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1 INTRODUCTION

This can be seen as follows. The scale factor E TF corresponds to 0. Figure 2. LSS electronic S e and nuclear S n stopping as a function of scaled energy. The energy axis for homoatomic recoils of 19 F is also shown. Nuclear stopping yields less ionization and electronic excitation per unit energy loss than does electronic stopping, implying that the W factor, defined as the energy loss required to create one ionization electron, will be larger for nuclear recoils.

Experimenters use empirical 'quenching factors' to describe the variation of energy per unit of ionization the ' W ' parameter compared with that from x-rays. The different microscopic distribution of ionization in tracks dominated by nuclear stopping can also lead to unexpected changes in the interactions of ionized and electronically excited target atoms e. Such interactions are important for particle identification signatures such as the quantity and pulse shape of scintillation light output, the variation of scintillation pulse shape with applied electric field, and the field variation of ionization charge collection efficiency.

The relatively small contribution of electronic stopping and the small variation in k for homoatomic recoils, makes the total scaled range for this case depend on the target and projectile almost entirely through E TF. Predictions for the actual range of homoatomic recoils can be obtained from the nearly universal scaled range curve as follows. This is of the order of a few millimeters for a monoatomic gas at 0.

As a consequence, tracking devices for the DM detection must provide accurate reconstruction of tracks with typical lengths between 1 and a few millimeters while operating at pressures of a small fraction of an atmosphere. Figure 3. LSS range as a function of scaled energy.

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The energy axes for homoatomic 19 F and Xe recoils are also shown. Multiply the vertical axis values by 3. When comparing LSS predictions with experimental results, two correction factors must be considered. On the other hand, many older experiments report 'extrapolated ranges', which are closer in magnitude to the path length than to the 'projected range'. This correction has generally been applied in the next section, where experimental data are discussed. In addition, it must be noted that the LSS calculations described above were obtained for solids. In condensed phases, the higher collision frequency results in a higher probability for stripping of excited electrons before they can relax, which leads to a higher energy loss rate than for gases.

This correction is rather uncertain and has generally not been applied in the following section of this paper. The literature of energy loss and stopping of fast particles in matter is vast and still growing [ 29 , 30 ]. The stopped particles were collected on segmented walls of the target chamber and later counted. Typical results were ranges of 2 3. Accuracy of agreement with the prediction from the SRIM code is about the same. As in all other cases discussed below, the direction of the deviation from LSS is as expected from the gas—solid effect mentioned in the previous section.

The device was simulated fitting the observed pulse height and event size distributions. Tracking readouts in gas TPC detectors are sensitive only to ionization of the gas. As noted above, both nuclear stopping and electronic stopping eventually contribute to both electronic excitations including ionization and to kinetic energy of target atoms, as primary and subsequent generations of collision products interact further with the medium. Several direct measurements of total ionization by very low-energy particles are available in the literature.

To summarize, most of the DM recoils expected from an isothermal galactic halo have very low energies, and therefore nuclear stopping plays an important role. Without applying any gas-phase correction, LSS-based estimates for range tend to be slightly shorter than those experimentally measured in gases.

The predicted ionization parameter W also tends to be slightly lower than the experimental data. This situation is adequate for initial design of detectors, but with the present literature base, each individual experiment will require its own dedicated calibration measurements. From the range-energy discussion in the previous section, we infer that track lengths of typical DM recoils will be only of the order of 0. The ancient mica etch pit technique was actually used to obtain DM limits.

However, recently the focus of directional DM detection has shifted to low-pressure gas targets, and that is the topic of the present review. The active target volume contains only the active gas, free of background-producing material. Only one wall of the active volume requires a readout system, leading to favorable cost-volume scaling.

Gaseous DM detectors have excellent background rejection capability for different kinds of backgrounds. First and foremost, direction sensitivity gives gas detectors the capability of statistically rejecting neutron and neutrino backgrounds. The energy loss rates for recoils discussed in the previous section are hundreds of times larger than those of electrons with comparable total energy.

The resulting much longer electron tracks are easily identified and rejected in any direction-sensitive detector. It can be shown that there is an optimum pressure for operation of any given direction sensitive WIMP recoil detector.

