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SUSY Searches at LEP Selected Topics

SUSY Searches at LEP Selected Topics. Outline. Introduction Standard SUSY and the LSP Gauge Mediated SUSY Breaking SUSY A taste of R-parity violating SUSY Conclusions. J.B. de Vivie, on behalf of the LEP collaborations. ICHEP’04, Beijing. Introduction.

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SUSY Searches at LEP Selected Topics

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  1. SUSY Searches at LEP Selected Topics Outline • Introduction • Standard SUSY and the LSP • Gauge Mediated SUSY Breaking SUSY • A taste of R-parity violating SUSY • Conclusions J.B. de Vivie, on behalf of the LEP collaborations ICHEP’04, Beijing

  2. Introduction • The LEP2data sample (/experiment) : L ~ 700 pb-1 at Ecm [130,209] GeV ~140pb-1 / expat Ecm 206 GeV • Detectors : • particleidentification e±, ±, , b • (especially upgraded vertex detectors at LEP2 •  Higgs boson searches) • good hermeticity Energy Flow • (e.g. ALEPH, (E) = 0.6(E/GeV) + 0.6 GeV) • Trigger efficiency ~ 100 % for Evis > 5 GeV

  3. e+e- collider  clean environment, s well known • intensive study of two and four fermion final states •  mW, W, (WW,ZZ,ff), single W, … - • The SM processes • Background well under control • good description by MC simulations • But we are looking for rare processes  Tails !!

  4. Why SUSY ? • Theoretical motivations • (e.g. stabilizes the hierarchy MPl/mEW  scalars are natural, • includes gravity in its local version, …) • Good agreement with EW precision data and gauge coupling unification (old top mass) …even in the simplest models • Some models provide a natural Cold Dark Matter candidate ~ • unfortunately, already from LEP1 (i.e. Z (invisible) width) : m  mZ/2

  5. Standard SUSY and the LSP • R-parity Rp = (-1)L+3B+2S conservation : SUSY particles pair-produced / Lightest SParticle (LSP) stable (CDM) • LSP = lightest neutralino (or sneutrino, but ) • Typical search : NLSP LSP + (SM particles), LSP undetected : Sensitivity : mNLSP ~ s /2 • Four main topologies covering most of the possible final states … … from slepton, squark, chargino and neutralino production • All topologies crucially depend on M = mNLSP - m  Visible Energy • SM background : low M :  process High M : 4 fermion processes with 

  6. The way to the mass limit for the lightest neutralino • The relevant parameters : LEP-MSSM • at mGUT, • * gaugino unified mass : m1/2 M1, M2 and M3 at mZ • * sfermion(not Higgs) unified mass : m0 • mA and free • trilinear couplings At, A (Ab) • tan • For the LSP : interplay of various searches • From charginos to the LSP, in a large part of the parameter space • m ~ M1 M2/2 ~ m/2 • From sfermionsto the LSP, • Miappear in their masses through the RGEs • From Higgs bosons to the LSP, • through stop masses in radiative corrections

  7. Let’s go : the ingredients and the recipe • The heavy sfermion case : high m0, only chargino and neutralino  Heavier neutralinos relevant at low tan and small||  Excluded domain in the (, M2) plane • Mass limit for the chargino : > ~ kinematic limit 103.5 GeV/c2 > even beyond with neutralinos • degradation at high M2 : M efficiency background (Similarly for neutralinos, m + mj almost at kinematic limit)

  8. The very low M loophole (I) : chargino searches At high M2, small ||, m ~ m : standard searches inefficient (or in non unified models, when |M2|<<|M1|, e.g. AMSB) • 2 specific searches : >M < 150 MeV/c2, long lived, highly ionizing particles > 150 MeV/c2 < M < 3 GeV/c2, ISR tagging analysis : require a high p photon to reduce events  M, m > 91.9 GeV/c2 Finally,  and isearches, ISR easily translated into a limit on m long lived > 39 GeV/c2 @ tan = 1

