arXiv subjects

Andreas van Hameren

Publications and source records attributed to Andreas van Hameren.

At least 19 recordsLinked to original sources

arXiv2025

Hybrid high-energy factorization and evolution at NLO from the high-energy limit of collinear factorization

We derive the scheme of NLO computations of generic observables in high-energy hadron-hadron collisions within the framework of high-energy factorization (HEF) with one off-shell initial-state parton, by taking a high-energy limit of the NLO computation in collinear factorization (CF). The NLO terms belonging to the projectile and target are identified and the ambiguity of projectile-target separation is related with the Collins-Soper scale ($μ_Y$). The NLO unintegrated PDF(UPDF) is constructed in terms of the usual PDFs, and its $μ_Y$-evolution reproduces the Collins-Soper-Sterman equation in the TMD limit ($|k_\perp|\ll μ_Y$). The resummation of high-energy logarithms is taken care of by the BFKL-Collins-Ellis evolution of the Green's function in the UPDF.

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arXiv2024

A new subtraction scheme at NLO exploiting the privilege of kT-factorization

We present a subtraction method for the calculation of real-radiation integrals at NLO in hybrid kT-factorization. The main difference with existing methods for collinear factorization is that we subtract the momentum recoil, occurring due to the mapping from an (n+1)-particle phase space to an n-particle phase space, from the initial-state momenta, instead of distributing it over the final-state momenta.

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arXiv2023

Forward $γ$+jet production in proton-proton and proton-lead collisions at LHC within the FoCal calorimeter acceptance

Using the small-$x$ Improved Transverse Momentum Dependent factorization (ITMD), which, for a general two-to-two massless scattering can be proved within the Color Glass Condensate (CGC) theory for transverse momenta of particles greater than the saturation scale, we provide predictions for isolated forward photon and jet production in proton-proton and proton-nucleus collisions within the planned ALICE FoCal detector acceptance. We study azimuthal correlations, $p_T$ spectra, as well as normalized ratios of proton-proton cross sections for different energies. The only TMD distribution needed for that process is the "dipole" TMD gluon distribution, which in our computations is based on HERA data and undergoes momentum space BK evolution equation with DGLAP corrections and Sudakov resummation. We conclude, that the process provides an excellent probe of the dipole TMD gluon distribution in saturation regime.

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arXiv2023

One-loop gauge invariant amplitudes with a space-like gluon

Nowadays the particle physics has entered an era where high precision calculations are required in order to compare the theoretical predictions with the experimental data. In this paper, we explicitly compute the virtual contributions for the space-like one-jet processes, $\procgg,\procqq,\procH$ and $\procee$ within the auxiliary parton method. Our results, which are expected to play an important role in high precision description of small $x$ physics, explicitly confirm the conjecture developed in Ref. \cite{vanHameren:2022mtk}, thus helping to bridge the gap between lowest order calculations and NLO corrections within hybrid $k_T$-factorization scheme.

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arXiv2022

Hybrid kT-factorization and impact factors at NLO

In the hybrid kT-factorization formula, one initial-state parton momentum is space-like and carries non-vanishing transverse components, while the other is on-shell. We promote this factorization formula to next-to-leading order. Studying the partonic cross section, we identify all soft and collinear divergencies in the real and virtual contribution, and recognize that all non-cancelling ones can be attributed to PDF evolution, evolution kernel, and target impact factors. In result, we construct a framework that may be used to compute NLO impact factors in general. In particular, we recover known expressions for inclusive NLO quark-and gluon impact factor corrections.

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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.

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arXiv2021

Probing gluon number density with electron-dijet correlations at EIC

We propose a novel way of studying the gluon number density (the so-called Weizsäcker-Williams gluon distribution) using the planned Electron Ion Collider. Namely, with the help of the azimuthal correlations between the total transverse momentum of the dijet system and the scattered electron, we examine an interplay between the effect of the soft gluon emissions (the Sudakov form factor) and the gluon saturation effects. The kinematic cuts are chosen such that the dijet system is produced in the forward direction in the laboratory frame, which provides an upper bound on the probed longitudinal fractions of the hadron momentum carried by scattered gluons. Further cuts enable us to use the factorization formalism that directly involves the unpolarized Weizsäcker-Williams gluon distribution. We find this observable to be very sensitive to the soft gluon emission and moderately sensitive to the gluon saturation.

