arXiv subjects

Javier Virto

Publications and source records attributed to Javier Virto.

At least 55 records · Page 3Linked to original sources

arXiv2016

Non-abelian gauge extensions for B-decay anomalies

We study the generic features of minimal gauge extensions of the Standard Model in view of recent hints of lepton-flavor non-universality in semi-leptonic $b \to s \ell^+ \ell^-$ and $b \to c \ell ν$ decays. We classify the possible models according to the symmetry-breaking pattern and the source of flavor non-universality. We find that in viable models the $\mathrm{SU(2)}_L$ factor is embedded non-trivially in the extended gauge group, and that gauge couplings should be universal, hinting to the presence of new degrees of freedom sourcing non-universality. Finally, we provide an explicit model that can explain the $B$-decay anomalies in a coherent way and confront it with the relevant phenomenological constraints.

hep-ph↗

arXiv2016

Assessing lepton-flavour non-universality from $B\to K^*\ell\ell$ angular analyses

The $B\to K^*μμ$ decay exhibits deviations with respect to Standard Model expectations and the measurement of the ratio $R_K$ hints at a violation of lepton-flavour universality in $B\to K\ell\ell$ transitions. Both effects can be understood in model-independent fits as a short-distance contribution to the Wilson coefficient $C_{9μ}$, with some room for similar contributions in other Wilson coefficients for $b\to sμμ$ transitions. We discuss how a full angular analysis of $B\to K^*ee$ and its comparison with $B\to K^*μμ$ could improve our understanding of these anomalies and help confirming their interpretation in terms of short-distance New Physics. We discuss several observables of interest in this context and provide predictions for them within the Standard Model as well as within several New Physics benchmark scenarios. We pay special attention to the sensitivity of these observables to hadronic uncertainties from SM contributions with charm loops.

hep-ph↗

arXiv2015

Three-Body Non-Leptonic B Decays and QCD Factorization

We extend the framework of QCD factorization to non-leptonic $B$ decays into three light mesons, taking as an example the decay $B^+ \to π^+ π^+ π^-$. We discuss the factorization properties of this decay in different regions of phase space. We argue that, in the limit of very large $b$-quark mass, the central region of the Dalitz plot can be described in terms of the $B \to π$ form factor and the $B$ and $π$ light-cone distribution amplitudes. The edges of the Dalitz plot, on the other hand, require different non-perturbative input: the $B \to ππ$ form factor and the two-pion distribution amplitude. We present the set-up for both regions to leading order in both $α_s$ and $Λ_{\rm QCD} / m_b$ and discuss how well the two descriptions merge. We argue that for realistic $B$-meson masses there is no perturbative center in the Dalitz plot, but that a systematic description might be possible in the context of two-pion states. As an example, we estimate the $B\toρπ$ branching fraction beyond the quasi-particle approximation. We also discuss the prospects for studies of three-body and quasi-two-body non-leptonic $B$ decays from QCD.

hep-ph↗

arXiv2015

Time dependence in $B\to V\ell\ell$ decays

We discuss the theory and phenomenology of $B_{d,s}\to V(\to M_1M_2)\ell\ell$ decays in the presence of neutral-meson mixing. We derive expressions for the time-dependent angular distributions for decays into CP eigenstates, and identify the relevant observables that can be extracted from time-integrated and time-dependent analyses with or without tagging, with a focus on the difference between measurements at $B$-factories and hadronic machines. We construct two observables of interest, which we call $Q_8^-$ and $Q_9$, and which are theoretically clean at large recoil. We compute these two observables in the Standard Model, and show that they have good potential for New Physics searches by considering their sensitivity to benchmark New Physics scenarios consistent with current $b\to s\ell\ell$ data. These results apply to decays such as $B_d\to K^*(\to K_Sπ^0)\ell\ell$, $B_s\to ϕ(\to K_SK_L)\ell\ell$ and $B_s\to ϕ(\to K^+K^-)\ell\ell$.

hep-ph↗

arXiv2015

Theoretical status of $B \to K^* μ^+μ^-$: The path towards New Physics

We summarize the status of the analysis of the 4-body angular distribution $B \to K^*(\to K π) μ^+μ^-$ using a basis of optimized observables. We provide a New Physics interpretation of the pattern of deviations observed in the 1fb$^{-1}$ dataset with the coefficient of the semileptonic operator $O_9$ being the main responsible for this pattern. Also other scenarios involving more Wilson coefficients are briefly discussed. Further, we present a detailed description of each of the hadronic uncertainties entering our predictions and suggest possible tests for alternative hadronic explanations. Our most accurate SM predictions for the optimized observables in various bins including all uncertainties are also provided.

hep-ph↗

arXiv2014

QCD uncertainties in the prediction of B --> K^* mu^+ mu^- observables

The recent LHCb angular analysis of the exclusive decay B --> K^* mu^+ mu^- has indicated significant deviations from the Standard Model expectations. In order to give precise theory predictions, it is crucial that uncertainties from non-perturbative QCD are under control and properly included. The dominant QCD uncertainties originate from the hadronic B --> K^* form factors and from charm loops. We present a systematic method to include factorisable power corrections to the form factors in the framework of QCD factorisation and study the impact of the scheme chosen to define the soft form factors. We also discuss charm-loop effects.

