Diffractive Final States Research Articles

Investigating the diffractive gluon jet production in lepton-ion collisions

We study the diffractive jet production in electron-ion collisions in the kinematical region where the mass $M_X$ of the diffractive final state is larger than $Q^2$. Based on parton saturation framework predictions are done for the kinematics of future or possible $eA$ machines as the EIC, LHeC, HE-LHeC and FCC-eA. We analyze the differential cross section as a function of jet (gluon) transverse momentum and from the experimental point of view this observable could be used to extract the saturation scale as a function of $x_{I\!\!P}$.

Soft and diffractive scattering with the cluster model in Herwig

We present a new model for soft interactions in the event-generator Herwig. The model consists of two components. One to model diffractive final states on the basis of the cluster hadronization model and a second component that addresses soft multiple interactions as multiple particle production in multiperipheral kinematics. We present much improved results for minimum-bias measurements at various LHC energies.

A New Model for Soft Interactions in Herwig

We present a new model for soft interactions in the Monte Carlo event-generator Herwig. The soft diffractive final states are modeled on the basis of the cluster hadronization model and interactions between soft particles are modeled as multiple particle production with multiperipheral kinematics. We further present much improved results of mininum-bias measurements at different energies.

Production of exclusive dijets in diffractive deep inelastic scattering at HERA

Production of exclusive dijets in diffractive deep inelastic $$e^\pm p$$ scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 372 pb $$^{-1}$$ . The measurement was performed for $$\gamma ^{*}$$ –p centre-of-mass energies in the range $$90< W < {250}\,{\mathrm{GeV}}$$ and for photon virtualities $$Q^2 > {25}\,{\mathrm{GeV}^{2}}$$ . Energy flows around the jet axis are presented. The cross section is presented as a function of $$\beta $$ and $$\phi $$ , where $$\beta =x/x_\mathrm{I\!P}$$ , x is the Bjorken variable and $$x_\mathrm{I\!P}$$ is the proton fractional longitudinal momentum loss. The angle $$\phi $$ is defined by the $$\gamma ^{*}$$ –dijet plane and the $$\gamma ^{*}$$ – $$e^\pm $$ plane in the rest frame of the diffractive final state. The $$\phi $$ cross section is measured in bins of $$\beta $$ . The results are compared to predictions from models based on different assumptions about the nature of the diffractive exchange.

Theoretical remarks on diffraction at the LHC

We comment on the nonperturbative nature of the total cross-section in pp scattering and on the importance of the survival probability for the understanding of diffractive final states in pp scattering.

Exclusive final states in diffractive excitation

In this paper we describe a formalism for generating exclusive final states in diffractive excitation, based on the optical analogy where diffraction is fully determined by the absorption into inelastic channels. The formalism is based on the Good--Walker formalism for diffractive excitation, and it is assumed that the virtual parton cascades represent the diffractive eigenstates defined by a definite absorption amplitude. We emphasize that, although diffractive excitation is basically a quantum-mechanical phenomenon with strong interference effects, it is possible to calculate the different interfering components to the amplitude in an event generator, add them and thus calculate the reaction cross section for exclusive diffractive final states. The formalism is implemented in the DIPSY event generator, introducing no tunable parameters beyond what has been determined previously in studies of non-diffractive events. Some early results from DIS and proton-proton collisions are presented, and compared to experimental data.

Erratum to “Saturation models of HERA DIS data and inclusive hadron distributions in p + p collisions at the LHC” [Nucl. Phys. A 850 (2011) 136

Unintegrated gluon distributions sensitive to the transverse spatial distribution of gluons in the proton are extracted from data on exclusive and diffractive final states at HERA in the dipole approach. These unintegrated gluon distributions can be used to compute inclusive hadron production in p+p collisions at the LHC. In this paper, we consider a number of saturation models with differering dynamical assumptions that give good fits to the available HERA data. We apply these models to study the rapidity and transverse momentum dependence of the LHC data up to $\sqrt{s} = 7$ TeV. We examine the sensitivity of these results to parameters that are not constrained by the HERA data and comment on similarities and differences with previous work. We compute the n-particle inclusive multiplicity distribution and show that the LHC p+p results are in agreement with predictions for multi-particle production in the Color Glass Condensate approach. This result has significant ramifications for the interpretation of multi-particle correlations in high multiplicity events at the LHC.

