We discuss the potential of combined LHC and ILC experiments for SUSY
searches in a difficult region of the parameter space, in which all sfermion
masses are above the TeV scale. Precision analyses of cross sections of light
chargino production and forward--backward asymmetries of decay leptons and
hadrons at the ILC, together with mass information on \tilde{\chi}^0_2 and
squarks from the LHC, allow us to fit rather precisely the underlying
fundamental gaugino/higgsino MSSM parameters and to constrain the masses of the
heavy virtual sparticles. For such analyses the complete spin correlations
between the production and decay processes have to be taken into account. We
also took into account expected experimental uncertainties.Comment: 4 pages, talk given at XLI Rencontres de Moriond on Electroweak
  Interactions and Unified Theories, La Thuile, Italy, 11-18 Mar 200

Desch, K.

Kalinowski, J.

Moortgat-Pick, G.

Rolbiecki, K.

English

arXiv

We discuss the potential of combined LHC and ILC experiments for SUSY searches in a difficult region of the parameter space, in which all sfermion masses are above the TeV scale. Precision analyses of cross sections of light chargino production and forward--backward asymmetries of decay leptons and hadrons at the ILC, together with mass information on \tilde{\chi}^0_2 and squarks from the LHC, allow us to fit rather precisely the underlying fundamental gaugino/higgsino MSSM parameters and to constrain the masses of the heavy virtual sparticles. For such analyses the complete spin correlations between the production and decay processes have to be taken into account. We also took into account expected experimental uncertainties.We discuss the potential of combined LHC and ILC experiments for SUSY searches in a difficult region of the parameter space, in which all sfermion masses are above the TeV scale. Precision analyses of cross sections of light chargino production and forward--backward asymmetries of decay leptons and hadrons at the ILC, together with mass information on \tilde{\chi}^0_2 and squarks from the LHC, allow us to fit rather precisely the underlying fundamental gaugino/higgsino MSSM parameters and to constrain the masses of the heavy virtual sparticles. For such analyses the complete spin correlations between the production and decay processes have to be taken into account. We also took into account expected experimental uncertainties.We discuss the potential of combined LHC and ILC experiments for SUSY searches in a difficult region of the parameter space, in which all sfermion masses are above the TeV scale. Precision analyses of cross sections of light chargino production and forward--backward asymmetries of decay leptons and hadrons at the ILC, together with mass information on \tilde{\chi}^0_2 and squarks from the LHC, allow us to fit rather precisely the underlying fundamental gaugino/higgsino MSSM parameters and to constrain the masses of the heavy virtual sparticles. For such analyses the complete spin correlations between the production and decay processes have to be taken into account. We also took into account expected experimental uncertainties.We discuss the potential of combined LHC and ILC experiments for SUSY searches in a difficult region of the parameter space, in which all sfermion masses are above the TeV scale. Precision analyses of cross sections of light chargino production and forward--backward asymmetries of decay leptons and hadrons at the ILC, together with mass information on \tilde{\chi}^0_2 and squarks from the LHC, allow us to fit rather precisely the underlying fundamental gaugino/higgsino MSSM parameters and to constrain the masses of the heavy virtual sparticles. For such analyses the complete spin correlations between the production and decay processes have to be taken into account. We also took into account expected experimental uncertainties

