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What We Learned from ATLAS at Les Rencontres de Moriond 2013
8 Apr 2013

Les Rencontres de Moriond, an important conference for the worldwide community of particle physicists, took place from 2-16 March 2013 in La Thuile, Italy. Of all the scientists present, 22 ATLAS physicists had been invited to reveal the experiment's latest findings. What did we learn from this new ATLAS physics harvest? A brief summary looks like this:
  1. The Higgs-like boson is a Higgs boson and it is compatible with a Standard Model Higgs boson.
  2. The predictions of the Standard Model stand firm.
  3. No physics beyond the Standard Model has been observed despite great efforts to find it.
  4. The best is yet to come: there are still plenty of areas to explore, many mysteries to elucidate, especially with the new collisions at higher energy that are expected in 2015.
At Les Rencontres de Moriond, ATLAS physicists for the first time showed results which included the whole 2012 dataset. This represents a significant increase of 60% compared to the number of 2012 proton-proton collisions covered in the results presented in December 2012. The data collected in 2012 at 8 TeV of energy represent about four times the data collected in 2011 at 7 TeV of collision energy. ATLAS and the other LHC experiments are deeply grateful to the LHC accelerator team for these excellent achievements. The efficient operation of the ATLAS detector—over 93% efficiency—and the hard work of the analysis teams were also essential ingredients in these 28 new ATLAS conferences notes.

1. The Higgs-like boson is a Higgs boson and it is compatible with a Standard Model Higgs boson.

On July 4th 2012, the CMS and ATLAS collaborations announced that they had observed a new particle with a mass of about 126 GeV: a boson consistent with the Higgs boson. In the following months, Higgs searches in ATLAS moved from discovery to measurement phase: The measurement of the particle's mass gained in precision. Its production rates, production modes as well as the ways it decays and how it interacts with other particles all point to a particle with behavior compatible with that predicted by the SM. Its spin (intrinsic quantum property of all particles) and parity (how its mirror image behaves) have been tested and they also favor the SM scenario (spin 0 and parity +1).

The new results shown at Les Rencontres de Moriond about the Higgs-like particle included optimized analysis techniques and the use of the full 2011 and 2012 datasets. The three main discovery channels were updated:

  • the Higgs boson decaying into two photons (H → γ γ ),
  • the Higgs boson decaying into two Z bosons which then decay into four leptons: electrons or muons (H→ZZ→llll) and
  • the Higgs boson decaying into two W bosons which then decay into leptons and neutrinos (H → WW → lνlν).
As it is important to study all the decay channels of the new particle, two new results looking for H→Zγ and H→μμ in the context of SM Higgs scenario were also presented.

Numerous ATLAS analyses also try to find decays of the Higgs-like particle, which would correspond to theories beyond the SM. In that context, new ATLAS searches were shown for H→ Invisible decay products and searches for an additional H→WW→ eμγγ ; no evidence of such behaviors have been established.

Given the new results presented at Les Rencontres de Moriond, particle physicists happily agreed to drop the "like" in the "Higgs-like boson" expression. The new particle is a Higgs boson. Is it the Standard Model Higgs boson? ATLAS physicists must continue their measurements to answer this question definitively.

2. The SM stands firm.

An important part of the ATLAS physics program is to make precise measurements within the Standard Model framework in order to validate analysis techniques and tools and to accurately evaluate many of the background events in the searches for Higgs bosons or physics beyond the SM.

Important new ATLAS SM results reported at Les Rencontres de Moriond were the detailed measurements of rare diboson production (WZ and ZZ) in proton-proton collisions. As these SM processes represent most of the background for key Higgs searches, their production rate has to be accurately measured and compared to the theoretical estimates. This can also help to test the electroweak theory at the TeV energy scale. Deviations from theory could mean new physics processes; no deviations were found.

3. No physics beyond the Standard Model has been observed despite great efforts to find it.

At Les Rencontres de Moriond, many new ATLAS results were related to supersymmetry, a model which predicts a supersymmetric partner for every particle in the SM. Supersymmetry commands much attention in the particle physics community as it may help to answer the so-called hierarchy problem which has long been puzzling scientists: Why is the weak force so much stronger than the force of gravity? The supersymmetry framework may also provide us with answers about dark matter, another enigma of our Universe.

Four new ATLAS results were presented of searches for the expected decay products of gluinos and squarks (superpartners of gluons and quarks) produced via the strong force. Two were optimized to find the stop particle (superpartner of the top quark), crucial to solving the hierarchy problem. With the large number of proton-proton collisions accumulated so far, the ATLAS collaboration also presented a new search looking for supersymmetry produced via the weak force. Finally, another new result reported investigations of long-lived supersymmetric particles decaying a few centimeters away from the origin of the collisions. No sign of supersymmetry was found in these searches, but the limits on the mass of the various superpartners were significantly improved.

Searches for the Z' and W', for heavy particles decaying in three leptons and for new heavy top quarks were also presented as part of the quest for new physics. None of these ATLAS analyses showed evidence of these new particles.

Event display of a heavy particle candidate decaying to WZ which sebsequently decay to one muon (blue line), two electrons (coloured yellow) plus missing transverese energy (red line)

4. The best is yet to come!

Does this mean that all the options have been explored, and that the triumph of the SM is complete? Here are three reasons why not.

First of all, as we write, ATLAS physicists are still scrutinizing 2012 collisions. Only a small fraction of the possible analyses has been completed so far and many unexplored areas still remain to be covered.

Secondly, we know that the SM is not the end of the particle physics story because many mysteries remain unsolved. Among them, most notable are probably the dark matter enigma, the puzzling difference in strength between the force of gravity and the other forces of nature, as well as the fact that most of the Universe is made of matter, not antimatter. Trying to resolve these mysteries remains one of the primary goals of the ATLAS experiment.

Les Rencontres de Moriond have now ended but the ATLAS collaboration is continuing its work on many fronts. ATLAS physicists are probing more precisely as many characteristics of the Higgs boson as possible. They are also making great efforts to push forward the experiment's rich physics program, including SM measurements and searches for physics beyond the SM framework. On top of that, ATLAS physicists are analyzing the proton-lead collisions taken earlier this year. And last but not least, many members of the ATLAS collaboration are involved in the major upgrade of the detector taking place in 2013-2014.

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