arXiv subjects

S. Alekhin

Publications and source records attributed to S. Alekhin.

At least 19 recordsLinked to original sources

arXiv2025

Determination of the strong coupling from high-energy data

We determine the strong coupling from high-energy data for the Drell-Yan (DY) process and top-quark hadro-production collected at the Large Hadron Collider and the Tevatron combined with the world data on deep-inelastic scattering (DIS) and fixed-target DY data. The theory description uses results at next-to-next-to-leading order in perturbative QCD in the $\overline{\mathrm{MS}}$-scheme together with leading order QED evolution. The DIS data are subject to stringent kinematic cuts to suppress the contribution of power corrections. We apply a cut on the hadronic invariant mass squared $W^2 \geq 12.5~$GeV$^2$ together with a series of cuts on momentum transfer squared $Q^2$. Discarding higher-twist terms we find that the value of strong coupling $α_s(m_Z)$ preferred by the data stabilizes at large enough cuts on $Q^2$, when low-$Q^2$ DIS data sensitive to power corrections are effectively removed. In particular, we extract the value of $α_s(m_Z,N_f=5)=0.1152 \pm 0.008$ for $N_f=5$ light flavors with the cut $Q^2>10~$GeV$^2$. In the absence of higher-twist terms less tight cuts on $Q^2$ show a clear deterioration of the fit and lead to rising values of the strong coupling, shifted upwards by about two standard deviations.

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arXiv2025

Status of QCD precision predictions for Drell-Yan rapidity distributions

We compute differential distributions for Drell-Yan processes at the LHC and the Tevatron colliders at next-to-next-to-leading order in perturbative QCD, including fiducial cuts on the decay leptons in the final state. The comparison of predictions obtained with four different codes shows excellent agreement, once linear power corrections from the fiducial cuts are included in those codes that rely on phase-space slicing subtraction schemes. For Z-boson production we perform a detailed study of the symmetric cuts on the transverse momenta of the decay leptons. Predictions at fixed order in perturbative QCD for those symmetric cuts, typically imposed in experiments, suffer from an instability. We show how this can be remedied by an all-order resummation of the fiducial transverse momentum spectrum, and we comment on the choice of cuts for future experimental analyses.

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arXiv2025

NNLO PDFs driven by top-quark data

We study the impact of state-of-the-art top-quark data collected at the Large Hadron Collider on parton distribution functions (PDFs). Following the ABMP methodology, the fit extracts simultaneously proton PDFs, the strong coupling $α_s(M_Z)$ and heavy-quark masses at next-to-next-to-leading order (NNLO) accuracy in QCD. It includes recent high-statistics data on absolute total inclusive cross sections for $t\bar{t}+X$, the sum of $(t + X)$ and $(\bar{t} + X)$ hadroproduction, and normalized inclusive data double-differential in the invariant mass and rapidity of the $t\bar{t}$ pair at $\sqrt{S}=13$ TeV. The gluon PDF at large $x$ and the top-quark mass value derived from these data are well compatible with the previous ABMP16 results, but with significantly smaller uncertainties, reduced by up to a factor of two. At NNLO in QCD we obtain for the strong coupling the value $α_s^{(n_f=5)}(M_Z)= 0.1150 \pm 0.0009$ and for the top-quark mass in the ${\overline{\mbox{MS}}}$-scheme $m_t(m_t) = 160.6 \pm 0.6$ GeV, corresponding to $m_t^{\rm pole} = 170.2 \pm 0.7$ GeV in the on-shell scheme. The new fit, dubbed ABMPtt, is publicly released in grids in LHAPDF format.

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arXiv2024

Top-quark pole mass extraction at NNLO accuracy

We describe our recent NNLO QCD extraction of the top-quark pole mass from fits to experimental data on total inclusive and normalized (multi)-differential cross sections for $t\bar{t} + X$ hadroproduction, using as input various modern PDF + $α_s(M_Z)$ sets. We find top-quark mass values compatible among each other and with the PDG 2024 preferred value.

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arXiv2024

NNLO fits of top-quark mass using total, single-differential and double-differential $t\bar{t}+X$ cross-section data

We describe the fits of the top-quark mass value at NNLO using as input the double-differential distributions in rapidity and invariant mass of $t\bar{t}$ pairs obtained by the ATLAS and CMS collaborations from unfolding of their experimental data to the parton level, compared to NNLO theory predictions. We consider different state-of-the-art PDF sets, finding results of the fits compatible among each other within uncertainties. On the other hand, we observe some tension among the fits to different datasets.

