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New particle searches with t at DELPHI

Introduction t ID in searches Higgs search SUSY Excited leptons Technicolor. New particle searches with t at DELPHI. F. Matorras IFCA, Santander, Spain DELPHI collaboration. Introduction. A different point of view of tau physics widely used at LEP (as will be in LHC)

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New particle searches with t at DELPHI

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  1. Introduction t ID in searches Higgs search SUSY Excited leptons Technicolor New particle searches with t at DELPHI • F. Matorras • IFCA, Santander, Spain • DELPHI collaboration New particles searches with t at DELPHI Nara, 17/09/2004

  2. Introduction • A different point of view of tau physics widely used at LEP (as will be in LHC) • look for new particles that decay to tau(s) leptons rather than studying the tau decay • In many models the most probable final state topologies include taus • In other cases it is important for some region of the parameters • Also used in other searches as complementary channel • when the new particle is expected to follow lepton universality or is accompanied by Z or W bosons • always significantly worst than mm or ee channels! • In general tau identification and reconstruction complicated • large variety of decays • hadronic decays • missing energy: lose mass information • Good knowledge of t physics is important! • people in searches often ignores “basic” properties of the taus New particles searches with t at DELPHI Nara, 17/09/2004

  3. Data sample • DELPHI • multipurpose detector at LEP e+e- Collider at CERN • see more details in A.Andreazza and D.Dedovich talks • First phase (LEP I) ‘89 to ‘95 • Ecm~Mz • >150 pb-1 • Mainly designed for Z physics • also for new particle searches • data was reanalyzed recently in view of new models • From ‘95 until 2000 (LEP II) • gradual increase of energy up to 208 GeV • ~ 700pb-1 above WW threshold • most searches rely on this data, especially the highest energy in 2000 New particles searches with t at DELPHI Nara, 17/09/2004

  4. Tau ID @ LEP2 • Not too different from ID @LEP1 • see A.Andreazza and D.Dedovich talks • higher energy →higher particle multiplicity in the event→separation more complicated • A t is typically seen as an isolated lepton or low multiplicity and narrow jet associated with missing energy • if lepton, its energy significantly smaller than that expected from a prompt lepton • usually for any background channel with t, there is the equivalent with e or m • but the lepton energy of prompt leptons is three times larger on average • high boost → very narrow jet w.r.t. quark jets • often just one-prong events are used to, reduce background from gluon/hadron jets • Different jet algorithms depending on the topology: Luclus, Durham, pure “geometrical”, mass clustering... • often refinements done to remove spurious tracks attached to the tau from quark jets • based on energy, angle with the jet and mass of the jet New particles searches with t at DELPHI Nara, 17/09/2004

  5. Tau event selection @ LEP2 • Large variety of selection procedures and background processes depending on the topology, mass of the particle... • Major background in most cases • qq(g)(g) for events containing jets • very high cross section • lost photons at low angle produce missing energy • gluon jets or tracks unattached mimic a t • basic to have a good t ID and good jet definition • radiative l+l- • as above, a g looks like a n • acoplanarity, angle of missing momentum • two-photon events (e+e-ff) • huge cross section • missing energy in electrons/positrons at low angle • transverse energy and transverse momentum • Often WW or ZZ decays remain as almost irreducible background • mass information • specific variables in each case New particles searches with t at DELPHI Nara, 17/09/2004

  6. Charged Higgs Eur. Phys. J. C34 (2004) • Many extensions of SM predict the existence of the Charged Higgs • studied here in the framework of general 2 Higgs Doublet Models (2HDM) • produced in pairs H+H- at LEP energies • cross section only dependent on the Ecm and mass • Type II models • decay to heaviest fermions available • cs or tn • BR free, but prejudice in favor of high tn BR • look for cscs, cstn and tntn New particles searches with t at DELPHI Nara, 17/09/2004

  7. Charged Higgs (type II) • Topology of these events: • in tntn: two acollinear t, with additional missing energy • in cstn: two hadronic jets, with cs content and one t • Remaining irreducible background, WW equivalent decays • Mass of the parent boson • for purely leptonic decays at least 4 n undetected, mass cannot be reconstructed • some sensitivity remains in acoplanarity and PT • for semileptonic decays mass can be obtained from kinematical fits New particles searches with t at DELPHI Nara, 17/09/2004