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THE CHALLENGE OF DETERMINING SUSY PARAMETERS IN FOCUS-POINT-INSPIRED CASES K. ROLBIECKI1a, K.  DESCH2, J. KALINOWSKI\ G. MOORTGAT-PICK3 1 Institute of Theoretical Physics, Warsaw University, PL-00681 Warsaw, Poland 2Phys. Institut, Albert-Ludwigs- Universitiit Freiburg, D-79101, Freiburg, Germany 3 TH Division, Physics Department, CERN, CH-121 1 Geneva 23, Switzerland We discuss the potential of combined LHC and ILC experiments for SUSY searches in a dif­ficult region of the parameter space, in which all sfermion masses are above the Te V scale. Precision analyses of cross sections of light chargino production and forward-backward asym­metries of decay leptons and hadrons at the ILC, together with mass information on xg and squarks from the LHC, allow us to fit rather precisely the underlying fundamental gaug­ino/higgsino MSSM parameters and to constrain the masses of the heavy virtual sparticles. For such analyses the complete spin correlations between the production and decay processes have to be taken into account. We also took into account expected experimental uncertainties. 1 Introduction Supersymmetry (SUSY) is one of the most promising extensions of the Standard Model (SM) since, among other things, it solves the hierarchy problem, provides a cold dark matter candidate, and enables gauge couplings unification. Because of the unknown mechanism of SUSY breaking, supersymmetric extensions of the Standard Model contain a large number of new parameters: 105 appear in the Minimal Supersymmetric Standard Model (MSSM) and have to be specified. Experiments at future accelerators, the LHC and the ILC, will have not only to discover SUSY but also to determine precisely the underlying scenario without theoretical prejudices on the SUSY breaking mechanism.  Particularly challenging are those scenarios where the scalar SUSY particle sector is heavy, as required e.g. in focus-point scenarios (FP) as well as in split SUSY (sS). Since it is not easy to determine experimentally cross sections for production processes, studies have been made to exploit the whole production-and-decay process. Angular and energy aspeaker, e-mail: Krzysztof.Rolbiecki@fuw.edu.pl 499 distributions of the decay products in production processes with subsequent three-body decays have been studied for chargino as well as for neutralino processes 1 . Since such observables depend strongly on the polarization of the decaying particle, the complete spin correlations between production and decay can have a lot of influence and must be taken into account. Exploiting such spin effects, it has been shown 2 that, once the chargino parameters are known, useful indirect bounds for the mass of the heavy virtual particles could be derived from forward­backward asymmetries of the final lepton AFB(f) . 2 Chosen Scenario: Focus-Point-Inspired Case In this section we take a FP-inspired mSUGRA scenario defined at the GUT scale3 .  However, in order to assess the possibility of unravelling such a challenging new physics scenario, our analysis is entirely performed at the EW scale without any reference to the underlying SUSY-breaking mechanism. The parameters at the EW scale are obtained with the help of the code SPheno 4. The low-scale gaugino/higgsino/gluino masses, as well as the derived masses of SUSY particles, are listed in Table 1. As can be seen, the chargino/neutralino sector, as well as the gluino, are rather light, whereas the scalar particles have masses of about 2 TeV (with the only exception of h ,  which is a SM-like light Higgs boson) . Mi M2 Ma µ tan ,6  mx± mx� mxo mxg mxg mx� 60 121  322 540 20 1 17 552 59 1 17  545 550 mh mH,A mH± m;; meR meL ffi7'1 mr2 milR mqL mt1 1 19 1934 1935 1994 1996 1998 1930 1963 2002 2008 1093 Table 1 :  Low-scale MSSM parameters and the particle masses in our scenario in GeV. 2.1 Expectations at the LHC m9 416 m12 1584 As can be seen from Table 1, all squarks are kinematically accessible at the LHC. The largest squark production cross section is for l1,2 - However, with stops decaying mainly to gt [with BR(t1,2 -> gt) � 66%] , where the background from top production will be large, the reconstruc­tion of the stops will be very challenging. The other squarks decay mainly via gq, but since they are very heavy, mi/L.R � 2 TeV, precise mass reconstruction will be difficult. Neverthe­less, the indication that the scalar quarks are very heavy will be very important in narrowing experimental uncertainty on the slepton sector from the ILC measurements. The gluino production is expected to have very high rates. Therefore several gluino decay channels can be exploited. The largest branching ratio for the gluino decay in our scenario is into neutralinos BR(g _, Xgbb) � 14% with a subsequent leptonic neutralino decay BR(x_g -> ;;je+e-) � 6%, e = e, µ. In this channel the dilepton edge will be clearly visible, since this process is practically background-free. The mass difference between the two light neutralino masses could be measured from the dilepton edge with an uncertainty of about 3 ( 1 )  2. 