arXiv subjects

Bogdan A. Dobrescu

Publications and source records attributed to Bogdan A. Dobrescu.

At least 73 records · Page 4Linked to original sources

arXiv2004

Massless Gauge Bosons other than the Photon

Gauge bosons associated with unbroken gauge symmetries, under which all standard model fields are singlets, may interact with ordinary matter via higher-dimensional operators. A complete set of dimension-six operators involving a massless U(1) field, gamma-prime, and standard model fields is presented. The mu to e gamma-prime decay, primordial nucleosynthesis, star cooling and other phenomena set lower limits on the scale of chirality-flip operators in the 1 - 15 TeV range, if the operators have coefficients given by the corresponding Yukawa couplings. Simple renormalizable models induce gamma-prime interactions with leptons or quarks at two loops, and may provide a cold dark matter candidate.

hep-ph↗

arXiv2004

Chiral Compactification on a Square

We study quantum field theory in six dimensions with two of them compactified on a square. A simple boundary condition is the identification of two pairs of adjacent sides of the square such that the values of a field at two identified points differ by an arbitrary phase. This allows a chiral fermion content for the four-dimensional theory obtained after integrating over the square. We find that nontrivial solutions for the field equations exist only when the phase is a multiple of π/2, so that this compactification turns out to be equivalent to a T^2/Z_4 orbifold associated with toroidal boundary conditions that are either periodic or anti-periodic. The equality of the Lagrangian densities at the identified points in conjunction with six-dimensional Lorentz invariance leads to an exact Z_8\times Z_2 symmetry, where the Z_2 parity ensures the stability of the lightest Kaluza-Klein particle.

hep-th↗

arXiv2004

Z-prime Gauge Bosons at the Tevatron

We study the discovery potential of the Tevatron for a Z-prime gauge boson. We introduce a parametrization of the Z-prime signal which provides a convenient bridge between collider searches and specific Z-prime models. The cross section for p pbar -> Z-prime X -> l^+ l^- X depends primarily on the Z-prime mass and the Z-prime decay branching fraction into leptons times the average square coupling to up and down quarks. If the quark and lepton masses are generated as in the standard model, then the Z-prime bosons accessible at the Tevatron must couple to fermions proportionally to a linear combination of baryon and lepton numbers in order to avoid the limits on Z--Z-prime mixing. More generally, we present several families of U(1) extensions of the standard model that include as special cases many of the Z-prime models discussed in the literature. Typically, the CDF and D0 experiments are expected to probe Z-prime-fermion couplings down to 0.1 for Z-prime masses in the 500--800 GeV range, which in various models would substantially improve the limits set by the LEP experiments.

hep-ph↗

arXiv2003

Analytic Estimates of the QCD Corrections to Neutrino-Nucleus Scattering

We study the QCD corrections to neutrino deep-inelastic scattering on a nucleus, and analytically estimate their size. For an isoscalar target, we show that the dominant QCD corrections to the ratio of the neutral- to charged-current events are suppressed by sin^4 theta_W, where theta_W is the weak mixing angle. We then discuss the implications for the NuTeV determination of sin^2 theta_W.

hep-ph↗

arXiv2003

Nonexotic Neutral Gauge Bosons

We study theoretical and experimental constraints on electroweak theories including a new color-singlet and electrically-neutral gauge boson. We first note that the electric charges of the observed fermions imply that any such Z' boson may be described by a gauge theory in which the Abelian gauge groups are the usual hypercharge along with another U(1) component in a kinetic-diagonal basis. Assuming that the observed quarks and leptons have generation-independent U(1) charges, and that no new fermions couple to the standard model gauge bosons, we find that their U(1) charges form a two-parameter family consistent with anomaly cancellation and viable fermion masses, provided there are at least three right-handed neutrinos. We then derive bounds on the Z' mass and couplings imposed by direct production and Z-pole measurements. For generic charge assignments and a gauge coupling of electromagnetic strength, the strongest lower bound on the Z' mass comes from Z-pole measurements, and is of order 1 TeV. If the new U(1) charges are proportional to B-L, however, there is no tree-level mixing between the Z and Z', and the best bounds come from the absence of direct production at LEPII and the Tevatron. If the U(1) gauge coupling is one or two orders of magnitude below the electromagnetic one, these bounds are satisfied for most values of the Z' mass.

