Tag Archives: Higgs

Another interesting and very successful year of the Oskar Klein Centre

Francois Englert just before Nobel lecture in Stockholm University’s Aula Magna. Photo: L. Bergström.

Hello and Happy Holidays to all friends of the OKC

As the year 2013 is now nearing its end, it is time to recapitulate the main events of the year from the OKC perspective. If I temporarily put on my Nobel hat (being the scientific secretary of the Nobel Committee for Physics) the main event from the Stockholm horizon is without doubt the Nobel Prize to Francois Englert and Peter Higgs for their almost 50-year old prediction from the early 1960’s that was so spectacularly confirmed by the ATLAS and CMS experiments at CERN’s LHC accelerator last year. Hats off for Englert and Higgs, and also for the many clever and hard-working experimentalists, in particular the ATLAS people of OKC such as Jonas Strandberg, who has been directly involved in the discovery of the Higgs particle in ATLAS. Of course we now look forward to the energy upgrade of LHC, which will increase the chances substantially to find the much awaited effects beyond the Standard Model that, hopefully, could give an indication of what the dark matter may consist of. At the Oskar Klein Centre we also have been searching in gamma-ray, positron and neutrino signals, without positive results (yet), but producing some of the best limits.

Peter Higgs signing the Nobel Poster (prepared with the help of OKC's Sara Strandberg and Oscar Stål). Photo: L. Bergström.

The OKC has now been in existence for 5 years, and we will soon encounter the international mid-review panel of the Science Council (VR) of Sweden. By September 1st we had to submit our self-assessment report, containing a detailed description This meant a lot of work for me and the OKC Steering Group: Christophe Clement, Jan Conrad, Claes Fransson, Ariel Goobar, Klas Hultquist, Garrelt Mellema, Mark Pearce, Sara Strandberg and Göran Östlin (and of course our great communications manager, Serena Nobili). We hope that we managed to convey our great enthusiasm for the scientific outcome of the OKC during its first 5 years, and that the evaluation committee will agree that it has been a great success. In fact, when we meet them January 30th, we will have quite a number of recent sucesses to report:

  • A generous grant of SEK 32 million from the K&A Wallenberg Foundation was given to groups in OKC (with J. Sollerman as PI) for contributing to the Zwicky Transient Facility (with S. Kulkarni of Caltech leading the team).
  • OKC Steering Group member Sara Strandberg has obtained both a young researcher’s grant from the VR and a Wallenberg Academy Fellow (WAF) grant – both in very strong competition.
  • Jan Conrad (also OKC Steering Group member) has in addition to his previous WAF grant also been given one of the new excellent junior investigator grants from VR. Congratulations to Sara and Jan!
  • The IceCube experiment has finally detected high-energy (PeV) cosmological neutrino events, with a surprising energy distribution.  This was declared the discovery of the year of the Physics World magazine. Congratulations to the OKC IceCube group (Chad Finley, P.O. Hulth, Klas Hultquist & al.) and the Uppsala group (with present IceCube spokesperson Olga Botner and her colleagues)!
  • The Fermi satellite project with large OKC contribution continues to make important discoveries. One concerns the discovery of a gamma-ray spectrum of two supernova remnants which clearly shows a hadronic origin (from decays of neutral pions), and thus is a proof that these sources accelerate protons, and are thus the long-sought-for sources of the Galactic cosmic rays. This was one of the runner-ups for the discovery of the year of Science magazine.

To conclude, with all the interesting science produced by OKC during its first half-life, one may only anticipate with great expectations what will come out of the second half!

Merry Holidays and a Happy New Year to all in the OKC research environment and all our followers!


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

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.