Production and Decays of Neutral Higgs Bosons in the 2HDM and MSSMin the 2HDM and MSSM

In the 2HDM the production of the light and heavy neutral CP-even Higgs bosons, h and H, through the Higgs-strahlung mechanism:

e⁺e⁻→Z^∗ →hZ(HZ) (3.6)

is complemented with the processes

e⁺e⁻ →Z^∗ →hA(HA). (3.7)

The cross sections of the processes (3.6) and (3.7) are related to the cross section of

35

Z e

⁻

e

⁺

h A

Figure 3.5: Associated pair production in the 2HDM: The CP-even Higgs boson h is produced together with the CP-odd Higgs boson A.

the Higgs-strahlung process in the SM,σ_HZ^SM, and the 2HDM parameters α and β in the following way:

σ_hZ= sin²(β−α)σ_HZ^SM (3.8)

σ_HZ = cos²(β−α)σ^SM_HZ (3.9)

σ_hA= cos²(β−α)¯λ_hAσ_HZ^SM (3.10) σHA= sin²(β−α)¯λHAσ_HZ^SM (3.11) The factor ¯λ, defined as

λ¯_hA,HA = λ^3/2_hA,HA

λ^1/2_hZ,HZ[12m²_Z/s +λ_hZ,HZ], (3.12) accounts for the correct suppression of the P-wave cross section near the kinematic threshold. The quantity λ_ij = [1−(m_i+ m_j)²/s] [1−(m_i−m_j)²/s] is the usual momen-tum factor of the two particle phase space. The angular distributions exhibit the stan-dard behaviour expected for the Higgs-strahlung mechanism and the spin-zero associated pair production process [44]:

dσ dcosΘ ∼







λ_hZ,HZsin²Θ + 8m²_Z/s for e⁺e⁻ →hZ(HZ), sin²Θ for e⁺e⁻ →hA(HA).

(3.13) The couplings of the Higgs bosons to fermions involve as a scale factor geometric func-tions of the parameters α and β as can be seen from Table 2.4. Consequently, the corresponding partial widths are proportional to the square of these factors and in the case of the type II model read:

Γ_{h``</sub> = <sup>sin</sup><sub>cos</sub><sup>2</sup>2<sup>α</sup>β Γ<sup>SM</sup><sub>h``}, ΓH`` = <sup>cos</sup><sub>sin</sub>2<sup>2</sup>β<sup>α</sup> Γ<sup>SM</sup><sub>h``</sub>, ΓA`` = tan<sup>2</sup>β Γ<sup>SM</sup><sub>h``</sub>,

Γ_hu¯u = ^cos_sin2²β^α Γ^SM_hu¯u, Γ_Hu¯u = _cos^sin22^αβ Γ^SM_hu¯u, ΓAu¯u = cot²βΓ^SM_hu¯u,

Γ_hd¯d = _cos^sin22^αβ Γ^SM_hd¯d, Γ_Hd¯d= ^cos_sin2^2αβ Γ^SM_hd¯d, Γ_Ad¯d= tan²βΓ^SM_hd¯d,

(3.14)

36 3.2 Production and Decays of Neutral Higgs Bosons in the 2HDM and MSSM

with Γ^SM_Hf¯f denoting the partial width of the SM Higgs boson decay into the corresponding fermion antifermion pair.

10^-2 10^-1 1

40 60 80 100

m

_h

(GeV)

σ (pb)

σ(hZ), tanβ=2

σ(hA), tanβ=25

σ(hA), tanβ=2

σ(hZ), tanβ=25 10^-2

10^-1 1

40 60 80 100

m

_h

(GeV)

σ (pb)

σ(hZ), tanβ=2

σ(hA), tanβ=25 σ(hA), tanβ=2

σ(hZ), tanβ=25

a) “No mixing” scenario b) “m_h-max” scenario Figure 3.6: The cross section of the hZ and hA production at √

s=206 GeV in a)

“no mixing” and b) “m_h-max” scenarios for tanβ = 2 and 25. Curves stop at values corresponding to the upper theoretical bounds on m_h for a given tanβ value. The point mh ≈ 45 GeV corresponds to mh+ mA ∼ mZ. This causes the sudden change in the e⁺e⁻ →hA cross section evolution as for m_h ≤45 GeV the hA production is dominated by the radiative return to the Z resonance, which cannot kinematically contribute beyond this point.

