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Paul Langacker

Publications and source records attributed to Paul Langacker.

At least 73 records · Page 4Linked to original sources
arXiv2005

Massive Neutrinos and (Heterotic) String Theory

String theories in principle address the origin and values of the quark and lepton masses. Perhaps the small values of neutrino masses could be explained generically in string theory even if it is more difficult to calculate individual values, or perhaps some string constructions could be favored by generating small neutrino masses. We examine this issue in the context of the well-known three-family standard-like Z_3 heterotic orbifolds, where the theory is well enough known to construct the corresponding operators allowed by string selection rules, and analyze the D- and F-flatness conditions. Surprisingly, we find that a simple see-saw mechanism does not arise. It is not clear whether this is a property of this construction, or of orbifolds more generally, or of string theory itself. Extended see-saw mechanisms may be allowed; more analysis will be needed to settle that issue. We briefly speculate on their form if allowed and on the possibility of alternatives, such as small Dirac masses and triplet see-saws. The smallness of neutrino masses may be a powerful probe of string constructions in general. We also find further evidence that there are only 20 inequivalent models in this class, which affects the counting of string vacua.

hep-th↗
arXiv2005

Neutrino Masses in Supersymmetric SU(3)_C x SU(2)_L x U(1)_Y x U(1)' Models

We consider various possibilities for generating neutrino masses in supersymmetric models with an additional U(1)' gauge symmetry. One class of models involves two extra U(1)' x U(1)'' gauge symmetries, with U(1)'' breaking at an intermediate scale and yielding small Dirac masses through high-dimensional operators. The right-handed neutrinos N^c_i can naturally decouple from the low energy U(1)', avoiding cosmological constraints. A variant version can generate large Majorana masses for N^c_i and an ordinary see-saw. We secondly consider models with a pair of heavy triplets which couple to left-handed neutrinos. After integrating out the heavy triplets, a small neutrino Majorana mass matrix can be generated by the induced non-renormalizable terms. We also study models involving the double-see-saw mechanism, in which heavy Majorana masses for N^c_i are associated with the TeV-scale of U(1)' breaking. We give the conditions to avoid runaway directions in such models and discuss simple patterns for neutrino masses.

hep-ph↗
arXiv2005

Electroweak Baryogenesis in a Supersymmetric U(1)' Model

We construct an anomaly free supersymmetric U(1)' model with a secluded U(1)'-breaking sector. We study the one-loop effective potential at finite temperature, and show that there exists a strong enough first order electroweak phase transition for electroweak baryogenesis (EWBG) because of the large trilinear term $A_h h S H_d H_u$ in the tree-level Higgs potential. Unlike in the MSSM, the lightest stop can be very heavy. We consider the non-local EWBG mechanism in the thin wall regime, and find that within uncertainties the observed baryon number can be generated from the $τ$ lepton contribution, with the secluded sector playing an essential role. The chargino and neutralino contributions and the implications for the Z' mass and electric dipole moments are briefly discussed.

hep-ph↗
arXiv2004

Z' Discovery Limits For Supersymmetric E_6 Models

We study how the exotic particles and supersymmetric partners would affect the discovery limit at the Tevatron and LHC for neutral gauge bosons in generic E_6 models. We examine the Z' decay in the extreme case that all of the particles are massless, then consider how the masses of non-standard model particles will affect the discovery limit. We also calculate the discovery limit for a supersymmetric E_6 model with a secluded sector as a concrete example of a model with a definite set of exotic particles. Its discovery limit is small compared with other E_6 models due to the U'(1) charge assignment.

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arXiv2004

Neutrino Physics (theory)

Nonzero neutrino masses are the first definitive need to extend the standard model. After reviewing the basic framework, I describe the status of some of the major issues, including tests of the basic framework of neutrino masses and mixings; the question of Majorana vs. Dirac; the spectrum, mixings, and number of neutrinos; models, with special emphasis on constraints from typical superstring constructions (which are not consistent with popular bottom-up assumptions); and other implications.

