Flavour T agging Methods used in the ATLAS B-Physics Programme - - PowerPoint PPT Presentation

Flavour T agging Methods used in the ATLAS B-Physics Programme

Patrick Jussel Institut für Astro- und T eilchenphysik Universität Innsbruck FAKT – Aflenz, 23.9.2008

Patrick Jussel, FAKT, 23.9.2008

Overview

• The ATLAS Detector
• ATLAS B-Physics Programme
• Flavour Tagging Methods: b or b?

➢ Jet Charge Tagging ➢ Soft Muon Tagging

• Austrian Federated Tier 2
• Summary and Outlook

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Patrick Jussel, FAKT, 23.9.2008

The ATLAS Detector

Inner detector Solenoidal magnetic field Pixel detector: 50 x 400 μm2 Semi Conductor Tracker: 17 μm Transition Radiation Tracker: 170 μm Precision measurement for tracks and vertices Muon System Toroidal magnetic field Standalone muon reconstruction Trigger Fast identification of interesting events, e.g. muons, jets, missing energy etc. Calorimeters LAr calorimeters @ -183°C

• EM calorimeter endcaps & centre
• Hadronic calorimeter endcaps

Tile Calorimeter

• Hadronic calorimeter barrel

Patrick Jussel, FAKT, 23.9.2008

The ATLAS Detector 2008-09-10 10:19

Patrick Jussel, FAKT, 23.9.2008

σ(bb) ≈ 500 μb 5·105 bb pairs/s @ L=1033cm-2s-1(low lumi) B @ ATLAS - features / drawbacks: + precise vertexing and tracking + high resolution calorimetry + good muon identification + flexible trigger scheme

• no kaon/pion separation

Physics Programme: Study heavy flavour mesons and baryons e.g. heavy quarkonium production Measure rare and very rare decays sensitive to new physics (e.g. Bs

0 → μ+ μ-)

CP Violation: parameters of the Bs

0 - Bs 0 meson system

→ here we need flavour tagging: b or b?

ATLAS B-Physics Programme

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σbb

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Patrick Jussel, FAKT, 23.9.2008

Bs

0 mixing parameter Δms:

Oscillations between the flavour eigenstates Bs

0 and Bs

are proportional to mass difference of the two mass eigenstates: Channels: Method: measure the flavour of the Bs meson at decay time and the proper decay time (Reconstruction), tag the flavour at production (Flavour Tagging), maximum likelihood fit method to extract Δms

Bs

0 D s  0 ,  −

Bs

0 D s  0 , a1 −

P±t~e

− t1±cos mst

_ At= N mixt−N nomixt N mixtN nomixt ∝cosmst

ATLAS B-Physics Programme

Patrick Jussel, FAKT, 23.9.2008

Bs

0 J /  −

Measurement of the weak mixing phase Φs= -2ηλ2: Method: angular distribution of the final state particles in is depending on Φs, 8 parameters extracted in a maximum likelihood fit SM Φs not accessible by ATLAS, but deviations CKM Fitter: Φs

SM = -0.036 +/- 0.003

ATLAS B-Physics Programme

Patrick Jussel, FAKT, 23.9.2008

• Opposite Side Tagging:
• Reconstruction
• Jet charge
• Lepton charge

Oscillations of the opposite side B hadron are possible!

• Same Side Tagging:
• Jet charge
• Kaon charge

Flavour Tagging Methods

Patrick Jussel, FAKT, 23.9.2008

• Principle: due to hadronisation a meson

with a b quark is accompanied more likely by negatively charged mesons.

• Jet charge:

… with

• Sum over all tracks in cone except

the decay products of the signal channel.

• pL found to be the best choice to use

in this case as pi

κ

Q jet=∑

 R

qi pi



∣pi

∣

 R=

2 2

Flavour Tagging Methods

Jet Charge Tagging

Patrick Jussel, FAKT, 23.9.2008

Wrong tag fraction 0.37 Dilution 0.25 Tagging Power 0.04 Bs

0 J / Expected Performance of the ATLAS Experiment, Detector, Trigger and Physics; ATLAS Collaboration; CERN-OPEN- 2008-020, Geneva, 2008, to appear.

= N wN g N wN gN no = N w N wN g D= N w−N g N wN g  D

2

When using a flavour tagger on events with well known flavour, count the events with a good tag (Ng), with a wrong tag (Nw) and the events without a tag (Nno), then we receive...

