Showing papers on "Gauge boson published in 2006"

Causality, analyticity and an IR obstruction to UV completion

Abstract: We argue that certain apparently consistent low-energy effective field theories described by local, Lorentzinvariant Lagrangians, secretly exhibit macroscopic non-locality and cannot be embedded in any UV theory whose S-matrix satisfies canonical analyticity constraints. The obstruction involves the signs of a set of leading irrelevant operators, which must be strictly positive to ensure UV analyticity. An IR manifestation of this restriction is that the “wrong” signs lead to superluminal fluctuations around non-trivial backgrounds, making it impossible to define local, causal evolution, and implying a surprising IR breakdown of the effective theory. Such effective theories can not arise in quantum field theories or weakly coupled string theories, whose S-matrices satisfy the usual analyticity properties. This conclusion applies to the DGP brane-world model modifying gravity in the IR, giving a simple explanation for the difficulty of embedding this model into controlled stringy backgrounds, and to models of electroweak symmetry breaking that predict negative anomalous quartic couplings for the W and Z. Conversely, any experimental support for the DGP model, or measured negative signs for anomalous quartic gauge boson couplings at future accelerators, would constitute direct evidence for the existence of superluminality and macroscopic non-locality unlike anything previously seen in physics, and almost incidentally falsify both local quantum field theory and perturbative string theory.

Electroweak gauge boson production at hadron colliders through O(α s 2 )

Abstract: We describe a calculation of the O({alpha}{sub s}{sup 2}) QCD corrections to the fully differential cross section for W and Z boson production in hadronic collisions. The result is fully realistic in that it includes spin correlations, finite width effects, {gamma}-Z interference and allows for the application of arbitrary cuts on the leptonic decay products of the W and Z. We have implemented this calculation into a numerical program. We demonstrate the use of this code by presenting phenomenological results for several future LHC analyses and recent Tevatron measurements, including the W cross section in the forward rapidity region and the central over forward cross section ratio.

Precise predictions for the Higgs-boson decay H ---> WW/ZZ ---> 4 leptons

Abstract: The decay of the standard model Higgs boson into four leptons via a virtual $W$- or $Z$-boson pair is one of the most important decay modes in the Higgs-boson search at the Large Hadron Collider. We present the complete electroweak radiative corrections of $\mathcal{O}(\ensuremath{\alpha})$ to these processes, including improvements beyond $\mathcal{O}(\ensuremath{\alpha})$ originating from heavy-Higgs effects and final-state radiation. The intermediate $W$- and $Z$-boson resonances are described (without any expansion or on-shell approximation) by consistently employing complex mass parameters for the gauge bosons (complex-mass scheme). The corrections to partial decay widths typically amount to some percent and increase with growing Higgs mass ${M}_{H}$, reaching about 8% at ${M}_{H}\ensuremath{\sim}500\text{ }\text{ }\mathrm{GeV}$. For not too large Higgs masses (${M}_{H}\ensuremath{\lesssim}400\text{ }\text{ }\mathrm{GeV}$) the corrections to the partial decay widths can be reproduced within $\ensuremath{\lesssim}2%$ by simple approximations. For angular distributions the corrections are somewhat larger and distort the shapes. For invariant-mass distributions of fermion pairs they can reach several tens of percent depending on the treatment of photon radiation. The discussed corrections have been implemented in a Monte Carlo event generator called Prophecy4f.

Electroweak precision constraints on the littlest Higgs model with T parity

Abstract: We compute the leading corrections to the properties of W and Z bosons induced at the one-loop level in the SU(5)/SO(5) Littlest Higgs model with T parity, and perform a global fit to precision electroweak data to determine the constraints on the model parameters. We find that a large part of the model parameter space is consistent with data. Values of the symmetry breaking scale f as low as 500 GeV are allowed, indicating that no significant fine tuning in the Higgs potential is required. We identify a region within the allowed parameter space in which the lightest T-odd particle, the partner of the hypercharge gauge boson, has the correct relic abundance to play the role of dark matter. In addition, we find that a consistent fit to data can be obtained for large values of the Higgs mass, up to 800 GeV, due to the possibility of a partial cancellation between the contributions to the T parameter from Higgs loops and new physics.

