arXiv subjects

T. N. Pham

Publications and source records attributed to T. N. Pham.

At least 37 records · Page 2Linked to original sources

arXiv2008

B, D and K decays

With the advent of the LHC, we will be able to probe New Physics (NP) up to energy scales almost one order of magnitude larger than it has been possible with present accelerator facilities. While direct detection of new particles will be the main avenue to establish the presence of NP at the LHC, indirect searches will provide precious complementary information, since most probably it will not be possible to measure the full spectrum of new particles and their couplings through direct production. In particular, precision measurements and computations in the realm of flavour physics are expected to play a key role in constraining the unknown parameters of the Lagrangian of any NP model emerging from direct searches at the LHC. The aim of Working Group 2 was twofold: on one hand, to provide a coherent, up-to-date picture of the status of flavour physics before the start of the LHC; on the other hand, to initiate activities on the path towards integrating information on NP from high-pT and flavour data.

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arXiv2007

Two-photon decay of heavy quarkonium from heavy-quark spin symmetry

With the recent measurements on $η_{c}$ and $η_{c}^{\prime}$ at CLEO, Babar and Belle, and with the prospect of finding the $η_{b}$ at the Tevatron, it seems appropriate to have another look at the two-photon decay of heavy quarkonium from the standpoint of an effective Lagrangian based on local operator expansion and heavy-quark spin symmetry. In this talk, I would like to discuss a recent work on the two-photon decay rates of ground states and excited states of $η_c$ and $η_b$ using the local operator expansion approach and heavy-quark spin symmetry and taking into account the binding-energy. We find that the predicted two-photon width for $η_c$ agrees well with experiment, but the predicted value for $η_c(2S)$ is twice larger than the CLEO estimation. We point out that the essentially model-independent ratio of $η_b$ two-photon width to the $Υ$ leptonic width and the $η_b $ two-photon width could be used to extract the strong coupling constant $α_s$ .

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arXiv2007

Non leptonic B decays to axial-vector mesons and factorization

We present an analysis of two-body B decays with a pseudoscalar (P) and an axial-vector meson (A) in the final state using naive factorization. We employ as inputs a limited number of experimental data, i.e. results for the B -> K1 gamma and B -> K* gamma radiative decays and the branching ratios for B -> pion+rho, pion+K*, K+rho, K+pion non leptonic decays. In this way we are able to make predictions on several B -> P A decays that might be used as a guide for experimental researches of these decays and as tests of factorization.

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arXiv2006

B -> pi pi decays

The branching ratios and CP asymmetries of $B\to ππ$ decays measured in B factory experiments indicate a large ratio of color-suppressed ($C$) to color-allowed ($T$) tree contributions and a large relative phase between the tree and penguin amplitude. In this talk, I would like to report on a recent analysis to see whether the large $C/T$ ratio can be explained within the QCD based model with i) a large contribution from the annihilation terms or with ii) large final-state-interaction phase between two different isospin amplitudes. We show that the current experimental data do not exclude either possibility but we may be able to distinguish these two effects in future measurements of direct CP asymmetry in $B\to π^{0}π^{0}$ decay.

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arXiv2006

Spin effects in rare $B \to X_s τ^+ τ^-$ and $B \to K^{(*)} τ^+ τ^-$ decays in a single Universal Extra Dimension scenario

We study the branching fractions and the lepton polarization asymmetries of rare $B \to X_s τ^+ τ^-$ and $B \to K^{(*)} τ^+ τ^-$ modes within the Standard Model and in the Appelquist-Cheng-Dobrescu model, which is a New Physics scenario with a single universal extra dimension. In particular, we investigate the sensitivity of the observables to the radius $R$ of the compactified extra-dimension, which is the only new parameter in this New Physics model. For the exclusive transitions we study the uncertainties due to the hadronic matrix elements using two set of form factors, finding that in many cases such uncertainties are small. In particular, we show that the $τ^-$ polarization asymmetries become free of hadronic uncertainties if Large Energy relations for the form factors, valid for low momentum transfer to the lepton pair, are used. We also consider $K^*$ helicity fractions and their sensitivity to the compactification parameter.

