Unpolarized Cross Section Research Articles

Temperature dependence of the cross sections for Meson-Meson nonresonant reactions in hadronic matter

We study the unpolarized cross sections for the endothermic nonresonant reactions, ππ → ρρ for I = 2, KK → K*K* for I = 1, KK* → K*K* for I = 1, πK → ρK* for I = 3/2, πK* → ρK* for I = 3/2, ρK → ρK* for I = 3/2, and πK* → ρK for I = 3/2, which take place in hadronic matter. We obtain the temperature-dependent meson masses from the Schrödinger equation with the potential that is given by perturbative quantum chromodynamics (QCD) with loop corrections at short distances, that becomes a distance-independent and temperature-dependent potential at long distances, and that has a spin-spin interaction with relativistic modifications. In the first Born approximation with the quark-interchange mechanism, the temperature dependences of the potential, the meson masses, and the mesonic quark-antiquark wave functions bring about a temperature dependence of the unpolarized cross sections for the seven nonresonant reactions. Noticeably, rapid changes in the π and K radii cause a rapid increase in peak cross sections, except for the πK* → ρK cross section, as the temperature approaches a critical temperature. Compared to in-vacuum cross sections, the medium modifications of the cross sections are remarkable.

Towards a resolution of the proton form factor problem: new electron and positron scattering data.

There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p) from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (ϵ) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ϵ at Q(2)=1.45 GeV(2). This measurement is consistent with the size of the form factor discrepancy at Q(2)≈1.75 GeV(2) and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV(2).

Subtracted dispersion relation formalism for the two-photon exchange correction to elastic electron-proton scattering: Comparison with data

We apply a subtracted dispersion relation formalism with the aim to improve predictions for the two-photon exchange corrections to elastic electron-proton scattering observables at finite momentum transfers. We study the formalism on the elastic contribution, and make a detailed comparison with existing data for unpolarized cross sections as well as polarization transfer observables.

Exclusive processes at JLab at 6 GeV

Deeply virtual exclusive reactions provide a unique opportunity to probe the complex internal structure of the nucleon. They allow to access information about the correlations between parton transverse spatial and longitudinal momentum distributions from experimental observables. Dedicated experiments to study Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP) have been carried out at Jefferson Lab using continuous electron beam with energies up to 6 GeV. Unpolarized cross sections, beam, target and double spin asymmetries have been measured for DVCS as well as for π 0 exclusive electroproduction. The data from Hall B provide a wide kinematic coverage with Q 2 =1-4.5 GeV 2 , xB=0.1-0.5, and −t up to 2 GeV 2 . Hall A data have limited kinematic range partially overlapping with Hall B kinematics but provide a high accuracy measurements. Scaling tests of the DVCS cross sections provide solid evidence of twist-2 dominance, which makes chiral-even GPDs accessible even at modest Q 2 . We will discuss the interpretation of these data in terms of Generalized Parton Distributions (GPDs) model. Successful description of the recent CLAS π 0 exclusive production data within the framework of the GPD-based model provides a unique opportunity to access the chiral-odd GPDs.

Polarization observables in lepton-deuteron elastic scattering including the lepton mass

Expressions for the unpolarized differential cross section and for various polarization observables in the lepton-deuteron elastic scattering, $\ell+D\to \ell+D$, $\ell=e$, $\mu$, $\tau$, have been obtained in one-photon-exchange approximation, taking into account the lepton mass. Polarization effects have been investigated for the case of a polarized lepton beam and polarized deuteron target which can have vector or tensor polarization. Numerical estimations of the lepton mass effects have been done for the unpolarized differential cross section and for some polarization observables and applied to the case of low energy muon deuteron elastic scattering.

Meson exchange effects in elasticepscattering at loop level and the electromagnetic form factors of the proton

A new form of two-photon exchange (TPE) effect is studied to explain the discrepancy between unpolarized and polarized experimental data in elastic $ep$ scattering. The mechanism is based on a simple idea that apart from the usual TPE effects from box and crossed-box diagrams, the mesons may also be exchanged in elastic $ep$ scattering by two-photon coupling at loop level. The detailed study shows such contributions to reduced unpolarized cross section (${\ensuremath{\sigma}}_{\mathrm{un}}$) and polarized observables (${P}_{t},{P}_{l}$) at fixed ${Q}^{2}$ are only dependent on proton's electromagnetic form factors ${G}_{E,M}$ and a new unknown universal parameter $g$. After combining this contribution with the usual TPE contributions from box and crossed-box diagrams, the ratio ${\ensuremath{\mu}}_{p}{G}_{E}/{G}_{M}$ extracted from the recent precise unpolarized and polarized experimental data can be described consistently.