1 INTRODUCTION

This optimum pressure depends on the fill gas, the halo parameter set and WIMP mass, and the expected track length threshold for direction measurement. The total sensitive mass, and hence the total number of expected events, increases proportionally to the product of the pressure P and the active volume V.

Searching for Dark Matter Among the Rocks

Operating at this optimum pressure, the track-able event rate still scales as P opt V , which increases linearly as the tracking threshold decreases. Achieving the shortest possible tracking threshold R min is seen to be the key to sensitive experiments of this type. Diffusion of track charge during its drift to the readout plane sets the ultimate limit on how short a track can be measured in a TPC.

Diffusion in gases has a rich phenomenology for which only a simplified discussion is given here. Here k is the Boltzmann constant, T the gas temperature, and L the drift distance. No pressure or gas dependence appears in this equation. The diffusion decreases inversely as the square root of the applied drift field. Increasing the drift field would appear to allow diffusion to be reduced as much as desired, allowing large detectors to be built while preserving good tracking resolution.

However, in reality diffusion is not so easily controlled. This condition amounts to requiring that the work done by the drift field on a charge carrier between collisions and not lost to collisions, must be much smaller than the carrier's thermal energy. If this condition is fulfilled it will ensure that the drifting carriers' random thermal velocity remains consistent with the bulk gas temperature. Importantly, E d max for electrons in a given gas generally scales inversely as the pressure, as would be expected from the presence of the mean free path in the 'low field' condition.

Feature - Sifting for dark matter

If the drift field exceeds E d max , the energy gained from the drift field becomes non-negligible. The average energy of drifting charge carriers begins to increase appreciably, giving them an effective temperature T eff , which can be orders of magnitude larger than that of the bulk gas. Diffusion stops dropping with increasing drift field and may rapidly increase in this regime, with longitudinal diffusion increasing more rapidly than transverse. This is because the ions' masses are comparable to the gas molecules, so the energy-exchange-efficiency factor f , which determines E d max is much larger than for electrons.

Ion—molecule scattering cross sections also tend to be larger than electron—molecule cross sections. The above outline shows that diffusion places serious constraints on the design of detectors with large sensitive mass and millimeter track resolution, particularly when using a conventional electron drift TPC. To be competitive, directional detectors should be able to use comparable exposures. However, integrating large exposures is particularly difficult for low-pressure gaseous detectors.

This mass of low-pressure gas would occupy thousands of cubic meters. It is, therefore, key to the success of the directional DM program to develop detectors with a low cost per unit volume. Since for standard gaseous detectors the largest expense is represented by the cost of the readout electronics, it follows that a low-cost readout is essential to make DM directional detectors financially viable.

These authors explicitly considered the directional signature, but they did not publish any experimental findings. This pioneering work used optical readout of light produced in a parallel plate avalanche counter PPAC located at the readout plane of a low-pressure TPC. The CS 2 gas fill allowed diffusion suppression by running with very high drift fields despite the low pressure. The detectors were calibrated with alpha particles, 55 Fe x-rays and Cf neutrons. Neutron exposures gave energy spectra in agreement with simulations when the energy per ion pair W was adjusted in accordance with the discussion of ionization yields given above.

The absence of nonzero spin nuclides in the CS 2 will require a very large increase in target mass or a change of gas fill in order to detect WIMPs with this device. It was shown above that the event rate and therefore the sensitivity of an optimized tracking detector improves linearly as the track length threshold gets smaller. In recent years, there has been widespread development of gas detectors achieving very high spatial resolution by using micropatterned gain elements in place of wires. The gas CF 4 also figures prominently in recent micropattern DM search proposals.

The ultraviolet part of the spectrum may also be seen by making use of a wavelength shifter. Finally, CF 4 is non-flammable and non-toxic, and, therefore, safe to operate underground. This group has recently published the first limit on DM interactions derived from the absence of a directional modulation during a 0.

Operation at higher-than-optimal gas pressure was chosen to enhance the HV stability of the gain structure. One array is connected to the central anode dots of the micro-well gain structure, and the other array to the surrounding cathodes. The strip amplifiers and position decoding electronics are on-board with the gain structures themselves, using an eight layer PCB structure. The detector was calibrated with a Cf neutron source. The advantages claimed for 3 He as a DM search target include nonzero nuclear spin, low mass and hence sensitivity to low WIMP masses, and a very low Compton cross section, which suppresses backgrounds from gamma rays.

