On behalf of the BABAR Collaboration Search for DARK Force at BABAR APS April Meeting 2018, Columbus, Ohio Introduction to the DARK force Search for muonic DARK force Search for invisible DARK photon decay Summary/Conclusions Richard Kass On behalf of the BABAR Collaboration 4/15/2018 Richard Kass

Search Motivation-I Ample evidence for DARK MATTER (DM) gravitational mass≠luminous mass Galactic Rotation Curves Fritz Zwicky (1933) Coma galaxy cluster Large Scale Structure CMB Bullet Cluster Generated by Amanda Yoho, Planck CMB simulator http://strudel.org.uk/planck/#. X-ray: NASA / CXC / CfA / M.Markevitch et al.; Optical: NASA / STScI; Magellan / U.Arizona / D.Clowe et al.; Lensing Map: NASA / STScI; ESO WFI; Magellan / U.Arizona / D.Clowe et al. Sloan Digital Sky Survey 1.25 Declination Slice 2013 Data” by M. Blanton and the Sloan Digital Sky Survey 4/15/2018 Richard Kass

Search Motivation-II Lots of indirect evidence of DM from Astronomy We’d like some direct evidence….. Non-accelerator DM experiments: detect DM hitting a target LUX, PandaX, CDMS, PICO, CRESST, etc Accelerator experiments: produce & detect DM CMS, ATLAS In spite of intense effort no direct detection of DM Maybe we should expand our search horizon…. 4/15/2018 Richard Kass

Can perform DS searches with BaBar The Dark Sector (DS) Move beyond the WIMP paradigm, go to the DARK sector What is the DARK sector? The DARK sector includes particles that couple weakly to the SM Theory motivated by BSMs, string theory, extra U(1) group Dark force mediated by DARK U(1) photon(s), A’ DARK matter could be part of the DARK sector No reason to believe that there is only one DM particle DARK sector could have a rich spectroscopy with DM just a piece of it Light DM models (sub-GeV) require new low mass mediators to explain the observed relic density. Can explain with DARK sector. DARK photon couples to SM fermion with strength: α’=ε2αEM Can perform DS searches with BaBar 4/15/2018 Richard Kass

Search for muonic DARK force @ BaBar > A DARK force that couples to 2nd and 3rd generation SM particles Z’/g-2: X. G. He et al., Phys. Rev. D 43, 22 (1991). X. G. He et al., Phys. Rev. D 44, 2118 (1991) New gauge boson, Z’, could be low mass (Shuve, Yavin, PRD 89, 075014 (2014)) > Could explain anomalies such as g-2, dark matter as sterile v’s, proton radius discrepancy. Search for Z’ via bremsstrahlung 4 muon final state CCFR: νμN®μ+μ-X Existing limits in region accessible to BaBar 0.1-10 GeV Trident: W. Altmannshofer etal., PRL. 113, 091801 (2014). A. Kamada and H. B. Yu, PRD 92, 113004 (2015). 4/15/2018 Richard Kass

Search for muonic DARK force @ BaBar Analysis Strategy > Use 514 fb-1 of data collected at Y(2S), Y(3S), Y(4S) > Select events with 4 charged tracks little/no extra energy (< 200MeV) > At least 2 same sign tracks passing muon criteria > |m4μ-ECM| ≤ 500 MeV > Veto Y(2S, 3S)-> ππ Y(1S), Y(1S)->μμ candidates > Kinematic fit with a beam energy constraint > Selection criteria optimized on 5% of the data, rest of analysis is blind Signal efficiency is function of mR, varies from 35% to 50% mR=(m2μ μ-4m2μ )½ blue line is a fit ρ ρ reflection > Signal region dominated by e+e-->4μ background Discrepancy due to no ISR in MC, PID efficiencies, track reconstruction. Discrepancy not important since looking for a peak over background 4/15/2018 Richard Kass

Results Search for muonic DARK force @ BaBar Calculate 90% CL Bayesian Upper limits for σ(e+e-->μ+μ-Z’, Z’->μ+μ-) Combine likelihoods from Y(2S), Y(3S), Y(4S) samples Includes systematic uncertainties: signal + bkg modeling, luminosity, etc Stringent upper limits on coupling constant ~ 7x10-4 near μ+μ- threshold Assume identical coupling strengths for muons, taus, corresponding neutrinos Can be interpreted as constraint on NP vector fields coupling to muons Rules out most of “g-2” favored region BaBar BaBar Published in Phys. Rev. D. 94,011102 2016 4/15/2018 Richard Kass

