Searches for Heavy Neutrinos with the ATLAS Detector

Maximiliano Sioli

(Bologna University and INFN)

on behalf the ATLAS collaboration

ICHEP 2018 – COEX, Seoul – July 7th, 2018

Outline

►Origin of neutrino masses

►Seesaw paradigm and new heavy states

►The ATLAS detector

►Type-I and LRSM seesaw searches with ATLAS

►Type-II seesaw searches with ATLAS

►Type-III seesaw searches with ATLAS

►Conclusions

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Origin of neutrino masses

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►Neutrino oscillations at least two n states have non-zero masses physics BSM

►Striking features of neutrinos:• Smallness of masses (sub-eV) suppressed w.r.t. charged fermions (which are tied to EW scale)• The simple introduction of a SM singlet neutral fermion (“sterile”) leads to tiny Yukawa’s:

►But neutrinos are electrically neutral can be Majorana fermions Seesaw

►Seesaw basic idea: new heavy states of mass Mgenerate tiny neutrino (Majorana) masses:

to have sub-eV neutrino masses and natural coupling, M has to point at GUT scales

ℒ ⊃ 𝑦𝐻𝜈𝑁 → 𝑦 ≈𝑚𝑎𝑡𝑚

𝑣𝑒𝑣≈ 10−13

“unnatural” and “unobservable”

𝑚𝜈 ≅ 𝑚𝐷𝑖𝑟𝑎𝑐

𝑚𝐷𝑖𝑟𝑎𝑐

𝑀

Seesaw mechanisms

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►The simplest parametrization to obtain n mass in the SM is through dim-5 operator:

“Open up” of dim-5 operator in all minimal

tree-level ways

Type-I: Fermion singlet Type-II: Scalar triplet Type-III: Fermion triplet

𝑚𝜈 = 𝑌𝑁𝑇1

𝑀𝑁𝑌𝑁𝑣

2 𝑚𝜈 = 𝑌Δ𝜇Δ

𝑀Δ2 𝑣

2 𝑚𝜈 = 𝑌Σ𝑇 1

𝑀Σ𝑌Σ𝑣

2

(1 RH neutrino) (no RH neutrinos) (1 RH neutrino + 2 charged heavy leptons)

Minimal Type-I seesaw phenomenology

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►In the (minimal) Type-I seesaw mechanism one right-handed NR is introduced

►Phenomenologically addressed through:

𝑝𝑝 → 𝑊∗ → 𝑙𝑁𝑅 → 𝑙𝑙𝑊 → 𝑙𝑙𝑗𝑗

►If NR is Majorana final states contain both OS and SS lepton pairs in equal amount

►Free parameters of the model: mN and mixing VnN

►Clean signature:Same-sign final stateClosed kinematics (no MET)Resonant W boson from mjj reconstruction

►At low masses (below mW) the intensity is high enough to explore low VnN (~10-5) via displaced vertexes

Heavy n searches in minimal Type-I seesaw

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JHEP07 (2015) 162

Run 1 searches at 𝑠 = 8 TeV (20.3 fb-1)• Only SS lepton pairs considered in lljj final states• Background from prompt SS leptons (diboson) and

prompt OS leptons (e.g. ttbar + charge-flip) • mjj as discriminant variable• Limits in the mixing-mN plane

high-massshort-lived

low-masslong-lived

energy / intensity frontier complementarity

Left-Right Symmetric Models

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►Add a new high-scale SU(2)R symmetry group to explain parity violation in weak decays

►Include extended gauge sector VR = {WR, Z’} with NR counterparts of RH charged leptons

►Assume perfect symmetry at high scales: RH gauge bosons are assumed to interact with SM particles as in the left sector, gR = gL and also assume (CKM)R = (CKM)L

►LRSM naturally embeds neutrino mass generation through seesaw Type-I after EW symmetry breaking

►Golden channel at colliders is the Keung-Senjanović (KS) process:

►If MWR >> MNR boosted topology

𝑝𝑝 → 𝑊𝑅 → 𝑙𝑁𝑅 → 𝑙𝑙 𝑊𝑅∗ → 𝑙𝑙𝑞𝑞

Phys. Rev. Lett. 50, 1427 (1983)

• Smoking gun to probe the Majorana nature of N(50% OS and 50% SS decays)

• High energy analogue to low energy 0nbb

• Can probe the (PMNS)R mixing matrix

Heavy n searches in LRSM

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►Similar analysis strategy w.r.t Type-I seesaw but addressing different kinematical regions Different background composition

and analysis optimization strategy

►Free parameters: mWR and mNR

►Previous limits from LEP using WR tb decays

►Run 1 searches at 𝑠 = 8 TeV (20.3 fb-1) • Only Majorana neutrino considered (SS dilepton + two jets)• Only e, m in the final state (no hadronic tau decays)• Limits reach ~3 TeV for mWR and ~1.8 TeV for mNR

►Run 2 searches at 𝑠 = 13 TeV ongoing• Both Majorana and Dirac cases considered (SS+OS dileptons)• Both mWR > mNR and mWR < mNR hierarchies considered

[ JHEP07 (2015) 162 ]

Probing Type-II seesaw at 13 TeV

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►Type-II seesaw does not provide RH neutrino states

►It can be experimentally addressed through the quest fortriplet Higgs scalars, e.g. through pp g* / Z* H++H--

llll

►A recent search at 13 TeV by ATLAS extends the mass limits up to ~870 GeV

►Limits provided as a function of BRs into light leptons

►Complementary search for 𝐻±± → 𝑊±𝑊± ongoing (high v value regime)

DL = 2

DL = 2

[ EPJC 78 (2018) 199 ]

Heavy n searches in Type-III seesaw

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►ATLAS Run 1 searches at 8 TeV► ll+jj final statesmN > 335 GeV [ PRD 92 (2015) 032001 ]

► lll final statesmN > 470 GeV [ JHEP 09 (2015) 108 ]

►Signature: OS or SS leptons + 2 jets, requiring mjj mW and MET [ Phys. Rev. D 88, 114018 (2013) ]

►Only light leptons considered (electron or muons) six channels: (ee, em, mm) × (OS, SS)

►Considered minimal type-III seesaw: (L+,L-,N0), where N0 is the heavy Majorana particle

►Dominant production via:

OS leptons

SS leptons

Analysis strategy and results

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►MET + HT (scalar sum of transverse momenta) used as discriminating variable

►Background estimation:►Prompt leptons from Monte Carlo simulation (ttbar, diboson)

► “Fake” leptons estimated using data-driven methods

►Charge-flip evaluated by Monte Carlo and corrected with data-driven scale-factors

►Simultaneous maximum-likelihood fit in all control and signal regions

►Combined result obtained assuming democratic mixing between SM and heavy leptons

ATLAS-C

ON

F-20

18

-02

0

Conclusions

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►Discovery of neutrino masses opened a gate toward BSM physics

►Neutrino mass scale and its possibile Majorana nature suggest the seesaw mechanism as a very promising paradigm►Direct connection to a new large energy scale

►Intriguing connections with Cosmology (Dark matter and BAU generation)

►Different realizations: Type-I, Type-II, Type-III accessible at colliders

►LR symmetric models at 13 TeV through Keung-Senjanović process►Would provide a spectacular signature to probe Majorana heavy neutrinos

►ATLAS fully covered the topics in Run 1 and produced new results in Run 2, with a rich program ahead!