[PPT] - Dark sector searches at BaBar and Belle and outlook for Belle II PowerPoint Presentation

SLIDE 1

Dark sector searches at BaBar and Belle and

utlook for Belle II

Christopher Hearty University of British Columbia / IPP March 23, 2017 U.S. Cosmic Visions: New Ideas in Dark Matter

SLIDE 2

Outline

Search for invisible decays of a dark photon produced in

e+e- collisions at BaBar.

Prospects for Belle II: single photon search; axion-like

particles.

Search for muonic dark forces at BaBar.
Belle: Search for a dark vector gauge boson decaying to

π+π- using η → π+π- γ decays .

2

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 3

BaBar single photon search

Optimized for and interpreted in terms of a dark photon

A′ decaying invisibly.

We assume on-shell A′ (mχ < mA′/2), so signal is a

monoenergetic photon.

analysis is otherwise not sensitive to mχ or to the

coupling of the χ to the A′.

3

E∗

γ =

√s 2 − m2

A0

2√s

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 4

Detector issues — trigger

Single photon trigger was implemented for final BaBar

running period: 

48 fb-1 for high mA′ (low Eγ), mostly Υ(2S) and Υ(3S) 
53 fb-1 for low mA′ (addition 5 fb-1 at the Υ(4S)).
Somewhat complicated, but nominal trigger threshold

GeV; analysis threshold GeV.  

trigger calibration issues.

4

E∗

γ > 1.5

E∗

γ > 1.8

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 5

Detector issues — hermeticity

e+e- → γ γ event 
Require |cosθ∗|<0.6 so that both γ are in the calorimeter.

5

photon shower detected in muon system (IFR) azimuthal gaps between crystals align with collision point no efficiency in muon system near sector boundaries

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 6

Backgrounds

e+e- → γ γ, 1 γ not detected.  
identical to the signal for mA′ < 1.6 GeV/c2  
difficult to quantify this background; not possible to

detect a signal.

e+e- → γ γ γ, 1 γ not detected, 2nd out of the detector

acceptance (typically ~0°).

e+e- → e+e- γ, both electrons out of acceptance.  
kinematic limitations on maximum photon energy.
Beam background photons do not mimic signal, but can

be the second photon in a signal event.

6

SLIDE 7

Event selection / BDT

Boosted Decision Tree to distinguish signal and
background. 12 items, including:
dφ between signal and 2nd photon;
angle between and IFR cluster, calorimeter

crystal edge, IFR sector boundary

Training: 3 fb-1 of Υ(3S) data, simulated signal with

uniform mA′.

7

~ pmiss

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 8

Mass regions

High A′ mass region (low γ energy) mA′ >5.5 GeV/c2 is

dominated by radiative Bhabha background smooth in recoil mass . Loose cuts on BDT output

8 )

2

(GeV

2 X

M 25 30 35 40 45 50 55 60 65 )

2

Events / ( 0.5 GeV

1 −

10 1 10

/df = 67.3/89

2

χ

25 30 35 40 45 50 55 60 65

Pull

2 − 2

background-only shape from BDT<0 events Υ(3S) data

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

mX

m2

X ≡ s − 2√sE∗ γ

BaBar

SLIDE 9

Low mass region has both peaking and smooth
backgrounds. Select data using two statistically

independent cuts on BDT and θ.

9

)

2

(GeV

2 X

M 5 10 15 20 25 30 35 )

2

Events / ( 1 GeV 2 4 6 8 10 12 14 16

/df = 29.7/38

2

χ

5 10 15 20 25 30 35

Pull

2 − 2 )

2

(GeV

2 X

M 5 10 15 20 25 30 35 )

2

Events / ( 1 GeV 1 2 3 4 5 6

/df = 14.1/38

2

χ

5 10 15 20 25 30 35

Pull

2 − 2

Υ(3S) data — tight selection Υ(3S) data — loose′ selection

peaking background from e+e- → γ γ background-only fit

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

BaBar BaBar

SLIDE 10

Signal extraction

Fit distribution; float signal, peaking background, and

smooth background yields. 166 mass hypotheses.  

simultaneous fit to 2S, 3S, 4S data, tight and loose.

10

m2

X

)

2

(GeV

2 X

M

25 30 35 40 45 50 55 60 )

2

Events / ( 0.5 GeV

1 −

10 1 10

/df = 69.0/77

2

χ

25 30 35 40 45 50 55 60

Pull 2 − 2

Fit to Υ(2S) + Υ(3S) data,   mA′ = 6.21 GeV/c2

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

BaBar

SLIDE 11

BaBar exclusion region for invisible decays of a dark photon

Region preferred by (g-2)μ excluded.

