Unitarity Triangle Analysis Research Articles

B-physics from Nf = 2 tmQCD: the Standard Model and beyond

We present a lattice QCD computation of the b-quark mass, the B and B_s decay constants, the B-mixing bag parameters for the full four-fermion operator basis as well as determinations for \xi and f_{Bq}\sqrt{B_i^{(q)}} extrapolated to the continuum limit and to the physical pion mass. We used N_f = 2 twisted mass Wilson fermions at four values of the lattice spacing with pion masses ranging from 280 to 500 MeV. Extrapolation in the heavy quark mass from the charm to the bottom quark region has been carried out on ratios of physical quantities computed at nearby quark masses, exploiting the fact that they have an exactly known infinite mass limit. Our results are m_b(m_b, \overline{\rm{MS}})=4.29(12) GeV, f_{Bs}=228(8) MeV, f_{B}=189(8) MeV and f_{Bs}/f_B=1.206(24). Moreover with our results for the bag-parameters we find \xi=1.225(31), B_1^{(s)}/B_1^{(d)}=1.01(2), f_{Bd}\sqrt{\hat{B}_{1}^{(d)}} = 216(10) MeV and f_{Bs}\sqrt{\hat{B}_{1}^{(s)}} = 262(10) MeV. We also computed the bag parameters for the complete basis of the four-fermion operators which are required in beyond the SM theories. By using these results for the bag parameters we are able to provide a refined Unitarity Triangle analysis in the presence of New Physics, improving the bounds coming from B_{(s)}-\bar B_{(s)} mixing.

Standard Model updates and new physics analysis with the Unitarity Triangle fit

We present the summer 2012 update of the Unitarity Triangle (UT) analysis performed by the UTfit Collaboration within the Standard Model (SM) and beyond. The increased accuracy on several of the fundamental constraints is now enhancing some of the tensions amongst and within the constraint themselves. In particular, the long standing tension between exclusive and inclusive determinations of the Vub and Vcb CKM matrix elements is now playing a major role. Then we present the generalisation the UT analysis to investigate new physics (NP) effects, updating the constraints on NP contributions to ΔF=2 processes. In the NP analysis, both CKM and NP parameters are fitted simultaneously to obtain the possible NP effects in any specific sector. Finally, based on the NP constraints, we derive upper bounds on the coefficients of the most general ΔF=2 effective Hamiltonian. These upper bounds can be translated into lower bounds on the scale of NP that contributes to these low-energy effective interactions.

Kaon mixing beyond the SM from Nf = 2 tmQCD and model independent constraints from the UTA

Abstract We present the first unquenched, continuum limit, lattice QCD results for the matrix elements of the operators describing neutral kaon oscillations in extensions of the Standard Model. Owing to the accuracy of our calculation on ∆S = 2 weak Hamiltonian matrix elements, we are able to provide a refined Unitarity Triangle analysis improving the bounds coming from model independent constraints on New Physics. In our nonperturbative computation we use a combination of N f = 2 maximally twisted sea quarks and Osterwalder-Seiler valence quarks in order to achieve both O(a)-improvement and continuum-like renormalization properties for the relevant four-fermion operators. The calculation of the renormalization constants has been performed non-perturbatively in the RI-MOM scheme. Based on simulations at four values of the lattice spacing and a number of quark masses we have extrapolated/interpolated our results to the continuum limit and physical light/strange quark masses.

Unitarity Triangle and New Physics

The Unitarity Triangle (UT) Analysis represents a powerful tool to test the SM and therefore to probe New Physics effects. After summarizing various tensions in the present UT analyses, we consider a number of Supersymmetric models showing their capability to solve these tensions. Moreover, we show how the characteristic patterns of correlations among flavour observables allow to distinguish between these different SUSY scenarios. We propose a “DNA-Flavour Test” of NP models, with the aim of showing a tool to distinguish between these NP scenarios, once additional data on flavour changing processes become available.

Unitarity Triangle Analysis: An Update

We present the update of the Unitarity Triangle analysis performed by the UTfit Collaboration within the Standard Model and beyond. Within the Standard Model, we find an overall consistency among the constraints, with some tension between the fit prediction and the experimental measurement of BR ( B → τ ν ) , sin 2 β and e K . Generalising the analysis beyond the Standard Model, we allow for New Physics effects in all Δ F = 2 processes simultanously. We obtain constraints on the parameters of the Cabibbo-Kobayashi-Maskawa matrix together with the New Physics ones. We find that a deviation from the Standard Model at the 3.1σ level in the B s - B ¯ s mixing phase is confirmed by the present update, which includes the new DO result on the dimuon charge asymmetry. However the new CDF measurement of ϕ s is not included yet as the experimental likelihood is not publicly available.

