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Search for heavy lepton partners of neutrinos
in the context of type III seesaw mechanism
in 2012 LHC CMS data
& CMS activities in LNL
Andrea Gozzelino
INFN LNL
LNL - April 16 th 2014
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Outline
 The Standard Model (SM) of fundamental interactions and its possible extensions
 Seesaw mechanism and signal simulation
 Overview of Compact Muon Solenoid (CMS) experiment at Large Hadron Collider (LHC)
 Physics objects and data samples
 Analysis overview
SM background processes
Offline selections
Background from asymmetric photon conversion
Background from non prompt leptons
Uncertainties
 Results and their interpretations
CMS activities @ LNL
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SM and beyond
Standard Model of particles
the best description of the phenomena involving elementary particles and fundamental interactions
Higgs boson discovered by ATLAS and CMS experiments at mH ≅ 126 GeV
Open issues:
 “hierarchy problem”
 the existing asymmetry in the universe between matter and anti-matter
 dark matter
 the neutrinos mass
Neutrinos oscillations
Neutrinos eigenstates mixed in flavors
Neutrinos mass ≅ order of eV (small)
Beyond SM searches in high energy physics experiments can explain the small neutrinos mass.
Test benchmark models with
exclusive and dedicated searches
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At Large Hadron Collider (LHC) exploration of
clean multi leptons final states
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The “Seesaw” mechanism
Neutrinos mass mν = YT M-1 Y (v2 /2)
where Y is the Yukawa coupling matrix
M is the heavy partners of neutrinos mass
v is the Higgs vacuum expectation value
Seesaw lagrangian L = LSM + LK + LY + LM
To obtain small mν
• O(M) = 1014 GeV, Yukawa coupling O(1)
• M «small», Yukawa coupling small
At the tree level, three different ways of production
Type I fermion singlet
Production and decay process: pp  Nl+  l+W-l+  l+l+jj
Signature: two same sign same flavor leptons and two jets (j)
Type II scalar weak triplet (hypercharge =2)
Production and decay process: pp  WR + X  Nl l1 X  l1l2 WR*  l1l2jj
Signature: two muons or two electrons (opposite or same sign), and two jets (j)
Type III fermion weak triplet (hypercharge = 0)
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Seesaw type III
Studied production and decay processes
Σ+, Σ0, Σ- seesaw type III heavy partners of neutrinos
Search for final states with exactly three charged leptons
Contributions from final states with four, five or six leptons negligible
(less than 10% respect three leptons case)
Final states with two leptons backgrounds dominated
Small Yukawa couplings (“natural value”) Vα = 10-6 (constraints from theory and measurements respected)
Benchmark in flavor democratic scenario Ve = Vμ = Vτ = Vα
Thesis update: removed the benchmark in the interpretations
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Seesaw type III signal
Σ0
• Production cross sections: σ (pp Σ+Σ0 ) ≈ 2 σ (pp Σ-Σ0)
• Production cross sections: range [123; 0.9] fb.
• Ten mass point values for Σ+ and Σ- : range [140; 340] GeV
• Processes involving Σ± have similar branching fraction (BR).
• The small contribution (below 10%) from decays involving Higgs boson are not implemented.
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Seesaw status
 CMS results at 7 TeV published
 ATLAS results at 8 TeV with partial dataset published
 CMS studies at 8 TeV in this thesis and in documents going to be published
Seesaw type III
searches status
Seesaw searches status @ 8 TeV
thesis
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Analysis overview
Seesaw type III signature
exactly three charged isolated leptons in the final state and charge sum ±1
Analyzed data
all 2012 proton-proton collisions data @ 8 TeV (19.7 fb-1),
online selected with triggers requiring at least two leptons with transverse momentum above
17 GeV and 8 GeV, respectively
Offline pre selections
 Exactly three charged isolated leptons (electrons or muons)
 Transverse momentum above 20, 10, 10 GeV, respectively
 Charge sum ±1
Background sources
 SM processes (di-boson, tri-boson)
 Asymmetric photon conversions (Dalitz)
 Non prompt leptons (“Fake”)
Seesaw type III signal search is performed doing a cut and count experiment.
