Searches for new physics in
B decays at BaBar
Richard Kass for the BaBar Collaboration
Outline of Talk
Searches for:
B0invisible (γ)
Charged Lepton Flavor Violation
Measurement of BD(*)τν
Richard Kass
CIPANP
1
Search for Binvisible (g)
Search for B decays with neutrinos or other non-interacting (e.g. SUSY)
particles in the final state.
In SM B vv is highly suppressed by ~(mv/mb)2
In SM BF(B vvγ) is ~10-9
In SUSY models BF(B invisible) can be enhanced to ~(0.1-1)x10-6
due to neutrino + neutralino production
A. Dedes, H. Dreiner, and P. Richardson, Phys. Rev.D65, 015001 (2001)
Previous Babar results with 88.5 fb-1 (B. Aubert et al., Phys.Rev.Lett.93:091802,2004 ):
BF(B invisible)<22x10-5, BF(B vvγ) <4.7x10-5
Richard Kass
CIPANP
2
Binvisible (γ) analysis details
Use BaBar data sample: 471x106 BB pairs taken at U(4S)
Divide the event into a signal B and a tag B
Use B0D(*)-l+v as the tag B. l=electron/muon
Semi-leptonic B tag has higher efficiency than
fully reconstructed B tag
B0 → D-lν D- → K+π-π-/KSπB0 → D*-lν D*- → D0π-/D-π0 D0 → K-π+/K-π+π0/K-π+π+πUse cosine between B & D(*)l assuming only 1 v is missing
common vertex for D(*)l system
Signal side of the event:
no charged tracks,
require photon with ECM>1.2 GeV for Binvisible γ
Use Neural Net to discriminate signal from background (B’s, qq, etc)
B → Invisible
BaBar Preliminary
Dtag : 9 variables
D*tag : 6 variables
B → Inv.+γ
Dtag : 6 variables
D*tag : 4 variables
Richard Kass
CIPANP
3
Binvisible (γ) analysis details
Extract event yields using unbinned Max. Likelihood fit
Discriminating variable is the extra energy in the EM calorimeter, Eextra
For B → invisible+γ the high energy γ is removed from Eextra.
Signal region has Eextra <1.2 GeV
Signal efficiencies: B0invisible =17.8x10-4, B0invisible+γ= 16.0x10-4
Preliminary
B0invisible
Preliminary
B0invisible+γ
Signal and background yields determined from ML fit
Richard Kass
CIPANP
4
Binv (γ) systematic errors & results
Use “unphysical”
B+inv (γ) as
a control sample
to check the analysis
Bayesian approach used to calculate 90% CL upper limits
UL

0

P( B)dB /  P( B)dB  0.9
0
P(B) assumes flat priors, systematics modeled as gaussians
BR(B0invisible) <2.4x10-5
BR(B0invisible + γ) <1.7x10-5
Richard Kass
CIPANP
5
Search for Charged Lepton Flavor
Violation in B decay
submitted to PRD arXiv:1204.2852v1
Look for charged B decays B±h±τl with h=π/K and l=e/μ
LFV highly suppressed in SM ~ (mv/mW)2
Possible to greatly enhance the rate in extensions to SM
2nd & 3rd generations favored if coupling ~ mass
NP
SM+v mixing
φs is a scalar
Sher & Yuan, Phys.Rev.D44:1461-1472,1991 consider φs and estimate:
B±h±τe < 3x10-3
B±h±τ μ < 3x10-4
BaBar: B±K±τ μ < 7.7x10-5 @90% CL, PRL 99, 201801 (2007)
Richard Kass
CIPANP
6
B±h±τl Analysis Details
Use BaBar data sample: 472x106 BB pairs taken at Y(4S)
Divide the event into a signal B and a tag B
Tag B reconstructed using hadronic decays B±D(*)0XX-=combination of ps & Ks
Signal B reconstructed from B±h± tl
3 charged tracks
h±= π± or K±, l=e or m
indirectly reconstruct the tau (use 1-prong taus: tm, te, , tnp0)p-):
pt=-ptag- ph- pl Et=Ebeam– Eh- El m2t=E2t- |pt|2  |mt-mt,PDG|<60 MeV/c2
Use BD(*)0lv, D0Kπ as a control sample for BF normalization
NDlv /ND*lv /ND**lv determined from ML fit to data
BD(*)0 μv
BaBar Preliminary
Richard Kass
D0
D*0
D**0
CIPANP
7
B±h±τl Analysis Details
Reject continuum (e+e-qq) background using a likelihood ratio
BaBar Preliminary
Variables include:
cosqthrust
neutral energy in calorimeter
muon quality (primary, tau daughter)
Extract event yields using an unbinned Max. Likelihood fit
Use poisson PDFs for the three tau channels (m, e, np0)p-)
Assume uniform 3-body phase space for the signal decays
Set 90% UL CL using Feldman-Cousins procedure
Richard Kass
CIPANP
8
B±h±τl Results
Look at 8 channels, NO signals, Only ULs
BaBar Preliminary
BaBar Preliminary
Combine modes assuming B(B+h+τl )= B(B-h-τl )
5X better model
independent bounds on
NP scale in μτ flavor
changing operators.
