Pasquale Noli XXII Cycle

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


Event rates production @ √s =14 TeV and L= 1034 cm-2 s-1


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


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


Calorimetric systemECAL :

PhD Noli Pasquale

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

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


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 pol. background


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


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

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


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.)


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


Systematics

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

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

4.Luminosity


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


LHC started on the 23th October 2009

PhD Noli Pasquale

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


Backup

PhD Noli Pasquale 27

Transverse slice through CMS



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


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


Accettanza geometrica


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


Track-Isolation Efficiency Correlation

Pasquale Noli XXII Cycle

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