Bottom Quark Pair Production Research Articles

B-hadron production at the LHC from bottom-quark pair production at NNLO+PS

The production of B hadrons is among the most abundant fundamental QCD processes measured at the LHC. We present for the first time predictions for this process accurate to next-to-next-to-leading order in QCD perturbation theory by simulating bottom-quark pair production at this accuracy matched to parton showers. Our novel results are in good agreement with experimental data for the production of different types of B hadrons from ATLAS, CMS and LHCb at 7 TeV and/or 13 TeV, including various fiducial cross sections as well as single- and double-differential distributions, and 13 TeV/7 TeV cross-section ratios.

Bottom-quark forward-backward and charge asymmetries at hadron colliders

Predictions are made for the forward-backward and charge asymmetries in bottom-quark pair production at hadron colliders. Tree-level exchanges of electroweak (EW) gauge bosons dominate the Standard Model (SM) contribution to the asymmetry near the $Z$-pole. The mixed EW-QCD corrections are computed in an approximate way, and are found to be small in magnitude. These SM predictions are consistent with experimental results from CDF, D0, and LHCb. In particular, CDF and LHCb find that the asymmetry in the invariant mass bin containing the $Z$-pole is larger than in the adjacent bins, as predicted. Several beyond the Standard Model scenarios proposed for the top-quark forward-backward asymmetry, including a 100 GeV axigluon, are disfavored by this combination of SM predictions and measurements. On the other hand, modified $Zb\bar{b}$ couplings can explain the 2$\sigma$ discrepancy in the bottom-quark forward-backward asymmetry at LEP1, while being consistent with the results of CDF and LHCb. It is also shown that $t$-channel $W$ exchange makes a non-negligible contribution to the charm-quark charge asymmetry.

First measurement of the forward-backward asymmetry in bottom-quark pair production at high mass

We measure the particle-level forward-backward production asymmetry in $b\bar{b}$ pairs with masses $m(b\bar{b})$ larger than 150 GeV/$c^2$, using events with hadronic jets and employing jet charge to distinguish $b$ from $\bar{b}$. The measurement uses 9.5/fb of ppbar collisions at a center of mass energy of 1.96 TeV recorded by the CDF II detector. The asymmetry as a function of $m(b\bar{b})$ is consistent with zero, as well as with the predictions of the standard model. The measurement disfavors a simple model including an axigluon with a mass of 200 GeV/$c^2$ whereas a model containing a heavier 345 GeV/$c^2$ axigluon is not excluded.

Z-pole observables in an effective theory

There are two $Z$-peak observables related to the pair production of bottom quarks that show a deviation of about $2.5\ensuremath{\sigma}$ each from Standard Model expectations. While the discrepancy in the forward-backward asymmetry is a long-standing one, the tension for the second observable, namely the ratio of the partial width for a $Z$ decaying to a pair of bottom quarks to the total hadronic decay width of the $Z$, has recently gone up due to a full two-loop evaluation of the Standard Model contributions. We show how both these discrepancies may be explained in the framework of new physics that couples only to the third generation of quarks. In the paradigm of effective operators, the Wilson coefficients of some of the possible operators are already very tightly constrained by flavor physics data. However, there still remain certain operators, particularly those involving right-chiral quark fields, which can successfully explain the anomalies. We also show how the footprints of such operators may be observed at the upgraded LHC.

Linear polarization of gluons and photons in unpolarized collider experiments

Abstract We study azimuthal asymmetries in heavy quark pair production in unpolarized electron-proton and proton-proton collisions, where the asymmetries originate from the linear polarization of gluons inside unpolarized hadrons. We provide cross section expressions and study the maximal asymmetries allowed by positivity, for both charm and bottom quark pair production. The upper bounds on the asymmetries are shown to be very large depending on the transverse momentum of the heavy quarks, which is promising especially for their measurements at a possible future Electron-Ion Collider or a Large Hadron electron Collider. We also study the analogous processes and asymmetries in muon pair production as a means to probe linearly polarized photons inside unpolarized protons. For increasing invariant mass of the muon pair the asymmetries become very similar to the heavy quark pair ones. Finally, we discuss the process dependence of the results that arises due to differences in color flow and address the problem with factorization in case of proton-proton collisions.

Top quark forward-backward asymmetry in R-parity violating supersymmetry

The interaction of bottom squark-mediated top quark pair production, occurring in the R-parity violating minimal supersymmetric standard model (MSSM), is proposed as an explanation of the anomalously large $t\bar{t}$ forward-backward asymmetry (FBA) observed at the Tevatron. We find that this model can give a good fit to top quark data, both the inclusive and invariant mass-dependent asymmetries, while remaining consistent (at the 2-$\sigma$ level) with the total and differential production cross-sections. The scenario is challenged by strong constraints from atomic parity violation (APV), but we point out an extra diagram for the effective down quark-Z vertex, involving the same coupling constant as required for the FBA, which tends to weaken the APV constraint, and which can nullify it for reasonable values of the top squark masses and mixing angle. Large contributions to flavor-changing neutral currents can be avoided if only the third generation of sparticles is light.