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arXiv2006

Two-photon width of eta_b, eta'_b and eta''_b from Heavy-Quark Spin Symmetry

We predict the two-photon width of the pseudoscalar bottomonia, Gamma(eta_b -> gamma gamma), Gamma(eta'_b -> gamma gamma) and Gamma(eta''_b -> gamma gamma) within a Heavy-Quark Spin-Symmetry setting following the same line as our recent work for the corresponding decays for charmonia. Binding-energy effects are included for excited states and are shown to shift up the eta'_b and eta''_b widths by 10 %. We point out that the essentially model independent ratio Gamma(eta_b -> gamma gamma)/Gamma(Upsilon -> ll) and the branching ratio BR(eta_b -> gamma gamma) obtained could be used to extract the coupling constant alpha_s.

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arXiv2006

Two-photon width of eta_c and eta'_c from Heavy-Quark Spin Symmetry

We evaluate the two-photon width of the pseudoscalar charmonia, Gamma(eta_c -> gamma gamma) and Gamma(eta'_c -> gamma gamma), within a Heavy-Quark Spin-Symmetry setting and show that whereas the former agrees with experiment, the latter is more than twice larger than the recent measurement by CLEO. When binding-energy effects are included in the eta'_c case, the discrepancy is worse, pointing out at a possible anomaly in the eta'_c decay.

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arXiv2006

CP asymmetry and branching ratio of B -> pi pi

We investigate the branching ratios and CP asymmetries of the B -> pi pi processes measured in B factory experiments. Fits to the experimental data of this process indicate a large ratio of color-suppressed (C) to color-allowed (T) tree contributions. We investigate whether the large C/T can be explained within the QCD based model computation with i) a large effect from the end-point singularity or with ii) large final-state-interaction phase between two different isospin amplitudes. We show that the current experimental data do not exclude either possibility but we may be able to distinguish these two effects in future measurements of direct CP asymmetry of B -> pi^0 pi^0.

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arXiv2006

Exclusive $B \to K^{()} \ell^+ \ell^-$, $B \to K^{()} ν{\bar ν}$ and $B \to K^* γ$ transitions in a scenario with a single Universal Extra Dimension

We analyze the exclusive rare $B \to K^{(*)} \ell^+ \ell^-$, $B \to K^{(*)} ν\bar ν$ and $B \to K^* γ$ decays in the Applequist-Cheng-Dobrescu model, which is an extension of the Standard Model in presence of universal extra dimensions. In particular, we consider the case of a single universal extra dimension. We study how spectra, branching fractions and asymmetries depend on the compactification parameter 1/R, and whether the hadronic uncertainty due to the form factors obscures or not such a dependence. We find that, using present data, it is possible in many cases to put a sensible lower bound to 1/R, the most stringent one coming from $B \to K^* γ$. We also study how improved experimental data can be used to establish stronger constraints to this model.

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arXiv2005

Relations for Direct CP asymmetries \\in $B\to \rm PP$ and $B\to \rm PV$ decays

The presence of additional strong phase from power corrections and other chirally enhanced terms makes it more difficult to predict direct CP asymmetries in two-body charmless B decays. In this talk, I would like to report on a recent work on QCD Factorisation and Power Corrections in Charmless B Decays. Using the measured branching ratios for $B\to \rm PV$, it is shown that power corrections in charmless B decays are probably large, at least for penguin dominated $\rm PV$ channels. Since the tree-penguin interference responsible for direct CP asymmetries in two-body charmless B decays are related by CKM factors and SU(3) symmetry, we find that, if power corrections other than the chirally enhanced power corrections and annihilation topology were negligible, QCD Factorisation would predict the direct CP asymmetry of $B \to π^+ π^-$ to be about 3 times larger than that of $B \to π^\pm K^\mp$, with opposite sign, in agreement with the latest measurement from Belle. Similar relations are also given for direct CP asymmetries in $B\to\rm PV$ .

