Search

A weighted average weak mixing angle θW derived from relatively low Q² experiments is compared with the standard model prediction obtained from precision measurements. The approximate 1.8 sigma discrepancy is fit with an intermediate mass (~10-35 GeV) "dark" Z boson Zd, corresponding to a U(l)d gauge symmetry of hidden dark matter, which couples to our world via kinetic and Z-Zd mass mixing. Constraints on such a scenario are obtained from precision electroweak bounds and searches for the rare Higgs decays H → ZZd → 4 charged leptons at the LHC. The sensitivity of future anticipated low Q² measurements of sin² θW (Q²) to intermediate mass Zd is also illustrated. This dark Z scenario can provide interesting concomitant signals in low energy parity violating measurements and rare Higgs decays at the LHC over the next few years. [ABSTRACT FROM AUTHOR]

The insular nature of the Standard Model may be explained if the Higgs mass parameter is only sensitive to quantum corrections from physical states. Starting from a scale-free electroweak sector at tree level, we postulate that quantum effects of heavy right-handed neutrinos induce a mass term for a scalar weak doublet that contains the dark matter particle. In turn, below the scale of heavy neutrinos, the dark matter sector sets the scale of the Higgs potential. We show that this framework can lead to a Higgs mass that respects physical naturalness, while also providing a viable scalar dark matter candidate, realistic light neutrino masses, and the baryon asymmetry of the Universe via thermal leptogenesis. The proposed scenario can remain perturbative and stable up to the Planck scale, thereby accommodating simple extensions to include a high-scale (~2×1016 GeV) inflationary sector, implied by recent measurements. In that case, our model typically predicts that the dark matter scalar is close to 1 TeV in mass and could be accessible in near future direct detection experiments. [ABSTRACT FROM AUTHOR]

In two-Higgs doublet extensions of the Standard Model, flavor-changing neutral current constraints can be addressed by introducing a U(1)' gauge symmetry, under which the Higgs doublets carry different charges. That scenario implies the presence of a H±W±Z' vertex at tree level. For the light "dark" Z model (Z' = Zd) with mZj < 10 GeV, such a coupling leads to the dominant decay mode H± W± + Zd (for mH± ≾ 175 GeV), rather than the usual type I model decay HH± → τ± ν, for a broad range of parameters. We find that current analyses do not place significant bounds on this scenario. Over much of the parameter space considered, the decay of a pair-produced t (t) into H+ b (H-b) provides the dominant H± production. Analysis of available LHC data can likely cover significant ranges of our parameters, if Zd → µ+µ- has a branching ratio of ~20%. If the Zd decays mainly invisibly then probing the entire relevant parameter space would likely require additional data from future LHC runs. We briefly discuss the phenomenology for mH± g 175 GeV. [ABSTRACT FROM AUTHOR]

We show that weak scale vectorlike fermions with order one couplings to the Higgs can lead to a novel mechanism for a strongly first-order electroweak phase transition through their tendency to drive the Higgs quartic coupling negative. These same fermions could also enhance the loop-induced branching fraction of the Higgs into two photons, as suggested by the recent discovery of a ~ 125 GeV Higgs-like state at the CERN LHC. Our results suggest that measurements of the diphoton decay rate of the Higgs and its self-coupling, at the LHC or perhaps at a future lepton collider, could probe the electroweak phase transition in the early Universe, with significant implications for the viability of electroweak baryogenesis scenarios. [ABSTRACT FROM AUTHOR]

We propose that the LHC hints for a Higgs diphoton excess and the muon g -- 2 (gµ - 2) discrepancy between theory and experiment may be related by vectorlike "leptons" charged under both U(1)Y hypercharge and a "dark" U(1)d. Quantum loops of such leptons can enhance the Higgs diphoton rate and also generically lead to U(1)y-U(1)d kinetic mixing. The induced coupling of a light U(1)d gauge boson Zd to electric charge can naturally explain the measured gµ - 2. We update Zd mass and coupling constraints based on comparison of the electron g -- 2 experiment and theory, and find that explaining gµ-2 while satisfying other constraints requires Zd to have a mass ∼20-100 MeV. We predict new Higgs decay channels γZd and ZdZd, with rates below the diphoton mode but potentially observable. The boosted Zd → e+e- in these decays would mimic a promptly converted photon and could provide a fraction of the apparent diphoton excess. More statistics or a closer inspection of extant data may reveal such events. [ABSTRACT FROM AUTHOR]

Precision data generally require the threshold for physics beyond the Standard Model to be at the deca-TeV (10 TeV) scale or higher. This raises the question of whether there are interesting deca-TeV models for which the LHC may find direct clues. A possible scenario for such physics is a 5D warped model of fermion masses and mixing, with Kaluza-Klein masses mKK ~10 TeV, allowing it to avoid tension with stringent constraints, especially from flavor data. Discovery of a Standard-Model-like Higgs boson, for which there are some hints at ~125 GeV at the LHC, would also require the Kaluza-Klein masses to be at or above 10 TeV. These warped models generically predict the appearance of a much lighter radion scalar. We find that, in viable warped models of flavor, a radion with a mass of a few hundred GeV and an inverse coupling of order mKK ~10 TeV could typically be accessible to the LHC experiments--with √S=14 TeV and ~100 fb-1 of data. The above statements can be applied, mutatis mutandis, to 4D dual models, where conformai dynamics and a dilaton replace warping and the radion, respectively. Detection of such a light and narrow scalar could thus herald the proximity of a new physical threshold and motivate experiments that would directly probe the deca-TeV mass scale. [ABSTRACT FROM AUTHOR]