PROBING THE LEFT–RIGHT TWIN HIGGS MODEL VIA PROCESS $e^{+}e^{-}\rightarrow b\bar{b}$ AT THE ILC

In the framework of the Left–Right Twin Higgs model, we studied the bottom-quark pair production via e+e- collision at the International Linear Collider. We calculated the bottom-quark pair production cross-section at tree level and the relevant differential distributions. The relative correction of the production cross-section is significant and the result will be a sensitive probe of the Left–Right Twin Higgs model.

New color-octet axial vector boson revisited

In this paper we reexamine how to utilize the previous proposed color-octet axial-vector boson ${Z}_{\mathrm{C}}$ to explain the $3.4\ensuremath{\sigma}$ anomaly of $t\overline{t}$ forward-backward (FB) asymmetry ${A}_{\mathrm{FB}}$ for ${m}_{t\overline{t}}>450\text{ }\text{ }\mathrm{GeV}$ observed by CDF. Our numerical results indicate that the best-fit parameters are ${g}_{A}^{q}=0.07$, ${g}_{A}^{Q}=3$, and ${M}_{\mathrm{C}}=440\text{ }\text{ }\mathrm{GeV}$, which are obtained by fitting the mass dependent ${A}_{\mathrm{FB}}$ and total cross section data provided by a recent CDF measurement. Here ${g}_{A}^{q}({g}_{A}^{Q})$ and ${M}_{\mathrm{C}}$ are the axial couplings among ${Z}_{\mathrm{C}}$ with the first two (the third) generation quarks, and ${Z}_{\mathrm{C}}$ mass, respectively. We also calculate one-side forward-backward asymmetry ${A}_{\mathrm{OFB}}$ for top and bottom quark pair production at the LHC, focusing on the new contributions from ${Z}_{\mathrm{C}}$. Our studies show that ${A}_{\mathrm{OFB}}$ can be utilized to measure the properties of new particle ${Z}_{\mathrm{C}}$.

Nonperturbative enhancement of heavy quark-pair production in a strong SU(2) color field

Non-perturbative charm and bottom quark-pair production is investigated in the early stage of heavy ion collisions. Following our earlier works, the time-dependent study is based on a kinetic description of fermion-pair production in strong non-Abelian fields. We introduce time-dependent chromo-electric external field with a pulse-like time evolution, which simulates the overlap of two colliding heavy ions. The calculations is performed in a SU(2) color model with finite current quark masses. Yields of heavy quark-pairs are compared to the ones of light and strange quark-pairs. We show that the small inverse duration time of the field pulse determines the efficiency of the quark-pair production. Thus we do not see the expected suppression for heavy quark production, as follows from the Schwinger formula for constant field, but rather an enhanced heavy quark production at ultrarelativistic energies. We convert pulse duration time-dependent results into collisional energy dependence and introduce flavour-dependent energy string tensions, which can be used in string based model calculations at RHIC and LHC energies.

Search for scalar bottom quarks and third-generation leptoquarks in [formula omitted] collisions at [formula omitted

We report the results of a search for pair production of scalar bottom quarks (b˜1) and scalar third-generation leptoquarks (LQ3) in 5.2 fb−1 of pp¯ collisions at the D0 experiment of the Fermilab Tevatron Collider. Scalar bottom quarks are assumed to decay to a neutralino (χ˜10) and a b quark, and we set 95% C.L. lower limits on their production in the (mb˜1, mχ˜10) mass plane such as mb˜1>247 GeV for mχ˜10=0 and mχ˜10>110 GeV for 160<mb˜1<200 GeV. The leptoquarks are assumed to decay to a tau neutrino and a b quark, and we set a 95% C.L. lower limit of 247 GeV on the mass of a charge-1/3 third-generation scalar leptoquark.

Search for the Production of Scalar Bottom Quarks inpp¯Collisions ats=1.96 TeV

We report on a search for direct scalar bottom quark (sbottom) pair production in $p \bar{p}$ collisions at $\sqrt{s}=1.96$~TeV, in events with large missing transverse energy and two jets of hadrons in the final state, where at least one of the jets is required to be identified as originating from a $b$ quark. The study uses a CDF Run~II data sample corresponding to 2.65~fb${}^{-1}$ of integrated luminosity. The data are in agreement with the standard model. In an R-parity conserving minimal supersymmetric scenario, and assuming that the sbottom decays exclusively into a bottom quark and a neutralino, 95$\%$ confidence-level upper limits on the sbottom pair production cross section of 0.1~pb are obtained. For neutralino masses below 70~GeV/$c^2$, sbottom masses up to 230~GeV/$c^2$ are excluded at 95$\%$ confidence level.

