Backward Asymmetry Research Articles

Forward–backward [formula omitted] asymmetry from anomalous stop pair production

We analyze a simple Standard Model (SM) extension with only two new light fields: a scalar partner of the top t˜ (with mass above mt) and a light neutral fermion χ0 (with mass of a few GeV), coupled to SM quarks via Yukawa interaction. We show that such model can lead to a significant enhancement of the forward–backward asymmetry in tt¯ production at the Tevatron via the additional tt¯ pairs produced from t˜t˜† decays. The model satisfies existing constraints on new-physics searches both at low and high energies, and could even address the cosmological dark-matter abundance. The implications for future searches at the LHC are briefly outlined.

Tevatron asymmetry of tops in a [formula omitted], [formula omitted] model

Recent results from the CDF Collaboration indicate that the top-pair forward–backward asymmetry is largest in regions of high rapidity difference |Δy| and invariant mass Mtt¯. We show that experimental observations can be explained by our previously proposed Asymmetric Left–Right Model (ALRM). The gauge symmetry U′(1)×SU′(2)×SU(2) is broken by a triplet Higgs in the primed sector. The W′ boson has a (t,d) right-handed coupling and the Z′ boson has diagonal fermion couplings. We determine the model parameters to be MW′=700 GeV, MZ′=1 TeV, and charged current coupling g2′=3. The W′ and Z′ total decay widths are of O(100 GeV). The signals from Z′→tt¯ and W′→tb at the LHC will test the model.

THE ELICITED PRODUCTION OF KOREAN RELATIVE CLAUSES BY HERITAGE SPEAKERS

In response to new theoretical claims and inconclusive empirical findings regarding relative clauses in East Asian languages, this study examined the factors relevant to relative clause production by Korean heritage speakers. Gap position (subject vs. object), animacy (± animate), and the topicality of head nouns (± topicalization) were manipulated as experimental variables, and factors that appear to have been confounded in previous studies (e.g., context, proficiency) were controlled for or measured. Data were collected from Korean native and heritage speakers using an elicited production task. Group results failed to show a subject advantage, but individual analysis revealed that the effect of gap position varied for speakers at different levels of Korean proficiency. Results from the topicalized lead-in condition as well as error analysis revealed two different sources related to these patterns: Traditional asymmetry (i.e., subject advantage) appears to reflect the noun phrase accessibility hierarchy effect, whereas backward asymmetry (i.e., object advantage) seems to stem from word order strategy. Proficiency scores exhibited a modest correlation with the different individual response patterns. However, an animacy effect was not observed. The implications of these findings are discussed in terms of a general learning strategy, language universals based on informational prominence, the role of proficiency, and the effect of task demands.

Signals of additional Z′ boson in e + e − → W + W − at the ILC with polarized beams

We consider the possibility of fingerprinting the presence of heavy additional Z' bosons that arise naturally in extensions of the standard model such as E_6 models and left-right symmetric models, through their mixing with the standard model Z boson. By considering a class of observables including total cross sections, energy distributions and angular distributions of decay leptons we find significant deviation from the standard model predictions for these quantities with right-handed electrons and left-handed positrons at \sqrt{s}=800 GeV. The deviations being less pronounced at smaller centre of mass energies as the models are already tightly constrained. Our work suggests that the ILC should have a strong beam polarization physics program particularly with these configurations. On the other hand, a forward backward asymmetry and lepton fraction in the backward direction are more sensitive to new physics with realistic polarization due to interesting interplay with the neutrino t- channel diagram. This process complements the study of fermion pair production processes that have been considered for discrimination between these models.

New measurement of the [formula omitted] decay

A sample of 3120 K±→π±μ+μ− decay candidates with (3.3±0.7)% background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a detailed study of the decay properties. The branching ratio was measured to be BR=(9.62±0.25)×10−8. The form factor W(z), where z=(Mμμ/MK)2, was parameterized according to several models. In particular, the slope of the linear form factor W(z)=W0(1+δz) was measured to be δ=3.11±0.57. Upper limits of 2.9×10−2 and 2.3×10−2 on possible charge asymmetry and forward–backward asymmetry were established at 90% CL. An upper limit BR(K±→π∓μ±μ±)<1.1×10−9 was established at 90% CL for the rate of the lepton number violating decay.

