Thomas Czank. September 12 th, 2017

Transcription

1 Z search at 017 秋季日本物理学会 Thomas Czank Tohoku University September 1 th, 017

2 Outline

3 Muonic Dark Photon Assuming MSM ( Minimal SM) with the L µ L τ model Xiao-Gang He, G. C. Joshi, H. Lew, and R. R. Volkas Phys.Rev.D 44, 118 Published 1 October 1991 L Z = g 1 Z µ 1 (ēγµe µγµµ) L Z = g Z µ (ēγµe τγµτ) 1 γ l µ µ + γ Z l γ muon coupling term tau coupling term not the usual Dark Photon model A model motivated by the g µ problem Z as cause for g µ excess may be rejected by BELLE analysis following results from NA64 3 / 16

4 Z Branching Ratios Model courtesy from Brian Shuve ν l ν l might allow for sterile neutrino probes 4 / 16

5 BABAR Leptophilic Dark Gauge Search Search for a muonic dark force at BABAR Phys.Rev.D e + e µ + µ Z µ + µ µ + µ search Z, µ coupled dark gauge L µ L τ model 5 / 16

6 Babar result for e + e µ + µ Z Highest significance at m Z = 0.79GeV, 4.9σ, locally 1.6σ globally no signal Belle larger data sample and better sensitivity could get a better result on the same channel analysis 6 / 16

7 Belle Z Search - MC production parameters and cuts e + + e Z µ + µ µ + µ µ + µ 000 events using MadGraph GeV GeV Dark Photon generation mass range Z defined as oppositely charged promptly decayed µ ± pair, while two other charged tracks are another µ ± pair also promptly decayed from initial interaction Background is e + e µ + µ µ + µ 4 charged tracks requirement which reduces the yield by 61% assuming loose muon inclusion, but nullifies all most of the non leptonic background Kinematic fitter used to guarantee mass, momentum and energy conservation improved resolution of Z invariant mass, constraining the initial state with the final state 7 / 16

8 µ id likelihood distribution µ id likelihood distribution entries mu_id_distro Entries Mean RMS Likelihood Looking at the peak over 1, it is clear that most of the signal comes from tight muons, but the signal could be boosted by adding the lower likelihood ones as well 8 / 16

9 Comparison for the Z inv mass with and without the use of the Kinematic Fitter m_z counts Kfitter no Kfitter h_mapall Entries 5900 Mean 4.07 Std Dev χ / ndf / 73 gaus_height ± 3.3 mean ± std ± x e+04 ± 7.577e+00 x e+04 ± 1.9e+00 x 4846 ± 0.5 x3 11 ± 0.1 h_mzpall Entries 7966 Mean 4 Std Dev χ / ndf 16.8 / 93 gaus_height ± 5.5 mean 4 ± 0.0 std ± x e+04 ± 4.668e+00 x e+04 ± 1.188e+00 x 4844 ± 0.3 x3 113 ± Z inv. mass[gev/c^] the Kfitter σ = while the no Kfitter σ= / 16

10 4µ final state background distribution with 0.5GeV signal counts m_z for kintest MC sig m = 0.5GeV Z µ µ µ µ background h_mzpkintest_0 Entries 6735 Mean 4.13 Std Dev Z inv. mass[gev/c ] This background is scaled by 1 3 due to its luminosity compared to Belle s. / 16

11 Z inv mass fit to extract signal efficiency Evaluation of inclusion of different amounts of loose muons, from left to right, top to bottom: 0,1,,3 counts m_z for kintest h_mzpkintest_0 Entries 6735 Mean Std Dev χ / ndf 767. / 319 gaus_height ± 7.9 mean 0.5 ± 0.0 std ± x ± x ± x ± x ± h_mzpkintest_1 Entries Mean Std Dev χ / ndf 3761 / 653 gaus_height ± 11.9 mean ± std ± x ± x ± x 0.53 ± x ± counts m_z for kintest Z inv. mass[gev/c ] Z inv. mass[gev/c ] counts m_z for kintest h_mzpkintest_ Entries 670 Mean Std Dev χ / ndf 5637 / 7154 gaus_height ± 14.0 mean ± std ± x0.43 ± x ± x ± x ± counts m_z for kintest h_mzpkintest_3 Entries Mean Std Dev χ / ndf / 50 p ± h_mzpkintest_3 Entries 7170 Mean 0.5 Std Dev χ / ndf 5875 / 734 gaus_height ± 14. mean ± std ± x0.48 ± x ± x ± x ± Z inv. mass[gev/c ] Z inv. mass[gev/c ] From top left (up to 0 loose muons) to bottom right ( up to 3 loose muons), the yield increases from 6735, 19350, 670 to 7170 events 11 / 16

12 figure of merit with B only figure of merit with S+B figure of merit following ponzi model FOM for different loose muons fom foma ponzi fom Entries 4 Entries 4 Entries Mean Mean Mean Std Dev Std Dev Std Dev # of loose muons # of loose muons # of loose muons The Figure of Merit is highest for up to 3 loose muon inclusion in the signal 1 / 16

13 Visible Cross Section Evaluation σ visible = N obs N bkg L B ɛ where: N obs is the number of observed events σ visible is the visible cross section L is the luminosity B is the Branching Ratio ɛ is the detection efficiency Find the maximum coupling at a particular mass for which the data does not allow a distinction between Z or no Z, null hypothesis N obs = N bkg Assuming null hypothesis is a good description of the data 13 / 16

14 Theoretical cross section and 90% CL on mc Z visible cross section up to GeV by Mass 1 for g' = 0.1 [pb] 4 [ab] σ vis σ th m [GeV/c Z' ] e + e µ + µ Z, Z µ + µ m [GeV/c Z' ] Mc background e + e µ + µ µ + µ 14 / 16

15 Mc g coupling with BaBar 016 up to GeV 3 g' m [GeV/c Z' ] Still no obvious reason why Belle results should be worse than BaBar for masses higher than 4.5GeV. 15 / 16

16 There are still ways to reduce the background, KLM acceptance angle cut, dimuon pairs difference cut, any other suggestions? Other sources of background to be included, ππππ, KK, and others Not clear why g for higher energies worse than BaBar s No analytical cross section yet 16 / 16