Search for Heavy Majorana Neutrinos

Transcription

1 Search for Heavy Majorana Neutrinos Workshop on Lepton Baryon Number Violation Madison, WI Anupama Atre Fermilab

2 Outline A Brief Introduction: What we know about neutrinos Simplest extension The Search for Majorana Neutrinos Laboratory searches/constraints ΔL = 2 processes : rare decays of tau and mesons Collider searches of like-sign di-leptons Summary

3 What we know Neutrinos are massive - oscillation experiments There are only three active light neutrinos N ν = ± 0.008, from Z pole at LEP-1. Direct lab bound: m β < 2.2 ev from Tritium beta decay Σ mν i < ev from WMAP, SDSS (Lyα spectra), SNIa. The absence of neutrinoless double beta decay bound on Majorana mass <m> ee < 1 ev

4 Absolute mass scale? What we don t know Hierarchy inverted or normal? δ m 2 a δ m 2 s m3 m2 m1 m2 m1 m3 δ m 2 s δ m 2 a Normal Inverted Dirac or Majorana?

5 Neutrino Masses : Dirac vs Majorana Simplest extension of the SM: Gauge invariant Yukawa interactions lead to Dirac neutrinos

6 If there are Majorana mass terms Then, the full neutrino mass terms read The diagonalized masses read Leads to Majorana neutrinos Where U is the MNSP matrix and typically

7 Cosmology Pulsar kicks, dark matter Structure formation, supernova, etc Experiment Laboratory, collider Precision, oscillation Testing Neutrino Physics Theory Seesaw, SUSY, ED, new ideas, Phenomenological considerations, etc

8 For practical purposes The charged currents: The neutral currents: U lm and Δm ν are from oscillation experiments V lm and m N are free parameters

9 Laboratory Constraints The search for heavy Majorana neutrinos depends on mass Thanks to s.pascoli for graphic

10 Laboratory Constraints Beta decay: Searches for kinks in electron beta decay spectra Sensitive for masses of 10 ev to 1 MeV Sensitive to mixing with electron neutrino Peak Searches: If heavy neutrino mixes with light ones it would modify the spectrum of leptons in meson decays For example, in pion and kaon decays a peak would appear

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12 AA, Han, Pascoli, Zhang 09

13 Laboratory Constraints Decays Heavy neutrinos that mix with active ones are produced in colliders, beams, etc ~ The heavy neutrinos decay ~ Look for SM decay products Example: These bounds less reliable than peak searches. If non-sm decays exist the bounds can be weakened or evaded. Present and future neutrino facilities can improve on these bounds due to large neutrino flux

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15 AA, Han, Pascoli, Zhang 09

16 Other constraints Precision electroweak tests ~ Fermi constant measured in muon decays, lepton universality, invisible Z decay width, etc Lepton Flavor Violation, muon-electron conversion, mu to 3e

17 Searches for Majorana Neutrinos The transition rates are proportional to: for light neutrino for resonant neutrino production generic diagram for heavy neutrino

18 * U Seljak et al., (2005), (2006), AA et al. (2005).

19 36 decay modes to look for lepton number violation We calculate ALL ΔL=2 modes via a heavy Majorana neutrino N 4 Compare with experimental bounds from direct searches Parameters for MC sampling Mass of neutrino m 4 : ~ resonant mass region ~ 100 MeV - 5 GeV Three mixings V e4, V µ4 and V τ4 :

20 Lepton Number Violating Tau Decays

21 Lepton Number Violating Meson Decays Real particle, propagates before decaying May exit the detector before decaying Limits weakened

22 Lepton Number Violating Meson Decays The branching ratios can be approximated as

23 Collider Searches of Heavy Neutrinos Explore signals at collider experiments via Rizzo; Heusch, Minkowski Constraint from neutrinoless double beta decay very strong At HERA At hadron colliders Flanz, Rodejohann, Zuber In top quark decays Keung, Senjanovic; Han, Zhang; AA, Han, Pascoli, Zhang Si, Wang; Bar-Shalom, Deshpande, Eilam, Jiang, Soni

24 Resonant Production Define a factorization: This is verified for σ 0 (m N < 3 TeV) narrow-width approximation valid AA, Han, Pascoli, Zhang

25 Consider A very clean channel : like-sign di-muons plus two jets no missing energies m(jj) = M W, m(jjµ) = m N Bare cross sections (scaled by Sµµ) AA, Han, Pascoli, Zhang (2009)

26 Tevatron Main backgrounds : After cuts : Rapidity : 2 and 1.5 muon rapidity acceptance at D0 and CDF AA, Han, Pascoli, Zhang 09

27 Current Sensitivity Tevatron Future Sensitivity AA, Han, Pascoli, Zhang 09

28 LHC Main backgrounds : After cuts : AA, Han, Pascoli, Zhang 09, A-Saavedra et al.

29 LHC Sensitivity AA, Han, Pascoli, Zhang 09, A-Saavedra et al.

30 Summary Important to test the Dirac or Majorana nature of neutrinos Need lepton number violating processes For light neutrinos 0νββ is the best bet For a heavy neutrino kinks in beta decay spectra, peaks in lepton energies in meson decays, decays in accelerator/collider experiments tau and rare meson decays sensitive to 140 MeV < m 4 < 5 GeV; 10-6(9) < V l 4 2 < 10-2 ; Tevatron searches sensitive to 10 GeV < m 4 < 180 GeV; 10-6 < V l4 2 < 10-2 ; LHC searches sensitive to 10 GeV < m 4 < 400 GeV; 10-7 < V l4 2 < 10-2 ;

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32 Supplementary Slides

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