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Michael S. Chanowitz

Publications and source records attributed to Michael S. Chanowitz.

At least 19 recordsLinked to original sources
arXiv2013

Electroweak Constraints on the Fourth Generation at Two Loop Order

If the Higgs-like particle at 126 GeV is the standard model Higgs boson, then SM4, the simplest four generation extension of the SM, is inconsistent with the most recent LHC data. However, 4G variations (BSM4) are possible if the new particle is not the SM Higgs boson and/or if other new quanta modify its production and decay rates. Since LHC searches have pushed 4G quarks to high mass and strong coupling where perturbation theory eventually fails, we examine the leading nondecoupling EW (electroweak) corrections at two loop order to estimate the domain of validity for perturbation theory. We find that the two loop hypercharge correction, which has not been included in previous EW fits of 4G models, makes the largest quark sector contribution to the rho parameter, much larger even than the nominally leading one loop term. Because it is large and negative, it has a big effect on the EW fits. It does not invalidate perturbation theory since it only first appears at two loop order and is large because it does not vanish for equal quark doublet masses, unlike the one loop term. We estimate that perturbation theory is useful for $m_Q\simeq 600$ GeV but begins to become marginal for $m_Q\, \gtap\, 900$ GeV. The results apply directly to BSM4 models that retain the SM Higgs sector but must be re-evaluated for non-SM Higgs sectors.

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arXiv2011

A heavy little Higgs and a light Z' under the radar

The original littlest Higgs model with universal fermion couplings is found to be consistent with precision electroweak data but is strongly constrained by Tevatron limits on the predicted centi-weak Z' boson. A possible signal observed by CDF at 240 GeV is consistent with the predicted Z' and a region below 150 GeV is largely unconstrained by collider data.

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arXiv2010

Higgs Mass Constraints on a Fourth Family: Upper and Lower Limits on CKM Mixing

Limits on the Higgs boson mass restrict CKM mixing of a possible fourth family beyond the constraints previously obtained from precision electroweak data alone. Existing experimental and theoretical bounds on the Higgs mass already significantly restrict the allowed parameter space. Zero CKM mixing is excluded and mixing of order the Cabibbo angle is allowed. Upper and lower limits on 3-4 CKM mixing are exhibited as a function of the Higgs mass. We use the default inputs of the Electroweak Working Group and also explore the sensitivity of both the three and four family fits to alternative inputs.

hep-ph↗
arXiv2009

Bounding CKM Mixing with a Fourth Family

CKM mixing between third family quarks and a possible fourth family is constrained by global fits to the precision electroweak data. The dominant constraint is from nondecoupling oblique corrections rather than the Zbb vertex correction used in previous analyses. The possibility of large mixing suggested by some recent analyses of FCNC processes is excluded, but 3-4 mixing of the same order as the Cabbibo mixing of the first two families is allowed.

hep-ph↗
arXiv2008

A Z' Boson and the Higgs Boson Mass

The Standard Model fit prefers values of the Higgs boson mass that are below the 114 GeV direct lower limit from LEP II. The discrepancy is acute if the 3.2 sigma disagreement for the effective weak interaction mixing angle from the two most precise measurements is attributed to underestimated systematic error. In that case the data suggests new physics to raise the predicted value of the Higgs mass. One of the simplest possibilities is a Z' boson, which would generically increase the prediction for the Higgs mass as a result of Z-Z' mixing. We explore the effect of Z-Z' mixing on the Higgs mass prediction, using both the full data set and the reduced data set that omits the hadronic asymmetry measurements of the weak mixing angle, which are more likely than the leptonic asymmetry measurements to have underestimated systematic uncertainty.

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arXiv2006

Hunting the Scalar Glueball: Prospects for BES III

The search for the ground state scalar glueball G_0 is reviewed. Spin zero glueballs will have unique dynamical properties if the amplitude is suppressed by chiral symmetry, as it is to all orders in perturbation theory: for instance, mixing of G_0 with \bar qq mesons would be suppressed, radiative J/psi decay would be a filter for new physics in the spin zero channel, and the decay G_0 \to \bar KK could be enhanced relative to G_0 \to ππ. These properties are consistent with the identification of f_0(1710) as the largely unmixed ground state scalar glueball, while recent BES data implies that f_0(1500) does not contain the dominant glueball admixture. Three hypotheses are discussed: that G_0 is 1) predominantly f_0(1500) or 2) predominantly f_0(1710) or 3) is strongly mixed between f_0(1500) and f_0(1710).