The study of the contribution of the LHT model to [formula omitted] coupling

In the framework of the Littlest Higgs model with T-parity (LHT), we study the contributions of the new particles to Z b b ¯ couplings at one-loop level. Based on these results, we further study the branching ratio R b and the unpolarized forward–backward asymmetry A FB b . We find that the correction of the new particles to Z b b ¯ couplings is mainly on the left-handed coupling and has small part of the parameter space to alleviate the deviation between theoretical predictions and experimental values. The precision measurement value of R b can give severe constraints on the relevant parameters. The constraints from the precision measurement value of A FB b are very weak.

Model independent analysis of the forward–backward asymmetry for the B→K 1 μ + μ − decay

The sensitivity of the zero position of the forward–backward asymmetry $\mathcal{A}_{\mathit{FB}}$ for the exclusive $B\rightarrow K_{1}(1270)\mu^{+}\mu^{-}$ decay is examined by using most general non-standard four-fermion interactions. Our analysis shows that the zero position of the forward–backward asymmetry is very sensitive to the sign and size of the Wilson coefficients corresponding to the new vector type interactions, which are the counter partners of the usual Standard Model operators but have opposite chirality. In addition to these, the other significant effect comes from the interference of scalar–pseudoscalar and tensor type operators. These results will not only enhance our theoretical understanding of the axial vector mesons but will also serve as a good tool to look for physics beyond the SM.

Probe a family non-universal [formula omitted] boson effects in [formula omitted] decay

Motivated by the recent measurement on B ( B ¯ s → ϕ μ + μ − ) by CDF Collaboration, we study the effects of a family non-universal Z ′ boson on rare semileptonic B ¯ s → ϕ μ + μ − decay. In our evaluations, we analyze the dependences of the dimuon invariant mass spectrum and normalized forward–backward asymmetry on Z ′ couplings and show that these observables are highly sensitive to new Z ′ contributions. Three limiting scenarios are presented in the detailed analyses. Numerically, within the allowed ranges of Z ′ couplings under the constraints from B ¯ s − B s mixing, B → π K , B ¯ d → ( X s , K , K ⁎ ) μ + μ − decays and so on, B ( B ¯ s → ϕ μ + μ − ) and A FB ( L ) ( B ¯ s → ϕ μ + μ − ) could be enhanced by about 96 % and 17 % ( 133 % ) respectively at most by Z ′ contributions. However, B ( B ¯ s → ϕ μ + μ − ) is hardly to be reduced. Furthermore, the zero crossing in A FB ( B ¯ s → ϕ μ + μ − ) spectrum at low dimuon mass always exists.

Forward–backward asymmetry of top quark in unparticle physics

The updated CDF measurement of the forward–backward asymmetry AFB in the top quark production pp¯→tt¯ at Tevatron (with s=1.96 TeV) shows a deviation of 2σ from the value predicted by the Standard QCD Model. We present calculation of this quantity in the scenario, where colored unparticle physics contributes to the s-channel of the process, and obtain the regions in the plane of the unparticle parameters λ and dU, which give the values of the AFB and of the total tt¯ production cross section compatible with the present measurements.

ON MASS LIMIT FOR CHIRAL COLOR SYMMETRY G'-BOSON FROM TEVATRON DATA ON $t\bar t$ PRODUCTION

The contributions of G'-boson predicted by the chiral color symmetry of quarks to the cross section [Formula: see text] and to the forward–backward asymmetry [Formula: see text] of [Formula: see text] production at the Tevatron are calculated with account of the difference of the strengths of the [Formula: see text] and [Formula: see text] interactions. The results are analyzed in dependence on two free parameters of the model, the mixing angle θG and G' mass mG'. The G'-boson contributions to [Formula: see text] and [Formula: see text] are shown to be consistent with the Tevatron data on [Formula: see text] and [Formula: see text], the allowed region in the mG' - θG plane is discussed and around mG' = 1.2 TeV , θG = 14° the region of 1σ consistency is found.

Heavy-to-light currents at NNLO in SCET and semi-inclusive [formula omitted] decay

We perform the two-loop matching calculation for heavy-to-light currents from QCD onto soft-collinear effective theory for the complete set of Dirac structures. The newly obtained matching coefficients enter several phenomenological applications, of which we discuss heavy-to-light form factor ratios and exclusive radiative decays, as well as the semi-inclusive decay B ¯ → X s ℓ + ℓ − . For this decay, we observe a significant shift of the forward–backward asymmetry zero and find q 0 2 = ( 3.34 − 0.25 + 0.22 ) GeV 2 for an invariant mass cut m X cut = 2.0 GeV .

