Top Forward-backward Asymmetry Research Articles

Looking for BSM physics using top-quark polarization and decay-lepton kinematic asymmetries

We explore beyond Standard Model (bSM) physics signatures in the $l+jets$ channel of $t\overline{t}$ pair production process at the Tevatron and the LHC. We study the effects of bSM physics scenarios on the top quark polarization and on the kinematics of the decay leptons. To this end, we construct asymmetries using the lepton energy and angular distributions. Further, we find their correlations with the top polarization, net charge asymmetry and top forward backward asymmetry. We show that used together, these observables can help discriminate effectively between SM and different bSM scenarios which can lead to varying degrees of top polarization at the Tevatron as well as the LHC. We use two types of coloured mediator models to demonstrate the effectiveness of proposed observables, an $s$-channel axigluon and a $u$-channel diquark.

Colored bosons on top FBA and angular cross section for [formula omitted] production

With full data set that corresponds to an integrated luminosity of 9.4 fb−1, CDF has updated the top-quark forward–backward asymmetry (FBA) as functions of rapidity difference |Δy| and tt¯ invariant mass Mtt¯. Beside the sustained inconsistency between experiments and standard model (SM) predictions at large |Δy| and Mtt¯, an unexpected large first Legendre moment with a1=0.39±0.108 is found. In order to solve the large top FBA, we study the contributions of color triplet scalar and color octet vector boson. We find that the top FBA at |Δy|>1 (Mtt¯>450 GeV) in triplet and octet model could be enhanced to be around 40% (30%) and 26% (20%), whereas the first Legendre moment is a1Di=0.38 and a1Axi=0.23, respectively.

Top-flavored dark matter and the forward-backward asymmetry

We propose a simple model where dark matter (DM) carries top flavor and couples to the Standard Model through the top quark within a framework of minimal flavor violation. Top-flavored DM can explain the anomalous top forward-backward asymmetry observed at the Tevatron, while remaining consistent with other top observables at colliders. By virtue of its large coupling to the top, DM acquires a sizable loop coupling to the $Z$ boson, and the relic density is set by annihilation through the $Z$. We also discuss constraints from current direct detection searches, emphasizing the role of spin-dependent searches to probe this scenario.

B → D (*) τν and B → τν in chiral U(1)′ models with flavored multi Higgs doublets

Abstract We discuss semileptonic and leptonic B decays, B → D (∗) τν and B → τν, in the chiral U(1)′ models which were proposed by the present authors in the context of the top forward-backward asymmetry $ \left( {A_{\mathrm{FB}}^t} \right) $ observed at the Tevatron. In these models, extra Higgs doublets with nonzero U(1)′ charges are required in order to make the realistic mass matrix for up-type quarks. Then the extra (pseudo)scalars contribute to $ A_{\mathrm{FB}}^t $ with large flavor-changing Yukawa couplings involving top quark. The contribution of the charged Higgs to $ A_{\mathrm{FB}}^t $ is negligible, but it may significantly affect B decays: especially, B → D (*) τν and B → τν. We investigate constraints on the B decays, based on the recent results in BaBar and Belle experiments, and discuss the possibility that the allowed parameter region in the B decays can achieve large $ A_{\mathrm{FB}}^t $ .

Open windows for a light axigluon explanation of $ A_{{^{FB}}}^t $

The top forward-backward asymmetry ( $ A_{{^{FB}}}^t $ ) measured at the Tevatron remains one of the most puzzling outstanding collider anomalies. After two years of LHC running, however, few models for $ A_{{^{FB}}}^t $ remain consistent with LHC data. In this paper we take a detailed look at the most promising surviving class of models, namely light ( $ {m_{{G\prime }}}\lesssim 450 $ GeV), broad axigluons. We show which models simultaneously satisfy constraints from Tevatron and LHC top measurements, hadronic resonance searches, and LEP precision electroweak (PEW) observables. We consider three flavor structures: flavor-universal; down-type nonuniversal, designed to ease constraints from LHC charge asymmetry measurements; and top-type nonuniversal, designed to ameliorate constraints from PEW. We compute contributions to the PEW observables from states in the minimal UV completion of the axigluon model and demonstrate that new heavy fermions make the constraints universally more stringent, while related contributions from new scalars are much smaller, but act to relax the constraints. Paired dijet searches from ATLAS and CMS rule out all narrow axiglue models, while the LHC charge asymmetry measurement is less constraining than expected due to the high central value measured by ATLAS. Excepting the tension with the CMS charge asymmetry measurement, a broad axigluon is consistent with all data over the entire mass range we consider (50 GeV $ \lesssim {m_{{G\prime }}}\lesssim 450 $ GeV) in the flavor-universal and top-type nonuniversal models, while it is consistent for $ {m_{{G\prime }}}\gtrsim 200 $ GeV in the down-type non-universal model. The LHC charge asymmetry remains the best avenue for excluding, or observing, these models.

