Backward Asymmetry Research Articles

Searching for new physics in c→u transitions with nonuniversal Z′ model

In recent years, it has been reported that rare charm decays have immense potential to probe physics beyond the standard model (SM). The Glashow–Iliopoulos–Maiani mechanism effectively suppresses the flavor changing neutral currents decays involving [Formula: see text] transitions which makes them sensitive to new physics (NP) effects. In this work, we study D meson decays undergoing [Formula: see text] transitions in nonuniversal [Formula: see text] model. We determine the NP coupling from [Formula: see text] mixing and [Formula: see text] process. We then estimate the upper limits of branching fractions and forward–backward asymmetries for the decays [Formula: see text], [Formula: see text] and [Formula: see text]. The upper limits of the branching fractions obtained in the [Formula: see text] model show a significant enhancement from their SM results.

Charge-symmetric and -asymmetric fragmentation dynamics of argon dimers in slow Ar8+–Ar2 collisions

We present an experimental study of multiple-electron capture-induced fragmentation dynamics of Ar2m+ (4 ≤m≤ 7) dimer ions in 4 keV/u Ar8+–Ar2 collisions. The fragment recoil ion pairs and the charge-changing projectiles are coincidentally measured using a double coincidence technique. The branching ratios between the different charge-sharing fragmentation channels show an inherent enhancement of the asymmetric channels. The kinetic energy release (KER) distributions for the associated electron capture process show a shift in the mean KER values toward the higher side with increasing capture stabilization. The interplay between the different projectile autoionization processes sheds light on the energy depositions to the system during collisions. The Coulomb potential energy curves give a physical insight into the role of the projectile final states in the dimer fragmentation dynamics. The dimer-axis orientation-dependent cross sections for the asymmetric fragmentation channels reveal a forward–backward asymmetry that arises from the geometry of the collision system. Our findings thus give insight into the impact parameter-controlled fragmentation dynamics of multiply charged Ar2m+ dimer ions in highly charged ion–dimer slow collisions.

Probing gauge-Higgs unification models at the ILC with quark–antiquark forward–backward asymmetry at center-of-mass energies above the Z mass

The International Linear Collider (ILC) will allow the precise study of e-e+→qq¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^{-}e^{+} \\rightarrow q\\bar{q}$$\\end{document} interactions at different center-of-mass energies from the Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z$$\\end{document}-pole to 1 TeV. In this paper, we discuss the experimental prospects for measuring differential observables in e-e+→bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^{-}e^{+} \\rightarrow b\\bar{b}$$\\end{document} and e-e+→cc¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^{-}e^{+} \\rightarrow c\\bar{c}$$\\end{document} at the ILC baseline energies, 250 and 500 GeV. The study is based on full simulation and reconstruction of the International Large Detector (ILD) concept. Two gauge-Higgs unification models predicting new high-mass resonances beyond the Standard Model are discussed. These models predict sizable deviations of the forward–backward observables at the ILC running above the Z mass and with longitudinally polarized electron and positron beams. The ability of the ILC to probe these models via high-precision measurements of the forward–backward asymmetry is discussed. Alternative scenarios at other energies and beam polarization schemes are also discussed, extrapolating the estimated uncertainties from the two baseline scenarios.

Electroweak Corrections to Forward–Backward Asymmetry in the Dilepton Production Via Photon Fusion at the LHC

Electroweak Corrections to Forward–Backward Asymmetry in the Dilepton Production Via Photon Fusion at the LHC

Semileptonic and nonleptonic weak decays of $$\psi (1S,2S)$$ and $$\eta _{c}(1S,2S)$$ to $$D_{(s)}$$ in the covariant light-front approach

