Dark Matter

Dec 5, 2018

Much of this material taken from Fall 2015 Physics 290esee: https://indico.physics.lbl.gov/indico/categoryDisplay.py?categId=8

Outline

• Dark Matter: Why do we need it?

• Astophysical Constraints

• Direct Detection

• Indirect Detection

• Collider Searches

Why Dark Matter?

• Strong evidence that ∃ matter in Universe that doesn’t shine

• Distribution of this matter is different from the luminous stuff

• Many independent observations using different techniques

• Among the most important are:I Study of rotation curves in galaxies and in clusters of galaxiesI Measurements of temperature of hot gas in clustersI Gravitational lensingI Baryon acoustic oscillations

Rotation Curves: Velocity tells us about the mass enclosed

Mass-to-light ratio

• Mass-to-light ratio defined astotal mass in solar massesdivided by luminosity in solarluminosities

• Observed mass-to-light ratiodepends on radius

• About 90% of mass within 100

kpc is dark matter

Using temperature to measure mass

• Assume hot gas in clusters in gravitational equilibrium

• Velocity related to temperature

v = (0.1 km/s)× (T/K)12

• Mass related to v:

M =v2r

G

Gravitational Lensing

• Light bends in the presence ofmatter

• Look at a galaxy using lightthat has passed through acluster

• See three images instead of one

• Graviational lensing affects

power spectrum measured in

CMB

Baryon Acoustic Oscillations

U. Seljak 290e

Consistent picture from all measurements

What kind of particle?

• BBN suggests that the DM is non-relativistic

• Gravitational lensing searches rule out massive dark baryonic DM

• A common model: Assume particles in thermal equilibriumI DM particles can annihilate and can pair produceI Freeze out occurs when annihilation rate ≈ expansion rateI Can calculate DM density

ΩX ∝1

σv∝ MX

g4X

I Using observed DM density if mX ∼ 100 GeV and gx ∼ 0.6then the predicted ΩX ∼ 0.1

I Weakly interacting particles with masses in the 100 GeV rangegive about the right Ω: The WIMP Miracle

Many experimental searches for such WIMPs

Three Complementary Approaches

Tim Tait: DarkMatter LHC 2013

Direct Searches

• Earth is in the halo of the galaxy

• DM is passing through at all times

• Look for scattering of DM particles from ordinary matterI Nuclear recoil in underground detectorsI Small signal, so excellent background rejection necessaryI Two types of interaction:

• Spin-independent: couples to charge of nucleusEnhanced (coherent) scattering from heavy nuclei that arespin independent

• Spin dependent: couples via, eg magnetic momentWeaker limits from direct detection

Direct Detection Status: Spin Independent Interactions

What Next?

Indirect Detection

• Term used to describeexperiments that searchevidence of DM through itsannihilation of decay products

• Possible sources of signalI Neutralino annhilation to

SM particlesI Sterile neutrinos decaying to

active neutrinos

• Photon line at half sterile

neutrino mass

I Superheavy DM annihilatingof decaying to SM particles

• Observed in cosmic ray

spectrum

Summary of Current and Future Experiments

Instruments Advantages ChallengesGamma-ray Fermi, ACTs, HESS, point back to source backgrounds,

photons VERITAS, MAGIC spectral signatures attenuationCTA, GAMMA- 400

Neutrinos IceCube, PINGU point back to source low statisicsANTARES, KM3NET spectral signatures backgrounds

Super-K, Hyper-KCharged Particles PAMELA, AMS antimatter diffusion

ATIC, ACTs, Fermi hard to produced don’t point backCTA, CALET, GAPS in Universe to sources

Multiwavelength telescopes good angular assumptionsEmission resolution, good stats about environment

different bckgnd

taken from J. Siegal-Gaskins’s 2014 SLAC Summer Institute Lectures

Indirect Detection: Current Status

• Many experimental results,including some anomolies

• No smoking gun pointingtowards DM source

• See Slac Summer Institute2015 Lectures for details

• And tune in for future results

LHC Dark Matter Searches

In an ideal world (for us), the LHC would:

• Produce DM candidates in LHC collisions

• Produce heavier exotic particles that are relatives of the DMcandidate

• Allowing us to:I Determine quantum numbers of the DM particlesI Determine how DM candidates interact with SM particlesI Relate observations to direct and indirect searches

Making Dark Matter at the LHC: the possibilities

• Create from decay of SM particleI Some possibilites are

• Z → χχ• h→ χχ• t→ cχχ

I Constraints on Z decays to DM severe

• Produce DM directly with SM particlesI Initial State Radiation of SM particle (γ, g, etc)I Associated Production of DM with tt, W/Z, etc

