inelastic scattering (DIS) data, to make predictions for leading twist nuclear shadowing of nuclear quark and gluon distributions in DIS on nuclei. We find that the HERA diffractive data are sufficiently precise to allow us to predict large nuclear shadowing for gluons and quarks, unambiguously. We performed detailed studies of nuclear shadowing for up and charm sea quarks and gluons within several scenarios of shadowing and diffractive slopes, as well as at central impact parameters. We compare these leading twist results with those obtained from the eikonal approach to nuclear shadowing (which is based on a very different space-time picture) and observe sharply contrasting predictions for the size and Q^2-dependence of nuclear shadowing. The most striking differences arise for the interaction of small dipoles with nuclei, in particular for the longitudinal structure function F_{L}^{A}.

hep-ph↗

arXiv2001

Revealing the black-body regime of small-x deep-inelastic scattering through final-state signals

We derive the major characteristics of inclusive and diffractive final states in deep-inelastic scattering off heavy nuclei for the-high energy (small-x) kinematics in which the limit of complete absorption is reached for the dominant hadronic fluctuations in the virtual photon (the black-body limit of the process). Both the longitudinal and transverse distributions of the leading hadrons are found to be strikingly different from the corresponding ones within the leading-twist approximation, and hence provide unambiguous signals for the onset of the black-body limit.

hep-ph↗

arXiv2001

Measuring Double Parton Distributions in Nucleons at Proton-Nucleus Colliders

We predict a strong enhancement of multijet production in proton-nucleus collisions at collider energies, as compared to a naive expectation of a cross section $\propto A$. The study of the process would allow to measure, for the first time, the double parton distribution functions in a nucleon in a model independent way and hence to study both the longitudinal and the transverse correlations of partons.

hep-ph↗

arXiv2001

Fading out of J/psi color transparency in high energy heavy ion peripheral collisions

We provide predictions for the J/psi coherent production in the peripheral heavy ion collisions at LHC and at RHIC using the leading twist model of nuclear shadowing based on the QCD factorization theorem for diffraction and the HERA hard diffraction data. We demonstrate that for the LHC kinematics this model leads to a bump-shape distribution in rapidity which is suppressed overall as compared to the expectations of the color transparency regime by a factor 6. This is significantly larger suppression than that expected within the impact parameter eikonal model. Thus we show that the interaction of spatially small wave package for which the total cross section of interaction with nucleons is small is still strongly shadowed by nuclear medium in high energy processes.

hep-ph↗

arXiv2001

Complete analysis of spin structure function g1 of He-3

We present a comprehensive analysis of the nuclear effects important in DIS on polarized He-3 over a wide range of Bjorken x, 10^{-4} \leq x \leq 0.8. Effects relevant for the extraction of the neutron spin structure function, g1^n, from the He-3 data are emphasized.

hep-ph↗

arXiv2001

Nuclear Broadening Effects on Hard Prompt Photons at Relativistic Energies

We calculate prompt photon production in high-energy nuclear collisions. We focus on the broadening of the intrinsic transverse momenta of the partons in the initial state from nuclear effects, and their influence on the prompt photon p_t distribution. Comparing to WA98 data from Pb+Pb collisions at SPS energy we find evidence for the presence of nuclear broadening at high p_t in this hard process. Below p_t=2.7 GeV the photon distribution is due to small momentum transfer processes. At RHIC energy, the effect of intrinsic transverse momentum on the spectrum of prompt photons is less prominent. The region p_t=3-4 GeV would be the most promising for studying the nuclear broadening effects at that energy. Below p_t=2-3 GeV the contribution from large momentum transfers flattens out, and we expect that region to be dominated by soft contributions.

hep-ph↗

arXiv2001

Electron-Nucleus Collisions at THERA

The nuclear option at THERA provides an ideal and unique opportunity to investigate the black body limit (BBL) of high energy Deep Inelastic Scattering of highly virtual photons off heavy nuclear targets and thereby probe QCD in a new regime. At high enough energies, whichever hadronic configuration the photon fluctuates into, the interaction at a given impact parameter with the heavy nuclear target will eventually reach its geometrical limit corresponding to the scattering from a black disk. An attractive feature of the BBL regime for a large nucleus is that the interaction is strong although the coupling constant is small. Predictions for longitudinal and transverse distributions of the leading hadrons in inclusive and diffractive channels and exclusive vector meson production are found to be strikingly different in BBL and the leading twist approximation. In particular, the multiplicity of leading hadrons in the current fragmentation region is strongly suppressed, while the cross section of diffractive vector meson production by longitudinally polarized photons is $ \propto 1/Q^2$. The x and $Q^2$ ranges where BBL maybe reached are calculated for the interaction of color triplet and color octet dipoles at central impact parameters. We conclude that for heavy nuclei THERA would reach deep into BBL for a wide range of $Q^2$. The connection between hard diffraction and leading twist nuclear shadowing is analysed. It is demonstrated that the current leading twist analyses of the HERA diffractive data lead to similar and very large leading twist shadowing for gluons. Several processes sensitive to the amount of nuclear shadowing for gluons are discussed.

