Dark sector searches at BaBar and Belle and outlook for Belle II Christopher Hearty University of British Columbia / IPP March 23, 2017 U.S. Cosmic Visions: New Ideas in Dark Matter
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 . C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 2
BaBar single photon search • Optimized for and interpreted in terms of a dark photon A ′ decaying invisibly. √ s 2 − m 2 A 0 E ∗ γ = 2 √ s • We assume on-shell A ′ (m χ < m A ′ /2), so signal is a monoenergetic photon. • analysis is otherwise not sensitive to m χ or to the coupling of the χ to the A ′ . C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 3
Detector issues — trigger • Single photon trigger was implemented for final BaBar running period: - 48 fb -1 for high m A ′ (low E γ ), mostly Υ (2S) and Υ (3S) - 53 fb -1 for low m A ′ (addition 5 fb -1 at the Υ (4S)). • Somewhat complicated, but nominal trigger threshold GeV; analysis threshold GeV. E ∗ E ∗ γ > 1 . 5 γ > 1 . 8 - trigger calibration issues. C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 4
Detector issues — hermeticity • e + e - → γ γ event photon shower detected in muon system (IFR) no efficiency in muon system near sector boundaries azimuthal gaps between crystals align with collision point • Require |cos θ ∗ |<0.6 so that both γ are in the calorimeter. C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 5
Backgrounds • e + e - → γ γ , 1 γ not detected. - identical to the signal for m A ′ < 1.6 GeV/c 2 - difficult to quantify this background; not possible to detect a signal. • e + e - → γ γ γ , 1 γ not detected, 2 nd 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
Event selection / BDT • Boosted Decision Tree to distinguish signal and background. 12 items, including: • d φ between signal and 2 nd photon; • angle between and IFR cluster, calorimeter ~ p miss crystal edge, IFR sector boundary • Training: 3 fb -1 of Υ (3S) data, simulated signal with uniform m A ′ . C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 7
Mass regions • High A ′ mass region (low γ energy) m A ′ >5.5 GeV/c 2 is dominated by radiative Bhabha background smooth in recoil mass . Loose cuts on BDT output m X Pull 2 0 Υ (3S) data − 2 25 30 35 40 45 50 55 60 65 ) 2 /df = 67.3/89 χ 2 Events / ( 0.5 GeV background-only 10 shape from BDT<0 events 1 BaBar X ≡ s − 2 √ sE ∗ 1 − 10 m 2 25 30 35 40 45 50 55 60 65 2 2 M (GeV ) γ X C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 8
• Low mass region has both peaking and smooth backgrounds. Select data using two statistically independent cuts on BDT and θ . Υ (3S) data — tight selection Υ (3S) data — loose ′ selection Pull Pull 2 2 0 0 2 2 − − 16 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 6 ) ) BaBar BaBar 2 /df = 14.1/38 2 /df = 29.7/38 χ χ 2 2 Events / ( 1 GeV Events / ( 1 GeV 14 peaking background 5 12 from e + e - → γ γ 4 10 8 3 6 2 4 1 2 0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 2 2 2 2 M (GeV ) M (GeV ) X X background-only fit C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 9
Signal extraction • Fit distribution; float signal, peaking background, and m 2 X smooth background yields. 166 mass hypotheses. - simultaneous fit to 2S, 3S, 4S data, tight and loose. Pull 2 Fit to Υ (2S) + Υ (3S) data, 0 m A ′ = 6.21 GeV/c 2 2 − 25 30 35 40 45 50 55 60 ) 2 /df = 69.0/77 χ 2 Events / ( 0.5 GeV 10 1 BaBar 1 − 10 25 30 35 40 45 50 55 60 2 2 M (GeV ) X C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 10
BaBar exclusion region for invisible decays of a dark photon • Region preferred by (g-2) μ excluded. 2 − 10 K → π ν ν ε (g-2) ± 5 σ µ B A B AR 2017 favored 3 − 10 (g-2) NA64 e BaBar collab., “Search for invisible decays of a dark photon produced in e + e − collisions at B A B AR ”, arXiv:1702.03327; submitted to PRL 4 − 10 3 − − 2 − 1 10 10 10 1 10 m (GeV) A' C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 11
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. C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 12
Belle II detector • Calorimeter is much more hermetic than BaBar: • gaps between barrel crystals are not projective • coverage is -0.94 < cos θ ∗ < 0.96 versus -0.92 < cos θ ∗ < 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. C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 13
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 ≥ 3 γ 4 ) (GeV) 10 Belle II simulation m A ′ -1 20 fb final θ selection, 2.4 5 not optimized CMS 3 10 × Entries / (bin E ← ~300 events → 5.2 4 2 10 7.0 3 10 e + e - → γ γ γ with 1 γ in backwards gap ← ~25k events → with E ∗ <3.9 GeV and 1 at θ ∗ ~ 0 8.3 2 1 50 100 150 irreducible irreducible (deg) θ lab C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 14
Projected Belle II exclusion region, 20 fb -1 • Assumes we can quantitatively predict background levels. - photon efficiency over barrel/endcap gaps. 2 − 10 ε (g-2) 2 ± σ E787, E949 µ 3 − 10 BaBar 2017 (g-2) vs α e NA64 -1 Belle II projection 20 fb Reach masses of 9.1–9.5 GeV/c 2 with lower trigger threshold (vs 8 GeV/c 2 for BaBar) 4 − 10 2 1 − − 10 10 1 10 m (GeV) A' better calorimeter hermeticity to suppress C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 15 e + e - → γ γ
Projected Belle II exclusion region, 20 fb -1 2 − 10 ε (g-2) 2 ± σ E787, E949 µ 3 − 10 BaBar 2017 (g-2) vs α e NA64 -1 Belle II projection 20 fb 4 − 10 0 1 2 3 4 5 6 7 8 9 m (GeV) A' C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 16
Belle II search for Axion-like particles (ALP , a) • Production mechanisms: e + b s d i u/c/t d j see backup slides g aW g a γ W a e - E. Izaguirre, T. Lin, and B. Shuve, “A New Flavor of Searches for Axion- K. Mimasu & V. Sanz, “ALPs at Colliders”, JHEP 1506 (2015) 173 Like Particles”, arXiv 1611.09355 • ALP decays to a pair of photons (but could be long lived). C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 17
• Decay photons in e + e - → a γ 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. Different experimental signatures for e + e - → a γ search level 1 trigger may not 5° 15° separate these two clusters 0 ]) -1 [GeV Overlapping Photons Merged Photons Separated Photons Opening Angle α : 90 1 − 5.0 ° γ a 10.0 ° (g 2 − 15.0 ° 10 log 30.0 ° 3 − Flight distance: 4 − 0.1cm Displaced Vertex 1.0cm Torben Ferber 10.0cm 5 − Single 100cm 1cm 100.0cm Photon Torben Ferber 6 − 1.0 0.5 0.0 0.5 1.0 − − log (m [GeV]) C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 18 a 10
Search for muonic dark forces at BaBar • Dark gauge boson Z ′ couples only to 2 nd and 3 rd generations. Results are much less constrained; could explain muon g-2. final state = 4 muons • 514 fb -1 mostly at Y(4S), but also Y(3S), Y(2S), off-peak. C. Hearty | Dark sector at BaBar, Belle, and Belle II | US Cosmic Visions 19
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