Searches for Dark Matter at the LHC

Jeanette Lorenz (LMU München)

Science Week, Origin Cluster, 02.12.2019

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 2

Dark Matter

[https://www.nature.com/articles/nphys4049/figures/1 from Nature Physics volume 13, pages 224–231 (2017)]

Existence of dark matter established by many cosmological and astrophysical observations, e.g.:

● Rotation curves of galaxies,● Gravitational lensing,● Measurements of anisotropy of of cosmic

microwave background.

Little known about properties of Dark Matter. Constraints by achieving the correct relic density.

Consequently many different candidates, including Weakly Interacting Massive Particles, Axions, ALPs etc.

[D. C

low

e et al., Astro

ph

ys. J. 64

8, L1

09

(20

06

)]

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 3

Interplay of experiments searching for Dark Matter

Fermi-LAT, MAGIC, Planck, H.E.S.S. ...

LHC

XENON1T, LUX, Panda-X, Picasso...

Three types of experiments:● Collider searches: Aim to detect Dark

Matter particles produced in particle collisions.

● Direct detection: Collisions of galactic Dark Matter with material in underground detectors.

● Indirect detection: Look for products of annihilating Dark Matter.

In case of a discovery, complementary information obtained from these different types of experiments:

● A discovery at colliders needs to be confirmed by (in)direct detection experiments to link to a cosmological origin.

● Colliders can give more information about the interaction between Standard Model (SM) and Dark Matter (DM) particles in case of a discovery in (in)direct detection experiments.

This close connection between experiments motivates a close between-experiments discussion and collaboration, also to define sensible benchmark scenarios to investigate.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 4

What can we do at the LHC?

Most searches at the LHC focused on WIMPs in the recent years:

● “WIMP miracle”● Weakly interacting.● Roughly weak-scale mass ~ O(100) GeV.● Yields roughly the observed relic density.● Primary focus also for upcoming data-taking periods, including HL-LHC.

Hidden sector particles/feebly interacting particles:

● MeV – GeV mass range.● Very feeble interaction with the SM; coupling in the order of 10-10

● Accessed through portals, like a dark Higgs, dark photon, …● Potentially accessible at LHC through searches for long-lived particles.● Also by future experiments including experiments like SHiP and FASER.

Search for Dark Matter particles with minimal assumptions on the underlying theory

● Simplified models with minimal extensions of the SM.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 5

Geneva

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 6

Excellent performance of LHC and detectors

● Proton-proton data taking in Run 2 finished.● 158 fb-1 proton-proton data (√s = 13 TeV) delivered by the LHC 2015 -2018.● About 140 fb-1 available for analyses.● Now in a shut-down with upgrades to collider and detectors. Run 3 to start 2021 (up to

300 fb-1) and then HL-LHC (up to 3000 fb-1).● Most analyses have not analyzed the full dataset yet.

[https://twiki.cern.ch/twiki/bin/view/AtlasPublic/LuminosityPublicResultsRun2]

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 7

Dark matter models at colliders

χ

χq

q

Dark matter particles invisible to LHC detectors→ this signature cannot directly be probed

χ

χq

q

W, Z, g, γ

Initial state emission → recoils against dark matter particles

Generic model good for sizable cross-sections, a priori no assumptions on specific model

Or detect dark matter particles in decay of other new particles→ specific models/extensions of SM (like supersymmetry)

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 8

Dark matter models with mediators

χ

χq

q

?q

q

χ

χ

Mediator particle

Mediator particle can be SM particle (Z or H) or a new particle – either spin 1 or 0 – e.g. Higgs portal vector-like particle or scalar-like, or Two-Higgs-Doublet Model

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 9

Search strategies for BSM mediator models

ET

miss

X Pure production of dark matter particles invisible, need some other SM particle the dark matter particles are recoiling against.

Two possibilities:● Radiation in the initial state. ● Emission of SM particle from

mediator.

