Implications of Precision Electroweak Measurements for the Standard Model Higgs Boson
We summarize the status of the Standard Model with special emphasis on the extraction of the Higgs boson mass using Bayesian inference.
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arXiv subjects
Jens Erler
Publications and source records attributed to Jens Erler.
Constraints on Extended Neutral Gauge Structures
Indirect precision data are used to constrain the masses of possible extra Z^prime bosons and their mixings with the ordinary Z. We study a variety of Z^prime bosons as they appear in E_6 and left-right unification models, the sequential Z boson, and the example of an additional U(1) in a concrete model from heterotic string theory. In all cases the mixings are severely constrained (sin theta < 0.01). The lower mass limits are generally of the order of several hundred GeV and competitive with collider bounds. The exception is the Z_psi boson, whose vector couplings vanish and whose limits are weaker. The results change little when the rho parameter is allowed, which corresponds to a completely arbitrary Higgs sector. On the other hand, in specific models with minimal Higgs structures the limits are generally pushed into the TeV region.
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Implications of Precision Electroweak Measurements for Physics Beyond the SM
We perform a global analysis of electroweak precision measurements to find constraints on physics beyond the Standard Model. In particular, we discuss oblique parameters, which are useful to constrain additional matter fields, as well as extra $Z$ bosons, and supersymmetry. We also summarize the present information on the Higgs boson mass.
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Calculation of the QED Coupling alpha (M_Z) in the Modified Minimal-Subtraction Scheme
I calculate the QED coupling, alpha, directly in the MS-bar scheme using an unsubtracted dispersion relation for the three light quarks, and perturbative QCD for charm and bottom quarks. Compact analytical expressions are presented, making this approach particularly suitable for electroweak fits. After alpha^(-1) (m_tau) = 133.513 +- 0.025 is obtained in a first step, I perform a 4-loop renormalization group evolution with 3-loop matching conditions to arrive at alpha^(-1) (M_Z) = 127.934 +- 0.026 for alpha_s (M_Z) = 0.120. The corresponding hadronic contribution to the on-shell coupling is Delta alpha_had^(5) (M_Z) = 0.02779 +- 0.00019. The error is mainly from m_c, and from experimental uncertainties in e^+ e^- annihilation into unflavored and strange hadrons and tau decay data.
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Status of the Standard Model
We review the experimental and theoretical status of the standard electroweak theory, and its fundamental parameters. We obtain the global best fit result for the Standard Model Higgs boson of M_H = 107^{+67}_{-45} GeV, and find the 95% upper limit of 255 GeV. Parameters describing physics beyond the Standard Model are discussed as well. Particular emphasis is given to implications for supersymmetric extensions of the Standard Model.
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Bounds on the Standard Higgs Boson
We review the status of precision electroweak physics with particular emphasis on the extraction of the Higgs boson mass. Global fit results depend strongly on the used value for the hadronic contribution to alpha(M_Z). We emphasize, however, that the general tendency for a light Higgs persists when using any of the recently obtained values for alpha(M_Z), and is also less dependent on deviating observables such as A_LR than in the past.
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Bounds on Supersymmetry from Electroweak Precision Analysis
The Standard Model global fit to precision data is excellent. The Minimal Supersymmetric Standard Model can also fit the data well, though not as well as the Standard Model. At best, supersymmetric contributions either decouple or only slightly decrease the total chi^2, at the expense of decreasing the number of degrees of freedom. In general, regions of parameter space with large supersymmetric corrections from light superpartners are associated with poor fits to the data. We contrast results of a simple (oblique) approximation with full one-loop results, and show that for the most important observables the non-oblique corrections can be larger than the oblique corrections, and must be taken into account. We elucidate the regions of parameter space in both gravity- and gauge-mediated models which are excluded. Significant regions of parameter space are excluded, especially with positive supersymmetric mass parameter mu. We give a complete listing of the bounds on all the superpartner and Higgs boson masses. For either sign of mu, and for all supersymmetric models considered, we set a lower limit on the mass of the lightest CP-even Higgs scalar, mh > 78 GeV. Also, the first and second generation squark masses are constrained to be above 280 (325) GeV in the supergravity (gauge-mediated) model.
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chi^2 Analysis of Supersymmetric Models
We discuss the results of a global fit to precision data in supersymmetric models. We consider both gravity- and gauge-mediated models. As the superpartner spectrum becomes light, the global fit to the data typically results in larger values of chi^2. We indicate the regions of parameter space which are excluded by the data. We discuss the additional effect of the B(B --> X_sγ) measurement. Our analysis excludes chargino masses below MZ in the simplest gauge-mediated model with mu>0, with stronger constraints for larger values of tan beta.
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Unification or Compositeness?
The status of precision electroweak data, tests of the standard model, determination of its parameters, and constraints on new physics, are surveyed.
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A wide scalar neutrino resonance and b\bar{b} production at LEP
In supersymmetric models with R-parity violation, scalar neutrinos may be produced as s-channel resonances in e^+e^- colliders. We note that within current constraints, the scalar neutrino may have a width of several GeV into b\bar{b} and be produced with large cross section, leading to a novel supersymmetry discovery signal at LEP II. In addition, if the scalar neutrino mass approximately equals m_Z, such a resonance necessarily increases R_b and reduces A_{FB}(b), significantly improving the fit to electroweak data. Bounds from B meson and top quark decays are leading constraints, and we stress the importance of future measurements.
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Asymmetric Orbifolds and Higher Level Models
I introduce a class of string constructions based on asymmetric orbifolds leading to level two models. In particular, I derive in detail various models with gauge groups $E_6$ and SO(10), including a four generation $E_6$ model with two adjoint representations. The occurrence of multiple adjoint representations is a generic feature of the construction. In the course of describing this approach, I will address the problem of twist phases in higher twisted sectors of asymmetric orbifolds.