  9. The light sfermion case : small m0 • light sneutrinos, selectrons (smuons and staus)  chargino production cross section  leptonic branching ratio ( WW background ) • difficult region : sneutrino-corridor Soft track : trigger ? background ?   invisible  • use sleptonsearches and At m ~ 40 GeV/c2 meR > 99.9 GeV/c2 ,  l  m > 39 GeV/c2 @ tan = 1 robust… (Another very low M loophole…)

  10.  Dedicated searches  Absolute eR mass limit meR > 73 GeV/c2 ~ The (very low M) loophole (II) : slepton searches • very soft lepton from  almost invisible final state • for selectron, the gap is closed by the single electron search • For smuons, back to the Z width • For staus, not even sufficient due to decoupling (stau-mixing) (DELPHI dedicated searches : m1 > 26.3 GeV/c2 any mixing, any M) • Loopholes when cascade decays : , (t-channel  exchange)

  11. Including the Higgs boson searches : low tan • From charginos, neutralinos and sleptons, LSP limit set at tan = 1 • TheHiggs cover the low tan and • protect against low m0at intermediate tan m > 47 GeV/c2 @ high tan, in the sneutrino-corridor • Model dependence? • Profit from the sound LEP environment • to exclude pathological regions •  experimental h limit : • OK except for very unnatural cases mtop = 180 GeV/c2 OK  ? • From experiment to interpretation : the excluded tan range has a strong dependence on mtop and the mh computation

  12. Best reach at Tevatron but LEP can improve at low M • Large mtop mixing maybe large : t may be the lightest squark (also in mSUGRA-type models, stop soft masses generically smaller that other squark masses) • acoplanar jets from ~ • = 56o, tan = 1.5  = -100 GeV/c2 The stop and the very low M loophole (III) : Very low M(< 5 GeV/c2) long lived stop-hadrons decay inside the tracking Dedicated generator for stop-hadron formation, interaction and decay for M ~ 40 GeV/c2 Mstop > 95 GeV/c2 acop. jets ~stable high impact parameter Also 3 body decay at small msneutrino 4 body decay  Mstop > 63 GeV/c2 M

  13. Higgs • staus ~ invisible • charginos ~ invisible • selectrons and smuons too heavy selectrons and staus Stable staus theory charginos LEP1 Model dependence ? The stau mixing • Until now, no stau mixing A =  tan.Does it matter ? YES ! • Impact in mSUGRA where stau mixing is built in mA and  no longer free, A0 fixesA  tan  new corridor at large tan : the stau-corridor Mstau ~ m Again, exploit the clean LEP events to search for difficult topologies : recycle the ISR tagging single tau or asymmetric taus Multi taus

  14. The stau-corridor is closed : no more holes in the (m0,m1/2) planes  < 0  > 0 stable slepton theory Higgs chargino Z width and the LSP mass limit in mSUGRA m > 50 GeV/c2 (mtop = 175 GeV/c2, any A0) Stau mixing is a delicate issue in a Very Constrained MSSM  worse in the LEP-MSSM ALEPH only • m > 29.7 GeV/c2 … • … for very unnatural A values (CCB ?) • For not too unnaturalA (<20 TeV/c2) 39 GeV/c2 (no Higgs, no mixing)  36.6 GeV/c2 (no Higgs, mixing)

  15. Mass limit for the lightest neutralino : Summary • In mSUGRA, m > 50 GeV/c2, any A0 but strong dependence on mtop • In LEP-MSSM, mtop < 180 GeV/c2, no stau mixing m > 47 GeV/c2 • With stau mixing, low tan delicate… in the most conservative case * no Higgs * stau mixing (|A| < 20 TeV/c2) m > 36.6 GeV/c2 • all this without any radiative corrections in the gaugino-higgsino sector  ~ 1-2 GeV/c2 uncertainty

