Quark Forward-backward Asymmetry Research Articles

Jet charge: A new tool to probe the anomalous [formula omitted] couplings at the EIC

We propose to probe the Zbb¯ interactions at the Electron-Ion Collider (EIC) by utilizing the average jet charge weighted single-spin asymmetry AebQ, which is induced by different cross sections of a left-handed and right-handed electron beam scattering off an unpolarized proton beam in the neutral current deeply-inelastic scattering processes with one observed b-tagged jet. This novel observable at the EIC is sensitive to the axial-vector component of the Zbb¯ coupling, providing similar information as the gg→Zh cross section measurement at the high-luminosity Large Hadron Collider, and is complementary to the single-spin asymmetry measurement at the EIC which is sensitive to the vector component of the Zbb¯ coupling. We show that the apparent degeneracy of the allowed Zbb¯ coupling in its vector and axial-vector components, implied by the Large Electron-Positron Collider (LEP) and the Stanford Linear Collider (SLC) precision electroweak data, can be broken by the AebQ measurement at the EIC. With a large enough integrated luminosity collected at the EIC, the measurement of AebQ could also resolve the long-standing discrepancy between bottom quark forward-backward asymmetry AFBb at the LEP and the Standard Model prediction, with a strong dependence on the b-tagging efficiency.

The anomalous [formula omitted] couplings at the HERA and EIC

To resolve the long-standing discrepancy between the precision measurement of bottom quark forward-backward asymmetry at LEP/SLC and the Standard Model prediction, we propose a novel method to probe the Zbb¯ coupling by measuring the single-spin asymmetry Aeb of the polarized lepton cross section in neutral current DIS processes with a b-tagged jet at HERA and EIC. Depending on the tagging efficiency of the final state b-jet, the measurement of Aeb at HERA can already partially break the degeneracy found in the anomalous Zbb¯ coupling, as implied by the LEP and SLC precision electroweak data. In the first year run of the EIC, the measurement of Aeb can already break the degeneracy, due to its much larger luminosity and higher electron beam polarization. With enough integrated luminosity collected at the EIC, it is possible to either verify or exclude the LEP data and resolve the AFBb puzzle. We also discuss the complementary roles between the proposed Aeb measurement at EIC and the measurement of gg→Zh cross section at the HL-LHC in constraining the anomalous Zbb¯ coupling.

Anomalous Zbb[over ¯] Couplings: From LEP to LHC.

The bottom quark forward-backward asymmetry (A_{FB}^{b}) data at LEP exhibits a long-standing discrepancy with the standard model prediction. We propose a novel method to probe the Zbb[over ¯] interactions through gg→Zh production at the LHC, which is sensitive to the axial-vector component of the Zbb[over ¯] couplings. The apparent degeneracy of the anomalous Zbb[over ¯] couplings implied by the LEP precision electroweak measurements seems to be resolved by the current 13TeV LHC Zh data, which is, however, dominated by the two data points with high transverse momentum of Z boson whose central values are in conflict with the standard model prediction. We also show the potential of the HL-LHC to either verify or exclude the anomalous Zbb[over ¯] couplings observed at LEP through measuring the Zh production rate at the HL-LHC, and this conclusion is not sensitive to the possible new physics contribution induced by top quark or Higgs boson anomalous couplings in the loop.

Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at sqrt{s} = 13 TeV

The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric left({hat{d}}_{mathrm{t}}right) and chromomagnetic left({hat{mu}}_{mathrm{t}}right) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb−1. The linearized variable {A}_{mathrm{FB}}^{(1)} is used to approximate the asymmetry. Candidate mathrm{t}overline{mathrm{t}} events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for mathrm{t}overline{mathrm{t}} final states. The values found for the parameters are {A}_{mathrm{FB}}^{(1)}={0.048}_{-0.087}^{+0.095}{left(mathrm{stat}right)}_{-0.029}^{+0.020}left(mathrm{syst}right),{hat{mu}}_{mathrm{t}}=-{0.024}_{-0.009}^{+0.013}{left(mathrm{stat}right)}_{-0.011}^{+0.016}left(mathrm{syst}right), and a limit is placed on the magnitude of left|{hat{d}}_{mathrm{t}}right| < 0.03 at 95% confidence level.

