Showing papers on "Electroweak interaction published in 2008"

Universality of quantum gravity corrections

Abstract: We show that the existence of a minimum measurable length and the related generalized uncertainty principle (GUP), predicted by theories of quantum gravity, influence all quantum Hamiltonians. Thus, they predict quantum gravity corrections to various quantum phenomena. We compute such corrections to the Lamb shift, the Landau levels, and the tunneling current in a scanning tunneling microscope. We show that these corrections can be interpreted in two ways: (a) either that they are exceedingly small, beyond the reach of current experiments, or (b) that they predict upper bounds on the quantum gravity parameter in the GUP, compatible with experiments at the electroweak scale. Thus, more accurate measurements in the future should either be able to test these predictions, or further tighten the above bounds and predict an intermediate length scale between the electroweak and the Planck scale.

CERN LHC phenomenology of an extended standard model with a real scalar singlet

Abstract: Gauge singlet extensions of the standard model (SM) scalar sector may help remedy its theoretical and phenomenological shortcomings while solving outstanding problems in cosmology. Depending on the symmetries of the scalar potential, such extensions may provide a viable candidate for the observed relic density of cold dark matter or a strong first order electroweak phase transition needed for electroweak baryogenesis. Using the simplest extension of the SM scalar sector with one real singlet field, we analyze the generic implications of a singlet-extended scalar sector for Higgs boson phenomenology at the Large Hadron Collider (LHC). We consider two broad scenarios: one in which the neutral SM Higgs and singlet mix and the other in which no mixing occurs and the singlet can be a dark matter particle. For the first scenario, we analyze constraints from electroweak precision observables and their implications for LHC Higgs phenomenology. For models in which the singlet is stable, we determine the conditions under which it can yield the observed relic density, compute the cross sections for direct detection in recoil experiments, and discuss the corresponding signatures at the LHC.

Electroweak and QCD corrections to Higgs production via vector-boson fusion at the CERN LHC

Abstract: The radiative corrections of the strong and electroweak interactions are calculated at next-to-leading order for Higgs-boson production in the weak-boson-fusion channel at hadron colliders. Specifically, the calculation includes all weak-boson fusion and quark-antiquark annihilation diagrams to Higgs-boson production in association with two hard jets, including all corresponding interferences. The results on the QCD corrections confirm that previously made approximations of neglecting s-channel diagrams and interferences are well suited for predictions of Higgs production with dedicated vector-boson fusion cuts at the LHC. The electroweak corrections, which also include real corrections from incoming photons and leading heavy Higgs-boson effects at two-loop order, are of the same size as the QCD corrections, viz. typically at the level of 5%-10% for a Higgs-boson mass up to {approx}700 GeV. In general, both types of corrections do not simply rescale differential distributions, but induce distortions at the level of 10%. The discussed corrections have been implemented in a flexible Monte Carlo event generator.

Effects of new leptons in electroweak precision data

Abstract: We obtain limits on generic vectorlike leptons at the TeV scale from electroweak precision tests. These limits are complementary to the ones obtained from lepton flavor violating processes. In general, the quality of the global electroweak fit is comparable to the one for the standard model. In the case of an extra neutrino singlet mixing with the muon or electron, the global fit allows for a relatively large Higgs mass (${M}_{H}\ensuremath{\lesssim}260\text{ }\text{ }\mathrm{GeV}$ at 90% C.L.), thus relaxing the tension between the direct CERN LEP limit and the standard model fit.

The nuMSM, leptonic asymmetries, and properties of singlet fermions

Abstract: We study in detail the mechanism of baryon and lepton asymmetry generation in the framework of the νMSM (an extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale). We elucidate the issue of CP-violation in the model and define the phase relevant for baryogenesis. We clarify the question of quantum-mechanical coherence, essential for the lepton asymmetry generation in singlet fermion oscillations and compute the relevant damping rates. The range of masses and couplings of singlet leptons which can lead to successful baryogenesis is determined. The conditions which ensure survival of primordial (existing above the electroweak temperatures) asymmetries in different leptonic numbers are analysed. We address the question whether CP-violating reactions with lepton number non-conservation can produce leptonic asymmetry below the sphaleron freeze-out temperature. This asymmetry, if created, leads to resonant production of dark matter sterile neutrinos. We show that the requirement that a significant lepton asymmetry be produced puts stringent constraints on the properties of a pair of nearly degenerate singlet fermions, which can be tested in accelerator experiments. In this region of parameters the νMSM provides a common mechanism for production of baryonic matter and dark matter in the universe. We analyse different fine-tunings of the model and discuss possible symmetries of the νMSM Lagrangian that can lead to them.

