arXiv subjects

Heather E. Logan

Publications and source records attributed to Heather E. Logan.

At least 73 records · Page 4Linked to original sources

arXiv2009

Charged Higgs phenomenology in the lepton-specific two Higgs doublet model

We study the "lepton-specific" two Higgs doublet model, in which one doublet Phi_l gives mass to charged leptons and the other Phi_q gives mass to both up- and down-type quarks. We examine the existing experimental constraints on the charged Higgs boson mass and the parameter tan(beta) \equiv / . The most stringent constraints come from LEP-II direct searches and lepton flavour universality in tau decays. The former yields M_H+ \geq 92.0 GeV; the latter yields two allowed regions, 0.61 tan(beta) GeV \leq M_H+ \leq 0.73 tan(beta) GeV or M_H+ \geq 1.4 tan(beta) GeV, and excludes parameter regions beyond the LEP-II bound for tan(beta) \gtrsim 65. We present the charged Higgs decay branching fractions and discuss prospects for charged Higgs discovery at the LHC in this model.

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arXiv2009

Higgs ID at the LHC

We make a complete catalog of extended Higgs sectors involving SU(2)_L doublets and singlets, subject to natural flavor conservation. In each case we present the couplings of a light neutral CP-even Higgs state h in terms of the model parameters, and identify which models are distinguishable in principle based on this information. We also give explicit expressions for the model parameters in terms of h couplings and exhibit the behaviors of the couplings in the limit where the deviations from the Standard Model Higgs couplings are small. Finally we discuss prospects for differentiation of extended Higgs models based on measurements at the LHC and ILC and identify the regions in which these experiments could detect deviations from the SM Higgs predictions.

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arXiv2009

Experimental constraints on nMSSM and implications on its phenomenology

We examine various direct and indirect experimental constraints on the nearly minimal supersymmetric standard model (nMSSM) and obtain the following observations: (i) Current experiments stringently constrain the parameter space, setting a range of 1-37 GeV for the lightest neutralino (LSP), 30-140 GeV (1-250 GeV) for the lightest CP-even (CP-odd) Higgs boson, and 1.5-10 for \tanβ; (ii) To account for the dark matter relic density, besides the s-channel exchange of a Z-boson, the s-channel exchange of a light A_1 (the lightest CP-odd Higgs boson) can also play an important role in LSP annihilation. Compared with the Z-exchange annihilation channel, the A_1 exchange channel is more favored by muon g-2 data and allows much broader regions for the parameters; (iii) In a large part of the allowed parameter space the SM-like Higgs boson may dominantly decay to LSP pair or A_1 pair and the conventional visible decays (e.g. into bottom quarks) are severely suppressed.

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arXiv2007

Physics impact of ILC Higgs coupling measurements: the effect of theory uncertainties

We study the effect of theoretical and parametric uncertainties on the ability of future Higgs coupling measurements at the International Linear Collider (ILC) to reveal deviations from the Standard Model (SM). To quantify the impact of these uncertainties we plot Delta chi^2 = 25 contours for the deviations between the SM Higgs couplings and the light Higgs couplings in the mh-max benchmark scenario of the Minimal Supersymmetric Standard Model (MSSM). We consider the theoretical uncertainties in the SM Higgs decay partial widths and production cross section and the parametric uncertainties in the bottom and charm masses and the strong coupling alpha_s. We find that the impact of the theoretical and parametric uncertainties is moderate in the first phase of ILC data-taking (500 fb-1 at 350 GeV centre-of-mass energy), reducing the "reach" in the CP-odd MSSM Higgs mass M_A by about 10% to ~500 GeV, while in the second phase (1000 fb-1 at 1000 GeV) these uncertainties are larger than the experimental uncertainties and reduce the reach in M_A by about a factor of two, from ~1200 down to ~600 GeV. The bulk of the effect comes from the parametric uncertainties in m_b and alpha_s, followed by the theoretical uncertainty in Gamma_b.

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arXiv2006

QCD corrections to neutralino annihilation

We report on two recent calculations of QCD corrections to neutralino annihilation cross sections: (1) the next-to-leading order corrections to chi chi -> g g, and (2) the contribution to the cross section for chi chi -> q qbar g arising from interference between the tree-level and loop-induced processes.

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arXiv2006

Neutralino annihilation to q qbar g

We compute the cross section for chi chi -> q qbar g at order alpha_s^2/M_{squark}^6 arising from interference between the tree-level and loop-induced processes. This interference term is the same order in alpha_s as chi chi -> gg; for mass degenerate squarks M_{squark_R} = M_{squark_L} = M_{squark} we find v_{rel} sigma_{int} = [-2 m_{chi}^2 / 3 M_{squark}^2] v_{rel} sigma(chi chi -> gg).

