Search Results (153 total)

We consider a new interaction between a heavy Majorana neutrino (N) and a charged Higgs boson (H^±), and show that it can have drastic implications on lepton-number-violating (LNV) signal of same-sign dileptons at the CERN LHC. The LNV signal of heavy Majorana neutrinos previously considered at the LHC, pp→ℓ⁺N→ℓ⁺ℓ⁺W^-, may be overwhelmed by pp→ℓ⁺N→ℓ⁺ℓ⁺H^-. With the subsequent decays H^-→t̅ b or H^-→W^-H⁰, the heavy Majorana neutrino production leads to the spectacular events of ℓ⁺ℓ⁺bb̅ +2 jets. We also explore the case m_N<m_H⁺, where the decay H⁺→ℓ⁺N can become the dominant N-production mechanism at the LHC. In particular, we show that the process gb→tH^- followed by t→bW⁺ and H^-→ℓ^-N→ℓ^-ℓ^-W⁺ could lead to another type of spectacular events of ℓ^-ℓ^-b+4 jets.

We present the first results for the K_l3 form factor from simulations with 2+1 flavors of dynamical domain wall quarks. Combining our result, namely, f₊(0)=0.964(5) with the latest experimental results for K_l3 decays leads to |V_us|=0.2249(14), reducing the uncertaintity in this important parameter. For the O(p⁶) term in the chiral expansion we obtain Δf=-0.013(5).

A linear relation between Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing parameters, η̅ =tan⁡Φ_3/2(ρ̅ -0.24±0.03), involving a 1σ range for Φ_3/2, 20°<Φ_3/2<115°, is obtained from B⁰→K^*π amplitudes measured recently in Dalitz plot analyses of B⁰→K⁺π^-π⁰ and B⁰(t)→K_Sπ⁺π^-. This relation is consistent within the large error on Φ_3/2 with other CKM constraints. We discuss the high sensitivity of this method to a new physics contribution in the ΔS=ΔI=1 amplitude.

We consider direct experimental verification of warped models, based on the Randall-Sundrum (RS) scenario, that explain gauge and flavor hierarchies, assuming that the gauge fields and fermions of the standard model (SM) propagate in the 5D bulk. Most studies have focused on the bosonic Kaluza-Klein (KK) signatures and indicate that discovering gauge KK modes is likely possible, yet challenging, while graviton KK modes are unlikely to be accessible at the CERN LHC, even with a luminosity upgrade. We show that direct evidence for bulk SM fermions, i.e. their KK modes, is likely also beyond the reach of a luminosity-upgraded LHC. Thus, neither the spin-2 KK graviton, the most distinct RS signal, nor the KK SM fermions, direct evidence for bulk flavor, seem to be within the reach of the LHC. We then consider hadron colliders with sqrt[s]=21, 28, and 60 TeV. We find that discovering the first KK modes of SM fermions and the graviton typically requires the Next Hadron Collider (NHC) with sqrt[s]≈60  TeV and O(1)  ab^-1 of integrated luminosity. If the LHC yields hints of these warped models, establishing that nature is described by them, or their 4D conformal field theory duals, requires an NHC-class machine in the post-LHC experimental program.

We present the first results for neutral-kaon mixing using (2+1)-flavors of domain-wall fermions. A new approach is used to extrapolate to the physical up and down quark masses from our numerical studies with pion masses in the range 240–420 MeV; only SU(2)_L×SU(2)_R chiral symmetry is assumed and the kaon is not assumed to be light. Our main result is B_K^MS̅ (2  GeV)=0.524(10)(28) where the first error is statistical and the second incorporates estimates for all systematic errors.