11

(GeV)

A'

m

3 −

10

2 −

10

1 −

10 1 10 ε

4 −

10

3 −

10

2 −

10

e

(g-2) NA64 ν ν π → K σ 5 ±

µ

(g-2) favored

BABAR 2017

BaBar collab., “Search for invisible decays of a dark photon produced in e+e− collisions at BABAR”, arXiv:1702.03327; submitted to PRL

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 12

Belle II projections for single photon analysis

Schedule (my best estimate, not official):
Phase 2 commissioning without vertex detectors:

February – July 2018; hope for 20 fb-1. Υ(6S) or 4S.

Phase 3 commissioning (full detector):  
Dec. 2018 — June 2019: 200 fb-1? Υ(3S)?
Goal is to produce a useful single photon measurement

using the Phase 2 data.

12

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 13

Belle II detector

Calorimeter is much more hermetic than BaBar:
gaps between barrel crystals are not projective
coverage is -0.94 < cosθ∗ < 0.96 versus  
0.92 < cosθ∗ < 0.89 for BaBar
However, there are gaps between the barrel and

endcaps in the calorimeter and the muon systems.

We are aiming for a trigger threshold of E∗ > 1 GeV.

13

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 14

Predicted backgrounds in Belle II single photon analysis for 20 fb-1. Loose selection, not optimized.

Final sample is almost entirely e+e- → γ γ (γ) with ≥3γ

14

(deg)

lab

θ 50 100 150 (GeV)

CMS

E 2 3 4 5 )

1

20 fb × Entries / (bin 1 10

2

10

3

10

4

10

irreducible irreducible e+e- → γ γ γ with 1γ in backwards gap and 1 at θ∗ ~ 0

← ~300 events → ← ~25k events →   with E∗ <3.9 GeV

final θ selection, not optimized mA′ 2.4 5.2 7.0 8.3

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

Belle II simulation

SLIDE 15

(GeV)

A'

m

2 −

10

1 −

10 1 10

ε

4 −

10

3 −

10

2 −

10

BaBar 2017 E787, E949 NA64 σ 2 ±

µ

(g-2) α vs

e

(g-2)

1

Belle II projection 20 fb

Projected Belle II exclusion region, 20 fb-1

Assumes we can quantitatively predict background levels. 
photon efficiency over barrel/endcap gaps.

15

better calorimeter hermeticity to suppress e+e- → γ γ Reach masses of 9.1–9.5 GeV/c2 with lower trigger threshold (vs   8 GeV/c2 for BaBar)

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 16

Projected Belle II exclusion region, 20 fb-1

16

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

(GeV)

A'

m 1 2 3 4 5 6 7 8 9

ε

4 −

10

3 −

10

2 −

10

BaBar 2017 E787, E949 NA64 σ 2 ±

µ

(g-2) α vs

e

(g-2)

1

Belle II projection 20 fb

SLIDE 17

Belle II search for Axion-like particles (ALP , a)

Production mechanisms:
ALP decays to a pair of photons (but could be long lived).

17

E. Izaguirre, T. Lin, and B. Shuve, “A

New Flavor of Searches for Axion- Like Particles”, arXiv 1611.09355

K. Mimasu & V. Sanz, “ALPs at

Colliders”, JHEP 1506 (2015) 173

e+ e-

di

dj

u/c/t

a

W

b s see backup slides

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

gaγ gaW

SLIDE 18

[GeV])

a

(m

10

log 1.0 − 0.5 − 0.0 0.5 1.0 ])

1

[GeV

γ a

(g

10

log 6 − 5 − 4 − 3 − 2 − 1 −

:

90

α Opening Angle ° 5.0 ° 10.0 ° 15.0 ° 30.0 Flight distance: 0.1cm 1.0cm 10.0cm 100.0cm

Overlapping Photons Merged Photons Separated Photons Single Photon Displaced Vertex

5° 15°

1cm 100cm

Decay photons in e+e- → aγ could overlap in the

trigger and look like γγ. Large prescales applied to existing BaBar and Belle data sets.

Belle II trigger should be able to keep these events.

18

Different experimental signatures for e+e- → aγ search

Torben Ferber level 1 trigger may not separate these two clusters Torben Ferber

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 19

Search for muonic dark forces at BaBar

19

Dark gauge boson Z′ couples only to 2nd and 3rd
generations. Results are much less constrained; could

explain muon g-2.

514 fb-1 mostly at Y(4S), but also Y(3S), Y(2S), off-peak.

final state = 4 muons

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 20

20

Plot all four μ+μ- mass combinations per event, and

look for a narrow peak on a smooth background.