Lattice QCD inputs to the CKM unitarity triangle analysis

We perform a global fit to the CKM unitarity triangle using the latest experimental and theoretical constraints. Our emphasis is on the hadronic weak matrix elements that enter the analysis, which must be computed using lattice QCD or other nonperturbative methods. Realistic lattice QCD calculations which include the effects of the dynamical up, down, and strange quarks are now available for all of the standard inputs to the global fit. We therefore present lattice averages for all of the necessary hadronic weak matrix elements. We attempt to account for correlations between lattice QCD results in a reasonable but conservative manner: whenever there are reasons to believe that an error is correlated between two lattice calculations, we take the degree of correlation to be 100%. These averages are suitable for use as inputs both in the global CKM unitarity triangle fit and other phenomenological analyses. In order to illustrate the impact of the lattice averages, we make Standard Model predictions for the parameters BK, |Vcb|, and |Vub|/|Vcb|. We find a (2-3) sigma tension in the unitarity triangle, depending upon whether we use the inclusive or exclusive determination of |Vcb|. If we interpret the tension as a sign of new physics in either neutral kaon or B mixing, we find that the scenario with new physics in kaon-mixing is preferred by present data.

Probing Yukawa unification with K and B mixing

We consider corrections to the unification of down-quark and charged-lepton Yukawa couplings in supersymmetric GUTs, which links the large ντ−νμ mixing angle to b → s transitions. These corrections generically occur in simple grand-unified models with small Higgs representations and affect s → d and b → d transitions via the mixing of the corresponding right-handed superpartners. On the basis of a specific SUSY-SO(10) model, we analyze the constraints from K− and Bd−d mixing on the additional R−R rotation angle θ. We find that K already sets a stringent bound on θ, θmax ~ (1°), indicating a very specific flavor structure of the correction operators. The impact of the large neutrino mixings on the unitarity triangle analysis is also briefly discussed, as well as their ability to account for the sizeable CP-violating phase observed recently in Bs → J/ψ decays.

Update of the Unitarity Triangle Analysis

We present the update of the Unitarity Triangle (UT) analysis within the Standard Model (SM) and beyond. Within the SM, combining the direct measurements on sides and angles, the UT turns out to be overconstrained in a consistent way, showing that the CKM matrix is the dominant source of flavour mixing and CP-violation and that New Physics (NP) effects can appear at most as small corrections to the CKM picture. Generalizing the UT analysis to investigate NP effects, constraints on b → s transitions are also included and both CKM and NP parameters are fitted simultaneously. While no evidence of NP effects is found in K- K ¯ and B d - B ¯ d mixing, in the B s - B ¯ s mixing an hint of NP is found at the 2.9 σ level. The UT analysis beyond the SM also allows us to derive bounds on the coefficients of the most general Δ F = 2 effective Hamiltonian, that can be translated into bounds on the NP scale.

Status of the unitarity triangle analysis in the standard model

We present the update of the Unitarity Triangle analysis in the Standard Model. Combining the direct measurements on sides and angles of the Unitarity Triangle, we determine the values of the CKM parameters ρ̄ and η̄. This set of information provides a very poweful test of the Standard Model in the fermion sector. Thanks to the fact that the fit is overconstraint, we can predict hadronic QCD parameters without relying on any theoretical calculation of hadronic elements.

Probing the TeV scale (and above) with flavor physics

Generalizing the unitarity triangle analysis to include new physics effects, we derive upper bounds on the coefficients of the most general δF = 2 effective Hamiltonian. These upper bounds can be translated into lower bounds on the scale of new physics that contributes to these low-energy effective interactions. We conclude that the scale of new physics in models that generate new δF = 2 operators, such as next-to-minimal flavor violation, has to be much higher than the scale of minimal flavor violation, and it most probably lies beyond the reach of direct searches at the LHC. Updated results are available on http://www.utfit.org.

Model-independent constraints on ΔF= 2 operators and the scale of new physics

We update the constraints on new-physics contributions to Delta F=2 processes from the generalized unitarity triangle analysis, including the most recent experimental developments. Based on these constraints, we derive upper bounds on the coefficients of the most general Delta F=2 effective Hamiltonian. These upper bounds can be translated into lower bounds on the scale of new physics that contributes to these low-energy effective interactions. We point out that, due to the enhancement in the renormalization group evolution and in the matrix elements, the coefficients of non-standard operators are much more constrained than the coefficient of the operator present in the Standard Model. Therefore, the scale of new physics in models that generate new Delta F=2 operators, such as next-to-minimal flavour violation, has to be much higher than the scale of minimal flavour violation, and it most probably lies beyond the reach of direct searches at the LHC.

