XXXIV Rencontres de Moriond , March 13-20, 1999

About this presentation

The present document does not aim at providing a detailed specialist account of the latest developments in particle physics presented and discussed in depth at the Rencontres de Moriond.
Our purpose is rather here to present in a form accessible to the layman some key points of the meeting, dealing both with the scientific content and with the specific format which makes the Rencontres de Moriond a unique venue.

We will thus refer the professional particle physicist directly to the scanned slides (or later to the proceedings) where the details of each presentation can be examined freely.
We also welcome enquiries from information professionals: beyond the ground material found below, we encourage them to contact the members of the program committee (the simplest is to proceed via the Moriond secretariat, see at end of this file).

In the following paragraphs, "background " information is printed in italics.

The Neutrinos steal the show…

While this meeting on unified electromagnetic and weak interactions of particles  traditionally opens with the latest searches for new particles at the highest energy reached (see below in this report), Neutrinos were this year in the limelight.

Amongst the most discreet existing particles, neutrinos were first conjectured to account for escaping momentum and energy in weak decays. Nowadays well established members of the standard model of electroweak interactions, they still keep some of their mystery. The central question is indeed their mass, or rather masses;
Most direct attempts at measuring the neutrino mass put lower limits which are consistent with a null value. Indirect experiments however indicate that the 3 neutrinos, each associated with a different lepton (electron, muon, neutrino) must possess slightly different masses. An important result of this is that neutrinos produced in association with a given lepton (say electron-type neutrinos produced in the thermonuclear reactions powering the sun) may, after propagating over large distances convert (oscillate) into neutrinos belonging to another species.
For a long time, evidence for such oscillations, although growing, was too uncertain. The main reason why those light, and therefore easy to produce, particles have so long escaped the limelight is of course their extremely weak interactions (we are crossed each second by billions of neutrinos originating from the Sun, but without any harmful effect: they actually cross easily the Earth! -- after all, they even crossed the Sun to reach us!).
picture caption:
The SuperKamiokande neutrino detector is constituted of a (very) large tank of water.
The bulk of the detector compensates for the very weak interaction probability.
Interations result typically in electron emission, which
in turn emit Cerenkov light, picked up by the large photomultipliers lining the detector.
The experiment is currently examining both the fine details of solar neutrinos,
and the neutrinos produced in the high atmosphere by
cosmic rays. In both cases, they show evidence of oscillation between neutrino types.
(picture credit:ICRR (Institute for Cosmic Ray Research), The University of Tokyo)
The development of larger detectors, the constant refinement of their techniques has now brought sufficient evident to force the decision, and suddenly, it is not 1 but 3 mechanisms of oscillations that we are facing. (solar, atmospheric and at the LSND experiment)Each of these observations needs to be examined and cross-examined in details:
Are the Karmen data compatible with the observations at LSND?
Couldn't the atmospheric oscillations call for a better description of the interaction of cosmic rays in the atmosphere?
Meanwhile the Super Kamiokande experiment is now busy unravelling the detailed fate of solar neutrinos as a function of their energy, and tries to find out whether the observed oscillation took place in the Sun itself or on the way here…

Definitely, perseverance in a difficult search for particularly discreet and elusive particles has brought its fruits, and if neutrinos were given a place of choice at this meeting, it is also for all the expected progress in the field, both from experiments and from theory.

We expect thus a thriving activity in this field in the coming years.

Time reversal and associated topics.

While we know that weak interactions do not respect parity (P) (in other words, the image seen in a mirror CANNOT be a physical world, nature CAN tell left from right), the status of  time reversibility (T) at the microscopic level stays open.
In the favoured theoretical framework (local lagrangian theories), Time reversal is equivalent to reversibility under the simultaneous operations of P and C (changing all particles into antiparticles). This CP symmetry, while almost exact, is known for a long time to suffer some minute exceptions (more below). We might thus conclude, given the existing theoretical prejudice that T should be violated as well: the 2 directions of the flow of time are already inequivalent at the particle level, without any connection to the notion of entropy!
Yet, in view of the crucial nature of this question, it is important to reach the conclusion as independently of the formalism as possible.
Figure caption:  In this schematic view of the CP LEAR detector, a pair of K particles is produced. The decay of the charged one is readily visible
at the center of the detector (tracks in blue) , while a neutral K (or anti-K) is not visible until its decay. The length of the path before decay
measures the time elapsed since its emission.Detailed study of the time evolution of the neutral K particle sheds light on the important issues of
the violation ofT  (time-reversal at the microscopic scale) and CP symmetries.
(picture credit: CP Lear /CERN)

Two experiments (one from Fermilab, the other from CERN) have reported, not exactly a direct observation of T violation, strong arguments in its favour.
For this purpose, consider for instance the disintegration of a heavy particle (a K meson for instance) into lighter ones. The process in general depends upon the speed of the outgoing particles, and T reversal changes the sign of these speeds. One of the experiments reports that the disintegration occurs at a different rate when all "speeds" are reversed. This is closer to the notion of  T invariance than the CP symmetry discussed before, yet, this is not quite T invariance: for this we should observe, not a decay but the fusion of the lighter particles into the K.
The same is true for a different approach, where the different evolution of K and anti-K mesons are compared.
Considerable discussion therefore took place on the extent of the extra assumptions relating these very fine and detailed observations to the establishment of T violation itself.
The observation of an electric dipole moment for the neutron would bring a completely clear-cut answer to this crucial question.

