Reminder of CUSO 3ème cycle, Spring 2016 [email protected] previous chapters http://dpnc.unige.ch/~mermod/Teaching/LLPs/ (LLPs at colliders)

• Neutral LLP ( etc)

Neutral, τ ≳ 50 ns • Neutral LLP (neutralino etc) • Charged LLP (slepton, R- etc)

Charged, Neutral, τ ≳ 50 ns τ ≳ 50 ns • Neutral LLP (neutralino etc) • Charged LLP (slepton, R-hadron etc) • High-charge LLP (monopole etc)

Highly ionising, τ ≳ 50 ns

Charged, Neutral, τ ≳ 50 ns τ ≳ 50 ns • Neutral LLP (neutralino etc) • Decaying LLP (restricted phase • Charged LLP (slepton, R-hadron etc) space, hidden • High-charge LLP (monopole etc) sector etc)

Displaced decay, τ ~ 0.01–1 ns

Highly ionising, τ ≳ 50 ns

Charged, Neutral, τ ≳ 50 ns τ ≳ 50 ns Where to look for new physics?

6 Where to look for new physics?

Massive new physics High-energy experiments Light new physics High-intensity experiments

7 Massive vs Light new physics Massive new physics Seen in previous chapter • Electroweak–TeV energy scale, otherwise would have been discovered at earlier colliders • LLP can be neutral or charged • LLP can be massive and produced directly in pairs or light and produced as a decay product of a massive , with a relatively large production cross section

Light new physics This chapter • keV–electroweak energy scale • In simplest models, light LLP is the only new particle • LLP must be neutral (“hidden”), otherwise would have been discovered at accelerator experiments • LLP generally produced through , Higgs or “portals” with very small production cross section Chapter 3: light LLPs History lesson – 1930s

• Beta decay was assumed to involve only and • Expect discrete energy (from energy and momentum conservation) ...continuous spectrum observed! • Pauli proposed a radical solution – the neutrino!

• Perfect example of a “” – New light particle – Electrically neutral, small interaction cross section – Manifests itself through a new interaction, or “portal”: weak interaction (W and Z ) Interactions, mediators, portals to hidden sector • Electromagnetic – mediator : photon γ – Possibility of new light LLP mixing with γ (vector portal) • Weak – mediators:

– Light LLP (νL) coupling only to W and Z

– Possibility of new light LLP mixing with νL (neutrino portal) • Strong – mediator: g • Higgs – mediator: H – Possibility of new light LLP coupling only to H (Higgs portal) • New interaction? – New heavy mediator could open portal to hidden sector – Possibility of new light LLPs Example scenarios of light new physics

No new particle ⪆ EW scale postulated

•

• Heavy neutral (right-handed neutrino)

• Dark photon

12 Example scenarios of light new physics

No new particle ⪆ EW scale postulated

• Axion – Strong CP AND well motivated! – LLP, dark • Heavy neutral lepton (right-handed neutrino) – Neutrino masses – Matter- asymmetry – LLP, • Dark photon – g-2 – Parity – LLP, dark matter 13 Experimental strategies to search for light new physics with beams

Chapter 3.2 • Fixed-target facilities – Electron beam – beam • High-intensity colliders – B factories – LHC This chapter

14 3.1 Light LLPs at colliders

• Dark photon (A') – BABAR (SLAC, 10 GeV e+e-) – ATLAS (LHC, 8 TeV pp) – LHCb (LHC, 14 TeV pp) • Heavy neutral lepton (HNL) – LEP1 (91 GeV e+e-) – LHC (13 TeV pp) – FCC (91 GeV e+e-, 100 TeV pp)

15 A' Dark photon Dark photon (γD) Hidden / heavy photon Mirror photon hidden sector charged under U(1)' Dark Z (ZD) U-boson, etc. • g-2, dark matter, excess, parity (mirror world),... • Production via kinetic mixing with the photon (“vector portal”) – Coupling to charged suppressed by ε • Decay to pairs – Search for resonances

16 Dark photon search with BABAR PRL 113, 201801 (2014) • Main channels – e⁺ e ⁻ → γ A' , A' → l⁺ l ⁻ (l = e,μ) – e⁺ e ⁻ → γ A' , A' → invisible • Probe mass range 0.02 – 10 GeV

17 Quiz

What is the detector signature in the e⁺ e ⁻ → γ A' , A' → invisible decay channel? A) A photon and a large missing mass B) A photon and two C) A photon and two forward-scattered electrons D) A bump in the two-photon mass spectrum

18 Quiz (answer)

What is the detector signature in the e⁺ e ⁻ → γ A' , A' → invisible decay channel? A) A photon and a large missing mass B) A photon and two leptons C) A photon and two forward-scattered electrons D) A bump in the two-photon mass spectrum Presumably, the dark photon decays into two dark charged under U(1)', which would be candidates for DM. The invisible particles go through the detector unseen, so the signature is a single photon with an imbalance in energy.

