Semi-inclusive Reactions Research Articles

Longitudinal and transverse polarizations of Λ hyperon in unpolarized SIDIS and e+e− annihilation

We make a systematic study of $\mathrm{\ensuremath{\Lambda}}$ hyperon polarizations in unpolarized lepton induced semi-inclusive reactions such as ${e}^{\ensuremath{-}}N\ensuremath{\rightarrow}{e}^{\ensuremath{-}}\mathrm{\ensuremath{\Lambda}}X$ and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}hX$. We present the general form of cross sections in terms of structure functions obtained from a general kinematic analysis. This already shows that the produced hyperons can be polarized in three orthogonal directions, i.e., the longitudinal direction along the hyperon momentum, the normal direction of the production plane, and the transverse direction in the production plane. We present the parton model results at the leading twist and leading order in perturbative QCD using transverse momentum dependent factorization and provide the expressions for these structure functions and polarizations in terms of three-dimensional parton distribution functions and fragmentation functions. We emphasize in particular that by studying the longitudinal polarization and the transverse polarization in the production plane, we can extract the corresponding chiral-odd fragmentation functions ${H}_{1Lq}^{\ensuremath{\perp}\mathrm{\ensuremath{\Lambda}}}$, ${H}_{1Tq}^{\mathrm{\ensuremath{\Lambda}}}$, and ${H}_{1Tq}^{\ensuremath{\perp}\mathrm{\ensuremath{\Lambda}}}$. We also present numerical results of rough estimates utilizing available parametrizations of fragmentation functions and approximations. We discuss how to measure these polarizations and point out in particular that they can be carried out in future EIC and/or ${e}^{+}{e}^{\ensuremath{-}}$ annihilation experiments such as Belle.

Light-front transverse momentum distributions for J=1/2 hadronic systems in valence approximation

The semi-inclusive correlator for a ${\cal J}$=1/2 bound-system, composed by A spin-1/2 fermions, is linearly expressed in terms of the light-front Poincar\'e covariant spin-dependent spectral function, in valence approximation. The light-front spin-dependent spectral function is fully determined by six scalar functions that allow for a complete description of the six T-even transverse-momentum distributions, suitable for a detailed investigation of the dynamics inside the bound system. The application of the developed formalism to a case with a sophisticated dynamical content, like $^3$He, reaches two goals: (i) to illustrate a prototype of an investigation path for gathering a rich wealth of information on the dynamics and also finding valuable constraints to be exploited from the phenomenological standpoint; (ii) to support for the three-nucleon system a dedicated experimental effort for obtaining a detailed 3D picture in momentum space. In particular, the orbital-angular momentum decomposition of the bound state can be studied through the assessment of relations among the transverse-momentum distributions, as well as the relevance of the relativistic effect generated by the implementation of macrocausality. A fresh evaluation of the longitudinal and transverse polarizations of the neutron and proton is also provided, confirming essentially the values used in the standard procedure for extracting the neutron structure functions from both deep-inelastic scattering and semi-inclusive reactions, in the same kinematical regime.

Neutrino-nucleus scattering in the SuSA model

The super-scaling approach (SuSA) model, based on the analogies between electron and neutrino interactions with nuclei, is reviewed and its application to the description of neutrino-nucleus scattering is presented. The contribution of both one- and two-body relativistic currents is considered. The model is validated with the (e,e') data, including also inelastic contributions for the inclusive reaction. A discussion of semi-inclusive reactions and their implications for charge-changing reactions of neutrinos is also presented. A selection of results for the inclusive neutrino reactions with change of charge is presented where theoretical predictions are compared with cross-section measurements from the main ongoing neutrino oscillation experiments.

