arXiv subjects

Sally Dawson

Publications and source records attributed to Sally Dawson.

At least 55 records · Page 3Linked to original sources

arXiv2018

Showcasing HH production: Benchmarks for the (HL-)LHC

Current projections suggest that the LHC will have only limited sensitivity to di-Higgs production in the Standard Model (SM), possibly even after the completion of its high luminosity phase. Multi-Higgs final states play a fundamental role in many extensions of the SM as they are intrinsically sensitive to modifications of the Higgs sector. Therefore, any new observation in multi-Higgs final states could be linked to a range of beyond the SM (BSM) phenomena that are not sufficiently addressed by the SM. Extensions of the Higgs sector typically lead to new phenomenological signatures in multi-Higgs final states that are vastly different from the SM expectation. In this work, we provide a range of signature-driven benchmark points for resonant and non-resonant BSM di-Higgs production that motivate non-SM kinematic correlations and multi-fermion discovery channels. Relying on theoretically well-motivated assumptions, special attention is devoted to the particular case where the presence of new physics will dominantly manifest itself in multi-Higgs final states.

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arXiv2018

Electroweak Corrections to Higgs to $γγ$ and $W^+W^-$ in the SMEFT

Higgs decays to gauge boson pairs are a crucial ingredient in the study of Higgs properties, with the decay $H\rightarrowγγ$ being particularly sensitive to new physics effects. Assuming all potential new physics occurs at energies much above the weak scale, deviations from Standard Model predictions can be parameterized in terms of the coefficients of an effective field theory (SMEFT). When experimental limits on the SMEFT coefficients reach an accuracy of a few percent, predictions must be done beyond the lowest order in the SMEFT in order to match theory and experimental accuracy. This paper completes a program of computing the one-loop electroweak SMEFT corrections to $H\rightarrow VV^\prime$, $V=W^\pm,Z,γ$. The calculation of the real contribution to $H\rightarrow W^+W^-γ$ is performed by mapping two-loop amplitudes to the $3-$ body phase space.

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arXiv2018

SMEFT Corrections to $Z$ Boson Decays

We compute the one-loop corrections to $Z$ decay properties from dimension-6 operators in the Standard Model Effective Field Theory (SMEFT) that contribute also to anomalous 3-gauge boson couplings and examine the relative sensitivity of the two processes to the anomalous couplings. The size of the contributions is of order a few percent, of the same size as Standard Model electroweak corrections. This is part of a program of computing electroweak quantities to one-loop in the SMEFT: these calculations are needed for a future global fit to limit the coefficients of the dimension-six Wilson coefficients consistently at one loop.

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arXiv2017

An NLO QCD effective field theory analysis of $W^+_{}W^-_{}$ production at the LHC including fermionic operators

We study the impact of anomalous gauge boson and fermion couplings on the production of $W^+W^-$ pairs at the LHC. Helicity amplitudes are presented separately to demonstrate the sources of new physics contributions and the impact of QCD and electroweak corrections. The QCD corrections have important effects on the fits to anomalous couplings, in particular when one $W$ boson is longitudinally polarized and the other is transversely polarized. In effective field theory language, we demonstrate that the dimension-6 approximation to constraining new physics effects in $W^+W^-$ pair production fails at $p_T\sim 500-1000$ GeV.

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arXiv2017

Standard Model EFT and Extended Scalar Sectors

One of the simplest extensions of the Standard Model is the inclusion of an additional scalar multiplet, and we consider scalars in the $SU(2)_L$ singlet, triplet, and quartet representations. We examine models with heavy neutral scalars, $m_H\sim 1-2$ TeV, and the matching of the UV complete theories to the low energy effective field theory. We demonstrate the agreement of the kinematic distributions obtained in the singlet models for the gluon fusion of a Higgs pair with the predictions of the effective field theory. The restrictions on the extended scalar sectors due to unitarity and precision electroweak measurements are summarized and lead to highly restricted regions of viable parameter space for the triplet and quartet models.

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arXiv2014

Unravelling an extended quark sector through multiple Higgs production?

