ATLAS Experiment CERN


ATLAS and the Higgs
Published October 2012 (Updated April 2013)

At a seminar held on 4 July 2012, the ATLAS experiment announced that it had observed a new particle: a boson consistent with the Higgs boson. The excess of signal over background was observed at a mass of around 126 GeV, and the level of confidence in the results was calculated to be 5 sigma. (You can find an explanation of GeV here and more information about sigma here).

At the same seminar, ATLAS' sister experiment on the LHC (Large Hadron Collider), CMS, announced very similar results. The similarity acts as verification: if one experiment saw something very different to the other, there would be doubts about the results.

In March 2013, in the light of the updated ATLAS and CMS results, CERN announced that the new particle was indeed a Higgs boson. Having analyzed two and a half times more data than was available for the discovery announcement in July, the confidence of observation has risen to 10 sigma. The experiments were also able to show that the properties of the particle as well as the ways it interacts with other particles were well-matched with those of a Higgs boson, which is expected to have spin 0 and parity +. Physicists have now to pursue their measurements to determine if this Higgs particle corresponds to the Standard Model Higgs boson or if it is part of a new physics scenario.

ATLAS' role

ATLAS is located at Point 1 of the LHC, which accelerates proton beams to high energy and then collides them head-on at four different points along its 27-km ring. As a "general purpose" detector, it is designed to identify and measure many different types of particles produced in these collisions. From the data captured, ATLAS physicists are able to study a broad variety of interesting physics topics and to search for new phenomena, such as the Higgs boson. (You can find a description of the ATLAS detector here).

Thanks to the particularly impressive performance of the LHC in producing collisions during 2012, and the detector's very high data-taking efficiency (nearly 96%), ATLAS was able to record nearly 22 inverse femtobarns of data during 2012 to add to the 4.8 inverse femtobarns it recorded in 2011.

To obtain the high quantity of data, the LHC attained very high instantaneous luminosities. This means that there were many more proton collisions occurring at essentially the same time in the detector, an effect known as "event pile-up", making it more complex to process and analyse the data. Fortunately, the quality of data taken during that time was excellent, so ATLAS physicists were able to take advantage of the additional data to make the discovery after little more than two years of LHC operation.

Why is this important to mankind?

This result is an important advance in our understanding of the basic forces holding the universe together. In particular this new boson provides support for the existence of the proposed Higgs field, which explains how some particles come to have mass and others don't. Without mass, all particles would fly around freely and matter as we know it would not exist.

Physicists work to a theory of fundamental particles and their interactions called the Standard Model, which was first proposed in the 1970s. So far experiments have been able to confirm the existence of nearly all its elements with a high degree of precision. The Higgs boson, however, had eluded detection until now, prompting speculation that the theory could be incomplete. The findings so far suggest a Higgs boson compatible with the Standard Model, but further studies are needed to confirm this.

There is a more philosophical reason for the importance of this observation. Human beings have a capacity for abstract thinking and reasoning that goes beyond solving only our immediate needs. This scientific investigation and the large, complex apparatus needed to make it happen are examples of our unique human ability and drive to find out "why?". This drive forms the basis of our civilisation, producing knowledge and tools for future generations.

Pursuit of new physics

Up to now, the more detailed studies of the newly discovered particle's properties reveal it to be compatible with the Standard Model Higgs boson. However, scientists are looking for more Higgs particles which, according to almost all high energy extensions of the Standard Model, should exist. Some of the most popular new models of physics are the so-called supersymmetry theories, which could potentially solve a number of problems in theoretical physics. The most minimalist supersymmetry theory predicts at least five (!) Higgs bosons: three neutral and two charged. So in the future if we detect more than one, we will know that we are looking at new physics!

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