Flavor Scheme Research Articles

Q 2-evolution of parton densities at small x values. Charm contribution in the combined H1 and ZEUS F 2 data

The Bessel-inspired behavior of parton densities at small x, obtained in the case of the flat initial conditions for DGLAP evolution equations, is used in the fixed flavor scheme to analyze precise H1 and ZEUS combined data on the structure function F 2.

Simulating b -associated production of Z and Higgs bosons with the Sherpa event generator

We compare four-- and five--flavour scheme predictions for $b$-associated production of $Z$ and Higgs bosons. The results are obtained with \Sherpa's MC@NLOimplementation for the four--flavour scheme, treating the $b$'s as massive, and with multijet merging at leading and next-to leading order for the five--flavour schemes. Comparison with data for $Z+b(\bar{b})$ production at the $7$ TeV LHC exhibit strengths and weaknesses of the different approaches and are used to validate predictions for $b$-associated Higgs-boson production at the 13 TeV Run II.

Anatomy of double heavy-quark initiated processes

A number of phenomenologically relevant processes at hadron colliders, such as Higgs and Z boson production in association with b quarks, can be conveniently described as scattering of heavy quarks in the initial state. We present a detailed analysis of this class of processes, identifying the form of the leading initial-state collinear logarithms that allow the relation of calculations performed in different flavour schemes in a simple and reliable way. This procedure makes it possible to assess the size of the logarithmically enhanced terms and the effects of their resummation via heavy-quark parton distribution functions. As an application, we compare the production of (SM-like and heavy) scalar and vector bosons in association with b quarks at the LHC in the four- and five-flavour schemes as well as the production of a heavy Z' in association with top quarks at a future 100 TeV hadron collider in the five- and six-flavour schemes. We find that, in agreement with a previous analysis of single heavy-quark initiated processes, the size of the initial-state logarithms is mitigated by a kinematical suppression. The most important effects of the resummation are a shift of the central predictions typically of about 20% at a justified value of the scale of each considered process and a significant reduction of scale variation uncertainties.

Pair production of scalar leptoquarks at the LHC to NLO parton shower accuracy

We present the scalar leptoquark pair production process at the LHC computed at the next-to-leading order in QCD, matched to the PYTHIA parton shower using the MC@NLO formalism. We consider the leading order decay of a leptoquark to a lepton ($e,\mu,\tau$ or $\nu_e,\nu_\mu,\nu_\tau$) and a jet and observe the effects of parton shower on the final states. For demonstration, we display the kinametical distributions of a selection of observables along with their scale uncertainties for the 13 TeV LHC. We also present pair production cross sections and $K$-factors using massless five-quark flavor scheme for different LHC center-of-mass energies. The complete stand-alone code is available online.

Neutrino mass determination using circulating heavy ions

We study process of radiative neutrino pair emission $|e\rangle \rightarrow |g \rangle + \gamma +\nu \bar{\nu}$ from coherently excited heavy ions (quantum mixture of two ionic states, the ground and an excited states) in circular motion. Determination of the neutrino mass is found to be possible with simultaneous detection of the photon and one of neutrinos in the pair down to the level of the smallest neutrino mass of order 5 meV in the three flavor scheme.

Measurements of vector boson plus jets from ATLAS

Measurements of single W, Z boson production in association with jets probe QCD in multi-scale environment. Measurements of W + jet and Z + jet production and their ratio, extending to high jet multiplicities, and a large set of kinematic distributions are presented for 7 TeV data. The results are compared to the predictions of modern Monte Carlo generators. The production of vector bosons plus heavy flavors is sensitive to the c and b quark parton distribution functions (PDFs) and gluon splitting effects. Measurements of W + c and Z + b (b ) are reported and compared to predictions based on various PDFs in 4 and 5 flavour schemes. An overview of the results is given.

Measurements of vector boson plus jets from ATLAS

Measurements of single W, Z boson production in association with jets probe QCD in multi-scale environment. Measurements of W + jet and Z + jet production and their ratio, extending to high jet multiplicities, and a large set of kinematic distributions are presented for 7 TeV data. The results are compared to the predictions of modern Monte Carlo generators. The production of vector bosons plus heavy flavors is sensitive to the c and b quark parton distribution functions (PDFs) and gluon splitting effects. Measurements of W + c and Z + b(b) are reported and compared to predictions based on various PDFs in 4 and 5 flavour schemes. An overview of the results is given.

