We present here a study on the determination of the effective weak mixing angle, $\sin^{2}θ^{lept}_{eff}$ from the measurement of the Forward-Backward Asymmetry with a high a statistical precision, 10$^{-4}$. To reach such a precision it is necessary to identify the electrons in the forward regions of the ATLAS detector. It is demonstrated that one can reach an electron-jet rejection of more than 100 with an efficiency on electron reconstruction better than 50%, by using a multivariate analysis.
The data set collected with the ALEPH detector from 1991 to 1995 at LEP has been analysed to measure the charm forward-backward asymmetry at the Z. Out of a total of 4.1 million hadronic Z decays, about 36000 high momentum D*+, D+ and D0 decays were reconstructed, of which 80% originate from Z -> ccbar events...
A top-quark reconstruction method for CLIC is described. The reconstructed events are used to measure the e+ e- -> t \bar{t} cross-section as well as the forward-backward asymmetry. The results suggest that the relative measurement accuracy for the cross-section will be 1.4 % and for the asymmetry 4.2 % for 1 ab^{-1} of data.
We report the first measurement of the forward-backward asymmetry and the ratios of Wilson coefficients $A_9/A_7$ and $A_{10}/A_7$ in $B \to K^* \ell^+ \ell^-$, where $\ell$ represents an electron or a muon. We observe a large integrated forward-backward asymmetry with a significance of 3.4$σ$. The results are obtained from a data sample containing 386 $\times 10^6$ $B\bar{B}$ pairs that were collected on the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider.
We present a new measurement of the forward-backward asymmetry in ttbar production in ppbar collisions in the lepton+jets channel at D0. We measure asymmetries from two different observables and unfold the data to allow the results to be compared to standard model predictions. For the unfolded asymmetry based on the Delta-y variable, we measure (19.6 +/- 6.5)%, compared with an MC@NLO-based prediction of (5.0 +/- 0.1)%. We also discuss the correlation between the asymmetry and the transverse momentum of the ttbar system.
We report measurements of the differential $q^2$ spectrum and the forward-backward asymmetry for $B \to K^{(*)} \ell^+\ell^-$, where $\ell$ represents an electron or a muon, with a data sample of 253 fb$^{-1}$ accumulated on the $Υ(4S)$ resonance with the Belle detector at KEKB. We also present measurements of the branching fractions and their ratios.
We present a measurement of forward-backward asymmetry in top quark-antiquark production in proton-antiproton collisions in the final state containing a lepton and at least four jets. Using a dataset corresponding to an integrated luminosity of $5.4\,\mathrm {fb}^{-1}$, collected by the \DZ\ experiment at the Fermilab Tevatron Collider, we measure the \ttbar\ forward-backward asymmetry to be $(9.2 \pm 3.7)$% at the reconstruction level. When corrected for detector acceptance and resolution, the asymmetry is found to be $(19.6 \pm 6.5)$%. We also measure a corrected asymmetry based on the lepto…
We report the first measurement of the forward-backward asymmetry and the ratios of Wilson coefficients in B --> K* l+ l- using 386 million BBbar pairs that were collected on the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We also present the first evidence of the decay B- --> tau- nu_tau-bar using 414 fb^-1 of data.
We have studied the measurement accuracy of the forward-backward asymmetry of the $e^+e^- \rightarrow t\bar{t}$ process in the 6-jet mode at $\sqrt{s} = 500$ GeV with the ILD detector. In the analysis the vertex charges of b-jets were used to distinguish t from $\bar{t}$ in each event. The distribution of the cosine of the reconstructed polar angle of so identified t or $\bar{t}$ showed a clear forward-backward asymmetry. After the correction for charge misidentification the forward-backward asymmetry was determined to be $A^t_{FB} = 0.334 \pm 0.0079$ for 500 fb$^{-1}$ with the beam polarizati…
We present the first measurement of the integrated forward-backward charge asymmetry in top-antitop quark pair (ttbar) production in proton-antiproton collisions in the lepton plus jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming ttbar+X production and decay, a sample of 0.9fb-1 of data, collected by the D0 experiment at the Fermilab Tevatron Collider, is used to measure the asymmetry for different jet multiplicities. The result is also used to set upper limits on ttbar+X production via a Z' resonance.