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LHC
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Large Hadron Collider at European Organization for Nuclear Research (CERN)
Accelerator ring 100 m underground (LEP tunnel) with circumference ≈ 27 km
Head-to-head hadron collisions
6700 magnets
1600 magnets superconducting at working temperature 1.9 K (liquid He) giving 8 T
In operation since 2008, now in Long Shutdown 1 period (since March 2013)
8 operative points
4 big experiments: ALICE, ATLAS, CMS, LHCb
Jura mountains
Meyrin
Geneva
lake
Geneva airport
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CMS experiment
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Trigger & DAQ
Run Control Monitoring System
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CMS event display
Typical event selected by seesaw type III analysis
MET is defined as the imbalance of
transverse momentum in the event.
HT is the transverse
momentum sum of jets
in the event.
LEPTON ISOLATION:
output variable of particle flow algorithm to determine if lepton is inside or outside a jet
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SM backgrounds
SM processes contributions evaluated with Monte Carlo simulations
SM Process
σ (pb)
Background
WZ
24.6
Irreducible background, three leptons in final states
ZZ
8.4
Z bosons decay leptonically, and one lepton is not detected
WWW
0.08
Irreducible background, production cross section small
WW
69.9
Reducible background through the physics objects selections
SM Process
σ (pb)
t - anti t
25.32
W+jets
35604
Z,γ+jets (mass> 50 GeV)
3503
Some SM processes and detectors effects give
“instrumental” background contributions, not
properly simulated.
Data-driven methods to determine Dalitz and Fake
events contributions.
All the cross sections are measured by CMS at 8 TeV, except for WWW.
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Offline selections
 Backgrounds reduction
 Signal preservation
 Thresholds common to all mass point values for both Σ+ and Σ-
 Pre selections: exactly three charged isolated leptons above pT thresholds and charge sum ±1
 Missing transverse energy (neutrinos in final state) MET > 50 GeV
Example in next slide
 Hadron activity (jets in final states) HT < 150 GeV
 Combined Secondary Vertex (CSV) b tagging variable value, for leading pT jet CSV < 0.244
 If opposite sign same flavor leptons pair (OSSF):
reduction low mass resonances contributions M(ll) > 12 GeV
Z removal M(ll) < 76 GeV or M(ll) > 106 GeV
Dedicated selection to reduce Dalitz events contribution
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Choice of MET threshold
The signal and SM backgrounds characterization drives
the choice about thresholds on physical quantities and variables.
Solid red line = signal mass point value 180 GeV, sign +
Stacked histograms = SM background processes
Legend
Selection threshold:
MET > 50 GeV
Selection on MET is the
most important.
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SM background-data comparison (I)
To preserve significant statistics in the plots, the distributions are shown
after MET, HT , b tag, and low mass resonance selections.
Z to ee
Control plots
Z to μμ
(data-MC)/MC
MET
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Distributions
of relevant
physics
variables
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HT
Events with HT = 0 GeV not included
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SM background-data comparison (II)
First lepton pT
Third lepton pT
Second lepton pT
The distributions show a good agreement between
data and MC. Dividing events into different
leptons categories, the agreement is also good,
within the statistical errors.