We study signals at the Large Hadron Collider (LHC) for Kaluza-Klein (KK) excitations of the electroweak gauge bosons in the framework with the standard model (SM) gauge and fermion fields propagating in a warped extra dimension. Such a framework addresses both the Planck-weak and flavor hierarchy problems of the SM. Unlike the often studied Z^′ cases, in this framework, there are three neutral gauge bosons due to the underlying SU(2)_L×SU(2)_R×U(1)_X gauge group in the bulk. Furthermore, couplings of these KK states to light quarks and leptons are suppressed, whereas those to top and bottom quarks are enhanced compared to the SM gauge couplings. Therefore, the production of light quark and lepton states is suppressed relative to other beyond the SM constructions, and the fermionic decays of these states are dominated by the top and bottom quarks, which are, though, overwhelmed by KK gluons dominantly decaying into them. However, as we emphasize in this paper, decays of these states to longitudinal W, Z and Higgs are also enhanced similarly to the case of top and bottom quarks. We show that the W, Z and Higgs final states can give significant sensitivity at the LHC to ∼2(3)  TeV KK scale with an integrated luminosity of ∼100  fb^-1 (∼1  ab^-1). Since current theoretical framework(s) favor KK masses ≳3  TeV, a luminosity upgrade of LHC is likely to be crucial in observing these states.

In the Randall-Sundrum model, setting the ratio of up- and down-quark masses m_u/m_d≪1, relevant to the strong CP problem, does not require chiral symmetry or fine-tuning, due to exponential bulk fermion profiles. We point out that such geometric suppression of the mass of a fermion magnifies the masses of its corresponding Kaluza-Klein (KK) states. In this sense, these KK states act as “microscopes” for probing light quark and lepton masses. In simple realizations, this hypothesis can be testable at future colliders, like the LHC, by measuring the spectrum of level-1 KK fermions. The microscope can then provide an experimental test for the vanishing of m_u in the ultraviolet, independently of nonperturbative determinations, by lattice simulations or other means, at hadronic scales. We also briefly comment on application of our microscope idea to other fermions, such as the electron and neutrinos.

An exploratory study of charmless 3-body decays of B mesons is presented using a simple model based on the framework of the factorization approach. The nonresonant contributions arising from B→P₁P₂ transitions are evaluated using heavy meson chiral perturbation theory (HMChPT). The momentum dependence of nonresonant amplitudes is assumed to be in the exponential form e^{-α_NRp_B·(p_i+p_j)} so that the HMChPT results are recovered in the soft meson limit p_i,p_j→0. In addition, we have identified another large source of the nonresonant signal in the matrix elements of scalar densities, e.g. ⟨KK̅ |s̅ s|0⟩, which can be constrained from the decay B̅ ⁰→K_SK_SK_S or B^-→K^-K_SK_S. The intermediate vector-meson contributions to 3-body decays are identified through the vector current, while the scalar meson resonances are mainly associated with the scalar density. Their effects are described in terms of the Breit-Wigner formalism. Our main results are: (i) All KKK modes are dominated by the nonresonant background. The predicted branching ratios of K⁺K^-K_S(L), K⁺K^-K^- and K^-K_SK_S modes are consistent with the data within errors. (ii) Although the penguin-dominated B⁰→K⁺K^-K_S decay is subject to a potentially significant tree pollution, its effective sin⁡2β is very similar to that of the K_SK_SK_S mode. However, direct CP asymmetry of the former, being of order -4%, is more prominent than the latter. (iii) For B→Kππ decays, we found sizable nonresonant contributions in K^-π⁺π^- and K̅ ⁰π⁺π^- modes, in agreement with the Belle measurements but larger than the BABAR result. (iv) Time-dependent CP asymmetries in K_Sπ⁰π⁰, a purely CP-even state, and K_Sπ⁺π^-, an admixture of CP-even and CP-odd components, are studied. (v) The π⁺π^-π⁰ mode is found to have a rate larger than π⁺π^-π^- even though the former involves a π⁰ in the final state. They are both dominated by resonant ρ contributions. (vi) We have computed the resonant contributions to 3-body decays and determined the rates for the quasi-two-body decays B→VP and B→SP. The predicted ρπ, f₀(980)K and f₀(980)π rates are in agreement with the data, while the calculated ϕK, K^*π, ρK and K₀^*(1430)π are in general too small compared to experiment. (vii) Sizable direct CP asymmetry is found in K⁺K^-K^- and K⁺K^-π^- modes.

