Among the possible decay modes of Higgs particles into supersymmetric states,
neutralino and chargino decays play a prominent r\^ole. The experimental
opportunities of observing such decay modes at LEP2 and at future e+e- linear
colliders are analyzed within the frame of the Minimal Supersymmetric extension
of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay
modes can be very important, while only a small window is open for the lightest
CP-even Higgs particle. If charginos/neutralinos are found at LEP2, such decay
modes can be searched for in a small area of the parameter space, and invisible
decays may reduce the exclusion limits of the lightest CP-even Higgs particle
slightly; if charginos/neutralinos are not found at LEP2 in direct searches,
the Higgs search will not be affected by the SUSY particle sector.Comment: 13 pages including 4 figures, uses latex and (e)psfig.st

A. Djouadi

Baer

Barger

Carena

Djouadi

El Kheishen

Giudice

Griest

Gunion

Haber

J. Kalinowski

Janot

P. Janot

P.M. Zerwas

Zerwas

Physics Letters B

English

arXiv

Among the possible decay modes of Higgs particles into supersymmetric states, neutralino and chargino decays play a prominent rôle. The experimental opportunities of observing such decay modes at LEP2 and at future e+e− linear colliders are analyzed within the frame of the Minimal Supersymmetric extension of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay modes can be very important, while only a small window is open for the lightest CP-even Higgs particle. If charginos/neutralinos are found at LEP2, such decay modes can be searched for in a small area of the parameter space, and invisible decays may reduce the exclusion limits of the lightest CP-even Higgs particle slightly; if charginos/neutralinos are not found at LEP2 in direct searches, the Higgs search will not be affected by the SUSY particle sector

Djouadi, A.

Janot, P.

Kalinowski, J.

Zerwas, P. M.

DESY

SUSY decays of Higgs particles

Among the possible decay modes of Higgs particles into supersymmetric states, neutralino and chargino decays play a prominent rôle. The experimental opportunities of observing such decay modes at LEP2 and at future $\ee$ linear colliders are analyzed within the frame of the Minimal Supersymmetric extension of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay modes can be very important, while only a small window is open for the lightest CP--even Higgs particle. If charginos/neutralinos are found at LEP2, such decay modes can be searched for in a small area of the parameter space, and invisible decays may reduce the exclusion limits of the lightest CP-even Higgs particle slightly; if charginos/neutralinos are not found at LEP2 in direct searches, the Higgs search will not be affected by the SUSY particle sector.Among the possible decay modes of Higgs particles into supersymmetric states, neutralino and chargino decays play a prominent r\~ole. The experimental opportunities of observing such decay modes at LEP2 and at future e+e- linear colliders are analyzed within the frame of the Minimal Supersymmetric extension of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay modes can be very important, while only a small window is open for the lightest CP-even Higgs particle. If charginos/neutralinos are found at LEP2, such decay modes can be searched for in a small area of the parameter space, and invisible decays may reduce the exclusion limits of the lightest CP-even Higgs particle slightly; if charginos/neutralinos are not found at LEP2 in direct searches, the Higgs search will not be affected by the SUSY particle sector.Among the possible decay modes of Higgs particles into supersymmetric states, neutralino and chargino decays play a prominent rôle. The experimental opportunities of observing such decay modes at LEP2 and at future e + e − linear colliders are analyzed within the frame of the Minimal Supersymmetric extension of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay modes can be very important, while only a small window is open for the lightest CP-even Higgs particle. If charginos/neutralinos are found at LEP2, such decay modes can be searched for in a small area of the parameter space, and invisible decays may reduce the exclusion limits of the lightest CP-even Higgs particle slightly; if charginos/neutralinos are not found at LEP2 in direct searches, the Higgs search will not be affected by the SUSY particle sector

Zerwas, P.M.