AbstractIn addition to the strong and electromagnetic decay modes, the $$\psi (1S,2S)$$ ψ ( 1 S , 2 S ) and $$\eta _{c}(1S,2S)$$ η c ( 1 S , 2 S ) can also decay via the weak interaction. Such weak decays can be detected by the high-luminosity heavy-flavor experiments. At present, some of the semileptonic and nonleptonic $$J/\Psi $$ J / Ψ weak decays have been measured at BESIII. Researching for these charmonium weak decays to $$D_{(s)}$$ D ( s ) meson can provide a platform to check of the standard model (SM) and probe new physics (NP). So we investigate the semileptonic and nonleptonic weak decays of $$\psi (1S,2S)$$ ψ ( 1 S , 2 S ) and $$\eta _{c}(1S,2S)$$ η c ( 1 S , 2 S ) to $$D_{(s)}$$ D ( s ) within the covariant light-front quark model (CLFQM). With form factors of the transitions $$\psi (1S,2S)\rightarrow D_{(s)}$$ ψ ( 1 S , 2 S ) → D ( s ) and $$\eta _{c}(1S,2S)\rightarrow D_{(s)}$$ η c ( 1 S , 2 S ) → D ( s ) calculated under the CLFQM, we predict and discuss some physical observables, such as the branching ratios, the longitudinal polarizations $$f_{L}$$ f L and the forward–backward asymmetries $$A_{FB}$$ A FB . One can find that the Cabibbo-favored semi-leptonic decay channels $$\psi (1\,S,2\,S)\rightarrow D_{s}^{-}\ell ^{+}\nu _{\ell }$$ ψ ( 1 S , 2 S ) → D s - ℓ + ν ℓ with $$\ell =e,\mu $$ ℓ = e , μ and the nonleptonic decay modes $$\psi (1S,2S)\rightarrow D_{s}^{-}\rho ^{+}$$ ψ ( 1 S , 2 S ) → D s - ρ + have relatively large branching ratios of the order $$\mathcal {O}(10^{-9})$$ O ( 10 - 9 ) , which are most likely to be accessible at the future high-luminosity experiments.

Study of forward–backward correlation and multiplicity fluctuations of pions produced in p–p collisions at recent LHC energies

The two-particle rapidity correlation among the pions produced in p–p collisions at recent LHC energies 2.76, 5.02, 7 and 13 TeV are studied in this paper by using the analysis of events generated by UrQMD-3.4, AMPT-2.26 and PYTHIA-8.3. Our investigation for bin-to-bin fluctuation using the variance of forward–backward asymmetry parameter in the forward–backward pseudo-rapidity windows is done through the simulated data. In addition to that, energy-wise variation of correlation strength (β), the probability distribution of roughness parameter () and number density fluctuation parameter (d ik ) are also incorporated in this investigation. Also, we have tried to study the same for the data of different impact parameters generated using the UrQMD model, and the comparison is made with its minimum biased events data-set. The analysis signifies, that the signature of dynamical fluctuation is not so prominent but increases a bit with the increase in energy. An increase in the correlation strength with the increase in energy signifies the contribution of long-range correlation is more in comparison to the short-range one. However, there is no such accountable effect of various impact parameters seen in the present study.

Semileptonic decays B_{c} meson to S-wave charmonia and X(3872) within the covariant light-front approach

In this work, we investigate the semileptonic decays of B_{c} meson to eta _{c}(1S,2S,3S), psi (1S,2S,3S) and X(3872) within the framework of covariant light-front quark model (CLFQM). We combine the helicity amplitudes via the corresponding form factors to obtain the branching ratios of the semileptonic decays B_{c}rightarrow eta _{c}(1S, 2S, 3S)ell nu _{ell }, B_{c}rightarrow psi (1S, 2S, 3S))ell nu _{ell } and B_{c}rightarrow X(3872)ell nu _{ell }, with ell =e,mu ,tau . In view of the R_{J/Psi } anomaly released by the LHCb collaboration, it is necessary to systematically calculate the ratios R_X with X=psi (1S,2S,3S),eta _c(1S,2S,3S),X(3872), which are helpful for checking the lepton flavor universality (LFU). We also take into account another two physical observables, the longitudinal polarization fraction f_{L} and the forward–backward asymmetry A_{FB}, which can provide new clues for understanding the R_{J /Psi } anomaly. Such theoretical predictions are necessary and interesting, and can be tested in future LHCb experiments.

Delving into new physics in semileptonic b→cτν̄ transitions

In recent times, there have been several observations suggesting deviations from lepton universality in semileptonic decays of [Formula: see text] mesons. These deviations have been noticed both in the neutral-current transitions ([Formula: see text]) and charged-current transitions ([Formula: see text]). Motivated by these intriguing findings, we investigate the semileptonic decays involving the quark-level transitions of [Formula: see text]. To explore the presence of new physics, we adopt a model-independent approach and analyze the processes [Formula: see text], [Formula: see text] and [Formula: see text], where [Formula: see text] represent the four lightest excited charm mesons. Considering a comprehensive effective Hamiltonian, we perform a global fit using various sets of new coefficients and incorporate measurements of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], as well as the upper limit on the branching ratio of [Formula: see text]. Subsequently, we examine the impact of constrained new couplings on the branching ratios, forward–backward asymmetry parameters, lepton nonuniversality (LNU) ratios, as well as lepton and hadron polarization asymmetries in these decay modes.

