Tag Archives: supersymmetry

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

Interview with Christopher Savage

Christopher SavageChristopher Savage is a Oskar Klein Fellow since the summer of 2009. He is working on direct detection of Dark Matter and seems to be very happy about it! I asked him to tell us more.

Why did you choose the OKC for doing a postdoc?
The broad focus on cosmology, with an emphasis on interaction between different areas, was very appealing. In addition, I had started working on capture of dark matter in stars and there is a lot of expertise in that area here in Stockholm (particularly Joakim Edsjö).

When I was given the offer, I was at a conference in Michigan with four OKC people in attendance (Marcus Berg and Joakim Edsjö plus PhD students Erik Lundström and Sara Rydbeck). That gave me a chance to see what the group was like and it made the decision an easy one.

What is your field of research?
My field of research is in the phenomenology of detecting dark matter, both directly and indirectly. The former case (direct detection) involves looking for interactions (scattering) of dark matter particles inside a detector, while the latter case (indirect detection) involves looking for products of dark matter particles that annihilate elsewhere.

Direct detection has long been a focus of my research and I continue to work in that area here at the OKC. I look at how various issues affect the signals seen in direct detection experiments, which might explain the apparent incompatibility between experiments that observe signals consistent with dark matter (CoGeNT, CRESST, and DAMA) and those that do not (CDMS and XENON, to name a few). The issues include how dark matter couples to ordinary matter, how the dark matter is distributed in the galaxy, and potential systematic issues in the experiments themselves (often involving energy calibrations). In addition, several of us (Yashar Akrami, Pat Scott, Jan Conrad, Joakim Edsjö and I) are looking at how direct detection results constrain supersymmetric models, which provide a natural candidate for a dark matter particle (the neutralino).
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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