hep-ph↗

arXiv2001

A fresh look at diffractive $J/ψ$ photoproduction at HERA, with predictions for THERA

We quantify perturbative and non-perturbative QCD effects in the exclusive $J/ψ$-photoproduction cross section, and in the shrinkage of the differential cross section with respect to momentum transfer, $t$. We predict that in the high energy THERA region there will always be a significant contribution to this process that rises quickly with energy. This implies that the taming of the rise of the cross section with energy, due to both the expansion of spatially-small fluctuations in the photon and to higher twist effects, is rather gradual.

hep-ph↗

arXiv2001

Color coherent phenomena on nuclei and the QCD evolution equation

We review the phenomenon of color coherence in quantum chromodynamics (QCD), its implications for hard and soft processes with nuclei, and its experimental manifestations. The relation of factorization theorems in QCD with color coherence phenomena in deep inelastic scattering (DIS) and color coherence phenomena in hard exclusive processes is emphasized. Analyzing numerically the QCD evolution equation for conventional and skewed parton densities in nuclei, we study the onset of generalized color transparency and nuclear shadowing of the sea quark and gluon distributions in nuclei as well as related phenomena. Such novel results as the dependence of the effective coherence length on $Q^2$ and general trends of the QCD evolution are discussed. The limits of the applicability of the QCD evolution equation at small Bjorken $x$ are estimated by comparing the inelastic quark-antiquark- and two gluon-nucleon (nucleus) cross sections, calculated within the DGLAP approximation, with the dynamical boundaries, which follow from the unitarity of the $S$ matrix for purely QCD interactions. We also demonstrate that principles of color coherence play an important role in the processes of soft diffraction off nuclei.

hep-ph↗

arXiv2001

Non-singlet structure function of the 3He-3H system and divergence of the Gottfried integral

We study shadowing and antishadowing corrections to the flavor non-singlet structure function F_2(3He)-F_2(3H) and show that the difference between the one-particle density distributions of 3He and 3H plays an important role at very small x. We find that the flavor non-singlet structure function in these mirror nuclei is enhanced at small x by nuclear shadowing, which increases the nuclear Gottfried integral, integrated from 10^{-4} to 1, by 11-36 %. When integrated from zero, the Gottfried integral is divergent for these mirror nuclei. It seems likely that, as a consequence of charge symmetry breaking, this may also apply to the proton-neutron system.

hep-ph↗

arXiv2001

Soft pion theorem for hard near threshold pion production

We prove new soft pion theorem for the near threshold pion production by a hard electromagnetic probe. This theorem relates various near threshold pion production amplitudes to the nucleon distribution amplitudes. The new soft pion theorem is in a good agreement with the SLAC data for F_2^p(W,Q^2) for W^2 < 1.4 GeV^2 and 7 < Q^2 < 30.7 GeV^2.

hep-ph↗

arXiv2000

Single particle strength restoration and nuclear transparency in high Q^2 exclusive (e,e'p) reactions

Quenching of the single particle strength which previously precluded unambiguous measurement of nuclear transparency in quasifree (e,e'p) reactions at high momentum transfer is evaluated from the cross sections of measured processes. We have demonstrated evidence of the single particle strength restoration with increase of the momentum transfer in exclusive (e,e'p) reactions and argue that in practically interesting cases of study the nuclear transparency in these processes quenching is weakly depends on the probability of short range nucleon correlations. It is shown that use of Glauber approximation well describes NE18 and E94-139 (e,e'p) data at the momentum transfers 1 GeV^2 < Q^2 < 3 GeV^2. This gives a further support to our observation that the quenching of nuclear levels strongly depends on the resolution (Q^2)and practically disappears at Q^2>1GeV^2.

