Teilchenphysik ohne Beschleuniger (Non Accelerator Particle Physics)

Experimental and theoretical research in the areas of
Double Beta Decay, Dark Matter Searches and Physics beyond the Standard Model

einige Zitate aus der jüngsten Literatur, die sich auf die Ergebnisse des HEIDELBERG-MOSKAU-Experiments beziehen:

E. Witten
E. Ma and M. Raidal
H. Minakata
H. Georgi and S.L. Glashow;
R. Adhikari and G. Rajasekaran R. Adhikari, E. Ma and G. Rajasekaran"
J. Ellis and S. Lola
H. Minakata and O. Yasuda,


 Modern Physics Letters A (MPLA)
The article entitled
"Evidence for Neutrinoless Double Beta Decay" by
H.V. Klapdor-Kleingrothaus, A. Dietz, H.L. Harney and
I.V. Krivosheina (Vol. 16, No. 37) has been published.

In this article the authors have found evidence for neutrinoless double beta decay for the first time. This result is extremely exciting from several grounds. The most important feature of this discovery is that it establishes lepton number violation in nature for the first time.

The following is the comment from Prof. E. Witten (Inst. Advanced Study, Princeton) regarding the article:

If the result mentioned in the article holds, it is a real landmark with the first experimental confirmation of lepton number nonconservation, giving us an important window on physics beyond the standard model, and the first experiment beginning to give us results about the neutrino masses, as opposed to mass differences.

There is something surprising about the result. Given the neutrino mass differences as measured from the various solar and atmospheric neutrino measurements, most theorists would have guessed that the effective Majorana mass of the electron neutrino would be nonzero but smaller than this experiment appears to suggest. Of course, most of us also expected neutrino mixing angles to be small, and they have turned out to be large! If this results holds, it points to a real surprise: either the different neutrinos are surprisingly close to having the same mass, or the mechanism of lepton number violation is more complicated than a simple Majorana mass. Surprises like this are very good, but of course I don't know at the moment which way this one would ultimately take us.

Since theoretical physicists guessed the roughly correct mass scale of neutrinos more than 20 years ago, in connection with Grand Unified Theories, and since lepton number violation was also suspected since this time, it is intriguing that these major successes of theory would be coupled with at least one big experimental surprise surprise (the large mixing angles) and maybe a second (if the value of the effective Majorana mass suggested by this experiment really holds).


 EDWARD WITTEN, Nature 415, 969 - 971 (2002)
High-energy physics: The mass question

Do the elementary particles known as neutrinos have mass?
Yes, according to recent experiments. But how much?
A surprising - and controversial - result suggests that the answer is not what we thought.....

.... There is now extensive evidence for neutrino oscillations, both from neutrinos produced by cosmic rays in the Earth's atmosphere [8,9] and from neutrinos produced by the Sun [10]. (The interpretation in terms of neutrino oscillations has resolved a longstanding discrepancy [11] between the number of neutrinos expected from the Sun and the number we actually detect.) In this fast-moving area, experiment is well ahead of theory, and many important measurements are expected in the next few years. The results so far support the rough range of possible neutrino masses that arises from grand-unification theory.
The experiments have also turned up a surprise: the measured 'mixing angles' (which determine the probability that neutrinos oscillate from one type to another) are much larger than theorists generally expected.
It seems logical to suspect that neutrino mass results from the non-conservation of lepton number. But the neutrino-oscillation measurements alone do not show that lepton number is not conserved. So can we do this in some other way? This is what Klapdor-Kleingrothaus et al. claim to have done, by observing the nuclear decay
76Ge --> 76Se + 2e-.
This reaction is called neutrinoless double-beta-decay, as the final state contains two electrons (historically known as beta-particles) and no antineutrinos - so the reaction violates the conservation of lepton number by two units. Taken together with the oscillation measurements, and assuming that the only relevant particles are the three known types of neutrino, the new result implies that the three neutrinos have approximately equal masses, probably a few tenths of an electron volt. This is a surprising result because other particle families, such as quarks and the charged leptons, do not have approximately equal masses (Fig. 1), and it will put a severe constraint on theories of the origin of neutrino masses.
Some caution is called for, however, because of the exceptionally difficult nature of the experiment. ....
....At any rate, planned future experiments using much larger quantities of 76Ge (or similar nuclei) will achieve much greater sensitivity. By extrapolating from the oscillation measurements, many physicists have guessed, prior to this claim, that a sensitivity 103 or 104 times greater than that of this experiment may be needed to conclusively observe the violation of lepton-number conservation. Such sensitivity suggests how difficult, as well as how potentially rewarding, future experiments are likely to be.


