Bottom-quark Mass Effects Research Articles

Bottom quark mass effects in associated WH production with the H→bb¯ decay through NNLO QCD

We present a computation of NNLO QCD corrections to the production of a Higgs boson in association with a $W$ boson at the LHC followed by the decay of the Higgs boson to a $b\bar{b}$ pair. At variance with previous NNLO QCD studies of the same process, we treat $b$ quarks as massive. An important advantage of working with massive $b$ quarks is that it makes the use of flavor jet algorithms unnecessary and allows us to employ conventional jet algorithms to define $b$ jets. We compare NNLO QCD descriptions of the associated $WH(b\bar{b})$ production with massive and massless $b$ quarks and also contrast them with the results provided by parton showers. We find ${\cal O}(5\%)$ differences in fiducial cross sections computed with massless and massive $b$ quarks. We also observe that much larger differences between massless and massive results, as well as between fixed-order and parton-shower results, can arise in selected kinematic distributions.

On the computation of finite bottom-quark mass effects in Higgs boson production

We present analytic results for the partonic cross-sections contributing to the top-bottom interference in Higgs production via gluon fusion at hadron colliders at NLO accuracy in QCD. We develop a method of expansion in small bottom-mass for master integrals and combine it with the usual infinite top-mass effective theory. Our method of expansion admits a simple algorithmic description and can be easily generalized to any small parameter. These results for the integrated cross-sections will be needed in the computation of the renormalization counter-terms entering the computation of finite bottom-quark mass effects at NNLO.

Power corrections in the dispersive model for a determination of the strong coupling constant from the thrust distribution

In the context of the dispersive model for non-perturbative corrections, we extend the leading renormalon subtraction to NNLO for the thrust distribution in $e^+e^-$ annihilation. Within this framework, using a NNLL+NNLO perturbative description and including bottom quark mass effects to NLO, we analyse data in the centre-of-mass energy range $\sqrt{s}=14-206$ GeV in view of a simultaneous determination of the strong coupling constant and the non-perturbative parameter $\alpha_0$. The fits are performed by matching the resummed and fixed-order predictions both in the R and the log-R matching schemes. The final values in the R scheme are $\alpha_s(M_Z) = 0.1131^{+0.0028}_{-0.0022}$, $\alpha_0(2 {\rm GeV}) = 0.538^{+0.102}_{-0.047}$.

Thrust atN3LLwith power corrections and a precision global fit forαs(mZ)

We give a factorization formula for the e+e- thrust distribution dsigma/dtau with tau=1-T based on soft-collinear effective theory. The result is applicable for all tau, i.e. in the peak, tail, and far-tail regions. The formula includes O(alphas^3) fixed-order QCD results, resummation of singular partonic alphas^j ln^k(tau)/tau terms with N^3LL accuracy, hadronization effects from fitting a universal nonperturbative soft function defined in field theory, bottom quark mass effects, QED corrections, and the dominant top mass dependent terms from the axial anomaly. We do not rely on Monte Carlo generators to determine nonperturbative effects since they are not compatible with higher order perturbative analyses. Instead our treatment is based on fitting nonperturbative matrix elements in field theory, which are moments Omega_i of a nonperturbative soft function. We present a global analysis of all available thrust data measured at center-of-mass energies Q=35 to 207 GeV in the tail region, where a two parameter fit to $\alpha_s(m_Z)$ and the first moment Omega_1 suffices. We use a short distance scheme to define Omega_1, called the R-gap scheme, thus ensuring that the perturbative dsigma/dtau does not suffer from an O(Lambda_QCD) renormalon ambiguity. We find alphas(mZ)=0.1135 \pm (0.0002)_{expt} \pm (0.0005)_{hadr} \pm (0.0009)_{pert}, with chi^2/dof=0.91, where the displayed 1-sigma errors are the total experimental error, the hadronization uncertainty, and the perturbative theory uncertainty, respectively. The hadronization uncertainty in alphas is significantly decreased compared to earlier analyses by our two parameter fit, which determines Omega_1=0.323 GeV with 16% uncertainty.

Helicity fractions ofWbosons from top quark decays at next-to-next-to-leading order in QCD

Decay rates of unpolarized top quarks into longitudinally and transversally polarized W bosons are calculated to second order in the strong coupling constant alpha_s. Including the finite bottom quark mass and electroweak effects, the Standard Model predictions for the W boson helicity fractions are F_L=0.687(5), F_+=0.0017(1), and F_-=0.311(5).

W- andZ-boson production with a massive bottom-quark pair at the Large Hadron Collider

We present total and differential cross sections for $Wb\overline{b}$ and $Zb\overline{b}$ production at the CERN Large Hadron Collider with a center-of-mass energy of $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, including next-to-leading order (NLO) QCD corrections and full bottom-quark mass effects. We also provide numerical results obtained with a center-of-mass energy of $\sqrt{s}=10\text{ }\text{ }\mathrm{TeV}$. We study the scale uncertainty of the total cross sections due to the residual renormalization- and factorization-scale dependence of the truncated perturbative series. While in the case of $Zb\overline{b}$ production the scale uncertainty of the total cross section is reduced by NLO QCD corrections, the $Wb\overline{b}$ production process at NLO in QCD still suffers from large scale uncertainties, in particular, in the inclusive case. We also perform a detailed comparison with a calculation that considers massless bottom quarks, as implemented in the Monte Carlo program MCFM. The effects of a nonzero bottom-quark mass (${m}_{b}$) cannot be neglected in phase-space regions where the relevant kinematic observable, such as the transverse-momentum of the bottom quarks or the invariant-mass of the bottom-quark pair, are of the order of ${m}_{b}$. The effects on the total production cross sections are usually smaller than the residual scale uncertainty at NLO in QCD.

NLO QCD corrections toZbb¯production with massive bottom quarks at the Fermilab Tevatron

We calculate the next-to-leading order (NLO) QCD corrections to $Zb\overline{b}$ production in hadronic collisions including full bottom-quark mass effects. We present results for the total cross section and the invariant mass distribution of the bottom-quark jet pair at the Fermilab Tevatron $p\overline{p}$ collider. We perform a detailed comparison with a calculation that considers massless bottom quarks, as implemented in the Monte Carlo program MCFM. We find that neglecting bottom-quark mass effects overestimates the total NLO QCD cross section for $Zb\overline{b}$ production at the Tevatron by about 7%, independent of the choice of the renormalization and factorization scales. Moreover, bottom-quark mass effects can impact the shape of the bottom-quark pair invariant mass distribution, in particular, in the low invariant mass region.

NLO QCD corrections toWboson production with a massiveb-quark jet pair at the Fermilab Tevatronpp¯collider

We calculate the next-to-leading order (NLO) QCD corrections to Wbb production including full bottom-quark mass effects. We study the impact of NLO QCD corrections on the total cross section and invariant mass distribution of the bottom-quark jet pair at the Fermilab Tevatron pp collider. We perform a detailed comparison with a calculation that considers massless bottom quarks. We find that neglecting bottom-quark mass effects overestimates the NLO total cross section for Wbb production at the Tevatron by about 8% independent of the choice of renormalization and factorization scale.

CTEQ6 parton distributions with heavy quark mass effects

Previously published CTEQ6 parton distributions adopt the conventional zero-mass parton scheme since the corresponding hard cross sections are universally available. For precision observables which are sensitive to charm and bottom quark mass effects, we provide in this paper an improved CTEQ6HQ parton distribution set determined in the more general variable flavor number scheme that incorporates heavy flavor mass effects. We describe in detail the QCD scheme and analysis procedure used, examine the predominant features of the new distributions, and compare them with previous distributions.