Showing papers on "Pseudoscalar published in 2013"

Study of the Mass and Spin-Parity of the Higgs Boson Candidate via Its Decays to Z Boson Pairs

Abstract: A study is presented of the mass and spin-parity of the new boson recently observed at the LHC at a mass near 125 GeV. An integrated luminosity of 17.3 fb^(-1), collected by the CMS experiment in proton-proton collisions at center-of-mass energies of 7 and 8 TeV, is used. The measured mass in the ZZ channel, where both Z bosons decay to e or μ pairs, is 126.2±0.6(stat)±0.2(syst) GeV. The angular distributions of the lepton pairs in this channel are sensitive to the spin-parity of the boson. Under the assumption of spin 0, the present data are consistent with the pure scalar hypothesis, while disfavoring the pure pseudoscalar hypothesis.

Meson vacuum phenomenology in a three-flavor linear sigma model with (axial-)vector mesons

Abstract: We study scalar, pseudoscalar, vector, and axial-vector mesons with nonstrange and strange quantum numbers in the framework of a linear sigma model with global chiral U(Nf)L×U(Nf)R symmetry. We perform a global fit of meson masses, decay widths, as well as decay amplitudes. The quality of the fit is, for a hadronic model that does not consider isospin-breaking effects, surprisingly good. We also investigate the question whether the scalar q¯q states lie below or above 1 GeV and find the scalar states above 1 GeV to be preferred as q¯q states. Additionally, we also describe the axial-vector resonances as q¯q states.

Decay constants of heavy-light vector mesons from QCD sum rules

Abstract: We revisit QCD sum rules for the decay constants of heavy-light mesons. In the sum rules for the vector mesons B^*_(s) and D^*_(s) we improve the accuracy of OPE, taking into account the O(alpha_s^2) terms in the perturbative part and calculating the O(alpha_s) corrections to the quark-condensate contribution. With this accuracy, we obtain the ratios of decay constants: f_B^*/f_B=1.02 +0.07 -0.03, f_D^*/f_D=1.20 +0.10 -0.07. The sum rule predictions for the decay constants of pseudoscalar mesons are updated with the results f_B=(207 +17 -9) MeV, f_B_s=(242 +17 -12) MeV, f_D=(201 +12 -13) MeV, f_D_s=(238 +13 -23) MeV. In order to assess the sensitivity of our calculation to the form of the sum rule, we consider alternative versions such as the power moments and Borel sum rules with different weights of the spectral density. We also investigated the heavy quark limit of the sum rules for vector and pseudoscalar mesons, estimating the violations of the heavy-quark spin and flavour symmetry.

Probing dark matter couplings to top and bottom quarks at the LHC

Abstract: Monojet searches are a powerful way to place model-independent constraints on effective operators coupling dark matter to the standard model For operators generated by the exchange of a scalar mediator, however, couplings to light quarks are suppressed and the prospect of probing such interactions through the inclusive monojet channel at the LHC is limited We propose dedicated searches, focusing on bottom and top quark final states, to constrain this class of operators We show that a search in mono $b$-jets can significantly improve current limits The mono-$b$ signal arises partly from direct production of $b$ quarks in association with dark matter, but the dominant component is from top quark pair production in the kinematic regime where one top is boosted A search for tops plus missing energy can strengthen the bounds even more; in this case signal and background have very different missing energy distributions We find an overall improvement by several orders of magnitude in the bound on the direct detection cross section for scalar or pseudoscalar couplings

Leptonic decays of charged pseudoscalar mesons - 2012

Abstract: We review the physics of purely leptonic decays of $\pi^\pm$, $K^\pm$, $D^{\pm}$, $D_s^\pm$, and $B^\pm$ pseudoscalar mesons. The measured decay rates are related to the product of the relevant weak-interaction-based CKM matrix element of the constituent quarks and a strong interaction parameter related to the overlap of the quark and antiquark wave-functions in the meson, called the decay constant $f_P$. The interplay between theory and experiment is different for each particle. Theoretical predictions of $f_B$ that are needed in the $B$ sector can be tested by measuring $f_{D^+}$ and $f_{D_s^+}$ in the charm sector. The lighter $\pi^{\pm}$ and $K^{\pm}$ mesons provide stringent comparisons between experiment and theory due to the accuracy of both the measurements and the theoretical predictions. This review was prepared for the Particle Data Group's 2012 edition.

