arXiv subjects

G. Hiller

Publications and source records attributed to G. Hiller.

29 records · Page 2Linked to original sources

arXiv2000

$b \to s \ell^+ \ell^-$ decays in and beyond the Standard Model

We briefly review the status of rare radiative and semileptonic $b \to s (γ, \ell^+ \ell^-)$, ($\ell=e,μ$) decays. We discuss possible signatures of new physics in these modes and emphazise the role of the exclusive channels. In particular, measurements of the Forward-Backward asymmetry in $B \to K^* \ell^+ \ell^-$ decays and its zero provide a clean test of the Standard Model, complementary to studies in $b \to s γ$ decays. Further, the Forward-Backward CP asymmetry in $B \to K^* \ell^+ \ell^-$ decays is sensitive to possible non-standard sources of CP violation mediated by flavor changing neutral current $Z$-penguins.

hep-ph↗

arXiv1999

A Comparative Study of the Decays $B \to (K,K^*) \ell^+ \ell^-$ in Standard Model and Supersymmetric Theories

Using improved theoretical calculations of the decay form factors in the Light Cone-QCD sum rule approach, we investigate the decay rates, dilepton invariant mass spectra and the forward-backward (FB) asymmetry in the decays $B \to (K,K^*) \ell^+ \ell^-$ ($\ell^\pm =e^\pm,μ^\pm,τ^\pm$) in the standard model (SM) and a number of popular variants of the supersymmetric (SUSY) models. Theoretical precision on the differential decay rates and FB-asymmetry is estimated in these theories taking into account various parametric uncertainties. We show that existing data on $B \to X_s γ$ and the experimental upper limit on the branching ratio ${\cal B}(B \to K^* μ^+ μ^-)$ provide interesting bounds on the coefficients of the underlying effective theory. We argue that the FB-asymmetry in $B \to K^* \ell^+ \ell^-$ constitutes a precision test of the SM and its measurement in forthcoming experiments may reveal new physics. In particular, the presently allowed large-$\tan β$ solutions in SUGRA models, as well as more general flavor-violating SUSY models, yield FB-asymmetries which are characteristically different from the corresponding ones in the SM.

hep-ph↗

arXiv1998

Perturbative QCD and Power Corrected Hadron Spectra and Spectral Moments in the Decays $B \to X_s \ell^+ \ell^-$

Leading order (in $α_s$) perturbative QCD and power ($1/m_b^2)$ corrections to the hadronic invariant mass and hadron energy spectra in the decay $B \to X_s \ell^+ \ell^-$ are reviewed in the standard model using the heavy quark expansion technique (HQET). In particular, the first two hadronic moments $ $ and $ $, $n=1,2$, are presented working out their sensitivity on the HQET parameters $λ_1$ and $\barΛ$. Data from the forthcoming B facilities can be used to measure the short-distance contribution in $B \to X_s \ell^+ \ell^-$ and determine the HQET parameters from the moments $ $. This could be combined with complementary constraints from the decay $B \to X \ell ν_\ell$ to determine these parameters precisely.

hep-ph↗

arXiv1998

A Theoretical Reappraisal of Branching Ratios and CP Asymmetries in the Decays $B \to (X_d,X_s) \ell^+ \ell^-$ and Determination of the CKM Parameters

We present a theoretical reappraisal of the branching ratios and CP asymmetries for the decays $ B \to X_q \ell^+ \ell^-$, with $q=d,s$, taking into account current theoretical uncertainties in the description of the inclusive decay amplitudes from the long-distance contributions, an improved treatment of the renormalization scale dependence, and other parametric dependencies. Concentrating on the partial branching ratios $Δ{\cal B}(B \to X_q \ell^+ \ell^-)$, integrated over the invariant dilepton mass region $1 {GeV}^2 \leq s \leq 6 {GeV}^2$, we calculate theoretical precision on the charge-conjugate averaged partial branching ratios $<Δ{\cal B}_q >= (Δ{\cal B}(B \to X_q \ell^+ \ell^-) + Δ{\cal B}(\bar{B} \to \bar{X}_q \ell^+ \ell^-))/2$, CP asymmetries in partial decay rates $(a_{CP})_q=(Δ{\cal B}(B \to X_q \ell^+ \ell^-) - Δ{\cal B}(\bar{B} \to \bar{X}_q \ell^+ \ell^-))/(2 < Δ{\cal B}_q >)$, and the ratio of the branching ratios $Δ{\cal R} = < Δ{\cal B}_d >/< Δ{\cal B}_s>$. For the central values of the CKM parameters, we find $<Δ{\cal B}_s > =(2.22^{+0.29}_{-0.30}) \times 10^{-6}$, $<Δ{\cal B}_d > =(9.61^{+1.32}_{-1.47}) \times 10^{-8}$, $(a_{CP})_s =-(0.19^{+0.17}_{-0.19})%$, $(a_{CP})_d =(4.40^{+3.87}_{-4.46})%$, and $Δ{\cal R} =(4.32 \pm 0.03)%$. The dependence of $<Δ{\cal B}_d>$ and $Δ{\cal R}$ on the CKM parameters is worked out and the resulting constraints on the unitarity triangle from an eventual measurement of $Δ{\cal R}$ are illustrated.