Entries / 0.1 (GeV) 1000 2000 3000 4000 5000

Data

µ

+

µ

µ

+

µ →

e

+

e

τ

+

τ →

e

+

e q=u,d,s,c q q →

e

+

e )

µ

+

µ → ( ψ J/

π

+

π →

e

+

e

(GeV) m

1 2 3 4 5 6 7 8 9 10

Data/MC

0.8 1 1.2

J/ψ → µ+µ- e+e- → π+π-ρ0; ρ0 → π+π-  e+e- → µ+µ-ρ0

ther candidates

in ρ0 events blue line = overall correction for ISR, tracking, PID etc mR ≡ q m2

µ+µ− − 4m2 µ (GeV)

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

BaBar

SLIDE 21

(GeV)

Z'

m

1

10 1 10 UL g'

3

10

2

10

1

10 Trident Borexino

g-2 favored

21

No significant signal. Excludes this model as an

explanation for muon g-2 for Z′ heavy enough to decay to muons.

Belle II will study e+e- → μ+μ-Z′ (Z′ → invisible), but no

results yet. Better hermeticity helps compared to BaBar

Production of µ+µ- in νµ scattering; CCFR Assumes equal coupling to τ, µ, νµ, ντ BABAR collab., “Search for a muonic dark force at BABAR”, Phys. Rev. D94, 011102(R) (2016)

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

BaBar

SLIDE 22

Search for a dark vector gauge boson U′ decaying to π+π– using η → π+π– γ decays by Belle

Dark sector vector gauge boson that couples

predominantly to quarks.

Decay chain:

22

S. Tulin, Phys. Rev. D 89, 114008 (2014)

D⇤+ → D0π+ D0 → Ksη η → U 0γ U 0 → π+π

sidebands for π+π- background subtraction slow pion and Ks   give a clean signature

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

Belle

SLIDE 23

Look for a peak in the π+π- mass distribution on top of

the smooth background. No evidence for signal.

23

sideband-subtracted mass distribution exclusion limits on coupling constant gU′ between U′ and quarks

αU 0 = g2

U 0/4π

Belle Collab, “Search for a dark vector gauge boson…”, Phys. Rev. D94, 092006 (2016) 400 Mev/c2 signal

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

Belle Belle

SLIDE 24

Summary

BaBar single photon search excludes g-2 region of

parameter space.

Belle II will achieve useful limits in this mode with initial

data set. Possibly for ALP searches as well. Improvements in searches for visible dark photon decays (backup slides) will require luminosity.

Large number of additional searches are possible; Z′, U′,

h′…  

dark Higgs searches are almost background-free

(backup slides); will improve linearly with luminosity.

24

C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions

SLIDE 25

Backup

25

SLIDE 26

Belle II collaboration

23 countries, 100 institutions, 750 collaborators,

including 380 PhD physicists & 260 graduate students.

26

Carnegie Mellon Univ. Indiana Univ. Kennesaw State Univ. Luther College Pacific Northwest National Laboratory(PNNL)

Univ. of Cincinnati
Univ. of Florida
Univ. of Hawaii
Univ. of Mississippi
Univ. of Pittsburgh
Univ. of South Alabama
Univ. of South Carolina

Virginia Polytechnic Institute and State Univ. Wayne State Univ. McGill Univ.

Univ. of Montreal
Univ. of British Columbia
Univ. of Victoria

SLIDE 27

SuperKEKB luminosity projection

27

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year 20 days/month

Integrated luminosity (ab-1) Peak luminosity (cm-2s-1)

Calendar Year

SLIDE 28

Event selection / BDT

Boosted Decision Tree to distinguish signal & background
quality and θ of γ
E and θ of 2nd γ, dφ with signal;
total extra energy
distance of to calorimeter crystal edge, IFR

sector boundary, IFR cluster

Training: 3 fb-1 of Υ(3S) data, signal MC with uniform mA′.

28

~ pmiss

SLIDE 29

Search for ALP in B decay

B → K(∗)a , a → invisible: BaBar has published

B → K(∗)J/ψ, J/ψ → invisible  

same dataset could be used for arbitrary a′ mass.
a → γ γ is similar to K(∗)π0, previously published.
Belle II will eventually repeat these with much higher statistics.

29

BABAR collab., “Search for B→ K(∗)νν and invisible quarkonium decays”,

Phys. Rev. D87, 112005 (2013)

SLIDE 30

Projected Belle II sensitivity for visible dark photon decays

No real analysis yet; projected limits scaled from BaBar,

assuming twice as good mass resolution.

