Nicola De Filippis & Marcello Maggi

Search for charginos nearly mass degenerate with the lightest neutralino

in e+e- collisions up to s=209 GeV

Nicola De Filippis & Marcello Maggi

Università Degli Studi di Bari and INFN

ALEPH collaboration

ALEPH SUSY TASK FORCE

Charginos and neutralinos in MSSM

and almost degenerate in mass

1χ~ 0

1χ~ Small values of

011 χ~χ~

-Δ mmm

In MSSM: small are possible:

• in higgsino region:

• in gaugino region: relaxing the relation between gaugino masses due to unification at GUT scale

2GeV/c 5m

2Mμ

2Mμ

In AMSB models due to 1MM 21 • and like a W-ino

• heavy gravitino

1χ~ 0

1χ~

Charginos production at LEP2

as a function of )χ~χ~eσ(e -11

- s and 1χ~m

Charginos decay

3 body decay channels:

• High m0 scenario: the exchange is dominant

• Low m0 scenario: the and exchange are dominant

l~ ~

W

In the range little energy is available for final decay products low trigger efficiency due to soft tracks

2GeV/c 3m

High m0 scenarioThe coupling is dominant:

• perturbative QCD

• sum over exclusive hadronic final states

W2GeV/c 2m2GeV/c 2m

with one or more pions

1χ~ 0

1χ~

High m0 scenario: signatures

Three interesting regions:

• : longlived stable particle analysis

• : small visible energy - low trigger efficiency

• : decay products detectable efficiently standard analysis

πmm

2GeV/c 3m

2π GeV/c 3m m

1χ~

In the range :ISR photons

The detection of an ISR energetic photon ensures the trigger because of the energy depositions in calorimeters

The signal consists of:

• a high pt photon

• small visible energy-soft tracks

• large missing energy

2π GeV/c 3m m

Current limits

• from the total width of Z measured at LEP1: 2mm Zχ~1

• using standard analysis up to kinematical limit for

2GeV/c 3Δm

2GeV/c 3Δm

2GeV/c 5Δm

decay parameters…from semileptonic decays: Kuhn-Santamaria

The best fit using ALEPH

data for

events

2 MeV/c301128 m1

a

MeV20523 1

a

2 MeV/c10773 m

MeV15145

2' MeV/c301370 m

MeV25510 '

Resonances:and’

a1

1χ~

ττee

Monte Carlo simulation

1. Generation of the signal events

2. Simulation of the interaction in the detector: GALEPHmodified

3. Reconstruction of the events: JULIA

11 χ~χ~ γeeSUSYGEN

•

•

•

•

•

100)c/GeV( m45 2

χ~1

208)GeV( s189

5)c/GeV( Q10 2value

5

80)cm( λ0

, χ~ (n)πχ~ 011 νμχ~χ~ μ

011

, νeχ~χ~ e011

MC signal event: (1)

cm 30λ

χ~πχ~GeV/c 0.145Δm

GeV/c 56m

GeV 208 s

011

2

2

χ~1


cm 30λ


GeV/c 56m

GeV 208 s

011

2

2

χ~1

cm 80λ


GeV/c 56m

GeV 208 s

011

2

2

χ~1


ISR simulation in SUSYGENE and pt

spectra in gaugino and higgsino region:

Differences related to the different coupling of charginos to Z:

they are rilevant for light charginos

“Radiative return to

Z”

REMT approach:

F.A. Berends R.Kleiss

REMT approach:

F.A. Berends R.Kleiss

Simulation of leptonic decay

Ee e cos spectra of theelectron from the decay at high and low m0:

01e1 χ~ νeχ~

Small differences in the cosspectrum due to the or exchange

W l~

SM processes

4 fermioni events:

2 photons events:

2 fermions events:

(Bhabha) ee ee ττ ,μμ ee

g qq ,qq ee

νν Z ,eZe ZZ, ν, We ee

qq ,ll γγ -

Events: KORALZνν γee

BHWIDE

KORALZ

KORALZ

PHOT02

PYTHIA

-WW ee KORALW

Data analysis: 1998-2000

Data samples: 208)GeV( s189

Total integrated luminosity:

-1pb 627L

Analysis with ISR photonsThe variables of the selection:

• visible energy,

• transverse visible momentum,

• energy andtransverse energy of ,

• impact parameter of photon

• isolation angle of photon

• total energy of the photons,

• recoil to photons mass,

• number of charged tracks,

• beam collision time signal

• invariant mass of a pair of tracks

visEvistp

tγ,γ E,E

mind

miss γ,m

chN

isolθ

0t

totγ,E

max0v

ISR analysis: selection cuts

Two kinds of cuts are distinguished :Topological cuts useful to• reject not simulated background events;• reject topologies too much different from the

signal

Signal to background discrimination cuts useful to:• distinguish signal from background with the same

topologies

ISR analysis: topological cuts• Preselection of photons (or pair conversions):

95.0θ cos GeV 1Eγ GeV 189 s

Radiative return to Z

bremsstrahlung collinear to fermions

cosmics and

cm 80d

ns 100 t

min

0

Rejection of cosmics

not simulated

Rejection of mrad 34θ

θ sin sE

min

mint,γ

events with many tracks

ISR analysis: topological cuts

GeV 189 s

cm 80d

ns 100 t

min

0

Rejection of cosmics

cosmics Events with many charged

tracks

10Nch

Rejection of events with

many charged tracks, 2f e 4f


GeV 189 s

Residual excess on the data at large transverse visible momentum

It’s due to events with a and many charged tracks inside a cone with a half- opening of 11.5o whose energy is added clustering effect of energy

oisol 30θ

This excess is eliminated requiring the isolation of the photon with respect to the nearest charged track:


GeV 189 s

The residual excess is due to events with an electron atnot simulated in BHWIDE

ee γeeo8.6 θ 2Nch

the residual excess is due to…

not simulated event ee γee

…the residual excess

discriminating signal to bkg

against radiative return to Z

Using additional selection cuts: 2

missγ, GeV/c 100m s 3.5%pvis

t

0NN pointconv against residual cosmics

The residual excess is due to:

• not simulated e+e- e+e- events with 2 or 3 tracks• not simulated events with two real with a not identified conv.•few cosmics

two real with a not identified conversion

GeV 189 s

ee not simulated in KORALZ

Selection of two photons:

•

•

•

•

•

•

mint,γ θsin sE

0NN pointconv

0Nch

cm 80d ns 100 t min0

s % 5.3pvist

s % 20 EE totγ,vis

ee

A better identification of conversion

0n

MeV/c 100v

ITC

20max



0n

MeV/c 100v

ITC

20max



0n

MeV/c 100v

ITC

20max


…non simulated e+e- e+e- events with an electron at low angle and a ray (2 tracks)

…non simulated e+e- e+e- events with an electron at low angle and a ray (2 tracks)

ISR analysis: discrimination cuts

max energy of a ISR fixed

and m1χ~

2

)(2m-s E 1

2

χ~

γ

s s

Cut from signal kinematics:

cm 0λat signal of pmaxEE

cm 0λat signal of pmaxpvisttotγ,vis

vist

vist

against residual dileptons events

Optimization of the analysis

The selection is optimized varying cuts to minimize the

variable:

)... 3!

)(b 7.75

2!

)(b 6.30 )(b 74.43(

)ε(

e)(N

32)b(

95 xx

xx

xx

95N

event background ofnumber b

efficiencyε

cuts ofvector

x

mean value over a large number of experiments of

the UL at 95% C.L.

Selection efficiency

momentum of at a level of generation

Sample of events with leptons and hadrons in the final state

Sample of events with leptons and hadrons in the final state

% 35 ε

cuts on the acceptance and E ,t

30 % cut on the isolation

15 % other cuts

The events are lost due to:


)λ Δm, ,m ,s(εε11111 χ~χ~χ~χ~χ~

)hadrons(BR ε)μ(BRε)e(BRεε hadronsμe

χ~χ~1χ~1χ~1χ~1χ~1χ~1χ~11

BR depends on the parameter space of MSSM

The efficiency depends on the kinematical variables and the performance of detector

Electrons, muons and pions are reconstructed with different efficiency

ISR is different in gaugino and higgsino region


011 χ~ πχ~

Due to the characteristics of the tracker there is a minimum value of the reconstructed momentum Reduction of the efficiency

at very low QvalueMeV/c 110pfor % 50ε

The effect of the reduction of the efficiency is not visibile at low and large m1χ~

m

Efficiency vs and m…competitive effects:• visible energy increases with • cuts on at = 0 cmtotγ,vis

vist EE and p

ISR is enhanced at small 1χ~

m

011 χ~ πχ~

Efficiency vs ECM

011 χ~ πχ~

Efficiency scales with the ratio for each value of andm

sm1χ~

The efficiency: decay channels

01e1 χ~ ν eχ~

The 3-body leptonic decays are reconstructed less efficiently than 1-pion decay for the same Qvalue because the neutrino escapes carrying energy.

