Neutrino properties deduced from the study of Lepton Number Violating processes

1. Neutrino properties deduced from the study of Lepton Number Violating processes at low and high energies Sabin Stoica FHH & IFIN-HH

2. Outline Introduction Neutrino properties and connection with the LNV processes LNV processes at low-energy: 0νββ decay - general presentation and current challenges - future prospects LNV processes at high-energies - general presentation and motivation of searching - signatures at colliders; specific channels to be studied at LHC - prospects: reconstruction of LNV channels a possible attractive direction of investigation at LHC Conclusions

3. Introduction There is a great interest nowadays in the study of neutrino properties. This is, on the one hand, related to the fact that fundamental properties of the neutrinos like their absolute mass, their character (are they Dirac or Majorana particles?), their mass hierarchy, the number of neutrino flavors, etc., are still unknown. The knowledge of these properties are of great importance, since neutrinos are very abundant in nature and play a key role in nuclear and particle physics, astrophysics and cosmology. On the other hand, the results of the neutrino oscillation experiments have convincingly showed that neutrinos have mass, oscillate, and mix, in contradiction with the Standard Model (SM) assumptions. Thus, the first clear evidence of beyond SM physics comes from neutrino studies, and further investigations are very needed to complete our understanding on the neutrino properties. In this context there is an increased interest in the study of the LNV processes, since they are capable to decide on the above mentioned neutrino properties. Since recently, the neutrinoless double beta decay was considered the only process able to distinguish between Dirac or Majorana neutrinos and to give a hint on the absolute mass of the electron neutrino. At present, the increased luminosity of the LHC experiments makes feasible the search for LNV processes at LHC as well. Besides the neutrino character, these studies can shed light on the existence of sterile neutrinos as well. In my talk, I’ll present briefly the current status of research on neutrino properties and I’ll show how our understanding on these properties can be improved through the study of LNV processes. Specifically, I’ll review the results and the prospects in neutrinoless double-beta decay searches and I’ll present the first attempts of analyzing LNV processes in hadron collider experiments, particularly at LHC. The study of LNV processes may open an interesting direction of investigation at CERN.

4. Neutrino properties The neutrino has half-integer spin (1/2) and is therefore a fermion. It is an electrically neutral lepton, interacting only through the weak force and gravity. Because the cross section in weak nuclear interactions is very small, neutrinos can pass through matter almost unhindered. Detection of neutrinos is therefore challenging, requiring large detection volumes or high intensity artificial neutrino beams.All neutrinos observed to date have left-handed helicity. SM neutrinos: m=0; neutrino ≠ antineutrino (Dirac) Recent results: neutrinos have mass, oscillate from one flavor to another and mix

5. Neutrino oscillations Neutrinos can transform from one flavor into another (neutrino oscillations) The three neutrino states that interact with the charged leptons in weak interactions are each a different superposition of the three neutrino states of definite mass. Idea of neutrino oscillation: 1957 by Bruno Pontecorvo Theory of  flavor oscillations: Maki, Nakagawa, and Sakata in 1962; Pontecorvo 1967. |> = Σi Ui|i> ; |i> = Σ U*i|>;  = e, , ; i = 1, 2, 3; |> = a neutrino with definite flavor; |i> = a neutrino with definite mass mi. UI = MNK matrix

6. Neutrino mixing matrix

7. Neutrino properties -present status Hierarchical  mass schemes • What we know • Neutrinos oscillate, have a mass and mix • Squared mass differences between eigenstates • Mixing angles ( θ12, θ23, θ13 (?) ) • Δm212 = Δm2sol~ 7.58 x 10-5 eV2; • tan2θ12~0.484 → θ12~350 : Solar  experiments • + KamLAND(r) • |Δm213|= |Δm232| =Δm2atm~ 2.40 x10-3 eV2; • sin2 2θ23~1.02 →θ23~450 : Atmospheric  experiments + K2K (r) + MINOS (acc) • sin2 2θ13 ~ 0.092; θ13 ~ 90 : • Daya Bay experiment (r) normal inverted • What we still do not know • Absolute mass scale • Mass hierarchy • Majorana vs. Dirac • Sterile neutrino(s)?