Kaluza–Klein gluon and b-jet forward–backward asymmetry

The forward–backward asymmetry of b-quark jets on the Z-pole measured at LEP/SLD experiments shows us −2.8-σ deviation from the Standard Model (SM) prediction. We examine a possibility of Kaluza–Klein (KK) gluon to explain the AFBb data in a scenario based on the warped extra dimension model by Randall and Sundrum. In this scenario, the KK gluon strongly couples to b-quark by an appropriate choice of the bulk quark mass terms. We find that the AFBb data could be explained if the KK gluon mass is few hundred GeV. Constraints on our scenario from the hadron collider experiments are discussed.

Charge asymmetries in [formula omitted] at the ϕ resonance

We consider the forward–backward pion charge asymmetry for the e+e−→π+π−γ process. At tree level we consider bremsstrahlung and double resonance contributions. Although the latter contribution is formally sub-leading, it is enhanced at low dipion invariant mass due to ρ resonant effects. We consider also four alternative models to describe the final state radiation at the loop level: Resonance Chiral Perturbation Theory, Unitarized Chiral Perturbation Theory, Kaon Loop Model and Linear Sigma Model. The last three models yield results compatible with experimental data. The Kaon Loop Model requires an energy dependent phase to achieve the agreement.

W polarisation beyond helicity fractions in top quark decays

We calculate the density matrix for the decay of a polarised top quark into a polarised W boson and a massive b quark, for the most general Wtb vertex arising from dimension-six gauge-invariant effective operators. We show that, in addition to the well-known W helicity fractions, for polarised top decays it is worth defining and studying the transverse and normal W polarisation fractions, that is, the W polarisation along two directions orthogonal to its momentum. In particular, a rather simple forward–backward asymmetry in the normal direction is found to be very sensitive to complex phases in one of the Wtb anomalous couplings. This asymmetry, which indicates a normal W polarisation, can be generated for example by a P-odd, T-odd transition electric dipole moment. We also investigate the angular distribution of decay products in the top quark rest frame, calculating the spin analysing powers for a general Wtb vertex. Finally we show that, using a combined fit to top decay observables and the tW cross section, at LHC it will be possible to obtain model-independent measurements of all the (complex) Wtb couplings as well as the single top polarisation. Implications for spin correlations in top pair production are also discussed.

NLO QCD corrections to top quark pair production in association with one hard jet at hadron colliders

We compute the QCD corrections to the production of a t t ¯ pair in association with one hard jet at the Tevatron and the LHC, using the method of generalized D-dimensional unitarity. Top quark decays are included at leading order in perturbative QCD. We present kinematic distributions of top quark decay products in lepton plus jets and dilepton final states at the Tevatron and the LHC, using realistic selection cuts. We confirm a strong reduction of the top quark forward–backward asymmetry in p p ¯ → t t ¯ j at next-to-leading order, first observed by Dittmaier, Uwer and Weinzierl. We argue that there is a natural way to understand this reduction and that it does not imply a breakdown of the perturbative expansion for the asymmetry.

Model independent analysis of the forward–backward asymmetry of top quark production at the Tevatron

Motivated by a possible anomaly in the forward–backward (FB) asymmetry of top quark (AFB) observed at the Tevatron, we perform a model independent analysis on qq¯→tt¯ using an effective Lagrangian with dim-6 four-quark operators. We derive necessary conditions on new physics structures and the couplings that are consistent with the tt¯ production cross section and AFB measured at the Tevatron, and discuss possible new physics scenarios that could generate such dim-6 operators.

Mass-degenerate heavy vector mesons at hadron colliders

We study the LHC phenomenology of two mass-degenerate heavy gauge bosons with the same quantum numbers as Z and γ. We give a leading-order estimate of the number of events expected in Drell–Yan processes in terms of the parameters of the model (mass and coupling) and of the LHC machine specifications (integrated luminosity and energy). We consider the feasibility of measuring a forward–backward asymmetry for various choices of the parameters and estimate the potential reach. We comment on how the results may affect future collider design and apply our results to a specific model of electro-weak symmetry breaking by way of example.

