MEASUREMENT OF RADIATIVE D0 Ñ

Universityof Ljubljana FacultyofMathematics and Physics

MEASUREMENT OF RADIATIVE

D ⁰ Ñ V γ DECAYS

PhD topic defense

Tara Nanut

Advisor: Prof. Dr. Boštjan Golob May 13th, 2014

Brief Introduction: Standard Model

The Standard Model of Particle Physics:

Theory describing elemental particles and their interactions.

Particles:

‚ fermions: quarks and leptons (building blocks of matter)

‚ gauge bosons: force carriers

‚ Higgs boson Forces:

‚ EM interaction

‚ weak interaction

‚ strong interaction

Extremely accurately predicts experimental results for energies up to „1 TeV.

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Brief Introduction

radiative charm decays

joint occurence of weak and EM interactions

D ⁰ Ñ V γ

mesons: q1q₂ charm: qi“c D⁰:cu

vector meson: J^P“1^´ φ:ss

K^˚0:sd ρ⁰:^{uu´d d?}

2

ω: ^{uu`d d?}

2

*

observed by Belle and BABAR (detectors ate^`e^´ colliders,

"B-Factories")

Brφ“ p2.7˘0.35q ˆ10^´5 Br_K˚0“ p3.27˘0.34q ˆ10^´4

Analysis goals

‚ improvement of Br measurements

‚ measurement of CP asymmetry

Brief Introduction

radiativecharmdecays

joint occurence of weak and EM interactions

D ⁰ Ñ V γ

mesons: q1q₂ charm: qi“c D⁰:cu

vector meson: J^P“1^´ φ:ss

K^˚0:sd ρ⁰:^{uu´d d?}

2

ω: ^{uu`d d?}

2

*

observed by Belle and BABAR (detectors ate^`e^´ colliders,

"B-Factories")

Brφ“ p2.7˘0.35q ˆ10^´5 Br_K˚0“ p3.27˘0.34q ˆ10^´4

Analysis goals

‚ improvement of Br measurements

‚ measurement of CP asymmetry

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Brief Introduction

radiative charm decays

joint occurence of weak and EM interactions

D ⁰ Ñ V γ

mesons: q1q₂ charm: qi“c D⁰:cu

vector meson: J^P“1^´ φ:ss

K^˚0:sd ρ⁰:^{uu´d d?}

2

ω: ^{uu`d d?}

2

*

observed by Belle and BABAR (detectors ate^`e^´ colliders,

"B-Factories")

Brφ“ p2.7˘0.35q ˆ10^´5 Br_K˚0“ p3.27˘0.34q ˆ10^´4

Analysis goals

‚ improvement of Br measurements

‚ measurement of CP asymmetry

Brief Introduction

radiativecharm decays

joint occurence of weak and EM interactions

D ⁰ Ñ V γ

mesons: q1q₂ charm: qi“c D⁰:cu

vector meson: J^P“1^´ φ:ss

K^˚0:sd ρ⁰:^{uu´d d?}

2

ω: ^{uu`d d?}

2

*

observed by Belle and BABAR (detectors ate^`e^´ colliders,

"B-Factories")

Brφ“ p2.7˘0.35q ˆ10^´5 Br_K˚0“ p3.27˘0.34q ˆ10^´4

Analysis goals

‚ improvement of Br measurements

‚ measurement of CP asymmetry

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Brief Introduction

radiative charm decays

joint occurence of weak and EM interactions

D ⁰ Ñ V γ

mesons: q1q₂ charm: qi“c D⁰:cu

vector meson: J^P“1^´ φ:ss

K^˚0:sd ρ⁰:^{uu´d d?}

2

ω: ^{uu`d d?}

2

*

observed by Belle and BABAR (detectors ate^`e^´ colliders,

"B-Factories")

Brφ“ p2.7˘0.35q ˆ10^´5 Br_K˚0“ p3.27˘0.34q ˆ10^´4

Analysis goals

‚ improvement ofBr measurements

‚ measurement of CP asymmetry

CP

A

symmetry

C: charge conjugation

‚ transforms particleÑ antiparticle

‚ violated in weak decays

P: parity

‚ transformsr Ñ ´r

‚ violated in weak decays

CP

believed to be a fundamental symmetry

1964:

VIOLA TION

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CP Asymmetry

C: charge conjugation

‚ transforms particleÑ antiparticle

‚ violated in weak decays

P: parity

‚ transformsr Ñ ´r

‚ violated in weak decays

CP

believed to be a fundamental symmetry

1964:

VIOLA TION

‚ We need CPV to explain matter-antimatter asymmetry of the universe.

