Determination of the top-quark pole mass using tt+1-jet events with the ATLAS detector

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Determination of the

top-quark pole mass using tt+1-jet events with the

ATLAS detector

Adrian Irles

VI CPAN days

Sevilla, 20 October 2014

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Outline

Introduction: Top-quark mass

Top-quark mass from jet rates: the R distribution Top quark pole mass determination

using ATLAS data

Prospects for future analysis

Conclusions

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The top-quark mass

Top quark: the heaviest particle in the SM.

Fundamental parameter for precision tests of the SM (or BSM) .

●

Enter in all loop corrections (reduce parametric uncertainty)

●

SM relation H, W, t mass → EW fit

●

Stability of the Higgs potential (driving the Higgs potential negative)

Quark masses are not observables

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Most precise top-quark mass measurements

Summary of the direct m

_t

measurements (CMS and combinations)

Precision of the world combination:

σ( m

_t

)= 0.76 GeV (~0.4 %) Unknown uncertainty

associated to the mass scheme interpretation:

~1 GeV

(Hoang, Stewart arXiv:0808.0222)

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Most precise top-quark mass measurements

m

_t^pole

determination from the inclusive tt cross section.

“Limited” theoretical sensitivity:

Δσ

_tt

/ σ

_tt

=3-4% Δ m

_t

Mass scheme unambiguosly defined

m

_t^pole

= 172.9

^+2.5

GeV June 2014

-2.6

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Top-quark pole mass from jet rates

Eur.Phys.J. C73 (2013) 2438

m

₀

=170 GeV

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- tt+1-jet differential cross section

- tt+1-jet normalized differential cross section:

Mass dependence enchanced in certain regions of the phase space

Cancelation and reduction of systematic uncertainties (theoretical and experimental)

Top-quark pole mass from jet rates

Eur.Phys.J. C73 (2013) 2438

m

₀

=170 GeV

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- NLO calculations:

●

Mass scheme fixed (pole mass)

●

Fixed NLO calculations available

(Dittmaier et al arXiv:0810.0452)

●

NLO+PS calculations available (Alioli et al, PowHeg-ttJ arXiv:1110.5251)

Differences between using one calculation or the other to extract the top-quark pole mass have been quantified in 0.3 GeV - well covered by the scale uncertainties -

Top-quark pole mass from jet rates

Eur.Phys.J. C73 (2013) 2438

m

₀

=170 GeV

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The observable Eur.Phys.J. C73 (2013) 2438

1 TeV 680 GeV 500 GeV 410 GeV

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Top-quark mass sensitivity Eur.Phys.J. C73 (2013) 2438

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Top-quark mass determination ATLAS-CONF-2014-053

√s= 7 TeV (L = 4.6 fb ¹), l+jets channel ⁻

Kinematical event reconstruction to identify:

tt candidates + "additional jet"

Event Selection:

1 lepton(e or μ), p

_T

>25 GeV

E

_T^miss

> 30 GeV (Transverse missing energy associated to the neutrino escaping)

2 bjets, p

_T

>25 GeV (tagged with the MV1 algorithm at a 70% of eff. working point)

At least 3 light jets, p

_T

> 25 GeV

m

_T^W

>30 GeV (leptonic W boson transverse

mass → reduce QCD bkg)

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√s= 7 TeV (L = 4.6 fb ¹), l+jets channel ⁻ kinematical event reconstruction to identify:

tt candidates (W- and t-candidates) + "additional jet" with p

_T

>50 GeV

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√s= 7 TeV (L = 4.6 fb ¹), l+jets channel ⁻ ρ s distribution at reconstructed level

Background subtraction (~5%)

Regularization matrix unfolding to correct for

detector effects → unfolding

to parton level

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Distribution at parton level .

Compared with tt+1-jet

@NLO+PS calculations (PowHeg+Pythia)

Mass extraction from a Chi² minimization:

R

_i

→ unfolded data

f(m) → theoretical mass dep.

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ZOOM Most sensitive bin

Naive estimation using one bin:

m

_t^pole

≈ 173.3 GeV

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No m

_t^MC

mass dependence due to MC based correction

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No m

_t^MC

mass dependence due to MC based correction

One matrix (m

_t^MC

=172.5 GeV) applied to different simulated R-distributions (with different m

_t^MC

).

The different unfolded distributions are fit and the extracted mass compared with

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m t pole =173.71

± 1.50 ( stat. )

± 1.43 ( syst. )

−0.49 +0.95

( theo.) GeV

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Statistical uncertainties

Experimental systematic uncertainties Dominant:

proton PDF → 0.54 GeV

ISR/FSR modeling → 0.72 GeV JES → 0.94 GeV

Theoretical systematic uncertainties Scale variations (2m

_t

,m

_t

/2)

PDF + α

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Most precise m

_t^pole

measurement so far.

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Future prospects

Current maximum sensitivity 0.675 <ρ

_s

<1

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Future prospects

Larger statistics may reduce the systematics uncertainties

More statistics

~ x5 using the 8 TeV sample

Preliminary studies for new

bin size:

0.725 <ρ

_s

<1

Larger

sensitivity

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Conclusions

We present the first results of the top-quark pole mass measurement with the full 7 TeV ATLAS data set using tt+1-jet final states.

The semileptonic top decay channel is studied:

lepton(electron and muon)+jets

A complete study of the systematic uncertainties has been performed.

Theoretical uncertainties due to the scale variations and to the PDF choice have been also considered

Final result

Future prospects: room for improvement and reduction of statistical and

systematic uncertainties

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Back up slides

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Uncertainties