The CDF and D0 experiments at the Fermilab Tevatron have measured the asymmetry between yields of forward- and backward-produced top and antitop quarks based on their rapidity difference and the asymmetry between their decay leptons. These measurements use the full data sets collected in proton-antiproton collisions at a center-of-mass energy of $\sqrt s =1.96$ TeV. We report the results of combinations of the inclusive asymmetries and their differential dependencies on relevant kinematic quantities. The combined inclusive asymmetry is $A_{\mathrm{FB}}^{t\bar{t}} = 0.128 \pm 0.025$. The combin…
A measurement of the forward-backward asymmetry (A[FB]) of Drell-Yan lepton pairs in pp collisions at sqrt(s) = 7 TeV is presented. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 5 inverse femtobarns. The asymmetry is measured as a function of dilepton mass and rapidity in the dielectron and dimuon channels. Combined results from the two channels are also presented. The A[FB] measurement in the dimuon channel and the combination of the two channels are the first such results obtained at a hadron collider. The measured asymmetries are consistent wit…
A measurement of the forward-backward charge asymmetry in top-antitop (t\bar{t}) pair production in proton-antiproton (p\bar{p}) collisions is presented. The asymmetry is measured for different jet multiplicities in the lepton+jets final state on 0.9 fb^{-1} of data collected by the D0 experiment at the Fermilab Tevatron Collider. The result is sensitive to new physics, which is demonstrated by setting an upper limit on t\bar{t} production via heavy neutral gauge bosons (Z^{\prime}).
We report on a measurement of the mass dependence of the forward-backward charge asymmetry, A_FB, and production cross section dsigma/dM for e+e- pairs with mass M_ee>40 GeV/c2. The data sample consists of 108 pb-1 of p-pbar collisions at sqrt(s)=1.8 TeV taken by the Collider Detector at Fermilab during 1992-1995. The measured asymmetry and dsigma/dM are compared with the predictions of the Standard Model and a model with an extra Z' gauge boson.
We present a measurement of the forward-backward asymmetry in the production of $B^{\pm}$ mesons, $A_{\rm FB}(B^{\pm})$, using $B^{\pm} \rightarrow J/ψK^{\pm}$ decays in 10.4 ${\rm fb}^{-1}$ of $p\bar{p}$ collisions at $\sqrt{s} = 1.96$ TeV collected by the D0 experiment during Run II of the Tevatron collider. A nonzero asymmetry would indicate a preference for a particular flavor, i.e., $b$ quark or $\bar{b}$ antiquark, to be produced in the direction of the proton beam. We extract $A_{\rm FB}(B^{\pm})$ from a maximum likelihood fit to the difference between forward- and backward-produced $B^…
It has been more than 15 years since the discovery of the top quark. Great strides have been made in the measurement of the top quark mass and the properties of it. Most results show consistency with the standard model. However, using 5 fb$^{-1}$ data, recent measurements of the asymmetry in the production of top and anti-top quark pair have demonstrated surprisingly large values at CDF. Using 4 fb$^{-1}$ data, D0 also has similar effect.
We measure the forward-backward asymmetry in the production of $Λ_b^0$ and $\overline Λ_b^0$ baryons as a function of rapidity in $p \overline p $ collisions at $\sqrt s =1.96$ TeV using $10.4$ fb$^{-1}$ of data collected with the D0 detector at the Fermilab Tevatron collider. The asymmetry is determined by the preference of $Λ_b^0$ or $\overline Λ_b^0$ particles to be produced in the direction of the beam protons or antiprotons, respectively. The measured asymmetry integrated over rapidity $y$ in the range $0.1<|y|<2$ is $A=0.04 \pm 0.07 {\rm (stat)} \pm 0.02 {\rm (syst)}$.