‚ Now: CPV part of the Standard Model (Kobayashi-Maskawa mechanism - Nobel prize 2008)

VCKM “

¨

˝

V_ud V_us V_ub Vcd Vcs Vcb

V_td Vts V_tb

˛

‚“

“

¨

˝

c₁₂c₁₃ s₁₂c₁₃ s₁₃e^´iδ

´s12c23´c12s23s13e^iδ c12c23´s12s23s13e^iδ s23c13

s12s23´c12c23s13e^iδ ´c12s23´s12c23s13e^iδ c23c13

˛

‚

‚ BUT Standard Model CPV not enough to explain the

experimentally observed asymmetry Ñ search for new sources of CPV (New Physics)

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CPV in charm

SM predictions for CPV in the charm sector are VERY SMALL.

small CKM elements

&

GIM cancellation

c u D⁰

d u

π^´ W^`

V_cd^˚

Vud d uπ^`

c

u D⁰

u

u q q d,s,b

W

CPV in charm has not been experimentally confirmed yet.

(A_CP thought to be discovered in 2011 by LHCb, but an updated analysis later showed a decrease in the value.)

Current world average:

∆A_CP “A_CPpK^´K^{`</sup>q ´A<sub>CP</sub>pπ<sup>´</sup>π<sup>`}q “ p´0.329˘0.121q % Possiblity of CPV sources beyond the Standard Model (New Physics)?

Difficulties in measurements of charm weak decays

Weak decays of D mesons exhibit significant hadron dynamical contributions.

Short distance

‚ on short distance scale M_W

‚ negligible in radiative decays

c

u

u

u d,s,b

W

γ

c

u

s

s W γ

Long distance

‚ on strong interaction scale

‚ NON-PERTURBATIVE ñ NON-ANALYTICAL CALCULATIONS

‚ dominant in radiative decays

‚ demanding calculations for theoretical predictions

c

u

s

u u s

γ ρ,ω W

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CPV in radiative charm decays

CPV in radiative charm decays has not been measured yet, but it could help our understanding of CPV in the charm sector.

In addition,BrpD⁰ ÑVγq is poorly or not known.

In some extensions of the SM, sizableACP can be expected:

‚ In D⁰ ÑK^{`</sup>K<sup>´</sup>γ, m(K<sup>`}K^´q «m(φ): A^φγ_CP «2ˆ10^´2.

‚ In D⁰ Ñπ^{`</sup>π<sup>´</sup>γ, m(π<sup>`}π^´q «m(ρ⁰): A^ρ_CP^0γ«10ˆ10^´2.

‚ A^V_CP^γą3ˆ10^´2 would be a clear signal of physics beyond the SM.

(Phys.Rev.Lett.109.171801 (2012))

A

measurement

A^f_CP ““

ΓpD⁰ Ñfq ´ΓpD⁰ Ñfq‰ {“

ΓpD⁰ Ñfq `ΓpD⁰ Ñfq‰ (1) BUT in actual experimentals, we measure

A^f_rec “ N_f ´N_f

N_f `N_f (2)

where other asymmetries contribute:

A^f_rec “A^f_CP`A<sub>FB</sub>`A^h_ε^` (3)

asymmetry from CP violation specific for decay

asymmetry in the reconstruction efficiency of˘charged particles forward-backward asymmetry

assumed to be same for all charm mesons

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A

measurement

A^f_rec “A^f_CP`A<sub>FB</sub>`A^h_ε^`

If we want to measureA_CP with effects of order 10^´2 or 10^´3, we need to evaluate these other contributions with great precision!

Solution: we choose a normalisation channel and calculate the difference.

Candidate channel for normalisation: D⁰ ÑK^`K^´ (Br andA_CP measured, with an accuracy better than forD⁰ ÑVγ).

