arXiv subjects

Heather E. Logan

Publications and source records attributed to Heather E. Logan.

At least 37 records · Page 2Linked to original sources

arXiv2017

Large scalar multiplet dark matter in the high-mass region

We study two models of scalar dark matter from "large" electroweak multiplets with isospin 5/2 (n=6 members) and 7/2 (n=8), whose scalar potentials preserve a $Z_2$ symmetry. Because of large annihilation cross sections due to electroweak interactions, these scalars can constitute all the dark matter only for masses in the multi-TeV range. For such high masses, Sommerfeld enhancement and co-annihilations play important roles in the dark matter relic abundance calculation, reducing the upper bound on the large multiplet's mass by almost a factor of two. We determine the allowed parameter ranges including both of these effects and show that these models are as yet unconstrained by dark matter direct detection experiments, but will be probed by currently-running and proposed future experiments. We also show that a Landau pole appears in these models at energy scales below $10^9$ GeV, indicating the presence of additional new physics below that scale.

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arXiv2017

Constraining the scalar septet model through vector boson scattering

The scalar septet model extends the Standard Model Higgs sector by an isospin septet with hypercharge chosen to preserve $ρ\equiv M_W^2/M_Z^2 \cos^2θ_W = 1$. In this paper we constrain the model at high septet masses using perturbative unitarity of longitudinal vector boson scattering amplitudes. We also apply the constraints from LHC searches for doubly-charged Higgs bosons produced in vector boson fusion, which constrain the model at lower septet masses. We point out some important differences between the septet model and extended Higgs models that preserve custodial symmetry in the scalar spectrum.

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arXiv2016

Real singlet scalar dark matter extension of the Georgi-Machacek model

The Georgi-Machacek model extends the Standard Model Higgs sector with the addition of isospin-triplet scalar fields in such a way as to preserve the custodial symmetry. The presence of higher-isospin scalars contributing to electroweak symmetry breaking offers the interesting possibility that the couplings of the 125 GeV Higgs boson to both gluons and vector boson pairs could be larger than those of the Standard Model Higgs boson. Constraining this possibility using measurements of Higgs production and decay at the CERN Large Hadron Collider is notoriously problematic if a new, non-Standard Model decay mode of the 125 GeV Higgs boson is present. We study an implementation of this scenario in which the Georgi-Machacek model is extended by a real singlet scalar dark matter candidate, and require that the singlet scalar account for all the dark matter in the universe. The combination of the observed dark matter relic density and direct detection constraints exclude singlet scalar masses below about 57 GeV. Higgs measurements are not yet precise enough to be very sensitive to $h\rightarrow SS$ in the remaining allowed kinematic region, so that constraints from Higgs measurements are so far the same as in the GM model without a singlet scalar. We also find that, above the Higgs pole, a substantial region of parameter space yielding the correct dark matter relic density can escape the near-future direct detection experiments DEAP and XENON~1T for dark matter masses as low as 120 GeV and even have a direct detection cross section below the neutrino floor for $m_S\gtrsim 150$ GeV. This is in contrast to the singlet scalar dark matter extension of the Standard Model, for which these future experiments are expected to exclude dark matter masses above the Higgs pole up to the multi-TeV range.

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arXiv2016

Constraining the Higgs couplings to up and down quarks using production kinematics at the CERN Large Hadron Collider

We study the prospects for constraining the Higgs boson's couplings to up and down quarks using kinematic distributions in Higgs production at the CERN Large Hadron Collider. We find that the Higgs $p_T$ distribution can be used to constrain these couplings with precision competitive to other proposed techniques. With 3000 fb$^{-1}$ of data at 13 TeV in the four-lepton decay channel, we find $-0.73 \lesssim \barκ_u \lesssim 0.33$ and $-0.88 \lesssim \barκ_d \lesssim 0.32$, where $\barκ_q = (m_q/m_b) κ_q$ is a scaling factor that modifies the $q$ quark Yukawa coupling relative to the Standard Model bottom quark Yukawa coupling. The sensitivity may be improved by including additional Higgs decay channels.

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arXiv2016

Probe of new light Higgs bosons from bottomonium chi_{b0} decay

We calculate the branching ratios of chi_{b0} --> tau+ tau- via an s-channel Higgs boson and estimate the sensitivity to this process from Upsilon --> gamma chi_{b0} --> gamma tau+ tau-. We show that future running at the Upsilon(3S) at a very high luminosity super B factory can put significant constraints on the Type-II two-Higgs-doublet model when the discovered 125 GeV Higgs boson is the heavier of the two CP-even scalars.

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arXiv2015

Automatic predictions in the Georgi-Machacek model at next-to-leading order accuracy

We study the phenomenology of the Georgi-Machacek model at next-to-leading order (NLO) in QCD matched to parton shower, using a fully-automated tool chain based on MadGraph5_aMC@NLO and FeynRules. We focus on the production of the fermiophobic custodial fiveplet scalars H_5^0, H_5^+/-, and H_5^++/-- through vector boson fusion (VBF), associated production with a vector boson (V H_5), and scalar pair production (H_5 H_5). For these production mechanisms we compute NLO corrections to production rates as well as to differential distributions. Our results demonstrate that the Standard Model (SM) overall K-factors for such processes cannot in general be directly applied to beyond-the-SM distributions, due both to differences in the scalar electroweak charges and to variation of the K-factors over the differential distributions.