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arXiv2005

B -> K1 gamma and tests of factorization for two-body non leptonic Bdecays with axial-vector mesons

The large branching ratio for B-> K1 gamma recently measured at Belle implies a large B -> K1 transition form factor and large branching ratios for non leptonic B decays involving an axial-vector meson. In this paper we present an analysis of two-body B decays with an axial-vector meson in the final state using naive factorization and the B -> K1 form factors obtained from the measured radiative decays. We find that the predicted B -> J/psi K1 branching ratio is in agreement with experiment. We also suggest that the decay rates of B -> K1 pi, B -> a1 K and B -> b1 K could be used to test the factorization ansatz.

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arXiv2005

$B \to χ_{c0,2} K$ decays: a model estimation

In this paper, we investigate the vertex corrections and spectator hard scattering contributions to $B \to χ_{c0,2}K$ decays, which has no leading contribution from naive factorization scheme. A non-zero binding energy $b=2m_c-M$ is introduced to regularize the infrared divergence of the vertex part. The spectator diagrams also contain logarithmic and linear infrared divergences, for which we adopt a model dependent parametrization. If we neglect possible strong phases in the hard spectator contributions, we obtain a too small branching ratio for $χ_{c0}K$ while too large one for $χ_{c2}K$, as can be seen from the ratio of the branching ratio of $B^+ \to χ_{c2}K^+$ to that of $B^+ \to χ_{c0} K^+$, which is predicted to be $2.15^{+0.63}_{-0.76}$ in our model, while experimentally it should be about 0.1 or even smaller. But a closer examination shows that, assuming large strong phases difference between the twist-2 and twist-3 spectator terms, together with a slightly larger spectator infrared cutoff parameter $Λ_h$, it is possible to accommodate the experimental data. This shows that, for $B\to χ_{c0,2}K$ decays with no factorizable contributions, QCDF seems capable of producing decay rates close to experiments, in contrast to the $B\to J/ψK$ decay which is dominated by the factorizable contributions.

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arXiv2004

Power corrections in charmless B decays

Power corrections seem to play an important role in charmless B decays as indicated by recent analysis using QCD Factorization. In this talk, I would like to report on a recent work on power corrections in charmless B decays. By using the ratio of the branching fraction of $B^+ \to π^+ K^{\ast 0}$ to that of $B^0 \to π^- ρ^+$, for which the theoretical uncertainties are greatly reduced, it is shown in a transparent manner that power corrections in charmless B decays are probably large and that the $B^0 \to K^- ρ^+$ decay could be explained with the annihilation term included. For ratios of direct CP asymmetries, QCD Factorisation with the annihilation terms included would predict the direct CP asymmetry of $B \to π^+ π^-$ to be about 3 times larger than that of $B \to π^\pm K^\mp$, with opposite sign. In particular, the large measured value for $B \to π^\pm K^\mp $ CP asymmetry implies naturally a corresponding large $B \to π^+ π^-$ CP asymmetry as observed by Belle. Experimentally any significant deviation from this prediction or possibly the importance of long-distance rescattering effects.

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arXiv2004

The Riddle of Polarization in $B \to VV$ Transitions

Measurements of polarization fractions in $B \to VV$ transitions, with $V$ a light vector meson, show that the longitudinal amplitude dominates in $B^0 \to ρ^+ ρ^-$, $B^+ \to ρ^+ ρ^0$, and $B^+ \to ρ^0 K^{*+}$ decays and not in the penguin induced decays $B^0 \to ϕK^{*0}$, $B^+ \to ϕK^{*+}$. We study the effect of rescattering mediated by charmed resonances, finding that in $B \to ϕK^*$ it can be responsible of the suppression of the longitudinal amplitude. For the decay $B \to ρK^*$ we find that the longitudinal fraction cannot be too large without invoking new effects.

