Heavy charged Higgs boson production at next generation $gammagamma$ colliders

a r X i v :h e p -p h /0211055v 1 5 N o v 2002

CERN-TH/2002-291

IPPP/02/59DCPT/02/118November 2002

Heavy charged Higgs boson production

at next generation γγcolliders

Stefano Moretti ?

Theory Division,CERN,CH-1211Geneva 23,Switzerland

and

Institute for Particle Physics Phenomenology,University of Durham,Durham DH13LE,UK

Shinya Kanemura ?

Theory Group,KEK,1–1Oho,Tsukuba,

Ibaraki 305–0801,Japan

Abstract

We investigate the scope of all relevant production modes of charged Higgs bosons in the MSSM,with mass larger than the one of the top quark,at future Linear Colliders operating in γγmode at the TeV energy scale.Final states with one or two H ±bosons are considered,as produced by both tree-and loop-level interactions.

Keywords:Higgs Physics,Supersymmetry,Linear Colliders,Photon-photon Interactions

In the Minimal Supersymmetric Standard Model(MSSM)it is not unnatural to assume that the typical mass of the Supersymmetric(SUSY)partners of ordinary matter is at the

TeV scale or above–well in line with current experimental bounds–this rendering the Higgs sector a privileged probe to access physics beyond the SM.In this respect,it would be intriguing to detect charged Higgs states(henceforth denoted by H±),as in this case

one would unquestionably be in presence of some non-standard phenomena.In fact,even the discovery of a(light)neutral Higgs boson,would leave open questions as to whether it belongs to the SM or else the MSSM,since in the so-called‘decoupling regime’of the latter(i.e.,when

a hierarchy exists among the masses of the?ve Higgs states:M H0～M A0～M H±?M h0) the fundamental properties of such a particle(quantum numbers,couplings,branching ratios,

etc.)would be the same in both models1.

Rumours of a possible evidence of light charged Higgs bosons being produced at LEP2 [1]have faded away.One is now left with a model independent limit on M H±,of order

M W±.However,within the MSSM,the current lower bound on a light Higgs boson state,of approximately120GeV(from LEP2),can be converted into a minimal value for the charged

Higgs boson mass,of order140GeV or so(at small values of tanβ,the ratio of the vacuum expectation values of the two Higgs doublet?elds2).In the mass interval140GeV<～M H±<～m t, charged Higgs bosons could well be found at Tevatron(Run2)[2],which has already begun

data taking at

√

s pp =14TeV)at CERN.Even there though,because of the dependence of

the production cross section of charged Higgs bosons upon tanβ,there is no certainty that these particles will be accessible to the experiments.This happens if tanβis in the so-called ‘intermediate’regime,starting at around6or7for M H±～m t and encompassing more and more parameter space as M H±grows larger,no matter the channels in which the charged Higgs boson decays to,as long as the latter only include ordinary SM objects and neutral Higgs states[4].Not coincidentally,over the same area of the(M H±,tanβ)parameter plane, there is no coverage through the neutral Higgs sector of the MSSM either.

Lowering the SUSY mass scale may induce new interactions among neutral/charged Higgs boson states and sparticles,so that the former may abundantly be produced in the decay of the latter(gluinos and squarks for example[5])or,alternatively,new Higgs decay channels into light SUSY particles may well open at pro?table rate(e.g.,into chargino-neutralino pairs[6]).This unfortunately implies a proliferation of MSSM parameters rendering the phenomenological analysis very cumbersome.

With the option of an e+e?Linear Collider(LC)[7]being possibly available within a few years of the beginning of the LHC,also operating in e±γandγγmodes(both at an energy scale similar to the one of the primary electron-positron design,i.e.,

√

1In practice,decoupling occurs for M H0,M A0and/or M H±around and above200GeV.

2Which,together with M H±,or M A0(the mass of the pseudoscalar Higgs boson),uniquely de?nes the MSSM Higgs sector at tree-level.

is very instructive to assess the potential of this kind of machine in complementing the LHC

in the quest for such elusive,yet crucial particles for understanding the Higgs mechanism. Besides,the ability to polarise the incoming particles,both electron3and photon beams4,is a de?nite advantage of future LCs with respect to the LHC.

