Showing papers on "Electroweak interaction published in 2004"
TL;DR: In this article, the rapidity distributions of W and Z bosons produced at the Tevatron and the LHC through next-to-next-to leading order in QCD are computed.
Abstract: We compute the rapidity distributions of W and Z bosons produced at the Tevatron and the LHC through next-to-next-to leading order in QCD. Our results demonstrate remarkable stability with respect to variations of the factorization and renormalization scales for all values of rapidity accessible in current and future experiments. These processes are therefore ''gold-plated'': current theoretical knowledge yields QCD predictions accurate to better than one percent. These results strengthen the proposal to use $W$ and $Z$ production to determine parton-parton luminosities and constrain parton distribution functions at the LHC. For example, LHC data should easily be able to distinguish the central parton distribution fit obtained by MRST from that obtained by Alekhin.
744 citations
TL;DR: In this article, the profile of the CKM matrix is updated by means of a global fit of the Unitarity Triangle and the apex of the unitarity triangle is given by a global fitting.
Abstract: We update the profile of the CKM matrix. The apex (rhobar,etabar) of the Unitarity Triangle is given by means of a global fit. We propose to include therein sin2alpha from the CP-violating asymmetries in B0->rho+rho-, using isospin to discriminate the penguin contribution. The constraint from epsilon'/epsilon is briefly discussed. We study the impact from the measurement of the rare decay K+->pi+nunu-bar, and from a future observation of KL->pi0nunubar. The B system is investigated in detail, beginning with 2beta+gamma and gamma from B0->D(*)+-pi-+ and B+->D(*)0K+. A significant part of this paper is dedicated to B decays into pipi, Kpi, rhopi and rhorho. Various phenomenological and theoretical approaches are studied. Within QCD Factorization we find a remarkable agreement of the pipi and Kpi data with the other UT constraints. A fit of QCD FA to all pipi and Kpi data leads to precise predictions of the related observables. We analyze separately the B->Kpi decays, and in particular the impact of electroweak penguins in response to recent phenomenological discussions. We find no significant constraint on electroweak nor hadronic parameters. We do not observe any unambiguous sign of New Physics, whereas there is some evidence for potentially large rescattering effects. Finally we use a model-independent description of a large class of New Physics effects in both BBbar mixing and B decays, namely in the b->d and b->s gluonic penguin amplitudes, to perform a new numerical analysis. Significant non-standard corrections cannot be excluded yet, however standard solutions are favored in most cases.
704 citations
TL;DR: In this article, it was shown that the proposed Higgsless models in 5D do not provide a viable description of electroweak symmetry breaking in their full range of parameters, when calculable.
596 citations
TL;DR: In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude.
Abstract: In this Letter we study pair annihilation processes of dark matter (DM) in the Universe, in the case that the DM is an electroweak gauge nonsinglet. In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude. We also discuss sensitivities in future searches for anomalous gamma rays from the galactic center, which originate from DM annihilation.
496 citations
TL;DR: The Stueckelberg mechanism was introduced in this paper to reveal a symmetry of a gauge-fixed theory, which led to applications in many areas not anticipated by the author, such as strings.
Abstract: In 1938, Stueckelberg introduced a scalar field which makes an Abelian gauge theory massive but preserves gauge invariance. The Stueckelberg mechanism is the introduction of new fields to reveal a symmetry of a gauge--fixed theory. We first review the Stueckelberg mechanism in the massive Abelian gauge theory. We then extend this idea to the standard model, stueckelberging the hypercharge U(1) and thus giving a mass to the physical photon. This introduces an infrared regulator for the photon in the standard electroweak theory, along with a modification of the weak mixing angle accompanied by a plethora of new effects. Notably, neutrinos couple to the photon and charged leptons have also a pseudo-vector coupling. Finally, we review the historical influence of Stueckelberg's 1938 idea, which led to applications in many areas not anticipated by the author, such as strings. We describe the numerous proposals to generalize the Stueckelberg trick to the non-Abelian case with the aim to find alternatives to the standard model. Nevertheless, the Higgs mechanism in spontaneous symmetry breaking remains the only presently known way to give masses to non-Abelian vector fields in a renormalizable and unitary theory.
419 citations
TL;DR: In this paper, a new symmetry, denoted as T-parity, under which all heavy gauge bosons and scalar triplets are odd, can remove all the tree-level contributions to the electroweak observables and therefore make the little Higgs theories completely natural.
