The Search for Dark Matter. Jim Musser

Transcription

1 The Search for Dark Matter Jim Musser

2 Composition of the Universe Dark Matter There is an emerging consensus that the Universe is made of of roughly 70% Dark Energy, (see Stu s talk), 25% Dark Matter, and 5 % normal matter.

3 What is Dark Matter Dark Matter is the name given to the unseen material that dominates the gravitational behavior of large scale astrophysical systems. We now think that its most likely that DM is a form of matter unlike that from which we are made. That is, it is a form of matter that is dark because it responds to the gravitational force, but not to the electromagnetic or strong force. This property of DM makes it extremely challenging to observe directly!

4 Dark Matter Candidates The type of dark matter we are discussing is called non-baryonic to distinguish it from DM made from normal matter (ie( asteroids, planets, etc). Non-Baryonic DM is further subdivided into two categories: light (hot), and heavy (cold) Primary Hot DM candidate: Neutrinos Primary Cold DM candidate: Lightest Super- Symmetric Particle (LSP)

5 Dark Matter Candidates: Neutrinos Neutrinos are particles emitted in the decay of a neutron to a proton and electron. (this is called beta decay). Historically, they were thought to be massless,, like the photon. Neutrino Neutron Proton Electron

6 Neutrino Mass Over the last decade, numerous experiments have indicated that neutrinos do have mass. These experiments exploit a quantum- mechanical phenomena the probability of a neutrino of one type (there are three types of neutrinos) to change to a neutrino of a second type. This phenomena occurs ONLY if neutrinos of different types have different non-zero masses.

7 Neutrino Oscillation Neutrinos that are generated as one type morph to neutrinos of one of the other two types as they travel. The probability that this transformation occurs depends upon the pathlength traveled, the energy of the neutrino, and the difference in mass between the two neutrino types.

8 A example: MINOS Veto Shield Coil

9 MINOS Evidence for Neutrino Mass Observe neutrinos that interact in the MINOS far detector. This is a RARE event only 216 in a year. The distribution of energies of these events is distorted by the oscillation process, and the number of observed events is reduced, if neutrinos have mass.

10 Neutrinos as Dark Matter Although it is now clear from MINOS and many other experiments that neutrinos have mass, this mass is very small. This means that astrophysical neutrinos have velocities close to the speed of light. This limits their ability to cluster on small scales (by small, we mean on the scale of galaxies and clusters of galaxies). Consequently, another, heavier, DM component is needed to explain the clumpiness of the Universe on this scale.

11 Heavy (Cold) Dark Matter (WIMPS) The most likely form of Cold Dark Matter - Super-Symmetric Symmetric Particles (SUSY) The theory of Super-Symmetry Symmetry postulates that for every normal particle there exists a super- symmetric partner.. This postulate fixes an asymmetry in the standard theory of fundamental particles, in which all particles that experience forces (quarks and leptons) have spins that come in units of ½,, while carriers of forces (e.g. the photon) have spins that come in integer units. In SUSY, all but the least massive SUSY particle is unstable, and would never be seen except at an accelerator. The lightest SUSY particle, however, is stable and charge neutral, and is an natural candidate for cold dark matter. It s s mass is expected to be large.

12 Techniques for Detecting WIMPs Direct: The DM particle interacts in the volume of a detector, colliding with a nucleus, causing the nucleus to recoil and release energy in the detector. The released energy is seen either as electric signals (solid state detectors, light (scintillators( scintillators), or heat (bolometers( bolometers) The expected rate of interactions is VERY small (<1 event per kilogram of detector per day.) Primary background is from normal neutral particles (ie( ie, neutrons and gamma rays), and so shielding from these is extremely important. These experiments are normally conducted deep underground, within heavy layers of shielding. (Lead recovered from sunken ships is usually used to make sure that the lead is not itself radioactive)

13 Sources of Background Detectors must effectively discriminate between Nuclear Recoils (Neutrons, WIMPs) Electron Recoils (gammas, betas)

14 Direct Cold Dark Matter Detection CDM particles are very heavy (100 s s of times heavier than a proton) they have a low velocity wrt the speed of light. That means, when we swim through the sea of CDM particles as the sun orbits the center of the galaxy, and the Earth orbits the sun, the t velocity of the CDM relative to us changes with the time of year. In June, the Sun s s motion in the Galaxy is aligned with the Earth s motion about the sun, and the apparent velocity of CDM particles increases. In December, the two velocities are opposed. This effects the event rates seen in Dark Matter detectors, and we expect this event rate to modulate with the time of year.

