We consider the possible role of rescattering effects in the determination of the gluon density for the LHC from DIS data. We discuss a method that uses results of s-channel calculations to estimate these effects, and comment on potential applications to diffractive and multi-parton interactions.
One of the most poorly understood phenomenon in hadron collisions, is the so called multiple parton interaction (MPI). Apart from one quark or gluon each from each colliding proton, additional quarks or gluons can interact as well, and these can not be calculated from first principles. The concept of rescattering has been introduced recently in Pythia8 event generator, where particles originating from these secondary interactions can interact again with quarks or gluons from incoming protons. In this paper, we look at events with a $Z$-boson, to find observables which can potentially be sensit…
We present a QCD-based model where rescattering between final state partons in deep inelastic scattering leads to events with large rapidity gaps and a leading proton. In the framework of this model the amplitude for multiple gluon exchanges is calculated in the eikonal approximation to all orders in perturbation theory. Both large and small invariant mass M_X limits are considered. The model successfully describes the precise HERA data on the diffractive deep inelastic cross section in the whole available kinematical range and gives new insight into the density of gluons at very small momentu…
We study the twist-3 fragmentation functions $H$ and $\tilde{H}$, by applying a spectator model. In the calculation we consider the effect of the gluon rescattering at one loop level. We find that in this case the hard-vertex diagram, which gives zero contribution to the Collins function, does contribute to the fragmentation function $H$. The calculation shows that the twist-3 T-odd fragmentation functions are free of light-cone divergences. The parameters of the model are fitted from the known parametrization of the unpolarized fragmentation $D_1$ and the Collins function $H_1^\perp$. We find…
The three-gluon contribution to $J/ψ$ hadroproduction is calculated within perturbative QCD in the $k_T$ factorization framework. This mechanism involves double gluon density and enters at a non-leading twist, but it is enhanced at large energies due to large double gluon density at small $x$. We obtain results for differential $p_T$-dependent cross-sections for all $J/ψ$ polarisations. The rescattering contribution is found to provide a significant correction to the standard leading twist cross-section at the energies of the Tevatron or the LHC at moderate $p_T$. We also discuss a possible co…
Large angle gluon radiations induced by multiple parton scatterings contribute to dijet production in deeply inelastic scattering off a large nucleus at the Electron-Ion Collider. Within the generalized high-twist approach to multiple parton scattering, such contributions at the leading order in perturbative QCD and large Bjorken momentum fraction $x_B$ can be expressed as a convolution of the multiple parton scattering amplitudes and the transverse momentum dependent (TMD) two-parton correlation matrix elements. We study this medium-induced dijet spectrum and its azimuthal angle correlation u…
The cross section for inclusive production of $f_2(1270)$ meson is calculated. We include both the mechanism of gluon-gluon fusion as well as the $ππ$ final-state rescattering. The contribution of the gluon-gluon fusion is calculated within the $k_t$-factorization approach with modern unintegrated gluon distribution functions (UGDFs). Some parameters for the $g^* g^* \to f_2$ vertex are extracted from the $γγ\to f_2(1270) \to ππ$ reactions. The results strongly depend on the parametrization of the $g^* g^* \to f_2(1270)$ form factor. Results of our model are compared to the ALICE preliminary d…
The transverse momentum dependent gluon density obtained with CCFM evolution is determined from a fit to the latest combined HERA structure function measurements.
We adapt the Schwinger-Keldysh formalism to study heavy-ion collisions in perturbative QCD. Employing the formalism, we calculate the two-point gluon correlation function $G_{22}^{aμ,bν}$ due to the lowest-order classical gluon fields in the McLerran-Venugopalan model of heavy ion collisions and observe an interesting transition from the classical fields to the quasi-particle picture at later times. Motivated by this observation, we push the formalism to higher orders in the coupling and calculate the contribution to $G_{22}^{aμ,bν}$ coming from the diagrams representing a single rescattering …
We study the effects of hadronic rescattering on hadron distributions in high-energy nuclear collisions by using an integrated dynamical approach. This approach is based on a hybrid model combining (3+1)-dimensional ideal hydrodynamics for the quark gluon plasma (QGP), and a transport model for the hadron resonance gas. Since the hadron distributions are the result of the entire expansion history of the system, understanding the QGP properties requires investigating how rescattering during the hadronic stage affects the final distributions of hadrons. We include multistrange hadrons in our stu…
We discuss how the stability of the theoretical prediction for exclusive $J/ψ$ photoproduction has been improved through a systematic taming of the known $\overline{\text{MS}}$ coefficient functions by accounting for a formally power suppressed, but numerically significant, correction encoded within a $Q_0$ cut. The phenomenological implications of this will be emphasised meaning, ultimately, the possibility to include the exclusive data into a global fitter framework to provide constraints on the small $x$ gluon.