Abstract: Little Higgs theories are an attempt to address the ``little hierarchy problem,'' ie, the tension between the naturalness of the electroweak scale and the precision electroweak measurements showing no evidence for new physics up to 5 – 10 TeV In little Higgs theories, the Higgs mass-squareds are protected at one-loop order from the quadratic divergences This allows the cutoff of the theory to be raised up to ~ 10 TeV, beyond the scales probed by the current precision data However, strong constraints can still arise from the contributions of the new TeV scale particles which cancel the one-loop quadratic divergences from the standard model fields, and hence re-introduces the fine-tuning problem In this paper we show that a new symmetry, denoted as T-parity, under which all heavy gauge bosons and scalar triplets are odd, can remove all the tree-level contributions to the electroweak observables and therefore makes the little Higgs theories completely natural The T-parity can be manifestly implemented in a majority of little Higgs models by following the most general construction of the low energy effective theory a la Callan, Coleman, Wess and Zumino In particular, we discuss in detail how to implement the T-parity in the littlest Higgs model based on SU(5)/SO(5) The symmetry breaking scale f can be even lower than 500 GeV if the contributions from the higher dimensional operators due to the unknown UV physics at the cutoff are somewhat small The existence of T-parity has drastic impacts on the phenomenology of the little Higgs theories The T-odd particles need to be pair-produced and will cascade down to the lightest T-odd particle (LTP) which is stable A neutral LTP gives rise to missing energy signals at the colliders which can mimic supersymmetry It can also serve as a good dark matter candidate
407 citations
TL;DR: In this article, the electroweak corrections due to light fermions to the production cross section σ(gg! H) and to the partial decay widths (H! γ γ) and ( H! g g) were computed in terms of generalized harmonic polylogarithms.
396 citations
TL;DR: In this article, the authors address the possible interplay between the Large Hadron Collider (LHC) and the International e+e- Linear Collider (ILC) in testing the Standard Model and in discovering and determining the origin of new physics.
Abstract: Physics at the Large Hadron Collider (LHC) and the International e+e- Linear Collider (ILC) will be complementary in many respects, as has been demonstrated at previous generations of hadron and lepton colliders. This report addresses the possible interplay between the LHC and ILC in testing the Standard Model and in discovering and determining the origin of new physics. Mutual benefits for the physics programme at both machines can occur both at the level of a combined interpretation of Hadron Collider and Linear Collider data and at the level of combined analyses of the data, where results obtained at one machine can directly influence the way analyses are carried out at the other machine. Topics under study comprise the physics of weak and strong electroweak symmetry breaking, supersymmetric models, new gauge theories, models with extra dimensions, and electroweak and QCD precision physics. The status of the work that has been carried out within the LHC / LC Study Group so far is summarised in this report. Possible topics for future studies are outlined.
334 citations
TL;DR: In this paper, a new symmetry, denoted as T-parity, under which all heavy gauge bosons and scalar triplets are odd, can remove all the tree-level contributions to the electroweak observables and therefore make the little Higgs theories completely natural.
Abstract: Little Higgs theories are an attempt to address the little hierarchy problem, i.e., the tension between the naturalness of the electroweak scale and the precision measurements showing no evidence for new physics up to 5-10 TeV. In little Higgs theories, the Higgs mass-squareds are protected to the one-loop order from the quadratic divergence. This allows the cutoff to be raised up to \~10 TeV, beyond the scales probed by the precision data. However, strong constraints can still arise from the contributions of the new TeV scale particles and hence re-introduces the fine-tuning problem. In this paper we show that a new symmetry, denoted as T-parity, under which all heavy gauge bosons and scalar triplets are odd, can remove all the tree-level contributions to the electroweak observables and therefore makes the little Higgs theories completely natural. The T-parity can be manifestly implemented in a majority of little Higgs models by following the most general construction of the low energy effective theory a la Callan, Coleman, Wess and Zumino. In particular, we discuss in detail how to implement the T-parity in the littlest Higgs model based on SU(5)/SO(5). The symmetry breaking scale f can be even lower than 500 GeV if the contributions from the unknown UV physics at the cutoff are somewhat small. The existence of $T$-parity has drastic impacts on the phenomenology of the little Higgs theories. The T-odd particles need to be pair-produced and will cascade down to the lightest T-odd particle (LTP) which is stable. A neutral LTP gives rise to missing energy signals at the colliders which can mimic supersymmetry. It can also serve as a good dark matter candidate.
