Backward Asymmetry Research Articles

Impact of torsion space–time on observables at hadron colliders

Starting from the effective torsion space-time model, we study its effects on the top pair production cross section at hadron colliders. We also study the effects of this model on top pair asymmetries at the Tevatron and LHC. We find that torsion space-time can explain forward–backward asymmetry according to a measured anomaly at the Tevatron. We find an allowed region in the parameter space which can simultaneously satisfy all observables measured at the Tevatron and LHC.

Forward–backward asymmetry study of semi-leptonic B decays

We study the resonant contributions in the process of B¯0→K−π+μ+μ− with the K−π+ invariant mass square mKπ2 involving strange mesons, K⁎(1410),K0⁎(1430),K2⁎(1430),K⁎(1680),K3⁎(1780) and K4⁎(2045). Applying the heavy quark expansion and the large energy limit, we derive the relations of form factors to cancel much of the hadronic uncertainties. Using the dependence of branching fractions on the mKπ, we find that the KJ⁎ resonance has a clear signature. Branching ratios, polarizations, forward–backward asymmetries and transversity amplitudes are predicted, from which we find a promising prospective to observe these channels in the future experiment. We update the constraints on effective Wilson coefficients and/or free parameters in these new physics scenarios by making use of the B→K⁎l+l− and b→sl+l− experimental data. Their impact on B→KJ⁎l+l− is subsequently explored and in particular the zero-crossing point for the forward–backward asymmetry in new physics scenarios can sizably deviate from the standard model.

RARE BARYONIC DECAYS Λb → Λl+l- IN THE TTM MODEL

In the framework of the top triangle moose model, we analyze the rare decays Λb → Λl+l-(l = e, μ, τ) by using the form factors calculated in full QCD. We calculate the contributions of the new particles predicted by this model to observables, such as the branching ratios, forward–backward asymmetry (A FB ) and polarizations related these decay processes. We find that, in wide range of the parameter space, the values of the branching ratios are enhanced by one order of magnitude comparing to the standard model predictions. This model can also produce significant corrections to A FB , normal polarization PN and transversal polarization PT.

Semileptonic transition of Σb → Σμ+μ− in the family non-universal Z′ model

Using newly available form factors obtained from light-cone QCD sum rules in full theory, we study the flavor-changing neutral current transition of Σb → Σμ+μ− decay in the family non-universal Z′ model. In particular, we evaluate the differential branching ratio and forward–backward asymmetry, as well as some related asymmetry parameters and polarizations. We compare the obtained results with predictions of the standard model and discuss the sensitivity of the observables under consideration to the family non-universal Z′ gauge boson. The order of the differential branching ratio shows that this decay mode can be checked at the LHC in the near future.

One-loop effects on MSSM parameter determination via chargino production at the LC

At a future linear collider very precise measurements, typically with errors of <1 %, are expected to be achievable. Such an accuracy gives sensitivity to the quantum corrections, which therefore must be incorporated in theoretical calculations in order to determine the underlying new physics parameters from prospective linear collider measurements. In the context of the chargino–neutralino sector of the minimal supersymmetric standard model, this involves fitting one-loop predictions to prospective measurements of the cross sections, forward–backward asymmetries and of the accessible chargino and neutralino masses. Taking recent results from LHC SUSY and Higgs searches into account we consider three phenomenological scenarios, each with characteristic features. Our analysis shows how an accurate determination of the desired parameters is possible, providing in addition access to the stop masses and mixing angle.