Coherent [formula omitted]-photoproduction on the deuteron near the [formula omitted]-production threshold including polarization observables

Coherent π0-photoproduction on the deuteron including polarization observables is studied in the energy region near the η-production threshold at backward center-of-mass angles of the outgoing pion. This work is motivated by the measurements of the CLAS Collaboration at Jefferson Lab, where a cusp-like structure in the energy dependence of the differential cross section has been observed at extremely backward pion angles. The present approach is based on the impulse approximation and first-order rescattering diagrams with intermediate production of both π- and η-mesons. Numerical results for unpolarized cross sections, the linear photon asymmetry (Σ), the vector (T11) and tensor (T2M, M=0, 1, 2) deuteron target asymmetries, and the double polarization E-asymmetry are predicted and compared with available experimental data and other theoretical models. The effect of first-order rescattering is found to be much larger in spin asymmetries than in the unpolarized cross sections. It reaches on average about 40% in the tensor target and E asymmetries. Compared to the experimental data from CLAS Collaboration, sizable discrepancies are found. This is not the case for the linear photon asymmetry, for which a better comparison with the data from YerPhI Collaboration is obtained.

The complete-experiment problem of photoproduction of pseudoscalar mesons in a truncated partial-wave analysis

The complete experiment problem in the truncated partial wave analysis of pseudoscalar meson photoproduction with suppressed t-channel exchanges is investigated. The focus is set to ambiguities of the group S observables with the unpolarized differential cross section, $\sigma_0$, and the three single-spin observables, $\Sigma$, $T$ and $P$. For this purpose, the approach and formalism already worked out by Omelaenko in 1981 is revisited in this work. A numerical study using multipoles of the PWA solution MAID2007 shows how only one additional double polarization observable can resolve all ambiguities. Therefore, the possibility emerges to perform a complete experiment with only five observables.

Phenomenology of spin observables in the reactions of meson production with the 1 S 0 Diproton pN → {pp} s X

Formalism for spin observables for production of pseudoscalar, scalar, and vectormesons in pN collision with formation of the 1S0 diproton {pp}s in the final state is considered in model-independent way assuming parity and angular momentum conservation. Production of the pseudoscalar mesons (Jπ = 0−) is described by two spin amplitudes and both of them can be completely determined by measurement of unpolarized cross section dσ0, vector analyzing power Ay, and spin-correlation coefficients Cx,x = −Cz,z> and Cx,z. Similar result is obtained for production of scalar mesons (Jπ = 0+). It is shown that measurement of the tensor polarization t20 of the vector meson V (Jπ = 1−) in the reaction pN → {pp}sV in collinear kinematics gives the same information as measurement of the spin-correlation coefficients Cz,z or Cy,y.

Pair production of doubly charged Higgs boson at photon linear collider in the Higgs triplet model

The characteristic feature of the Higgs Triplet Model (HTM) is the existence of the doubly charged Higgs bosons H±±. In this paper, we study the pair production of doubly charged Higgs bosons in γγ collisions at the International Linear Collider (ILC). We present the production cross-sections and the distributions of the various observables, i.e. the distributions of the transverse momenta, the rapidity distributions for doubly charged Higgs bosons and the production angle distributions of the charged Higgs boson pair. Our numerical results show that, the values of the unpolarized cross-sections can reach a few hundreds of fb. We also study the possible final state for the decay mode H±±→W±W± and relevant Standard Model (SM) background. Due to high produced rate and small SM background, the possible signals of H±± might be detected via this process in the future ILC experiments.

Investigations on the tensor analyzing powert20in the reaction \vec{d} + p →3He +η

The reaction dp → 3 He η has been investigated at the COSY-ANKE experiment with both an unpolarised and a vector/tensor polarised deuteron beam in an excess energy range of Q = 0 MeV up to Q = 10 MeV. The determination of the tensor analysing power t 20 along with the unpolarised cross section allows for the investigation of the s -wave production amplitudes near threshold. The weak energy dependence of t 20 supports a strong final state interaction of the η 3 He system which might lead to a quasi-bound η-mesic state. Furthermore, the angular dependence of the t 20 might reveal further insight into the production amplitude.

