Phase transitions between different states of matter can be associated with a specific type of excitation called the "Higgs excitation". This phenomenon has now been ob-served in a two-dimensional quantum gas at temperatures near absolute zero. In physics spontaneous symmetry breaking is a fundamental feature of transitions between different states of matter.

An example of this phenomenon is the abrupt alignment of spin orientation in a ferromagnetic substance when the material is cooled below the so-called Curie temperature.

Phase transitions introduce a new degree of order into the system, which may in turn provoke a specific type of excitation that causes the ensemble of particles to behave in a coordinated fashion. If their collective motion conforms to rules akin to those of the theory of relativity, a so-called Higgs excitation may arise.

Tracking transient excitations
The Higgs excitation plays a key role in the Standard Model of Particle Physics, where it is associated with the famous Higgs boson. But Higgs excitations can also develop in solid-state-like systems.

The problem is that, as in particle physics, they are difficult to detect ex-perimentally because they rapidly decay.

Higgs excitations are expected to have particularly short lifetimes in low-dimensional atomic systems. Indeed, some physicists have doubted whether they could be observed in such systems at all.

An unpredictable phenomenon
A team of researchers led by Professor Immanuel Bloch, LMU physicist and a Director at the Max-Planck-Institute of Quantum Optics, in close collaboration with theorists at several American institutions, has now experimentally detected Higgs excitations in low-dimensional systems for the first time.

For the experiments, they used an ultracold two-dimensional gas made up of rubidium atoms. This system is in the vicinity of a phase transition, in a state that behaves in accordance with relativistic field theories.

"We are excited to study phenomena close to absolute zero temperature that usually occur at the highest energies", says Bloch. Furthermore, the observations allow the researchers to characterize a phenomenon that is, as yet, not fully understood theoretically. This makes the new data still more valuable. (Nature, 2012) MPQ/god

Publication: Manuel Endres, Takeshi Fukuhara, David Pekker, Marc Cheneau, Peter Schaub, Christian Grob, Eugene Demler, Stefan Kuhr, and Immanuel Bloch The 'Higgs' Amplitude Mode at the Two-Dimensional Superfluid-Mott Insulator Tran-sition Nature, 26. July, 2012.