Scalar Resonances Research Articles

Search for new resonances in Wγ and Zγ final states in pp collisions at [formula omitted] with the ATLAS detector

This Letter presents a search for new resonances decaying to final states with a vector boson produced in association with a high transverse momentum photon, Vγ, with V=W(→ℓν) or Z(→ℓ+ℓ−), where ℓ=e or μ. The measurements use 20.3 fb−1 of proton–proton collision data at a center-of-mass energy of s=8 TeV recorded with the ATLAS detector. No deviations from the Standard Model expectations are found, and production cross section limits are set at 95% confidence level. Masses of the hypothetical aT and ωT states of a benchmark Low Scale Technicolor model are excluded in the ranges [275,960] GeV and [200,700]∪[750,890] GeV, respectively. Limits at 95% confidence level on the production cross section of a singlet scalar resonance decaying to Zγ final states have also been obtained for masses below 1180 GeV.

Comprehensive amplitude analysis ofγγ→π+π−,π0π0andK¯Kbelow 1.5 GeV

In this paper we perform an amplitude analysis of essentially all published pion and kaon pair production data from two-photon collisions below 1.5 GeV. This includes all the high statistics results from Belle, as well as older data from Mark II at SLAC, CELLO at DESY, and Crystal Ball at SLAC. The purpose of this analysis is to provide as close to a model-independent determination of the $\ensuremath{\gamma}\ensuremath{\gamma}$ to meson pair amplitudes as possible. Having data with limited angular coverage, typically $|\mathrm{cos}\ensuremath{\theta}|<0.6--0.8$, and no polarization information for reactions in which spin is an essential complication, the determination of the underlying amplitudes might appear an intractable problem. However, imposing the basic constraints required by analyticity, unitarity, and crossing symmetry makes up for the experimentally missing information. Above 1.5 GeV multimeson production channels become important, and we have too little information to resolve the amplitudes. Nevertheless, below 1.5 GeV the two-photon production of hadron pairs serves as a paradigm for the application of $S$-matrix techniques. Final state interactions among the meson pairs are critical to this analysis. To fix these, we include the latest $\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$, $\overline{K}K$ scattering amplitudes given by dispersive analyses, supplemented in the $\overline{K}K$ threshold region by the recent precision Dalitz plot analysis from BABAR. With these hadronic amplitudes built into unitarity, we can constrain the overall description of $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$ and $\overline{K}K$ data sets, both integrated and differential cross sections, including the high statistics charged and neutral pion, as well as ${K}_{s}{K}_{s}$, data from Belle. Since this analysis invokes coupled hadronic channels, having data on both $\ensuremath{\pi}\ensuremath{\pi}$ and $\overline{K}K$ reduces the solution space to essentially a single form in the region where these channels saturate unitarity. For the $\ensuremath{\pi}\ensuremath{\pi}$ channel, the separation of isospin-0 and -2 and helicity-0 and -2 components is complete. We present the partial wave amplitudes, show how well they fit all the available data, and give the two-photon couplings of scalar and tensor resonances that appear. These partial waves are important inputs into forthcoming dispersive calculations of hadronic light-by-light scattering.

Status of the Zee–Babu model for neutrino mass and possible tests at a like-sign linear collider

We provide an updated scan of the allowed parameter space of the two-loop Zee–Babu model for neutrino mass. Taking into account most recent experimental data on μ→eγ as well as the mixing angle θ13 we obtain lower bounds on the masses of the singly and doubly charged scalars of between 1 and 2 TeV, with some dependence on perturbativity and fine-tuning requirements. This makes the scalars difficult to observe at LHC with 14 TeV even with optimistic assumptions on the luminosity, and would require a multi-TeV linear collider to see the scalar resonances. We point out, however, that a sub-TeV linear collider in the like-sign mode may be able to observe lepton flavor violating processes such as e−e−→μ−μ− due to contact interactions induced by the doubly charged scalar with masses up to around 10 TeV. We investigate the possibility to distinguish the Zee–Babu model from the Higgs triplet model using such processes.