Λbd>11 TeV, Λbs>15 TeV
D. Black, et al, PRD 66, 053002 (2002)
more details in “extra slides”
Richard Kass
CIPANP
9
BD(*)τν
D(*)tn
H-,W-
t-
arXiv:1205.5442
submitted to PRL
Large mass of tau adds sensitivity to additional helicity
amplitude. For BD(*)τν we have:
We can compare our rate measurements with the SM predictions for:
B( B  Dtn )
R ( D) 
 0.293  0.017
B( B  Dn )
B( B  D*tn )
R( D ) 
 0.252  0.003
B( B  D*n )
*
S. Fajfer, J. F. Kamenik, I. Nisandzic, arXiv:1203.2654, , J. F. Kamenik, F. Mescia, PRD 78 014003 (2008)
These ratios are sensitive to physics beyond the SM
Richard Kass
CIPANP
10
Analysis Details
Use BaBar data sample: 471x106 BB pairs taken at Y(4S)
Divide the event into a signal B and a tag B
Fully reconstructed tag B using hadronic decays
Signal B reconstructed from a lepton (te/m) & D(*)
No additional charged particles allowed in event
Require q2 =(pB-pD(*))2 > 4 GeV2
Backgrounds are suppressed using Boosted Decision Trees
Extract event yields using an unbinned 2-D Max. Likelihood fit
Perform fit using lepton momentum in B rest frame (pl*) & (missing mass)2
mm2=(pe+e--ptag-pD(*) -pl)2
Signal Samples: D0l, D*0l, D+l, D*+l with l= e or m
Fixed backgrounds: B0-B+ cross feed, combinatorial, continuum
Use data control samples for corrections & validation: BD**lν, D(*)lν
control samples
BaBar Preliminary
BaBar Preliminary
Richard Kass
CIPANP
11
BD*τν Yields
D*0tn
D*+tn
D*tn
Nsig
693±62
245±27
888±63
Stat. sig. (s)
11.3
11.6
16.4
R(D*)
0.322±0.032
0.355±0.039
0.332±0.024
1.74±0.19
1.76±0.13
B(BD*τν ) % 1.71±0.17
fit assumes
R(D*0)=R(D*+)
statistical errors only
D*0
D*+
m2miss (GeV2)
Richard Kass
D*0
Events/100MeV
MeV
Events/100
Events/25 MeV
BaBar Preliminary
Free yields
D*+
Fixed yield
pl* (GeV)
CIPANP
12
BDτν Yields
D0tn
D+tn
Dtn
Nsig
314±60
177±31
489±63
Stat. sig. (s)
5.5
6.1
8.4
R(D)
0.429±0.082
0.469±0.084
0.440±0.058
B(BDτν ) %
0.99±0.19
1.01±0.18
1.02±0.13
fit assumes
R(D0)=R(D+)
statistical errors only
D0
D0
D+
Events/100 MeV
Events/25 MeV
BaBar Preliminary
Free yields
D+
Fixed yield
M2miss (GeV2)
Richard Kass
pl* (GeV)
CIPANP
13
Systematic Errors & Results
Source
R(D) %
R(D*) %
r
D**lv bkg
5.8
3.7
0.62
MC statistics
5.0
2.5
-0.48
Cont. & BB bkg
4.9
2.7
-0.30
esig/enorm
2.6
1.6
0.22
System. Uncert.
9.5
5.3
0.05
Statist. Uncert.
13.1
7.1
-0.45
Total Uncertainty
16.2
9.0
-0.27
r=correlation coeff.
R(D)=0.440±0.058±0.042
R(D*)=0.332±0.024±0.018
SM
Average
does not include
this measurement
Richard Kass
CIPANP
14
Comparison with SM
BABAR
SM
D
R(D)
0.440 +/- 0.071
0.293 +/- 0.017
2.0 s
R(D*)
0.332 +/- 0.029
0.252 +/- 0.003
2.7 s
The combination of the two measurements (-0.27 correlation)
yields 2=14.6 for 2 DOF and p-value = 6.9x10-4
Data differ from SM rate by 3.4 s
Richard Kass
CIPANP
15
Charged Higgs?