CERN Document Server

CERN PPE/96-34DESY 95{212IFT{96{05KA{TP{08{96SUSY Decays of Higgs ParticlesA. Djouadi1;2, P. Janot3, J. Kalinowski4yand P.M. Zerwas11Deutsches Elektronen{Synchrotron DESY, D{22603 Hamburg/FRG.2Inst. Theor. Physik, Univ. Karlsruhe, D{76128 Karlsruhe/FRG.3PPE Division, CERN, CH{1211, Geneva 23/Switzerland.4Institute of Theoretical Physics, Warsaw University, PL{00681 Warsaw/Poland.AbstractAmong the possible decay modes of Higgs particles into supersymmetric states,neutralino and chargino decays play a prominent ro^le. The experimental op-portunities of observing such decay modes at LEP2 and at future e+e linearcolliders are analyzed within the frame of the Minimal Supersymmetric exten-sion of the Standard Model. For heavy Higgs particles, the chargino/neutralinodecay modes can be very important, while only a small window is open for thelightest CP{even Higgs particle. If charginos/neutralinos are found at LEP2,such decay modes can be searched for in a small area of the parameter space,and invisible decays may reduce the exclusion limits of the lightest CP-evenHiggs particle slightly; if charginos/neutralinos are not found at LEP2 in directsearches, the Higgs search will not be aected by the SUSY particle sector.Supported by Deutsche Forschungsgemeinschaft DFG (Bonn).ySupported in part by Committee for Scientic Research and by the EC under contract CHRX-CT92-0004.1. New light will be shed on the Higgs sector of supersymmetric theories in the forthcom-ing LEP2 experiments [1]. In fact, if the parameter tan  in the Minimal SupersymmetricStandard Model (MSSM) is realized within the range between 1 and 3 as suggestedby unication of b{ Yukawa couplings [2], prospects of discovering at least the lightestCP{even neutral Higgs boson h at LEP2 are very good. At future e+e linear colliders,the entire parameter space can be explored. The lightest Higgs boson h can be discoveredwith certainty, and the Higgs bosons H;A and Hare accessible in major parts of theparameter space.Most theoretical predictions of Higgs decay properties have focussed in the past onnal states built up by the electroweak gauge bosons, standard quarks/leptons and cascadedecays [3]. While decays to squarks and sleptons are expected to be less important andare in general forbidden kinematically for fairly light Higgs bosons, decays to neutralinosand charginosh;H;A ! 0i0jand +i j;H! 0ij; (1)[i; j = 1{4 for neutral, and i; j = 1; 2 for charged states], in particular to the light01states, could play a potentially important ro^le [4, 5]. The nal states would haveinteresting topologies. If R{parity is conserved and 01is the lightest supersymmetricstable particle (LSP), the 0101nal states are invisible. In the other modes, 0iand idecay cascades [5, 6] would come with large missing energies in the events. For instance,the decays h;A ! 0102could lead to nal states 0101ff , with the two LSPs escapingundetected.These novel decay modes can only be relevant if the standard decay modes are notoverwhelming. Prime interest is therefore restricted to low to moderate tan  values sinceb and  decays are otherwise dominant. It turns out a posteriori that when  decaychannels are open, the branching ratios can be close to 100%, opening opportunities forthe experimental analysis of the fundamental parameters in supersymmetric theories.In this note, possible  decays of Higgs particles are analyzed in the parameter rangeof LEP2 and future e+e linear colliders which should eventually run with energies up to 2TeV, sweeping the entireMSSM Higgs spectrum up to masses of order 1 TeV [7]. Theseunconventional Higgs decays, in particular the invisible 0101nal states, could changethe experimental analyses for discovering MSSM Higgs particles at the LHC ratherdramatically, making the search of these particles much more dicult. This problem ismuch less severe at e+e colliders so that theMSSM Higgs parameter space can still becovered completely [8]. It would only be more dicult to detect the CP{odd A particlefor small tan where it is produced only in association with the light CP{even particleh, leading to invisible events altogether if both particles decay into LSP nal states.However, since the branching ratios for 0101is always less than unity [especially whenother  decays are kinematically possible], such a problem can be solved by increasingthe integrated luminosity.2In