Helicity-flip Amplitudes Research Articles

Helicity-flip transitions and the t-dependence of exclusive photoproduction of rho meson

We calculate the differential cross section dσ/dt for the diffractive photoproduction process γp → ρp and compare our results with experimental data. Our model is based on two-gluon exchange in the nonperturbative domain. We take into account both helicity conserving and often neglected helicity-flip amplitudes in the γ → V transition, which can contribute at finite t. The shape of the differential cross section as well as the role of helicity-flip processes is strongly related to the dependence of the unintegrated gluon distribution on transverse momenta in the nonperturbative region.

Exclusive production of ρ meson in gamma-proton collisions: dσ/dt and the role of helicity flip processes

We calculate the differential cross section dσ/dt for the diffractive photoproduction process γp→ρp and compare to recent data extracted by the CMS collaboration from ultraperipheral proton-lead collisions. Our model is based on two-gluon exchange in the nonperturbative domain. We take into account both helicity conserving and often neglected helicity-flip amplitudes in the γ→V transition. The latter can contribute at finite t. The shape of the differential cross section as well as the role of helicity flip processes is strongly related to the dependence of the unintegrated gluon distribution on transverse momenta in the nonperturbative region.

Deeply Virtual Compton Scattering Cross Section at High Bjorken xB

We report high-precision measurements of the deeply virtual Compton scattering (DVCS) cross section at high values of the Bjorken variable x_{B}. DVCS is sensitive to the generalized parton distributions of the nucleon, which provide a three-dimensional description of its internal constituents. Using the exact analytic expression of the DVCS cross section for all possible polarization states of the initial and final electron and nucleon, and final state photon, we present the first experimental extraction of all four helicity-conserving Compton form factors (CFFs) of the nucleon as a function of x_{B}, while systematically including helicity flip amplitudes. In particular, the high accuracy of the present data demonstrates sensitivity to some very poorly known CFFs.

Spin-dependent Pomeron and Odderon in elastic proton\u2013proton scattering

We introduce a new model of near-forward elastic proton–(anti)proton scattering at high energy based on the modern formulation of Pomeron and Odderon in terms of Wilson lines and generalized transverse momentum dependent distributions. We compute the helicity-dependent elastic amplitudes phi _{1,2,3,4,5} in this model and study their energy dependence from the nonlinear small-x evolution equations. While both Pomeron and Odderon contribute to helicity-flip processes in general, in the forward limit t=0 only the double helicity-flip amplitude phi _2, dominated by the spin-dependent Odderon, survives. This may affect the extraction of the rho parameter as well as the total cross section in the LHC energy domain and beyond.

Wide angle Compton scattering on the proton: study of power suppressed corrections

We study the wide angle Compton scattering process on a proton within the soft-collinear factorization (SCET) framework. The main purpose of this work is to estimate the effect due to certain power suppressed corrections. We consider all possible kinematical power corrections and also include the subleading amplitudes describing the scattering with nucleon helicity flip. Under certain assumptions we present a leading-order factorization formula for these amplitudes which includes the hard- and soft-spectator contributions. We apply the formalism and perform a phenomenological analysis of the cross section and asymmetries in the wide angle Compton scattering on a proton. We assume that in the relevant kinematical region where $$-t,-u>2.5$$ GeV $$^{2}$$ the dominant contribution is provided by the soft-spectator mechanism. The hard coefficient functions of the corresponding SCET operators are taken in the leading-order approximation. The analysis of existing cross section data shows that the contribution of the helicity-flip amplitudes to this observable is quite small and comparable with other expected theoretical uncertainties. We also show predictions for double polarization observables for which experimental information exists.

ON THE LARGE-t ELASTIC SCATTERING AT $\sqrt{s} = 7~{\rm TeV}$

We discuss discrimination of the scattering mechanisms on the basis of functional dependencies of the large-t elastic scattering suggested by the recent data from the TOTEM experiment. It is shown that Orear exponent is in a better agreement with the data than the power-like dependence used by the TOTEM. This implies that the collective dynamics is dominating over the point-like mechanism related to the scattering of the proton constituents. We also emphasize that vanishing of the helicity-flip amplitudes contributions at the LHC energies would result in appearance of the dip–bump structures at higher values of transferred momenta.