We study the production and decay of Kaluza-Klein (KK) gravitons at the CERN Large Hadron Collider (LHC), in the framework of a warped extra dimension in which the standard model (SM) fields propagate. Such a scenario can provide solutions to both the Planck-weak hierarchy problem and the flavor puzzle of the SM. In this scenario, the production via qq̅ annihilation and decays to the conventional photon and lepton channels are highly suppressed. However, we show that graviton production via gluon fusion followed by decay to longitudinal Z/W can be significant; vector boson fusion is found to be a subdominant production mode. In particular, the golden ZZ decay mode offers a distinctive 4-lepton signal that could lead to the observation at the LHC with 300  fb^-1 (SLHC with 3  ab^-1) of a KK graviton with a mass up to ∼2 (∼3) TeV for the ratio of the AdS₅ curvature to the Planck scale modestly above unity. We argue that (contrary to the lore) such a size of the curvature scale can still be within the regime of validity of the framework. Upgrades beyond the SLHC luminosity are required to discover gravitons heavier than ∼4  TeV, as favored by the electroweak and flavor precision tests in the simplest such models.

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 present results for light meson masses and pseudoscalar decay constants from the first of a series of lattice calculations with 2+1 dynamical flavors of domain wall fermions and the Iwasaki gauge action. The work reported here was done at a fixed lattice spacing of about 0.12 fm on a 16³×32 lattice, which amounts to a spatial volume of (2  fm)³ in physical units. The number of sites in the fifth dimension is 16, which gives m_res=0.00308(4) in these simulations. Three values of input light sea quark masses, m_l^sea≈0.85m_s, 0.59m_s and 0.33m_s were used to allow for extrapolations to the physical light quark limit, while the heavier sea quark mass was fixed to approximately the physical strange quark mass m_s. The exact rational hybrid Monte Carlo algorithm was used to evaluate the fractional powers of the fermion determinants in the ensemble generation. We have found that f_π=127(4)  MeV, f_K=157(5)  MeV and f_K/f_π=1.24(2), where the errors are statistical only, which are in good agreement with the experimental values.

We present results for the static interquark potential, light meson and baryon masses, and light pseudoscalar meson decay constants obtained from simulations of domain wall QCD with one dynamical flavour approximating the s quark, and two degenerate dynamical flavours with input bare masses ranging from m_s to m_s/4 approximating the u and d quarks. We compare these quantities obtained using the Iwasaki and DBW2 improved gauge actions, and actions with larger rectangle coefficients, on 16³×32 lattices. We seek parameter values at which both the chiral symmetry breaking residual mass due to the finite lattice extent in the fifth dimension and the Monte Carlo time history for topological charge are acceptable for this set of quark masses at lattice spacings above 0.1 fm. We find that the Iwasaki gauge action is best, demonstrating the feasibility of using QCDOC to generate ensembles which are good representations of the QCD path integral on lattices of up to 3 fm in spatial extent with lattice spacings in the range 0.09–0.13 fm. Despite large residual masses and a limited number of sea quark-mass values with which to perform chiral extrapolations, our results for light hadronic physics scale and agree with experimental measurements within our statistical uncertainties.

In our previous paper we applied U-spin symmetry to charmless hadronic B^±→M⁰M^± decays for the purpose of precise extraction of the unitarity angle γ. In this paper we extend our approach to neutral B⁰ and B_s→M₁M₂ decays. A very important feature of this method is that no assumptions regarding relative sizes of topological decay amplitudes need to be made. As a result, this method avoids an uncontrollable theoretical uncertainty that is often related to the neglect of some topological diagrams (e.g., exchange and annihilation graphs) in quark-diagrammatic approaches. In charged B^± decays, each of the four data sets, P⁰P^±, P⁰V^±, V⁰P^± and V⁰V^±, with P≡pseudoscalar and V≡vector, can be used to obtain a value of γ. Among neutral decays, only experimental data in the B⁰, B_s→P^-P⁺ subsector is sufficient for a U-spin fit. Application of the U-spin approach to the current charged and neutral B decay data yields: γ=(80_-8⁺⁶)°. In this method, which is completely data driven, in a few years we should be able to obtain a model-independent determination of γ with an accuracy of O(few degrees).

