arXiv subjects

Sven-Olaf Moch

Publications and source records attributed to Sven-Olaf Moch.

At least 55 records · Page 3Linked to original sources

arXiv2022

Uncertainties on the theoretical input for $t\bar{t}j$ experimental analyses at the LHC

The precise measurement of the top-quark mass constitutes one of the main goals of the LHC top-quark physics program. One possibility to extract this parameter uses the $ρ_{\mathrm{s}}$ distribution, which depends on the invariant mass of the $t\bar{t}j$ system. To fully take advantage of the experimental accuracy achievable in measuring top quark production cross sections at the LHC, the theory uncertainties need to be well under control. We present a study of the effect of varying the input parameters of the theoretical calculation on the predicted $ρ_{\mathrm{s}}$ distribution. Thereby we studied the influence of the $R$ parameter in the jet reconstruction procedure, as well as the effect of various renormalization and factorization scale definitions and different PDF sets. A behaviour similar to the one presented here for the $ρ_{\mathrm{s}}$-distribution was also found for other differential distributions.

hep-ph↗

arXiv2021

Theory input for $t\bar{t}j$ experimental analyses at the LHC

The precise measurement of the top quark mass, which is a fundamental SM parameter, constitutes one of the main goals of the LHC top physics program. One approach to measure this quantity uses the $ρ_\mathrm{s}$ distribution, an observable depending on the invariant mass of the $t\bar{t}j$ system. To fully exploit the experimental accuracy achievable in measuring top quark production cross sections at the LHC, the theory uncertainties associated to these measurements need to be well under control. To this end we present a study of the effect of varying the theoretical input parameters in the calculation of differential cross sections of the $t\bar{t}j$ process. Thereby we studied the influence of the jet reconstruction procedure, as well as the effect of various renormalization and factorization scale definitions and different PDF sets. The variation of the $R$ parameter in the jet reconstruction algorithm was found to have negligible influence on the scale variation uncertainty. A strong reduction of scale uncertainties and a better behaviour of the NLO/LO ratios using selected dynamical scales instead of a static one in the high energy tails of differential distributions was observed. This is particularly interesting in the context of the top quark mass measurements through the $ρ_\mathrm{s}$ distribution, in which the perturbative stability can be improved by applying the proposed dynamical scale definition.

hep-ph↗

arXiv2021

Cross-Sections for $\text{t}\bar{\text{t}}\text{H}$ production with the Top Quark $\overline{\text{MS}}$ Mass

We study the impact of the top quark mass renormalized in the $\overline{\text{MS}}$ scheme on the $\text{t}\bar{\text{t}}\text{H}$ production cross-sections as an alternative to theory predictions with the conventionally used pole mass scheme. The differential cross-sections at next-to-leading order in perturbative QCD with stable top quarks show a moderate decrease in scale uncertainties for the $\overline{\text{MS}}$ mass renormalization scheme compared to the on-shell one. The shape of the differential distributions is not affected much, the largest differences being observed in the invariant mass distributions of the $\text{t}\bar{\text{t}}$ and the $\text{t}\bar{\text{t}}\text{H}$ systems.

hep-ph↗

arXiv2021

Hypergeometric Functions and Feynman Diagrams

The relationship between Feynman diagrams and hypergeometric functions is discussed. Special attention is devoted to existing techniques for the construction of the $ε$-expansion. As an example, we present a detailed discussion of the construction of the epsilon-expansion of the Appell function $F_3$ around rational values of parameters via an iterative solution of differential equations. As a by-product, we have found that the one-loop massless pentagon diagram in dimension $d=3-2ε$ is not expressible in terms of multiple polylogarithms. Another interesting example is the Puiseux-type solution involving a differential operator generated by a hypergeometric function of three variables. The holonomic properties of the $F_N$ hypergeometric functions are briefly discussed.

hep-th↗

arXiv2020

Soft corrections to inclusive deep-inelastic scattering at four loops and beyond

We study the threshold corrections for inclusive deep-inelastic scattering (DIS) and their all-order resummation. Using recent results for the QCD form factor, related anomalous dimensions and Mellin moments of DIS structure functions at four loops we derive the complete soft and collinear contributions to the DIS Wilson coefficients at four loops. For a general $SU(n_c)$ gauge group the results are exact in the large-$n_c$ approximation and for QCD with $n_c=3$ we present precise approximations. We extend the threshold resummation exponent $G^N$ in Mellin-$N$ space to the fifth logarithmic (N$^4$LL) order collecting the terms $α_{\rm s}^{\,3} (α_{\rm s} \ln N)^n$ to all orders in the strong coupling constant $α_{\rm s}$. We study the numerical effect of the N$^4$LL corrections using both the fully exponentiated form and the expansion of the coefficient function in towers of logarithms. As a byproduct, we derive a numerical result for the complete pole structure of the QCD form factor in the parameter of dimensional regularization $\varepsilon$ at four loops.