the subsequent analysis, the fact is taken into account that neither charginos norneutralinos have been found at LEP1.5 [9].2. Besides the conventional parameters of the MSSM Higgs sector, the  masses andcouplings to the Higgs particles are determined by the Higgs{higgsino mass parameter and the SU(2) gaugino mass parameter M2.The general chargino mass matrix [10],MC="M2p2MWsin p2MWcos #; (2)is diagonalized by two matrices U and V , leading to the 1;2massesm1;2=1p2nM22+ 2+ 2M2Wh(M22  2)2+ 4M2Wcos22 + 4M2W(M22+ 2+ 2M2 sin 2)i1212: (3)When one of the two parameters  or M2is large, one of the charginos corresponds toa pure gaugino state while the other corresponds to a pure higgsino state; in these casesthe masses approach the asymptotic values:jj MZand M2MZ: m1M2; m2 jj;jj MZand M2MZ: m1 jj ; m2M2: (4)The four-dimensional neutralino mass matrix depends on the same two mass param-eters if the SUGRA relation M1=53tan2WM2'12M2[10] is used. In the bino{wino{higgsino basis ( i~B; i~W3;~H01;~H02) the matrix has the formMN=26664M10  MZsin Wcos MZsin Wsin0 M2MZcos Wcos  MZcos Wsin MZsin Wcos  MZcos Wcos 0  MZsin Wsin  MZcos Wsin    037775: (5)It can be diagonalized analytically [11] by a single matrix Z; as the nal results for themasses are rather involved, they should not be recorded here. Again, for large values ofone of the parameters  or M2, two neutralinos are pure gaugino states while the twoothers are pure higgsino states, and the 0masses are given in these limits byjj MZand M2MZ: m01M1; m02M2; m03 m04 jj;jj MZand M2MZ: m01 m02 jj ; m03M1; m04M2: (6)A typical set of the neutralino/chargino masses [the lightest of which could be observedat LEP2] is shown as a function of  in Fig.1a for M2= 150 GeV and tan = 1:6.The non{observation of chargino and neutralino production e+e ! +1 1and 0201at3LEP1.5 [9] roughly translates to lower limits on 2m+1and m02+m01of 135 GeV, so thatthe range  40 GeV<<140 GeV is ruled out forM2= 150 GeV and tan  = 1:6. [For = 0 and also for moderate  values, the lightest neutralino and chargino states couldhave been massless|these values are of course already ruled out by the negative searchof chargino states at LEP1.]3. The decay widths of the Higgs bosons (Hk) = (H;h;A;H) into neutralino andchargino pairs are given by [4] (Hk! ij) =GFM2W2p2MHk1=21 + ij"(F2ijk+ F2jik) 1  m2iM2Hk m2jM2Hk! 4kijFijkFjikmimjM2Hk#; (7)where 1;2;4= +1, 3=  1 and ij= 0 unless the nal state consists of two identical(Majorana) neutralinos in which case ii= 1; i= 1 is the sign of the neutralino masseigenvalue [i= 1 for charginos];  = (1 m2i=M2Hk m2j=M2Hk)2  4m2im2j=M4Hkis theusual two{body phase space function.In the case of neutral Higgs boson decays, the coecients Fijkare related to theelements of the matrices U; V for charginos and Z for neutralinos,Hk! +i j: Fijk=1p2[ekVi1Uj2  dkVi2Uj1] ;Hk! 0i0j: Fijk=12(Zj2  tan WZj1) (ekZi3+ dkZi4) + i$ j; (8)with the coecients ekand dkgiven bye1=d1= cos=   sin ; e2=d2= sin= cos  ; e3=d3=   sin = cos : (9)The partial widths of the neutral Higgs particles for decays to the lightest neutralino orchargino states can be written in a particularly simple form, (Hk! 11) =GFM2W2p2MHk"1 4m21M2Hk#p2k; (10)with p = 3 for h;H and p = 1 for A, corresponding to P{ and S{wave nal states asrequired by the CP= quantum numbers of the Higgs bosons.For the light scalar and the pseudoscalar Higgs decays to the lightest neutralinos,h=A! 0101, the coecients kare given byh= (Z12  tan WZ11) (sinZ13+ cosZ14) ;A= (Z12  tan WZ11) (  sin Z13+ cos Z14) : (11)4For the decays to the lightest charginos, h=A! +1 1, the coecients kare of the formh=1p2(sin V11U12  cosV12U11) ;A=1p2(  sin V11U12+ cos V12U11) : (12)It follows from Eq.(11) that the Higgs bosons couple to mixtures of gaugino and higgsinocomponents of the lightest neutralino, corresponding to the matrix elements Z11; Z12andZ13; Z14respectively. If the 01were either a pure gaugino or a pure higgsino state, theh0101and A0101couplings would vanish. The matrix elements are displayed for the sameset of parameters as for the neutralino/chargino masses in Fig.1b. It is obvious from thegure that decays to LSPs play a less prominent ro^le for  < 0 than for  > 0 since fornegative , the 01state tends to be either gaugino{ or higgsino{like. In this case, onlyin a small  window around  M1do the couplings reach the level of a few percent, asis shown in Fig.1c. [For the values tan  = 1:6 and MA= 100 GeV that were chosen forillustration, the couplings hand Aare approximately equal.]For the charged Higgs boson, the decay widths into neutralino/chargino pairs [4] aregiven by Eq.