XLIInd Rencontres de Moriond , March 10-17th, 2007

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 summary talks  by Antonio Masiero and Bruno Mansoulie, and the detailed 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.

"Basics of" ... a new (experimental) Moriond feature?
 Moriond is an exceptional conference, notably by the wide scope of the subjects discussed (from flavour to symmetry breaking, dark matter to extra dimensions, particle searches to neutrino and cosmic rays,...). For this reason, we have asked prominent speakers to provide mini-reviews, or elaborate introductions to the various topics. We went further this year, with 3 "basics of" commissonned talks, aimed at refreshing the concepts at a more elementary level. For these "service" talks, we also lifted experimentally some of our usual umplicit rules (organizers don't speak,  participants don't speak 2 years in a row). Two of the "basics" were commissionned well in advance (Supersymmetry, by Pierre Fayet, Neutrinos, by Boris Kayser). A third was commissionned on the spot, due to the announcement of Dzero mixing (see later): Patricia Ball accepted to prepare with short notice a critical review of the theoretical litterature. The experiment was generally considered very positive, and it is a possiblity which will be kept in mind for future editions.

Searches and preparation for the Brout-Englert Higgs particle / Theoretical variants

One of the highlights was certainly the presentation of the current search results for the scalar bosons associated to electroweak symmetry breaking.
Direct searches by the Tevatron groups (CDF and D0) don't reach the sensitivity to confirm or exclude the minimum standard model, but graphs showing the exclusion zone as a function of mass reach cross sections typically one order of magnitude above it. Considerable progress has been made in the analysis of individual channels, and further significance is expected in the future from:
current limitsIt is interesting to see that already some variants (for instance with just one extra generation) can already be excluded for a relevant range of parameters. How far this will lead before the start of LHC is certainly an exciting suspense!

In parallel, the LHC teams prepare their analysis, and plan for the first years.

Meanwhile, theoretical models are diversifying, and in some way simplifying!
Besides the heavy machinery of the Supersymmetry fits, other TeV energy completion schemes are considered. In particular, the possibility that the Brout-Englert-Higgs excitations would be spread over 2 or several peaks (by the simple addition of singlets), or even over a continuous mass distribution was reminded by J Van Der Bij. In that case, some of the events observed at LEPII might even be part of the signal. This could however be quite frustrating for collider searches.
Other models try to include "dark matter" in a simple way as extra scalar particles associated to a discrete symmetry.

indirect limitsIndirect limits are constantly improving (with the "1 sigma" ellipse now only tangent to the SM expectations!) , with progress in the determination of the W and top mass.


Heavy flavours at the center of attention…and the D-anti D mixing.

Heavy flavours made an essential part of the meeting.
After the impressive evidence last year for Bs oscillations from the Tevatron, we had a more varied, but no less rich set of new data in 2007.

First of all, the top studies are really entering into force, with more precisely reconstructed decays, and increased precision on the mass.
This is of course an essential element for indirect tests of the Standard Model. But we see also the first evidence coming for single top production. (which will in the future lead to intersting helicity tests of the couplings)

The surprise : D°-anti D° mixing.
mixingThis first evidence (from different analysis in Belle and BaBar) came a bit "out of the blude". It is not unfair to say that theorists had almost given up on this (a notoriously difficult evaluation, with the decay of the charm quark playing an important rôle, and the mixing dominated in standard model by long distance, real K-pion intermediary stated), and estmations span orders of magnitude. The currernt experimental number (see the Belle and BaBar talks, and the review- Basic of by P. Ball), came at the upper limit of those estimations.
Even so, perspectives for finding "beyond the Standard Model" effects in this channel are difficult, precisely due to the difficulty of the evaluations; one would most likely need to favour CP violating effects for such a search.
As goes without saying, this "Moriond First" called for some celebration (which the summary speaker dubbed the "D° mix cocktail")

mixing diagrams

B physics and CKM fits
ckm fit
Achievements in individual and rare decays of B's keep adding. Meanwhile, the issue of the CP mixing angles is not completely sorted out.
In particular, some tension exists between the "gold plated" determination of sin(2 beta) , and that coming through "Penguin modes". (one should also mention the attention to direct measurement of Vub).

Let us note that some tension also persists in the unitarity of the CKM matrix, (the "u" column)

K physics
Here also, precision measurements abound, see details in the summary talks.

Neutrinos consolidation and waiting…

<>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.
At the time of the Moriond meeting, the MiniBoone data were not available yet ( but we got a detailed study of the systematics and expectations) -- (at the time of writing this summary it is now known that they don't agree with LSND in an oscillation context)

Meanwhile, neutrinos have also become an "astronomical" tool, with large neutino telescopes entering
a new era (the larger IceCube is developing Amanda at the South Pole, Antares as started its deployment and seen first light). 

An interesting approach to more general neutrino mixing was brought by B. Gavela, in a genric non-unitary mixing analysis (it is well known that mixing with heavy or sterile states can induce some non-unitarity in the 3X3 reduced matrix, but we are dealing here with  a generic approach)

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! -- more about this later.

Improved limits are also reported on neutrinoless double beta decay, despite a continuiing controversy.

Beyond the Standard Model,
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,   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. 

Matter-antimatter asymmetry, dark matter, cosmological constant, high energy cosmic rays.

With the "concordance" model now generally accepted (but with some reserves, see for instance the presentation by ), 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). The defeat of antimatter (in a Universe thought to be initially symmetrical) is also an important feature, linked to CP violation.

Leptogenesis is obviously a favourite model for generating the matter-antimatter asymmetry, but in a way, it becomes even too successfull and robust, at the point of being difficult to infirm.
While it was initially argued that serious constraints on the (observable) light neutrino mass spectrum were implied by leptogenesis, it appears now that much more effects can come in play. In particular, several heavy neutrinos can come into play, as well as scalar particles coupled to them. A recently stressed aspect (flavour leptogenesis) also considers the possibility that part of the lepton number be sequestered in family flavours, which makes it resistant to later wash-out by lighter Majorana exchanges.
As a result, and while some calculations need to be re-considered, the impression is that the mechanism becomes even more robust, up to the point that devising low-energy tests becomes a real challenge.

Meanwhile, other alternatives continue to be explored, notably the electroweak-scale baryogenesis (impossible in the Standard Model, but approached typically in a supersymmetric context).

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  approaching the required sensitivity. At the time of Moriond, the PVLAS results were still in conflict with other searches in a traditional axion searches. As a result, a number of new experiments are either starting or being planned.

The AUGER experiment is largely deployed by now, but the final verdict on the GZK cut-off events is still to come.
(note: in particular, the relative calibration of the air showers vs ground detectors is clearly an issue to be solved, and for which
new experimental work is in progress, but on which final results were not yet reported)

Outstanding questions and precision tests
The muon "g-2" measurement remains somewhat of a puzzle.
Here, much of the debate comes from the extraction of the vacuum polarization effects from data. Traditional methods (accelerators) yield a 3.4 sigma difference, but tau decay only 1 sigma... this stays an open question, with the theory behind experiment.

Gravity at short distance, quantum coherence in the B system are other examples of such tests.


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 Electroweak2007; 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 :