NLO QCD corrections to tW ′ and tZ ′ production in forward-backward asymmetry models

We consider Z' and W' models recently proposed to explain the top forward-backward asymmetry at the Tevatron. We present the next-to-leading order QCD corrections to associated production of such vector bosons together with top quarks at the Large Hadron Collider, for centre-of-mass energies of 7 and 8 TeV. The corrections are significant, modifying the total production cross-section by 30-50%. We consider the effects of the corrections on the top and vector-boson kinematics. The results are directly applicable to current experimental searches, for both the ATLAS and CMS collaborations.

Top A FB at the Tevatron vs. charge asymmetry at the LHC in chiral U(1) flavor models with flavored Higgs doublets

We consider the top forward–backward (FB) asymmetry at the Tevatron and top charge asymmetry at the LHC within chiral U(1)′ models with flavor-dependent U(1)′ charges and flavored Higgs fields, which were introduced in Ko et al. (J. High Energy Phys. 1201:147, 2012). In this model, one has to include the flavor-changing couplings of the Higgs bosons as well as the Z′ to the up-type quarks. The models could enhance not only the top forward–backward asymmetry at the Tevatron, but also the top charge asymmetry at LHC, without too large same-sign top-quark pair production rates. Also the \(m_{t\bar{t}}\) distribution at high \(m_{t\bar{t}}\) shows less deviations from the SM predictions. We identify parameter spaces for the U(1)′ gauge boson and (pseudo) scalar Higgs bosons where all the experimental data could be accommodated, including the case with about 125 GeV Higgs boson, as suggested recently by ATLAS and CMS.

Top Forward-backward asymmetry at the Tevatron vs. Charge asymmetry at the LHC in chiralU(1)’ models with flavored Higgs fields P.

An extra $U(1)'$ model with $Z'$ coupled only to the right-handed (RH) up-type quarks has been one of the popular models for the Tevatron top forward-backward asymmetry (FBA), and has been excluded by the same-sign top-pair productions at the LHC. However, the original $Z'$ model is not physical, since the up-type quarks are massless including the top quark. This disaster can be evaded if the Higgs sector is extended by including new Higgs doublets with nonzero $U(1)'$ charges. We find that some parameter regions could achieve not only the top FBA at the Tevatron, but also the charge asymmetry at the LHC without exceeding the upper limit of the same-sign top-quark pair production at the LHC. The lesson is that it is mandatory to extend the Higgs sector whenever one considers chiral gauge symmetries beyond the SM gauge group. Otherwise some fermions remain massless, and thus it is meaningless to work on phenomenology without the extra Higgs doublets with new chiral gauge charges.

Comment on the CMS search for charge-asymmetric production of [formula omitted] boson in [formula omitted] events

A reanalysis is presented on the CMS result on a search for a W′ boson that couples to the top and down quarks. The model is motivated by the Tevatron results on the forward–backward asymmetry of tt¯ pair production. In the evaluation of the theoretical cross section of pp→tt¯j, the interference effect between the SM and W′ amplitudes is shown to be important, though it is ignored in the CMS analysis. The lower mass bound on the W′ boson is relaxed from 840 GeV to 740 GeV at the 95% C.L. due to the interference effect at the leading order calculation of the cross section. The bound is also compared to the top forward–backward asymmetry.

Predictions from heavy new physics interpretation of the top forward-backward asymmetry

Abstract We derive generic predictions at hadron colliders from the large forward- backward asymmetry observed at the Tevatron, assuming the latter arises from heavy new physics beyond the Standard Model. We use an effective field theory approach to characterize the associated unknown dynamics. By fitting the Tevatron $ t\overline{t} $ data we derive constraints on the form of the new physics. Furthermore, we show that heavy new physics explaining the Tevatron data generically enhances at high invariant masses both the top pair production cross section and the charge asymmetry at the LHC. This enhancement can be within the sensitivity of the 8TeV run, such that the 2012 LHC data should be able to exclude a large class of models of heavy new physics or provide hints for its presence. The same new physics implies a contribution to the forward-backward asymmetry in bottom pair production at low invariant masses of order a permil at most.

