Showing papers by "Miguel S. Costa published in 2020"

$\gamma^* \gamma$ and $\gamma^* p$ scattering in IHQCD

Abstract: We perform joint fits using measurements of the proton structure functions $F_2(x,Q^2)$ and $F_L(x,Q^2)$, of the photon structure function $F_2^\gamma(x,Q^2)$ and of the total cross-sections $\sigma\left( \gamma p \rightarrow X\right)$ and $\sigma \left( \gamma \gamma \rightarrow X\right)$. The data is gathered from several sources including HERA and LEP. The kinematical range considered is wide with a photon virtuality $Q^2 \leq400 \, {\rm GeV}^2$, Bjorken variable $x 4 \, {\rm GeV}$. This is done by considering a $U\left(1\right)$ vector gauge field minimally coupled to the graviton Regge trajectory in improved holographic QCD. We find good agreement with the data for a joint fit of $F_2^\gamma$ and $\sigma\left(\gamma \gamma \rightarrow X\right)$ with a $\chi_{d.o.f.}^2 = 1.07$. For a joint fit of $F_2$, $F_L$ and $\sigma\left( \gamma p \rightarrow X\right)$ we find $\chi_{d.o.f.}^2 = 1.40$. A joint fit of all observables for both processes $\gamma^* \gamma$ and $\gamma^* p$ gives a $\chi_{d.o.f.}^2 = 1.38$. The gravitational couplings of the $U\left(1\right)$ vector gauge field and of the proton with the Reggeons of the graviton Regge trajectory are given as an output of the fitting procedure. An approximate relation between the structure functions, valid at NLO QCD, is used to estimate the parton distribution function of the gluon inside the proton.

Unpolarized DIS and Gluon PDFs in IHQCD

Abstract: We perform a joint fit to HERA measurements of the unpolarized deep inelastic scattering structure functions $F_2 \left(x, Q^2\right)$ and $F_L\left(x, Q^2\right)$ over a very large kinematical range in the photon virtualily $Q^2 \leq 400 \, { \rm GeV}^2$ and Bjorken variable $x < 0.01$. This is done by considering a $U\left(1\right)$ vector gauge field minimally coupled to the graviton Regge trajectory in improved holographic QCD. We find good agreement with the data from HERA achieving a $\chi^2$ of $1.4$ without removing presumed outliers existing in data. If we consider only data with $Q^2 \leq 10 \, { \rm GeV}^2$ the quality of the fit improves to a $\chi^2$ of $1.3$. An approximate relation between the structure functions, valid at NLO QCD, is used to estimate the parton distribution function of the gluon inside the proton. Our PDFs are in agreement with the NLO results presented by CTEQ and NNPDF collaborations.

The perturbative CFT optical theorem and high-energy string scattering in AdS at one loop

Abstract: We derive an optical theorem for perturbative CFTs which computes the double discontinuity of conformal correlators from the single discontinuities of lower order correlators, in analogy with the optical theorem for flat space scattering amplitudes. The theorem takes a purely multiplicative form in the CFT impact parameter representation used to describe high-energy scattering in the dual AdS theory. We use this result to study four-point correlation functions that are dominated in the Regge limit by the exchange of the graviton Regge trajectory (Pomeron) in the dual theory. At one-loop the scattering is dominated by double Pomeron exchange and receives contributions from tidal excitations of the scattering states which are efficiently described by an AdS vertex function, in close analogy with the known Regge limit result for one-loop string scattering in flat space at finite string tension. We compare the flat space limit of the conformal correlator to the flat space results and thus derive constraints on the one-loop vertex function for type IIB strings in AdS and also on general spinning tree level type IIB amplitudes in AdS.