Z' Physics at the LHC
The existing limits on Z' gauge bosons and prospects for discovery and diagnostic studies at the LHC are briefly reviewed.
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arXiv subjects
Paul Langacker
Publications and source records attributed to Paul Langacker.
The Physics of New U(1)' Gauge Bosons
Additional Z' gauge bosons are predicted by a wide variety of extensions of the standard model (SM). Possibilities include TeV-scale bosons with electroweak coupling, very light bosons which nearly decouple from the standard model particles, and bosons which communicate with a quasi-hidden sector. A broad survey is given of the theoretical possibilities and of the physics implications for particle physics and cosmology. Several novel examples, including light Z's suggested by PAMELA, Stueckelberg Z's, and Z's associated with the mediation of supersymmetry breaking, are described.
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The Physics of Heavy Z' Gauge Bosons
The U(1)' symmetry associated with a possible heavy Z' would have profound implications for particle physics and cosmology. The motivations for such particles in various extensions of the standard model, possible ranges for their masses and couplings, and classes of anomaly-free models are discussed. Present limits from electroweak and collider experiments are briefly surveyed, as are prospects for discovery and diagnostic study at future colliders. Implications of a Z' are discussed, including an extended Higgs sector, extended neutralino sector, and solution to the mu problem in supersymmetry; exotic fermions needed for anomaly cancellation; possible flavor changing neutral current effects; neutrino mass; possible Z' mediation of supersymmetry breaking; and cosmological implications for cold dark matter and electroweak baryogenesis.
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Electroweak Baryogenesis, CDM and Anomaly-free Supersymmetric U(1)-prime Models
We construct two anomaly-free supersymmetric $U(1)'$ models with a secluded $U(1)'$-breaking sector. For the one with $E_6$ embedding we show that there exists a strong enough first order electroweak phase transition for electroweak baryogenesis (EWBG) because of the large soft trilinear terms in the Higgs potential. Unlike the Minimal Supersymmetric Standard Model (MSSM), the stop masses can be very heavy. We then discuss possible CP violation in the Higgs sector, which can be both spontaneous and explicit, even at tree level. The spontaneous violation provides a direct source for baryogenesis, while its magnitude is mediated by an explicit phase from the secluded sector. These new CP sources do not introduce significant new contributions to electric dipole moments. EWBG in the thin wall ($τ$ leptons) and thick wall regimes (top squarks, charginos and top quarks) are systematically discussed. We find that the CP-violating stop and chargino currents are very different from those obtained in the MSSM. Due to the space-dependence of the relevant CP phases, they do not require a variation of $\tan β$ in the bubble wall to have a non-trivial structure at the lowest order of Higgs insertion. In addition to $τ$ leptons, top squarks and charginos, we find that top quarks can also play a significant role. Numerical results show that the baryon asymmetry is large enough to explain the cosmological observation today. We illustrate that EWBG and neutralino cold dark matter can be accommodated in the same framework, i.e., there exists parameter space where a strong enough first order EWPT, large CP phase variations across the bubble wall, a reasonable baryon asymmetry as well as an acceptable neutralino dark matter relic density can be achieved simultaneously.
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Improved Constraints on Z' Bosons from Electroweak Precision Data
We analyze various models with an extra U(1) gauge symmetry in addition to the Standard Model (SM) gauge group at low energies, and impose limits on the mass of the neutral Z' boson, M_Z', predicted in all such models, and on the Z-Z' mixing angle, theta_ZZ'. The precision electroweak data strongly constrain theta_ZZ' to very small values and for most models we find lower limits on M_Z' of order 1 TeV. In one case we obtain a somewhat better fit than in the SM (although this is only marginally statistically significant) and here we find a weak upper limit at the 90% C.L.
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$b \to s$ Transitions in Family-dependent $U(1)^\prime$ Models
We analyze flavor-changing-neutral-current (FCNC) effects in the $b\to s$ transitions that are induced by family non-universal $U(1)'$ gauge symmetries. After systematically developing the necessary formalism, we present a correlated analysis for the $ΔB =1, 2$ processes. We adopt a model-independent approach in which we only require family-universal charges for the first and second generations and small fermion mixing angles. We analyze the constraints on the resulting parameter space from $B_s - \bar B_s$ mixing and the time-dependent CP asymmetries of the penguin-dominated $B_d \to (π, ϕ, η', ρ, ω, f_0)K_S$ decays. Our results indicate that the currently observed discrepancies in some of these modes with respect to the Standard Model predictions can be consistently accommodated within this general class of models.
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Family Non-universal $U(1)^\prime$ Gauge Symmetries and $b\to s$ Transitions
We present a correlated analysis for the $ΔB =1, 2$ processes which occur via $b\to s$ transitions within models with a family non-universal $U(1)^\prime$. We take a model-independent approach, and only require family universal charges for the first and second generations and small fermion mixing angles. The results of our analysis show that within this class of models, the anomalies in $B_s - \bar B_s$ mixing and the time-dependent CP asymmetries of the penguin-dominated $B_d \to (π, ϕ, η', ρ, ω, f_0)K_S$ decays can be accommodated.
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Complex Singlet Extension of the Standard Model
We analyze a simple extension of the Standard Model (SM) obtained by adding a complex singlet to the scalar sector (cxSM). We show that the cxSM can contain one or two viable cold dark matter candidates and analyze the conditions on the parameters of the scalar potential that yield the observed relic density. When the cxSM potential contains a global U(1) symmetry that is both softly and spontaneously broken, it contains both a viable dark matter candidate and the ingredients necessary for a strong first order electroweak phase transition as needed for electroweak baryogenesis. We also study the implications of the model for discovery of a Higgs boson at the Large Hadron Collider.