μ+μ+μDominant SM process background
WZ
e+e+e-
μ+μ+eμ+e+μe+e+μ-
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μ+e+e-
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Offline dedicated selection
 Pre selections: exactly three charge leptons above pT thresholds and charge sum ±1
 Missing transverse energy (neutrinos in final state) MET > 50 GeV
 Hadron activity (jets in final states) HT < 150 GeV
 Combined Secondary Vertex (CSV) b tagging variable value, for leading pT jet CSV < 0.244
 If opposite sign same flavor leptons pair:
reduction low mass resonances contributions M(ll) > 12 GeV
Z removal M(ll) < 76 GeV or M(ll) > 106 GeV
Dedicated selection to reduce Dalitz events contribution
M(lll) < 76 GeV or M(lll) > 106 GeV
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Photon asymmetric conversion
W or Z bosons decays + final state radiation (FSR) γ + γ asymmetric conversions
Feynman’s diagrams for asymmetric
photons conversions in FSR
Scatter plots
three bodies invariant mass
versus the OSSF lepton pair
invariant mass
at pre selection
FSR
FSR
μe+eApril 16th 2014
Remove Dalitz events or
estimate through a data driven method
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μ+μ- l (l is mostly e)
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Dalitz contribution
C(e) = N(eel)/N(eeγ) = (2.1 ± 0.3)%
C(μ) = N(μμl)/N(μμγ) = (0.7± 0.1)%
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Evaluate the conversions factors in control regions
Estimate the Dalitz events contribution in signal region for each category
Dalitz events contribution independent on the sign
Dedicated selection applied when the third lepton not in OSSF pair is e
Estimation added to background when the third lepton not in OSSF pair is μ
Category
Dalitz events
Category
Dalitz events
μ+μ+μ-
1.4 ± 0.7 (52%)
μ-μ-μ+
1.4 ± 0.7 (52%)
e+e+e-
Removed
e-e-e+
Removed
μ+μ+e-
-
μ-μ-e+
-
μ+e+μ-
Removed
μ-e-μ+
Removed
e+e+μ-
-
e-e-μ+
-
μ+e+e-
3.1 ± 1.6 (52%)
μ-e-e+
3.1 ± 1.6 (52%)
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Non prompt leptons background
“fake” leptons = non prompt isolated leptons from top-antitop, W+jets, WW+jet, Z+jets or other
SM processes, not coming from primary interaction vertex.
The Fake Rate (FR) is defined as the probability for a “loose lepton” (looser criteria on
isolation variable and the track impact parameter) to pass the “tight identification” selection in
samples where the presence of prompt isolated leptons is suppressed, and therefore almost all
leptons are candidate fakes.
STEP 1: FR determination
Control data sample: events with high hadron activity
Trigger: at least one jet with pT > 60 GeV or HT > 200 GeV, and single lepton with pT > 17 GeV
Offline selections to reduce the contamination from prompt leptons due to electroweak processes (W, Z)
• One lepton and one jet in opposite direction;
• Jet pT > 60 GeV;
• MET < 20 GeV;
• MT(MET, pT(l)) < 20 GeV;
• If OSSF lepton pair, M(ll) < 76 GeV or M(ll) > 106 GeV.
Electron
pT > 10 GeV
Isolation < 0.15
FR = 0.30 ± 0.04 (stat.)
Muon
pT > 10 GeV
Isolation < 0.10
FR = 0.15 ± 0.01 (stat.)
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Fake leptons contribution
STEP 2: FR application
Consider the analysis data samples and the loose leptons definition, four possible combinations of objects are:
• LLL: events with three loose, not-tight leptons;
this contribution is dominated by QCD multi lepton events; it is negligible due to the requirements;
• TLL: events with two loose, not-tight leptons and one tight lepton,
this contribution is dominated by W+jets events;
• TTL: events with one loose, not-tight lepton and two tight leptons,
this contribution contains mainly WW+jet events and is the most important one;
• TTT: events with three tight isolated leptons,
which define the signal region for the seesaw type III exclusive search.
Category
Fake events
Category
Fake events
μ+μ+μ-
4.3 ± 2.2 (51%)
μ-μ-μ+
2.1 ± 1.2 (55%)
e+e+e-
3.5 ± 2.0 (56%)
e-e-e+
5.4 ± 2.9 (53%)
μ+μ+e-
6.0 ± 3.1 (51%)
μ-μ-e+
4.7 ± 2.4 (52%)
μ+e+μ-
6.1 ± 3.2 (51%)
μ-e-μ+
7.2 ± 3.7 (51%)
e+e+μ-
3.5 ± 1.9 (54%)
e-e-μ+
5.3 ± 2.7 (52%)
μ+e+e-
7.6 ± 3.9 (51%)
μ-e-e+
6.1 ± 3.1 (51%)
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Total background contributions
Category
SM events
Dalitz events
Fake events
Total background events
μ+μ+μ-
8.5
1.4
4.3
14.2 ± 2.4 (17%)
e+e+e-
3.5
Removed
3.5
7.1 ± 2.0 (28%)
μ+μ+e-
0.9
-
6.0
6.9 ± 3.1 (45%)
μ+e+μ-
10.5
Removed
6.1
16.7 ± 3.3 (19%)
e+e+μ-
1.2
-
3.5
4.7 ± 1.9 (39%)
μ+e+e-
7.2
3.1
7.6
18.0 ± 4.3 (23%)
μ-μ-μ+
5.4
1.4
2.1
8.9 ± 1.5 (16%)
e-e-e+
2.7
Removed
5.4
8.2 ± 2.9 (35%)
μ-μ-e+
0.7
-
4.7
5.4 ± 2.4 (46%)
μ-e-μ+
5.0
Removed
7.2
12.3 ± 3.7 (30%)
e-e-μ+
1.0
-
5.3
6.3 ± 2.8 (43%)
μ-e-e+
5.3
3.1
6.1
14.5 ± 3.6 (24%)
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Systematic uncertainties
Simulation: parton distribution function knowledge, leptons momentum scale and resolution,
jets energy corrections, missing transverse energy scale and resolution, pileup effects
 negligible
Data versus simulation correction factors: trigger efficiencies, objects reconstruction,
identification (ID) and isolation efficiencies.
