XLI Rencontres de Moriond , March 11-18, 2006

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 rather general terms  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.

Fermilab shines with Flavour Physics;
The Mixing diagram gets more colours...
The TOP quark is at the center of attention,
All prepare for the Brout-Englert Higgs particle

After several years where the most impressive progress was taking place in areas like neutrino physics (with the confimation of oscillations, and also their relation to
the matter-antimatter asymmetry), this year, the meeting opened with impressive new results in flavour physics.
Not only does this bring essential information and constraints on theoretical models (in particular those based on Supersymmetry), it also demonstrate the impressvie progress
made by the Fermilab experimental teams (CDHS and D0). Indeed , careful identification of B mesons, and charge determination are tricky businesses, (which will also prove
essential in the search for the Brout-Englert-Higgs scalar, responsible for symmetry breaking, and the separation between weak and electromagnetic interactions).

A very stunning measurement was the mass difference between Bs and  anti Bs ("strange and beautiful mesons") , a particularly tricky measurement.
The value measured is in very good agreement with the "bare" Standard Model, and thus constrains severely any additional contribution which could originate
from new source of flavour changning neutral interactions, in particular in Supersymmetry.

Many more detailed measurement of  Flavour properties, in particular a hoard of exclusive B decay modes , (BELLE, BABAR)  and precise measurements of CP violation in B decays has lead to an impressive progress in the fitting of Qarks Mixings and CP violation parameters, usually represented as the "Unitarity Triangle", which becomes seriously (over) constrained.
This has reached the level where CKM fitting becomes almost a speciality in itself , slowly moving from the theory to the experimental realm.
Once again, the (bare) Standard Model expectations account remarkably for the measurements, which leaves little room for CP violation from other sources
(see in particular the review by F. Borzumati)

This figure summarizes an impressive amount of work form numerous teams working on very different experiments, to determne precisely the parameters of fundamental interactions, see the summary talks for a first impression, and the individual talks for more details


The TOP quark is also coming "to age" , and we start to see the first signs of detailed "top physics decays" experiments, which are exttemely promising from a theoretical
point of vue. Also, a precise determination of the top quark mass is essential in the Standard Model fits, because it constrains, via the ever-improving Z and W mass measurements,
the mass of the still missing Brout-Englert-Higgs particle .  Here also, the graph synthethizes impressive efforts by  experiments, both precise analysis of LEP data, and improvements in the measurements at the Tevatron collider

Neutrinos still at the center of attention…

<>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, it is now established that they have mass, and that the various flavours mix (as do the quarks, but with very diffetrent characteristics). The mass differences are estimated by "oscillations" , while the absolute masses (and their  nature : Dirac , i.e. lepton-number conserving or Majorana) are not establised.  Whether the presence of neutrino masses (which implies either right handed neutrinos or extra scalar fields) is departure from the "Standard Model' is merely a matter of definition or history; except maybe for Majorana masses, the changes are minimal , and similar to those introduced when generalising the Standard Model to include more quark families; in any cases, the "gauge structure" is unaffected.

Striking changes in our conception of neutrinos have been brought in the past few years, and the present period is more a time for consolidation, with however some impressive progress.

Much is expected from the results of the MiniBoone experiment, designe to check the disputed LSND results, but some more patience will be needed.
Earth-based oscillations neutrinos are progressing, and dedictated experiments are aimed at finer understanding of the initial beams (in the case of accelerators, see HARP),
while other aparatus considers bounds on the (still badly known) geological neutrino flux, yielding a new view on the energy-producing reactions in Earth.

 Detailed fits where presented,as well as theoretical  discussion of neutrino mass patterns  in preparation for the next range of experiments. Meanwhile, neutrinos have also become an "astronomical" tool, with large neutino telescopes entering
a new era (for instance, the move from the prototype AMANDA to the currently installing full-size IceCube in the South Pole). 

The understanding of neutrino masses, mixing and CP violation is also critical for our understanding of Leptogenesis, a possible pathway to explain the victory of matter against antimatter in our observed Universe!

Beyond the Standard Model,
Alternatives to Brout-Englert-Higgs symmetry breaking,
Supersymmetry and string-inspired models
Extra- dimensions

With the increasing luminosity of  Fermilab's Tevatron, and the coming of CERN's LHC, the next question is clearly our understanding of the symmetry breaking which differentiates dramatically the "unified" forces governing week interactions and electromagnetism. While the simplest mechanism (but not necessarily the most elegant) uses the famed (but still to be found) Brout-Englert-Higgs partile, a number of alternatives have been proposed, from dynamical symmetry breaking to very intricate models, which typically extend towards the higher energies the search domain.
Supersymmetry, while usually associated with the minimal version of the Standard model,
can even involve such new mechanims, notably as an additional way to stabilize the electroweak scale, but stays mostly the "typical" extension used in numerous analysis to prepare the upcoming experiments.

We had impressive reviews of  Supersymmetry, including new breaking patterns, inclusion of non-standard electroweak breaking, and a detailed review of its effetcs on flavour physics,  compared to the latest results showing the strong experimental constraints on such contributions.

An important tendency is the study of neutral flavour-changing interactions, now considerably extended to the leptonic sector, where very sensitive tests not only exist, but will be considerably developped.

Extra dimensions, long the province of string theories  and characterized by unreachable scales  have become a studied possibilty, even at the TeV ranges, which makes them open to experimental investigation. 

Exemples where the mass spectra are related to 5 or 6-dimensional theories have been discussed, with special attention again to spectacular flavour-changing predictions at high energy, or the mass of the scalar boson..

Matter-antimatter asymmetry, dark matter, cosmological constant.

With the "concordance" model now generally accepted (but with some reserves, see for instance the presentation by KOLB), theorist's attention turns to
explaining why the contributions of baryonic matter, dark matter, and dark energy (with furher speculations about its nature) are currently of comparable sizes. (or at least not vastly different).

A number of models discusses the origin of Matter-antimatter asymmetry (leptogenesis) , in relation to low-energy observation, but also try to link its value to that of dark matter,
searching for a common origin. This is clearly a path of future development.

Direct searches for dark matter, either in the form of "WIMPS", like the Lightest Supersymmetric Particles, or as the less conventional Axions, is also a domain
where experiments are reporting results (mostly negative for the moment), approaching the required sensitivity. These complement the indirect tests (annihilation into neutrinos), and the collider searches.


A special word about the Rencontres de Moriond

For  40 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 . 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 Electroweak2006; remarks and criticisms are welcome :
J.-M. Frère : frere@ulb.ac.be

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

Rencontres de Moriond :