Forward-backward Charge Asymmetry Research Articles

Measurement of the e+e−→π+π− cross section from threshold to 1.2 GeV with the CMD-3 detector

The cross section of the process e+e−→π+π− has been measured in the center of mass energy range from 0.32 to 1.2 GeV with the CMD-3 detector at the electron-positron collider VEPP-2000. The measurement is based on a full dataset collected below 1 GeV during three data taking seasons, corresponding to an integrated luminosity of about 62 pb−1. In the dominant ρ-resonance region, a systematic uncertainty of 0.7% has been reached. At energies around ϕ-resonance the π+π− production cross section was measured for the first time with high beam energy resolution. The forward-backward charge asymmetry in the π+π− production has also been measured. It shows a strong deviation from the theoretical prediction based on the conventional scalar quantum electrodynamics framework, and it is in good agreement with the generalized vector-meson-dominance and dispersive-based predictions. The impact of the presented results on the evaluation of the hadronic contribution to the anomalous magnetic moment of muon is discussed. Published by the American Physical Society 2024

Factorization of the forward-backward asymmetry and measurements of the weak mixing angle and proton structure at hadron colliders

The forward-backward charge asymmetry (${A}_{\mathrm{FB}}$) at hadron colliders is sensitive to both the electroweak (EW) symmetry breaking represented by the effective weak mixing angle ${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{\mathrm{eff}}^{\ensuremath{\ell}}$, and the proton structure information in the initial state modeled by the parton distribution functions (PDFs). Due to their strong correlation, the precisions of determination on ${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{\mathrm{eff}}^{\ensuremath{\ell}}$ and PDFs using the measured ${A}_{\mathrm{FB}}$ spectrum are limited. In this paper, we define a set of structure parameters which factorize the unique proton information of the relative difference between quarks and antiquarks in the ${A}_{\mathrm{FB}}$ observation. Other than the conventional way of extracting ${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{\mathrm{eff}}^{\ensuremath{\ell}}$ from the convolution of PDF and EW calculations, we propose a new method to simultaneously determine the value of ${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{\mathrm{eff}}^{\ensuremath{\ell}}$ and the proton structure terms by fitting to the observed ${A}_{\mathrm{FB}}$ distribution, and point out the necessity of specifying additional observations to further reduce the uncertainties on the proton structure terms, respectively, so that model-independent high precision ${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{\mathrm{eff}}^{\ensuremath{\ell}}$ and proton structure measurements can be achieved at the future LHC experiments.

Using Drell–Yan forward–backward asymmetry to reduce PDF uncertainties in the measurement of electroweak parameters

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. $$\sin ^2\theta _{eff}(M_Z)$$ , $$\sin ^2\theta _{W}=1-M_W^2/M_Z^2$$ 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.

Measurement of the forward-backward asymmetry in Z/γ∗ → μ + μ − decays and determination of the effective weak mixing angle

The forward-backward charge asymmetry for the process $q\bar{q} \rightarrow Z/\gamma^{\ast} \rightarrow \mu^{+}\mu^{-}$ is measured as a function of the invariant mass of the dimuon system. Measurements are performed using proton proton collision data collected with the LHCb detector at $\sqrt{s} = 7$ and 8\tev, corresponding to integrated luminosities of $1$fb$^{-1}$ and $2$fb$^{-1}$ respectively. Within the Standard Model the results constrain the effective electroweak mixing angle to be $$sin^{2}\theta_{W}^{eff} = 0.23142 \pm 0.00073 \pm 0.00052 \pm 0.00056 $$ where the first uncertainty is statistical, the second systematic and the third theoretical. This result is in agreement with the current world average, and is one of the most precise determinations at hadron colliders to date.

Measurement of the Effective Weak Mixing Angle in pp[over ¯]→Z/γ^{*}→e^{+}e^{-} Events.

We present a measurement of the fundamental parameter of the standard model, the weak mixing angle sin^{2}θ_{eff}^{ℓ} which determines the relative strength of weak and electromagnetic interactions, in pp[over ¯]→Z/γ^{*}→e^{+}e^{-} events at a center of mass energy of 1.96 TeV, using data corresponding to 9.7 fb^{-1} of integrated luminosity collected by the D0 detector at the Fermilab Tevatron. The effective weak mixing angle is extracted from the forward-backward charge asymmetry as a function of the invariant mass around the Z boson pole. The measured value of sin^{2}θ_{eff}^{ℓ}=0.23147±0.00047 is the most precise measurement from light quark interactions to date, with a precision close to the best LEP and SLD results.

