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Register a new userLeading order corrections to the Bethe-Heitler process in the $gamma prightarrow l^+l^-p$ reaction
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The ratio of di-lepton production cross sections on a proton, using the $gamma prightarrow l^+ l^- p$ process, above and below di-muon production threshold allows to extract the effective lepton-proton interaction, which is required to be identical for electrons and muons if lepton universality is exact. To test for a scenario of broken universality at the percent level, of the size which could explain the different proton charge radii extracted from electron scattering and from muonic hydrogen spectroscopy, we evaluate all one-loop QED corrections to this process, including the full lepton mass dependencies. We furthermore show that two-photon exchange processes with both photons attached to the proton line vanish after averaging over di-lepton angles, and estimate the relatively small radiation off the proton. We compare the full one-loop calculation with a soft-photon approximation of the same order, and present estimates for a planned experiment.
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We develop the full finite lepton mass formalism for the production of real photons via the Bethe-Heitler reaction of unpolarized leptons off unpolarized nucleons. Genuine lepton mass effects are described, in particular their dependence upon the lepton mass and the initial beam energy, as well as their sensitivity to the nucleon isospin. In the minimum momentum transfer region, these effects dominate the muon induced proton cross section and become significant for electron scattering at small $x_B$.
A calculation of the loop-induced gluon-fusion process gg --> Z(photon)Z(photon) --> l anti-l l anti-l is presented, which provides an important background for Higgs boson searches in the H --> ZZ channel at the LHC. We find that the photon contribution is important for Higgs masses below the Z-pair threshold and that the gg-induced process yields a correction of about 15% relative to the NLO QCD prediction for the q anti-q-induced process when only a M(l anti-l), M(l anti-l) > 5 GeV cut is applied.
We present the cross section for $e^{+}e^{-}to hZ$ with arbitrary sets of electron and $Z$ boson polarizations at the full next-to-leading order in various extended Higgs models, such as the Higgs singlet model (HSM), the inert doublet model (IDM) and the two Higgs doublet model (2HDM). We systematically perform complete one-loop calculations to the helicity amplitudes in the on-shell renormalization scheme, and present the full analytic results as well as numerical evaluations. The deviation $Delta R^{hZ}$ in the total cross section from its standard model (SM) prediction is comprehensively analyzed, and the differences among these models are discussed in details. We find that new physics effects appearing in the renormalized $hZZ$ vertex almost govern the behavior of $Delta R^{hZ}$, and it takes a negative value in most cases. The possible size of $Delta R^{hZ}$ reaches several percent under the theoretical and experimental bounds. We also analyze the deviation $Delta R^{hZ}_{XY}$ in the total cross section times decay branching ratios of the discovered Higgs boson by utilizing the $texttt{H-COUP}$ program. It is found that the four types of 2HDMs can be discriminated by analyzing the correlation between $Delta R^{hZ}_{tautau}$ and $Delta R^{hZ}_{bb}$ and those between $Delta R^{hZ}_{tautau}$ and $Delta R^{hZ}_{cc}$. Furthermore, the HSM and the IDM can be discriminated from the 2HDMs by measuring $Delta R^{hZ}_{WW}$. These signatures can be tested by precision measurements at future Higgs factories such as the International Linear Collider.
We calculate O(alpha_s) two-loop virtual corrections to the differential decay width dGamma(B --> X_s l^+ l^-)/ds, where s is the invariant mass squared of the lepton pair. We also include those contributions from gluon bremsstrahlung which are needed to cancel infrared and collinear singularities present in the virtual corrections. Our calculation is restricted to the range 0.05 < s/m_b^2 < 0.25 where the effects from resonances are small. The new contributions drastically reduce the renormalization scale dependence of existing results for dGamma(B --> X_s l^+ l^-)/ds. For the corresponding branching ratio (restricted to the above s-range) the renormalization scale uncertainty gets reduced from +/- 13% to +/- 6.5%.
In this work, the two-photon-exchange (TPE) effects in $eprightarrow enpi^+$ at small $-t$ are discussed within a hadronic model. The TPE contributions to the amplitude and the unpolarized differential cross section are both estimated and we find that the TPE corrections to the unpolarized differential cross section are about $-4%sim-15%$ at $Q^2=1$GeV$^2sim1.6$GeV$^2$. After considering the TPE corrections to the experimental data sets of unpolarized differential cross section, we analyse the TPE corrections to the separated cross sections $sigma_{textrm{L,T,LT,TT}}$. We find that the TPE corrections (at $Q^2=1$GeV$^2sim1.6$GeV$^2$) to $sigma_{textrm{L}}$ are about $-10%sim -20%$, to $sigma_{textrm{T}}$ are about $20%$ and to $sigma_{textrm{LT,TT}}$ are much larger. By these analysis, we conclude that the TPE contributions in $eprightarrow enpi^+$ at small $-t$ are important to extract the separated cross sections $sigma_{textrm{L,T,LT,TT}}$ and the electromagnetic magnetic form factor of $pi^+$ in the experimental analysis.
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