Higgs decay

The complete arbitrariness of the Higgs meson mass has many consequences. Having some idea of its mass would be instrumental in devising the proper experiment to find it, and its value influences some theoretical estimates. 

In what follows we consider briefly three aspects of Higgs searches (i) Higgs decay, (ii) Higgs production at the mass, and (iii) limits on the Higgs mass. 

The fundamental vertices for the Higgs coupling to fermions and to the weak gauge bosons are 

Prom (5.1.2), we can rewrite 9wwH = 2(v Gr) / M. This shows that the Higgs coupling to the W is proportional to the square of the boson mass as compared with the linear dependence on the fermion mass 

Because of its simple coupling, the calculation of its decay width into a fermion-antifermion pair can be carried out in an analogous fashion to the treatment of the W in Section 5.3.1 we have (Ellis et al, 1976) 

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The Higgs decay into two gluons can be obtained from the above formulae (6.2.5-6.2.8) keeping just the quark loop contributions and replacing by (note, however, that the one-loop calculation can only be trusted in this case if the Higgs is suflSciently heavy, say above 1 GeV, so that is suflSciently small). Keeping only the main contribution (coming from heavy quarks), a reasonable estimate for T H — GG) is then... 

A summary of all two-body (leptonic, photonic, gluonic and partonic) Higgs decay widths is shown in Fig. 6.1 (from Pranzini et al, 1989) where one has taken... 

The prediction of a heavy boson has received preliminary empirical support [92,96] from an anomaly in Z decay widths that points toward the existence of Z bosons with a mass of 812 GeV 1 33j [92,96] within the SO(l) grand unified field model, and a Higgs mechanism of 145 GeV4gj3. This suggests that a new massive neutral boson has been detected. Analysis of the hadronic peak cross sections obtained at LEP [96] implies a small amount of missing invisible width in Z decays. The effective number of massless neutrinos is 2.985 0.008, which is below the prediction of 3 by the standard model of electroweak interactions. The weak charge Qw in atomic parity violation can be interpreted as a measurement of the S parameter. This indicates a new Qw = 72.06 0.44, which is found to be above the standard model pre-... 

Another decay mode, especially relevant at LEP energies for light Higgs, is > 27 which goes via one-loop diagrams involving W bosons, quarks and leptons ... 

Fig. 6.2. The lifetime and width of the Higgs versus its mass taking into account the decays displayed in Fig. 6.1. The jumps come from the imperfect treatment of the threshold, i.e. cutting ofiF qq decays at the quarkonium mass.

Assuming that mu < Mz a number of rare Z decays into Higgs particles are possible such as Z Hff, Z Hj or, if the Higgs is sufficiently low in mass, even Z HHff etc. The rates for these decays may be rather low as compared with the dominant Z decays (i.e. Z Z QQ) but if some 10 Z events were... 

Only supercolliders, either hadronic such as SSC (v/i 40 TeV), LHC ( /i 16 — 20 TeV) or e" e , such as CLIC y/s 2 TeV) could pursue the hunt for Higgs particles of mass higher than the Z. Up to 2M v or, roughly, up to 200 GeV/c the main decay mode would still be that into the heaviest pair of quarks allowed by phase space whereas for m/f > 2Mw, or mn 200 GeV/c, the Higgs will decay predominantly into W W or as we have already mentioned. 

If, finally, the Higgs is very heavy and mn > 2mt, the most promising channel appears to be H Z Z —> ffi O with / = e or p. This avoids all QCD background problems and has no additional difiiculties in the event that mt > Mw (as we now expect) since the main t-quark decay mode would then be t Wb and each tt pair produced would just increase the H W W background. Because of the small branching ratio ( 8 X10 ) of the reaction H —> —> ffi/u, the estimate is that... 

The subject of heavy flavours has e3q>anded tremendously in recent years stretching from the static properties (mainly spectroscopy, i.e. energy levels, lifetimes, branching ratios, decays, mixing etc.) of hadrons with one or more heavy quarks, e.g. bottom or charm, to more dynamical properties (like fragmentation, structure functions, jets etc.) and on to more exotic topics, e.g. production and decay of as yet undiscovered flavours like top, or speculations on a fourth generation or imphcations on Higgs or on non-standard effects and so on. 

A new chapter on heavy flavour physics has been initiated at LEP in its studies of decays. The main motivation for heavy flavour searches, i.e. the quest for top (and Higgs), has been frustrating so far and we have repeatedly commented about the top mass limits set by LEP [eqn (8.6.20)] ... 

It should be noted that the heavy mass of the r (and, therefore, its ability to decay weakly into ordinary hadrons) makes it a unique probe of the coupling to the standard quarks and leptons of any new weak boson. Also, the T may represent one of the dominant and more tractable decay modes of charged Higgs bosons. 

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