SEARCHES FOR NEW PHYSICS AT BABAR

Transcription

1 SEARCHES FOR NEW PHYSICS AT BABAR Bertrand Echenard California Institute of Technology on behalf of the BABAR Collaboration p. 1

2 The BABAR experiment at SLAC PEP-II IR2 p. 2

3 The BABAR experiment at SLAC Γ1S,2S,3S~ kev Beam energy spread ~ 5 MeV BB threshold e+e- γ* ϒ(nS) CUSB BABAR data sample contains Γ4S~ 20 MeV ~470 x 106 ~120 x 106 ~100 x 106 ~ 18 x 106 ϒ(4S) ϒ(3S) ϒ(2S) ϒ(1S) (10x Belle) (10x CLEO) from ϒ(2S) π+π ϒ(1S) p. 3

4 BABAR publication statistics Four years after the end of data taking, BABAR is still very productive!!! So far, we have published almost 500 papers, and we expect to publish another ~80 analyses these next years. p. 4

5 Selected highlights from BABAR Searches for T violation and new sources of CP violation Observation of Time Reversal Violation in the B 0 meson system, PRL 109 (2012) Search for CP Violation in the Decay τ- π- Ks ( 0π0) ν, PRD 85 (2012) , PRD 85 (2012) Search for CP violation in the decays D± KsK±, D±s KsK± and D±s Ksπ±, preliminary. Searches for B decays with unique B factory signatures Evidence for an excess of B D(*) τν decays, PRL 109 (2012) Evidence of B τν decays with hadronic tags, submitted to PRD-RC, arxiv: B0 Decays to Invisible Final States and to ννγ, PRD 86 (2012) Branching fraction measurement of B ω l ν decays, submitted to PRD, arxiv: Searches for New physics Search for Dark Higgs boson, PRL 108 (2012) Search for Light Scalar Higgs Boson Decaying to Tau Pairs and Muon Pairs in Single-Photon decays of Y(1S), submitted to PRD-RC, arxiv: and arxiv: p. 5

6 Time reversal violation in B0 meson decays p. 6

7 Time reversal violation Time reversal is a discrete symmetry Exchanges in> and out> states, t -t. Time reversal violating processes A non-zero value of a T-odd observable in a stationary state, (e.g EDM) or a difference in the probability of a b from b a in an oscillation process at a given time t (e.g., νe νµ vs. νµ νe). - Not observed yet. Exchange in> and out> states in unstable systems. EDM - Tricky to prepare the initial state. The CP and T symmetries are connected via the CPT theorem Observe CP violation T violation (assuming CPT) Can we observe direct T violation? p. 7

8 Time reversal violation in B0B0 decays Exploit quantum entanglement of the B0B0 pair produced at the Y(4S) to overcome the problem of irreversibility Method described in J. Bernabeu et al. JHEP 1208 (2012) 064 Flavor eigenstates B0 / B0 CP eigenstates BCP+ / BCP- Tagging: - flavor eigenstate, e.g. the sign of a prompt lepton in B0 l+x; B0 l-x decays - CP eigenstate, reconstructing the final state J/ψ KL (CP+) or J/ψ KS (CP-) p. 8

9 Time reversal processes We can build 4 independent T comparisons (and 4 CP and 4 CPT) t TRV test imply comparison of - Opposite t sign - Opposite CP states - Opposite tag states B0 BCP+ (l-,j/ψ KL) 0 - B BCP- (l,j/ψ KS) T BCP+ B0 (J/ψ KS,l+) BCP- B0 (J/ψ KL,l+) B0 BCP+ (l+,j/ψ KL) BCP+ B0 (J/ψ KS,l-) B0 BCP- (l+,j/ψ KS) BCP- B0 (J/ψ KL,l-) p. 9

10 Time reversal violating parameters Measure time dependent decay rates α = B0, B0 β = J/ψ Ks, J/ψ KL τ = ± t > 0 Unbinned maximum likelihood fit to extract the 8 (S±,C±) values, including resolution, imperfect tagging and background. Combine these 8 (S±,C±) values to form a set of independent ( S±T,CP,CPT, C±T,CP,CPT) differences sensitive to T, CP and CPT violations t Any non-zero S/ C value corresponds to a symmetry violation p. 10

11 Independent T asymmetries PRL 109 (2012) Asymmetries of the 4 transitions studied BCP+ B0 B0 BCP- Projection of fit without TRV Projection of fit with TRV BABAR BCP- B0 B0 BCP+ p. 11

