arXiv subjects

S. Taheri Monfared

Publications and source records attributed to S. Taheri Monfared.

At least 19 recordsLinked to original sources

arXiv2025

Determination of Parton Densities for QCD partons and Electroweak Bosons

Parton densities are obtained from a solution of the extended DGLAP-type evolution equation that includes both QCD and electroweak contributions. The equations are solved using the Parton-Branching (PB) approach, and the evolution is performed at next-to-leading order for QCD partons and leading order for electroweak bosons. The initial QCD parton distributions are fitted to HERA deep inelastic scattering data, while photon and weak-boson densities are generated perturbatively and validated against $dσ/dQ^2$. The resulting collinear and transverse-momentum dependent (TMD) densities are provided in LHApdf and TMDlib formats for direct phenomenological use.

hep-ph↗

arXiv2025

Soft-gluon coupling and the TMD parton branching Sudakov form factor

The evolution of transverse momentum dependent (TMD) distributions in Quantum Chromodynamics (QCD) can be formulated in a parton branching (PB) framework. We show that next-to-next-to-leading-logarithm (NNLL) accuracy can be achieved in this framework by using the concept of soft-gluon physical coupling. We present results for the TMD distributions and for the Collins-Soper kernel controlling rapidity evolution. The results pave the way for PB predictions at NNLL level for physical observables at the Large Hadron Collider (LHC) and future colliders.

hep-ph↗

arXiv2025

Collinear and TMD distributions with dynamical soft-gluon resolution scale

Soft-gluon resolution scales characterize parton branching Monte Carlo implementations of the evolution equations for parton distribution functions in Quantum Chromodynamics (QCD). We examine scenarios with dynamical, i.e., branching-scale dependent, resolution scale, and discuss physical implications for both collinear and transverse-momentum dependent (TMD) distributions. We perform the first determination of parton distributions with dynamical resolution scale, at next-to-leading order (NLO) in perturbation theory, from fits to precision deep-inelastic scattering measurements from HERA. We present an application of TMD distributions with dynamical resolution scale to Drell-Yan lepton-pair transverse momentum spectra at the LHC, and comment on the extraction of non-perturbative intrinsic-kT parameters from Drell-Yan data at small transverse momenta.

hep-ph↗

arXiv2025

A parton shower consistent with parton densities at LO and NLO: PDF2ISR

We present a method for obtaining an initial-state parton shower model where the (backward) evolution fully consistent with the (forward) evolution of the collinear parton density used. As a proof-of-concept we use parton densities obtained with the Parton Branching (PB) approach, and modify the default initial-state shower in PYTHIA8 with this method to be consistent with them. PB is ideally suited for checking the validity of our method since, in addition to producing collinear parton densities, it also produces the corresponding transverse-dependent (TMD) ones, and these can then be directly compared to the transverse momentum distribution obtained from the parton shower. We show that TMD distributions which we in this way obtain from our modified PYTHIA8 shower using leading order (LO) parton densities and splitting functions are fully consistent with the corresponding leading order TMD densities. At next-to-leading order (NLO) it is not possible to achieve the same consistency using the built-in LO splitting functions in the shower, but we show that by introducing NLO splitting functions using a reweighting procedure, we can achieve consistency also at NLO. The method presented here, which we have named PDF2ISR, can be easily extended to any collinear parton densities, as long as the exact conditions for the evolution are known. With the PDF2ISR method we obtain an initial-state parton shower which in principle has no free parameters, and is fully consistent with collinear parton densities at LO and NLO.

hep-ph↗

arXiv2025

The small kt-region in Drell-Yan production at next-to-leading order with the Parton Branching Method

