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E. Aprile: Direct Searches of Dark Matter

March 3, 2015

We know something about DM: it is neutral, and mostly cold; it is stable wrt the time scale of the universe lifetime, and not strongly self-interacting. It is non baryonic, and it is not due to a standard model particle.

The theory gives a variety of models and candidates. We will focus on the weak scale, the neutralino, WIMPS, and direct detection of the same.

There is the possibility that these particles, if they exist, will scatter off normal matter. We can search for their atomic collisions. We may calculate how much energy they will impart to nuclei, and this amounts to tens of MeV. So we need detectors with very low energy thresholds, and ultra-low backgrounds as the interaction rates are small. We also need large masses, to study the lowest event rates that these processes could yield.

The expected rate favor the choice of heavy nuclei. For a 100 GeV dark matter particle the predictable cross section is in the 10^-47 cm^2 range. Despite the difficulty of measuring such tiny interaction rates, direct searches are widespread around the world, with scores of experiments. The upper limits on the WIMP-nucleon cross section are nowadays reaching 10^-45 cm^2 for masses in the 10-100 GeV range, for lower masses they are much higher.

The interest in the low-mass region is triggered by Dama but also by CoGeNT, CRESST, CDMS-Si. Excesses of events above known backgrounds. The interpretation of these signals as light dark matter is disfavored by results from other experiments, like CDMS-Ge, XENON-100, LUX, CDEX and others.

Light dark matter is still a viable candidate after direct detection constraints. There are several next-generation cryogenic experiments in construction or planned. SuperCDMS at SNOlab is one. It will focus on the low mass range between 0.3 and 10 GeV. The start of data taking is predicted in 2018, with an ultimate goal of a sensitivity to 8×10^-47 cm^2.

Small detectors with solid targets are also promising. CoGeNT will reach a ultra-low threshold, below the keV, with masses on the scale of kilograms. They expect backgrounds of the order of 1 event per kg of mass per day of data taking.

Then we keep increasing the sensitivity to heavy WIMP with multi-ton scale noble liquid experiments. Recently the increase has been a factor of 10 every two years, so this looks promising. These detectors are either with a single phase or a double phase. An example of the first is XMASS at Kamioka mine, a liquid Xenon vessel. 850kg, only 100 of which are fiducial mass. They expect results this summer. A new phase will increase to 5 ton the total mass (3 fiducial), predicting to reach sensitivity at 10^-47 cm^2 for 50 GeV mass of the WIMP.

In the liquid argon camp another single phase detector is DEAP3600 at SNOLAB. It has 3.6 tonnes of liquid, with 1Ton fiducial, designed for less than 0.2 background events per year. They expect to reach 10^-46 cm^2 sensitivity for 100-GeV particles with few years of exposure. DEAP plans to go to 150T (50T fiducial) mass.

In the two-phase camp there are many TPCs running at present. XENON100 is in operation. LUX at SURF also started a new run at the end of 2014. PandaX, ArDM and DarkSide are others.

In the remainder of her talk the speaker discussed XENON1T, which is a big promise in the field. In 2 years should reach the 10^-47 cm^2 sensitivity. The collaboration has grown to 120 scientists from 17 institutions. The challenges for this detector include the x20 increase in mass, while decreasing by two orders of magnitude the specific backgrounds. They also need to improve the liquid xenon purification speed. Also, going from 30cm to 1 meter drift in liquid xenon is not trivial, as it has never been done. Operating these detectors with 100 kV and these long drifts will entail several challenges.

The construction has been going on steadily. The tank is flanked by a service building, a tower where on the top floor the cryogenics are olcated, below is electronics, and at the bottom there are two instrumentation pieces. The collaboration is very proud of this construction.

The detector has a water cherenkov muon veto; inside the cryostat is a double-walled vacuum insulated cage made from selected low-radiactivity steel, 2.4 by 1.6m diameter. The vessel is already installed but the TPC is still missing, they plan to install it in June this year.

At the bottom of the service tower lays the ReStoX system, for recovery and storage of Xenon in case of emergencies. It is a double-walled high-pressure sphere of 2.1 m diameter. It will store 7.6 tons of Xenon either in gas or liquid/solid phase under high-purity conditions.

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