12 Results PRL 109 (2012) No TRV BABAR Time reversal violation with a significance > 10σ p. 12

13 CP and CPT violation PRL 109 (2012) CP violating parameters CPT violating parameters BABAR Clear evidence of CP violation BABAR and no evidence of CPT violation p. 13

14 Summary No TRV BABAR Observe time reversal violation with a significance > 10σ And many other citations in scientific / popular journals p. 14

15 B D(*)τν and B τν p. 15

16 Motivation Tree-level semileptonic decay mediated by a W± τ mode sensitive to New Physics contributions, e.g. charged Higgs boson H± in type-ii Two Higgs Doublet Model (2HDM) Reduce sensitivity to hadronic form factor and Vcb by measuring the ratio signal l=e,µ normalization Previous measurements from B-factories Above Standard Model predictions, but limited significance New result from BABAR Based on full dataset (2x more statistics than previous result1) ) Improved B reconstruction Signal yield increased by more than a factor 3 Main challenge: many neutrinos in the final state!!! 1) Z. Phys. C46, 93 (1990), PRD 78, (2008), PRD 85, (2012) p. 16

17 Event selection Tagged analysis exclusive reconstruction of one B meson, the B tag The beam-energy substituted mass mes = ( (E*beam )2 - (p*tag)2)1/2 - peaks at the B mass for signal with a resolution ~ 2.5 MeV The energy difference E = E*tag E*beam - peaks at zero for signal with a resolution of ~ 18 MeV Example of mes distribution The rest of the event is assigned to the signal B Must be compatible with B D(*) τ ντ / B D(*) l νl hypothesis Reconstruct D/D* candidate Identify leptonic decays of τ lepton (final states have identical content except neutrinos) BABAR No additional charged particles and no significant extra neutral energy Extract signal by a 2D unbinned maximum likelihood fit to Invariant mass of undetected particles and lepton momentum in Bsig rest frame Simultaneous fit with B D(*) π0 lν to constrain the contribution of B D** τν + B D** lν p. 17

18 Results for B D* τ ν PRL 109 (2012) B D* π0 lν BABAR D**lν Free in fit Nsig Fixed Significance R(D*) D*0τν D*+τν D* τν 639 ± ± ± ± ± ± Only statistical uncertainties p. 18

19 Results for B D τ ν PRL 109 (2012) B D π0 lν BABAR D**lν fb o n o i t serva b o σ 5 First Free in fit Nsig Fixed Significance R(D*) D τν D0τν D+τν D τν 314 ± ± ± ± ± ± Only statistical uncertainties p. 19

20 Comparison with previous measurements and SM predictions PRL 109 (2012) Consistent with previous measurements Average does not include this result R(D) R(D*) The SM prediction is excluded at 3.4σ [*] J.F. Kaminik and F. Mescia, Phys. Rev. D 78, (2008) S. Fajfer et al., PRD 85, (2012). p. 20

21 Implication for 2HDM type-ii PRL 109 (2012) Constraints on the value tanβ /mh in 2HDM models to explain the observations BABAR 2HDM type-ii Constraints on tanβ / mh plane SM 2HDM type-ii excluded at 99.8% CL on the whole range for H+ mass > ~10 GeV, and excluded below 10 GeV by B Xs γ measurements p. 21

22 Connection with B τν decays arxiv: B τν is also a tree level decay mediated by a W±, sensitive to New Physics contributions such as H±. Analysis similar to that of B D τ ν Reconstruct Btag, assign rest of events to Bsig Reconstruct leptonic and hadronic tau decays Extract signal as UML fit to the extra neutral energy of the event All channels combined Signal Bkg Combine result with statistically independent measurement in semileptonic tagging (PRD 81 (2010) ) B (B+ τ+ν) = (1.79 ± 0.48) x 10-4 (stat+syst) Recent result from Belle (arxiv: ) BABAR prel. B (B+ τ+ν) = ( ± 0.11 ) x 10-4 Tension between BABAR / Belle p. 22

23 Connection with B τν decays arxiv: BABAR result x10-4 B (B+ τ+ν) = 1.79 ± 0.48 SM Model predictions x10-4 (depends on Vub) BSM (B+ τ+ν) = 0.62 ± 0.12 BSM (B+ τ+ν) = 1.18 ± 0.16 Vub = 3.13 ± 0.14 ± 0.27 (Vub from exclusive measurement1)) Vub = 4.33 ± 0.24 ± 0.15 (Vub from inclusive measurement2)) 2HDM type-ii predictions BABAR Constraints on tanβ / mh plane BABAR prel. 2HDM Vub low 2HDM Vub high BABAR prel. Tension with the SM (1.6σ / 2.4σ) and constraints on the 2HDM type-ii Preferred values of tanβ / mh different from preferred by B D(*) τν Belle measurement in agreement with the SM predictions 1) M.F. Sevilla, PoS (EPS-HEP2011)155 (2011) 2) PRD 86, (2012) p. 23