Saturation models of HERA DIS data and inclusive hadron distributions in [formula omitted] collisions at the LHC

Unintegrated gluon distributions sensitive to the transverse spatial distribution of gluons in the proton are extracted from data on exclusive and diffractive final states at HERA in the dipole approach. These unintegrated gluon distributions can be used to compute inclusive hadron production in p + p collisions at the LHC. In this paper, we consider a number of saturation models with differing dynamical assumptions that give good fits to the available HERA data. We apply these models to study the rapidity and transverse momentum dependence of the LHC data up to s = 7 TeV . We examine the sensitivity of these results to parameters that are not constrained by the HERA data and comment on similarities and differences with previous work. We compute the n-particle inclusive multiplicity distribution and show that the LHC p + p results are in agreement with predictions for multi-particle production in the Color Glass Condensate approach. This result has significant ramifications for the interpretation of multi-particle correlations in high multiplicity events at the LHC.

Inclusive Diffraction and Diffractive Final States at HERA

Recent results are presented on diffractive ep scattering at HERA as measured by the H1 and ZEUS experiments.

Unified description of diffractive deep inelastic scattering with saturation

We propose a new description of inclusive diffraction in deep inelastic scattering. The diffractive structure functions are expressed in the dipole picture and contain heavy-quark contributions. The dipole scattering amplitude, a saturation model fitted on inclusive deep inelastic scattering data, features a saturation scale ${Q}_{s}(x)$ larger than 1 GeV for $x={10}^{\ensuremath{-}5}$. The $q\overline{q}g$ contribution to the diffractive final state is modeled in such a way that both the large-${Q}^{2}$ and small-$\ensuremath{\beta}$ limits are implemented. In the regime ${x}_{\mathbb{P}}l0.01$ in which saturation is expected to be relevant, we obtain a parameter-free description of the HERA data with ${\ensuremath{\chi}}^{2}/\mathrm{points}=1.2$.

Diffractive final states with the H1 detector at HERA

Recent measurements of diffractive dijet and charm quark production in electron-proton collisions using the H1 detector at HERA are presented, where the exchanged photon is either almost real or highly virtual. The data are compared with leading and next-to-leading order QCD calculations based on the diffractive parton distributions obtained from a recent DGLAP QCD analysis of H1 inclusive diffractive deep-inelastic scattering data, thus testing QCD factorization in diffractive ep interactions.

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.

Diffractive structure functions in the model of the stochastic vacuum

We present a calculation for the diffractive structure functions χ p F L D(3) and χ p F L D(3) at low values of Q 2 using an eikonal approximation. The non-perturbative model of the stochastic QCD vacuum is used to evaluate the interactions between the quarks (antiquarks) and the color field. The diffractive final state contains only a quark-antiquark pair. We show the behavior of the diffractive structure functions against β and Q 2.

Leading nucleons from peripheral processes in lepton deep inelastic scattering and the nucleon structure

