A Quantum Diaries Survivor

Carlo Rubbia gave the last talk at the Neutrino Oscillation conference in Venice last Friday. Below are some notes I collected during the event; but first let me post a picture of Milla Baldo-Ceolin giving her final remarks and inviting everybody to next year’s event. Carlo observes at her left:

Rubbia started with a prayer -which sounded more like a benign order- to theorists: stop talking about supersymmetric dark matter and think about telling us what dark energy is: because that would be extremely important.

As far as dark matter is concerned, we already have many evidences of it. The most important one is gravitational lensing: the gravitational mass of a galaxy is measured from the focussing effect induced by a distant, passing star. This evidence confirms the WMAP results of the fluctuations in the cosmic microwave radiation for a so-far unknown component of matter. It is very clear that plasma and gravitational lensing distributions in the bullet cluster of galaxies are different. This is the proof that we are dealing with a huge amount of matter which dominates the matter density in the Universe.

Dark energy is another problem. It will stay with us for a long time to come. But at least dark matter is about to be studied by direct and indirect detection experiments. Admittedly, all present evidence of dark matter is limited to gravitational effects. The main question is that if other types of interactions are connected to dark matter, like a weak coupling. One then has to consider the possible candidates on the market, and there is a long list of them: primordial black holes, split SUSY, neutralinos, branons, messenger states in GMSB, heavy neutrinos, braneworlds DM, cryptons, D-matter, mirror matter, q-balls, you name it.

Supersymmetrical (SUSY) particles are those most studied in the list. In order to protect the mass of the Higgs from higher order conrrections, we need an extremely precise graph cancellation to compensate for the divergence of known fermions. Supersymmetry can provide that cancellation. A low higgs mass tells us that the mass reange of SUSY partners must not be too far away. A discovery of a low mass Higgs, if elementary, may become an important  hint to the existence of an extremely rich realm of new physics, a real blessing for LHC.  A doubling of the number of elementary particles would be a result of gigantic magnitude.
However, if the explanation of dark matter is SUSY, we need to postulate some strictly conserved quantum number, R-symmetry. This is because the lifetime of the universe is at least 14 billions of years, but that of otherwise permitted SUSY particle decays is 10^-18 seconds: R-symmetry would need to be conserved to a fantastic degree to allow the neutralino to be the source of present-day dark matter.

The real question is the following: is the relic density of weak interacting massive particles (WIMP) the source of non-baryonic matter ?  Current experiments are providing conflicting results. We have to find a third possibility to explain why DAMA sees a signal and other experiments do not. If you take the graph of energy components of the excess from DAMA-LIBRA you see the recoil energy in keV.  Shapes look quite similar to what it should be observed from a WIMP. We are dealing with a high-statistics effect. Its source is systematical. How can we make a test ?

One test to suggest is that if DAMA were repeated in the southern emisphere, by going to South Africa or Argentina, then if the effect observed is of cosmological nature one would get the same result,  while if it is of seasonal nature (a seasonal variation in the number of neutrons, or others),  then one would see something of the opposite sign in the southern emisphere. So DAMA should take everything and carry their whole setup to a place which is on the other side of the Earth.

Several increasingly accurate astronomical observations have strenghtened the evidence that today’s Universe is dominated by an exotic energy density with negative pressure. The simplest candidate is a cosmological term in Einstein’s field equations. However, it turns out to be way too small by particle physics standards. It is thus a profound mystery. And there is a connected mystery: since the vacuum energy density is constant in time, while the matter energy density decreases as the Universe expands, why are the two comparable at about the present time, tiny in the early Universe and very large in the distant future ?

Concluding, Carlo Rubbia noted that the man of the past had discovered that earth, air, fire, and water are the elements which constitute the world. Now physicists say one should rather list baryons, neutrino, dark matter,  and dark energy. Despite that, we do not know the identity of 95% of the Universe: is it composed of dull particles, or is it something much richer than that ?