XLIIInd Rencontres de Moriond , March 1st-8th, 2008

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.
Considerable work relating to the preparation and analysis of future experiments was also presented,
even though space lacks to review it here.

We will thus refer the professional particle physicist directly to the summary talks  by Ken Peach (exp),  Marcela Carena (th) and the detailed scanned slides (or later to the proceedings) where the details of each presentation can be examined freely. The illustrations below are extracted from those summary talks, where the reference to the respective presentations can be found.
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.


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

While  the LHC will constitute the ideal tool for the production and detection of the (Brout Englert Higgs) scalar boson, (the missing piece in the Standard Model), experiments at Tevatron are accumulting data (although at lower energy and luminosity), and refining considerably their analysis.
For the first time, their sensitivity this year extends to the level expected in the Standard Model (although only for relatively heavy scalars). (remember that, at the difference of its couplings, the scalar boson is not predicted by the Standard Model, although constraints from radiative corrections and indirect experimental data favour rather light scalars).


















Theoretical models are evolving in expectation for the LHC !
In the domain of the Scalar sector, and its variants, attention has long concentrated on the Supersymmetric approach (which foresees a doubling of the scalar structure, and generally a rather light scalar (<130 GeV), except in the NMMS extension. Many alternate models are however currently proposed, notably based on extra dimensions (invoking or not a duality between strongly and weakly coupled sectors à la AdS/CFT). In particular, in warped models (Randall-Sundrum), the fermions and scalars can be placed according to various scenarions on the boundaries or in the bulk. An obvious prediction is particular flavour effects for the Top quark family.

Other possibilities (in the line discussed last year by J Vander Bij) involve mixing the canonical scalar with other states, possibly ending
in an "unparticle" configuration (in practice, a continuous effective mass distribution, rather than a single peak ), much harder to spot at a collider.






Link with Dark Matter
Even very simple models in the scalar sectors can introduce dark matter (it is sufficient to have a conserved "parity", like Z2, similar in a way to the much more evolved R parity in SuperSymmetry) ) . An interesting possibility is that precisely this sector triggers the symmetry breaking.

Flavour Physics

Single top production , and Top quark properties
Precision increases in both the Top mass and its properties.



Both FNAL experiments now see the single top production (thus by weak interactions), and start to study its properties in some detail, This is also an opportunity to serach for abnormal couplings of the Top








 

Bottom and Charm  physics , Cabibbo-Kobayashi-Maskawa fits


Abundant data emerge from B factories (and Tevatron), not only on the general oscillation/mixing parameters but also on
 particular decay modes. The results are too numerous to review here, and we send the reader to the aforementioned summary talks, or the original presentations.

Glitches in the picture ?

An intriguiing result is the measurement of time-dependant CP violation in the Kshort Pi Pi mode for B decays. While the Standard Model expects results similar to the usual J/Psi Kshort,
the newest data (still quite preliminary) stand at 2 sigma from that expectation.

The summary talk also mentioned some discrepancies in the Bs decay phases recently claimed by the  UTfit team.(but too late for presentation at this meeting)

The potential for detecting "beyond standard model", particularly in radiative decays of B into s , is also the subject of methodical and extensive theoretical investigation

The D° mixing has now been confirmed, and progress on CKM fits continues .


From Charm physics to Glueballs

Charm physics continues to be studied in depth. We will not review the details here, but stress an interesting development, namely the study (at BES) of the "gluon -rich" domain of radiative J/Psi decay (  the decay indeed can involve the intermediary state: photon + 2 gluons ). This long-unraveled field starts to give important results, whch will soon be ampliified with more sensitive equipment, and are thus an area to watch in the future!






Neutrinos

<>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 a 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.

MiniBoone sparks theoy

With the MiniBoone data now published, one might have hoped for the situation to settle in terms of neutrino mixings.
Yet, those very result (which at face value exclude the oscillation interpretation of the former LSND data) have suscited considerable theoretical interest.


On the one hand, while the MiniBoone data don't find any excess in the expected domain of energy (and thus disprove the interpretation of LSND as naïve oscillations), they also include an unexpected excess at very low energy (below their initially planned energy cut). Possible explanations for this excess have been suggested, in particular, the contribution of anomalies to neutrino X -> neutrino X photon diffusion. (the photon being misindentified as an electron). Such contritutions certainly exist, but their  exact importance must be studied; ore generally low-energy neutrino diffusion processes are re-investigated.

