Tag Archives: LHC

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; http://arxiv.org/abs/1502.01518 (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.



The Nobel Prize in Physics 2013

Today’s Nobel Prize awarded jointly to François Englert and Peter W. Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”.

Here at OKC we are so delighted to see this prize. It confirms the importance of last year’s discovery of the mechanism and the particle imagined by Englert and Higgs. To tell the truth, although the Higgs particle was only discovered recently it has been part of some of our calculations here at OKC for some time. Some theories of dark matter assume the existence of a Higgs particle. So it was important to confirm this with the ATLAS and CMS experiments, since the discovery we know we are on the right track.
But not until a short time before the discovery annoucement did we really know that the Higgs particle existed. Not so long before the discovery some experimentalists and theorists would get a bit nervous, wondering what would we do if no Higgs particle was found… one would have to start from scratch, change the theory, go back to the drawing board, invent something new but what?

François Englert. Photo: Pnicolet via Wikimedia Commons
Peter W. Higgs. Photo: G-M Greuel via Wikimedia Commons

Thanks to the hard physical discovery of the Higgs particle at CERN we can now move forward, while many other theories without a Higgs particle have faded away into history.
That’s science at work. The Higgs boson is the last missing piece of the so-called standard model of particle physics. Good we got that sorted out!
But we know that the standard model is not the full story, the Higgs particle does not give mass to the neutrinos, nor do we know what is dark matter, as the standard model does not contain any such particles.

The CERN programme with the ATLAS, CMS and LHC experiments is still to provide about 200 times more data than was needed to find the Higgs particle. This is by no mean a guarantee that we will find something new, but it is only by covering new ground with some ingenious new instruments, that there is a chance to learn something new about Nature. The LHC project and the ATLAS and CMS experiments are just fantastic instruments built for that purpose. It is a great privilege to work on the ATLAS experiment and see the Nobel Prize going to particle physics today, after a bit of excitement, here at OKC, we will go back to analysing the data from the ATLAS experiment and see if we can solve another mystery of Nature.

Christophe Clement – researcher at the Oskar Klein Centre

Interview with Oscar Stål

Oscar Stål is one of the OKC fellows working at the Cosmology, Particle astrophysics and String theory group (CoPS) since August 2012. He is doing his second postdoc and his filed of interest is particle physics phenomenology. He is Swedish and studied both as undergraduate and for his PhD at Uppsala University, before moving to Hamburg.

Can you tell us a bit of yourself? Where are you from?
I am 30 years old, and this is my second Postdoc. Before joining the OKC I spent two very nice years in the theory group at DESY, Hamburg. Originally I am from Enköping, which is a small town about 80 km west of Stockholm. I am married and we have a 2-year old son, Anton, who takes up most of my free time.

What is your field of research?
Broadly speaking, my area of research is particle physics phenomenology, that is theoretical work in close connection to experiment. The main experiment we consider at present (and probably for many years to come) is the LHC at CERN. To be somewhat more specific, my main interests lie in the phenomenology of physics beyond the standard model (SM), such as supersymmetry, with its interesting connections to electroweak symmetry breaking (the Higgs!) and also, of course, the dark matter. Continue reading Interview with Oscar Stål

Grids, Clouds and the Big (CHEP) Apple

Stephan Zimmer is a PhD students in the OKC. This is his report from the CHEP conference last week.

Alvarez illustrating our fights when trying to connect to a video conference and what we always think but never dare to say
Do you care about computing? Probably not, probably you are happy just knowing that all your stuff just works. But what “Does work” actually means? Let me try to give you a few reasons why you should actually care… and why it matters.

Last week I was at the CHEP conference where the latest and greatest news of computing in high energy nuclear and particle physics were discussed. CHEP is an international conference with about 500 scientists, computing experts and business professionals, reviewing the current set of Clouds, Grids and technologies for the upcoming challenges, first and foremost those posed by the LHC experiments. I believe most sessions were actually recorded, so have a look and see if there’s anything that fancies your interest.

