Howard E. Haber - Santa Cruz Institute for Particle Physics

Howard E. Haber
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Howard E. Haber
Santa Cruz Institute for Particle Physics

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High Energy Physics - Phenomenology (50)
High Energy Physics - Experiment (8)
High Energy Physics - Theory (5)
High Energy Astrophysical Phenomena (1)

Publications Authored By Howard E. Haber

The most general two-Higgs doublet model (2HDM) includes potentially large sources of flavor changing neutral currents (FCNCs) that must be suppressed in order to achieve a phenomenologically viable model. The flavor alignment ansatz postulates that all Yukawa coupling matrices are diagonal when expressed in the basis of mass-eigenstate fermion fields, in which case tree-level Higgs mediated FCNCs are eliminated. In this work, we explore models with the flavor alignment condition imposed at a very high energy scale, which results in the generation of Higgs-mediated FCNCs via renormalization group running from the high energy scale to the electroweak scale. Read More

We examine some of the theoretical and phenomenological implications of the Higgs boson discovery and discuss what these imply for future Higgs studies at the LHC and future colliders. In particular, one of the outstanding unanswered questions is whether additional scalars beyond the observed Higgs boson are present in the spectrum of fundamental particles. Any theory of a non-minimal Higgs sector must possess a scalar state whose properties are approximately those of the Standard Model Higgs boson. Read More

We perform a parameter scan of the phenomenological Minimal Supersymmetric Standard Model (pMSSM) with eight parameters taking into account the experimental Higgs boson results from Run I of the LHC and further low-energy observables. We investigate various MSSM interpretations of the Higgs signal at 125 GeV. First, we consider the case where the light CP-even Higgs boson of the MSSM is identified with the discovered Higgs boson. Read More

We study some aspects of perturbation theory in $N=1$ supersymmetric abelian gauge theories with massive charged matter. In general gauges, infrared (IR) divergences and nonlocal behavior arise in 1PI diagrams, associated with a $1/k^4$ term in the propagator for the vector superfield. We examine this structure in supersymmetric QED. Read More

It is possible that the electroweak scale is low due to the fine-tuning of microscopic parameters, which can result from selection effects. The experimental discovery of new light fundamental scalars other than the Standard Model Higgs boson would seem to disfavor this possibility, since generically such states imply parametrically worse fine-tuning with no compelling connection to selection effects. We discuss counterexamples where the Higgs boson is light because of fine-tuning, and a second scalar doublet is light because a discrete symmetry relates its mass to the mass of the Standard Model Higgs boson. Read More

In the alignment limit of a multi-doublet Higgs sector, one of the Higgs mass eigenstates aligns in field space with the direction of the scalar field vacuum expectation values, and its couplings approach those of the Standard Model (SM) Higgs boson. We consider CP-conserving Two-Higgs-Doublet Models (2HDMs) of Type I and Type II near the alignment limit in which the heavier of the two CP-even Higgs bosons, $H$, is the SM-like state observed with a mass of 125 GeV, and the couplings of $H$ to gauge bosons approach those of the SM. We review the theoretical structure and analyze the phenomenological implications of this particular realization of the alignment limit, where decoupling of the extra states cannot occur given that the lighter CP-even state $h$ must, by definition, have a mass below 125 GeV. Read More

The Next-to-Minimal Supersymmetric extension of the Standard Model (NMSSM) with a Higgs boson of mass 125 GeV can be compatible with stop masses of order of the electroweak scale, thereby reducing the degree of fine-tuning necessary to achieve electroweak symmetry breaking. Moreover, in an attractive region of the NMSSM parameter space, corresponding to the "alignment limit" in which one of the neutral Higgs fields lies approximately in the same direction in field space as the doublet Higgs vacuum expectation value, the observed Higgs boson is predicted to have Standard-Model-like properties. We derive analytical expressions for the alignment conditions and show that they point toward a more natural region of parameter space for electroweak symmetry breaking, while allowing for perturbativity of the theory up to the Planck scale. Read More

We introduce a strategy to study the parameter space of the general, CP-conserving, two-Higgs-doublet Model (2HDM) with a softly broken Z_2-symmetry by means of a new "hybrid" basis. In this basis the input parameters are the measured values of the mass of the observed Standard Model (SM)-like Higgs boson and its coupling strength to vector boson pairs, the mass of the second CP-even Higgs boson, the ratio of neutral Higgs vacuum expectation values, and three additional dimensionless parameters. Using the hybrid basis, we present numerical scans of the 2HDM parameter space where we survey available parameter regions and analyze model constraints. Read More