Asymmetric interface band alignments of Cu2O/ZnO and ZnO/Cu2O heterojunctions

Abstract X-ray photoelectron spectroscopy was used to investigate the valence-band offsets (VBOs) of the Cu2O–ZnO heterojunctions fabricated by magnetron sputtering. A significant forward–backward asymmetry was observed in the band alignments of Cu2O/ZnO–ZnO/Cu2O heterojunctions. The valence band offsets of 2.91 eV and 2.52 eV were achieved in Cu2O/ZnO and ZnO/Cu2O heterojunctions respectively. The asymmetry was attributed to the lattice distortion of the Cu2O at the interface of Cu2O/ZnO heterojunction. The compressed Cu2O lattices lead to the upward shift of the top of the valence-band of Cu2O, and thus the measured VBO of Cu2O/ZnO is larger than that of ZnO/Cu2O. Considering the band alignments, the ZnO/Cu2O structure is expected to have more advantage in photovoltaic application than Cu2O/ZnO structure.

The form factors of -&gt; K ( ) and the predictions for CP violation beyond the standard model

We study the hadronic form factors of the τ lepton decays |$\tau \rightarrow K \pi (\eta) \nu $|⁠. We compute one-loop corrections to the form factors using the chiral Lagrangian including vector mesons. The counterterms that subtract the divergence of the one-loop amplitudes are determined by using the background field method. In the vector form factor, K* resonance behavior is reproduced because a diagram with a vector meson propagator is included. We fit the data of the hadronic invariant mass spectrum measured by Belle by determining some of the counterterms of the Lagrangian. Besides the hadronic invariant mass spectrum, forward–backward asymmetry is predicted. We also study the effect of CP violation of a two Higgs doublet model. In the model, CP violation of the neutral Higgs sector generates the mixing of CP-even Higgs and CP-odd Higgs. We show how the mixing leads to direct CP violation of the τ decays and predict the CP violation of the forward–backward 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.

THE CHIRAL COLOR SYMMETRY OF QUARKS AND G′-BOSON CONTRIBUTIONS TO CHARGE ASYMMETRY IN $t \bar t$-PRODUCTION AT THE LHC AND TEVATRON

The contributions of G′-boson predicted by the chiral color symmetry of quarks to the charge asymmetry [Formula: see text] in [Formula: see text]-production at the LHC and to the forward–backward asymmetry [Formula: see text] in [Formula: see text]-production at the Tevatron are calculated and analyzed in dependence on two free parameters of the model, the G′ mass mG′ and mixing angle θG. The mG′ - θG regions of 1 σ consistency with the CMS data on the cross-section [Formula: see text] and on the charge asymmetry [Formula: see text] are found and compared with those resulted from the CDF data on the cross-section [Formula: see text] and on the forward–backward asymmetry [Formula: see text] of [Formula: see text]-production at the Tevatron with account of the current SM predictions for [Formula: see text].

Monte Carlo event generators &amp; the top quark forward-backward asymmetry

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.

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.

Constraining unparticles from top physics at the Tevatron

We study and analyze the recent observations of the top pair production $\sigma (p\bar p\to t \bar t)$ at TeVatron through flavor conserving and flavor violating channels ${\it via}$ vector and tensor unparticles. The unparticle sector is considered with the possibility of being a color singlet or octet. The modified unparticle propagator is used to investigate the contribution of these unparticles to the observed $A_{FB}^{t\bar t}$ (forward backward asymmetry in top pair production) and the spin correlation at TeVatron. We have also studied the impact of the flavor violating couplings of unparticles to the third generation quarks on (a) pair production of same sign tops/antitops $\sigma (p\bar p\to tt+\bar t \bar t)$ at TeVatron and (b) the partial top decay width for $\Gamma_{\cal U}(t\to u\,{\cal U}^V)$. We find that a large region of parameter space is consistent with the measurements of \tt production cross-section, \afbt and spin correlation coefficient at TeVatron and observe that the top decay width measurement constrains the flavor violating coupling of vector unparticles more severely than the same sign top/antitop production at TeVatron. We also predict the best point-set in the model parameter space for specific choices of \du corresponding to $\chi^2_{\rm min}$ evaluated using the \mttb spectrum of \afbt from the data set of Run II of TeVatron at the integrated luminosity 8.7 fb$^{-1}$. Our results and analysis are consistent even with unparticle theories having broken scale invariance as long as the infrared cut-off scale is much less than the top pair production threshold.

