Ian Forster - U. of Liverpool IoP 2005

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B0 Ks; Branching Fraction and Time Dependent CP Analysis at

BaBar

Ian ForsterUniversity of Liverpool

Ian Forster - U. of Liverpool IoP 2005

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Introduction and Overview

Working with David Payne (University of Liverpool)

CKM Matrix, CP violation and the Unitarity Triangle

Event selection Maximum Likelihood fit Branching fraction results CP fit validation

Ian Forster - U. of Liverpool IoP 2005

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CKM Matrix and Unitarity Triangle

b

s

d

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Requirement of unitarity

This relation is depicted asa triangle in the complex plane

CKM matrix relates rotated quark states to physical states

Angles related to CKM matrix elements

Ian Forster - U. of Liverpool IoP 2005

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f

ff A

A

p

qmS )sin()cos( mtSmtCACP

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])([])([

])([])([)(

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CPCP

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Ratio of CKM elementsIn Feynman diagrams

BB Mixing

(Equivalent factor for KK mixing if decay involves KS)

Particular B decays have physically interesting values of Im() eg: B0 J/Ks and decay modes dominated by ‘b-s penguin’ diagrams

CP violation due to Interference between Mixing and Decay

Ian Forster - U. of Liverpool IoP 2005

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Measuring Sin2 From b-s Penguins

3.6 from s-penguin to sin2 (cc) No sign of direct CP asymmetry

b-s penguins showing signs of measuring a different value for b to cc decays (eg B0 JKs)

Interesting area of research at the moment

fS C= (1 - ||2) / (1 + ||2)

Ian Forster - U. of Liverpool IoP 2005

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B0 0Ks

The two leading order Feynman diagrams. Penguin decay has weak phase able to measure the Unitarity Triangle

angle. We extract a measurement of by performing a time dependent CP analysis

using a maximum likelihood fit.

CKM suppressedTree diagram

Gluonic penguindominates

Ian Forster - U. of Liverpool IoP 2005

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Initial Event Selection

|E| < 0.3 GeV mES > 5.18 GeV Cos(Thrust angle)<0.9 T < 20 ps, T < 2.5 ps

E = EB - EBEAM

Energy of the B

Energy of the beam

2*2BBEAMES pEm Center of mass

Momentum of B

Rest of event

B

(CM frame)

• BB pair hardly moving in CM frame• If something other than BB pair, decay will form back to back jets• A cut on the thrust of the event selects BB pairs

T is the time between the decay of the B’s

Thrust

Ian Forster - U. of Liverpool IoP 2005

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Ntuple Level Selection

The 2 tracks from the are required to fail tight PID selections for e, p and K.

Invariant mass of these two tracks must be within 0.375GeV of the PDG mass (0.776GeV)

mES > 5.23GeV (on peak data), mES > 5.21GeV (off peak data)

Cosine of pointing angle < 0.997 Ks vertex must be displaced from the 0 vertex by

more than 3x the error on the distance. Mass of Ks must be within 0.13GeV of the PDG value

for the K0 (0.498GeV) Veto against D+- and K*+- by ensuring mass of Ks

combination are more than 0.055GeV and 0.04GeV away from the PDG masses of D+ and K*+.

Efficiency ~ 25%

Pointing angle is theAngle between the Displacement vectorBetween the 0 and The Ks and the Ks Momentum vector

Ian Forster - U. of Liverpool IoP 2005

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Backgrounds

Split into two fundamental categories; Charmed and Charmless. Charmed Backgrounds

Identified with MC. Split up into B+ and B0 with all sub-modes collected together

into two non-parametric PDF’s Charmless modes contributing < 1 event included in the PDF’s

Charmless Backgrounds Identified with Generic MC all modes contributing > 1 event. Modeled with non-parametric PDF.

Almost all of the events in our selection are continuum ~99%!!

B Backgrounds

Continuum Background

Ian Forster - U. of Liverpool IoP 2005

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Maximum Likelihood Fit

mES E Neural Net Output

(NNO) Cos(qHELICITY) T Mass

NNO: Inputs to the neural net are:• Fisher: combines neutral and charged, 0th and 2nd order Legendre monomials• Sum of the transverse momentum• Cosine of angle between direction of B and the thrust axis• Cosine of angle between direction of B and the z axis• Cosine of angle between thrust axis of B and the z axis

Discriminating Variables

Ian Forster - U. of Liverpool IoP 2005

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Results of BF Fit

BF(B0 0Ks) = (5.1 +/-1.0+/-1.2) x 10-6

Float a non-resonant B0 Ks component with flat helicity distribution

Evidence for B0 0Ks at the 3.6s level Presented at ICHEP 2004

(Dataset – 227 million BB pairs corresponding to 205 fb-1 int. luminosity)

Ian Forster - U. of Liverpool IoP 2005

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CP Fit Validations

Total Continuum Continuum + BBkg

B0 J/ Ks studyFit CP parameters to values consistent with theory

B0 D+- studyAlso fit CP parameters to values consistent with theory

Validation fits use the nominal fit but selecting the mass of The J/ or D+ instead of the 0

Plots of variableMes

Ian Forster - U. of Liverpool IoP 2005

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Summary

Have measured evidence for the decay B0 0Ks

Result went to ICHEP in the summer PRL in pipeline

Time dependent CP analysis Fit two control samples and appear to

get expected CP result Aim to have CP analysis for summer

Ian Forster - U. of Liverpool IoP 2005

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Types of CP violation

Direct CP Violation CP violation in decay

Indirect CP Violation CP violation in mixing – when CP is conserved the

mass eigenstates are CP eigenstates.

CP Violation in the interference between mixing and decay – The one we are interested in for this analysis!

1f

f

A

A Ratio of decay amplitudes

Ian Forster - U. of Liverpool IoP 2005

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Which BSM models could account for the difference in Sin(2)?

Assume difference comes from New Physics 3 quark SM naturally produces a CP odd phase in

the CKM matrix There is no reason to think only one CP violating

phase exists outside the SM 2 Higgs doublets – 1 new phase SUSY – 10’s of new phases LR symmetry – up to 6 new phases

Browder and Soni: hep-ph/0410192

Ian Forster - U. of Liverpool IoP 2005

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Vertexing

Reconstruct Brec vertex from charged Brec daughters

Determine BTag vertex from charged tracks not belonging to Brec Brec vertex and momentum beam spot Y(4S) momentum

Average Dz resolution is 180 mm From measurement of Dz we calculate

DT, the time between the decay of the B’s This gives us the time dependent analysis

Beam spot

Interaction Point

BREC Vertex

BREC daughters

BREC direction

BTAG direction

TAG Vertex

TAG tracks

z

Ian Forster - U. of Liverpool IoP 2005

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B Flavour Tagging

Lepton: Charge of fastest electron (muon) with p* > 1.0 (1.1) GeV/c

Kaon : Net charge of identified kaons 0 (Also non-identified leptons and kaons, soft

pions from D*’s, etc)

b sc

e- , -

K-

Overall 67.5 +/- 0.5 % efficient

Ian Forster - U. of Liverpool IoP 2005

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B Background modes