Investigating the physics case of running a B-factory at theUpsilon(5S)resonance

We discuss the physics case of a high luminosity B-Factory running at the Y(5S) resonance. We show that the coherence of the B meson pairs is preserved at this resonance, and that Bs can be well distinguished from Bd and charged B mesons. These facts allow to cover the physics program of a traditional B-Factory and, at the same time, to perform complementary measurements which are not accessible at the Y(4S). In particular we show how, despite the experimental limitations in performing time-dependent measurements of Bs decays, the same experimental information can be extracted, in several cases, from the determination of time-integrated observables. In addition, a few examples of the potentiality in measuring rare Bs decays are given. Finally, we discuss how the study of Bs meson will improve the constraints on New Physics parameters in the Bs sector, in the context of the generalized Unitarity Triangle analysis.

Lattice QCD confronts the Unitarity Triangle Fit

Before the starting of the B factories, the Unitarity Triangle (UT) analysis relied on the results of lattice QCD simulations to relate the experimental determinations of semileptonic B decays, K 0 − K ¯ 0 and B d , s 0 − B ¯ d , s 0 mixing to the CKM parameters. In the last years, much more information has been obtained from the direct determination of the UT angles from non-leptonic B decays. In this talk, we compare the results of the “classical” UT analysis (UTlattice) with the analysis based on the angles determinations (UTangles). We show that the recent measurement of Δ m s , combined with Δ m d and e K , allows from this comparison a quite accurate extraction of the values of the lattice parameters, B ˆ K , f B s B ˆ B s 1 / 2 , ξ and f B . These values, obtained “experimentally” by assuming the validity of the Standard Model, can be compared with the theoretical predictions from lattice QCD. We also discuss the different determinations of V u b and show that current data do not favor the value measured in inclusive decays. Finally, we present updated predictions for the leptonic branching fraction B R ( B → τ ν τ ) , and compare them with the recent experimental determination. We take the opportunity of this write up to update the results of ref. [M. Bona et al. [UTfit Collaboration], arXiv:hep-ph/0606167 ] with the most recent results presented at the ICHEP'06 conference.

B-meson mixing and lifetimes

$B_d$ and $B_s$ meson systems play a fundamental role to test and improve our understanding of Standard Model flavor dynamics. The mixing parameters $\Delta m_{d,s}$ represent important constraints in the unitarity triangle analysis. Their theoretical estimates require non-perturbative calculations of B-meson decay constants and B-parameters; accurate results, recently obtained from the lattice, are reviewed. Other phenomenologically interesting quantities are the beauty hadron lifetime ratios and, the width differences and CP-violation parameters in $B_d$ and $B_s$ systems. We discuss their theoretical predictions which, in the last four years, have been improved thanks to accurate lattice calculations and next-to-leading order perturbative computations.

The UTfit collaboration report on the status of the unitarity triangle beyond the Standard Model I. Model-independent analysis and minimal flavour violation

Starting from a (new physics independent) tree level determination of and , we perform the Unitarity Triangle analysis in general extensions of the Standard Model with arbitrary new physics contributions to loop-mediated processes. Using a simple parameterization, we determine the allowed ranges of non-standard contributions to |ΔF| = 2 processes. Remarkably, the recent measurements from B factories allow us to determine with good precision the shape of the Unitarity Triangle even in the presence of new physics, and to derive stringent constraints on non-standard contributions to |ΔF| = 2 processes. Since the present experimental constraints favour models with Minimal Flavour Violation, we present the determination of the Universal Unitarity Triangle that can be defined in this class of extensions of the Standard Model. Finally, we perform a combined fit of the Unitarity Triangle and of new physics contributions in Minimal Flavour Violation, reaching a sensitivity to a new physics scale of about 5 TeV. We also extrapolate all these analyses into a ``year 2010'' scenario for experimental and theoretical inputs in the flavour sector. All the results presented in this paper are also available at the URL http://www.utfit.org, where they are continuously updated.