Introduction

The characteristic n , p capture interaction with slow neutrons gives a strong signature for the presence of slow neutrons. The detector is read out by an array of CCD cameras and photomultipliers PMTs mounted outside the vessel to reduce the amount of radioactive material in the active volume. The CCD cameras image the visible and near infrared photons that are produced by the avalanche process in the amplification region, providing a projection of the 3D nuclear recoil on the 2D amplification plane.

The 3D track length and direction of the recoiling nucleus is reconstructed by combining the measurement of the projection along the amplification plane from pattern recognition in the CCD with the projection along the direction of drift, determined from the waveform of the signal from the PMTs.

The correlation between the energy of the recoil, proportional to the number of photons collected in the CCD, and the length of the recoil track provides an excellent rejection of all electromagnetic backgrounds. In a second design the copper plate was replaced with two additional woven meshes.

This design has the advantage of creating a transparent amplification region, which allows a substantial cost reduction since a single CCD camera can image tracks originating in two drift regions located on either side of a single amplification region. Gas gain is obtained using the mesh-plate design described above. The detector is read out by two CCD cameras, each imaging one drift region. This apparatus is currently being operated above ground with the goal of characterizing the detector response and understanding its backgrounds.

A second liter module is being constructed for underground operations at the waste isolation pilot plant WIPP in New Mexico. The performance of the DMTPC detector in determining the sense and direction of nuclear recoils has been evaluated by studying the recoil of fluorine nuclei in interaction with low-energy neutrons.


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The Q factor represents the effective fraction of reconstructed recoils with head—tail information, and the error on the head—tail asymmetry scales as. The apparatus consists of a stainless steel vessel of 1. Nine CCD cameras and nine PMTs are mounted on each of the top and bottom plates of the vessel, separated from the active volume of the detector by an acrylic window.

The detector consists of two optically separated regions. Each of these regions is equipped with a triple-mesh amplification device, located between two symmetric drift regions. Each drift region has a diameter of 1. A gas system recirculates and purifies the CF 4. Directional detectors can provide an unambiguous positive observation of DM particles even in presence of insidious backgrounds, such as neutrons or neutrinos. In the past decade, several groups have investigated new ideas to develop directional DM detectors.

Low-pressure TPCs are best suited for this purpose if an accurate sub-millimeter 3D reconstruction of the nuclear recoil can be achieved.


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A good tracking resolution also allows for an effective rejection of all electromagnetic backgrounds, in addition to statistical discrimination against neutrinos and neutrons based on the directional signature. The choice of different gaseous targets makes these detectors well suited for the study of both spin-dependent CS 2 or spin-independent CF 4 and 3 He interactions.

The challenge for the field of directional DM detection is now to develop and deploy very sensitive and yet inexpensive readout solutions, which will make large directional detectors financially viable. The authors are grateful to D Dujmic and M Morii for useful discussions and for proofreading the manuscript.

CJM is supported by Fermilab. A homoatomic molecular entity is a molecular entity consisting of one or more atoms of the same element. The scale factors are in cgs-Gaussian units : ,. The parameter becomes substantially larger only for light recoils in heavy targets. IOPscience Google Scholar. Crossref Google Scholar. Crossref PubMed Google Scholar. Google Scholar. It is named after Enrich Fermi, an Italian-American scientist who did pioneering work in highenergy physics. Theories of dark matter, mainly invented by Particle Physicists, propose that it is a new sector of matter very different than normal matter.

In a group of physicists at Stanford came up with an idea. They wanted to develop a large telescope to see the entire universe and the gamma ray sky in order to find out more about dark matter. With this new Gamma Ray Telescope, the largest ever launched in space, this group of physicists wanted to find out more about dark matter and the hi energy universe we cannot see. Now after ten years in space, the scientists have some results. And they have done way beyond what they initially thought they could do.

The LCH searches for dark matter by attempting to produce it using its high energy circulating beams of protons in proton-proton collisions. Except for indications from gravitational interactions, all of these techniques have so far come up empty with no observation of dark matter, just limits. Currently, theory indicates that we should be searching using all three experimental techniques as one or the other may be more sensitive, though unpredictably so.