24 Search for light CP-odd Higgs boson p. 24

25 Search for light CP-odd Higgs boson Dermisek et al., PRD 81, (2010) Light Higgs boson tanβ=10, µ=150 GeV, M1,2,3 =100,200,300 GeV Many SM extensions, such as the NMSSM, NMSSM proposed to solve the µ problem, adding one CP-odd Higgs, one CP-even Higgs and one neutralino to MSSM content A light CP-odd Higgs A0 with mass lower than 2mb is not excluded by LEP constraints BF(Υ γ A0) x BF(A0 µµ) include the possibility of a light Higgs boson Radiative decays ϒ(nS) γ A0 (n=1,2,3) offer an ideal environment to search for light Higgs: - Fully reconstructed in A0 µ+µ- Partially reconstructed in A0 τ+τ-,qq - Invisible decay A0 χ1 χ1 if ma0 > 2mχ Can have a very large branching fraction A0 = cosθa AMSSM + sinθa AS 0 < ma0 < 2mτ 2mτ < ma0 < 7.5 GeV 7.5 < ma0 < 8.8 GeV 8.8 < ma0 < 9.2 GeV p. 25

26 Search for light CP-odd Higgs boson Dermisek et al., PRD 81, (2010) Branching fractions Light Higgs boson µµ Many SM extensions, such as the NMSSM, include the possibility of a light Higgs boson NMSSM proposed to solve the µ problem, adding one CP-odd Higgs, one CP-even Higgs and one neutralino to MSSM content A light CP-odd Higgs A with mass lower than 2mb 0 ττ is not excluded by LEP constraints Radiative decays ϒ(nS) γ A0 (n=1,2,3) offer an ideal environment to search for light Higgs: - Fully reconstructed in A0 µ+µ- Partially reconstructed in A0 τ+τ-,qq - Invisible decay A0 χ1 χ1 if ma0 > 2mχ Can have a very large branching fraction gg p. 26

27 Search for A0 in ϒ(1S) γ A0, A0 µ+µsearch for a light CP-odd Higgs in ϒ(2S,3S) π+π- ϒ(1S), ϒ(1S) γ A0, A0 µ+µanalysis highlights Tag the Y(1S) from the dipion transition Require the γµµ system to be compatible with a Y(1S) meson Fit for a narrow peak to the reduced dimuon mass mred = (m2a0-4m2µ)1/2 mred distributions fit at mred = 5.93 GeV Y(2S) BABAR prel. Y(2S) BABAR prel. Y(3S) p. 27

28 Search for A0 in ϒ(1S) γ A0, A0 µ+µarxiv: BABAR prel. Y(2S) Y(2S) Y(2S) Y(3S) Y(2S,3S) Y(3S) Y(3S) BABAR prel. fy: effective b-quark - A0 Yukawa coupling No significant signal, improved limits in the low mass range p. 28

29 Search for A0 in ϒ(1S) γ A0, A0 τ+τarxiv: Search for a light CP-odd Higgs in ϒ(2S,3S) π+π- ϒ(1S), ϒ(1S) γ A0, A0 τ+τanalysis highlights Tag the Y(1S) from the dipion transition Use 1-prong decay of tau (τ µνν, τ eνν, τ πνν) Fit for a narrow peak to the photon recoil mass in the Y(1S) system BABAR prel. BABAR prel. ma=8.93 GeV, Local significance = 3.0σ Global significance = 1.4σ p. 29

30 Search for light CP-odd Higgs - results A0 µ+µ- A0 τ+τ- A0 µ+µ- CMS (PRL 109 (2012) ) χb region excluded tanβ=10, µ=150 GeV, M1,2,3 =100,200,300 GeV 0 < ma0 < 2mτ Upper limits 2mτ < ma0 < 7.5 GeV (90% CL) 7.5 < ma0 < 8.8 GeV 8.8 < ma0 < 9.2 GeV ϒ(2,3S) γ A0; A0 µ+µprl 103, (2009) ϒ(1S) γ A0; A0 invisible PRL 107, (2011) ϒ(3S) γ A0; A0 τ+τprl 103, (2009) ϒ(2,3S) γ A0; A0 hadrons PRL 107, (2011) Substantial fraction of the parameter space is excluded p. 30