An inflationary model with small scalar and large tensor nongaussianities

Abstract: We study a model of inflation where the scalar perturbations are almost gaussian while there is sizable (equilateral) nongaussianity in the tensor sector. In this model, a rolling pseudoscalar gravitationally coupled to the inflaton amplifies the vacuum fluctuations of a vector field. The vector sources both scalar and tensor metric perturbations. Both kinds of perturbations are nongaussian, but, due to helicity conservation, the tensors have a larger amplitude, so that nongaussianity in the scalar perturbations is negligible. Moreover, the tensors produced this way are chiral. We study, in the flat sky approximation, how constraints on tensor nongaussianities affect the detectability of parity violation in the Cosmic Microwave Background. We expect the model to feature interesting patterns on nongaussianities in the polarization spectra of the CMB.

Parity violation in the CMB bispectrum by a rolling pseudoscalar

Abstract: We investigate parity-violating signatures of temperature and polarization bispectra of the cosmic microwave background (CMB) in an inflationary model where a rolling pseudoscalar produces large equilateral tensor non-Gaussianity. By a concrete computation based on full-sky formalism, it is shown that resultant CMB bispectra have nonzero signals in both parity-even (l1+l2+l3 = even) and parity-odd (l1+l2+l3 = odd) spaces, and are almost uncorrelated with usual scalar-mode equilateral bispectra. These characteristic signatures and polarization information help to detect such tensor non-Gaussianity. Use of both temperature and E-mode bispectra potentially improves of 400% the detectability with respect to an analysis with temperature bispectrum alone. Considering B-mode bispectrum, the signal-to-noise ratio may be able to increase by 3 orders of magnitude. We present the 1σ uncertainties of a parameter depending on a coupling constant and a rolling condition for the pseudoscalar expected in the Planck and the proposed PRISM experiments.

The impact of heavy-quark loops on LHC dark-matter searches

Abstract: If only tree-level processes are included in the analysis, LHC monojet searches give weak constraints on the dark matter-proton scattering cross section arising from the exchange of a new heavy scalar or pseudoscalar mediator with Yukawa-like couplings to quarks. In this letter we calculate the constraints on these interactions from the CMS 5.0 fb−1 and ATLAS 4.7 fb−1 searches for jets with missing energy including the effects of heavy-quark loops. We find that the inclusion of such contributions leads to a dramatic increase in the predicted cross section and therefore a significant improvement of the bounds from LHC searches.

Cosmological perturbation in f(T) gravity revisited

Abstract: We perform detailed investigation of cosmological perturbations in f(T) theory of gravity coupled with scalar field. Our work emphasizes on the way to gauge fix the theory and we examine all possible modes of perturbations up to second order. The analysis includes pseudoscalar and pseudovector modes in addition to the usual scalar, vector, and tensor modes. We find no gravitational propagating degree of freedom in the scalar, pseudoscalar, vector, as well as pseudovector modes. In addition, we find that the scalar and tensor perturbations have exactly the same form as their counterparts in usual general relativity with scalar field, except that the factor of reduced Planck mass squared Mpl2≡1/(8πG) that occurs in the latter has now been replaced by an effective time-dependent gravitational coupling −2(df/dT)|T = T0, with T0 being the background torsion scalar. The absence of extra degrees of freedom of f(T) gravity at second order linear perturbation indicates that f(T) gravity is highly nonlinear. Consequently one cannot conclusively analyze stability of the theory without performing nonlinear analysis that can reveal the propagation of the extra degrees of freedom.