In many new physics scenarios, the particle content of the Standard Model is extended and the Higgs couplings are modified, sometimes without affecting single Higgs production. We analyse two models with additional quarks. In these models, we compute double Higgs production from gluon fusion exactly at leading-order, and present analytical results in the heavy-quark mass ap- proximation. The experimental bounds from precision electroweak measurements and from the measured rate of single Higgs production combine to give significant restrictions for the allowed deviation of the double Higgs production rate from the Standard Model prediction as well as on the branching ratio for the Higgs decay into photons. The two models analysed eventually present a similar Higgs phenomenology as the Standard Model. We connect this result to the magnitude of the dimension six operators contributing to the gluon-fusion Higgs production.

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arXiv2014

Yukawa Corrections to Higgs Production in Top Partner Models

Higgs production from gluon fusion is sensitive to the properties of heavy colored fermions and to the Yukawa couplings, (Y_F M_F)/v, of these particles to the Higgs boson. We compute the two--loop, ${\cal O}((Y_F M_F)^3/v^3)$ contributions of new high mass fermions to Higgs production. In the Standard Model, these contributions are part of the well-known electroweak corrections and are negligible. However, in models with TeV scale fermions, such as top partner or composite models, Yukawa corrections are enhanced by effects of ${\cal O}((Y_F M_F)^3/v^3)$ and are potentially significant due to the large mass of the new quarks. We examine the size of these top partner Yukawa corrections to Higgs production for parameter choices which are allowed by precision electroweak constraints.

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arXiv2013

CP violating anomalous couplings in $Wγ$ and $Zγ$ production at the LHC

The ATLAS and CMS collaborations have recently published new limits on CP conserving anomalous couplings from the $Wγ$ and $Zγ$ production processes. We study the corresponding limits that can be placed on the CP violating anomalous couplings $κ_tilde_γ$ and $h^1_{γ, Z}$ at the LHC. We find that the process $pp --> Wγ$ at 14 TeV can place the 95% CL limit $|κ_tilde_γ| <~ 0.05$ with 10 fb^{-1} which is comparable to the existing LHC bound on the CP conserving anomalous couplings $κ_γ$. Similarly, the process $pp --> Zγ$ can place the 95% CL limits $|h^1_γ| <~ 20$ and $|h^1_Z| <~ 40$ respectively. None of these limits is derived from a truly CP-odd observable so that it is not possible to separate the effects of the CP violating anomalous couplings from the rest.

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arXiv2009

Hiding the Higgs Boson with Multiple Scalars

We consider models with multiple Higgs scalar gauge singlets and the resulting restrictions on the parameters from precision electroweak measurements. In these models, the scalar singlets mix with the SU(2) Higgs doublet, potentially leading to reduced couplings of the scalars to fermions and gauge bosons relative to the Standard Model Higgs boson couplings. Such models can make the Higgs sector difficult to explore at the LHC. We emphasize the new physics resulting from the addition of at least two scalar Higgs singlets.

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arXiv2008

One-loop Corrections to the S Parameter in the Four-site Model

We compute the leading chiral-logarithmic corrections to the S parameter in the four-site Higgsless model. In addition to the usual electroweak gauge bosons of the Standard Model, this model contains two sets of heavy charged and neutral gauge bosons. In the continuum limit, the latter gauge bosons can be identified with the first excited Kaluza-Klein states of the W^\pm and Z bosons of a warped extra-dimensional model with an SU(2)_L \times SU(2)_R \times U(1)_X bulk gauge symmetry. We consider delocalized fermions and show that the delocalization parameter must be considerably tuned from its tree-level ideal value in order to reconcile experimental constraints with the one-loop results. Hence, the delocalization of fermions does not solve the problem of large contributions to the S parameter in this class of theories and significant contributions to S can potentially occur at one-loop.

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arXiv2008

Higgs Triplets, Decoupling, and Precision Measurements

Electroweak precision data has been extensively used to constrain models containing physics beyond that of the Standard Model. When the model contains Higgs scalars in representations other than SU(2) singlets or doublets, and hence rho not equal to one at tree level, a correct renormalization scheme requires more inputs than the three needed for the Standard Model. We discuss the connection between the renormalization of models with Higgs triplets and the decoupling properties of the models as the mass scale for the scalar triplet field becomes much larger than the electroweak scale. The requirements of perturbativity of the couplings and agreement with electroweak data place strong restrictions on models with Higgs triplets. Our results have important implications for Little Higgs type models and other models with rho not equal to one at tree level.

