ATLAS and CMS experiments present Higgs search status
In a seminar held at CERN on 13 December, the ATLAS and CMS experiments presented the status of their searches for the Standard Model Higgs boson. Their results are based on the analysis of considerably more data than those presented at the summer conferences, sufficient to make significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the elusive Higgs. The main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 116-130 GeV by the ATLAS experiment, and 115-127 GeV by CMS. Tantalising hints have been seen by both experiments in this mass region, but these are not yet strong enough to claim a discovery.
Higgs bosons, if they exist, are very short lived and can decay in many different ways. Discovery relies on observing the particles they decay into rather than the Higgs itself. Both ATLAS and CMS have analysed several decay channels, and the experiments see small excesses in the low mass region that has not yet been excluded.
Taken individually, none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row. What is interesting is that there are multiple independent measurements pointing to the region of 124 to 126 GeV. It’s far too early to say whether ATLAS and CMS have discovered the Higgs boson, but these updated results are generating a lot of interest in the particle physics community.
"We have restricted the most likely mass region for the Higgs boson to 116-130 GeV, and over the last few weeks we have started to see an intriguing excess of events in the mass range around 125 GeV," explained ATLAS experiment spokesperson Fabiola Gianotti. "This excess may be due to a fluctuation, but it could also be something more interesting. We cannot conclude anything at this stage. We need more study and more data. Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012."
"We cannot exclude the presence of the Standard Model Higgs between 115 and 127 GeV because of a modest excess of events in this mass region that appears, quite consistently, in five independent channels," explained CMS experiment Spokesperson, Guido Tonelli. "The excess is most compatible with a Standard Model Higgs in the vicinity of 124 GeV and below but the statistical significance is not large enough to say anything conclusive. As of today what we see is consistent either with a background fluctuation or with the presence of the boson. Refined analyses and additional data delivered in 2012 by this magnificent machine will definitely give an answer."
Over the coming months, both experiments will be further refining their analyses in time for the winter particle physics conferences in March. However, a definitive statement on the existence or non-existence of the Higgs will require more data, and is not likely until later in 2012.
The Standard Model is the theory that physicists use to describe the behaviour of fundamental particles and the forces that act between them. It describes the ordinary matter from which we, and everything visible in the Universe, are made extremely well. Nevertheless, the Standard Model does not describe the 96% of the Universe that is invisible. One of the main goals of the LHC research programme is to go beyond the Standard Model, and the Higgs boson could be the key.
A Standard Model Higgs boson would confirm a theory first put forward in the 1960s, but there are other possible forms the Higgs boson could take, linked to theories that go beyond the Standard Model. A Standard Model Higgs could still point the way to new physics, through subtleties in its behaviour that would only emerge after studying a large number of Higgs particle decays. A non-Standard Model Higgs, currently beyond the reach of the LHC experiments with data so far recorded, would immediately open the door to new physics, whereas the absence of a Standard Model Higgs would point strongly to new physics at the LHC’s full design energy, set to be achieved after 2014. Whether ATLAS and CMS show over the coming months that the Standard Model Higgs boson exists or not, the LHC programme is opening the way to new physics.
Involvement Nikhef
Nikhef is involved in the ATLAS experiment at CERN. Many Nikhef researchers are looking for signals in various decay pathways of the Higgs particle. For example, a Nikhef researcher developed the statistical interpretation of the ATLAS results collected.
Prof. Frank Linde, director Nikhef: "Ever since I drove excitedly in the middle of the night to CERN in 1991 with an (unfortunately failed) idea to find the Higgs particle in the LEP data, the Higgs particle has been a top item on my wish list. We did not find it with LEP: with the LHC we are now getting extremely close … and my hope is that in the future we will be able to study it in detail using the ILC or CLIC!"
Prof. Nicolo de Groot, researcher in the ATLAS group at Nikhef and a professor at Radboud University Nijmegen. "Since the summer, when the data suddenly started to pour in rapidly, it has been hectic. It was fantastic to see how groups who initially viewed each other as competitors started to work closely together to obtain the results quickly. In 1988 I went to CERN to find the Higgs particle. Knowing that the end of the quest is almost in sight is therefore a unique feeling."
Prof. Stan Bentvelsen, programme leader of the ATLAS group at Nikhef and professor at the University of Amsterdam. "The real surprise of today is that in the enormous quantity of complex collisions, the ATLAS and CMS detectors are capable of measuring very subtle effects in such a short timeframe. Fantastic! The zeal and enthusiasm needed to realise all of this together is inspiring. And whether we have indeed found the Higgs particle now? That would seem to be the case but we will not know for sure until next year."
Prof. Sijbrand de Jong, researcher at Nikhef and professor at Radboud University Nijmegen. "The net around the Higgs particle is closing. Now we know where we should look and we have also caught a first glimpse of it. We don’t have the hard evidence yet but it won’t be long before we are certain."
Background information
http://press.web.cern.ch/press/background/B10-Higgs_evolution_or_revolution_en.html
http://www.quantumdiaries.org/2011/12/13/higgs-liveblog/
About Nikhef
The National Institute for Subatomic Physics (Nikhef) is an institute that carries out research in the area of (astro)particle physics. Scientists and engineers work together on research into the smallest building blocks of matter and the forces that act between them. These miniscule particles are studied in collision processes using large particle accelerators, including those of CERN near Geneva, as well as in interactions of high-energy cosmic particles in the Earth's atmosphere or in seawater.
Nikhef is a partnership between the Foundation for Fundamental Research on Matter (FOM) and four universities: Radboud University Nijmegen, University of Amsterdam, Utrecht University and VU University Amsterdam. Nikhef is located at Science Park Amsterdam.
www.nikhef.nl
More information
Communication Department Nikhef,
Melissa van der Sande
+31 (020) 592 50 75
Prof. dr. Frank Linde, managing director Nikhef
Prof. dr. Nicolo de Groot, Nikhef en Radboud University Nijmegen
Prof. dr. Stan Bentvelsen, Nikhef en University of Amsterdam
CERN-persbericht
Click here for the original CERN press release: http://press.web.cern.ch/press/PressReleases/List.html
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