Quark Forward-backward Asymmetry Research Articles

Rare B decays and Tevatron top-pair asymmetry

The recent Tevatron result on the top quark forward–backward asymmetry, which deviates from its standard model prediction by 3.4σ, has prompted many authors to build new models to account for this anomaly. Among the various proposals, we find that those mechanisms which produce via the t- or u-channel can have a strong correlation to the rare B decays. We demonstrate this link by studying a model with a new charged gauge boson, W′. In terms of the current measurements on B → πK decays, we conclude that the branching ratio for is affected most by the new effects. Furthermore, using the world average branching ratio for the exclusive B decays at the 2σ level, we discuss the allowed values for the new parameters. Finally, we point out that the influence of the new physics effects on the direct CP asymmetry in B decays is insignificant.

Top quark forward-backward asymmetry, flavor-changing neutral-current decays, and like-sign pair production as a joint probe of new physics

The anomaly of the top quark forward-backward asymmetry $A^t_{FB}$ observed at the Tevatron can be explained by the t-channel exchange of a neutral gauge boson (Z') which has sizable flavor changing coupling for top and up quarks. This gauge boson can also induce the top quark flavor-changing neutral-current (FCNC) decays and the like-sign top pair production at the LHC. In this work we focus on two models which predict such a Z', namely the left-right model and the U(1)_X model, to investigate the correlated effects on $A^t_{FB}$, the FCNC decays $t -> u V$ ($V=g,Z,\gamma$) and the like-sign top pair production at the LHC. We also pay special attention to the most recently measured $A^t_{FB}$ in the large top pair invariant mass region. We find that under the current experimental constraints both models can alleviate the deviation of $A^t_{FB}$ and, meanwhile, enhance the like-sign top pair production to the detectable level of the LHC. We also find that the two models give different predictions for the observables and their correlations, and thus they may even be distinguished by jointly studying these top quark observables.

The axigluon, a four-site model and the top quark forward–backward asymmetry at the Tevatron

Very recently, it has been shown by other authors that the CDF measurement of the top quark forward–backward asymmetry can be explained by means of a heavy and broad axigluon. In order to work, this mechanism needs that the axigluon coupling to the top quark must be different than the coupling to light quarks and both must be stronger than the one predicted in classical axigluon models. In this Letter, we argue that this kind of axigluon can be accommodated in an extended chiral color model we proposed previously. Additionally, we show that the desired features can be derived from a simple four-site model with delocalized fermions.

Direct detection and CMB constraints on the light dark matter scenario of top quark asymmetry and dijet excess at the Tevatron

We study in detail the model by Isidori and Kamenik that is claimed to explain the top quark forward-backward asymmetry at Tevatron, provide GeV-scale dark matter (DM), and possibly improve the agreement between data and theory in Tevatron $W+jj$ events. We compute the DM thermal relic density, the spin-independent DM-nucleon scattering cross section, and the cosmic microwave background constraints on both Dirac and Majorana neutralino DM in the parameter space that explains the top asymmetry. A stable light neutralino is not allowed unless the local DM density is 3--4 times smaller than expected, in which case Dirac DM with mass around 3 GeV may be possible, to be tested by the Planck mission. The model predicts a too broad excess in the dijet distribution and a strong modification of the missing ${E}_{T}$ distribution in $W+jj$ events.

B physics constraints on a flavor symmetric scalar model to account for the [formula omitted] asymmetry and Wjj excess at CDF

Recently Nelson et al. proposed an interesting flavor symmetric model to account for the top quark forward–backward asymmetry and the dijet anomaly at CDF simultaneously with just three parameters: a coupling constant of order one, and two scalar masses of 160 GeV and 220 GeV. However these fiducial values of the parameters lead to the branching ratio of a almost pure penguin B→πK decay about one hundred times larger than the experimental results. Consider also the precision electroweak constraints, the scalar masses should be at least around 500 GeV. Actually with the coupling constant larger than one, it is impossible to explain either of the two CDF measurements consistently in this model. But one may raise the charged scalar mass to, for example, 250 GeV and reduce the coupling strength to 0.6 to meet the B physics constraints. With this parameter set, the Wjj cross section is found to be in the right range. But due to the scalar mass splitting, its correction to T-parameter is about 3σ away from the precision electroweak constraints. In addition, the top quark forward–backward asymmetry should be well below 0.1 with this small coupling constant.

