arXiv subjects

Melih A. Ozcelik

Publications and source records attributed to Melih A. Ozcelik.

14 recordsLinked to original sources

arXiv2026

Light-by-light scattering at three loops in massless QCD and QED: amplitudes and cross sections

We present the calculation of three-loop massless QCD and QED helicity amplitudes for light-by-light scattering. We make use of Lorentz tensor decomposition in the 't Hooft-Veltman dimensional regularisation scheme to reduce the complexity of the computation. Our analytic amplitude results are remarkably compact and can be efficiently evaluated numerically. We employ them to compute the corresponding NNLO differential cross-section predictions in the invariant mass and rapidity distributions of the di-photon system, for which we find agreement with the experimental ATLAS data from ultra-peripheral heavy-ion collisions.

hep-ph↗

arXiv2026

Transition Form-Factor for $η_Q$ at NNLO in the strong coupling $α_s$ and with all-order $v^2$ resummation

In this work, we discuss both relativistic and perturbative QCD corrections to the transition form-factor ${\cal F}_{η_Q}{(t_1,t_2)}$ for the process $γ^*(q_1) γ^*(q_2) \leftrightarrow η_Q(P)$ with dependencies on the normalised photon virtualities $t_1=q_1^2/m_Q^2$ and $t_2=q_2^2/m_Q^2$, where $m_Q$ is the heavy quark mass. We resum a class of relativistic corrections to all orders in the relativistic parameter $\langle v^2 \rangle_{η_Q}$. In addition, we include perturbative QCD corrections up to Next-to-Next-to-Leading Order (NNLO) in the strong coupling constant $α_s$. This involves the computation of two-loop amplitudes with two off-shell photons. We explore three different phenomenological applications of our transition form-factors for the charmonium case. We first study the ratio $\vert {\cal F}_{η_c}{(t_1,0)} \vert/ \vert {\cal F}_{η_c}{(0,0)} \vert$ for the single space-like photon case and compare our results with the existing $η_c$ measurements from the BaBar collaboration. Secondly, we consider observables for the case of double space-like photons and discuss the impact of the different corrections. The NNLO corrections for this case are presented here for the first time in the literature. Finally, we revisit the decay width $Γ[η_c \rightarrow γγ]$ and compare it with the existing PDG value.

hep-ph↗

arXiv2025

Two-loop form factors for $P$-wave quarkonium production and decay

We present the analytical results for the two-loop form factors needed for $χ_{Q,J}$ production and decay. We consider the two-loop corrections to the process $γγ\leftrightarrow {^3 P_J^{[1]}}$, that has been known only numerically before, and the processes $gg \leftrightarrow {^3 P_J^{[1]}}$, $γg \leftrightarrow {^3 P_J^{[8]}}$ and $gg \leftrightarrow {^3 P_J^{[8]}}$, which have not been computed before. We observe that the NRQCD pole structure of the two-loop amplitude in the $gg$ channel is more involved for the spin-triplet $P$-wave case than for the pseudo-scalar $S$-wave case. It involves in addition to the standard Coulomb singularity also a new singularity whose cancellation requires the inclusion of the $gg \leftrightarrow {^3S_1^{[8]}}$ form factor. We give the high precision numerical results for the hard functions that can be used to compute $χ_{Q,J}$ production and decay up to NNLO accuracy.

hep-ph↗

arXiv2025

Prospective report of the French QCD community to the ESPPU 2025 with respect to the program of the LHC Run 5 and beyond and future colliders at CERN

This document summarizes the prospective physics plans of the French QCD and Heavy-Ion community, including the experimental programs at the LHC Run 5 and beyond and future colliders at CERN, within the context of the French contribution to the update of the European Strategy in Particle Physics (ESPPU 2025), as discussed in the workshop on European Strategy for Particle Physics Update 2025 organised by the QCD GdR in Oct. 2024.

hep-ph↗

arXiv2024

Linear power corrections to single top production and decay at the LHC in the narrow width approximation

We consider top quark production and decay in the narrow width approximation and study if the polarisation effects, that manifest themselves in correlations of angular distributions of particles from top quark decays and final state jets in the production sub-process, are affected by linear power corrections. We find that, in general, the answer to this question is affirmative. We also discuss how these non-perturbative corrections affect polarisation observables used to study single top production at the LHC. Finally, we point out that generic kinematic distributions of leptons from top quark decays are affected by linear power corrections, which may have implications for proposals to extract the top quark mass from such leptonic observables. On the other hand, we demonstrate that the distribution of the ``out-of-collision-plane'' component of the positron momentum is free from linear power corrections, making it an interesting candidate for the top quark mass measurement.