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arXiv2004

The No-Higgs Signal: Strong WW Scattering at the LHC

Strong WW scattering at the LHC is discussed as a manifestation of electroweak symmetry breaking in the absence of a light Higgs boson. The general framework of the Higgs mechanism -- with or without a Higgs boson -- is reviewed, and unitarity is shown to fix the scale of strong WW scattering. Strong WW scattering is also shown to be a possible outcome of five-dimensional models, which do not employ the usual Higgs mechanism at the TeV scale. Precision electroweak constraints are briefly discussed. Illustrative LHC signals are reviewed for models with QCD-like dynamics, stressing the complementarity of the W^{\pm}Z and like-charge W^+W^+ + W^-W^- channels.

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arXiv2003

The Direct Limit on the Higgs Mass and the SM Fit

Because of two $3σ$ anomalies, the Standard Model (SM) fit of the precision electroweak data has a poor confidence level, $CL= 0.02$. Since both anomalies involve challenging systematic issues, it might appear that the SM could still be valid if the anomalies resulted from underestimated systematic error. Indeed the $CL$ of the global fit could then increase to 0.71, but that fit predicts a small Higgs boson mass, $m_H=45$ GeV, that is inconsistent at 95% CL with the lower limit, $m_H>114$ GeV, established by direct searches. The data then favor new physics if the anomalous measurements are both excluded or both retained, and the Higgs boson mass cannot be predicted until the new physics is understood. The validity of the SM could however be maintained by a propitious combination of statistical fluctuation and systematic error. The current data do not allow a definitive conclusion.

hep-ph↗
arXiv2002

Electroweak Data and the Higgs Boson Mass: A Case for New Physics

Because of two 3 sigma anomalies, the Standard Model (SM) fit of the precision electroweak data has a poor confidence level, CL= 0.010. Since both anomalies involve challenging systematic issues, it might appear that the SM could still be valid if the anomalies resulted from underestimated systematic error. Indeed the CL of the global fit could then increase to 0.65, but that fit predicts a small Higgs boson mass, m_H=43 GeV, that is only consistent at CL=0.035 with the lower limit, m_H > 114 GeV, established by direct searches. The data then favor new physics whether the anomalous measurements are excluded from the fit or not, and the Higgs boson mass cannot be predicted until the new physics is understood. Some measure of statistical fluctuation would be needed to maintain the validity of the SM, which is unlikely by broad statistical measures. New physics is favored, but the SM is not definitively excluded.

hep-ph↗
arXiv2002

The Z ->anti-b b decay asymmetry: lose-lose for the Standard Model

Combining precision measurements and the Higgs boson search limit, the electroweak data has evolved to a point where new physics is favored whether the 3.2 sigma A_{FB}^b anomaly is genuine or not. Such new physics could greatly alter the inferred value of the Higgs boson mass.

hep-ph↗
arXiv2000

New Physics and the Landau Pole

In scalar field theories the Landau pole is an ultraviolet singularity in the running coupling constant that indicates a mass scale at which the theory breaks down and new physics must intervene. However, new physics at the pole will in general affect the running of the low energy coupling constant, which will in turn affect the location of the pole and the related upper limit (``triviality'' bound) on the low energy coupling constant. If the new physics is strongly coupled to the scalar fields these effects can be significant even though they are power suppressed. We explore the possible range of such effects by deriving the one loop renormalization group equations for an effective scalar field theory with a dimension 6 operator representing the low energy effects of the new physics. As an independent check we also consider a renormalizable model of the high-scale physics constructed so that its low energy limit coincides with the effective theory.

