Partonic Description Research Articles

Coherent γ*-nucleus scattering and coherent nuclear states

In the context of a McLerran-Venugopalan (MV) model for a large nucleus, coherent scattering of a virtual photon on that nucleus is evaluated in the A− = 0 gauge, the gauge appropriate for the target nucleus. The evaluation of the scattering in A− = 0 gauge is very intricate compared to the usual A+ = 0 gauge evaluation natural for the scattering process, but has the advantage of directly giving the scattering in terms of a partonic description of the nucleus. In the limit where a tagged forward jet puts the dipole-nucleus scattering in the saturation regime the coherent reactions are equal to the inelastic reactions. In terms of the nuclear wave function the coherent reactions come from color singlet and zero total transverse momentum quark-antiquark pairs in the wave function and in the saturation regime the nuclear wave function is a coherent state for these pairs. In the saturation region half of all quarks (or antiquarks) come from zero momentum and color charge pairs.

EpIC: novel Monte Carlo generator for exclusive processes

We present the EpIC Monte Carlo event generator for exclusive processes based on generalised parton distributions. EpIC utilises the PARTONS framework, which provides a flexible software architecture and a variety of modelling options for the partonic description of the nucleon. The generator offers a comprehensive set of features, including simulation of a variety of exclusive processes and radiative corrections. It may be used both in the analysis of experimental data, as well as for impact studies, especially for the future electron-ion colliders.

Decays of a NMSSM CP-odd Higgs in the low-mass region

A popular regime in the NMSSM parameter space involves a light CP-odd Higgs A1. This scenario has consequences for e.g. light singlino Dark Matter annihilating in the A1-funnel. In order to confront the pseudoscalar to experimental limits such as flavour observables, Upsilon decays or Beam-Dump experiments, it is necessary to control the interactions of this particle with hadronic matter and derive the corresponding decays. The partonic description cannot be relied upon for masses close to {m_A}_{{}_1} ∼ 1GeV and we employ a chiral lagrangian, then extended to a spectator model for somewhat larger masses, to describe the interplay of the CP-odd Higgs with hadrons. Interestingly, a mixing can develop between A1 and neutral pseudoscalar mesons, leading to substantial hadronic decays and a coupling of A1 to the chiral anomaly. Additionally, quartic A1-meson couplings induce tri-meson decays of the Higgs pseudoscalar. We investigate these effects and propose an estimate of the Higgs widths for masses below {m_A}_{{}_1} ≲ 3 GeV. While we focus on the case of the NMSSM, our results are applicable to a large class of models.

Phenomenology of hadron structure — why low energy physics matters

The description of the internal structure of hadrons is one of the main goal of QCD. At moderate energy scales, the hadronic representation succeeds to the partonic description, rendering challenging the description of the dynamics of scattering processes and hadronic structure. The information on the hadron structure is embodied in the long distance contributions which are defined as Parton Distribution Functions (PDFs). PDFs are a key framework for connecting the low and high-energy regimes, in that the knowledge on nonperturbative QCD carries important consequences at the high-energy level. We here review recent progress in the description of the proton, from complementary approaches such as fits of PDFs, phenomenological analyses and experimental predictions in view of the JeffersonLab upgrade and applications for high-energy colliders.

Chiral dynamics and peripheral transverse densities

In the partonic (or light-front) description of relativistic systems the electromagnetic form factors are expressed in terms of frame-independent charge and magnetization densities in transverse space. This formulation allows one to identify the chiral components of nucleon structure as the peripheral densities at transverse distances b = O(M_pi^{-1}) and compute them in a parametrically controlled manner. A dispersion relation connects the large-distance behavior of the transverse charge and magnetization densities to the spectral functions of the Dirac and Pauli form factors near the two-pion threshold at timelike t = 4 M_pi^2. Using relativistic chiral effective field theory in the leading-order approximation, we (a) derive the asymptotic behavior (Yukawa tail) of the isovector transverse densities in the "chiral" region b = O(M_pi^{-1}) and the "molecular" region b = O(M_N^2/M_pi^3); (b) perform the heavy-baryon expansion; (c) explain the relative magnitude of the peripheral charge and magnetization densities in a simple mechanical picture; (d) include Delta intermediate states and study the densities in the large-N_c limit of QCD; (e) quantify the spatial region where the chiral components are numerically dominant; (f) calculate the chiral divergences of the b^2-weighted moments of the transverse densities (charge and magnetic radii) and determine their spatial support. Our approach provides a concise formulation of the spatial structure of the nucleon's chiral component and offers new insights into basic properties of the chiral expansion. It relates the information extracted from low-t elastic form factors to the generalized parton distributions probed in peripheral high-energy scattering processes.