  16. The most important hypothesis : Gaugino mass Unification What if M1and M2 are NOT unified at mGUT ? • If at mGUT, |M1/M2| < 1 chargino constraints less stringent… m> ? e.g. |M1/M2| = 1/3, In the worst case, heavy sleptons, |M2|, || >> |M1| no limit from LEP ! • If at mGUT, |M1/M2| > 1, m> 45 GeV/c2 should hold (the ISR-tagging analysis is very relevant to go beyond)

  17. Solve the FCNC problem of generic Gravity mediated models • The LSP is the Gravitino G • Experimental topologies depend on the nature of the NLSP and its lifetime, determined by the Gravitino mass : • In minimal models, the lightest neutralino or the sleptons are in general the only Sparticles relevant for LEP2 searches (+ ) ~ ~ G Gauge Mediated SUSY breaking SUSY • A curiosity : in non minimal models the gluino can be the NLSP or LSP  Search for light stable gluino at LEP1 (DELPHI, ALEPH) Search for R-hadrons in mgluino > 26.9 GeV/c2 Z width : mgluino > 6.9 GeV/c2

  18. GMSBinterpretation of CDF ee event: (much weaker if open) NeutralinoNLSP : • Short lifetime : acoplanar photons • Intermediate lifetime : single photon with high impact parameter (“non pointing photon”)  Results for the acoplanar photons at last dead ! • Long lifetime : indirect from charginos and sleptons (OPAL increased the sensitivity at short lifetime with sleptons and charginos, e.g. )

  19. SleptonNLSP : ~ (High cross-section since eR light, 50% events with 2 high E Same Sign leptons) • At small tan, all sleptons are mass-degenerate: co-NLSP • At large tan,the stau is lighter due to large mixing ( m  tan) • Short lifetime : MSSM slepton searches for very high M •  acoplanar leptons • Intermediate lifetime : • sleptons decay inside the tracking volume (kinks) • or give tracks with high impact parameters • Long lifetime : search for pair-produced • heavy stable charged particles  combining the three searches, for a stau NLSP Mstau > 86.9 GeV/c2 • increase sensitivity by looking for

  20. Interpretation in minimal models :   5.5 parameters needed • F, the SUSY breaking scale (lifetime), • tan, sign() • Soft masses determined from • , the universal mass scale of SUSY particles • N, the effective number of messenger pairs • Mmess, the mean messenger mass • Excluded domain in the (m,m) plane … from which one can infer a lower limit on  as a function oftan  slepton, 0 for slepton NLSP slepton, ± for 0 NLSP  In these models MNLSP > 54 GeV/c2  > 16 TeV/c2(N5)

  21. A taste of R-parity violating SUSY The General MSSM allows lepton and baryon number violating couplings: •  45 new couplings (some of them constrained by low E processes) •  LSP can be any Sparticle and is unstable • Sparticles can be singly produced • Searches assuming a single coupling is dominant • Lots of topologies covered: from 2 leptons (slepton production) to many jets, many leptons and missing energy (up to 10 quarks from chargino production) • Very good test of the standard model with a very broad range of final states studied !

  22. Example of 6 jet event in ALEPH An example: single sneutrino production with e.g.122  improvement over low energy constraint up to msneutrino = 189 GeV/c2

  23. Conclusions • Lots of SUSY searches performed by the four LEP experiments • large class of models studied, • many analyses dedicated to potential loopholes : limits are robust • No signal from SUSY : sfermion, chargino masses > 100 GeV/c2 • In LEP-MSSM with reasonable assumptions, m > 47 GeV/c2 • The LEP legacy : e.g. in mSUGRA • minimal unified models : hard for Tevatron (trilepton very relevant !) • but still room for discovery at CDF/D0 Standard unification relations may not hold, Higgs coverage dependence on mtop, A0, … • eagerly wait for more Tevatron results and LHC start !

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