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arXiv2021

All-plus helicity off-shell gauge invariant multigluon amplitudes at one loop

We calculate one loop scattering amplitudes for arbitrary number of positive helicity on-shell gluons and one off-shell gluon treated within the quasi-multi Regge kinematics. The result is fully gauge invariant and possesses the correct on-shell limit. Our method is based on embedding the off-shell process, together with contributions needed to retain gauge invariance, in a bigger fully on-shell process with auxiliary quark or gluon line.

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arXiv2021

Forward Higgs production within high energy factorization in the heavy quark limit at next-to-leading order accuracy

We use Lipatov's high energy effective action to determine the next-to-leading order corrections to Higgs production in the forward region within high energy factorization making use of the infinite top mass limit. Our result is based on an explicit calculation of real corrections combined with virtual corrections determined earlier by Nefedov. As a new element we provide a proper definition of the desired next-to-leading order coefficient within the high energy effective action framework, extending a previously proposed prescription. We further propose a subtraction mechanism to achieve for this coefficient a stable cancellation of real and virtual infra-red singularities in the presence of external off-shell legs. Apart from its relevance for direct phenomenological studies, such as high energy resummation of Higgs + jet configurations, our result will be further of use for the study of transverse momentum dependent factorization in the high energy limit.

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arXiv2020

In-medium transverse momentum broadening effects on di-jet observables

Heavy ion collisions at high energies can be used as an interesting way to recreate and study the medium of the quark-gluon plasma (QGP). We particularly investigate the jets produced in hard binary collisions and their interactions with a tentative medium. These jets were obtained numerically from the Monte-Carlo simulations of hard collisions using the KATIE-algorithm [1], where parton momenta within the colliding nucleons were describe by means of unintegrated parton distribution functions (uPDF). We evolved these jets within a medium that contains both, transverse kicks (yielding a broadening in momentum transvers to the jet-axis) as well as medium induced radiation within the MINCAS-algorithm [2] following the works of [3,4]. We produce qualitative results for the decorrelation of dijets. In particular, we study deviations from a transverse momentum broadening that follows a Gaussian distribution. [1] A. van Hameren, Comput.Phys.Commun. 224 (2018) 371-380 [2] K. Kutak, W. Płaczek, R. Straka, Eur.Phys.J. C79 (2019) no.4, 317 [3] J.-P. Blaizot, F. Dominguez, E. Iancu, Y. Mehtar-Tani, JHEP 1301 (2013) 143 [4] J.-P. Blaizot, F. Dominguez, E. Iancu, Y. Mehtar-Tani, JHEP 1406 (2014) 075

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arXiv2020

Forward trijet production in p-p and p-Pb collisions at LHC

We calculate various azimuthal angle distributions for three jets produced in the forward rapidity region with transverse momenta $p_T>20\,\mathrm{GeV}$ in proton-proton (p-p) and proton-lead (p-Pb) collisions at center of mass energy $5.02\,\,\mathrm{TeV}$. We use the multi-parton extension of the so-called small-$x$ Improved Transverse Momentum Dependent factorization (ITMD). We study effects related to change from the standard $k_T$-factorization to ITMD factorization as well as changes as one goes from p-p collision to p-Pb. We observe rather large differences in the distribution when we change the factorization approach, which allows to both improve the small-$x$ TMD gluon distributions as well as validate and improve the factorization approach. We also see significant depletion of the nuclear modification ratio, indicating a possibility of searches for saturation effects using trijet final states in a more exclusive way than for dijets.

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arXiv2020

Single-jet inclusive rates with exact color at $\mathcal{O}(α_s^4)$

Next-to-next-to-leading order QCD predictions for single-, double- and even triple-differential distributions of jet events in proton-proton collisions have recently been obtained using the NNLOjet framework based on antenna subtraction. These results are an important input for Parton Distribution Function fits to hadron-collider data. While these calculations include all of the partonic channels occurring at this order of the perturbative expansion, they are based on the leading-color approximation in the case of channels involving quarks and are only exact in color in the pure-gluon channel. In the present publication, we verify that the sub-leading color effects in the single-jet inclusive double-differential cross sections are indeed negligible as far as phenomenological applications are concerned. This is the first independent and complete calculation for this observable. We also take the opportunity to discuss the necessary modifications of the sector-improved residue subtraction scheme that made this work possible.

