(Semi)-leptonic charm decays at BaBar

(Semi)-leptonic charm decays at BaBar

Paul D Jackson

The Ohio State University

Charm 2006, Beijing, China.

June 5th, 2006

Outline

Ds+→μ+ν → extract Branching Fraction and fDs

D0→K-e+ν → measure form factor and q2 D→Xl+l- → search for FCNC decays Describe analysis strategies Conclude

2Paul D. Jackson ([email protected]) Charm 2006

QCD Parameterization by Form FactorsQCD Parameterization by Form Factors

Strong interaction is asymptotically free: Large energies (small distances): αs(q2) small →

perturbation in powers of αs

Large distances (size of hadron): αs≈1 → no perturbative treatment

Form factors F(q2,ε) parameterize soft QCD effects (hadronization)

Decay of pseudoscalar mesons Mq constant, ε=0: F(q2,ε) → fM

llD

FDπ(q2)

q2

ll

DfD

q2=mD2

factor)(known )( 2 dqqFBR

3Paul D. Jackson Charm 2006

Extraction of fM through leptonic decays: M+ → l+νl

Cleanest source 1% measurements for π+,K+

Heavy D(s)+,B(s)

+: low branching ratio

Partial Width of M+→l+ν pifpMQq M)(0 5

DS+

(J=0)

+

Helicity Suppression(disfavours l=e, favours l=τ)

Phasespace(disfavours l=τ)

CKM Mixing(disfavours M = B, Bs, D)

2

2

2222

2

18

M

llMMQq

F

M

mmMfV

G

Measurement of fM (M+=π+,K+,D(s)+,B(s)

+)Measurement of fM (M+=π+,K+,D(s)+,B(s)

+)


Ds+→μ+ν

Using charm tagging technique to extract fDs


Analysis OverviewAnalysis Overview Goal: Identify Ds → μνμ decays in cc events Identify cc events: ‘Charm-Tagging’

Reconstruct charm mesons D0, D+, Ds+,

and D*+ in hadronic mode – the ‘tag’ High tag momentum close to kinematic

limit from B decays Search for Ds

*+ → γDs+ → γμ+ν in recoil

Advantages: Reduction of uds, BB, background Better ν resolution Flavour correlation of tag and recoil

Disadvantages Loss in efficiency due to tagging

M(Ds*)-M(Ds)

Ds+→μ+ν


Tagging StrategyTagging Strategy Fully reconstructed D in 13 hadronic decay modes

Tag momentum above 2.35 GeV/c Fit tag mass peak: estimate μ, σ Define tag signal region μ±2σ,

and sidebands between 3 and 6σ

D0 → K-π+, K-π+π0, K-π+π+π-

D+ → K-π+π+(π0), KS0π+(π0), Ks

0π+π+π-,K+K-π+, KS

0K+

DS+ → KS

0K+, φρ+

D*+ → D0π+, D0 →KS0π+π-(π0), KS

0K+K-, KS0π0

D0 → K-π+, K-π+π0, K-π+π+π-

D+ → K-π+π+(π0), KS0π+(π0), Ks

0π+π+π-,K+K-π+, KS

0K+

DS+ → KS

0K+, φρ+

D*+ → D0π+, D0 →KS0π+π-(π0), KS

0K+K-, KS0π0

Modes allow identification of the charm quark flavour

Modes allow identification of the charm quark flavour

Ds+→μ+ν


Tagging removes bb, uds, and ττ background, left with signal and large cc background

Identify kinematic quantities which distinguish signal

Signal SelectionSignal Selection

Used also Emiss, angle (μ,Ds+), θν

Photon energy pcorr = |pmiss|-|pν| Ds*+ momentum

Ds+→μ+ν


Ds+→μ+ν

Control SamplesControl Samples

9

Validate signal simulation: D0* → γD0→ γK-π+

Remove π+ and treat K- as μ-

In particular: pmiss reconstruction, neutrino fit (pcorr)

Validate electron efficiency correction: D*+ → π+D0 → K-l+ν, l+ = e+ and μ+ Apply electron weighting (data PID

tables) and phase space correction (3%) μ+ minus e+ subtracted ΔM distribution

Paul D. Jackson Charm 2006

Signal YieldSignal Yield

Binned 2-fit Vary: signal and

background yield Fix: relative

background sizes

Nμν=489 ± 55 2/d.o.f. = 31/22

Signal, leptonic background D→μν, and fake muon combinatoric background (shapes from simulation) remain

Ds+→μ+ν


Comparison to Previous MeasurementsComparison to Previous Measurements

All (except BES) normalized to

BR(Ds+→φπ+) = (3.6±0.9)%

(PDG)

Average:

fDs=267±33 MeV (6.1%stat+sys)

For comparison, our measurement (similarly normalized):

fDs=248±35 MeV (6.5%stat+sys)

Dominated by

BR(Ds+→φπ+) (12.5% on fDs)

New average: fDs = (261 ± 31) MeV (6.1% → 4.4%stat+sys)(based on BR(Ds+→φπ+) = (3.6±0.9)%)

Ds+→μ+ν


D0→K-l+ν

Measurement of form factor from semileptonic D decay


Several measurements need precise Lattice calculations of hadronic effects in weak interactions (fB,….)

