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Discrepancy in Standard Model Prediction
for Z0 Asymmetry Measurements at SLAC and CERN?

The weak mixing angle can be determined from asymmetry measurements in Z0 interactions with leptons and also from asymmetry measurements in Z0 interactions with quarks.  These two techniques give results that differ by 3.5 standard deviations (see summary plot of weak mixing angle results).  The reason for this discrepancy could be a statistical fluctuation, an experimental technique problem, or the influence of new physics beyond the Standard Model.  

     The probability for 2 measurements to disagree by 3.5 standard deviations (3.5s) is much less than 0.1%.  This means that in a sample of 1000 measurements, it is unlikely for any 2 of the 1000 measurements to disagree by 3.5s or more.  And for any random comparison of 2 measurements, it is highly unlikely to observe such a large disagreement.
     The leptonic asymmetry results measure the lepton-asymmetry parameter, Al, which is very sensitive to the weak mixing angle.  There is a similar b-quark asymmetry parameter, Ab, that describes the asymmetry in the strength of the Z0 interaction between left-handed b quarks and right-handed ones. Ab is insensitive to the value of the weak mixing angle and is predicted to be 0.935 by the Standard Model.  With the availability of a polarized electron beam, the SLD experiment makes a direct measurement of this parameter.  At CERN, the 4 LEP experiments do not have a polarized electron beam, but can measure a forward-backward asymmetry in Z0 decays to b quarks, AbFB, and this asymmetry is proportional to the product of Ab times Al.   The observed discrepancy in the weak mixing angle determination between the lepton and quark asymmetry measurements could result from new physics affecting Ab.   In this case it is very interesting to examine the self-consistency of the Al, Ab and AbFB measurements.  This is done in the plot below.

     SLAC and CERN results for asymmetry measurements in the interactions of Z0 particles with b-quarks and leptons.  The SLAC and CERN measurements for the leptonic asymmetries, Al, are combined and are represented by the red band.  SLD measures the b-quark asymmetry parameter, Ab, directly (because of its polarized electron beam) and this result is shown as the green band.   The CERN experiments measure a forward-backward asymmetry in Z0 decays to b-quark pairs and this asymmetry is proportional to the product of Ab times Al.  This CERN result is shown as the blue band.  The Standard Model predicts that Ab should be equal to 0.935, and that Al should be in the range from 0.136 (for a Higgs mass of 1000 GeV) to 0.147 (for a Higgs mass of 113 GeV).  The widths of the bands shown represent +/- 1 standard deviation.
     SLAC and CERN's Al results favor a light (Standard Model) Higgs and SLD's Ab result is consistent with the Standard Model prediction.  CERN's forward-backward b-quark asymmetry result favors a heavy (Standard Model) Higgs.  The self-consistency of these three data measurements and the Standard Model predictions is marginal and has only an 0.2% probability to give the large observed c2/dof=12.4/2.  Of the three data measurements, the simplest and most reliable one experimentally is the Al result.  In particular, SLD's very precise measurement of Al from its left-right asymmetry (ALR) measurement results from a simple counting measurement and is very robust.

Last updated 04-09-2001