Leptonic, semi-leptonic and rare decays at B-Factories Gabriele - - PowerPoint PPT Presentation

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Leptonic, semi-leptonic and rare decays at B-Factories Gabriele Simi University and INFN Padova On behalf of BaBar and BELLE collaborations Outline Introduction D 0 gg NEW [Belle PRD 93 (2016) 051102] [BaBar PRD 85,

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Leptonic, semi-leptonic and rare decays at B-Factories

Gabriele Simi University and INFN Padova On behalf of BaBar and BELLE collaborations

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Outline

  • Introduction
  • D0

→ gg

– [Belle PRD 93 (2016) 051102] – [BaBar PRD 85, 091107(R) (2012)]

  • Ds

l → n

– [Belle JHEP09 (2013) 139] – [BaBar PRD 82, 091103(R) (2010)]

  • D0

→ pln

– [Belle PRL 97, 061804 (2006)] – [BaBar PRD 91, 052022 (2015)]

NEW

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Introduction

  • Semileptonic and leptonic decays have a are very clean signature

from an experimental point of view: a single lepton in the final state

  • Are also relatively simple from a theoretical perspective provide a

mean of both measuring fundamental standard model parameters (CKM, lepton universality) and perform detailed studies of the decay dynamics (QCD)

  • Deviations of the parameters (for instance lepton universality) from

the expectation can be a clear sign of new physics

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Introduction

  • Semileptonic and leptonic decays have a are very clean signature

from an experimental point of view: a single lepton in the final state

  • Are also relatively simple from a theoretical perspective provide a

mean of both measuring fundamental standard model parameters (CKM, lepton universality) and perform detailed studies of the decay dynamics (QCD form factors, decay constants)

  • Deviations of the parameters (for instance lepton universality) from

the expectation can be a clear sign of new physics

  • Rare and forbidden modes can be a window in new physics trough

the effect of virtual particles in the loop diagrams

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D0 → gg

  • Flavour changing neutral current forbidden at tree level
  • Sensitive probe for NP because highly GIM suppressed in SM
  • Short distance contribution including QCD corrections

– BF~O(10-11) [Fajfer et. al. (2001), Burdman et. al. (2002)]

  • Inluding long distance effects

– BF~(1-3) 10-8 (ex. S. W. Bosch and G. Buchalla (2002))

  • Possible BF~10-6

– due to NP (ex. exchange of gluinos in S. Prelovsek and D. Wyler 2001)

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D0 → gg

  • BaBar (470.5 fb-1) and Belle (832 fb-1)

analysis use similar strategies

  • To reduce large combinatorial

background require D*- D →

0p-

  • Reduce QED background requiring > 4

tracks and >4 photons

  • Largest remaining background from

D0→p0p0,p0h,hh

– Measure p0p0 (with respect to Ksp0) and use

the p0p0 shape in gg background

– Implement p0,h veto

  • Measure gg with respect to ks0p0 to

remove D* production uncertainty and cancel systematics

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D0 → gg

  • BaBar (470.5 fb-1) and Belle (832 fb-1)

analysis use similar strategies

  • To reduce large combinatorial

background require D*- D →

0p-

  • Reduce QED background requiring > 4

tracks and >4 photons

  • Largest remaining background from

D0→p0p0

– Measure p0p0 (with respect to Ksp0) and use

the p0p0 shape in gg background

– Implement p0,h veto

  • Measure gg with respect to ks0p0 to

remove D* production uncertainty and cancel systematics

[BaBar PRD 85, 091107(R) (2012)] combinatoric p0p0 background D0→gg signal + background

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D0 → gg

  • BaBar (470.5 fb-1) and Belle (832 fb-1)

analysis use similar strategies

  • To reduce large combinatorial

background require D*- D →

0p-

  • Reduce QED background requiring > 4

tracks and >4 photons and in-time sig.

  • Largest remaining background from

D0→p0p0,p0h,hh

– Measure p0p0 (with respect to Ksp0) and use

the p0p0 shape in gg background

– Implement p0,h veto

  • Measure gg with respect to ks0p0 to

remove D* production uncertainty and cancel systematics

[Belle PRD 93 (2016) 051102]

Mgg

combinatoric peaking

signal

combinatoric peaking

signal ΔM

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D0 → gg

  • Results for 90% CL upper limit

– BaBar: Nsig=-6 +/-15; BF<2.2 10-6 – Belle: Nsig= 4 +/-15; BF< 8.5 10-7

  • No statistically significant signal
  • Belle result more stringent than

previous limits

  • Already posing contraints in new

physics models

  • Will be significantly improved in

the next generation Belle-II experiment

[Belle PRD 93 (2016) 051102 BaBar PRD 85, 091107(R) (2012)]