Extracting three-body breakup observables from continuum-discretized coupled-channels calculations with core excitations

Core-excitation effects in the scattering of two-body halo nuclei have been investigated in previous works. In particular, these effects have been found to affect in a significant way the breakup cross sections of neutron-halo nuclei with a deformed core. To account for these effects, appropriate extensions of the continuum-discretized coupled-channels (CDCC) method have been recently proposed. We aim to extend these studies to the case of breakup reactions measured under complete kinematics or semi-inclusive reactions in which only the angular or energy distribution of one of the outgoing fragments is measured. We use the standard CDCC method as well as its extended version with core excitations (XCDCC), assuming a pseudo-state basis for describing the projectile states. Two- and three-body observables are computed by projecting the discrete two-body breakup amplitudes, obtained within these reaction frameworks, onto two-body scattering states with definite relative momentum of the outgoing fragments and a definite state of the core nucleus. The presented method provides a tool to compute double and triple differential cross sections for outgoing fragments following the breakup of a two-body projectile, and might be useful to analyze breakup reactions with other deformed weakly-bound nuclei, for which core excitations are expected to play a role. We have found that, while dynamical core excitations are important for the proton target at intermediate energies, they are very small for the Zn target at energies around the Coulomb barrier.

Meson Form Factors and Deep Exclusive Meson Production Experiments

Pion and kaon electroproduction data play a unique role in Nature and our understanding of them is essential for explaining hadron structure. Precision longitudinaltransverse separated pion and kaon cross sections are of particular interest. They allow for the extraction of meson form factors and validation of understanding of hard exclusive and semi-inclusive reactions (π+ , K + , π0 , γ ) towards 3D hadron imaging and potential future flavor decomposition. We review recent data and present prospects for deep exclusive pion and kaon electroproduction at the 12 GeV Jefferson Lab including the prospects to use projected charged- and neutral pion data to further determine the spin, charge-parity and flavor of GPDs, including the helicity-flip GPDs.

Neutrino Interactions with Nucleons and Nuclei: Importance for Long-Baseline Experiments

This article reviews our present knowledge of neutrino interactions with nucleons and discusses the interactions with nuclei, the target material of all presently running and planned long-baseline experiments. I emphasize descriptions of semi-inclusive reactions and full descriptions of the final state; the latter are needed to reconstruct the incoming neutrino energy from final-state observations. I then discuss Monte Carlo generator and more advanced transport-theoretical approaches in connection with experimental results on various reaction mechanisms. Finally, I describe the effects of uncertainties in the reconstruction of the incoming neutrino energy on oscillation parameters. The review argues that the precision era of neutrino physics also needs precision-era generators.

Three Dimensional Imaging of the Nucleon — TMD (Theory and Phenomenology)

This is intend to provide an overview of the theory and phenomenology parts of the TMD (Transverse Momentum Dependent parton distribution and fragmentation functions) studies. By comparing with the theoretical framework that we have for the inclusive deep inelastic lepton-nucleon scattering and the one-dimensional imaging of the nucleon, I try to outline what we need to do in order to construct a comprehensive theoretical framework for semi-inclusive reactions and the three dimensional imaging of the nucleon. After that, I try to give an overview of what we have already achieved and make an outlook for the future.

Three-dimensional imaging of the nucleon and semi-inclusive high-energy reactions

We present a short overview on the studies of transverse momentum dependent parton distribution functions of the nucleon. The aim of such studies is to provide a three dimensional imagining of the nucleon and a comprehensive description of semi-inclusive high energy reactions. By comparing with the theoretical framework that we have for the inclusive deep inelastic lepton-nucleon scattering and the one-dimensional imaging of the nucleon, we summarize what we need to do in order to construct such a comprehensive theoretical framework for semi-inclusive processes in terms of three dimensional gauge invariant parton distributions. After that, we present an overview of what we have already achieved with emphasize on the theoretical framework for semi-inclusive reactions in leading order perturbative QCD but with leading and higher twist contributions. We summarize in particular the results for the differential cross section and the azimuthal spin asymmetries in terms of the gauge invariant transverse momentum dependent parton distribution functions. We also briefly summarize the available experimental results on semi-inclusive reactions and parameterizations of transverse momentum dependent parton distributions extracted from them and make an outlook for the future studies.

A PbWO4-based Neutral Particle Spectrometer in Hall C at 12 GeV JLab

The Neutral Particle Spectrometer is a standalone electromagnetic calorimeter capable of detecting high energy photons from, for instance, DVCS or π0 decay with good energy and spatial resolution in a high rate environment. It can be used together with the Hall C high-momentum spectrometers for a suite of experiments with the underlying scientific objective of studying quark dynamics through exclusive and semi-inclusive reactions.