hep-ph↗
arXiv2004

D6-brane Splitting on Type IIA Orientifolds

We study the open-string moduli of supersymmetric D6-branes, addressing both the string and field theory aspects of D6-brane splitting on Type IIA orientifolds induced by open-string moduli Higgsing (i.e., their obtaining VEVs). Specifically, we focus on the Z_2 x Z_2 orientifolds and address the symmetry breaking pattern for D6-branes parallel with the orientifold 6-planes as well as those positioned at angles. We demonstrate that the string theory results, i.e., D6-brane splitting and relocating in internal space, are in one to one correspondence with the field theory results associated with the Higgsing of moduli in the antisymmetric representation of Sp(2N) gauge symmetry (for branes parallel with orientifold planes) or adjoint representation of U(N) (for branes at general angles). In particular, the moduli Higgsing in the open-string sector results in the change of the gauge structure of D6-branes and thus changes the chiral spectrum and family number as well. As a by-product, we provide the new examples of the supersymmetric Standard-like models with the electroweak sector arising from Sp(2N)_L x Sp(2N)_R gauge symmetry; and one four-family example is free of chiral Standard Model exotics.

hep-th↗
arXiv2004

Neutralino Relic Density in a Supersymmetric U(1)' Model

We study properties of the lightest neutralino (χ) and calculate its cosmological relic density in a supersymmetric U(1)' model with a secluded U(1)' breaking sector (the S-model). The lightest neutralino mass is smaller than in the minimal supersymmetric standard model; for instance, m_χ< 100 GeV in the limit that the U(1)' gaugino mass is large compared to the electroweak scale. We find that the Z-χ-χcoupling can be enhanced due to the singlino components in the extended neutralino sector. Neutralino annihilation through the Z-resonance then reproduces the measured cold dark matter density over broad regions of the model parameter space.

hep-ph↗
arXiv2004

Solution to the B -> pi K Puzzle in a Flavor-Changing Z' Model

Recent experiments suggest that certain B -> pi K branching ratios are inconsistent with the standard model expectations. We show that a flavor-changing Z' provides a solution to the problem. Electroweak penguin amplitudes are enhanced by the Z' boson for select parameters. We discuss implications for the Z' mass and its couplings to the standard model fermions. We also show that the solution is consistent with constraints from the CP asymmetries of the B -> phi K_S decay.

hep-ph↗
arXiv2004

The Higgs Sector in a $U(1)^\prime$ Extension of the MSSM

We consider the Higgs sector in an extension of the MSSM with extra SM singlets, involving an extra $U(1)^\prime$ gauge symmetry, in which the domain-wall problem is avoided and the effective $μ$ parameter is decoupled from the new gauge boson $Z^\prime$ mass. The model involves a rich Higgs structure very different from that of the MSSM. In particular, there are large mixings between Higgs doublets and the SM singlets, significantly affecting the Higgs spectrum, production cross sections, decay modes, existing exclusion limits, and allowed parameter range. Scalars considerably lighter than the LEP2 bound (114 GeV) are allowed, and the range $\tan β\sim 1$ is both allowed and theoretically favored. Phenomenologically, we concentrate our study on the lighter (least model-dependent, yet characteristic) Higgs particles with significant SU(2)-doublet components to their wave functions, for the case of no explicit CP violation in the Higgs sector. We consider their spectra, including the dominant radiative corrections to their masses from the top/stop loop. We computed their production cross sections and reexamine the existing exclusion limits at LEP2. We outline the searching strategy for some representative scenarios at a future linear collider. We emphasize that gaugino, Higgsino, and singlino decay modes are indicative of extended models and have been given little attention. We present a comprehensive list of model scenarios in the Appendices.

hep-ph↗
arXiv2004

B_s-anti-B_s Mixing in Z' Models with Flavor-Changing Neutral Currents

In models with an extra U(1)' gauge boson family non-universal couplings to the weak eigenstates of the standard model fermions generally induce flavor-changing neutral currents. This phenomenon leads to interesting results in various B meson decays, for which recent data indicate hints of new physics involving significant contributions from b -> s transitions. We analyze the B_s system, emphasizing the effects of a Z' on the mass difference and CP asymmetries.

hep-ph↗
arXiv2004

Beyond the MSSM

Specific quasi-realistic superstring constructions often predict the existence of new TeV-scale physics which may be different in character from bottom-up motivated constructions. I describe examples of such beyond the MSSM physics, with special emphasis on heavy Z' gauge bosons and associated exotic particles. Implications of an extra U(1)' include a viable scienario for electroweak baryogenesis, a highly non-standard light Higgs sector, and Z'-mediated contributions to rare B decays.

