Search Results (51 total)

A charged Higgs pair can be produced at an ee collider through a t-channel exchange of a heavy neutrino (N) via e⁺e^-→H⁺H^- and, if N is a Majorana particle, also via the lepton number violating (LNV) like-sign reaction e^±e^±→H^±H^±. Assuming no a priori relation between the effective eNH⁺ coupling (ξ) and light neutrino masses, we show that this interaction vertex can give a striking enhancement to these charged Higgs pair production processes. In particular, the LNV H^-H^- signal can probe N at the International Linear Collider (ILC) in the mass range 100  GeV≲m_N≲10⁴  TeV and with the effective mixing angle ξ in the range 10^-4≲ξ²≲10^-8—well within its perturbative unitarity bound and the ββ_0ν limit. The lepton number conserving e⁺e^-→H⁺H^- mode can be sensitive to, e.g., an O(10)  TeV heavy Majorana neutrino at a 500 GeV ILC, if ξ²≳0.001.

We show that in a decay of the form B_d or B_s→P₁P₂γ (where P₁ and P₂ are pseudoscalar mesons), through a flavor changing dipole transition, time dependent oscillations can reveal the presence of physics beyond the standard model. If P₁ and P₂ are CP eigenstates (e.g. as in B_d→K_Sπ⁰γ), then to leading order in the effective Hamiltonian, the oscillation is independent of the resonance structure. Thus data from resonances as well as from nonresonant decays can be included. This may significantly enhance the sensitivity to new physics of the method. If P₁ is a charged particle, and P₂ its antiparticle (e.g. as in B_d→π⁺π^-γ), one has the additional advantage that both the magnitude and the weak phase of any new physics contribution can be determined from a study of the angular distribution. These signals offer excellent ways to detect new physics because they are suppressed in the standard model. We also show that the potential contamination of these signals originating from the standard model annihilation diagram gives rise to photons with, to a very good approximation, the same helicity as the dominant penguin graph and thus causes no serious difficulty. The formalism which applies to the case where P₁ and P₂ are C eigenstates also further generalizes to the case of final states containing multiple C eigenstates and a photon. This suggests several additional channels to search for new physics, such as K_Sη^′(η)γ, ϕK_Sγ etc. We also emphasize that the contribution of nondipole interactions can be monitored by the dependence of the mixing-induced CP asymmetry of nonresonant modes on the Dalitz variables. Furthermore, using a number of different final states can also provide important information on the contribution from nondipole effects.

Heavy meson pairs produced in the decays of heavy quarkonium resonances at e⁺e^- machines (beauty and tau-charm factories) have the useful property that the two mesons are in the CP-correlated states. We show that this leads to time-independent correlations allowing the extraction of the lifetime difference ΔΓ_D and other mixing parameters. In particular, for the decay ψ(3770)→D⁰D̅ ⁰ the correlation of a flavor specific decay of one D with a CP-specific decay of the other D is linearly sensitive to the D⁰ lifetime difference. The utility of this method is considered at CLEO-c as well as future threshold charm factories. We include the impact of possible CP-violating effects and present the complete results for time-integrated CP-entangled decay rates with CP violation taken into account. We comment on the utility of using this method to extract the lifetime difference of neutral B mesons at future high luminosity B factories.

The implementation of various methods for the determination of γ through direct CP violation arising in the interference of b→c and b→u processes in charged as well as neutral B meson decays are considered. We show that the inclusion of D⁰ resulting from D^*0→D⁰+π⁰(γ), say, via B→KD^*0 makes a significant difference in the attainable accuracy for γ. Both exclusive and inclusive decays of the B^± and B⁰ to states containing D⁰/D̅ ⁰ followed by both inclusive and exclusive decays of the D⁰ are discussed. It is shown that with statistics which might be obtained at B factories (5–10×10⁸ B pairs) a 1σ determination of γ to ≈±8° may be possible, depending on the efficiency of reconstruction, backgrounds, and the details of the decay amplitudes involved. The role of data from a charm factory as well as effects of D⁰ mixing are discussed. Extraction of γ with accuracy that is roughly commensurate with the intrinsic theory error of these methods (i.e., around 0.1%), which is an important goal, will require >10¹⁰ B pairs, namely, a super B factory.

Assuming that a nonstandard neutral Higgs boson with an enhanced Yukawa coupling to a bottom quark is observed at future hadron experiments, we propose a method for a better understanding of the Higgs sector. Our procedure is based on “counting” the number of events with heavy jets (where “heavy” stands for a c or b jet) versus b jets, in the final state of processes in which the Higgs boson is produced in association with a single high p_T c or b jet. We show that an observed signal of the type proposed, at either the Fermilab Tevatron or CERN Large Hadron Collider (LHC), will rule out the popular two Higgs doublet model of type II as well as its supersymmeric version, the minimal supersymmetric standard model, and may provide new evidence in favor of some more exotic multi-Higgs-boson scenarios. As an example, we show that in a version of a two Higgs doublet model which naturally accounts for the large mass of the top quark, our signal can be easily detected at the LHC within that framework. We also find that such a signal may be observable at the upgraded Tevatron run III, if the neutral Higgs boson in this model has a mass around 100 GeV and tanβ≳50 and if the efficiency for distinguishing a c jet from a light jet reaches the level of ∼50%.