ESTIMATING THE SIGN AND SIZE OF THE TRANSVERSE SINGLE TARGET-SPIN ASYMMETRY IN INCLUSIVE DIS

The single-spin asymmetry of unpolarized leptons scattering deep-inelastically off transversely polarized nucleons is discussed. This observable is generated by a two-photon exchange between lepton and nucleon. In a partonic description of the asymmetry the non-perturbative part is given in terms of multiparton correlations: quark-gluon correlation functions and quark-photon correlation functions. Using input from models and data fits for these twist-3 correlation functions, estimates for the single-spin asymmetries for a proton and a neutron are presented.

Partonic description of the transverse target single-spin asymmetry in inclusive deep-inelastic scattering

The single-spin asymmetry of unpolarized leptons scattering deep-inelastically off transversely polarized nucleons is studied in a partonic picture within a collinear twist-3 framework. Since this observable is generated by multi-photon exchanges between lepton and nucleon a partonic description of the asymmetry contains essential elements of a full next-to-leading order calculation for single-spin asymmetries in perturbative Quantum Chromodynamics. In particular it is shown how nontrivial cancellations between kinematical and dynamical twist-3 contributions lead to a well-behaved and finite formula. This final result can be expressed in terms of multipartonic quark-gluon and quark-photon correlation functions. Hence, a measurement of the transverse target single-spin asymmetry may provide new constraints on these multipartonic correlations.

Spin Observables in Transition-Distribution-Amplitude Studies

Exclusive hadronic reactions with a massive lepton pair (ℓ+ℓ−) in the final state will be measured with PANDA at GSI-FAIR and with Compass at CERN, both in p + p̄ → ℓ+ℓ− + π and π + N → N′ + ℓ+ℓ−. Similarly, electroproduction of a meson in the backward region will be studied at JLAB. We discuss here how the spin structure of the amplitude for such processes will enable us to disentangle various mechanisms. For instance, target-transverse-spin asymmetries are specific of a partonic description, where the amplitude is factorised in terms of baryon to meson or meson to baryon Transition Distribution Amplitudes (TDAs) as opposed to what is expected from baryon-exchange contributions.

Partonic description of a supersymmetric p-brane

We consider supersymmetric extensions of a recently proposed partonic description of a bosonic p-brane which reformulates the Nambu-Goto action as an interacting multi-particle action with Filippov-Lie algebra gauge symmetry. We construct a worldline supersymmetric action by postulating, among others, a p-form fermion. Demanding a local worldline supersymmetry rather than the full worldvolume supersymmetry, we circumvent a known no-go theorem against the construction of a Ramond-Neveu-Schwarz supersymmetric action for a p-brane of p>1. We also derive a spacetime supersymmetric Green-Schwarz extension from the preexisting kappa-symmetric action.

Investigating the exclusive protoproduction of dileptons at high energies

Using the high energy color dipole approach, we study the exclusive photoproduction of lepton pairs $\ensuremath{\gamma}N\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}(\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}})N$ (with $N=p$, $A$). We use simple models for the elementary dipole-hadron scattering amplitude that captures main features of the dependence on atomic number $A$, on energy and on momentum transfer $t$. This investigation is complementary to conventional partonic description of timelike Compton scattering, which considers quark handbag diagrams at leading order in ${\ensuremath{\alpha}}_{s}$ and simple models of the relevant generalized parton distributions. These calculations are input in electromagnetic interactions in $pp$ and $AA$ collisions to measured at the LHC.

Deep inelastic scattering at strong coupling from gauge/string duality: the saturation line

For gauge theories which admit a dual string description, we analyze deep inelastic scattering at strong 't Hooft coupling and high energy, in the vicinity of the unitarity limit. We discuss the onset of unitarity corrections and determine the saturation line which separates weak scattering from strong scattering in the parameter space of rapidity and photon virtuality. We discover that the approach towards unitarity proceeds through two different mechanisms, depending upon the photon virtuality Q^2 : single Pomeron exchange at relatively low Q^2 and, respectively, multiple graviton exchanges at higher Q^2. This implies that the total cross-section at high energy and large Q^2 is dominated by diffractive processes. This is furthermore suggestive of a partonic description where all the partons have transverse momenta below the saturation momentum and occupation numbers of order one.

Singleton strings

In this note, we briefly introduce singleton representations and discuss their relevance to the study of string and brane configurations in AdS space, their tensionless limit and the connection with higher-spin gauge theory. We then discuss the main properties of subcritical SO(D-1,2) WZW models featuring singletons and their composites as primary fields. After a suitable gauging, these models are good candidates for a fundamental, partonic description of tensionless branes in AdS. Their massless sector provides an affine Lie-algebraic setting for the study of higher-spin symmetries. Based on hep-th/0508124, hep-th/0701051 and work in progress.