There are no strong theoretical arguments favouring a particular choice of the pa-rameters α and β and thus predicting specific branching fractions for the Higgs bosons in 2HDM. Due to the fact that up-type and down-type fermions couple to the Higgs bosons in a different way, scenarios are possible in 2HDM in which couplings of the Higgs bosons to down-type fermions are reduced while couplings to up-type fermions are enhanced. An extreme scenario is realised by setting tanβ to very low values, tanβ 1, cotβ 1. In this case h and A decays into c¯c and via charm and top quark loops into a gluon pair are significantly enhanced and supplant decays to b¯b and τ⁺τ⁻.

The structure of the Higgs sector in the MSSM corresponds to the 2HDM of type II.

Hence the production mechanisms for the neutral Higgs bosons of the MSSM are identical to those described above. The dependence of the e⁺e⁻ →hZ and e⁺e⁻→hA cross sections on m_h is shown in Figure 3.6 at two representative tanβ values, 2 and 25, for the “m_h-max” and “no mixing” scenarios. At tanβ &10 and m_h .100 GeV, the quantity sin²(β−α) is close to zero and only associated Higgs boson pair production contributes

37

Figure 3.7: The dependence of a) h mass, b) h decay branching fractions and c) A decay branching fractions on m_Ain the “no mixing” scenario at tanβ =0.4. At mA.42 GeV, m_h >2m_A and the h→AA channel opens. In addition due to a small value of tanβ the A→c¯c and A→gg decay modes supplant A→b¯b.

to the signal. For these models, m_h is predicted to be close to m_A, m_h ≈ m_A. The difference in mass reaches not more than 5 GeV. With decreasing tanβ and increasing m_h, the quantity sin²(β−α) increases leading to the rise of the e⁺e⁻ →hZ cross section and fall of the e⁺e⁻→hA cross section. As a consequence, at a certain point the Higgs-strahlung mechanism becomes dominant. The e⁺e⁻ →hZ process prevails, for instance, in the mass range m_h & 90 GeV at tanβ ≈ 8 and m_h & 60 GeV at tanβ ≈ 4.

For a larger part of MSSM parameter space the decay of h and A to b¯b is dominant followed by the decay toτ⁺τ⁻. However, at certain conditions other decay modes can be enhanced. As an example, Figure3.7illustrates the situation when the channel h→AA opens and in addition A→c¯c and A→gg decay modes supplant A→b¯b.

In the “large-µ” scenario the maximum allowed value of m_h is less than 108 GeV for any tanβ value, thus making the light Higgs boson, h, kinematically accessible at the highest LEP energies over the entire (tanβ,m_A) plane. For some choices of m_A and tanβ , the e⁺e⁻→hZ cross section is suppressed by a small value of sin²(β−α), and the e⁺e⁻ →hA process is kinematically inaccessible. For these models, however, the heavy

38 3.3 Limits on the Higgs Boson Mass

Higgs boson, H, has the mass less than 109 GeV and can be produced with relatively high cross section via the Higgs-strahlung process as illustrated in Figure3.8a. The detection of Higgs boson signal in the “large-µ” scenario is complicated by the pathological regions in the (tanβ,m_A) plane where the decay of either h or H to b¯b is suppressed due to large corrections from SUSY loop processes [48,49]. This situation is illustrated in Figure 3.8b. The suppression of the h(H)→b¯b decay mode is accompanied by an enhancement of the h(H)→c¯c,gg,WW^∗, τ⁺τ⁻ channels. For many of these models, the decay into a tau-lepton pair is also suppressed providing an additional experimental challenge.

10^-2 10^-1

80 100 120

m

_A

(GeV)

σ (pb)

Large-µ scenario tanβ=15 σ(hZ) σ(HZ)

σ(hA)

10 ^-2 10 ^-1 1

90 95 100 105 110 115

m

_A

(GeV)

Br(H)

Large-µ scenario tanβ=15

H→bb H→τ⁺τ -H→cc+gg H→WW^*

a) b)

Figure 3.8: a) The cross sections of the processes contributing to the Higgs boson signal at√

s = 206 GeV and b) the branching fractions of the heavy Higgs boson H as a function of m_Ain the “large-µ” scenario at tanβ = 15. Atm_A .110 GeV, m_H is about 108 GeV and H becomes accessible at LEP via the e⁺e⁻→HZ process. At m_A values around 94 GeV, the H → b¯b decay is suppressed whereas H →c¯c, gg, W W^∗ decays are enhanced.

Im Dokument Search for neutral higgs bosons in e+e- collisions (Seite 48-52)