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arXiv2023

Impact of SeaQuest data on sea-quark PDFs at large $x$

We investigate the impact of the recently released FNAL-E906 (SeaQuest) data on the ratio of proton-deuteron and proton-proton Drell-Yan production cross-sections on the sea quark PDFs. We find that they have constraining power on the light-quark sea isospin asymmetry $(\bar{d}-\bar{u})(x)$ and on the $(\bar{d}/\bar{u})(x)$ ratio at large longitudinal momentum fraction $x$ values, and that their constraints turn out to be compatible with those from Drell-Yan data in collider experiments (Tevatron and Large Hadron Collider) and in the old fixed-target experiment by the FNAL-E866 collaboration. We study the impact of nuclear corrections due to the deuteron target, finding them within 1% in most of the kinematic region covered by SeaQuest. We perform a new proton PDF fit, including SeaQuest data, using the ABMP16 methodology and we compare it to the ABMP16 baseline.

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arXiv2023

Impact of SeaQuest data on PDF fits at large $x$

We evaluate the impact of recent SeaQuest (E906 experiment) data on dimuon production in proton-deuteron and proton-proton collisions on parton distribution functions (PDFs). We find these data in a good agreement with the QCD predictions based on PDFs fitted to the Tevatron and LHC data on forward production of $W$ and $Z$ bosons. As a basis for this study we use the ABMP16 PDF fits and show that they turn out to be compatible with the SeaQuest data, and that these data have constraining power, allowing to reduce the uncertainties on the isospin asymmetry of the light-sea-quark distribution at large longitudinal momentum fraction $x$. We discuss the nuclear corrections needed to describe the deuteron and show that they affect the theoretical description of the proton-deuteron Drell-Yan cross section at the level of $\mathcal{O}(0.5 - 1)$\%. We also comment on the compatibility of the SeaQuest results with other state-of-the-art PDF fits and show that these data are in clear disagreement with models proposing an SU(3)-flavor symmetric quark sea. Finally, we perform a comparison between the second Mellin moments of the light-quark PDFs and recent results from various lattice QCD computations, which demonstrates good compatibility, albeit limited by the uncertainties inherent in current lattice QCD simulations.

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arXiv2021

Precision studies for Drell-Yan processes at NNLO

We present a detailed comparison of the fixed-order predictions computed by four publicly available computer codes for Drell-Yan processes at the LHC and Tevatron colliders. We point out that while there is agreement among the predictions at the next-to-leading order accuracy, the predictions at the next-to-next-to-leading order (NNLO) differ, whose extent depends on the observable. The sizes of the differences in general are at least similar, sometimes larger than the sizes of the NNLO corrections themselves. We demonstrate that the neglected power corrections by the codes that use global slicing methods for the regularization of double real emissions can be the source of the differences. Depending on the fiducial cuts, those power corrections become linear, hence enhanced as compared to quadratic ones that are considered standard.

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arXiv2021

The Large Hadron-Electron Collider at the HL-LHC

The Large Hadron electron Collider (LHeC) is designed to move the field of deep inelastic scattering (DIS) to the energy and intensity frontier of particle physics. Exploiting energy recovery technology, it collides a novel, intense electron beam with a proton or ion beam from the High Luminosity--Large Hadron Collider (HL-LHC). The accelerator and interaction region are designed for concurrent electron-proton and proton-proton operation. This report represents an update of the Conceptual Design Report (CDR) of the LHeC, published in 2012. It comprises new results on parton structure of the proton and heavier nuclei, QCD dynamics, electroweak and top-quark physics. It is shown how the LHeC will open a new chapter of nuclear particle physics in extending the accessible kinematic range in lepton-nucleus scattering by several orders of magnitude. Due to enhanced luminosity, large energy and the cleanliness of the hadronic final states, the LHeC has a strong Higgs physics programme and its own discovery potential for new physics. Building on the 2012 CDR, the report represents a detailed updated design of the energy recovery electron linac (ERL) including new lattice, magnet, superconducting radio frequency technology and further components. Challenges of energy recovery are described and the lower energy, high current, 3-turn ERL facility, PERLE at Orsay, is presented which uses the LHeC characteristics serving as a development facility for the design and operation of the LHeC. An updated detector design is presented corresponding to the acceptance, resolution and calibration goals which arise from the Higgs and parton density function physics programmes. The paper also presents novel results on the Future Circular Collider in electron-hadron mode, FCC-eh, which utilises the same ERL technology to further extend the reach of DIS to even higher centre-of-mass energies.

