Direct detection of dark matter
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Direct Detection of Dark Matter. Ankur Deep Bordoloi , Cristian Gaidau. Outline. Cold Dark Matter (WIMP hypothesis) Why? What? Direct detection techniques What to detect Detector types Cryogenic Dark Matter Detection Experimental method Results. Indirect means:.

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Direct Detection of Dark Matter

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Direct Detection of Dark Matter

Ankur Deep Bordoloi, CristianGaidau


Outline

  • Cold Dark Matter (WIMP hypothesis)

    • Why? What?

  • Direct detection techniques

    • What to detect

    • Detector types

  • Cryogenic Dark Matter Detection

    • Experimental method

    • Results

Indirect means:

Indirect means


Motivation

1.) Galactic rotation curves

Indirect means:

Indirect means

2.)X-ray emission from inter-cluster medium


Why Cold Dark Matter?

  • Non-baryonic dark matter is classified as:

    • Hot Dark Matter (relativistic matter; e.g. neutrino)

    • Cold Dark Matter (non-relativistic matter; axion, neutralino)

COLD DARK MATTER


Candidates

  • Astrophysical observations require the cold dark matter particle to be massive, ~ GeV.

  • As a result, neutrinos and axions are ruled out.

  • Potential cold dark matter candidates are provided by SUSY. The lightest SUSY particle is the superpartner of neutrino – the neutralino – with predicted invariant mass in the range :


WIMPs

Relic density depends on annihilation cross section of the particle:

For non-baryonic DM,

At typical order of weak cross-section : 0.1 pb

Any stable non-baryonic massive particle with weak interaction is a DM candidate


Experimental Techniques

  • Inelastic scattering

    • The WIMP will produce an excited nuclear or electronic state or ionize the atom

    • Background cosmic ray μ, υ, high energy e, p and n from radioactivity

    • Difficult to isolate WIMP from background

  • Elastic scattering

    • The WIMP exchanges energy with the nucleus as a whole, observable recoil.

    • Background consists of radioactive neutrons

    • Easier to isolate from background.


Elastic Scattering

  • Expected scattering rate

Target cross-section

Particle velocity

WIMP particle density

  • Expected energy deposit

Decays exponentially as E


What to look for?

  • WIMP signatures

    • A characteristic recoil spectrum

    • Uniformity in detector

    • Site independent WIMP parameters

    • Annular modulation in recoil rate & event spectrum


Background

  • The low energy regime is dominated by background event (gamma ray, radio-activity)

  • 3 classes of sensitivity:

    • Background free

      • Background noise is estimated as zero

    • Background subtraction

      • Pre-estimated background, later subtracted

    • Background limited

      • No idea about background, reduced sensitivity


Direct Detection Techniques

  • Direct detection techniques are based on 3 secondary effects from nuclear recoil:

    • Ionization (electrons)

    • Scintillation (photons)

    • Heat energy (phonons)

  • The more of these effects being detected, the more is the SNR.


Direct Detection Techniques

  • Solid State detectors (CDMS)

  • CRESST (employs CaWO4 crystals, only phonon detection)

  • DRIFT (directional information)

  • DAMA (NaIscintillator; no event by event discrimination)

  • KIMS (similar to DAMA, used CsI)

  • Xenon (noble liquid target)


DAMA/LIBRA

  • Basic technique: scintillation of thallium doped NaI.

  • Signal extracted from background by using the annular modulation of the dark matter flux.

  • In 2008 reported a positive dark matter signal, corresponding to a 60 GeV WIMP.

  • The only collaboration to report a positive signal.


DAMA/LIBRA

  • Annular modulation of counting rate on DM particle and earthbound target was monitored

  • Results found were claimed to be consistent with expected signal from standard halo model.

  • Not consistent with other experimental results


CDMS

Electron recoil

Ge, Si crystal : 7.6 cm φ, 1 cm thick 240 (100) g;

2 ionization channel 4 phonon sensors

Nuclear recoil

  • Currently, the most sensitive DM detection experiment.


Operation Principle

  • A particle interacts with ZIP through e-recoil (Compton scattering) or n-recoil.

  • Interaction deposits energy in the crystal.

  • A portion of energy (6-33% depending on recoil/material) is converted into ionization then into phonons.

  • An electric field drifts away the charge carriers; collected at the bottom surface.

  • Phonons are collected at the top surface


Operation Principle (cont.)

  • The WIMP particle will interact with the nuclei.

  • Ordinary matter will mostly interact with the electron gas.

  • Thus, identifying the characteristics of an electron vs nucleon recoil provides a very powerful method of discriminating the background.

  • Exception: neutrons. This is the main source of backgrounds in CDMS. Energetic neutrons will produce nuclear recoils indistinguishable from WIMP recoils.


Sources of background noise

  • Radioactive contamination

    • U, Th, 40K decay in the cavern

    • Contamination from the detector & shield

    • Radon contamination

  • Cosmogenicbackgrond

    • High energy muons

    • Neutrino background


Reducing background

  • Background shielding

    • The ~700 m layer of dirt above the cavern reduce the muon flux by a factor of 10^4.

    • The outer layer veto shield for charged particles. This shield covers 99% of the detector’s total surface.

    • Followed by a polyethylene layer and lead to moderate neutrons.

    • Radioactive backgrounds are also suppressed by known gamma ray spectra of the radioactive sources

  • Radon reduction

    • All detectors and electronics are run under a purge


Background estimate

  • CDMS can discriminate nuclear recoil from electron recoil.

  • Measures the ratio, ionization to phonon energy (ionization yield)

  • A timing cut (based on phonon pulse time) is applied as a filter

  • A blind procedure (calibration based masking) was performed to avoid bias.


Search Results

  • WIMP search efficiency

    • Nuclear recoil efficiency vs. phonon recoil efficiency

    • Application of band cuts reduced the efficiency to 30% for Ge and 40% for Si detectors above 20 KeV phonon recoil energy.


Search Results

  • After unblinding the data

No favorable event was found

Before ublinding

After ublinding


Search Results

  • Mass / cross section sensitivity

Better sensitivity

at higher mass

Same minimum cross section as XENON10

  • Soudan detectors have best sensitivity over wide mass range


Search Results

  • Two favorable events were found in 2009!

Could not be interpreted as significant WIMP interaction given 23 % probability of it being from background fluctuation


Conclusion

  • CDMS is the most powerful detector in terms of sensitivity.

  • On 12.17.2009, the CDMS collaboration reported the detection of two events which met the WIMP criteria. However, because of such a small number of events, these could not be declared as true WIMP events.

  • Except for these two possible WIMP candidates detected by CDMS and the controversial results of DAMA, all of the DM experiments have reported null results on WIMP detection.


Future Experiments

  • SuperCDMS, a proposed successor of CDMS II. - more detector towers- improved design of the thermal sensor

  • This will increase the sensitivity by an order of magnitude.

  • Super-Kamiokande is proposed as an independent check of the DAMA results.


References

  • Bruch, Tobias (2010) Dissertation, A Search for Weakly Interacting Particles with the Cryogenic Dark Matter Search Experiment, University of Zurich.

  • Qiu, Xinjie (2009) Dissertation, Advanced Analysis and Background Techniques for Cryogenic Dark Matter Search, University of Minnesota.

  • Sumner, Timothy. J. (2002) Experimental Searches for Dark Matter, Living Reviews in Relativity, Vol. 5

  • Spooner, Neil, Direct Search for Dark Matter

  • Cerdeno D. G, Green A M (2010), Direct Detection of WIMPs, arXiv 1002. 1912v1

  • Longair, Malcolm, Galax Formation (1998) Springer


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