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arXiv2015

Hiding a Higgs width enhancement from off-shell gg (--> h*) --> ZZ measurements

Measurements of the off-shell Higgs boson production cross section in gg (--> h*) --> ZZ have recently been used by the CMS and ATLAS collaborations to indirectly constrain the total width of the Higgs boson. I point out that the interpretation of these measurements as a Higgs width constraint can be invalidated if additional neutral Higgs boson(s) are present with masses below about 350 GeV.

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arXiv2015

All the generalized Georgi-Machacek models

The Georgi-Machacek model adds two SU(2)_L-triplet scalars to the Standard Model in such a way as to preserve custodial SU(2) symmetry. We study the generalizations of the Georgi-Machacek model to SU(2)_L representations larger than triplets. Perturbative unitarity considerations limit the possibilities to models containing only SU(2)_L quartets, quintets, or sextets. These models are phenomenologically interesting because they allow the couplings of the 125 GeV Higgs boson to WW and ZZ to be larger than their values in the Standard Model. We write down the most general custodial SU(2)-preserving scalar potentials for these models and outline their phenomenology. We find that experimental and theoretical constraints on the fermiophobic custodial-fiveplet states present in each of the models lead to absolute upper bounds on the 125 GeV Higgs boson coupling strength to WW and ZZ.

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arXiv2015

LHC constraints on large scalar multiplet models with a $Z_2$ symmetry

We study the LHC search constraints on models that extend the Standard Model (SM) with an inert, complex scalar electroweak multiplet, $Σ$, with isospin T=5/2 (sextet) or T=7/2 (octet) and identical hypercharge to the SM Higgs doublet. Imposing a global $Z_2$ symmetry under which $Σ\to -Σ$, the lightest member of $Σ$ is stable and we require that it be neutral ($ζ^{0,r}$) to avoid cosmological constraints from charged relics. Pair production of scalars by electroweak interactions followed by cascade decays to $ζ^{0,r}$ through W and Z emission produces signatures similar to those of supersymmetric electroweak gauginos, and we constrain the models by recasting a collection of such searches made with data from the 8 TeV run of the LHC. We find that there is no sensitivity from these searches to the compressed spectrum regime, in which the mass splittings between the lightest and heaviest states in $Σ$ are less than about 20 GeV. In the remaining parameter space, we find significant exclusions for $m_{ζ^{0,r}} \sim$ 80-180 GeV in the sextet model, and $m_{ζ^{0,r}} \sim$ 80-120 GeV in the octet model.

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arXiv2015

Two viable large scalar multiplet models with a Z2 symmetry

We study models in which the Higgs sector is extended by a single scalar electroweak multiplet Z with isospin T=5/2 (sextet) or 7/2 (octet) and the same hypercharge as the Standard Model Higgs doublet, in which Z is odd under a global Z2 symmetry. This Z2 symmetry keeps the lightest (neutral) member of Z stable and has interesting implications for phenomenology. We determine the constraints on these models from precision electroweak measurements and Higgs boson decays to two photons. We compute the thermal relic density of the stable member of Z and show that, for masses below 1 TeV, it can make up at most 1% of the dark matter in the universe. We also show that current dark matter direct detection experiments do not constrain the models, but future ton-scale experiments will probe their parameter space.

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arXiv2015

Enhanced hVV couplings in the Georgi-Machacek model and beyond

In this talk I discuss extended Higgs sectors in which the 125 GeV Higgs boson couplings to W and Z bosons can be larger than in the Standard Model. Constraints from perturbative unitarity and the electroweak rho parameter limit the number of possible models to a tractable few. Focusing on generalizations of the Georgi-Machacek model and taking advantage of the custodial symmetry, I show that existing experimental and theoretical constraints can be combined to set an upper limit on the enhancement of the Higgs couplings to W and Z bosons. This talk is based mostly on arXiv:1502.01275.

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arXiv2014

GMCALC: a calculator for the Georgi-Machacek model

The Georgi-Machacek model adds scalar triplets to the Standard Model Higgs sector in such a way as to preserve custodial SU(2) symmetry in the scalar potential. This allows the triplets to have a non-negligible vacuum expectation value while satisfying constraints from the rho parameter. Depending on the parameters, the 125~GeV neutral Higgs particle can have couplings to WW and ZZ larger than in the Standard Model due to mixing with the triplets. The model also contains singly- and doubly-charged Higgs particles that couple to vector boson pairs at tree level (WZ and like-sign WW, respectively). GMCALC is a self-contained FORTRAN program that, given a set of input parameters, calculates the particle spectrum and tree-level couplings in the Georgi-Machacek model, checks theoretical and indirect constraints, and computes the branching ratios and total widths of the scalars. It also generates a param_card.dat file for MadGraph5 to be used with the corresponding FeynRules model implementation.