(7), withFij4= cos "Zj4Vi1+1p2(Zj2+ tan WZj1)Vi2#;Fji4= sin "Zj3Ui1 1p2(Zj2+ tan WZj1)Ui2#: (13)4. Examples of branching ratios are shown in Fig.2a for Higgs boson masses Mhand MAtypically accessible at LEP2. The partial decay widths to R{even particles are treatedaccording to the current knowledge [3]. Whenever the 0101decay is kinematically allowed,the branching ratio is close to 100% for positive  values. For  < 0 the branching rationever exceeds the 20% level. As pointed out previously, the branching ratios becomesmaller for increasing tan, except when h reaches its maximal mass value: the hbbcoupling is no longer enhanced in this case. These Mhvalues are not accessible at LEP2,but they would be accessible at future colliders.The branching ratios for the sum into all possible neutralino and chargino statesare shown in Fig.2b for (H;A;H) particles which can be produced at high{energy e+e colliders. Mixing [12] in the Higgs sector has been treated properly for  6= 0, At=p6MS[so-called \maximal mixing"] and Ab= 0, with MS= 1 TeV. These branching ratios arealways large except in three cases: (i) For H in the mass range between 140 and 200 GeV,especially if  > 0, due to the large value of BR(H ! hh); (ii) For small A masses andnegative  values as discussed above; and (iii) For Hjust above the tb threshold if notall the decay channels into heavy  states are open. [A similar pattern holds for the other5values of  and M2discussed above.]Even above the thresholds of decay channels including top quarks, the branching ratiosfor the decays into charginos and neutralinos are sizeable. For very large Higgs bosonmasses, they reach a common value of  40% for tan  = 1:6. In fact, as a consequenceof the unitarity of the U; V and Z matrices, the total widths of the three Higgs bosondecays to charginos and neutralinos do not depend on M2,  or tan  in the asymptoticregime MHk m, (Hk!Xi;jij) =3GFM2W4p2MHk1 +13tan2W; (14)giving rise to the branching ratioBR(Hk!Xi;jij) =1 +13tan2WM2W1 +13tan2WM2W+m2tcot2 +m2btan2; (15)Only the leading tt, bb modes for neutral and the tb modes for the charged Higgs bosonsneed be included in the total widths. This branching ratio is shown in Fig.2c as a functionof tan . It is always large, even for extreme values of tan  1 or 50, where it still is atthe 20% level.5. To discuss the impact of invisible decays on the search of Higgs particles at LEP2, twocases must be distinguished.(i) The region in the [;M2] parameter space in which CP-even h Higgs boson decaysinto the lightest neutralino 0101can be observed at LEP2, is illustrated in Fig. 3a for themassMh= 90 GeV. The same mixing pattern as before has been considered:  6= 0, At=p6MSand Ab= 0, withMS= 1 TeV. To nd the region which is experimentally accessibleby the search for invisible decays of the Higgs boson, the product Rinv= BR(h! 0101)sin2(   ) has to exceed 0.45 [13] under the LEP2 luminosity conditions. The smallparameter range in which the invisible Higgs decays could be detected is shown in Fig. 3atogether with the corresponding [;M2] range which can be investigated by direct charginoand neutralino searches at LEP2, e+e ! +i jand 0i0j. For the latter, a minimumtotal cross-section of 100 fb [14], relevant for a total integrated luminosity of 150 pb 1that should be delivered each year to the four LEP experiments, was required for largemass splitting between the lightest neutralino and the charginos and other neutralinos,and the unavoidable loss of eciency for small mass dierences was parameterized in arealistic manner [15].It can immediately be seen from this example that the region in which the light Higgsboson h could be detected in the 0101decay mode is small if the LEP1.5 results are takeninto account, and it is embedded completely in the area covered by direct neutralino andchargino searches. Since this holds true for any Higgs boson mass at all centre-of-massenergies of LEP2, the [;M2] area cannot be extended through the Higgs boson search.