Bottom Quark Pair Research Articles

Search for neutral MSSM Higgs bosons decaying into a pair of bottom quarks

A search for neutral Higgs bosons decaying into a b-bbar quark pair and produced in association with at least one additional b quark is presented. This signature is sensitive to the Higgs sector of the minimal supersymmetric standard model (MSSM) with large values of the parameter tan(beta). The analysis is based on data from proton-proton collisions at a center-of-mass energy of 8 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.7 inverse femtobarns. The results are combined with a previous analysis based on 7 TeV data. No signal is observed. Stringent upper limits on the cross section times branching fraction are derived for Higgs bosons with masses up to 900 GeV, and the results are interpreted within different MSSM benchmark scenarios, m[h,max], m[h,mod+], m[h,mod-], light-stau and light-stop. Observed 95% confidence level upper limits on tan(beta), ranging from 14 to 50, are obtained in the m[h,mod+] benchmark scenario.

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.

Study of the top-quark pair production in association with a bottom-quark pair from fast simulations at the LHC

A large number of top quarks will be produced at the Large Hadron Collider (LHC) for Run II period. This will allow us to measure the rare processes from the top sector in great details. We present the study of the top-quark pair production in association with a bottom-quark pair (ttbb) from fast simulations for the Compact Muon Solenoid (CMS) experiment. The differential distributions of ttbb are compared with the top-quark pair production with two additional jets (ttjj) and with the production in association with the Higgs (ttH), where the Higgs decays to a bottom-quark pair. The significances of ttbb process in the dileptonic and semileptonic decay mode are calculated with the data corresponding to an integrated luminosity of 10 fb-1, which is foreseen to be collected in the early Run II period. This study will be an important input in searching for new physics beyond the standard model as well as in searching for ttH process where the Yukawa coupling with the top quark can be directly measured.

Search for the standard model Higgs boson produced through vector boson fusion and decaying tobb¯

A first search is reported for a standard model Higgs boson (H) that is produced through vector boson fusion and decays to a bottom-quark pair. Two data samples, corresponding to integrated luminosities of 19.8 inverse femtobarns and 18.3 inverse femtobarns of proton-proton collisions at sqrt(s) = 8 TeV were selected for this channel at the CERN LHC. The observed significance in these data samples for a H to b bbar signal at a mass of 125 GeV is 2.2 standard deviations, while the expected significance is 0.8 standard deviations. The fitted signal strength mu = sigma/sigma[SM] = 2.8 + 1.6 - 1.4. The combination of this result with other CMS searches for the Higgs boson decaying to a b-quark pair, yields a signal strength of 1.0 +/- 0.4, corresponding to a signal significance of 2.6 standard deviations for a Higgs boson mass of 125 GeV.

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.

SUSY implications from WIMP annihilation into scalars at the Galactic Center

An excess in gamma-rays emanating from the galactic centre has recently been observed in the Fermi-LAT data. This signal can be interpreted as resulting from WIMP annihilation, with the spectrum well-fit by dark matter annihilating dominantly into either bottom-quark or Higgs pairs. Supersymmetric models provide a well-motivated framework to study the implications of this signal in these channels. With a neutralino dark matter candidate, the gamma-ray excess cannot be easily accommodated in the minimal supersymmetric model, which in any case requires tuning below the percent level to explain the observed Higgs mass. Instead we are naturally led to consider the next-to-minimal model with a singlet superfield. This not only allows for the annihilation channel into bottom-quark pairs to be implemented, but also provides new possibilities for annihilation into Higgs-pseudoscalar pairs. We show that the fit to the gamma-ray excess for the Higgs-pseudoscalar channel can be just as good as for annihilation into bottom-quark pairs. Moreover, in the parameter range of interest, the next-to-minimal supersymmetric model solves the mu-problem and can explain the 125 GeV Higgs mass with improved naturalness. We also consider an extension by adding a right-handed neutrino superfield with the right-handed sneutrino acting as a dark matter candidate. Interestingly, this allows for the annihilation into pseudoscalar pairs which also provide a good fit to the gamma-ray excess. Furthermore, in the case of a neutralino LSP, the late decay of a sneutrino NLSP can non-thermally produce the observed relic abundance. Finally, the WIMP annihilation into scalar pairs allows for the possibility of detecting the Higgs or pseudoscalar decay into two photons, providing a smoking-gun signal of the model.

