Tag jets

 As described in section 2.2 the Higgs production for a heavy Higgs is through gluon fusion and vector boson fusion. The Higgs events produced in vector boson fusion are despite the lower cross section the interesting events for the identification of a heavy Higgs particle. As illustrated in fig. 8.6 the quarks that emit the vector bosons will generate jets in the forward direction. The forward jets can be used as tag jets for the vector boson fusion events and strongly reduce the background.

In the ATLAS detector hadronic calorimeters will cover the region down to |$\eta$| = 4.9 . Details on the calorimeter design can be found in section 4.3. The tag jets will have a transverse momentum of the order of m_W/2 . In table 8.3 the effect of requiring either one or two tag jets is shown. The jets are required to have 2.5 < |$\eta$| < 5.0 and to be in opposite forward regions for the two jet case.

 

E_T cut Efficiency 1 tag jet (%) Efficiency 2 tag jets (%)

$$\rightarrow \rightarrow$$ (GeV) jj jj

$$\pm \pm \pm \pm$$ 15

$$\pm \pm \pm \pm$$ 20

$$\pm \pm \pm \pm$$ 30

$$\pm \pm \pm \pm$$ 50

 

The probability p for a single tag jet in the forward or the backward direction of the detector is the same. If the two tag jets in double tag events are uncorrelated the probability for a double tag is p ² while the probability for a single tag, in either the forward or the backward direction, is 1 - (1 - p)² and not p as a would be the naïve expectation.

From the tag jet efficiencies for single and double tag jets listed in table 8.3 it is seen that the Higgs events have uncorrelated tag jets while the QCD-jet events show a positive correlation. The Higgs events have uncorrelated tag jets since the quarks emitting the vector bosons are uncorrelated (see fig. 8.6).