8. Current challenges in neutrino physics • Absolute neutrino masses • Mass hierarchy • Precision measurements of Δm2 and • mixing angles: bi-maximal or tri-maximal mixing? • CP –violation • Mechanism of neutrino masses • Searches for other neutrino flavors

9. Motivation for searching of LNV processes • LNC: although is experimentally verified to a very high precision, it is not a consequence of a known gauge symmetry In SM L is conserved • LNV: was invoked in connection with the neutrino properties; particularly, with neutrinoless double-beta decay mode (A,Z)  (A, Z+2) + 2 e-0 Check: absolute mv ν Dirac or Majorana particle (?)

10. Double  decay is a nuclear natural decay by which a e-e nucleus transforms into another e-e nucleus with its nuclear charge changed by two units. It occurs whatever single  decay can not occur due to energetical reasons or it is highly forbidden by angular momentum selection rules (a) and (d) are stable against  decay, but unstable against   decay: - - for (a) and ++ for (d) There are 35 --isotopes in nature

11. Double  decay modes

12. One can distinguish the two modes by measuring the sum electron energy

13. Challenging issues in double beta decay • 1) TH. : extraction of the information regarding the ν mass scale from observation. This is not trivial, since one needs an accurate calculation of the NMEs involved and the determination of the mechanisms responsible for the occurrence of 0νββ decay • 2) Exp.: observation of 0νββ decay: improvements of experimental set-ups and techniques  large isotopically enriched sources; the reducing of background; detectors with high energy resolution, improved techniques of detection, etc.

14. Challenging issues in double beta decay light Majorana neutrino exchange and SM RH interactions • Calculation of the nuclear matrix elements • Evaluation of the transition ME: construction of eff. SR transition operators • Construction of the many-body w.f.:one needs the w.f. of the initial and final states (if the closure approx. is not adopted) • Several methods: • a) pnQRPA (different versions) • b) large basis interacting Shell model (ISM) • c) IBM • d) Energy density functional method • e) Projected HFB

15. ISM: considers only a limited no. of orbits close to the Fermi level, but all the possible correlations; pp, nn and pn pairing correlations are treated exactly with the conservation of the p and n numbers. The eff. Interactions are constructed from the monopole corrected G matrices which are further adjusted to describe sets of exp. levels. The progress of computers and of ShM codes allows at present to manage valence spaces of size as large as ~ 1011. pnQRPA: large valence spaces but a limited no. of correlations. S.p. states are usually constructucted from a Wood-Saxon potential; one is possible to include to each orbit of the QRPA model, also the spin-orbit partner which guarantees that the ISM is fulfilled; this is an important advantage, since the GT transitions are in this way, correctly described. IBM: the low lying states are described in terms of s (L=0) and d (L=2) bosons. Thus one is restricted to 0+ and 2+ neutron pairs transfering into two protons. The bosons interact through 1- and 2-body forces giving rise to bosonic wave functions. PHFB:w.f. of good N&J are projected from axially symmetric intrinsic HFB states. H is restricted to a quadrupole interaction; with a real B transformation, without parity mixing, one can describe only neutron pairs with even J and positive parity. EDF: is considered as an improvement of PHFB: the density functional methods based on Gony functional is considered. N&J projections for mother and daughter nuclei are performed and configuration mixing within the generating coordinate method is included. A large s.p. basis (11 major oscillator shells) is considered and the results are for all nuclei of interest.

16. Differences between the methods come from: • - different ways of using the mean field s.p. occupancies of individual orbitals differ significantly from each-other • the residual interactions are of various origin and renormalized in different ways • - the model spaces differ in size and structure • - different many-body approximations are used in the diagonalization of the H • Ingredients of calculation of the NME • the closure approximation:energies of the intermediate states (En-Ei) are replaced by an average value E ~ 10MeV simplifies much the calculations and the error is ≤ 5% • tensor contribution to the eff. transition operator: ~ 5% for the mechanisms involving only light Majorana  exchange • higher order terms of nucleon currents: ~ 20-30% contribution • FNS and SRC effects: both reduce the magnitude of NME. • FNS: form factors are introduced to take into account the finite size of nucleons. SRC: the relative two-nucleons is modified: Ψnl= [1 + fc] Ψnl ; fc = -ce-ar(1-br2) is parametrized in different ways: Miller-Spencer, UCOM, CCM (differences ~ (20-30)%) • deformation: induces the suppression of the NME • quenching of the gA: (1.269  1.0) to account for the differences between the experimental and the calculated GT strengths.