Probing New Physics with the B_s to {\mu}+ {\mu}- Time-Dependent Rate

Abstract: The B_s to mu+ mu- decay plays an outstanding role in tests of the Standard Model and physics beyond it. The LHCb collaboration has recently reported the first evidence for this decay at the 3.5 sigma level, with a branching ratio in the ballpark of the Standard Model prediction. Thanks to the recently established sizable decay width difference of the B_s system, another observable, A^mumu_DeltaGamma, is available, which can be extracted from the time-dependent untagged B_s to mu+ mu- rate. If tagging information is available, a CP-violating asymmetry, S_mumu, can also be determined. These two observables exhibit sensitivity to New Physics that is complementary to the branching ratio. We define and analyse scenarios in which these quantities allow us to discriminate between model-independent effective operators and their CP-violating phases. In this context we classify a selection of popular New Physics models into the considered scenarios. Furthermore, we consider specific models with tree-level FCNCs mediated by a heavy neutral gauge boson, pseudoscalar or scalar, finding striking differences in the predictions of these scenarios for the observables considered and the correlations among them. We update the Standard Model prediction for the time-integrated branching ratio taking the subtle decay width difference effects into account. We find (3.56 +/- 0.18) x 10^-9, and discuss the error budget.

An inflationary model with small scalar and large tensor nongaussianities

Abstract: We study a model of inflation where the scalar perturbations are almost gaussian while there is sizable (equilateral) nongaussianity in the tensor sector. In this model, a rolling pseudoscalar gravitationally coupled to the inflaton amplifies the vacuum fluctuations of a vector field. The vector sources both scalar and tensor metric perturbations. Both kinds of perturbations are nongaussian, but, due to helicity conservation, the tensors have a larger amplitude, so that nongaussianity in the scalar perturbations is negligible. Moreover, the tensors produced this way are chiral. We study, in the flat sky approximation, how constraints on tensor nongaussianities affect the detectability of parity violation in the Cosmic Microwave Background. We expect the model to feature interesting patterns on nongaussianities in the polarization spectra of the CMB.

Mesons in large-N QCD

Abstract: We present the results of a systematic, first-principles study of the spectrum and decay constants of mesons for different numbers of color charges N, via lattice computations. We restrict our attention to states in the non-zero isospin sector, evaluating the masses associated with the ground-state and first excitation in the pseudoscalar, vector, scalar, and axial vector channels. Our results are based on a new set of simulations of four dimensional SU(N) Yang-Mills theories with the number of colors ranging from N = 2 to N =17;thespectraandthedecayconstantsarecomputedinthequenchedapproximation (which becomes exact in the ’t Hooft limit) using Wilson fermions. After discussing the extrapolations to the chiral and large-N limits, we present a comparison of our results to some of the numerical computations and analytical predictions available in the literature — including, in particular, those from holographic computations.

The couplings of the Higgs boson and its CP properties from fits of the signal strengths and their ratios at the 7+8 TeV LHC

Abstract: Using the full set of the LHC Higgs data from the runs at 7 and 8 TeV center of mass energies that have been released by the ATLAS and CMS collaborations, we determine the couplings of the Higgs particle to fermions and gauge bosons as well as its parity or CP composition. We consider ratios of production cross sections times decay branching fractions in which the theoretical (and some experimental) uncertainties as well as as some ambiguities from new physics cancel out. A fit of both the signal strengths in the various search channels that have been conducted, H -> Z Z, W W, gamma gamma, tau tau and b b, and their ratios shows that the observed ~126 GeV particle has couplings to fermions and gauge bosons that are Standard Model-like already at the 68% confidence level (CL). From the signal strengths in which the theoretical uncertainty is taken to be a bias, the particle is shown to be at most 68% CP-odd at the 99%CL and the possibility that it is a pure pseudoscalar state is excluded at the 4 sigma level when including both the experimental and theoretical uncertainties. The signal strengths also measure the invisible Higgs decay width which, with the same type of uncertainty analysis, is shown to be Gamma_H^inv / Gamma_H^SM < 0.52 at the 68%CL.

Decay of the pseudoscalar glueball into scalar and pseudoscalar mesons

Abstract: We study a chiral Lagrangian that describes the two- and three-body decays of a pseudoscalar glueball into scalar and pseudoscalar mesons. The various branching ratios are a parameter-free prediction of our approach. We compute the decay channels for a pseudoscalar glueball with a mass of 2.6 GeV, as predicted by lattice QCD in the quenched approximation, which is in the reach of the PANDA experiment at the upcoming Facility for Antiproton and Ion Research. For completeness, we also repeat the calculation for a glueball mass of 2.37 GeV that corresponds to the mass of the resonance $X(2370)$ measured in the BESIII experiment.