Top quark asymmetry from a non-Abelian horizontal symmetry

Motivated by the persistence of a large measured top quark forward-backward asymmetry at the Tevatron, we examine a model of non-Abelian flavor gauge symmetry. The exchange of the gauge bosons in the $t$-channel can give a large ${A}_{\mathrm{FB}}^{t}$ due to the forward Rutherford scattering peak. We address generic constraints on non-Abelian $t$-channel physics models including flavor diagonal resonances and potentially dangerous contributions to inclusive top pair cross sections. We caution on the general difficulty of comparing theoretical predictions for top quark signals to the existing experimental results due to potentially important acceptance effects. The first signature at the Large Hadron Collider can be a large inclusive top pair cross section, or like-sign dilepton events, although the latter signal is much smaller than in Abelian models. Deviations of the invariant mass distributions at the LHC will also be promising signatures. A more direct consistency check of the Tevatron asymmetry through the LHC asymmetry is more likely to be relevant at a later stage.

Explaining the $ t\overline t $ forward-backward asymmetry without dijet or flavor anomalies

We consider new physics explanations of the anomaly in the top quark forward-backward asymmetry measured at the Tevatron, in the context of flavor conserving models. The recently measured LHC dijet distributions strongly constrain many otherwise viable models. A new scalar particle in the antitriplet representation of flavor and color can fit the t tbar asymmetry and cross section data at the Tevatron and avoid both low- and high-energy bounds from flavor physics and the LHC. An s-channel resonance in uc to uc scattering at the LHC is predicted to be not far from the current sensitivity. This model also predicts rich top quark physics for the early LHC from decays of the new scalar particles. Single production gives t tbar j signatures with high transverse momentum jet, pair production leads to t tbar j j and 4 jet final states.

Top Quark Forward-Backward Asymmetry and Same-Sign Top Quark Pairs

The top quark forward-backward asymmetry measured at the Tevatron collider shows a large deviation from standard model expectations. Among possible interpretations, a nonuniversal Z' model is of particular interest as it naturally predicts a top quark in the forward region of large rapidity. To reproduce the size of the asymmetry, the couplings of the Z' to standard model quarks must be large, inevitably leading to copious production of same-sign top quark pairs at the energies of the Large Hadron Collider (LHC). We explore the discovery potential for tt and ttj production in early LHC experiments at 7-8 TeV and conclude that if no tt signal is observed with 1 fb⁻¹ of integrated luminosity, then a nonuniversal Z' alone cannot explain the Tevatron forward-backward asymmetry.

Top quark forward-backward asymmetry at the Tevatron and its implications at the LHC

The measurement of forward-backward asymmetry in the top and anti-top quark ($t \bar{t}$) production has been recently reconfirmed by the CDF Collaboration and shows a more than $3\sigma$ deviation from the Standard Model(SM) prediction in the large $t \bar{t}$ invariant mass region. Models with new W$^{\prime}$ or Z$^{\prime}$ bosons have been invoked to explain this deviation. In the context of these models we perform a $\chi^2$ analysis with all the available experimental numbers in different $\Delta Y$ and $M_{t \bar{t}}$ bins. We show that for the $Z^{\prime}$ model the region of parameter space which explain the Tevatron asymmetry can be probed in the same sign top production channel by Tevatron itself. Moreover, we consider a recently proposed observable, the one sided forward-backward asymmetry ($A_{OFB}$) at the LHC and conclude that both the W$^{\prime}$ and Z$^{\prime}$ models can lead to sizable $A_{OFB}$ even at the LHC running at a center of mass energy of 7 TeV for the model parameters consistent with the Tevatron measurements.