We study the inclusive semileptonic rare decay b→sl⁺l^- in the minimal supergravity model. If tan β is large, down-type quark mass matrices and their Yukawa couplings cannot be diagonalized on the same basis. This induces flavor violating neutral Higgs boson couplings. These couplings contribute significantly to the decay b→sμ⁺μ^- and b→sτ⁺τ^-, but negligibly to the b→se⁺e^- decay because of its negligible m_e mass. The ratio R≡B(b→sμ⁺μ^-)/B(b→se⁺e^-) can be very different from its corresponding value in the standard model. We find that part of the parameter space can accommodate a large R value, and that the maximum R value can be larger than 2. We also present our results in the b→sτ⁺τ^- decay channel. Although it cannot be detected now, it is potentially a new channel for the future observation of new physics.

Oscillation effects in B⁰→K_SD⁰ and related processes are considered to determine δ≡β-α+π=2β+γ. We suggest that D⁰ decays to CP eigenstates used in concert with inclusive D⁰ decays provide a powerful method for determining δ cleanly. The CP asymmetry is expected to be ≲40% for D⁰ decays to non-CP eigenstates and ≲80% for decays to CP eigenstates. This method can lead to a fairly accurate determination of δ with O(10⁸-10⁹) B mesons.

In this paper we study the impact of data that can be obtained from a charm factory on the determination of the Cabibbo-Kobayashi-Maskawa parameter γ from decays of the form B→D⁰K where the D⁰ decays to specific inclusive and exclusive final states. In particular, for each exclusive final state f, the charm factory can determine the strong phase difference between D⁰→f and D⁰→f by exploiting correlations in ψ(3770)→D⁰D⁰. This provides crucial input to the determination of γ via the interference of B^±→K^±D⁰→f with B^±→K^±D⁰→f. We discuss how the method may be generalized to inclusive final states and illustrate with a toy model how such methods may offer one of the best means to determine γ with O(10⁸–10⁹) B mesons.

The flavor changing decay Z→d_Id_J is investigated with special emphasis on the bs̅ final state. Various models for flavor violation are considered: two Higgs doublet models (2HDM’s), supersymmetry (SUSY) with flavor violation in the up- and down-type squark mass matrices and SUSY with flavor violation mediated by R-parity-violating interaction. We find that, within the SUSY scenarios for flavor violation, the branching ratio for the decay Z→bs̅ can reach 10^-6 for large tan β values, while the typical size for this branching ratio in the 2HDM’s considered is about two orders of magnitude smaller at best. Thus, flavor changing SUSY signatures in radiative Z decays such as Z→bs̅ may be accessible to future “Z factories” such as a giga-Z version of the DESY TESLA design.

A method for the clean determination of the unitarity angles α and γ is proposed that uses only direct CP violation and does not require any time dependent measurements. The method takes advantage of the helicity amplitudes for B_u, B_d, and B_s decay to two vector mesons and can be used, at any B facility, in conjunction with a large number of modes. It also allows for experimental tests of the theoretical approximations involved.

Various methods are discussed for obtaining the CKM angle γ through the interference of the charged B-meson decay channels B^-→K^-D⁰ and B^-→K^-D⁰ where the D⁰ and D⁰ decay to common final states. It is found that choosing final states which are not CP eigenstates can lead to large direct CP violation which can give significant bounds on γ without any theoretical assumptions. If two or more modes are studied, γ may be extracted with a precision on the order of ±15° given ∼10⁸ B-mesons. We also discuss the case of three body decays of the D where additional information may be obtained from the distribution of the D decay products and consider the impact of DD̅ oscillations.

Recently, a scenario has been proposed in which the gravitational scale could be as low as the TeV scale, and extra dimensions could be large and detectable at the electroweak scale. Although supersymmetry is not a requirement of this scenario, it is nevertheless true that its best-motivated realizations arise in supersymmetric theories (such as M theory). We argue here that supersymmetry can have robust, and in some instances fatal, implications for the expected experimental signature for TeV-scale gravity. The signature of the supersymmetric version of the scenario differs most dramatically from what has been considered in the literature because mass splittings within the gravity supermultiplet in these models are extremely small, implying in particular the existence of a very light spin-one superpartner for the graviton. We compute the implications of this graviphoton, and show that it can acquire dimension-four couplings to ordinary matter which can strongly conflict with supernova bounds.

