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Michael Trott

Publications and source records attributed to Michael Trott.

At least 73 records · Page 4Linked to original sources

Electroweak Baryogenesis in Two Higgs Doublet Models and B meson anomalies

Motivated by 3.9 sigma evidence of a CP-violating phase beyond the standard model in the like-sign dimuon asymmetry reported by DO, we examine the potential for two Higgs doublet models (2HDMs) to achieve successful electroweak baryogenesis (EWBG) while explaining the dimuon anomaly. Our emphasis is on the minimal flavour violating 2HDM, but our numerical scans of model parameter space include type I and type II models as special cases. We incorporate relevant particle physics constraints, including electroweak precision data, b to s gamma, the neutron electric dipole moment, R_b, and perturbative coupling bounds to constrain the model. Surprisingly, we find that a large enough baryon asymmetry is only consistently achieved in a small subset of parameter space in 2HDMs, regardless of trying to simultaneously account for any B physics anomaly. There is some tension between simultaneous explanation of the dimuon anomaly and baryogenesis, but using a Markov chain Monte Carlo we find several models within 1 sigma of the central values. We point out shortcomings with previous studies that reached different conclusions. The restricted parameter space that allows for EWBG makes this scenario highly predictive for collider searches. We discuss the most promising signatures to pursue at the LHC for EWBG-compatible models.

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Flavor Symmetric Sectors and Collider Physics

We discuss the phenomenology of effective field theories with new scalar or vector representations of the Standard Model quark flavor symmetry group, allowing for large flavor breaking involving the third generation. Such field content can have a relatively low mass scale \lesssim TeV and O(1) couplings to quarks, while being naturally consistent with both flavor violating and flavor diagonal constraints. These theories therefore have the potential for early discovery at LHC, and provide a flavor safe "tool box" for addressing anomalies at colliders and low energy experiments. We catalogue the possible flavor symmetric representations, and consider applications to the anomalous Tevatron t-tbar forward backward asymmetry and B_s mixing measurements, individually or concurrently. Collider signatures and constraints on flavor symmetric models are also studied more generally. In our examination of the t-tbar forward backward asymmetry we determine model independent acceptance corrections appropriate for comparing against CDF data that can be applied to any model seeking to explain the t-tbar forward backward asymmetry.

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Forward-backward asymmetry in t anti-t production from flavour symmetries

We show that the forward-backward asymmetry in top quark pair production can be enhanced by fields that transform nontrivially under the flavour group and satisfy Minimal Flavour Violation, while at the same time the constraints from associated effects on the d σ(t anti-t)/d M_{t anti-t} distribution, dijet resonance searches, same sign top pair production and other phenomenology are satisfied. We work out two examples in detail, one where a scalar colour anti-sextet field, that is also an anti-sextet of SU(3)_U, enhances the forward-backward asymmetry, and one where the enhancement arises from a vector colour octet field that is also an octet of SU(3)_U.

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On theories of enhanced CP violation in B_s,d meson mixing

The DO collaboration has measured a deviation from the standard model (SM) prediction in the like sign dimuon asymmetry in semileptonic b decay with a significance of 3.2 sigma. We discuss how minimal flavour violating (MFV) models with multiple scalar representations can lead to this deviation through tree level exchanges of new MFV scalars. We review how the two scalar doublet model can accommodate this result and discuss some of its phenomenology. Limits on electric dipole moments suggest that in this model the coupling of the charged scalar to the right handed u-type quarks is suppressed while its coupling to the d-type right handed quarks must be enhanced. We construct an extension of the MFV two scalar doublet model where this occurs naturally.

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Prospects and Constraints for Vector-like MFV Matter at LHC

We examine the prospects for LHC discovery of SU(2)_L singlet vector-like quarks that obey Minimal Flavour Violation (MFV) and are consistent with lower energy phenomenology. We study models where the vector-like quarks have the same quantum numbers as u_R or d_R, allowing mixing, which generally leads to significant low energy constraints. We find that there are two naturally phenomenologically viable MFV models of this type when the Weyl spinor components of the vector-like quarks are flavour triplets. We examine direct production bounds, flavour and electroweak precision data constraints for these models and determine the cross section for allowed values of the model parameters at LHC. For the models we identify as naturally phenomenologically viable, large amounts of parameter space afford a significant early discovery reach at LHC while being consistent with lower energy phenomenology.

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High pT Production of b\bar{b} at LHC and New SU(3) Bosons

We discuss the potential of measurements of σ(p p -> b\bar{b}) to constrain new bosonic degrees of freedom at the LHC when pT > \sqrt{s}/11 TeV for a pseudorapidity cut |η| < 2.4. By suppressing the NLO QCD production of b\bar{b} pairs through simple kinematic constraints we show how to more efficiently exploit CMS's reach out to 1.5 TeV in pT for B mesons in searches for new physics. Using this technique we investigate the potential for discovery of new massive spin one and spin zero SU_c(3) octet bosons by analyzing their effect on the dσ(p p -> b\bar{b})/d pT^2 distribution. In some cases, the tree level contributions are small and the largest effect of new physics on the pT tail of the distribution is due to one-loop resonant s-channel production or interference effects with the LO standard model production mechanisms q\bar{q}, g g -> b\bar{b}. We explore this possibility in some detail when the standard model is extended with an (8,2)_{1/2} scalar motivated by Minimal Flavor Violation.

