pasquale noli xxii cycle
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Determination of the pp ZX μ+μ−X
inclusive cross section with a simultaneous fit ofZ 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
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Event rates production @ √s =14 TeV and L= 1034 cm-2 s-1
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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
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The Compact Muon SolenoidThe Compact Muon Solenoid is a highgranularity detector built around andinside 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 HzPhD 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
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Calorimetric systemECAL :
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HB
HE
HO
HCAL :
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%
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RPC
CSC
DT
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Z candidates: Reco vs Monte Carlo
• Good agreement w.r.t MC sample• Low longitudinal momentum• Low transverse momentum• Flat distribution in 𝜙
10/26
Generator cut
z
z
pE
pEY
log2
1
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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/c2S/B ≈ 0.1 %86 % only 1 Z
0.4 % no Z 13.6% >1 Z
11/26
Z events selection
– || < 2, pt > 20 GeV/c
– 60 < m < 120 GeV/c2
– Track Isolation : pt < 3 GeV/c
– HLT single not-isolated muon :HLT_Mu15
1) Z->µ µ : two global muons, 2 HLT matches2) Z->µ µ : two global muons, 1 HLT match3) Z->µ s : global + stand-alone, global µ HLT matched4) Z->µ t : global + track, global µ HLT matched, 5) Z->µ µ : two global muons, at least 1 HLT match
Both muonsisolated
at least one muonnot isolated
5 statisticaly indipendent event categories allow to fit 5 parameters.
Two muon candidates (or 1 muon + 1 track) with:
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
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Di-muon categories
Five independentcategories(golden, stand-alone, tracks, ...)
∫Ldt = 45 pb-1∫Ldt = 45 pb-1
μμ, 2HLTμμ, 2HLT μμ, 1HLTμμ, 1HLT μtμt
μsμs μμ, no-isoμμ, no-iso
Signal peak Negligible bkg
Signal peak Negligible bkg
Signal peak pol. background
Signal peak Negligible bkg
Signal peak pol. background
CMS AN -2009/005PhD Noli Pasquale 13
Efficiencies from data
μμ, 2HLTμμ, 2HLT μμ, 1HLTμμ, 1HLT μtμt
μsμs μμ, no-isoμμ, 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
Signal yield expressions
Background shapes fittedas exponential polynomial
Differential event yields
• fpeak(m) : Z➝μ+μ- = Z➝μ+μ-2HLT + Z➝μ+μ-1HLT
mass spectra• f s
peak(m): taken from Z➝μ+μ- removing the track component to one of the two muons to mimic a standalone muon
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Data driven signal shapes
15/26
Signal shapes
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In good agreement
16/26
f speak(m)
fpeak(m)
Chi-squared definition
• Five observables• Five unknown signal parameters
– plus background shapes and yields
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Other estimators, for example Poissonian Likelihood Ratio,lead to very similar results
17/26
Fit Results @ 10 TeV, 45 pb-1
μtμt
μμ, no-isoμμ, no-iso
μsμs
CMS AN -2009/005PhD Noli Pasquale 18/26
Fit stability: 133 to 5 pb-1
CMS PAS -2009/001
Z yield (normalized)
Z yield (normalized)εtrkεtrk εisoεiso
εHLTεHLT εs.a.εs.a.
Fit stable down to ~ 5 pb-1
PhD Noli Pasquale 19/26
Toy Monte Carlo study• 1000 Toy experiments generated for 45 pb-1 • True parameter taken from our full Monte Carlo fit
Mean 0.36±0.04σ = 1.14±0.03
Mean 0.12±0.03σ =0.99 ±0.02
Mean 0.08±0.03σ = 1.01±0.03
Mean 0.04±0.04σ = 1.03±0.03 Mean: 0.0680.034
σ: 1.021 0.028
CMS AN -2009/005
εtrkεtrk
εisoεiso εHLTεHLT
εs.a.εs.a.
Z yield Z yield
Small biasfor Zs(low stat.)
PhD Noli Pasquale 20/26
Toy Monte Carlo @ 133 pb-1
Mean 0.26 ± 0.24σ = 1.03 ± 0.02
Mean 0.0955± 0.023σ = 0.99± 0.02
Mean 0.085 ±0.024σ = 1.04 ± 0.02
Mean 0.007±0.024σ = 1.03±0.02
εtrkεtrk
εisoεisoεHLTεHLT
εs.a.εs.a.
No biaswhen Zshas larger stat.
Bias decreasing w.r.t @ 45pb-1
Mean: 0.010.02σ: 0.98 0.02
Z yield Z yield
CMS AN -2009/005
PhD Noli Pasquale 21/26
Systematics
Main systematic uncertainties addressed1.Background estimation2.Efficiency correlation3.Acceptance:
• Choise of generator type• PDF uncertainties• Muon scale and resolution
4.Luminosity
PhD Noli Pasquale 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
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LHC started on the 23th October 2009
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CMS Control room
2010
2009
Global machine checkout
450 GeV Collisions
Trial rump up
Pilot physics
Rump up commissioning to 1.2 TeV
Rump up commissioning to 3.5 TeV
Collisions at √s = 7 TeV
Xmas
First data are taken and analyzed
24/26
CMS first data
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The analysis chain is able to reconstruct the particle in the event !!!
CMS Event display:J/Ψ candidate in pp collision
at √s =2.36 TeV
Two muons in forward part of CMS J/Ψ candidate with
invariant mass 3.032 GeV/c2
Grazie dell’attenzione
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Backup
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Transverse slice through CMS
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Signal-Background
Number of candidates in each category after the selection with an invariantmass 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.
Isolation variable
∆Rveto = 0.015
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Fit Results @ 10 TeV, 10 pb-1
Misure di Efficienza dai dati
Problemi• Alta statistica necessaria per evitare
un binning rozzo• Contaminazione residua dei fondi
ε = # probe passanti la selzione
# tutti i probe corrisp. a un tag
Tag & probe Campione di eventi Z e e (μ μ )
TAG : elettrone(muone) selezionato con criteri molto stringenti
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
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TAG
PROBE
T&P – Fit Comparison
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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&PNzµµ =8962 ± 97Consistent with fit resultNzµµ =8827 ± 98
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Accettanza geometrica
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Sistematiche Accettanza geometrica
PDF and Generator
Pt Scale
Correlations (I)
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N0 = total number of produced eventsp1 p2 muons 3-momentaf0 probability density function
(1)
(2)
(3)
(4)
Correlations (II)
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(5)
(6)
(7)
Standalone-HLT Efficiency Correlation
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Track-Isolation Efficiency Correlation
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