Two photon exchange and transverse spin asymmetries in the A4 experiment

In the present review, we discuss different aspects of the two-photon exchange (TPE) physics in elastic ep scattering at high Q2, as well as at low Q2. The imaginary part of the TPE amplitude gives rise to beam and target single-spin asymmetries. Different theoretical approaches to the calculation of these observables are considered. The real part of the TPE amplitude influences the unpolarized cross-section and double-spin observables and is, most likely, responsible for the discrepancy between two methods of measurements of the proton form factors. We review different methods of calculations of the TPE amplitudes in the framework of the “hadron” and “quark-gluon” approaches. We discuss the dispersion approach suitable for low and intermediate Q2, which includes elastic and inelastic intermediate hadronic states, as well as the connection of TPE with the proton radius puzzle. The present situation with direct experimental searches for the TPE amplitude in the e+p/e−p charge asymmetry is also discussed, as well as attempts to extract the TPE amplitudes from existing experimental data obtained by the Rosenbluth and double polarization techniques. The TPE physics in other processes such as elastic мp, e-nucleus, and еп scattering is also reviewed.

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SINGLE SPIN ASYMMETRIES IN INCLUSIVE DIS AND MULTI-PARTON CORRELATIONS IN THE NUCLEON

International Journal of Modern Physics Conference Series ◽

10.1142/s2010194512009178 ◽

2012 ◽

Vol 20 ◽

pp. 137-144

Author(s):

ANDREAS METZ ◽

DANIEL PITONYAK ◽

ANDREAS SCHÄFER ◽

MARC SCHLEGEL ◽

WERNER VOGELSANG ◽

...

Keyword(s):

Experimental Data ◽

Spin Asymmetry ◽

Recent Experimental Data ◽

Hard Scattering ◽

Photon Exchange ◽

Spin Asymmetries ◽

Single Spin ◽

Two Photon ◽

Single Spin Asymmetries ◽

Dependent Relation

Transverse single spin asymmetries in inclusive deep-inelastic lepton-nucleon scattering can be generated through multi-photon exchange between the leptonic and the hadronic part of the process. Here we consider two-photon exchange, and mainly focus on the transverse target spin asymmetry. In particular, we investigate the case where two photons couple to different quarks. Such a contribution involves a quark-photon-quark correlator in the nucleon, which has a (model-dependent) relation to the Efremov-Teryaev-Qiu-Sterman quark-gluon-quark correlator TF. Using different parameterizations for TF we compute the transverse target spin asymmetry for both a proton and a neutron target, and compare the results to recent experimental data. Potential implications on our general understanding of single spin asymmetries in hard scattering processes are discussed as well.

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ESTIMATING THE SIGN AND SIZE OF THE TRANSVERSE SINGLE TARGET-SPIN ASYMMETRY IN INCLUSIVE DIS

International Journal of Modern Physics Conference Series ◽

10.1142/s201019451460012x ◽

2014 ◽

Vol 25 ◽

pp. 1460012 ◽

Cited By ~ 1

Author(s):

MARC SCHLEGEL

Keyword(s):

Correlation Functions ◽

Spin Asymmetry ◽

Photon Correlation ◽

Photon Exchange ◽

Spin Asymmetries ◽

Single Spin ◽

Two Photon ◽

Single Spin Asymmetries ◽

Single Target ◽

Perturbative Part

The single-spin asymmetry of unpolarized leptons scattering deep-inelastically off transversely polarized nucleons is discussed. This observable is generated by a two-photon exchange between lepton and nucleon. In a partonic description of the asymmetry the non-perturbative part is given in terms of multiparton correlations: quark-gluon correlation functions and quark-photon correlation functions. Using input from models and data fits for these twist-3 correlation functions, estimates for the single-spin asymmetries for a proton and a neutron are presented.

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Two-photon-exchange effects in ep→enπ+ and their corrections to separated cross sections σL,T,LT,TT at small −t

Physical Review C ◽

10.1103/physrevc.101.055201 ◽

2020 ◽

Vol 101 (5) ◽

Author(s):

Hui-Yun Cao ◽

Hai-Qing Zhou

Keyword(s):

Cross Sections ◽

Photon Exchange ◽

Two Photon ◽

Exchange Effects

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Global analysis of the Sivers functions at NLO+NNLL in QCD

Journal of High Energy Physics ◽

10.1007/jhep01(2021)126 ◽

2021 ◽

Vol 2021 (1) ◽

Cited By ~ 2

Author(s):

Miguel G. Echevarria ◽

Zhong-Bo Kang ◽

John Terry

Keyword(s):

Pair Production ◽

Global Analysis ◽

Vector Boson ◽

Lepton Pair ◽

Transverse Spin ◽

Dglap Evolution ◽

Spin Asymmetries ◽

Lepton Pair Production ◽

Asymmetry Data ◽

The Impact

Abstract We perform global fit to the quark Sivers function within the transverse momentum dependent (TMD) factorization formalism in QCD. We simultaneously fit Sivers asymmetry data from Semi-Inclusive Deep Inelastic Scattering (SIDIS) at COMPASS, HERMES, and JLab, from Drell-Yan lepton pair production at COMPASS, and from W/Z boson at RHIC. This extraction is performed at next-to-leading order (NLO) and next-to-next-to leading logarithmic (NNLL) accuracy. We find excellent agreement between our extracted asymmetry and the experimental data for SIDIS and Drell-Yan lepton pair production, while tension arises when trying to describe the spin asymmetries of W/Z bosons at RHIC. We carefully assess the situation, and we study in details the impact of the RHIC data and their implications through different ways of performing the fit. In addition, we find that the quality of the description of W/Z vector boson asymmetry data could be strongly sensitive to the DGLAP evolution of Qiu-Sterman function, besides the usual TMD evolution. We present discussion on this and the implications for measurements of the transverse-spin asymmetries at the future Electron Ion Collider.

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