Introduction
Unavoidable evidence of gravitational interactions between baryonic and a new form of unseen matter can be identified on both the cosmological and astronomical scales. Its nature, almost unpredictable. A common assumption is made that elementary particles could be the constituents of this dark matter. These new particles in the account of the dark matter seemed in various theories distinct from the standard model of particle physics. Numerous experiments have been conducted over the past decades. With the aim to detect these massive particles through their scattering in a detector medium and to give information about particle mass and its interaction with ordinary matter¹..
Structure dark matter halos²
Fig 1. -These experiments are trying to explore the weakly interacting massive particles(WIMPs). In future, direct dark matter detection experiments are expected to have sensitivity in detecting neutrinos. The main interest of experiments is to identify the impact of neutrino signals on apparent WIMPs³ .
This Review article focuses on -
1. Migdal Effect in Direct detection experiments.
2.Background in dark matter direct detection experiments (DMDDE).
3.Current status of direct DDE.
4.Future prospects of direct detection experiments.
5.Conclusion.
1. Migdal Effect in DDE
Detectors are used for dark matter direct search are generally optimized for elastically scattering off dark matter(DM). Due to less energy is transferred from dark matter to the recoiling nucleus, detectors typically lose sensitivity for the dark matter with masses less than a few GeV.
But when the dark matter-nucleus scattering is carried out by non-reducible simultaneous emission of ‘Migdal’ electron or by interactive dark matter electron scattering.
In theory, the Migdal effect still tied to isolated scattering of the dark matter (DM), for which inner-shell electrons should show an expected signal rate⁴. In both, Migdal and electron scattering effects, incoming and outgoing states are same as
Dark matter (DM) plus bound atom and dark matter particle plus free electron. The incoming and outgoing dark matter both are assumed to be a plane wave⁵.
2. Background in direct detection experiments
For a clear understanding of dark matter interaction, ultra-low background experiment conditions are required. It includes both ultra-low and internal radiation background.
Fig.2 – Muon flux as function of km water eq. for many labs of dark matter⁶.
2.1 External backgrounds
Much of the radiation from gamma decays emerges from the decays in thorium, uranium, K, Co and Cs (i.e decays of common isotopes) present in nearby material. The Uranium and Thorium chains have alpha and beta decays with the emission of several gamma rays having energy from 10KeV to 20 MeV. Gamma radiation with nearly matching to sensitive volume of dark matter detector can be decreased by selecting matter with low radioactive traces. Gamma- Spectrometry is effective technique used to check radio pure material. The inevitable gamma activity present in natural radioactivity outside the experimental setup can be armature by covering the detector with a material with bigger atomic number and high density. Analysis tools are also used in reducing the background reduction rate.
2.2 Internal background
Though external background is there almost in all types of detectors but it is not so with internal backgrounds, it differs on the state, i.e for crystal and liquid targets.
2.2.1 Crystalline Detectors
Crystalline detectors like Germanium are grown out from purity powder. In the process of growth, impurities are effectively rejected because their ionic radius doesn’t meet the space in crystal gridline. And the growth of crystal itself reduces internal defilement.
2.2.2 Liquid Detector
For both solids and liquids, surface have its special role. For example, Radium deposited at target surface or with any liquid can give rise to background. But the surface treatment by acid and electropolishing has proven efficient in erasing radioactive impurity at surfaces⁷.
3. Current status of dark matter direct detection experiments
In this section, efforts are made to see the recent progress in dark matter direct detection experiments.
Almost a century ago, Astronomers predicted dark matter existence , invisible in entire electromagnetic spectrum. On galactic level, astronomers believe that our milky way is covered by an diffusive and extended dark matter halo. In recent years, considerable experimental progress has made in direct detection of weakly interactive massive particles( WIMPs).
To date, except controversial claims, no signal of weakly interactive massive particle has found in these detection experiments. But these experiments are successful in the direction to constraint variety of theories.
At present, a fight race is going between a few xenon experiments.
In Stanford Underground Research Facility (SURF)29, USA, started taking physics data on LUX28, a 250 kg xenon experiment. The half-ton scale PandaX-11 experiment done at Jinping underground Laboratory (CJPL) in china⁸.
4. Future prospects of Direct Detection experiments
The future direct detection experiments are expected with more
Sensitive and bigger experiments which are in developing phase. Mainly, this is true for light DM due to experiments are developing with low energy threshold like SuperCDMS2 and revision in knowing the response of xenon-based experiments with energies lower than 3KeV. The sensitivity will improve in coming years because the background presence from solar neutrino will reduce⁹.
However, using the elastic scattering, the gravitational capture of dark matter is possible inside the Sun. Neutrinos interacting with nuclei will be constraining the DME sensitivity for the dark matter searches of cross section 10^-49cm2. Possibly there are techniques to conquer neutrino background at these cross section, A dark matter theory is expected to be appear before this neutrinos challenging background.
5. Conclusion
Various technologies are trying to find dark matter interactions using direct detection. In general, direct detection experiments propose high sensitivity at heavy weakly interactive massive particle masses because of the limited COM energy in collider. It is noticed that the collider findings can’t scale the DM particle and its life time. So, the final confirmation of such detection can only be done in addition with direct detection results.
In addition to it, a demand for the comparison of indirect searches with direct one has raised from the time when earlier approach is only sensitive to the thermally averaged self annihilation cross section of dark matter. And it is necessary to lessen the backgrounds from natural radioactivity and of cosmogenic origin. However, the detection of dark matter is possibly with gravitational capture of dark matter particles in the sun with elastic scattering.
To date, although no sure proofs of dark matter has seen but great progress has been made in direct dark matter searches ¹⁰.
References
[1,6,7,10] - Undagoitia T.M and Rauch, L. Dark matter direct detection experiments.
2. Navarro, J.F. The Structure of Cold Dark Matter Halos, 255-258, 1996
3. Billard, J. and Feliciano, E.F. Implication of neutrino backgrounds on the reach of next generation dark matter direct detection experiments, 89(2) 023524,2014
4. Essig, R., Pradler, J., Sholapurkar, M. and Yu, T.T. Relation between the Migdal Effect and Dark Matter- Electron Scattering in Isolated Atoms and Semiconductors, 124(2), 021801,2020
5. Baxter, D. , Kahn, Y. and Krnjaic. G, Electron ionisation via dark matter- electron scattering and the migdal effect.
8. Liu. J, Chen. X and Ji X., Current Status of direct dark matter detection experiments, 212- 216, 2017.
9. Davis J. H., The past and future of light dark matter direct detection, 30(15), 1530038,2015.
By Deepak Kumar
deepakyadav101.com@gmail.com
Wonderfull 💯👏👏