hep-ph↗

arXiv1998

Phenomenological Profiles of the Inclusive Hadron Spectra in the Decay $B \to X_s \ell^+ \ell^-$

Hadron spectra and hadronic moments in the decay $B \to X_s \ell^+ \ell^-$ are calculated taking into account both the short-distance and long-distance contributions in the decay amplitude using a Fermi motion (FM) model to incorporate the $B$-meson wave-function effects. The measured branching ratios for the inclusive decays $B \to X_s+ (J/ψ,ψ^\prime,...)\to X_s \ell^+ \ell^-$ are used to fix the normalization of the long-distance contribution. The momentum distribution of the $J/ψ$ measured by the CLEO collaboration is fitted in the FM model which is then used to calculate the hadronic spectra from the resonant contribution also away from the $J/ψ$-resonance. We also study the effect of various descriptions of the resonant and non-resonant $c\bar{c}$ contributions in $B \to X_s \ell^+ \ell^-$ existing in the literature on the hadron energy and invariant mass spectra, and in the Forward-Backward asymmetry. Selective cuts on the hadron and dilepton invariant masses can be used to reduce the $B\bar{B}$ background and resonant contribution and, as an example, we work out the hadron spectra with the experimental cuts used by the CLEO collaboration in searching for the decay $B \to X_s \ell^+ \ell^-$. We show that data from the forthcoming B facilities could be used effectively to measure the short-distance contribution in $B \to X_s \ell^+ \ell^-$, enabling precise determination of the FM model and heavy quark effective theory parameters $λ_1$ and $\barΛ$.

hep-ph↗

arXiv1998

Perturbative QCD- and Power-Corrected Hadron Spectra and Spectral Moments in the Decay $B \to X_s \ell^+ \ell^-$

We compute the leading order (in $α_s$) perturbative QCD and power ($1/m_b^2)$ corrections to the hadronic invariant mass and hadron energy spectra in the decay $B \to X_s \ell^+ \ell^-$ in standard model. This is done both by using the heavy quark expansion technique (HQET) and a perturbative-QCD improved Fermi motion (FM) model which takes into account $B$-meson wave-function effects. The corrections in the hadron energy ($E_H$) spectrum are found to be small over a good part of this spectrum in both the methods. However, the expansion in $1/m_b$ in HQET fails near the lower kinematic end-point and at the $c\bar{c}$ threshold. The hadronic invariant mass ($S_H$) spectrum is calculable only over a limited range $S_H > \barΛm_B$ in the heavy quark expansion, where $\barΛ \simeq m_B-m_b$. We also present results for the first two hadronic moments $< S_H^n>$ and $< E_H^n>$, $n=1,2$, working out their sensitivity on the HQET and FM model parameters. For equivalent values of these parameters, the moments in these methods are remarkably close to each other. Using the FM model, we study the effect of the experimental cuts, used recently by the CLEO collaboration in searching for the decay $B \to X_s \ell^+ \ell^-$, on the hadron spectra and hadronic invariant mass moments. The constraints following from assumed values of $< S_H^n>$ on the HQET parameters $λ_1$ and $\barΛ$ are worked out. Data from the forthcoming B facilities could be used to measure the short-distance contribution in $B \to X_s \ell^+ \ell^-$ and determine the HQET parameters $λ_1$ and $\barΛ$. This could be combined with complementary constrains in $B \to X \ell ν_\ell$ to determine them precisely.

hep-ph↗

arXiv1998

Hadron Spectra and Spectral Moments in the Decay B \to X_s \ell^+ \ell^- using HQET