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Introduction to the Standard Model and Electroweak Physics
A concise introduction is given to the standard model, including the structure of the QCD and electroweak Lagrangians, spontaneous symmetry breaking, experimental tests, and problems.
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Scalar Potentials and Accidental Symmetries in Supersymmetric U(1)' Models
We address two closely related problems associated with the singlet scalars' potential that are often present in supersymmetric U(1)' models, especially those which maintain the gauge unification of the MSSM in a simple way. The first is the possibility of an accidental global symmetry which results in a light Goldstone boson. The second is the problem of generating a vacuum expectation value for more than one field without reintroducing the $μ$ problem. We give sufficient conditions for addressing both issues and provide a concrete example to generate them.
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D-Instanton Generated Dirac Neutrino Masses
We present a stringy mechanism to generate Dirac neutrino masses by D-instantons in an experimentally relevant mass scale without fine-tuning. Within Type IIA string theory with intersecting D6-branes, we spell out specific conditions for the emergence of such couplings and provide a class of supersymmetric local SU(5) Grand Unified models, based on the Z_2 x Z'_2 orientifold compactification, where perturbatively absent Dirac neutrino masses can be generated by D2-brane instantons in the experimentally observed mass regime, while Majorana masses remain absent, thus providing an intriguing mechanism for the origin of small neutrino masses due to non-perturbative stringy effects.
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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. The current status and the implications for the standard model and beyond are discussed.
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Aspects of Z'-mediated Supersymmetry Breaking
In a recent paper, we proposed the possibility that supersymmetry breaking is communicated dominantly via a U(1)' vector multiplet. We also required that the U(1)' plays a crucial role in solving the mu problem. We discuss here in detail both the construction and the phenomenology of one class of such models. The low energy spectrum generically contains heavy sfermions, Higgsinos and exotics ~10-100 TeV; an intermediate M_Z' ~ 3-30 TeV; light gauginos ~100-1000 GeV, of which the lightest can be wino-like; a light Higgs with a mass of ~140 GeV; and a singlino which can be very light. We present a set of possible consistent charge choices. Several benchmark models are used to demonstrate characteristic phenomenological features. Special attention is devoted to interesting LHC signatures such as gluino decay and the decay patterns of the electroweak-inos. Implications for neutrino masses, exotic decays, R-parity, gauge unification, and the gravitino mass are briefly discussed.
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Z'-mediated Supersymmetry Breaking
We consider a class of models in which supersymmetry breaking is communicated dominantly via a U'(1) gauge interaction, which also helps solve the μproblem. Such models can emerge naturally in top-down constructions and are a version of split supersymmetry. The spectrum contains heavy sfermions, Higgsinos, exotics, and Z' ~ 10-100 TeV; light gauginos ~ 100-1000 GeV; a light Higgs ~ 140 GeV; and a light singlino. A specific set of U'(1) charges and exotics is analyzed, and we present five benchmark models. Implications for the gluino lifetime, cold dark matter, and the gravitino and neutrino masses are discussed.
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Dirac Neutrino Masses from Generalized Supersymmetry Breaking
We demonstrate that Dirac neutrino masses in the experimentally preferred range are generated within supersymmetric gauge extensions of the Standard Model with a generalized supersymmetry breaking sector. If the usual superpotential Yukawa couplings are forbidden by the additional gauge symmetry (such as a U(1)'), effective Dirac mass terms involving the "wrong Higgs" field can arise either at tree level due to hard supersymmetry breaking fermion Yukawa couplings, or at one-loop due to nonanalytic or "nonholomorphic" soft supersymmetry breaking trilinear scalar couplings. As both of these operators are naturally suppressed in generic models of supersymmetry breaking, the resulting neutrino masses are naturally in the sub-eV range. The neutrino magnetic and electric dipole moments resulting from the radiative mechanism also vanish at one-loop order.
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Theory and Phenomenology of Exotic Isosinglet Quarks and Squarks
Extensions of the MSSM often predict the existence of new fermions and their scalar superpartners which are vectorlike with respect to the standard model gauge group but may be chiral under additional gauge factors. In this paper we explore the production and decay of an important example, i.e., a heavy isosinglet charge -1/3 quark and its scalar partner, using the charge assignments of a 27-plet of E6 for illustration. We emphasize that, depending on the symmetries of the low energy theory, such exotic particles may decay by the mixing of the fermion with the d, s, or b quarks; may decay by leptoquark or diquark couplings (which may nevertheless preserve a form of R-parity); or may be stable with respect to renormalizable couplings but decay by higher-dimension operators on cosmological times scales. We discuss the latter two possibilities in detail for various assumptions concerning the relative masses of the exotic fermions, scalars, and the lightest neutralino, and emphasize the necessity of considering the collider signatures in conjunction with the normal MSSM processes. Existing and projected constraints from colliders, indirect experiments, proton decay, and big bang nucleosynthesis are considered.
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A T-odd observable sensitive to CP violating phases in squark decay
We present a new observable sensitive to a certain combination of CP violating phases in supersymmetric extensions of the Standard Model, viz. a triple product of momenta in the cascade decay of a heavy squark via an on-shell neutralino and off-shell slepton. We investigate the regions of parameter space in which the signal is strong enough to be detectable at the LHC with $\sim \bigl(10^2-10^3\bigr)/\sin^2(2Δϕ)$ identified events, where $Δϕ$ is a certain combination of phases in the MSSM presented in the text.
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LHC Phenomenology of an Extended Standard Model with a Real Scalar Singlet
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.
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