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Four-Fermi Operators in $e^+$ $e^-$ Annihilation Experiments and Uncertainties in $Z$ Boson Properties
We investigate the effects of possible new four-Fermi operators on $e^+$ $e^-$ annihilation experiments. They represent a class of new physics, which has the potential to change the parameters of the $Z$ boson and at the same time interferes with their determination from the data. We show that in the presence of such operators the $Z$ parameters obtained from lineshape fits can change significantly. Another important property of these operators is that they spoil the factorization of the expression for the left-right asymmetry, $A_{LR}$, into initial and final state couplings. Factorization and subsequent cancellation of the final state coupling occurs in the Standard Model (after correcting for photonic amplitudes) and is crucial for the interpretation of $A_{LR}$ as a measurement of the effective weak mixing angle. Four-Fermi operators may thus provide an explanation for the high value of the polarization asymmetry as observed at SLC. However, the data from lower energy $e^+$ $e^-$ annihilation severely constrains this class of operators and virtually closes this possible loophole on how new physics might explain the SLC/LEP discrepancy. We point out that if the surplus of observed $b$-pairs at LEP is real and caused by these operators, there may be a significant effect on the forward-backward asymmetry into $b$-quarks. It is this quantity which presently gives the most precise determination of the weak mixing angle. We present compact analytical expressions for the treatment of initial state radiation in the presence of new contact operators.
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Implications of High Precision Experiments and the CDF Top Quark Candidates
We discuss the consequences of recent experimental results from CDF, SLC, LEP and elsewhere for the Standard Model and for new physics. A global fit to all indirect precision data yields $m_t = 175 \pm 11^{+17}_{-19}$~GeV, $\sin^2θ_{MS} = 0.2317(3)(2)$, and $α_s = 0.127(5)(2)$, where the central values are for $M_H = 300$ GeV and the second uncertainties are for $M_H \rightarrow 1000$ GeV (+) and 60 GeV (-). The $m_t$ value is in remarkable agreement with the value $m_t=174 \pm 16$ GeV suggested by the CDF candidate events. There is a slight preference for a light Higgs with $M_H < 730$ (880) GeV at 95\% c.l. if the CDF $m_t$ value is (not) included. The sensitivity is, however, due almost entirely to the anomalously large observed values for the $Z \rightarrow b\bar{b}$ width and left-right asymmetry. The value of alpha_s (from the lineshape) is clean theoretically assuming the Standard Model, but is sensitive to the presence of new physics contributions to the $Z \rightarrow b\bar{b}$ vertex. Allowing a vertex correction $δ_{b\bar{b}}^{\rm new}$ one obtains the significantly lower value alpha_s = 0.111 \pm 0.009, in better agreement with low energy determinations, and $δ_{b\bar{b}}^{\rm new} = 0.023 \pm 0.011$. There is now enough data to perform more general fits to parameters describing new physics effects and to separate these from $m_t$ and $M_H$. Allowing the parameter $ρ_0$, which describes sources of $SU(2)$ breaking beyond the Standard Model, to be free one finds $ρ_0=1.0012 \pm 0.0017 \pm 0.0017$, remarkably close to unity. One can also separate the new physics contributions to the oblique parameters $S_{\rm new}$, $T_{\rm new}$ and $U_{\rm new}$, which all take values consistent with zero. The effects of supersymmetry on the determination of the SM parameters are discussed.
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The SLD Asymmetry in View of the LEP Results
LEP determines $\sin^2 θ_{\rm eff}$, by combining the various asymmetry measurements to be $0.2321 \pm 0.0005$. On the other hand, the left-right asymmetry as measured at SLC corresponds to \seff = $0.2292 \pm 0.0010$. I will discuss the possibilities of this being i) a statistical fluctuation, ii) a hint for New Physics or iii) an experimental discrepancy.
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Breakdown of Duality in (0,2) Superstring Models
After pointing out the role of the compactification lattice for spectrum calculations in orbifold models, I discuss modular discrete symmetry groups for $Z_N$ or\-bi\-folds. I consider the $Z_7$ orbifold as a nontrivial example of a (2,2) model and give the generators of the modular group for this case, which does not contain $[SL(2,{\bf Z})]^3$ as had been speculated. I also discuss how to treat cases where quantized Wilson lines are present. I consider in detail an example, demonstrating that quantized Wilson lines affect the modular group in a nontrivial manner. In particular, I show that it is possible for a Wilson line to break $SL(2,{\bf Z})$.}
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Anomaly Cancellation in Six Dimensions
I show that anomaly cancellation conditions are sufficient to determine the two most important topological numbers relevant for Calabi-Yau compactification to six dimensions. This reflects the fact that K3 is the only non-trivial CY manifold in two complex dimensions. I explicitly construct the Green-Schwarz counterterms and derive sum rules for charges of additional enhanced U(1) factors and compare the results with all possible Abelian orbifold constructions of K3. This includes asymmetric orbifolds as well, showing that it is possible to regain a geometrical interpretation for this class of models. Finally, I discuss some models with a broken $E_7$ gauge group which will be useful for more phenomenological applications.
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Comment on the Generation Number in Orbifold Compactifications
There has been some confusion concerning the number of $(1,1)$-forms in orbifold compactifications of the heterotic string in numerous publications. In this note we point out the relevance of the underlying torus lattice on this number. We answer the question when different lattices mimic the same physics and when this is not the case. As a byproduct we classify all symmetric $Z_N$-orbifolds with $(2,2)$ world sheet supersymmetry obtaining also some new ones.
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