Searches for electroweak neutralino and chargino production in channels with Higgs,Z, andWbosons inppcollisions at 8 TeV

Searches for supersymmetry (SUSY) are presented based on the electroweak pair production of neutralinos and charginos, leading to decay channels with Higgs, Z, and W bosons and undetected lightest SUSY particles (LSPs). The data sample corresponds to an integrated luminosity of about 19.5 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV collected in 2012 with the CMS detector at the LHC. The main emphasis is neutralino pair production in which each neutralino decays either to a Higgs boson (h) and an LSP or to a Z boson and an LSP, leading to hh, hZ, and ZZ states with missing transverse energy (ET(miss)). A second aspect is chargino-neutralino pair production, leading to hW states with ET(miss). The decays of a Higgs boson to a bottom-quark pair, to a photon pair, and to final states with leptons are considered in conjunction with hadronic and leptonic decay modes of the Z and W bosons. No evidence is found for supersymmetric particles, and 95% confidence level upper limits are evaluated for the respective pair production cross sections and for neutralino and chargino mass values.

The Hb b ¯ $$ Hb\overline{b} $$ form factor to three loops in QCD

We compute the three-loop QCD corrections to the vertex function for the Yukawa coupling of a Higgs boson to a pair of bottom quarks in the limit of vanishing quark masses. This QCD form factor is a crucial ingredient to third-order QCD corrections for the production of Higgs bosons in bottom quark fusion, and for the fully differential decay rate of Higgs bosons to bottom quarks. The infrared pole structure of the form factors agrees with the prediction from infrared factorization in QCD.

NMSSMCALC: A program package for the calculation of loop-corrected Higgs boson masses and decay widths in the (complex) NMSSM

We present the program package NMSSMCALC for the calculation of the loop-corrected NMSSM Higgs boson masses and decay widths in the CP-conserving and CP-violating NMSSM. The full one-loop corrections to the Higgs boson masses are evaluated in a mixed renormalisation scheme of on-shell and DR¯ conditions. The Higgs decay widths include the dominant higher order QCD corrections, and the decays into bottom quarks, strange quarks and τ leptons are supplemented by higher order SUSY corrections through effective couplings. All relevant off-shell decays into two massive gauge bosons, gauge and Higgs boson and Higgs pair final states as well as into heavy quark pairs are computed. The input and output files feature the SUSY Les Houches Accord so that the program can easily be linked with existing computer tools. Program summaryProgram title: NMSSMCALCCatalogue identifier: AEUE_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEUE_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 53070No. of bytes in distributed program, including test data, etc.: 329132Distribution format: tar.gzProgramming language: Fortran77.Computer: Any with a Fortran77 system.Operating system: Linux, Unix.RAM: 0.5MBClassification: 11.1.Nature of problem:Computation of the NMSSM Higgs mass spectrum including higher order corrections in a mixed renormalisation scheme of on-shell and DRbar conditions, and numerical calculation of the decay widths and branching ratios, both in the CP-conserving and in the CP-violating NMSSM. The decay widths include the dominant higher order QCD corrections and, for the neutral Higgs boson decays into a bottom quark pair, the higher order SUSY–QCD and the approximate SUSY–electroweak (elw) corrections up to one-loop accuracy. The decays into a strange quark pair include the dominant resummed SUSY–QCD corrections and the one into a τ pair the dominant resummed SUSY–elw corrections. Analogously for the charged Higgs boson the higher order SUSY corrections have been implemented for the decays into fermion pairs. In the real NMSSM, the decays into stop and sbottom pairs, respectively, contain the SUSY–QCD corrections. All relevant off-shell decays into massive gauge bosons, into gauge and Higgs bosons, into Higgs pairs and into heavy quark pairs have been taken into account. The input and output files feature the SUSY Les Houches Accord (SLHA).Solution method:The necessary input values are set in the two input files inp.dat (in the SLHA format) and bhdecay.in. The file inp.dat, where the choice between the real and the complex NMSSM can be made, is read in by CalcMasses.F, which calculates the one-loop corrected NMSSM Higgs mass spectrum. CalcMasses.F writes out all necessary parameters, masses and mixing angles in an SLHA format file slha.in, which is read in by bhdecay.f (by bhdecay c.f in the complex case). This Fortran routine then computes the decay widths and branching ratios and writes them out in an SLHA format file slha decay.out. Furthermore it writes out all parameters and mixing angles.Restrictions:At present the NMSSM Higgs spectrum is calculated at one-loop accuracy, the Higgs self-couplings at leading order, and no renormalisation group running of the input parameters is included. However, we are about to implement the two-loop corrections to the NMSSM Higgs boson masses and the Higgs self-couplings. Furthermore, the renormalisation group equations for the input parameters shall be provided in the near future. The program does not provide any distributions.Running time:Less than one second per point

