phy326/426 dark matter and the universe/file/phy326-2014... · phy326/426 dark matter and the...

PHY326/426 Dark Matter and the Universe

Dr. Vitaly Kudryavtsev F9b, Tel.: 0114 2224531

[email protected]

Dr. Vitaly Kudryavtsev Dark Matter and the Universe slide 2

Indirect searches for dark matter WIMPs


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/cm3 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 ~10-1000 GeV/c2 - similar to heavy atoms. •  Velocities ~200 km/s; kinetic energies - up to a few hundred keV. •  Expected WIMP-proton cross-section: 10-10-10-8 pb (10-46-10-44 cm2). •  If WIMPs are responsible for all dark matter in the Galactic halo, then their flux at

the Earth should be about 104-105 particles/cm2/s (depends on WIMP mass).


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.


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.


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, Eγ = mχ or Eγ = mχ (1 - mZ

2 / 4 mχ2 ) (about 100-1000 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.


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).


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).


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.


Gamma excess in EGRET data

Gamma excess is consistent with neutralino annihilations: mχ ~50-100 GeV


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 Nγ ~ Z 2 .

•  Electromagnetic calorimeter - total energy and type of particles.

•  An anti-coincidence veto counter notices stray particles sneaking through AMS sideways.



Pictures from AMS web-site


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).


Positrons and PAMELA data

Adriani et al. Nature 458, 607-609 (2009)


Antiprotons and PAMELA data


AMS: positrons


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.


Neutrinos

•  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).

ν ν χ are slowing down by interactions with nuclei and are trapped in the core

χ

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.


Detection of neutrinos

•  Underground (underwater, under-ice) 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.


Muon in ANTARES (animation)


Limits from neutrino telescopes

•  Data from existing underground (under-ice) detectors already limit parameters of the SUSY models.

•  Large (1 km3) 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.


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.


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.

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