Category Archives: Food for the mind

Still no Dark Matter in the latest analysis of LHC data…

Last night the ATLAS Collaboration released its latest search for dark matter and other beyond the standard model theories [1] based on the full dataset from the LHC Run I (2010-2012).

By looking for proton-proton collisions where jets of hadronic particles are produced only in one direction (Figure 1), violating conservation of momentum only in appearance, we use ATLAS to search for  weakly interacting massive particles (WIMPs), such as dark matter particles. Because they are weakly interacting, the WIMPs escape ATLAS undetected and lead to what looks like missing momentum.

Fig1Figure 1: A proton-proton collision recorded by ATLAS in November 2012, viewed in the transverse plane perpendicular to the beam axis. It shows a 1.2 TeV jet of hadronic particles (tracks and green bars) on one side, one can note the absence of activity on the other side, mimicking a violation of momentum conservation by as much as 1.2 TeV.

Neutrinos do exist and are often produced in LHC proton-proton collisions. These collisions look exactly like dark matter candidates and make up most of the background. We need to precisely predict the expected number of background collisions with outgoing neutrinos and compare that with the observed number of events in ATLAS data. Only a significant excess over the expected background allows a claim for a new physics signal.

The new ATLAS result [1] explores collisions with very high missing momentum, beyond any previous LHC search and we observe a good agreement between the number of events actually detected and the background alone prediction.

How well do we know the backgrounds? This is the most central question of this work. The sensitivity to dark matter is dominated by systematic uncertainties on the background. This is why at Stockholm University we decided to focus on the background calculations.

In the fall of 2014 Olof Lundberg, PhD student at the Stockholm university physics department, defended his licentiate thesis [2] where in particular he presented the development of a new technique to compute the backgrounds.  All the gory details of the calculations are in his licentiat, all the work that went into defining and understanding our new background control region and the mechanics to extrapolate to the signal regions. This 1.5 year long effort has really made a difference. With our new control region we were able to significantly increase the sensitivity to dark matter and other exotic signals. Figure 2 shows our new limits on the WIMP-WIMP annihilation cross section as function of the WIMP mass for various explored models.

Figure2Figure 2: Exclusion upper limits on WIMP-WIMP annihilation cross section as function of the WIMP mass, from the ATLAS monojet analysis [1] in various signal scenarios. D5 corresponds to an effective field theory where the WIMP dark matter is a Dirac particle interacting via a massive spin-1 vector particle. D8 corresponds to a scenario where the WIMP would interact via an axial-vector interaction. This graph also illustrates the complementarity with astrophysical searches for WIMP-WIMP annihilation with HESS and FERMI-LAT.

The paper came out last night, but to tell the truth the data analysis has been ready and unblinded for almost six months, albeit embargoed at the time of Olofs licentiat thesis. The theoretical interpretation took a long time. To be able to translate ATLAS absence of new physics signal into WIMP-WIMP annihilation cross sections a specific model has to be used. Several models have been investigated, Figure 2 is based on an effective field theory which is only valid under certain assumptions. The validity of the approach depends on the momenta of the initial partons involved in the proton-proton collisions and the exchanged momentum. In the end we chose a very conservative approach and for Figure 2 we simply assumed that we had zero sensitivity to dark matter signals when the validity limit was broken. This weakened our limits but at least the limits feel more robust.

We are of course disappointed we did not find anything new. As Figure 2 shows, a WIMP with a mass below 20 GeV and interacting via a vector particle could no longer explain on its own the whole relic dark matter density provided to us by fits to cosmological data.

On the other hand our little Stockholm monojet analysis team: Gabriele Bertoli (grad student), Olof Lundberg (grad student), Valerio Rossetti (postdoc), Christophe Clément (faculty) is stronger than ever and we are already working on new developments for the upcoming 2015 ATLAS data. This spring LHC is restarting stronger than before, with a much higher center of mass energy and much more data. So we are certainly looking forward a very exciting second ride with the ATLAS data.

– Christophe Clément

[1] ATLAS Collaboration, Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √s=8 TeV with the ATLAS detector; (submitted to Eur. Phys. J. C).

[2] Olof Lundberg, Searches for Dark Matter and Extra Dimensions in Monojet Final States with the ATLAS Experiment, Licentiat thesis  Stockholm University, October 2014.



An ATLAS art installation at AlbaNova

Since early Autumn the ATLAS Art Installation is on the first floor of Stockholm House of Science at AlbaNova.
“In ATLAS we have made rather artistic animations of particle physics. In addition an artist has made the mural painting at CERN on one of the ATLAS buildings. The ATLAS events have also been rather artistic and made it to the many magazines and newspapers. I think that is why the installation is called the ATLAS Art Installation. I prefer to call it the People of ATLAS, as it is very much about the ATLAS people.” says Erik Johansson, professor at the Oskar Klein Centre.

The installation is composed of four video projectors showing different aspects of the ATLAS Experiment at the CERN LHC. “it shows that the people of ATLAS have a lot of other interests than physics. And we are not dressed in white laboratory clothes. We sing and play and climb mountains and many other things – as most people.”
Erik Johansson has been involved as the co-coordinator of ATLAS Education and Outreach group at the time, and he is also responsible to bring it here to Stockholm and House of Science. Continue reading An ATLAS art installation at AlbaNova

What is the perfect size for a research group?

If you are thinking about your research group and are wondering on whether to include more people you better read this.

Ralph Kenna from the University of Coventry and Bertrand Berche from the University of Nancy (France) have analyzed the correlation between quality and research group size for different areas, based on data collected for the UK’s 2008 Research Assessment Exercise (RAE). Their findings are very interesting. While one could think that more people would get better results together, it seems that this is not true indefinitely. There is a linear relation between group size and quality of the scientific results, and they correlate until reaching a breaking point, which is different for different areas. In experimental physics the breaking point is 25 people while in theoretical physics is 13. Once reached the breaking point, if you keep increasing the group size, quality does not increase anymore, and the trend flatten out.
The analysis shows that there is also a lower critical mass calculated as a minimum of 2 persons for theorists and 13 for experimentalists to guarantee an healthy scientific outcome.

This is probably something to keep in mind when deciding about future strategies. If you are interested in reading the original article you can find it here:


Brainstorming is one of the most known and diffused creativity boosting techniques used for getting fresh ideas. While brainstorming can also be done by individuals, it is most effective when done in a group of 10-12 people.
The technique is in principle very simple.

A topic of discussion should be clarified and well defined at the beginning of a brainstorming session, possibly in terms of a question to answer. The definition of the task is important and one of the key of a successful brainstorming session. It should contain the least of possible information, and definitely no indication about possible solutions. Continue reading Brainstorming

Thinking outside the box

Have you ever wanted to have a thinking cap?
Really! I sometimes wish I had one helping me out of my fixed thoughts. Unless you know Allan Snyder, director of the University of Sydney’s Centre for the Mind, who has just invented one, you need to get thinking outside the box using some other techniques.
How often do you get stuck in your research and feel like you need new ideas to get back on track again?
Science and creativity is not an oxymoron.
Indeed good scientific achievements are often the results of a vision that other people missed to see.

But what is creativity? Continue reading Thinking outside the box