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arXiv2006

Quantum corrections to the MSSM h^0 b b-bar vertex: Decoupling limit

We consider the leading one-loop Yukawa-coupling corrections to the h^0 b b-bar coupling at O(m_t^2)in the MSSM in the decoupling limit. The decoupling behavior of the corrections from the various MSSM sectors is analyzed in the case of having some or all of the supersymmetric mass parameters and/or the CP-odd Higgs mass large as compared to the electroweak scale.

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arXiv2006

Smoking-gun signatures of little Higgs models

Little Higgs models predict new gauge bosons, fermions and scalars at the TeV scale that stabilize the Higgs mass against quadratically divergent one-loop radiative corrections. We categorize the many little Higgs models into two classes based on the structure of the extended electroweak gauge group and examine the experimental signatures that identify the little Higgs mechanism in addition to those that identify the particular little Higgs model. We find that by examining the properties of the new heavy fermion(s) at the LHC, one can distinguish the structure of the top quark mass generation mechanism and test the little Higgs mechanism in the top sector. Similarly, by studying the couplings of the new gauge bosons to the light Higgs boson and to the Standard Model fermions, one can confirm the little Higgs mechanism and determine the structure of the extended electroweak gauge group.

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arXiv2005

Neutralino annihilation beyond leading order

High-precision measurements of the relic dark matter density and the calculation of dark matter annihilation branching fractions in the sun or the galactic halo today motivate the computation of the neutralino annihilation cross section beyond leading order. We consider neutralino annihilation via squark exchange and parameterize the effective annihilation vertex as a dimension-six operator suppressed by two powers of the squark mass and related to the divergence of the axial vector current of the final-state quarks. Since the axial vector current is conserved at tree level in the limit of massless quarks, this dimension-six operator contains a suppression by the quark mass. The quark mass suppression can be lifted in two ways: (1) by corrections to the dimension-six operator involving the anomalous triangle diagram, and (2) by going to dimension-eight. We address the first of these possibilities by evaluating the anomalous triangle diagram, which contributes to neutralino annihilation to gluon pairs. We relate the triangle diagram via the anomaly equation to the decay of a pseudoscalar into two gluons and use the Adler-Bardeen theorem to extract the next-to-leading order (NLO) QCD corrections to chi chi --> gg from the known corrections to pseudoscalar decay. The strong dependence of the dominant chi chi --> q qbar cross section on the relative velocity of the neutralinos makes these NLO corrections unimportant at chi decoupling but significant today.

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arXiv2005

Neutrino masses and lepton-number violation in the Littlest Higgs scenario

We investigate the sources of neutrino mass generation in Little Higgs theories, by confining ourselves to the "Littlest Higgs" scenario. Our conclusion is that the most satisfactory way of incorporating neutrino masses is to include a lepton-number violating interaction between the scalar triplet and lepton doublets. The tree-level neutrino masses generated by the vacuum expectation value of the triplet are found to dominate over contributions from dimension-five operators so long as no additional large lepton-number violating physics exists at the cut-off scale of the effective theory. We also calculate the various decay branching ratios of the charged and neutral scalar triplet states, in regions of the parameter space consistent with the observed neutrino masses, hoping to search for signals of lepton-number violating interactions in collider experiments.

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arXiv2005

Discovering an Invisibly Decaying Higgs at Hadron Colliders

A Higgs boson lighter than 2 m_W that decays mostly into invisible channels (e.g., dark matter particles) is theoretically well-motivated. We study the prospects for discovery of such an invisible Higgs, h_inv, at the LHC and the Tevatron in three production modes: (1) in association with a Z, (2) through Weak Boson Fusion (WBF), and (3) accompanied by a jet. In the Z+h_inv channel, we show that the LHC can yield a discovery signal above 5 sigma with 10 fb-1 of integrated luminosity for a Higgs mass of 120 GeV. With 30 fb-1 the discovery reach extends up to a Higgs mass of 160 GeV. We also study the extraction of the h_inv mass from production cross sections at the LHC, and find that combining WBF and Z+h_inv allows a relatively model-independent determination of the h_inv mass with an uncertainty of 35-50 GeV (15-20 GeV) with 10 (100) fb-1. At the Tevatron, a 3 sigma observation of a 120 GeV h_inv in any single channel is not possible with less than 12 fb-1 per detector. However, we show that combining the signal from WBF with the previously-studied Z+h_inv channel allows a 3 sigma observation of h_inv with 7 fb-1 per detector. Because of overwhelming irreducible backgrounds, h_inv+j is not a useful search channel at either the Tevatron or the LHC, despite the larger production rate.