Flavoured Large N Gauge Theory in an External Magnetic Field

Abstract: We consider a D7-brane probe of AdS$_{5}\times S^5$ in the presence of pure gauge $B$-field. In the dual gauge theory, the $B$-field couples to the fundamental matter introduced by the D7-brane and acts as an external magnetic field. The $B$-field supports a 6-form Ramond-Ramond potential on the D7-branes world volume that breaks the supersymmetry and enables the dual gauge theory to develop a non-zero fermionic condensate. We explore the dependence of the fermionic condensate on the bare quark mass $m_{q}$ and show that at zero bare quark mass a chiral symmetry is spontaneously broken. A study of the meson spectrum reveals a coupling between the vector and scalar modes, and in the limit of weak magnetic field we observe Zeeman splitting of the states. We also observe the characteristic $\sqrt{m_{q}}$ dependence of the ground state corresponding to the Goldstone boson of spontaneously broken chiral symmetry.

Spin-dependent macroscopic forces from new particle exchange

Abstract: Long-range forces between macroscopic objects are mediated by light particles that interact with the electrons or nucleons, and include spin-dependent static components as well as spin- and velocity-dependent components. We parametrize the long-range potential between two fermions assuming rotational invariance, and find 16 different components. Applying this result to electrically neutral objects, we show that the macroscopic potential depends on 72 measurable parameters. We then derive the potential induced by the exchange of a new gauge boson or spinless particle, and compare the limits set by measurements of macroscopic forces to the astrophysical limits on the couplings of these particles.

Z' Phenomenology and the LHC

Abstract: A brief pedagogical overview of the phenomenology of Z' gauge bosons is presented. Such particles can arise in various electroweak extensions of the Standard Model (SM). We provide a quick survey of a number of Z' models, review the current constraints on the possible properties of a Z' and explore in detail how the LHC may discover and help elucidate the nature of these new particles. We provide an overview of the Z' studies that have been performed by both ATLAS and CMS. The role of the ILC in determining Z' properties is also discussed.

Probing the Randall-Sundrum geometric origin of flavor with lepton flavor violation

Abstract: The anarchic Randall-Sundrum model of flavor is a low energy solution to both the electroweak hierarchy and flavor problems Such models have a warped, compact extra dimension with the standard model fermions and gauge bosons living in the bulk, and the Higgs living on or near the TeV brane In this paper we consider bounds on these models set by lepton flavor-violation constraints We find that loop-induced decays of the form l{yields}l{sup '}{gamma} are ultraviolet sensitive and incalculable when the Higgs field is localized on a four-dimensional brane; this drawback does not occur when the Higgs field propagates in the full five-dimensional space-time We find constraints at the few TeV level throughout the natural range of parameters, arising from {mu}-e conversion in the presence of nuclei, rare {mu} decays, and rare {tau} decays A tension exists between loop-induced dipole decays such as {mu}{yields}e{gamma} and tree-level processes such as {mu}-e conversion; they have opposite dependences on the five-dimensional Yukawa couplings, making it difficult to decouple flavor-violating effects We emphasize the importance of the future experiments MEG and PRIME These experiments will definitively test the Randall-Sundrum geometric origin of hierarchies in the lepton sector at the TeV scale

Low scale B-L extension of the Standard Model at the LHC

Abstract: The fact that neutrinos are massive indicates that the Standard Model (SM) requires extension. We propose a low energy (

The flavor of a little Higgs with T-parity

Abstract: We analyze flavor constraints in the littlest Higgs model with T-parity. In particular, we focus on neutral meson mixing in the K, B, and D systems due to one loop contributions from T-parity odd fermions and gauge bosons. We calculate the short distance contributions to mixing for a general choice of T-odd fermion Yukawa couplings. We find that for a generic choice of textures, a TeV scale GIM suppression is necessary to avoid large contributions. If order one mixing angles are allowed in the extended flavor structure, the mass spectrum is severely constrained, and must be degenerate at the 1-5% level. However, there are still regions of parameter space where only a loose degeneracy is necessary to avoid constraints. We also consider the B(s) system, and identify a scenario in which the mixing can be significantly enhanced beyond the standard model prediction while still satisfying bounds on the other mixing observables. We present both analytical and numerical results as functions of the T-odd fermion mass eigenvalues.

