XENON

Many independent elements of evidence point to a Universe composed mainly of unknown parts: ~70% "Dark Energy", ~25% Dark Matter, and ~5% ordinary matter. Our research group at NYUAD  is part of the international  XENON collaboration that , since 2004, has designed and operated three "liquid xenon time projection chambers" (LXeTPCs): extremely sensitive detectors for the search of "weakly interacting massive particles" (WIMPs), a well motivated class of particles that could be the mainly constituents of that 25% of matter in the universe whose gravitational effects have been measured in several ways: Dark Matter (DM).

The detectors have all been operated at the Gran Sasso National Laboratory in Italy (LNGS), and they were named XENON10, XENON100 and XENON1T. At the time of their respective operation, each of them was the most sensitive of its time for WIMP search 

The last detector,  XENONnT, concluded its operation in 2019. It utilized 3.2 tonnes of liquid xenon, making it the largest and most sensitive detector for WIMP-like DM particles. It was installed in the hall B of LNGS and immersed in a tank with 700 tonnes of purified water. About 80 photomultipliers, installed inside the tank, detected the passage of any charged particle (typically, energetic cosmic ray muons with other accompanying particles), and provided a veto system to avoid mistaking any externally generated event for a signal caused by DM interactions.

A substantial upgrade of the experiment, named XENONnT, was completed in 2020, bringing the total xenon mass to 8 tonnes. The upgrade is making use of all the existing infrastructure: only the inner part of the detector has been rebuilt to host the increased amount of target.

Our contributions to the XENON Program 

Prof. Francesco Arneodo began working on XENON in 2004 when he was researcher at the LNGS. In 2013 he moved to NYUAD together with Dr Adriano Di Giovanni, also former staff member of LNGS. Together they initiated the XENON activities at NYUAD, which became formally a XENON institution in 2014. Below, a list of the main contributions to XENON of our group:

Direct search of WIMP Dark  Matter 

Introduction

In the framework of the Cosmological Standard Model (CSM), it is well assessed the presence in the Universe of a sizable fraction of matter composed by particles not belonging to the Standard Model (SM) of Particle Physics.  The discussion on the possible presence of an unexpected matter component of the Universe started in 1922 with Jacobus Cornelius Kapteyn [1] which was the first to realize that the Milky Way (MW) rotates as contrary to the common belief, of that time, according to which the stars were moving randomly. Subsequently, in 1932 Jan Oort determined the MW center of rotation overpassing definitively the idea that the Sun was the center of the MW. Finally, in 1933 Fritz Zwicky realized, by measuring the velocity of galaxies in the Coma clusters, that those were larger than the escape velocity accounted by visible matter [2]. Thus, he came to the hypothesis of the presence of “Dunkle Materie”, that today is indicated as Dark Matter (DM), which would provide the gravitation potential to keep together the visible matter.

The measurements of temperature anisotropy of the Cosmological Microwave Background (CMB) have been of crucial importance in the process of narrowing down the features of DM. The most recent measurements have been provided by WMAP and PLANCK experiments in [4] and [5], respectively. The CMB anisotropy imposes that a component of matter, not interacting with the ordinary matter by means of electromagnetic forces, exists and it must have been present also in the early stages of the Universe. If DM is assumed to be stable it turns out to contribute to the Universe energy balance by 25% (85% if normalized to the matter content). The CMB measurements allowed to definitively establish that DM is not a form of non-luminous ordinary matter but some form of particles weakly interacting with those of the  SM.DM was only known to be some form of matter, non-emitting light and interacting gravitationally. Later on, a similar measurement was realized by Vera Rubin [3] on the M31 galaxy, showing that the rotational velocity of stars was higher than what she was able to predict given the visible matter. Also at galaxy scale, the presence of DM is needed to explain the velocity of stars. The data in Fig 1 show the rotation velocity of stars as a function of the distance from the galactic center (galaxy M31). This dependency shows a behavior which conflicts with what expected according to the Newton Gravitational law. In fact, when measuring the star velocities outside of the bulk of visible matter, the rotational velocity should decrease like 1Sqrt[r] while it is almost constant. This type of measurement was confirmed in the following years and high precision measurements are reported in [6] where also the dark halo distribution surrounding the visible matter is measured.