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Comment on Higgs Inflation and Naturalness

We rebut the recent claim (arXiv:0912.5463) that Einstein-frame scattering in the Higgs inflation model is unitary above the cut-off energy Lambda ~ Mp/xi. We show explicitly how unitarity problems arise in both the Einstein and Jordan frames of the theory. In a covariant gauge they arise from non-minimal Higgs self-couplings, which cannot be removed by field redefinitions because the target space is not flat. In unitary gauge, where there is only a single scalar which can be redefined to achieve canonical kinetic terms, the unitarity problems arise through non-minimal Higgs-gauge couplings.

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Light Octet Scalars, a Heavy Higgs and Minimal Flavour Violation

It is widely believed that existing electroweak data requires a Standard Model Higgs to be light while electroweak and flavour physics constraints require other scalars charged under the Standard Model gauge couplings to be heavy. We analyze the robustness of these beliefs within a general scalar sector and find both to be incorrect, provided that the scalar sector approximately preserves custodial symmetry and minimal flavour violation (MFV). We demonstrate this by considering the phenomenology of the Standard Model supplemented by a scalar having SU(3)_c x SU(2)_L x U(1)_Y quantum numbers (8,2)_(1/2) which has been argued to be the only kind of exotic flavour singlet scalar allowed by MFV that couples to quarks. We examine constraints coming from electroweak precision data, direct production from LEPII and the Tevatron, and from flavour physics, and find that the observations allow both the Standard Model Higgs and the new scalars to be simultaneously light, with masses ~ 100 GeV, and in some cases lighter. The discovery of such light coloured scalars could be a compelling possibility for early LHC runs, due to their large production cross section, ~100 pb. But the observations equally allow all the scalars to be heavy (including the Higgs), with masses ~ 1 TeV, with the presence of the new scalars removing the light-Higgs preference that normally emerges from fits to the electroweak precision data.

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Scalar Representations and Minimal Flavor Violation

We discuss the representations that new scalar degrees of freedom (beyond those in the minimal standard model) can have if they couple to quarks in a way that is consistent with minimal flavor violation. If the new scalars are singlets under the flavor group then they must be color singlets or color octets. In this paper we discuss the allowed representations and renormalizable couplings when the new scalars also transform under the flavor group. We find that color \bar{3} and 6 representations are also allowed. We focus on the cases where the new scalars can have renormalizable Yukawa couplings to the quarks without factors of the quark Yukawa matrices. The renormalizable couplings in the models we introduce automatically conserve baryon number.

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Power-counting and the Validity of the Classical Approximation During Inflation

We use the power-counting formalism of effective field theory to study the size of loop corrections in theories of slow-roll inflation, with the aim of more precisely identifying the limits of validity of the usual classical inflationary treatments. We keep our analysis as general as possible in order to systematically identify the most important corrections to the classical inflaton dynamics. Although most slow-roll models lie within the semiclassical domain, we find the consistency of the Higgs-Inflaton scenario to be more delicate due to the proximity between the Hubble scale during inflation and the upper bound allowed by unitarity on the new-physics scale associated with the breakdown of the semiclassical approximation within the effective theory. Similar remarks apply to curvature-squared inflationary models.

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R-parity preserving super-WIMP decays

We point out that when the decay of one electroweak scale super-WIMP state to another occurs at second order in a super-weak coupling constant, this can naturally lead to decay lifetimes that are much larger than the age of the Universe, and create observable consequences for the indirect detection of dark matter. We demonstrate this in a supersymmetric model with Dirac neutrinos, where the right-handed scalar neutrinos are the lightest and next-to-lightest supersymmetric partners. We show that this model produces a super-WIMP decay rate scaling as m_nu^4/(weak scale)^3, and may significantly enhance the fraction of energetic electrons and positrons over anti-protons in the decay products. Such a signature is consistent with the observations recently reported by the PAMELA experiment.

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Top quark induced vacuum misalignment in little higgs models

We show that the effect of the top quark can dominate over the effect of the gauge sector in determining the vacuum alignment in little higgs (LH) models. We demonstrate that in the littlest LH model and the SU(2)xSU(2)xU(1) LH model, ensuring that the correct vacuum alignment is chosen requires that a subset of the gauge sector couplings be large to overcome the effect of the top quark. We quantify this effect by deriving bounds on the couplings in the gauge sector and demonstrate that these bounds provide a compelling theoretical reason for the gauge coupling constant hierarchy in the SU(2)xSU(2)xU(1) model that reduces the Goldstone decay constant scale to a TeV. We also argue that for a class of LH models with T parity the top quark drives the correct vacuum alignment and therefore all gauge couplings can be small.