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.

Lepton polarization effects in [formula omitted] decay in family non-universal [formula omitted] model

Possible manifestation of the family non-universal Z′ boson effects in lepton polarization in rare, exclusive baryonic Λb→Λℓ+ℓ− decay is examined. It is observed that the double lepton polarizations PNN, PTT and PTN are sensitive to the Z′ contribution. Moreover, it is found that the values of the polarized forward–backward asymmetry AFBLL are different in Standard Model (SM) and family non-universal Z′ model in different regions of q2, and therefore can serve as an efficient tool for establishing new physics beyond the SM.

Electron DDCS in ionization of O2 by 4.5 MeV/u bare O-ions: A comparative study of the angular asymmetry between O2 and H2

We have measured double differential cross-sections (DDCS) of low energy electrons emitted from O2 in collisions with 72MeV bare O ions. The derived forward backward asymmetry parameter spectrum shows monotonically increasing behavior. No oscillation due to interference was observed in the forward-backward angular asymmetry unlike that for molecular hydrogen for which oscillation was unambiguous.

Forward–backward asymmetry of Drell–Yan lepton pairs in pp collisions at [formula omitted

A measurement of the forward–backward asymmetry (AFB) of Drell–Yan lepton pairs in pp collisions at s=7 TeV is presented. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 5 fb−1. 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 presented, and are compared with the standard model predictions. The AFB 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 with the standard model predictions.

Top polarization and exploration of $\mathbf{\emph{t}} \boldsymbol{\bar t}$ forward–backward asymmetry†

The large forward–backward asymmetry in top pair-production observed at the Tevatron has generated much excitement in recent times. Many different models have been proposed as possible explanations for this anomaly. We discuss how the measurement of the polarization of the top quark can be used to establish the role of new physics in generating the observed asymmetry and to characterize and discriminate among various models.

Energy and angular distributions of the low-energy electron emission in collisions of 4 MeV/u bare F ions with He atoms

The energy and angular distributions of double-differential cross sections (DDCS) of electron emission from He in collisions with 4 MeV/u F9 + ions are reported. The derived single-differential distributions and the total cross sections are also reported. The measured distributions of the low-energy electrons between 1 and 400 eV over a wide angular range between 20° and 160° are compared with the state-of-the-art quantum mechanical models. The first Born (B1) and the continuum distorted wave-eikonal initial state (CDW-EIS) approximations are used for this purpose. The DDCS for a given angle was found to fall by a few orders of magnitude over the electron energy range studied. The CDW-EIS model provides excellent agreement with the energy distributions and the angular distributions. The electron energy dependence of the forward–backward asymmetry parameter shows monotonically increasing behaviour. This has been explained very well in terms of the CDW-EIS model, which includes the two-centre effect. A large deviation from the B1 is also observed. We have also derived the single-differential distributions in terms of the angle as well as the electron energy. These distributions are also well reproduced by the CDW-EIS model.

COMPLETE STUDY OF THE SEMILEPTONIC B TO $K_2^*(1430)$ DECAYS IN THE STANDARD MODEL WITH FOURTH GENERATION

The [Formula: see text] decays are analyzed in the standard model (SM) extended to fourth generation of quarks (SM4). The decay rate, forward–backward asymmetry, lepton polarization asymmetries, the helicity fractions of the final state [Formula: see text] meson and CP asymmetry are discussed using the form factors calculated in the light-cone sum rules (LCSR) approach. We utilized the constraints on different fourth generation parameters obtained from the experimental information on K, B and D decays and from their recent searches at the ATLAS and CMS experiments to explore their impact on the [Formula: see text] decays. We find that the values of above mentioned physical observables deviate significantly from their minimal SM predications. In the present study, we will also perform the analysis of CP asymmetry in [Formula: see text] decays within SM4, which is one of the most interesting observable in the study of SM4.