Dynamical SUSY Breaking at Meta-Stable Minima from D-branes at Obstructed Geometries

Abstract: We study the existence of long-lived meta-stable supersymmetry breaking vacua in gauge theories with massless quarks, upon the addition of extra massive flavors. A simple realization is provided by a modified version of SQCD with N_{f,0} < N_c massless flavors, N_{f,1} massive flavors and additional singlet chiral fields. This theory has local meta-stable minima separated from a runaway behavior at infinity by a potential barrier. We find further examples of such meta-stable minima in flavored versions of quiver gauge theories on fractional branes at singularities with obstructed complex deformations, and study the case of the dP_1 theory in detail. Finally, we provide an explicit String Theory construction of such theories. The additional flavors arise from D7-branes on non-compact 4-cycles of the singularity, for which we find a new efficient description using dimer techniques.

Resonances from two universal extra dimensions

Abstract: Standard model gauge bosons propagating in two universal extra dimensions give rise to heavy spin-1 and spin-0 particles. The lightest of these, carrying Kaluza-Klein numbers (1,0), may be produced only in pairs at colliders, whereas the (1,1) modes, which are heavier by a factor of {radical}(2), may be singly produced. We show that the cascade decays of (1,1) particles generate a series of closely-spaced narrow resonances in the tt invariant mass distribution. At the Tevatron, s-channel production of (1,1) gluons and electroweak bosons will be sensitive to tt resonances up to masses in the 0.5-0.7 TeV range. Searches at the LHC for resonances originating from several higher-level modes will further test the existence of two universal extra dimensions.

Recursion relations for gauge theory amplitudes with massive vector bosons and fermions

Abstract: We apply the on-shell tree-level recursion relations of Britto, Cachazo, Feng and Witten to a variety of processes involving internal and external massive particles with spin. We show how to construct multi-vector boson currents where one or more off-shell vector bosons couples to a quark pair and number of gluons. We give compact results for single vector boson currents with up to six partons and double vector boson currents with up to four partons for all helicity combinations. We also provide expressions for single vector boson currents with a quark pair and an arbitrary number of gluons for some specific helicity configurations. Finally, we show how to generalise the recursion relations to handle massive particles with spin on internal lines using gg→t as an example.

Dynamical generation of fuzzy extra dimensions, dimensional reduction and symmetry breaking

Abstract: We present a renormalizable 4-dimensional SU(N) gauge theory with a suitable multiplet of scalar fields, which dynamically develops extra dimensions in the form of a fuzzy sphere S 2 N . We explicitly find the tower of massive KaluzaKlein modes consistent with an interpretation as gauge theory on M 4 × S 2 , the scalars being interpreted as gauge fields on S 2 . The gauge group is broken dynamically, and the low-energy content of the model is determined. Depending on the parameters of the model the low-energy gauge group can be SU(n), or broken further to SU(n1) × SU(n2) × U(1), with mass scale determined by the size of the extra dimension.

Localized U(1) Gauge Fields, Millicharged Particles, and Holography

Abstract: We consider U(1) gauge fields in a slice of ${\mathrm{AdS}}_{5}$ with bulk and boundary mass parameters. The zero mode of a bulk U(1) gauge field can be localized either on the UV or IR brane. This leads to a simple model of millicharged particles in which fermions can have arbitrarily small electric charge. In the electroweak sector we also discuss phenomenological implications of a localized $\mathrm{U}(1{)}_{Y}$ gauge boson. Using the AdS/CFT correspondence we present the 4D holographic interpretation of the 5D model. In particular the photon is shown to be a composite particle when localized near the IR brane, whereas it is elementary when localized near the UV brane. In the dual interpretation the millicharge results from an elementary fermion coupling to a composite photon via a vector current with large anomalous dimension.