hep-ph↗

arXiv2000

Suppression of Quarkonium Production in Heavy Ion Collisions at RHIC and LHC

A model for the production of quarkonium states in the midrapidity region at RHIC and LHC energy range is presented which explores well understood properties of QCD only. An increase of the quarkonium hadronisation time with the initial energy leads to a gradual change of the most important phenomena from fixed target- to collider-energies. We evaluate nuclear effects in the quarkonium production due to medium modification of the momentum distribution of the heavy quarks produced in the hard interactions, i.e. due to the broadening of the transverse momentum distribution. Other nuclear effects, i.e. nuclear shadowing and parton energy loss, are also evaluated.

nucl-th↗

arXiv2000

Role of the Delta (1232) in DIS on polarized $^3$He and extraction of the neutron spin structure function $g_{1}^{n}(x,Q^2)$

We consider the effect of the transitions $n \to Δ^{0}$ and $p \to Δ^{+}$ in deep inelastic scattering on polarized $^3$He on the extraction of the neutron spin structure function $g_{1}^{n}(x,Q^2)$. Making the natural assumption that these transitions are the dominant non-nucleonic contributions to the renormalization of the axial vector coupling constant in the A=3 system, we find that the effect of $Δ$ increases $g_{1}^{n}(x,Q^2)$ by $10 ÷40$% in the range $0.05 \le x \le 0.6$, where our considerations are applicable and most of the data for $g_{1}^{n}(x,Q^2)$ exist.

hep-ph↗

arXiv2000

DVCS amplitude in the parton model

We compute amplitude of deeply virtual Compton scattering in the parton model. We found that the amplitude up to the accuracy O(1/Q) depends on new skewed parton distributions (SPD's). These additional contributions make the DVCS amplitude explicitly transverse.

hep-ph↗

arXiv2000

Unitarity and the QCD-improved dipole picture

As a consequence of QCD factorization theorems, a wide variety of inclusive and exclusive cross sections may be formulated in terms of a universal colour dipole cross section at small $x$. It is well known that for small transverse size dipoles this cross section is related to the leading-log gluon density. Using the measured pion-proton cross section as a guide, we suggest a reasonable extrapolation of the dipole cross section to the large transverse size region. We point out that the observed magnitude and small $x$ rise of the gluon density from conventional fits implies that the DGLAP approximation has a restricted region of applicability. We found that `higher twist' or unitarity corrections are required in, or close to, the HERA kinematic region, even for small `perturbative' dipoles for scattering at central impact parameters. This means that the usual perturbative leading twist description, for moderate virtualities, $1 < Q^2 < 10$ GeV$^2$, has rather large `higher twist' corrections at small $x$. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles ($b \gsim 0.4$ fm) to $F_2$, and also to $F_L$. This also leads to deviations from the standard leading twist DGLAP results, at small $x$ and moderate $Q^2$. Our model also describes the low $Q^2$ data very well without any further tuning. We generalize the Gribov unitarity limit for the structure functions of a hadron target to account for the blackening of the interaction at central impact parameters and to include scattering at peripheral impact parameters which dominate at extremely large energies.

hep-ph↗

arXiv2000

The Transparency of 12C for Protons

Existing cross-section data for 1p-shell knockout in the reaction 12C(e,e'p) - as obtained under different kinematic conditions - are shown to be mutually consistent, apart from a recent measurement performed in Mainz. New data have been collected at the Amsterdam Pulse Stretcher that confirm the normalization of the older measurements. An analysis of the world's 12C(e,e'p) data has yielded precise values of the spectroscopic factor for 1p-shell and 1s-shell knockout from 12C. These values have been used to evaluate the transparency of the 12C nucleus for 1p-shell and 1s-shell protons separately on the basis of recent high-energy 12C(e,e'p) data taken at a four-momentum transfer squared of 1.1 GeV/c^2. As the resulting average value of the nuclear transparency, 0.81(0.04), is considerably higher than the value obtained from previous analyses and theoretical estimates, the high Q^2 data were used instead for an independent determination of the spectroscopic strength for 1p + 1s knockout. Combining these results with the low Q^2 data the spectroscopic factors appear to be momentum-transfer dependent. Possible explanations of these surprising results in terms of reaction-mechanism effects or a possible breakdown of the quasi-particle concept at high Q^2 are discussed as well.

nucl-ex↗

arXiv1999

Hard exclusive electroproduction of decuplet baryons in the large N_c limit

The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N_c limit and found to be comparable to that of octet baryons. Large N_c 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.

hep-ph↗