Searches for heavy long-lived particles at LHCb

Transcription

1 Searches for heavy long-lived particles at LHCb Marcin Chrząszcz SUSY 2015, Tahoe City, August, / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 1/20 20

2 Why long-lived particles? We all know here that the SM is incomplete Unfortunately we do no know what is the scale of NP NP still can come from the Higgs sector not all properties are yet constrained There is a long list of theoretical models that predict the existence of new particles that couple to the SM sector by mixing with the Higgs Inflaton, axion-like, dark matter mediator models also predict the new boson to be light SUSY models also can have stable long living particles like q, l 2 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 2/20 20

3 LHCb detector - tracking Excellent Impact Parameter (IP) resolution (20 µm) Identify secondary vertices from heavy flavour decays Proper time resolution 40 fs Good separation of primary and secondary vertices Excellent momentum (δp/p 04 06%) and inv mass resolution Low combinatorial background 3 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 3/20 20

4 LHCb detector - particle identification Excellent Muon identification ϵ µ µ 97%, ϵ π µ 1 3% Good K π separation via RICH detectors, ϵ K K 95%, ϵ π K 5% Reject peaking backgrounds High trigger efficiencies, low momentum thresholds Muons: p T > 176GeV at L0, p T > 10GeV at HLT1, B J/ψX: Trigger 90% 3 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 3/20 20

5 Data taken by LHCb In 2011 and 2012 LHCb has gathered 3 fb 1 of pp collisions 4 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 4/20 20

6 B K χ(µµ) search Search for displaced di-muon vertex coming form B meson B 0 K χ(µ µ + ) If χ mixes with the Higgs and it is light: Γ(K πχ) m 4 t λ 5 Γ(D πχ) m 4 b λ5 Γ(B Kχ) m 4 t λ 2 In addition; K K + π helps in vertex reconstruction High B(χ µ µ + ) 5 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 5/20 20

7 B K χ(µµ) motivation Discussed models: 1 Inflaton: PhysLett B736 (2014) 494 τ χ = s m χ O(1 GeV) B(B Kχ) 10 6 effective couplings to SM particles: m g f Y v EW, g Y = sin θ 2 Axion portal: PhysRevD81:034001,2010 Prompt decay Large allowed masses Axion decay constant: f χ 1 3 TeV Coupling m f f χ All those particles have width much smaller than resolution of LHCb detector 6 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 6/20 20

8 Signal properties Depending on the coupling of the hidden sector we can identify two lifetime regimes: Long lifetime (> 02 ps) Inflaton JHEP 1005:010 Displaced vertex Almost background free Lower reconstruction efficiency Short lifetime ( 02 ps) Dark matter mediator Phys Lett B727 Axion PhysRevD81 Prompt decay Contaminated via SM decay 7 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 7/20 20

9 Selection Trigger on muons Multivariate selection: µbdt JINST 8(2013) µbdt ensures flat efficiency in lifetime of χ Optimized on Punzi figure-of-merit: S P a = B, with S and B are signal and background yields Factorize lifetime into two components: L = L prompt L displaced Prompt: τ < 3σ τ SM background of B 0 K µ µ + Displeased: τ > 3σ τ Almost background free 8 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 8/20 20

10 Search strategy B 0 mass constrained Di-muon mass resolution σ m = 1 7 MeV Scan m test in steps of 05 σ m Wide resonances can t affect the search Narrows resonances we veto Calculations performed in each m test window 9 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 9/20 20

11 Results Grey regions correspond to vetoed regions where narrow resonances are expected Largest deviation seen in m χ = 253 MeV Not statistically significant: local p-value = 02 LHCb-PAPER in preparation 10 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 10/20 20

12 Branching fraction exclusion limit No deviations from background only hypothesis is observed We set a 95% CL upper limit as function of mass and lifetime of the new particle (in the LHCb accessible range) Lower lifetimes have better limit due to higher reconstruction efficiency 11 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 11/20 20

13 Benchmark models Interpretation of the results in two specific models: 12 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 12/20 20

14 Long living charged particles like τ Long living particles can also be produced in the PV This kind of particles would be produce in relatively low velocities and could be identified by their time -of-flight, de/dx or in Cherenkov detectors LHCb performed a search for long living τ particles τ + τ produced by Drell-Yan process 13 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 13/20 20

15 τ analysis strategy Search performed τ in mass range of GeV After the loose preselection to reduce normal Drell-Yan production After the preselection an Neural Net is trained based on Cherenkov detectors to calculate to further suppress the remaining background 14 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 14/20 20

16 τ results No significant signal yield has been observed 95 % upper limit has been set 15 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 15/20 20

17 Hidden valley searches A possible extensions of the SM are models where the new particles have a small couplings to the SM particles Such models are: Lightest SUSY B/LNV Gravity mediated SUSY Hidden Valleys LHCb have performed a search for π ν particles that are pair produced from Higgs like SM particle They have a long lifetime and decay to pair of jets 16 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 16/20 20

18 Analysis strategy Efficient trigger for long living particles Reconstruction of two jets MVA used for vertex search Search performed in different regions of displaced vertexes (R xy ) 04 < R xy < 4 mm, removes heavy flavour and material interaction backgrounds Candidates / (1 GeV/c 2 ) Data LHCb m = 25 GeV/c 2 π v m = 35 GeV/c 2 π v = 50 GeV/c 2 m π v mass [GeV/c 2 ] p [GeV/c] T 17 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 17/20 20 Candidates / (1 GeV/c) Data LHCb m = 25 GeV/c 2 π v m = 35 GeV/c 2 π v = 50 GeV/c 2 m π v

19 Di-jet distribution Signal component fit result, Background component 18 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 18/20 20

20 Results σ(h) B (H πvπv) [pb] m πv = 25 GeV/c 2 m πv = 35 GeV/c 2 m πv = 43 GeV/c 2 m πv = 50 GeV/c 2 m πv m πv = 35 GeV/c 2, π v c c = 35 GeV/c 2, π v s s LHCb lifetime [ps] 19 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 19/20 20

21 Conclusion A search for a dark boson in the decay channel B 0 K µ µ + has been presented No deviations from SM observed Results are the most constraining exclusion limit on the process LHCb is suited for search for long lived particles Stay tuned, more searches like this are on they way 20 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 20/20 20

22 Backup 21 / Marcin Chrząszcz (Universität Zürich) Searches for heavy long-lived particles at LHCb 21/20 20