Theoretical efforts have also aimed at reconciling the LSND and MiniBoone results, either by adding more sterile neutrinos (a difficult approach), or by using the subtle differences between the experiments. Indeed, LSND and MiniBoone strive for studiyng the same domain of L/E (distance/energy), but with different L and E. If the oscillation can be shown to depend on energy (as is the case when interaction with matter, à la MSW intervenes), this can be a way to reconcile things (the new name "chameleon" has been forged to apply to such situation ). The cost however seems to introduce new interactions involving more light particles.



Generic departures and Lepton mixing
An increasingly important theoretical approach deals with departures from standard processes in leptonic interactions.
On the one hand, a systematic study evaluates the possible low-energy effects of mixing neutrinos with heavier (unseen) partners.
More generally Lepton Flavour violation is seen as a possible signal of physics beyond the Standard Model, including extra dimensions, supersymmetry ? ...

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

Neutrinos have also become an "astronomical" tool, with large neutino telescopes entering
a new era. The larger IceCube is successfully developing Amanda at the South Pole,  but its searches for point sources have this far given negative results. Antares now pursues its developments and identifies neutrino candidates.
Their search also puts bounds on dark matter candidates, through their possible annihilation (in the center of the Earth, Sun, or of the Galaxy) into neutrinos .

Direct searches for dark matter
They complement the indirect searches (see above), and the collider searches for long-lived particles.
See the review talks for a detailed review of the current situation.
An interesting development is the use of an "old" tool , namely small bubble chambers in the search for dark matter (the COUPP experiment)







Cosmic Rays after Auger results









  Cosmic rays , paticularly the very high energy ones, could teach us a lot baout our Universe, but also about particle physics and fundamental interactions in general.

One of the outstanding issues  is the so-called "GZK" cut-off: above some energy, the collisions of cosmic rays (protons) with the cosmic background (relic photons)  reduces dramatically their effective range, resulting in an expected sharp cut-off.
Another issue, particularly with very high energy cosmic rays, is their physical origin (what are the cosmic accelerators involved), and, in an associated way, their exact composition.

Answers to these questions are coming now, and we are probably at a turning point in high energy cosmic rays astronomy and astrophysice. (note that  many of these questions also arise for the neutrino telescopes, mentioned at the beginning of this  section.

Very high energy cosmic rays (and also very high energy neutrinos) are a rare occurence, requiring the current large instruments (from 1 to several squared kilometers) to achieve any statistics. Even so, localizing sources remains an elusive task, since the identification of a source by any given experiment would require several events originiating from that direction, .. a sensitivity not yet reached.
Meanwhile, alternate strategies are based on catalog searches. In simple terms, this involves pooling together a number of potential sources (on the basis of theoretical prejudice, for instance active galactic nuclei),  to associate a small area around each of the potential sources, and to consider the total surface constituted from those patches. If this area receives significantly more events than an equivalent area not associated to the potential sources, a correlation is established... This correlation establishes a link between the observed cosmic rays and at least a subset of the considered catalog of potential sources (or possibly other obtject closely associated).
As we will see below, it does not, at the present stage validate the composition of the whole catalog.

The AUGER experiment (even still only partially deployed) has made impressive breakthroughs ,  and for this reason was the subject of a mini-session.

First of all, the experiment has now confirmed the presence of the GZK cut-off.

On a more technical level, considerable work (and progress) is devoted to study the calibration of the "air showers' (the ratio of the light detected to the energy deposited
in the high atmosphere by incoming cosmic rays), a complicated issue which also requires a good knowledge of the atmosphere.

The most exciting result however was the established correlation between high-energy cosmic rays and a catalog involving Active Galactic Nuclei sources.
This achievement is impressive.
But physicists are impatient people, and, this result barely obtained, they want to go further... and are already  venturing into the following step, namely, the unraveling of the catalogs.
Indeed, establishing a correlation with a catalog in no way means that all the potential sources involved indeed contribute. Acutally, even the presence of a few (one?) powefull source in the catalog could induce such a general correlation. The next steps will involve detailing such catalogs, or reversing the procedure, and looking for parts of the catalog contributing less than naïvely expected. We look forward to exciting times in this, ... but of course the best indication will come when experiments will localize individual sources based on their data alone.
 


Preparing for the LHC ... and further Tevatron results

Several presentations dealt with the current searches (supersymmetry, extra dimensions, Kaluza-Klein particles, ..) at the Tevatron, but also with the final preparation for the LHC start-up. We don't have the space to enter here in a detailed presentation, but this augures in the best way for the next Moriond meeting!.



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 :
http://moriond.in2p3.fr/