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(Semi)-leptonic charm decays at BaBar. Paul D Jackson The Ohio State University Charm 2006, Beijing, China. June 5 th , 2006. Outline. D s + → μ + ν → extract Branching Fraction and f Ds D 0 →K - e + ν → measure form factor and q 2 D→Xl + l - → search for FCNC decays - PowerPoint PPT Presentation
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(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)
+)
4Paul D. Jackson Charm 2006
Ds+→μ+ν
Using charm tagging technique to extract fDs
5Paul D. Jackson Charm 2006
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
6Paul D. Jackson Charm 2006
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
7Paul D. Jackson Charm 2006
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
8Paul D. Jackson Charm 2006
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
10Paul D. Jackson Charm 2006
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
11Paul D. Jackson Charm 2006
D0→K-l+ν
Measurement of form factor from semileptonic D decay
12Paul D. Jackson Charm 2006
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
13Paul D. Jackson Charm 2006
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
14Paul D. Jackson Charm 2006
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
15Paul D. Jackson Charm 2006
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
16Paul D. Jackson Charm 2006
D→Xl+l-
Searches for Flavour Changing Neutral Currents with di-lepton in
final state
17Paul D. Jackson Charm 2006
Why FCNC charm decays?Why FCNC charm decays?
18Paul D. Jackson Charm 2006
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
Paul D. Jackson Charm 2006 20
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
Paul D. Jackson Charm 2006 21
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
Paul D. Jackson Charm 2006 22
Signal FitsSignal Fits
23Paul D. Jackson Charm 2006
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
Paul D. Jackson Charm 2006 24
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
26Paul D. Jackson Charm 2006
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.
27Paul D. Jackson Charm 2006
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
Paul D. Jackson Charm 2006 29