Historically,with some exceptions,it was mainly the pair production modes of charged Higgs boson states,i.e.,e+e?→H?H+,e±γ→e±H?H+andγγ→H?H+,that were considered in some detail[10,11,12].However,the exploitation of these channels alone may clearly be insu?cient to clarify the real potential of future LCs in investigating the Higgs sector,especially considering that in the MSSM framework twice the heavy H±mass values

may mean that the rest mass of H?H+pairs is already comparable to the minimal energy

√

foreseen for these machines.Needless to say,whenever2M H±exceeds

3Some proposals also exist for polarising positrons[9].

4See,e.g.,Ref.[13]for an example of new physics e?ects which can be probed by using polarisedγ-beams to produce H±Higgs states.

5In the case of photon-photon collisions,charged Higgs bosons can also be produced as virtual states,e.g., in the loop enteringγγ→Higgs processes.Such channels can be used as a means to distinguish between various possible Higgs scenarios,e.g.:SM,MSSM and/or a general Two-Higgs Doublet Model(2HDM)[16].

Diagrams by FeynmanDraw

A

A H

H H

graph 1

1

2

3

4

A

A

H H

H

graph 2

1

2 3

4

A

A

H

H

graph 3

1

2

3

4

Figure 1:Feynman diagrams for process (1).The labels A and H refer to a photon and a charged

Higgs boson,respectively.

integrations over the ?nal state phase space (and photon momentum fractions,see below)have been performed by a variety of methods,for cross-checking purposes:by using VEGAS [19],RAMBO [20]and Metropolis [21].In the case of process (1),we have found agreement with previous literature.

For the 2HDM parameters,we assumed the MSSM throughout.For the SM ones,we adopted the following:m b =4.25GeV,m t =175GeV,m e =0.511MeV,m τ=1.78GeV,m ν=0,M W

The back-scattered photon ?ux has been worked out in [8],where all details of the deriva-tion can be found.For brevity,we do not reproduce here those formulae,rather we simply recall to the un-familiar reader the basic features of γγscatterings initiated by laser light at e +e ?LCs.We assume that the laser back-scattering parameter z of [8]assumes its maximum value,z ≡z max =2(1+

√s e +e ?,where M X is the rest mass in the ?nal state of (1)–(7).Finally,

one can cast the production cross sections in the following form:

σe +e ?→γγ→X (s )=

dx +dx ?F γ+(x +)F γ

?(x ?)?σγγ→X (?s ),

(8)

where x +(?)is the electron(positron)momentum fraction carried by the emerging photon,

x +x ?=?s γγ/s e +e ?,with s e +e ?(?

s γγ)being the centre-of-mass (CM)energy squared of the e +e ?(γγ)system,and F γ

±(x ±)the photon distribution functions,de?ned in terms of x ±.(As γ-structure functions we have used those of Ref.[8].)

Diagrams by FeynmanDraw

1

2

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1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

Figure 2:Feynman diagrams for processes of the type (2)–(3).The labels D/U,A and H refer to

a d /u -type (anti)quark,a photon and a charged Higgs boson,respectively.

Diagrams by FeynmanDraw

A

A

H W Phi W W

graph 1

1

2

5

4 3 A

A

H

W Phi H

H graph 2

1

2

5

4 3

A

A

H

W

Phi W

H graph 3

1

2

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4 3A

A

H W

Phi W

W graph 4

1

2

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4

3 A

A

H

W Phi H H

graph 5

1

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3 A

A

H W

Phi W

H

graph 6

1

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3A

A

H W

Phi W

graph 7

1

2

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3 A

A

H

W

Phi H

graph 8

1

2

5 4 3

A

A

H W

Phi W

graph 9

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3 A

A

H W

Phi W

graph 10

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1

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3A

A

W H

Phi H

graph 11

1

2

4

5

3 A

A

W H

Phi H

graph 12

2

1

4

5

3Figure 3:Feynman diagrams for processes of the type (4)–(6).The labels A,W and H(Phi)refer

to a photon,a W ±gauge boson and a charged(neutral)Higgs boson,respectively.

