Determination of the
pp ZX  μ+μ−X
inclusive cross section with a simultaneous fit of
Z yield, muon reconstruction,
Isolation cut and High Level Trigger efficiencies.
Pasquale Noli
XXII Cycle
Tutors:
Chiar.mo Prof. C.Sciacca
Dr L. Lista – Dr F. Fabozzi
Outline
• LHC and CMS description
• Event selection
– Fiducial and kinematical cuts
– Di-muons categories
• Fit strategy
• Analysis results
• Fit stability
– Comparison of results at different Luminosity scenarios
– Toy Monte Carlo study
• Systematics of cross-section measurement
• Conclusions
PhD Noli Pasquale
2/26
The Large Hadron Collider
•
Energy: √s = 14 TeV
– 7 times larger than TEVATRON
– Search for new massive particles up to 5 TeV/c2
•
•
L = 1034 cm-2 s-1
Biggest cryogenic system in the word:
– 1232 superconducting dipoles working at 1.9 K
to provide a magnetic field B= 8.3 Tesla
•
•
Cost: ≈ 4 biliion€ (accelerator + experiments)
Human resources: > 5000 peoples invloved
PhD Noli Pasquale
3/26
Event rates production @ √s =14 TeV and
L= 1034 cm-2 s-1
Process
PhD Noli Pasquale
Event/s
w.r.t.
Wlν
600
103 Tevatron
105 LEP
Zll
60
102 LEP
bb
107
103 Babar
103 Belle
tt
8
104 Tevatron
Higgs (m=120GeV)
0.4
-
Higgs (m= 800GeV)
0.005
-
4/26
LHC Physics Goals
• Precise Standard Model measurements:
– QCD jet cross section and αs
– Top quark (factory !): mass, couplings and decay properties
– Search of Standard Model Higgs boson in the range 115 GeV/c2 < mH < 1 TeV/c2
• Search for physics beyond the Standard Model:
– SUSY
– Extradimentions
– Technicolor
• B-physics
– Mainly at LHCb: CP-violation in the B-channel
• Heavy ions
– Mainly at ALICE: phase transition from hadronic to quark-gluon plasma
PhD Noli Pasquale
5/26
The Compact Muon Solenoid
The Compact Muon Solenoid is a high
granularity detector built around and
inside a superconducting solenoid that
provides a strong magnetic field of 4 T.
• Inner Tracker
– Silicon Pixel and Microstrips
•
Electromagnetic Calorimeter ECAL
– Scintillating lead tungstate crystals
•
Hadronic Calorimeter HCAL
– Scintillator brass sandwich
•
Muon system
– Drift tube (BARREL)
– Cathode Strip Chambers (ENDCAP)
– Resistive Plate Chambers (BARRL-ENDCAP)
•
Trigger system:
– L1 (custom electronic) 40MHz  100 kHz
– HLT (processors farm) 100kHz  100 Hz
PhD Noli Pasquale
6/26
The Inner Tracker
Pixel Tracker:
• Made by 100  150 μm cells
• Resolution of 10 μm in the r-𝜙 plane and 20 μm in the r-z plane
Microstrip Tracker:
• Divided in 4 different parts TIB,TOB, TID, and TEC
• Resolution: 25 μm in the r-𝜙 plane and 230 μm in the r-z plane TIB
• Resolution: 32-52 μm in the r-𝜙 plane and 530 μm in the r-z plane TID, TOB, and TEC
PhD Noli Pasquale
7/26
Calorimetric system
ECAL :
HCAL :
HB
HE
HO
PhD Noli Pasquale
The Muon system
•
•
•
•
Muon Trigger
Muon identification
BX identification
Pt measuraments
– Standalone resolution: 9% (up to 200 GeV/c) , 15-40 % (1 TeV/c)
depending on η
– with Tracker resolution improve: 5% at 1 TeV/c
• Correct charge assignment 99%
DT
RPC
CSC
PhD Noli Pasquale
Z candidates: Reco vs Monte Carlo
Generator cut
Y
• Good agreement w.r.t MC sample
• Low longitudinal momentum
• Low transverse momentum
• Flat distribution in 𝜙
1
E  pz
log
2
E  pz
PhD Noli Pasquale
10/26
Z candidates: no MC matched
Combinatorial background is
peaked at low values of
invariant mass
It decreases rapidly (log scale)
for invariant mass increasing
M μμ∈[60, 120] GeV/c2
S/B ≈ 0.1 %
86 % only 1 Z
0.4 % no Z
13.6% >1 Z
PhD Noli Pasquale
11/26
Z events selection
Two muon candidates (or 1 muon + 1 track) with:
–
–
–
–
|| < 2, pt > 20 GeV/c
60 < m < 120 GeV/c2
Track Isolation : pt < 3 GeV/c
HLT single not-isolated muon :HLT_Mu15
5 statisticaly indipendent event categories allow to fit 5 parameters.