Generalized parton distributions and rapidity gap survival in exclusive diffractiveppscattering

We study rapidity gap survival (RGS) in the production of high-mass systems ($H=\mathrm{\text{dijet}}$, heavy quarkonium, Higgs boson) in double-gap exclusive diffractive $pp$ scattering, $pp\ensuremath{\rightarrow}p+(\mathrm{\text{gap}})+H+(\mathrm{\text{gap}})+p$. Our approach is based on the idea that hard and soft interactions are approximately independent because they proceed over widely different time and distance scales. We implement this idea in a partonic description of proton structure, which allows for a model-independent treatment of the interplay of hard and soft interactions. The high-mass system is produced in a hard scattering process with exchange of two gluons between the protons, whose amplitude is calculable in terms of the gluon generalized parton distribution (GPD), measured in exclusive $ep$ scattering. The hard scattering process is modified by soft spectator interactions, which we calculate neglecting correlations between hard and soft interactions (independent interaction approximation). We obtain an analytic expression for the RGS probability in terms of the phenomenological $pp$ elastic scattering amplitude, without reference to the eikonal approximation. Contributions from inelastic intermediate states are suppressed. The onset of the black-disk limit in $pp$ scattering at TeV energies strongly suppresses diffraction at small impact parameters and is the main factor in determining the RGS probability. Correlations between hard and soft interactions (e.g. due to scattering from the long-range pion field of the proton or due to possible short-range transverse correlations between partons) further decrease the RGS probability. We also investigate the dependence of the diffractive cross section on the transverse momenta of the final-state protons (``diffraction pattern''). By measuring this dependence one can perform detailed tests of the interplay of hard and soft interactions and even extract information about the gluon GPD in the proton. Such studies appear to be feasible with the planned forward detectors at the Large Hadron Collider.

γγ*production of twoρ0mesons

We present a theoretical estimation for the cross-section of exclusive two $\rho$-meson production in two photon collision when one of the initial photons is highly virtual. The compatibility of our analysis with recent experimental data obtained by the L3 Collaboration at LEP is discussed. We show that these data prove the scaling behavior of the exclusive production amplitude. They are thus consistent with a partonic description of the exclusive process $\gamma\gamma ^*\to\rho ^0\rho ^0$ when $Q^2\sim 2 - 20 {\rm GeV^2}$.

Exclusive two ϱ0-mesons production in collision

We give a theoretical description of the L3 Collaboration data on the cross-section of the exclusive two $\rho$-mesons production in $\gamma\gamma ^*$ collision with large photon virtuality. These data prove the scaling behaviour of the leading twist generalized distribution amplitude of vector mesons. They thus prove the relevance of a partonic description of the exclusive process $\gamma\gamma ^*\to\rho ^0\rho ^0$ at $Q^2\geq 1.2 {\rm GeV^2}$ and $W\leq 3.0 {\rm GeV}.$

STRUCTURE, FRAGMENTATION AND FRACTURE FUNCTIONS

We address the partonic description of the proton, the photon and the "color singlet," as seen in inclusive and semi-inclusive DIS, in e+e-collisions, and in diffractive processes, respectively. Their formal treatment using structure, fragmentation, and fracture functions is outlined giving an insight into the perturbative QCD framework for these functions. Examples and comparisons with experimental data from LEP, HERA, and Tevatron are also covered.

Constituent quarks and parton distributions

The high energy parton distribution when evolved to a low energy scale appears to indicate that a valence picture of hadron structure arises. We have developed a formalism based on a laboratory partonic description which connects the parton distributions with the momentum distributions of a quark model. The formalism uses Next to Leading Order evolution and has been defined to produce the right support for the parton distributions. In this scheme we have analyzed the polarized and unpolarized data and shown that well-known Quark Models lead to a qualitative description of the data. However, if one aims at a quantitative agreement, these conventional low energy models have to be changed to include higher momentum and angular momentum components. Moreover, if the present gluon distributions, despite their indirect extraction, are taken to be an accurate description of nature, the need of primordial gluons or constituent quark substructure is an outcome of our calculation.

Can one probe the structure function of the pomeron?

We discuss whether the diffractive structure functions defined by current experiments at HERA are indeed probing the partonic structure function of the pomeron. We observe that the pseudorapidity cuts commonly employed require that the struck parton in the pomeron be far off mass shell in sizeable regions of parameter space. As a result an interpretation in terms of constituent partons within the pomeron is inadequate. One may nevertheless use a partonic description for the amplitude for virtual photon-pomeron scattering to compute a diffractive structure function for pseudorapidity gap events. The resulting form may have significant scaling violation.

Quark exchange in deep inelastic scattering

We use a model for baryons that links the constituent structure to the deep inelastic (current) properties. The approach consists in a laboratory partonic description (based on a model of hadron structure), to which a low momentum scaleQ 0 is adscribed, which is evolved to high momenta by means of the renormalization group. A generalization of the model by means of the hadronic quark cluster decomposition, provides a description of the structure functions of nuclei and is the starting point to study the effects that the antisymmetrization at the quark level has on the structure function of a model deuteron. The analysis contains conventional and high momentum partonic components. We next study quark Pauli blocking scenarios by including exotic delta nuclei in our analysis. By assuming that nuclei are composed of quasi-deuteron clusters, i.e., the deuteron parameters are density dependent, we analyze these effects on the structure functions of heavier nuclei.