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F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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New particle searches with at DELPHI
• Introduction
• ID in searches
• Higgs search
• SUSY
• Excited leptons
• Technicolor
F. Matorras
IFCA, Santander, Spain
DELPHI collaboration
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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• 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 or ee channels!
• In general tau identification and reconstruction complicated
– large variety of decays
– hadronic decays
– missing energy: lose mass information
• Good knowledge of physics is important!
– people in searches often ignores “basic” properties of the taus
Introduction
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Data sample
• DELPHI– multipurpose detector at LEP ee 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
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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 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 , there is the equivalent
with e or – 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
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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 for events containing jets• very high cross section• lost photons at low angle produce missing energy• gluon jets or tracks unattached mimic a • basic to have a good ID and good jet definition
– radiative ll
• as above, a looks like a • acoplanarity, angle of missing momentum
– two-photon events (eeff)• 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
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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 HH at LEP energies
– cross section only dependent on the Ecm and mass
• Type II models
– decay to heaviest fermions available
• cs or • BR free, but prejudice in favor of high BR
• look for cscs, cs and
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Charged Higgs (type II)
• Topology of these events:
– in : two acollinear , with additional missing energy
– in cs: two hadronic jets, with cs content and one • Remaining irreducible background, WW equivalent decays
• Mass of the parent boson
– for purely leptonic decays at least 4 undetected, mass cannot be reconstructed
• some sensitivity remains in acoplanarity and PT
– for semileptonic decays mass can be obtained from kinematical fits
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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 Easunknown 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 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
cmEE
p 0
cs
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Charged Higgs (type II)
• Additional differences come from the spin of the parent boson:
– differential polar angle cross-section
– polarization of the produced • given the fixed helicity
• from scalar H bosons have P1
• from vector W bosons have P1
• physics relevant for the search!
• simple decay ID– en, others
• use “optimal polarization estimators” used to measure Pat LEP I
• build a likelihood function with that variables
– firstly proposed by DELPHI
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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HH/WW separation in
• These four variables are combined into a single likelihood function with significant WW discrimination
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Charged Higgs (type I)
• Within type I models H can also decay to W*A, if the pseudoscalar A is light– WAWA, WA, WAcs
channels can be accessible– A decays to bb if
kinematically allowed• WA topology similar to cs ,
but jets with bb content– Neural Nets used to optimize
the separation• tau ID vars• b tagging vars• kinematical vars
– WW largely suppressed
5C-fit mass
NN
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Doubly Charged HiggsPhys. Lett. B522 (2003)
• Predicted in extensions of SM– left-right symmetric models– could be relatively light– produced in pairs HH at LEP– decay to pairs of leptons with the same
charge • not necessarily of the same flavor• coupling Hthe less bounded
by indirect measurement• 4 topologies searched for
– no prompt – almost no background (only ZZ4,
with good ID)– despite having ≥4, mass
reconstructed with high precision with 4C-fit
– excluded below 99.1 GeV at 95% CL
• one candidate left with mass compatible with the hypothesis of two equal mass charge 2 objects
• also compatible with ZZ with usual pairing!
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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
h>10
h<10
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Neutral Higgs Eur. Phys. J. C32 (2004)/ Phys.Lett.B499(2001)
• SM/MSSM neutral Higgs
– H decay to is second highest BR at LEP mass range, 1/10 of BR(bb)
– qq in HZ (SM and MSSM) as a complementary channel
– more important in MSSM, hA production (1/5 or more) with at least 1
• Good mass reconstruction with 4C/5C fit
• Final discrimination with a likelihood function
• No signal-like candidates at the highest energy
ID refined with likelihood
Event likelihood
radiative evt rejection
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Neutral Higgs Eur. Phys. J. C32 (2004)/ Phys.Lett.B499(2001)
• SM/MSSM neutral Higgs
– H decay to is second highest BR at LEP mass range, 1/10 of BR(bb)
– qq in HZ (SM and MSSM) as a complementary channel
– more important in MSSM, hA production (1/5 or more) with at least 1
• Good mass reconstruction with 4C/5C fit
• Final discrimination with a likelihood function
• No signal-like candidates at the highest energy
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Neutral Higgs in extended modelsCERN-PH-EP 2003-061 (acc. by Eur. Phys. J. C)
• Decays with , even more interesting in the framework of general 2HDM
– BR to or bb is a free parameter– search for hA and hZ
• Search for 4– identical to HH for high mass,
except mass pairing (neutral bosons)
– if one of the bosons is light, its boost is large and both 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– bb, 4
• look for low mass radiated or bb pair
Limits set on a scale factor on the SM cross section
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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SUSYEur. Phys. J. C31 (2003)
• 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) and a pair of LSP
0~~
neutralino multilepton search
e
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Stau search• Final state topology is two and
two neutralinos– neutralinos escape undetected– if neutralinos significantly
lighter than staus (high M ) the topology and analysis is identical to HH
– Otherwise similar, but taking care of a slower (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 M, 26.3
GeV<M<81.9 GeV excluded• M>81.9 if M>15 or MLSP<68
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Extensions of MSSM
• 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 channelhadronic interaction fakes a
decay at 20 cmhadronic interaction fakes a
decay at 20 cm
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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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*→l or lZ or W, (charged excited leptons)
– *→ or lW or Z, (neutral excited leptons)
– present in many channels for
and
• Cleaner signature in channels– preselection based on and
ID + kinematic cuts
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Excited leptons
• In * channels mass reconstructed with kinematical fits
• No evidence observed
– m*>102.7/101.0 GeV
– mv*>94.2/101.9 GeV
**→*→W
*→
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Limits on excited leptons
• Cross section and couplings limits
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Technicolor models Eur. Phys. J. C22 (2001)
• DELPHI has studied the possibility to search for technimesons, T,T
• technirho decay to technipions or W
• technipions decay to bc or
• search identical to HH in type I models
– in fact search developed for this channel
• limits set over a wide range of parameter space
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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Conclusions
• DELPHI has searched a large variety of new physics/particles, whose decays included 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 standard 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
F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004
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charged higgs cross section limits