arXiv subjects

Sebastian Sapeta

Publications and source records attributed to Sebastian Sapeta.

At least 19 recordsLinked to original sources

arXiv2023

Effects of gluon kinematics and the Sudakov form factor on the dipole amplitude

We investigate effects of exact gluon kinematics on the parameters of the Golec-Biernat-W\"usthoff, and Bartels-Golec-Biernat-Kowalski saturation models. The resulting fits show some differences, particularly, in the normalization of the dipole cross section $\sigma_0$. The refitted models are used for the dijet production process in DIS to investigate effects of the Sudakov form factor at Electron Ion Collider energies.

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arXiv2022

Sudakov effects and the dipole amplitude

In this study we incorporate the Sudakov form factor into the dipole factorization formula, where the hard scale of the former is provided by the photon virtuality $Q^2$. We obtain a general formula which we then apply to the well-known GBW and BGK saturation models. Parameters of the above Sudakov-improved models are successfully fitted to the $F_2$ data from HERA. We observe, in particular, that inclusion of the Sudakov factor on top of the GBW model improves description of data at large $Q^2$.

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arXiv2022

Complete collection of one-loop triple-collinear splitting operators for dimensionally-regulated QCD

We provide results for the one-loop triple-collinear color/spin-space splitting operators for the five possible processes, $q \to qq'\bar{q}'$, $q \to qq\bar{q}$, $q \to qgg$, $g \to gq\bar{q}$ and $g \to ggg$. The expressions are exact in dimensionally-regulated massless QCD up to a single integral, which we expand to second order in the dimensional-regularisation parameter. We also evaluate the related splitting functions. Our results are both sufficient and indispensable for the construction of subtraction and integrated-subtraction terms for triple-collinear singularities of one-loop double-real-emission cross-section contributions as part of a next-to-next-to-next-to leading order subtraction scheme.

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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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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

NNLO soft function for top quark pair production at small transverse momentum

The cross section for top quark pair production factorizes at small transverse momentum of the heavy quark pair, $q_T$. One of the key ingredients that appears in the factorization formula is the soft function, which mediates soft gluon exchanges between particles and gives rise to colour correlations. We present the complete result for the small-$q_T$ soft function at the next-to-next-to-leading order. This is the last missing element needed to calculate the NNLO cross section for top quark pair production by means of the $q_T$ slicing method. In order to evaluate divergent integrals appearing in the calculation, we develop methods based on sector decomposition and differential equations. We present an extensive validation of our framework. In particular, we recover results predicted by the renormalization group, which constitutes a direct demonstration of validity of the small-$q_T$ factorization at NNLO. We provide complete results for the real and imaginary part of the soft function, which are ready for application in the calculation of the $t\bar t$ cross section at NNLO.

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arXiv2018

Saturation model of DIS : an update

We present the results of new fits to the recently extracted data on $F_2$ at low $x$ with the GBW saturation model and its modification to cover high values of $Q^2$. We find that the model stands the test of time and gives a good description of the data with slightly modified parameters. All the essential elements of the model, especially the saturation scale, are retained.

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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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arXiv2016

Anomalous couplings in WZ production beyond NLO QCD

We study WZ production with anomalous couplings at $\bar{n}$NLO QCD using the LoopSim method in combination with the Monte Carlo program VBFNLO. Higher order corrections to WZ production are dominated by additional hard jet radiation. Those contributions are insensitive to anomalous couplings and should thus be removed in analyses. We do this using a dynamical jet veto based on the transverse energy of the QCD and EW final state particles. This removes jet dominated events without introducing problematic logs like a fixed $p_{\text{T}}$ jet veto.

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arXiv2016

QCD and Jets at Hadron Colliders

We review various aspects of jet physics in the context of hadron colliders. We start by discussing the definitions and properties of jets and recent development in this area. We then consider the question of factorization for processes with jets, in particular for cases in which jets are produced in special configurations, like for example in the region of forward rapidities. We review numerous perturbative methods for calculating predictions for jet processes, including the fixed-order calculations as well as various matching and merging techniques. We also discuss the questions related to non-perturbative effects and the role they play in precision jet studies. We describe the status of calculations for processes with jet vetoes and we also elaborate on production of jets in forward direction. Throughout the article, we present selected comparisons between state-of-the-art theoretical predictions and the data from the LHC.

