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Joaquim Matias

Publications and source records attributed to Joaquim Matias.

72 records · Page 4Linked to original sources
arXiv2007

Extracting alpha from B0(d) --> K0 anti-K0 Decays

We propose a new method for obtaining the CP phase alpha, based on measurements of B0(d) --> K0 anti-K0, along with theoretical input. Due to the similarities of QCD factorization (QCDf) and perturbative QCD (pQCD), this input is basically the same for each of these models. Although the theoretical error is large at present, many of the contributing quantities will be better known in the future, leading to a smaller error. One outstanding question is the extent to which the precision on quark masses, especially m_c/m_b, can be improved. If one assumes that new physics is not present, the method can be used to predict the values of CP asymmetries in B0(d) --> K0 anti-K0: 0.02 <= A_{dir}^2 + A_{mix}^2 <= 0.125. A result outside this range would signal the existence of long-distance effects beyond those included in models of nonleptonic decays based on factorization (such as QCDf and pQCD), or the presence of new physics.

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arXiv2007

Merging Flavour Symmetries with QCD Factorisation for B-->KK Decays

The interplay between flavour symmetries connecting Bs-->KK decays with the recently measured Bd--> K0 anti-K0 decay and QCD Factorisation opens new strategies to describe the decays Bs--> K0 anti-K0 and Bs--> K+ K- in the SM and in supersymmetry. A new relation, emerging from the sum-rule for the Bs--> K0 anti-K0 decay mode, is presented offering a new way to determine the weak mixing angle phi_s of the Bs system.

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arXiv2006

Bs-->K+K- and Bs-->K0 anti-K0 Decays within Supersymmetry

We compute the supersymmetric (SUSY) contributions to the observables in Bs-->K+K- and Bs-->K0 anti-K0 decays. The hadronic parameters in the standard-model (SM) amplitudes are obtained from the Bd-->K0 anti-K0 decay using a recent approach that combines flavor SU(3) symmetry and a controlled input from QCD factorization. The latest experimental data for BR(Bs-->K+K-) is in agreement with the SM prediction. We study how the branching ratios and the direct and mixing-induced CP asymmetries of both Bs-->KK decay modes are affected with the inclusion of SUSY, after imposing constraints from BR(B--> Xs gamma), B--> pi K and Delta Ms over the parameter space. While the branching ratios remain unaffected by SUSY, we identify the CP asymmetries of the Bs-->KK decays as the most promising observables to look for large deviations from the SM.

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arXiv2006

Exploring B_{d,s}-> KK decays through flavour symmetries and QCD-factorisation

We present a new analysis of B_{d,s}->KK modes within the SM, relating them in a controlled way through SU(3)-flavour symmetry and QCD-improved factorisation. We propose a set of sum rules for B_{d,s}-> K^0{\bar K}^0 observables. We determine B_s-> KK branching ratios and CP-asymmetries as functions of A_{dir}(B_d-> K^0{\bar K}^0), pointing out a conflict between BR(B_s-> K+K-) in the SM and data. Finally, we predict the amount of U-spin breaking between B_d-> pi+pi- and B_s-> K+K-.

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arXiv2006

Supersymmetric Contributions to Bs->K+K-

Inspired by the existing calculation of B->piK decays in supersymmetry (SUSY), we evaluate the dominant SUSY contributions to Bs->K+K-. We show that the observables of this process can be significantly modified in the presence of SUSY. In particular, the branching ratio can be increased considerably compared to the prediction of the standard model (SM). The effect is even more dramatic for the CP-violating asymmetries A_dir and A_mix. These asymmetries, expected to be small in the SM (A_dir is predicted to take only positive values), change drastically with SUSY contributions. The measurement of these observables can therefore be used to detect the presence of physics beyond the SM, and put constraints on its parameters.

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arXiv2005

The Angular Distribution of B0-> K*0(->K-pi+)l+l- at Large Recoil in and Beyond the SM

We discuss, in detail, the K* polarization states in the exclusive B meson decay B0-> K*0(-> K-pi+)l^+l^- (l=e, μ, τ) in the low dilepton mass region. We focus on the study of the angular distribution of this decay that provides valuable information on the K* spin amplitudes A_perp, A_par, A0. This can give us a handle on non-standard interactions that cannot be proved through measurements of the branching ratio and lepton forward-backward asymmetry. We explore the transverse asymmetries A_T^1(s), A_T^2(s), K* polarization parameter α_{K*}(s), the fraction of K* polarization F_L(s) and F_T(s) and the corresponding integrated observables at NLL order, including factorizable and non-factorizable corrections. We find, in particular, that the dependence on hadronic uncertainties for the transverse asymmetries turns out to be very small. This allow us to distinguish which observables are better suited to look for physics beyond the SM. Finally, we study in a model independent way the implications of New Physics for these observables.

