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Home , Neutralino

PHY323:Lecture 11 SUSY and UED

•Higgs and •The Neutralino •Extra Dimensions •How WIMPs interact

Candidates for Dark III The New Zoo Here are a few of the candidates on a plot showing cross section vs. mass. An enormous range. We will focus on WIMPs

thanks to L. Roszkowski (Sheffield) Freeze Out of Thermal DM WIMP Candidate 1 Supersymmetric

Each particle gets a “sparticle” counterpart. get and vice versa. e.g. W Wino Z Zino etc The Lightest Supersymmetric Particle (LSP) is predicted to be stable. This is called the NEUTRALINO. Supersymmetry Theory What we are aiming to do, e.g.:

At higher energies, where symmetries are unbroken, you might expect a unified theory should have a single coupling constant Supersymmetry Theory and the Higgs To make things simpler, it would be nice if all the forces of nature were unified under the same theoretical framework. The energy at which this is likely called the Planck energy (1019 GeV). This was started in the 1970s - the result is the electroweak theory. The theory is intricate and complicated, partly because the photon is massless, but the W & Z are heavy. The electroweak theory posits that the very different carriers, and therefore properties, of these forces at energy scales present in nature today are actually the result of taking a much more symmteric theory at higher energies, above the ‘electroweak scale’ of 90GeV (the W and Z rest energy) and ‘spontaneously breaking’ it. The theoretical mechanism for spontaneous symmetry breaking requires yet another new particle, a zero particle called the HIGGS . Massive Force Carriers Extra Note The carrier of the electromagnetic force is a massless photon.

The carriers of the weak force are heavy, having a rest energy of roughly 90GeV. They are weakly interacting massive particles.

Can these things be dark matter ? No, actually, because they are unstable. A Are there other weakly interacting, massive particles which ARE stable, and therefore could be a candidate for dark matter ? Yes, but not within the . The “Higgs” is unstable, so it can’t be the dark matter itself The Higgs endows the carriers of the weak force with their high mass, resulting in a feeble force with a short range. But now the standard model contains another particle, and when you look in to the properties of the Higgs, you hit problems. For example, the Higgs in electroweak theory has a coupling to two and a four Higgs self-coupling.

Because of the Higgs mechanism, the standard model of is internally inconsistent. To remove the inconsistencies, extensions to the theory are needed. SUPERSYMMETRY is a popular candidate extension. Higgs Self Energy

The quantum mechanical amplitude for a Higgs to travel from A to B, summed over all contributing processes is...

H Notes Fixing Higgs Self Energy Problem In supersymmetry, each particle in the standard model has a supersymmetric partner with spin angular momentum differing by hbar/2. So for every there is a supersymmetric partner boson and vice-versa. The extra diagrams for the Higgs self energy where virtual are formed and destroyed cancel the divergent diagrams in the standard model sector, rendering the Higgs self- energy finite. In fact, it’s more complicated than this. The supersymmetric partners, none of which have been detected, mix together quantum mechanically, so that the actual supersymmetric objects we might detect in the LHC will be MIXTURES of the supersymmetric partners of the known particles. A Lot to Buy Into ? Supersymmetry is a lot to swallow. Twice the number of particles we have already found, and none of the superpartners detected yet ? To solve one puny Higgs self-energy problem?

Well, supersymmetry could do much more than this. (1) Some supersymmetric extensions, particularly supergravity, allow the possibility of quantizing the gravitational force, the biggest unsolved problem in quantum mechanics. (2) Supersymmetry eases extrapolation to high energy unification (3) The lightest supersymmetric particle might be stable. Lots of supersymmetric theories conserve something called R-parity, in which case a supersymmetric particle cannot decay into a set of non-supersymmetric particles. Supersymmetric Partner Mixing Lots of very clever people are working very hard on detecting the supersymmetric partners of ordinary particles, and if they do then they will sort out the mess of what supersymmetric states are actually observed in nature, and what their properties are.

