arXiv subjects
Publications and source records attributed to Heather E. Logan.
We discuss the complementarity between the LHC and a low energy photon collider. We mostly consider the scenario, where the first linear collider is a photon collider based on dual beam technology like CLIC.
The little Higgs idea is a new way to solve the little hierarchy problem by protecting the Higgs mass from quadratically divergent one-loop corrections. In this talk I describe the phenomenology of one particular realization of the little Higgs idea: the "Littlest Higgs" model.
We study the loop-induced process e+e- --> W+H- in the Minimal Supersymmetric Standard Model (MSSM). This process allows the charged Higgs boson to be produced in e+e- collisions when its mass is larger than half the center-of-mass energy, so that e+e- --> H+H- is kinematically forbidden. By scanning over the MSSM parameters subject to experimental constraints we examine the range of values possible for this cross section. We find that, in regions of parameter space where this cross section is large enough to be of interest, the contributions from supersymmetric particles typically increase the cross section by 50-100% compared to the non-supersymmetric two Higgs doublet model result. Choosing a few typical MSSM parameter sets, we show the regions in the m_{H^{\pm}}-tan(beta) plane in which at least 10 W^{\pm}H^{\mp} events would be produced at the e+e- collider for m_{H^{\pm}} >= sqrt(s)/2. We also show that including radiative corrections to the MSSM Higgs sector has only a small effect on the cross section.
We study the associated production of the charged Higgs boson and W^{\pm} gauge boson in high energy e+e- collisions in the Minimal Supersymmetric Standard Model (MSSM). This associated production, which first arises at the one loop level, offers the possibility of producing the charged Higgs boson at the e+e- collider with mass more than half the center-of-mass energy, when the charged Higgs pair production is kinematically forbidden. We present analytic and numerical results for the cross section for e+e- --> W+ H- in the full MSSM, taking into account the previously uncalculated contributions from supersymmetric (SUSY) particles. We find that the contributions of the SUSY particles enhance the cross section over most of SUSY parameter space, especially when the SUSY particles are light, ~200 GeV. With favorable SUSY parameters, at small tan beta, this process can yield more than ten W^{\pm}H^{\mp} events for m_{H^{\pm}} <~ 350 GeV in 500 fb-1 at a 500 GeV e+e- collider, or m_{H^{\pm}} <~ 600 GeV in 1000 fb-1 at a 1000 GeV collider. 80% left-handed polarization of the e- beam improves these reaches to m_{H^{\pm}} <~ 375 GeV and m_{H^{\pm}} <~ 670 GeV, respectively.
We study the production and decays of top squarks (stops) at the Tevatron collider in models of low-energy supersymmetry breaking. We consider the case where the lightest Standard Model (SM) superpartner is a light neutralino that predominantly decays into a photon and a light gravitino. Considering the lighter stop to be the next-to-lightest Standard Model superpartner, we analyze stop signatures associated with jets, photons and missing energy, which lead to signals naturally larger than the associated SM backgrounds. We consider both 2-body and 3-body decays of the top squarks and show that the reach of the Tevatron can be significantly larger than that expected within either the standard supergravity models or models of low-energy supersymmetry breaking in which the stop is the lightest SM superpartner. For a modest projection of the final Tevatron luminosity, L = 4 fb-1, stop masses of order 300 GeV are accessible at the Tevatron collider in both 2-body and 3-body decay modes. We also consider the production and decay of ten degenerate squarks that are the supersymmetric partners of the five light quarks. In this case we find that common squark masses up to 360 GeV are easily accessible at the Tevatron collider, and that the reach increases further if the gluino is light.
We survey techniques for finding a CP-odd Higgs boson, A, at the Linear Collider that do not depend upon the presence of other light Higgs bosons. The potential reach in $[m_A,\tanβ]$ parameter space for various production/discovery modes is evaluated and regions where discovery might not be possible at a given $\sqrt{s}$ are delineated. We give, for the first time, results for $\epem\to ν\antiνA$ one-loop $W$ boson fusion production.
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. The general expectation is that the Higgs couplings to W+W-, ZZ, c cbar and t tbar should quickly approach their SM values for increasing CP-odd Higgs mass mA, while the couplings to b bbar and tau+ tau- do so more slowly. Using the expected experimental and theoretical accuracy in determining SM branching ratios and partial widths, we demonstrate the sensitivity of measurements at the LC to variations in the MSSM parameters, with particular attention to the decoupling limit. For a wide range of MSSM parameters, the LC is sensitive to mA ~ 600 GeV almost independently of tan beta. For large values of tan beta and some specific choices of MSSM parameters [e.g., A_t mu < 0 and |A_t| ~ |mu| ~ O(M_S)], one of the CP-even Higgs bosons can be SM-like independent of the value of mA. In the case of large deviations from the SM, we present a procedure using Higgs coupling measurements to extract the supersymmetric correction to the relation between the b quark mass and Yukawa coupling.