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arXiv2007

SUSY QCD Corrections to Higgs Pair Production from Bottom Quark Fusion

We present a complete next-to-leading order (NLO) calculation for the total cross section for inclusive Higgs pair production via bottom-quark fusion at the CERN Large Hadron Collider (LHC) in the minimal supersymmetric standard model (MSSM) and the minimal supergravity model (mSUGRA). We emphasize the contributions of squark and gluino loops (SQCD) and the decoupling properties of our results for heavy squark and gluino masses. The enhanced couplings of the b quark to the Higgs bosons in supersymmetric models with large tanb yield large NLO SQCD corrections in some regions of parameter space.

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arXiv2006

QCD Corrections to Higgs Pair Production in Bottom Quark Fusion

We present a complete next-to-leading order (NLO) calculation for the total cross section of inclusive Higgs pair production via bottom-quark fusion ($b\bar{b} \to hh$) at the CERN Large Hadron Collider (LHC) in the Standard Model. The NLO QCD corrections lead to less dependence on the renormalization scale ($μ_R$) and the factorization scale ($μ_F$) than the leading-order (LO) cross section, and they significantly increase the LO cross section. The rate for inclusive Higgs pair production is small in the Standard Model, but can be large in models with enhanced couplings of the $b$ quark to the Higgs bosons.

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arXiv2006

Constraining New Models with Precision Electroweak Data

Electroweak precision data have been extensively used to constrain models containing physics beyond that of the Standard Model. When the model contains Higgs scalars in representations other than singlets or doublets, and hence rho not equal to 1 at tree level, a correct renormalization scheme requires more inputs than the three commonly used for the Standard Model case. In such cases, the one loop electroweak results cannot be split into a Standard Model contribution plus a piece which vanishes as the scale of new physics becomes much larger than M_W. We illustrate our results by presenting the dependence of M_W on the top quark mass in a model with a Higgs triplet and in the SU(2)_L x SU(2)_R left-right symmetric model. In these models, the allowed range for the lightest neutral Higgs mass can be as large as a few TeV.

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arXiv2006

Higgs Triplets and Limits from Precision Measurements

In this letter, we present our results on a global fit to precision electroweak data in a Higgs triplet model. In models with a triplet Higgs boson, a consistent renormalization scheme differs from that of the Standard Model and the global fit shows that a light Higgs boson with mass of 100-200 GeV is preferred. Triplet Higgs bosons arise in many extensions of the Standard Model, including the left-right model and the Little Higgs models. Our result demonstrates the importance of the scalar loops when there is a large mass splitting between the heavy scalars. It also indicates the significance of the global fit.

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arXiv2004

The Littlest Higgs Model and One-Loop Electroweak Precision Constraints

We present in this talk the one-loop electroweak precision constraints in the Littlest Higgs model, including the logarithmically enhanced contributions from both fermion and scalar loops. We find the one-loop contributions are comparable to the tree level corrections in some regions of parameter space. A low cutoff scale is allowed for a non-zero triplet VEV. Constraints on various other parameters in the model are also discussed. The role of triplet scalars in constructing a consistent renormalization scheme is emphasized.

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arXiv2004

One-loop Radiative Corrections to the rho Parameter in the Littlest Higgs Model

We perform a one-loop analysis of the rho parameter in the Littlest Higgs model, including the logarithmically enhanced contributions from both fermion and scalar loops. We find the one-loop contributions are comparable to the tree level corrections in some regions of parameter space. The fermion loop contribution dominates in the low cutoff scale f region. On the other hand, the scalar loop contribution dominates in the high cutoff scale f region and it grows with the cutoff scale f. This in turn implies an upper bound on the cutoff scale. A low cutoff scale is allowed for a non-zero triplet VEV. Constraints on various other parameters in the model are also discussed. The role of triplet scalars in constructing a consistent renormalization scheme is emphasized.

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arXiv2004

Physics Opportunities with a TeV Linear Collider

We discuss the physics motivations for building a 500 - 1 TeV electron-positron linear collider. The state of the art collider technologies and the physics-driven machine parameters are discussed. A survey of some of the phenomena well suited to study at a linear collider are described, including Higgs bosons, supersymmetry, other extensions to the Standard Model, and cosmology.

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