We consider the production of high invariant mass jet pairs at hadron colliders as a test for TeV scale gravitational effects. We find that this signal can probe effective Planck masses of about 10 TeV at the CERN LHC with a center of mass energy of 14 TeV and 1.5 TeV at the Fermilab Tevatron with a center of mass energy of 2 TeV. These results are compared to analogous scattering processes at leptonic colliders.

We consider the production of gravitons via two photon and electron-photon fusion in Kaluza-Klein theories which allow TeV scale gravitational interactions. We show that at electron-positron colliders, the processes l⁺l^-→l⁺l^-+graviton, with l=e, μ, can lead to a new signal of low energy gravity of the form l⁺l^-→l⁺l^-+missing energy which is well above the standard model background. For example, with two extra dimensions, at the Next Linear Collider with a center of mass energy of 500 or 1000 GeV, hundreds to thousands such l⁺l^-+graviton events may be produced if the scale of the gravitational interactions, M_D, is around a few TeV. At a gamma-electron collider, more stringent bounds may be placed on M_D via the related reaction e^-γ→e^-G. For instance, if a 1 TeV e⁺e^- collider is converted to an electron-photon collider, a bound of ∼10(14) TeV may be placed on the scale M_D if the number of extra dimensions δ=2, while a bound of ∼4(5) TeV may be placed if δ=4, with unpolarized (right polarized) electron beams.

We consider the scattering amplitudes of the form V₁V₂→V₃V₄, where V_i=γ, Z, W or g  (=gluon) are the standard model gauge bosons, due to graviton exchange in Kaluza-Klein theories with large extra dimensions. This leads to a number of experimentally viable signatures at high energy leptonic and hadronic colliders. We discuss the observability or attainable limits on the scale of the gravitational interactions (m_D), that may be obtained at an e⁺e^- Next Linear Collider (NLC) and at the CERN LHC, by studying some of these types of gauge-boson scattering processes. We find that the attainable limits through these type of processes are m_D≳3 TeV at the NLC and m_D≳6 TeV at the LHC.

We derive constraints for B→V₁V₂ modes (V_1,2=vector meson) that allow a model-independent quantitative assessment of the contributions from electroweak penguin diagrams and/or new physics. The interplay of direct CP with oscillation studies then may lead to the extraction of the angle α, using B→K^*ω(ρ) and B→ρω(φ). Any reservation one may have can be explicitly verified in a model-independent way by assuming only isospin conservation. We also briefly mention how the method can be used to extract γ via B_s decays: B_s→K^*ρ, K^*K^*.

A systematic experimental search for two-body hadronic decays of the b quark of the type b→quark+meson is proposed. These reactions have a well-defined experimental signature and they should be theoretically cleaner compared to exclusive decays. Many modes have appreciable branching ratios, and partial rate asymmetries may also be quite large (about 8%– 50%) in several of them. In a few cases electroweak penguins appear to be dominant and may be measurable. CP-violating triple correlation asymmetries provide a clean test of the standard model.

We consider the strong rescattering effects that can occur in decays such as B→Kπ, K^*π, Kρ,…, and their impact on direct CP violation in these modes. First we discuss, in general, how the CPT theorem constrains the resulting pattern of partial rate asymmetries (PRA’s) leading to different brands of direct CP violation. Traditional discussions have centered around the absorptive part of the penguin graph which has ΔI=0 in b→s transitions and as a result causes “simple” CP violation; long-distance final state rescattering effects, in general, will lead to a different pattern of CP violation: “compound” CP violation. Predictions of simple CP violation are quite distinct from that of compound CP violation. Final state rescattering phases in B decays are unlikely to be small possibly causing large compound CP-violating partial rate asymmetries in these modes. The CPT theorem requires a cancellation of PRA’s due to compound CP violation among the Kπ states themselves; thus there can be no net cancellation with other states such as K^*π, Kρ, etc. Therefore, each class of such modes, namely, Kπ, Kρ, K^*π, Ka₁, etc., can have large direct CP violation emanating from rescattering effects. Various repercussions for the angle γ are also discussed.

