06 Apr NEXT researchers make technological breakthrough in pursuit of zero-background experiment
Discovery experiments involving a handful of very rare events, as those resulting from the elastic interaction of weakly interacting massive particles (WIMPs) with nuclei, neutrinoless double-beta decay (ββ0ν), proton decay or axion-photon conversion, increase their sensitivity with the square root of the measurement time. This law of diminishing returns stems from the underlying Poisson statistics of both signal and signal-like events, and gets further exacerbated when the magnitude of interest has a slower than linear dependence with the production rate (e.g. the neutrino mass for ββ0ν experiments, or the axion-photon coupling in axion detectors). Halving an existing upper limit to the neutrino mass via ββ0ν decay, for instance, if that is derived when the experiment goes through its second year, would near a decade.
Requirements for this kind of experiments are daunting: with one to a handful of events expected over the experiment’s lifetime, any signal-like event is a deadly hazard. Underground operation and selection of radiopure materials is mandatory, however these invariably fall too short at suppressing physical backgrounds. Topology, calorimetry and even more exotic means like temporal information of the event can be used to suppress these unwanted ‘fakers’ to minute levels, but even that is usually not enough. The prize for a zero-background experiment is not to be taken lightly, provided sensitivity improves linearly as a function of time in that case. Clearly, besides the power of two gain in the discovery potential, an experiment with a demonstrated zero background will be able to make an unambiguous (systematic-free) discovery claim.
Now, researchers from IGFAE and the NEXT experiment (Neutrino Experiment with a Xenon TPC) at the Canfranc Undeground Lab in the Pyrenees, have taken the first steps towards making this holy grail a reality for the elusive ββ0ν decay. NEXT, the only ββ0ν-experiment that relies on gas as an active medium, seeks the decay of 136Xe to 136Ba plus two electrons, with no neutrino in the final state. The process violates lepton number conservation (that can be directly linked to one of the three Sakharov conditions through the mechanism called ‘leptogenesis’) and gives direct access to the neutrino mass scale, besides establishing the nature of the neutrino as a Majorana particle. This extremely rare decay, with half-lives in excess of 1026 years, takes place at a well-defined energy corresponding to the mass difference between both nuclei (2.45MeV), given that no missing energy is carried off by neutrinos. Despite the excellent imaging and calorimetric response of NEXT, gamma rays from natural radionuclides contaminating the detector materials prevent the detector (and any ββ0ν experiment in fact) from achieving its full potential, until now.
But what if the 136Ba nucleus could be simultaneously detected, together with the two emitted electrons?. NEXT co-spokesperson Dave Nygren was pondering that possibility during a talk by Nobel Laureate Steve Chu on Single Molecule Fluorescent Imaging (SMFI) when he had one of those rare eureka moments. SMFI (awarded with the Nobel Prize in chemistry in 2014) is a technique that allows imaging individual Ca dications (Ca++) during metabolic processes in cells, with nm-scale resolution. The idea is to dissolve a fluorescent dye (fluor) that undergo a shape transformation in the presence of a free Ca++ ion, becoming fluorescent (a process dubbed ‘chelation’). By repeatedly interrogating the chelated molecule in a suitable wavelength, the resulting fluorescence can be used to localizing the Ca ions with super-resolution, near the Abbes diffraction limit (~1 nm in practice).
Provided barium and calcium are congeners, Nygren thought, it was likely that a good deal of know-how could be put to good use, this time in a completely different field. Researchers at UTA (University of Texas at Arlington), under his lead, started to investigate this possibility and found a number of dyes that exhibit strong fluorescence to Ba++ chelation as well. Indeed, Ba++ is the equilibrium charge state of the Ba nucleus in xenon atmosphere, provided the electron affinity of Ba++ is a mere 10 eV, 2 eV below the ionization potential of Xe. Thereby, drifting 136Ba++ along m-long distances through an ultra-pure xenon detector is a realistic possibility, as the most recent microscopic calculations based on density functional theory confirm.
In his latest work, published recently in physical review letters, the NEXT collaboration has demonstrated that SMFI can be achieved for Ba++ ions embedded in an aqueous solution, when the fluor is fixed to a transparent sample that is back illuminated using the TIRF technique (through-objective internal reflection fluorescence). The hallmark of SMFI is a localized spot (~1 nm position resolution), displaying fluorescence in the expected fluor band, and that comes suddenly to a halt (Figs 1,2). Such a steep fluorescent trajectory results from photo-bleaching of the chelated molecule (usually assisted by impurities) and represents an unambiguous confirmation that single-molecules were being interrogated.
With this milestone, NEXT demonstrates the viability of this novel barium-tagging technique, in a configuration that is technically adequate for the experiment. Further work is needed to demonstrate that chelation can be produced efficiently in a xenon atmosphere, and that the 136Ba++ ion can be transported to the interrogation station unscathed. This crucial milestone is currently under investigation in a dedicated setup, with a number of new dyes being explored as well. If finally achieved, ββ0ν-searches will move into an entirely new sensitivity domain.
Fig. 1. Left: fluorescence field, displaying spots corresponding to near-surface (bright) and deeper (dim) fluorescent molecules upon Ba-chelation. Right: fluorescent trajectory of one of the spots, deactivated by photo-bleaching at about 100s time.
 A. D. McDonald et al., (NEXT collaboration), Phys. Rev. Lett. 120, 132504 (2018).
 J. P. Jones et al., J. Instrum. 11, P12011 (2016).