(ii) The exclusion limits of the lightest CP{even Higgs boson h are aected by invisibledecay modes. This is shown at tan  = 1:6 for a few examples of the parameters  and6M2in Fig. 3b. It is well-known that LEP2 covers the entire h mass range for tan<2 ifh decays only into SM particles [13]. However, it is apparent from the examples in Fig. 3bthat for  and M2values in a small strip, adjacent to the upper right boundary of thecrossed range in Fig. 3a, the exclusion limit is signicantly smaller than the theoreticalupper Higgs mass limit. For these values of  and M2, both Rinvand Rvis= BR(h !bb) sin2( ) drop below the lower limits, given in Figs.31 and 32 of Ref.[13], for whichHiggs events could be detected in either of the two channels. Thus, in a small area of theSUSY parameter space Higgs bosons could escape undetected under the LEP2 runningconditions even for low tan.7References[1] Report \Higgs Physics at LEP2", M. Carena, P.M. Zerwas (conv.) et al, Proceedingsof the LEP2 Workshop, G. Altarelli, T. Sjostrand and F. Zwirner (eds), CERN 1995.[2] M. Carena, S. Pokorski and C.E.M. Wagner, Nucl. Phys. B406 (1993) 59; V. Barger,M. Berger and P. Ohmann, Phys. Rev. D47 (1993) 1093 and 2030.[3] For recent updates, see A. Djouadi, M. Spira and P.M. Zerwas, Report DESY 95{210 (hep{ph{9511344), Z. Phys. C (in press); A. Djouadi, J. Kalinowski and P.M.Zerwas, Report DESY 95{211 (hep{ph{9511342), Z. Phys. C (in press).[4] H. Baer, D. Dicus, M. Drees and X. Tata, Phys. Rev. D37 (1987) 1367; J.F. Gunionand H.E. Haber, Nucl. Phys. B307 (1988) 445; K. Griest and H.E. Haber, Phys.Rev. D37 (1988) 719.[5] A. Djouadi, J. Kalinowski and P.M. Zerwas, Z. Phys. C57 (1993) 569.[6] J.F. Gunion and H.E. Haber, Nucl. Phys. B272 (1986) 1; ibid. B278 (1986) 449.[7] P. Janot, \Higgs boson search at a 500 GeV e+e collider", LAL 93-38 (1993) andProceedings of the Second Workshop on Physics and Experiments with Linear e+e Colliders; P.M. Zerwas, Proceedings, International EPS Conference on High-EnergyPhysics, Marseille 1994; A. Djouadi, Int. J. Mod. Phys. A10 (1995) 1.[8] P. Janot, \Testing the neutral Higgs sector of theMSSM with a 300-500 GeV e+e collider", LAL 91-61 (1991) and DESY 92-123A (1992) 107.[9] D. Buskulic et al. (ALEPH Coll.), \Search for supersymmetric particles in e+e collisions of centre-of-mass energies of 130 and 136 GeV", CERN PPE/96{10 (subm.to Phys. Lett. B); forthcoming reports of the other collaborations on the LEP1.5results.[10] H.E. Haber and G. Kane, Phys. Rep. C117 (1985) 75.[11] M. El Kheishen, A. Shak and A. Aboshousha, Phys. Rev. D45 (1992) 4345.[12] M. Carena, J. Espinosa, M. Quiros and C. Wagner, Phys. Lett. B355 (1995) 209.[13] P. Janot et al., in Ref.[1].[14] Report \Searches for New Physics at LEP2", G.F. Giudice, R. Ruckl (conv.) et al,Proceedings of the LEP2 Workshop, G. Altarelli, T. Sjostrand and F. Zwirner (eds),CERN 1995.[15] L. Duot and M. Schmitt, private communication.8Fig. 1: (a) Chargino [dashed lines] and neutralino [solid lines] masses as a function of; (b) the mixing matrix elements Z1iof the lightest neutralino as a function of ; (c) thecouplings hand Aof the h;A Higgs bosons to 01as a function of . M2is xed to 150GeV and tan  = 1:6.9Fig. 2a: Branching ratios of the decays of the h and A bosons into the lightest neutralinopair 0101as a function of the Higgs masses for a set of  and M2values; tan  = 1:6.10Fig. 2b/c: Branching ratios of the decays of the heavy A [solid], H [dashed] and H[dot{dashed] Higgs bosons into the sum of neutralino and chargino pairs as a functionof the Higgs mass [for a set of =M2values and tan  xed to 1.6] in (b1) and (b2).The inclusive  decay branching ratio as a function of tan in the asymptotic region[MAMHMH= 1 TeV  m] in (c).11Fig. 3a: In the [;M2] parameter space, ranges covered at LEP2, with a centre-of-massof 192 GeV and an integrated luminosity of 150 pb 1delivered to each of the four ex-periments, by the direct chargino and neutralino searches, and by the search for invisibleHiggs decays h! 0101for Mh= 90 GeV and tan  = 1:6, (maximal mixing conditions).The dash{dotted lines represent the boundary of [;M2] values after the LEP1.5 resultsare taken into account.12Fig. 3b: Exclusion limits on the mass of the lightest CP{even Higgs boson h for a setof [;M2] values and tan = 1:6. The full lines refer to the search based on the invisible0101decay channel, the dashed lines to bb decays of h. The invisible decays are dominantwhenever this channel is open, the fast fall{o corresponds to the kinematical boundary.13

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