We report on a precision measurement of the effective weak mixing angle using the forward-backward asymmetry, $A_{FB}$, in Drell-Yan ($ee$ and $μμ$) events in pp collisions at $\sqrt{s}=8~\mathrm{TeV}$ at the CMS experiment at the Large Hadron Collider (LHC). The results are compared to other hadron collider measurements published by ATLAS, LHCb, CDF, D0 and Tevatron combination.
The uncertainties in Parton Distribution Functions (PDFs) are the dominant source of the systematic uncertainty in precision measurements of electroweak parameters at hadron colliders (e.g. $\sin^2θ_{eff}(M_Z)$, $\sin^2θ_{W}=1-M_W^2/M_Z^2$ and the mass of the W boson). We show that measurements of the forward-backward charge asymmetry ($A_{FB}(M,y)$) of Drell-Yan dilepton events produced at hadron colliders provide a new powerful tool to reduce the PDF uncertainties in these measurements.
We present a measurement of the forward--backward asymmetry in top quark-antiquark production using the full Tevatron Run II dataset collected by the D0 experiment at Fermilab. The measurement is performed in lepton+jets final states using a new kinematic fitting algorithm for events with four or more jets and a new partial reconstruction algorithm for events with only three jets. Corrected for detector acceptance and resolution effects, the asymmetry is evaluated to be 10.6+-3.0 %. Results are consistent with the standard model predictions which range from 5.0% to 8.8%. We also present the de…
We study B->K(*)l+l- decays based on a large data sample of 657 million BB pairs collected with the Belle detector at the KEKB e+e- collider. The differential branching fraction, the isospin asymmetry, the K* polarization, and the forward-backward asymmetry (A_FB) as functions of q^2 are reported. The fitted A_FB spectrum tends to be shifted toward the positive side from the SM expectation. The measured branching fractions and lepton flavor ratios (electron/muon) are B(B->K*l+l-)=(10.8+1.1-1.0+-0.9)*10^-7, B(B->Kl+l-)=(4.8+0.5-0.4+-0.3)*10^-7, R_K*=1.21+-0.25+-0.08, and R_K=0.97+-0.18+-0.06, r…
We report a measurement of the ratios of Wilson coefficients $A_9/A_7$ and $A_{10}/A_7$ in $B \to K^* \ell^+ \ell^-$. The result is obtained from a data sample containing 386 million $B\bar{B}$ pairs that was collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider.
We report a measurement of the forward-backward charge asymmetry of electrons from W boson decays in p anti-p collisions at s**(1/2) = 1.96 TeV using a data sample of 170 pb-1 collected by the Collider Detector at Fermilab. The asymmetry is measured as a function of electron rapidity and transverse energy and provides new input on the momentum fraction dependence of the u and d quark parton distribution within the proton.