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arXiv2005

Probing New Physics Via the Transverse Amplitudes of B0-> K*0(-> K- pi+) l+l- at Large Recoil

We perform an analysis of the K* polarization states in the exclusive B meson decay B0->K*0(-> K- pi+)l+ l- (l=e, mu, tau) in the low dilepton mass region, where the final vector meson has a large energy. Working in the transversity basis, we study various observables that involve the K* spin amplitudes Aperp, Apar, A0 by exploiting the heavy-to-light form factor relations in the heavy quark and large-E_K* limit. We find that at leading order in 1/m_b and alpha_s the form-factor dependence of the asymmetries that involve transversely polarized K* completely drops out. At next-to-leading logarithmic order, including factorizable and non-factorizable corrections, the theoretical errors for the transverse asymmetries turn out to be small in the standard model (SM). Integrating over the lower part of the dimuon mass region, and varying the theoretical input parameters, the SM predicts A_T^(1)=0.9986+/- 0.0002 and A_T^(2)=-0.043+/- 0.003. In addition, the longitudinal and transverse polarization fractions are found to be (69+/-3)% and (31+/-3)% respectively, so that Gamma_L/Gamma_T = 2.23+/-0.31. Beyond the SM, we focus on new physics that mainly gives sizable contributions to the coefficients C_7^eff' of the electromagnetic dipole operators. Taking into account experimental data on rare B decays, we find large effects of new physics in the transverse asymmetries. Furthermore, we show that a measurement of longitudinal and transverse polarization fractions will provide complementary information on physics beyond the SM.

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arXiv2005

Bd^0(t)->pi^+pi^- and Bs^0(t)-> K^+ K^- Decays: A Tool to Measure New-Physics Parameters

If physics beyond the standard model is present in B decays, experimental measurements seem to suggest that it principally affects those processes with significant b->s penguin amplitudes. It was recently argued that, in general, such new-physics (NP) effects can be parametrized in terms of a single NP amplitude A^q and phase Φ_q, for q=u,d,s,c. In this paper, we show that the study of the decays Bs(t) -> K^+ K^- and Bd(t) -> π^+π^- allows one to measure the NP parameters A^u and Φ_u. We examine the implications for this method of the latest experimental results on these decays. If NP is found in Bs(t) -> K^+ K^-, it can be partially identified through measurements of Bs(t)-> K^0 \bar K^0.

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arXiv2004

Testing the Standard Model with Bs-> K^+ K^- Decays

In the limit of flavor SU(3) symmetry, the amplitudes for Bd->pi^+π^- and Bs-> K^+ K^- are related to one another. Taking the values of the weak phases beta and gamma from independent measurements, the ratio Rds=BR(Bs-> K^+K^-)/BR(Bd->pi^+ pi^-) and the two Bd->pi^+pi^- CP asymmetries Adir and Amix depend on only two quantities. By measuring these three observables, one can test for the presence of physics beyond the standard model. In this paper, using present data, and including theoretical uncertainties due to SU(3) breaking, we perform such an analysis. The experimental errors are still very large, but with improved measurements from Babar, Belle and CDF this will be a useful method to search for new physics.

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arXiv2003

CKM angles from non-leptonic B decays using SU(3) flavour symmetry

We discuss the determination of the CKM angles gamma and alpha using recent data from non-leptonic B decays together with flavour symmetries. Penguin effects are controlled by means of the CP-averaged branching ratio B_d \to π^\pm K^\mp. The information from A_{CP}(B_d \to J/ΨK_S) (two solutions for phi_d), Rb and gamma allow us to determine beta, even in presence of New Physics not affecting Delta B=1 amplitudes. In this context we address the question of to what extent there is still space for New Physics.

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arXiv2003

Shedding Light on the "Dark Side" of $B^0_d$--$\bar B^0_d$ Mixing through $B_d\toπ^+π^-$, $K\toπν\barν$ and $B_{d,s}\toμ^+μ^-$

In a wide class of NP models, which can be motivated through generic arguments and within SUSY, we obtain large contributions to $B^0_d$--$\bar B^0_d$ mixing, but not to $ΔB=1$ processes. If we assume such a scenario, the solutions $ϕ_d\sim 47^\circ\lor 133^\circ$ for the $B^0_d$--$\bar B^0_d$ mixing phase implied by $A_{CP}^{mix}(B_d\to J/ψK_S)$ cannot be converted directly into a constraint in the $ρ$--$η$ plane. However, we may complement $ϕ_d$ with $|V_{ub}/V_{cb}|$ and the recently measured CP asymmetries in $B_d\toπ^+ π^-$ to determine the unitarity triangle, with its angles $α$, $β$ and $γ$. To this end, we have also to control penguin effects, which we do by means of the $B_d\toπ^\mp K^\pm$ branching ratio. Interestingly, the present data show a perfectly consistent picture not only for the ``standard'' solution of $ϕ_d\sim 47^\circ$, but also for $ϕ_d\sim 133^\circ$. In the latter case, the preferred region for the apex of the unitarity triangle is in the second quadrant, allowing us to accommodate conveniently $γ>90^\circ$, which is also favoured by other non-leptonic B decays such as $B\toπK$. Moreover, also the prediction for BR$(K^+\toπ^+ν\barν})$ can be brought to better agreement with experiment. Further strategies to explore this scenario with the help of $B_{d,s}\toμ^+μ^-$ decays are discussed as well.