hep-ph↗

arXiv2024

Linear power corrections to top quark pair production in hadron collisions

We compute, in the framework of renormalon calculus, the ${\cal O}(Λ_{\rm QCD})$ corrections to the production of $t\bar{t}$ pairs in hadron collisions under the assumption that $q \bar q \to t \bar t$ is the dominant partonic channel. This assumption is not applicable to top quark pair production at the LHC but it is valid for the Tevatron where collisions of protons and anti-protons were studied. We show that the linear power correction to the total $t \bar t$ production cross section vanishes provided one uses a short-distance scheme for the top quark mass. We also derive relatively simple formulas for the power corrections to top quark kinematic distributions. Although small numerically, these power corrections exhibit interesting dependencies on top quark kinematics.

hep-ph↗

arXiv2023

Curing the high-energy perturbative instability of vector-quarkonium-photoproduction cross sections at order $αα_s^3$ with high-energy factorisation

We cure the perturbative instability of the total-inclusive-photoproduction cross sections of vector $S$-wave quarkonia observed at high photon-proton-collision energies ($\sqrt{s_{γp}}$) in Next-to-Leading Order (NLO) Collinear-Factorisation (CF) computations. This is achieved using High-Energy Factorisation (HEF) in the Doubly-Logarithmic Approximation (DLA), which is a subset of the Leading-Logarithmic Approximation (LLA) of HEF which resums higher-order QCD corrections proportional to $α_s^n \ln^{n-1} (\hat{s}/M^2)$ in the Regge limit $\hat{s}\gg M^2$ with $M^2$ being the quarkonium mass and $\hat{s}$ is the squared partonic-center-of-mass energy. Such a DLA is strictly consistent with the NLO and NNLO DGLAP evolutions of the Parton Distribution Functions. By improving the treatment of the large-$\hat{s}$ asymptotics of the CF coefficient function, the resummation cures the unphysical results of the NLO CF calculation. The matching is directly performed in $\hat{s}$ space using the Inverse-Error Weighting matching procedure which avoids any possible double counting. The obtained cross sections are in good agreement with data. In addition, the scale-variation uncertainty of the matched result is significantly reduced compared to the LO results. Our calculations also yield closed-form analytic limits for $\hat{s}\gg M^2$ of the NLO partonic CF and numerical limits for contributions to those at NNLO scaling like $α_s^2 \ln(\hat{s}/M^2)$.

hep-ph↗

arXiv2023

Linear power corrections to single top production processes at the LHC

We discuss the linear power corrections to the electroweak production of top quarks at the LHC using renormalon calculus. We show how such non-perturbative corrections can be obtained using the Low-Burnett-Kroll theorem, which provides the first subleading term to the expansion of the real-emission amplitudes around the soft limit. We demonstrate that there are no linear power corrections to the total cross sections of arbitrary processes of a single top production type provided that these cross sections are expressed in terms of a short-distance top quark mass. We also derive a universal formula for the linear power corrections to generic observables that involve the top-quark momentum.

hep-ph↗

arXiv2023

Two-loop form factors for pseudo-scalar quarkonium production and decay

We present the analytic expressions for the two-loop form factors for the production or decay of pseudo-scalar quarkonia, in a scheme where the quarks are produced at threshold. We consider the two-loop amplitude for the process $γγ\leftrightarrow {^1S_0^{[1]}}$, that was previously known only numerically, as well as for the processes $gg \leftrightarrow {^1S_0^{[1]}}$, $γg \leftrightarrow {^1S_0^{[8]}}$ and $gg \leftrightarrow {^1S_0^{[8]}}$, which have not been computed before. The two-loop corrections to $gg \leftrightarrow {^1S_0^{[1]}}$ are the last missing ingredients for a full NNLO calculation of $η_Q$ hadro-production. We discuss how the singularity structure of the amplitudes is affected by the threshold kinematics, which in particular introduces Coulomb singularities. In this context, we first show how the usual structure of the infrared singularities degenerates at threshold kinematics, and then extract the anomalous dimensions governing the Coulomb singularities for colour-singlet and octet channels, the latter being presented here for the first time. We give high-precision numerical results for the hard functions, which can be used for phenomenological studies of $η_Q$ production and decay at NNLO.

hep-ph↗

arXiv2023

Revisiting NLO QCD corrections to total inclusive J/psi and Upsilon photoproduction cross sections in lepton-proton collisions