Tagging Effciency: Wrong tag fraction: Dilution: Tagging Power:

Jet Charge Tagging

Flavour Tagging Methods

Patrick Jussel, FAKT, 23.9.2008

• Principle:
• pposite side b hadron

decaying semi- leptonically

• Search for highest pT muon in event, muon charge → signal Bs

0 flavour

• Used in the channel: Bs

0 → Ds

• (Φ,π)π+ and: Bs

0 → Ds

• (Φ,π)a1

+

• Need to understand the sources of wrong tags:

– b → c → μ decay chain – additional bb and cc pairs – muons from from J/ψ, τ, ρ, … – oscillations of neutral opposite side b hadrons (Bd

0, Bs 0)

Soft Muon Tagging

Flavour Tagging Methods

Patrick Jussel, FAKT, 23.9.2008

Luminosity 10 fb-1 10 fb-1

• Rec. Events

6657 3368 Efficiency 0.988 0.985 Wrong tag Fraction 0.223 0.233 Dilution 0.554 0.25 Tagging Power 0.30 0.28

Bs

0 Ds  0 , − − Bs 0 Ds  0 , −a1 − Expected Performance of the ATLAS Experiment, Detector, Trigger and Physics; ATLAS Collaboration; CERN- OPEN-2008-020, Geneva, 2008, to appear.

BsDsPi BsDsa1

Soft Muon Tagging

Comparision of the two signal channels and histograms showing the wrong tag fractions depending on the tagging muons transverse momentum.

Flavour Tagging Methods

Patrick Jussel, FAKT, 23.9.2008

• different distributions, here wrong tag fraction (9500 Bs

0 → Ds

• (Φ,π)π+ )

– distributions depending on e.g. pT(μ), pT

rel(μ) and pT(BS 0)

– distributions are in different WTF ranges, e.g. 14%-27% for pT

rel(μ)

• PROBLEM: we would like the wrong tag fraction for each event...
• QUESTION: Which wrong tag fraction should go into the fit? What is the “real”

event by event wrong tag fraction? How can all this information be combined?

• ANSATZ: combine variables with TMVA

Flavour Tagging Methods

Patrick Jussel, FAKT, 23.9.2008

• TMVA = Toolkit for multivariate data analysis in ROOT
• Easy to use framework to train, test and use a classifier with a common

interface for each technique

• Techniques available: Fisher Discriminant, Multidimensional Likelihood

Estimator, various Artificial Neural Networks, …

• Here: combine pT(μ), pT

rel(μ) and pT(BS 0) to one tmva-classifier by:

– train classifier with a small fraction of events – evaluate the classifier – test resulting classifier with remaining events

• Knowing the truth from Monte Carlo, calculate the wrong tag fraction

depending on the tmva-classifier …

( more on http://tmva.sourceforge.net/ )

Flavour Tagging Methods

Patrick Jussel, FAKT, 23.9.2008

H1 H0 x1 x2 H1 H0 x1 x2

Wrong tag Good tag Axis

• Geometric classification: search for axis in

hyperspace pushing the means of the projections as far as possible from each other

Flavour Tagging Methods

Fisher Discriminant

Patrick Jussel, FAKT, 23.9.2008

• Combine variables in a Clermont-Ferrand

Neural Network

Flavour Tagging Methods

Neural Network

Patrick Jussel, FAKT, 23.9.2008

Flavour Tagging Methods

next steps...

• Combine Jet Charge Tagger and Soft Muon Tagger to one combined

Flavour Tagger to calculate one event by event wrong tag fraction

• Add additional tagging methods:

– soft electron tagger – same side K*(892) tagger → – same side lambda tagger

• Prepare software for calibration

with real data

MC study

Patrick Jussel, FAKT, 23.9.2008

• LHC experiments produces a high

data rate (more then 1 GB/s)

• Worldwide LHC Computing Grid (WLCG)
• embedded: Austrian Federated Tier 2

Some words on Grid Computing...

What is the Austrian Federated Tier 2?

Austrian Federated Tier 2

Patrick Jussel, FAKT, 23.9.2008

• Grid Pilot Project in Innsbruck (2002)
• Austrian Grid Phase I (2004-2006)
• Austrian Grid Phase II (2007-2010)

Status of the Austrian Federated Tier 2 until end of 2008: Cost estimate: 1.053k€, funded by Infrastructure and manpower to be provided by participating Institutions

Vienna Innsbruck CPU > 1000 cores > 200 cores Storage > 100 TB ~ 50 TB

Austrian Federated Tier 2

Patrick Jussel, FAKT, 23.9.2008

• Jet Charge Tagger and Soft Muon Taggers for the

measurements of Bs

0 channels

• Sources of wrong tags are understood (in theory...)
• Multivariate Analysis used for an event by event

wrong tag fraction

• Waiting for data, need calibration

Summary

Patrick Jussel, FAKT, 23.9.2008

FIN

Patrick Jussel, FAKT, 23.9.2008

Likelihood function of Φs measurement:

ATLAS B-Physics Programme