Study of new physics effects in lepton flavor violating B→K2∗(1430)l1l2 decays

Lepton flavor violation (LFV) is one of the most trending topics to probe new physics (NP). The powerful accelerators have enhanced their intensities to observe the LFV decays very precisely. In this situation, the theorists are also interested in studying these decays in various NP models and in model-independent ways to get precise results. Motivated by these results we have studied [Formula: see text] decays in non-universal [Formula: see text] model. Here, we have structured the two-fold angular distribution of the decays in terms of transversity amplitudes and the transversity amplitudes are formed with NP Wilson coefficients. The variation of the branching ratios and forward–backward asymmetries show the sensitivity of NP. The observables calculated in this work are very interesting and might provide a new way towards NP.

Investigation of Bd(s)∗→Dd+(Ds+)τ−v̄τ decays in W′ model and scalar leptoquark model

Inspired by recent anomalies associated with the [Formula: see text] hadron decays, a theoretical investigation on [Formula: see text] decays has been performed in [Formula: see text] model and scalar leptoquark model. To implicate these models, new coupling parameters are fixed by [Formula: see text] fitting using recent experimental results from [Formula: see text] transition as constraints. We have estimated branching fraction, lepton flavor universality (LFU) parameter, forward–backward asymmetry and lepton-spin asymmetry in the standard model (SM) as well as above two new physics (NP) models. Except lepton-spin asymmetry, significant deviations from the SM predictions are observed in [Formula: see text] model for both channels. In comparison to other observables, lepton-spin asymmetry shifts slightly from the SM in scalar leptoquark model. Our results may be helpful to the experimental communities to explore these decays and [Formula: see text] boson and leptoquark at the colliders.

Lepton flavor violating decays

A number of measurements in decays induced by the semileptonic b → s and b → c transitions hint towards a possible role of new physics in both sectors. Motivated by these anomalies, we investigate the lepton flavor violating decays. We calculate the two-fold angular distribution of decay in the presence of vector, axial-vector, scalar and pseudo-scalar new physics interactions. We then compute the branching fraction and lepton forward–backward asymmetry in the framework of vector leptoquark which is a viable solution to the current B anomalies. We find that the upper limits are and at 90% C.L.

Footprints of New Physics in the angular distribution of [formula omitted] decays

We investigate the angular decay distribution of the four-fold Bc→Ds⁎(→Dsγ)μ+μ−, and Bc→Ds⁎(→Dsπ)μ+μ− decays that proceed through b→sμ+μ− quark level transition. We use the model independent effective Hamiltonian with vector and axial vector new physics operators to formulate the angular observables and study the implications of different latest new physics scenarios, taken from the global fits to all the b→s data, on these observables. We also give Standard Model and new physics predictions of several observables such as differential branching ratios, forward backward asymmetry, longitudinal polarization fraction of Ds⁎, and the unpolarized and polarized lepton flavor universality violating ratios. Future measurements of the predicted angular observables, both at current and future high energy colliders, will add to the useful complementary data required to clarify the structure of new physics in b→sℓℓ neutral current decays.

Breakdown of dipole Born approximation and the role of Rydberg’s predissociation for the electron-induced ion-pair dissociation to oxygen in the presence of background gases

We study the electron-induced ion-pair dissociation to gas-phase oxygen molecules using a state-of-the-art velocity-map ion-imaging technique. The analysis is entirely based on the conical time-gated wedge-shaped velocity slice images of O-/O2 nascent anionic fragments, and the resulting observations are in favor of Van Brunt et al.'s report [R. J. Van Brunt and L. J. Kieffer, J. Chem. Phys. 60, 3057 (1974)]. A new image reconstruction method, Jacobian over parallel slicing, is introduced to overcome the drawback of ion exaggeration in determining the kinetic energy distribution from the time-gated parallel slicing technique, which offers an alternative approach to the wedge slicing method. Most importantly, the role of the quintet-heavy Rydberg state has been drawn out to the complex ion-pair formalism. The extracted kinetic energy and angular distributions from the wedge slice images reveal a high momentum transfer during the ion-pair dissociation process, which could be the finest rationale to observe the breakdown of dipole Born approximation driven by multipole moment associated with the incident electron beam. Three distinct dissociative momentum bands have been precisely identified for O- dissociation. However, radiationless Rydberg's predissociation continuum (≥15%) has become an inherent character of electron-induced ion-pair dissociation, which could be dealt with using the beyond Born-Oppenheimer treatment. The incoherent sum of Σ and Π symmetric-associated ion-pair final states has been precisely identified by modeling the angular distribution of O-/O2 for each of the kinetic energy bands. A negligibly small amount of forward-backward asymmetry is observed in the angular distribution of O-/O2, which might be explained by the dissociative state-specific quantum coherence mechanism as reported [Krishnakumar et al., Nat. Phys. 14, 149 (2018); Kumar et al., arXiv:2206.15024 (2022)] by Prabhudesai et al.