hep-ph↗

arXiv2014

On the impact of power corrections in the prediction of B->K*mu+mu- observables

The recent LHCb angular analysis of the exclusive decay B->K^*mu+mu- has indicated significant deviations from the Standard Model expectations. Accurate predictions can be achieved at large K*-meson recoil for an optimised set of observables designed to have no sensitivity to hadronic input in the heavy-quark limit at leading order in alpha_s. However, hadronic uncertainties reappear through non-perturbative Lambda_QCD/m_b power corrections, which must be assessed precisely. In the framework of QCD factorisation we present a systematic method to include factorisable power corrections and point out that their impact on angular observables depends on the scheme chosen to define the soft form factors. Associated uncertainties are found to be under control, contrary to earlier claims in the literature. We also discuss the impact of possible non-factorisable power corrections, including an estimate of charm-loop effects. We provide results for angular observables at large recoil for two different sets of inputs for the form factors, spelling out the different sources of theoretical uncertainties. Finally, we comment on a recent proposal to explain the anomaly in B->K*mu+mu- observables through charm-resonance effects, and we propose strategies to test this proposal identifying observables and kinematic regions where either the charm-loop model can be disentangled from New Physics effects or the two options leave different imprints.

hep-ph↗

arXiv2013

Optimizing the basis of B -> K* l+l- observables and understanding its tensions

We discuss the impact of recent LHCb data on the B -> K* mu+mu- decay for New Physics searches. We use an optimised set of observables with a limited sensitivity to hadronic inputs (form factors), leading to a significant deviation from the Standard Model for the semileptonic operator O9. We ascertain the robustness of our results with respect to models of power corrections, charm-loop effects, factorisation approaches, possible effects from resonance tails at low-recoil, choice of observables and binnings. We explain some contradictory results obtained for the chirality-flipped operator O9prime by different analyses by focusing on the sensitivity of observables at low recoil afflicted by the psi(4160) and possibly other resonances for B+-> K+ mu+mu- and illustrating the need for a finely binned analysis at large recoil (rather than a wide [1-6] bin). Finally, we present updated results including experimental correlations in B -> K* mu+ mu- observables, confirming the deviation seen with respect to the SM hypothesis (4.2 sigma for large-recoil data, 3.5 sigma for large- and low-recoil data).

hep-ph↗

arXiv2013

Understanding the B->K*mu+mu- Anomaly

We present a global analysis of the B->K*(->K pi)mu+mu- decay using the recent LHCb measurements of the primary observables P_{1,2} and P'_{4,5,6,8}. Some of them exhibit large deviations with respect to the SM predictions. We explain the observed pattern of deviations through a large New Physics contribution to the Wilson coefficient of the semileptonic operator O9. This contribution has an opposite sign to the SM one, i.e., reduces the size of this coefficient significantly. A good description of data is achieved by allowing for New Physics contributions to the Wilson coefficients C7 and C9 only. We find a 4.5 sigma deviation with respect to the SM prediction, combining the large-recoil B->K*(->K pi)mu+mu- observables with other radiative processes. Once low-recoil observables are included the significance gets reduced to 3.9 sigma. We have tested different sources of systematics, none of them modifying our conclusions significantly. Finally, we propose additional ways of measuring the primary observables through new foldings.

hep-ph↗

arXiv2013

Optimizing the basis of B->K*ll observables in the full kinematic range

We discuss the observables for the B->K*(->K pi)ll decay, focusing on both CP-averaged and CP-violating observables at large and low hadronic recoil with special emphasis on their low sensitivity to form-factor uncertainties. We identify an optimal basis of observables that balances theoretical and experimental advantages, which will guide the New Physics searches in the short term. We discuss some advantages of the observables in the basis, and in particular their improved sensitivity to New Physics compared to other observables. We present predictions within the Standard Model for the observables of interest, integrated over the appropriate bins including lepton mass corrections. Finally, we present bounds on the S-wave contribution to the distribution coming from the B->K_0*ll decay, which will help to establish the systematic error associated to this pollution.

hep-ph↗

arXiv2013

B ->K*ll: The New Frontier of New Physics searches in Flavor

The exclusive decay mode B ->K*ll has become one of the key players in the search for New Physics in flavor. An increasing number of observables are being studied experimentally, and with increasing precision. Its theoretical description is well under control, and an interesting set of clean observables have been identified, reducing further the theoretical uncertainties. Model-independent analyses show that in the near future this mode will either reveal NP through a pattern of correlated deviations from the SM, or pose very stringent constraints on radiative and semileptonic operators.

hep-ph↗

arXiv2012

Implications from clean observables for the binned analysis of B -> K*ll at large recoil