One-loop matching ofΔS=2four-quark operators with improved staggered fermions

We compute O({alpha}{sub s}) lattice-to-continuum perturbative matching coefficients for the {delta}S=2 flavor changing four-quark operators for the Asqtad improved staggered action. In conjunction with lattice simulations with three flavors of light, dynamical quarks, our results yield an unquenched determination of B{sub K}, the parameter that determines the amount of indirect CP violation in the neutral kaon system. Its value is an important input for the unitarity triangle analysis of weak decays.

Upper bounds on rare K and B decays from minimal flavour violation

We study the branching ratios of rare K and B decays in models with minimal flavour violation, using the presently available information from the universal unitarity triangle analysis and from the measurements of Br ( B → X s γ ) , Br ( B → X s l + l − ) and Br ( K + → π + ν ν ¯ ) . We find the following upper bounds: Br ( K + → π + ν ν ¯ ) < 11.9 × 10 −11 , Br ( K L → π 0 ν ν ¯ ) < 4.6 × 10 −11 , Br ( K L → μ + μ − ) SD < 1.4 × 10 −9 , Br ( B → X s ν ν ¯ ) < 5.2 × 10 −5 , Br ( B → X d ν ν ¯ ) < 2.2 × 10 −6 , Br ( B s → μ + μ − ) < 7.4 × 10 −9 , Br ( B d → μ + μ − ) < 2.2 × 10 −10 at 95% probability. We analyze in detail various possible scenarios with positive or negative interference of Standard Model and new physics contributions, and show how an improvement of experimental data corresponding to the projected 2010 B factory integrated luminosities will allow to disentangle and test these different possibilities. Finally, anticipating that subsequently the leading role in constraining this kind of new physics will be taken over by the rare decays K + → π + ν ν ¯ , K L → π 0 ν ν ¯ and B s , d → μ + μ − , that are dominated by the Z 0 -penguin function C, we also present plots for several branching ratios as functions of C. We point out an interesting triple correlation between K + → π + ν ν ¯ , B → X s γ and B → X s l + l − present in MFV models.

The 2004 UTfit collaboration report on the status of the unitarity triangle in the standard model

Using the latest determinations of several theoretical and experimental parameters, we update the Unitarity Triangle analysis in the Standard Model. The basic experimental constraints come from the measurements of |V_ub/V_cb|, Delta M_d, the lower limit on Delta M_s, epsilon_k, and the measurement of the phase of the B_d - anti B_d mixing amplitude through the time-dependent CP asymmetry in B^0 to J/psi K^0 decays. In addition, we consider the direct determination of alpha, gamma, 2 beta + gamma and cos(2 beta) from the measurements of new CP-violating quantities, recently performed at the B factories. We also discuss the opportunities offered by improving the precision of the various physical quantities entering in the determination of the Unitarity Triangle parameters. The results and the plots presented in this paper can also be found at http://www.utfit.org, where they are continuously updated with the newest experimental and theoretical results.

Exploring flavor structure of supersymmetry breaking from rareBdecays and the unitarity triangle

We study effects of supersymmetric particles in various rare B decay processes as well as in the unitarity triangle analysis. We consider three different supersymmetric models, the minimal supergravity, $\mathrm{SU}(5)$ SUSY GUT with right-handed neutrinos, and the minimal supersymmetric standard model with $U(2)$ flavor symmetry. In the $\mathrm{SU}(5)$ SUSY GUT with right-handed neutrinos, we consider two cases of the mass matrix of the right-handed neutrinos. We calculate direct and mixing-induced CP asymmetries in the $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\gamma}$ decay and CP asymmetry in ${B}_{d}\ensuremath{\rightarrow}\ensuremath{\varphi}{K}_{S}$ as well as the ${B}_{s}--{B}_{s}$ mixing amplitude for the unitarity triangle analysis in these models. We show that large deviations are possible for the $\mathrm{SU}(5)$ SUSY GUT and the $U(2)$ model. The patterns and correlations of deviations from the standard model will be useful to discriminate the different SUSY models in future B experiments.

Chiral corrections and lattice QCD results for fBs/fBd and ΔmBs/ΔmBd

It has been argued recently that the inclusion of the chiral logarithms in extrapolation of the lattice data can shift the value of the hadronic parameter ξ=fBsBBs/fBdBBd, from 1.16(6) to 1.32(10) and even higher. If true, that would considerably change the theoretical estimate for the ratio of oscillation frequencies in the B0s- and B0d-systems, and would affect the standard CKM unitarity triangle analysis. In this Letter we show that fBs/fBd≈fK/fπ, and thus the uncertainty due to the missing chiral logs is smaller than previously thought. By combining the NLO chiral expansion with the static heavy quark limit we obtain ξ=1.22(9).