F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004 1 New particle searches with at DELPHI Introduction ID in searches.

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


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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


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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


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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


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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


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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


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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


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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


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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


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HH/WW separation in

• These four variables are combined into a single likelihood function with significant WW discrimination


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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


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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


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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!


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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


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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


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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


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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


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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


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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


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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


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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


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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

*→


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Limits on excited leptons

• Cross section and couplings limits


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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


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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


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charged higgs cross section limits


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More limits on charged higgs

F.Matorras, IFCA, Spain New particles searches with at DELPHI Nara, 17/09/2004 1 New particle searches with at DELPHI Introduction ID in searches.

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