Resonance effects in pion and kaon decay constants

In this article we study impact of the lightest vector and scalar resonance multiplets in the pion and kaon decay constants up to next-to-leading order in the $1/N_C$ expansion, i.e., up to the one-loop level. The $F_\pi$ and $F_K$ predictions obtained within the framework of Resonance Chiral Theory are confronted with lattice simulation data. The vector loops (and the $\rho-\pi\pi$ coupling $G_V$ in particular) are found to play a crucial role in the determination of the Chiral Perturbation Theory couplings $L_4$ and $L_5$ at next-to-leading order in $1/N_C$. Puzzling, values of $G_V < 40$ MeV seem to be necessary to agree with current phenomenological results for $L_4$ and $L_5$. Conversely, a value of $G_V > 60$ MeV compatible with standard $\rho-\pi\pi$ determinations turns these chiral couplings negative. However, in spite of the strong anti-correlation with $L_4$, the $SU(3)$ chiral coupling $F_0$ remains stable all the time and stays within the range $78 \sim 86$ MeV when $G_V$ is varied in a wide range, from $40$ up to $70$ MeV. Finally, we would like to remark that the leading order expressions used in this article for the $\eta-\eta'$ mixing, mass splitting of the vector multiplet masses and the quark mass dependence of the $\rho(770)$ mass are found in reasonable agreement with the lattice data.

String Phenomenology: Past, Present and Future Perspectives

The observation of a scalar resonance at the Large Hadron Collider (LHC), compatible with perturbative electroweak symmetry breaking, reinforces the Standard Model (SM) parameterisation of all subatomic data. The logarithmic evolution of the SM gauge and matter parameters suggests that this parameterisation remains viable up to the Planck scale, where gravitational effects are of comparable strength. String theory provides a perturbatively consistent scheme to explore how the parameters of the Standard Model may be determined from a theory of quantum gravity. The free fermionic heterotic string models provide concrete examples of exact string solutions that reproduce the spectrum of the Minimal Supersymmetric Standard Model. Contemporary studies entail the development of methods to classify large classes of models. This led to the discovery of exophobic heterotic-string vacua and the observation of spinor-vector duality, which provides an insight to the global structure of the space of (2,0) heterotic-string vacua. Future directions entail the study of the role of the massive string states in these models and their incorporation in cosmological scenarios. A complementary direction is the formulation of quantum gravity from the principle of manifest phase space duality and the equivalence postulate of quantum mechanics, which suggest that space is compact. The compactness of space, which implies intrinsic regularisation, may be tightly related to the intrinsic finite length scale, implied by string phenomenology.

Constraining minimalU(1)B−Lmodel from dark matter observations

We study the $B-L$ gauge extension of the Standard Model which contains a singlet scalar and three right-handed neutrinos. The vacuum expectation value of the singlet scalar breaks the $U(1)_{B-L}$ symmetry. Here the third generation right-handed neutrino is qualified as the dark matter candidate, as an artifact of $Z_2$-charge assignment. Relic abundance of the dark matter is consistent with WMAP9 and PLANCK data, only near scalar resonances where dark matter mass is almost half of the scalar boson masses. Requiring correct relic abundance, we restrict the parameter space of the scalar mixing angle and mass of the heavy scalar boson of this model. Besides this, the maximum value of spin-independent scattering cross-section off nucleon is well-below the {\sc Xenon100} and recent LUX exclusion limits and can be probed by future {\sc Xenon1T} experiment. In addition, we compute the annihilation of the dark matter into two photon final state in detail and compare with the Fermi-LAT upper bound on $\langle\sigma v\rangle_{\gamma \gamma}$ for NFW and Einasto profile.

Parameters of scalar resonances from the combined analysis of data on processesππ→ππ,KK¯,ηηandJ/ψdecays

A combined analysis of data on isoscalar S-wave processes $\pi\pi\to\pi\pi,K\overline{K},\eta\eta$ and on decays $J/\psi\to\phi\pi\pi,\phi K\overline{K}$ from the DM2, Mark III and BESIII collaborations is performed to study $f_0$ mesons. The method of analysis is based on analyticity and unitarity and uses an uniformization procedure. In the analysis limited only to the multi-channel $\pi\pi$-scattering data, two possible sets of parameters of the $f_0(500)$ were found: in both cases the mass was about 700 MeV but the total width was either about 600 or 930 MeV. The extension of the analysis using only the DM2 and Mark III data on the $J/\psi$ decays does not allow to choose between these sets. However, the data from BESIII on the di-pion mass distribution in the decay $J/\psi\to\phi\pi^+\pi^-$ clearly prefers the wider $f_0(500)$ state. Spectroscopic implications from results of the analysis are also discussed.

