Rare Charm Decays

Rare FCNC decay modes of charmed mesons are reviewed. The standard model predictions, including both short and long distance contributions, are summarized. Several new physics options that can give detectable signals are described.Comment: 4 pages. Presented at the 6th International Conference on Hyperons, Charm and Beauty Hadrons, IIT, Chicago, june 27-july 3, 2004. To be published in the Proceeding

Scalars from Top-condensation Models at Hadron Colliders

We study the production and decay of neutral scalars and pseudo-scalars at hadron colliders, in theories where the top-quark mass is the result of a $t\bar t$ condensate. We show that the dominant decay channel for masses below the $t\bar t$ threshold is the flavor changing mode $tc$. This is a consequence of the non-universal nature of the underlying interactions in all top-condensation models and provides a model-independent signature of these scenarios. We show that an upgraded Tevatron is sensitive to a sizeable region of the interesting parameter space and that the LHC will highly constrain these models through this flavor violating channel.Comment: 4 pages, 4 figures. Minor changes in figures for readibility. final version to appear in PR

First study of B→πB \to \pi semileptonic decay form factors using NRQCD

We present a quenched calculation of the form factors of the semileptonic weak decay $B \to \pi l \bar{\nu}$ with $O(1/m_Q)$ NRQCD heavy quark and Wilson light quark on a $16^3 \times 32$ lattice at $\beta=5.8$. The form factors are evaluated at six heavy quark masses, in the range of $m_Q \sim 1.5-8$ GeV. $1/m_Q$ dependence of matrix elements are investigated and compared with HQET predictions. We observe clear signal for the form factors near $q^2_{max}$, even at the $b$-quark mass range. $f^0(q^2_{max})$ is compared with $f_B/f_{\pi}$ based on the soft pion theorem and significant difference is observed.Comment: 3 pages, 5 ps figures, uses espcrc2.sty and epsf.sty, Talk presented at Lattice'9

Top quark associated production of the neutral top-pion at high energy e+e−e^{+}e^{-} colliders

In the context of topcolor-assisted technicolor (TC2) models, we calculate the associated production of the neutral top-pion $\pi_{t}^{0}$ with a pair of top quarks via the process $e^{+}e^{-}\longrightarrow t\bar{t}\pi_{t}^{0}$. We find that the production cross section is larger than that of the process $e^{+}e^{-}\longrightarrow t\bar{t}H$ both in the standard model (SM) and in the minimal supersymmetric SM. With reasonable values of the parameters in TC2 models, the cross section can reach $20fb$. The neutral top-pion $\pi_{t}^{0}$ may be direct observed via this process.Comment: Latex files, 10 pages and 3 figure

A Possible Arena for Searching New Physics - the Γ(D0→ρ0γ)/Γ(D0→ωγ)\Gamma(D^0 \to \rho^0 \gamma)/\Gamma (D^0 \to \omega \gamma) Ratio

We propose to investigate flavour changing neutral currents in the $c \to u \gamma$ transition through the measurement of the difference between $\Gamma (D^0 \to \rho^0 \gamma)$ and $\Gamma (D^0 \to \omega \gamma)$. This is based on the observation that $D^0 \to (d \bar d) \gamma$ is due to long distance physics while $D^0 \to (u \bar u) \gamma$ arises from the $c \to u \gamma$ transition. The effect of $\rho - \omega$ mixing is included. A difference in the decay widths of more than about 30% would be indicative of new physics.Comment: 7 pages, 1 figure include

Anomalous couplings of the third generation in rare B Decays

We study the potential effect of anomalous couplings of the third generation quarks to gauge bosons in rare B decays. We focus on the constraints from flavor changing neutral current processes such as $b\to s\gamma$ and $b\to s \ell^+\ell^-$. We consider both dimension-four and dimension-five operators and show that the latter can give large deviations from the standard model in the still unobserved dilepton modes, even after the bounds from $b\to s\gamma$ and precision electroweak observables are taken into account