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arXiv1999

The Z -> bb decay asymmetry and flavor changing neutral currents

The measured value of A_b, the Z \bar bb asymmetry parameter, disagrees with the Standard Model at 99% confidence level. If genuine the discrepancy could indicate new interactions unique to third generation quarks, implying enhanced Z penguin amplitudes. Enhanced rates are predicted for rare K and B decays, such as K^{+} --> pi^{+}\bar nu nu, K_{L} --> pi^{0}\bar nu nu, B --> X_{s}\bar nu nu, and B_{s} --> \bar mu mu. Measurements of epsilon^{\prime}/epsilon then imply QCD penguin amplitudes must also be similarly enhanced. The Higgs sector of an SU(2)_L x SU(2)_R gauge theory has some of the features needed to explain these phenomena and would also imply right-handed penguin amplitudes.

hep-ph↗
arXiv1999

Quantum corrections from nonresonant WW scattering

An estimate is presented of the leading radiative corrections to low energy electroweak precision measurements from strong nonresonant WW scattering at the TeV energy scale. The estimate is based on a novel representation of nonresonant scattering in terms of the exchange of an effective scalar propagator with simple poles in the complex energy plane. The resulting corrections have the form of the corrections from the standard model Higgs boson with the mass set to the unitarity scale for strong WW scattering.

hep-ph↗
arXiv1998

Higgs boson mass constraints from precision data and direct searches

Two of the nine measurements of $sin^{2}θ^{lepton}_{eff}$, the effective weak interaction mixing angle, are found to be in significant conflict with the direct search limits for the Standard Model (SM) Higgs boson. Using a scale factor method, analogous to one used by the Particle Data Group, we assess the possible effect of these discrepancies on the SM fit of the Higgs boson mass. The scale factor fits increase the value of $sin^{2}θ^{lepton}_{eff}$ by as much as two standard deviations. The central value of the Higgs boson mass increases as much as a factor of two, to $\simeq 200$ GeV, and the 95% confidence level upper limit increases to as much as 750 GeV. The scale factor is based not simply on the discrepant measurements, as was the case in a previous analysis, but on an aggregate goodness-of-fit confidence level for the nine measurements and the limit. The method is generally applicable to fits in which one or more of a collection of measurements are in conflict with a physical boundary or limit. In the present context, the results suggest caution in drawing conclusions about the Higgs boson mass from the existing data.

hep-ph↗
arXiv1998

Combining real and virtual Higgs boson mass constraints

Within the framework of the standard model we observe that there is a significant discrepancy between the most precise $Z$ boson decay asymmetry measurement and the limit from direct searches for Higgs boson production. Using methods inspired by the Particle Data Group we explore the possible effect on fits of the Higgs boson mass. In each case the central value and the 95% confidence level upper limit increase significantly relative to the conventional fit. The results suggest caution in drawing conclusions about the Higgs boson mass from the existing data.

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arXiv1996

Tree-unitary sigma models and their application to strong WW scattering

Sigma models are exhibited which have tree amplitudes for Goldstone boson scattering that satisfy elastic unitarity exactly. The models have imaginary coupling constants and the scalar propagators have poles on the imaginary axis in the complex p^2 plane. They are equivalent to K-matrix models, which are ad hoc unitarizations of low energy theorems for Goldstone boson scattering that have been used recently to describe strong WW scattering. The sigma model formulation of the K-matrix models may be used to estimate directly the effect of strong WW scattering on low energy radiative corrections.

hep-ph↗
arXiv1996

Gauge invariant formulation of strong WW scattering

Models of strong $WW$ scattering in the $s$-wave can be represented in a gauge invariant fashion by defining an effective scalar propagator that represents the strong scattering dynamics. The $σ(qq \ra qqWW)$ signal may then be computed in U-gauge from the complete set of tree amplitudes, just as in the standard model, without using the effective $W$ approximation (EWA). The U-gauge ``transcription'' has a wider domain of validity than the EWA, and it provides complete distributions for the final state quanta, including experimentally important jet distributions that cannot be obtained from the EWA. Starting from the usual formulation in terms of unphysical Goldstone boson scattering amplitudes, the U-gauge transcription is verified by using BRS invariance to construct the complete set of gauge and Goldstone boson amplitudes in $R_ξ$ gauge.

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