Further cosmological evidence of DM come from the gravitation lensing where the light pattern (images) are deformed by the gravitation potential, generated by visible and not visible matter, across the line of sight between the source of the image and the observer. Once subtracted the effect of visible matter, the evidence of DM is obtained [7],[8]. Another interesting phenomenon requiring the presence of DM is the measurement of bullet clusters where two clusters of galaxies cross each other. The measurements of such an event show halos of gaseous matter that emit X-ray, because of the interaction, and are shaped like shock waves. While gravitational measurements show dark-halos accounting for the largest fraction of matter crossing each other whose motion remains unperturbed after the scattering. This is another evidence of the presence of something weakly interacting and accounting for the largest fraction of the mass. The bullet clusters are also very effective to rule out the models called MOdified Newtonian Dynamics (MOND)[9], which describe the DM as an effect of a modification of the Newton Law. Indeed, the bullet clusters can’t be understood in the framework of a MOND model while they can be very well understood the existence of weakly interacting particles, i.e.  DM, is assumed.

It is worth noticing that the CMB measurement constraint also the baryonic matter (protons and other nuclei) content of the Universe, this implies that DM has to be non-baryonic. Thus, neutrinos can naturally be a good candidate to solve the DM problem, as they are not emitting light, have low interaction- rate with matter and are expected to be very abundant in the Universe. The problem associated to neutrinos is that the upper bound on the sum of the mass eigenstate (m, 0.15 eV [10]) allows them to be non-relativistic only in the recent era of the Universe. This would have affected the halo made out of DM as being much larger than what is actually measured nowadays. So neutrinos, having small mass are too ”Hot” and incompatible with the measured size of the halo, while a good candidate for DM has to be ”Cold”, meaning large mass and low kinetic energy.

Principle of Dark Matter Direct Detection

A seminal paper where the DM detection was envisaged is [11]. The authors considered several possible candidates and among those also the neutrino’s super- partner in a Supersymmetric extension of the SM, that is today the most commonly assumed DM constituent.  In this framework, DM particles are indicated as Weakly Interacting Massive Particle (WIMP). For a detailed treatment of this topic see [12]. In [11] the authors discuss the coherent scattering of DM particles and the possibility to detect very small energy release due to the scattering of WIMPs, with mass in the range of 1−102 GeV. The cross sections of DM particles with ordinary matter is evaluated in both cases, spin-dependent and spin independent. In [11] energy release of few keV up to tens of keV is considered and this is what today is named as DM direct detection. It differs from the indirect detection, where secondary products of the WIMP and anti-WIMP annihilation are detected in outer-space detectors, focused on the detection of γ, p/p¯, e−/e+), or in ground-based ones more focused on ν and anti-ν detection.

The WIMP interaction rates with ordinary matter have been calculated in different theoretical frameworks, [12], all ending up with very low interaction rate and none of them confirmed by any measurement, yet. It is commonly accepted that the scale of the cross sections of WIMPs is one of the weak interactions. To support this assumption one can look at [13] where it is deduced the formula connecting the WIMP density, the annihilation cross-section of the WIMP and anti-WIMP at the freeze-out (when the WIMP annihilation rate became smaller than the Universe expansion rate, then the WIMPs decoupled from the primordial plasma) and the contribution of the DM to the critical density of the Universe.

The low interaction cross section is also confirmed by the most recent experiments on direct DM search where the negative results impose that the DM rate must be lower than 1 event/kg/yr.

The low interaction rate and the very little energy released impose experimental challenges. In fact, suitable detection techniques must have an energy threshold as low as few keV.

The low expected rate imposes also to install the apparatus in a clean and “silent” environment where the flow of cosmic rays is significantly reduced like in an underground laboratory which the LNGS (laboratory Nazionale del Gran  Sasso, L'Aquila, Italy) which proved 1440 m overburden of rocks.

Another important signature of DM interaction is the time variation of the interaction rate which must show a seasonal modulation because of the Earth motion and so for the experiment in the galaxy.  The Earth speed is almost 10% of one of the solar systems in the galaxy. Thus, the composition of motions generates a variation of the relative velocity between experiment and galaxy and so of the rate of DM in a possible experiment.

Liquid XENON Target: The Most Effective Target Used So Far

In many targets suitable for DM experiments a WIMP scattering provokes the production of phonons (heat), photons and ionization electrons. An optimal experiment should measure the three forms of energy but unfortunately, this is never the case. The illustration in Fig. 2 groups experiments according to the different forms energy that is released in it. The field of DM search is populated by experiments able to measure one or two forms of energy.