 Science & Technology, Feb 14th (2002)
Rioactive Disputes
Potentially Nobel-prize winning discovery. Or maybe not

IF IT proves true, remember that you read it here first. Hans Klapdor-Kleingrothaus and his colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg have just reported the first recorded instance of neutrinoless double beta decay.
That might not sound like news worth holding the front page for, but to those interested in fundamental physics, it is. If it turns out to be correct, it will require a substantial rewriting of the Standard Model, the current repository of all knowledge and wisdom about particle physics. ....
Dr Klapdor-Kleingrothaus and his colleagues looked for the tell-tale signal in ten years' worth of data collected from a radioactive-decay experiment being carried out at a laboratory inside Gran Sasso, a mountain in central Italy. In a paper in Modern Physics A, they say they have found it.
If they have, the result would not only violate the conservation of lepton number, it would also mean that anti-neutrinos and neutrinos are actually the same thing; in other words a neutrino is its own antiparticle. It would also have cosmological consequences, since it would make neutrinos into objects a lot more massive than is currently believed. Since neutrinos are extremely abundant, that would go some way towards explaining the so-called exotic dark matter in the universe, which observation shows is there, but is not made of ordinary atoms.
Not surprisingly, the announcement has provoked a backlash. ... Whether that result is indeed a lemon, or the letter proves to be sour grapes, remains to be seen.


 CERN Courier (April, 2002)
Team reports neutrinoless double beta decay

Scientists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have reported evidence of neutrinoless double beta decay. This result comes from members of the long-running Heidelberg-Moscow double beta decay experiment at Italy's Gran Sasso underground laboratory.
Neutrinoless double beta decay violates lepton number conservation, a fundamental tenet of particle physics. If confirmed, the consequences for particle physics will be profound.
In the conventional picture of particle physics, neutrinos and antineutrinos are distinct. Beta decay proceeds via the transformation of a neutron into a proton with the emission of a neutrino and an electron. However, if the neutrino is its own antiparticle, a rather more exotic process becomes
possible. In this case, two successive neutron decays could occur, with the neutrino emitted by the first being absorbed by the second. Two electrons would emerge from the nucleus with no neutrinos, leaving a nucleus that contains two more protons than the original.
In the latest paper from the Heidelberg-Moscow collaboration (2001 Modern Physics Letters A 16 (37) 2409-2420), the authors claim evidence for neutrinoless double beta decay with a half-life of around 1.5 x 1025 years from 10 years of running with an enriched sample of germanium-76. This would be compatible with a neutrino mass of around 0.4 eV, which is difficult to reconcile with neutrino mass results from other experiments and would require a special neutrino mass scenario. A previous limit from this experiment was reported in an earlier issue of CERN Courier (January/February 2000 p8).