Dark Matter, the Invisible Component. The largest all sky telescope, Fermi has answered persistent questions about super massive blackhole systems, pulsars, the origin of cosmic rays, and searches for signals of new physics. Eric Charles who has been on the project since , three years before it launched said that he came to the project because it was a new kind of telescope and incredibly appealing.

Our eyes have evolved to create a view that makes sense, but there is so much more to see. Pulsars are rapidly spinning neutron stars, super dense objects forged when a massive star collapses and explodes as a supernova. Young neutron stars spin dozens of times a second and gradually slow with age.


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Dizzying spin coupled with super strong magnetic and electric fields make pulsars superb natural particle accelerators, nearly 1, times more powerful than any machine on Earth. Originally discovered nearly 50 years ago by their radio emissions, more than 2, pulsars have been identified to date. With a big budget increase for particle physics this year million , future scientists searching for answers to the mysterious dark matter can go back to fundamental science says Dr. Applied science was doing very well, he says, but science was dying.

Fundamental science is the long term. The LAT collaboration includes more than scientists and students at more than 90 universities and laboratories in 12 countries. This picture is an all sky intensity map derived from 9 years of Fermi-LAT data, August 4, - August 4, time integrated all-sky image based on Pass 8 Source class, PSF3 event type-internal collaboration notation.

The map is also integrated above 1 GeV that is 1 billion electron volt energy gamma rays and above up to about 1 trillion electron volt energy. The intensity scale is false color with low intensity black and high intensity white. The map shows the entire universe in standard astronomical Galactic coordinates in what is called a Hammer-Aitoff projection — not simple , these coordinates and projection have the center of our galaxy at the center of the picture very bright , and the anticenter of the galaxy at the edges of the picture centerline the anticenter is in the direction out from the Galactic center along the Galactic center-sun axis The images are smoothed with a 0.

The maps are in intensity units. The images have a logarithmic scaling, from about 3 x gamma rays per cm-2 s-1 sr-1 to 1 x gamma rays per cm-2 s-1 sr The actual maximum intensity in the map is 0. The images are x 0. The band that you see horizontally across the picture is the Milky Way Galaxy our galaxy. Above this plane and below this plane are gamma ray sources from the rest of the universe and typically many millions to billions of light years away from us. These structures are thought to be remnants of energetic jet activity powered by the central black hole of our galaxy in times long past and were discovered using Fermi-LAT data.

Picture 2: Diffuse - in a complex analysis we can isolate the Galactic diffuse signal and it looks as in this picture.

Feature - Sifting for dark matter

This gamma ray signal mostly originates in the interaction of Galactic cosmic rays with the gas of the Galaxy. Picture 3: Point sources - In different complex analysis we can extract the point and near point sources of gamma rays. Picture 4: Extra Galactic diffuse — In yet a different complex analysis we can extract the diffuse gamma ray radiation coming from beyond our galaxy, likely from the furthest reaches of the universe.

This diffuse is uniform from all directions. Picture 5: Use Dwarf Galaxies to search for dark matter - This picture is similar to the point sources in which I try to demonstrate how we can set limits on gamma rays coming from dark matter particle decay or annihilationwith another DM particle. There are now about 60 dwarf satellite galaxies of the Milky Way that have been discovered and new ones are discovered every year via ground based optical telescope surveys of the sky.

One can measure from the stellar motions in these galaxies using big optical telescopes that they are heavily dark matter dominated. The rest of the energy density of the universe is made up of dark energy. What is it? The Fermi-LAT community has be searching for emission of gamma -rays from dark matter that has been predicted by theoretical models of what might make up the dark matter new types of elementary particles.

This last picture shows as a green dot a hypothetical dwarf galaxy located where there are no gamma ray point sources this is typical in our data, though this sky location is hypothetical. Optical telescopes would measure the presence of a small galaxy with not too many stars, and also indicate that the dwarf galaxy was dark matter dominated. Fermi knows where to look as the optical telescopes precisely establish the location of this dwarf galaxy. When the Fermi — LAT does its observations and analysis of this spot it finds no emission of gamma-rays, and then we can set limits on the gamma ray intensity from this dwarf.