Probing new physics with the B s → μ + μ − time-dependent rate

Abstract: The B s → μ + μ − decay plays an outstanding role in tests of the Standard Model and physics beyond it. The LHCb collaboration has recently reported the first evidence for this decay at the 3.5 σ level, with a branching ratio in the ballpark of the Standard Model prediction. Thanks to the recently established sizable decay width difference of the B s system, another observable, $ \mathcal{A}_{{\varDelta \varGamma}}^{{\mu \mu }} $ , is available, which can be extracted from the time- dependent untagged B s → μ + μ − rate. If tagging information is available, a CP-violating asymmetry, S μμ , can also be determined. These two observables exhibit sensitivity to New Physics that is complementary to the branching ratio. We define and analyse scenarios in which these quantities allow us to discriminate between model-independent effective operators and their CP-violating phases. In this context we classify a selection of popular New Physics models into the considered scenarios. Furthermore, we consider specific models with tree-level FCNCs mediated by a heavy neutral gauge boson, pseudoscalar or scalar, finding striking differences in the predictions of these scenarios for the observables considered and the correlations among them. We update the Standard Model prediction for the time-integrated branching ratio taking the subtle decay width difference effects into account. We find (3.56 ± 0.18) × 10−9, and discuss the error budget.

SU(2) chiral perturbation theory low-energy constants from 2+1 flavor staggered lattice simulations

Abstract: We extract the next-to-leading-order low-energy constants ${\overline{\ensuremath{\ell}}}_{3}$ and ${\overline{\ensuremath{\ell}}}_{4}$ of SU(2) chiral perturbation theory, based on precise lattice data for the pion mass and decay constant on ensembles generated by the Wuppertal-Budapest Collaboration for QCD thermodynamics. These ensembles feature $2+1$ flavors of two-fold stout-smeared dynamical staggered fermions combined with Symanzik glue, with pion masses varying from 135 to 435 MeV, lattice scales between 0.7 and 2.0 GeV, while ${m}_{s}$ is kept fixed at its physical value. Moderate taste splittings and the scale being set through the pion decay constant allow us to restrict ourselves to the taste pseudoscalar state and to use formulas from continuum chiral perturbation theory. Finally, by dropping the data points near 135 MeV from the fits, we can explore the range of pion masses that is needed in SU(2) chiral perturbation theory to reliably extrapolate to the physical point.

The Full $B \to D^{*} \tau^{-} \bar{\nu_\tau}$ Angular Distribution and CP violating Triple Products

Abstract: We perform a comprehensive study of the impact of new-physics operators with different Lorentz structures on $\BDstarellnu$ decays, $(\ell = e, \mu, \tau $) involving the $b \to c l u_\ell$ transition. We present the full three angle and $q^2$ angular distribution with new physics operators with complex couplings. Various observables are constructed from the angular distribution with special focus on the CP violating triple product asymmetries which vanish in the Standard Model without any hadronic complications. Two of the three triple products are only sensitive to vector/axial vector new physics operators. %and do not depend on pseudoscalar new physics. Hence, the measurements of non-zero triple-product asymmetries will be a clear sign of new physics and a strong signal for vector/axial vector new physics operators. Even though we focus on $\tau$ final state, one can use the triple-products to search for new physics with $e$ and $\mu$ in the final state.

Parity violation in the CMB bispectrum by a rolling pseudoscalar

Abstract: We investigate parity-violating signatures of temperature and polarization bispectra of the cosmic microwave background (CMB) in an inflationary model where a rolling pseudoscalar produces large equilateral tensor non-Gaussianity. By a concrete computation based on full-sky formalism, it is shown that resultant CMB bispectra have nonzero signals in both parity-even $(\ell_1 + \ell_2 + \ell_3 = {\rm even})$ and parity-odd $(\ell_1 + \ell_2 + \ell_3 = {\rm odd})$ spaces, and are almost uncorrelated with usual scalar-mode equilateral bispectra. These characteristic signatures and polarization information help to detect such tensor non-Gaussianity. Use of both temperature and E-mode bispectra potentially improves of $400\%$ the detectability with respect to an analysis with temperature bispectrum alone. Considering B-mode bispectrum, the signal-to-noise ratio may be able to increase by 3 orders of magnitude. We present the $1\sigma$ uncertainties of a parameter depending on a coupling constant and a rolling condition for the pseudoscalar expected in the ${\it Planck}$ and the proposed PRISM experiments.

