New particle searches with t at DELPHI Introduction t ID in searches Higgs search SUSY Excited leptons Technicolor

F. MatorrasIFCA, Santander, SpainDELPHI collaboration

IntroductionA 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 decayIn many models the most probable final state topologies include tausIn other cases it is important for some region of the parametersAlso used in other searches as complementary channelwhen the new particle is expected to follow lepton universality or is accompanied by Z or W bosonsalways significantly worst than mm or ee channels!In general tau identification and reconstruction complicatedlarge variety of decayshadronic decaysmissing energy: lose mass informationGood knowledge of t physics is important!people in searches often ignores basic properties of the taus

Data sampleDELPHImultipurpose detector at LEP e+e- Collider at CERNsee more details in A.Andreazza and D.Dedovich talksFirst phase (LEP I) 89 to 95Ecm~Mz>150 pb-1 Mainly designed for Z physicsalso for new particle searchesdata was reanalyzed recently in view of new modelsFrom 95 until 2000 (LEP II) gradual increase of energy up to 208 GeV~ 700pb-1 above WW thresholdmost searches rely on this data, especially the highest energy in 2000

Tau ID @ LEP2Not too different from ID @LEP1see A.Andreazza and D.Dedovich talkshigher energy higher particle multiplicity in the eventseparation more complicatedA t is typically seen as an isolated lepton or low multiplicity and narrow jet associated with missing energyif lepton, its energy significantly smaller than that expected from a prompt leptonusually for any background channel with t, there is the equivalent with e or mbut the lepton energy of prompt leptons is three times larger on averagehigh boost very narrow jet w.r.t. quark jetsoften just one-prong events are used to, reduce background from gluon/hadron jetsDifferent 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 jetsbased on energy, angle with the jet and mass of the jet

Tau event selection @ LEP2Large variety of selection procedures and background processes depending on the topology, mass of the particle...Major background in most casesqq(g)(g) for events containing jetsvery high cross sectionlost photons at low angle produce missing energygluon jets or tracks unattached mimic a tbasic to have a good t ID and good jet definitionradiative l+l-as above, a g looks like a nacoplanarity, angle of missing momentumtwo-photon events (e+e-ff)huge cross sectionmissing energy in electrons/positrons at low angletransverse energy and transverse momentumOften WW or ZZ decays remain as almost irreducible backgroundmass informationspecific variables in each case

Charged Higgs Eur. Phys. J. C34 (2004)Many extensions of SM predict the existence of the Charged Higgsstudied here in the framework of general 2 Higgs Doublet Models (2HDM) produced in pairs H+H- at LEP energiescross section only dependent on the Ecm and massType II modelsdecay to heaviest fermions availablecs or tnBR free, but prejudice in favor of high tn BRlook for cscs, cstn and tntn

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 tRemaining irreducible background, WW equivalent decaysMass of the parent bosonfor purely leptonic decays at least 4 n undetected, mass cannot be reconstructedsome sensitivity remains in acoplanarity and PTfor semileptonic decays mass can be obtained from kinematical fits

Kinematical fitsWidely used in channels with taus and/or neutrinos (also in others)Basic ideas and approximations:treat primary neutrino momentum as unknowntreat Et as unknown 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 Energysometimes it becomes just a systemif applicable use additional constrain in mass (5C-fit) two particles of equal massesone of the jets with Z mass in other channelscstn

Charged Higgs (type II)Additional differences come from the spin of the parent boson:differential polar angle cross-sectionpolarization 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 IDenn, mnn, p, p+ng, 3p, othersuse optimal polarization estimators used to measure Pt at LEP Ibuild a likelihood function with that variables firstly proposed by DELPHI

H+H-/W+W- separation in tntn These four variables are combined into a single likelihood function with significant WW discrimination

Charged Higgs (type I)Within type I models H can also decay to W*A, if the pseudoscalar A is lightWAWA, WAtn, WAcs channels can be accessibleA decays to bb if kinematically allowedWAtn topology similar to cstn , but jets with bb contentNeural Nets used to optimize the separationtau ID varsb tagging varskinematical varsWW largely suppressed5C-fit massNN