hep-ph↗

arXiv2002

Neutrinos Vis-a-vis the Six-Dimensional Standard Model

We examine the origin of neutrino masses and oscillations in the context of the six-dimensional standard model. The space-time symmetries of this model explain proton stability and forbid Majorana neutrino masses. The consistency of the six-dimensional theory requires three right-handed neutrinos, and therefore Dirac neutrino masses are allowed. We employ the idea that the smallness of these masses is due to the propagation of the right-handed neutrinos in a seventh, warped dimension. We argue that this class of theories is free of gravitational anomalies. Although an exponential hierarchy arises between the neutrino masses and the electroweak scale, we find that the mass hierarchy among the three neutrino masses is limited by higher-dimension operators. All current neutrino oscillation data, except for the LSND result, are naturally accommodated by our model. In the case of the solar neutrinos, the model leads to the large mixing angle, MSW solution. The mechanism employed, involving three right-handed neutrinos coupled to a scalar in an extra dimension, may explain the features of the neutrino spectrum in a more general class of theories that forbid Majorana masses.

hep-ph↗

arXiv2001

Proton Stability in Six Dimensions

We show that Lorentz and gauge invariance explain the long proton lifetime within the standard model in six dimensions. The baryon-number violating operators have mass dimension 15 or higher. Upon TeV-scale compactification of the two universal extra dimensions on a square $T^2/Z_2$ orbifold, a discrete subgroup of the 6-dimensional Lorentz group continues to forbid dangerous operators.

hep-ph↗

arXiv2001

Linear Collider Physics

We report on a study of the physics potential of linear $e^+e^-$ colliders. Although a linear collider (LC) would support a broad physics program, we focus on the contributions that could help elucidate the origin of electroweak symmetry breaking. Many extensions of the standard model have a decoupling limit, with a Higgs boson similar to the standard one and other, higher-mass states. Mindful of such possibilities, we survey the physics of a (nearly) standard Higgs boson, as a function of its mass. We also review how measurements from an LC could help verify several well-motivated extensions of the standard model. For supersymmetry, we compare the strengths of an LC with the LHC. Also, assuming the lightest superpartner explains the missing dark matter in the universe, we examine other places to search for a signal of supersymmetry. We compare the signatures of several scenarios with extra spatial dimensions. We also explore the possibility that the Higgs is a composite, concentrating on models that (unlike technicolor) have a Higgs boson with mass of a few hundred GeV or less. Where appropriate, we mention the importance of high luminosity, for example to measure branching ratios of the Higgs, and the importance of multi-TeV energies, for example to explore the full spectrum of superpartners.

hep-ex↗

arXiv2001

Bounds on Universal Extra Dimensions

We show that the bound from the electroweak data on the size of extra dimensions accessible to all the standard model fields is rather loose. These "universal" extra dimensions could have a compactification scale as low as 300 GeV for one extra dimension. This is because the Kaluza-Klein number is conserved and thus the contributions to the electroweak observables arise only from loops. The main constraint comes from weak-isospin violation effects. We also compute the contributions to the S parameter and the $Zb\bar{b}$ vertex. The direct bound on the compactification scale is set by CDF and D0 in the few hundred GeV range, and the Run II of the Tevatron will either discover extra dimensions or else it could significantly raise the bound on the compactification scale. In the case of two universal extra dimensions, the current lower bound on the compactification scale depends logarithmically on the ultra-violet cutoff of the higher dimensional theory, but can be estimated to lie between 400 and 800 GeV. With three or more extra dimensions, the cutoff dependence may be too strong to allow an estimate.

hep-ph↗

arXiv2001

Universal Extra Dimensions and the Muon Magnetic Moment

We analyze the muon anomalous magnetic moment in the context of universal extra dimensions. Our computation shows that the bound from electroweak data on the size of these dimensions allows only a small shift in the muon magnetic moment given by Kaluza-Klein modes of standard model fields. In the well-motivated case of two universal extra dimensions, additional contributions arising from physics at scales where the effective 6-dimensional standard model breaks down, given by dimension-ten operators, have a natural size comparable to the sensitivity of the muon (g-2) experiment at BNL.

hep-ph↗

arXiv2001

Number of Fermion Generations Derived from Anomaly Cancellation

We prove that global anomaly cancellation requires more than one generation of quarks and leptons, provided that the standard model fields propagate in two universal extra dimensions. Furthermore, if the fermions of different generations have the same gauge charges and chiralities, then global anomaly cancellation implies that there must be three generations.

