Gravitational Waves from Phase Transitions at the Electroweak Scale and Beyond

If there was a first order phase transition in the early universe, there should be an associated stochastic background of gravitational waves. In this paper, we point out that the characteristic frequency of the spectrum due to phase transitions which took place in the temperature range 100 GeV - 10^7 GeV is precisely in the window that will be probed by the second generation of space-based interferometers such as the Big Bang Observer (BBO). Taking into account the astrophysical foreground, we determine the type of phase transitions which could be detected either at LISA, LIGO or BBO, in terms of the amount of supercooling and the duration of the phase transition that are needed. Those two quantities can be calculated for any given effective scalar potential describing the phase transition. In particular, the new models of electroweak symmetry breaking which have been proposed in the last few years typically have a different Higgs potential from the Standard Model. They could lead to a gravitational wave signature in the milli-Hertz frequency, which is precisely the peak sensitivity of LISA. We also show that the signal coming from phase transitions taking place at T ~ 1-100 TeV could entirely screen the relic gravitational wave signal expected from standard inflationary models.Comment: 18 pages, 24 figure

(Dys)Zphilia or a custodial breaking Higgs at the LHC

Electroweak precision measurements established that custodial symmetry is preserved to a good accuracy in the gauge sector after electroweak symmetry breaking. However, recent LHC results might be interpreted as pointing towards Higgs couplings that do not respect such symmetry. Motivated by this possibility, we reconsider the presence of an explicitly custodial breaking coupling in a generic Higgs parameterization. After briefly commenting on the large UV sensitivity of the T parameter to such a coupling, we perform a fit to results of Higgs searches at LHC and Tevatron, and find that the apparent enhancement of the ZZ channel with respect to WW can be accommodated. Two degenerate best-fit points are present, which we label `Zphilic' and `dysZphilic' depending on the sign of the hZZ coupling. Finally we highlight some measurements at future linear colliders that may remove such degeneracy.Comment: 16 pages, 10 figure

Disguising the Oblique Parameters

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.Comment: 12 pages. v2: two references adde

Alternatives to an Elementary Higgs

We review strongly coupled and extra dimensional models of electroweak symmetry breaking. Models examined include warped extra dimensions, bulk Higgs, "little" Higgs, dilaton Higgs, composite Higgs, twin Higgs, quantum critical Higgs, and "fat" SUSY Higgs. We also discuss current bounds and future LHC searches for this class of models.Comment: 42 pages, 36 figure

Up Asymmetries From Exhilarated Composite Flavor Structures

We present a class of warped extra dimension (composite Higgs) models which conjointly accommodates the t\bar t forward-backward asymmetry observed at the Tevatron and the direct CP asymmetry in singly Cabibbo suppressed D decays first reported by the LHCb collaboration. We argue that both asymmetries, if arising dominantly from new physics beyond the Standard Model, hint for a flavor paradigm within partial compositeness models in which the right-handed quarks of the first two generations are not elementary fields but rather composite objects. We show that this class of models is consistent with current data on flavor and CP violating physics, electroweak precision observables, dijet and top pair resonance searches at hadron colliders. These models have several predictions which will be tested in forthcoming experiments. The CP asymmetry in D decays is induced through an effective operator of the form (\bar u c)_{V+A}(\bar s s)_{V+A} at the charm scale, which implies a larger CP asymmetry in the D^0\to K^+K^- rate relative the D^0\to \pi^+\pi^- channel. This prediction is distinctive from other Standard Model or dipole-based new physics interpretation of the LHCb result. CP violation in D-\bar D mixing as well as an an excess of dijet production of the LHC are also predicted to be observed in a near future. A large top asymmetry originates from the exchange of an axial resonance which dominantly produces left-handed top pairs. As a result a negative contribution to the lepton-based forward-backward asymmetry in t\bar t production, as well as O(10%) forward-backward asymmetry in b\bar b production above m_{b\bar b}\simeq 600GeV at the Tevatron is expected.Comment: 35 pages, 7 fig