Comparison of neural network and hadronic model predictions of the two-photon exchange effect

Predictions for the two-photon exchange (TPE) correction to unpolarized $ep$ elastic cross section, obtained within two different approaches, are confronted and discussed in detail. In the first one the TPE correction is extracted from experimental data by applying the Bayesian neural network (BNN) statistical framework. In the other the TPE is given by box diagrams, with the nucleon and the $P_{33}$ resonance as the hadronic intermediate states. Two different form factor parametrizations for both the proton and the $P_{33}$ resonance are taken into consideration. Proton form factors are obtained from the global fit of the full model (with the TPE correction) to the unpolarized cross section data. Predictions of both methods agree well in the intermediate $Q^2$ range, $(1,3)$ GeV$^2$. Above $Q^2=3$ GeV$^2$ the agreement is on $2\sigma$ level. Below $Q^2=1$ GeV$^2$ the consistency between both approaches is broken. The values of the proton radius extracted within both models are given. In both cases predictions for VEPP-3 experiment have been obtained and confronted with the preliminary experimental results.

Transverse momentum dependent evolution: Matching semi-inclusive deep inelastic scattering processes to Drell-Yan andW/Zboson production

We examine the QCD evolution for the transverse momentum dependent observables in hard processes of semi-inclusive hadron production in deep inelastic scattering and Drell-Yan lepton pair production in $pp$ collisions, including the spin-average cross sections and Sivers single transverse spin asymmetries. We show that the evolution equations derived by a direct integral of the Collins-Soper-Sterman evolution kernel from low to high Q can describe well the transverse momentum distribution of the unpolarized cross sections in the Q^2 range from 2 to 100 GeV^2. In addition, the matching is established between our evolution and the Collins-Soper-Sterman resummation with b*-prescription and Konychev-Nodalsky parameterization of the non-perturbative form factors, which are formulated to describe the Drell-Yan lepton pair and W/Z boson production in hadronic collisions. With these results, we present the predictions for the Sivers single transverse spin asymmetries in Drell-Yan lepton pair production and $W^\pm$ boson production in polarized pp and $\pi^- p$ collisions for several proposed experiments. We emphasize that these experiments will not only provide crucial test of the sign change of the Sivers asymmetry, but also provide important opportunities to study the QCD evolution effects.

Overview of recent results from the HERMES experiment

HERMES has taken a wealth of deep-inelastic scattering (DIS) data using the 27.6 GeV polarized lepton beam at HERA and various pure gas targets, both unpolarized and polarized, which opened the door to several unique results. Among them are the first evidences for the naive-T-odd Sivers and Collins effects but also the recent first measurements of azimuthal modulations in the unpolarized semi-inclusive DIS cross section for charged kaons and pions and of beam-helicity asymmetries in exclusive leptoproduction of real photons using recoil-proton detection. An overview of HERMES results is given with emphasis on the exploration of the three-dimensional structure of the nucleon.

The form factors of the nucleons

There has been much activity in the measurement of the elastic electromagnetic proton and neutron form factors in the last decade, and the quality of the data has been greatly improved by performing double-polarization experiments, in comparison with with previous unpolarized cross section data. Here we will review the experimental data base in view of the new results for the proton and the neutron, obtained at MIT-Bates, JLab and MAMI. The rapid evolution of phenomenological models triggered by these high-precision experiments will be discussed. In particular, the possibility that the proton is non-spherical in its ground state, and that the transverse charge density are model independently defined in the infinite momentum frame. Likewise, flavor decomposition of the nucleon form factors into dressed u and d quark form factors, may give information about the quark-diquark structure of the nucleon. The current proton radius “crisis” will also be discussed.

Formula omitted] production at the B factories under the principle of maximum conformality

Under the conventional scale setting, the renormalization scale uncertainty usually constitutes a systematic error for a fixed-order perturbative QCD estimation. The recently suggested principle of maximum conformality (PMC) provides a principle to eliminate such scale ambiguity in a step-by-step way. Using the PMC, all non-conformal terms in perturbative expansion series are summed into the running coupling, and one obtains a unique, scale-fixed, scheme-independent prediction at any finite order. In the paper, we make a detailed PMC analysis for both the polarized and the unpolarized cross sections for the double charmonium production process, e++e−→J/ψ(ψ′)+χcJ with (J=0,1,2). The running behavior for the coupling constant, governed by the PMC scales, are determined exactly for the specific processes. We compare our predictions with the measurements at the B factories, BaBar and Belle, and the theoretical estimations obtained in the literature. Because the non-conformal terms are different for various polarized and unpolarized cross sections, the PMC scales of these cross sections are different in principle. It is found that all the PMC scales are almost independent of the initial choice of renormalization scale. Thus, the large renormalization scale uncertainty usually adopted in the literature up to ∼40% at the NLO level, obtained from the conventional scale setting, for both the polarized and the unpolarized cross sections are greatly suppressed. It is found that the charmonium production is dominated by J=0 channel. After PMC scale setting, we obtain σ(J/ψ+χc0)=12.25−3.13+3.70 fb and σ(ψ′+χc0)=5.23−1.32+1.56 fb, where the squared average errors are caused by bound state parameters as mc, |RJ/ψ(0)| and |RχcJ′(0)|, which are non-perturbative error sources in different to the QCD scale setting problem. In comparison to the experimental data, a more accurate theoretical estimation shall be helpful for a precise testing of QCD and for determining whether there is new physics beyond the Standard Model.