Real-time b-jet identification in ATLAS

There are interesting physics processes studied at the ATLAS detector at the LHC, whose signatures contain quark jets, but no charged lepton: top quark pair production where both quarks decay hadronically, Higgs boson produced in vector boson fusion and decaying to a bottom quark pair, or supersymmetric signatures with no charged lepton. Data for these processes can only be collected using jet-based triggers. As LHC instantaneous luminosity increases, jet transverse-energy thresholds need to be raised, lowering the signal acceptance for a wide range of signatures. However, the need to raise jet thresholds can be made less pressing if jets originating from bottom quarks are identified as such (b-tagging) already at the trigger stage if at least one jet originates from a bottom quark. ATLAS started using b-jet real-time identification in 2011, increased their performance in 2012 and plans to use even more advanced taggers in 2015.

Phenomenology of E 6-inspired leptophobic Z ′ boson at the LHC

We study collider phenomenology of a leptophobic $Z'$ boson existing in eight scenarios of the $E_6$ grand unified theory, differing in particle embeddings. We first review the current bound on the $Z'$ mass $m_{Z'}$ based upon the LHC data of $pp\to t\bar{t}$ process at 8 TeV collisions with an integrated luminosity of 19.6 fb$^{-1}$. Most scenarios have a lower bound of about 1 TeV. However, this constraint does not apply to the case where $m_{Z'} < 2 m_t$, and other methods need to be employed for this lower mass regime. Using existing UA2 constraints and dijet data at the LHC, we find that only one of the eight scenarios is excluded at 95\% confidence level. No bound can be obtained from $Wjj$ and $Zjj$ measurements. We propose to use the photon associated production of the $Z'$ boson that subsequently decays into a pair of bottom quarks, $pp\to Z'\gamma \to b\bar{b}\gamma$, at the LHC to explore the constraints in the lower mass regime. We compute the expected signal significance as a function of $m_{Z'}$ using detailed simulations of signal and irreducible background events. We find constraints for two more scenarios using the 8-TeV data and taking appropriate kinematical cuts. We also show the discovery reach for each scenario at the 14-TeV LHC machine.

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 partner probes of extended Higgs sectors

Natural theories of the weak scale often include fermionic partners of the top quark. If the electroweak symmetry breaking sector contains scalars beyond a single Higgs doublet, then top partners can have sizable branching ratios to these extended Higgs sector states. In fact, top partner decays may provide the most promising discovery mode for such scalars, especially given the large backgrounds to direct and associated production. In this paper, we present a search strategy for top partner decays to a charged Higgs boson and a bottom quark, focusing on the case where the charged Higgs dominantly decays to third-generation quarks to yield a multi-b final state. We also discuss ways to extend this search to exotic neutral scalars decaying to bottom quark pairs.