Event-by-event multiplicity fluctuations and correlations in ring-like and jet-like events in 197Au–AgBr collisions at 11.6 A GeV/c

Event-by-event (ebe) multiplicity fluctuations and correlations amongst the charged particles emitted in the forward–backward symmetric pseudorapidity ([Formula: see text]) windows of varying widths and positions are investigated by analyzing the experimental data on [Formula: see text]Au–AgBr collisions at 11.6[Formula: see text]A[Formula: see text]GeV/[Formula: see text]. The findings are compared with the predictions of relativistic transport model, urqmd and independent particle emission (or mixed event) model. It is observed that the fluctuations in ebe mean pseudorapidity values and those reflected from the fluctuations strength measure, [Formula: see text] are relatively higher as compared to those expected from the statistically independent particle emission model. The study of the variance, [Formula: see text] of a suitably defined forward–backward asymmetry variable [Formula: see text] as a function of [Formula: see text] window width and position indicates the presence of strong short-range correlations, which might arise due to isotropic decay of cluster-like objects either in forward or backward [Formula: see text] region. Furthermore, analyses of events having ring-like and jet-like substructures, carried out separately, suggest that the major contribution to the observed fluctuations in the data sample are due to ring-like events, while the contributions from the jet-like events appear to be rather small. The observed difference in the behavior of correlation strengths from the two types of events might be due to the enhanced emission of Cherenkov gluons, giving rise to the ring-like substructure. The mixed event analysis further confirms that the observed fluctuations are the distinct feature of the data, which disappear after event mixing.

Light quark decays of doubly heavy baryons in light front approach

We explore the semileptonic and nonleptonic decays of doubly heavy baryons (Omega _{cc}^{(*)+},Omega _{bb}^{(*)0},Omega _{bc}^{(*)-},Omega _{bc}^{prime 0}) induced by the srightarrow u transition. Hadronic form factors are parametrized by transition matrix elements and are calculated in the light front quark model. With the form factors, we make use of helicity amplitudes and analyze semileptonic and nonleptonic decay modes of doubly heavy baryons. Benchmark results for partial decay widths, branching fractions, forward–backward asymmetries and other phenomenological observables are derived. We find that typical branching fractions for semileptonic decays into ell bar{nu } are at the order 10^{-7}-10^{-8} and the ones for nonleptonic decays are at the order 10^{-5}, which are likely detectable such as in LHCb experiment. With the potential data accumulated in future, our results may help to shape our understanding of the decay mechanism in the presence of two heavy quarks.

Parton distributions and new physics searches: the Drell–Yan forward–backward asymmetry as a case study

We discuss the sensitivity of theoretical predictions of observables used in searches for new physics to parton distributions (PDFs) at large momentum fraction x. Specifically, we consider the neutral-current Drell–Yan production of gauge bosons with invariant masses in the TeV range, for which the forward–backward asymmetry of charged leptons from the decay of the gauge boson in its rest frame is a traditional probe of new physics. We show that the qualitative behaviour of the asymmetry depends strongly on the assumptions made in determining the underlying PDFs. We discuss and compare the large-x behaviour of various different PDF sets, and find that they differ significantly. Consequently, the shape of the asymmetry observed at lower dilepton invariant masses, where all PDF sets are in reasonable agreement because of the presence of experimental constraints, is not necessarily reproduced at large masses where the PDFs are mostly unconstrained by data. It follows that the shape of the asymmetry at high masses may depend on assumptions made in the PDF parametrization, and thus deviations from the traditionally expected behaviour cannot be taken as a reliable indication of new physics. We demonstrate that forward–backward asymmetry measurements could help in constraining PDFs at large x and discuss the accuracy that would be required to disentangle the effects of new physics from uncertainties in the PDFs in this region.

