Electron’s Lifetime Spans Unheard of Numbers


Raşit Gürdilek

At least 66,000 “yottayears,” 5 “quintillion” times the universe’s age...

 

At the end of an experiment  designed to determine whether electron is indeed an elementary particle, physicists at an underground observatory in Italy arrived at the conclusion that it was probably stable, that is, it cannot decay. According to the findings,  the lower limit for electron’s lifetime is tens of thousands times longer than the highest  decimal  unit in the metric system. This corresponds to about five-quintillion (billion times billion) times the 13.8 billion year age of the universe!

According to the theory called the Standard Model which explains the particles and forces interacting at the subatomic  scale, electron is among the elementary particles, which are those that are not formed of subparticles and so, cannot  split or decay. The result found by the physicists who carried out the experiment  at  Gran Sasso underground partiicle physics laboratory in Italy is that any electron currently existing  in  the universe will  also be  around   at least 66.000 “yottayears”  (6.6 X 1028 years) , or 660.000.000.000.000.000.000.000.000.000 years from now. Being the biggest decimal unit in the metric system, yotta corresponds to 1024 (trillion times trillion). This means that electron’s lifespan equals 5 quintillion times the current age of the universe which is 13.8 billion years  (1 quintillion = 1018 or 1000.000.000.000.000.000).

If you are not dizzy from counting and recounting the zeros, we can go on.

In a more detailed description, the experiment conducted by the Gran Sasso physicists was designed to see whether electron heralds a new physics beyond the Standard Model by decaying into a photon and a neutrino and thereby violating the principle of the conservation of electric charge. In the Standart Model, photon, or the light particle, which carries the electromagnetic force has no mass. And  neutrino, which is a particle with a tiny mass, does not feel  the strong force (which binds together the  particles inside atom nuclei) is also impervious to the  electromagnetic force (because it  carries no electric charge) interacts only with the weak force which causes atoms and subatomic particles to turn into other atoms or particles. 

In physics known to us, electron is the particle with smallest mass which carries the negative electric charge. So, if it was to decay, particles with even smaller masses likeneutrinos would have to emerge.  But since, as noted above, none of the particles smaller than the electron carries electric charge, in a hypothetical decay electron’s charge would have to disappear, too, in  violation of the principle of the conservation of electrical charge.  Therefore, the Standard Model bans electron decay. But  despite its experimentally verified successful predictions, the Standard Model  cannot explain all physical phenomena. So, since the decay of an electron would mean a new physics which provides a more consistent picture of the nature, researchers   were looking for that decay.

Borexino Detector

The main task of the detector built 1.4 km below the surface under Mt. Gran Sasso in Italy, is identifying neutrinos produced by different fusion reactions in the core of the sun. The 1.4-km-thick rock mass above the detector largely blocks the muon flux.

To further reduce it, the spherical inner detector is protected by an outer detector shaped like a domed cylinder 16.9 m high and 18 m wide, filled with 2100 tons of purified water and equipped with 208 photomultiplier tubes (PMTs or light sensors). This shield identifies the Cherenkov radiation special to the cosmic rays which could penetrate through, enabling their cancellation, besides absorbing the neutrons produced by radioactive decays in the rock mass. The inner detector contains organic fluid filling two concentric nylon spheres. While 200  tons of fluid in the outer case functions as an extra shield catching unwanted particles which penetrate the outer shield, the inner case, filled with 100 tons of organic fluid and surrounded by 2212 light sensors, allows the physicists to determine which neutrinos are products of which reactions by loooking at variations in the Cherenkov  radiation.

Cherenkov Radiation

The Cherenkov radiation, named after Russian physicist Pavel Alekseyevich Cherenkov who discoverd the phenomenon, is based on the the facts that light, traveling at 300.000 km per second in a vacuum, slows down in denser media (e.g. air, water, glass) and that electrically charged particles, traveling faster than light in a dense medium, emit radiation. In neutrino detectors, as particles with negative electric charge (typically electrons  produced in rare instances when neutrinos interact with atoms in the stored liquid), or muons which are heavier types of electrons (produced when cosmic rays which are mostly protons collide with molecules in  the air) travel in the liquid with a speed grater than light’s speed in that medium, the turbulence they leave in their wake radiates as a shock wave in smooth phase. And this radiation callled “Cherenkov radiation” can be picked up by the light.

Hunt for the marker  photon

The instrument researchers used in the experiment is a detector below the Gran Sasso laboratory, built under a 1.5 kilometer thick rock mass of a mountain to screen it from the cosmic rays.

Originally built to capture and analyse the  elusive neutrinos coming from the sun, the detector is comprised of two concentric spheres filled with 300 tons of organic liquid surrounded by 2212 light amplifying tubes which capture  a special light called “Cherenkov radiation” and boost its intensity. 

What the Gran Sasso searched for was a photon of 256 keV (256 thousand electronvolts)  energy, predicted to emerge together with a neutrino in a hypothetical electron decay. In case that photon interacted with an electron in the liquid, it would cause Cherenkov radiation (see box) which would be captured by light sensors.

Combing through the signals caught by the light sensors in the period from January 2012 to May 2013, physicists looked for  photons with that 256 keV energy level. But after the photons of same energy originating from the decay of some radioactive materials in the liquid or from the interaction of neutrinos with atoms in the liquid were canceled out, no sign of electron decay was observed in the specified period.

Since it was calculated that the organic liquid in the Borexino detector contained  1032 (100 quadrillion times quadrillion) electrons, researchers concluded that the minimum value for the “average” lifespan of an electron was 6.6 X 1028 year .

According to Borexino researchers, further screening of  unwanted radiation sources in the detector could  raise the minimum lifespan of the electron even to 1031 years. What’s more, the improved detector could also probe the possibility of electrons disappearing into hypothetical  “extra dimensions”. 

REFERENCES

  • 1. “Electron lifetime is at least 66,000 yottayears”, Physics World, 9 Decembre 2015
  • 2. “Real-time solar neutrino spectroscopy atlow energies”, Max-Planck-Institut für Kernphysik, (Particle & Astroparticle Physics), https://www.mpi-hd.mpg.de/lin/research_bx.en.html
  • 3. “Electron”, Wikipedia, https://en.wikipedia.org/wiki/Electron