Rare Charmless B Decays

Looking Looking Towards Towards

sinsin2α2α

G. Hamel de Monchenault – Experimental Aspects of CP Violation

B→ π+π− in a NutshellSuperb PID

DIRC

strangeless-charmless two-body decays: transitions

π+π− is a CP eigenstate with very small branching fraction: ∼10−6

if Tree amplitude dominates

… but Penguin amplitude cannot be neglected

and

to extract αisospin analysis: might not be possible experimentallymodels (e.g. QCD factorization): needs independent validations

B → ππ : Penguins & Trees

Tree

Color-suppressed tree

Gluonic penguin

contributes to

contributes to

contributes to

B → Kπ : Penguins & Trees

Tree

Color-suppressed tree

Gluonic penguin

contributes to

contributes to

contributes to

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Penguins are Significant!

Tcs&P

T&PT&Tcs

P’&T’cs

P’&T’&T’cs

P’P’&T’

pure tree

pure penguin

control of re-scattering

effects

connectedvia

isospinsymmetry

connectedto ππvia

SU(3)symmetry

Without penguins,expect:

Measurements show:

penguins cannot be neglected

!!

Use Kπ & SU(3) to estimate

penguin pollution in ππ

why the hell do you call these Penguin diagrams?

They don’t look like penguins!

I’ve never seen a Feynman diagram

that looks like you ☺

a controversy…

mirror image of Richard Feynman

G. Hamel de Monchenault – Experimental Aspects of CP Violation

B → ππ : Decay Amplitudesreceives T and P contributions

by convention, use unitarity

CP violatingparameter

no penguin general case

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Background FightingExpect huge background

from QED continuum!B produced (almost) at rest

in Y(4S) frame

Isotropic B Jetty Continuum

Combine into a Fisher (or NN)

Signalu,d,s,cbackground

Fisher Discriminant

Arb

itrar

y U

nits

Monte Carlo

BG

Signal

Increasedpurity atlarge |∆t| flavor-

taggingalso helps!

exploit event topology

signal purity improves at large |∆t|

characterize background from sidebands

study off-resonance data (~10%)

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Select 2-Body B Decays

Identify candidate 2-body B decays with expected invariant massand CM energy, e.g. for B0 → h+h- (h=K,p)

Evts

/MeV

/c2

P*B ~ 340 MeVσ(mES) ~ 2.6 MeV

(MC)

B0 → h+h-

BGmES GeV/c2

BG

σ(∆Ε) ~26 MeV

(MC)

Two-body decays cannot be disentangled cleanly using kinematics only

Requires excellent Particle Identification capability (up to largest momenta)

BABAR: the DIRC System

G. Hamel de Monchenault – Experimental Aspects of CP Violation

BABAR PID Performance

8σ separationat 2 GeV/c

2.5σ separationat kin. endpt.

DIRC

from D*control sample

Simulateous ππ/Kπ analysis

Cherenkov angle as a discriminating variable

in the Maximum Likelihood Fit

dE/dx DIRC

B → ππ

B → DK

Tagging Kaons

dE/dx DIRC

dE/dx DIRC

Note: in two-body decays, full dip angle/momentum correlation due to boostNote

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Belle PID Performance

B (1.5Tesla) 3" FM-PMT2.5" FM-PMT

2" FM-PMT

17

°

127 ° 34°

Endcap ACC

885

R(BACC/inner)

1145

R(EACC/outer)

1622 (BACC)

1670 (EACC/inside)

1950 (EACC/outside)

854 (BACC)

1165

R(BACC/outer)

n=1.02860mod.

n=1.020240mod.

n=1.015240mod.

n=1.01360mod.

n=1.010360mod.

Barrel ACC

n=1.030228mod.