We study $Λ$ and $\barΛ$ production asymmetries in $p \bar{p} \rightarrow Λ(\barΛ) X$, $p \bar{p} \rightarrow J/ψΛ(\barΛ) X$, and $p \bar{p} \rightarrow μ^\pm Λ(\barΛ) X$ events recorded by the D0 detector at the Fermilab Tevatron collider at $\sqrt{s} = 1.96$ TeV. We find an excess of $Λ$'s ($\barΛ$'s) produced in the proton (antiproton) direction. This forward-backward asymmetry is measured as a function of rapidity. We confirm that the $\barΛ/Λ$ production ratio, measured by several experiments with various targets and a wide range of energies, is a universal function of "rapidity loss", i.…
We present a new measurement of the inclusive forward-backward t-tbar production asymmetry and its rapidity and mass dependence. The measurements are performed with 5.3 fb^{-1} of p-pbar collisions at \sqrt{s} = 1.96 TeV, recorded with CDF II at the Fermilab Tevatron. Significant inclusive asymmetries are observed in the laboratory and t-tbar rest frame, and are consistent with CP conservation under interchange of t and tbar. In the t-tbar rest frame, the asymmetry increases with the t-tbar rapidity difference, Δ(y), and with the invariant mass M_{t-tbar} of the t-tbar system. Parton-level asy…
Reconstructable final state kinematics and charge assignment in the reaction ppbar->ttbar allows tests of discrete strong interaction symmetries at high energy. We define frame dependent forward-backward asymmetries for the outgoing top quark in both the ppbar and ttbar rest frames, correct for experimental distortions, and derive values at the parton-level. Using 1.9/fb of ppbar collisions at sqrt{s}=1.96 TeV recorded with the CDF II detector at the Fermilab Tevatron, we measure forward-backward top quark production asymmetries in the ppbar and ttbar rest frames of A_{FB,pp} = 0.17 +- 0.08 an…
The measurements of Rb = sigma(e+e- -> bb~)/sigma(e+e- -> qq~) and of the b quark forward-backward charge asymmetry, A^b_fb, at centre-of-mass energies above the Z pole are described. The measurement of Rb is performed at \root{s} between 130 and 189 GeV using a b-tagging method that exploits the relatively large decay length of b-hadrons. The measurement of A^b_fb is performed using the large statistics event sample collected at \root{s}=189 GeV with a lepton-tag analysis based on the selection of prompt muons and electrons. The results at \root{s}=189 GeV are: Rb = 0.163 +/- 0.013 (stat.) +/…
Recently the CDF and the D0 Collaborations presented the data on the top forward-backward (FB) asymmetry A_FB as functions of M_(t t^bar) and Δy = y_t - y_t^bar. We study these observables in the effective Lagrangian approach with dimension-6 q q^bar t t^bar 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-sigma away. If this discrepancy remains in the future data with better statistics, it would imply that the effec…
A measurement of the forward-backward asymmetry A[FB] of oppositely charged lepton pairs (mu mu and e e) produced via Z/gamma* boson exchange in pp collisions at sqrt(s) = 8 TeV is presented. The data sample corresponds to an integrated luminosity of 19.7 inverse femtobarns collected with the CMS detector at the LHC. The measurement of A[FB] is performed for dilepton masses between 40 GeV and 2 TeV and for dilepton rapidity up to 5. The A[FB] measurements as a function of dilepton mass and rapidity are compared with the standard model predictions.
We present a measurement of the forward-backward charge asymmetry ($A_{FB}$) in $p\bar{p} \to Z/γ^{*}+X \to e^+e^-+X$ events at a center-of-mass energy of 1.96 TeV using 1.1 fb$^{-1}$ of data collected with the D0 detector at the Fermilab Tevatron collider. $A_{FB}$ is measured as a function of the invariant mass of the electron-positron pair, and found to be consistent with the standard model prediction. We use the $A_{FB}$ measurement to extract the effective weak mixing angle $sin^2Theta^{eff}_W = 0.2327 \pm 0.0018 (stat.) \pm 0.0006 (syst.)$.