Dynamics of non-renormalizable electroweak symmetry breaking

Abstract: We compute the complete one-loop finite temperature effective potential for electroweak symmetry breaking in the Standard Model with a Higgs potential supplemented by higher dimensional operators as generated for instance in composite Higgs and Little Higgs models. We detail the resolution of several issues that arise, such as the cancellation of infrared divergences at higher order and imaginary contributions to the potential. We follow the dynamics of the phase transition, including the nucleation of bubbles and the effects of supercooling. We characterize the region of parameter space consistent with a strong first-order phase transition which may be relevant to electroweak baryogenesis. Finally, we investigate the prospects of present and future gravity wave detectors to see the effects of a strong first-order electroweak phase transition.

A precision constraint on multi-Higgs-doublet models

Abstract: We derive a general expression for Δρ (or, equivalently, for the oblique parameter T) in the SU(2) × U(1) electroweak model with an arbitrary number of scalar SU(2) doublets, with hypercharge ±1/2, and an arbitrary number of scalar SU(2) singlets. The experimental bound on Δρ constitutes a strong constraint on the masses and mixings of the scalar particles in that model.

CERN LHC signals from warped extra dimensions

Abstract: We study production of Kaluza-Klein (KK) gluons at the Large Hadron Collider (LHC) in the framework of a warped extra dimension with the standard model fields propagating in the bulk. We show that the detection of the KK gluon is challenging since its production is suppressed by small couplings to the proton’s constituents. Moreover, the KK gluon decays mostly to top pairs due to an enhanced coupling and hence is broad. Nevertheless, we demonstrate that for MKKG?4??TeV, 100??fb-1 of data at the LHC can provide discovery of the KK gluon. We utilize a sizable left-right polarization asymmetry from the KK gluon resonance to maximize the signal significance, and we explore the novel feature of extremely highly energetic “top-jets.” We briefly discuss how the detection of electroweak gauge KK states (Z/W) faces a similar challenge since their leptonic decays (golden modes) are suppressed. Our analysis suggests that other frameworks, for example, little Higgs, which rely on UV completion via strong dynamics might face similar challenges, namely, (1) suppressed production rates for the new particles (such as Z?), due to their “light-fermion-phobic” nature, and (2) difficulties in detection since the new particles are broad and decay predominantly to third generation quarks and longitudinal gauge bosons

Discovering the top partners at the LHC using same-sign dilepton final states

Abstract: A natural, non-supersymmetric solution to the hierarchy problem generically requires fermionic partners of the top quark with masses not much heavier than 500 GeV. We study the pair production and detection at the LHC of the top partners with electric charge Q=5/3 (T_{5/3}) and Q=-1/3 (B), that are predicted in models where the Higgs is a pseudo-Goldstone boson. The exotic T_{5/3} fermion, in particular, is the distinct prediction of a LR custodial parity invariance of the electroweak symmetry breaking sector. Both kinds of new fermions decay to Wt, leading to a t\bar{t}WW final state. We focus on the golden channel with two same-sign leptons, and show that a discovery could come with less than 100 pb^{-1} (less than 20 fb^{-1}) of integrated luminosity for masses M=500 GeV (M=1TeV). In the case of the T_{5/3}, we present a simple strategy for its reconstruction in the fully hadronic decay chain. Although no full mass reconstruction is possible for the B, we still find that the same-sign dilepton channel offers the best chances of discovery compared to other previous searches that used final states with one or two opposite-sign leptons, and hence suffered from the large t\bar{t} background. Our analysis also directly applies to the search of 4th generation b' quarks.

Some cosmological implications of hidden sectors

Abstract: We discuss some cosmological implications of extensions of the standard model with hidden-sector scalars coupled to the Higgs boson. We put special emphasis on the conformal case, in which the electroweak symmetry is broken radiatively with a Higgs mass above the experimental limit. Our refined analysis of the electroweak phase transition in this kind of models strengthens the prediction of a strongly first-order phase transition as required by electroweak baryogenesis. We further study gravitational wave production and the possibility of low-scale inflation as well as a viable dark matter candidate.

Revisiting the Global Electroweak Fit of the Standard Model and Beyond with Gfitter