Radiative and semileptonic B decays involving higher K-resonances in the final states

We study the radiative and semileptonic B decays involving a spin-J resonant \(K_{J}^{(*)}\) with parity (−1)J for \(K_{J}^{*}\) and (−1)J+1 for K J in the final state. Using large energy effective theory (LEET) techniques, we formulate \(B\to K_{J}^{(*)}\) transition form factors in the large recoil region in terms of two independent LEET functions \(\zeta_{\perp}^{K_{J}^{(*)}}\) and \(\zeta_{\parallel}^{K_{J}^{(*)}}\), the values of which at zero momentum transfer are estimated in the BSW model. According to the QCD counting rules, \(\zeta_{\perp,\parallel}^{K_{J}^{(*)}}\) exhibit a dipole dependence in q 2. We predict the decay rates for \(B\to K_{J}^{(*)}\gamma\), \(B\to K_{J}^{(*)}\ell^{+}\ell^{-}\) and \(B\to K_{J}^{(*)}\nu \bar{\nu}\). The branching fractions for these decays with higher K-resonances in the final state are suppressed due to the smaller phase spaces and the smaller values of \(\zeta^{K_{J}^{(*)}}_{\perp,\parallel}\). Furthermore, if the spin of \(K_{J}^{(*)}\) becomes larger, the branching fractions will be further suppressed due to the smaller Clebsch–Gordan coefficients defined by the polarization tensors of the \(K_{J}^{(*)}\). We also calculate the forward–backward asymmetry of the \(B\to K_{J}^{(*)}\ell^{+}\ell^{-}\) decay, for which the zero is highly insensitive to the K-resonances in the LEET parametrization.

A simple event weighting technique: optimizing the measurement of the forward-backward asymmetry of Drell-Yan dilepton and top-antitop pairs at hadron colliders

We describe a simple technique for optimizing the extraction of the forward-backward asymmetry ($A_{fb}$) of Drell-Yan lepton pairs ($e^+e^-$,$ \mu^+\mu^-$) produced in $\bar{p}p$ and $pp$ collisions at hadron colliders. The method employs simple event weights which are functions of the rapidity and $cos\theta$ decay angle of the lepton pair. It yields the best estimate of the acceptance corrected parton level ($\bar{q}q$) forward backward asymmetry as a function of final state dilepton mass ($M_{\ell\ell}$). Typically, when compared to the simple count method, the technique reduces the statistical errors by 20% for $\bar{p}p$, and 40% for $pp$ collisions, respectively. The technique can be used to search for new high mass and large width Z' bosons which may be best detected through the observation of deviations from the Standard Model expectation for the forward-backward asymmetry. In addition, we derive expressions for the QCD angular coefficients for Drell-Yan events.

TeV scale seesaw model and a flavorfulZ′at the LHC

Small neutrino masses and their large mixing angles can be generated at the TeV scale by augmenting the standard model with an additional generation-dependent, anomaly-free $U(1{)}_{\ensuremath{\nu}}$ symmetry, in the presence of three right-handed neutrinos. The ${Z}^{\ensuremath{'}}$ gauge boson associated with the breaking of the $U(1{)}_{\ensuremath{\nu}}$ symmetry can be produced at the LHC. The flavorful nature of the ${Z}^{\ensuremath{'}}$ can be established by measuring its nonuniversal couplings to the charged leptons as determined by the lepton's $U(1{)}_{\ensuremath{\nu}}$ charges, which also govern the neutrino flavor structure. While the LHC has the potential of discovering the ${Z}^{\ensuremath{'}}$ up to ${M}_{{Z}^{\ensuremath{'}}}=4.5\text{ }\text{ }\mathrm{TeV}$ with $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data at the center of mass energy $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, to establish the flavorful nature of the ${Z}^{\ensuremath{'}}$ requires much higher integrated luminosity. For our benchmark parameters that are consistent with neutrino oscillation data, at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, a $5\ensuremath{\sigma}$ distinction between the dielectron and dimuon channels for ${M}_{{Z}^{\ensuremath{'}}}=3\text{ }\text{ }\mathrm{TeV}$ requires $500\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data. We find that the forward backward asymmetry distributions can also be useful in distinguishing the dielectron and dimuon channels in the low invariant mass and transverse momentum regions.