 Bild der Wissenschaft, online,
Teilchen-/Atomphysik, 14.02.2002, Stefan Maier
Deutsche Physiker wollen erstmals
neutrinofreien Doppelbetazerfall beobachtet haben

Ein Physikerteam des Max-Planck Instituts für Kernphysik in Heidelberg könnte in einer Untersuchung von Kernzerfällen des Elements Germanium Anzeichen für einen Doppelbetazerfall gefunden haben. Dieser bisher nur theoretisch vorhergesagte Kernzerfall ist mit der Umwandlung von zwei Neutronen in Protonen verbunden und läuft im Gegensatz zum herkömmlichen Betazerfall ohne die Aussendung von Neutrinos ab. Darüber berichten die Wissenschaftler im Fachmagazin Modern Physics Letters A (Band 16 Seite 2409). Sollte sich ihre bisher in der Fachwelt noch umstrittene Arbeit als richtig erweisen, so könnte sie eine Revolution der Elementarteilchenphysik einleiten - die Überwindung des Standardmodells.
In ihrem in dem Gran Sasso Laboratorium in Italien durchgeführten Experiment untersuchten die Wissenschaftler um Hans Klapdor-Kleingrothaus Kernzerfälle in einem 11.5 kg schweren Körper aus Germanium 76. Die Analyse der Energien der bei den Kernzerfällen freigesetzten Elektronen zeigte laut den Forschern starke Indizien für einen neutrinofreien Betazerfall. Damit würde die Arbeit den größten Durchbruch in der jüngeren Geschichte der Elementarteilchenphysik darstellen.
Während sich beim herkömmlichen Betazerfall ein Neutron eines instabilen Kerns unter Aussendung eines Elektrons und eines Antineutrinos in ein Proton umwandelt, wandeln sich bei dem von Elementarteilchenphysikern postulierten Doppelbetazerfall zwei Neutronen gleichzeitig in Protonen um. Dies ist mit der Aussendung zweier Elektronen mit genau definierten Energien verbunden - ohne Aussendung von Neutrinos.
Der Doppelbetazerfall verstößt damit gegen die von dem Standardmodell der Elementarteilchenphysik geforderte Erhaltung der Leptonenzahl bei Kernreaktionen. Elektronen und Elektron-Neutrinos weisen eine Leptonenzahl von 1 auf, während Positronen und Elektron-Antineutrinos eine Leptonenzahl von -1 zugeordnet wird. Damit bleibt die Leptonenzahl beim Betazerfall erhalten - beim Doppelbetazerfall jedoch nicht. Sollte dieser tatsächlich in der Natur vorkommen, so würde sich damit auch das Neutrino als sein eigenes Antiteilchen erweisen, was Aufschlüsse über dessen Masse zulassen würde.
Ob die deutschen Physiker nun tatsächlich diesen Zerfall beobachtet haben, ist in der Fachwelt sehr umstritten. Kritiker weisen daraufhin, dass die von dem Team in ihrem Experiment bestimmte Neutrinomasse mit 0.39 Elektronenvolt um ein Vielfaches größer ist als die bisherige aus Neutrinooszillationen bestimmte Obergrenze. Derartig schwere Neutrinos könnten einen Großteil der Dunklen Materie im Weltraum ausmachen.
.... Weitere Meldungen zum Thema Betazerfall finden Sie im Archiv von


 H. V. Klapdor-Kleingrothaus,
Physik in unserer Zeit 33, No. 4, 155-155 (2002)
Teilchenphysik. - Ist das Neutrino ein Majorana-Teilchen?

Die Leptonenzahl ist eine fundamentale Größe, die bislang bei allen bekannten Teilchenreaktionen erhalten geblieben ist. J etzt kam von dem gegenwärtig empfindlichsten Experiment zum neutrinolosen Doppelbetazerfall der erste Hinweis auf eine Verletzung der Leptonenzahlerhaltung [1]. Die Daten stammen von unserer deutsch-russischen Kollaboration, deren Experiment im Gran-Sasso-Untergrundlabor bei Rom läuft. Dies ist gleichzeitig ein Hinweis darauf, dass Neutrinos Majorana-Teilchen sind oder zumindest eine Majorana-Komponente enthalten.