In the alignment limit of a multi-doublet Higgs sector, one of the Higgs mass eigenstates aligns with the direction of the scalar field vacuum expectation values, and its couplings approach those of the Standard Model (SM) Higgs boson. We consider CP-conserving Two-Higgs-Doublet Models (2HDMs) of Type I and Type II near the alignment limit in which the lighter of the two CP-even Higgs bosons, $h$, is the SM-like state observed at 125 GeV. In particular, we focus on the 2HDM parameter regime where the coupling of $h$ to gauge bosons approaches that of the SM. Read More

We examine the constraints on the two Higgs doublet model (2HDM) due to the stability of the scalar potential and absence of Landau poles at energy scales below the Planck scale. We employ the most general 2HDM that incorporates an approximately Standard Model (SM) Higgs boson with a flavor aligned Yukawa sector to eliminate potential tree-level Higgs-mediated flavor changing neutral currents. Using basis independent techniques, we exhibit robust regimes of the 2HDM parameter space with a 125 GeV SM-like Higgs boson that is stable and perturbative up to the Planck scale. Read More

Precision measurements of the Higgs boson properties at the LHC provide relevant constraints on possible weak-scale extensions of the Standard Model (SM). In the context of the Minimal Supersymmetric Standard Model (MSSM) these constraints seem to suggest that all the additional, non-SM-like Higgs bosons should be heavy, with masses larger than about 400 GeV. This article shows that such results do not hold when the theory approaches the conditions for "alignment independent of decoupling", where the lightest CP-even Higgs boson has SM-like tree-level couplings to fermions and gauge bosons, independently of the non-standard Higgs boson masses. Read More

A sign change in the Higgs couplings to fermions and massive gauge bosons is still allowed in the framework of two-Higgs doublet models (2HDM). In this work we discuss the possible sign changes in the Higgs couplings to fermions and gauge bosons, while reviewing the status of the 8-parameter CP-conserving 2HDM after the Large Hadron Collider 8 TeV run. Read More

We confront the most common CP-conserving 2HDM with the LHC data analysed so far while taking into account all previously available experimental data. A special allowed corner of the parameter space is analysed - the so-called wrong-sign scenario where the Higgs coupling to down-type quarks changes sign relative to the Standard Model while the coupling to the massive vector bosons does not. Read More

We consider the two-Higgs-doublet model as a framework in which to evaluate the viability of scenarios in which the sign of the coupling of the observed Higgs boson to down-type fermions (in particular, $b$-quark pairs) is opposite to that of the Standard Model (SM), while at the same time all other tree-level couplings are close to the SM values. We show that, whereas such a scenario is consistent with current LHC observations, both future running at the LHC and a future $e^+ e^-$ linear collider could determine the sign of the Higgs coupling to $b$-quark pairs. Discrimination is possible for two reasons. Read More

The Higgs data analyzed by the ATLAS and CMS Collaborations suggest that the scalar state discovered in 2012 is a Standard Model (SM)--like Higgs boson. Nevertheless, there is still significant room for Higgs physics beyond the Standard Model. Many approaches to electroweak symmetry breaking possess a decoupling limit in which the properties of the lightest CP-even Higgs scalar approach those of the SM Higgs boson. Read More

Recently, it has been argued that in the supersymmetric extension of the seesaw-extended Standard Model, heavy right-handed neutrinos and sneutrinos may give corrections as large as a few GeV to the mass of the lightest neutral CP-even Higgs boson, even if the soft supersymmetry-breaking parameters are of order the electroweak scale. The presence of such large corrections would render precise Higgs masses incalculable from measurable low-energy parameters. We show that this is not the case: decoupling is preserved in the appropriate sense and right-handed (s)neutrinos, if they exist, have negligible impact on the physical Higgs masses. Read More

The analysis of the Higgs boson data by the ATLAS and CMS Collaborations appears to exhibit an excess of h --> gamma\gamma events above the Standard Model (SM) expectations; whereas no significant excess is observed in h --> ZZ* --> {four lepton} events, albeit with large statistical uncertainty due to the small data sample. These results (assuming they persist with further data) could be explained by a pair of nearly mass-degenerate scalars, one of which is a SM-like Higgs boson and the other is a scalar with suppressed couplings to W+W- and ZZ. In the two Higgs doublet model, the observed \gamma\gamma and ZZ* --> {four lepton} data can be reproduced by an approximately degenerate CP-even (h) and CP-odd (A) Higgs boson for values of \sin(\beta-\alpha) near unity and 0. Read More