Excitation of the [formula omitted] isobar in deuteron charge exchange on hydrogen at 1.6, 1.8, and 2.3 GeV

The charge-exchange break-up of polarised deuterons d→p→{pp}sn, where the final {pp}s diproton system has a very low excitation energy and hence is mainly in the S01 state, is a powerful tool to probe the spin-flip terms in the proton–neutron charge-exchange scattering. Recent measurements with the ANKE spectrometer at the COSY storage ring at 1.6, 1.8, and 2.27 GeV have extended these studies into the pion-production regime in order to investigate the mechanism for the excitation of the Δ(1232) isobar in the d→p→{pp}sX reaction. Values of the differential cross section and two deuteron tensor analysing powers, Axx and Ayy, have been extracted as functions of the momentum transfer to the diproton or the invariant mass MX of the unobserved system X. The unpolarised cross section in the high MX region is well described in a model that includes only direct excitation of the Δ isobar through undistorted one pion exchange. However, the cross section is grossly underestimated for low MX, even when Δ excitation in the projectile deuteron is included in the calculation. Furthermore, direct Δ production through one pion exchange only reproduces the angular dependence of the difference between the two tensor analysing powers.

Energy evolution for the Sivers asymmetries in hard processes

We investigate the energy evolution of the azimuthal spin asymmetries in semi-inclusive hadron production in deep inelastic scattering (SIDIS) and Drell-Yan lepton pair production in pp collisions. The scale dependence is evaluated by applying an approximate solution to the Collins-Soper-Sterman (CSS) evolution equation at one-loop order which is adequate for moderate Q^2 variations. This describes well the unpolarized cross sections for SIDIS and Drell-Yan process in the $Q^2$ range of 2.4-100GeV^2. A combined analysis of the Sivers asymmetries in SIDIS from HERMES and COMPASS experiments, and the predictions for the Drell-Yan process at RHIC at \sqrt{S}=200GeV are presented. We further extend to the Collins asymmetries and find, for the first time, a consistent description for HERMES/COMPASS and BELLE experiments with the evolution effects. We emphasize an important test of the evolution effects by studying di-hadron azimuthal asymmetry in e^+e^- annihilation at moderate energy range, such as at BEPC at \sqrt{S}=4.6GeV.

Dissecting deuteron Compton scattering I: The observables with polarised initial states

A complete set of linearly independent observables in Compton scattering with arbitrarily polarised real photons off an arbitrarily polarised spin-1 target is introduced, for the case that the final-state polarisations are not measured. Adopted from the one widely used, e.g., in deuteron photo-dissociation, it consists of 18 terms: the unpolarised cross section, the beam asymmetry, 4 target asymmetries and 12 asymmetries in which both beam and target are polarised. They are expressed by the helicity amplitudes and —where available— related to observables discussed by other authors. As application to deuteron Compton scattering, their dependence on the (isoscalar) scalar and spin dipole polarisabilities of the nucleon is explored in Chiral Effective Field Theory with dynamical Δ(1232) degrees of freedom at order e 2 δ 3. Some asymmetries are sensitive to only one or two dipole polarisabilities, making them particularly attractive for experimental studies. At a photon energy of 100 MeV, a set of 5 observables is identified from which one may be able to extract the spin polarisabilities of the nucleon. These are experimentally realistic but challenging and mostly involve tensor-polarised deuterons. Relative to Compton scattering from a nucleon, sensitivity to the “mixed” spin polarisabilities γ E1M2 and γ M1E2 is increased because of the interference with the D wave component of the deuteron and with its pion-exchange current. An interactive Mathematica 9.0 notebook with results for all observables at photon energies up to 120 MeV is available from hgrie@gwu.edu.

Two-photon exchange in theℓ+p→ℓ+pprocess with a massive lepton

Stimulated by the proton size puzzle and the proposal of $\ensuremath{\mu}p$ scattering at the Paul Scherrer Institute (PSI), we study the two-photon exchange corrections to the massive-lepton--proton scattering. The leptonic helicity broken caused by lepton mass introduces new form factors. We estimate the two-photon exchange corrections to these form factors as well as to the unpolarized differential cross sections.