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K. Kovarik

Publications and source records attributed to K. Kovarik.

At least 37 records · Page 2Linked to original sources
arXiv2014

One-loop corrections to gaugino (co-)annihilation into quarks in the MSSM

We present the full $\mathcal{O}(α_s)$ supersymmetric QCD corrections for gaugino annihilation and co-annihilation into light and heavy quarks in the Minimal Supersymmetric Standard Model (MSSM). We demonstrate that these channels are phenomenologically relevant within the so-called phenomenological MSSM. We discuss selected technical details such as the dipole subtraction method in the case of light quarks and the treatment of the bottom quark mass and Yukawa coupling. Numerical results for the (co-)annihilation cross sections and the predicted neutralino relic density are presented. We show that the impact of including the radiative corrections on the cosmologically preferred region of the parameter space is larger than the current experimental uncertainty from Planck data.

hep-ph↗
arXiv2014

NLO Monte Carlo predictions for heavy-quark production at the LHC: pp collisions in ALICE

Next-to-leading order (NLO) QCD predictions for the production of heavy quarks in proton-proton collisions are presented within three different approaches to quark mass, resummation and fragmentation effects. In particular, new NLO and parton shower simulations with POWHEG are performed in the ALICE kinematic regime at three different centre-of-mass energies, including scale and parton density variations, in order to establish a reliable baseline for future detailed studies of heavy-quark suppression in heavy-ion collisions. Very good agreement of POWHEG is found with FONLL, in particular for centrally produced D^0, D^+ and D^*+ mesons and electrons from charm and bottom quark decays, but also with the generally somewhat higher GM-VFNS predictions within the theoretical uncertainties. The latter are dominated by scale rather than quark mass variations. Parton density uncertainties for charm and bottom quark production are computed here with POWHEG for the first time and shown to be dominant in the forward regime, e.g. for muons coming from heavy-flavour decays. The fragmentation into D_s^+ mesons seems to require further tuning within the NLO Monte Carlo approach.

hep-ph↗
arXiv2013

A $N_F$-Dependent VFNS for Heavy Flavors: Merging the FFNS and VFNS

We introduce a Hybrid Variable Flavor Number Scheme (H-VFNS) for heavy flavors, which incorporates the advantages of both the traditional Variable Flavor Number Scheme (VFNS) as well as the Fixed Flavor Number Scheme (FFNS). We include an explicit dependence on number of active flavors $N_F$ in both the Parton Distribution Functions (PDFs) and the strong coupling constant $α_{S}$. This results in sets of coexisting PDFs and $α_{S}$ for $N_F=\{3,4,5,6\}$, that are related analytically by the $\overline{\rm MS}$ matching conditions. The H-VFNS resums the heavy quark contributions and provides the freedom to choose the optimal $N_F$ for each particular data set. Thus, we can fit selected HERA data in a FFNS framework, while retaining the benefits of the VFNS to analyze LHC data at high scales. We illustrate how such a fit can be implemented for the case of both HERA and LHC data.

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arXiv2013

A Hybrid Scheme for Heavy Flavors: Merging the FFNS and VFNS

We introduce a Hybrid Variable Flavor Number Scheme for heavy flavors, denoted H-VFNS, which incorporates the advantages of both the traditional Variable Flavor Number Scheme (VFNS) as well as the Fixed Flavor Number Scheme (FFNS). By including an explicit $N_F$-dependence in both the Parton Distribution Functions (PDFs) and the strong coupling constant $α_S$, we generate coexisting sets of PDFs and $α_S$ for $N_F=\{3,4,5,6\}$ at any scale $μ$, that are related analytically by the $\overline{\text{MS}}$ matching conditions. The H-VFNS resums the heavy quark contributions and provides the freedom to choose the optimal $N_F$ for each particular data set. Thus, we can fit selected HERA data in a FFNS framework, while retaining the benefits of the VFNS to analyze LHC data at high scales. We illustrate how such a fit can be implemented for the case of both HERA and LHC data.

hep-ph↗
arXiv2013

CTEQ nuclear parton distribution functions

We show for the first time preliminary results of nuclear parton distribution function analysis of charged lepton DIS and Drell-Yan data within the CTEQ framework including error PDFs. We compare our error estimates to estimates of different nPDF groups.

hep-ph↗
arXiv2013

Neutralino-stop co-annihilation into electroweak gauge and Higgs bosons at one loop

We compute the full O(alpha_s) supersymmetric QCD corrections for neutralino-stop co-annihilation into electroweak gauge and Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM). We show that these annihilation channels are phenomenologically relevant within the so-called phenomenological MSSM, in particular in the light of the observation of a Higgs-like particle with a mass of about 126 GeV at the LHC. We present in detail our calculation, including the renormalization scheme, the infrared treatment, and the kinematical subtleties to be addressed. Numerical results for the co-annihilation cross sections and the predicted neutralino relic density are presented. We demonstrate that the impact of including the corrections on the cosmologically preferred region of parameter space is larger than the current experimental uncertainty from WMAP data.

hep-ph↗
arXiv2013

Impact of SUSY-QCD corrections on neutralino-stop co-annihilation and the neutralino relic density

We have calculated the full O(alpha_s) supersymmetric QCD corrections to neutralino-stop co-annihilation into electroweak vector and Higgs bosons within the Minimal Supersymmetric Standard Model (MSSM). We performed a parameter study within the phenomenological MSSM and demonstrated that the studied co-annihilation processes are phenomenologically relevant, especially in the context of a 126 GeV Higgs-like particle. By means of an example scenario we discuss the effect of the full next-to-leading order corrections on the co-annihilation cross section and show their impact on the predicted neutralino relic density. We demonstrate that the impact of these corrections on the cosmologically preferred region of parameter space is larger than the current experimental uncertainty of WMAP data.