Six-dimensional gauge theory on the chiral square

Abstract: We construct gauge theories in two extra dimensions compactified on the chiral square, which is a simple compactification that leads to chiral fermions in four dimensions. Stationarity of the action on the boundary specifies the boundary conditions for gauge fields. Any six-dimensional gauge field decomposed in Kaluza-Klein modes includes a tower of heavy spin-1 particles whose longitudinal polarizations are linear combinations of the extra-dimensional components, and a tower of heavy spin-0 particles corresponding to the orthogonal combinations. These linear combinations depend on the Kaluza-Klein numbers, and are independent of the gauge fixing. If the gauge symmetry is broken by the vacuum expectation value of a six-dimensional scalar, at each Kaluza-Klein level three spinless fields in the adjoint representation mix to provide the longitudinal polarization of the spin-1 mode, leaving the orthogonal states as two spin-0 particles. We derive the interactions of the Kaluza-Klein modes for generic gauge theories, laying the groundwork for the Standard Model in two universal extra dimensions, and more generally for future model building and phenomenological studies.

D-terms, unification, and the Higgs mass

Abstract: We study gauge extensions of the MSSM that contain non-decoupling D-terms, which contribute to the Higgs boson mass. These models naturally maintain gauge coupling unification and raise the Higgs mass without fine-tuning. Unification constrains the structure of the gauge extensions, limiting the Higgs mass in these models to mh 150 GeV. The D-terms contribute to the Higgs mass only if the extended gauge symmetry is broken at energies of a few TeV, leading to new heavy gauge bosons in this mass range.

Smoking-gun signatures of little Higgs models

Abstract: Little Higgs models predict new gauge bosons, fermions and scalars at the TeV scale that stabilize the Higgs mass against quadratically divergent one-loop radiative corrections. We categorize the many little Higgs models into two classes based on the structure of the extended electroweak gauge group and examine the experimental signatures that identify the little Higgs mechanism in addition to those that identify the particular little Higgs model. We find that by examining the properties of the new heavy fermion(s) at the LHC, one can distinguish the structure of the top quark mass generation mechanism and test the little Higgs mechanism in the top sector. Similarly, by studying the couplings of the new gauge bosons to the light Higgs boson and to the Standard Model fermions, one can confirm the little Higgs mechanism and determine the structure of the extended electroweak gauge group.

A non-perturbative study of 4d U(1) non-commutative gauge theory — the fate of one-loop instability

Abstract: Recent perturbative studies show that in 4d non-commutative spaces, the trivial (classically stable) vacuum of gauge theories becomes unstable at the quantum level, unless one introduces sufficiently many fermionic degrees of freedom. This is due to a negative IR-singular term in the one-loop effective potential, which appears as a result of the UV/IR mixing. We study such a system non-perturbatively in the case of pure U(1) gauge theory in four dimensions, where two directions are non-commutative. Monte Carlo simulations are performed after mapping the regularized theory onto a U(N) lattice gauge theory in d = 2. At intermediate coupling strength, we find a phase in which open Wilson lines acquire non-zero vacuum expectation values, which implies the spontaneous breakdown of translational invariance. In this phase, various physical quantities obey clear scaling behaviors in the continuum limit with a fixed non-commutativity parameter θ, which provides evidence for a possible continuum theory. The extent of the dynamically generated space in the non-commutative directions becomes finite in the above limit, and its dependence on θ is evaluated explicitly. We also study the dispersion relation. In the weak coupling symmetric phase, it involves a negative IR-singular term, which is responsible for the observed phase transition. In the broken phase, it reveals the existence of the Nambu-Goldstone mode associated with the spontaneous symmetry breaking.