Results and perspective on TMDs and GPDs at COMPASS

COMPASS is a fixed target experiment at the CERN SPS dedicated to the study of the nucleon structure in the framework of the most complete descriptions based on transverse momentum dependent parton distribution functions (TMDs) and generalised parton distributions (GPDs). TMDs have been accessed so far with semi-inclusive deep inelastic reactions using a 160GeV muon beam off transversely polarized targets and they will be further investigated with Drell-Yan reactions using a 190GeV pion beam off a transversely polarized proton target. GPDs will be studied with deeply virtual Compton scattering and hard exclusive meson production using muon beams. A selection of the results as well as the projections for the future program are discussed.

MICROMEGAS TRACKERS FOR HADRONIC PHYSICS

Modern experiments in hadronic physics require excellent detection and tracking capabilities in very high flux environment. Probing the quark and gluon content of the nucleon indeed necessitates semi-inclusive or exclusive reactions with very small cross-sections. Micromegas detectors have therefore been rapidly identified as natural candidates to equip the hottest region of the COMPASS spectrometer at CERN. Benefitting from several years of smooth operation at COMPASS, the Micromegas have been proposed to equip the future central tracker of the CLAS12 experiment at the Jefferson Laboratory. This project triggered an important R&D on the bulk and resistive technologies and inspired several projects involving cylindrical trackers. This paper focuses on the design and performance of the COMPASS and CLAS12 Micromegas, and reviews the R&D made on the discharge reduction in large hadron fluxes.

Breakup of an oxygen nucleus to light fragments of mass number in the range A ⩽ 4 in 16Op interactions at a momentum of 3.25 GeV/c per nucleon

New experimental data on the correlations of the yields of 4He and 2H nuclei in semi-inclusive reactions involving by the production of 3He and 3H mirror nuclei in 16Op collisions at a momentum of 3.25 GeV/c per nucleon are presented. The mean multiplicities of protons and neutrons appearing as fragments are found to be independent of the number of associated deuterons, this indicating that a considerable part of these fragments originate from the breakup of the alpha-particle clusters forming the oxygen nucleus.

Partonic Pole Matrix Elements for Fragmentation

A model-independent analysis of collinear three-parton correlation functions for fragmentation is performed. By investigating their support properties it is shown, in particular, that the so-called partonic pole matrix elements vanish. This sheds new light on the understanding of transverse single spin asymmetries in various hard semi-inclusive reactions. Moreover, it gives additional strong evidence for the universality of transverse-momentum-dependent fragmentation functions.

Nonvanishing of the Collins mechanism for single spin asymmetries

The Collins mechanism provides a non-perturbative explanation for the large single spin asymmetries found in hard semi-inclusive reactions involving a transversely polarized nucleon. However, there are seemingly convincing reasons to suspect that the mechanism vanishes, and indeed it does vanish in the naive parton model where a quark is regarded as an essentially 'free' particle. We give an intuitive analysis which highlights the difference between the naive picture and the realistic one, and shows how the Collins mechanism arises when the quark is described as an off-shell particle by a field in interaction. A typographical error is corrected in this version.

Hadron polarization in semi-inclusive reactions involving photons

A phenomenological model which has had some success in explaining polarization phenomena and left-right asymmetry in inclusive proton-proton scattering is considered for reactions involving photons. In particular, the reactions (a) gamma + p -> H + X; (b) gamma + p(<IMG SRC="/img/revistas/bjp/v34n3a/a46img01.gif">) <FONT FACE=Symbol>® p</FONT>± + X and (c) p(<IMG SRC="/img/revistas/bjp/v34n3a/a46img01.gif">) -> + p <FONT FACE=Symbol>® g</FONT> + X are considered where gamma = resolved photon and hyperon H = lambda, sigma, xi etc. Predictions for hyperon polarization in (a) and the asymmetry (in (b) and (c)) provide further tests of this particular model. Feasibility of observing (b) at HERA and the effect of the polarization of the sea in the proton in p(<IMG SRC="/img/revistas/bjp/v34n3a/a46img01.gif">) + p <FONT FACE=Symbol>® p</FONT>± + X is briefly discussed.