A combined approach to the analysis of space and ground experimental data within a simplified E_6SSM

Within the Exceptional Supersymmetric Standard Model (E_6SSM), we investigate signatures at the Large Hadron Collider (LHC) for a long-lived charged inert higgsino, which is degenerate with the inert neutralino at tree level and a small mass splitting is generated at the loop level, resulting in a lifetime mathcal{O}(0.02) nanoseconds. We focus on the most sensitive search for long-lived charged inert higgsino decays to the lightest neutral inert higgsino dark matter and very soft charged leptons, which are eventually stopped in the detector resulting in a disappearing-track signal. Furthermore, we study the displaced vertex signature of the inert chargino in the case where it is produced via the Z^{prime } portal. We illustrate how difficult it is to construct displaced vertices in this class of models, though some evidence could be gained at the High Luminosity LHC. Finally, we compare the spin independent and spin dependent cross sections of the lightest inert higgsino DM to those of current direct detection experiments, proving that it is possible to gain sensitivity to the active DM component of this scenario in the near future. The combination of these signatures with the one emerging from Z' production and decay via Drell–Yan, which can be characterised as belonging to the E_6SSM via both the cross section and Forward–Backward Asymmetry, could point uniquely to this non-minimal realisation of Supersymmetry.

Scrutinizing new physics in semi-leptonic B c → J/ψτν decay

We perform a global analysis of the b → cτν data using the recent lattice results on the B c → J/ψ vector and axial-vector form factors. To explore the effects from the tensor operator of new physics beyond the Standard Model, we determine the tensor form factors by using the non-relativistic QCD (NRQCD) relations between tensor and (axial-)vector form factors. Based on the lattice + NRQCD form factors, we fit the Wilson coefficients and the new physics couplings in R 2, S 1 and U 1 leptoquark models by including the recently measured R(Λ c ) and imposing the relaxed constraint in light of the recent studies on LEP1 data and B c lifetime. We give predictions for the experimental observables including R(J/ψ), P τ (J/ψ), F L (J/ψ) and as well as their q 2 distribution in new physics scenarios/models. Our results suggest that the longitudinal τ polarization fraction P τ (J/ψ) and the forward–backward asymmetry are useful for testing the R 2 leptoquark model.

Multiplicity distribution of backward shower particle in total disintegrated events revisited with Weibull distribution

The well-known statistical distribution function, Weibull distribution function, has been employed to study the multiplicity distribution and multiplicity moments for the backward shower particles produced in case of total disintegrated events of [Formula: see text]O–AgBr, [Formula: see text]Ne–AgBr and [Formula: see text]Si–AgBr interactions at (4.1–4.5) AGeV/c. Multiplicity fluctuation of backward shower particles in case of total disintegrated events by the method of scaled variance and the forward–backward asymmetry parameter had also been investigated.

Phenomenology analysis of bar{B}^* rightarrow V tau ^-bar{nu }_{tau } decays in and beyond the Standard Model

Motivated by the persistent anomalies reported in the brightarrow c tau bar{nu } data, we investigate the semileptonic decays bar{B}^* rightarrow V tau ^-bar{nu }_{tau } (V=D^*_{u,d,s},J/psi ), within the Standard Model and beyond. The relevant transition form factors, being calculated in the covariant light-front quark model, is the main source of theoretical uncertainties. Using various best-fit solutions for the new operator Wilson coefficients, we report numerical results on various observables related to the processes bar{B}^* rightarrow V tau ^-bar{nu }_{tau }, such as the branching ratios, the ratio of branching fractions, the hadron and lepton polarization asymmetries, the forward–backward asymmetries and the convexity parameter. We also predict the q^2 distribution of these observables both within the Standard Model and in various new physics scenarios. The accessible observables are studied in order to assess their sensitivity to the different new physics scenarios. We can find that some of the new physics scenarios can be differentiated from each other by using these observables and their correlations. We also find that some observables of these decay modes are sizeable and deviate significantly (for new physics coupling parameter S_R especially) from their corresponding standard model values, which are expected to be within the reach of Run III of Large Hadron Collider experiment. And experimental information on these distributions which will be testified in the future can help to disentangle the dynamical origin of the current anomalies.