All posts by Christophe

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.



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

Fresh Summer Harvest of ATLAS Results: Top quarks, Sleptons and Gauginos

Since monday the last of a string of summer High Energy Physics (HEP) conferences is unwinding in Beijing, SUSY-12 and before that in Melbourne ICHEP-2012. Some results with leading contributions from the Stockholm HEP group figure high in the topics of discussion. Finally ATLAS physicists can breathe a  little bit, with most results out for now. The Higgs boson discovery got a lot of attention, and this is great because it is not every day that there is a discovery of that magnitude, but there is an other important reason: simply said, the existence of the Higgs boson is at the basis of much of the research being carried out at Stockholm University and in ATLAS.

One of the mysteries of the Standard Model is the top quark which has a mass (172 GeV) much much higher than the other quarks, this is also much heavier than expected initially and still remains unexplained. But thanks to the Higgs mechanism we know that the mass of a particle is set by how strongly it interacts with the Higgs field, so for some unknown reason the top quark very strongly couples with the Higgs. This is why we decided to study the top quark properties in details. It could be that some of the top quark properties deviate slightly from the standard model of particle physics, or that some of the top quarks are produced via new mechanisms inside the proton-proton collisions at LHC.

Normalised measured top differential cross section as function of the top quark pair invariant mass and theoretical predictions.

New heavy particles could decay into pairs of top quarks. The HEP group in Stockholm has lead the effort of a measurement of the so-called top quark pair differential cross section (see ), which would for instance be able to detect the presence of new particles decaying into top quarks pairs in the invariant mass distribution of the two top quarks. Similar measurements were performed at Tevatron, but thanks to the very high center of mass of the LHC, it is possible to probe new particles to much higher masses. No new particle turned up, and this measurement will have to be redone in 2015 when higher, 14TeV of energy LHC data will be available (and allow to probe even higher particle masses). For now this completely new measurement will allow theorists to tune and improve Monte Carlo simulation programs so that they agree with this new data in a previously unexplored region of phase space.
Continue reading Fresh Summer Harvest of ATLAS Results: Top quarks, Sleptons and Gauginos

ATLAS and CMS experiments observe new particle consistent with long-sought Higgs boson

Today the ATLAS and CMS experiments have reported the observation of a strong excess of proton-proton collision events compatible with the Higgs boson.

The observed excess is obtained by combining 5 channels in the case of CMS to reach a level of 4.9 sigma of statistical significance. ATLAS has presented so far the result from two channels and observes an excess of 5 sigma. The number of events and the type of decays observed are both compatible with the standard model Higgs boson with a mass of about 125 GeV, and given the statistical significance of both ATLAS and CMS observations this can no longer be a statistical fluctuation. So today we have the discovery of a new particle.

So that’s it we got a Higgs boson! Yes! The atmosphere at CERN right now is pretty amazing and there is a palpable feeling we are living a historical moment, one which will be mentioned in text books. While a few other fundamental particles have been discovered in the 1990’s such as the top quark and the tau neutrino, we probably have to go back to the discovery of the J/Psi in 1974 which validated the quark model, to find a discovery of today’s significance.

Mass distribution for the two-photon channel. The strongest evidence for this new particle comes from analysis of events containing two photons. The smooth dotted line traces the measured background from known processes. The solid line traces a statistical fit to the signal plus background. The new particle appears as the excess around 126.5 GeV. The full analysis concludes that the probability of such a peak is three chances in a million.

So this has taken almost 40 years. It was deeply moving to see and hear the comments from Peter Higgs and Francois Englert right after the presentation, see the people queueing to the CERN conference room at 2 in the morning….

The observations by ATLAS and CMS are just enough to state a discovery of a new particle compatible with the Higgs boson, but it is not yet enough to precisely measure the properties of this new particle (well we know its mass already pretty well). Is it just a standard model Higgs boson? In supersymmetry there are five Higgs bosons, could it be one of them?

To determine this we need to now measure the Higgs production in all its possible decay channels, into two photons, two Z bosons, two WW, into two tau leptons, into two b quarks and so on. Today not even all these channels have been observed or presented yet, let alone measuring precisely enough the branching ratios into the various channels. So that is the next important step. In 2012 the LHC should provide 3 times more data than we have analysed so far so we should get a bit on the way towards checking all these channels. In some sense we are lucky that the Higgs boson has a mass of just 125 GeV since around that mass in particular it decays into so many channels. This will give us some extra help to analyse its properties in details. We also have to see whether there could be additional Higgs bosons, as predicted by Supersymmetry and other theories of physics beyond the Standard Model.
Continue reading ATLAS and CMS experiments observe new particle consistent with long-sought Higgs boson

LHC Experiments ATLAS and CMS to update their Higgs boson hunt results

CERN has announced that the two experiments leading the search for the Higgs boson, ATLAS and CMS will update their results concerning the search for the Higgs boson tomorrow on July 4th.

Last December the ATLAS and CMS experiments reported they excluded a Higgs boson in the mass range above 130 GeV and up to 500 GeV and observed a modest excess of collisions compatible with a Higgs boson at about 125 GeV, but with a low statistical significance.

The probability for the observed December 2011 excess to be the result of a statistical fluctuation (rather than a Higgs boson) is about one chance in a thousand.

With current computer technologies, physicists can easily look at thousands of distributions while trying to find a handful potential Higgs boson events among billions of proton-proton collisions. In clear: there is always a chance that a few mundane proton proton collisions will look like collisions in which a Higgs boson was produced and then decayed. It is for this reason that particle physics needs to have very strict criteria to assert whether an effect is real or is just a fluke of statistics. To assert with certitude that a certain outcome is not just the result of a statistical fluctuation we require that the probability that a fluctuation would explain the observation to be lower than one in a few millions.
Continue reading LHC Experiments ATLAS and CMS to update their Higgs boson hunt results

One big step closer to finding or excluding the Higgs boson.

Today the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) have presented the results of the analysis of all their most recent data. One tricky thing about the Higgs boson is that we do not know what is its mass, and so one needs to look for it in all its possible decay channels. ATLAS and CMS show that there is no Higgs boson with a mass above about 130 GeV and below 115 GeV (it could still be heavier than about 500 GeV but this is not favored by the theory).

Whether the Higgs boson exists or not, the ATLAS and CMS experiments moved today one big step closer to the answer.
Continue reading One big step closer to finding or excluding the Higgs boson.

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

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 ….