CERN, 13th December 2011
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The Higgs boson is the only particle predicted by the Standard Model (SM) of particle physics that has not yet been experimentally observed. Its observation would be a major step forward in our understanding of how particles acquire mass. Conversely, not finding the SM Higgs boson at the LHC would be very significant and would lead to a greater focus on alternative theories that extend beyond the Standard Model, with associated Higgs-like particles.
Today the CMS Collaboration presented their latest results in the search for the Standard Model Higgs boson, using the entire data sample of proton-proton collisions collected up to the end of 2011. These data amount to 4.7 fb-1 of integrated luminosity, meaning that CMS can study Higgs production in almost the entire mass range above the limit from CERN’s Large Electron Positron (LEP) collider of 114 GeV/c2 (or 114 GeV in natural units ) and up to 600 GeV. Our results were achieved by combining searches in a number of predicted Higgs “decays channels” including: pairs of W or Z bosons, which decay to four leptons; pairs of heavy quarks; pairs of tau leptons; and pairs of photons (Figure 1).
Our preliminary results, for several statistical confidence levels , exclude the existence of the SM Higgs boson in a wide range of possible Higgs boson masses:
- 127 – 600 GeV at 95% confidence level, as shown in Figure 2a; and
- 128 – 525 GeV at 99% confidence level.
A mass is said to be “excluded at 95% confidence level” if the Standard Model Higgs boson with that mass would yield more evidence than that observed in our data at least 95% of the time in a set of repeated experiments.
We do not exclude a SM Higgs boson with a mass between 115 GeV and 127 GeV at 95% confidence level. Compared to the SM prediction there is an excess of events in this mass region (see Figure 2b), that appears, quite consistently, in five independent channels.
With the amount of data collected so far, it is inherently difficult to distinguish between the two hypotheses of existence vs non-existence of a Higgs signal in this low mass region. The observed excess of events could be a statistical fluctuation of the known background processes, either with or without the existence of the SM Higgs boson in this mass range. The larger data samples to be collected in 2012 will reduce the statistical uncertainties, enabling us to make a clear statement on the possible existence, or not, of the SM Higgs boson in this mass region.
The excess is most compatible with an SM Higgs hypothesis in the vicinity of 124 GeV and below, but with a statistical significance of less than 2 standard deviations (2σ) from the known backgrounds, once the so-called Look-Elsewhere Effect  has been taken into account. This is well below the significance level that traditionally has been associated with excesses that stand the test of time.
If we explore the hypothesis that our observed excess could be the first hint of the presence of the SM Higgs boson, we find that the production rate (“cross section” relative to the SM, σ/σSM) for each decay channel is consistent with expectations, albeit with large uncertainties. However, the low statistical significance means that this excess can reasonably be interpreted as fluctuations of the background.
More data, to be collected in 2012, will help ascertain the origin of the excess.
UPDATE: CMS publishes Higgs boson search results using 2010–2011 data
Event images and animations of real CMS data
Short movies about the Higgs
Seminar at CERN, 13th December 2011
More about the Higgs at CMS
About CMS Physics
More information may be found on the CMS web site: http://cern.ch/cms.
CMS is one of two general-purpose experiments at the LHC that have been built to search for new physics. It is designed to detect a wide range of particles and phenomena produced in the LHC's high-energy proton-proton and heavy-ion collisions and will help to answer questions such as: "What is the Universe really made of and what forces act within it?" and "What gives everything substance?" It will also measure the properties of well-known particles with unprecedented precision and be on the lookout for completely new, unpredicted phenomena. Such research not only increases our understanding of the way the Universe works, but may eventually spark new technologies that change the world in which we live as has often been true in the past.
The conceptual design of the CMS experiment dates back to 1992. The construction of the gigantic detector (15 m diameter by nearly 29 m long with a weight of 14000 tonnes) took 16 years of effort from one of the largest international scientific collaborations ever assembled: more than 3100 scientists and engineers from 169 institutions and research laboratories distributed in 39 countries all over the world.
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-  http://news.stanford.edu/news/2004/july21/femtobarn-721.html
-  The electron volt is a unit of energy. In particle physics, where mass and energy are often interchanged, it is common to use eV/c2 as a unit of mass (from E = mc2, where c is the speed of light in vacuum). Even more common is to use a system of natural units with c set to 1 (hence, E = m), and simply use eV as a unit of mass.
-  Confidence level is a statistical measure of the percentage of test results that can be expected to be within a specified range. For example, a confidence level of 95% means that the result of an action will probably meet expectations 95% of the time.
-  http://cms.web.cern.ch/news/should-you-get-excited-your-data-let-look-el...