Top quark forward-backward asymmetry and W ′-boson with general couplings

The measured forward-backward asymmetry in top pair events at the Fermilab Tevatron collider deviates significantly from the standard model expectation. Several models have been proposed to describe the observed asymmetry which grows with the rapidity difference of the top pairs and also with the $t\bar{t}$ invariant mass. The presence of a heavy charged gauge boson $W'$ with the coupling $W'-t-d$ (left-handed, right-handed, and a mixture of left and right-handed couplings) could generate the desired top forward-backward asymmetry keeping the top pair cross section consistent with the standard model prediction. Such $W'$-boson makes contribution to the electric dipole moment of the neutron through contribution to the $d$-quark electric dipole moment, recently measured charge asymmetry ($A_{C}$) by the LHC experiments, and the total cross section of top pair at the LHC and Tevatron. We show that the upper bounds on neutron and top electric dipole moments disfavour any $W'$ with a mass below 240 GeV which could explain the Tevatron forward-backward asymmetry. It is shown that the charge asymmetry provides an allowed region in the parameters space with no overlap with the allowed region where the asymmetry could be described.

Top forward-backward asymmetry and the CDFWjjexcess in leptophobicU(1)′flavor models

We construct anomaly-free leptophobic U(1)' flavor models with light Z'(~ 145 GeV). In order to allow renormalizable Yukawa interactions for the standard model chiral fermions, new Higgs doublets with nonzero U(1)' charges are introduced. Then the neutral (pseudo)scalar Higgs bosons as well as Z' contribute to the t\bar{t} and the same sign top pair productions [\sigma(t\bar{t}]) and \sigma(tt)], and one can evade the strong constraint from \sigma(tt). The top forward-backward asymmetry (A_{FB}) and Wjj excess at CDF could be accommodated by A_{FB}^{New} = 0.084 ~ 0.12 and \sigma(W j j) \lesssim O(10) pb \times \sin^2 2 \beta.

Very light axigluons and the top asymmetry

We show that very light (50--90 GeV) axigluons with flavor-universal couplings of order ${g}_{s}/3$ may explain the anomalous top forward-backward asymmetry reported by both CDF and D0 collaborations. The model is naturally consistent with the observed $t\overline{t}$ invariant mass distribution and evades bounds from light Higgs searches, LEP event shapes, and hadronic observables at the $Z$ pole. Very light axigluons can appear as resonances in multijet events, but searches require sensitivity to masses below current limits.

Mass dependent top forward–backward asymmetry in the effective Lagrangian approach: Addendum to “Model independent analysis of the forward–backward asymmetry of top quark production at the Tevatron”

Recently the CDF and the D0 Collaborations presented the data on the top forward–backward (FB) asymmetry AFB as functions of Mtt¯ and Δy≡yt−yt¯. We study these observables in the effective Lagrangian approach with dimension-6 qq¯tt¯ contact interactions, and compare with the CDF and D0 data. When we stay within the validity region of the effective Lagrangian approach, the mass dependent top FB asymmetry turns out to be smaller than the CDF data, more than 2-σ away. If this discrepancy remains in the future data with better statistics, it would imply that the effective Lagrangian approach is not adequate for the top FB asymmetry, and a new physics scale around a few hundred GeV in the t- or u-channel may be responsible for the observed top FB asymmetry.

Chiral U(1) flavor models and flavored Higgs doublets: the top FB asymmetry and the W jj

We present U(1) flavor models for leptophobic Z' with flavor dependent couplings to the right-handed up-type quarks in the Standard Model, which can accommodate the recent data on the top forward-backward (FB) asymmetry and the dijet resonance associated with a W boson reported by CDF Collaboration. Such flavor-dependent leptophobic charge assignments generally require extra chiral fermions for anomaly cancellation. Also the chiral nature of U(1)' flavor symmetry calls for new U(1)'-charged Higgs doublets in order for the SM fermions to have realistic renormalizable Yukawa couplings. The stringent constraints from the top FB asymmetry at the Tevatron and the same sign top pair production at the LHC can be evaded due to contributions of the extra Higgs doublets. We also show that the extension could realize cold dark matter candidates.