We propose a new set of observables that can be used as experimental null tests of the Standard Model in charged and neutral B decays. The CP asymmetries in hadronic decays of charged B mesons into inclusive final states containing at least one of the following mesons: K_S,L, η^′, cc̅ bound states or neutral K^* or D mesons, for all of which a U-spin rotation is equivalent to a CP conjugation, are CKM suppressed and furthermore vanish in the exact U-spin limit. We show how this reduces the theoretical error by using Soft Collinear Effective Theory to calculate the CP asymmetries for K_S,LX_s+d, K^*X_s+d and η^′X_s+d final states in the endpoint region. For these CP asymmetries only the flavor and not the charge of the decaying B meson needs to be tagged up to corrections of NLO in 1/m_b, making the measurements more accessible experimentally.

Hadronic matrix elements of operators relevant to nucleon decay in grand unified theories are calculated numerically using lattice QCD. In this context, the domain-wall fermion formulation, combined with nonperturbative renormalization, is used for the first time. These techniques bring reduction of a large fraction of the systematic error from the finite lattice spacing. Our main effort is devoted to a calculation performed in the quenched approximation, where the direct calculation of the nucleon to pseudoscalar matrix elements as well as the indirect estimate of them from the nucleon to vacuum matrix elements are performed. First results, using two flavors of dynamical domain-wall quarks for the nucleon to vacuum matrix elements, are also presented to address the systematic error of quenching, which appears to be small compared to the other errors. Our results suggest that the representative values for the low-energy constants from the nucleon to vacuum matrix elements are given as |α|≃|β|≃0.01  GeV³. For a more reliable estimate of the physical low-energy matrix elements, it is better to use the relevant form factors calculated in the direct method. The direct method tends to give a smaller value of the form factors, compared to the indirect one, thus enhancing the proton lifetime; indeed, for the π⁰ final state the difference between the two methods is quite appreciable.

We study top quark flavor violation in the framework of a warped extra dimension with the Standard Model (SM) fields propagating in the bulk. Such a scenario provides solutions to both the Planck-weak hierarchy problem and the flavor puzzle of the SM without inducing a flavor problem. We find that, generically, tcZ couplings receive a huge enhancement, in particular, the right-handed ones can be O(1%). This results in BR(t→cZ) at or above the sensitivity of the Large Hadron Collider (LHC). At the International Linear Collider (ILC), single top production, via e⁺e^-→tc̅ , can be a striking signal for this scenario. In particular, it represents a physics topic of critical importance that can be explored even with a relatively low energy option, close to the tc threshold. At both the LHC and the ILC, angular distributions can probe the above prediction of dominance of right-handed couplings.

Recently Ciuchini, Pierini, and Silvestrini proposed a method for constraining CKM parameters in B→Kππ and B_s→Kππ through phase measurements of amplitudes involving I=3/2 K^*π final states. We show that complementary information on CKM parameters may be obtained by studying the phases of ΔI=1 B→(K^*π)_I=1/2, B_s→(K^*K̅ )_I=1 and B_s→(K̅ ^*K)_I=1 amplitudes. Hadronic uncertainties in these constraints from electroweak penguin operators O₉ and O₁₀, studied using flavor SU(3), are shown to be very small in B→Kππ and B_s→Kππ and somewhat larger in B_s→KK̅ π. The first processes imply a precise linear relation between ρ̅ and η̅ , with a measurable slope and an intercept at η̅ =0 involving a theoretical error of 0.03. The decays B_s→Kππ permit a measurement of γ involving a theoretical error below a degree. We note that while time-dependence is required when studying B⁰ decays at the Υ(4S), it may not be needed when studying B_s decays at hadronic colliders.

