arXiv subjects
Publications and source records attributed to Gustavo Burdman.
The little Higgs model provides an alternative to traditional candidates for new physics at the TeV scale. The new heavy gauge bosons predicted by this model should be observable at the Large Hadron Collider (LHC). We discuss how the LHC experiments could test the little Higgs model by studying the production and decay of these particles.
We review the current status of flavor-changing neutral currents in the charm sector. We focus on the standard-model predictions and identify the main sources of theoretical uncertainties in both charm mixing and rare charm decays. The potential of these observables for constraining short-distance physics in the standard model and its extensions is compromised by the presence of large nonperturbative effects. We examine the possible discovery windows in which short-distance physics can be tested and study the effects of various extensions of the standard model. The current experimental situation and future prospects are reviewed.
We construct realistic theories in which the Higgs fields arise from extra dimensional components of higher dimensional gauge fields. In particular, we present a minimal 5D SU(3)_C x SU(3)_W model and a unified 5D SU(6) model. In both cases the theory is reduced to the minimal supersymmetric standard model below the compactification scale, with the two Higgs doublets arising from the 5D gauge multiplet. Quarks and Leptons are introduced in the bulk, giving Yukawa couplings without conflicting with higher dimensional gauge invariance. Despite the fact that they arise from higher dimensional gauge interactions, the sizes of these Yukawa couplings can be different from the 4D gauge couplings due to wave-function profiles of the matter zero modes determined by bulk mass parameters. All unwanted fields are made heavy by introducing appropriate matter and superpotentials on branes, which are also the source of intergenerational mixings in the low-energy Yukawa matrices. The theory can accommodate a realistic structure for the Yukawa couplings as well as small neutrino masses. Scenarios for supersymmetry breaking and the mu-term generation are also discussed.
We derive constraints on the Randall-Sundrum scenario with the standard model fields in the bulk. These result from tree level effects associated with the deformation of the zero mode wave-functions of the W and the Z once electroweak symmetry is broken. Recently Csáki, Erlich and Terning pointed out that this implies large contributions to electroweak oblique parameters. Here we find that when fermions are allowed in the bulk the couplings of the $W$ and the $Z$ to zero-mode fermions are also affected. We perform a fit to electroweak observables assuming universal bulk fermion masses and including all effects and find constraints that are considerably stronger than for the case with fermions localized in the low energy boundary. These put the lowest Kaluza-Klein excitation out of reach of the Large Hadron Collider. We then relax the universality assumption and study the effects of flavor violation in the bulk and its possible signatures.
We perform a comprehensive study of a number of rare charm decays, incorporating the first evaluation of the QCD corrections to the short distance contributions, as well as examining the long range effects. For processes mediated by the $c\to u\ell^+\ell^-$ transitions, we show that sensitivity to short distance physics exists in kinematic regions away from the vector meson resonances that dominate the total rate. In particular, we find that $D\toπ\ell^+\ell^-$ and $D\toρ\ell^+\ell^-$ are sensitive to non-universal soft-breaking effects in the Minimal Supersymmetric Standard Model with R-parity conservation. We separately study the sensitivity of these modes to R-parity violating effects and derive new bounds on R-parity violating couplings. We also obtain predictions for these decays within extensions of the Standard Model, including extensions of the Higgs, gauge and fermion sectors, as well as models of dynamical electroweak symmetry breaking.
I discuss recent progress in our understanding of exclusive rare and semileptonic decays. I show the impact of HQET when combined with the predictions in the Large Energy Limit of QCD, focusing first on applications to $B\to K^*\ell^+\ell^+$. I also discuss the constraints on semileptonic form-factors that appear in HQET/LEET with the use of radiative decay data, and update these to include the effects of next-to-leading order contributions in $B\to K^*γ$, as well as the latest data.
We argue that extended technicolor augmented with topcolor requires that all mixing between the third and the first two quark generations resides in the mixing matrix of left-handed down quarks. Then, the anti-B_d--B_d mixing that occurs in topcolor models constrains the coloron and Z' boson masses to be greater than about 5 TeV. This implies fine tuning of the topcolor couplings to better than 1%.
Making use of the measurement of the $B\to K^*γ$ branching ratio together with the relations following from the limit of high recoil energy, we obtain stringent constraints on the values of the form-factors entering in heavy-to-light $B\to V\ell\ell'$ processes such as $B\to K^*\ell^+\ell^-$, $B\to K^*ν\barν$ and $B\to ρ\ellν$ decays. We show that the symmetry predictions, when combined with the experimental information on radiative decays, specify a severely restricted set of values for the vector and axial-vector form-factors evaluated at zero momentum transfer, $q^2=0$. These constraints can be used to test model calculations and to improve our understanding of the $q^2$-dependence of semileptonic form-factors. We stress that the constraints remain stringent even when corrections are taken into account.
We investigate collider signals for gauged flavor symmetries that have been proposed in models of dynamical electroweak symmetry breaking and fermion mass generation. We consider the limits on the masses of the gauge bosons in these models which can be extracted from Tevatron Run I data in dijet production. Estimates of the Run II search potential are provided. We show that the models also give rise to significant signals in single top production which may be visible at Run II. In particular we study chiral quark family symmetry and SU(9) chiral flavor symmetry. The Run I limits on the gauge bosons in these models lie between (1.5-2) TeV and should increase to about 3 TeV in Run II. Finally, we show that an SU(12) enlargement of the SU(9) model, including leptonic interactions, is constrained by low energy atomic parity violation experiments to lie outside the reach of the Tevatron.