Vector-like quark interpretation for the CKM unitarity violation, excess in Higgs signal strength, and bottom quark forward-backward asymmetry

Due to a recent more precise evaluation of Vud and Vus, the unitarity condition of the first row in the Cabibbo-Kobayashi-Maskawa (CKM) matrix: |Vud|2 +|Vus|2 +|Vub|2 = 0.99798 ± 0.00038 now stands at a deviation more than 4σ from unity. Furthermore, a mild excess in the overall Higgs signal strength appears at about 2σ above the standard model (SM) prediction, as well as the long-lasting discrepancy in the forward-backward asymmetry {mathcal{A}}_{mathrm{FB}}^b in Zto boverline{b} at LEP. Motivated from the above three anomalies we investigate an extension of the SM with vector-like quarks (VLQs) associated with the down-quark sector, with the goal of alleviating the tension among these datasets. We perform global fits of the model under the constraints coming from the unitarity condition of the first row of the CKM matrix, the Z -pole observables {mathcal{A}}_{mathrm{FB}}^b , Rb and Γhad, Electro-Weak precision observables ∆S and ∆T , B-meson observables {B}_d^0-{overline{B}}_d^0 mixing, B+→ π+ℓ+ℓ− and B0→ μ+μ− and direct searches for VLQs at the Large Hadron Collider (LHC). Our results suggest that adding VLQs to the SM provides better agreement than the SM.

Predictions for the top-quark forward-backward asymmetry at high invariant pair mass using the principle of maximum conformality

The D0 collaboration at FermiLab has recently measured the top-quark pair forward-backward asymmetry in $\bar p p \to t \bar{t} X$ reactions as a function of the $t\bar{t} $ invariant mass $M_{t\bar{t}}$. The D0 result for $A_{\rm FB}(M_{t\bar{t}}>650\, {\rm GeV})$ is smaller than $A_{\rm FB}(M_{t\bar{t}})$ obtained for small values of $M_{t\bar{t}}$, which may indicate an "increasing-decreasing" behavior for $A_{\rm FB}(M_{t\bar{t}}>M_{\rm cut})$. This behavior is not explained using conventional renormalization scale-setting, even by a next-to-next-to-leading order (N$^2$LO) QCD calculation -- one predicts a monotonically increasing behavior. In the conventional scale-setting method, one simply guesses a single renormalization scale $\mu_r$ for the argument of the QCD running coupling and then varies it over an arbitrary range. However, the conventional method has inherent difficulties. ...... In contrast, if one fixes the scale using the Principle of Maximum Conformality (PMC), the resulting pQCD predictions are renormalization-scheme independent since all of the scheme-dependent $\{\beta_i\}$-terms in the QCD perturbative series are resummed into the QCD running couplings at each order. ...... In this paper we show that if one applies the PMC to determine the top versus anti-top quark forward-backward asymmetry by properly using the pQCD predictions up to N$^2$LO level, one obtains the predictions without renormalization scheme or scale ambiguities. ...... In addition, the PMC prediction for $A_{\rm FB}(M_{t\bar{t}}> M_{\rm cut})$ shows an "increasing-decreasing" behavior for increasing values of $M_{\rm cut}$ which is not observed in the NLO and N$^2$LO predictions for $A_{\rm FB}(M_{t\bar{t}}> M_{\rm cut})$ with conventional scale-setting. This behavior could be tested by the future more precise measurements at the LHC.

Top quark pair production at NNLO in the quark-antiquark channel

We present the derivation of the NNLO two-parton final state contributions to top pair production in the quark-antiquark channel proportionnal to the leading colour factor $N_c^2$. Together with the three and four-parton NNLO contributions presented in a previous publication, this enables us to complete the phenomenologically most important NNLO corrections to top pair hadro-production in this channel. We derive this two-parton contribution using the massive extension of the NNLO antenna subtraction formalism and implement those corrections in a parton-level event generator providing full kinematical information on all final state particles. In addition, we also derive the heavy quark contributions proportional to $N_h$. Combining the new leading-colour and heavy quark contributions together with the light quark contributions derived previously, we present NNLO differential distributions for LHC and Tevatron. We also compute the differential top quark forward-backward asymmetry at Tevatron and find that our results are in good agreement with the measurements by the D0 collaboration.

Top quark forward-backward asymmetry at approximateNLO3

I calculate the top quark forward-backward asymmetry at the Tevatron in both the laboratory frame and the $t{\bar t}$ rest frame. I show that soft-gluon corrections are the dominant contribution to the asymmetry and closely approximate exact results through next-to-next-to-leading order (NNLO). I present a calculation of the asymmetry including approximate next-to-next-to-next-to-leading-order (N$^3$LO) soft-gluon contributions from next-to-next-to-leading-logarithm (NNLL) resummation as well as electroweak corrections. Thus approximate N$^3$LO (aN$^3$LO) results are obtained, which significantly enhance and improve previous NNLO results. The theoretical aN$^3$LO result for the top quark forward-backward asymmetry at the Tevatron in the laboratory frame is ($6.8 \pm 0.3$)\%, and in the $t{\bar t}$ rest frame it is ($10.0 \pm 0.6$)\% which is in excellent agreement with recent Tevatron data.

Top quark forward-backward asymmetry in e+ e- annihilation at next-to-next-to-leading order in QCD.