The efficiency in gaugino and higgsino region

ISR enhanced in gaugino region small differences at low 1χ~

m011 χ~ πχ~

Parameterization of the efficiency for any kinematical configuration of the final states

Systematics uncertanties

• on the efficiency% 7σε

dominated from statistics

% 15σb


• on the estimate of the background

• on the estimate of the minimum reconstructed momentum: Data and MC are in agreement

MeV/c5 50pmin

Systematics uncertanties

• on the efficiency% 7σε


% 15σb


• on the estimate of the background

Systematics on photon reconstruction are derived from the single photon analysis G. Taylor studiesG. Taylor studies

Observed candidates

The number of observed events is in good agreement with the expected number of the SM events.

GeV 208-189s

No background subtraction is performed

The number of observed events

GeV/c 87mfor 0N 2

χ~obs1

The sliding cuts on the visible energy and total transverse momentum depend on is

different in the plane

) Δm,m (1χ~

obsN Δm and m χ~

1χ~

m Δm obsN

Candidate at ECM =196 GeV

GeV 21Eγ

Event compatible with the production of with: and for any

1χ~ GeV/c 84m 2

χ~1 Δm

Candidate at ECM=189 GeV

GeV 8Eγ

Event compatible with the production of with: and for any

1χ~ GeV/c 51m 2

χ~1 Δm

Interpretation of results in MSSM

MSSM with the following hypothesis:

•R-parity conservation

•The LSP is

•Unification of scalar masses at GUT scale,

•Unification of trilinear couplings,

•No mixing in the heavy sfermion sector

21 M M 0mμ βtan

0m

0A

01χ~

Two different scenario:

• High m0

• Low m0

Parameters:

High m0 scenario

1. The is larger in gaugino region

2. ISR is enhanced in gaugino region

3. The is larger in higgsino region

)χ~χ~ee(σ 11

Comparison between gaugino and higgsino region:

It is distinguished the exclusion in gaugino and higgsino region.

At large m0, in both regions:

• the decay of goes through the W-exchange

• for the decay is dominant.

1χ~

πmm 011 χ~ πχ~

High m0: gaugino region-scan on tan is very large for some tanvalues; it’s due to the vanishing of couplings

011 χ~Wχ~

The position of peaksdepends on m and m

Chargino behaves as a stable particle in these region.

High m0 scenario: gaugino regionExcluded region in the planeat 95 % C.L:

)Δm ,m(1χ~

20βtan 2

χ~GeV/c 88m

1

max

offor1βtan

OR

Reduction of the efficiency

due to the threshold of momentum

The intersection between the ISR and the stable particle analysis occurs for cm 30λ

High m0 scenario: gaugino regionMinimal excluded region in the planeat 95 % C.L:

)Δm ,m(1χ~

Region excluded from the standard analysis for any tan

Region excluded from the ISR analysis for any tan

Region excluded from the stable particle analysis for any tan

Region excluded or from ISR analysis or from the stable particle

analysis for any tan

C.L. % 95at GeV/c 88m 2

χ~1

High m0 scenario: higgsino regionMinimal excluded region in the planeat 95 % C.L:

)Δm ,m(1χ~

C.L. % 95at GeV/c 80m 2

χ~1

The exclusion limit is worse than the gaugino one due to:

• reduction of cross section

• ISR not enhanced

Region excluded from the standard analysis, the ISR analysis, the stable particle and from the OR for any tan

The intersection between the ISR and the stable particle analysis occurs for cm 30λ

• reduction of in gaugino region

• the leptonic decays are dominant

• shorter lifetime,

Low m0 scenario

cm 0λ

1χ~ν~,l

~ mmFor 01e1 χ~ ν eχ~

λ and )χ~χ~eσ(e -11

- are minimal at the same

time at points of minimum m0 with respect to tan

Low m0 scenario

At low m0 it’s not possible to set a lower limit on indipendent from m

1χ~

m

The only available limit comes from the precise measurement of the Z total width at LEP1

C.L. % 95at 2mm1χ~

Conclusions:

Lower limits on chargino mass at 95 % C.L.

C.L. % 95at 2mm1χ~

C.L. % 95at GeV/c 80m 2

χ~1

C.L. % 95at GeV/c 88m 2

χ~1Gaugino

Higgsino

High m0 scenario:

Low m0 scenario:

The other collaborations

DELPHI

L3

In high m0 scenario and in gaugino region:202)GeV( s189

Documents

Nicola De Filippis & Marcello Maggi