328 citations
TL;DR: In this paper, the standard model expectations for the rare radiative decays B->K^* gamma, B->rho gamma and B-> omega gamma at the next-to-leading order (NLO) were provided.
Abstract: We provide Standard Model expectations for the rare radiative decays B->K^* gamma, B->rho gamma and B-> omega gamma, and the electroweak penguin decays B->K^* l^+ l^- and B->rho l^+ l^- at the next-to-leading order (NLO), extending our previous results to b->d transitions. We consider branching fractions, isospin asymmetries and direct CP asymmetries. For the electroweak penguin decays, the lepton-invariant mass spectrum and forward-backward asymmetry is also included. Radiative and electroweak penguin transitions in b->d are mainly interesting in the search for new flavour-changing neutral current interactions, but in addition the B->rho gamma decays provide constraints on the CKM parameters (\bar\rho,\bar\eta). The potential impact of these constraints is discussed.
307 citations
TL;DR: In this paper, T-parity invariant extensions of the littlest Higgs model were constructed, in which only linear representations of the full symmetry group are employed, without recourse to the non-linear representations introduced by Coleman, Callan, Wess, and Zumino (CCWZ).
Abstract: We construct T-parity invariant extensions of the littlest Higgs model, in which only linear representations of the full symmetry group are employed, without recourse to the non-linear representations introduced by Coleman, Callan, Wess, and Zumino (CCWZ). These models are based on the symmetry breaking pattern SU(5)l × Hr/SO(5), where Hr can be SO(5) or other larger symmetry groups. The structure of the models in the SU(5)l sector is identical to the littlest Higgs model based on SU(5)/SO(5). Since the full symmetry group is realized linearly, these models can be thought of as possible UV extensions of the T-invariant model using non-linear representations via CCWZ, with whom they share similar low energy phenomenology. We also comment on how to avoid constraints from four-fermion operators on T-invariant models with or without CCWZ construction. The electroweak data therefore place a very weak bound on the symmetry breaking scale, f ≥ 450 GeV.
TL;DR: In this paper, the authors present the implementation of the radiative corrections of the Higgs sector in three public computer codes for the evaluation of the particle spectrum in the Minimal Supersymmetric Standard Model, Softsusy, Spheno and SuSpect.
Abstract: We present the implementation of the radiative corrections of the Higgs sector in three public computer codes for the evaluation of the particle spectrum in the Minimal Supersymmetric Standard Model, Softsusy, Spheno and SuSpect. We incorporate the full one-loop corrections to the Higgs boson masses and the electroweak symmetry breaking conditions, as well as the two-loop corrections controlled by the strong gauge coupling and the Yukawa couplings of the third generation fermions. We include also the corrections controlled by the tau Yukawa coupling that we derived for completeness. The computation is consistently performed in the DRbar renormalisation scheme. In a selected number of MSSM scenarios, we study the effect of these corrections and analyse the impact of some higher order effects. By considering the renormalisation scheme and scale dependence, and the effect of the approximation of zero external momentum in the two-loop corrections, we estimate the theoretical uncertainty on the lighter Higgs boson mass to be 3 to 5 GeV. The uncertainty on Mh due to the experimental error in the measurement of the SM input parameters is approximately of the same size. Finally, we discuss the phenomenological consequences, using the latest value of the top quark mass. We find, in particular, that the most conservative upper bound on the lighter Higgs boson mass in the general MSSM is Mh < 152 GeV and that there is no lower bound on the parameter tan(beta) from non-observation of the MSSM Higgs bosons at LEP2.
TL;DR: In this article, the authors examined the possibility of electroweak baryogenesis and dark matter in the nMSSM, a minimal extension of the MSSM with a singlet field.
Abstract: We examine the possibility of electroweak baryogenesis and dark matter in the nMSSM, a minimal extension of the MSSM with a singlet field. This extension avoids the usual domain wall problem of the NMSSM, and also appears as the low energy theory in models of dynamical electroweak symmetry breaking with a so-called fat-Higgs boson. We demonstrate that a strong, first order electroweak phase transition, necessary for electroweak baryogenesis, may arise in regions of parameter space where the lightest neutralino provides an acceptable dark matter candidate. We investigate the parameter space in which these two properties are fulfilled and discuss the resulting phenomenology. In particular, we show that there are always two light CP-even and one light CP-odd Higgs bosons with masses smaller than about 250 GeV. Moreover, in order to obtain a realistic relic density, the lightest neutralino mass tends to be smaller than ${M}_{Z}/2,$ in which case the lightest Higgs boson decays predominantly into neutralinos.
