Light Higgs search in SUSY cascades
First Results
Alain Romeyer (Mons - Belgium)
Filip Moortgat (CERN)
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Introduction
Simulated events
Reconstruction framework
Part I : trigger
Part II : b tagging efficiency
Part III : invariant mass reconstruction
Part IV : jet calibration
Conclusion
Alain Romeyer - Nov. 2004
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Introduction
χ 10
Signature :
q/g
χ 02
LSP
MET
b
h0
B tagging
b
It has been shown in
CMS-note 1997/070 :
• signal can be seen with
only few months of data
• main bkg : SUSY
• SM bkg very low
• discrimination between
SUSY signal and SUSY bkg
is difficult
Alain Romeyer - Nov. 2004
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Introduction…
MSUGRA 5 parameters : m0, m1/2, A, tan b, sign(µ)
S. Abdullin – L. Pape
Update II of the new MSUGRA
test points proposal
Benchmark point LM5
Tan b = 10
AO = 0
µ>0
m g ~ 860 GeV
M1/2 = 360 GeV
M0 = 230 GeV
m ~ 800 GeV
q
m χ 0 ~ 273 GeV
m χ 0 ~ 142 GeV
m H0 ~ 570 GeV
m h 0 ~ 116 GeV
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χ 10
Signature :
q/g
χ 02
1
LSP
MET
b
h0
Alain Romeyer - Nov. 2004
B tagging
b
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FULL SIMULATION CHAIN
Simulated events
generator
Isajet 7.69
+
Pythia 6220
+
CMKIN
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Production of signal events and pile-up
Signal events : all SUSY channel (easier
for later event reweighting)
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simulation
digitisation
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OSCAR
2_4_6
ORCA
7_6_1
Private production of 10 000 evts @ Mons
(~25 days)
~20% contains our decay chain
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Digitisation : merge pile-up and signal
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Start to use the 100 000 evts production
Low statistic -> only a first try
Alain Romeyer - Nov. 2004
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« Reconstruction » strategy
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Simulated
events
ORCA_8_1_3 + patch for muon reconstruction
Reconstruction
I – L1 +HLT
Signal
Efficiency ?
+ Background :
• SUSY
• SM : top, QCD…
Analysis
• event selection
• IV - b ̅b inv mass reco.
II - Jet Reconstruction :
• Iterative Cone Algo.
• cone size : 0.4
• tower Et thr. : 0.5 GeV
• seed Et thr. : 1 GeV
Jet sel.
Pt > 10 GeV
III - B tagging :
• By Secondary Vertex
• Regional reconstruction
• cone size : 0.4
• Vertex algo. : principal
• Min. tracks from sec. vertex : 3
MET :
• MET from EcalHcalTower
Alain Romeyer - Nov. 2004
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Part I : trigger
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L1 : 99.9 % - HLT : 94 %
Trigger
Condition (GeV)
% of evts
1e
Et > 26
16.67
2e
Et > 14.5
1.14
1 ph
Et > 80
7.08
2 ph
Et > 35
1.90
1µ
Pt > 19
7.45
2µ
Pt > 7
2.61
1 jet
Et > 571
23.50
3 jets
Et > 209
21.70
4 jets
Et > 122
32.67
B jet
1 Jet Et > 237
77.40
MET calo
1 jet > 180 GeV + MET
> 123
79.53
MET jets
1 jet > 180 GeV + MET
> 123
79.77
Alain Romeyer - Nov. 2004
Caveat :
~ same efficiency
for SUSY bkg.
~ all events !!!
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Part II : b tagging efficiency
By S.V. - Cone size : 0.4
p T (b-b) = 72.5GeV
<> ~ 51,7 %
<> ~ 9,3 %
Alain Romeyer - Nov. 2004
h
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Part III: inv. mass reco.
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Look for the h0 peak in the b -b invariant mass distribution
Jet pairing : crucial point
Evts with 6 b jets
2 from higgs decay
I.P.
No simple « a priori »
kinematic constrainsts
Select most probable b jets (biased  high energy jets ~ better proba )
Select closest b jets (higgs boost not so high… pT (h 0 ) =175.3GeV )
try all the possible combinations of reconstructed b tagged jets
Alain Romeyer - Nov. 2004
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Jet pairing strategy
Reco jet + B tagging
MC jet Energy and momentum
Try all combinations
seems the best
Bkg due to combinatorial
S/√B @ 1 s @ 2 s
@3s
Best b tag proba
12.4
12.3
10.5
Closest b jets
14.2
14
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All combi.
25.7
25.4
21.7
Alain Romeyer - Nov. 2004
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Likelihood fit
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Follow the Z’  µµ strategy (CMS AN 2004/018)
Unbinned method
NS
NS
P(mh )=
Ps (m h ,σ) + [1 ] Pbg (m h )
N tot.
N tot.
Fraction of signal
Normalised
Gaussian
Extract from fit out of peak
NS
3 parameters : mh, s and
N tot.
No constraints on the absolute
background level  only the shape
Alain Romeyer - Nov. 2004
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Likelihood fit…
Reco jet + B tagging
True jet Energy and momentum
m h =113,51± 0.48
σ = 2.71± 0.45
 Ns
 N = 0.04 ± 0.06
 tot
  6.6
S = ln  L S+B
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L
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B 

SUSY bg + signal
mh = 116 GeV
Alain Romeyer - Nov. 2004
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Inv. mass reco with Rec. Info.
Reco jet + B tagging
Reco. jet Energy and momentum
No calibration
mh = 116 GeV
• no peak
• jet energy calibration -> hand made
Alain Romeyer - Nov. 2004
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Part IV : jet calibration
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Jet calibration method following CMS-note 1999/066
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Association between rec. Jets and generated partons before
hadro. (cone < 0.3)
2 types of jets : b jets and other
Etruejet = sum(Equarks,gluons)
Correction Factor C = Etruejets / Erecjets
Fit of the correction factor surface in the E vs |h| plane :
f(|η|)
C  η, E  = a1+ a 2
E
f(|η|) = b1 + b 2 |η|+ b3 |η| + b 4 |η|
2
Alain Romeyer - Nov. 2004
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Correction factor Emc / Erec
Non B jets
C(h, E)
C(h, E)
B jets
|h|
Ejet
|h|
Alain Romeyer - Nov. 2004
Ejet
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Inv. mass reco with Hand made
Jet calibration
Reco jet + B tagging
Reco. jet Energy and momentum
Hand made calibration (CMS-note 1999/066)
mh = 116 GeV
No peak !!!
Alain Romeyer - Nov. 2004
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Test with Z  bb
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Iterative Cone Algorithm not adapted ?
MidPoint ?
Test jet reconstruction and calibration with a Zbb
sample (production of 50 000 events)
Very usefull in order to check :
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Position of the peak
Jet Energy calibration
Energy resolution
Important Validation Sample
Alain Romeyer - Nov. 2004
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Conclusion
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First results with 10k events :
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Trigger efficiency 94 %
Higgs peak visible if using the MC information
Fit strategy already working
Jet energy reconstruction is not working
Try other algorithms
Optimize B tagging
Move to more recent ORCA version
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Future results with 100k event production
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In addition, now we will start :
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Study the background : SUSY and SM (Z, top…)
Define several selection criteria
Alain Romeyer - Nov. 2004
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