hep-ph↗

arXiv2019

Soft corrections to inclusive DIS at four loops and beyond

We study the threshold corrections to the structure functions in deep-inelastic scattering (DIS) at the fifth logarithmic (N$^4$LL) order of the soft-gluon exponentiation in massless perturbative QCD. Using recent results for the splitting functions and the quark form factor, we derive the fourth-order contribution to the coefficient $f^{\rm q}$ of the form factor and from it the N$^4$LL part of the exponentiation coefficient $B^{\rm DIS}$ in the limit of a large number of colours. An approximation scheme is shown that leads to sufficiently accurate N$^4$LL results for full QCD. The N$^4$LL corrections are small and lead to a further stabilization of the perturbative expansion for the soft-gluon exponent.

hep-ph↗

arXiv2018

Top-Quark Physics at the LHC

We report on the precision determination of the top-quark mass to next-to-next-to-leading order in QCD in well-defined renormalization schemes using data from the Large Hadron Collider for single-top and top-quark pair production. We also discuss the calibration of the so-called Monte Carlo top-quark mass parameter which is determined from a comparison to events with top-quark decay products. The implications of the measured value of the top-quark mass for conclusions about the stability of the electroweak vacuum state of our Universe are illustrated. At future lepton colliders, we provide for the first time matched exclusive calculations valid both at the top threshold and in the continuum, also fully differentially. In addition, we calculate fully off-shell top-pair production (also with an associated Higgs boson) at next-to-leading order in QCD, which allows to extract the top-Yukawa coupling with an unprecedented precision.

hep-ph↗

arXiv2018

Threshold and jet radius joint resummation for single-inclusive jet production

The QCD predictions for single-inclusive jet production are computed with joint resummation of threshold and jet radius logarithms. The results are compared to those based on fixed order perturbation theory up to next-to-next-to-leading order and to data by the CMS collaboration measured in proton-proton collisions at the LHC at $\sqrt{S}=8$ TeV. The joint resummation results are in remarkable agreement with the data.

hep-ph↗

arXiv2018

Phenomenology of single-inclusive jet production with jet radius and threshold resummation

We perform a detailed study of inclusive jet production cross sections at the LHC and compare the QCD theory predictions based on the recently developed formalism for threshold and jet radius joint resummation at next-to-leading logarithmic accuracy to inclusive jet data collected by the CMS collaboration at $\sqrt{S} = 7$ and $13$TeV. We compute the cross sections at next-to-leading order in QCD with and without the joint resummation for different choices of jet radii $R$ and observe that the joint resummation leads to crucial improvements in the description of the data. Comprehensive studies with different parton distribution functions demonstrate the necessity of considering the joint resummation in fits of those functions based on the LHC jet data.

hep-ph↗

arXiv2017

Threshold and jet radius joint resummation for single-inclusive jet production

We present the first threshold and jet radius jointly resummed cross section for single-inclusive hadronic jet production. We work at next-to-leading logarithmic accuracy and our framework allows for a systematic extension beyond the currently achieved precision. Longstanding numerical issues are overcome by performing the resummation directly in momentum space within Soft Collinear Effective Theory. We present the first numerical results for the LHC and observe an improved description of the available data. Our results are of immediate relevance for LHC precision phenomenology including the extraction of parton distribution functions and the QCD strong coupling constant.

hep-ph↗

arXiv2016

Single-top production in the s-channel and the top-quark mass

We use a fixed-order expansion of resummed soft-gluon corrections to determine an approximate NNLO formula for the partonic cross section of single-top production in the $s$-channel. This formula is implemented in the program Hathor for the numerical evaluation of hadronic cross sections. With the resulting code, we perform a fit of the top-quark mass to Tevatron cross section data. Results for $m_t$ are given in the pole-mass scheme and in the $\overline{\mathrm{MS}}$ scheme.

hep-ph↗

arXiv2016

Calibration of the Top-Quark Monte-Carlo Mass

We present a method to establish experimentally the relation between the top-quark mass $m_t^{MC}$ as implemented in Monte-Carlo generators and the Lagrangian mass parameter $m_t$ in a theoretically well-defined renormalization scheme. We propose a simultaneous fit of $m_t^{MC}$ and an observable sensitive to $m_t$, which does not rely on any prior assumptions about the relation between $m_t$ and $m_t^{MC}$. The measured observable is independent of $m_t^{MC}$ and can be used subsequently for a determination of $m_t$. The analysis strategy is illustrated with examples for the extraction of $m_t$ from inclusive and differential cross sections for hadro-production of top-quarks.