START 2002


 E. Ma and M. Raidal - hep-ph/0102255
We show that unless the neutrino mass matrix is almost degenerate, i.e. with 3 nearly equal mass eigenvalues, the a\mu measurement is in conflict with the \tau ---> \mu \gamma rate....
We predict the relative decay rates of \mu ---> e \gamma, \tau ---> e \gamma,
and \tau ---> \mu \gamma in terms of neutrino oscillation data, and show that these processes constrain the common neutrino mass scale and the solar neutrino oscillation solution in a very interesting range....
We note that at m\nu = 0.2 eV, B (\mu --> e \gamma)
is at its present upper limit of 1.2 x 10-11.
From letter of Prof. Marti Raidal (10 Mai, 2001):"
We found that neutrinos must be almost degenerate with the common mass scale of 0.2 eV or higher (!) in order to explain (g-2) and to satisfy \mu ---> e \gamma.

 H. Minakata - hep-ph/0101148
``The SMA MSW solutions, as well as LMA solution at its CHOOZ allowed parameters are not compatible with the double beta constraint (Heidelberg-Moscow Collab. 2001) at 90% CL.
The vacuum solution is consistent with the double beta constraint.''


 H. Georgi und S.L. Glashow -
Phys. Rev. D 61(2000)097301 and hep-ph/9808293)
``Fact 6. .....In this paper we adopt the strongest published bound (for neutrinoless double beta decay), Mee < 0.46 eV (Heidelberg-Moscow Collab. 1997).
These facts dramatically constrain the form of the neutrino mass matrix........they constrain solar neutrino oscillations to be nearly maximal [and rule out the small angle Mikheyev-Smirnov-Wolfenstein (MSW) explanation of solar neutrino observations] if relic neutrinos comprise at least three percent of the critical mass density of the universe.''

 R. Adhikari and G. Rajasekaran -
Phys. Rev. D 61(2000)031301 and hep-ph/9812361 )
``By combining the inputs from neutrinoless double beta decay (Heidelberg-Moscow Collab. 1997) and the fits of cosmological models of dark matter with solar and atmospheric neutrino data, we obtain constraints on two of the mixing angles of Majorana neutrinos.... These constraints are strong enough to rule out Majorana neutrinos if the small angle solution of the solar neutrino puzzle is borne out.''
 R. Adhikari, E. Ma and G. Rajasekaran -
Phys. Lett. B 486(2000)134 and hep-ph/0004197 )
``...the radiative splitting is able to account for the solar data, but only with the large-angle MSW solution .... and resulting in an effective neutrino mass for neutrinoless double beta decay close to the present experimental limit .....''


 J. Ellis and S. Lola
Phys. Lett. B 458(1999)310 and hep-ph/9904279
...considering ``the possibility that the masses of the three light neutrinos of the Standard Model might be almost degenerate ... ... the cancellations required by the latest upper limit on neutrinoless double beta decay enforce near-maximal mixing that may be compatible only with the vacuum-oscillation scenario for solar neutrinos........ in particular, the new upper limit on neutrinoless double beta decay (Heidelberg-Moscow coll. 1999) in combination with the rest of the solar-neutrino data, seems to exclude even the large-angle MSW solution to the solar neutrino problem, and thus degenerate neutrinos may be compatible only with vacuum oscillations........''


 H. Minakata and O. Yasuda -
Phys. Rev. D 56(1997)1692 and hep-ph/9609276
``...we have discussed the almost degenerate three-flavor neutrino scenario as a simultaneous solution to the solar, atmospheric and dark matter problems. The neutrinoless double beta decay constraint (Heidelberg-Moscow Collab. 1997) imposed in ADN (almost degenerate neutrino) scenarios makes the small angle MSW solution untenable in this scenario. We also briefly addressed the question of uncertainty of the nuclear matrix elements in double beta decay and the issue of stability of our conclusions against the uncertainty. We have shown that our conclusion, disfavour of the small angle MSW solution in the ADN scenario, remains unaltered.''

Last modification I.Krivosheina 31/12/2007