Category
Trigger
ID and Isolation
Total
μ+μ+μ-
0.02
0.030
0.036
e+e+e-
0.02
0.075
0.078
μ+μ+e-
0.02
0.032
0.038
μ+e+μ-
0.02
0.032
0.038
Background source
Value (%)
e+e+μ-
0.02
0.051
0.055
WZ
6
μ+e+e-
0.02
0.051
0.055
ZZ
12
WWW
50
Dalitz
50
Fake
50
Background yields
Uncertainty on integrated luminosity 2.6 %
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Results
Signal predictions
Category
Background events
Data
140
280
340
μ+μ+μ-
14.2 ± 2.4
22
14.9
1.2
0.6
e+e+e-
7.1 ± 2.0
8
6.6
0.8
0.4
μ+μ+e-
6.9 ± 3.1
4
24.4
1.3
0.6
μ+e+μ-
16.7 ± 3.3
17
28.8
2.4
1.0
e+e+μ-
4.7 ± 1.9
4
20.8
1.2
0.5
μ+e+e-
18.0 ± 4.3
12
28.5
2.1
1.0
μ-μ-μ+
8.9 ± 1.5
11
5.8
0.5
0.2
e-e-e+
8.2 ± 2.9
7
2.4
0.4
0.2
μ-μ-e+
5.4 ± 2.4
2
9.8
0.6
0.2
μ-e-μ+
12.3 ± 3.7
11
11.0
1.1
0.5
e-e-μ+
6.3 ± 2.8
1
8.3
0.5
0.2
μ-e-e+
14.5 ± 3.6
9
11.2
1.0
0.4
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CONCLUSIONS:
 No excess of events in
data with respect to the
total background is
found.
 The analysis shows no
evidence for seesaw type
III model signal in 2012
CMS data.
 Interpretations of results
are upper limits on
production cross sections
(σ) times the branching
fraction (BR) for the
considered processes, at
95% confidence level
(CL).
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Upper limit Σ+ case
The expected (dashed line) and
observed (black points) exclusion
limits at 95% CL on σ × BR as a
function of the fermion mass MΣ,
assuming natural mixing value in
flavor democratic scenario is shown.
The thick blue theory curve
represents the Next-to-NextLeading Order type III seesaw
model prediction.
σ X BR
expected
σ X BR
observed
Σ+ mass
expected
Σ+ mass
observed
19 fb
19 fb
230 GeV
240 GeV
Yellow (68% CL) and
green (95% CL) bands reflect
the combined statistical and
systematic uncertainties on the
background contributions.
CMS observed limit @ 7 TeV = 179 GeV  improvement
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Upper limit Σ- case
The mass limit for the
negative charge Σ is expected
to be lower with respect to
the positive charge Σ because
signal and SM background
cross sections scale down of
about 50 %, but Fake leptons
and Dalitz events are
independent on the charge.
σ X BR
expected
σ X BR
observed
Σ- mass
expected
Σ- mass
observed
16 fb
8 fb
190 GeV
230 GeV
Not investigated in CMS analysis @ 7 TeV
In CMS first limitA. Gozzelino - CMS seesaw type III search
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Going beyond the scenario …
Thesis update
PROCEDURE:
where v2lN =
• Assuming efficiencies equal for the same final states but coming from different decays chains,
the number of events for the categories Nijk are calculated.
• N(tot) is the event number sum of the six categories contributions at mass value 240 GeV.