Diagrams by FeynmanDraw A W A

H ...............................................graph 1

1

2

4

3

Figure 4:Feynman diagrams for process (7).The labels A,W and H refer to a photon,a W ±

gauge boson and a charged Higgs boson,respectively.The ‘blob’signi?es all possible one-loop

contributions,as seen in Figs.1–2of [18].

The cross sections of processes (1)–(7)can be found in Figs.5–6,respectively,for four reference choices of tan β.In all our plots and in the discussion,charge conjugated (c.c.)contributions are always included.For brevity,we limit ourselves to the representative case of √s γγ≈0.8

√

s γγ>2M H ±,independently of tan β,as expected.At and above the

threshold point

√

s γγ<～

2M H ±are not very large,as they never exceed the fraction of femtobarns.After 1ab ?1of accumulated luminosity,one should expect at best 100events or so,both at small and large tan β.Moreover,given the dependence upon this parameter of the three leading modes,the intermediate tan βregion (i.e.,around 7or so)would have little coverage,only through charged Higgs production in association with a W ?boson,yielding typical production rates that are one order of magnitude smaller than those seen for extreme values of this parameter (1.5and 40).

Such small cross sections inevitably require one to select the dominant decay channel of

√

Figure5:Total cross sections for process(1)at

s

e+e?≈2M H±,with statistical signi?cances between3σand5σ,in correspondence of1and5ab?1of accumulated luminosity.Given that the starting signal-to-background ratio(S/B)is here not much di?erent from the case of the corresponding e+e?initiated process(the signal here also being burdened by top-antitop production and decay as dominant background),one should expect the same happening in the context of photon-photon collisions,albeit with a reduced charged Higgs mass scope,since the CM energy is smaller in this case(assuming a contemporaneous running in e+e?andγγmodes).

Figure6:Total cross sections for(clockwise)processes(2),(3),(4),(5),(6)[here,the four curves

√

in the plot coincide within graphical resolution]and(7)plus c.c.at

s

e+e?the more(less)relevant process(3)becomes with respect to reactions(2)and(7).

In summary,total cross sections of heavy charged Higgs bosons with mass similar to or larger than approximately half the collider CM energy and produced viaγγmodes compare well to the corresponding e+e?ones in most cases.In absolute terms,the latter are larger

√

at smaller energies whereas the former grows relatively with

,the latter being singly produced at a rate

s

e+e?/γγ

of O(10?1fb)at best.It will presumably be the interplay between the typical mass scale of the charged Higgs bosons(that one could,e.g.,either have a direct hint of from data or else estimate indirectly within the MSSM from the measured value of M h0at Tevatron and/or the LHC)and the machine performance in producing mono-chromatic Compton back-scattered photons that will eventually dictate whether to put more e?ort inγγor e+e?analyses in the quest for such particles at next generation LCs.

However,the running time to be spent on each mode will most likely depend on the measured value of M h0.On the one hand,it should be recalled that in electron-positron annihilations the CM energy is typically higher but the lightest Higgs boson is always pro-duced in association with some other particles(hence,with a phase space suppression):a Z0(Higgs-strahlung),aνeˉνe/e+e?pair(W+W/Z0Z0fusion)or the pseudoscalar Higgs state (pair production).On the other hand,in photon-photon scatterings,h0states are produced singly,via a loop of charged(s)particles,but with a reduced energy and possibly,if the Higgs width is rather small,also o?-resonance.Whichever the case,should the close investigation of h0(and,possibly,H0and A0)signatures need to be supported by the detection of charged Higgs states in order to clarify the nature of the EW symmetry breaking,a LC with the option of photon beams will be well placed in pursuing this task,over a considerable M H±range,provided the value of tanβis either large or small.

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