Disguising the oblique parameters

Abstract: We point out a set of operator identities that relate the operators corresponding to the oblique corrections to operators that modify fermion couplings to the gauge bosons as well as operators that modify triple gauge boson couplings Such identities are simple consequences of the equations of motion Therefore the contributions from new physics to the oblique parameters can be disguised as modifications of triple gauge boson couplings provided the fermion couplings to the gauge bosons are suitably modified by higher-dimensional operators Since the experimental constraints on triple gauge boson couplings are much weaker than the constraints on the oblique parameters this observation allows extra room for model building We derive operator relations in effective theories of the standard model with the electroweak symmetry either linearly or nonlinearly realized and discuss applications of our results

Weak boson fusion production of supersymmetric particles at the CERN LHC

Abstract: We present a complete calculation of weak boson fusion production of colorless supersymmetric particles at the LHC, using the new matrix element generator susy-madgraph . The cross sections are small, generally at the attobarn level, with a few notable exceptions which might provide additional supersymmetric parameter measurements. We discuss in detail how to consistently define supersymmetric weak couplings to preserve unitarity of weak gauge boson scattering amplitudes to fermions, and derive sum rules for weak supersymmetric couplings.

Gauge-Higgs unification and quark-lepton phenomenology in the warped spacetime

Abstract: In the dynamical gauge-Higgs unification of electroweak interactions in the Randall-Sundrum warped spacetime, the Higgs boson mass is predicted in the range 120-290 GeV, provided that the spacetime structure is determined at the Planck scale. Couplings of quarks and leptons to gauge bosons and their Kaluza-Klein excited states are determined by the masses of quarks and leptons. All quarks and leptons other than top quarks have very small couplings to the Kaluza-Klein excited states of gauge bosons. The universality of weak interactions is slightly broken by magnitudes of 10{sup -8}, 10{sup -6}, and 10{sup -2} for {mu}-e, {tau}-e and t-e, respectively. Yukawa couplings become substantially smaller than those in the standard model, by a factor cos(1/2){theta}{sub W} where {theta}{sub W} is the non-Abelian Aharonov-Bohm phase (the Wilson line phase) associated with dynamical electroweak symmetry breaking.

Post-Sphaleron Baryogenesis

Abstract: We present a new mechanism for generating the baryon asymmetry of the universe directly in the decay of a singlet scalar field $S_r$ with a weak scale mass and a high dimensional baryon number violating coupling. Unlike most currently popular models, this mechanism, which becomes effective after the electroweak phase transition, does not rely on the sphalerons for inducing a nonzero baryon number. CP asymmetry in $S_r$ decay arises through loop diagrams involving the exchange of $W^\pm$ gauge bosons, and is suppressed by light quark masses, leading naturally to a value of $\eta_B \sim 10^{-10}$. We show that the simplest realization of this mechanism, which uses a six quark $\Delta B=2$ operator, predicts colored scalars accessible to the LHC, and neutron--antineutron oscillation within reach of the next generation experiments.

Neutrino Majorana Masses from String Theory Instanton Effects

Abstract: Finding a plausible origin for right-handed neutrino Majorana masses in semirealistic compactifications of string theory remains one of the most difficult problems in string phenomenology. We argue that right-handed neutrino Majorana masses are induced by non-perturbative instanton effects in certain classes of string compactifications in which the $U(1)_{B-L}$ gauge boson has a St\"uckelberg mass. The induced operators are of the form $e^{-U} u_R u_R$ where $U$ is a closed string modulus whose imaginary part transforms appropriately under $B-L$. This mass term may be quite large since this is not a gauge instanton and $Re U$ is not directly related to SM gauge couplings. Thus the size of the induced right-handed neutrino masses could be a few orders of magnitude below the string scale, as phenomenologically required. It is also argued that this origin for neutrino masses would predict the existence of R-parity in SUSY versions of the SM. Finally we comment on other phenomenological applications of similar instanton effects, like the generation of a $\mu$-term, or of Yukawa couplings forbidden in perturbation theory.