The experimental information on nucleon production in lepton deep inelastic scattering (DIS) is rather scarce. Recently there is a growing interest in understanding the mechanism of the production of baryons in DIS, stimulated by recent results on leading protons and neutrons from electron-proton scattering at HERA. I review on different peripheral mechanisms of nucleon (proton or neutron) production in lepton DIS and discuss their role in understanding the spectra of nucleons for both fixed target and collider experiments. In DIS ep → e′Xh, the QCD hardness scale gradually diminishes from the hard scale, Q 2, in the virtual photon (current) fragmentation region to the soft, hadronic, scale in the proton (target) fragmentation region. This suggests a similarity of the inclusive spectra of leading protons and neutrons, h = p, n, in high energy hadron-proton collisions and in lepton DIS at small Bjorken-χ. The semi-inclusive cross section for production of slow protons in charged-current deep inelastic (anti)neutrino scattering on protons and neutrons is discussed. The standard hadronization models based upon the color neutralization mechanism appear to underestimate the rate of slow proton production on hydrogen. The presence of the virtual mesons (pions) in the nucleon leads to an additional mechanism for proton production, referred to as spectator process. The results of the calculations are compared with the CERN bubble chamber (BEBC) data. I discuss the structure of deep-inelastic events induced by the pione-xchange at HERA kinematics. Most of the event characteristics do not provide a possibility to distinguish the ordinary DIS fragmentation events from those induced by the pion-exchange mechanism. The only exception is the energy distribution of outgoing neutrons. The pion cloud model predicts energy distribution of neutrons which differs significantly from the standard hadronization models which can in principle be verified experimentally. A detailed analysis of semi-exclusive diffractive processes in ep DIS at HERA, with the diffractive final states in the forward direction is presented. The contributions of the subleading f 0; ω,a 2, ρ reggeons and the pion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the ( a 2, ρ) reggeons are entirely responsible for the forward neutron production at χ IP < 10 −3. The π N production in the forward region is estimated using the Deck mechanism. The significance of this reaction for the processes measured at HERA is discussed. In the DIS production of leading protons DIS off neutral pions must be supplemented by a contribution from isoscalar reggeon ( f0) exchange extrapolated down to moderate values of χ L . I comment on the χ and Q 2 dependence of leading proton production as a probe of a universal pattern of the χ, Q 2 evolution of the nucleon and meson (reggeon) structure functions at small χ.

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states.

Multiplicity structure of the hadronic final state in diffractive deep-inelastic scattering at HERA

The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type ep -> eXY, where Y is a hadronic system with mass M_Y< 1.6 GeV and where the squared momentum transfer at the pY vertex, t, is limited to |t|<1 GeV^2, is studied as a function of the invariant mass M_X of the system X. Results are presented on multiplicity distributions and multiplicity moments, rapidity spectra and forward-backward correlations in the centre-of-mass system of X. The data are compared to results in e+e- annihilation, fixed-target lepton-nucleon collisions, hadro-produced diffractive final states and to non-diffractive hadron-hadron collisions. The comparison suggests a production mechanism of virtual photon dissociation which involves a mixture of partonic states and a significant gluon content. The data are well described by a model, based on a QCD-Regge analysis of the diffractive structure function, which assumes a large hard gluonic component of the colourless exchange at low Q^2. A model with soft colour interactions is also successful.

High-p⊥ jets in diffractive electroproduction

The diffractive production of high-$p_{\perp}$ jets in deep-inelastic scattering is studied in the semiclassical approach. The $p_{\perp}$-spectra of $q {\bar q}$ and $q {\bar q} g$ diffractive final states are found to be qualitatively different. For $q {\bar q}$ final states, which are produced by `hard' colour-singlet exchange, the $p_{\perp}$-spectrum is much softer than for $q {\bar q} g$ final states, where the colour neutralization is `soft'. Furthermore, the two different final states can be clearly distinguished by their diffractive mass distributions.

Reggeon and pion contributions in semiexclusive diffractive processes at DESY HERA

A detailed analysis of semi-exclusive diffractive processes in ep DIS at HERA, with the diffractive final states in the forward direction is presented. The contributions of the subleading f2, !, a2, � reggeons and the pion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the (a2,�) reggeons are entirely responsible for the forward neutron production

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states.

Charm as a key to diffractive processes

The diffractive production of open charm in deep-inelastic scattering is studied in the semiclassical approach which has been proposed recently. In this approach, the leading order process contains a charm quark pair and an additional gluon in the diffractive final state. The p⊥-spectrum and the diffractive mass distribution are evaluated and compared with predictions based on perturbative two-gluon exchange calculations for charm quark pair production. It is shown that the p⊥-spectrum provides a clear test of the underlying partonic process whereas the diffractive mass distribution reflects the non-perturbative mechanism of colour neutralization.