• Chosen another mass point, the search for the mixing that gives same N(tot) is performed.
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Results interpretations
Thesis update
Lines at fixed mass
Red = 140 GeV
Green = 180 GeV
Blue = 240 GeV
Purple = 260 GeV
Brown = 280 GeV
The grey region is excluded by the
requirement of unitarity:
CONCLUSIONS
 CMS limit, shown before assuming flavor democratic scenario, are the points on the 240 GeV
blue lines.
 In case the Σ mix almost completely with e and μ and not with τ (i.e. close to the black line on
the left plot), the analyzed data fixes that the bound on the mass is at 280 GeV.
Significant improvements on limits with respect to the 7 TeV results interpretations
Common way to show interpretations of results with theoretical team
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Thesis contributions
Seesaw type III searches status
Seesaw searches status @ 8 TeV
PhD thesis
CMS AN-13-135
CMS PAS EXO-14-001
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Summary
ANALYSIS RESULTS
No significant excess observed in 2012 CMS data with respect to the expected background.
Limits set in the context of the seesaw type III model.
For the first time in CMS, the investigation involves processes with both the signs.
 Significant limits improvements respect to CMS 7 TeV data analysis are obtained.
ACTIVITIES in CMS collaboration
 Development of collaboration with theoretical team to generate signal and to give results
interpretations
 Contact person for the analysis, author of the Analysis Note and
the Physics Analysis Summary (going to be published)
 Deep involvement in Run I data taking period, as online central
Data AcQuisition and Drift Tube detector on call expert shifter
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Appendix
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GRID for LHC
T1_IT_CNAF (Bologna)
T2_IT_Pisa |T2_IT_LNL |
T2_IT_Roma | T2_IT_Bari
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Tier2 LNL-Padova
The Legnaro-Padova Tier-2 is a computing facility serving in particular the ALICE and CMS LHC experiments.
Its unique characteristic is its topology: the computational resources are spread in two different sites, about 15 km apart:
the INFN Legnaro National Laboratories and the INFN Padova unit.
Nevertheless these resources are seamlessly integrated and are exposed as a single computing facility.
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Tier 2 @ LNL
T2_IT_LNL is located near experimental room 3.
It is T2 for CMS since 2006 and for ALICE since 2011. Prototype born in 2001.
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Computing room @ LNL
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Site administration tools
and solutions adopted
DOCET is the main tool used for the logging and synchronization of activities among the people working on the Tier-2
administration. It is also used at INFN CNAF Tier 1.
GANGLIA, NAGIOS, CACTI are the three Tier 2 monitoring tools, all with customized scripts and configurations.
In addition to the standard tools commonly used for farm monitoring, a couple have been locally developed to meet specific
needs related to Cooling and Power Infrastructure Monitor (a custom application to monitor chillers, rack coolers, power
distribution lines and UPS) and the LSF Monitor.
CMS UI clusters is an efficient system for the analysis activities of the local CMS community. It is currently composed by
13 servers which all mount a shared storage organized in home directories, a large working area and some scratch space.
This UI storage is a mix of old and new disk systems, all aggregated in a Lustre distributed file-system, for a total of about
30TB.
Virtualization Infrastructure
Site performance and results
* It has always been among the top sites in the availability
ranking measured by CMS and ALICE and in the official
WLCG “Availability and Reliability Report” the site
averages for the last two years were respectively 99.7% and
99.2%.
* The site contribution to the computing activities of the two
VOs is usually larger than their quota of resources thanks to
the dynamic sharing of WNs.
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CMS spin off @ LNL
RCMS and/or DAQ systems
•EUROBALL
•PRISMA
•PRISMA+CLARA
•AGATA
•EXOTIC
•PISOLO
•GALILEO (on going)
Computing cluster and storage
Improvement in cooling systems
Mu-Steel experiment in LAE (closed in 2013)
European project on muon tomography | RFSR-CT-2010-000033 partnership
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THANK YOU VERY MUCH
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Additional slides
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Standard Model
new boson observed @ LHC: mH ≅ 126 GeV
Standard Model of particles gives the best description of
the phenomena involving elementary particles and
fundamental interactions.