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arXiv2016

Single and double inclusive forward jet production at the LHC at $\sqrt{s}$ = 7 and 13 TeV

We provide a description of the transverse momentum spectrum of single inclusive forward jets produced at the LHC, at the center-of-mass energies of 7 and 13 TeV, using the high energy factorization (HEF) framework. We subsequently study double inclusive forward jet production and, in particular, we calculate contributions to azimuthal angle distributions coming from double parton scattering. We also compare our results for double inclusive jet production to those obtained with the Pythia Monte Carlo generator. This comparison confirms that the HEF resummation acts like an initial state parton shower. It also points towards the need to include final state radiation effects in the HEF formalism.

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arXiv2015

ZZ production at high transverse momenta beyond NLO QCD

We study the production of the four-lepton final state $l^+ l^- l^+ l^-$, predominantly produced by a pair of electroweak Z bosons, ZZ. Using the LoopSim method, we merge NLO QCD results for ZZ and ZZ+jet and obtain approximate NNLO predictions for ZZ production. The exact gluon-fusion loop-squared contribution to the ZZ process is also included. On top of that, we add to our merged sample the gluon-fusion ZZ+jet contributions from the gluon-gluon channel, which is formally of N^3LO and provides approximate results at NLO for the gluon-fusion mechanism. The predictions are obtained with the VBFNLO package and include the leptonic decays of the Z bosons with all off-shell and spin-correlation effects, as well as virtual photon contributions. We compare our predictions with existing results for the total inclusive cross section at NNLO and find a very good agreement. Then, we present results for differential distributions for two experimental setups, one used in searches for anomalous triple gauge boson couplings, the other in Higgs analyses in the four charged-lepton final state channel. We find that the approximate NNLO corrections are large, reaching up to 20% at high transverse momentum of the Z boson or the leading lepton, and are not covered by the NLO scale uncertainties. Distributions of the four-lepton invariant mass are, however, stable with respect to QCD corrections at this order.

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arXiv2013

WW production at high transverse momenta beyond NLO

Pair production of W gauge bosons is an important process at the LHC entering many experimental analyses, both as background in new-physics searches or Higgs measurements and as signal in precision studies and tests of the Standard Model. Therefore, accurate predictions for this class of processes are of great interest in order to exploit the full potential of LHC measurements. We use the LoopSim method to combine NLO QCD results for WW and WW+jet, as well as the loop-squared gluon-fusion contribution, to obtain approximate NNLO predictions for WW production. The cross sections are calculated with VBFNLO and include leptonic decays of the W bosons as well as finite-width and off-shell effects. We find that the size of the additional corrections beyond NLO can be significant and well outside of the NLO error bands given by renormalization and factorization scale variation. Applying a jet veto, we observe further negative corrections at NNLO, which we relate to the presence of large Sudakov logarithms.

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arXiv2013

Simulated NNLO for high-pT observables in vector boson + jets production at the LHC

We present a study of higher order QCD corrections beyond NLO to processes with an electroweak vector boson, W or Z, in association with jets. We focus on the regions of high transverse momenta of commonly used differential distributions. We employ the LoopSim method to merge NLO samples of different multiplicity obtained from MCFM and from BLACKHAT+SHERPA in order to compute the dominant part of the NNLO corrections for high-pT observables. We find that these corrections are indeed substantial for a number of experimentally relevant observables. For other observables, they lead to significant reduction of scale uncertainties.

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arXiv2013

A comparative study of jet substructure taggers

We explicitly study how jet substructure taggers act on a set of signal and background events. We focus on two-pronged hadronic decay of a boosted Z boson. The background to this process comes from QCD jets with masses of the order of m_Z. We find a way to compare various taggers within a single framework by applying them to the most relevant splitting in a jet. We develop a tool, TOY-TAG, which allows one to get insight into what happens when a particular tagger is applied to a set of signal or background events. It also provides estimates for significance and purity. We use our tool to analyze differences between various taggers and potential ways to improve the performance by combining several of them.

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arXiv2013

Di-boson and Tri-boson production at the LHC

The status of di-boson and tri-boson production is shortly review. Using the VBFNLO and the LOOPSIM package, approximated results at NNLO QCD are given for WZ production. Results for waa + jet at NLO QCD are also shown.

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arXiv2013

WZ and W+jets production at large transverse momenta beyond NLO

We present a study of higher order QCD corrections beyond NLO to processes with electroweak vector bosons. We focus on the regions of high transverse momenta of commonly used differential distributions. We employ the LoopSim method, combined with NLO packages, VBFNLO and MCFM, to merge the NLO samples with different multiplicities, in order to compute the dominant part of the NNLO corrections at high pT. We find that these corrections are indeed substantial, in the 30%-100% range, for a number of experimentally relevant observables. For other observables, they lead to significant reduction of scale uncertainties.

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