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Register a new userDiffraction and sigma_{gamma* p}
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D. Schildknecht
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The empirical scaling law, wherein the total photoabsorption cross section depends on the single variable eta=(Q^2+m_0^2)/Lambda^2(W^2), provides empirical evidence for saturation in the sense of sigma_{gamma* p}(W^2,Q^2)/sigma_{gamma p}(W^2) --> 1 for W^2 --> infinity at fixed Q^2. The total photoabsorption cross section is related to elastic diffraction in terms of a sum rule. The excess of diffractive production over the elastic component is due to inelastic diffraction that contains the production of hadronic states of higher spins. Motivated by the diffractive mass spectrum, the generalized vector dominance/color dipole picture (GVD/CDP) is extended to successfully describe the DIS data in the full region of x=<0.1, all Q^2>=0, where the diffractive two-gluon-exchange mechanism dominates.
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Though the neutron mass is larger than the proton mass and obviously one expects in $e^+e^-$ annihilation a creation of more $p bar p$ pairs in comparison with $n bar n$ at the same energy, just the opposite inequality for the corresponding total cross sections $sigma_{tot}(e^+e^- to n bar n) > sigma_{tot}(e^+e^- to p bar p)$ has been revealed in the analysis of only the present time existing proton electromagnetic form factors data by the unitary and analytic approach.
The most recent high-precision data on spin observables $Sigma$, $T$, $P$, $E$, $F$ and $H$ reported by the CLAS Collaboration together with the previous data on differential cross sections and spin-density-matrix elements reported by the CLAS, A2, GRAAL, SAPHIR and CBELSA/TAPS Collaborations for the reaction $gamma p to omega p$ are analyzed within an effective Lagrangian approach. The reaction amplitude is constructed by considering the $t$-channel $pi$ and $eta$ exchanges, the $s$-channel nucleon and nucleon resonances exchanges, the $u$-channel nucleon exchange and the generalized contact current. The latter accounts effectively for the interaction current and ensures that the full photoproduction amplitude is gauge invariant. It is shown that all the available CLAS data can be satisfactorily described by considering the $N(1520)3/2^-$, $N(1700)3/2^-$, $N(1720)3/2^+$, $N(1860)5/2^+$, $N(1875)3/2^-$, $N(1895)1/2^-$ and $N(2060)5/2^-$ resonances in the $s$-channel. The parameters of these resonances are extracted and compared with those quoted by PDG.
We propose a new method of the determination of $R^{D}=sigma_{L}^{D}/sigma_{T}^{D}$ from the dependence of the diffractive cross section on the azimuthal angle between the electron scattering and proton scattering planes. The method is based on our finding of the model independence of the ratio of the $LT$ interference and transverse diffractive structure functions. The predicted azimuthal asymmetry is substantial and can be measured at HERA. We show that the accuracy of our reconstruction of $R^{D}$ is adequate for a reliable test of an important pQCD prediction of $R^{D}gsim 1$ for large $beta$.
The production of hidden-bottom pentaquark $P_{b}$ states via $gamma p$ and $pi ^{-}p$ scatterings is studied within an effective Lagrangian approach and the vector-meson-dominance mechnism. For the $P_{b}$ production in the process $gamma prightarrow Upsilon p$, the dipole Pomeron model is employed to calculate the background contribution, and the experimental data can be well described. For the process $pi ^{-}prightarrow Upsilon n$, the Reggeized $t$-channel with $pi $ exchange is considered as the main background for the $P_{b}$ production. Near the threshold, two-peak structure from the states $P_{b (11080)$ and $P_{b}(11125)$ can be observed if energy bin width is chosen as 0.01 GeV, and the same result is obtained in the $pi ^{-}p$ scattering. Moreover, by taking the branching ratio of Br$[{P_{b}rightarrow pi N}]simeq 0.05%$, the numerical result shows that the average value of cross section from the $P_{b}(11080)$ state produced in the $gamma p$ or $pi ^{-}p$ scattering reaches at least 0.1 nb with a bin of 0.1 GeV. Even if we reduce the branching ratio of the $P_{b}$ state into $pi N$ channel by one order, the theoretical average of the cross section from $P_{b}(11080)$ production in $pi ^{-}p$ scattering can reach the order of 0.01 nb with a bin of 0.1 GeV, which means that the signal can still be clearly distinguished from the background. The experimental measurements and studies on the hidden-bottom pentaquark $P_{b}$ state production in the $gamma p $ or $pi ^{-}p$ scattering near-threshold energy region around $Wsimeq 11$ GeV are strongly suggested, which are accessible at COMPASS and JPARC. Particularly, the result of the photoproduction suggests that it is very promising to observe the hidden-bottom pentaquark at proposed EicC facility in China.
We compute 10 radiative three-body decays of charged charmed mesons $D^+ to P^+ P^0 gamma$ and $D_s to P^+ P^0 gamma$, $P=pi, K$, in leading order QCDF, HH$chi$PT and the soft photon approximation. We work out decay distributions and asymmetries in the standard model and with new physics in the electromagnetic dipole operators. The forward-backward asymmetry is suitable to probe the QCD frameworks, in particular the $s$-channel dependent weak annihilation contributions in QCDF against the markedly different resonance structure in HH$chi$PT. These studies can be performed with Cabibbo-favored modes $D_s to pi^+ pi^0 gamma$, $D^+ to pi^+ overline{K}^0 gamma$ and $D_s to K^+ overline{K}^0 gamma$ with ${cal{O}}(10^{-4}-10^{-3})$-level branching ratio, which are standard model-like and induced by different hadronic dynamics. Understanding of the latter can therefore be improved in a data-driven way and sharpens the interpretation of standard model tests. Singly Cabibbo-suppressed modes such as $ D^+ to pi^+ pi^0 gamma$, $D_s to pi^+ K^0 gamma$, $D_s to K^+ pi^0 gamma$ with branching ratios within $sim 10^{-5}-10^{-4}$ are sensitive to new physics that can be signalled in the forward-backward asymmetry and in the CP-asymmetry of the rate, ideally in the Dalitz region but also in single differential distributions. Results complement those with neutral $D^0 to PP gamma$ decays.
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