Dirac dynamical resonance states around Schwarzschild black holes

Recently, a novel kind of scalar wigs around Schwarzschild black holes---scalar dynamical resonance states were introduced in [Phys. Rev. D 84, 083008 (2011)] and [Phys. Rev. Lett. 109, 081102 (2012)]. In this paper, we investigate the existence and evolution of Dirac dynamical resonance states. First we look for stationary resonance states of a Dirac field around a Schwarzchild black hole by using the Schr\"{o}dinger-like equations reduced from the Dirac equation in Schwarzschild spacetime. Then Dirac pseudo-stationary configurations are constructed from the stationary resonance states. We use these configurations as initial data and investigate their numerical evolutions and energy decay. These dynamical solutions are the so-called "Dirac dynamical resonance states". It is found that the energy of the Dirac dynamical resonance states shows an exponential decay. The decay rate of energy is affected by the resonant frequency, the mass of Dirac field, the total angular momentum, and the spin-orbit interaction. In particular, for ultra-light Dirac field, the corresponding particles can stay around a Schwarzschild black hole for a very long time, even for cosmological time-scales.

The Scalar--Isovector Sector in the Extended Linear Sigma Model

We study basic properties of scalar hadronic resonances within the so-called extended Linear Sigma Model (eLSM), which is an effective model of QCD based on chiral symmetry and dilatation invariance. In particular, we focus on the mass and decay width of the scalar isovector state $a_{0}(1450)$ and perform a numerical study of the propagator pole(s) on the unphysical Riemann sheets. In this work, this meson is understood as a seed state explicitly included in the eLSM. Our results show that the inclusion of hadronic loops does not modify much the previously obtained tree-level results. Moreover the $a_{0}(980)$ cannot be found as a propagator pole generated by hadronic loop contributions.

Branching fractions and directCPviolation in charmless three-body decays ofBmesons

Charmless three-body decays of B mesons is studied using a simple model based on the framework of the factorization approach. Hadronic three-body decays receive both resonant and nonresonant contributions. Dominant nonresonant contributions to tree-dominated three-body decays arise from the $b\to u$ tree transition which can be evaluated using heavy meson chiral perturbation theory valid in the soft meson limit. For penguin-dominated decays, nonresonant signals come mainly from the penguin amplitude governed by the matrix elements of scalar densities $<M_1M_2|\bar q_1 q_2|0>$. We use the measurements of $\bar B^0\to K_SK_SK_S$ to constrain the nonresonant component of $<K\bar K|\bar ss|0>$. The intermediate vector meson contributions to three-body decays are identified through the vector current, while the scalar meson resonances are mainly associated with the scalar density. While the calculated direct CP violation in $B^-\to K^+K^-K^-$ and $B^-\to \pi^+\pi^-\pi^-$ decays agrees well with experiment in both magnitude and sign, the predicted CP asymmetries in $B^-\to \pi^- K^+K^-$ and $B^-\to K^-\pi^+\pi^-$ are wrong in signs when confronted with experiment. It has been conjectured recently that a possible resolution to this CP puzzle may rely on final-state rescattering of $\pi^+\pi^-$ and $K^+K^-$. Assuming a large strong phase associated with the matrix element $<K\pi|\bar sq|0>$ arising from some sort of power corrections, we fit it to the data of $K^-\pi^+\pi^-$ and find a correct sign for $\pi^- K^+K^-$.

Composite and elementary nature of a resonance in theσmodel

We analyze the mixing nature of the low-lying scalar resonance consisting of the $\ensuremath{\pi}\ensuremath{\pi}$ composite and the elementary particle within the $\ensuremath{\sigma}$ model. A method to disentangle the mixing is formulated in the scattering theory with the concept of the two-level problem. We investigate the composite and elementary components of the $\ensuremath{\sigma}$ meson by changing a mixing parameter. We also study the dependence of the results on model parameters such as the cut-off value and the mass of the elementary $\ensuremath{\sigma}$ meson.

The vector resonance triplet with the direct coupling to the third quark generation

The effective Lagrangian with scalar and vector resonances that might result from new strong physics beyond the SM is formulated and studied. In particular, the scalar resonance representing the recently discovered 125-GeV boson is complemented with the SU(2) L+R triplet of hypothetical vector resonances. Motivated by experimental and theoretical considerations, the vector resonance is allowed to couple directly to the third quark generation only. The coupling is chiral-dependent and the interaction of the right top quark can differ from that of the right bottom quark. To estimate the applicability range of the effective Lagrangian the unitarity of the gauge boson scattering amplitudes is analyzed. The experimental fits and limits on the free parameters of the vector resonance triplet are investigated.