Top quark forward-backward asymmetry in the large invariant mass region

The forward-backward asymmetry (FBA) in top-pair production that was observed in 2008 gets a boost in a recent CDF publication. Not only has the FBA further been confirmed, but also distributional preferences are shown. Strikingly, the FBA is the most sizable in the large $M_{t\bar t}$ invariant mass region and in the large rapidity difference $|\Delta y|$ region. Here we used our previously proposed $t$-channel exchanged $W'$ boson to explain the new observations. We show that a new particle exchanged in the $t$-channel generically gives rise to such observations. Furthermore, we show that the proposed $W'$ can be directly produced in association with a top quark at the Tevatron and the LHC. We perform a signal-background analysis and show that such a $W'$ is readily observed at the Tevatron with a 10 fb$^{-1}$ luminosity and at the LHC-7 with just a 100 pb$^{-1}$ luminosity.

Top quark forward-backward asymmetry from the 3-3-1 model

The forward-backward asymmetry $A_{FB}$ in top quark pair production, measured at the Tevatron, is probably related to the contribution of new particles. The Tevatron result is more than a $2\sigma$ deviation from the standard model prediction and motivates the application of alternative models introducing new states. However, as the standard model predictions for the total cross section $\sigma_{tt}$ and invariant mass distribution $M_{tt}$ for this process are in good agreement with experiments, any alternative model must reproduce these predictions. These models can be placed into two categories: One introduces the s-channel exchange of new vector bosons with chiral couplings to the light quarks and to the top quark and another relies on the t-channel exchange of particles with large flavor-violating couplings in the quark sector. In this work we employ a model which introduces both s- and t-channel nonstandard contributions for the top quark pair production in proton antiproton collisions. We use the minimal version of the $SU(3)_C \otimes SU(3)_L \otimes U (1)_X$ model (3-3-1 model) that predicts the existence of a new neutral gauge boson, called $Z^\prime$. This gauge boson has both flavor-changing couplings to up and top quarks and chiral coupling to the light quarks and to the top quark. This very peculiar model coupling can correct the $A_{FB}$ for top quark pair production for two ranges of $Z^\prime$ mass while leading to cross section and invariant mass distribution quite similar to the standard model ones. This result reinforces the role of the 3-3-1 model for any new physics effect.

Axigluons cannot explain the observed top quark forward-backward asymmetry

We study an SU(3)^2 axigluon model introduced by Frampton, Shu, and Wang to explain the recent Fermilab Tevatron observation of a significant positive enhancement in the top quark forward-backward asymmetry relative to standard model predictions. First, we demonstrate that data on neutral B_d-meson mixing excludes the region of model parameter space where the top asymmetry is predicted to be the largest. Keeping the gauge couplings below the critical value that would lead to fermion condensation imposes further limits at large axigluon mass, while precision electroweak constraints on the model are relatively mild. Furthermore, by considering an extension to an SU(3)^3 color group, we demonstrate that embedding the model in an extra-dimensional framework can only dilute the axigluon effect on the forward-backward asymmetry. We conclude that axigluon models are unlikely to be the source of the observed top quark asymmetry.

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.

Explorations of the top quark forward-backward asymmetry at the Tevatron

We consider the recent measurement of the top quark forward-backward asymmetry at the Fermilab Tevatron, which shows a discrepancy of slightly more than $2\ensuremath{\sigma}$ compared to the standard model prediction. We find that $t$-channel exchange of a color sextet or triplet scalar particle can explain the measurement, while leaving the cross section for $t\overline{t}$ production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of $t\overline{t}+\mathrm{\text{jets}}$ at the LHC.

Top quark forward-backward asymmetry at the Tevatron: A comparative study in different new physics models

The top quark forward-backward asymmetry ${A}_{\mathrm{FB}}^{t}$ measured at the Tevatron is above the standard model prediction by more than $2\ensuremath{\sigma}$ deviation, which might be a harbinger for new physics. In this work we examine the contribution to ${A}_{\mathrm{FB}}^{t}$ in two different new physics models: one is the minimal supersymmetric model without $R$ parity which contributes to ${A}_{\mathrm{FB}}^{t}$ via sparticle-mediated $t$ channel process $d\overline{d}\ensuremath{\rightarrow}t\overline{t}$; the other is the third-generation enhanced left-right model which contributes to ${A}_{\mathrm{FB}}^{t}$ via ${Z}^{\ensuremath{'}}$-mediated $t$ channel or $s$ channel processes. We find that in the parameter space allowed by the $t\overline{t}$ production rate and the $t\overline{t}$ invariant mass distribution at the Tevatron, the left-right model can enhance ${A}_{\mathrm{FB}}^{t}$ to within the $2\ensuremath{\sigma}$ region of the Tevatron data for the major part of the parameter space, and in optimal case ${A}_{\mathrm{FB}}^{t}$ can reach 12% which is slightly below the $1\ensuremath{\sigma}$ lower bound. For the minimal supersymmetric model without $R$ parity, only in a narrow part of the parameter space can the ${\ensuremath{\lambda}}^{\ensuremath{'}\ensuremath{'}}$ couplings enhance ${A}_{\mathrm{FB}}^{t}$ to within the $2\ensuremath{\sigma}$ region while the ${\ensuremath{\lambda}}^{\ensuremath{'}}$ couplings just produce negative contributions to worsen the fit.