CP-nonconserving effects in the reaction pp̅ →tb̅ +X→W⁺bb̅ +X, driven by the supersymmetric CP-odd phase of the top squark trilinear soft breaking term arg(A_t), are studied. We discuss the CP-nonconserving effects in both production and the associated decay amplitudes of the top quark. We find that, within a plausible low energy scenario of the MSSM and keeping the neutron electric dipole moment below its current limit, a CP-violating cross-section asymmetry as large as 2-3 % can arise if some of the parameters lie in a favorable range. A partial rate asymmetry originating only in the top quark decay t→W⁺b is found to be, in general, below the 0.1% level which is somewhat smaller than previous claims. For a low tanβ of order one the decay asymmetry can reach at the most ∼0.3%. This (few) percent level overall CP-violating signal in pp̅ →tb̅ +X→W⁺bb̅ +X might be within the reach of the future 2(4) TeV pp̅ Fermilab Tevatron collider that may be able to produce ∼10000 (∼30000) such tb̅ events with an integrated luminosity of 30 fb^-1. In particular, it may be used to place an upper bound on arg(A_t) if indeed arg(μ)→0, as implied from the present experimental limit on the neutron electric dipole moment. The partial rate asymmetry in the top quark decay (∼few×10^-3) may also be within the reach of the CERN LHC with ∼10⁷ pairs of tt̅ produced, provided detector systematics are sufficiently small. We also show that if the GUT scale universality of the soft breaking trilinear A terms is relaxed, then the phases associated with arg(A_u) and arg(A_d) can take values up to ∼few×10^-1 even with squarks and gluino masses of several hundred GeV’s without contradicting the experimental limit on the neutron electric dipole moment.

Attributing the recent CLEO discovery of B→η^′+X_s to originate (primarily) from the fragmentation of an off-shell gluon (g^*) via b→s+g^*, g^*→g+η^′, we emphasize that many such states (X_g) should materialize. Indeed the hadronic fragments (X_g) of g^* are closely related to those seen in J/ψ→γ(φ, ω)+X_g. A particular final state of considerable interest is X_g  =  K⁺K^-. Signals from such states in B decays may be combined to provide a very sensitive search for CP violating phase(s) from nonstandard physics. The method should work even if the contribution of these source(s) to the rates is rather small (∼10%).

In the standard model (SM) the photon in radiative B̅ ⁰ and B̅ _s decays is predominantly left handed. Thus, mixing-induced CP asymmetries in b→sγ and b→dγ are suppressed by m_s/m_b and m_d/m_b, respectively, and are very small. In many extensions of the SM, such as the left-right symmetric model (LRSM), the amplitude of right-handed photons grows proportional to the virtual heavy fermion mass, which can lead to large asymmetries. In the LRSM, asymmetries larger than 50% are possible even when radiative decay rate measurements agree with SM predictions.

The Gronau-London-Wyler method extracts the Cabibbo-Kobayashi-Maskawa angle γ by measuring B^± decay rates involving D⁰/D̅ ⁰ mesons. CP violation can be greatly enhanced for decays to final states common to both D⁰ and D̅ ⁰ that are not CP eigenstates. Large asymmetries are possible for final states f such that D⁰→f is doubly Cabibbo suppressed while D̅ ⁰→f is Cabibbo allowed. The measurement of interference effects in such modes allows the clean extraction of γ.

A comprehensive phenomenological analysis of a two Higgs doublet model, with flavor-changing scalar currents at the tree level, called model III, is presented. Constraints from existing experimental information especially on ΔF=2 processes are systematically incorporated. Constraints emerging from rare B decays, Z→bb̅ , and the ρ parameter are also examined. Experimental implications for e⁺e^-(μ⁺μ^-)→tc̅ +t̅ c, t→cγ(Z,g), D⁰-D̅ ⁰, and B_s⁰-B̅ _s⁰ oscillations, and for e⁺e^-(Z)→bs̅ +b̅ s are investigated and experimental effort towards these is stressed. We also emphasize the importance of clarifying the experimental issues pertaining to Z→bb̅ .

The reaction pp̅ →tb̅ X is found to be rather rich in exhibiting several different types of CP asymmetries. The spin of the top quark plays an important role. Asymmetries are related to form factors arising from radiative corrections of the tbW production vertex due to nonstandard physics. As illustrations, effects are studied in two Higgs doublet models and in supersymmetric models; asymmetries up to a few percent may be possible.

A study of R_b and R_c is presented in the context of a two-Higgs-doublet model with flavor-changing scalar currents (FCSC's). Implications of the model for the ρ parameter and for b→sγ are also considered. The experimental data on R_b places stringent constraints on the model parameters that are difficult to reconcile with the constraints from b→sγ and ρ. If we treat the couplings of the model as purely phenomenological, then the model can still survive albeit in a rather narrow region of the parameter space. Noting that aspects of the experimental analysis for R_b and R_c may be of some concern, we also disregard R_b^expt and R_c^expt and give predictions for these using constraints from b→sγ and the ρ parameter only. We emphasize the theoretical and experimental advantages of the observable R_b+c≡Γ(Z→bb̅  or cc̅ ) / Γ(Z→hadrons). We also stress the role of R_l≡Γ(Z→hadrons) / Γ(Z→l⁺l^-) in testing the standard model despite its dependence on QCD corrections. Noting that in models with FCNC's the amplitude for Z→cc̅ receives a contribution which grows with m_t² the importance and uniqueness of precision Z→cc̅ measurements for constraining flavor-changing tc̅ currents is underscored.