Heavy Higgs Searches: Flavour Matters

We point out that the stringent lower bounds on the masses of additional electrically neutral and charged Higgs bosons crucially depend on the flavour structure of their Yukawa interactions. We show that these bounds can easily be evaded by the introduction of flavour-changing neutral currents in the Higgs sector. As an illustration, we study the phenomenology of a two Higgs doublet model with a Yukawa texture singling out the third family of quarks and leptons. We combine constraints from low-energy flavour physics measurements, LHC measurements of the 125 GeV Higgs boson rates, and LHC searches for new heavy Higgs bosons. We propose novel LHC searches that could be performed in the coming years to unravel the existence of these new Higgs bosons.Comment: 41 pages, 11 figures and 4 tables (v2: References added. Comment on associated production with a top quark added. Matched published version.

Lifting degeneracies in Higgs couplings using single top production in association with a Higgs boson

Current Higgs data show an ambiguity in the value of the Yukawa couplings to quarks and leptons. Not so much because of still large uncertainties in the measurements but as the result of several almost degenerate minima in the coupling profile likelihood function. To break these degeneracies, it is important to identify and measure processes where the Higgs coupling to fermions interferes with other coupling(s). The most prominent example, the decay of $h \to \gamma \gamma$, is not sufficient to give a definitive answer. In this Letter, we argue that $t$-channel single top production in association with a Higgs boson, with $h\to b\bar b$, can provide the necessary information to lift the remaining degeneracy in the top Yukawa. Within the Standard Model, the total rate is highly reduced due to an almost perfect destructive interference in the hard process, $W b \rightarrow t h$. We first show that for non-standard couplings the cross section can be reliably computed without worrying about corrections from physics beyond the cutoff scale $\Lambda\gtrsim 10\,\mathrm{TeV}$, and that it can be enhanced by more than one order of magnitude compared to the SM. We then study the signal $p p \rightarrow t h j (b)$ with 3 and 4 $b$'s in the final state, and its main backgrounds at the LHC. We find the 8 TeV run dataset to be sensitive to the sign of the anomalous top Yukawa coupling, while already a moderate integrated luminosity at 14 TeV should lift the degeneracy completely.Comment: 17 pages, 7 figures. v2: inclusive signal cross sections at NLO in QCD added; new comment on sensitivity of the analysis to t tbar h process. Matches version accepted by JHE

Scaling and tuning of EW and Higgs observables

We study deformations of the SM via higher dimensional operators. In particular, we explicitly calculate the one-loop anomalous dimension matrix for 13 bosonic dimension-6 operators relevant for electroweak and Higgs physics. These scaling equations allow us to derive RG-induced bounds, stronger than the direct constraints, on a universal shift of the Higgs couplings and some anomalous triple gauge couplings by assuming no tuning at the scale of new physics, i.e. by requiring that their individual contributions to the running of other severely constrained observables, like the electroweak oblique parameters or $\Gamma(h \rightarrow \gamma\gamma)$, do not exceed their experimental direct bounds. We also study operators involving the Higgs and gluon fields.Comment: v2: 41 pages, 12 tables, 4 figures. Plots of the RG-induced bounds from S and T added, presentation of our approach in sections 2 and 4 improved, a few typos fixed, references added, conclusions and analysis unchanged. Version to appear in JHE

Renormalization Group Scaling of Higgs Operators and \Gamma(h -> \gamma \gamma)

We compute the renormalization of dimension six Higgs-gauge boson operators that can modify \Gamma(h -> \gamma \gamma) at tree-level. Operator mixing is shown to lead to an important modification of new physics effects which has been neglected in past calculations. We also find that the usual formula for the S oblique parameter contribution of these Higgs-gauge boson operators needs additional terms to be consistent with renormalization group evolution. We study the implications of our results for Higgs phenomenology and for new physics models which attempt to explain a deviation in \Gamma(h -> \gamma \gamma). We derive a new relation between the S parameter and the \Gamma(h -> \gamma \gamma) and \Gamma(h ->Z \gamma) decay rates.Comment: 20 pp. 2 fi