We study single superparticle productions at the linear collider, putting
particular emphasis on resonant processes. We find that there exists a wide
region of model parameters where single chargino and neutralino productions
dominate over R-violating fermion-antifermion final states. For certain values
of mu and M_2 it is possible to produce even the heavier charginos and
neutralinos at significant rates, amplifying the total cross section and
obtaining interesting chains of cascade decays. Effects from initial-state
radiation are also included.Comment: 7 pages, 3 figures. Presented at the 2nd ECFA/DESY study on Linear
  Colliders, Frascati, November 1998 (alternative theories working group).
  Typos correcte

Lola, S.

English

arXiv

We study single superparticle productions at the linear collider, putting particular emphasis on resonant processes. We find that there exists a wide region of model parameters where single chargino and neutralino productions dominate over R-violating fermion-antifermion final states. For certain values of mu and M_2 it is possible to produce even the heavier charginos and neutralinos at significant rates, amplifying the total cross section and obtaining interesting chains of cascade decays. Effects from initial-state radiation are also included

DESY

Resonant single chargino and neutralino versus fermion anti-fermion production at the linear collider

We study single superparticle productions at the linear collider, putting particular emphasis on resonant processes. We find that there exists a wide region of model parameters where single chargino and neutralino productions dominate over R-violating fermion-antifermion final states. For certain values of $\mu$ and $M_2$ it is possible to produce even the heavier charginos and neutralinos at significant rates, amplifying the total cross section and obtaining interesting chains of cascade decays. Effects from initial-state radiation are also included.We study single superparticle productions at the linear collider, putting particular emphasis on resonant processes. We find that there exists a wide region of model parameters where single chargino and neutralino productions dominate over R-violating fermion-antifermion final states. For certain values of mu and M_2 it is possible to produce even the heavier charginos and neutralinos at significant rates, amplifying the total cross section and obtaining interesting chains of cascade decays. Effects from initial-state radiation are also included