Large effort from Lattice community (FNAL, MILC,…) to improve computation methods (unquenched, staggered fermions…) → impressive accuracy

Semileptonic decays of charm hadrons provide a way to validate those results through the measurement of form factors

Physics outlinePhysics outline

hadronic effectshadronic effects parameterized by parameterized by form factorsform factors

D0→K-e+ν


From continuum events: eFrom continuum events: e++ee-- cc cc (( =1.3 nb) =1.3 nb)

Using the D*+ D0 +

decay channeldecay channel

Reduce the backgroundReduce the background Fisher discriminant (bb and cc events)Fisher discriminant (bb and cc events)

DetermineDetermine qq2 2 = (p= (pDD – p – pKK ) )2 2 = (p= (p + + pp ) )2 2

Extract the form factorExtract the form factor

two constrained fitstwo constrained fits

Unfolding: SVD methodUnfolding: SVD method

Principle of the AnalysisPrinciple of the Analysis


D0→K-e+ν

)()( 00 DmDmm 222 )()( KDe ppppq

e

)()( 00 DmDmm

Analysis is based on the reconstruction of D*+ mesons produced in cc events and in which the D0 meson decays semileptonically.

In this decay one can define a q2 as following:

222 )()( KDe ppppqe

Form factor in D0→K-e+νForm factor in D0→K-e+ν


polemq

fqf

2

22

1

)0(|)(|

)1)(1(

)0(|)(|

**2

2

2

2

2

ss D

pole

D m

q

mq

fqf

polemq

fqf

2

22

1

)0(|)(|

A preliminary q2 distribution of the form factor, corrected for effects from reconstruction efficiency and finite resolution.

mpole = (1.854 ± 0.016 ± 0.020) GeV/c2

αpole=0.43 ± 0.03 ± 0.04 αpole(lattice) = 0.50 ± 0.04

Form factor in D0→K-e+νForm factor in D0→K-e+ν


D→Xl+l-

Searches for Flavour Changing Neutral Currents with di-lepton in

final state


Why FCNC charm decays?Why FCNC charm decays?


FCNC decays only occur in loop diagrams in SM:

Charm decays heavily GIM suppressed in SM: BF(cull)~10-8

New physics can introduce new particles into loopSome models increaseBF(cull) to 10-6—10-5

In range of possiblemeasurement!

SM, short distance only

SM+SUSY

SM with LD effects

RPV SUSY

PRD 66, 014009 (2002)

Significant LD contributions from intermediate resonancesThis analysis excludes the region around resonance

Several papers have studied SM and NP (mainly SUSY)contributions to the D++l+l

– decay mode

m(µ+µ–) (GeV/c2)m(e+e–) (GeV/c2)

(

1/

)d

/dm

2

(1/

)d

/dm

2

(

1/

)d

/dm

2

SM with LD effects

D++µ+µ– D++e+e–

Theory predictionsTheory predictions

Paul D. Jackson Charm 2006 19

FCNC decays: Both quarks change flavor:

*

*

**

**

**

*

*

* Lepton-flavor violating decays

BaBar has previously searched for D0e+e–, e+µ– and µ+µ–

with BF limits of 1.2x10-6, 0.8x10-6 and 1.3x10-6 respectively

PDG ULPDG UL

PRL93, 191801 (2004)

DØ:<4.7

BF (x10-6)

BF (x10-6)

Decay modes investigatedDecay modes investigated


Total E vs pT of event

+e+e–

Signal MC

bb MC

Number of Tracks

+e+e–

Reconstruct h+l+l- candidatesStringent lepton PIDRequire large pCM(h+l+l–)Semileptonic bb decays

suppressed by event shape and vertexing requirementsRadiative Bhabhas and

other QED events suppressed by event shape variablesRemove 's from +l+l–

decaysSignal efficiency 0.3%-4.5%,depending on decay mode

Background SuppressionBackground Suppression


D++l+l- selection c+p+l+l- selection

Calculate BR by normalizing signal yields to hadronic charm decays:

Decays selection same as for signal modes except PID - Cancels most non-PID systematic errors

pK-+K+K-

D+

Ds+

K+K- )

BF (x10-3)

NormalizationNormalization


Signal FitsSignal Fits


No signal forFCNC charmdecays foundLimits on yieldsextracted withunbinned likelihood fits

Largest “signal”is ~1.5 in

c+pµ+µ– decays

D+e+e– D+µ+µ–

Ds+Ke+e– D

s+Kµ+µ–

c+pe+e–

c+pµ+µ–

Unblinded Mass DistributionsUnblinded Mass Distributions


BF Upper LimitsBF Upper Limits

25

Existing BF (x10-6)Limits (90%CL)

<7.4 CLEO-c <11<8.8 FOCUS <24<34 E791 <11<34 E791 <6

<270 E791 <8<26 FOCUS <19

<610 E791 <22<610 E791 <14<6.2 CLEO-c <5<9.2 FOCUS <14<68 E791 <4<68 E791 <4

<1600 E791 <7<36 FOCUS <25

<630 E791 <6<630 E791 <4

- <4<340 E653 <40

- <9- <8

Yields converted to limits on branching fractions:BaBar BF (x10-6)Limits (90%CL)

Paul D. Jackson Charm 2006

Branching Fraction LimitsBranching Fraction Limits


Improved limits in 17 modes, more than order magnitude in 12 modes

(preliminary)

Upper limits on BF (x10-6) at 90% CL

Summary

Rich and varied program in (semi)leptonic charm decays

Measured Ds+→μ+ν and normalise to Ds

+→φπ Extract fDs to ~6% Measured D0→K-e+ν and form factors Searched for 20 FCNC D→Xl+l- modes. No

measurements but improved limits in 17 cases.


Backup Slides

28

Extraction of the q2 dependence of the form factor:

Unfolding the measured q2 distribution

Single Value Decomposition* approach:

* SVD; A. Höcker, V. Kartvelishvili [hep-ph/9509307]

algorithm imposing a minimum curvature condition

SVD of the reconstruction matrix S[q2

rec,q2sim]

MC

results tested on a toy generator

Unfolding ProcedureUnfolding Procedure


(Semi)-leptonic charm decays at BaBar

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