A^φγ_rec “A^φγ_CP `A<sub>FB</sub>`A^π_ε^{s`</sup> A<sup>KK</sup><sub>rec</sub> “A<sup>KK</sup><sub>CP</sub> `A_FB`A<sup>π</sup><sub>ε</sub><sup>`s}

A^φγ_rec´A^KK_rec “A^φγ_CP´A^KK_CP “∆A_CP (4)

Experimental setup: KEKB collider

‚ asymmetrical e^`e^´ collider

‚ Tsukuba, Japan

‚ precision measurements

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Experimental setup: Belle detector

Reconstruction of the decay

Final state particles: e,µ,K,π,p,γ

We reconstruct decayed particles by calcualting theinvariant mass:

m“E²´ |p|² “ ÿ

i

`b

m²_i ´ |pi|²˘₂

´ | ÿ

i

pi|² (5) Our signal decays:

D⁰ ÝÑφγ

ëK^`K^´ D⁰ ÝÑK^˚0γ

ëπ^`K^´

D⁰ÝÑρ⁰γ ëπ^`π^´

(D⁰ ÝÑωγ - included inρ⁰ mode) ëπ^`π^´

(Analysis of charge conjugated modes is implied.) D⁰{D⁰ tag:

D^{˚`</sup>ÑD<sup>0</sup>π<sub>s</sub><sup>`} D^˚´ÑD⁰π_s^´

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Monte Carlo study

When reconstructing or signal decay, we also get a lot ofincorrectly reconstructed events, which represent background.

Possible reasons are:

‚ missing final state particle(s)

‚ other decays with the same final state particles

‚ misidentification of a final state particle (πØK)

‚ ...

Our signal decays are rareÑ background will prevail.

We need to devise a method forrecognizing signal among background Ñwe use a Monte Carlo simulation, which includes the “true"

information about the decay.

We need to perform some systematic cross-checksto verify that the MC simulation describes real data well.

Only after the analysis procedure is fully determined and verified on the MC we proceed with the analysis of real data.

Dominant background: missing photon

The dominant sources of background in our analysis are decays including aπ⁰ (η) instead of a photon,

π⁰ Ñγγ (ηÑγγ) and we miss one photon in the reconstruction.

Examples:

‚ all channels: D⁰ÑVπ⁰ ëγ

Sγ; V=signal meson

‚ K^˚0 channel:

˝ D⁰ÑK^´ρ^{`</sup> ëπ<sup>`}π⁰

ëγ A γ

˝ D⁰ÑK^´π^`π⁰non-resonant ëγ

A γ

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) (GeV) m(D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

/0.0039eventsN

0 10 20 30 40 50 60

(signal) γ φ ->

D0

) (GeV) m(D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

/0.0039eventsN

0 20 40 60 80 100 120

π0 φ ->

D0

) (GeV) m(D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

/0.0039eventsN

0 2 4 6 8 10 12

KS φ ->

D0

) (GeV) m(D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

/0.0039eventsN

0 100 200 300 400 500 600 700 800 900

gamma (signal) K*0 ->

D0

) (GeV) m(D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

/0.0039eventsN

0 200 400 600 800 1000 1200 1400 1600

π0 K*0 ->

D0

) (GeV) m(D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

/0.0039eventsN

0 500 1000 1500 2000 2500 3000

ρ -> K D0

We devise aπ⁰ (η) veto for determining the probability for a photon to be coming from aπ⁰ (η).

Analysis

We reduce background in our reconstructed sample by limiting some parameters (invariant mass of a reconstructed particle, p_CMS,E_γ, ...).

We optimize theseselection criteria so that the figure of merit is maximal:

FOM “ N_sig aNsig `Nbkg

“max. (6)

On the so obtained data sample (signal + background), we perform a 2D fit inmpD⁰q and cospθ_Hq to extract the individual components.

V

D⁰ γ

f2

f1

θH

(We do all this on the MC simulation.)

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2D Fit on MC

‚ We perform a 2D fit in mpD⁰qandcospθ_Hq with the aim to extract thesignal yield, which we need for the Br andACP calculations.

‚ We determine the shape of the 1D PDFs for the signal and different background components on MC, then perform the 2D fit (product of PDFs).

‚ The free parameters of the fit are the yields of individual components and some parameters of the PDFs (width, slope).