Measurements of the energy dependence of the analyzing power in pp elastic scattering in the CNI region

We present new measurements of the analyzing power AN in proton-proton elastic scattering in the Coulomb-Nuclear Interference region at √s = 7.7 and 21.7 GeV obtained with the polarized atomic hydrogen jet target at RHIC. These measurements complement our earlier results at √s = 6.8 and 13.7 GeV confirming the presence of a hadronic helicity flip amplitude contribution in proton-proton elastic scattering at lower energies (√s <8 GeV) while higher energy data (√s >13 GeV) are consistent with no hadronic helicity flip contribution.

Forward pion-nucleon charge exchange reaction and Regge constraints

We present our recent study of pion-nucleon charge exchange amplitudes above 2 GeV. We analyze the forward pion-nucleon charge exchange reaction data in a Regge model and compare the resulting amplitudes with those from the Karlsruhe–Helsinki and George-Washington-University partial-wave analyses. We explore possible high-energy constraints for theoretical baryon resonance analyses in the energy region above 2 GeV. Our results show that for the pion-nucleon charge exchange reaction, the appropriate energy region for matching meson-nucleon dynamics to diffractive scattering should be around 3 GeV for the helicity flip amplitude.

Double spin asymmetries ANN and ASS at s=200 GeV in polarized proton–proton elastic scattering at RHIC

We present the first measurements of the double spin asymmetries ANN and ASS at s=200 GeV, obtained by the PP2PP experiment using polarized proton beams at the Relativistic Heavy Ion Collider (RHIC). The data were collected in the four momentum transfer t range 0.01⩽|t|⩽0.03(GeV/c)2. The measured asymmetries, which are consistent with zero, allow us to estimate upper limits on the double helicity-flip amplitudes ϕ2 and ϕ4 at small |t| as well as on the difference ΔσT between the total cross sections for transversely polarized protons with antiparallel or parallel spin orientations.

Scattering amplitudes with massive fermions using BCFW recursion

We study the QCD scattering amplitudes for \(\bar{q}q\to{gg}\) and \(\bar{q}q\to{ggg}\) where q is a massive fermion. Using a particular choice of massive fermion spinor we derive compact expressions for the partial spin amplitudes for the 2→2 process. We then investigate the corresponding 2→3 amplitudes using the BCFW recursion technique. For the helicity conserving partial amplitudes we present new expressions, but we were unable to treat the helicity-flip amplitudes recursively, except for the case where all the gluon helicities are the same. We therefore evaluate the remaining partial amplitudes using standard Feynman diagram techniques.

Beam normal spin asymmetry in the quasireal Compton scattering approximation

he two-photon exchange contribution to the single spin asymmetries with the spin orientation normal to the reaction plane is discussed for elastic electron-proton scattering in the equivalent photon approximation. In this case, the hadronic part of the two-photon exchange amplitude describes real Compton scattering (RCS). We show that in the case of the beam normal spin asymmetry this approximation selects only the photon helicity flip amplitudes of RCS. At low energies, we make use of unitarity and estimate the contribution of the piN multipoles to the photon helicity flip amplitudes. In the Regge regime, the quasi-RCS (QRCS) approximation allows for a contribution from two-pion exchange, and we provide an estimate of such contributions.

Normal Spin Asymmetries in Elastic Electron-Proton Scattering

We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the single spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions. We further discuss this observable in the quasi-RCS kinematics which may be dominant at certain kinematical conditions and find it to be governed by the photon helicity-flip RCS amplitudes.

The role of double helicity-flip amplitudes in small-angle elastic pp-scattering

We asses a role of the double helicity-flip amplitudes in small-angle elastic pp-scattering and obtain a new unitary bound for the double helicity-flip amplitude F2 in elastic pp-scattering at small values of t on the basis of the U-matrix method of the s-channel unitarization.