Neutrino magnetic moment,CPviolation, and flavor oscillations in matter

We consider collective oscillations of neutrinos, which are emergent nonlinear flavor evolution phenomena instigated by neutrino-neutrino interactions in astrophysical environments with sufficiently high neutrino densities. We investigate the symmetries of the problem in the full three flavor mixing scheme and in the exact many-body formulation by including the effects of CP violation and neutrino magnetic moment. We show that, similar to the two flavor scheme, several dynamical symmetries exist for three flavors in the single-angle approximation if the net electron background in the environment and the effects of the neutrino magnetic moment are negligible. Moreover, we show that these dynamical symmetries are present even when the CP symmetry is violated in neutrino oscillations. We explicitly write down the constants of motion through which these dynamical symmetries manifest themselves in terms of the generators of the SU(3) flavor transformations. We also show that the effects due to the CP-violating Dirac phase factor out of the many-body evolution operator and evolve independently of nonlinear flavor transformations if neutrino electromagnetic interactions are ignored. In the presence of a strong magnetic field, CP-violating effects can still be considered independently provided that an effective definition for neutrino magnetic moment is used.

The effect on PDFs and [Formula: see text] due to changes in flavour scheme and higher twist contributions.

I consider the effect on MSTW partons distribution functions (PDFs) due to changes in the choices of theoretical procedure used in the fit. I first consider using the 3-flavour fixed flavour number scheme instead of the standard general mass variable flavour number scheme used in the MSTW analysis. This results in the light quarks increasing at all relatively small x values, the gluon distribution becoming smaller at high values of x and larger at small x, the preferred value of the coupling constant alpha _S(M_Z^2) falling, particularly at NNLO, and the fit quality deteriorates. I also consider lowering the kinematic cut on W^2 for DIS data and simultaneously introducing higher twist terms which are fit to data. This results in much smaller effects on both PDFs and alpha _S(M_Z^2) than the scheme change, except for quarks at very high x. I show that the structure function one obtains from a fixed input set of PDFs using the fixed flavour scheme and variable flavour scheme differ significantly for x sim 0.01 at high Q^2, and that this is due to the fact that in the fixed flavour scheme there is a slow convergence of large logarithmic terms of the form (alpha _Sln (Q^2/m_c^2))^n relevant for this regime. I conclude that some of the most significant differences in PDF sets are largely due to the choice of flavour scheme used.

Top Quark Induced Backgrounds to Higgs Production in theWW(*)→llννDecay Channel at Next-to-Leading-Order in QCD

We present the complete NLO contributions to the pp -> e^+ v_e mu^- vbar_m b bbar + X process in the four flavour scheme, i.e. with massive b quarks, and its contribution to the H -> WW -> llvv measurement in the 1-jet bin at the LHC. This background process includes top pair, single top and non-top quark-resonant contributions. The uncertainty at NLO from renormalisation and factorisation scale dependence is about +30% -20%. We show that the NLO corrections are relatively small, and that separating this background in top pair, Wt and b-quark associated llvv is a fair approximation.

Combination and QCD analysis of charm production cross section measurements in deep-inelastic ep scattering at HERA

Measurements of open charm production cross sections in deep-inelastic ep scattering at HERA from the H1 and ZEUS Collaborations are combined. Reduced cross sections \(\sigma_{\rm red}^{c\bar{c}}\) for charm production are obtained in the kinematic range of photon virtuality 2.5≤Q2≤2000 GeV2 and Bjorken scaling variable 3⋅10−5≤x≤5⋅10−2. The combination method accounts for the correlations of the systematic uncertainties among the different data sets. The combined charm data together with the combined inclusive deep-inelastic scattering cross sections from HERA are used as input for a detailed NLO QCD analysis to study the influence of different heavy flavour schemes on the parton distribution functions. The optimal values of the charm mass as a parameter in these different schemes are obtained. The implications on the NLO predictions for W± and Z production cross sections at the LHC are investigated. Using the fixed flavour number scheme, the running mass of the charm quark is determined.