Search for invisible decay of a DARK photon (A’) DARK photon will decay to invisible (undetectable) states if lighter DS states exist At BaBar we search for e+e--> γA’, A’-> invisible by tagging the photon BaBar collected ~53fb-1 of data with dedicated single photon triggers mostly at Y(2S) and Y(3S) energies Signature is an event with missing energy and missing momentum BaBar is almost hermetic Select single-photon events, look for bump in missing mass, MX or Eγ Main backgrounds are from QED sources: e+e-->γγ, e+e-->e+e-γ where one or more of particles is outside acceptance 4/15/2018 Richard Kass

Analysis details: invisible decay of a DARK photon Divide data into two subsets by photon triggers: Low mass: -4 <MX2 <36 GeV2 High mass: 36 <MX2 <69 GeV2 Detected photon criteria: energy, angles in lab and CM Zero charged tracks with CM momentum above: 1 GeV for low mass 0.1 GeV for high mass Minimum extra energy in calorimeter Missing 4-vector Direction of missing vector Fiducial cuts Angular distance between missing 4-vector and EMC crystal edge Hadron calorimeter + muon veto Train BDT to discriminate signal/background in low/high mass regions Low mass: residual bkgd from e+e-->γγ limits sensitivity High mass: smooth background BaBar 4/15/2018 Richard Kass

Search for invisible decay of a DARK photon, A’ More Analysis Details Need to further reduce peaking contribution from e+e-->γγ near mX ~0 Define several signal regions in 2D plot, photon angle vs BDT output Tight Cut: reduce peaking contribution from e+e-->γγ near mX ~0 Loose Cut: optimize for observation (smooth bkdg) Background: pure background region Use 4 non-overlapping regions for each data set Low-mass+tight cut Low-mass+loose-cut and Not tight cut High mass+loose cut Background Total of 9 low-mass and 4 high mass data sets Signal efficiency varies as a function of mA’ lowest 0.2%, highest 3.8% Signal Search Extract number of signal events by a simultaneous ML fit to the independent regions for each data set (Y(2S),Y(3S), Y(4S)) 166 mass hypotheses For each fit: bkg shape is fixed, signal yield, peaking & contin. bkgs floated 4/15/2018 Richard Kass

Results: No significant signal Search for invisible decay of a DARK photon, A’ Results: No significant signal Large improvement over previous searches Rules out entire region preferred by (g-2)μ anomaly 90% CL on ε2 vs MA’ 90% CL on ε vs MA’ BaBar PRL 119, 131804 (2017) 4/15/2018 Richard Kass

Summary/Conclusions Lack of direct evidence of DM  Consider DARK force Older BaBar searches looking for DARK particles: Search for dark photon in e+e-→γ A‘, A'→ e+e-, μ+μ- PRL 113 (2014) 201801 Search for dark Higgs boson in e+e- → h'A' h' → AA‘ PRL 108 (2012) 211801 Search for dark boson in e+e- → γA'→W'W‘ arXiv:0908.2821 Today reported on latest results from two searches: Muonic DARK force Phys. Rev. D. 94,011102 2016 Invisible decay of a DARK photon (A’) PRL 119, 131804 (2017) No evidence for signal(s), report 90 % CL ULs in mass range < 10 GeV Ongoing BaBar efforts in this area: Search for DARK scalar (φ) Search for self-interacting DM: darkonuim  multi muon final states 4/15/2018 Richard Kass

Extra Slides 4/15/2018 Richard Kass

BaBar Data Set Took data 1999-2008 at PEP-II asymmetric e+e- collider: ~1.3x109 e+e- →cc, ~0.9x109 e+e- →τ+τ-, ~0.5x109 e+e- →BB, ~0.2x109 e+e- →bb CUSB 4/15/2018 Richard Kass