hep-ph↗

arXiv2015

A facility to Search for Hidden Particles at the CERN SPS: the SHiP physics case

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (Search for Hidden Particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $τ\to 3μ$ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the Standard Model and describe interactions between new particles and four different portals - scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the Standard Model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation

hep-ph↗

arXiv2015

Impact of heavy-flavour production cross sections measured by the LHCb experiment on parton distribution functions at low x

The impact of recent measurements of heavy-flavour production in deep inelastic $ep$ scattering and in $pp$ collisions on parton distribution functions is studied in a QCD analysis in the fixed-flavour number scheme at next-to-leading order. Differential cross sections of charm- and beauty-hadron production measured by LHCb are used together with inclusive and heavy-flavour production cross sections in deep inelastic scattering at HERA. The heavy-flavour data of the LHCb experiment impose additional constraints on the gluon and the sea-quark distributions at low partonic fractions $x$ of the proton momentum, down to $x \sim 5 \times 10^{-6}$. This kinematic range is currently not covered by other experimental data in perturbative QCD fits.

hep-ph↗

arXiv2015

Top-quark pair production at hadron colliders: differential cross section and phenomenological applications with DiffTop

The results of phenomenological studies of top-quark pair production in proton-proton collisions are presented. Differential cross sections are calculated in perturbative QCD at approximate next-to-next-to-leading order ${\cal O}(α_s^4)$ by using methods of threshold resummation beyond the leading logarithmic accuracy. Predictions for the single-particle inclusive kinematics are presented for transverse momentum and rapidity distributions of final-state top quarks. Uncertainties related to the description of proton structure, top-quark mass and strong coupling constant are investigated in detail. The results are compared to the recent measurements by the ATLAS and CMS collaborations at the LHC at the center of mass energy of 7 TeV. The calculation presented here is implemented in the computer code \textsc{Difftop} and can be applied to the general case of heavy-quark pair production at hadron-hadron colliders. For the first time, a fit of parton distribution functions at NNLO is performed by using the differential cross sections of top-quark pair production together with other data sets. The impact of the top-pair production on the precision of the gluon distribution at high scales is illustrated.

hep-ph↗

arXiv2014

QCD Corrections to Semi-Inclusive Hadron Production in Electron-Positron Annihilation at Two Loops

We report on the analytic calculation of the second-order QCD corrections to semi-inclusive hadron production in electron-positron annihilation. The calculation of the single-particle inclusive cross-section in time-like kinematics is performed in Mellin N-space and uses an algorithmic evaluation of inclusive phase-space integrals, based on the unitarity cutting rules and integration-by-parts. We obtain splitting functions and coefficient functions up to second order in the strong coupling alpha_s. Our results are in agreement with earlier calculations in the literature by Rijken and van Neerven.

hep-ph↗

arXiv2014

HYPERDIRE: HYPERgeometric functions DIfferential REduction: MATHEMATICA based packages for differential reduction of generalized hypergeometric functions: $F_D$ and $F_S$ Horn-type hypergeometric functions of three variables

HYPERDIRE is a project devoted to the creation of a set of Mathematica based programs for the differential reduction of hypergeometric functions. The current version includes two parts: the first one, FdFunction, for manipulations with Appell hypergeometric functions $F_D$ of $r$ variables; and the second one, FsFunction, for manipulations with Lauricella-Saran hypergeometric functions $F_S$ of three variables. Both functions are related with one-loop Feynman diagrams. The published version includes also Chapter 5 with two theorems about structure of coefficients of epsilon-expansion of the Horn-type hypergeometric functions. As illustration, the first three coefficients of epsilon-expansion for the Appell hypergeometric function FD of r-variables are explicitly evaluated.

math-ph↗

arXiv2014

Differential cross sections for top pair production at the LHC

We present results of phenomenological studies for top-quark pair production at the LHC at the center of mass energy $\sqrt{S} = 7$ TeV. The transverse momentum and rapidity distributions for final-state top quarks are calculated in perturbative QCD at approximate next-to-next-to-leading order ${\cal O}(α_s^4)$ by using methods of threshold resummation beyond the leading logarithmic accuracy. The theoretical predictions are obtained by using the computer code \textsc{DiffTop} and are compared to recent measurements by the ATLAS and CMS collaborations. \textsc{Difftop} can be employed in the general case of heavy-quark pair production at hadron-hadron colliders and provides a basis for applications in QCD analyses for parton distribution functions determination.

hep-ph↗