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arXiv2012

Associated production of charged Higgs bosons and top quarks with POWHEG

The associated production of charged Higgs bosons and top quarks at hadron colliders is an important discovery channel to establish the existence of a non-minimal Higgs sector. Here, we present details of a next-to-leading order (NLO) calculation of this process using the Catani-Seymour dipole formalism and describe its implementation in POWHEG, which allows to match NLO calculations to parton showers. Numerical predictions are presented using the PYTHIA parton shower and are compared to those obtained previously at fixed order, to a leading order calculation matched to the PYTHIA parton shower, and to a different NLO calculation matched to the HERWIG parton shower with MC@NLO. We also present numerical predictions and theoretical uncertainties for various Two Higgs Doublet Models at the Tevatron and LHC.

hep-ph↗
arXiv2012

Heavy Quark Production in the ACOT Scheme beyond NLO

We compute the structure functions F2 and FL in the ACOT scheme for heavy quark production. We use the complete ACOT results to NLO, and make use of the MSbar massless results at NNLO and N3LO to estimate the higher order mass-dependent corrections. We show numerically that the dominant heavy quark mass effects can be taken into account using massless Wilson coefficients together with an appropriate rescaling prescription. Combining the exact NLO ACOT scheme with these expressions should provide a good approximation to the full calculation in the ACOT scheme at NNLO and N3LO.

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arXiv2012

Update and Comparison of Nuclear Parton Distribution Functions and Neutrino DIS

We compare the nuclear corrections factors from neutrino deep-inelastic scattering (DIS) with the ones coming from a standard analysis of nuclear parton distribution functions (nPDF). We focus on a discrepancy between the most precise neutrino DIS data from NuTeV and the nuclear PDF coming from the analysis of charged lepton DIS and Drell-Yan data.

hep-ph↗
arXiv2012

Heavy Quark Production in the ACOT Scheme Beyond NLO

We analyze the properties of the ACOT scheme for heavy quark production and make use of the MS-Bar massless results at NNLO and N3LO for the structure functions F2 and FL in neutral current deep-inelastic scattering to estimate the higher order corrections. The dominant heavy quark mass effects at higher orders can be taken into account using the massless Wilson coefficients together with an appropriate slow-rescaling prescription implementing the phase space constraints. Combining the exact ACOT scheme at NLO with these expressions should provide a good approximation to the full calculation in the ACOT scheme at NNLO and N3LO.

hep-ph↗
arXiv2012

Heavy Quark Production in the ACOT Scheme at NNLO and N3LO

We analyze the properties of the ACOT scheme for heavy quark production and make use of the \MSbar massless results at NNLO and N3LO for the structure functions F2 and FL in neutral current deep-inelastic scattering to estimate the higher order corrections. For this purpose we decouple the heavy quark mass entering the phase space from the one entering the dynamics of the short distance cross section. We show numerically that the phase space mass is generally more important. Therefore, the dominant heavy quark mass effects at higher orders can be taken into account using the massless Wilson coefficients together with an appropriate slow-rescaling prescription implementing the phase space constraints. Combining the exact ACOT scheme at NLO with these expressions should provide a good approximation to the missing full calculation in the ACOT scheme at NNLO and N3LO.

hep-ph↗
arXiv2012

Strange Quark PDFs and Implications for Drell-Yan Boson Production at the LHC

Global analyses of Parton Distribution Functions (PDFs) have provided incisive constraints on the up and down quark components of the proton, but constraining the other flavor degrees of freedom is more challenging. Higher-order theory predictions and new data sets have contributed to recent improvements. Despite these efforts, the strange quark PDF has a sizable uncertainty, particularly in the small x region. We examine the constraints from experiment and theory, and investigate the impact of this uncertainty on LHC observables. In particular, we study W/Z production to see how the s-quark uncertainty propagates to these observables, and examine the extent to which precise measurements at the LHC can provide additional information on the proton flavor structure.

hep-ph↗
arXiv2012

A Survey of Heavy Quark Theory for PDF Analyses

We survey some of the recent developments in the extraction and application of heavy quark Parton Distribution Functions (PDFs). We also highlight some of the key HERA measurements which have contributed to these advances.

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arXiv2011

Compatibility of global NPDF analyses of neutrino DIS and charged-lepton DIS data

The neutrino deep inelastic scattering (DIS) data is very interesting for global analyses of proton and nuclear parton distribution functions (PDFs) since they provide crucial information on the strange quark distribution in the proton and allow for a better flavor decompositon of the PDFs. In order to use neutrino DIS data in a global analysis of proton PDFs nuclear effects need to be understood. We study these effects with the help of nuclear PDFs extracted from global analyses of charged-lepton DIS, Drell-Yan and neutrino DIS data at next-to-leading order in QCD.

hep-ph↗
arXiv2011

Probing gluon nuclear PDF with direct photon production in association with a heavy quark

We investigate a possible use of direct photon production in association with a heavy quark in $pA$ collisions at the large hadron collider to constrain the nuclear gluon parton distribution function. This process is sensitive to both, the nuclear heavy quark and gluon parton distribution functions and is a very promising candidate to help determine the gluon nuclear PDF which is still largely untested.

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