1) Z->µ µ : two global muons, 2 HLT matches
2) Z->µ µ : two global muons, 1 HLT match
Both muons
3) Z->µ s : global + stand-alone, global µ HLT matched isolated
4) Z->µ t : global + track, global µ HLT matched,
at least one muon
5) Z->µ µ : two global muons, at least 1 HLT match
not isolated
 Z->µ s contributes to track efficiency estimate
 Z->µ t contributes to muon system efficiency estimate
 Z-> µ µ not iso contributes to isolation cut efficiency estimate
PhD Noli Pasquale
12/26
Di-muon categories
μμ, 2HLT
μs
μμ, 1HLT
μt
μμ, no-iso
Five independent
categories
(golden, standalone, tracks, ...)
PhD Noli Pasquale
CMS AN -2009/005
∫Ldt = 45 pb-113
Efficiencies from data
μμ, 2HLT
μs
μμ, 1HLT
μt
μμ, no-iso
Data driven simultaneous
estimate of
• Z yield
• eff. of tracking reconstruction
• eff. of muon reconstruction
• eff. trigger
• eff. Isolation cut
PhD Noli Pasquale
14/26
Fit model
Differential event yields
Background shapes fitted
as exponential  polynomial
Data driven signal shapes
Signal yield expressions
• fpeak(m) : Z➝μ+μ- = Z➝μ+μ-2HLT + Z➝μ+μ-1HLT
mass spectra
• f speak(m): taken from Z➝μ+μ- removing the track
component to one of the two muons to
mimic a standalone muon
PhD Noli Pasquale
15/26
Signal shapes
fpeak(m)
In good agreement
f speak(m)
PhD Noli Pasquale
16/26
Chi-squared definition
• Five observables
• Five unknown signal parameters
– plus background shapes and yields
Other estimators, for example Poissonian Likelihood Ratio,
lead to very similar results
PhD Noli Pasquale
17/26
Fit Results @ 10 TeV, 45 pb-1
μt
μs
μμ, no-iso
PhD Noli Pasquale
CMS AN -2009/005
18/26
Fit stability: 133 to 5 pb-1
εtrk
Z yield
(normalized)
εiso
CMS PAS -2009/001
εHLT
PhD Noli Pasquale
Fit stable down to
~ 5 pb-1
εs.a.
19/26
Toy Monte Carlo study
• 1000 Toy experiments generated for 45 pb-1
• True parameter taken from our full Monte Carlo fit
Z yield
Mean 0.36±0.04
σ = 1.14±0.03
εtrk
Mean 0.04±0.04
σ = 1.03±0.03
εiso
PhD Noli Pasquale
εs.a.
Small bias
for Zs
(low stat.)
Mean 0.12±0.03
σ =0.99 ±0.02
Mean: 0.0680.034
σ: 1.021  0.028
CMS AN -2009/005
εHLT
Mean 0.08±0.03
σ = 1.01±0.03
20/26
Toy Monte Carlo @ 133 pb-1
Mean 0.26 ± 0.24
σ = 1.03 ± 0.02
εtrk
Z yield
No bias
when Zs
has larger stat.
εs.a.