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hep-ph/9912217CERN-TH/99-375RESONANT SINGLE CHARGINO AND NEUTRALINOVERSUS FERMION-ANTIFERMION PRODUCTIONAT THE LINEAR COLLIDER aS. LOLACERN Theory Division, CH-1211, Geneva 23, SwitzerlandWe study single superparticle productions at the linear collider, putting partic-ular emphasis on resonant processes. We nd that there exists a wide region ofmodel parameters where single chargino and neutralino productions dominate overR-violating fermion-antifermion nal states. For certain values of µ and M2, it ispossible to produce even the heavier charginos and neutralinos at signicant rates,amplifying the total cross section and obtaining interesting chains of cascade de-cays. Eects from initial-state radiation are also included.Although the main working tool for supersymmetry searches has been the Min-imal Supersymmetric Standard Model, the most general SU(3)cSU(2)LU(1)Yinvariant superpotential with the minimal eld content also contains the termsW = λijkLiLj Ek + λ0ijkLiQj Dk + λ00ijkUi Dj Dk (1)where L (Q) are the left-handed lepton (quark) superelds while E, D, and U arethe corresponding right-handed elds. If both lepton- and baryon-number violatingoperators were present at the same time in the low energy Lagrangian, they wouldlead to unacceptably fast proton decay; to avoid this, a symmetry that forbids theterms in (1), R-parity 1, has been invoked. However, it has been shown that thereexist symmetries which allow the violation of only a subset of these operators 2, re-sulting in a very rich phenomenology 3: single superparticle productions are allowed,while for couplings > 10−6, the lightest supersymmetric particle decays inside thedetector 4. In both cases, the standard missing energy signature is substituted bymultilepton and/or multijet events.There are three basic categories of new signals: Pair superparticle productions and subsequent decays via R-violating operators.Such processes are favoured for small R-violating couplings. For reasonably large R-violating couplings, single superparticle productions mayoccur. In this case, the mass reach can be considerably larger than for MSSMprocesses at the same machine. Virtual eects, from sparticle exchanges. These provide the optimal signals for avery heavy superparticle spectrum.Here, we put particular emphasis on resonant scalar-neutrino production andits subsequent decay to either sfermions or a single chargino or neutralino 6,7,8,9. Inparticular, we study the processese+e− ! (~ν) ! f f 0 and e+e− ! (~ν) !{`i ~χνi ~χ0aPresented at the 2nd ECFA/DESY study on Linear Colliders, Frascati, November 1998 (Al-ternative Theories working group).1ijk λijk Sources121 0.05 (0.5) charged current universality131 0.06 (0.6) Γ(τ ! eνν)/Γ(τ ! µνν)Table 1: Upper limits on couplings for mf˜ = 100 (1000) GeV.and identify for which regions of the supersymmetric parameter space each channelis expected to dominate.For a collider operating in the e+e− mode, the only couplings that involve twoelectrons are L1L2 E1 and L1L3 E1 (remember that from SU(2) invariance, the twolepton doublets cannot have the same flavour) b. The bounds for these couplings(Table 1) scale proportionally to the superparticle masses 5,6 and therefore, for aheavy sparticle spectrum, the couplings can be quite large.Close to the resonance (where the t- and u- channel exchanges can be neglectedin comparison to the s-channel pole), the cross section productions can be approx-imated by a Breit-Wigner formula. For instance, for single neutralino production,σ =8pism2ν˜Γ(~ν ! f f) Γ(~ν ! ν ~χ0)(s−m2ν˜)2 + m2ν˜Γ2total[s−m2χ˜0m2ν˜ −m2χ˜0]2! 8pim2ν˜B(~ν ! f f)B(~ν ! ν ~χ0) , as s ! m2ν˜ (2)Similar expressions arise for the other processes. The resonant cross sections canthus be deduced by the appropriate branching fractions.Ignoring contributions to the vertices of the MSSM from mass terms, the latterare given by the following formulas:Γ(~ν ! νχ0i ) =g232pi(Ni2 − tan θW Ni1)2 mν˜(1−m2χ0im2ν˜)2Γ(~ν ! `χi ) =g2V 2j116pimν˜ (1−m2χ±im2ν˜)2, Γ(~ν ! f f) = λ2ijk16pimν˜In the above, λijk is the appropriate R-parity violating Yukawa coupling generatingthe decay ~ν ! f f ,while Vi1 and Ni1, Ni2 are the relevant matrix elements in themixing matrix for charginos and neutralinos respectively.Conclusively, whether single chargino and neutralino nal states will dominateover the resonant fermion-antifermion productions depends on (i) the SUSY pa-rameter space and (ii) the strength of λ. This is indicated in Figs. 1,2 where thebranching ratio of the sneutrino decay to fermions is presented for the regions ofbIn 2! 