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arXiv2019

Broadening and saturation effects in dijet azimuthal correlations in p-p and p-Pb collisions at $\mathbf{\sqrt{s_{NN}}} = $ 5.02 TeV

We demonstrate that the recent forward-forward dijet correlation data measured by the ATLAS collaboration for proton-proton and proton-lead collisions are consistent with the broadening effects due to both the gluon saturation and the resummation of large logarithms of the hard scale (the so-called Sudakov logarithms). We find that both effects are necessary to describe the experimental results.

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arXiv2018

One-loop amplitudes for k_T-dependent factorization

I report on progress in the calculation of one-loop amplitudes with a space-like gluon. Amplitudes with space-like partons are relevant for factorization procedure s that require initial-state partons with non-vanishing transverse momentum components.

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arXiv2017

Calculations with off-shell matrix elements, TMD parton densities and TMD parton showers

A new calculation using off-shell matrix elements with TMD parton densities supplemented with a newly developed initial state TMD parton shower is described. The calculation is based on the KaTie package for an automated calculation of the partonic process in high-energy factorization, making use of TMD parton densities implemented in TMDlib. The partonic events are stored in an LHE file, similar to the conventional LHE files, but now containing the transverse momenta of the initial partons. The LHE files are read in by the CASCADE package for the full TMD parton shower, final state shower and hadronization from PYTHIA where events in HEPMC format are produced. We have determined a full set of TMD parton densities and developed an initial state TMD parton shower, including all flavors following the TMD distribution. As an example of application we have calculated the azimuthal de-correlation of high pt dijets as measured at the LHC and found very good agreement with the measurement when including initial state TMD parton showers together with conventional final state parton showers and hadronization.

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arXiv2017

Single- and double-scattering production of four muons in ultraperipheral PbPb collisions at the Large Hadron Collider

We discuss production of two $μ^+μ^-$ pairs in ultraperipheral ultrarelativistic heavy ion collisions at the LHC. We take into account electromagnetic (two-photon) double-scattering production and for a first time direct $γγ$ production of four muons in one scattering. We study the unexplored process $γγ\to μ^+μ^-μ^+μ^-$. We present predictions for total and differential cross sections. Measurable nuclear cross sections are obtained and corresponding differential distributions and counting rates are presented.

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arXiv2017

QCD amplitudes with 2 initial spacelike legs via generalised BCFW recursion

We complete the generalisation of the BCFW recursion relation to the off-shell case, allowing for the computation of tree level scattering amplitudes for full High Energy Factorisation (HEF), i.e. with both incoming partons having a non-vanishing transverse momentum. We provide explicit results for color-ordered amplitudes with two off-shell legs in massless QCD up to 4 point, continuing the program begun in two previous papers. For the 4-fermion amplitudes, which are not BCFW-recursible, we perform a diagrammatic computation, so as to offer a complete set of expressions. We explicitly show and discuss some plots of the squared $2 \rightarrow 2$ matrix elements as functions of the differences in rapidity and azimuthal angle of the final state particles.

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arXiv2016

Search for optimal conditions for exploring double-parton scattering in four-jet production: $k_t$-factorization approach

In the present paper we discuss how to maximize the double-parton scattering (DPS) contribution in four-jet production by selecting kinematical cuts. Here both single-parton and double-parton scattering effects are calculated in the $k_T$-factorization approach, following our recent developments of relevant methods and tools. Several differential distributions are shown and discussed in the context of future searches for DPS effects, such as rapidity of jets, rapidity distance, and azimuthal correlations between jets. The dependences of the relative DPS amount is studied as function of those observables. The regions with an enhanced DPS contribution are identified. Future experimental explorations could extract more precise values of $σ_{eff}$ and its potential dependence on kinematical variables.

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