A method for tuning parameters in Monte Carlo generators is described and applied to a specific case. The method works in the following way: each observable is generated several times using different values of the parameters to be tuned. The output is then approximated by some analytic form to describe the dependence of the observables on the parameters. This approximation is used to find the values of the parameter that give the best description of the experimental data. This results in significantly faster fitting compared to an approach in which the generator is called iteratively. As an ap…
The isospin-breaking decay eta {\to} 3pi is an ideal tool to extract information on light quark mass ratios from experiment. For a precise determination, however, a detailed description of the Dalitz plot distribution is necessary. In that respect, in particular the slope parameter alpha of the neutral decay channel causes some concern, since the one-loop prediction from chiral perturbation theory misses the experimental value substantially. We use the modified non-relativistic effective field-theory, a dedicated framework to analyze final-state interactions beyond one loop including isospin-b…
Jet production at hadron colliders provides constraints on the parton distribution functions (PDFs) of the proton, in particular on the gluon distribution. In the present paper, the impact on PDF of CMS inclusive differential jet cross sections at center-of-mass energy of $\sqrt s = 8$ TeV for jets with low momentum $p\mathrm{_T}$ and produced in the forward direction is investigated at next-to leading order in perturbative quantum chromodynamics (QCD). The results of the QCD global analysis are compared with theoretical predictions. The impact of low-$p\mathrm{_T}$ jet measurements on the det…
The small-$x$ gluon in global fits of parton distributions is affected by large uncertainties from the lack of direct experimental constraints. In this work we provide a precision determination of the small-$x$ gluon from the exploitation of forward charm production data provided by LHCb for three different centre-of-mass (CoM) energies: 5~TeV, 7~TeV and 13~TeV. The LHCb measurements are included in the PDF fit by means of normalized distributions and cross-section ratios between data taken at different CoM values, $R_{13/7}$ and $R_{13/5}$. We demonstrate that forward charm production leads t…
The gluon condensate, $\langle \frac{α_s}π G^2 \rangle$, i.e. the leading order power correction in the operator product expansion of current correlators in QCD at short distances, is determined from $e^+ e^-$ annihilation data in the charm-quark region. This determination is based on finite energy QCD sum rules, weighted by a suitable integration kernel to (i) account for potential quark-hadron duality violations, (ii) enhance the contribution of the well known first two narrow resonances, the $J/ψ$ and the $ψ(2S)$, while quenching substantially the data region beyond, and (iii) reinforce the…
We determine the non-perturbative gluon condensate of four-dimensional SU(3) gauge theory in a model independent way. This is achieved by carefully subtracting high order perturbation theory results from non-perturbative lattice QCD determinations of the average plaquette. No indications of dimension two condensates are found. The value of the gluon condensate turns out to be of a similar size as the intrinsic ambiguity inherent to its definition.
We give an update on our lattice determination of r_0 LambdaMS for different Nf. Our calculations employ the strong coupling constant in the minimal MOM scheme for QCD in Landau gauge, and we report here on our progress towards a quantitative understanding of the intrinsic lattice discretization artifacts at large momenta. This is important for a high-precision analysis, in particular for the unquenched calculations for which the access to small lattice spacings is restricted by the available gauge configurations.
We present the most precise value for the Higgs boson cross-section in the gluon-fusion production mode at the LHC. Our result is based on a perturbative expansion through N$^3$LO in QCD, in an effective theory where the top-quark is assumed to be infinitely heavy, while all other Standard Model quarks are massless. We combine this result with QCD corrections to the cross-section where all finite quark-mass effects are included exactly through NLO. In addition, electroweak corrections and the first corrections in the inverse mass of the top-quark are incorporated at three loops. We also invest…
We give the hyperasymptotic expansion of the plaquette with a precision that includes the terminant associated to the leading renormalon. Subleading effects are also considered. The perturbative series is regulated using the principal value prescription for its Borel integral. We use this analysis to give a determination of the gluon condensate in SU(3) pure gluodynamics that is independent of the scale and renormalization scheme used for the coupling constant: $\langle G^2 \rangle_{\rm PV} (n_f=0)=3.15(18)\; r_0^{-4}$.