  8. Kinematical fits • Widely used in channels with taus and/or neutrinos (also in others) • Basic ideas and approximations: • treat primary neutrino momentum as unknown • treat Etasunknown and tau direction as that of the resultant of the decay products (good approximation if boost is high) • use energy and momentum conservation • fit mass and all measured quantities (4C-fit) • precise estimation of t Energy • sometimes it becomes just a system • if applicable use additional constrain in mass (5C-fit) • two particles of equal masses • one of the jets with Z mass in other channels cstn New particles searches with t at DELPHI Nara, 17/09/2004

  9. Charged Higgs (type II) • Additional differences come from the spin of the parent boson: • differential polar angle cross-section • polarization of the produced t • given the fixed n helicity • t from scalar H bosons have Pt=+1 • from vector W bosons have Pt=-1 • t physics relevant for the search! • simple t decay ID • enn, mnn, p, p+ng, 3p, others • use “optimal polarization estimators” used to measure Ptat LEP I • build a likelihood function with that variables • firstly proposed by DELPHI New particles searches with t at DELPHI Nara, 17/09/2004

  10. H+H-/W+W- separation in tntn • These four variables are combined into a single likelihood function with significant WW discrimination New particles searches with t at DELPHI Nara, 17/09/2004

  11. Charged Higgs (type I) • Within type I models H can also decay to W*A, if the pseudoscalar A is light • WAWA, WAtn, WAcs channels can be accessible • A decays to bb if kinematically allowed • WAtn topology similar to cstn , but jets with bb content • Neural Nets used to optimize the separation • tau ID vars • b tagging vars • kinematical vars • WW largely suppressed NN 5C-fit mass New particles searches with t at DELPHI Nara, 17/09/2004

  12. Limits • No excess from SM expectation was found, limits set on MH at 75-90 GeV depending on the model parameters (95% CL) limit for pure decay to tn New particles searches with t at DELPHI Nara, 17/09/2004

  13. Doubly Charged HiggsPhys. Lett. B522 (2003) • Predicted in extensions of SM • left-right symmetric models • could be relatively light • produced in pairs H++H-- at LEP • decay to pairs of leptons with the same charge • not necessarily of the same flavor • coupling H++tt the less bounded by indirect measurement • 4t topologies searched for • no prompt n • almost no background (only ZZ4t, with good t ID) • despite having ≥4n, mass reconstructed with high precision with 4C-fit • excluded below 99.1 GeV at 95% CL • one candidate left with tt mass compatible with the hypothesis of two equal mass charge 2 objects • also compatible with ZZ with usual pairing! New particles searches with t at DELPHI Nara, 17/09/2004

  14. Doubly Charged Higgs limits • DELPHI extended the search to the case of couplings so small that the Higgs flies • taus produced far from the IP but inside the tracking system • seen as “kinks” (described later in SUSY) • charged higgs reaching the calorimeter • anomalous dE/dx • Excluded for masses below 97.3 GeV for any assumption on the model htt>10-7 htt<10-9 New particles searches with t at DELPHI Nara, 17/09/2004

  15. Neutral Higgs Eur. Phys. J. C32 (2004)/ Phys.Lett.B499(2001) t ID refined with likelihood radiative evt rejection Event likelihood • SM/MSSM neutral Higgs • H decay to tt is second highest BR at LEP mass range, 1/10 of BR(bb) • qqtt in HZ (SM and MSSM) as a complementary channel • more important in MSSM, hA production (1/5 or more) with at least 1 t • Good mass reconstruction with 4C/5C fit • Final discrimination with a likelihood function • No signal-like candidates at the highest energy New particles searches with t at DELPHI Nara, 17/09/2004

  16. Neutral Higgs Eur. Phys. J. C32 (2004)/ Phys.Lett.B499(2001) • SM/MSSM neutral Higgs • H decay to tt is second highest BR at LEP mass range, 1/10 of BR(bb) • qqtt in HZ (SM and MSSM) as a complementary channel • more important in MSSM, hA production (1/5 or more) with at least 1 t • Good mass reconstruction with 4C/5C fit • Final discrimination with a likelihood function • No signal-like candidates at the highest energy New particles searches with t at DELPHI Nara, 17/09/2004