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arXiv2020

Heavy-flavor PDF evolution and variable-flavor number scheme uncertainties in deep-inelastic scattering

We consider a detailed account on the construction of the heavy-quark parton distribution functions for charm and bottom, starting from $n_f=3$ light flavors in the fixed-flavor number (FFN) scheme and by using the standard decoupling relations for heavy quarks in QCD. We also account for two-mass effects. Furthermore, different implementations of the variable-flavor-number (VFN) scheme in deep-inelastic scattering (DIS) are studied, with the particular focus on the resummation of large logarithms in $Q^2/m_h^2$, the ratio the virtuality of the exchanged gauge-boson $Q^2$ to the heavy-quark mass squared $m_h^2$. A little impact of resummation effects if found in the kinematic range of the existing data on the DIS charm-quark production so that they can be described very well within the FFN scheme. Finally, we study the theoretical uncertainties associated to the VFN scheme, which manifest predominantly at small $Q^2$.

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arXiv2019

Resummation of large logarithms in the VFN scheme for DIS heavy-quark production

We consider the impact of the resummation of large logarithms, which appear in the QCD evolution of the heavy-quark distributions, on the phenomenology of deep-inelastic heavy-quark production. The heavy-quark PDFs are derived using the fixed-order matching conditions as a boundary for the QCD evolution and the result obtained is compared to the distributions defined by the matching conditions at all scales. With such an approach, the effect of heavy-quark PDF evolution is found to be sizable at LO and dramatically reduces at NLO. The NNLO evolved distributions are not very different from the NLO ones at large scales, however, show substantial differences at low virtualities, i.e. where the additional large logarithms are numerically not important, while a mismatch between the NLO accuracy of the matching conditions and the NNLO accuracy in the evolution kernels causes a substantial excess in the heavy-quark distributions. This excess propagates into the variable flavor number (VFN) scheme predictions for the deep-inelastic structure functions and has to be compensated by a decrease in the small-$x$ gluon distribution determined from PDF fits based on the VFN scheme, which should be considered as a theoretical uncertainty in VFN PDF fits and reaches $\sim 30\%$ for the small-$x$ gluon distribution extracted from the data on deep-inelastic charm-quark production.

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arXiv2019

An Update of the ABMP16 PDF Fit

We present an updated version of the ABMP16 nucleon PDFs, which is tuned by using recent precise data on $W$- and $Z/γ^*$-production at the LHC and the final HERA data on DIS $c$- and $b$-quark production and by imposing a stringent $Q^2$-cut on the inclusive DIS data in order to avoid the impact of higher twist terms at small $x$ at HERA. The new $W$- and $Z$-boson production data, in particular the updated version of the ATLAS data at the c.m.s. energy 7 TeV, are well accommodated into the present fit. The strange sea distribution obtained is consistent with the average of the up and down quark ones at small $x$. However, it is still suppressed with respect to the non-strange one by a factor of $\sim 0.5$ at moderate $x$. The small-$x$ gluon distribution is enhanced as compared to the previous ABMP16 fit, in line with updated data on the DIS $c$-quark production. Finally, a good description of the non-resonant $γ^*/Z$-production data, which are included into the ABM analysis for the first time, is achieved provided the photon-initiated lepton pair production is taken into account.

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arXiv2019

$α_s$(2019): Precision measurements of the QCD coupling

This document collects a written summary of all contributions presented at the workshop "$α_s$(2019): Precision measurements of the strong coupling" held at ECT* (Trento) in Feb. 11--15, 2019. The workshop explored in depth the latest developments on the determination of the QCD coupling $α_s$ from the key categories where high precision measurements are available: (i) lattice QCD, (ii) hadronic $τ$ decays, (iii) deep-inelastic scattering and parton distribution functions, (iv) event shapes, jet cross sections, and other hadronic final-states in $e^+e^-$ collisions, (v) Z boson and W boson hadronic decays, and (vi) hadronic final states in p-p collisions. The status of the current theoretical and experimental uncertainties associated to each extraction method, and future perspectives were thoroughly reviewed. Novel $α_s$ determination approaches were discussed, as well as the combination method used to obtain a world-average value of the QCD coupling at the Z mass pole.