TL;DR: In this article, the boundary conditions at the ends of an extra dimensional interval were found consistent with the variational principle and explained which ones arise in various physical circumstances, and they were applied to Higgsless models of electroweak symmetry breaking, where the symmetry is not broken by a scalar vacuum expectation value, but rather by the boundary condition of the gauge fields.
Abstract: We consider fermions on an extra dimensional interval. We find the boundary conditions at the ends of the interval that are consistent with the variational principle, and explain which ones arise in various physical circumstances. We apply these results to Higgsless models of electroweak symmetry breaking, where electroweak symmetry is not broken by a scalar vacuum expectation value, but rather by the boundary conditions of the gauge fields. We show that it is possible to find a set of boundary conditions for bulk fermions that would give a realistic fermion mass spectrum without the presence of a Higgs scalar, and present some sample fermion mass spectra for the standard model quarks and leptons as well as their resonances.
TL;DR: In this article, the authors investigated new models of dynamical electroweak symmetry breaking resulting from the condensation of fermions in higher representations of the technicolor group, which lie close to the conformal window, and are free from the flavor-changing neutral current problem despite small numbers of flavors and colors.
TL;DR: In this paper, the SU(3) little Higgs model has a region of parameter space in which electroweak symmetry breaking is natural and in which corrections to precision electroweak observables are sufficiently small.
Abstract: We show that the SU(3) little Higgs model has a region of parameter space in which electroweak symmetry breaking is natural and in which corrections to precision electroweak observables are sufficiently small. The model is anomaly free, generates a Higgs mass near 150 GeV, and predicts new gauge bosons and fermions at 1 TeV.
TL;DR: In this article, the anomalous magnetic moment of the muon a μ has been evaluated in the Standard Model without the approximation of a heavy Higgs-boson mass, finding corrections up to 0.2 × 10 − 10 for a light Higgs boson.
23 Sep 2004
TL;DR: In this article, it was shown that the flavor problem in theories with dynamical electroweak symmetry breaking can be effectively decoupled if the physics above the TeV scale is strongly conformal.
Abstract: We point out that the flavor problem in theories with dynamical electroweak symmetry breaking can be effectively decoupled if the physics above the TeV scale is strongly conformal, and the electroweak order parameter has a scaling dimension d = 1 + epsilon with epsilon \simeq 1/few There are many restrictions on small values of epsilon: for epsilon << 1, electroweak symmetry breaking requires a fine-tuning similar to that of the standard model; large-N conformal field theories (including those obtained from the AdS/CFT correspondence) require fine-tuning for d < 2; `walking technicolor' theories cannot have d < 2, according to gap equation analyses However, strong small-N conformal field theories with epsilon \simeq 1/few avoid all these constraints, and can give rise to natural dynamical electroweak symmetry breaking with a top quark flavor scale of order 10^{1/epsilon} TeV, large enough to decouple flavor Small-N theories also have an acceptably small Peskin-Takeuchi S parameter This class of theories provides a new direction for dynamical electroweak symmetry breaking without problems from flavor or electroweak precision tests A possible signal for these theories is a prominent scalar resonance below the TeV scale with couplings similar to a heavy standard model Higgs
TL;DR: In this article, the two-loop electroweak corrections due to the top quark to the gluon fusion production cross section of an intermediate-mass Higgs boson were determined.
TL;DR: In this article, it was shown that the electroweak symmetry can be broken by boundary conditions, leading to a new type of Higgsless theories in 5 dimensions, which could in principle improve on the 4D case by extending the perturbative domain to energies higher than 4πv.
TL;DR: In this paper, a new non-supersymmetric framework for electroweak symmetry breaking (with or without Higgs) involving SU(2)_L * SU( 2)_R * U(1)_{B-L} in higher dimensional warped geometry has been suggested.
Abstract: In the past year, a new non-supersymmetric framework for electroweak symmetry breaking (with or without Higgs) involving SU(2)_L * SU(2)_R * U(1)_{B-L} in higher dimensional warped geometry has been suggested. In this work, we embed this gauge structure into a GUT such as SO(10) or Pati-Salam. We showed recently (in hep-ph/0403143) that in a warped GUT, a stable Kaluza-Klein fermion can arise as a consequence of imposing proton stability. Here, we specify a complete realistic model where this particle is a weakly interacting right-handed neutrino, and present a detailed study of this new dark matter candidate, providing relic density and detection predictions. We discuss phenomenological aspects associated with the existence of other light (<~ TeV) KK fermions (related to the neutrino), whose lightness is a direct consequence of the top quark's heaviness. The AdS/CFT interpretation of this construction is also presented. Most of our qualitative results do not depend on the nature of the breaking of the electroweak symmetry provided that it happens near the TeV brane.