17. Observations: • - NME calculated with ISM are systematically smaller than calculated with other methods • differences between different calculations ~ a factor of 2 • the disagreement between different NME diminishes due to the th. effort made over the last years • an accurate determination with uncertainty of less than 30% is needed to establish the •  mass spectrum and CP violating phases of the  mixing

18. A measurement of the 0νββ decay rate combined with neutrino oscillation data and a reliable calculation of the NMEs, would yield insight into all three neutrino mass eigenstates. Based on the present data one can extract limits for the neutrino mass scale. NH: IH: NH IH:

19. Allowed range of values for |<m>| as a function of the lowest mass eigenstate m0 using the three standard neutrinos for the NH and IH cases. Obs. In the IH there is a bound which means that in that scenarios 0νββ decay should definitively be observed if the experiments will reach the required sensitivity

20. DBD Experiments

21. 0ββprovides a broader potential to search for beyond SM physics: any ∆L=2 process can contribute to 0 • Existence of RH components in the weak interaction (λ, η) ~ 10-5 • L-R theories: 0 mediated by heavy RH bosons. Absence of 0 • provides a lower mass limit for these bosons • SUSY theories: 0 occurs via exchange of supersymmetric • particles and can be used to restrict R parity violating SUSY models • Majoron: 0 may appear by a spontaneous breaking of a global • symmetry (B-L),due to a Majoron (a light or massless boson which can • couple to ) • Leptoquarks (bosons carrying both lepton and baryon number) • appearing in some GUTs scenarious could mediate 0 • Compositeness: possible substructure of quarks and leptons at an • energy scale of ~ TeV. A possible low energy manifestation of • compositeness could be 0 mediated by a composite heavy Majorana .

22. Diagrams that can contribute to the 0nbb decay amplitude d u d u WL WR Light Majorana neutrinos. Model extended to include right-handed WR. Mixing extended between the left and right-handed neutrinos.This is the mode where the rate ~2 or 2 Light Majorana neutrino, only SM LH weak interactions. Decay rate ~<m>2 e- e- n n e- e- WL WL d u d u d u d u WR e (selectron) Supersymmetry with R-parity violation. Many new particles invoked. e- e- Heavy Majorana neutrino interacting with WR. Model extended to include right-handed current interactions.  (neutralino) nheavy e- e- WR e (selectron) d u d u

23. Prospects in 0 • Discovery of 0  is a Majorana particle; existence of LNV processes • hints on  absolute mass and hierarchy • Once discovered  understand the mechanism of its occurrence • Subject of interest: derive the LNV parameters associated with different mechanisms • |T0|-1 = ∑_k |ήk|2 |M0k|2 G0(E0,Z) ; k ≡ mechanism of occurrence • |ήk| ≡ LNV parameter and |M0k| depend on the mechanism • Contribution from one mechanism: Light  Majorana exchange and only SM LH WI • |ήk| = |ήν| = |<m>|/me • α_i ≡ complex phases which can produce • cancellation • Contribution from two mechanisms: exchange of light and heavy Majorana  |T0|-1 = |ήν|2 |M0ν|2 + |ήNR|2 |M0NR|2 G0(E0,Z)  to restrict both |ήν| and |ήNR| one needs measurements of 0 in two different nuclei

24. Search of LNV processes at high energy:like sign dilepton processes Interest for searching LNV processes at hadron colliders Motivation: i) pressing need to check BSM physics in the lepton sector one verify several GUTs scenarios in which heavy s might play an important role ii) the increased luminosity of the present LHC experiments and future superB factories Check: - lepton number violation - neutrino character: Dirac or Majorana - existence of heavy sterile neutrino(s): when the heavy  mass is kinematically accessible, a process may undergo a resonant production of the heavy  and, at present and future luminosity of LHC experiments, there is a chance to be observed