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arXiv2018

NLO PDFs from the ABMP16 fit

We perform a global fit of parton distribution functions (PDFs) together with the strong coupling constant $α_s$ and the quark masses $m_c$, $m_b$ and $m_t$ at next-to-leading order (NLO) in QCD. The analysis applies the $\overline{\mathrm{MS}}\, $ renormalization scheme for $α_s$ and all quark masses. It is performed in the fixed-flavor number scheme for $n_f=3, 4, 5$ and uses the same data as the previous fit of the ABMP16 PDF at next-to-next-to-leading order (NNLO). The new NLO PDFs complement the set of ABMP16 PDFs and are to be used consistently with NLO QCD predictions for hard scattering processes. At NLO we obtain the value $α_s^{(n_f=5)}(M_Z) = 0.1191 \pm 0.0011$ compared to $α_s^{(n_f=5)}(M_Z) = 0.1147 \pm 0.0008$ at NNLO.

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arXiv2018

NNLO PDFs for the LHC

We consider some trends, achievements and a series of remaining problems in the precision determination of parton distribution functions. For the description of the scaling violations of the deep-inelastic scattering data, forming the key ingredients to all PDF fits, a solid theoretical framework is of importance. It is provided by the fixed flavor number scheme in describing the heavy-quark contributions which is found in good agreement with the present experimental data in a very wide range of momentum transfers. In this framework also a consistent determination of the heavy-quark masses is possible at high precision. The emerging Drell-Yan data measured at hadron colliders start to play a crucial role in disentangling the quark species, particularly at small and large values of $x$. These new inputs demonstrate a good overall consistency with the earlier constraints on the PDFs coming from fixed-target experiments. No dramatic change is observed in the PDFs in case of a consistent account of the higher-order QCD corrections and when leaving enough flexibility in the PDF shape parameterization.}

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arXiv2018

Strange and non-strange distributions from the collider data

We check the stability of the ABMP16 fit with respect to modifications of quark PDFs suggested in the recent literature: the strange sea enhancement and a positive non-vanishing $d/u$ ratio at $x \to 1$. These possibilities are examined using test versions of the ABMP16 PDF fit which demonstrate no need of those changes. Furthermore, we localize peculiar features in other analyses which are responsible for a different behaviour of the PDFs obtained. The strange sea enhancement can be explained by a choice of the PDF shapes being not flexible enough and therefore leading to an over-suppressed $d$-quark distribution. This suppression has to be compensated by a corresponding rise by the $s$-quark distribution. As a result, an unusually large strange sea suppression factor is obtained. The non-vanishing value of $d/u\vert _{x=1}$ becomes consistent with zero in case the higher-order QCD corrections to the DØ $W$-asymmetry data, which drive its value, are taken into account. Furthermore, the related $e$-asymmetry sample prefers a slightly negative value of $d/u\vert _{x=1}$, although it is consistent with zero. These clarifications support confidence in the PDF shapes used in the ABMP16 analyses.

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arXiv2017

Strange sea determination from collider data

We consider determinations of the strange sea in the nucleon based on the QCD analyses of data collected at the LHC with focus on the recent high-statistics ATLAS measurement of the $W^\pm$- and $Z$-boson production. We study the effect of different functional forms for parameterization of the parton distribution functions and the combination of various data sets in the analysis. We compare to earlier strange sea determinations and discuss ways to improve them in the future.

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arXiv2017

Parton Distribution Functions, $α_s$ and Heavy-Quark Masses for LHC Run II

We determine a new set of parton distribution functions (ABMP16), the strong coupling constant $α_s$ and the quark masses $m_c$, $m_b$ and $m_t$ in a global fit to next-to-next-to-leading order (NNLO) in QCD. The analysis uses the $\overline{\mathrm{MS}}$ scheme for $α_s$ and all quark masses and is performed in the fixed-flavor number scheme for $n_f=3, 4, 5$. Essential new elements of the fit are the combined data from HERA for inclusive deep-inelastic scattering (DIS), data from the fixed-target experiments NOMAD and CHORUS for neutrino-induced DIS, and data from Tevatron and the LHC for the Drell-Yan process and the hadro-production of single-top and top-quark pairs. The theory predictions include new improved approximations at NNLO for the production of heavy quarks in DIS and for the hadro-production of single-top quarks. The description of higher twist effects relevant beyond the leading twist collinear factorization approximation is refined. At NNLO we obtain the value $α_s^{(n_f=5)}(M_Z) = 0.1147 \pm 0.0008$.

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