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arXiv2014

Indirect constraints on the Georgi-Machacek model and implications for Higgs couplings

We update the indirect constraints on the Georgi-Machacek model from $B$-physics and electroweak precision observables, including new constraints from $b \to s γ$ and $B^0_s \to μ^+ μ^-$. We illustrate the effect of these constraints on the couplings of the Standard Model-like Higgs boson by performing scans using the most general scalar potential, subject to vacuum stability and perturbativity constraints. We find that simultaneous enhancements of all the Higgs production cross sections by up to 39\% are still allowed after imposing these constraints. LHC rate measurements on the Higgs pole could be blind to these enhancements if unobserved non-standard Higgs decays are present.

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arXiv2014

The decoupling limit in the Georgi-Machacek model

We study the most general scalar potential of the Georgi-Machacek model, which adds isospin-triplet scalars to the Standard Model (SM) in a way that preserves custodial SU(2) symmetry. We show that this model possesses a decoupling limit, in which the predominantly-triplet states become heavy and degenerate while the couplings of the remaining light neutral scalar approach those of the SM Higgs boson. We find that the SM-like Higgs boson couplings to fermion pairs and gauge boson pairs can deviate from their SM values by corrections as large as $\mathcal{O}(v^2/M_{\rm new}^2)$, where $v$ is the SM Higgs vacuum expectation value and $M_{\rm new}$ is the mass scale of the predominantly-triplet states. In particular, the SM-like Higgs boson couplings to $W$ and $Z$ boson pairs can decouple much more slowly than in two Higgs doublet models, in which they deviate from their SM values like $\mathcal{O}(v^4/M_{\rm new}^4)$. Furthermore, near the decoupling limit the SM-like Higgs boson couplings to $W$ and $Z$ pairs are always larger than their SM values, which cannot occur in two Higgs doublet models. As such, a precision measurement of Higgs couplings to $W$ and $Z$ pairs may provide an effective method of distinguishing the Georgi-Machacek model from two Higgs doublet models. Using numerical scans, we show that the coupling deviations can reach 10% for $M_{\rm new}$ as large as 800~GeV.

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arXiv2013

Scrutinizing h(125) in Two Higgs Doublet Models at the LHC, ILC, and Muon Collider

The discovery at the LHC of a scalar particle with properties that are so far consistent with the SM Higgs boson is one of the most important advances in the history of particle physics. The challenge of future collider experiments is to determine whether its couplings will show deviations from the SM Higgs, as this would indicate new physics at the TeV scale, and also to probe the flavor structure of the Yukawa couplings. As a benchmark alternative to the SM Higgs, we consider a generic two Higgs doublet model (2HDM) and analyze the precision to which the LHC14, an ILC250, 500, 1000 GeV and a 125 GeV Muon Collider (MC) can determine the gauge and Yukawa couplings. We allow for correlations among the couplings. We include the impact of a Higgs total width measurement, indirectly at the LHC and ILC and by a direct scan at the MC. We also discuss pattern relations among the couplings that can test for singlet or doublet Higgs extensions of 2HDMs.

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arXiv2013

Learning what the Higgs boson is mixed with

The Standard Model Higgs boson may be mixed with another scalar that does not couple to fermions. The electroweak quantum numbers of such an additional scalar can be determined by measuring the quartic Higgs-Higgs-vector-vector couplings, which contribute---along with the coveted triple Higgs coupling---to double Higgs production in $e^+e^-$ collisions. We show that simultaneous sensitivity to the quartic Higgs-Higgs-vector-vector coupling and the triple Higgs coupling can be obtained using measurements of the double Higgs production cross section at two different e+e- center-of-mass energies. Kinematic distributions of the two Higgs bosons in the final state could provide additional discriminating power.

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arXiv2013

Discriminators of 2 Higgs Doublets at the LHC14, ILC and MuonCollider(125): A Snowmasss White Paper

The historic LHC discovery of the 125 GeV particle with properties that closely resemble the Standard Model (SM) Higgs boson verifies our understanding of electroweak symmetry breaking, but solidifies the need for a resolution to the hierarchy problem. Many extensions of the SM that address the hierarchy problem contain a non-minimal Higgs sector. Therefore, as a benchmark alternative to the SM Higgs mechanism, we study a general 2 Higgs doublet model (2HDM-G) framework for evaluating future sensitivity to Higgs couplings. We study how well it can be distinguished from the SM Higgs boson by future measurements at LHC14, ILC (250, 500,1000 GeV) and a Muon Collider (125 GeV). Additionally, our study bears on singlet Higgs extensions of two Higgs doublet models through predicted coupling relationships.

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arXiv2013

Constraining models with a large scalar multiplet

Models in which the Higgs sector is extended by a single electroweak scalar multiplet X can possess an accidental global U(1) symmetry at the renormalizable level if X has isospin T greater or equal to 2. We show that all such U(1)-symmetric models are excluded by the interplay of the cosmological relic density of the lightest (neutral) component of X and its direct detection cross section via Z exchange. The sole exception is the T=2 multiplet, whose lightest member decays on a few-day to few-year timescale via a Planck-suppressed dimension-5 operator.

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