We present a measurement of the mass dependence of the forward-backward charge asymmetry (A_{FB}) for electron-positron pairs produced via an intermediate Z/gamma with mass Mee > 40 GeV/c**(2). We study the constraints on the Z-quark couplings imposed by our measurement. We analyze an integrated luminosity of 72 pb-1 collected by the CDF II detector in proton anti-proton collisions at s**(1/2) = 1.96 TeV at the Fermilab Tevatron. A comparison of the uncorrected A_{FB} between data and Standard Model Monte Carlo gives good agreement with a chi^2/DOF of 15.7/15. The couplings measurements are al…
We present recent results on top quark pair production cross section and forward-backward asymmetry at the Tevatron. Three new cross section measurements from CDF and one new measurement from DO are presented that utilize the full dataset available. A new DO top cross section combination gives a ttbar production cross section of sigma_ttbar = 7.83 + 0.46-0.45 (stat) + 0.64-0.53 (syst) +-0.48 (lumi). The new CDF cross section combination for ttbar production is found to be 7.0 +- 0.3 (stat) +- 0.4 (syst) +- 0.4 (lumi) pb giving a total uncertainty of 9%, very close to the that of the current be…
This thesis describes a new a measurement of the tau lepton polarization and its forward-backward asymmetry at the Z resonance using the OPAL detector. This measurement is based on analyses of the tau->electron, muon and pion (Kaon) decays from a sample of 30663 tau-pair events collected in the polar angle range of Abs(cos(theta)) <0.68 during the 1990-1992 data taking period. Taking then the Standard Model with the V-A structure of the tau lepton decay, we measure the average tau polarization to be <p_tau>=(-13.5+/-2.9+/-2.2)% and the tau polarization forward-backward asymmetry to be AFBpol=(…
We measure the mass dependence of the forward-backward charge asymmetry in 157,553 $p\bar{p} \rightarrow Z/γ^{*} \rightarrow e^+e^-$ interactions, corresponding to 5.0 fb$^{-1}$ of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider at $\sqrt{s}=1.96$ TeV. The effective weak mixing angle ($\sin^2θ_{eff}^{lept}$) from this process involving predominantly the first generation of quarks is extracted as $\sin^2θ_{eff}^{lept} = 0.2309 \pm 0.0008 ({stat.}) \pm 0.0006 ({syst.})$. We also present the most precise direct measurement of the vector and axial-vector coup…
The measurements of the top quark charge asymmetry performed by the ATLAS and CMS experiments at the LHC are presented. The focus is on the single lepton and dilepton decay channels of the tt system. Both inclusive and differential top quark charge asymmetry measurements are shown.
We have studied inclusive muon events using all the data collected by the TOPAZ detector at sqrt(s)=58 GeV with an integrated luminosity of 273pb-1. From 1328 inclusive muon events, we measured the ratio R_qq of the cross section for qq-bar production to the total hadronic cross section and forward-backward asymmetry A^q_FB for b and c quarks. The obtained results are R_bb = 0.13+-0.02(stat)+-0.01(syst), R_cc = 0.36+-0.05(stat)+-0.05(syst), A^b_FB = -0.20+-0.16(stat)+-0.01(syst) and A^c_FB = -0.17+-0.14(stat)+-0.02(syst), in fair agreement with a prediction of the standard model.
We present measurements of the forward-backward asymmetry in the angular distribution of leptons from decays of top quarks and antiquarks produced in proton-antiproton collisions. We consider the final state containing a lepton and at least three jets. The entire sample of data collected by the D0 experiment during Run II of the Fermilab Tevatron Collider, corresponding to 9.7 inverse fb of integrated luminosity, is used. The asymmetry measured for reconstructed leptons is $A_{FB}^l = \big(2.9 \pm 2.1(stat.) ^{+1.5}_{-1.7}(syst.) \big)$%. When corrected for efficiency and resolution effects wi…
The electroweak contributions to the forward-backward asymmetry in the production of top-quark pairs at the Tevatron are evaluated at O(alpha^2) and O(alpha*alphas^2). We perform a detailed analysis of all partonic channels that produce an asymmetry and combine them with the QCD contributions. They provide a non-negligible fraction of the QCD-induced asymmetry with the same overall sign, thus enlarging the Standard Model prediction and diminishing the observed deviation. For the observed mass-dependent forward-backward asymmetry a 3-sigma deviation still remains at an invariant-mass cut of M_(…