Mean 0.085 ±0.024
σ = 1.04 ± 0.02
εiso
PhD Noli Pasquale
Mean 0.0955± 0.023
σ = 0.99± 0.02
Mean 0.007±0.024
σ = 1.03±0.02
Bias decreasing w.r.t @ 45pb-1
Mean: 0.010.02
σ: 0.98  0.02
εHLT
CMS AN -2009/005
21/26
Systematics
Main systematic uncertainties
addressed
1. Background estimation
2. Efficiency correlation
3. Acceptance:
• Choise of generator type
• PDF uncertainties
• Muon scale and resolution
4. Luminosity
PhD Noli Pasquale
Systematic source
Error
Background
0.3%
Efficiency correlation
< 0.1%
Order of generator
∼3%
PDF uncertainties
∼4%
pt muon scale
∼0.5%
LHC Luminosity
~10%
22/26
Conclusions…
• Analysis strategy is able to determine from data:
–
–
–
–
–
•
•
•
•
Z yield
eff. of tracking reconstruction
eff. of muon system reconstruction
eff. Trigger
eff. Isolation cut
Allows to measure cross section with the very early data (few pb-1)
Needs a smaller amount of statistics w.r.t. the Tag & Probe
Complementary to Tag &Probe with high statistics
Accurate and fast method
– Run a single analysis step + a single fast fit
– Suitable for prompt applications:
• DQM
• ‘Z counting’ for luminosity monitoring
• Analysis is approved by CMS and an “early paper ” is done
• Waiting for the real data
PhD Noli Pasquale
23/26
LHC started on the 23th October 2009
Global machine checkout
2009
450 GeV Collisions
CMS Control room
Trial rump up
Xmas
Rump up commissioning
to 1.2 TeV
Rump up commissioning
to 3.5 TeV
2010
Collisions at √s = 7 TeV
Pilot physics
PhD Noli Pasquale
First data are taken and analyzed
24/26
CMS first data
CMS Event display:
J/Ψ candidate in pp collision
at √s =2.36 TeV
Two muons in forward part of CMS
The analysis chain is able to reconstruct
the particle in the event !!!
PhD Noli Pasquale
J/Ψ candidate with
invariant mass
3.032 GeV/c2
Grazie dell’attenzione
PhD Noli Pasquale
26/26
Backup
PhD Noli Pasquale
27
Transverse slice through CMS
PhD Noli Pasquale
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Signal-Background
Number of candidates in each category after the selection with an invariant
mass in the range [60-120] GeV=c2. Here Z = Z1HLT + Z2HLT .
The separate contributions from signal and background processes are shown.
An integrated luminosity of 45 pb-1 is assumed.
PhD Noli Pasquale
29
Isolation variable
∆Rveto = 0.015
Fit Results @ 10 TeV, 10 pb-1
Fit
MC -truth
εHLT
0.913 ±0.004
0.917±0.002
εIso
0.982±0.003
0.9785±0.0009
εsa
0.988±0.002
0.9907±0.0006
εTk
0.9988±0.0008
0.9986±0.0002
Parameter
Nzµµ
c2
PhD Noli Pasquale
3818±65
1.203
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Misure di Efficienza dai dati
Tag & probe
 Campione di eventi Z e e (μ μ )
TAG
TAG : elettrone(muone) selezionato con
criteri molto stringenti
PROBE
PROBE : elettrone(muone) selezionato
con dei criteri più larghi
dipendenti dalle selezioni
richieste nelle varie analisi
 Massa invariante Tag-Probe in una finestra
di massa intorno alla massa della Z
 Mappa delle efficienze in funzione di pt, η,
Φ
ε=
# probe passanti la selzione
# tutti i probe corrisp. a un tag
Problemi
•
•
Alta statistica necessaria per evitare
un binning rozzo
Contaminazione residua dei fondi
32
T&P – Fit Comparison
We reweigh Zmumu invariant mass
distributions using :
• Fit recontruction efficiency values(black)
• T&P recontruction efficiency values(red)
The two distributions are in good agreement
to each other
Yield with T&P
Nzµµ =8962 ± 97
Consistent with fit result
Nzµµ =8827 ± 98
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Accettanza geometrica
PhD Noli Pasquale
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Sistematiche Accettanza geometrica
PDF and Generator
Pt Scale
PhD Noli Pasquale
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Correlations (I)
(1)
N0 = total number of produced events
p1 p2 muons 3-momenta
f0 probability density function
(2)
(3)
(4)
36
Correlations (II)
(5)
(6)
(7)
37
Standalone-HLT Efficiency Correlation
Track-Isolation Efficiency Correlation
PhD Noli Pasquale
38