Constraints on short-range spin-dependent interactions from scalar spin-spin coupling in deuterated molecular hydrogen.

Abstract: A comparison between existing nuclear magnetic resonance measurements and calculations of the scalar spin-spin interaction (J coupling) in deuterated molecular hydrogen yields stringent constraints on anomalous spin-dependent potentials between nucleons at the atomic scale (∼ 1 A). The dimensionless coupling constant g(P)(p)g(P)(N)/4 π associated with the exchange of pseudoscalar (axionlike) bosons between nucleons is constrained to be less than 3.6 × 10(-7) for boson masses in the range of 5 keV, representing improvement by a factor of 100 over previous constraints. The dimensionless coupling constant g(A)(p)g(A)(N)/4 π associated with the exchange of an axial-vector boson between nucleons is constrained to be g(A)(p)g(A)(N)/4 π<1.3 × 10(-19) for bosons of mass ≲ 1000 eV, improving constraints at this distance scale by a factor of 100 for proton-proton couplings and more than 8 orders of magnitude for neutron-proton couplings.

Probing the two light Higgs scenario in the NMSSM with a low-mass pseudoscalar

Abstract: In this article we propose a simultaneous collider search strategy for a pair of scalar bosons in the NMSSM through the decays of a very light pseudoscalar. The massive scalar has a mass around 126 GeV while the lighter one can have a mass in the vicinity of 98 GeV (thus explaining an apparent LEP excess) or be much lighter. The successive decay of this scalar pair into two light pseudoscalars, followed by leptonic pseudoscalar decays, produces clean multi-lepton final states with small or no missing energy. Furthermore, this analysis offers an alternate leptonic probe for the 126 GeV scalar that can be comparable with the ZZ * search channel. We emphasize that a dedicated experimental search for multi-lepton final states can be an useful probe for this scenario and, in general, for the NMSSM Higgs sector. We illustrate our analysis with two representative benchmark points and show how the LHC configuration with 8 TeV center-of-mass energy and 25 fb−1 of integrated luminosity can start testing this scenario, providing a good determination of the light pseudoscalar mass and a relatively good estimation of the lightest scalar mass.

Determining the Higgs spin and parity in the diphoton decay channel.

Abstract: We calculate the diphoton distribution in the decay of arbitrary spin-0 and spin-2 bosons produced from gluon fusion, taking into account the fact that gluons inside an unpolarized proton are generally linearly polarized. The gluon polarization brings about a difference in the transverse momentum distribution of positive and negative parity states. At the same time, it causes the azimuthal distribution of the photon pair to be nonisotropic for several spin-2 coupling hypotheses, allowing one to distinguish these from the isotropic scalar and pseudoscalar distributions.

Sommerfeld enhancements with vector, scalar and pseudoscalar force-carriers

Abstract: The first AMS-02 measurement confirms the existence of an excess in the cosmic-ray positron fraction previously reported by the PAMELA and Fermi-LAT experiments. If interpreted in terms of thermal dark matter (DM) annihilation, the AMS-02 result still suggests that the DM annihilation cross section in the present day should be significantly larger than that at freeze-out. The Sommerfeld enhancement of the DM annihilation cross section is a possible explanation, which is however subject to the constraints from DM thermal relic density, mainly due to the annihilation of DM particles into force-carrier particles introduced by the mechanism. We show that the effects of the Sommerfeld enhancement and the relic density constraints depend significantly on the nature of the force carrier. Three scenarios where the force carrier is a vector boson, scalar, and pseudoscalar particle are investigated and compared. The results show that for the case with a vector force carrier, the Sommerfeld enhancement can marginally account for the AMS-02 data for the DM particle annihilating into 2 mu final states, while for the scalar force carrier the allowed Sommerfeld enhancement factor can be larger by a factor of 2. For the case with a pseudoscalar force carrier, the Sommerfeld enhancement factor can be very large in the resonance region, and it is possible to accommodate the AMS-02 and Fermi-LAT result for a variety of DM annihilation final states.