25. LHC 2012 Run at 8 TeV has started (19 February 2012) • The LHC 2012 run at a beam energy of 4 TeV has started, corresponding to a collision energy of 8 TeV, compared with the 7 TeV runs in 2010 and 2011. The data target for 2012 is 15 inverse femtobarns for ATLAS (and CMS), three times larger than the total until now. The LHC is scheduled to enter a long technical stop at the end of 2012 to prepare for running at its full design energy of around 7 TeV per beam. Both the increased energy and the increased data will significantly extend the reach and therefore the opportunities to explore new physics. "By the time the LHC goes into its first long stop at the end of this year, we’ll either know that a Higgs particle exists or have ruled out the existence of a Standard Model Higgs". • Similarly, searches for supersymmetric particles and other new particles with masses in the TeV range will benefit significantly from the increased energy and data. • Main LHC experiments: ATLAS, CMS, LHCb, ALICE

26. Classification of the LNV processes • dd −> uu W*-W*- −> uu e-e- : 0ββ • Σ- −> Σ+ e-e- ; Ξ-−> p - - : hyperon decays • Ξ+c −> Ξ- p + +; Λc+−> Σ-+ + • c) -−> l+ M-1 M-2 -μ+μ-μ- : tau decays • d) M±1−> l±1l±2 M-/+2 : rare meson decays (B, D, K,..) • e) t −> b l+1 l+2 W- W- : top-quark decay • f) pp −> l+1 l+2 X : same sign dileptonic production • g) H±± −> l±1 l±2 X : double-charged Higgs decays

27. Lepton flavor violation processes (LFV) They are part of the physics program at superB factories and began now to be studied at LHC as well There are three kinds of LFV processes: i) e → μ; ii) μ → τ; iii) τ → e From their study one can extract useful bounds for VlN (l=e, μ, ) and use them for the search of LNV processes

28. Theoretical approach • In a simplest extension of the SM one includes: • - three active left-handed SM SU(2)Lflavors (e ,  , ) • one (heavy) sterile flavor (N4) • with the mixing relations between the gauge and • mass eigenstates: • L = Um mL + V4 Nc4L (=e, μ,τ) f ≡ q, n, p; M ≡ B, D, K, π; X ≡ π, ρ, K, K*

29. σ0~ tens of femtobarns Goals: discover a LNV process  decide on the  character (D or M) get constraints on mN mass and on the mixing parameters

30. Branching fractions Calculations are done with constraints for Ul4 and Vl4 taken from different experiments. The recent progress in 0νββ decay or other low-energy processes, might reconsider these calculations and provide new BR bounds for high-energy searches

31. Search of LNV processes @ LHC Collider signatures: ATLAS & CMS: search of isolated same-sign dilepton pairs σ [pp −> W± W ± −> l ±1 l ±2 X] = [2- δ(l1,l2)] |Vl,4 Vl,4|2 σ0(mN) σ0(mN) = “bare” cross section independent of the mixing parameters

32. References LHCb: [B+ (π-, K-) + + ] (PRL108, 2012) [B- (D(*,0)+ (s), π+) - - ] (PRD85, 2012) [− μ+μ−μ− ] LHCb-CONF-2012-015 ATLAS: JHEP 10 (2011) 107 [pp  l+1 l+2 X; l1,2 = e, ] CMS: JHEP06 (2011) 077 [pp  l+1 l+2 X; l1,2 = e, , ] BELLE: arXiv 1107.0642 [hep-ex] (B+  same channels as LHCb)

33. CMS:JHEP06 (2011) 077 [pp  l+1 l+2 X; l1,2 = e, , ] Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy at the LHC Events with same-sign isolated lepton pairs from hadron collisions are very rare in the SM but appear very naturally in many new physics scenarios. In particular, they have been proposed as signatures of supersymmetry (SUSY), universal extra dimensions, pair production of T (a fermionic partner of the top quark), heavy Majorana neutrinos, etc. In this paper we describe searches for new physics with same-sign isolated dileptons (ee, e, , and ), missing transverse energy (E Tmiss), and hadronic jets The searches use an integrated luminosity of 35 pb-1 of pp collision data at a ECM of 7 TeV collected by the CMS experiment at the LHC. The observed numbers of events agree with the SM predictions, and no evidence for new physics is found. To facilitate the interpretation of our data in a broader range of new physics scenarios, information on our event selection, detector response, and efficiencies is provided. pp −> P1±± −> l+1 l+2 X X = j j’