30

(GeV)

A'

m

2 −

10

1 −

10 1 10

ε

4 −

10

3 −

10

2 −

10

BaBar KLOE 2014 KLOE 2016 KLOE 2015 KLOE 2013 WASA HADES APEX E774 E141 α vs

e

(g-2) σ 2 ±

µ

(g-2) favored Phenix NA-48

1

Belle II 500 fb

1

50 ab

lifetime is non- negligible here; requires some work BaBar collab., PRL 113, 201801 (2014), 514 fb-1 Upper limit on ε  scales as luminosity L0.25 Bertrand Echenard  Chris Hearty

SLIDE 31

Sources of calorimeter inefficiency (in order of importance)

31

90 60

290

310 R1145 ARICH R1180 BECL flange inner R

CDC

Pre-amp

1 7 °

2650(CDC) 2980 20 20 IDS 60(SC w/ cover)

145

Backward IDS and SC detail : Scale 1/2 Forward IDS and SC detail : Scale 1/2

20
CDC-SC(Support cylinder)

IDS(Inner detector support)

CDC inner support ring-f R1111.4 Outer most sense wires R290.5 R436.5 580 320 929.5

1590(CDC)

735(CDC) 1000(CDC) 1650(CDC) 265(SC w/ cover) R410 R452

R202.5

105

22

60

Preamp board H=180,W=200

Inner cover Middle cover QCSL QCSR R168 Inner most sense wires 5t CFRP Outer cylinder Small cell end plate-f Joint Small cell end plate-b Middle support ring Normal cell end plate-f(conical) Normal cell end plate-b(conical) Middle support ring Joint Joint Joint R1130 CDC end plate and CDC-SC

ARICH TOP

R220 R122.5

R142.5

2 4 4 7 251.2 Outer cover Inner cover 1646.6 R265 Normal cell end plate-b(main) Normal cell end plate-f(main) 20 103

R443

CDC inner support ring-b

R173
PMT access window

CDC support cylinder R1190.5 M6 Bolts R1250 BECL inner R 4 628 QCS 633.5 QCS 700 QCS R372.3 30° R273.8 R463.4 34 R371 R347 R160

End cap shield End cap shield

ECL VTX

1. barrel/endcap gap
2. barrel/endcap gap
3. projective φ

cracks in endcaps

3. projective φ

cracks in endcaps

4. 1.5mm structure at 90°
5. γ non-conversion 3 × 10-6

SLIDE 32

Sources of muon detector inefficiency

32 20 40 60 80 100 120 140 160 150 − 100 − 50 − 50 100 150 1 10

2

10

φlab vs θlab of all KLM clusters in e+e- → γ γ (γ)

φ lab (deg) θ lab (deg)

chimney endcap sector   boundary barrel sector   boundary endcap/barrel transition no KLM behind innermost ECL endcaps no KLM behind innermost ECL endcaps

SLIDE 33

Signal extraction

Correlation between Eγ and θ in backgrounds produces

pseudo peaks in the mX distribution.

Essential to study photon efficiency near endcap/barrel

gaps to establish this correlation.

33

)

2

(GeV

2

M 40 45 50 55 60 65 70 Entries / bin 200 400 600

2

0.018) GeV ± = (49.089 µ

2

0.011) GeV ± = (1.152 σ = 7 GeV)

A'

Simulation (m Novosibirsk fit

)

2

(GeV

2

M 40 45 50 55 60 65 70 Entries / bin 50 100 150

Signal, mA′ = 7 GeV/c2 Background, 20 fb-1

SLIDE 34

34

Four tracks (two identified as muons) consistent

with √s.

) (GeV) µ m(4 6 7 8 9 10 11 Entries / 0.06 (GeV) 10

2

10

3

10

4

10

5

10

Data

µ

+

µ

µ

+

µ →

e

+

e

τ

+

τ →

e

+

e q=u,d,s,c q q →

e

+

e )

µ

+

µ → ( ψ J/

π

+

π →

e

+

e

MC (DIAG36) does not include initial state radiation. In general, neither does MadGraph (signal MC) Initial state radiation causes low-side tail in data Dominant background is e+e- → µ+µ-µ+µ- Y(4S) data only signal region

SLIDE 35

Searches for dark Higgs at e+e- colliders

3A′ analysis is almost

background-free. Comparing Belle and BaBar, limits on αDε2 improve as 1/ℒ. Belle II will have very good sensitivity.

35

h′ → A′A′ (6 track final state) or

  h′ is long lived, if mh′ < mA′

BaBar collab, Phys. Rev. Lett. 108, 211801 (2012) Belle collab, Phys. Rev. Lett. 114, 211801 (2015) KLOE collab, Phys. Lett. B 747, 365 (2015)

]

2

c [GeV/

cand

A'

m 1 2 3 ]

2

c [GeV/

cand

A'

cand

A'

m 5 10

)

e

+

3(e )

µ

+

µ 3( )

π

+

π 3(

µ

+

µ )

e

+

2(e

π

+

π )

e

+

2(e

e

+

)e

µ

+

µ 2(

π

+

π )

µ

+

µ 2(

e

+

)e

π

+

π 2(

µ

+

µ )

π

+

π 2(

π

+

π

µ

+

µ

e

+

e )X

e

+

2(e )X

µ

+

µ 2( X

µ

+

µ

e

+

e

(a)

PRL 114, 211801 (2015) PRL 114, 211801