hep-ph↗
arXiv2004

An NMSSM Without Domain Walls

We consider the Higgs sector in an extension of the MSSM involving an extra U(1)' gauge symmetry and SM singlet U(1)' charged scalars, in which the effective $μ$ parameter is decoupled from the Z' mass. There are large mixings between Higgs doublets and singlets, significantly affecting the Higgs spectrum, production cross sections, decay modes, exclusion limits, and allowed parameters ranges. Scalars considerably lighter than 114 GeV are allowed, and the range $\tan β\sim 1$ is both allowed and theoretically favored. We concentrate on the lighter (least model dependent) Higgs particles with significant SU(2)-doublet components to their wave functions, for the case of no explicit CP violation in the Higgs sector. We consider their spectra, including the dominant radiative corrections to their masses in the large $\tilde t$ mass limit; production cross sections and exclusion limits at LEP and a future linear collider; and decay properties.

hep-ph↗
arXiv2004

Big Bang Nucleosynthesis Constraints on Z' Properties

In models involving new TeV-scale Z' gauge bosons, the new U(1)' symmetry often prevents the generation of Majorana masses needed for a conventional neutrino seesaw, leading to three super-weakly interacting right-handed neutrinos, the Dirac partners of the ordinary neutrinos. These can be produced prior to Big Bang Nucleosynthesis (BBN) by the Z' interactions, leading to a faster expansion rate and too much ^4He. We quantify the constraints on the Z' properties from nucleosynthesis for Z' couplings motivated by a class of E6 models parametrized by an angle theta_E6. The decoupling temperature, which is higher than that of ordinary left-handed neutrinos due to the large Z' mass, is calculated, and the equivalent number of extra weakly interacting neutrinos, Delta N_nu, is obtained numerically as a function of the Z' mass, couplings, and the Z-Z' mixing angle. The ^4He abundance from BBN gives the most stringent limit on M_Z' unless Z' coupling to the right-handed neutrinos are small.

hep-ph↗
arXiv2003

Z' Mediated Flavor Changing Neutral Currents in B Meson Decays

We study the effects of an extra U(1)' gauge boson with flavor changing couplings with fermion mass eigenstates on certain B meson decays that are sensitive to such new physics contributions. In particular, we examine to what extent the current data on B_d -> ϕK, B_d -> η' K and B_s -> μ^+ μ^- decays may be explained in such models, concentrating on the example in which the flavor changing couplings are left-chiral. We find that within reasonable ranges of parameters, the Z' contribution can readily account for the anomaly in S_{ϕK_S} but is not sufficient to explain large branching ratio of B_d -> η' K with the same parameter value. S_{ϕK_S} and S_{η' K_S} are seen to be the dominant observables that constrain the extra weak phase in the model.

hep-ph↗
arXiv2003

Electroweak Physics

The results of high precision weak neutral current (WNC), Z-pole, and high energy collider electroweak experiments have been the primary prediction and test of electroweak unification. The electroweak program is briefly reviewed from a historical perspective. Current changes, anomalies, and things to watch are summarized, and the implications for the standard model and beyond discussed.

hep-ph↗
arXiv2003

TeV Physics from the Top Down

Concrete semi-realistic string/M theory constructions often predict the existence of new physics at the TeV scale, which may be different in character from the bottom-up ideas that are motivated by specific problems of the standard model. I describe two examples, heterotic and open string, of such constructions. The latter has a particularly interesting strongly-coupled quasi-hidden sector, which may lead to composite states at low energy and dynamical supersymmetry breaking. General issues, such as grand unification versus direct compactification, additional U(1)' gauge symmetries and other exotics, and flat directions are discussed.

hep-ph↗
arXiv2003

Hiding relativistic degrees of freedom in the early universe

We quantify the extent to which extra relativistic energy density can be concealed by a neutrino asymmetry without conflicting with the baryon asymmetry measured by the Wilkinson Microwave Anisotropy Probe (WMAP). In the presence of a large electron neutrino asymmetry, slightly more than seven effective neutrinos are allowed by Big Bang Nucleosynthesis (BBN) and WMAP at 2σ. The same electron neutrino degeneracy that reconciles the BBN prediction for the primordial helium abundance with the observationally inferred value also reconciles the LSND neutrino with BBN by suppressing its thermalization prior to BBN.

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