Can also search for decays of mediator particle to SM particles.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 10

Common to most of these searches: ET

miss

[Jet Goodson]

Invisible particles to the detector (like neutrinos or dark matter particles) result in a momentum imbalance in the transverse plane to the proton-proton collision => missing transverse momentum (E

Tmiss)

Calculated using the x- and y-components:

The soft term is composed of all tracks or energy deposits not associated to a reconstructed particle.

ET

miss can also arise from mis-measurements → for reliable searches this fake E

Tmiss needs

to be understood and minimized.

[htt

ps:// atla s.w

eb

.cern

. ch/A

tlas /G

RO

UP

S /PH

YSICS/P

LOT S/JE TM

- 20

19

-0

3/]

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 11

Examples for BSM mediator searches

Emission of particle from initial state→ e.g. mono-jet

Search for a di-jet resonance

[Phys. Rev. D 97 (2018) 092005] [arXiv:1911.03947]

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 12

Mono-h[ATLAS-CONF-2018-039]

Search for dark matter produced in association with a SM Higgs boson decaying to bb

Signal regions for the resolved (two small-R jets) and merged regime (one large-R jet)

Signal region without leptons, main backgrounds:

W+jets, tt, Z+jets. Binned in b-jet multiplicity and E

Tmiss to increase

sensitivity, simultaneous fit in mass of Higgs candidate.

No excess seen.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 13

Innovations

Object- based ET

miss Significance provides information on how likely the measured E

Tmiss is

due to a resolution fluctuation.→ Suppression of di-jet backgrounds.

Improved identification of b-jets from boosted Higgs bosons by using jets of a variable cone size.

[ATLAS-CONF-2018-039]

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 14

Exclusion limits[ATLAS-CONF-2018-039]

Limits set on mass of mediator (Z’) and boson A. Dark matter mass fixed, as well as coupling strength and mass of other Higgs bosons.

Improvements due to use of VR track jets.

Region where object-based E

Tmiss

significance gets relevant.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 15

Reinterpreation of the mono-h search for dark Higgs models[ATL-PHYS-PUB-2019-032]

Decays s → WW, ZZ, HH topic of future analyses

Dark Higgs model:

● Additional Higgs boson s.● Motivated by need to generate

masses in DM sector.● Can relax DM relic abundance

constraints by opening up additional annihilation channel.

Decays depend on mass of s – for small masses s → bb

Thus reinterpretation of mono-h search possible.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 16

Reinterpreation of the mono-h search for dark Higgs models[ATL-PHYS-PUB-2019-032]

RECAST/CERN analysis preservation efforts

Mono-h search preserved in RECAST – one of the first analyses preserved at ATLAS!

Allows analysis of any other signal model/scenario in the future using the original (preserved) analysis software with minimal effort.

Exclusion limits up to a Z’ mass of 3.2 TeV.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 17

Mono-h searches at CMS[arXiv:1908.01713 [hep-ex]]

● Searches for h → bb + ET

miss in case of low Z’ masses in certain models not sensitive (as then relatively low E

Tmiss needed).

→ Study other channels like h → ττ and h → γγ, and also h → ZZ and h→ WW + E

Tmiss

● Combination of all channels.

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 18

Summary of searches for dark matter in BSM mediator models[JHEP 05 (2019) 142]

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 19

Comparison to non-collider dark matter searches[JHEP 05 (2019) 142]

Comparison only valid for a very specific model with specific parameters!

For specific models and parameter assumptions comparison between collider and direct detection experiments possible → collider experiments cover dark matter masses down to 1 GeV in these models

02.12.2019 J. Lorenz, Searches for Dark Matter at the LHC 20

Summary

● Rich program to search for Dark Matter particles (mostly WIMPs) at the LHC.

Searches for BSM mediator Dark Matter models. Higgs portal models. SUSY, long-lived particle searches,... So far no significant excess seen.

● Complementary to (in)direct searches for Dark Matter.● Discussion between experiments thus essential to use synergies and e.g.

define common benchmarks.

● Upcoming experiments (including HL-LHC) will have large impact on the parameter space for Dark Matter!