XXXV Rencontres de Moriond , March 11-18, 2000

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.

Neutrinos still at the center of attention…

Impressive progress in the experimentally difficult field of neutrino physics has placed them again  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;

figure caption: The image of the Sun reconstructed from its neutrinos detected on Earth (slide presented by the SuperKamiokande collaboration)
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: (slide presented by the K2K collaboration)
The first "long baseline" high energy event: muonic neutrinos produced at KEK are detected in the Superkamiokande detector, 250 km away!
This is the prototype of a series of long baseline experiments planned or in construction (with neutrino sources at Fermilab or CERN).
There is currently evidence for neutrino oscillation (or disappearance) in 3 different channels:
- a deficit in the solar enutrino flux is observed by several experiments (Homestake, Gallex and Sage, SuperKamiokande)
- a deficit of  atmospheric muonic neutrinos is observed by SuperKamiokande
- an evidence for the oscillation from muonic to electronic neutrinos is provided by LSND

We start with  the last issue. Both LSND and the similar KARMEN experiment have refined their measurements, and their conclusions are unchanged: LSND still claims a signal, while KARMEN finds no evidence. There is not necessarily a contradiction however, since the parameter ranges covered are (slightly) different.

The issue of atmospheric neutrinos is pursued by SuperKamiokande, which now investigates the fate of the missing muonic neutrinos; its findings suggest that they are converted into another kind of "active" neutrinos (most likely those associated to the tau lepton), rather than into completely sterile particles.
Whilewe do not control  the atmospheric neutrino flux  and must largely rely on simulations, a direct experiment is being conducted wiht a controlled man-made muonic neutrino beam, produced at KEK (see fiugre caption). The first events were presented, and they show indications of a similar deficit.
It is to be noted that the detemining factor is the ratio L/E of the distance to the beam energy. In this respect, the CHOOZ experiment, whose final results were given can be seen as the counterpart of  KtoKfor electronic neutrinos, and indeed this experiment provides some of the strongest constraints on model building.

The spectrum of solar neutrinos in the Boron region are now investigated in detail by SuperKamiokande. This experiment now puts limits on the contribution from neutrinos from the p- 3He reaction. It is quite striking that the results of SuperKamiokande converge to a lack of dependence of the observed spectrum both on energy and on distance (seasonal effects). These data may need more theoretical investigations.

Theory was very present on the neutrino front. On one hand, the phenomenological discussion in terms of oscillations was reconsidered to show that matter oscillations impose to consider the full range of mixing angles (rather than limiting to 45°). Detailed fits to the data are constatntly being updated.
On a more theoretical side, the understanding of the origin of neutrino masses, its possible link to supersymmetry, but also the use of neutrinos as probe of the possible existence of extra dimensions are the subject of much attention.

Data start to be coming from large neutrino arrays (AMANDA) while prototypes are being fine-tuned (ANTARES).
Even neutrino lensing was evoqued in this perspective.

More than 4 dimensions

Extra dimensions, long the province of string theories  and characterized by unreachable scales  have suddenly become a studied possibilty, even at the TeV ranges, which makes them open to experimental investigation.  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. The neutrinos, in particular right-handed components would be the only other particles to escape this fate...
The subject was extensively covered in this meeting, with various theoretical approaches compared, and their phenomenological consequences studied, both for neutrinos and for rare processes
Clearly this activity will continue developping boht in its experimental and theoretical aspects.

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 .
CP violation is now  better understood, with the observation of  "explicit" CP violation in the K decays, which offers a second independent CP violation observable (called e'), but the question now turns to its quantitative understanding. Namely, is the description in terms of the Standard model and the Kobayashi-Maskawa mixing matrix sufficient?
The accuracy of the measurement of e' improves, and both   at CERN and FERMILAB. Theoretical debate is raging to determine whether these new measurements can be fitted within the Standard Model, and the question is difficult, since it touches both strong interactions and long-distance effects. It was hotly debated in Moriond. Decisive arguments may have to come from the observtion of more signals of CP violation, particularly in  heavy mesons, specially the B and Bs.
Promising preliminary data have been shown by both BABAR abd BELLE dedicated experiments, while  data from general-purpose apparatus (Tevatron and LEP) already gives a glimpse of what awaits us.

A sure indication of the need for CP violation beyond the Standard Model is the current asymmetry between matter and antimatter; for this, "harder" sources of CP violation are needed. The cosmological implications on baryon asymmetry were discussed in details, with several mechanisms now on the market.

The Higgs  hunt 
(and other searches)

figure caption:
an exemple of data available from the scanned transparencies (here from the talk:  W mass and Electroweak fits  by A. Straessner 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?

Compared to last year, the forbidden area has been restricted both by improved indirect precision measurements and by pushing the direct lower  bound from 90 to 107.7 GeV.

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. Compared to last year, the forbidden area has been restricted both by improved indirect precision measurements and by pushing the direct lower  bound from 90 to 107.7 GeV.

The suspense is high, as the bounds are getting closer, but the shutdown of the LEPII facility also approaches...

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.

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 Electroweak2000; 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 :