We study the potential effect of anomalous couplings of the third generation quarks to gauge bosons in rare B decays. We focus on the constraints from flavor changing neutral current processes such as $b\to sγ$ and $b\to s \ell^+\ell^-$. We consider both dimension-four and dimension-five operators and show that the latter can give large deviations from the standard model in the still unobserved dilepton modes, even after the bounds from $b\to sγ$ and precision electroweak observables are taken into account.
Gauged flavor symmetries at low energies have been proposed in models of dynamical electroweak symmetry breaking and fermion mass generation. The massive flavor gauge bosons give rise to corrections to precisely measured electroweak quantities. We perform a fit to collider electroweak data and place indirect limits on such new physics. In particular we study several models from the literature: universal coloron; chiral top color; chiral, quark family symmetry; SU(9) and SU(12) chiral, flavor symmetry. The 95% exclusion limits on the mass of the heavy gauge bosons for these models, at their critical coupling for chiral symmetry breaking, typically lie between 1-3 TeV. We discuss the robustness of these bounds with respect to changes in higgs mass, b-quark asymmetry data, the SLD left right asymmetry data, or additional new physics.
We study the production and decay of neutral scalars and pseudo-scalars at hadron colliders, in theories where the top-quark mass is the result of a $t\bar t$ condensate. We show that the dominant decay channel for masses below the $t\bar t$ threshold is the flavor changing mode $tc$. This is a consequence of the non-universal nature of the underlying interactions in all top-condensation models and provides a model-independent signature of these scenarios. We show that an upgraded Tevatron is sensitive to a sizeable region of the interesting parameter space and that the LHC will highly constrain these models through this flavor violating channel.
I discuss some theoretical aspects of rare $K$, $D$ and $B$ decays focusing mainly on their potential as tests of the one loop structure of the standard model. I concentrate on flavor changing neutral current processes and compare our ability to extract short distance physics in the three cases. Finally, I give some examples of the sensitivity of these decays to extensions of the standard model.
The top condensate see-saw mechanism of Dobrescu and Hill allows electroweak symmetry to be broken while deferring the problem of flavour to an electroweak singlet, massive sector. We provide an extended version of the singlet sector that naturally accommodates realistic masses for all the standard model fermions, which play an equal role in breaking electroweak symmetry. The models result in a relatively light composite Higgs sector with masses typically in the range of (400-700)~GeV. In more complete models the dynamics will presumably be driven by a broken gauged family or flavour symmetry group. As an example of the higher scale dynamics a fully dynamical model of the quark sector with a GIM mechanism is presented, based on an earlier top condensation model of King using broken family gauge symmetry interactions (that model was itself based on a technicolour model of Georgi). The crucial extra ingredient is a reinterpretation of the condensates that form when several gauge groups become strong close to the same scale. A related technicolour model of Randall which naturally includes the leptons too may also be adapted to this scenario. We discuss the low energy constraints on the massive gauge bosons and scalars of these models as well as their phenomenology at the TeV scale.
We examine the sensitivity of flavor changing neutral current (FCNC) processes to anomalous triple gauge boson couplings. We show that in the non-linear realization of the electroweak symmetry breaking sector these processes are very sensitive to two CP conserving anomalous couplings. A clean separation of their effects is possible in the next round of experiments probing $b\to sγ$ and $b\to s\ell^+\ell^-$ processes, as well as kaon decays such as $K^+\toπ^+ν\barν$. The obtained sensitivity is found to be competitive with that of direct measurements at high energy colliders. In particular, for one of the $WWZ$ couplings the one-loop FCNC effects are enhanced by a logarithmic dependence on the scale of new physics. We also explore the potential signals of CP violating anomalous triple gauge boson couplings in rare $B$ decays.
We discuss the constraints from rare B and K decays on the Electroweak Symmetry Breaking (EWSB) sector, as well as on theories of fermion masses. We focus on models involving new strong dynamics and show that transitions involving Flavor Changing Neutral Currents (FCNC) play an important role in disentangling the physics in these scenarios. In a model-independent approach to the EWSB sector, the information from rare decays is complementary to precision electroweak observables in bounding the contributions to the effective lagrangian. We compare the pattern of deviations from the standard model that results from these sources, with the deviations associated with the mechanism for generating fermion masses.
We derive a condition the short distance coefficients governing $b\to (s,d)\ell^+\ell^-$ transitions must satisfy in order for the forward-backward asymmetry to vanish in the exclusive modes $B\to (K^*,ρ)\ell^+\ell^-$. This relation, which is satisfied in the standard model, involves the coefficient entering in $b\to sγ$ transitions as well as one of the additional Wilson coefficients present in the leptonic modes. We show that the resulting relation is largely free of hadronic uncertainties, thus constituting a reliable test of the standard model in exclusive rare B decays.
I briefly discuss the derivation of dispersive sum rules constraining semileptonic form-factors. I outline the use of these constraints in the context of charmless semileptonic decays and suggest how, in combination with other theoretical and experimental information, they may reduce the uncertainties in the extraction of $V_{ub}$.