We report a study describing the charm quark by a domain-wall fermion (DWF) in lattice quantum chromodynamics (QCD). Our study uses a quenched gauge ensemble with the DBW2 rectangle-improved gauge action at a lattice cutoff of a^-1∼3  GeV. We calculate masses of heavy-light (charmed) and heavy-heavy (charmonium) mesons with spin-parity J^P=0^∓ and 1^∓, leptonic decay constants of the charmed pseudoscalar mesons (D and D_s), and the D⁰-D̅ ⁰ mixing parameter. The charm quark mass is found to be m_c^MS̅ (m_c)=1.24(1)_stat(18)_syst  GeV. The mass splittings in charmed-meson parity partners Δ_q,J=0 and Δ_q,J=1 are degenerate within statistical errors, in accordance with experiment, and they satisfy a relation Δ_q=ud,J>Δ_q=s,J, also consistent with experiment. Using our lattice calculation of the splitting between h_c and χ_c1 and the experimental χ_c1 mass, we obtain a parity-odd axial-vector charmonium state m_hc=3533(11)_stat(336)_syst  MeV, with a systematic error dominated by heavy quark discretization at order (am_c)². However, in this regard, we emphasize significant discrepancies in the calculation of hyperfine splittings on the lattice. The leptonic decay constants of D and D_s mesons are found to be f_D=232(7)_stat(+6 / -0)_chiral(17)_syst  MeV and f_Ds/f_D=1.05(2)_stat(+0 / -2)_chiral(2)_syst, where the first error is statistical, the second is systematic due to chiral extrapolation, and the third error is a combination of other known systematics. The D⁰-D̅ ⁰ mixing bag parameter, which enters the ΔC=2 transition amplitude, is found to be B_D(2  GeV)=0.845(24)_stat(+24 / -6)_chiral(105)_syst. All the above systematic errors include our estimates of quenching errors.

We calculate the vector form factor in K→πlν semileptonic decays at zero momentum transfer f₊(0) from numerical simulations of two-flavor QCD on the lattice. Our simulations are carried out on 16³×32 at a lattice spacing of a≃0.12  fm using a combination of the DBW2 gauge and the domain-wall quark actions, which possesses excellent chiral symmetry even at finite lattice spacings. The size of fifth dimension is set to L_s=12, which leads to a residual quark mass of a few MeV. Through a set of double ratios of correlation functions, the form factor calculated on the lattice is accurately interpolated to zero momentum transfer, and then is extrapolated to the physical quark mass. We obtain f₊(0)=0.968(9)(6), where the first error is statistical and the second is the systematic error due to the chiral extrapolation. Previous estimates based on a phenomenological model and chiral perturbation theory are consistent with our result. Combining with an average of the decay rate from recent experiments, our estimate of f₊(0) leads to the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V_us|=0.2245(27), which is consistent with CKM unitarity. These estimates of f₊(0) and |V_us| are subject to systematic uncertainties due to the finite lattice spacing and quenching of strange quarks, though nice consistency in f₊(0) with previous lattice calculations suggests that these errors are not large.

We discuss the implementation and properties of the quenched approximation in the calculation of the left-right, strong penguin contributions (i.e. Q₆) to ϵ^′/ϵ. The coefficient of the new chiral logarithm, discovered by Golterman and Pallante, which appears at leading order in quenched chiral perturbation theory is evaluated using both the method proposed by those authors and by an improved approach which is free of power divergent corrections. The result implies a large quenching artifact in the contribution of Q₆ to ϵ^′/ϵ. This failure of the quenched approximation affects only the strong penguin operators and so does not affect the Q₈ contribution to ϵ^′/ϵ nor ReA₀, ReA₂ and thus, the ΔI=1/2 rule at tree level in chiral perturbation theory.

We present numerical results for the kaon B-parameter, B_K, determined in the quenched approximation of lattice QCD. Our simulations are performed using domain-wall fermions and the renormalization group improved, DBW2 gauge action which combine to give quarks with good chiral symmetry at finite lattice spacing. Operators are renormalized nonperturbatively using the RI/MOM scheme. We study scaling by performing the simulation on two different lattices with a^-1=1.982(30) and 2.914(54) GeV. We combine this quenched scaling study with an earlier calculation of B_K using two flavors of dynamical, domain-wall quarks at a single lattice spacing to obtain B_K^{MS̅ NDR}(μ=2  GeV)=0.563(21)(39)(30), were the first error is statistical, the second systematic (without quenching errors) and the third estimates the error due to quenching.