Azimuthal decorrelation intt¯production at hadron colliders

We present a new observable, $\Delta \phi$, an azimuthal angle difference between $t$ and $\bar{t}$ quarks in $t\bar{t}$ pair production, at hadron colliders as an interesting probe of the radiative quantum chromodynamics process as well as a high-order correction in the high-mass regime. This variable also enables good discrimination on some new physics models that may explain the forward-backward charge asymmetry of $t\bar{t}$ production measured at the Tevatron. With a reliable estimation of the dataset obtained up to 2011 at the Tevatron and Large Hadron Collider, we present an opportunity for testing the standard model as well as searching new physics models with the $\Delta \phi$ observable.

Searches for new neutral gauge boson at the Tevatron and LHC in the left-right twin Higgs model

In the framework of the left-right twin Higgs (LRTH) model, we study the possibilities to detect the new Z′ boson at the Tevatron and LHC. First, using \(p\bar p\) collision data collected by the D0 and CDF II detectors, we find that the LRTH Z′ boson is excluded with masses below 940 GeV. Then we search for signatures of the Z′ boson at the LHC from the analysis of some distributions for \(p\bar p \to \mu ^ + \mu ^ - + X\), such as the number of events, the differential cross section of the dimuon invariant mass, the distributions of the transverse momentum and the forward-backward charge asymmetry. We do our calculation for two typical values of the LHC center of mass energy (7 and 14 TeV). The numerical results show that, by applying convenient cuts on some of the observables, the dimuon invariant mass and final particle pT distributions can reveal the presence of the heavy neutral gauge boson Z′ contribution in the LRTH model.

Measurement ofsin2θeffℓandZ-light quark couplings using the forward-backward charge asymmetry inpp¯→Z/γ*→e+e−events withL=5.0 fb−1ats=1.96 TeV

We measure the mass dependence of the forward-backward charge asymmetry in 157 553 pp¯→Z/γ*→e+e− interactions, corresponding to 5.0 fb−1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider at s=1.96 TeV. The effective weak mixing angle (θeffℓ) from this process involving predominantly the first generation of quarks is extracted as sin2θeffℓ=0.2309±0.0008(stat.)±0.0006(syst.). We also present the most precise direct measurement of the vector and axial-vector couplings of u and d quarks to the Z boson.2 MoreReceived 25 April 2011DOI:https://doi.org/10.1103/PhysRevD.84.012007© 2011 American Physical Society

Effects of the littlest Higgs model on the process e + e − → µ+µ− at a Z 0-factory

We investigate the possibilities of finding the effects of the littlest Higgs (LH) model on the process e + e − → µ+µ− at the leading-order at a possible Z 0-factory. We study the impacts of the LH model on the observables, such as, the total cross section, the distribution of the transverse momentum of µ−, and the forward-backward charge asymmetry of µ− in polarized and unpolarized electron-positron collision modes. Besides, we analyze the relations between the observables and the LH model parameters, and discuss the observables of process e + e − → µ+µ− in different incoming electron-positron polarization modes. We conclude that one could observe the effects contributed by the LH model on the process e + e − → µ+µ− in a reasonable parameter space, or might put more stringent constraints on the LH parameters in the future experiments at the possible Z 0-factory.

Muon-pair production in the left-right symmetric model at a Z 0-factory

The potential for probing the impacts of the left-right symmetric model (LRM) on the process e+e− → µ+µ− at a proposed Z0-factory is investigated. Under different electron-positron polarization collision modes, we calculate and compare the deviations of various observables in the framework of the LRM from the standard model (SM) predictions, such as the total cross section, the distribution of the transverse momentum of µ−, and the forward-backward charge asymmetry of µ−. In addition, we present the relations between the observables and the LRM parameters. We conclude that it is possible to observe the effects of the LRM on the process e+e− → µ+µ− by using optimal observables and favorable parameters at a Z0-factory.