AAMQS: A non-linear QCD analysis of new HERA data at small-x including heavy quarks

We present a global analysis of available data on inclusive structure functions and reduced cross sections measured in electron-proton scattering at small values of Bjorken-x, x<0.01, including the latest data from HERA on reduced cross sections. Our approach relies on the dipole formulation of DIS together with the use of the non-linear running coupling BK equation for the description of the small-x dynamics. We improve our previous studies by including the heavy quark (charm and beauty) contribution to the reduced cross sections, and also by considering a variable flavor scheme for the running of the coupling. We obtain a good description of data, the fit parameters remaining stable with respect to our previous analyses where only light quarks were considered. The inclusion of the heavy quark contributions resulted in a good description of available experimental data for the charm component of the structure function and reduced cross section provide the initial transverse distribution of heavy quarks was allowed to differ (more specifically, to have a smaller radius) from that of the light flavors.

RareBsdecays toηandη′final states

We study exclusive ${B}_{s}$ decays to final states with $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$, induced by the rare $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $b\ensuremath{\rightarrow}s\ensuremath{\nu}\overline{\ensuremath{\nu}}$ transitions. Differential decay rates and total branching fractions are predicted in the standard model, adopting the flavor scheme for the description of the $\ensuremath{\eta}\ensuremath{-}{\ensuremath{\eta}}^{\ensuremath{'}}$ mixing. We discuss the theoretical uncertainty related to the hadronic matrix elements. We also consider these decay modes in a new physics scenario with a single universal extra dimension, studying the dependence of branching ratios and decay distributions on the compactification scale ${R}^{\ensuremath{-}1}$ of the extra dimension.

Vector mesonω−ϕmixing and their form factors in the light-cone quark model

The vector meson $\omega$-$\phi$ mixing is studied in two alternative scenarios with different numbers of mixing angles, i.e., the one-mixing-angle scenario and the two-mixing-angle scenario, in both the octect-singlet mixing scheme and the quark flavor mixing scheme. Concerning the reproduction of experimental data and the $Q^2$ behavior of transition form factors, one-mixing-angle scenario in the quark flavor scheme performs better than that in the octet-singlet scheme, while the two-mixing-angle scenario works well for both mixing schemes. The difference between the two mixing angles in the octet-singlet scheme is bigger than that in the quark flavor scheme.

Parametric resonance for antineutrino conversions using LSND best-fit results with a3+1flavor scheme

An analytical solution to a parametric resonance effect for antineutrinos in a $3+1$ flavor ($\mathrm{\text{active}}+\mathrm{\text{sterile}}$) scheme using multiple nonadiabatic density shifts is presented. We derive the conditions for a full flavor conversion for antineutrino oscillations ${\overline{\ensuremath{\nu}}}_{\ensuremath{\alpha}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{s}$ $(\ensuremath{\alpha}=e,\ensuremath{\mu},\ensuremath{\tau})$ under the assumption that LSND best-fits for the mixing parameters are valid in a short-baseline accelerator experiment. We show that the parametric resonance effect can be exploited to increase the effective antineutrino oscillation length by a factor of 10--40, thus sustaining a high oscillation probability for a much longer period of time than in the vacuum scenario. We propose a realistic experimental setup that could probe for this effect which leaves a signature in terms of a specific oscillation probability profile. Moreover, since the parametric resonance effect is valid in any 2 or $1+1$ flavor approximation, our results could be suggestive for future short-baseline accelerator neutrino detection experiments.

Reactor Neutrino Oscillations: KamLAND and KASKA

Nuclear reactors generate a huge number of low energy ν ¯ e 's . The reactor neutrinos have been used to study properties of neutrinos since its discovery a half century ago. Recently, KamLAND group finally discovered reactor neutrino oscillation with average baseline 180 km. According to the 3 flavor scheme of standard theory and measured oscillation parameters so far, the reactor neutrino is expected to perform another type of small oscillation at a baseline 1.8 km. KASKA experiment is a project to detect this small oscillation and to measure the last neutrino mixing angle θ 13 by using the world most powerful reactor complex, Kashiwazaki-Kariwa nuclear power station. In this proceedings, phenomena of neutrino oscillation and the two reactor oscillation experiments, KamLAND and KASKA, are introduced.