15 An Proto-typical typical DM Experiment - DAMA

16 DAMA Claims CDM Observation! DAMA - Dark Matter experiment located in the underground lab at Gran Sasso,, Italy, reported the observation of an annual modulation of the rates observed in their detector (NaI( NaI-based). This result is currently under dispute, as more sensitive experiments (CDMS) have not seen a signal, and have much better sensitivity and background rejection than DAMA.

17 CDMS the current state of the art CDMS (Cold Dark Matter Search), is a solid state detector now taking data in the Soudan Mine in northern Minnesota. The CDMS detector is unique in it s s ability to distinguish signals from Dark Matter particles from gamma signals originating for radioactive decay of nearby material.

18 CDMS II Limit Event rates in DM detection depends up mass of DM particle, and strength of interaction between DM particle and normal matter. Neither are known, and so when no signal is seen the experiment places limits on allowed possibilities for mass and interaction strength. The CDMS experiment has ruled out the range of values allowed by the earlier DAMA result.

19 Future Direct Searches DUSEL (US Deep Underground Science and Engineering Laboratory) Likely site for future DM searches. Planned experiments will cover all of the range of mass and interaction strength allowed by current theories.

20 DUSEL Deep Underground Science and Engineering Laboratory DUSEL will be sited at existing mine TBD. Construction will begin ~ 2010+

21 The Future of Direct Dark Matter Searches Cryogenic Detectors Solid state detectors like CDMS hard to scale up in size. Future e detectors are likely to use cryogenic liquids (Xe( or Ar) ) as the active volume. Next generation detectors will be 100 kg active mass, a factor of 10 larger than CDMS. The ultimate goal for DUSEL is tonne class detectors, that will be factor of ~1000 more sensitive than CDMS. This field is now moving very rapidly, and chances are extremely good that dark matter will be detected in the lab within the next years.

22 Indirect DM Detection Dark matter can be observed as well by observing the products resulting from the annihilation of two dark matter particles colliding in the interstellar medium. Dark matter particles have the unusual property that they are their own anti-particle, so a dark matter-dark matter collision is also a matter-anti anti-matter collision. (think Star Trek!) The energy released is in the form of normal matter that we can then detect with balloon or satellite-based experiments. The detection of dark matter through the observation of annihilation products is called indirect detection. DM DM Before positron electron gamma ray After

23 Signal e + χ ~ e χ ~

24 Background p p π ± or μ ± e ± p

25 Diffuse Galactic Gamma Ray Flux Space-based gamma ray detectors observe more diffuse gamma rays (gamma rays not coming from a point source) than can be explained by known sources. However, it s hard to know that you haven t just forgotten to include something! A stronger case can be made if the annihilation products being observed have a characteristic energy. The is in fact the case for positrons.

26 HEAT High Energy Antimatter Telescope Balloon-borne experiment designed to measure the abundance of anti-electrons (positrons) as a function of energy. HEAT-e ± Primary goal determine whether the positron abundance is consistent with expectations from background processes, and whether features exist in the positron spectrum that would be consistent with DM annihilation.

27 Scientific Ballooning Launch 9 am Crane hook-up 4 am Balloon at float, 12 pm Balloon inflation 8 am

28 HEAT Positron measurements detection of DM? HEAT was flown three times, and in each flight observed an excess in positron rates in a restricted energy interval. This is roughly what one expects for a dark matter signature. HOWEVER: Several other explanations of this measurement have been offered, involving positron production in various astrophysical sites. More definitive evidence is needed to claim the observation of DM, and our collaboration did not claim the observation of DM. HEAT Moskalenko & Strong, ApJ 493, 694(9 Galactic diffusi calculation

29 THE AMS EXPERIMENT

30 DARK MATTER SEARCH Sad to say, AMS was canceled last month by NASA due to lack of launch opportunities on the Shuttle. (The ~$100M detector is completely built and ready to go!)

31 Conclusion Although there have been claims of the detection of DM (DAMA), and several suggestive measurements, including HEAT, the smoking gun observation has not been made. Future experiments, in particular, future direct measurements at DUSEL using cryogenic detectors, hold great promise for detecting DM in the next decade. NOTE: In addition to the searches I ve I described, one of the goals of the LHC high energy physics program at CERN is the discovery of SUSY. If successful, this will led great weight to the notion that cold dark matter is composed of super-symmetric symmetric particles. STAY TUNED!!