Possibility of very exciting new physics if supersymmetry is detected at the LHC. Dan Tovey, Stathes Paganis, Davide Costanzo here at Sheffield. SUSY and R Parity SUSY introduces a new quantun number called R parity, in order to prevent decay and make distinguishable some states otherwise identical. Introducing R parity and giving the value of 1 and -1 to the SM particles and their supersymmetric partners respectively, the two states become distinguishable. The R parity is defined as where B, L are the and number operators and S is the spin. R parity is a multiplicative symmetry, so sparticles are produced in pairs. If R parity is broken, then there are no special selection rules to prevent the decay of those supersymmetric particles in the spectrum with masses of order of few GeV. In particular, the theory would possess no natural candidate for particles. The Neutralino The neutralino is a possibility for the lightest supersymmetric particle, and hence WIMPs. What is it? Well, it’s quite complicated. The neutralino is a quantum mechanical superposition of the bino, wino and two SUSY DM Summary • Supersymmetry first formulated in the 1970s. • Symmetry between fermions and bosons. • Each SM particle has a ‘’ • R-Parity introduced to prevent rapid proton decay - must be conserved. • Leads to existence of a stable Lightest Supersymmetric Particle (LSP)

For SUSY to agree with • Most important SUSY particle for Standard Model: DM is the Neutralino. 10GeV M 104 GeV • If this is the LSP, it is an ideal < χ < WIMP candidate. Searches for charginos and • In most realistic SUSY models (i.e. squarks at LEP and Tevatron: no charged LSP), Neutralino IS LSP M χ > 50GeV

A natural candidate not invented to solve Dark Matter Notes WIMP Candidate 2 Kaluza-Klein Dark Matter This important candidate comes from the theory of Universal Extra Dimensions (UED) where all standard model particles propagate into flat dimensions. It produces a WIMP called the Lightest Kaluza-Klein particle (LKP). Extra Dimensions Extra dimensions is used to lower the fundamental Planck scale, MP ~ 1019 GeV, close to the electroweak scale by allowing gravity to propagate in (3 + 1) + n dimensions whilst confining the Standard Model particles to the 3 + 1 (Minkowski subspace) In this case, the apparent Planck scale is related to the n/2 fundamental scale MF via MP ~ MF (RMF ) size of the extra dimensions As these extra dimensions modify the usual inverse-square law of gravity at distances < R, their size must be < 1mm in order not to conflict with the results from current short range gravitational experiments. Any extra dimensions must therefore be compactified, and so R represents their compactification radius. A significant feature of extra dimensional theories is heavy ‘Kaluza- Klein’ particles that can propagate in the extra dimensions. KK Dark Matter Candidate For 1 dimension plus 1 extra compact dimension, the compactification can be visualised as an infinite garden hose with a small but finite radius. At large distances only the length of the wire can be perceived, but sufficiently close to the wire the finite distance around the wire can also be resolved.

Consider 1 extra spatial dimensions curled up in a small circle Particles moving in extra dimensions appear as a set of copies of normal particles.

LKP (lightest KK particle) is stable –dark matter! KK Dark Matter Relic Density Some predictions from UED for KK dark matter particles, showing that they could make up all the dark matter

KK Rather like R-parity in SUSY there is KK-parity (−1) in UED Summary on DM particles

We like CDM and WIMPs and Neutralinos

(1) CDM favours structure formation (2) Such particles match well generic arguments about freeze-out in the (3) There are well motivated candidates predicted independently from particle physics

So WIMPs are the best generic candidate and of these we like Neutralinos from SUSY Standard Model Particle Physics Extra Note This is a model that accounts for the vast majority of the known behaviour of the known particle types, either present in nature or discovered in accelerators. It supercedes older classical field theories like classical electromagnetism, which means that it explains all the phenomena explained by these theories and then also explains some outside the context of those theories. It does NOT superscede classical gravitation ! The basic idea is that there is a set of fundamental spin-1/2 particles that interact with each other by the exchange of some fundamental spin-1 particles. The word particle is used loosely here - these are quantum mechanical objects having both particle and wave-like properties. EG: Quantum Electrodynamics - The quantum theory of electromagnetism Extra Note We know that electromagnetic radiation is emitted and absorbed by electrically charged particles being accelerated. What is the quantum foundation of this classical theory? The quantum theory of electromagnetic interactions is called quantum electrodynamics. It was invented by Julian Schwinger, Richard Feynmann, and others in the 1950s. Quantum electrodynamics is integral to the standard model of particle physics. Classical vs. Quantum Extra Note Electrodynamics