A measurement is presented of the effective leptonic weak mixing angle ($\sin^2θ^{\ell}_{\text{eff}}$) using the forward-backward asymmetry of Drell-Yan lepton pairs ($μμ$ and ee) produced in proton-proton collisions at $\sqrt{s}=$ 8 TeV at the CMS experiment of the LHC. The data correspond to integrated luminosities of 18.8 and 19.6 fb$^{-1}$ in the dimuon and dielectron channels, respectively, containing 8.2 million dimuon and 4.9 million dielectron events. With more events and new analysis techniques, including constraints obtained on the parton distribution functions from the measured forw…
We propose a charged, electrically neutral, and flavour changing Z' model to conciliate the apparent disagreement between the important excess found in the top-antitop Tevatron forward-backward asymmetry and the null -compatible with negative- results found in the LHC charge asymmetry. We show that this model contributes positively to the forward-backward asymmetry, whereas naturally a new cancellation is turned on at the LHC, yielding a null, or even negative, charge asymmetry. We found the region in parameter space that is simultaneously allowed by the stringent Tevatron and LHC observables.…
The b quark forward-backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0 -> bbbar 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: A^b_FB = 0.0582 +- 0.0153 +- 0.0012 at sqrt(s) = 89.50 GeV A^b_FB = 0.0977 +- 0.0036 +- 0.0018 at sqrt(s) = 91.26 GeV A^b_FB = 0.1221 +- 0.0123 +- 0.0025 at sqrt(s) = 92.91 GeV where the first error is statistical and the secon…
Both D0 and CDF at Tevatron reported the measurements of forward-backward asymmetry in top pair production, which showed possible deviation from the standard model QCD prediction. In this paper, we explore how to examine the {\em same} higher-order QCD effects at the more powerful Large Hadron Collider.
I present results for the top quark rapidity distribution at LHC and Tevatron energies, including higher-order corrections from threshold resummation. The next-to-next-to-leading-order (NNLO) soft-gluon corrections at next-to-next-to-leading-logarithm (NNLL) level are added to the NLO result. Theoretical predictions are shown for the rapidity distribution, including the scale dependence of the distributions. The forward-backward asymmetry at the Tevatron is also calculated.
The dijet signature at the LHC is studied in new physics models for the top forward-backward asymmetry at the Tevatron. In the t-mediator models, flavor-changing interactions contribute to the dijet production cross section as well as the asymmetry at least at the one-loop level. It is found that the LHC dijet measurements at 36pb^(-1) have constrained the Z' coupling larger than 2.5-3. The sensitivity is expected to be improved significantly as the integrated luminosity increases in the LHC.
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 and can be used to calculate any infrared-safe observable. Especially we calculated the next-to-next-to-leading order corrections to 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 a precision determination of the top quark electroweak couplings at future $e…
The Standard Model (SM) prediction of the top quark forward backward asymmetry is shown to be slightly enhanced by a correction factor of 1.05 due to electroweak Sudakov (EWS) logarithms of the form (α/sin^2 θ_W)^n log^{m< 2n} (s/M_{W,Z}^2). The EWS effect on the dijet and t \bar{t} invariant mass spectra is significant, reducing the SM prediction by ~20, 10 % respectively for the highest invariant masses measured at the LHC, and changing the shape of the high-mass tail of the spectrum. These corrections significantly affect measurements of the top quark invariant mass spectrum and the search …
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.
The forward-backward asymmetry measurement in top-pair production at the Tevatron is about $2σ$ from the Standard Model prediction. We propose an asymmetric left-right model, which includes a $W'$ boson with a right-handed coupling of down to top quark, and a $Z'$ boson with diagonal couplings to the up, top, and down quarks with $M_{W'}\approx175$ GeV and $M_{Z'}\approx 900$ GeV. The model accounts for the asymmetry while remaining consistent with the top-pair total cross-section and invariant mass distribution.
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$σ$ compared to the SM 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 $t \bar{t}$ production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of $t \bar{t}$+jets at the LHC.