The precision measurements of Higgs boson observables will be critical in the interpretation of the dynamics responsible for electroweak symmetry breaking. The capabilities of the ILC and CLIC for precision Higgs studies are well documented. In this talk, a theoretical framework is presented for interpreting phenomena that can arise in the two-Higgs-doublet model (2HDM). Read More

We formulate the necessary conditions for a scalar potential to exhibit spontaneous CP violation. Associated with each complex scalar field is a U(1) symmetry that may be explicitly broken by terms in the scalar potential (called spurions). In order for CP-odd phases in the vacuum to be physical, these phases must be related to spontaneously broken U(1) generators that are also explicitly broken by a sufficient number of inequivalent spurions. Read More

In the Standard Model, custodial symmetry is violated by the hypercharge U(1) gauge interactions and the Yukawa couplings, while being preserved by the Higgs scalar potential. In the two-Higgs doublet model (2HDM), the generic scalar potential introduces new sources of custodial symmetry breaking. We obtain a basis-independent expression for the constraints that impose custodial symmetry on the 2HDM scalar potential. Read More

Discovering the Higgs boson is one of the primary goals of both the Tevatron and the Large Hadron Collider (LHC). The present status of the Higgs search is reviewed and future prospects for discovery at the Tevatron and LHC are considered. This talk focuses primarily on the Higgs boson of the Standard Model and its minimal supersymmetric extension. Read More

Supersymmetric monojets may be produced at the Large Hadron Collider by the process qg -> squark neutralino_1 -> q neutralino_1 neutralino_1, leading to a jet recoiling against missing transverse momentum. We discuss the feasibility and utility of the supersymmetric monojet signal. In particular, we examine the possible precision with which one can ascertain the neutralino_1-squark-quark coupling via the rate for monojet events. Read More

In the two-Higgs-doublet model (THDM), generalized-CP transformations (phi_i--> X_{ij} phi_j^* where X is unitary) and unitary Higgs-family transformations (phi_i--> U_{ij} phi_j) have recently been examined in a series of papers. In terms of gauge-invariant bilinear functions of the Higgs fields phi_i, the Higgs-family transformations and the generalized-CP transformations possess a simple geometric description. Namely, these transformations correspond in the space of scalar-field bilinears to proper and improper rotations, respectively. Read More

The minimal supersymmetric standard model involves a rather restrictive Higgs potential with two Higgs fields. Recently, the full set of classes of symmetries allowed in the most general two Higgs doublet model was identified; these classes do not include the supersymmetric limit as a particular class. Thus, a physically meaningful definition of the supersymmetric limit must involve the interaction of the Higgs sector with other sectors of the theory. Read More

Many models of meta-stable supersymmetry (SUSY) breaking lead to a very light scalar pseudo-Nambu Goldstone boson (PNGB), P, associated with spontaneous breakdown of a baryon number like symmetry in the hidden sector. Current particle physics data provide no useful constraints on the existence of P. For example, the predicted decay rates for both K --> pi + P, b--> s + P and Upsilon --> photon + P are many orders of magnitude below the present experimental bounds. Read More

We consider the impact of imposing generalized CP symmetries on the Higgs sector of the two-Higgs-doublet model, and identify three classes of symmetries. Two of these classes constrain the scalar potential parameters to an exceptional region of parameter space which respects either a Z_2 discrete flavor symmetry or a U(1) symmetry. We exhibit a basis-invariant quantity that distinguishes between these two possible symmetries. Read More

Two-component spinors are the basic ingredients for describing fermions in quantum field theory in four space-time dimensions. We develop and review the techniques of the two-component spinor formalism and provide a complete set of Feynman rules for fermions using two-component spinor notation. These rules are suitable for practical calculations of cross-sections, decay rates, and radiative corrections in the Standard Model and its extensions, including supersymmetry, and many explicit examples are provided. Read More

In the minimal supersymmetric extension of the Standard Model (MSSM), if the two Higgs doublets are lighter than some subset of the superpartners of the Standard Model particles, then it is possible to integrate out the heavy states to obtain an effective broken-supersymmetric low-energy Lagrangian. This Lagrangian can contain dimension-four gauge invariant Higgs interactions that violate supersymmetry (SUSY). The "wrong-Higgs" Yukawa couplings generated by one-loop radiative corrections are a well known example of this phenomenon. Read More