We perform a frequentist analysis of q^2-dependent B-> K*(->Kpi)ll angular observables at large recoil, aiming at bridging the gap between current theoretical analyses and the actual experimental measurements. We focus on the most appropriate set of observables to measure and on the role of the q^2-binning. We highlight the importance of the observables P_i exhibiting a limited sensitivity to soft form factors for the search for New Physics contributions. We compute predictions for these binned observables in the Standard Model, and we compare them with their experimental determination extracted from recent LHCb data. Analyzing b->s and b->sll transitions within four different New Physics scenarios, we identify several New Physics benchmark points which can be discriminated through the measurement of P_i observables with a fine q^2-binning. We emphasise the importance (and risks) of using observables with (un)suppressed dependence on soft form factors for the search of New Physics, which we illustrate by the different size of hadronic uncertainties attached to two related observables (P_1 and S_3). We illustrate how the q^2-dependent angular observables measured in several bins can help to unravel New Physics contributions to B-> K*(->Kpi)ll, and show the extraordinary constraining power that the clean observables will have in the near future. We provide semi-numerical expressions for these observables as functions of the relevant Wilson coefficients at the low scale.

hep-ph↗

arXiv2012

Natural SUSY and Kaon Mixing in view of recent results from Lattice QCD

Lattice results are available for Delta S=2 matrix elements for the first time in full QCD, which improve considerably the status of hadronic uncertainties in K-Kbar mixing with respect to earlier phenomenological studies. Using an average of the ETMC and RBC results, we analyze epsilonK in Natural SUSY. This scenario arises as a consistent BSM framework after the latest results from the LHC. The analysis is improved with respect to previous studies including next-to-leading order matching conditions of order (alpha_s)^3. We derive new bounds for SUSY mass insertions in the scenario with a light third generation and study the implications for squark and gluino masses, compared with direct searches at the LHC. Assuming natural values for the flavor violating SUSY couplings of both chiralities, we find that the sbottom must be heavier than 3 TeV for a gluino mass up to 10 TeV. In this scenario no natural values for squark and gluino masses can satisfy the flavor bounds.

hep-ph↗

arXiv2012

New Physics constraints from optimized observables in B -> K*ll at large recoil

B -> K*ll angular observables have become a key ingredient in global model-independent analyses of b->s transitions. However, as experimental precision improves, the use of theoretically clean quantities becomes a crucial issue. Global analyses that use clean observables integrated in small bins are already a reality, opening up a new chapter in our quest for New Physics.

hep-ph↗

arXiv2012

Complete Anatomy of B -> K*ll and its angular distribution

We present a complete and optimal set of observables for the exclusive 4-body B meson decay B -> K*(->K pi) l+l- in the low dilepton mass region, that contains a maximal number of clean observables. This basis of observables is built in a systematic way. We show that all the previously defined observables and any observable that one can construct, can be expressed as a function of this basis. This set of observables contains all the information that can be extracted from the angular distribution in the cleanest possible way. We provide explicit expressions for the full and the uniangular distributions in terms of this basis. The conclusions presented here can be easily extended to the large-q^2 region. We study the sensitivity of the observables to right-handed currents and scalars. Finally, we present for the first time all the symmetries of the full distribution including massive terms and scalar contributions.

hep-ph↗

arXiv2011

An analysis of B_{d,s} mixing angles in presence of New Physics and an update of Bs -> K0* anti-K0*

We discuss a simple approach to measure the weak mixing angles phi_s and phi_d of the Bs and Bd systems in the presence of New Physics. We present a new expression that allows one to measure directly the New Physics mixing angles if New Physics contributes significantly to the mixing only. We apply the method to specific penguin-mediated B->PP, B->PV and B ->VV modes. We provide a very stringent and simple bound on the direct CP asymmetries of all these modes, the violation of which is a signal of New Physics in decay. Within the same theoretical framework, an updated prediction for the branching ratio of Bs->K0* anti-K0* is presented, which can be compared with a recent LHCb analysis.

hep-ph↗

arXiv2011

Top mass dependent alpha_s^3 corrections to B-meson mixing in the MSSM

We compute the top mass dependent NLO strong interaction matching conditions to the Delta F=2 effective Hamiltonian in the general MSSM. We study the relevance of such corrections, comparing its size with that of previously known NLO corrections in the limit mt->0, in scenarios with degeneracy, alignment, and hierarchical squarks. We find that, while these corrections are generally small, there are regions in the parameter space where the contributions to the Wilson coefficients C1 and C4 could partially overcome the expected suppression m_t/M_SUSY.

hep-ph↗

arXiv2010

On the Characterization of Polyhedra in Hyperbolic 3-Space

We review several results related to the characterization of polyhedra in hyperbolic 3-space. In particular we present Rivin's theorem that gives a characterization of compact convex hyperbolic polyhedra, and Hodgson's proof of the Adreev's theorem. We also review the analogous characterization of ideal polyhedra, and give a family of counter-examples that proves that hyperbolic polyhedra are not determined by edge lengths.

math.MG↗