Comparison between variable flavor number schemes for charm quark electroproduction

A comparison is made between two variable flavor number schemes which describe charm quark production in deep inelastic electron-proton scattering. In these schemes the coefficient functions are derived from mass factorization of the heavy quark coefficient functions presented in a fixed flavor number scheme. Since the coefficient functions in the variable flavornumber schemes have to be finite in the limit m → 0 we have defined a prescription for those processes where the virtual photon is attached to a light quark. Furthermore one has to construct a parton density set with four active flavors (u,d,s,c) out of a set which only contains three light flavors (u,d,s). In order � 2 the two sets are discontinuous at µ = mc which follows from mass factorization of the heavy quark coefficient functions. The charm component ofthe structure function F2,c is insensitive to the different variable flavor number schemes. In particular in the threshold region they both agree with the description in fixed order perturbation theory presented in a three flavor scheme. However one version does not lead to a correct description of the threshold behavior of the longitudinal structure function FL,c. This happens when one requires a non-vanishing zeroth order longitudinal coefficient function.

Charged-current leptoproduction ofDmesons in the variable flavor scheme

We present formulae for the momentum (z) distributions of D-mesons produced in neutrino deep-inelastic scattering off strange partons. The expressions are derived within the variable flavor scheme of Aivazis et al. (ACOT-scheme), which is extended from its fully inclusive formulation to one-hadron inclusive leptoproduction. The dependence of the results on the assumed strange quark mass m_s is investigated and the m_ \to 0 limit is compared to the corresponding MSbar results. The importance of O(alpha_s) quark-initiated corrections is demonstrated for the m_s=0 case.

Scheme and scale dependence of charm production in neutrino scattering

We discuss some theoretical uncertainties in the calculation of the cross section for charm production in charged current deep inelastic neutrino scattering related to ambiguities in the treatment of terms which are singular in the limit of a vanishing charm mass. In particular we compare the so-called variable flavour scheme where these terms are absorbed in the parton distribution functions containing the charm as an active flavour, with the so-called fixed flavour scheme with no charm mass subtraction where the charm appears only in the final state of fixed-order scattering matrix elements. Using available parametrizations of parton distribution functions we find that the two schemes lead to largely differing results for separate structure functions whereas the differences cancel to a large extent in the total cross section in that kinematical region which has been measured so far.

Cold plus hot dark matter cosmology in the light of solar and atmospheric neutrino oscillations.

We explore the implications of possible neutrino oscillations, as indicated by the solar and atmospheric neutrino experiments, for the cold plus hot dark matter scenario of large-scale structure formation. We find that there are essentially three distinct schemes that can accommodate the oscillation data and which also allow for dark matter neutrinos. These include (i) three nearly degenerate (in mass) neutrinos, (ii) nondegenerate masses with ${\ensuremath{\nu}}_{\mathrm{\ensuremath{\tau}}}$ in the eV range, and (iii) a nearly degenerate ${\ensuremath{\nu}}_{\mathrm{\ensuremath{\mu}}}$-${\ensuremath{\nu}}_{\mathrm{\ensuremath{\tau}}}$ pair (in the eV range), with the additional possibility that the electron neutrino is cosmologically significant. The last two schemes invoke a ``sterile'' neutrino which is light (\ensuremath{\lesssim}eV). We discuss the implications of these schemes for \ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$-\ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ and ${\ensuremath{\nu}}_{\mathrm{\ensuremath{\mu}}}$-${\ensuremath{\nu}}_{\mathrm{\ensuremath{\tau}}}$ oscillation, and find that scheme (ii), in particular, predicts them to be in the observable range. As far as structure formation is concerned we compare the one neutrino flavor case with a variety of other possibilities, including two and three degenerate neutrino flavors. We show, both analytically and numerically, the effects of these neutrino mass scenarios on the amplitude of cosmological density fluctuations. With a Hubble constant of 50 km ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{Mpc}}^{\mathrm{\ensuremath{-}}1}$, a spectral index of unity, and ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{baryon}}$=0.05, the two and three flavor scenarios fit the observational data marginally better than the single flavor scheme. However, taking account of the uncertainties in these parameters, we show that it is premature to pick a clear winner. \textcopyright{} 1996 The American Physical Society.