The Parton Branching (PB) method describes the evolution of transverse momentum dependent (TMD) parton distributions, covering all kinematic regions from small to large transverse momenta kT. The small kT-region is very sensitive both to the contribution of the intrinsic motion of partons (intrinsic kT) and to the resummation of soft gluons taken into account by the PB TMD evolution equations. We study the role of soft-gluon emissions in TMD as well as integrated parton distributions. We perform a detailed investigation of the PB TMD methodology at next-to-leading order (NLO) in Drell-Yan (DY) production for low transverse momenta. We present the extraction of the nonperturbative "intrinsic-kT" distribution from recent measurements of DY transverse momentum distributions at the LHC across a wide range in DY masses, including a detailed treatment of statistical, correlated and uncorrelated uncertainties. We comment on the (in)dependence of intrinsic transverse momentum on DY mass and center-of-mass energy, and on the comparison with other approaches.

hep-ph↗

arXiv2025

Interplay of intrinsic motion of partons and soft gluon emissions in Drell-Yan production studied with PYTHIA

Understanding the intrinsic transverse momentum (intrinsic-$k_T$) of partons within colliding hadrons, typically modeled with a Gaussian distribution characterized by a specific width (the intrinsic-$k_T$ width), has been an extremely challenging issue. This difficulty arises because event generators like Pythia require an intrinsic-$k_T$ width that unexpectedly varies with collision energy, reaching unphysical values at high energies. This paper investigates the underlying physics behind this energy dependence in Pythia, revealing that it arises from an interplay between two non-perturbative processes: the internal transverse motion of partons and non-perturbative soft gluon emissions. These contributions are most constrained in the production of Drell-Yan pairs with very low transverse momentum, where soft gluon effects become increasingly prominent with rising collision energy-contrary to initial expectations. Through a detailed analysis of the non-perturbative Sudakov form factor and its influence on intrinsic-$k_T$ width, we clarify the observed energy scaling behavior in Pythia, providing insight into a longstanding issue in parton shower modeling.

hep-ph↗

arXiv2025

Center-of-mass energy dependence of intrinsic-$k_T$ distributions obtained from Drell-Yan production

The internal motion of partons inside hadrons has been studied through its impact on very low transverse momentum spectra of Drell-Yan (DY) pairs created in hadron-hadron collisions. We study DY production at next-to-leading order using the Parton Branching (PB) method which describes the evolution of transverse momentum dependent parton distributions. The main focus is on studying the intrinsic transverse momentum distribution (intrinsic-$k_T$) as a function of the center-of-mass energy $\sqrt s$. While collinear parton shower Monte Carlo event generators require intrinsic transverse momentum distributions strongly dependent on $\sqrt s$, this is not the case for the PB method. We perform a detailed study of the impact of soft parton emissions. We show that by requiring a minimal transverse momentum, $q_0$, of a radiated parton, a dependence of the width of the intrinsic-$k_T$ distribution as a function of $\sqrt{s}$ is observed. This dependence becomes stronger with increasing $q_0$.

hep-ph↗

arXiv2024

On the role of soft gluons in collinear parton densities and parton shower event generators

The role of soft (non-perturbative) gluons in collinear parton densities and parton shower event generators is investigated with the Parton Branching method as a solution of the DGLAP evolution equations. It is found that soft gluons play a significant role. Within the Parton Branching frame, the Sudakov form factor can be split into a perturbative and non-perturbative part. The non-perturbative part can be calculated analytically under certain conditions. It is shown that the inclusion of soft (non-perturbative) gluons in the parton density evolution is essential for the proper cancellation of divergent terms. It is argued that the non-perturbative part of the Sudakov form factor has its correspondence in Transverse Momentum Dependent parton distributions. Within the Parton Branching approach, this non-perturbative Sudakov form factor is constrained by fits of inclusive, collinear parton densities. We show that the non-perturbative Sudakov form factor and soft gluon emissions are essential for inclusive distributions (collinear parton densities and Drell-Yan transverse momentum spectra). We also show by using Parton Branching TMD parton shower, that the effect of soft gluons plays essentially no role in final state hadron spectra and jets.

hep-ph↗

arXiv2024

What can we learn from the Parton Branching method in QCD?