fundamental coupling

Scattering of charged particles by each other (Moller scattering)

scattering of light by charged particles (Compton scattering) The Weak Interactions Defining characteristics: Extra Note (a) Much feebler than the electromagnetic interactions, at least when the energies of the interacting particles are much less than 90GeV. At 90GeV, they dominate over the electromagnetic interactions, and above 90GeV the strengths of the two interactions are compatible. Hence the name... (b) Short range - particles separated by more than the size of roughly an will not be affected. Because of these characteristics, weak interactions were not noticed until the latter half of the 20th century, and there was never any call for a classical theory of weak interactions.

So how do you make an interaction feeble and short range ? Summary on SUSY Particle theorists like supersymmetry because they could solve the higgs self energy problem, the problem of quantizing gravity and the problem of unifying the fundamental forces at high energies (in the early Universe) in one go! Particle experimentalists like supersymmetry because supersymmetric particles can be searched for in machines we can (just about) afford to build and run, and finding them may allow experiment to jump ahead of theory by providing sorely needed experimental data. Particle Astrophysicists like supersymmetry because it yields a class of dark matter candidates of about the right mass to close the universe with standard astrophysics phenomenology based on the standard model. BUT making supersymmetric particles in accelerators does not prove they are dark matter ! Basic Neutralino/WIMP Properties

Main characteristics of the particles • Mass in the range • Number density in the range per litre • De Broglie wavelength in the range Main characteristics of the collisions • Short range • Elastic - same particles come out as go in to collision. • Rare - cross section upper limits are currently ~10-[7,8] pb

Rare processes mean that we need as many ‘detectors’ as possible. Use solid matter, where each nucleus is a potential detector. Look for materials having CHARACTERISTIC SIGNATURES that are detectable by existing technology. Notes e.g. describe the relationship between the terms CDM, WIMP, Neutralino, LSP and LKP. Neutralino Dark Matter Properties REST ENERGY:

Lower limit because at this Upper limit is that if the energy mas they would have been scale for supersymmetric detected in accelerators but partners is this high, they no in fact have not been. longer rid the standard model of infinities. Thye may also over-close the Universe. Neutralino Interactions Rememeber the term WIMP encompases any particle with the properties of CDM above. Neutralinos are the SUSY candidate for WIMPs, the best motivated from particle physics. NEUTRALINO

if we assume the WIMP is a neutralino we make firmer predictions of the interactions

QUARK FROM THE NUCLEUS Both the Z zero and the q tilde particles are weakly interacting, heavy exchange particles. Therefore the neutralino is a STABLE, WEAKLY INTERACTING MASSIVE PARTICLE. WIMPs could be neutralinos ! And they could collide with nuclei and cause them to scatter ! How do Neutralinos/WIMPs interact?

HALO WIMP WIMP rest energy, 10-1000 GeV. Velocity about 220km/s

ELASTIC SCATTERING NUCLEUS OF A WIMP OFF rest energy, A NUCLEUS 10-200 GeV. At rest in lab.

VERY IMPROBABLE FOR NUCLEUS RECOILS ANY SINGLE NUCLEUS DUE TO FEEBLE NATURE OF WEAK INTERACTIONS How do Neutralinos/WIMPs interact?

(1) Weakly Interacting, so don’t interact much despite the number of them

(2) Neutral, so not going to interact with the electrons in nuclei, going to get elastic scatters (not inelastic)

(3) Non-relativistic, so we can use classical kinematics

(4) Slow moving, with mass comparable to that of nuclei, so turns out the energy released is quite low, keV region