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.
-
On the experimental side, new detectors start to look for "astrophysical
neutrinos", in particular high-energy neutrinos from distant sources (for
instance the Amanda detector in the Antarctic), while plans for neutrino
factories are developing!.
-
On the theory side, the presence of 3 mechanisms of oscillations seems
hard to reconcile with only 3 physical neutrinos. The theorists are back
to the drawing board to understand the mass patterns, to check whether
more (sterile) neutrinos would stay compatible with existing limits.
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 : 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://www.lal.in2p3.fr/CONF/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
FRANCE
or by Email to : moriond@qcd.th..u-psud.fr