This work reviews recent developments in the Parton Branching (PB) method, focusing on its application to Transverse Momentum Dependent (TMD) parton distributions and the implementation of TMD evolution equations in Monte Carlo generators. Key advancements include the inclusion of photon and heavy electroweak boson radiation in the evolution equations and their impact on collinear and TMD distributions. A detailed comparison of PB and Collins-Soper-Sterman formalisms highlights improvements in the accuracy of PB Sudakov form factors. The role of soft gluons, intrinsic transverse momentum, and the $z_M$ parameter in modelling non-perturbative effects is emphasized, with implications for inclusive distributions and Drell-Yan transverse momentum spectra. This review also addresses challenges in achieving consistency between forward and backward evolution.

hep-ph↗

arXiv2024

Recent progress in transverse momentum dependent (TMD) Parton Densities and corresponding parton showers

The parton branching method is crucial for Monte Carlo generators, which are essential for high-energy physics predictions. We examine the impact of soft gluons on inclusive collinear and Transverse Momentum Dependent (TMD) parton densities. By applying the Parton-Branching (PB) method, we identify the non-perturbative Sudakov form factor with the integration range $z \to 1$, which is often neglected in collinear parton shower approaches. The significance of soft gluons is demonstrated through the transverse momentum spectrum of Drell-Yan lepton pairs, resulting in an intrinsic-$k_t$ distribution width that remains independent of $\sqrt(s)$, contrary to observations in collinear parton shower approaches. This behavior is attributed to the non-perturbative Sudakov form factor.

hep-ph↗

arXiv2024

The Parton Branching evolution package uPDFevolv2

uPDFevolv2 is a software package designed for evolving collinear and Transverse Momentum Dependent (TMD) parton densities using the DGLAP evolution equation. A comprehensive description of both the theoretical framework and technical implementation is given, accompanied by a detailed guide on program usage, focusing on customizable parameters. This report is as a technical release note for uPDFevolv version 2.5.03.

hep-ph↗

arXiv2024

NLO Analysis of Small-$k_T$ Region in Drell-Yan Production with Parton Branching

The Parton-Branching Method (PB) facilitates the determination of Transverse Momentum Dependent (TMD) parton densities across a wide \kt\ range, spanning small to large transverse momentum scales. In the small $k_T$ region, both intrinsic parton motion and resummed ultra-soft gluons are significant contributors. Our analysis highlights their crucial role in shaping integrated and TMD parton densities. Using PB-derived TMD parton densities and a NLO calculation in MC@NLO style, we compute the transverse momentum spectrum of Drell-Yan pairs across a broad mass range. The spectrum's sensitivity to the intrinsic $k_T$ distribution allows us to fine-tune parametric parameters. Starting from the PB-NLO-HERAI+II-2018 set2 TMD parton distributions, we determine the intrinsic $k_T$ distribution width, resulting in a slightly wider profile than the default set. Importantly, this width remains independent of Drell-Yan pair mass and center-of-mass energy ($\sqrt{s}$), distinguishing our approach.

hep-ph↗

arXiv2023

High-$p_T$ Azimuthal Correlations of Z+jet and Multi-jet Production

In this study, we present our latest findings regarding azimuthal distributions in vector boson + jets and multi-jet production at the Large Hadron Collider (LHC). These findings result from matching next-to-leading order (NLO) perturbative matrix elements with transverse momentum dependent (TMD) parton branching. We conduct a comprehensive comparative analysis of azimuthal correlations between Z boson-jet and jet-jet systems in the back-to-back region. These distinct azimuthal correlation patterns can help identify potential factorization-breaking effects in this region. Such effects depend on the different color and spin structures of the final states and their interactions with the initial states.

hep-ph↗

arXiv2022

Boson-jet and jet-jet azimuthal correlations at high transverse momenta

We discuss our recent results on azimuthal distributions in vector boson + jets and multi-jet production at the LHC, obtained from the matching of next-to-leading order (NLO) perturbative matrix elements with transverse momentum dependent (TMD) parton branching. We present a comparative analysis of boson-jet and jet-jet correlations in the back to-back region, and a study of the theoretical systematic uncertainties associated with the matching scale in the cases of TMD and collinear parton showers.