31 Search for dark forces p. 31

32 Dark sector in a nutshell Models introducing a new dark force mediated by a new gauge boson with a mass around a GeV have been proposed to explain the observations of PAMELA, FERMI, DAMA/LIBRA, CREST,... Wimp-like TeV-scale dark matter particles can annihilate into pairs of dark bosons, which subsequently decay to lepton pairs (protons are kinematically forbidden). Other explanations of these anomalies have been proposed, but the possibility of a hidden MeV/GeVscale sector is poorly constrained and worth exploring. FERMI DAMA PAMELA FERMI Collab., PRL 108, (2012), arxiv: FERMI HAZE DAMA Collab., Eur. Phys. J. C (2010) 67: E. Mocchiutti, International workshop on positrons in Astrophysics p. 32

33 Dark sector and dark forces J.D. Bjorken et al., PRD 80 (2009) New dark sector with a U(1)D gauge group Constraints on α'/α = ε2 New gauge boson: dark photon A', which could have O(GeV) mass Interaction with the SM is via kinetic mixing with a mixing strength ε. The dark photon couples to fermions with a 2 charge εe, characterized by α' = α ε α'/α = ε2 ε Fµν Bµν From Y(2S,3S) γµ+µ- A dark photon can be readily produced in e+e- γ A', A' ff ma' (GeV) The limits on e+e- Y(2S,3S) γµ+µ- can be reinterpreted as limits on dark photon production (full dataset is currently analyzed) Excluded region Planned experiments g-2 favored region p. 33

34 Dark higgs boson B. Batell et al., PRD 79 (2009) R. Essig et al., PRD 80 (2009) Dark photon mass is generated via the Higgs mechanism, adding a dark Higgs boson (h') to the theory. A minimal scenario has a single dark photon and a single dark Higgs boson. The dark Higgs mass could be at the GeV scale The Higgs-strahlung process Higgs decay topology mh < m A off-shell boson / 2 body loop induced decays e+e- A'* h' A', h' A' A' is very interesting, as it is only suppressed by ε2 and is expected to have a very small background. Also sensitive to the dark sector gauge coupling gd. ε σ ε2 αd γ αd MH (GeV) prompt decay h' Analyze prompt decays A'* A' Look for three resonances with similar masses αd = g / 4π gd is the dark sector gauge coupling 2 D prompt decay ma = 2 GeV displaced / invisible decay 2 mh > 2mA p. 34

35 Dark Higgs boson - analysis PRL 108 (2012) Six events are selected from the full BABAR dataset (~500 fb-1) Signal candidates Three entries for each event, corresponding to the three possible assignments of the h A'A' decay BABAR Estimate background from - wrong-sign combinations, e.g. e+e- (e+e+) (e-e-) (µ+µ-) - sidebands from final sample - rate for 6 leptons ~ 100x rate for 4π+2l above 1.5 GeV Wrong-sign combinations BABAR Likely e+e- e+e- ρρ,ωω or e+e- 6π No events with 6 leptons, consistent with the pure background hypothesis p. 35

36 Dark Higgs boson - results 90% CL upper limit on αdε PRL 108 (2012) BABAR Limit on ε2=α'/α assuming αd=αem for various Higgs mass 90% CL upper limit on αdε2 BABAR excluded mh' = 3 GeV mh' = 5 GeV mh' = 7 GeV Substantial improvement over existing limits for mh' < 5-7 GeV if light dark Higgs boson exists p. 36

37 Conclusion BABAR is still producing many interesting results, and more are to come. See you soon... p. 37

38 Extra material p. 38

39 Time reversal violation T CP CPT sin(2β) = ± (HFAG Winter'12) p. 39

40 Search for invisible B decays p. 40

41 Motivation Invisible B decays In the SM, the B0 νν decay is helicity suppressed ~ (mν/mb)2 BF(B0 ννγ) ~ Can be enhanced by several order of magnitude in NP scenarios PRD 65 (2002) Only existing limits are from BABAR1) using 82 fb-1 of data BF(B0 νν) < BF(B0 ννγ) < 4.7 x 10-5 More data and improved reconstruction, calibration,... 1) PRL 93 (2004) p. 41

42 Signal reconstruction Exploit clean e+e- environment Identify one B through its semileptonic decay (B D l ν) Combine variables in a neural network to identify the signal B (variables describing the global event, the tag B, the missing mass of the event) NN output BABAR p. 42

43 Results PRD 86 (2012) Extract signal by a fit to the extra neutral energy deposited in the calorimeter (signal peaks near zero) Fit functions from MC Use D sidebands to assess systematic uncertainty on background shapes Bayesian upper 90% CL p. 43