BaBar Detector Took data 1999-2008 at PEP-II asymmetric e+e- collider ~1.3x109 e+e- →cc, ~0.9x109 e+e- →τ+τ-, ~0.5x109 e+e- →BB, ~0.2x109 e+e- →bb 1.5 T Solenoid Electromagnetic Calorimeter (EMC) Detector of Internally Recflected Cherenkov Light (DIRC) Instrumented Flux Return (IFR) Silicon Vertex Tracker (SVT) Drift Chamber (DCH) e- (9 GeV) e+ (3.1 GeV) 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 Efficient trigger for DS searches NIM A479, 1 (2002) NIM A729, 615 (2013) 4/15/2018 Richard Kass

Search for muonic DARK force @ BaBar Analysis Strategy > Use 514 fb-1 of data collected at Y(2S), Y(3S), Y(4S) > Select events with 4 charged tracks little/no extra energy (< 200MeV) > At least 2 same sign tracks passing muon criteria > |m4μ-ECM| ≤ 500 MeV > Veto candidates with |m2μ-mY(1S)| ≤ 10 MeV to reject events from Y(2S), Y(3S) data sets, i.e. Y(2S, 3S)-> ππ Y(1S), Y(1S)->μμ > Kinematic fit with a beam energy constraint > Selection criteria optimized on 5% of the data, rest of analysis is blind Signal efficiency is function of mR, varies from 35% to 50% Y(4S) Signal region mR=(m2μ μ-4m2μ )½ blue line is a fit ρ ρ reflection > Signal region dominated by e+e-->4μ background Discrepancy due to no ISR in MC, PID efficiencies, track reconstruction. Discrepancy not important since looking for a peak over background 4/15/2018 Richard Kass

Search for muonic DARK force @ BaBar Separately calculate signal yields as function mZ’ for Y(2S, 3S, 4S) data sets Perform unbinned ML using reduced mass, mR=(m2μ μ-4m2μ )½ 2219 fits in range 0 <mR< 10 GeV, mass window ~50X MZ’ resolution for mR > 0.2 GeV mass range 0-0.3 GeV for mR < 0.2 GeV Signal shape from MC Exclude region around J/ψ σ(e+e-->μ+μ-Z’, Z’->μ+μ-)=Nsig /(Lεsig) εsig from MC corrected with data Z’ simulated with MadGraph + Pythia for several masses interpolate between masses Calculate significance SS=sgn(Nsig)(2lnL/L0)½ L=signal+background likelihood, L0=background only likelihood J/ψ->μ+μ- 4/15/2018 Richard Kass

Search for invisible decay of a DARK photon, A’ More Analysis Details Need to further reduce peaking contribution from e+e-->γγ near mX ~0 Define several signal regions in 2D plot, photon angle vs BDT output Tight Cut: reduce peaking contribution from e+e-->γγ near mX ~0 Loose Cut: optimize for observation (smooth bkdg) Background: pure background region Use 4 non-overlapping regions for each data set Low-mass+tight cut Low-mass+loose-cut and Not tight cut High mass+loose cut Background Total of 9 low-mass and 4 high mass data sets Signal efficiency varies as a function of mA’ lowest 0.2%, highest 3.8% BDT photon angle 4/15/2018 Richard Kass

Search for invisible decay of a DARK photon, A’ Look at data as a function of M2X Blue curve: Bkdg only fit Low mass region Y(3S) Loose-Not Tight Y(3S) Tight Cut Irreducible peaking bkdg from e+e-->γγ M2X M2X High mass region Y(3S) Loose Y(2S) Loose M2X M2X 4/15/2018 Richard Kass

Search for invisible decay of a DARK photon, A’ Signal Search Extract number of signal events by a simultaneous ML fit to the independent regions for each data set (Y(2S),Y(3S), Y(4S)) 166 mass hypotheses For each fit background shape is fixed, signal yield, peaking and continuum bkgs are floated Local signal significance Most significant fit, mA’=6.22 GeV M2X mA’ Local significance: 3.1σ Global significance: 2.6σ Global p-value ~ 1% 4/15/2018 Richard Kass

Previous BaBar DARK force results Search for dark photon in e+e-→γ A‘, A'→ e+e-, μ+μ- Search for dark Higgs boson in e+e- → h'A' h' → A' A' PRL 113 (2014) 201801 PRL 108 (2012) 211801 Search for dark boson in e+e- → γA' → W'W' arXiv:0908.2821 4/15/2018 Richard Kass