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arXiv2002

Forward-backward and isospin asymmetry for B -> K* l+ l- decay in the standard model and in supersymmetry

We discuss two dedicated observables in exclusive B -> K* l+ l- decay that can be used to study effects of physics beyond the standard model, namely the forward-backward asymmetry in the lepton spectrum and the isospin-asymmetry between decays of charged and neutral B mesons. We consider the region of large recoil-energy (i.e. small invariant mass of the lepton pair), and employ the QCD factorization approach to exclusive B meson decays. Sub-leading effects in the heavy quark mass expansion have been taken into account for the calculation of the isospin-asymmetry. We give predictions for decay asymmetries in the standard model, and its supersymmetric extension with minimal flavor violation, using parameter values allowed by current experimental constraints on B->X_s gamma decay.

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arXiv2002

Impact of B physics on model building and vice versa: an example

We motivate that the start-up of the B factories has opened a new precision flavour physics era, with an important effect on model building. Using as an example a left-right model with spontaneous CP violation, we will show how the inclusion of the new experimental data on B physics observables, together with the old observables coming from kaon physics, has significantly widened our capacity to strongly constrain the parameter space up to the point to exclude models. On the contrary, using certain hypotheses, mainly concerning isospin, we discuss how theory may help us to `test' the data on charged, neutral and mixed $B \to πK$ decays once experimental errors will be reduced.

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arXiv2002

Exploring CP Violation through Correlations in B --> pi K, B_d --> pi^+pi^-, B_s --> K^+K^- Observable Space

We investigate allowed regions in observable space of B --> pi K, B_d --> pi^+pi^- and B_s --> K^+K^- decays, characterizing these modes in the Standard Model. After a discussion of a new kind of contour plots for the $B\toπK$ system, we focus on the mixing- induced and direct CP asymmetries of the decays B_d --> pi^+pi^- and B_s--> K^+K^-. Using experimental information on the CP-averaged B_d --> pi^{+/-}K^{+/-} and B_d --> pi^+pi^- branching ratios, the relevant hadronic penguin parameters can be constrained,implying certain allowed regions in observable space. In the case of B_d --> pi^+pi^-, an interesting situation arises now in view of the recent B-factory measurements of CP violation in this channel, allowing us to obtain new constraints on the CKM angle gamma as a function of the B^0_d--\bar{B^0_d} mixing phase phi_d=2beta, which is fixed through A_{CP}^{mix}(B_d --> J/psi K_S) up to a twofold ambiguity. If we assume that A_{CP}^{mix}(B_d --> pi^+pi^-) is positive, as indicated by recent Belle data, and that phi_d is in agreement with the ``indirect'' fits of the unitarity triangle, also the corresponding values for gamma around 60 degrees can be accommodated. On the other hand, for the second solution of phi_d, we obtain a gap around gamma ~ 60 degrees. The allowed region in the space of A_{CP}^{mix}(B_s --> K^+K^-) and A_{CP}^{dir}(B_s --> K^+K^-) is very constrained in the Standard Model, thereby providing a narrow target range for run II of the Tevatron and the experiments of the LHC era.

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arXiv2001

Model independent sum rules for B-> pi K decays

We provide a set of sum rules relating CP-averaged branching ratios and CP-asymmetries of the $B \to πK$ modes. They prove to be useful as a mechanism to `test' experimental data given our expectations of the size of isospin breaking. A set of observables emerges providing a simpler interpretation of data in terms of isospin breaking. Moreover, the derivation is done in a completely model independent way, i.e., they can accommodate also New Physics contributions.

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arXiv2000

A screening mechanism for extra W and Z gauge bosons

We generalize a previous construction of a fermiophobic model to the case of more than one extra $W$ and $Z$ gauge bosons. We focus in particular on the existence of screening configurations and their implication on the gauge boson mass spectrum. One of these configurations allows for the existence of a set of relatively light new gauge bosons, without violation of the quite restrictive bounds coming from the $ρ_{\rm NC}$ parameter. The links with Bess and degenerate Bess models are also discussed. Also the signal given here by this more traditional gauge extension of the SM could help to disentangle it from the towers of Kaluza-Klein states over $W$ and $Z$ gauge bosons in extra dimensions.

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arXiv1999

Searching for New Physics in Non-Leptonic B Decays

We present allowed regions in the space of observables of certain non-leptonic B-meson decays that characterize these modes within the Standard Model. A future measurement of observables lying significantly outside of these regions would indicate the presence of new physics. Making use of SU(3) arguments, we give the range for $B\toπK$ decays, and for the system of $B_d\toπ^+π^-$, $B_s\to K^+K^-$ modes.

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