Production representation of partial wave scattering amplitudes and the f 0(600) particle

The establishment of production representation of partial wave scattering amplitudes is reviewed in the context of quantum field theory. Its relation to the production representation, or Ning Hu representation in quantum mechanical scattering theory is pointed out. One of the most important application of the production representation is the physics of the f0(600) and K(700) scalar hadron resonances, on which we also give a brief review. It is emphasized that all evidences accumulated so far are consistent with the picture that the f0(600) meson is the chiral partner of the Nambu-Goldstone bosons in a linear realization of chiral symmetry.

Stückelberg mechanism in the presence of physical scalar resonances

We show that it is possible to accommodate physical scalar resonances within a minimal nonlinearly realized electroweak theory in a way compatible with a natural Hopf algebra selection criterion (Weak Power Counting) and the relevant functional identities of the model (Local Functional Equation, Slavnov-Taylor identity, ghost equations, b-equations). The Beyond-the-Standard-Model (BSM) sector of the theory is studied by BRST techniques. The presence of a mass generation mechanism \`a la St\"uckelberg allows for two mass invariants in the gauge boson sector. The corresponding 't Hooft gauge-fixing is constructed by respecting all the symmetries of the theory. The model interpolates between the Higgs and a purely St\"uckelberg scenario. Despite the presence of physical scalar resonances, we show that tree-level violation of unitarity in the scattering of longitudinally polarized charged gauge bosons occurs at sufficiently high energies, if a fraction of the mass is generated by the St\"uckelberg mechanism. The formal properties of the physically favoured limit after LHC7-8 data, where BSM effects are small and custodial symmetry in the gauge boson sector is respected, are studied.

Isospin violation inJ/Ψ→ϕπ0ηdecay and thef0−a0mixing

The isospin violating $J/\Psi\to \phi \pi^0 \eta$ decay is thought to be dominated by the mixing of the $f_0(980)$ and $a_0(980)$ scalar resonances. We make a theoretical evaluation of the $J/\Psi\to \phi \pi^0 \eta$ decay extending our own previous model for other $J/\Psi$ decays into one vector meson and two pseudoscalars using the techniques of the chiral unitary approach. The scalar resonances are dynamically generated through the final state interaction of the pseudoscalar mesons implementing unitarity from the lowest order ChPT amplitudes. Besides the direct $J/\Psi\phi PP$ vertex, other mechanisms like the sequential exchange of vector and axial-vector mesons are shown to be important in order to obtain the actual strength of the mixing. We get a very good agreement with the $\pi^0 \eta$ invariant mass distribution and branching ratio with recent BESIII data. Quantification of the $f_0(980)-a_0(980)$ mixing is done and compared with results from several experiments.

Probing the indefiniteCPnature of the Higgs boson through decay distributions in the processe+e−→tt¯Φ

The recently discovered scalar resonance at the LHC is now almost confirmed to be a Higgs Boson, whose CP properties are yet to be established. At the ILC with and without polarized beams, it may be possible to probe these properties at high precision. In this work, we study the possibility of probing departures from the pure CP-even case, by using the decay distributions in the process $e^+ e^- \to t \bar{t} \Phi$, with $\Phi$ mainly decaying into a $b\bar b$ pair. We have compared the case of a minimal extension of the SM case (Model I) with an additional pseudoscalar degree of freedom, with a more realistic case namely the CP-violating Two-Higgs Doublet Model (Model II) that permits a more general description of the couplings. We have considered the ILC with $\sqrt{s}=800$\,GeV and integrated luminosity of $300\, {\rm fb}^{-1}$. Our main findings are that even in the case of small departures from the CP-even case, the decay distributions are sensitive to the presence of a CP-odd component in Model II, while it is difficult to probe these departures in Model I unless the pseudoscalar component is very large. Noting that the proposed degrees of beam polarization increases the statistics, the process demonstrates the effective role of beam polarization in studies beyond the Standard Model. Further, our study shows that an indefinite CP Higgs would be a sensitive laboratory to physics beyond the SM.