34. ATLAS: JHEP 10 (2011) 107 [pp  l+1 l+2 X; l1,2 = e, ] Inclusive search for same-sign dilepton signatures in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector An inclusive search is presented for new physics in events with two isolated leptons (electron or muon) having the same electric charge. The data are selected from events collected from pp collisions at sqrt(s) = 7 TeV by the ATLAS detector and correspond to an integrated luminosity of 34 pb-1. The spectra in dilepton invariant mass, missing transverse momentum and jet multiplicity are presented and compared to SM predictions. In this event sample, no evidence is found for contributions beyond those of the SM. Limits are set on the cross-section in a fiducial region for new sources of same-sign high-mass dilepton events in the di-electron, di-muon and electron-muon channels. Four models predicting same-sign dilepton signals are constrained: two descriptions of Majorana neutrinos, a cascade topology similar to supersymmetry or universal extra dimensions, and fourth generation down-type quarks. Assuming a new physics scale of 1 TeV, Majorana neutrinos produced by an effective operator V with masses below 460 GeV are excluded at 95% CL. A lower limit of 290 GeV is set at 95% CL on the mass of fourth generation down type quarks.

35. LHCb: arXiv1110.0730 [hep-ph]: [B+ (π-, K-) + + ] (PRL108, 2012)

36. LHCb:[B- (D(*,0)+ (s), π+) - - ] (PRD85, 2012)

37. processes similar to 0ββ : sensitive to Majorana  irrespective of m • processes in which a heavy  (N) is produced and it further decays: N - W+ • A prolific dileptonic decay of B- ; it has never been calculated before, nor has been proposed for investigation/measurement

38. Analysis 1. Data sample ECM = 7 TeV All data sets collected in 2010 at (36.1 ± 1.3) pb-1 + data sets collected before July 2011 at (341 ± 12)pb-1 Forπ+- - channel both signal and reference channels are contained in the DiMuon stream of the stripped sample For the the other (charmed) channels one uses the DiMuon stream for reconstructing the reference channel and SemiLeptonic stream for the signal channels

39. Reference channel

41. LHCb-CONF-2012-015 (J. Albrecht, M. Calvi) A search is made for the lepton flavour violating decay − μ+μ−μ− using 1.0 fb−1 of data collected at sqrt(s) = 7TeV by LHCb in 2011. In LHCb, − leptons are copiously produced, almost exclusively from decays of B, Ds and D0 mesons. In the analysis of the data, the − production rate is normalized to the control channel D−s (μ+μ−)π−. The observed number of events is consistent with the background expectation, and an upper limit B(− μ+μ−μ−) < 7.8(6.3) × 10−8 is set at 95% (90%) confidence level.

42. Proposed channels Σ- −> Σ+ e-e- Ξ- −> p - -hyperon decays Ξ+c −> Ξ- p + + Λc+−> Σ-+ +  -−> l+ M-1 M-2tau decays  -- >  l- l- X+ (π, ρ, K, K*) t −> b l+1 l+2 W- W- top-quark decay M±1 −> l±1 l±2 M-/+2meson decays(B±, Bc ; l± = e± , ±)

43. Strategy • re-evaluate the bounds of the neutrino mixing parameters |Vα4| from different actual low-energy measurements, including 0νββ decay recent developments • use these bounds, through the corresponding decay rates/widths to constraint the Br. • choose specific channels for analysis combining the Br predictions with particular experimental constraints • get new limits for the neutrino mixing parameters and heavy neutrino mass

44. Conclusions • Recent  oscillation experiments have conclusively shown that s are massive and they mix: the first observation of BSM physics. The large majority of BSM theories involve massive Majoranas. • Precision measurements for  mixing parameters are still needed, especially for the θ13 • We still don’t know: the absolute neutrino mass, the neutrino character (D or M), • number of neutrino flavors, the mass hierarchy • LNV processes may shed light the first three issues  strong interest for searching these • processes • An extensive experimental effort for improving the set-ups and techniques for searching the 0ββ decay mode • An important theoretical effort for computing accurately the NMEs relevant for 0ββ decayand for understanding the mechanism of its occurrence • Great opportunity for searching LNV processes at LHC, particularly at LHCb: the very large “effective luminosity” of 0ββ compensated by the increased luminosity at LHC. • Advantage: expertise in using information and interpretation from low-E LNV processes. • Discovery of non-SM neutrinos understanding of fundamental issues as: mechanism of neutrino mass generation, DM composition, LSS formation, BB nucleosynthesis, etc. • Understanding of the neutrino properties is of great interest nowadays