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

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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

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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

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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

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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

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Signal shapes

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In good agreement

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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

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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

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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.)

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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

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Systematics

Main systematic uncertainties addressed1.Background estimation2.Efficiency correlation3.Acceptance:

• Choise of generator type• PDF uncertainties• Muon scale and resolution

4.Luminosity

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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

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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