An effectiveZ′

We describe a method to couple Z' gauge bosons to the standard model (SM), without charging the SM fields under the U(1)', but instead through effective higher dimension operators. This method allows complete control over the tree-level couplings of the Z' and does not require altering the structure of any of the SM couplings, nor does it contain anomalies or require introduction of fields in non-standard SM representations. Moreover, such interactions arise from simple renormalizable extensions of the SM - the addition of vector-like matter that mixes with SM fermions when the U(1)' is broken. We apply effective Z' models as explanations of various recent anomalies: the D0 same-sign dimuon asymmetry, the CDF W+di-jet excess and the CDF top forward-backward asymmetry. In the case of the W+di-jet excess we also discuss several complementary analyses that may shed light on the nature of the discrepancy. We consider the possibility of non-Abelian groups, and discuss implications for the phenomenology of dark matter as well.

Polarized view of the top asymmetry

Recent experimental results from the CDF collaboration which study the top forward-backward asymmetry have strengthened the case that new physics is playing a role in t-tbar production. Here we propose a set of measurements, built from the charged lepton kinematics in semileptonic and fully leptonic t-tbar events, designed to further probe the underlying causes of this asymmetry both at the Tevatron and at the LHC. Using a set of conservative reference models we find that measurements of the charged lepton asymmetry, top polarization, and t-tbar spin correlation can establish the existence of new physics and distinguish between competing models both at the Tevatron and the LHC. At the Tevatron, discrimination between models is possible at the three sigma level. At the LHC, we demonstrate that a top forward-backward asymmetry can be established at roughly three sigma in the first five inverse-femtobarns of data, and show how competing explanations can be further disentangled.

Shaping the top asymmetry

We study different profiles of the distribution of the top forward–backward asymmetry, as a function of the invariant mass of the tt¯ pair. We show that they can be reproduced by one or more light colour octets, while keeping moderate departures of the tt¯ cross section and invariant mass distributions with respect to the Standard Model predictions at Tevatron and LHC.

Color & weak triplet scalars, the dimuon asymmetry in B s decay, the top forward-backward asymmetry, and the CDF dijet excess

The new physics required to explain the anomalies recently reported by the D0 and CDF collaborations, namely the top forward-backward asymmetry (FBA), the like-sign dimuon charge asymmetry in semileptonic b decay, and the CDF dijet excess, has to feature an amount of flavor symmetry in order to satisfy the severe constrains arising from flavor violation. In this paper we show that, once baryon number conservation is imposed, color & weak triplet scalars with hypercharge $Y=1/3$ can feature the required flavor structure as a consequence of standard model gauge invariance. The color & weak triplet model can simultaneously explain the top FBA and the dimuon charge asymmetry or the dimuon charge asymmetry and the CDF dijet excess. However, the CDF dijet excess appears to be incompatible with the top FBA in the minimal framework. Our model for the dimuon asymmetry predicts the observed pattern $h_d\ll h_s$ in the region of parameter space required to explain the top FBA, whereas our model for the CDF dijet anomaly is characterized by the absence of beyond the SM b-quark jets in the excess region. Compatibility of the color & weak triplet with the electroweak constraints is also discussed. We show that a Higgs boson mass exceeding the LEP bound is typically favored in this scenario, and that both Higgs production and decay can be significantly altered by the triplet. The most promising collider signature is found if the splitting among the components of the triplet is of weak scale magnitude.

Top forward-backward asymmetry with generalZ′couplings

The measurement of the top forward-backward asymmetry in $\ttbar$ production measured at the Tevatron shows deviation from the Standard Model prediction. A $ u \to t$ transition via a flavor changing $Z^{\prime}$ can explain the data. We show that left handed $t_Lu_LZ^{\prime}$ couplings can be constrained from $B_{d,s}$ mixing while the constrains on the right handed couplings $t_R u_R Z^{\prime}$ vanish in the limit of $m_u \to 0$. We then consider the most general form of the $t uZ^{\prime}$ interaction which includes vector-axial vector as well as tensor type couplings and study how these couplings affect the top forward-backward asymmetry.