2 Expectations at the IL G  At the ILC with vs =  500 GeV, only light charginos and neutralinos are kinematically accessible. However, in this scenario the neutralino sector is characterized by very low production cross sections, below 1 fb, so that it might not be fully exploitable. Only the chargino pair production 500 I y's/GeV 350 (-90%, +60%) 1062.5±4.0 4.42±0.29 (+90%, -60%) 14.6±0.7 4.6±2.5 500 (-90%, +60%) 521 .6±2.3 4.62±0.41 ( +90%, -60%) 6.9±0.4 4.9±3.6 Table 2: Cross sections for the process e+ e- --+ X.iX.1 and forward-backward asymmetries for this process followed by X.1 --+ x_�e-;;,, with e =  e, µ, for different beam polarization P,- ,  P,+ configurations at the center of mass energies .JS = 350 Ge V and 500 Ge V at the ILC. Errors include la statistical uncertainty assuming luminosity £:, = 200 fb- 1 for each polarization configuration, efficiency £ = 503 and the beam polarization uncertainty of 0.5%. BR "°'  0.34, see Section 2.2. process has high rates at the ILC, see Table 2, and all information obtainable from this sector has to be used. In the following we study the production process (2) followed by leptonic and hadronic decays of the charginos, for which the analytical formulae including the complete spin correlations are given in a compact form 1 . The production process occurs via "I and Z exchange in the s-channel and De exchange in the t-channel, and the decay processes get contributions from w± and De, fL (leptonic decays) or iih , iiuL excliange (hadronic decays) .  The light chargino has a leptonic branching ratio of about BR(x1 -t x�e-ve) � 1 1% for each family and a hadronic brancliing ratio of about BR(x1 -t x�qdl[u) � 33%. In our analysis we use cross sections multiplied by the branching ratios of semileptonic chargino decays: o-(e+e- -t xix1) x BR, with BR = 2 x BR(xi -t X.�i:J.dqu) x BR(x1 -t x�e-ve) + [BR(x1 -t x�e-ve)]2 � 0.34, e = e, µ,, q,, = u, c, Qd = d, s. From the ILC scan at the threshold 5 ,  because of the steep s-wave excitation curve in xi x1 production, the determination of the light chargino mass will be possible with an accuracy of about 6 mx� = 117. 1 ± 0. 1  GeV. (3) The mass of the lightest neutralino mx� can be derived either from the energy distribution of the lepton e- or, in hadronic decays, from the invariant mass distribution of the two jets from xt decays. We therefore assume 3 that mx� = 59.2 ± 0.2 GeV. (4) Together with the information from the LHC, Eq. ( 1 ) ,  a mass uncertainty for the second lightest neutralino of about mxg = 117 .1 ± 0.5 GeV (5) can be assumed. 3 Determination of Parameters Following the method proposed in 2 we include in the fit the forward-backward asymmetries of the final leptons. As £plained before, this observable is very sensitive to the mass of the exchanged scalar particles, even for rather heavy masses. Since in the decay process also the left selectron exchange contributes, the SU(2) relation between the left selectron and sneutrino masses: m�L = mE, + m1 cos(2p)(-l + sin2 Bw) has been assumed. In principle this assumption could also be relaxed 3 .  501 Applying the 5-parameter x2 fit procedure with the leptonic forward-backward asymmetries included leads to 3:  Mi = 60.00 ± 0.35 GeV, M2 = 121.0 ± 1 . 1  GeV, 500 :S tL :S 610 GeV, m;:,e = 1995 ± 100 GeV, 14 :S tan p :S 31 .  (6) Including forward-backward asymmetries in the multiparameter fit provides strong constraints for the mass of the heavy virtual particle, m;;;, ,  and decreases its error by a factor of about 2 with respect to the fit without FB asymmetry 3. The constraints for the gaugino mass parameters lv!i and lvf2 are improved by a factor of about 5, thanks to the constraint on the value of tan ,8. It is clear that in order to improve considerably the bounds for the parameters tt and tanp, the measurement of the heavy higgsino-like chargino and/or neutralino masses will be necessary at the second phase of the ILC with .jS � 1000 GeV. 4 Conclusions We have demonstrated a method for constraining heavy virtual particles and for determining the SUSY parameters in focus-point-inspired scenarios. These appear very challenging since only little experimental information on the SUSY sector is accessible at both the LHC and the ILC at its first energy stage of .jS = 500 GeV. However, we show that a careful exploitation of the data leads to significant constraints on the unknown parameters. The most powerful tool in this kind of analysis turns out to be the forward-backward asymmetry. A proper treatment of spin correlations between the production and the decay is indispensable in that context. This asymmetry is strongly dependent on the mass of the exchanged heavy particle. We want to stress the important role of the LHC/ILC interplay since none of these colliders alone can provide us with the data needed to perform the determination of the SUSY parameters in focus-like scenarios. Acknowledgements The author would like to thank the organizers for financial support and the opportunity to give this talk. JK and KR have been supported by the Polish Ministry of Science and Higher Education Grant No 1 P03B 108 30. References 502 1 .  G .  Moortgat-Pick, H.  Fraas, A. Bartl and W. Majerotto, Eur. Phys. J. C 7, 113 ( 1999) [arXiv:hep-ph/9804306]; G. Moortgat-Pick and H. Fraas, Phys. Rev. D 59, 015016 (1999) [arXiv:hep-ph/9708481]. 2. G. Moortgat-Pick and H. Fraas, Acta Phys. Polon. B 30, 1999 (1999) [arXiv:hep­ph/9904209]; G. Moortgat-Pick, A. Bartl, H. Fraas and W. Majerotto, Eur. Phys. J. C 18, 379 (2000) [arXiv:hep-ph/0007222]. 3. K. Desch, J.  Kalinowski, G.  l'vfoortgat-Pick, K.  Rolbiecki and J. Stirling, in preparation. 4. W. Porod, Comput. Phys. Commun. 153, 275 (2003) [arXiv:hep-ph/0301101] .  5. W. J. Stirling, Nucl. Phys. B 456, 3 (1995) [hep-ph/9503320]. 6. T. Abe et al. [Amepcan Linear Collider Working Group Collaboration] , in Proc. of the APS/DPF/DPB Summer Study on the Future of Particle Physics (Snowmass 2001) ed. N. Graf, arXiv:hep-ex/0106056; J. A. Aguilar-Saavedra et al. [ECFA/DESY LC Physics Working Group Collaboration] , arXiv:hep-ph/0106315; K .  Abe et al. [ACFA Linear Col­lider Working Group Collaboration] , arXiv:hep-ph/0109166. 

The Challenge of Determining SUSY Parameters in Focus-Point-Inspired Cases

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