Optimization of the analysis of single top-quark production at the Large Hadron Collider (LHC)

Several steps of an investigation of single top-quark production at the Large Hadron Collider (LHC) are discussed. A number of general problems associated with basic steps of this analysis are discussed. The investigations are performed for the CMS (Compact Muon Solenoid detector) Collaboration, and detailed results will be presented in CMS publications. Three event generators used by the CMS Collaboration in simulating single top-quark production (SingleTop, MC@NLO, and a generator based on the MadGraph package) are compared in the present study. Schemes for optimizing the observation of a single top quark on the basis of the neural-network method are presented. The first steps in searches for possible anomalous vector couplings in the Wtb vertex that manifest themselves in processes of single top-quark production are described.     

QCD effects and b-tagging at LEP I

We analyze the impact of using b-tagged samples in studying non-Abelian effects due to QCD in e ⁺ e ^? → 4jet events at ${sqrt s=M_{Z^0}}$, using angular variable analyses and comparisons with e ⁺ e ^? → 3jetγ events. We find that QCD effects are largely enhanced in b-quark samples with respect to ‘unflavoured’ ones, where energy-ordering is used to distinguish between gluon and quark jets. We show that the b-quark mass influences the angular distributions significantly and should not be neglected.     

Large charmless yield inB decays and inclusiveB decay puzzles

In recent studies of inclusiveB decays, it has been suggested that eitherB mesons decay much more copiously to final states with no open charm than currently assumed, orB(D ⁰ → K^?π⁺) has to be reduced significantly. This note takes the experimentalB(D ⁰ → K^?π⁺) at its face value and estimatesB(b → no open charm) using complementary methods: one accounts for thec quark inb → c transitions, the other accounts for the c quark inb → ccs transitions. Through cancellation of errors, the average gives our best estimate ofB(b → no open charm), and the difference measures the consistency. The results of the methods are consistent with each other, strongly suggesting a much enhancedB(b → no open charm). This observation indicates that non-perturbative QCD effects are probably causing a sizable fraction of theb → ccs transitions to be seen as charmlessb → s processes, contrary to smaller traditional expectations. This mechanism has generally been overlooked and may explain the existing experimental data within the framework of the standard model. We then briefly discuss implications on baryon production governed byb → ccs processes, rare hadronicB decays and CP violation studies.     

QCD effects andb-tagging at LEP I

We analyze the impact of usingb-tagged samples in studying non-Abelian effects due to QCD ine ⁺ e ^? → 4jet events at √s=M _z ⁰, using angular variable analyses and comparisons withe ⁺ e ^? → 3jetγ events. We find that QCD effects are largely enhanced inb-quark samples with respect to ‘unflavoured’ ones, where energy-ordering is used to distinguish between gluon and quark jets. We show that theb-quark mass influences the angular distributions significantly and should not be neglected.     

Light-front quark model and analysis of the spectrum of photons from inclusive B → X s γ decays

The electroweak-decay width Γ(B → X _s γ) is investigated in a light-front (LF) constituent quark model. A new partonlike formula is derived that establishes a simple relation between Γ(B → X _s γ) and the b → sγ decay width. A treatment of the b quark as an on-mass-shell particle and the inclusion of effects that arise from the transverse motion of the b quark in the B meson are basic features of this approach. Adopting different b-quark LF distribution functions, both phenomenological ones and those that are derived from constituent quark models, and neglecting perturbative corrections, we compute the photon energy spectra and the moments of the shape function. It is shown that the LF approach can be matched completely with a heavy-quark expansion (HQE), provided that the constituent b-quark mass is redefined in a way similar to that used in HQE to define the pole mass of the b quark. In this way, the correction to first order in 1/m _b can be eliminated from the total width in agreement with the general statement of HQE. We also show that the photon energy spectra calculated in the LF approach agree well with those obtained in the model of Altarelli et al., provided that the same distribution function is used as an input in both cases. Despite the simplicity of the model, our results are in fairly good agreement both with HQE predictions and with available experimental data.     

CombinedB → X s Ψ andB → X s ν c decays as a test of factorization

We calculate the inclusive decaysB → X _s Ψ andB → X _s ν _c using factorization assumption. To investigate the bound state effect of the decaying B meson in these inclusive decays we take into account the motion of theb quark using a Gaussian momentum distribution model. The resulting correction to free quark decay approximation is around 6% at most. Utilizing a potential model evaluation of the ratio of the decay constantsf _νc ²/f _Ψ ², it is shown that the ratioR = Γ(B → X _sν_c)/Γ(B → X_sΨ) can be used a possible test of factorization assumption.     