We compute the leading order (in alpha_s) perturbative QCD and power (1/m_b^2) corrections to the hadronic invariant mass and hadron energy spectra in the decay B \to X_s \ell^+ \ell^- in standard model using the heavy quark expansion technique (HQET). Results for the first two hadronic moments < S_H^n> and , n=1,2, are presented here working out their sensitivity on the HQET parameters lambda_1 and \barΛ. Data from the forthcoming B facilities can be used to measure the short-distance contribution in B \to X_s \ell^+ \ell^- and determine the HQET parameters from the moments . This can be combined with the analysis of semileptonic decays B \to X \ell ν_\ell to determine them precisely.

hep-ph↗

arXiv1998

Leading logarithmic QCD corrections to the B_s --> γγdecay rate including long-distance effects through B_s --> ϕγ--> γγ

We present leading logarithmic QCD corrections to the decay B_s --> γγin the Standard Model. Further, the form factor F_{1}(0) of B_s --> ϕγis calculated in the framework of QCD sum rules and found to be in agreement with the result existing in the literature. Using Vector Meson Dominance model, the amplitude for B_s --> ϕγ-->γγis calculated as an estimate of the O_7-type contributions to the long-distance effects in the B_s --> γγdecay. The resulting branching ratio B(B_s --> γγ)_{SD+LD_{O_7}} is analysed in view of its strong dependence on the non-perturbative parameter $\barΛ_s$, describing bound state effects, and the renormalization scale $μ$. .

hep-ph↗

arXiv1997

Leading logarithmic QCD corrections to the B_s --> γγdecays in the two Higgs doublet model

We calculate the leading logarithmic QCD corrections to the decay B_s --> γγin the two Higgs doublet model (2HDM) including O_7 type long distance effects and estimate the restrictions of the 2HDM parameters, tanβand m_H, using the experimental data of B --> X_s γdecay provided by the CLEO collaboration. A lower bound for the charged Higgs mass m_H as a function of the renormalization scale μis given for 2HDM model II. We further present the dependencies of the branching ratio Br(B_s --> γγ) and the ratio |A^{+}|^2/|A^{-}|^2 on m_H and tanβincluding leading logarithmic QCD corrections. The dependence on the renormalization scale is found to be strong for both ratios. An additional uncertainty arises from the variation of the parameters of the bound state model, (m_b, \barΛ_s). We see, that to look for charged Higgs effects the measurement of the branching ratio Br(B_s --> γγ) is promising.

hep-ph↗

arXiv1997

Estimate of the long-distance contribution through b --> s ψto the B_{s} --> γγdecay rate

The B_{s} --> ϕψdecay is modeled through the inclusive b --> sψdecay. Using the Vector Meson Dominance model, the amplitude for the chain process B_{s} --> ϕψ--> ϕγ--> γγis estimated and it is found to be at most 4 % of the corresponding amplitude from the O_7 type LD contribution. The intermediate amplitude for the process B_{s} --> ϕγis compared with the corresponding one obtained by a different approach based on the interaction of the virtual charm quark loop with soft gluons \cite{Ruckl}. Both amplitudes are found to agree within 10 %. Further the influence on the branching ratio B(B_{s} --> γγ)_{SD+LD_{O7}} from inclusive b --> s ψis estimated as less than 1 %.

hep-ph↗

arXiv1997

Power Corrections in the Decay Rate and Distributions in $B \to X_s \ell^+ \ell^- $ in the Standard Model

We investigate the leading power corrections to the decay rates and distributions in the decay $B \rightarrow X_s \ell^+ \ell^- $ in the standard model (SM) using heavy quark expansion (HQE) in $(1/m_b)$ and a phenomenological model implementing the Fermi motion effects of the b quark bound in the B hadron. In the HQE method, we find that including the leading power corrections the decay width decreases by about 4% and the branching ratio by about 1.5% from their (respective) parton model values. The dilepton invariant mass spectrum is found to be stable against power corrections over a good part of this spectrum. However, near the high-mass end-point this distribution becomes negative with the current value of the non-perturbative parameter $λ_2$ (the $λ_1$-dependent corrections are found to be inoccouos), implying the breakdown of the HQE method in this region. Our results are at variance with the existing ones in the literature in both the decay rate and the invariant dilepton mass distribution calculated in the HQE approach. As an alternative, we implement the non-perturbative effects in the decay $B \rightarrow X_s \ell^+ \ell^- $ using a phenomenologically motivated Gaussian Fermi motion model. We find small corrections to the branching ratio, but the non-perturbative effects are perceptible in both the dilepton mass distribution and the Forward-Backward asymmetry in the high dilepton mass region. Using this model for estimating the non-perturbative effects, we present the decay rates and distributions for the inclusive process $B \rightarrow X_s \ell^+ \ell^- $ in the SM.

hep-ph↗