Abstract: The global fit of the Standard Model to electroweak precision data, routinely performed by the LEP electroweak working group and others, demonstrated impressively the predictive power of electroweak unification and quantum loop corrections. We have revisited this fit in view of (i) the development of the new generic fitting package Gfitter, (ii) the insertion of constraints from direct Higgs searches at LEP and the Tevatron, and (iii) a more thorough statistical interpretation of the results. Gfitter is a modular fitting toolkit, which features predictive theoretical models as independent plugins, and a statistical analysis of the fit results using toy Monte Carlo techniques. The state-of-the-art electroweak Standard Model is fully implemented, as well as generic extensions to it. This paper introduces the Gfitter project, and presents state-of-the-art results for the global electroweak fit in the Standard Model, and for a model with an extended Higgs sector (2HDM). Numerical and graphical results for fits with and without including the constraints from the direct Higgs searches at LEP and Tevatron are given. Perspectives for future colliders are analysed and discussed. Including the direct Higgs searches, we find M_H=(116.4 +18.3 -1.3) GeV, and the 2sigma and 3sigma allowed regions [114,145] GeV and [[113,168] and [180,225]] GeV, respectively. For the strong coupling strength at fourth perturbative order we obtain alpha_S(M_Z)=0.1193 +0.0028 -0.0027(exp) +- 0.0001(theo). Finally, for the mass of the top quark, excluding the direct measurements, we find m_t=(178.2 +9.8 -4.2) GeV. In the 2HDM we exclude a charged-Higgs mass below 240 GeV at 95% confidence level. This limit increases towards larger tan(beta), where e.g., M_H+-<780 GeV is excluded for tan(beta)=70.

Top Quark Compositeness: Feasibility and Implications

Abstract: In models of electroweak symmetry breaking in which the SM fermions get their masses by mixing with composite states, it is natural to expect the top quark to show properties of compositeness. We study the phenomenological viability of having a mostly composite top. The strongest constraints are shown to mainly come from one-loop contributions to the T parameter. Nevertheless, the presence of light custodial partners weakens these bounds, allowing in certain cases for a high degree of top compositeness. We nd regions in the parameter space in which the T -parameter receives moderate positive contributions, favoring the electroweak t of this type of models. We also study the implications of having a composite top at the LHC, focusing on the process pp ! t tt t(b b) whose cross-section is enhanced at high-energies.

Flavor physics in the Randall-Sundrum model I. Theoretical setup and electroweak precision tests

Abstract: A complete discussion of tree-level flavor-changing effects in the Randall-Sundrum (RS) model with brane-localized Higgs sector and bulk gauge and matter fields is presented. The bulk equations of motion for the gauge and fermion fields, supplemented by boundary conditions taking into account the couplings to the Higgs sector, are solved exactly. For gauge fields the Kaluza-Klein (KK) decomposition is performed in a covariant Rξ gauge. For fermions the mixing between different generations is included in a completely general way. The hierarchies observed in the fermion spectrum and the quark mixing matrix are explained naturally in terms of anarchic five-dimensional Yukawa matrices and wave-function overlap integrals. Detailed studies of the flavor-changing couplings of the Higgs boson and of gauge bosons and their KK excitations are performed, including in particular the couplings of the standard W± and Z0 bosons. A careful analysis of electroweak precision observables including the S and T parameters and the Z0b couplings shows that the simplest RS model containing only Standard Model particles and their KK excitations is consistent with all experimental bounds for a KK scale as low as a few TeV, if one allows for a heavy Higgs boson (mh 1 TeV) and/or for an ultra-violet cutoff below the Planck scale. The study of flavor-changing effects includes analyses of the non-unitarity of the quark mixing matrix, anomalous right-handed couplings of the W± bosons, tree-level flavor-changing neutral current couplings of the Z0 and Higgs bosons, the rare decays t → c(u)Z0 and t → c(u)h, and the flavor mixing among KK fermions. The results obtained in this work form the basis for general calculations of flavor-changing processes in the RS model and its extensions.

Naturally Speaking: The Naturalness Criterion and Physics at the LHC

Abstract: A non-technical discussion of the naturalness criterion and its implications for new physics searches at the LHC To be published in the book "LHC Perspectives", edited by G Kane and A Pierce

A Model of Lepton Masses from a Warped Extra Dimension

Abstract: In order to explain the non-hierarchical neutrino mixing angles and the absence of lepton flavor violating processes in the context of warped extra dimensions one needs to introduce bulk flavor symmetries. We present a simple model of lepton masses and mixings in RS models based on the A4 non-abelian discrete symmetry. The virtues of this choice are: (i) the natural appearance of the tri-bimaximal mixing pattern; (ii) the complete absence of tree-level flavor violations in the neutral sector; (iii) the absence of flavor gauge bosons; (iv) the hierarchies in the charged lepton masses are explained via wave-function overlaps. We present the minimal field content and symmetry breaking pattern necessary to obtain a successful model of this type. The bounds from electroweak precision measurements allow the KK mass scale to be as low as 3 TeV. Tree-level lepton flavor violation is absent in this model, while the loop induced mu -> e gamma branching fraction is safely below the experimental bound.