hep-ph↗

arXiv2022

Back-to-back azimuthal correlations in Z+jet events at high transverse momentum in the TMD parton branching method at next-to-leading order

Azimuthal correlations in Z+jet production at large transverse momenta are computed by matching Parton - Branching (PB) TMD parton distributions and showers with NLO calculations via MCatNLO. The predictions are compared with those for dijet production in the same kinematic range. The azimuthal correlations $Δϕ$ between the Z boson and the leading jet are steeper compared to those in dijet production at transverse momenta ${\cal O}(100)$ GeV, while they become similar for very high transverse momenta ${\cal O}(1000)$ GeV. The different patterns of Z+jet and dijet azimuthal correlations can be used to search for potential {\it factorization - breaking} effects in the back-to-back region, which depend on the different color and spin structure of the final states and their interferences with the initial states. In order to investigate these effects experimentally, we propose to measure the ratio of the distributions in $Δϕ$ for Z+jet - and multijet production at low and at high transverse momenta, and compare the results to predictions obtained assuming factorization. We examine the role of theoretical uncertainties by performing variations of the factorization scale, renormalization scale and matching scale. In particular, we present a comparative study of matching scale uncertainties in the cases of PB-TMD and collinear parton showers.

hep-ph↗

arXiv2021

TMDs from Monte Carlo event generators

Transverse Momentum Dependent (TMD) parton distributions are a very powerful concept for the description of low and high transverse momentum effects in high energy collisions. The Parton Branching (PB) method provides TMD distributions which can be used in parton shower simulations, as already implemented in \cascade\ Monte Carlo event generator. This report gives a description of the work done during the DESY summer student program 2021, young scientists from very different time zones connecting from remote twice a day for 8 weeks, to develop a method, PS2TMD, that allows to determine effective TMDs from the standard Monte Carlo parton showers. This method is validated and implemented to successfully reconstruct the PB-TMDs with different configuration settings. We also discuss kinematic shifts in longitudinal momentum distributions from initial state showering and point out the sizable influence of different reconstruction definitions on both collinear and transverse momentum PDFs. %the nonperturbative dependence on longitudinal and transverse degrees of freedom is fully coupled.

hep-ph↗

arXiv2021

Rivet, RivetHZTool and HERA -- A validation effort for coding HERA measurements for Rivet

During the DESY summer student program 2021, young scientists from more than 13 different countries worked together, connecting from remote, to provide computer codes within the Rivet framework for 19 HERA measurements. Most of these measurements were originally available within the HZTool package, but no longer accessible for modern analysis packages such as Rivet. The temporary RivetHZTool interface was used to validate most of the new Rivet plugins.

hep-ph↗

arXiv2021

Azimuthal correlations of high transverse momentum jets at next-to-leading order in the parton branching method

The azimuthal correlation, $Δϕ_{12}$, of high transverse momentum jets in pp collisions at $\sqrt{s}=13$ TeV is studied by applying PB-TMD distributions to NLO calculations via MCatNLO together with the PB-TMD parton shower. A very good description of the cross section as a function of $Δϕ_{12}$ is observed. In the back-to-back region of $Δϕ_{12} \to π$, a very good agreement is observed with the PB-TMD Set 2 distributions while significant deviations are obtained with the PB-TMD Set 1 distributions. Set 1 uses the evolution scale while Set 2 uses transverse momentum as an argument in $α_s$, and the above observation therefore confirms the importance of an appropriate soft-gluon coupling in angular ordered parton evolution. The total uncertainties of the predictions are dominated by the scale uncertainties of the matrix element, while the uncertainties coming from the PB-TMDs and the corresponding PB-TMD shower are very small. The $Δϕ_{12}$ measurements are also compared with predictions using MCatNLO together PYTHIA8, illustrating the importance of details of the parton shower evolution.

hep-ph↗