SM is a Quantum Field Theory based on the gauge group:
SU(3)color,C×SU(2)weak isospin,T×U(1)hypercharge, Y
A scalar SU(2) doublet is introduced in SM taking a nonzero vacuum expectation value: the electroweak symmetry
is broken and SM particles acquire mass through the Higgs
mechanism; a new massive scalar particle is predicted and
discovered (Higgs boson).
To date there is no known discrepancy between the SM and experimental observation. Open issues are
• the smallness of Higgs mass respect the Plank scale (“hierarchical problem”);
• the existing asymmetry in the universe between matter and anti-matter;
• the neutrinos mass.
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Neutrinos physics at a glance
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Neutrino was first postulated by Pauli in 1930 to preserve the conservation of energy, momentum and
angular momentum in the β transition of a neutron decaying into a proton and an electron.
Neutrino flavor oscillations was suggested by Pontecorvo in 1957.
Their mass eigenstates are mixed in flavors.
The sum of neutrinos masses is small (order of eV).
Five oscillations parameters are known, measured in different experiments.
Despite great progress in the field, neutrino physics has many open questions to be investigated.
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Seesaw type III signal
Σ±
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Constraints coming from LEP and theory
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Signal processes
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CMS sub systems
Detailed view of a CMS slice in the transverse plan
Uniform solenoidal magnetic field of 3.8 T in the tracker volume and 1.8 T in the return yokes.
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Muon and electron
MUON (tight)
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The muon track is globally reconstructed in all detectors;
The muon pseudo rapidity is inside the geometrical regions covered by the muon detectors;
The normalize χ2 of the global muon track fit is below 10;
The number of silicon layers activated is above 5;
The number of valid stand alone pixel hits is not null;
The number of matched muon stations is above 1;
The number of muon chamber hit in global fit is not null;
The impact parameter with respect to primary vertex in the transverse plane is below 0.005 cm;
The impact parameter is below 0.5 cm;
The isolation after the pileup correction in the cone size 0.3 is below 0.10.
ELECTRON (tight)
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The electron pseudo rapidity is inside the geometrical regions covered by the ECAL detector;
The first defined matching variable is below 0.15 (0.10);
The second defined matching variable is below 0.007 (0.009);
The defined cluster shape covariance variable is below 0.01 (0.03);
The energy ratio variable is below 0.12 (0.10);
The conversion rejection parameters is applied to solve electron-photon disambiguation;
The impact parameter with respect to primary vertex in the transverse plane is below 0.01 (0.01) cm;
The impact parameter is below 0.2 (0.2) cm;
The difference between the inverse quantities related to corrected energy is below 0.05 (0.05);
The isolation after the pileup correction in the cone size 0.3 is below 0.15 (0.15).
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Jet, b-jet, isolation
•
•
•
•
•
•
•
•
JET
The fraction of jet energy carried by neutral hadron is < 0.99;
The fraction of jet energy carried by neutral electromagnetic (photon) is < 0.99;
The fraction of jet energy carried by charged hadron fraction is > 0;
The fraction of jet energy carried by charged electromagnetic fraction is < 0.99;
Particles constituents are more than one;
At least one charged particle constituent is present;
Jet pseudo rapidity η is inside the geometrical regions covered by detectors: |η| < 2.4;
The jet transverse momentum is above 30 GeV.
Combining Secondary Vertex (CSV) b-jet identification
single discriminating variable for each jet is obtained combining:
• secondary vertices identified inside the jet;
• track-based lifetime information.
Features:
• highest b-efficiency for < 3% light-flavor misidentification;
• best c-jet rejection.
LEPTON ISOLATION with particle flow algorithm
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Particle Flow algorithm
Particle Flow: aims at reconstructing and identifying all stable particles in the event
(e, μ, γ, charged/neutral hadrons) with a thorough combination of all CMS sub detectors
towards an optimal determination of their direction, energy and type.
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Pile up (PU)
Exactly three charged isolated leptons in final states  pile up effect is negligible
Monte Carlo simulation is produced with a given PU scenario that does not necessarily match
the real one in data.
Re-weight procedure on the Monte Carlo to match the PU distribution in data is applied.
