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## IV. LEP ELECTROWEAK PRECISION DATA : A b FB AND R$_{b}$ 

LEP experiment is known as one of the most successful ones which confirm the Standard Model (SM) at a very high precision, one per mil. This acts as the main source of the frustration for any physics beyond the SM. In this paper we take a viewpoint that 3 sigma deviation of A b FB between the prediction and the measurement existing in LEP data might be a hint for a new physics. We discuss whether light Higgs scenario might be relevant to reduce the tension. 

Forward-backward asymmetry of bottom quark produced from electron-positron pair, 

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has been measured at LEP I and LEP II and it is the electroweak observable which shows the largest discrepancy compared to the standard model prediction: 

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The difference 0 . 0045 corresponds to 2 . 8 σ deviation with the experimental error or 2 . 5 σ deviation with the combined error. The strong constraints on the bottom quark pair pro- duction at the Z pole from R$_{b}$ measurements makes hard to resolve this deviation. 

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where q represents five quarks excluding top quark. The observed value and the standard model prediction agrees well: 

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