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arXiv2004

The Littlest Higgs boson at a photon collider

We calculate the corrections to the partial widths of the light Higgs boson in the Littlest Higgs model due to effects of the TeV-scale physics. We focus on the loop-induced Higgs coupling to photon pairs, which is especially sensitive to the effects of new particles running in the loop. This coupling can be probed with high precision at a photon collider in the process gamma gamma -> H -> b bbar for a light Higgs boson with mass 115 GeV <= MH <~ 140 GeV. Using future LHC measurements of the parameters of the Littlest Higgs model, one can calculate a prediction for this process, which will serve as a test of the model and as a probe for a strongly-coupled UV completion at the 10 TeV scale. We outline the prospects for measuring these parameters with sufficient precision to match the expected experimental uncertainty on gamma gamma -> H -> b bbar.

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arXiv2004

Loop induced decays of the Little Higgs: H --> gg, gamma gamma

We analyze the loop induced decays of the Higgs boson into pairs of gluons and photons in the Littlest Higgs model. We find that the deviation of the partial widths for these decays relative to their Standard Model values scales with 1/f^2, where f ~ TeV is the mass scale of the new heavy particles in the model. For f = 1 TeV, Gamma(H -> gg) is reduced by 6-10% and Gamma(H -> gamma gamma) is reduced by 5-7% compared to their Standard Model values. While the LHC and a linear e+e- collider would be sensitive to these deviations only for relatively low values of f <~ 650 GeV, a photon collider could probe the deviation in Gamma(H -> gamma gamma) up to f <~ 1.1 (0.7) TeV at the 2 (5) sigma level.

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arXiv2004

Extracting Higgs Boson Couplings from LHC Data

We show how LHC Higgs boson production and decay data can be used to extract the gauge and fermion couplings of the Higgs boson. Incomplete input data leads to parameter degeneracies, which can be lifted by imposing theoretical assumptions. We show how successive theoretical assumptions affect the parameter extraction, starting with a general multi-Higgs doublet model and finishing with specific supersymmetric scenarios.

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arXiv2004

The Little Higgs boson at a photon collider

We calculate the partial widths of the light Higgs boson in the Littlest Higgs model. The loop-induced Higgs coupling to photon pairs, which is especially sensitive to effects of the new TeV-scale particles running in the loop, can be probed with high precision at a photon collider in the process gamma gamma --> H --> b bbar. Using the parameters of the Littlest Higgs model measured at the LHC one can calculate a prediction for this process, which will serve as a test of the model and as a probe for a strongly-coupled UV completion at the 10 TeV scale.

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arXiv2004

Phenomenology of the Little Higgs Model

We study the low energy phenomenology of the little Higgs model. We first discuss the linearized effective theory of the "littlest Higgs model" and study the low energy constraints on the model parameters. We identify sources of the corrections to low energy observables, discuss model-dependent arbitrariness, and outline some possible directions of extensions of the model in order to evade the precision electroweak constraints. We then explore the characteristic signatures to test the model in the current and future collider experiments. We find that the LHC has great potential to discover the new SU(2) gauge bosons and the possible new U(1) gauge boson to the multi-TeV mass scale. Other states such as the colored vector-like quark T and doubly-charged Higgs boson Phi^{++} may also provide interesting signals. At a linear collider, precision measurements on the triple gauge boson couplings could be sensitive to the new physics scale of a few TeV. We provide a comprehensive list of the linearized interactions and vertices for the littlest Higgs model in the appendices.

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arXiv2004

e+e- --> H+ e- nubar in the two-Higgs-doublet model

We calculate the cross section for e+e- --> H+ e- nubar in the two-Higgs-doublet model from one-loop diagrams involving top and bottom quarks. This process offers the possibility of producing the charged Higgs boson at the e+e- collider when its mass is more than half the center-of-mass energy, so that charged Higgs pair production is kinematically forbidden. The cross section receives contributions from both s-channel and t-channel processes; the s-channel contribution dominates for center-of-mass energies of 1 TeV and below. About 80% of the s-channel contribution comes from the resonant process e+e- --> H+ W-, with W- --> e- nubar. The cross section is generally small, below 0.01 fb for tan beta > 2, and falls with increasing tan beta.

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arXiv2003

Little Higgs Phenomenology

Recently a new class of models has emerged that addresses the naturalness problem of a light Higgs boson. In these ''little Higgs'' models, the Standard Model Higgs boson is a pseudo-Nambu-Goldstone boson of an approximate global symmetry. The Higgs boson acquires mass radiatively only through ''collective breaking'' of the global symmetry, so that more than one interaction is required to give the Higgs a mass. This protects the Higgs mass from receiving quadratically divergent radiative corrections at one-loop. These models contain new vector bosons, fermions and scalars at the TeV scale that cancel the quadratic divergences in the Higgs mass due to the Standard Model gauge, top quark, and Higgs boson loops. In this talk I review the phenomenology of the little Higgs models, focusing on collider signatures and electroweak precision constraints.

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