Discovering the top partners at the LHC using same-sign dilepton final states

Abstract: A natural, non-supersymmetric solution to the hierarchy problem generically requires fermionic partners of the top quark with masses not much heavier than 500GeV. We study the pair production and detection at the LHC of the top partners with electric charge Qe = 5/3 (T5/3) and Qe = −1/3 (B), that are predicted in models where the Higgs is a pseudo-Goldstone boson. The exotic T 5/3 fermion, in particular, is the distinct prediction of a LR custodial parity invariance of the electroweak symmetry breaking sector. Both kinds of new fermions decay to W t, leading to a tW final state. We focus on the golden channel with two same-sign leptons, and show that a discovery could come with less than 100pb −1 (less than 20fb −1 ) of integrated luminosity for masses M = 500GeV (M = 1TeV). In the case of the T5/3, we present a simple strategy for its reconstruction in the fully hadronic decay chain. Although no full mass reconstruction is possible for the B, we still find that the same-sign dilepton channel offers the best chances of discovery compared to other previous searches that used final states with one or two opposite-sign leptons, and hence suffered from the large tbackground.

Higgs boson mass, sparticle spectrum, and the little hierarchy problem in an extended MSSM

Abstract: We investigate the impact of TeV-scale matter belonging to complete vectorlike multiplets of unified groups on the lightest Higgs boson in the MSSM. We find that consistent with perturbative unification and electroweak precision data the mass m{sub h} can be as large as 160 GeV. These extended MSSM models can also render the little hierarchy problem less severe, but only for lower values of m{sub h} < or approx. 125) GeV. We present estimates for the sparticle mass spectrum in these models.

NLO QCD corrections to tri-boson production

Abstract: We present a calculation of the NLO QCD corrections for the production of three vector bosons at the LHC, namely ZZZ, W+W−Z, W+ZZ, and W+W−W+ production. The virtual corrections are computed using the recently proposed method of reduction at the integrand level (OPP reduction). Concerning the contributions coming from real emission we used the dipole subtraction to treat the soft and collinear divergences. We find that the QCD corrections for these electroweak processes are in the range between 70 and 100 percent. As such they have to be considered in experimental studies of triple vector boson production at the LHC.

Seeking Sgluons

Abstract: Scalar gluons -- or sgluons -- are color octet scalars without electroweak charges They occur in supersymmetric models of Dirac gauginos as the scalar partners of the gluino and carry Standard-Model type R charge This allows them to interact with ordinary matter and to be produced at the LHC, singly as well as in pairs Sgluons dominantly decay into gluons, top pairs, and a top quark plus a light quark A pair of sgluons decaying into like-sign tops would provide a striking signature at the LHC In our discussion of this channel we especially focus on the proper treatment of QCD jets

The muon g-2 and the bounds on the Higgs boson mass

Abstract: After a brief review of the muon g-2 status, we analyze the possibility that the present discrepancy between experiment and the standard model (SM) prediction may be due to hypothetical errors in the determination of the hadronic leading-order contribution to the latter. In particular, we show how an increase of the hadroproduction cross section in low-energy e{sup +}e{sup -} collisions could bridge the muon g-2 discrepancy, leading however to a decrease on the electroweak upper bound on M{sub H}, the SM Higgs boson mass. That bound is currently M{sub H} 114.4 GeV (95% C.L.). By means of a detailed analysis we conclude that this solution of the muon g-2 discrepancy is unlikely in view of current experimental error estimates. However, if this turns out to be the solution, the 95% C.L. upper bound on M{sub H} is reduced to about 130 GeV which, in conjunction with the experimental lower bound, leaves a narrow window for the mass of this fundamental particle.

A Model of Lepton Masses from a Warped Extra Dimension

Abstract: In order to explain the non-hierarchical neutrino mixing angles and the absence of lepton flavor violating processes in the context of warped extra dimensions one needs to introduce bulk flavor symmetries. We present a simple model of lepton masses and mixings in RS models based on the A4 non-abelian discrete symmetry. The virtues of this choice are: (i) the natural appearance of the tri-bimaximal mixing pattern; (ii) the complete absence of tree-level flavor violations in the neutral sector; (iii) the absence of flavor gauge bosons; (iv) the hierarchies in the charged lepton masses are explained via wave-function overlaps. We present the minimal field content and symmetry breaking pattern necessary to obtain a successful model of this type. The bounds from electroweak precision measurements allow the KK mass scale to be as low as ∼ 3TeV. Tree-level lepton flavor violation is absent in this model, while the loop induced � → eγ branching fraction is safely below the experimental bound.

Higgs self-coupling as a probe of the electroweak phase transition

Abstract: We argue that, within a broad class of extensions of the standard model, there is a tight correlation between the dynamics of the electroweak phase transition and the cubic self-coupling of the Higgs boson: Models which exhibit a strong first-order electroweak phase transition predict a large deviation of the Higgs self-coupling from the standard model prediction, as long as no accidental cancellations occur. Order-one deviations are typical. This shift would be observable at the Large Hadron Collider if the proposed luminosity or energy upgrades are realized, as well as at a future electron-positron collider such as the proposed International Linear Collider. These measurements would provide a laboratory test of the dynamics of the electroweak phase transition.