Spin physics and more with HERMES

A review of recent results obtained by the HERMES experiment is given. Inclusive measurements on polarized and unpolarized targets provide precise information on the polarized structure functions g 1 d and g 1 p and the isoscalar unpolarized structure function ratio F 2 A /F 2 d . The geometrical acceptance of the HERMES detector and the good particle identification capabilities allow the study of semi-inclusive reactions as well. Using polarized targets, the polarized quark distribution functions $\Delta q$ can be extracted as well as a first indication of the transversity distributions $\delta q$ . On unpolarized heavy targets, hadron formation in a nuclear environment is studied. Finally, data on Deeply Virtual Compton Scattering, the hard exclusive electroproduction of real photons, are presented, which are closely linked to the novel framework of Generalised Parton Distributions. For this reaction HERMES has measured the asymmetry main beam charge as well as in-beam spin.

Lambda polarization measured at HERMES and lambda spin structure

Polarizations of δ and \(\bar \Lambda \) hyperons have been measured in leptoproduction by the HER-MES Experiment. Data were accumulated in years 1996 and 1997 using the 27.5 GeV polarized HERA positron beam and unpolarized gas targets. In semi-inclusive reactions with Q2>1 GeV2, the preliminary result is Pδ=0.03±0.06 (stat) ±0.03 (syst) at an average δ fractional energy 〈z〉=0.446 and Feynman scaling variable 〈xF〉=0.3. The spin transfer coefficient from the virtual photon to the δ was found to be DLL′δ=0.11±0.17 (stat) ±0.09 (syst). This result is consistent with both the naive expectation of zero polarization of the up quark in the δ (DLL′δ=0), and the Burkardt-Jaffe prediction of DLL′δ=−0.2 for the up quark contribution. Inclusive reactions with only the δ or \(\bar \Lambda \) detected in the final state were also studied. In this case most δ's and \(\bar \Lambda \)'s are produced in the photoproduction regime with Q2<0.08 (GeV/c)2. Transverse δ and \(\bar \Lambda \) polarizations perpendicular to the production plane have been measured. Averaged over the spectrometer acceptance for the transverse δ and \(\bar \Lambda \) momentum 0<pt<1.7 GeV, the preliminary results are Pn=0.066±0.011 (stat)±0.025 (syst) for δ and Pn=−0.066±0.037 (stat)±0.025 (syst) for \(\bar \Lambda \). This is the first clear indication of transverse δ polarization in leptoproduction. Theoretical analysis of these results is underway.

Quark Distributions in the Nucleon by the Semi-Inclusive Reaction on the Proton and Helium-3 Target

We investigate the spin asymmetries which will be measured by semi-inclusive reactions on proton targets and helium-3 targets with tagged charged pions and kaons. By using the quark parton model, we relate the spin asymmetries to the quark spin distributions in the nucleon. We also use the parametrized fragmentation functions and the existing parametrizations of polarized and unpolarized quark distributions. Then we predict the spin asymmetries. We show that we can study the quark distributions in the neutron with the helium-3 target. We find useful properties for experimental analysis: The spin asymmetry changes little if kaons are taken into account, and it is almost independent of the z-cut.

Some remarks on off-shell scattering in the eikonal approximation and applications to semi-inclusive reactions

Using the Abel inversion for the eikonal phase as function of the interaction we derive simple integral relations between half and on-shell eikonal phases. A frequently used short-range approximation for the half off-shell phase and profile appears supported by the above-mentioned relation. We work out some examples and also address the half off-shell eikonal phase pertinent to a diffractive amplitude. The latter is relevant for a calculation of selected transparencies T of nuclei for a proton, knocked-out in selected semi-inclusive A( e, e′ p) X reactions. Some numerical results for T are given.

Inclusive and semi-inclusive reactions

Inclusive and semi-inclusive reactions