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arXiv2004

Power Corrections in Charmless B Decays

In this paper, we focus on the role of power corrections in QCD factorization(QCDF) method in charmless two-body nonleptonic $B$ meson decays. We use the ratio of the branching fraction of $B^+ \to π^+ K^{\ast 0}$ to that of $B^0 \to π^- ρ^+$, for which the theoretical uncertainties are greatly reduced, to show clearly that the power corrections in charmless B decays are probably large. With other similar ratios considered, for example, for the $B^0 \to K^- ρ^+$ decay, it is very likely that, among various sources of power corrections, annihilation topology plays an indispensable role at least for penguin dominated $\rm PV$ channels. We also consider some selective ratios of direct CP asymmetries. Among these, we find that, if power corrections other than the chirally enhanced power corrections and annihilation topology were negligible, QCDF would predict the direct CP asymmetry of $B \to π^+ π^-$ to be about 3 times larger than that of $B \to π^\pm K^\mp$, with opposite sign. Experimentally any significant deviation from this prediction would suggest either new physics or possibly the importance of long-distance rescattering effects.

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arXiv2003

Non factorizable Effetcs in $B \to χ_{c0} K^- $ from Charmed Meson Rescattering

The $B^{-} \to χ_{c0} K^{-}$ decay which has no visible factorizable amplitude, could be induced by the Cabibbo-allowed, color-favored $B^{-} \to D_{s}D^{*0}$, $B^{-} \to D^{*0}_{s}D^{0}$ and $B \to D_{s}^{*}D^{*}$ via the rescattering of these charmed mesons to $χ_{co} K^{-}$. In this talk I would like to discuss a recent calculation \cite{Colangelo} of these effects for $B^{-} \to χ_{c0} K^{-}$ and $B^{-} \to J/ψK^{-}$ decays. We find that charmed scattering effects seem capable of producing a large $B^{-} \to χ_{c0} K^{-}$ branching ratio measured by the Babar and Belle Collaboration and make a significant contribution to the $B^{-} \to J/ψK^{-}$ branching ratios in agreement with experiments.

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arXiv2003

Nonfactorizable contributions in B decays to charmonium: the case of $B^- \to K^- h_c$

Nonleptonic $B$ to charmonium decays generally show deviations from the factorization predictions. For example, the mode $B^- \to K^- χ_{c0}$ has been experimentally observed with sizeable branching fraction while its factorized amplitude vanishes. We investigate the role of rescattering effects mediated by intermediate charmed meson production in this class of decay modes, and consider $B^- \to K^- h_c$ with $h_c$ the $J^{PC}=1^{+-}$ $\bar c c$ meson. Using an effective lagrangian describing interactions of pairs of heavy-light $Q{\bar q}$ mesons with a quarkonium state, we relate this mode to the analogous mode with $χ_{c0}$ in the final state. We find ${\cal B}(B^- \to K^- h_c)$ large enough to be measured at the $B$ factories, so that this decay mode could be used to study the poorly known $h_c$.

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arXiv2003

CP asymmetry in $B^- \to π^+ π^- K^-$ and $B^- \to K^+ K^- K^-$ decays

The near equality between the measured value for $|V_{ub}|/|V_{cb}|$ and its unitarity triangle lower bound from $\sin 2β$ measurements suggests that $α\approx 90^{\circ}$ and $\sinγ\approx \cosβ= 0.91$ and a large direct CP violation in B decays would be possible. In this talk, I would like to discuss a recent analysis of the non resonant and resonant CP asymmetry for the decays $B^- \to π^+ π^- K^-$ and $B^- \to K^+ K^- K^-$ where the CP asymmetry at the $χ_{c0}$ resonance comes from the interference of the non resonant amplitude with the resonant amplitude $B^\pm \to χ_{c0} K^\pm $ $ \toπ^+ π^- K^\pm $ and $B^\pm \to χ_{c0} K^\pm$ $ \to K^+ K^- K^\pm$. Using the the Babar and Belle measurements of the $B^-\to χ_{c0} K^- $ branching ratios, we predict that partial width asymmetry at the $χ_{c0}$ resonance could reach 10% for the $B^- \to π^+ π^- K^-$ decay and 16% for the $B^- \to K^+ K^- K^-$decay.

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