Direct photon production beyond leading order in QCD

The significance of the quark-quark scattering process (quark+quark→quark+quark+photon) for the production of large-q_T real photons is discussed in the framework of perturbative QCD. To extract the finite contribution of this process to the differential cross section dσ/dy d²q_T (hadron 1+hadron 2→photon+anything) we define the gluon distribution and the quark-to-photon fragmentation function beyond the leading approximation. The calculations are performed consistently in the dimensional regularization scheme. Our numerical estimates show the resulting finite qq→qqγ contribution to be a small (order α_s/2π) correction in comparison with the basic QCD subprocesses.     

Thread-scalable evaluation of multi-jet observables

We have implemented the leading-color n-gluon amplitudes using the Berends–Giele recursion relations on a multi-threaded GPU. Speed-up factors between 150 and 300 are obtained compared to the CPU-based implementation of the same event generator. In this first paper, we study the feasibility of a GPU-based event generator with an emphasis on the constraints imposed by the hardware. Some studies of Monte Carlo convergence and accuracy are presented for PP→2,…,10 jet observables using of the order of 10¹¹ events.     

Is SUSY accessible by direct dark matter detection?

We performed a combined analysis of the parameter space of the Minimal Supersymmetric Standard Model (MSSM) taking into account cosmological and accelerator constraints including those from the radiative b → sγ decay measured by the CLEO collaboration. Special attention is paid to the event rate, R, of direct dark matter neutralino detection. We have found domains of the parameter space with R - 5 ? 10 events/kg/day. This would be within the reach of current dark matter experiments. The b → sγ data do not essentially reduce these large event rate domains of the MSSM parameter space.     

Spin,helicity and flavour patterns in exclusive decays of bottom mesons into baryon-antibaryon pairs

I use the constituent quark model to calculate the exclusive decays of bottom mesons into ground state baryon-antibaryon pairs mediated by the weakb→c and b→u transitions. TheW-exchange contributions to these decays are found to be negligible. For theW-decay contributions I find that the simple colour-flavour structure of the 1/2⁺ and 3/2⁺ ground state baryons together with the current-current nonleptonic Hamiltonian leads to many simple and testable predictions. Notable among these is the predicted vanishing of decays involving 3/2⁺ baryons as in \(\overline {B^0 } (B^ - ) \to 3/2^ + \overline {1/2^ + } \) and \(\overline {B^0 } (B^ - ) \to 3/2^ + \overline {3/2^ + } \) . An experimental verification of these selection rules would test the chirality of the basicb→c andb→u transitions. I also derive predictions for the polarization of the final state baryons depending on the assumed mechanism of quark pair creation. These predictions can be checked in the cases where the final state baryons are weakly decaying spin 1/2 hyperons and charmed baryons.     

Exclusive c → s,d Semileptonic Decays of Spin-1/2 and Spin-3/2 cb Baryons

We present results for exclusive semileptonic decay widths of ground state spin-1/2 and spin-3/2  cb baryons corresponding to a c → s, d transition at the quark level. The relevance of hyperfine mixing in spin-1/2 cb baryons is shown. Our form factors are compatible with heavy quark spin symmetry constraints obtained in the infinite heavy quark mass limit.     

Measurement of the top quark mass in dileptonic top quark pair decays with √s = 7 TeV ATLAS data

The top quark mass in dileptonic top quark pair decays was measured using 4.7 fb^–1 of √s = 7 TeV proton-proton (pp) collision data recorded by the ATLAS experiment at the LHC in 2011. The event topology is characterized by the presence of two charged leptons, at least two neutrinos and several jets, two of which originate from bottom quarks. Using the template method and the m _?b observable, defined as the average invariant mass of the two charged lepton plus b-jet pairs in each event, the top quark mass is measured to be 173.09 ± 0.64(stat) ± 1.50(syst) GeV. This proceeding is based on a preliminary result, which has been superseded meanwhile.     