Simultaneous enhancement in $\gamma \gamma, b\bar{b}$ and $\tau^{+} \tau^{-}$ rates in the NMSSM with nearly degenerate scalar and pseudoscalar Higgs bosons

Abstract: We propose an experimental test of a scenario in the next-to-minimal supersymmetric standard model in which both the lightest scalar and the lightest pseudoscalar Higgs bosons have masses around 125 GeV. The pseudoscalar can contribute significantly to the $\ensuremath{\gamma}\ensuremath{\gamma}$ rate at the LHC due to light Higgsino-like charginos in its effective one-loop coupling to two photons. Such charginos are obtained for small values of the ${\ensuremath{\mu}}_{\mathrm{eff}}$ parameter, which also results in enhanced $b\overline{b}$ and ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ rates compared to those expected for a standard model (SM) Higgs boson. This scenario should result in a clear discrepancy between the observed rates in these three decay channels and those in the $WW$ and $ZZ$ channels, since the pseudoscalar does not couple to the $W$ and $Z$ bosons. However, in the dominant gluon fusion production mode, the pseudoscalar will stay hidden behind the SM-like scalar Higgs boson, and in order for it to be observable, the associated $b\overline{b}h$ production mode has to be considered, the cross section for which is tiny in the SM but $\mathrm{tan} \ensuremath{\beta}$-enhanced in supersymmetry. We analyze the constrained next-to-minimal supersymmetric standard model with nonuniversal Higgs sector parameters and identify regions of its parameter space where the lightest pseudoscalar with mass around 125 GeV and strongly enhanced $\ensuremath{\gamma}\ensuremath{\gamma}$ (up to 60%), $b\overline{b}$ and ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ rates in the $b\overline{b}h$ mode can be obtained.

Pion momentum distributions in the nucleon in chiral effective theory

Abstract: We compute the light-cone momentum distributions of pions in the nucleon in chiral effective theory using both pseudovector and pseudoscalar pion-nucleon couplings. For the pseudovector coupling we identify $\ensuremath{\delta}$-function contributions associated with end-point singularities arising from the pion-nucleon rainbow diagrams, as well as from pion bubble and tadpole diagrams which are not present in the pseudoscalar model. Gauge invariance is demonstrated, to all orders in the pion mass, with the inclusion of Kroll-Ruderman couplings involving operator insertions at the $\ensuremath{\pi}NN$ vertex. The results pave the way for phenomenological applications of pion cloud models that are manifestly consistent with the chiral symmetry properties of QCD.

Ultraviolet asymptotics of glueball propagators

Abstract: We point out that perturbation theory in conjunction with the renormalization group (RG) puts a severe constraint on the structure of the large-N non-perturbative glueball propagators in SU(N) pure Y M, in QCD and in $ \mathcal{N} $ = 1 SU SY QCD with massless quarks, or in any confining asymptotically-free gauge theory massless in perturbation theory. For the scalar and pseudoscalar glueball propagators in pure Y M and QCD with massless quarks we check in detail the RG-improved estimate to the order of the leading and next-to-leading logarithms by means of a remarkable three-loop computation by Chetyrkin et al. We investigate as to whether the aforementioned constraint is satisfied by any of the scalar or pseudoscalar glueball propagators computed in the framework of the AdS String/ large-N Gauge Theory correspondence and of a recent proposal based on a Topological Field Theory underlying the large-N limit of Y M . We find that none of the proposals for the scalar or the pseudoscalar glueball propagators based on the AdS String/large-N Gauge Theory correspondence satisfies the constraint, actually as expected, since the gravity side of the correspondence is in fact strongly coupled in the ultraviolet. On the contrary, the Topological Field Theory satisfies the constraint that follows by the asymptotic freedom.