Experimental results on Higgs searches

 The experimental bounds on the Higgs mass come from the experiments at the LEP e ⁺e ^- storage ring. The mass range accessible continues to rise as the centre of mass energy in the collisions increases during the LEP2 upgrade. The decay channel investigated is the so called Higgs-strahlung process shown in fig. 2.11.

  

Figure 2.11: Higgs-strahlung, the dominant production process for a standard model Higgs at LEP2.

The best Higgs mass limits at the moment are from data collected with a maximal centre of mass energy of 172 GeV. In articles submitted for publication the limits are: 70.7 GeV (ALEPH) [22], 69.5 GeV (L3) [23] and 65.0 GeV (OPAL) [24]. The ultimate Higgs mass reach for LEP2, at a collision energy of 192 GeV and with an integrated luminosity of 150 pb ^{- 1} for each experiment, is estimated to be 95 GeV [25]. No other experiments are expected to raise this limit further before the start of the LHC.

As can be seen the overlap between LEP2 and the LHC for Higgs masses is rather small and that makes it important that the detectors at the LHC will have ultimate performance in the difficult region of an intermediate mass Higgs.

Before the discovery of the top quark at the Tevatron its mass was quite well known from the top quark effect on lower energy processes through radiative corrections. The same is to some extent true for the Higgs. While the electroweak radiative corrections at the one loop level are proportional to

 

$${\frac{m_{t}^2-m_{b}^2}{m_{W}^2}} {\frac{m_{t}^2}{m_{Z}^2}}$$ (37)

they are for radiative corrections involving the Higgs proportional to

 

$${\frac{m_{H}}{m_{Z}}}$$ (38)

thus making the uncertainties in the Higgs mass predictions much larger than was the case for the top quark mass before its direct discovery.

Precision measurements of sin $\theta_{W}^{}$ , m_t , $\alpha_{s}^{}$ , m_Z and $\alpha$ give an over-determined system from which limits on the Higgs mass can be extracted. Assuming a standard model Higgs sector it is in [26] shown that a Higgs mass of the order 130 GeV gives the best fit to data and an upper limit of 430 GeV is claimed as a 95% confidence limit. Combining precision measurements from the LEP and Tevatron experiments gives the indirect measurement m_H = 115^{+ 116}_{- 66} GeV with 420 GeV as a 95% confidence limit [27]. As an interesting remark it can from (2.80) be seen that the indirect measurement m_H $\simeq$ m_Z is equivalent to the conclusion that no radiative corrections involving the Higgs particle have been measured.