Full electroweak one-loop corrections to [formula omitted] production at the ILC

The precise investigation of the W+W−Z0 production at the e+e− International Linear Collider (ILC) is of crucial importance in probing the couplings between massive vector gauge bosons and discovering the signature of new physics beyond the standard model (SM). We study the full one-loop EW effects on the observables, such as, the total cross section, the differential cross section of the invariant mass of W-pair, the distribution of the angle between W-pair, the production angle distributions of W- and Z0-boson, the distributions of the transverse momenta of final W- and Z0-boson, and the forward–backward charge asymmetry of W−-boson. Our numerical results show that the EW relative correction to the total cross sections (δew) varies from −18.9% to −5.7% when mH=120 GeV and s goes up from 300 GeV to 1 TeV.

Hadronic top-quark pair production in association with a hard jet at next-to-leading order QCD: phenomenological studies for the Tevatron and the LHC

We report on the calculation of the next-to-leading order QCD corrections to the production of top–antitop-quark pairs in association with a hard jet at the Tevatron and at the LHC. Results for integrated and differential cross sections are presented. We find a significant reduction of the scale dependence. In most cases the corrections are below 20% indicating that the perturbative expansion is well under control. Moreover, the forward–backward charge asymmetry of the top quark, which is analyzed at the Tevatron, is studied at next-to-leading order. We find large corrections, suggesting that the definition of the observable has to be refined.

Measurement of the Forward-Backward Charge Asymmetry and Extraction ofsin2θWeffinpp¯→Z/γ*+X→e+e−+XEvents Produced ats=1.96 TeV

We present a measurement of the forward-backward charge asymmetry (A FB) in pp-->Z/gamma* + X-->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 sin2thetaWeff = 0.2326+/-0.0018(stat)+/-0.0006(syst).

New physics in top quark events at the Fermilab Tevatron Collider

We present two new measurements from the Fermilab Tevatron Collider: the CDF collaboration presents a search for flavor changing neutral currents in top quark pair decay, and the D0 collaboration presents the first measurements of the integrated forward-backward charge asymmetry in top quark pair production. The later is used as a probe for resonant production.

Measurement of the Forward-Backward Charge Asymmetry in Top-Quark Pair Production

We present the first measurement of the integrated forward-backward charge asymmetry in top-quark-top-antiquark pair (tt) production in proton-antiproton (pp) collisions in the lepton+jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming tt + X production and decay, a sample of 0.9 fb(-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 tt+X production via a Z' resonance.

Next-to-Leading-Order QCD Corrections tott¯+jetProduction at Hadron Colliders

We report on the calculation of the next-to-leading-order QCD corrections to the production of top-quark--top-antiquark pairs in association with a hard jet at the Fermilab Tevatron and the CERN Large Hadron Collider. We present results for the $t\overline{t}+\mathrm{\text{jet}}$ cross section and the forward-backward charge asymmetry. The corrections stabilize the leading-order prediction for the cross section. The charge asymmetry receives large corrections.

NLO forward–backward charge asymmetries in [formula omitted] production at large hadron colliders

We consider the next-to-leading order corrections, O(αs), to forward–backward charge asymmetries for lepton-pair production in association with a large transverse momentum jet at large hadron colliders. We find that the leading order results are essentially confirmed. Although experimentally challenging and in practice with large backgrounds, these observables could provide a new determination of the weak mixing angle sin2θefflept(MZ2) with a statistical precision for each lepton flavour of ∼10−3 (7×10−3) at LHC (Tevatron), and if b jets are identified, of the b quark Z asymmetry AFBb with a statistical precision of ∼2×10−3 (4×10−2) at LHC (Tevatron).

Drell-Yan Production at the Fermilab Tevatron

Using data from [Formula: see text] collisions at [Formula: see text] collected by the CDF and DØ detectors, we present measurements of the Drell-Yan inclusive differential cross section and forward-backward charge asymmetry as a function of invariant mass. We compare these measurements with Standard Model predictions, and present searches for new physics that could appear as deviations from the Standard Model in the Drell-Yan differential cross section.

Measurement of the forward-backward charge asymmetry fromW→eνproduction inpp¯collisions ats=1.96 TeV

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.

Measurement of the forward-backward charge asymmetry of electron-positron pairs inp¯pcollisions ats=1.96 TeV

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 also consistent with Standard Model predictions.