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Electroweak Baryogenesis with a Pseudo-Goldstone Higgs

We examine the nature of electroweak Baryogenesis when the Higgs boson's properties are modified by the effects of new physics. We utilize the effective potential to one loop (ring improving the finite temperature perturbative expansion) while retaining parametrically enhanced dimension six operators of O(v^2/f^2) in the Higgs sector. These parametrically enhanced operators would be present if the Higgs is a pseudo-goldstone boson of a new physics sector with a characteristic mass scale Lambda ~ TeV, a coupling constant (4 pi) > g > 1 and a strong decay constant scale f = Lambda/g. We find that generically the effect of new physics of this form allows a sufficiently first order electro-weak phase transition so that the produced Baryon number can avoid washing out, and has enhanced effects due to new sources CP violation. We also improve the description of the electroweak phase transition in perturbation theory by determining the thermal mass eigenstate basis of the standard model gauge boson fields. This improves the calculation of the finite temperature effects through incorporating mixing in the determination of the vector boson thermal masses of the standard model. These effects are essential to determining the nature of the phase transition in the standard model and are of interest in our Pseudo-Goldstone Baryogenesis scenario.

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The Higgs Decay Width in Multi-Scalar Doublet Models

We show that there are regions of parameter space in multi-scalar doublet models where, in the first few hundred inverse femtobarns of data, the new charged and neutral scalars are not directly observable at the LHC and yet the Higgs decay rate to b bbar is changed significantly from its standard model value. For a light Higgs with a mass less than 140 GeV, this can cause a large change in the number of two photon and tau tau Higgs decay events expected at the LHC compared to the minimal standard model. In the models we consider, the principle of minimal flavor violation is used to suppress flavor changing neutral currents. This paper emphasizes the importance of measuring the properties of the Higgs boson at the LHC; for a range of parameters the model considered has new physics at the TeV scale that is invisible, in the first few hundred inverse femtobarns of integrated luminosity at the LHC, except indirectly through the measurement of Higgs boson properties.

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New Physics Effects in Higgs Decay to Tau Leptons

We study the possible effects of TeV scale new physics (NP) on the rate for Higgs boson decays to charged leptons, focusing on the tau tau channel which can be readily studied at the Large Hadron collider. Using an SU(3)_C X SU(2)_L X U(1)_Y invariant effective theory valid below a NP scale Lambda, we determine all effective operators up to dimension six that could generate appreciable contributions to the decay rate and compute the dependence of the rate on the corresponding operator coefficients. We bound the size of these operator coefficients based on the scale of the tau mass, naturalness considerations, and experimental constraints on the tau anomalous magnetic moment. These considerations imply that contributions to the decay rate from a NP scale Lambda ~ TeV could be comparable to the prediction based on the SM Yukawa interaction. A reliable test of the Higgs mechanism for fermion mass generation via the h-> tau tau channel is possible only after such NP effects are understood and brought under theoretical control.

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A Higgs-Higgs bound state due to New Physics at a TeV

We examine the effects of new physics effecting the Higgs sector of the standard model, focusing on the effects on the Higgs self couplings. We demonstrate that a low mass higgs, m_h < 2 m_t, can have a strong effective self coupling due to the effects of a new interaction at a TeV. We investigate the possibility that the first evidence of such an interaction could be a higgs-higgs bound state. To this end, we construct an effective field theory formalism to examine the physics of such a low mass higgs boson. We explore the possibility of a non relativistic bound state of the higgs field (Higgsium) at LHC and construct a non relativistic effective field theory of the higgs sector that is appropriate for such studies (NRHET).

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Jets in Effective Theory: Summing Phase Space Logs

We demonstrate how to resum phase space logarithms in the Sterman-Weinberg (SW) dijet decay rate within the context of Soft Collinear Effective theory (SCET). An operator basis corresponding to two and three jet events is defined in SCET and renormalized. We obtain the RGE of the two and three jet operators and run the operators from the scale $μ^2 = Q^2$ to the phase space scale $ μ^2_δ= δ^2 Q^2$. This phase space scale, where $δ$ is the cone half angle of the jet, defines the angular region of the jet. At $ μ^2_δ$ we determine the mixing of the three and two jet operators. We combine these results with the running of the two jet shape function, which we run down to an energy cut scale $μ^2_β$. This defines the resumed SW dijet decay rate in the context of SCET. The approach outlined here demonstrates how to establish a jet definition in the context of SCET. This allows a program of systematically improving the theoretical precision of jet phenomenology to be carried out.

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Towards the Anomalous Dimension to Order Lambda_QCD/m_b for Phase Space Restricted B --> X(u) ell anti-nu and B --> X(s) gamma

We examine the anomalous dimension matrix appropriate for the phase space restricted B --> X(u) ell anti-nu and B --> X(s) gamma decay spectra to subleading nonperturbative order. The effects of the time ordered products of the HQET Lagrangian with the leading order shape function operator are calculated, as are the anomalous dimensions of subleading operators. We establish the renormalizability and closure of a subset of the non-local operator basis, a requirement for the establishment of factorization theorems at this order. Operator mixing is found between the operators which occur to subleading order, requiring the subleading operator basis be extended. We comment on the requirement for new shape functions to be introduced to characterize the matrix elements of these new operators, and the phenomenological consequences for extractions of $|V_{ub}|$.

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