Number of Primary Vertex
(typical in 2012)
< PV > = 25
Number of Primary Vertex (PV)
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CMS performances
•
•
•
•
Reconstruction and identification
efficiency for muons (as a function
of transverse momentum)
Reconstruction and identification
efficiency for electrons (as a
function of transverse momentum)
Remarkable agreement between
data and Monte Carlo and high
reconstruction efficiencies
Di muon invariant mass in
endcaps and barrel to
reconstruct the Z peak mass
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Data and on line selections
In 2012, LHC provided proton-proton collisions with increasing instantaneous luminosity.
Frequency of pp interactions with total inelastic pp cross-section σ~80 mb:
Events rate (Hz) = σ * Linstantaneous ∈ [15,320] MHz
CMS is able to store ~ 400 Hz of data.
Level1 and High Level Trigger (HLT) selections reduce data flow on line.
The Data Acquisition system (DAQ) merges information coming out from the detectors, controls and
monitors the experiment during data taking period.
HLT paths for seesaw type III analysis:
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di-muon, di-electron, muon-electron
pT(lepton1) > 17 GeV, pT(lepton2) > 8 GeV
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Choice of HT threshold
The signal and SM backgrounds characterization drives
the choice about thresholds on physical quantities and variables.
Solid red line = signal mass point value 180 GeV, sign +
Stacked histograms = SM background processes
Legend
Selection threshold:
HT < 150 GeV
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Choice of CSV threshold
The signal and SM backgrounds characterization drives
the choice about thresholds on physical quantities and variables.
Solid red line = signal mass point value 180 GeV, sign +
Stacked histograms = SM background processes
Legend
Selection threshold:
CSV < 0.244
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SM background-data comparison (III)
To preserve significant statistics in the plots, the distributions are shown
after MET, HT , b tag, and low mass resonance selections.
Distributions
of relevant
physics
variables
LT
MT,1
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ST
MT,2
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SM background contributions
After the complete set of the offline selections
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Photon asymmetric conversion
Photon conversions in presence of W or Z give significant contribution, estimated via data driven method.
External conversions: a photon radiates in the magnetic field of the detector or interacts with the material
in the detector and generates an opposite sign, same flavor lepton pair (primarily, e +e− pair). The ratio of the
rate of external conversions from this final state radiation to e+e− versus μ+μ− is between 6.0*104 and
2.0*105. A selection on the opposite sign same flavor invariant mass reduces the external conversion
contribution.
Internal asymmetric conversions: decays of virtual photon from final state radiation (FSR) in Z decays in
which one of the leptons takes almost the whole photon energy, while the other lepton is soft and not
detected. Electrons and muons have almost the same probability to be involved. The internal
asymmetric conversions are studied in detail (Dalitz).
ISR
ISR
FSR
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electron
Scatter plots showing the
three bodies invariant mass
versus the two opposite sign
same flavor leptons invariant
mass for the preselected
events
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FSR
muon
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Dalitz contribution
A set of clean examples of events with final state radiation is selected by examining three bodies
masses near the Z peak in channels with both electrons and muons. The ratio of the number of l +l−l± events
to the number of l+l−γ events containing a real photon detected in the apparatus, when the three
bodies invariant mass is consistent with the Z mass, gives a conversion factor C. The conversion
factor is estimated in one control region (i.e. pre selected events) to have enough three
bodies events.
C(e) = (2.1 ± 0.3)%
C(μ) = (0.7± 0.1)%
The number of real photons in the signal region multiplied by the conversion factor gives the
estimation of the virtual photons in the signal region.
In the four categories where Dalitz contribution is the most relevant, the events are removed by
the three leptons invariant mass veto (Z Dalitz veto).
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Fake leptons in formula
STEP 2
Fake events prediction
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Fake leptons in numbers
STEP 1
STEP 2
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Signal events
Sign +
Sign -
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Results
Signal predictions for different mass values in GeV
CONCLUSIONS:
 No excess of events in data with respect to the total background is found.
 The analysis shows no evidence for seesaw type III model signal in 2012 CMS data.
 Interpretations of results are upper limits on production cross sections times the branching fraction
(BR) for the considered processes, at 95% confidence level (CL).
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Test statistic for upper limits
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The expected limit
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ATLAS results
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CMS results @ 7 TeV
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CMS results interpretations @ 7 TeV
Interpretation coming out 7 TeV CMS analysis
Theoretical team work and paper
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The end
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…
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…
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