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CERN | Geneva | Switzerland

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Much food for thought at major physics conference

The first week of the biggest winter conference, the Rencontres de Moriond held in La Thuile in Italy closed on March 10, leaving all attendants both impressed and puzzled by all the new results presented.

The situation is the following: Theorists know that the current theoretical model, the Standard Model of particle physics, has its limits and that it is probably the most accessible part of a more complex but unknown theory. Think of it as for mathematics: arithmetic is all most of us need for every day tasks even though we know geometry, algebra and calculus are needed for more complex applications.

Physicists expect to see new phenomena that are referred to as “new physics”, which would tell us which one of the many new theories currently proposed is the right one. And everybody hopes the Large Hadron Collider (LHC) experiments will soon discover something to set us in the right direction.

Hence, the focus of this conference was to assess the impact of the all the latest experimental results on new models, particularly supersymmetry (SUSY) and extra dimensions. And there were plenty of new results on searches for the Higgs boson, SUSY particles and dark matter, as well as new precision measurements and neutrino physics.

The first excitement came from the LHCb, CMS and ATLAS experiments operating at the LHC with new measurements of how often a Bs meson decays into two muons. This decay occurs so rarely in the context of the Standard Model that even small contributions from new physics could be detected. LHCb is setting the best limit to date, with less than 4.5 x 10-9, barely above the Standard Model prediction of around 3.5 x 10-9. This leaves very little room for new physics. However, David Straub, a theorist affiliated with Scuola Normale Superiore and INFN in Pisa, showed that finding less than what is predicted by the Standard Model would also open the door to new physics, something that has previously received little attention but is now becoming possible with the increase in precision from the LHC experiments.

With stringent limits on rare decays such as Bs or Bd to two muons, many supersymmetric models have very little parameter space still allowed, as shown by the small rectangle in the bottom left corner. The rest is what was still allowed a year ago.

On the search for the Higgs boson, now, four separate experiments see faint signs of what could be Higgs bosons in four different channels. It is a bit like hearing a rumor from four trust-worthy people who all got very similar information from different reputable sources. Although it does not prove anything, we can all start thinking seriously about it. All experiments see an excess compatible with a Higgs boson mass of 125 GeV, even though the strength of the signal is still too weak to be convincing. ATLAS and CMS will resume data-taking next week and should have a clear and final verdict this year.

While all four collaborations – ATLAS, CMS, CDF and D0 – insisted that it was too early to jump to conclusions about the Higgs boson, theorists have already been checking the effects of the mass of the Higgs. Nazila Mahmoudi, a theorist from CERN, showed that the currently allowed range for the Higgs boson mass is already putting constraints on SUSY models.

The values of tan β and mA, two important parameters of supersymmetric models, still allowed if a Higgs boson is found with a mass of 125 GeV. The red points are disqualified by b-physics results. Everything above the yellow curve is excluded by direct searches for SUSY particles obtained by the CMS collaboration. And if the Higgs boson is found around 125 GeV, only the green band would still be allowed under certain constraints.

The Universe content: 96% of it comes from some absolutely unknown types called dark matter and dark energy.

Josef Pradler from the Perimeter Institute in Canada addressed a long-standing and controversial result reported several years ago by the DAMA/LIBRA experiment. The group has been claiming for years the observation of a very strong signal for “dark matter”, a mysterious and unknown type of matter that accounts for about 23% of all the content of the Universe while regular matter (all stars and galaxies) amounts to only 4%. The remaining 73% comes from some unknown type of energy called “dark energy”.

From various gravitational measurements, astronomers have shown that dark matter is more concentrated galactic halo, i.e. outskirt of the galaxy. As the Earth orbits around the Sun on its annual cycle, it encounters more WIMPs (Weakly Interacting Massive Particles, a nickname for a dark matter candidate) than in December when moving away from the dark matter source. It is very much like getting a head-wind in the summer and a tail wind in the winter.

The DAMA/LIBRA detector counts more interactions with WIMPS in the summer than in the winter, hence the annual modulation in the number of particles detected (vertical axis) as a function of time (horizontal axis).

The problem is that other experiments cannot quite confirm this result, so some people have suggested that this could simply be due to cosmic muons. Josef Pradler and his colleagues just showed that the DAMA/LIBRA data are inconsistent with the cosmic muon hypothesis at 99% CL. The mystery remains.

Possible signs of a Higgs boson being produced and decaying just like the Standard Model predicts and no signs of new physics despite extremely precise tests, left all participants rather puzzled.

Theorists know that the Standard Model does not describe everything we observe. So what is the real theory that would explain everything? Lisa Randall from Harvard University reminded the audience that whatever the new theory is, it will have to address both the symmetry breaking (why some particles associated with the electroweak force are massive, others massless) and the hierarchy problem (why is the top quark so much heavier than the electron?). Much food for thought there.

Pauline Gagnon

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