LimitsNo 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

Doubly Charged HiggsPhys. Lett. B522 (2003)Predicted in extensions of SMleft-right symmetric modelscould be relatively lightproduced in pairs H++H-- at LEPdecay to pairs of leptons with the same charge not necessarily of the same flavorcoupling H++tt the less bounded by indirect measurement4t topologies searched for no prompt nalmost no background (only ZZ4t, with good t ID)despite having 4n, mass reconstructed with high precision with 4C-fitexcluded below 99.1 GeV at 95% CLone candidate left with tt mass compatible with the hypothesis of two equal mass charge 2 objectsalso compatible with ZZ with usual pairing!

Doubly Charged Higgs limitsDELPHI extended the search to the case of couplings so small that the Higgs flies taus produced far from the IP but inside the tracking systemseen as kinks (described later in SUSY)charged higgs reaching the calorimeteranomalous dE/dxExcluded for masses below 97.3 GeV for any assumption on the modelhtt>10-7htt

Neutral Higgs Eur. Phys. J. C32 (2004)/ Phys.Lett.B499(2001) SM/MSSM neutral HiggsH 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 channelmore 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 functionNo signal-like candidates at the highest energy

Neutral Higgs Eur. Phys. J. C32 (2004)/ Phys.Lett.B499(2001) SM/MSSM neutral HiggsH 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 channelmore 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 functionNo signal-like candidates at the highest energy

Neutral Higgs in extended modelsCERN-PH-EP 2003-061 (acc. by Eur. Phys. J. C)Decays with t , even more interesting in the framework of general 2HDMBR to tt or bb is a free parametersearch for hA and hZSearch for 4tidentical 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 topologyif both are light, 2 jetsReanalysis of LEP1 data to look for Yukawa productionradiation of a higgs from a ff final statebbtt, 4t look for low mass radiated tt or bb pairLimits set on a scale factor on the SM cross section

SUSYEur. Phys. J. C31 (2003)Assume here MSSM with R-parity conservationLSP (lightest supersymmetric particle) is stables-particles are produced in pairsmany scenarios depending on the model parametersIn 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 LSPneutralino multilepton searchte,m

Stau searchFinal state topology is two t and two neutralinosneutralinos escape undetectedif 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 ZNo evidence of sparticles foundAny mixing and any DM, 26.3 GeV15 or MLSP

Extensions of MSSMLight gravitino Eur. Phys. J. C27 (2003)gravitino is LSPdecays are not immediate and particles might flylook for taus not produced in the IPRp is violated Eur. Phys. J. C32 (2004)LSP can decay to fermionsmany s-particle cascade decays include tauStop, Sneutrino, Sleptons, gauginos...Anomaly Mediated SUSY Breaking Eur. Phys. J. C34 (2004)results interpreted also in AMSBNo excess found in any channelhadronic interaction fakes a decay at 20 cm

Excited leptons(Preliminary)Excited leptons expected in models with fermion substructureProduced 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 channelspreselection based on t and g ID + kinematic cuts

Excited leptonsIn t* channels mass reconstructed with kinematical fitsNo evidence observed mt*>102.7/101.0 GeVmv*>94.2/101.9 GeV

Limits on excited leptonsCross section and couplings limits

Technicolor models Eur. Phys. J. C22 (2001)DELPHI has studied the possibility to search for technimesons, pT, rTtechnirho decay to technipions or Wtechnipions decay to bc or tn search identical to H+H- in type I modelsin fact search developed for this channellimits set over a wide range of parameter space

ConclusionsDELPHI has searched a large variety of new physics/particles, whose decays included t leptonscharged and neutral Higgssleptonsexcited leptonsseveral new ideas proposed by DELPHI and adopted by other experiments extended the range of the searchmany ideas taken from standard t physics contributed to this improvementsMost results are final or hopefully become final in short termUnfortunately, no signal of new physics was foundlimits set on many modelsfor a large range of parameters modelfor most of the kinematically allowed mass range

charged higgs cross section limits

More limits on charged higgs