We report on a complete calculation of electroweak production of top-quark pairs in e+ e- annihilation at next-to-next-to-leading order in quantum chromodynamics. Our setup is fully differential in phase space and can be used to calculate any infrared-safe observable. Especially we calculated the next-to-next-to-leading-order corrections to the top-quark forward-backward asymmetry and found sizable effects. Our results show a large reduction of the theoretical uncertainties in predictions of the forward-backward asymmetry, and allow for a precision determination of the top-quark electroweak couplings at future e+ e- colliders.

Top-quark processes at NLO in production and decay

We describe the implementation of top production and decay processes in the parton-level Monte Carlo program MCFM. By treating the top quark as being on-shell, we can factorize the amplitudes for top-pair production, s-channel single-top production, and t-channel single-top production into the product of an amplitude for production and an amplitude for decay. In this way we can retain all spin correlations. Both the production and the decay amplitudes are calculated consistently at next-to-leading order in . The full dependence on the b-quark mass is also kept. Phenomenological results are presented for various kinematic distributions at the LHC and for the top quark forward–backward asymmetry at the Tevatron.

Top quark forward-backward asymmetry in the little Higgs model

A sizable difference in top quark pair forward backward asymmetry (AFB) is observed at Tevatron. The discrepancy triggers many new physics beyond the standard model (SM) and then constrains the parameter spaces in them. In this article we calculate the AFB of the top-pair production at Tevatron up to next to leading order (NLO) in the little Higgs model (LHM). We find that the contribution of ZH can be large enough to make up the gap between SM prediction and experimental data. Then, the parameter space for the couplings between ZH and quarks are constrained. Thus, this model can fulfill the experimental data, both in AFB and in cross section.

Tracking down the top quark forward-backward asymmetry with monotops

We revisit the possibility that the top quark forward-backward asymmetry arises from the on-shell production and decay of scalar top partners to $t\overline{t}+{\mathrm{E\ensuremath{\llap{\not\;}}}}_{T}$. Although the asymmetry is produced by $t$-channel exchange of a light mediator, the model remains unconstrained by low energy atomic parity violation tests. An interesting connection to the active neutrino sector through a type-I seesaw operator helps to evade stringent monojet constraints and opens up a richer collider phenomenology. After performing a global fit to top data from both the Tevatron and the LHC, we obtain a viable region of parameter space consistent with all phenomenological and collider constraints. We also discuss the discovery potential of a predicted monotop signal and related lepton charge asymmetry at the LHC.

On physics beyond standard model

In this review we do not try to cover all the aspects of physics beyond the standard model (BSM), instead our latest understanding on the BSM will be presented: i) The Higgs sector is likely related to BSM, which can be confirmed at current running large hadron collider (LHC) or the future colliders. Furthermore we pointed out that spontaneous CP violation can be closely related to the lightness of the Higgs boson. ii) Top quark forward-backward asymmetry, which was measured by Tevatron, might be the sign of BSM. We proposed a new color-octet particle ZC to account for the observation and ZC can be further studied at the LHC. iii) If dark matter (DM) is utilized to accommodate astrophysical observations, it ought to be observed at the high energy LHC and DM produced at colliders should be the smoking gun signal. iv) Lithium puzzle might also be the sign of the BSM. We briefly review the newly proposed solution to Lithium puzzle, i.e., the existence of non-thermal component during the big bang nuclei-synthesis (BBN). The possible origins of the non-thermal component can be dark matter or the new accelerating mechanism of normal particles.

One-loop effects on top pair production in the littlest Higgs model with T-parity at the LHC

In this work, we systematically investigate the one-loop corrections to $t\bar{t}$ production in the littlest Higgs model with T-parity (LHT) at the LHC for $\sqrt{s}=8,14$ TeV. We focus on the effects of LHT particles on $t\bar{t}$ cross section, polarization asymmetries, spin correlation and charge asymmetry at the LHC. We also study the top quark forward-backward asymmetry at Tevatron and its correlations with the LHC observables. We found that: (1) the contributions of the LHT particles to $t\bar{t}$ production can only reach about 1% at the 14 TeV LHC. Meanwhile, the anomalous top quark forward-backward asymmetry at Tevatron is also hardly to be explained in the LHT model. (2) the parity violating asymmetries in $t\bar{t}$ production, such as left-right asymmetry $|A_{LR}|$ and the polarization $|P_t|$ can respectively reach 1.1% and 0.5%, which may have the potential to provide a signal of LHT at the LHC.

Mass and rapidity dependent top quark forward-backward asymmetry in the effective Lagrangian approach

We study the invariant mass and rapidity dependent top quark forward-backward asymmetry from the effective Lagrangian viewpoint. The Wilson coefficients are constrained by the experimental observations and the concrete models that reproduce the low energy effective Lagaragians are considered. Some of them are disfavored and others relatively favored. For each cases, we estimate the appropriacy of the effective Lagrangian approach.