  17. Neutral Higgs in extended modelsCERN-PH-EP 2003-061 (acc. by Eur. Phys. J. C) Limits set on a scale factor on the SM cross section • Decays with t , even more interesting in the framework of general 2HDM • BR to tt or bb is a free parameter • search for hA and hZ • Search for 4t • identical to H++H -- for high mass, except mass pairing (neutral bosons) • if one of the bosons is light, its boost is large and both t seen as a single jet, look for 3 jet topology • if both are light, 2 jets • Reanalysis of LEP1 data to look for Yukawa production • “radiation” of a higgs from a ff final state • bbtt, 4t • look for low mass radiated tt or bb pair New particles searches with t at DELPHI Nara, 17/09/2004

  18. SUSYEur. Phys. J. C31 (2003) neutralino multilepton search • Assume here MSSM with R-parity conservation • LSP (lightest supersymmetric particle) is stable • s-particles are produced in pairs • many scenarios depending on the model parameters • In some scenarios, the stau is the lightest charged SUSY particle and the lightest neutralino is the LSP • Other scenarios predict production of neutralino pairs, with cascade decays to several (≥2) t and a pair of LSP e,m t New particles searches with t at DELPHI Nara, 17/09/2004

  19. Stau search • Final state topology is two t and two neutralinos • neutralinos escape undetected • if neutralinos significantly lighter than staus (high DM ) the topology and analysis is identical to H+H- • Otherwise similar, but taking care of a slower t (smaller energy and smaller boost) • Light stau at LEP1, data reanalyzed to exclude stau with suppressed couplings to Z • No evidence of sparticles found • Any mixing and any DM, 26.3 GeV<M<81.9 GeV excluded • M>81.9 if DM>15 or MLSP<68 New particles searches with t at DELPHI Nara, 17/09/2004

  20. Extensions of MSSM hadronic interaction fakes a decay at 20 cm • Light gravitino Eur. Phys. J. C27 (2003) • gravitino is LSP • decays are not immediate and particles might fly • look for taus not produced in the IP • Rp is violated Eur. Phys. J. C32 (2004) • LSP can decay to fermions • many s-particle cascade decays include tau • Stop, Sneutrino, Sleptons, gauginos... • Anomaly Mediated SUSY Breaking Eur. Phys. J. C34 (2004) • results interpreted also in AMSB • No excess found in any channel hadronic interaction fakes a decay at 20 cm New particles searches with t at DELPHI Nara, 17/09/2004

  21. Excited leptons(Preliminary) • Excited leptons expected in models with fermion substructure • Produced in pairs (l*l*) • Or accompanied by a normal lepton (l*l) • Expected decays: • l*→lg or lZ or nW, (charged excited leptons) • n*→ng or lW or nZ, (neutral excited leptons) • t present in many channels for nt*and t* • Cleaner signature in g channels • preselection based on t and g ID + kinematic cuts New particles searches with t at DELPHI Nara, 17/09/2004

  22. Excited leptons t*t→ttg t*t*→ttgg t*t→tnW • In t* channels mass reconstructed with kinematical fits • No evidence observed • mt*>102.7/101.0 GeV • mv*>94.2/101.9 GeV New particles searches with t at DELPHI Nara, 17/09/2004

  23. Limits on excited leptons • Cross section and couplings limits New particles searches with t at DELPHI Nara, 17/09/2004

  24. Technicolor modelsEur. Phys. J. C22 (2001) • DELPHI has studied the possibility to search for technimesons, pT, rT • technirho decay to technipions or W • technipions decay to bc or tn • search identical to H+H- in type I models • in fact search developed for this channel • limits set over a wide range of parameter space New particles searches with t at DELPHI Nara, 17/09/2004

  25. Conclusions • DELPHI has searched a large variety of new physics/particles, whose decays included t leptons • charged and neutral Higgs • sleptons • excited leptons • several new ideas proposed by DELPHI and adopted by other experiments extended the range of the search • many ideas taken from standardt physics contributed to this improvements • Most results are final or hopefully become final in short term • Unfortunately, no signal of new physics was found • limits set on many models • for a large range of parameters model • for most of the kinematically allowed mass range New particles searches with t at DELPHI Nara, 17/09/2004

  26. charged higgs cross section limits New particles searches with t at DELPHI Nara, 17/09/2004

  27. More limits on charged higgs New particles searches with t at DELPHI Nara, 17/09/2004

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