Dispersive Approach to Semileptonic Form-Factors in Heavy-to-Light Meson Decays

We study the semileptonic decays of heavy mesons into light pseudoscalars by making use of dispersion relations. Constraints from heavy quark symmetry, chiral symmetry and perturbative QCD are implemented into a dispersive model for the form-factors. Large deviations from $B^*$-pole dominance are observed in $B\to\pi\ell\nu$. We discuss the model prediction for this mode and its possible impact on the extraction of $|V_{ub}|$.Comment: 30 pages, including 5 Postcript figure

Dominance of the light-quark condensate in the heavy-to-light exclusive decays

Using the QCD {\it hybrid} (moments-Laplace) sum rule, we show $semi$-$analytically$ that, in the limit M_b \rar \infty, the $q^2$ and $M_b$ behaviours of the heavy-to-light exclusive (\bar B\rar \rho~(\pi) semileptonic as well as the B\rar \rho\gamma rare) decay--form factors are $universally$ dominated by the contribution of the soft light-quark condensate rather than that of the hard perturbative diagram. The QCD-analytic $q^2$ behaviour of the form factors is a polynomial in $q^2/M^2_b$, which mimics quite well the usual pole parametrization, except in the case of the $A_1^B$ form factor, where there is a significant deviation from this polar form. The $M_b$-dependence of the form factors expected from HQET and lattice results is recovered. We extract with a good accuracy the ratios: $V^B(0)/A^B_1(0) \simeq A^B_2(0)/A^B_1(0) \simeq 1.11\pm 0.01$, and $A^B_1(0)/F^B_1(0) \simeq 1.18 \pm 0.06$; combined with the ``world average" value of $f^B_+(0)$ or/and $F^B_1(0)$, these ratios lead to the decay rates: $\Gamma_{\bar B\rar \pi e\bar \nu} \simeq (4.3 \pm 0.7)Comment: 10 pages, CERN-TH 7237/94 (the previous version contains numerical errors). Latex file (run twice) 3 ps.figures available by mai

Constraints on the Bulk Standard Model in the Randall-Sundrum Scenario

We derive constraints on the Randall-Sundrum scenario with the standard model fields in the bulk. These result from tree level effects associated with the deformation of the zero mode wave-functions of the W and the Z once electroweak symmetry is broken. Recently Cs\'{a}ki, Erlich and Terning pointed out that this implies large contributions to electroweak oblique parameters. Here we find that when fermions are allowed in the bulk the couplings of the $W$ and the $Z$ to zero-mode fermions are also affected. We perform a fit to electroweak observables assuming universal bulk fermion masses and including all effects and find constraints that are considerably stronger than for the case with fermions localized in the low energy boundary. These put the lowest Kaluza-Klein excitation out of reach of the Large Hadron Collider. We then relax the universality assumption and study the effects of flavor violation in the bulk and its possible signatures.Comment: 18 pages, 2 ps figure

Is the LHC Observing the Pseudo-scalar State of a Two-Higgs Doublet Model ?

The ATLAS and CMS collaborations have recently shown data suggesting the presence of a Higgs boson in the vicinity of 125 GeV. We show that a two-Higgs doublet model spectrum, with the pseudo-scalar state being the lightest, could be responsible for the diphoton signal events. In this model, the other scalars are considerably heavier and are not excluded by the current LHC data. If this assumption is correct, future LHC data should show a strengthening of the $\gamma\gamma$ signal, while the signals in the $ZZ^{(*)}\to 4\ell$ and $WW^{(*)}\to 2\ell 2\nu$ channels should diminish and eventually disappear, due to the absence of diboson tree-level couplings of the CP-odd state. The heavier CP-even neutral scalars can now decay into channels involving the CP-odd light scalar which, together with their larger masses, allow them to avoid the existing bounds on Higgs searches. We suggest additional signals to confirm this scenario at the LHC, in the decay channels of the heavier scalars into $AA$ and $AZ$. Finally, this inverted two-Higgs doublet spectrum is characteristic in models where fermion condensation leads to electroweak symmetry breaking. We show that in these theories it is possible to obtain the observed diphoton signal at or somewhat above of the prediction for the standard model Higgs for the typical values of the parameters predicted.Comment: 6 pages, 5 figures, final version in Physical Review