Testing the littlest Higgs model withTparity in bottom quark pair production at high energy photon colliders

We have calculated the cross section of the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}b\overline{b}$ in the littlest Higgs model with $T$ parity (LHT). We find that, for the favorable parameters, the total cross section $\ensuremath{\sigma}({e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}b\overline{b})$ is sensitive to the breaking scale $f$, mixing parameter ${x}_{L}$, the masses of the mirror quarks ${m}_{Hi}$, and the relative correction of the LHT model is a few percent to dozens of percent. The cross section is significantly larger than the corresponding results in the standard model and in the other typical new physics models. Therefore the prediction in the LHT model is quite different from the predictions in other new physics models and such a process is really interesting in searching for the signs of the LHT model.

Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks inpp¯collisions ats=1.96 TeV

We search for direct pair production of supersymmetric top quarks and supersymmetric bottom quarks in proton-antiproton collisions at sqrt(s)=1.96 TeV, using 295 pb^-1 of data recorded by the Collider Detector at Fermilab (CDF II) experiment. The supersymmetric top (supersymmetric bottom) quarks are selected by reconstructing their decay into a charm (bottom) quark and a neutralino, which is assumed to be the lightest supersymmetric particle. The signature of such processes is two energetic heavy-flavor jets and missing transverse energy. The number of events that pass our selection for each search process is consistent with the expected standard model background. By comparing our results to the theoretical production cross sections of the supersymmetric top and supersymmetric bottom quarks in the minimal supersymmetric standard model, we exclude, at a 95% confidence level in the frame of that model, a supersymmetric top quark mass up to 132 GeV/c^2 for a neutralino mass of 48 GeV/c^2, and a supersymmetric bottom quark mass up to 193 GeV/c^2 for a neutralino mass of 40 GeV/c^2.

Precision determination of heavy quark masses and the strong coupling constant

We present a new QCD sum rule with high sensitivity to the continuum regions of charm and bottom quark pair production. Combining this sum rule with existing ones yields very stable results for the MS quark masses, mc(mc) and mb(mb). We introduce a phenomenological parametrization of the continuum interpolating smoothly between the pseudoscalar threshold and asymptotic quark regions. Comparison of our approach with recent BES data allows for a robust theoretical error estimate. The parametric uncertainty due to αs is reduced by performing a simultaneous fit to the most precise sum rules and other high precision observables. This includes a new evaluation of the lifetime of the τ lepton, ττ, serving as a strong constraint on αs. Our results are mc(mc)=1.289+0.040−0.045 GeV, mb(mb)=4.207+0.030−0.031 GeV (with a correlation of 29%), and αs(MZ)[ττ]=0.1221−0.0023+0.0026.

Large Yukawa coupling corrections to scalar quark pair production in e+e- annihilation

We calculate the large Yukawa coupling corrections to the top and bottom scalar quark pair production in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation within the minimal supersymmetric standard model. We include the vertex corrections and the corrections to the gauge boson propagator enhanced by large masses. We find that the total corrections are quite significant. In some regions of the parameter space the corrections are larger than 10%.

Search for scalar top and scalar bottom quarks in pp collisions at square root s=1.8 TeV.

We have searched for direct pair production of scalar top and scalar bottom quarks in 88 pb-1 of pp collisions at sqrt[s]=1.8 TeV with the CDF detector. We looked for events with a pair of heavy flavor jets and missing energy, consistent with scalar top (bottom) quark decays to a charm (bottom) quark and a neutralino. The numbers of events that pass our selections show no significant deviation from standard model expectations. We compare our results to the next-to-leading order scalar quark production cross sections to exclude regions in scalar quark-neutralino mass parameter space.

Parity violation in bottom quark pair production at polarized hadron colliders

Parity violation induced by the chromo-anapole form factor of the bottom quark, generated from weak corrections, is studied in polarized hadron collisions. The forward-backward asymmetry in the bottom quark pair production at polarized pp and p p colliders is evaluated in the Standard Model and in a two Higgs doublet model to examine the effects of parity violation. In the models studied, promising results are found for polarized p p colliders.

Yukawa corrections to the bottom pair production in photon-photon collisions.

The {ital O}({alpha}{ital m}{sup 2}{sub {ital t}}/{ital m}{sup 2}{sub {ital W}}) Yukawa corrections to bottom quark pair production via photon-photon fusion in TeV {ital e}{sup +}{ital e}{sup {minus}} colliders are calculated in both the general two-Higgs-doublet model and minimal supersymmetric model. We find that the corrections to the cross section of the subprocess {gamma}{gamma}{r_arrow}{ital b{bar b}} can be a few percent. The corrections to the total cross section of the process {ital e}{sup +}{ital e}{sup {minus}}{r_arrow}{gamma}{gamma}{r_arrow}{ital b{bar b}} in the beamstrahlung photon fusion mode are rather small and can be ignored at the one-loop level; however, the corrections in the Compton backscattering photon fusion mode may approach 0.1{percent} for favorable parameter values. {copyright} {ital 1996 The American Physical Society.}