π+π−andπ0π0pair production in photon-photon scattering and ultraperipheral ultrarelativistic heavy-ion collisions

We discuss ${\gamma}{\gamma}\to {\pi}{\pi}$ reactions, starting from the two-pion threshold up to about $\sqrt{s_{{\gamma}{\gamma}}}$ = 6 GeV. We include the dipion continuum due to pion exchange (for ${\pi}^+{\pi}^-$) and ${\rho}^\pm$ exchange (for ${\pi}^0{\pi}^0$) as well as the dipion ${\sigma}(600)$, $f_0(980)$ and $f_2(1270)$ s-channel resonances. We discuss also a possible contribution of less pronounced scalar resonances $f_0(1500)$ and $f_0(1710)$ as well as the tensor resonances $f'_2(1525)$, $f_2(1565)$ and $f_2(1950)$. We find that the inclusion of the spin-four $f_4(2050)$ resonant state improves the situation at $\sqrt{s_{{\gamma}{\gamma}}} \approx$ 2 GeV. We estimate the relevant diphoton partial decay width to be ${\Gamma}_{f_4 \to {\gamma}{\gamma}} \approx$ 0.7 keV. At higher energies, Brodsky-Lepage and hand-bag mechanisms are included in addition. We nicely describe the world data for ${\gamma}{\gamma}\to {\pi}{\pi}$ for the first time both for the total cross section and angular distributions. The cross section for the production of two-pions in ultraperipheral ultrarelativistic heavy ion collisions is calculated in the impact parameter space Equivalent Photon Approximation (EPA). We obtain the nuclear cross section of 46.7 mb for ${\pi}^+{\pi}^-$ and 8.7 mb for ${\pi}^0{\pi}^0$ at the LHC energy $\sqrt{s_{NN}} =$ 3.5 TeV and minimal cut on pion transverse momentum $p_t>$ 0.2 GeV. Differential distributions in impact parameter, dipion invariant mass, single pion and dipion rapidity, pion transverse momentum as well as pion pseudorapidity are shown. The ${\gamma}{\gamma}\to {\pi}^+ {\pi}^-$ subprocess constitutes a background to $A A \to A {\rho}^0 (\to {\pi}^+ {\pi}^-) A$ process.

CPviolation inB±→π±π+π−in the region with low invariant mass of oneπ+π−pair

Recently, the large CP asymmetries in $B^\pm\to\pi^\pm\pi^+\pi^-$ decays were found by the LHCb Collaboration to localize in the region $m_{\pi^+\pi^-}^2<0.4 \text{GeV}^2$. We find such large localized CP asymmetries may be due to the interference between a light scalar and $\rho^0(770)$ intermediate resonances. Consequently, we argue that the distribution of CP asymmetries in the Dalitz plots of three-body B decays could be very helpful for identifying the presence of the scalar resonance.

Scalar resonances in the non-linearly realized electroweak theory

We introduce a physical scalar sector in a SU(2)xU(1) electroweak theory in which the gauge group is realized non linearly. By invoking theoretical as well as experimental constraints, we build a phenomenologically viable model in which a minimum of four scalar resonances appear, and the mass of the CP even scalar is controlled by a vacuum expectation value; however, the masses of all other particles (both matter as well as vector boson fields) are unrelated to spontaneous symmetry breaking and generated by the St\"uckelberg mechanism. We evaluate in this model the CP-even scalar decay rate to two photons and use this amplitude to perform a preliminary comparison with the recent LHC measurements. As a result, we find that the model exhibits a preference for a negative Yukawa coupling between the top quark and the CP-even resonance.

The role of [formula omitted] in the ϕω threshold peak of [formula omitted

We study the process J/Ψ→γϕω, measured by the BES experiment, where a neat peak close to the ϕω threshold is observed and is associated to a scalar meson resonance around 1800 MeV. We make the observation that a scalar resonance coupling to ϕω unavoidably couples strongly to KK¯, but no trace of a peak is seen in the KK¯ spectrum of the J/Ψ→γKK¯ at this energy. This serves us to rule out the interpretation of the observed peak as a signal of a new resonance. After this is done, a thorough study is performed on the production of a pair of vector mesons and how its interaction leads necessarily to a peak in the J/Ψ→γϕω reaction close to the ϕω threshold, due to the dynamical generation of the f0(1710) resonance by the vector–vector interaction. We then show that both the shape obtained for the ϕω mass distribution, as well as the strength are naturally reproduced by this mechanism. The work also explains why the ϕω peak is observed in the BES experiment and not in other reactions, like B±→K±ϕω of Belle.