Perturbative and non-perturbative QCD corrections to wide-angle Compton scattering

We investigate corrections to the handbag approach for wide-angle Compton scattering off protons at moderately large momentum transfer: the photon-parton subprocess is calculated to next-to-leading order QCD and contributions from the generalized parton distribution E} are taken into account. Photon and proton helicity flip amplitudes are non-zero due to these corrections which leads to a wealth of polarization phenomena in Compton scattering. Thus, for instance, the incoming photon asymmetry or the transverse polarization of the proton are non-zero although small.

Power corrections to exclusive processes in QCD

In practice applicability of twist expansion crucially depends on the magnitude to power corrections to the leading-twist amplitude. I illustrate this point by considering explicit examples of two hard exclusive processes in QCD. In the case of γ *γ → ππ amplitude power corrections are small enough such that it should be possible to describe current experimental data by the leading-twist QCD prediction. The photon helicity-flip amplitude in DVCS on a nucleon receives large kinematical power corrections which screen the leading-twist prediction up to large values of the hard photon virtuality.

Hard electroproduction of mesons: Helicity flip amplitudes with tensor gluon contributions

We consider hard electroproduction of mesons at twist-3 level and focus our attention on helicity flip amplitudes corresponding to the production of transversely polarized mesons by transverse photons. We demonstrate that helicity-flip amplitude for the transversely polarized particles are closely related to the contributions of the matrix elements of the tensor gluon operator which can serve as pure gluonic probe of the hadrons. Using standard factorisation approach and Wanzura-Wilczek approximation for the twist-3 matrix elements we find that such amplitudes do not have divergencies from the endpoint regions which appear in the other twist-3 amplitudes.

Bounds in proton-proton elastic scattering at low momentum transfer

We present a bound on the imaginary part of the single helicity-flip amplitude for spin 1/2-spin 1/2 scattering at small momentum transfer. The variational method of Lagrange multipliers is employed to optimize the single-flip amplitude using the values of the total cross section, the elastic cross section and diffraction slope as equality constraints in addition to the inequality constraints resulting from unitarity. Such bounds provide important information related to the determination of polarization of a proton beam. In the case of elastic proton collisions the analyzing power at small scattering angles inside the Coulomb Nuclear Interference region offers a method of measuring the polarization of a proton beam, the accuracy of the polarization measurement depending on the single helicity-flip amplitude. The bound obtained on the imaginary part of the single helicity-flip amplitude indicates that the analyzing power for proton-proton collisions in the Coulomb nuclear interference region should take positive nonzero values at high energies.

Twist-4 photon helicity-flip amplitude in DVCS on a nucleon in the Wandzura-Wilczek approximation

We computed twist-4 part of the photon spin-flip amplitude in deeply virtual Compton scattering on a nucleon in the Wandzura-Wilczek approximation. We found a factorizable contribution, which arises from photon scattering on quarks with non-zero angular momentum along the collision axis. As the genuine twist-2 amplitude arises at the NLO, for moderate virtualities of the hard photon, $Q^2 \le 10$ GeV$^2$, kinematical twist-4 correction can give numerically important contribution to the photon helicity-flip amplitude.

Optimisation of the single helicity-flip amplitude in elastic pp collisions

The optimisation method of Lagrange multipliers is used to bound the proton proton single helicity-flip amplitude modified by a kinematical factor. The system is constrained by the elastic cross section and by partial wave unitarity. Using this technique an upper bound on the modified single helicity-flip amplitude close to the Coulomb Nuclear Interference (CNI) region is obtained.

Elastic electroproduction of

The elastic electroproduction of $\rho$ mesons is studied at HERA with the H1 detector for a photon virtuality in the range $1 < Q^2 < 60 {\rm GeV^2}$ and for a hadronic centre of mass energy in the range $30 < W < 140$ GeV. The shape of the ( $\pi \pi$ ) mass distribution in the $\rho$ resonance region is measured as a function of $Q^2$ . The full set of $\rho$ spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of $8 \pm 3\%$ of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on $Q^2$ , W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large $Q^2$ , the $\gamma^*p$ cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron–hadron cross sections.