PHY326/426 Dark Matter and the Universe. Dr. Vitaly Kudryavtsev F9b, Tel.:

Transcription

1 PHY326/426 Dark Matter and the Universe Dr. Vitaly Kudryavtsev F9b, Tel.:

2 Indirect searches for dark matter WIMPs Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 2

3 Weekly interacting massive particles (WIMPs) Non-baryonic dark matter should be: Stable; Neutral; Weakly interacting; Should have been produced in large numbers at early stages of the Universe; The mass density should be about 0.3 GeV/cm 3 at the Earth orbit. WIMPs (Weakly Interacting Massive Particles) are currently the best candidates. They are predicted by the particle physics theory - Supersymmetry (SUSY). Mass ~ GeV/c 2 - similar to heavy atoms. Velocities ~200 km/s; kinetic energies - up to a few hundred kev. Expected WIMP-proton cross-section: pb ( cm 2 ). If WIMPs are responsible for all dark matter in the Galactic halo, then their flux at the Earth should be about particles/cm 2 /s (depends on WIMP mass). Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 3

4 What is the indirect detection? Direct detection - WIMP (neutralino) scattering in the target material within the detector, nuclear recoils can be detected. Indirect detection - WIMPs produce secondary particles somewhere else (Sun, Earth, Galactic centre, halo) and these secondary particles (gammas, neutrinos, positrons, antiprotons) can be detected in a detector. Production of secondary particles: WIMP s annihilations (WIMPs are Majorana particles - a particle is equal to its antiparticle). In this case enough of energy is converted into other particles. If a simple scattering occurs outside the detector, then recoils have too small energy to reach the detector. (In direct detection techniques - recoils are produced within the detector). Secondary particles produced: hadrons which then decay (eventually) into gammas, neutrinos and other stable particles; also gammas. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 4

5 Detectors In space: EGRET (completed), PAMELA, Fermi Gamma Ray Telescope, AMS (running), CALET (future) - gamma fluxes, positrons, antiprotons. On the ground: Air Cherenkov Telescopes - need low energy threshold - gamma fluxes. Underwater (under-ice): neutrinos. Only gammas, neutrinos, positrons and antiprotons from neutralino annihilations can be discriminated from much larger background of cosmic rays. Protons, electrons (produced in pairs with antiprotons and positrons) and other particles cannot be discriminated from the background. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 5

6 Monochromatic gamma-rays Gamma-ray lines: Initiated by the processes χ χ γ γ or χ χ Z γ. Clear signature - a line superimposed on the continuum spectrum of background gamma rays, E γ = m χ or E γ = m χ (1 - m Z 2 / 4 m χ 2 ) (about GeV). Should come from the regions where the WIMP concentration is high (Galactic Centre - due to the gravitational field of the massive black hole) - directionality. Rather low fluxes. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 6

7 Gamma-ray lines Example of the expected signal - simulated signal from neutralino (78 GeV rest mass) annihilations in the Galactic halo. This particular example is a simulation for CALET experiment (collaboration of Japanese universities) at the International Space Station (in preparation). Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 7

8 Continuum gamma-rays Continuum: Initiated by the decay of hadrons: χ χ π 0 γ γ. Lower energy (compared to gamma-ray lines). Higher rates / annihilation (the process of hadron production is favoured compared to the production of a pair of photons). But no clear signature (such as a line). Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 8

9 Gamma-rays from the Galactic Centre Gamma rays from the Galactic Centre as seen by the Air Cherenkov Telescopes. Energy spectra agree with neutralino annihilation hypothesis but can be something else. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 9

10 Gamma excess in EGRET data Gamma excess is consistent with neutralino annihilations: m χ ~ GeV Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 10

11 Alpha Magnetic Spectrometer - AMS-02 Transition radiation detector gamma-factor of the highest-energy particles: ~E / m. The silicon tracker - particle's path. Superconducting magnet makes the particle's path curved: r ~ p / Ze ; charge sign. Two time-of-flight counters - velocity of lower-energy particles. Two star tracker cameras - orientation in space. Underneath AMS, a ring-imaging Cherenkov detector - accurate velocity measurement for fast particles: β = 1 / ( n cos θ ); also N γ ~ Z 2. Electromagnetic calorimeter - total energy and type of particles. An anti-coincidence veto counter notices stray particles sneaking through AMS sideways. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 11

12 Alpha Magnetic Spectrometer - AMS-02 Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 12

13 Alpha Magnetic Spectrometer - AMS-02 Pictures from AMS web-site Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 13

14 Antiprotons Excess over the background should be significant for high mass WIMPs. Simulated diffuse background and antiproton spectrum from neutralino annihilation in the halo (from Morselli s talk at IDM2004). Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 14

15 Positrons and PAMELA data Adriani et al. Nature 458, (2009) Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 15

16 Antiprotons and PAMELA data Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 16

17 AMS: positrons Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 17

18 Fermi telescope data (satellite) Gamma-ray flux from the Galactic Centre. A line at 130 GeV has been found. Consistent with 130 GeV mass WIMPs. Not confirmed by other experiments (which may not be sensitive enough). It appeared to be an instrumental effect. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 18

19 Neutrinos ν χ ν χ are slowing down by interactions with nuclei and are trapped in the core χ χ W + W -, W ± H ±, Z 0 H 0, Z 0 Z 0, τ + τ -, q q These particles decay into others eventually producing neutrinos. Only neutrinos can travel long distances inside dense material. WIMP s velocity ~ 200 km/s. If a WIMP interacts with a nuclei, it loses energy and can be trapped by a gravitational field (if it is strong - massive objects: Sun, Earth, other planets and stars, Galactic Centre). Trapped WIMPs are accumulated in the centres of massive objects and can annihilate with each other if the density is high enough - similar to the case of the annihilations in the halo, but the rate is enhanced since the density of WIMPs is higher (accumulation of WIMPs due to the gravitational field). Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 19

20 Detection of neutrinos Underground (underwater, underice) detectors: Super-Kamiokande, AMANDA-IceCube, ANTARES. Best technique: large water Cherenkov detectors (telescopes) like ANTARES or IceCube - using natural water reservoirs as a target and a detector. Muon neutrinos produce muons which are detected using Cherenkov light. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 20

21 Muon in ANTARES (animation) Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 21

22 Limits from neutrino telescopes Data from existing underground (under-ice) detectors already limit parameters of the SUSY models. Large (1 km 3 ) underwater (under-ice) Cherenkov detectors will be capable of probing the region of parameter space favoured by SUSY models and complement the efforts of direct searches. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 22

23 Summary of indirect searches Signal Rate Clear signature Gamma lines Low Yes Gamma continuum High No Antiprotons from halo High No Positrons from halo Low No Neutrinos from the Sun High Yes Neutrinos from the Earth Low Yes Signal from Galactic Centre High Yes/No Direct detection High Yes High rate means possible detection in the nearest future (5-10 years). Clear signature - easy discrimination from the background. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 23

24 Summary Indirect detection of WIMPs can be done via observation of an excess of gammas, neutrinos, positrons and antiprotons, in particular from certain astrophysical objects expected to have higher neutralino concentrations (Galactic Centre, Sun, centre of the Earth ). Several experiments have been designed and are or will be looking for WIMP annihilations. Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 24