We present a study of the neutron electric dipole moment (d→_N) within the framework of lattice QCD with two flavors of dynamical light quarks. The dipole moment is sensitive to the topological structure of the gauge fields, and accuracy can only be achieved by using dynamical, or sea quark, calculations. However, the topological charge evolves slowly in these calculations, leading to a relatively large uncertainty in d→_N. It is shown, using quenched configurations, that a better sampling of the charge distribution reduces this problem, but because the CP even part of the fermion determinant is absent, both the topological charge distribution and d→_N are pathological in the chiral limit. We discuss the statistical and systematic uncertainties arising from the topological charge distribution and unphysical size of the quark mass in our calculations and prospects for eliminating them. Our calculations employ the RBC collaboration two flavor domain wall fermion and DBW2 gauge action lattices with inverse lattice spacing a^-1≈1.7  GeV, physical volume V≈(2  fm)³, and light quark mass roughly equal to the strange quark mass (m_sea=0.03 and 0.04). We determine a value of the electric dipole moment that is zero within (statistical) errors, from which we obtain the bound |d→_N|≲0.02e-θ-fm. Satisfactory results for the magnetic and electric form factors of the proton and neutron are also obtained and presented.

We analyze the lepton sector of a left-right model based on the gauge group SU(2)_L×SU(2)_R×U(1), concentrating mainly on neutrino properties. Using the seesaw mechanism and a horizontal symmetry, we keep the right-handed symmetry breaking scale relatively low, while simultaneously satisfying phenomenological constraints on the light neutrino masses. We take the right-handed scale to be of order 10’s of TeV and perform a full numerical analysis of the model’s parameter space, subject to experimental constraints on neutrino masses and mixings. The numerical procedure yields results for the right-handed neutrino masses and mixings and the various CP-violating phases. We also discuss phenomenological applications of the model to neutrinoless double beta decay, lepton-flavor-violating decays (including decays such as τ→3μ) and leptogenesis.

We present results from the first large-scale study of two-flavor QCD using domain wall fermions (DWF), a chirally symmetric fermion formulation which has been proven to be very effective in the quenched approximation. We work on lattices of size 16³×32, with a lattice cutoff of a^-1≈1.7  GeV and dynamical (or sea) quark masses in the range m_strange/2≲m_sea≲m_strange. After discussing the algorithmic and implementation issues involved in simulating dynamical DWF, we report on the low-lying hadron spectrum, decay constants, static quark potential, and the important kaon weak matrix element describing indirect CP violation in the standard model, B_K. In the latter case we include the effect of nondegenerate quark masses (m_s≠m_u=m_d), finding B_K^MS̅ (2  GeV)=0.495(18).

Decay rates and time-dependent and direct CP asymmetries in the decays B⁰→K⁺K^-K_S(L) and K_SK_SK_S(L) are studied. Resonant and nonresonant contributions to the three-body decays are carefully investigated. Nonresonant effects on two-body and three-body matrix elements are constrained by QCD counting rules. The predicted branching ratios are consistent with the data within the theoretical and experimental errors, though the theoretical central values are somewhat smaller than the experimental ones. Owing to the presence of color-allowed tree amplitudes in B⁰→K⁺K^-K_S(L), this penguin-dominated mode may be subject to a potentially significant tree pollution and the deviation of the mixing-induced CP asymmetry from that measured in B→J/ψK_S, namely, Δsin⁡2β_K⁺K^-KS(L)≡sin⁡2β_K⁺K^-KS(L)-sin⁡2β_J/ψKS, can be as large as O(0.10). In contrast, the K_SK_SK_S(L) modes appear theoretically very clean in our picture with negligible theoretical errors in Δsin⁡2β_KSKSKS(L). Direct CP asymmetries in K⁺K^-K_S(L) and K_SK_SK_S(L) modes are found to be very small.