We report on a study of the physics potential of linear $e^+e^-$ colliders. Although a linear collider (LC) would support a broad physics program, we focus on the contributions that could help elucidate the origin of electroweak symmetry breaking. Many extensions of the standard model have a decoupling limit, with a Higgs boson similar to the standard one and other, higher-mass states. Mindful of such possibilities, we survey the physics of a (nearly) standard Higgs boson, as a function of its mass. We also review how measurements from an LC could help verify several well-motivated extensions of the standard model. For supersymmetry, we compare the strengths of an LC with the LHC. Also, assuming the lightest superpartner explains the missing dark matter in the universe, we examine other places to search for a signal of supersymmetry. We compare the signatures of several scenarios with extra spatial dimensions. We also explore the possibility that the Higgs is a composite, concentrating on models that (unlike technicolor) have a Higgs boson with mass of a few hundred GeV or less. Where appropriate, we mention the importance of high luminosity, for example to measure branching ratios of the Higgs, and the importance of multi-TeV energies, for example to explore the full spectrum of superpartners.
In the minimal supersymmetric extension of the Standard Model (MSSM), fermion masses and Yukawa couplings receive radiative corrections at one loop from diagrams involving the supersymmetric particles. The corrections to the relation between down-type fermion masses and Yukawa couplings are enhanced by tan beta, which makes them potentially very significant at large tan beta. These corrections affect a wide range of processes in the MSSM, including neutral and charged Higgs phenomenology, rare B meson decays, and renormalization of the CKM matrix. We give a pedagogical review of the sources and phenomenological effects of these corrections.
We study the prospects for distinguishing the CP-even Higgs boson of the minimal supersymmetric extension of the Standard Model (MSSM) from the Standard Model (SM) Higgs boson by measuring its branching ratios at an e+e- linear collider. The regions of the M_A - tan beta plane in which an MSSM Higgs boson can be distinguished from the SM Higgs boson depend strongly upon the supersymmetric parameters that enter the radiative corrections to the Higgs mass matrix and the Higgs couplings to fermions. In some regions of parameter space it is possible to extract the supersymmetric correction to the relation between the b quark mass and its Yukawa coupling from Higgs branching ratio measurements.
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. However, the onset of decoupling is delayed when $\tanβ$ is large. In addition, the one-loop SUSY-QCD corrections decouple if the masses of either the bottom squarks or the gluino are separately taken large; although the approach to decoupling is significantly slower in the latter case.
We compute the branching fractions of B_{s,d} -> l+ l- in the type-II two-Higgs-doublet model with large tan(beta). We find that the parameters of the neutral Higgs sector of the two-Higgs-doublet model cancel in the result, so that the branching fractions depend only on the charged Higgs mass and tan(beta). For large values of tan(beta) and a charged Higgs mass above the bound from b -> s gamma, we find that the branching fractions can be enhanced by up to an order of magnitude or suppressed by up to a factor of two compared to the Standard Model result. We point out that previous calculations in the literature are gauge-dependent due to the omission of an important diagram, which gives the dominant contribution in the 't Hooft-Feynman gauge. We have analyzed in detail the region of the (M_{H^+}, tan(beta)) plane to be probed by searches for B_s -> mu+ mu- in Run II of the Tevatron. Since the branching fraction increases like tan^4(beta), this decay mode is complementary to b -> s gamma and efficiently probes the large tan(beta) region. For tan(beta)=60, an integrated luminosity of 20 fb^{-1} in an extended Run II will probe charged Higgs masses up to 260 GeV, if the background to B_s -> mu+ mu- is small. For the same value of tan(beta), the LHC may be able to explore charged Higgs masses up to 1 TeV using this decay.
We explore the radiative corrections to the process $Z \to b \bar b$ in models with extended Higgs sectors. The observables $R_b = Γ(Z \to b \bar b)/Γ(Z \to \rm{hadrons})$ 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. We find that in models containing only doublets, singlets, or larger multiplets constrained by a custodial $SU(2)_{c}$ symmetry so that $M_W = M_Z \cosθ_W$ at tree level, the one-loop corrections due to virtual charged Higgs bosons always worsen agreement with experiment. The $R_{b}$ measurement can be used to set lower bounds on the charged Higgs masses. Constraints on models due to the one-loop contributions of neutral Higgs bosons are also examined.
We explore the radiative corrections to the process Z --> b b-bar in models with extended Higgs sectors. The observables R_b (the Z hadronic branching fraction to b b-bar) and the b-quark asymmetry A_b = (g_L^2 - g_R^2)/(g_L^2 + g_R^2) are sensitive to these corrections. We find that in models containing only Higgs doublets, singlets, or larger multiplets constrained by a custodial SU(2) symmetry, the radiative corrections to R_b involving charged Higgs bosons always worsen the Standard Model fit to electroweak data. Thus, the present R_b data can be used to set lower bounds on the charged Higgs masses in such models. Radiative corrections to R_b involving light neutral Higgs bosons in models with enhanced couplings to b b-bar can improve the Standard Model fit to present data. The Higgs sector alone cannot explain a significant deviation of A_b from its Standard Model prediction. We present general formulas for the corrections to R_b and A_b in an SU(2)xU(1) electroweak model with an arbitrary extended Higgs sector, and derive explicit results for a number of specific models.