Unpolarized Cross Section Research Articles

Polarimetry for a storage-ring electric-dipole-moment measurement

These proceedings summarize the progress which has been made within the Jülich Electric Dipole Moment Investigations (JEDI) Collaboration on deuteron beam polarimetry for a storage-ring electric-dipole-moment (EDM) measurement. The design of a dedicated EDM polarimeter requires a precise database for optimization purposes. To address this need, a measurement of a deuteron-carbon scattering database has been performed with the forward part of the WASA-at-COSY detector. Preliminary results for vector analyzing powers and unpolarized differential cross sections for different deuteron beam energies are presented.

Probing Generalized Parton Distributions Through the Photoproduction of a $\gamma \pi $ Pair

We study in the framework of collinear QCD factorization the photoproduction of a $\gamma\,\pi$ pair with a large invariant mass and a small transverse momentum, as a new way to access generalized parton distributions. In the kinematics of JLab 12-GeV, we demonstrate the feasibility of this measurement and show the extreme sensitivity of the unpolarized cross section to the axial quark GPDs.

Measurement of unpolarized and polarized cross sections for deeply virtual Compton scattering on the proton at Jefferson Laboratory with CLAS

This paper reports the measurement of polarized and unpolarized cross sections for the ep -> e'p' reaction, which is comprised of Deeply Virtual Compton Scattering (DVCS) and Bethe-Heitler (BH) processes, at an electron beam energy of 5.88 GeV at the Thomas Jefferson National Accelerator Facility using the Large Acceptance Spectrometer CLAS. The unpolarized cross sections and polarized cross section differences have been measured over broad kinematics, 0.10 < x_B < 0.58, 1.0 < Q^2 < 4.8 GeV^2, and 0.09 < -t < 2.00 GeV^2. The results are found to be consistent with previous CLAS data, and these new data are discussed in the framework of the generalized parton distribution approach. Calculations with two widely used phenomenological models, denoted VGG and KMSC, are approximately compatible with the experimental results over a large portion of the kinematic range of the data.

Semi-inclusive back-to-back production of a hadron pair and a single hadron in e+e\u2212 annihilation

Inclusive hadron production in e+e− annihilation has long been used to study both single hadron fragmentation functions (FF) and dihadron fragmentation functions (DiFF). In particular, the polarized DiFFs can be accessed in electron-positron annihilation by measuring azimuthal correlations between two back-to-back pairs of hadrons in the center of mass system, where the relevant structure functions can be expressed as convolutions of two (polarized) DiFFs. Here we explore the advantages of measuring the inclusive back-to-back production of a single hadron on one side against a hadron pair on the opposite side of the detector in two jet events. The leading twist cross section for this process contains convolutions of the corresponding single hadron FFs on one side and the DiFFs for the hadron pair on the other side, which furnishes several interesting new opportunities. A measurement of the unpolarized cross section with a number of different types of observed hadrons will help in untangling the quark flavor dependence of the unpolarized DiFFs, when the results are analyzed together with the inclusive measurements of dihadron pairs, such as those recently performed by the BELLE collaboration. Even more interesting, with a polarized hyperon on one side we can study the quark spin-dependent DiFFs of an unpolarized hadron pair on the other side. This, in turn, will allow us to test the universality of the spin-dependent DiFFs entering the cross sections of electron-positron annihilation and semi-inclusive deep inelastic scattering processes.

Experimental and theoretical study of deuteron–proton elastic scattering for proton kinetic energies between Tp = 882.2 MeV and Tp = 918.3 MeV

New precise unpolarised differential cross sections of deuteron–proton elastic scattering have been measured at 16 different deuteron beam momenta between pd=3120.17MeV/c and pd=3204.16MeV/c at the COoler SYnchrotron COSY of the Forschungszentrum Jülich. The data, which were taken using the magnetic spectrometer ANKE, cover the equivalent range in proton kinetic energies from Tp=882.2MeV to Tp=918.3MeV. The experimental results are analysed theoretically using the Glauber diffraction model with accurate nucleon–nucleon input. The theoretical cross section at Tp=900MeV agrees very well with the experimental one at low momentum transfers t. There are, however, significant deviations for |t|>0.2(GeV/c)2 that must be investigated further.

N* Photoexcitation Results from Mainz

The A2 Collaboration at MAMI performs a manifold research program with real photons with the Crystal Ball/TAPS detector at the MAMI accelerator facility in Mainz. In order to probe the internal structure of the nucleon, the spectrum of baryon resonances is studied via measurements of unpolarized cross sections and various polarization observables in single and double meson photoproduction. In this paper, recent results from the baryon spectroscopy program performed by the A2 Collaboration at MAMI are presented.