• Create BSM particles that decay to SM candidateI SUSY has many such possibilitiesI Exotic resonance E → χχ

Dark Matter Interactions with SM Particles

• We know DM particles feel gravity

• We don’t know what other interactions they have with SMparticles

• Only SM mediators possible are Z and Higgs

• Also possible that exotic BSM particles mediate SM-DMinteractions

I If these mediators light, could be produced at LHCI If they are heavy need to parameterize the interaction

phenomenologicallyI Possible approaches:

• Effective field theory (high mass mediator)• Simplified model (defined by interaction and exchanged mass)• A full theoretical model (eg SUSY)

I All 3 approaches used

• Will be very important if DM is seenI Required to compare LHC limits with direct and indirect

detection

Searches we will review today

• Higgs decays to DM

• DM pairs in association with SM particles

• SUSY

Dark Matter and the Higgs

• A simple, well defined modelI Higgs decays to pair of DM particlesI DM only couples to SM particles via Higgs

• “Higgs portal”• Probes models relevant for current and next generation direct

detection experiments (see comments by Kathryn Zurek fromSept 2, 2015 Physics 290e)

• Measurements of Higgs properties can contrain this:I DM contributes to “invisible Higgs decays”I Requires MDM < Mh/2I Bound depends on DM spinI Powerful for low MDM

Higgs to “Invisible”

• Associated production of Higgs with a Z

• Signature is Z with nothing balancing pT• Background dominated by ZZ with one Z → νν

I All backgrounds well studied for Higgs search

• Additional analyses look for Higgs plus hadronically decayingZ or Higgs produced with tagged jets from vector bosonfusion

Invisible Higgs Results: VBF Search

• Search only sensitive if Higgs to invisible BR large

• But, this is still good enough to constrain Higgs portal models!

Invisible Higgs: Global Fit

• Translate limit to coupling of Higgs to DM

• Three options: scalar, majorana fermion or vector

• Can compare to direct detection experiments

• LHC results very powerful for low mass WIMP

SUSY and Dark Matter

• SUSY models can provide DM candidatesI R-parity: Lighest SUSY particle (LSP) stableI In most models LSP is weakly interactingI Must be neutral to be DM candidate

• Strongly interacting SUSY particles heavier than LSPI Large production cross sectionsI Decay chains with LSP at the bottom

• Classic signature: missing momentum + many jets

Example: Jets and Missing Et

SUSY Limits and DM Constraints

• Tranlating limits on SUSY parameters into a form where wecan compare with direct and indirect DM searchs requires amodel

• Most general SUSY model has 100 parameters: impossible toturn into constraints

• Simplest model (CMSSM) with 5 parameters largely ruled out

• Next simplest model (PMSSM): 19 parametersI Difficult to characterize using a 2D plotI Favored space: large DM mass

CMSSM Limits

PSSM Constraints

Cahill-Rowley et al, arXiv:1305.6921

Effective Operators: Parameterization of the new physics

• Parameterize production byI DM massI Interaction Strength M∗

• AssumptionsI Only SM and DM producted

• No other new particles

I Interaction is treated as a point• M∗ > kinematics of production

Characterizing DM production: Effective Operators

Beyond Effective Operators: Simpified Models

⇒

• Effective operators assume mass of mediator high• Assumption not necessarily true at LHC where momentum

transfer can be largeI For direct detection, this is not an issue

• While form of allow operators won’t change, cross sectionswill if mediator mass comparable to momentum transfer

• Can be important if when comparing limits from LHC todirect detection limits

• Will be important if something is found in either place

Associated Production (ISR)

• Partons in proton can radiate before interacting

• Radiation can be g, γ, W , Z, etc

• If interaction produces DM pair, this pair recoils against theISR particle

• DM exits detector unobservedI Mono-X

• Depending on couplings of DM interaction, relative rates fordifferent ISR particles will change

Example I: Monojets

CMS:arXiv1408.3583

• Search for high pT jet recoiling against nothing• Background dominated by decays with ν (eg W/Z+jets andtt)

Example II: Mono-photon

• Search for high pT photon recoiling against nothing

• Background dominated by decays with ν (eg W/Z+jets andtt)

Limits: Spin Independent Operators

Limits: Spin Dependent Operators

Comparisons with Indirect Detection

DM in Top-Pair events

• Sensitive to C1 operator:

mq

M∗3 qqχχ

DM with tt: Search Strategy

• Search for tt events whereone or both t’s decay toleptons

• Require missing momentumrecoiling agains top pair

• Look for excess at largemissing momentum

Top-Pair Results