Top quark forward-backward asymmetry from newt-channel physics

Motivated by recent measurements of the top quark forward-backward asymmetry at the Tevatron, we study how t-channel new physics can contribute to a large value. We concentrate on a theory with an abelian gauge boson possessing flavor changing couplings between up and top quarks, but satisfies flavor physics constraints. Collider constraints are strong, but can be consistent with the aid of small flavor diagonal couplings. We find that M_Z' ~ 160 GeV can yield a total lab-frame asymmetry of ~18% without being in conflict with other observables. There are implications for future collider searches, including exotic top quark decays, like-sign top quark production, and detailed measurements of the top production cross section. An alternate model with a gauged non-Abelian flavor symmetry would have similar phenomenology, but lacks the like-sign top signal.

Measurement of the b quark forward–backward asymmetry around the Z0 peak using an inclusive tag

The b quark forward–backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0→bb̄ decays were selected using a combination of secondary vertex and lepton tags, and the sign of the b quark charge was determined using an inclusive tag based on jet, vertex and kaon charges. The results, corrected to the quark level, are: AFBb=0.0582±0.0153±0.0012ats=89.50GeV,AFBb=0.0977±0.0036±0.0018ats=91.26GeV,AFBb=0.1221±0.0123±0.0025ats=92.91 GeV, where the first error is statistical and the second systematic in each case. Within the framework of the Standard Model, the result is interpreted as a measurement of the effective weak mixing angle for electrons of sin2θeff,eW=0.23205±0.00068.

Measurement of the strange quark forward-backward asymmetry around the Z $^{0}$ peak

A precise measurement of the strange quark forward-backward asymmetry used 3.2M multihadronic events around the Z\(^0\) peak collected by the DELPHI experiment from 1992 to 1995. The ring imaging Cherenkov detectors in the barrel and end-cap regions identify high energy charged kaons which tag the s quark. The s quark asymmetry was measured at different centre-of-mass energies; in particular at the Z\(^0\) pole

Measurements of the b quark forward-backward asymmetry around the Z $^0$ peak using jet charge and vertex charge

The b quark forward-backward asymmetry has been measured using approximately four million hadronic Z(0) decays collected with the OPAL detector at LEP. Both jet charge and vertex charge were used to estimate whether the b quark was produced in the forward or backward thrust hemisphere. The measured values corrected to the hadron-level thrust axis areA(FB)(b) = 0.041 +/-0.021 +/-0.002 root s = 89.34 GeVA(FB)(b) = 0.0994 +/-0.0052 +/-0.0044 root s = 91.21 GeVA(FB)(b) = 0.145 +/-0.017 +/-0.007 root s = 92.91 GeVwhere in each case the first error is statistical and the second systematic. Within the framework of the Standard Model these measurements correspond to an effective weak mixing angle of sin(2) theta(W)(eff,e) = 0.2308 +/- 0.0013. The measurements are in agreement with the Standard Model and other measurements at LEP.

The forward-backward asymmetry for charm quarks at the Z pole

From 1.4 million hadronic Z decays collected by the ALEPH detector at LEP, an enriched sample of Z → cc̄ events is extracted by requiring the presence of a high momentum D ∗±. The charm quark forward-backward charge asymmetry at the Z pole is measured to be A FB 0. c = (8.0 ± 2.4) % corresponding to an effective electroweak mixing angle of sin 2 θ W eff = 0.2302 ± 0.0054.