Charged Higgs pair production in association with the Z0 boson at electron-positron colliders

In this paper, the production of the charged Higgs pair associated with the $Z^0$ boson is analyzed in the minimal extension of the standard model the so-called two-Higgs-doublet model (2HDM). The process $e^+e^- \rightarrow H^+H^-Z^0$ is calculated at the tree level including all the possible diagrams in 2HDM. The numerical analysis is performed in consideration of the current experimental constraints and various scenarios for the free parameters of the model. The results are presented as a function of center-of-mass energy, the charged Higgs mass ($m_{H^\pm}$), and the ratio of the vacuum expectation values ($t_\beta$). The unpolarized cross section, taking into account the results in the flavor physics, gets up to $0.278\text{ fb}$ for $m_{H^\pm}=175\text{ GeV}$, and it declines with decreasing $m_{H^\pm}$ in Type-I. However, it gets down to $0.073\text{ fb}$ for $m_{H^\pm}=500\text{ GeV}$ in Type-II. Further, the calculation is also carried out in the non-alignment scenario and low-$m_{h^0}$ scenario. The effect of the polarized incoming $e^+$ and $e^-$ beams shows that the cross section is enhanced by a factor up to 2.5 at P(+0.60,-0.80) polarization configuration. Decay channels of the charged Higgs, possible final states of the process, and some differential distributions belong to the charged Higgs and $Z^0$ boson are examined for each scenario. The analysis shows that some channels have higher branching ratio such as $H^+\rightarrow t \bar{b}$, $H^+\rightarrow W^+h^0$, and $H^+\rightarrow W^+H^0$. These decay channels are essential for the charged Higgs searches in the lepton colliders regarding the scenarios interested. The detection of the charged Higgs is a powerful sign for the extended scalar sectors, and the results show the potential of a future lepton collider.

Determination of the Polarization Observables $${{\varvec{C}}}_{{\varvec{x}}}$$, $${{\varvec{C}}}_{{\varvec{z}}}$$, and $${{\varvec{P}}}_{{\varvec{y}}}$$ for the Quasi-Free Mechanism in the Reaction $${\varvec{\overrightarrow{\gamma }}}{\varvec{d}}\varvec{\rightarrow } {\varvec{K}}^{\varvec{+}}{\varvec{\overrightarrow{\varLambda }}} {{\varvec{n}}}$$

The study of the excited-nucleon ( $$N^*$$ ) spectrum is fundamental for the quest to understand non-perturbative quantum chromodynamics. Measured kinematical evolutions of experimental observables, such as polarizations and unpolarized cross sections, for exclusive meson photoproduction off proton and neutron, are the foundation of theoretical and phenomenological analyses that identify the $$N^*$$ states excited in these reactions and determine their parameters. While free proton targets are readily available, experiments typically use bound neutrons, such as in the deuteron, to estimate observables for photoproduction off the free neutron. Even though exclusivity allows to select event samples dominated by quasi-free scattering (QF), it is not clear if the obtained observables are unbiased estimates of the observables for photoproduction off the free neutron. Studies assessing the possible sources of bias, such as the neutron binding and final-state interactions (FSI), are sparse and primarily focused on the unpolarized cross sections. The comparison between observables for production off the free and off the bound proton in high-statistics reactions, seems to be the natural way to study and to quantify these effects experimentally. Alternatively, one can map the evolution of the QF observables with target-neutron momentum and potentially extrapolate it to the “free”-neutron point of zero momentum. In this work, we determine the recoil polarization $$P_y$$ of the $$\varLambda $$ hyperon and the polarization transfers $$C_x$$ and $$C_z$$ from circularly-polarized photons to the $$\varLambda $$ for the quasi-free mechanism of the reaction $$\overrightarrow{\gamma }d\rightarrow K^+\overrightarrow{\varLambda } n$$ , i.e., for photoproduction off the bound proton. The data were taken with the CLAS detector at the Thomas Jefferson National accelerator facility (JLab) during the E06-103 (g13a) experiment and cover $$E_{\gamma }$$ between 0.9 and 2.6 GeV and $$\cos \theta _{KCM}$$ between $$-\,0.3$$ and 0.8. For several kinematic bins, the dependence of the polarizations on the target-proton momentum is studied. Our work can be used as a benchmark to develop analysis strategies to reduce potential bias due to FSI and the binding of the neutron in the deuteron in reported polarization observables for quasi-free meson photoproduction off the bound neutron.

