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
MC signal event: (2)
cm 30λ
χ~πχ~GeV/c 0.145Δm
GeV/c 56m
GeV 208 s
011
2
2
χ~1
cm 80λ
χ~πχ~GeV/c 0.145Δm
GeV/c 56m
GeV 208 s
011
2
2
χ~1
MC signal event: (3)
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
ISR analysis: topological cuts
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:
ISR analysis: topological cuts
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
two real with a not identified conversion
A better identification of conversion
0n
MeV/c 100v
ITC
20max
two real with a not identified conversion
A better identification of conversion
0n
MeV/c 100v
ITC
20max
two real with a not identified conversion
…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:
Selection efficiency
)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
Selection efficiency
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
dominated from statistics
• 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
dominated from statistics
% 15σb
dominated from statistics
• 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
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