The NEXT Experiment

Photo taken by David Ibáñez, from canfranc.es.

The NEXT experiment is an international collaboration that searches for the neutrinoless double-beta decay at the Canfranc Underground Laboratory (LSC) in Canfranc, Huesca (Spain), under mount Tobazo. NEXT stands for Neutrino Experiment with a Xenon TPC, therefore the detection concept considered consists in a Time Projection Chamber filled with high-pressure gaseous Xenon (HPXe-TPC) that exploits the Electroluminescence effect, with separated-function capabilities for calorimetry and tracking.

A bit of history...

The idea that would result in NEXT (as a HPXe-TPC experiment) was proposed to A. Bettini, then director of the LSC, circa 2008 by J.J. Gómez Cadenas. Shortly after, he heard that Dave Nygren, the famous inventor of the TPC technology was also interested in developing an experiment based on an HPXe TPC. Knowing that, JJ called Dave, and invited him to visit Spain and join the incipient NEXT collaboration. Dave accepted, and they started a collaboration and friendship that has lasted for more than 12 years. The third major player in the early story of NEXT was the late Jame White, at the time possibly the greatest World expert in high-pressure noble gas chambers. Dave referred to White as “my pal James”, and JJ referred to him as “Saint James”, since his help with the construction of the first IFIC prototype, NEXT-DEMO was essential for the development of the project.

J.J., James and Dave in Berkeley (2009).

The key elements that made NEXT a powerful ββ0v detector were: a) the use of electroluminescence, proposed by Dave, which permits reaching energy resolutions below 1% FWHM (DBDM, the prototype that Dave built at LBNL, reached resolutions in the Cs-137 peak that would extrapolate to 0.5% FWHM at the energies of the ββ0v decay) and b) the adoption of a pixelized tracking plane, based in SiPMs, an idea proposed by JJ, which allows the powerful reconstruction of electron tracks. A few years later, Dave would propose the ground-breaking idea of using Single Molecule Fluorescence Imaging, SMFI, to detect the Ba2++ ion produce in the ββ0v decay of xenon, a technique, that, if demonstrated, would make possible a NEXT detector (which we call NEXT-BOLD) capable of fully suppressing the background, and thus, with a huge discovery potential.

Back to the present

Due to the extreme rareness of the neutrinoless double beta decay searches, two ingredients have crucial importance: energy resolution and background suppression. Concerning the first one, the NEXT-White detector (2015-2021) showed a superb energy resolution below 1% FWHM in the Qββ. On the other hand, the addition of the Richardson-Lucy deconvolution algorithm within the reconstruction chain provides us with a highly defined representation of the events, which allows us to achieve an impressive signal/background discrimination.

Currently, we are about to finish the NEXT-100 detector construction, which will be taking data at some point in 2023 with 100kg of Xenon. Comprising an intermediate state before the tonne-scale generation, it is meant to understand technical solutions at large scale, validate the background model, and provide an additional physics measurement for  ββ0ν searches, among other issues.

However, the null results in the ββ0ν search by current-generation experiments point towards sensitivities that can only be achieved with both huge exposures and quasi-background-free experiments. Within the NEXT program, two different approaches will try to fulfill these conditions:

NEXT-HD will follow NEXT-100, increasing by a factor of 10 its fiducial mass and implementing some changes in the design: it will be a symmetric device comprising two twin back-to-back TPCs.

• The NEXT-Bold program will also include a barium tagging technique, that would allow to detect the presence of Ba++ ions, the daughter nuclei in 136-Xe decays. This achievement, in combination with an excellent energy resolution, would convert NEXT into an almost background-free experiment.

Clicking on the image above, you'll find a video that explains, in a really clarifying way, the motivation behind the NEXT experiment, as well as the relevance of the barium tagging technique in the field.


Why choosing Xenon?

Among the noble gases, Xenon is the only one which has an isotope that decays ββ (Xe136), whose natural abundance is quite high (9%) and can be easily enriched by centrifugation. Also, its Qββ value is acceptably high (~ 2458 keV), so that most of the background is left outside the region of interest.

What have we been doing lately?

Go to the NEWS section or follow any of our social media if you don't want to miss a thing! Besides, the most recent official status of NEXT can be found in the latest papers and talks

Where are we from?

Even though the experiment currently operates at the LSC in Canfranc, NEXT is an international collaboration that includes groups from Spain, Portugal, U.K., Israel, Germany, and the USA. As a matter of fact, more than 100 scientists are involved in the project. In case you want to take a look at the full list of people and institutions that take part in the experiment, click here. Among them, Juan José Gómez Cadenas (ERC Synergy Grant) corresponds to the spokesperson of the experiment.