TOF/TSC

CDC

B → ππ

B → DK

Tagging Kaons

dE/dxTOF

ACC BarrelACC Endcap

dE/dxTOF

ACC BarrelACC Endcap

dE/dxTOF

ACC BarrelACC Endcap

~0 f

or

π

~1 f

or

K

Mom

entu

m [G

eV/c

]

Prob(K) = L(K)/(L(K) + L(π))

* 0( )D D K ππ− +++ → →

input sample to the likelihood fit

flavor-tagged sample

lepton-tagged sample

The selected ππ/Kπ fit sample isstill dominated by QED continuum

background

Flavor-tagginghelps reducing

continuum background

Ample sidebands are used

to characterize backgrounds

Lepton-tagged sample the purest,

but statistically limited

Likelihood Projections

Note: signal-enriched plots after optimized cut

on likelihood ratio, with projection of fitted likelihood superimposed

ππ Signal

continuumrareB decays

continuum

Cross-check: B0->K+π– mixing

consistent with expected behavior

G. Hamel de Monchenault – Experimental Aspects of CP Violation

ππ Raw Asymmetries

-1

0

1

∆t (ps)

Asym

metr

y

(d)

B → π+π−0

10

20

30

40

∆t (ps)

π+π− y

ield

(c) q = +1q = −1

Belle: interpret as evidence of CP violation in B→ππ (3.4 σ)

direct CP violation (2.2 σ)

background subtracted

0B

0Bbkg

bkg

no evidence for CP violation

“Constraints” in the Sππ-Cππ Plane

-1 +1-1

+1

α = 97o

|P/T|

δ

α = 76o

α = 127o

At least five times more data needed to resolve the inconsistency…

Constraints on P/T

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Isospin Analysis

2αeff

2α

See Jerôme Charles’ lectures

isospin decomposition (as in the kaon system)

from wichone deducesisospinrelations(Gronau & London)

B → π+π0 : Clear Signals

G. Hamel de Monchenault – Experimental Aspects of CP Violation

B → π0π0 : Recent Observation

signal

ρπ bkg

continuum bkg

Summer 2003

“observation”

“evidence”

B → π0π0

a Candidate

π0

π0 π0

π0

BABAR

σ = 5.0%

π0

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Isospin Analysis: Pieces of the Puzzle

See lectures by Jerome Charles

full analysis requires and

Grossman-Quinn no very useful limit so far(don’t expect big improvement)

if significantly different from 1 suggest either large color-suppressed large FSI

G. Hamel de Monchenault – Experimental Aspects of CP Violation

B→ ρπ → π+π−π0

the decay receives a number of contributions that interfere in the Dalitz Plot

0Eπ

−πE+πE

Dalitz PlotStrong interest

“Low” statistics solution

Challenging task!

linked to angle α as ππ,but penguin contributions expected to be somewhat smaller

perform “simplified” quasi two-body analysis

select the and bands in the Dalitz Plot, excluding interfering region, where charge asignment is ambiguous

following Snyder & QuinnPhys. Rev D 48 (1993)

see also: BABAR Physics Book

Eventually: full time-dependent Dalitz analysis

G. Hamel de Monchenault – Experimental Aspects of CP Violation

B→ ρπ/ρK : Branching Fractions

Experimental challenge

mass spectrum

presence of neutral pionin final state

cope with both continuum and BB background

89 million BB pairs

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Β → ρπ Time-DependentGlobal charge asymmetry

Direct CP-violating

Mixing/decay interference CP-violating

Dilution parameter

Linked to vs

Fit parameters

New BABAR analysis:

Hint of Direct CPV?

new results with

Note: consistent with naïve

factorization (~0.4)

G. Hamel de Monchenault – Experimental Aspects of CP Violation

Hint of Direct CP Violation?

another very promising modes: ρρnow measuredindications that penguins are small

a effect of direct CP violation

a bit surprising since penguin areexpected small in this mode

Triangle Side Measurements

Measurements of Vubusing fully-reconstructed B mesons

near the lepton spectrum endpoint

Lep

ton

s / (

50 M

eV/c

)

5000

2500

1500

3000

0

0

3.002.752.502.252.00Momentum (GeV/c)

(a)

(b)

CLEO

Many methods,large theoretical errors,good consistency