It was quite an exciting conference for me, given that we (physicists) usually don’t attend this kind of conference.

During the plenary talk we were shown some of the greatest highlights of all LHC experiments, along with the latest developments. The bottom line (but of course the ATLAS folks at OKC know that already) is that by the end of this year, we’ll probably have either killed or confirmed the intriguing hint of a signal in both CMS and ATLAS at roughly 125 GeV.

All of us who work in HEP know of the pain with C++ and ROOT and all the other goodies we (have to) use in the community. Those of us that DON’T use ROOT, please skip this paragraph.
We were promised by Fons Rademakers, the new Mr. ROOT, after Rene Brun (you all know him! By the way: he gave a nice talk covering computing in HEP since the 1970s and was honored by standing ovations during the closing session of CHEP) to see ROOT v6 by November and from my little experience of C++ this will bring quite some interesting changes, among others a giant boost of performance (and lots of more support for iOS on various levels). Axel Nauman from CERN detailed Cling and Clang in a blog post on the ROOT website (http://tinyurl.com/75wsfm9). Continue reading Grids, Clouds and the Big (CHEP) Apple

New Limits on Higgs and Supersymmetry from ATLAS

ATLAS Experiment © 2011 CERN
The ATLAS experiment has almost completed the analysis of the first 2 inverse femtobarns(*) of data provided by the Large Hadron Collider (LHC) until July this year.

On the forefront of the search for the Higgs boson, ATLAS and CMS, two of the LHC experiments aimed at measuring the Higgs boson signal, did not detect any signal excess so far. The combination of the ATLAS and CMS results was just presented at the Hadron Collider Physics Symposium in Paris and it turns out that the mass range between 140 and 480 GeV is excluded at 95% confidence level. Still the Higgs could just be hiding in the remaining low mass region, above the LEP limit at 115 GeV and below the 140 GeV excluded by LHC experiments. Continue reading New Limits on Higgs and Supersymmetry from ATLAS

Interview with Abram Krislock

Abram Krislock is a postdoc at the Oskar Klein Centre. He started working with Joakim Edsjö on Dark Matter just a couple of months ago. Let’s hear from him how things are going for him.

Hi Abram! how it’s going so far?
So far, everything is going great. I really like the office, and the other professors and researchers here are all very friendly and helpful. Living in Stockholm has so far been extremely fulfulling. I really like it here.

What brought you to Europe for a postdoc?
Before coming to the OKC, I had my undergrad studies in my hometown at the University of Regina, Saskatchewan, Canada. I did my PhD in Physics at Texas A&M University. I came to Europe for a postdoc partly because the competition for physics postdocs is very tough, partly because the US seems to be cutting a lot of funding for science in general. Also, it seems like physics beyond the Standard Model is getting a little more attention in Europe than in the states. It benefits my career to be in a place where many people are interested in my area of research. Continue reading Interview with Abram Krislock

OKC is also about LHC and LHC is also about Dark Matter ….

From a particle physicist point of view the search for dark matter is just the search for yet another exotic particle. But the search for a possible dark matter candidate in particle physics experiments has definitely a special place on a par with the search for the famous Higgs boson. If the particle making up dark matter is light enough it could well be produced in the lab, at the european lab called CERN located close to Geneva in Switzerland. CERN’s new accelerator, the Large Hadron Collider collides proton beams at a center of mass energy of 7 TeV and will later on go up to 14TeV. Being able to produce the dark matter candidate in the lab would be the perfect opportunity to study it. In fact with enough data one could even imagine measure its properties in great detail, and for instance determine its spin. Such measurements would allow us to discriminate between different classes of theories predicting dark matter particles, such as extra-dimension and supersymmetry. While this is in the domain of the possible, it is still a long way down the road. But in the end, only the complementarity of the dark matter searches in the sky and in the lab will allow us to pin down what is exactly dark matter. Continue reading OKC is also about LHC and LHC is also about Dark Matter ….