  • S. W. Bosch and G. Buchalla,

Belle BaBar BES III CLEO

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Purely leptonic decays

  • Helicity suppressed
  • fDs can be compare to lattice QCD predictions
  • fDs feeds into Bs mixing by affecting the calculation of fBs ( and therefore |

Vts|)

  • Search for new physics

– Some new physics scenarios could affect differently D and Ds leptonic decays [for

example A.G. Akeroyd, hep-ph/0308260] c d(s) l+ n W+ D+(S)

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Belle Ds

  • l

→

  • n
  • Method: fully reconstruct
  • First inclusively select events

based on system recoiling against Dtag Kfrag Xfrag g

– – Xfrag=nothing,np (np<4, np0<2)

  • Ninc~94k events

cc Kfrag Xfrag D*-

s

Dtag l- n D--

s

g JHEP09 (2013) 139

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Belle Ds

  • l

→

  • n
  • Reconstruction of Ds

→ m-n

– Add single track identified as m – Select events based on missing

(neutrino) mass

– N(Ds→m-n) = 492 ± 26 – BF(Ds→m-n) = (0.531 ± 0.028 ± 0.020)%

  • Reconstruction of Ds

→ t-n

– Add single track identified as e,m or p – Select events based on missing

(neutrino) mass

– N(Ds→t-n) = 2217 ± 83 – BF(Ds→t-n) = (5.70 ± 0.21 ±0.30)%

  • Decay constant results:

Ds

+→ m+n

Ds

+→ t+n

JHEP09 (2013) 139

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BaBar Ds

  • l

→

  • n
  • Using an inclusive reconstruction method

similar to Belle e+e- c → c→DKXDs*-, Ds*- D →

sg

  • Reconstruction of Ds

→ m-n

– Add single track identified as m with

Eextra<1GeV/c2

– Select events based on missing (neutrino)

mass mr(DKXgm)

– N(Ds→m-n) = 275±17 – BF(Ds→m-n) = (0.602 ± 0.038 ± 0.034)%

  • Reconstruction of Ds

→ t-n

– Add single track identified as e,m – Select events based on missing (neutrino)

mass Mr2(DKX µ)>0.5 GeV γ

2/c4 (Remove µ+ν µ)

– BF(Ds→t-n) = (5.00 ± 0.35 ±0.49)%

  • fDs+ = (258.6 ± 6.4(stat) ± 7.5(syst)) MeV

PRD 82, 091103(R) (2010) Ds

+→ m+n

Muonic mode Ds

+→ t+n

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Comparison of decay constants

  • HFAG averages fDs+|Vcs| and extracts fDs+ assuming |Vcs|; fDs+=257.4

± 4.6 MeV

  • Value is in agreement with lattice QCD calculation fDs+=(248.6 ± 2.7)

http://www.slac.stanford.edu/xorg/hfag/charm/Vcs/december14/results.html

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Why D semileptonics decays

  • D

Pl → n decay rate can be used to:

– Determine Vcx using the form factors from lattice QCD: dΓ

∝|Vcx|

2FF2

– Determine Vub and Vcb and validate LQCD using the form factors

from CKM fits

  • D

PP'l → n can give information on the hadronic system without additional hadrons in the final state

  • Decay rate = Short distance x (form factor)

2 x |CKM| 2

Experiment Ds→ lnu D→ K(p)lnu D→ l+l- Known Use unitarity of CKM Martrix to extract Decay constant Form factors Test QCD Use theory to predict form factors and extract Vcd and Vcs

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D semileptonic decays reconstruction

  • @Y(4S) charm cross section is large

(1.3nb)

  • Jet like events from off-peak and

continuum provide better reconstruction of missing neutrino than secondary D from B

  • Divide event in two hemispheres
  • Use partial reconstruction on tag side

– Large statistics – Non negligible background – Poor resolution on kinematical variables

  • Background and resolutions can be derived

from data On peak Off peak Signal side Opposite side

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Belle approach: e+e- D →

(*) tag D* sig X

  • Signal D0 is tagged using reconstruction of all other particles

– D(*)tag reconstructed in D*+

D →

0 , D

π

+ π 0 and D*0

D →

0 π 0 , D0 with

γ D0/+ K(n ) [n=1,2,3] → π

– high resolution on decay kinematic variables (∆q2~0.015 GeV2),

allows absolute BR measurements, low background

– negative : low efficiency (~1%)