Azimuthal correlation among jets produced in association with a bottom or top quark pair at the LHC

Angular correlation of jets produced in association with a massive scalar, vector or tensor boson is crucial in the determination of their spin and CP properties. We study jet angular correlations in events with a high mass bottom quark pair or a top quark pair and two jets at the LHC, whose cross-section is dominated by the virtual gluon fusion sub-processes when appropriate kinematic selection cuts (vector-boson fusion cuts) are applied. We evaluate helicity amplitudes for sub-processes initiated by qq, qg and gg collisions in the limit where the intermediate gluons are collinear to the initial partons. We first obtain a general expression for the azimuthal angle correlations among the dijets and t t-bar or b b-bar, in terms of the gg to t t-bar or b b-bar helicity amplitudes in the real gluon approximation of the full matrix elements, and find simple analytic expressions in the two kinematic limits, the production of a heavy quark pair near the threshold, and in the relativistic limit where the invariant mass of the heavy quark pair is much larger than the quark mass. For b b-bar + 2 jets we find strong azimuthal angle correlations which are distinct from those expected for events with a CP-even or odd scalar boson which may decay into a b b-bar pair. For t t-bar + 2 jets we find that the angular correlations are similar to that of a CP-odd scalar+2 jets near the threshold M_(t t-bar) ~ 2 m_t, while in the relativistic limit they resemble the distribution for b b-bar + 2 jets. These correlations in the standard QCD processes will help establish the experimental technique to measure the spin and CP properties of new particles produced via gluon fusion at the LHC.

Search for the Standard Model Higgs boson produced in association with top quarks and decaying to $b\bar b$ at √s= 7 TeV

A search for a Higgs boson produced in association with a pair of top quarks (t¯) and decaying into a pair of bottom quarks (H→ b¯ b) is presented. The analysis uses 4.7 fb −1 of pp collision data at √ s = 7 TeV collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The search is focused on the semileptonic decay mode of the t¯ t system which, together with the H → bdecay, typically results in a final state signature characterised by one high transverse momentum isolated electron or muon, high transverse missing momentum and six jets, of which four jets originate from b quarks. Events are classified in nine different topologies depending on their jet and b-tagged jet multiplicities in order to improve the sensitivity of the search. No significant excess of events above the background expectation is observed and 95% confidence- level upper limits on the production cross section times branching ratio, �(t¯ tH)× BR(H→ b¯), are derived for a Higgs boson with a mass between 110 and 140 GeV. For a Higgs boson with a mass of 125 GeV, an observed (expected) 95% confidence-level upper limit of 13.1 (10.5) times the Standard Model cross section is obtained.

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.

J/ψ Measurements with the ALICE Experiment at the LHC

The ALICE detector is the dedicated heavy-ion experiment at the Large Hadron Collider (LHC) at CERN. It provides excellent capabilities to study quarkonium production in proton-proton (pp) and heavy-ion (A-A) collisions in the kinematic range of |y| < 0.9 and 2.5 < y < 4.0 down to a transverse momentum of zero. Quarkonia, bound states of heavy (charm or bottom) quark anti-quark pairs such as the J/ψ are expected to be produced by hard processes. Thus they will provide insights into the earliest and hottest stages of A-A collisions where the formation of a Quark-Gluon Plasma is expected. Results of the inclusive and prompt J/psi production cross section in pp collisions at =2.76 and 7 TeV will be presented. A polarization measurement in the forward region and the dependence of the J/ψ yield on the charged particle multiplicity in pp collisions will be discussed. The nuclear modification factor (RAA) at forward rapidity in Pb-Pb collisions at =2.76 TeV will be shown and its implications discussed.

Theoretical predictions for charm and bottom production at the LHC

Abstract We present predictions for a variety of single-inclusive observables that stem from the production of charm and bottom quark pairs at the 7 TeV LHC. They are obtained within the FONLL semi-analytical framework, and with two “Monte Carlo + NLO” approaches, MC@NLO and POWHEG. Results are given for final states and acceptance cuts that are as close as possible to those used by experimental collaborations and, where feasible, are compared to LHC data.

Search for the Standard Model Higgs Boson Decaying to abb¯Pair in Events with Two Oppositely Charged Leptons Using the Full CDF Data Set

We present a search for the standard model Higgs boson produced in association with a Z boson in data collected with the CDF II detector at the Tevatron, corresponding to an integrated luminosity of 9.45 fb(-1). In events consistent with the decay of the Higgs boson to a bottom-quark pair and the Z boson to electron or muon pairs, we set 95% credibility level upper limits on the ZH production cross section times the H→bb branching ratio as a function of Higgs boson mass. At a Higgs boson mass of 125 GeV/c(2), we observe (expect) a limit of 7.1 (3.9) times the standard model value.