Compare our R(D) & R(D*) measurements with Type II 2HDM
red=theory ±1s
blue=exp ± 1s
SM:tanb/mH=0
PDF shapes and efficiencies are recalculated vs tanb/mH
Data match the 2HDM model for:
R(D): tanb/mH=0.44 ±0.02 & R(D*): tanb/mH=0.75 ±0.04
Exclude Type II 2HDM at 99.8% CL
Richard Kass
CIPANP
16
Summary & Conclusions
*Search for B0invisible (γ): improved upper limits:
BR(B0invisible) <2.4x10-5
BR(B0invisible + γ) <1.7x10-5
*Search for Charged Lepton Flavor Violation in B decay
BR(B+K+ τμ) <4.8x10-5
BR(B+K+ τe) <3.0x10-5
BR(B+π+ τμ) <7.2x10-5
BR(B+π+ τe) <7.5x10-5
improved
new
submitted to PRD
arXiv:1204.2852v1
*Measurement of BD(*)τν: Larger than SM predictions
R(D)
R(D*)
BABAR 0.440 +/- 0.071 0.332 +/- 0.029
SM
0.293 +/- 0.017 0.252 +/- 0.003
2.0 s
2.7 s
D
submitted to PRL
arXiv:1205.5442
Exclude Type II 2HDM at 99.8% CL
Richard Kass
CIPANP
17
Extra slides
Richard Kass
CIPANP
18
B±h±τl Analysis Details
Use BaBar data sample: 472x106 BB pairs taken at Y(4S)
Divide the event into a signal B and a tag B
The tag B is fully reconstructed using hadronic decays B±D(*)0Xwith X-=n1π, n2K, n3Ks, n4π0, n1+n2 ≤ 5, n3 ≤2, n4 ≤2
The signal B is reconstructed from B±h± tl  3 charged tracks
h= π or K
l=e or m
indirectly reconstruct the tau (use 1-prong taus: tm, te, , tnp0)p-):
pt=-ptag- ph- pl Et=Ebeam– Eh- El m2t=E2t- |pt|2  |mt-mt,PDG|<60 MeV/c2
Use BD(*)0lv, D0Kπ as a control sample for BF normalization
NDlv /ND*lv /ND**lv determined from ML fit to data
BD(*)0
μv
BaBar Preliminary
Richard Kass
D0
D*0
D**0
N htl
 N Dln
 Bhtl e htl 
 BDln e Dln
CIPANP
  e tag
  Dln
  e tag
htl

  Bhtl e htl S 0
 ≈1
19
B±h±τl Results
submitted to PRD arXiv:1204.2852v1
Richard Kass
CIPANP
20
BD(*)τν Yields
arXiv:1205.5442
Richard Kass
CIPANP
21
PEP-II at SLAC
asymmetric e+e− collider: 9 GeV (e-)/3.1 GeV (e+)
PEP-II Peak Luminosity 1.2 x 1034 cm-2s-1
BaBar recorded 429 fb-1 at Y(4S)
4.7 x 108 U(4S)→BB events
Richard Kass
CIPANP
22
BaBar Detector
Electromagnetic
Calorimeter
(EMC)
1.5 T Solenoid
Detector of
Internally
Recflected
Cherenkov
Light (DIRC)
Drift Chamber
(DCH)
Instrumented
Flux Return
(IFR)
Silicon Vertex
Tracker (SVT)
SVT, DCH: charged particle tracking: vertex & mom. resolution, K0s/Λ
EMC: electromagnetic calorimeter: g/e/π0/η
DIRC, IFR, DCH: charged particle ID: π/μ/K/p
Highly efficient trigger for B mesons
Richard Kass
CIPANP
23
Analysis Technique
Threshold kinematics: we know the initial energy (E*beam) of the Y(4S) system
Therefore we know the energy & magnitude of momentum of each B meson
*2
mES  Ebeam
- p*B2
Signal
*
DE  E B* - E beam
Event topology
Signal
(spherical)
Background
Background
(jet-structure)
Also, use neural networks + unbinned maximum likelihood fits
Richard Kass
CIPANP
24
BaBar K/p ID
D*+ → D0p+
D0→ K+ p-
BaBar DIRC
Richard Kass
CIPANP
25