hep-ph↗

arXiv2001

Unscrambling New Models for Higher-Energy Physics

There is strong evidence that new physical degrees of freedom and new phenomena exist and may be revealed in future collider experiments. The best hints of what this new physics might be are provided by electroweak symmetry breaking. I briefly review certain theories for physics beyond the standard model, including the top-quark seesaw model and universal extra dimensions. A common feature of these models is the presence of vector-like quarks at the TeV scale. Then I discuss the role of a linear $e^+e^-$ collider in disentangling this new physics.

hep-ph↗

arXiv2000

Self-Breaking of the Standard Model Gauge Symmetry

If the gauge fields of the Standard Model propagate in TeV-size extra dimensions, they rapidly become strongly coupled and can form scalar bound states of quarks and leptons. If the quarks and leptons of the third generation propagate in 6 or 8 dimensions, we argue that the most tightly bound scalar is a composite of top quarks, having the quantum numbers of the Higgs doublet and a large coupling to the top quark. In the case where the gauge bosons propagate in a bulk of a certain volume, this composite Higgs doublet can successfully trigger electroweak symmetry breaking. The mass of the top quark is correctly predicted to within 20%, without the need to add a fundamental Yukawa interaction, and the Higgs boson mass is predicted to lie in the range 165 - 230 GeV. In addition to the Higgs boson, there may be a few other scalar composites sufficiently light to be observed at upcoming collider experiments.

hep-ph↗

arXiv2000

Electroweak Symmetry Breaking and Extra Dimensions

Electroweak symmetry can be naturally broken by observed quark and gauge fields in various extra-dimensional configurations. No new {\it fundamental} fields are required below the quantum gravitational scale ($\sim$ 10 - 100 TeV). We examine schemes in which the QCD gauge group alone, in compact extra dimensions, forms a composite Higgs doublet out of (t,b)_L and a linear combination of the Kaluza-Klein modes of t_R. The effective theory at low energies is the Standard Model. The top-quark mass is controlled by the number of active t_R Kaluza-Klein modes below the string scale, and is in agreement with experiment.

hep-ph↗

arXiv2000

Light Axion within the Next-to-Minimal Supersymmetric Standard Model

We analyze the Higgs sector in the Next-to-Minimal Supersymmetric Standard Model, emphasizing the possibility of a light CP-odd scalar (axion) in the spectrum. We compute the coupling of the Standard-Model-like Higgs boson to a pair of axions, and show that it can be large enough to modify the Higgs branching fractions, with a significant impact on the Higgs searches. We delineate the range of parameters relevant for this scenario, and also derive analytic expressions for the scalar masses and couplings in two special cases - a decoupling limit where all scalars other than the axion are heavier than the Standard-Model-like Higgs boson, and the large tan beta limit.

hep-ph↗

arXiv2000

Higgs Boson Decays to CP-odd Scalars at the Tevatron and Beyond

In extended Higgs models, the Higgs boson may decay into a pair of light CP-odd scalars, with distinctive collider signatures. We study the ensuing Higgs signals at the upgraded Tevatron, considering the subsequent decays of the scalars into pairs of gluons or photons. For CP-odd scalars lighter than a few GeV, the Higgs boson manifests itself as a diphoton resonance and can be discovered up to masses of a few hundred GeV. For heavier CP-odd scalars the reach extends at most up to $M_h\sim 120$ GeV. We also discuss the capabilities of the LHC and lepton colliders in these channels.

hep-ph↗

arXiv2000

Electroweak symmetry breaking by extra dimensions

Electroweak symmetry breaking may be naturally induced by the observed quark and gauge fields in extra dimensions without a fundamental Higgs field. We show that a composite Higgs doublet can arise as a bound state of $(t, b)_L$ and a linear combination of the Kaluza-Klein states of $t_R$, due to QCD in extra dimensions. The top quark mass depends on the number of active $t_R$ Kaluza-Klein modes, and is consistent with the experimental value.

hep-ph↗

arXiv1999

Minimal Composite Higgs Model with Light Bosons

We analyze a composite Higgs model with the minimal content that allows a light Standard-Model-like Higgs boson, potentially just above the current LEP limit. The Higgs boson is a bound state made up of the top quark and a heavy vector-like quark. The model predicts that only one other bound state may be lighter than the electroweak scale, namely a CP-odd neutral scalar. Several other composite scalars are expected to have masses in the TeV range. If the Higgs decay into a pair of CP-odd scalars is kinematically open, then this decay mode is dominant, with important implications for Higgs searches. The lower bound on the CP-odd scalar mass is loose, in some cases as low as $\sim$ 100 MeV, being set only by astrophysical constraints.

hep-ph↗