The seesaw-extended MSSM provides a framework in which the observed light neutrino masses and mixing angles can be generated in the context of a natural theory for the TeV-scale. Sneutrino-mixing phenomena provide valuable tools for connecting the physics of neutrinos and supersymmetry. We examine the theoretical structure of the seesaw-extended MSSM, retaining the full complexity of three generations of neutrinos and sneutrinos. Read More

In the most general two-Higgs-doublet model (2HDM), there is no distinction between the two complex hypercharge-one SU(2) doublet scalar fields, Phi_a (a=1,2). Thus, any two orthonormal linear combinations of these two fields can serve as a basis for the Lagrangian. All physical observables of the model must therefore be basis-independent. Read More

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. Read More

The most general Higgs potential of the two-Higgs-doublet model (2HDM) contains three squared-mass parameters and seven quartic self-coupling parameters. Among these, one squared-mass parameter and three quartic coupling parameters are potentially complex. The Higgs potential explicitly violates CP symmetry if and only if no choice of basis exists in the two-dimensional Higgs ``flavor'' space in which all the Higgs potential parameters are real. Read More

In the most general two-Higgs-doublet model (2HDM), unitary transformations between the two Higgs fields do not change the functional form of the Lagrangian. All physical observables of the model must therefore be independent of such transformations (i.e. Read More

A brief overview is given of the theory of Higgs bosons and electroweak symmetry breaking that is relevant for the Higgs physics program at the Linear Collider. Read More

We review the theory of Higgs bosons, with emphasis on the Higgs scalars of the Standard Model and its non-supersymmetric and supersymmetric extensions. After surveying the expected knowledge of Higgs boson physics after the Tevatron and LHC experimental programs, we examine in detail expectations for precision Higgs measurements at a future e+e- linear collider (LC). A comprehensive phenomenological profile can be assembled from LC Higgs studies (both in e+e- and gamma-gamma collisions). Read More

A short review of the theory and phenomenology of Higgs bosons is given, with focus on the Standard Model (SM) and the minimal supersymmetric extension of the Standard Model (MSSM). The potential for Higgs boson discovery at the Tevatron and LHC, and precision Higgs studies at the LHC and a future e+e- linear collider are briefly surveyed. The phenomenological challenge of the approach to the decoupling limit, where the properties of the lightest CP-even Higgs boson of the MSSM are nearly indistinguishable from those of the SM Higgs boson is emphasized. Read More

In the decoupling limit of a non-minimal Higgs sector, the lightest CP-even Higgs boson (h) is indistinguishable from the Standard Model (SM) Higgs boson. In the two-Higgs-doublet sector of the MSSM, the approach to the decoupling limit (for m_A>>m_Z) persists, even in the presence of potentially large (tan(beta)-enhanced) radiative corrections to the hbb coupling. Radiative corrections can also generate an accidental cancellation between tree-level and one-loop terms, resulting in a SM-like Higgs boson for moderate m_A outside the decoupling regime. Read More

In lepton-number-violating supersymmetric models, there is no natural choice of basis to distinguish the down-type Higgs and lepton superfields. We employ basis-independent techniques to identify the massless majoron and associated light scalar in the case of spontaneously-broken lepton number (L). When explicit L-violation is added, these two scalars can acquire masses of order the electroweak scale and can be identified as massive sneutrinos. Read More

Precision electroweak data presently favors a weakly-coupled Higgs sector as the mechanism responsible for electroweak symmetry breaking. Low-energy supersymmetry provides a natural framework for weakly-coupled elementary scalars. In this review, we summarize the theoretical properties of the Standard Model (SM) Higgs boson and the Higgs sector of the minimal supersymmetric extension of the Standard Model (MSSM). Read More

A CP-even neutral Higgs boson with Standard-Model-like couplings may be the lightest scalar of a two-Higgs-doublet model. We study the decoupling limit of the most general CP-conserving two-Higgs-doublet model, where the mass of the lightest Higgs scalar is significantly smaller than the masses of the other Higgs bosons of the model. In this case, the properties of the lightest Higgs boson are nearly indistinguishable from those of the Standard Model Higgs boson. Read More