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 …
We show that the forward-backward asymmetry in top quark pair production can be enhanced by fields that transform nontrivially under the flavour group and satisfy Minimal Flavour Violation, while at the same time the constraints from associated effects on the d σ(t anti-t)/d M_{t anti-t} distribution, dijet resonance searches, same sign top pair production and other phenomenology are satisfied. We work out two examples in detail, one where a scalar colour anti-sextet field, that is also an anti-sextet of SU(3)_U, enhances the forward-backward asymmetry, and one where the enhancement arises fro…
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…
We compute the order $α_s^2$ QCD corrections to the $b$-quark forward-backward asymmetry in $e^+e^-\to b{\bar b}$ collisions at the $Z$ boson resonance, taking the non-zero mass of the $b$ quark into account. We determine these corrections with respect to both the $b$-quark axis and the thrust axis definition of the asymmetry. We compute also the distributions of these axes with respect to the electron beam. If one neglects the flavor singlet contributions to the $b$-quark asymmetry, as was done in previous computations for massless $b$ quarks, then the second-order QCD corrections for $m_b\ne…
The top quark forward-backward asymmetry measured at the Tevatron collider shows a large deviation from standard model expectations. Among possible interpretations, a non-universal $Z^\prime$ 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 $Z^\prime$ 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…
We construct flavor-dependent chiral U(1)' models with a Z' boson which couples to the right-handed up-type quarks in the standard model (SM). To make the models have realistic renormalizable Yukawa couplings, we introduce new Higgs doublets with nonzero U(1)' charges. Anomaly-free condition can be satisfied by adding extra chiral fermions. We show that these models could analyze the discrepancy between the SM prediction and empirical data in the top forward-backward asymmetry at the Tevatron.
The forward-backward asymmetry in top pair production at the Tevatron has long been in tension with the Standard Model prediction. One of the only viable new physics scenarios capable of explaining this anomaly is an s-channel axigluon-like resonance, with the quantum numbers of the gluon but with significant axial couplings to quarks. While such a resonance can lead to a clear bump or excess in the ttbar or dijet mass spectra, it may also simply be too broad to cleanly observe. Here, we point out that broad ttbar resonances generally lead to net top and antitop polarizations transverse to the…
The top quark forward-backward asymmetry measured at the Fermilab Tevatron collider deviates from the standard model prediction. A W^prime boson model is described, where the coupling W^prime-t-d is fixed by the tt_bar forward-backward asymmetry and total cross section at the Tevatron. We show that such a W^prime boson would be produced in association with a top quark at the CERN Large Hadron Collider (LHC), thus inducing additional tt_bar+j events. We use measurements of tt_bar+n-jet production from the LHC to constrain the allowed W^prime-t-d couplings as a function of W^prime boson mass. We…
We describe a simple technique for optimizing the extraction of the forward-backward asymmetry ($A_{fb}$) of Drell-Yan lepton pairs ($e^+e^-$,$ μ^+μ^-$) produced in $\bar{p}p$ and $pp$ collisions at hadron colliders. The method employs simple event weights which are functions of the rapidity and $cosθ$ 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 err…
The leading-order accurate description of top quark pair production, as usually employed in standard Monte Carlo event generators, gives no rise to the generation of a forward--backward asymmetry. Yet, non-negligible -- differential as well as inclusive -- asymmetries may be produced if coherent parton showering is used in the hadroproduction of top quark pairs. In this contribution we summarize the outcome of our study of this effect. We present a short comparison of different parton shower implementations and briefly comment on the phenomenology of the colour coherence effect at the Tevatron…
A color-singlet neutral vector boson is considered as a possible common origin of the top forward-backward asymmetry and the CDF dijet resonance. We identify chiral and flavor structures of the couplings of this new vector boson to the standard model quarks for which one could accommodate both data. We also demonstrate that non-vanishing observables involving longitudinal top polarizations can provide useful criteria for the possible existence of parity violating new physics in the q q^bar -> t t^bar process, and discrimination between the flavor-conserving and flavor-violating cases.