Mass spectrum for Dsig candidates For ~ 56k tagged D0

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Belle approach: e+e- D →

(*) tag D* sig X

  • BF measured normalizing to

the total number of D0 tags

– BF(D0

k →

  • l+n)=3.45+/-0.07+/-

0.02)%

– BF(D0→p-l+n)=0.255+/-

0.019+/-0.016)%

  • For D→pln the unfolded q2

distribution can be described by simple pole model

D→pln ~230 signal D→kln ~2500 signal

[Belle PRL 97, 061804 (2006)]

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D0 → pen signal

  • Partial reconstruction from 347.2

fb-1 of e+ e- cc events at the (4S) → Υ

  • D*+

D →

0π + with the D0 →π

  • e+ν
  • q2=(pe+pn)2=(pD-pp)2
  • Since The ν

e momentum is

unmeasured two kinematic fits are performed, imposing in turn the D0 and D*+ mass constraint.

  • Use Fisher discriminant to suppress

background from B events and other semileptonic decays from continuum

  • Signal events selected in

m=m δ

D*+ -mD0<0.150 GeV/c2

Signal 5.3k S/B~0.5

[BaBar PRD 91, 052022 (2015)]

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D0 → pen form factor

  • me~0 =>only one form factor

f+(q2)

– 2 or 3 poles are used to

parametrize the FF

  • mD*=2.01 GeV/c2
  • mD*'=2.61 GeV/c2
  • mD*''=3.10 GeV/c2 or effective mass

[BaBar PRD 91, 052022 (2015)]

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D0 → pen form factor

  • From the unfolded q2

distribution extract

– Branching fraction ratio

using D k → p (BF from PDG) as normalization mode

– Test form factor

paramtereization

  • D*(2010) and D*'(2650) two

pole parameterization cannot explain the data

  • → three pole ansatz needed

– Form factor normalization

[BaBar PRD 91, 052022 (2015)]

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D0 → pen form factor

  • From the unfolded q2

distribution extract

– Branching fraction ratio

using D k → p (BF from PDG) as normalization mode

– Test form factor

paramtereization

  • D*(2010) and D*'(2650) two

pole parameterization cannot explain the data

  • → three pole ansatz needed

– Form factor normalization

[BaBar PRD 91, 052022 (2015)]

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D0 → pen form factor

  • From the unfolded q2

distribution extract

– Branching fraction ratio

using D k → p (BF from PDG) as normalization mode

– Test form factor

paramtereization

  • D*(2010) and D*'(2650) two

pole parameterization cannot explain the data

  • → three pole ansatz needed

– Form factor normalization

[BaBar PRD 91, 052022 (2015)]

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D0 → pen & extraction of |Vub|

  • Vub can extracted from the measured differential decay rate

for B and D decays measured at the same w (i.e. same pion energy) because effect of the light quark cancels out and the ratio of form factors can be predicted more precisely that the individual factors

  • can be obtained

– from lattice assuming a constant fB+(wB)/fD+(wD)

  • – From phenomenological models using three pole model for B→pln
  • RBD=|f +

B→ p|

|f +

D→p|

At wD=wB WB,D=E*p/mp

RBD=|f +

B→ p|

|f +

D→p|

experim FF ratio experim gH*Hp

[BaBar PRD 91, 052022 (2015)]

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D0 → pen & extraction of |Vub|

  • BaBar systematics of different origin, expected to be reduced by Lattice calculations:
  • →

f+B(q2)/ f+D(q2) form factor ratio as function of Eπ (or w)

  • →

g H*Hπ couplings

[BaBar PRD 91, 052022 (2015)]

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Conclusions

  • BaBar and Belle have been able to perform precise measurements
  • f rare leptonic and semileptonic decays of charm mesons with

charm samples far smaller than hadronic machines thanks to their excellent neutral reconstruction, cleaner environment and kinematical constraints

  • No significant excess is found in rare D0→gg decays
  • The Ds decay constant has been measured compatible with lattice

QCD predictions

  • The D

→ pln BF, form factor normalization and form factor q2 dependence has been measured precisely

  • Vub has been determined from the B

→ pln and D → pln form factors relation using a different method with respect to the inclusive/exclusive determination

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Backup

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D0 k → pln

  • BaBar, 347.5 fb-1 (4s), PRD 83, 072001 (2011)

Υ

  • Measurements of K resonant and non-resonant contributions:

π S-wave, search of radially excited P-wave and D-wave

  • Accurate measurements of K* (892) modes: resonance

parameters, form factors

  • K S-wave phase versus the K mass

π π

  • Differential decay rate has 5 degrees of freedom:

– m2, of the k system

π

– q2, of the e system

ν

– cos(θ

k)

– cos(θ

e)

– χ