Heavy flavor production at high energyep colliders

The total production rates for heavy quark pairs due to gauge boson fusion processes at high energyep colliders are evaluated. At HERA,b \(\bar t\) production dominates overt \(\bar t\) production form _t ≧60 GeV and is observable up tom _t ?80(90)GeV where the number of expectedb \(\bar t\) events is about 15(10) for ∝L=200pb^?1. Including the contributions fromep→WX→btX the total number of expectedbt events amounts to about 50 events form _t ?80GeV. The influence of thresholds for heavy quark pair production is also studied for the relevant structure functionsF _i (x,Q ²) and shown to contribute to the measured scaling violations. All these effects are sensitive to the heavy quark massesand to the shape of the gluon distribution which can thus be tested experimentally by analyzing heavy quark pair signals.     

Energy dependence of the differences between the quark and gluon jet fragmentation

Three jet events arising from decays of the Z boson, collected by the DELPHI detector, were used to measure differences in quark and gluon fragmentation. Gluon jets were anti-tagged by identifying b quark jets. Unbiased quark jets came from events with two jets plus one photon. Quark and gluon jet properties in different energy ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same energy in symmetric three jet event topologies were also compared. Using three independent methods, the average value of the ratio of the mean charged multiplicities of gluon and quark jets is $$< r >=1.241 pm 0.015 (stat.)pm 0.025 (syst.).$$ Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent energy. The string effect has been observed in fully symmetric three jet events. The measured ratio R_γ of the charged particle flow in the qq? inter-jet region of the qq?g and qq?γ samples agrees with the perturbative QCD expectation. The dependence of the mean charged multiplicity on the hadronic center-of-mass energy was analysed in photon plus n-jet events. The value for α_s(M _Z) determined from these data using a QCD prediction with corrections at leading and next-to-leading order is $$←pha_s(M_Z)=0.116pm 0.003 (stat.)pm 03009 (syst.).$$     

Spotting glueballs in collinear gluon jets

We suggest looking for pure gluon hadronic states (glueballs) in ?→ 3g → low spherically final state (collinear gluon jets). In these events one gluon has the maximum energy, $\text{M?}\text{2}$, favouring fragmentation into a glueball. The signature for a true C = G + glueball is its prominence at the ? resonance in e⁺e^? →? → (glueball → four charged pions) + … and its absence in q$\text{q}$ jets off (we do not expect significant quark fragmentation into glueballs).     

High-spin states in 22Ne populated in the 14C(12C, α)reaction

The CDF Collaboration reported an excess in the production of two jets in association with a W . We discuss constraints on possible new particle state interpretations of this excess. The fact of no statistically significant deviation from the SM expectation for Z +dijet events in CDF data disfavors the new particle explanation. We show that the nucleon intrinsic strange quarks provide an important contribution to the W boson production in association with a single top quark production. Such W +t single top quark production can contribute to the CDF W +dijet excess, thus the nucleon intrinsic quarks can provide a possible explanation to the CDF excess in W +dijet but not in Z +dijet events.     

Chiral SU(2)×SU(2) model with infrared quark confinement

An SU(2)×SU(2) chiral quark model describing the properties and interaction of pions and scalar and vector mesons is considered. The confinement of quarks is introduced in the model by means of an infrared cut-off in the one-loop quark diagrams. This cutoff gives rise to the elimination of the unphysical thresholds of the quark-antiquark pair production. The π-a ₁ transitions are taken into account. The model conserves all low-energy theorems. The masses of mesons and the widths of the decays ρ → 2π and σ → 2π are calculated.     

CP violation in t → W + b decays in two-Higgs doublet model

Due to the large mass of top quark, CP violation in the top-quark decay is sensitive to the interactions mediated by Higgs bosons. We consider CP violation in t → W ⁺ b decay by calculating consistently in unitary gauge the CP-violating up-down asymmetry of the leptons from W boson decays in t → W ⁺ b, defined by Grz?dkowski and Gunion, in the two-Higgs doublet model with CP-violating neutral sector. The asymmetry is shown to be at most of the order of (1–4) × 10^-4 for tan β = 1.0, where tan β is the ratio of vacuum expectation values for the two neutral Higgs bosons.     

Quark and gluon jets in the breit frame of lepton-nucleon scattering

Gluon bremsstrahlung,q→Gq, and quark pair production from gluons,laggy, in deep inelastic reactions is investigated in the Breit frame (moving alongQ in the laboratory). These QCD effects diminish the overall forward momentum. There are also events with asingle largep _⊥ forward jet. One spectacular class of events is predicted in which no forward going hadrons emerge, in the Breit frame. These effects are not mimicked by nonperturbative (limitedp _⊥) parton jets at large but attainableQ ².