Zero and one jet combined next-to-leading order analysis of the top quark forward-backward asymmetry

We present an analysis of the forward-backward asymmetry in the production of top quark pairs at the Tevatron collider. We use novel Monte Carlo methods for merging matrix elements and parton showers to combine NLO QCD predictions for tt and tt+jet production. Theoretical uncertainties are quantified in detail. We find agreement with experimental data on the transverse momentum dependence of the asymmetry.

Measurement of the top quark forward-backward production asymmetry and its dependence on event kinematic properties

We present new measurements of the inclusive forward-backward ttbar production asymmetry, AFB, and its dependence on several properties of the ttbar system. The measurements are performed with the full Tevatron data set recorded with the CDF II detector during ppbar collisions at sqrt(s) = 1.96 TeV, corresponding to an integrated luminosity of 9.4 fb^(-1). We measure the asymmetry using the rapidity difference Delta-y=y_(t)-y_(tbar). Parton-level results are derived, yielding an inclusive asymmetry of 0.164+/-0.047 (stat + syst). We observe a linear dependence of AFB on the top-quark pair mass M(ttbar) and the rapidity difference |Delta-y| at detector and parton levels. Assuming the standard model, the probabilities to observe the measured values or larger for the detector-level dependencies are 7.4*10^(-3) and 2.2*10^(-3) for M(ttbar) and |Delta-y| respectively. Lastly, we study the dependence of the asymmetry on the transverse momentum of the ttbar system at the detector level. These results are consistent with previous lower-precision measurements and provide additional quantification of the functional dependencies of the asymmetry.

Full $\mathcal{O}(\alpha)$ electroweak radiative corrections to $e^{+}e^{-} \rightarrow t \bar{t} \gamma$ with GRACE-Loop

We present the full \(\mathcal{O}(\alpha)\) electroweak radiative corrections to the process \(e^{+}e^{-} \rightarrow t \bar{t} \gamma\) at the International Linear Collider (ILC). The computation is performed with the help of the GRACE-Loop system. We present the total cross-section and the top quark forward–backward asymmetry (A FB) as a function of the center-of-mass energy and compare them with the process \(e^{+}e^{-} \rightarrow t \bar{t}\). We find that the value of A FB in \(t \bar{t} \gamma\) production is larger than A FB in \(t\bar{t}\) production. It is an important result for the measurement of the top quark forward–backward asymmetry at the ILC. Applying a structure function method, we also subtract the QED correction to gain the genuine weak correction in both the α scheme and the G μ scheme (\(\delta_{W}^{G_{\mu}}\)). We obtain numerical values for \(\delta_{W}^{G_{\mu}}\) which are changing from 2 % to −24 % when we vary the center-of-mass energy from 360 GeV to 1 TeV.

Same-sign tops: a powerful diagnostic test for models of new physics

We study the connection between the same sign top (SST) and the top quark forward-backward asymmetry $A^t_{FB}$. We find that a large class of new physics models that have been proposed to account for the $A^t_{FB}$ lead to SST quark production rate much larger than the observed rate at the LHC and consequently are severely constrained or ruled out. Our model independent, general, operator analysis shows that none of the tree-level flavor-changing operators are able to explain $A^t_{FB}$ and simultaneously remain consistent with the same-sign top-quark production constraints from the LHC data.

Data driving the top quark forward-backward asymmetry with a lepton-based handle

We propose that, within the standard model, the correlation between the $t\overline{t}$ forward-backward asymmetry ${A}_{t\overline{t}}$ and the corresponding lepton-based asymmetry ${A}_{l}$---at the differential level---is strong and rather clean both theoretically and experimentally. Hence a combined measurement of the two distributions as a function of the lepton ${p}_{T}$, a direct and experimentally clean observable, would lead to a potentially unbiased and normalization-free test of the standard model prediction. To check the robustness of our proposal, we study how the correlation is affected by mismeasurement of the $t\overline{t}$ system transverse momenta, acceptance cuts, and scale dependence and compare the results of mcfm, powheg (with and without pythia showering), and sherpa's csshower in first-emission mode. We find that the shape of the relative differential distribution ${A}_{l}({p}_{T}^{l})[{A}_{t\overline{t}}({p}_{T}^{l})]$ is only moderately distorted, hence supporting the usefulness of our proposal. Beyond the first emission, we find that the correlation is not accurately captured by lowest-order treatment. We also briefly consider other differential variables such as the system transverse mass and the canonical $t\overline{t}$ invariant mass. Finally, we study new physics scenarios where the correlation is significantly distorted and therefore can be more readily constrained or discovered using our method.