Still under the same heading, observation of CP violation itself has gone one step further with the report by the experiment KTeV (Fermilab) of its measurement of "explicit" CP violation. This measurement now confirms (with a lower error bar) the result from a previous CERN experiment (NA31) with which the older Fermilab data were in disagreement. While somewhat more technical than the previous issue, this proves that the K and anti-K mesons indeed  do have different decay mechanisms (although the same lifetime!). While such a result was expected, even in the standard model, the size of the effect is somewhat larger than the dominant predictions, and has brought a re-examination of possible extensions to the theory, and of the -particularly delicate- calculations of this detailed process.

Particularly promising to elucidate alternate mechanisms of CP violation is the decay of heavy mesons, specially the B and Bs. While we wait for the first results of dedicated machines and experiments, data from general-purpose apparatus already gives a glimpse of what awaits us, in particular the first measurements of mass differences between B0 states and the first signs of CP violation in their decay to J/Psi Kshort.

Precision measurements heat up the hunt for the Higgs 
(and other searches)



figure caption:
an exemple of data available from the scanned transparencies (here from the talk:  W mass and LEP energy calibration by I. Riu for the LEP collaborations)
The red and green colored loops are constraints derived from current experiments,
and clearly favour a light Higgs particle : the yellow bands correspond to the standard model expectations for
various Higgs masses (increasing from left to right).
The low values favoured increase the suspense: is the Higgs particle (the last predicted element of the standard model) within reach of the currently running LEP II accelerator,
or will we have to wait until the next experimental run at Fermilab, and maybe the LHC at CERN?

Last but not least, precision measurement of sensitive parameters, for instance the mass of the well known W and Z bosons (the equivalent of the photon for weak interactions) show that they are sensitive to the existence of heavier (not yet discovered) particles.After allowing the prediction of the top quark mass, these measurements, which represent an astounding sum of work, allow now to narrow the mass range for the much sought after "Higgs boson", the missing piece in the standard model puzzle. The numbers indicate a "low" value, close to the reach of the LEPII accelerator, currently ramping up in energy around 200 GeV. The suspense is high…

Another type of on-going searches is for supersymmetric particles. Long predicted to explain the mass scale difference between gravity (10 19 GeV) and weak interactions (100 GeV), supersymmetric particles should be characterized by an energy scale of a few TeV, but most models predict that some of their components will have significantly lower mass (they also interestingly predict a light Higgs boson).
Although no discovery has been made yet, current experiments are now moving into the most interesting energy range.

Unification and extra dimensions

Extra dimensions, long the province of string theories  and characterized by unreachable scales,  have suddenly emerged as a possibility even at relatively low mass scales.
Their study would thus bring observable phenomena already at energies in the TeV range, a rich prospect for developing experiments. One of the possible tricks here is to assume that gravity forces propagate in 4+n dimensions, while the other interactions are confined to our 4-dim world. While electroweak interactions are tested at the elementary particle scale, the same is not true of gravity, for which explicit tests barely reach down to the millimetre scale.
This effects offers the very serious possibility of studying experimentally the existence of extra dimensions in  the next 10 years.

A special word about the Rencontres de Moriond

For more than 30 years now, the Rencontres de Moriond, initiated by a small group of physicists around Professor Tran Thanh Van, have brought together scientists from around the world in a unique conference format.
The size of the meeting is voluntarily limited, to ensure a maximum of personal contact, and to avoid parallel sessions: all the presentations occur in plenary sessions, with strict instructions for experimenters to aim their talks at theorists and vice versa. Considerable time is foreseen for general discussions between the talks, and special extended discussions are set up by the organizers as the need arises (this year on the CP issue). More important however are the private discussions, in particular between theorists and experimenters, where projects can develop. An extended break in a long working day, and the setting in a winter sports resort do a lot to promote a relaxed and confident atmosphere, which facilitates such communication.
Another striking feature is the wide age range of participants, but here, the senior staff tends to stay in the audience and bring comments and suggestions while presentations are made by the young scientists who conducted the detailed analysis. Often this is their first international meeting, (and for this European support plays a crucial role) and the quality of their presentations is impressive.

Further Contacts

The present review is by essence a subjective presentation of the highlights of  the Rencontres de Moriond Electroweak 99; remarks and criticisms are welcome :
J.-M. Frère :

detailed in formation on this year's "Rencontres de Moriond" and on future related events can be obtained from:

Rencontres de Moriond :

Rencontres de Moriond            Phone : 33 (0)1 69 15 82 16
LPT - Batiment 211            Fax : 33 (0)1 69 15 82 87
Universite Paris-Sud
91405 Orsay Cedex

or by Email to :