Sivers effect in inelastic J/ψ photoproduction in ep↑ collision in color octet model

The prediction of single-spin asymmetry in inelastic photoproduction of $J/\psi$ in $ep^\uparrow$ collision is presented. At next-to-leading order, the dominating process is photon-gluon fusion, $\gamma+g\rightarrow J/\psi+g$ for the production of $J/\psi$ in $e+p^\uparrow\rightarrow J/\psi+X$, which directly probes the gluon Sivers function. Using the non-relativistic QCD based color octet model, the color octet states $\leftidx{^{3}}{S}{_1}^{(8)}$, $\leftidx{^{1}}{S}{_0}^{(8)}$ and $\leftidx{^{3}}{P}{_{J(0,1,2)}}^{(8)}$ contribution to $J/\psi$ production is calculated. Sizable asymmetry is estimated as a function of transverse momentum $P_T$ and energy fraction $z$ of $J/\psi$ in the range $0<P_T\leq 1$ GeV and $0.3<z\leq 0.9$. The unpolarized differential cross section of inelastic $J/\psi$ photoproduction is found to be in good agreement with H1 and ZEUS data.

Dispersion relation formalism for the two-photon exchange correction to elastic muon\u2013proton scattering: elastic intermediate state

We evaluate the two-photon exchange correction to the unpolarized cross section in the elastic muon–proton scattering within dispersion relations. One of the six independent invariant amplitudes requires a subtraction. We fix the subtraction function to the model estimate of the full two-photon exchange at one of three MUSE beam energies and make a prediction for the two other energies. Additionally, we present single and double polarization observables accounting for the lepton mass.

Photo- and Electro-excitation of Bound Neutrons and Protons

Data for pion photo-productions off the neutron ($\gamma n\to \pi N$) have been primarily extracted from deuteron-target data ($\gamma d\to \pi NN$) by applying kinematical cuts thereby isolating the quasi-free samples. We critically examine if the neutron-target data obtained through this conventional procedure can be contaminated by final state interactions (FSI) and/or the kinematical cuts. The analysis is conducted with a theoretical model for $\gamma d\to \pi NN$ that takes account of the impulse mechanisms supplemented by the $NN$ and $\pi N$ rescattering mechanisms. We show that the FSI effects still visibly remain in the extracted $\gamma$`$n$'$\to\pi N$ unpolarized cross sections and polarization asymmetries $E$ even after the kinematical cuts are applied. We also find the FSI effects on $\gamma$`$n$'$\to\pi^0n$ can be somewhat different from those on $\gamma$`$p$'$\to\pi^0p$.

Cross sections for inelastic meson-meson scattering

We study two kinds of inelastic meson-meson scattering. The first kind is inelastic 2-to-2 meson-meson scattering that is governed by quark interchange as well as quark-antiquark annihilation and creation. Cross-section formulas are provided to get unpolarized cross sections for $\pi K \to \rho K^\ast$ for $I=1/2$, $\pi K^\ast \to \rho K$ for $I=1/2$, $\pi K^\ast \to \rho K^\ast$ for $I=1/2$, and $\rho K \to \rho K^\ast$ for $I=1/2$. Near threshold, quark interchange dominates the reactions near the critical temperature. The second kind is 2-to-1 meson-meson scattering with the process that a quark in an initial meson and an antiquark in another initial meson annihilate into a gluon and subsequently the gluon is absorbed by the other antiquark or quark. The transition potential for the process is derived. Four Feynman diagrams at tree level contribute to the 2-to-1 meson-meson scattering. Starting from the $S$-matrix element, the isospin-averaged unpolarized cross section with transition amplitudes is derived. The cross sections for $\pi \pi \to \rho$ and $\pi K \to K^*$ decrease with increasing temperature.

Hard photoproduction of a diphoton with a large invariant mass

The electromagnetic probe has proven to be a very efficient way to access the $3-$dimensional structure of the nucleon, particularly thanks to the exclusive Compton processes. We explore the hard photoproduction of a large invariant mass diphoton in the kinematical regime where the diphoton is nearly forward and its invariant mass is the hard scale enabling to factorize the scattering amplitude in terms of generalized parton distributions. This amplitude has a very peculiar and interesting analytic structure. We calculate unpolarized cross sections and the angular asymmetry triggered by a linearly polarized photon beam.