In this summary report of the 2001 Snowmass Electroweak Symmetry Breaking Working Group, the main candidates for theories of electroweak symmetry breaking are surveyed, and the criteria for distinguishing among the different approaches are discussed. The potential for observing electroweak symmetry breaking phenomena at the upgraded Tevatron and the LHC is described. We emphasize the importance of a high-luminosity $e^+e^-$ linear collider for precision measurements to clarify the underlying electroweak symmetry breaking dynamics. Read More

The decoupling properties of the Higgs sector in the Minimal Supersymmetric Standard Model (MSSM) imply that a light CP-even Higgs boson discovered at the Tevatron or LHC may closely resemble the Standard Model (SM) Higgs boson. In this paper, we investigate how precision measurements of Higgs properties at a Linear Collider (LC) can distinguish between a CP-even Higgs boson of the MSSM and the SM Higgs boson. We review the expected theoretical behavior of the partial widths and branching ratios for decays of the neutral MSSM Higgs bosons with significant couplings to the W and Z bosons, including the leading radiative corrections to the mixing angle alpha and tan beta-enhanced vertex corrections. Read More

The structure of low-energy supersymmetric models of fundamental particles and interactions is reviewed, with an emphasis on the minimal supersymmetric extension of the Standard Model (MSSM) and some of its variants. Various approaches to the supersymmetry-breaking mechanism are considered. The implications for the phenomenology of Higgs bosons and supersymmetric particles at future colliders are discussed. Read More

A light CP-even Higgs boson (h) with a mass of about 10 GeV could explain the recent BNL measurement of the muon anomalous magnetic moment, in the framework of a general CP-conserving two-Higgs-doublet extension of the Standard Model with no tree-level flavor-changing neutral Higgs couplings. However, the allowed Higgs mass window is quite small and the corresponding model parameters are very constrained. The Higgs sector can contribute significantly to the observed BNL result for g-2 without violating known experimental constraints only if the hZZ coupling (approximately) vanishes and m_h lies between the Upsilon mass and 2m_B. Read More

We analyze the supersymmetric (SUSY) QCD contribution to the $h^0 b \bar{b}$ coupling at one loop in the Minimal Supersymmetric Model (MSSM) in the decoupling limit. Analytic expressions in the large SUSY mass region are derived and the decoupling behavior of the corrections is examined in various limiting cases, where some or all of the SUSY mass parameters become large. We show that in the decoupling limit of large SUSY mass parameters and large CP-odd Higgs mass, the $h^0 b \bar b$ coupling approaches its Standard Model value at one loop. Read More

In R-parity-violating supersymmetric models (with a conserved baryon number), there are no quantum numbers that distinguish the lepton-doublet and down-type Higgs supermultiplets. As a result, the R-parity-violating parameters depend on the basis choice for these superfields, although physical observables are independent of the choice of basis. This paper presents a basis-independent computation of the sneutrino/antisneutrino squared-mass splitting in terms of basis-independent quantities. Read More

We explore the radiative corrections to the process $Z \to b \bar b$ in models with extended Higgs sectors. The observables $R_b = \Gamma(Z \to b \bar b)/\Gamma(Z \to \rm{hadrons})$ and the $Z b \bar b$ coupling asymmetry, $A_b = (g_L^2 - g_R^2)/(g_L^2 + g_R^2)$, are sensitive to these corrections. We present general formulae for the one-loop corrections to $R_b$ and $A_b$ in an arbitrary extended Higgs sector, and derive explicit results for a number of specific models. Read More

R-parity-violating supersymmetry with a conserved baryon number B provides a framework for particle physics with lepton number (L) violating interactions. Two important probes of the L-violating physics are neutrino masses and sneutrino-antisneutrino mass-splittings. We evaluate these quantities in the context of the most general CP-conserving, R-parity-violating B-conserving extension of the minimal supersymmetric standard model. Read More

The upper bound on the mass of the light CP-even Higgs boson of the minimal supersymmetric model (MSSM) depends on the supersymmetric particle spectrum via radiative loop effects. At present, complete one-loop results and partial two-loop results are known. Some simple analytic approximations have been obtained which are quite accurate over a large portion of the MSSM parameter space. Read More

R-parity-violating supersymmetry (with a conserved baryon number B) provides a framework for particle physics with lepton number (L) violating interactions. We examine in detail the structure of the most general R-parity-violating (B-conserving) model of low-energy supersymmetry. We analyze the mixing of Higgs bosons with sleptons and the mixing of charginos and neutralinos with charged leptons and neutrinos, respectively. Read More