The reason why we linger on the topic of the forms of released energy is due to the fact that while photons are proportional to the energy, the response of the detector to photons or ionization electrons strongly depends on dE/dx. This gives the possibility to distinguish between events originated by gammas or electrons and by those originated by alpha or nuclear recoils. Two measurable quantities are sufficient to provide the capability to distinguish among events with different dE/dx.

One of the most effective technologies used to search for Dark Matter is the so-called double phase liquefied Xe gas technology which exploits liquid Xe as DM target. When energy is released in the active volume,  create scintillation and ionization. A drift E-fields prevent the full electrons recombination with the parent nuclei and drift the electrons to the liquid-gas interface. A higher amplitude E-field extracts the electrons to the gaseous phase, accelerate them and generate a second light pulse. The two light pulses are detected by PMTs and the time difference between the two pulses allow to measure the depth or the coordinate across the drift field. The most suited geometry is a cylindrical one where the active volume is defined by a cathode and an anode and shapers are used to make the drift field as uniform as possible. The light pattern of the second pulse allows reconstructing the transverse coordinates, orthogonal to the drift field direction. Thus a complete space reconstruction of events is possible. This detector takes the name of double phase Time Projection Chamber (TPC).

The XENON collaboration, of which the NYUAD astroparticle physics group is an active member, lead the field of LXe double phase TPC, building and running several detectors, ([15]) at the LNGS, of increasing sensitivity, XENON10, and XENON100. The last one has been the detector with the best sensitivity until 2013, while the third one, XENON1T, is expected to lead the field for the next years. The XENON1T detector, with 3.5 tonnes of instrumented mass and more than 1 tonnes of fiducial mass, is presently this is the last candidate. The next esc will revert to uncompleted text taking physics data taking and soon a paper is expected where the results of more than 1 year of data taking will be shown. We firmly believe that also with XENON1T near future results will come back to lead the field. The expected sensitivity is such that the minimum cross 2 Power[10,-47]Power[cm,2] section for a WIMP mass of 50 GeV will be reached in two years of DM data- taking. It is worth mentioning that the XENON1T has embedded in its design the possibility to upgrade the instrumented mass to 7 tonnes (XENONnT), reutilizing all the ancillaries devices of XENON1T, replacing only the vessel containing LXe, building a larger TPC with 100 more PMTs with respect to XENON1T. The construction of the XENONnT detector is already quite advanced and the NYUAD group is involved in the coordination, in fact, one of the two technical coordinators od the XENONnT is a member of the NYUAD-XENON   group.

References

[1] C. Kapteyn, Astrophys. J. 55, 302 (1922).

[2] Zwicky, Helv. Phys. Acta 6, 110 (1933).

[3] C. Rubin and W. K. Ford Jr, Astrophys. J. 159, 379 (1970).

[4] Hinshaw et al. (WMAP Collaboration), Astrophys. J. Suppl. 208, 1 (2013).

[5] Adam et al. (PLANCK Collaboration), Astro. and Astrophys., 594, 1 (2016).

[6] G. Begeman, A.H. Broeils and R.H. Sanders, Mon. Not. R. Astron. Sot. 249 (1991).

[7] Van Waerbeke, Y. Mellier, and H. Hoekstra,” Astron. Astrophys.  429 (2005).

[8] Bartelmann and P. Schneider, Phys. Rept. 340, (2001).

[9] Milgrom, Astrophys. J. 270 (1983). [10] JCAP 1502 (2015) no.02, 045.

[10] JCAP 1502 (2015) no.02, 045.

[11] W. Goodman and E. Witten, Phys. Rev. D 31, 3059  (1985).

[12] Jungman, M. Kamionkowski and K. Griest, Phys. Rept. 267, 195 (1996).

[13] Dodelson, Modern Cosmology, Academic Press (2003).

[14] Marroda´n Undagoitia and L. Rauch, J. Phys. G 43, 013001 (2016).

[15] E. Aprile et al. (XENON collaboration), Astropart.Phys. 35 (2012).

[16] E. Aprile et al. (XENON collaboration), Science 349, 851 (2015).

[17] E. Aprile et al. (XENON Collaboration), Phys. Rev. Lett. 115, (2015).

[18] D. S. Akerib et al. (LUX Collaboration), Phys. Rev. Lett. 116, (2016).

[19] A. Tan et al. (Panda-X II Collaboration), Phys. Rev. Lett. 117, (2016).

[20] E. Aprile et al. (XENON Collaboration), JCAP 04, 27 (2016).