19 Dark photon search with ATLAS JHEP 11, 088 (2014)

• Production in H decay (“Higgs portal”) • Search for collimated leptons from hidden photon decay (“lepton-jets”) – Prompt and displaced decays – Use special displaced decay triggers (seen in chapter 2.4) • Interpretation depends on a number of assumptions – Light hidden fermion in addition to the dark photon – Higgs decay branching ratio

20 Current hidden photon limits at the LHC

ATLAS ArXiv:1511.05542 (2015), CMS arXiv:1506.00424 (2015)

Remember : ATLAS limit relies on more assumptions

21 LHCb Projected sensitivities (PRD 92, 115017 (2015)) • Dark produced in charm decays D*0→D0γ (γ mixing with A') • Identification of prompt and displaced vertices of low- momentum lepton pairs, requiring: – Precise tracking – Triggerless readout planned for Run-3

22 N Heavy neutral lepton (HNL) Heavy neutral lepton (HNL) Right-handed neutrino Heavy neutrino right-handed neutrino Majorana neutrino , etc. • Neutrino masses, dark matter, X-ray astronomy, matter-antimatter asymmetry • Production via mixing to (“neutrino portal”) – No interaction except through coupling to the Higgs field • Decay to llν or l+hadron(s)

N stable dark matter 1 N long-lived, mass in 2,3 0.2-100 GeV range 23 HNLs from W and Z decays at colliders

• Can probe masses up to 90 GeV small • LEP1 (Z resonance) : Delphi search mixing long lifetime – ~ 106 νs from Z decays low – displaced HNL decays for masses < 4 GeV mass • Tevatron (2 TeV) – ~10⁷ νs from W decays – Low mass or prompt HNL decay → large backgrounds, no search • LHC Run1 (8 TeV) – ~109 νs from W decays in ATLAS and CMS – Displaced HNL decays for masses < ~30 GeV – no search • LHC Run2 (14 TeV) – ~10⁹ νs for each 25 fb⁻ ¹ from W decays in each experiment – search in progress 24 Quiz

Why cannot HNLs from Z decays be searched for at hadron colliders? A)The Z production cross section is too small B)There are far too much QCD backgrounds C)There are no efficient triggers

25 Quiz (answer)

Why cannot HNLs from Z decays be searched for at hadron colliders? A)The Z production cross section is too small B)There are far too much QCD backgrounds C)There are no efficient triggers A is true at the but not at the LHC (although it is true that the W production cross section is several times larger). B is not true if one consider the signature of a displaced vertex. C is true: the Z decays to a neutrino and an HNL and in the absence of displaced-vertex triggers one cannot achieve sufficient rate reduction at such energies. 26 HNL search with Delphi Z. Phys. C 74, 57 (1997) Search for heavy neutral leptons in Z decays (Z → υN) – 3•10⁶ Zs – Short-lived (high mass) N: jets and missing energy – Long-lived (low mass) N: displaced vertex

27 Quiz Why was no direct search for HNL from W decay made at the LHC Run1 using a displaced vertex signature? A)The integrated luminosity was insufficient to probe couplings which were not already excluded by LEP B)The integrated luminosity was insufficient to probe couplings small enough to feature displaced decays C)Just because of prejudice – the LHC was built as a high- energy machine for discovering massive new physics and nobody thought about using it as a high-intensity machine for discovering light new physics

28 Quiz (answer) Why was no direct search for HNL from W decay using a displaced vertex signature made at the LHC Run1? A)The integrated luminosity was insufficient to probe couplings which were not already excluded by LEP B)The integrated luminosity was insufficient to probe couplings small enough to feature displaced decays C)Just because of prejudice – the LHC was built as a high- energy machine for discovering massive new physics and nobody thought about using it as a high-intensity machine for discovering light new physics In fact, an analysis of 7 and 8 TeV LHC data could already surpass LEP limits for masses between 2 and 30 GeV. Part of the explanation may be that B was true at the Tevatron and most particle physicists joining the LHC experiments came from Tevatron experiments. 29 HNL search at the LHC using W decays

• Previous searches for displaced vertices in inner detectors (PLB 707, 478; PLB 719, 280; ATLAS-CONF-2013-092; JHEP 02, 085; PRL 114, 061801 (2015); PRD 91, 052012 (2015) ; PRD 91, 012007 (2015); PRD 92, 072004 (2015)) – Not sensitive to HNLs due to high energy thresholds (based on models of heavy new physics) – Adequate track and vertex reconstruction tools, similar backgrounds • Requirements on displaced vertex can reduce backgrounds to negligible levels • Dedicated HNL search possible and needed – Phys. Rev. D 89, 073005 (2014), arXiv:1312.2900 – arXiv:1504.02470

30 HNL signature in ATLAS and CMS

31 HNL signature in ATLAS and CMS

Essential for Prompt lepton triggering

32 HNL signature in ATLAS and CMS

Prompt lepton

Essential for background rejection

Displaced vertex

33 HNL signature in ATLAS and CMS

Prompt lepton

W invariant mass

Displaced vertex

34 HNL signature in ATLAS and CMS

Prompt lepton

W invariant mass

Displaced vertex

Flavour analysis possible!

35 LHC – expected sensitivity to HNLs assumes 50 fb⁻ ¹ @ 14 TeV in both ATLAS and CMS

Dark matter requirement in addition to BAU

36 HNLs at future circular colliders

• CERN Future Circular Collider (FCC) design study – ~100 km ring – Conceptual design report to be prepared for 2018 • FCC-ee: 10¹³ Zs, Higgs factory... • FCC-hh: 100 TeV pp collisions

• Dedicated displaced vertex analyses at FCC can achieve several orders of magnitude improvement in sensitivities to HNLs in mass range 2–90 GeV – arXiv:1411.5230

37 Light new physics at colliders – summary

• Given lack of evidence for massive new physics, light new physics is a hot topic – Hidden sectors, dark photons etc... – Direct search for right-handed neutrinos – etc... • Complementary searches can be made at colliders and dedicated fixed-target experiments – Surveyed collider searches in this lecture

Next lecture (3.2): light LLPs at non-collider facilities

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