We revisit inclusive J/psi and Upsilon photoproduction at lepton-hadron colliders, namely in the limit when the exchanged photon is quasi real. Our computation includes the next-to-leading-order (NLO) alpha_s corrections to the leading-order contributions in v. Similarly to the case of NLO charmonium-hadroproduction processes, the resulting cross sections obtained in the MS-bar factorisation scheme are sometimes found to be negative. We show that the scale-fixing criteria which we derived in a previous study of eta(c) production successfully solves this problem from the EicC all the way up to the FCC-eh energies. We then elaborate on how to study a scale uncertainty akin to that derived by scale variations when one fixes a scale. In turn, we investigate where both J/psi and Upsilon photoproduction could be used to improve our knowledge of gluon content of the proton at scales as low as a couple of GeV.

hep-ph↗

arXiv2022

Two-loop master integrals for pseudo-scalar quarkonium and leptonium production and decay

We compute the master integrals relevant for the two-loop corrections to pseudo-scalar quarkonium and leptonium production and decay. We present both analytic and high-precision numerical results. The analytic expressions are given in terms of multiple polylogarithms (MPLs), elliptic multiple polylogarithms (eMPLs) and iterated integrals of Eisenstein series. As an application of our results, we obtain for the first time an analytic expression for the two-loop amplitude for para-positronium decay to two photons at two loops.

hep-ph↗

arXiv2022

Matching next-to-leading-order and high-energy-resummed calculations of heavy-quarkonium-hadroproduction cross sections

The energy dependence of the total hadroproduction cross section of pseudo-scalar quarkonia is computed via matching Next-to-Leading Order (NLO) Collinear-Factorisation (CF) results with resummed higher-order corrections, proportional to $α_s^{n}\ln^{n-1}(1/z)$, to the CF hard-scattering coefficient, where $z=M^2/\hat{s}$ with $M$ and $\hat{s}$ being the quarkonium mass and the partonic center-of-mass energy squared. The resummation is performed using High-Energy Factorisation (HEF) in the Doubly-Logarithmic (DL) approximation, which is a subset of the leading logarithmic $\ln (1/z)$ approximation. Doing so, one remains strictly consistent with the NLO and NNLO DGLAP evolution of the PDFs. By improving the treatment of the small-$z$ asymptotics of the CF coefficient function, the resummation cures the unphysical results of the NLO CF calculation. The matching is directly performed in the $z$-space and, for the first time, by using the Inverse-Error Weighting (InEW) matching procedure. As a by-product of the calculation, the NNLO term of the CF hard-scattering coefficient proportional to $α_s^2\ln(1/z)$ is predicted from HEF.

hep-ph↗

arXiv2022

White Paper on Forward Physics, BFKL, Saturation Physics and Diffraction

The goal of this whitepaper is to give a comprehensive overview of the rich field of forward physics. We discuss the occurrences of BFKL resummation effects in special final states, such as Mueller-Navelet jets, jet gap jets, and heavy quarkonium production. It further addresses TMD factorization at low x and the manifestation of a semi-hard saturation scale in (generalized) TMD PDFs. More theoretical aspects of low x physics, probes of the quark gluon plasma, as well as the possibility to use photon-hadron collisions at the LHC to constrain hadronic structure at low x, and the resulting complementarity between LHC and the EIC are also presented. We also briefly discuss diffraction at colliders as well as the possibility to explore further the electroweak theory in central exclusive events using the LHC as a photon-photon collider.

hep-ph↗

arXiv2021

Prospects for quarkonium studies at the high-luminosity LHC

Prospects for quarkonium-production studies accessible during the upcoming high-luminosity phases of the CERN Large Hadron Collider operation after 2021 are reviewed. Current experimental and theoretical open issues in the field are assessed together with the potential for future studies in quarkonium-related physics. This will be possible through the exploitation of the huge data samples to be collected in proton-proton, proton-nucleus and nucleus-nucleus collisions, both in the collider and fixed-target modes. Such investigations include, among others, those of: (i) J/psi and Upsilon produced in association with other hard particles; (ii) chi(c,b) and eta(c,b) down to small transverse momenta; (iii) the constraints brought in by quarkonia on gluon PDFs, nuclear PDFs, TMDs, GPDs and GTMDs, as well as on the low-x parton dynamics; (iv) the gluon Sivers effect in polarised-nucleon collisions; (v) the properties of the quark-gluon plasma produced in ultra-relativistic heavy-ion collisions and of collective partonic effects in general; and (vi) double and triple parton scatterings.

hep-ph↗