2 single superparticle productions one may generically probe only a subset of operators.All 45 operators can be simultaneously probed by going to a 3-body nal state, for instance in rareZ0 decays; however, this process is phase-space suppressed and was found to be more relevant forhadron colliders 10.2the supersymmetry parameter space that are interesting for LEP (Fig. 1) and LC(Fig. 2) c. Here, the U(1) gaugino mass M1 is determined from the SU(2) gauginomass M2 by the unication relation M1 = (5/3) tan2 θW M2. For lower values ofM2, µ, a larger number of charginos and neutralinos can be produced at the nalstate, while the phase space suppression for their production is small. The picturestarts changing as we pass to larger M2, µ and this is indicated in the increase of thesneutrino decay rate to fermions. However, we can see that for a wide range of M2and µ the production of charginos and neutralinos at the LC tends to dominate.Moreover, there exist bands of the parameter space where the production of theheavier charginos and neutralinos may occur at a signicant level. This is shown inTable 2, where we present the branching ratios for the production of each charginoand neutralino separately.M2 µ Γ1 Γ2 Γ3u Γ4 Γ01 Γ02200. -1000. 0.53 1.49 { { 2.93 {200. -600. 0.51 1.48 { { 2.92 {200. -200. 0.44 0.51 0.08 0.89 2.11 0.81200. 200. 1.09 0.28 0.01 0.58 2.09 0.88200. 600. 0.72 1.38 { { 3.06 {200. 1000. 0.66 1.45 { { 3.04 {300. -1000. 0.49 0.85 { { 1.68 {300. -600. 0.48 0.84 { { 1.68 {300. -200. 0.46 0.01 0.06 0.75 0.41 1.30300. 200. 0.82 0.01 0.01 0.50 0.92 0.87300. 600. 0.62 0.85 { { 1.88 {300. 1000. 0.57 0.87 { { 1.82 {400. -1000. 0.43 0.26 { { 0.52 {400. -600. 0.42 0.26 { { 0.52 {400. -200. 0.24 0.04 0.15 0.22 0.18 0.40400. 200. 0.53 0.01 0.05 0.15 0.42 0.27400. 600. 0.51 0.33 { { 0.74 {400. 1000. 0.48 0.30 { { 0.63 {500. -1000. 0.34 { { { { {500. -600. 0.34 { { { { {500. -200. 0.05 0.03 0.27 { 0.10 {500. 200. 0.31 { 0.14 { 0.23 {500. 600. 0.41 0.03 { { 0.08 {500. 1000. 0.38 { { { 0.01 {Table 2: All units in the table are in GeV. We chose tanβ = 2, λ = 0.1 and mν˜ = 500 GeV. Γiare the decay rates for the four neutralinos, while Γ′i the decay rates for the two charginos. Forthis choice of parameters, the R-violating decay rate is 0.1 GeV.Subsequently, the charginos and neutralinos will decay to an R-parity evennal state, with the possibility of an interesting chain of cascade decays with multi-lepton events and explicit lepton-number violation at the nal state. The lightestcFor squark decays, analogous results have been presented in 11.3-1000 -500 0 500 1000m0100200400500600M2-1000 -500 0 500 10000100200400500600-1000 -500 0 500 1000m0100200400500600M2-1000 -500 0 500 10000100200400500600Figure 1: B for ~ν decay to fermions for parameters most relevant for LEP. We present contours forB  0.9, 0.3, 0.1, from the darker to the lighter areas respectively. We chose tanβ = 2.0, mν˜ = 200GeV and λ = 0.04 (left) and 0.1 (right). The LEP 2 bound on charginos has been implemented.80010001200M2-1000 -500 0 500 1000800100012004neutralino decays via~χ01 ! f(e, li , νe), (e, e, νi)gFor the charginos and the heavier neutralinos, there exist two possible decay modes:The rst is the cascade decay via the lightest neutralino and the second the directdecay via the R-violating coupling(s), as discussed in 12. For instance, for the lighterchargino we have the channels~χ−1 ! ~χ01 + (W−) ! ~χ01 + f f 0where f f 0 are the decay fermions of the (virtual) W-boson, or~χ−1 ! (e−e+l−i , νeνie−) (3)In this latter case the signal would be even more distinct since it involves four leptonsat the nal state (three being in the same semi-plane) without any missing energy,unlike the cascade decay which always involves neutrinos at the nal state d. It turnsout however that the charginos as well as the heavier neutralinos dominantly decayto χ01 and fermions for a wide region of the parameter space.In all cases, the signals should be clearly visible at an e+e− collider, providedthe cross-section is suciently large; the latter mainly depends on how large theunknown