Archive for October, 2015
The Higgs Boson in 2015
The Higgs boson has been discovered in 2012 at CERN’s Large Hadron Collider (LHC), or more precisely at the LHC experiments ATLAS and CMS. The discovery of the Higgs resonance is definitely a milestone in particle physics and two of the fathers of the Higgs mechanism, Peter Higgs and Francois Englert, were awarded the nobel prize in physics in 2013.
After the discovery three years ago, there was immediately a decisive question to be answered: Has the discovered particle all the properties which are predicted by the Standard Model (SM) of particle physics. Or turning the question around in more scientific terms: Are there any statistically significant deviations from the SM predictions which can be identified with the recorded proton-proton collisions. Any such deviation would of course call for physics beyond the SM. Until the end of LHC run 1 at the beginning auf 2013, there have been great efforts to collect as many Higgs collisions as possible. And similar efforts have been invested in recent years to extract as much information as possible from the recorded data.
Signal strength measurements for different Higgs-production channels, where a signal strenath of 1 is the SM expectation (taken from ATLAS CONF-2015-044)
Only recently the Higgs legacy of run 1 has been finalized performing the combination of the Atlas and the CMS data. The so-called signal strength for different production channels and the global signal strength is shown in the diagram on the right relative to the SM prediction. So far, the discovered particle does not give any hints for new physics beyond the SM. It simply looks more or less as predicted decades ago.
For these and similar measurements, the interplay between theory predictions and the experimental analysis is most crucial. Mid of October, experimentalists and theorist working on Higgs physics have met at the conference “Higgs Couplings 2015” which was hosted by the IPPP in Durham and took place in the beautiful medieval Lumley Castle close to Durham. The latest run 1 measurements have been presented and discussed. But run 1 is already part of the past. Everybody is looking forward to seeing the measurements from run 2 and gearing up for the upcoming analyses.
Run 2 has already started this year with the record breaking proton-proton energy of 13 TeV (run 1 has provided 7 and 8 TeV collisions). In 2015, a year to learn how to operate at the record-breaking energy and with collisions every 25 nano-secons, there will be not enough data recorded to make a major step forward in the precision of Higgs measurements (this is very different for other new physics searches, e.g. for multi-TeV resonances). However, the coming years of run 2 will be exciting for Higgs physics for sure.
So far, measurements are still statistically limited, i.e. by the relatively small number of recorded Higgs-bosons. However, residual uncertainties within the theoretical predictions will soon enter as a major player in the quest for ultimate measurements of Higgs properties, and therefore also in the quest for physics beyond the SM in the Higgs sector. Hence, improving theory predictions and making them available for the analysis of the data is more important than ever in the field of Higgs physics, and one of the research topics at our institute. Conferences like the “Higgs Couplings 2015” are providing an important forum for discussions on these topics between experimentalists and theorists.
So, let’s see what nature will teach us about the Higgs in the coming years.
Exploring dark matter with IceCube and the LHC
Various astrophysical and cosmological observations point towards the existence of dark matter, possibly a novel kind of fundamental particle, which does not emit or reflect light, and which only interacts weakly with ordinary matter.
If such a dark matter particle exists, it can be searched for in different ways: direct detection looks for the elastic scattering of dark matter with nuclei in highly sensitive underground experiments, as Earth passes through our galaxy’s dark matter halo. Indirect detection experiments on Earth or in space look for cosmic rays (neutrinos, photons, or antiparticles) from the annihilation of dark matter particles in the centre of the Galaxy or in the Sun. And last but not least, if dark matter interacts with ordinary matter, it may be produced in high-energy proton collisions at the LHC.
To explore the nature of dark matter, and to be able to combine results from direct, indirect and collider searches, one can follow a more model-independent approach and describe dark matter and its experimental signatures with a minimal amount of new particles, interactions and model parameters. Such simplified or minimal models allow to explore the landscape of dark matter theories, and serve as a mediator between the experimental searches and more complete theories of dark matter, like supersymmetry.
About 1027 dark matter particles per second may pass through the Sun. They can loose some energy through scattering off protons and eventually be captured in the core of the Sun by the Sun’s gravitational pull. Dark matter particles in the Sun would annihilate with each other and produce ordinary particles, some of which decay into neutrinos. Neutrinos interact weakly with matter, can thus escape the Sun and could be observed by the neutrino telescope IceCube near the South Pole. Neutrinos therefore provide a way to search for dark matter in the core of the Sun.
At the LHC, dark matter may be produced in high-energy proton collisions. As dark matter particles interact at most weakly with ordinary matter, they would leave no trace in the LHC detectors. However, dark matter (and other novel weakly interacting particles) can be detected by looking at exotic signatures, where a single spray of ordinary particles is seen, without the momentum and energy balance characteristic for standard particle collisions (so-called mono-jet events, see right figure).
We have recently joined forces with members of the RWTH IceCube team to explore dark matter searches from neutrinos in the Sun and through dark matter production at the LHC, see http://arxiv.org/abs/1411.5917 and http://arxiv.org/abs/1509.07867. We have considered a minimal dark matter model where we only add two new particles to the ordinary matter: a new dark matter fermion, and a new force particle, which mediates the interaction between the dark matter fermion and the ordinary matter. As no signal for dark matter has been observed, we can place limits on the masses of the dark matter particle and the new force particle, see figure to the left. We find a strinking complementarity of the different experimental approaches, which probe particular and often distinct regions of the model parameter space.
Thus only the combination of future collider, indirect and direct searches for dark matter will allow a comprehensive test of minimal dark matter models.