If fermion condensation is a main source of electroweak symmetry breaking, an ultra-heavy Higgs doublet of mass ∼ 108 GeV can yield naturally small Dirac neutrino masses. We show that such a scenario can lead to a new leptogenesis mechanism based on the decays of the ultra-heavy Higgs. Given its very large mass, the requisite Higgs doublet can be considered an elementary particle and would point to a cutoff scale ∼1010 GeV. We outline how our scenario can also naturally lead to composite asymmetric dark matter. Some potential signals of this scenario are discussed. [ABSTRACT FROM AUTHOR]

General consequences of mass mixing between the ordinary Ζ boson and a relatively light Z,d boson, the "dark" Z, arising from a U(1)d gauge symmetry, associated with a hidden sector such as dark matter, are examined. New effects beyond kinetic mixing are emphasized. Z-Zd mixing introduces a new source of low energy parity violation well explored by possible future atomic parity violation and planned polarized electron scattering experiments. Rare K(B) meson decays into π(Κ)ℓ+ℓ-(ℓ = e, μ) and π(K)vv̄ are found to already place tight constraints on the size of Z-Zd mixing. Those sensitivities can be further improved with future dedicated searches at Κ and Β factories as well as binned studies of existing data. Z-Zd mixing can also lead to the Higgs decay Η → ZZd, followed by Ζ → ℓ+1ℓ-1 and Zd → ℓ+2ℓ-2 or "missing energy," providing a potential hidden sector discovery channel at the Large Hadron Collider. An illustrative realization of these effects in a 2 Higgs doublet model is presented. [ABSTRACT FROM AUTHOR]

Pseudoscalar particles, often referred to as axionlike-particles (ALPs), arise in a variety of theoretical contexts. The phenomenology of such states is typically studied while assuming flavor-conserving interactions, yet they can in principle have flavor-violating (FV) couplings to fermions. We consider this general possibility while focusing on models where the ALP has non-negligible coupling to the Standard Model Higgs boson h. For a lepton FV ALP a of mass ma≳2 GeV, a→τℓ, where ℓ≠τ is a charged lepton, could have an Ϭ(1) branching fraction, leading to a potentially detectable h→aa→τℓτℓ at the LHC and its future program. We examine this possibility, in light of existing bounds on FV processes, in a general effective theory. We obtain constraints on the effective couplings from both prompt and long-lifetime searches at the LHC; some projections for envisioned measurements are also provided. The implications of the recently announced first results of the muon g-2 measurement at Fermilab for the ALP interactions considered in our work are also briefly discussed. [ABSTRACT FROM AUTHOR]

A modest extension of the Standard Model by two additional Higgs doublets--the Higgs troika model--can provide a well-motivated scenario for successful baryogenesis if neutrinos are Dirac fermions. Adapting the "spontaneous flavor violation" framework, we consider a version of the troika model where light quarks have significant couplings to the new multi-TeV Higgs states. Resonant production of new scalars leading to dijet or top-pair signals are typical predictions of this setup. The initial- and final-state quarks relevant to the collider phenomenology also play a key role in baryogenesis, potentially providing direct access to the relevant early-Universe physics in high-energy experiments. Viable baryogenesis generally prefers some hierarchy of masses between the observed and the postulated Higgs states. We show that there is a complementarity between direct searches at a future 100 TeV pp collider and indirect searches at flavor experiments, with both sensitive to different regions of parameter space relevant for baryogenesis. In particular, measurements of D-̅D mixing at LHCb probe much of the interesting parameter space. Direct and indirect searches can uncover the new Higgs states up to masses of O(10) TeV, thereby providing an impressive reach to investigate this model. [ABSTRACT FROM AUTHOR]

To explain the baryon asymmetry of the Universe, we extend the Standard Model (SM) with two additional Higgs doublets with small vacuum expectation values. The additional Higgs fields interact with SM fermions through complex Yukawa couplings, leading to new sources of C P violation. We propose a simple flavor model with O (1) or smaller Yukawa couplings for quarks and charged leptons, consistent with current flavor constraints. To generate neutrino masses and the baryon asymmetry, right-handed neutrinos in the ∼ 0.1 - 10     TeV range couple to the "Higgs troika." The new Higgs doublet masses are at or above the TeV scale, allowing for asymmetric decays into SM lepton doublets and right-handed neutrinos. The asymmetry in lepton doublets is then processed into a baryon asymmetry, similar to leptogenesis. Since the masses of the new fields could be near the TeV scale, there is potentially a rich high energy collider phenomenology, including observable deviations in the 125 GeV Higgs decay into muons and taus, as well as detectable low energy signals such as the electron electric dipole moment or μ → e γ. Hence, this is in principle a testable model for the generation of baryon asymmetry. [ABSTRACT FROM AUTHOR]