coupling λ will be. We study the relevant cross section, at and away fromthe resonance. Ignoring contributions to the vertices of the MSSM from mass terms,we have two channels present (s and t) for chargino production and all three (s, tand u) for neutralino production. For the s-channel diagram we take into accountthe contribution due to the decay width of the scalar neutrino.In Fig. 3, we show the cross sections for single chargino and neutralino produc-tions, including eects from initial state radiation (ISR), for two dierent sneutrinomasses. To illustrate the eects from the production of many charginos and neu-tralinos we chose a point of the parameter space where all several of these statesare produced. Indeed, for the choice of parameters tha appears in the gure, thechargino masses are 201.9 and 273.1 GeV respectively, while the neutralino massesare 127.8, 192.9, 217.5 and 272.1 respectively.As expected, the eect of initial state radiation is to lower the peak but widenthe resonance. For instance, in our example we nd that, forps = 500 GeV (whereall charginos and neutralinos may be produced) and mν˜ = 450 GeV, IR enhancesthe cross section by almost an order of magnitude. Actually, in this example, theheavier charginos and neutralinos may arise with large cross sections. Indeed, thepartial cross sections that we nd for the four neutralinos (from the lighter to theheavier), forps = 500 and mν˜ = 450 GeV, are: 1.03 pb, 0.22 pb, 0.15 pb, 1.9 pbwhile for the charginos 1.8pb and 2.9 pb respectively.dWhich of the two processes will appear, clearly depends on (i) the strength of the R-parityviolating operator: the strongest the operator the larger the decay rate for a direct decay of thechargino. (ii) the relative mass of chargino-neutralino: if the mass gap between the two states isvery small, then the cascade decay is suppressed by phase space.5Cross sections for Rpv Sneutrino S-channel Resonance10-210-111010 210 3200 250 300 350 400 450 500√s (GeV)σ (pb) e+e- to Chargino Lepton10-210-111010 210 3200 250 300 350 400 450 500√s (GeV)σ (pb) e+e- to Neutralino NeutrinoFigure 3: The parameters for this plot are the following: mν˜ = 300 and 450 GeV respectively;me˜ = 1000 and 450 GeV; tan β = 2, λ = 0.1, µ = −200, M2 = 250.0. In this case, all charginosand neutralinos are produced.6From the above discussion, we conclude that single chargino and neutralino pro-ductions arise with signicant cross sections and provide an interesting possibilityfor looking for R-violating supersymmetry at the Linear Collider.Acknowledgement: I would like to thank P. Morawitz for collaborating atthe rst stages of this work and A. Belyaev, for his help with PAW.1. P.Fayet, Phys.Lett. 69B (1977) 489.2. L. Ibanez and G.G. Ross, Nucl. Phys. B368 (1992) 3; S. Lola and G.G.Ross,Phys. Lett. B314 (1993) 336; J. Ellis, S. Lola and G.G. Ross, Nucl. Phys.B526 (1998) 1153. For some of the earlier references, see: F. Zwirner, Phys. Lett. B132 (1983)103 ; L. Hall and M. Suzuki, Nucl. Phys. B231 (1984) 419 ; J. Ellis et al,Phys. Lett. B150 (1985) 142 ; S. Dawson, Nucl. Phys. B261 (1985) 297 ; R.Barbieri and A. Masiero, Nucl. Phys. B267 (1986) 679.4. H. Dreiner and G.G. Ross, Nucl. Phys. B365 (1991) 597.5. For reviews, see for example: H. Dreiner, ’Perspectives on Supersymmetry’,Ed. by G.L. Kane, World Scientic; G. Bhattacharyya, hep-ph/9709395, pre-sented at Workshop on Physics Beyond the Standard Model, Tegernsee, Ger-many, 8-14 Jun 1997.6. V. Barger, G. F. Giudice and T. Han, Phys. Rev. D 40 (1989) 2987.7. H. Dreiner and S. Lola, EE500 Workshop, Hamburg, September 1991 DESY92-123, Part B, 707; Workshop for e+e− collisions at TEV energies, DESY-96-123D; G.F.Giudice et al., hep-ph/9602207.8. J. Kalinowski, R. Ruckl, H. Spiesberger and P.M. Zerwas, Phys. Lett. B406(1997) 314; M. Heyssler, R. Ruckl and H. Spiesberger, hep-ph/9908319.9. J.L.Feng, J.F.Gunion and T.Han, Phys. Rev. D58, 071701.10. S. Lola and J. McCurry, Nucl. Phys. B 381 (1992) 559.11. G. Altarelli et al., Nucl. Phys. B506 (1997) 3; J. Ellis, S. Lola and K. Sridhar,Phys. Lett. B408 (1997) 252; S. Lola, hep-ph/9706519.12. H. Dreiner, S. Lola and P. Morawitz, hep-ph/9606364. Phys. Lett. B389(1996) 62.7

Resonant single chargino and neutralino versus fermion-antifermion production at the Linear Collider

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