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T. Stavreva

Publications and source records attributed to T. Stavreva.

At least 19 recordsLinked to original sources
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

Searching for intrinsic charm in the proton at the LHC

Despite rather long-term theoretical and experimental studies, the hypothesis of the non-zero intrinsic (or valence-like) heavy quark component of the proton distribution functions has not yet been confirmed or rejected. The LHC with $pp$-collisions at $\sqrt{s}=$ 7--14 TeV will obviously supply extra unique information concerning the above-mentioned component of the proton. To use the LHC potential, first of all, one should select the parton-level (sub)processes (and final-state signatures) that are most sensitive to the intrinsic heavy quark contributions. To this end inclusive production of $c(b)$-jets accompanied by photons is considered. On the basis of the performed theoretical study it is demonstrated that the investigation of the intrinsic heavy quark contributions looks very promising at the LHC in processes such as $pp\rightarrow γ+ c(b)+X$.

hep-ph↗
arXiv2013

Prompt photon in association with a heavy-quark jet in Pb-Pb collisions at the LHC

We present a phenomenological study of the associated production of a prompt photon and a heavy quark jet (charm or bottom) in Pb-Pb collisions at the LHC. This channel allows for estimating the amount of energy loss experienced by the charm and bottom quarks propagating in the dense QCD medium produced in those collisions. Calculations are carried out at next-to-leading order (NLO) accuracy using the BDMPS-Z heavy-quark quenching weights. The quenching of the single heavy-quark jet spectrum reflects fairly the hierarchy in the heavy quark energy loss assumed in the perturbative calculation. On the contrary, the single photon spectrum in heavy-ion collisions is only modified at low momenta, for which less heavy-quark jets pass the kinematic cuts. On top of single particle spectra, the two-particle final state provides a range of observables (photon-jet pair momentum, jet asymmetry, among others) which are studied in detail. The comparison of the photon-jet pair momentum, from p-p to Pb-Pb collisions, is sensitive to the amount of energy lost by the heavy-quarks and could therefore be used in order to better understand parton energy loss processes in the heavy quark sector.

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.

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.

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arXiv2012

The SM and NLO Multileg and SM MC Working Groups: Summary Report

The 2011 Les Houches workshop was the first to confront LHC data. In the two years since the previous workshop there have been significant advances in both soft and hard QCD, particularly in the areas of multi-leg NLO calculations, the inclusion of those NLO calculations into parton shower Monte Carlos, and the tuning of the non-perturbative parameters of those Monte Carlos. These proceedings describe the theoretical advances that have taken place, the impact of the early LHC data, and the areas for future development.

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 nuclear parton densities and parton energy loss processes through photon + heavy-quark jet production in p-A and A-A collisions

We present a detailed phenomenological study of the associated production of a prompt photon and a heavy-quark jet (charm or bottom) in proton-nucleus (p-A) and nucleus-nucleus (A-A) collisions. The dominant contribution to the cross-section comes from the gluon--heavy-quark (gQ) initiated subprocess, making this process very sensitive to the gluon and the heavy quark nuclear parton densities. We show that the future p-A data to be collected at the LHC should allow one to disentangle the various nPDF sets currently available. In heavy-ion collisions, the photon transverse momentum can be used to gauge the initial energy of the massive parton which is expected to propagate through the dense QCD medium produced in those collisions. The two-particle final state provides a range of observables (jet asymmetry, photon-jet pair momentum, among others), through the use of which a better understanding of parton energy loss processes in the massive quark sector can be achieved, as shown by the present phenomenological analysis carried out in Pb-Pb collisions at the LHC.

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.

hep-ph↗
arXiv2011

Nuclear corrections in neutrino-nucleus DIS and their compatibility with global NPDF analyses

We perform a global chi^2-analysis of nuclear parton distribution functions using data from charged current neutrino-nucleus deep-inelastic scattering (DIS), charged-lepton-nucleus DIS, and the Drell-Yan (DY) process. We show that the nuclear corrections in nu-A DIS are not compatible with the predictions derived from l^+A DIS and DY data. We quantify this result using a hypothesis-testing criterion based on the chi^2 distribution which we apply to the total chi^2 as well as to the chi^2 of the individual data sets. We find that it is not possible to accommodate the data from nu-A and l^+A DIS by an acceptable combined fit. Our result has strong implications for the extraction of both nuclear and proton parton distribution functions using combined neutrino and charged-lepton data sets.

hep-ph↗
arXiv2011

Probing gluon and heavy-quark nuclear PDFs with photon + heavy quark production in pA collisions

We present a detailed phenomenological study of direct photon production in association with a heavy-quark jet in pA collisions at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) at next-to-leading order in QCD. The dominant contribution to the cross-section comes from the gluon--heavy-quark (gQ) initiated subprocess, making γ+ Q production a process very sensitive to both the gluon and the heavy-quark parton distribution functions (PDFs). Additionally, the RHIC and LHC experiments are probing complementary kinematic regions in the momentum fraction x_2 carried by the target partons. Thus, the nuclear production ratio R^{γ+Q}_{pA} can provide strong constraints, over a broad x-range, on the poorly determined nuclear parton distribution functions which are extremely important for the interpretation of results in heavy-ion collisions.

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