1. On behalf of the BABAR Collaboration
Search for light CP-odd Higgs
decay with a charm tag at BABAR
Richard Kass
Ohio State University
On behalf of the BABAR Collaboration
Phys. Rev. D. 91, (2015)
8/5/2015
Richard Kass

2. Search Motivation
Many extensions of the Standard Model (SM) include a CP-odd Higgs boson
If light enough a CP-odd Higgs (A0) can be produced via bottomonium decays: ϒ→γA0
The region mA0 < 2mb not constrained by LEP results
Good region for B-factories
Y(3S) →γA0, A0→invisible
arXiv: [hep-ex]
Y(3S) →γA0, A0→τ+τ-
PRL 103, (2009)
Y(3S) →γA0, A0→μ+μ-
PRL 103, (2009)
Y(2S,3S) →γA0, A0→hadrons
PRL 107, (2011)
Y(1S) →γA0, A0→invisible
PRL 107, (2011)
Y(1) →γA0, A0→μ+μ-
PRD 87, (2013)
Y(1S) →γA0, A0→τ+τ-
PRD 88, (2013)
Y(1S) →γA0, A0→gg, ss
PRD 88, (2013)
Many previous A0 BaBar searches
8/5/2015
Richard Kass

3. Light CP-odd Higgs ●mA0<2mτ ●2mτ<mA0<7.5 GeV
In next-to-MSSM there are 7 scalar fields including a CP odd Higgs:
A0 = cosθAAMSSM+sinθAAs
Branching fraction can be “large”: BR(Y->γA0) =>
For small values of tanβ & mA0 > 2mc BR(A0->cc) can be large:
R. Dermisek et al. PRD 76, (2007)
R. Dermisek et al. PRD 81, (2010)
BR(Y->γA0) A0
BR(A0 ->cc)
BR(A0 ->cc)
mA0 GeV
Also predictions for τ+τ-, μ+μ-, gg, ss
●2mτ<mA0<7.5 GeV
●mA0<2mτ
●8.8 GeV<mA0<9.2GeV
●7.5 GeV<mA0<8.8 GeV
8/5/2015
Richard Kass

4. BaBar Detector
1.5 T Solenoid
Electromagnetic Calorimeter (EMC)
Detector of Internally Recflected Cherenkov Light (DIRC)
e+ (3.1 GeV)
e- (9 GeV)
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,c meson, qq, tau events
NIM A479, 1 (2002)
NIM A729, 615 (2013)
Richard Kass 4
8/5/2015
Richard Kass

5. BaBar Data Set
Took data at PEP-II asymmetric e+e- collider: This analysis uses data taken at Y(2S)
98x106 events, 13.6fb−1 on peak, 1.4fb−1 off peak
CUSB
8/5/2015
Richard Kass

6. Selection of Events-I Get a clean sample of Υ(1S) decays:
Tag Υ(2S) -> Υ(1S) π+π-
Gives a cleaner sample of events than using Υ(2S) -> gA0
Recoiling mass (mR) against dipions consistent with Υ(1S)
|m Υ(1S) –mR|< 10 MeV/c2
Search for Υ(1S) -> γ A0
A0 -> cc with a reconstructed charm meson (“tag”)
Charm tags: D0->K-π+ /D0->K-π+ π+π-/D0->Ksπ+π-
D+->K-π+ π+/D*+->π+ D0 [D0->K-π+π0]
8/5/2015
Richard Kass

7. Off resonance data (pts)
Selection of Events-II
Use a Boosted Decision Tree (BDT) to separate
signal & background
24 variables input into BDT:
2 event variables
10 kinematic variables (main power)
e.g. D0 mass, g energy…
3 vertex variables
3 event shape
6 CM opening angle variables
e.g. angle between D&g….
Divide data into low & high mass
Off resonance data (pts) Vs
e+e-->qq MC (hist.)
Low mass
High mass
Low mass: 4.00 ≤ mX ≤ 8.00 GeV/c2
High mass: 7.50 ≤ mX ≤ 9.25 GeV/c2
8/5/2015
Richard Kass

8. Analysis Details-I NO obvious peaks…. Search for a narrow peak in the
Low mass: 4.00 ≤ mX ≤ 8.00 GeV/c2
Search for a narrow peak in the
low and high mX regions
low mass 9.8k candidates
high mass 7.4M candidates
Exclude 8.95 ≤ mX ≤ 9.1 GeV/c2
to avoid Y(2S)->χB->Y(1S)
NO obvious peaks….
High mass: 7.50 ≤ mX ≤ 9.25 GeV/c2
χB removed
8/5/2015
Richard Kass

9. Analysis Details-II
A0 signal mX is modeled with a crystal ball (CB) function
Width of Gaussian component of CB functions varies with mX
Natural A0 width is expected to be small compared to detector resolution
Mass resolution improves with increasing (decreasing) mX (Eγ).
Reconstruction efficiency varies with mX:
GeV/c2, 2.6% 9.25 GeV/c2
Includes: cc hadronization, BFs, detector acceptance, BDT selection
Systematic error is dominated by cc hadronization ~10%
Also: signal PDF shape, D mass PDF, dipion BF & pdf, MC stats, γ eff., NY(2S)
8/5/2015
Richard Kass

10. Results from fits NO significant signal
Perform binned extended maximum likelihood fits
in the low & high mX regions:
10 MeV steps for low mX
2 MeV steps for high mX
Signal PDF: Crystal Ball (fixed)
Bkgd PDF: 2nd order polynomial (float)
Output of fit:
Nsig, Nbkg
3 polynomial parameters
NO significant signal
Largest “signal”: 2.3σ (low mass region)
local significance.
Low mass: 4.00 ≤ mX ≤ 8.00 GeV/c2
4.145 GeV/c2
2.3σ
High mass: 7.50≤mX≤9.25 GeV/c2
8.411 GeV/c2
2.0σ
8/5/2015
Richard Kass

11. A0->cc Search Results
Produce 90% CL upper limits for B(Υ(1S) -> γ A0)xB(A0->cc)
90% CL upper limits range from 7.4x10-5 to 2.4x10-3
8/5/2015
Richard Kass

12. Summary Significant constraints on new physics from B-factories
Many BaBar searches for low mass CP-odd Higgs, A0
Y(3S) →γA0, A0→invisible
arXiv: [hep-ex]
Y(3S) →γA0, A0→τ+τ-
PRL 103, (2009)
Y(3S) →γA0, A0→μ+μ-
PRL 103, (2009)
Y(2S,3S) →γA0, A0→hadrons
PRL 107, (2011)
Y(1S) →γA0, A0→invisible
PRL 107, (2011)
Y(1) →γA0, A0→μ+μ-
PRD 87, (2013)
Y(1S) →γA0, A0→τ+τ-
PRD 88, (2013)
Y(1S) →γA0, A0→gg, ss
PRD 88, (2013)
Latest search: No evidence for A0->cc via Y(1S) decay
Details in: Phys. Rev. D. 91, (2015)
8/5/2015
Richard Kass

13. Extra Slides
8/5/2015
Richard Kass

14. A0 branching fractions
Dermisek & Gunion, Phys. Rev. D 81, (2010)
8/5/2015
Richard Kass