2D fit φγ

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Events / ( 0.0039 GeV )

0 20 40 60 80 100 120 140 160

180 signal

π0 φ other resonant combinatoric

H) θ cos(

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Events / ( 0.02 )

0 20 40 60 80 100

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Pull

-2 0 2

H) θ cos(

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Pull

-2 0 2

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2D fit φγ : Signal window

Projection to the signal window: 1.8 GeVămpD⁰q ă1.9 GeV,´0.2ăcospθ_Hq ă0.2

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Events / ( 0.0039 GeV )

0 5 10 15 20 25

30 signal

π0 φ other resonant combinatoric

H) θ cos(

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Events / ( 0.02 )

0 10 20 30 40 50 60

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Pull

-2 0 2

θ) cos(

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Pull

-2 0 2

2D fit K

γ

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Events / ( 0.0039 GeV )

0 1000 2000 3000 4000 5000

H) θ cos(

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Events / ( 0.016 )

0 500 1000 1500 2000 2500 3000 3500

signal π0

* 0 K η

* 0 Kππ K

η π K

ρ

K π

K(1430)*

combinatorial π K* ρ, rad. ρ K FSR other resonant

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Pull

-2 0 2

H) θ cos(

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Pull

-5 0 5

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2D fit K

γ: Signal window

Projection to the signal window: 1.8 GeVămpD⁰q ă1.9 GeV,´0.2ăcospθ_Hq ă0.2

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Events / ( 0.0039 GeV )

0 100 200 300 400 500 600

H) θ cos(

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Events / ( 0.016 )

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

signal π0

* 0 K⁰* η K

π π K

η π K ρ K

π K(1430)*

combinatorial π K*

ρ , rad.

ρ K FSR other resonant

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Pull

-2 0 2

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Pull

-2 0 2

2D fit ρ

γ

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Events / ( 0.0039 GeV )

0 200 400 600 800 1000

H) θ cos(

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Events / ( 0.016 )

0 200 400 600 800

1000 signal

π0 ρ0

π0

* 0 K⁺π- ρ^-π+ ρ other resonant combinatorial

ρ , rad.

π ρ FSR

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Pull

-5 0 5

H) θ cos(

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Pull

-2 0 2

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2D fit ρ

γ : Signal window

Projection to the signal window: 1.8 GeVămpD⁰q ă1.9 GeV,´0.2ăcospθ_Hq ă0.2

(GeV) mass D0

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Events / ( 0.0039 GeV )

0 20 40 60 80 100

H) θ cos(

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Events / ( 0.016 )

0 100 200 300 400

500 signal

π0 ρ0

π0

* 0 K π- ρ+

π+ ρ- other resonant combinatorial

ρ , rad.

π ρ FSR

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

Pull

-2 0 2

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Pull

-5 0 5

Signal yields

The signal yields obtained with the 2D fit on MC are:

Signal yield True value Expected σBr EstimatedσA_CP

φγ 621˘39 592 6% 9%

K^˚0γ 9537˘212 9795 2% 3%

ρ⁰γ 895˘82 920 10% 13%

Fitted yield is consistent with MC truth within uncertainties.

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Conclusion

‚ Analysis goal: measure the branching fraction and CP asymmetry in decays D⁰ ÑVγ,V “φ,K^˚0,ρ⁰.

‚ V “φ,K^˚0 modes: observed,Br measured, however a much larger data sample is available now.

‚ V “ρ⁰ mode: not observed yet; we expect to observe it in this analysis (using the full Belle data sample).

‚ ACP in radiative charm decays has not been measured yet.

‚ V “φ,ρ⁰ mode: we curently estimate to measure ACP with a 10%

error. A greater precision will be achieved with the upgrade of the collider and detector (Super-KEKB and Belle II), scheduled to start collecting data in 2016.

π

veto

‚ We want to check if a photon actually comes from a π⁰Ñγγ decay.

‚ The basis of our veto is a mass veto: we combine a photon with all others and calculate the invariant mass, remembering the

combination which lies closest to the π⁰ mass.

‚ We do this for different mass cuts on the second photon.

‚ Based on the invariant mass distribution, we can exclude the combinations in a determined range around the π⁰ mass.

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