TL;DR: In this article, the nuclear inclusive electron scattering model was extended to the study of electroweak Charged Current (CC) induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments.
Abstract: The Quasi-Elastic (QE) contribution of the nuclear inclusive electron scattering model developed in Nucl. Phys. A627 (1997) 543 is extended to the study of electroweak Charged Current (CC) induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for, among other nuclear effects, long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections. Predictions for the inclusive muon capture in $^{12}$C and the reaction $^{12}$C $(
u_\mu,\mu^-)X$ near threshold are also given. RPA correlations are shown to play a crucial role and their inclusion leads to one of the best existing simultaneous description of both processes, with accuracies of the order of 10-15% per cent for the muon capture rate and even better for the LSND measurement.
TL;DR: In this paper, the mechanism of electroweak symmetry breaking in the Little Higgs Models is analyzed in an effective field theory approach, which enables us to identify observable effects irrespective of the specific structure and content of the heavy degrees of freedom.
Abstract: The mechanism of electroweak symmetry breaking in Little Higgs Models is analyzed in an effective field theory approach. This enables us to identify observable effects irrespective of the specific structure and content of the heavy degrees of freedom. We parameterize these effects in a common operator basis and present the complete set of anomalous contributions to gauge-boson, Higgs, and fermion couplings. If the hypercharge assignments of the model retain their standard form, electroweak precision data are affected only via the S and T parameters and by contact interactions. As a proof of principle, we apply this formalism to the minimal model and consider the current constraints on the parameter space. Finally, we show how the interplay of measurements at LHC and a Linear Collider could reveal the structure of these models.
TL;DR: In this article, the second in a series of two papers, the authors used a model Lagrangean, in which fermions couple to scalars through Yukawa interactions and approximate the self-energies by the one-loop expressions, and computed the CP-violating contributions to both the scalar and the fermionic collision term.
TL;DR: In this paper, a modified minimal moose model with global symmetries protecting the Higgs boson mass was proposed. But the model is not suitable for high-energy Higgs doublets.
Abstract: In this paper we present a little Higgs model that has custodial $\mathrm{SU}(2)$ as an approximate symmetry. This theory is a simple modification of the ``minimal moose'' model with $\mathrm{SO}(5)$ global symmetries protecting the Higgs boson mass. This allows for a simple limit where TeV physics makes small contributions to precision electroweak observables. The spectrum of particles and their couplings to standard model fields are studied in detail. At low energies this model has two Higgs doublets and it favors a light Higgs boson from precision electroweak bounds, though for different reasons than in the standard model. The limit on the breaking scale, f, is roughly 700 GeV, with a top partner of 2 TeV, ${W}^{\ensuremath{'}}$ and ${B}^{\ensuremath{'}}$ of 2.5 TeV, and heavy Higgs partners of 2 TeV. These particles are easily accessible at hadron colliders.
Posted Content•
TL;DR: In this paper, it was shown that a dramatic improvement of the fine-tuning is indeed a very common feature of these scenarios for wide ranges of tanβ and the Higgs mass.
Abstract: As is well known, electroweak breaking in the MSSM requires substantial fine-tuning, mainly due to the smallness of the tree-level Higgs quartic coupling, λtree. Hence the fine tuning is efficiently reduced in supersymmetric models with larger λtree, as happens naturally when the breaking of SUSY occurs at a low scale (not far from the TeV). We show, in general and with specific examples, that a dramatic improvement of the fine tuning (so that there is virtually no fine-tuning) is indeed a very common feature of these scenarios for wide ranges of tanβ and the Higgs mass (which can be as large as several hundred GeV if desired, but this is not necessary). The supersymmetric flavour problems are also drastically improved due to the absence of RG cross-talk between soft mass parameters.
TL;DR: In this paper, a flavor-changing Z boson was proposed to solve the problem of inconsistent B → π K branching ratios with the standard model expectations, and the solution is consistent with constraints from the CP asymmetries of the B → ε K S decay.
Journal Article•
TL;DR: In this paper, the authors propose a method to solve the problem of "10.10.2010.0" and "11.11.2010" in the following order:
Abstract: 10.