Dissociation of large-pT prompt J/ψ produced in Pb–Pb collisions at the LHC

A collision of a light meson and a charmonium produces quarks and antiquarks first, then the charm quark fragments into a charmed hadron, and finally three or more mesons are produced. This is the mechanism that we consider and propose to understand the prompt-J/ψ nuclear modification factor in the range of J/ψ transverse momentum from 10 GeV/c to 18 GeV/c, as measured by the CMS Collaboration and the ATLAS Collaboration. Unpolarized cross sections are derived and calculated for the reactions π+charmonium→Hc+X with Hc being D+, D0, Ds+, D⁎+, or D⁎0. Numerical cross sections are parametrized and used to calculate the dissociation rate of charmonium in the interaction with pions in hadronic matter. The momentum dependence of the rate obtained is so as to lead to a decreasing charmonium nuclear modification factor with increasing transverse momentum.

Double-longitudinal spin asymmetry in single-inclusive lepton scattering at NLO

We calculate the double-spin asymmetries $A_{\mathrm{LL}}$ for the processes $\ell N\to h X$ and $\ell N\to {\mathrm{jet}}\, X$ at next-to-leading order accuracy in perturbative QCD. We compare our theoretical results for $A_{\mathrm{LL}}$ to data from the SLAC E155 experiment, finding only partially satisfactory agreement. We conclude that measurements of $A_{\mathrm{LL}}$ and the relevant polarized and unpolarized cross sections should be performed at the present-day fixed-target lepton scattering experiments, as well as at a future electron ion collider, in order to verify our understanding of this process. We present predictions of the longitudinal double-spin asymmetries for these experiments.

Probing left-right seesaw models using beam polarization at an e+e− collider

We show that the longitudinal beam polarization option at a future electron-positron collider provides an unambiguous distinction between low-scale seesaw models of neutrino mass. This is possible due to the fact that the pair production cross section of the heavy neutrinos in seesaw models is sensitive to the polarization of the initial lepton beams, and for a suitable choice of the polarization, shows a clear enhancement over the unpolarized cross section. More interestingly, the choice of the beam polarization for which the enhancement is maximum is governed by the size of the light-heavy neutrino mixing parameter. We also find that using this effect, one can probe a previously uncharted parameter space of the left-right seesaw model, which is complementary to the existing searches at both energy and intensity frontiers.

First measurement of the polarization observable E and helicity-dependent cross sections in single π0 photoproduction from quasi-free nucleons

The double-polarization observable E and the helicity-dependent cross sections σ1/2 and σ3/2 have been measured for the first time for single π0 photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π0N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ1/2 and σ3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.

Transverse single-spin asymmetries in ℓp↑→hX within a TMD approach: Role of quasireal photon exchange

We present an updated study of transverse single-spin asymmetries for the inclusive large-$P_T$ processes $\ell \, p^\uparrow \to h\, X$ and $\ell\, p^\uparrow \to {\rm jet}\,X$, within a transverse momentum-dependent approach, including the contribution of quasireal (Weizs\"acker-Williams) photons. In the spirit of a unified transverse momentum-dependent scheme, predictions are obtained adopting the Sivers and transversity distributions and the Collins fragmentation functions as extracted from fits to the azimuthal asymmetries measured in semi-inclusive deep inelastic scattering and $e^+e^-$ annihilation processes. The description of the available data is extremely good, showing a clear general improvement with respect to the previous leading-order analysis. Predictions for unpolarized cross sections and single-spin asymmetries for ongoing and future experiments are also given.

Tree-level equivalence between a Lorentz-violating extension of QED and its dual model in electron\u2013electron scattering

S-matrix amplitudes for the electron–electron scattering are calculated in order to verify the physical equivalence between two Lorentz-breaking dual models. We begin with an extended Quantum Electrodynamics which incorporates CPT-even Lorentz-violating kinetic and mass terms. Then, in a process of gauge embedding, its gauge-invariant dual model is obtained. The physical equivalence of the two models is established at tree level in the electron–electron scattering and the unpolarized cross section is calculated up to second order in the Lorentz-violating parameter.

Charmonium dissociation in collisions with ϕ mesons in hadronic matter**Supported by National Natural Science Foundation of China (11175111)

The ϕ-charmonium dissociation reactions in hadronic matter are studied. Unpolarised cross sections for , or , , , or , , or and are calculated in the Born approximation, in the quark-interchange mechanism and with a temperature-dependent quark potential. The potential leads to remarkable temperature dependence of the cross sections. With the cross sections and the ϕ distribution function we calculate the dissociation rates of the charmonia in interactions with the ϕ meson in hadronic matter. The dependence of the rates on temperature and charmonium momentum is relevant to the influence of ϕ mesons on charmonium suppression.