CMS search for the Standard Model Higgs Boson in LHC data from 2010 and 2011
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.
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).
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; and
- 128 – 525 GeV at 99% confidence level.
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, 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.
At CP3, our group is actively involved in data analysis of the CMS experiment. The research centre CP3 consists of about 50 researchers in theoretical physics and experimental physics who work closely together to unravel the mysteries of the infinitely small and the infinitely large.
UCL is a member of the CMS collaboration since its inception in 1992. It was at that time a letter of intent proposing the construction of the CMS detector. Since that time, the experimental design has evolved based on new technologies available. In this context, one of the subdetectors called CMS tracker was developed in 2000. The tracker is a device literally at the heart of the observation of particles produced in collisions at the LHC. It allows you to pinpoint the charged particles produced in interactions of protons in the heart of the LHC CMS. This is a detector with more than 200m^2 of silicon sensors, with 10 million electronic channels providing a spatial resolution of 20 microns and a temporal resolution about 2 ns.
With its experience in experimental physics detectors, the UCL group contributed to very significantly. These developments necessitated the construction of clean rooms and test devices of high precision and a team of UCL was detached over a year at CERN to complete the assembly and test the device before its integration into CMS. The group at UCL then contributed significantly to the commissioning and to the detection of the first particles.
The group at UCL has also been involved in the IT tools necessary to complete the reconstruction of events and the simulation of physical reactions and detector response which are essential to make analysis accurate. The proper functioning of this software requires phenomenal computing power. In this context, CP3 hosts one of the two Belgian computer centres connected to what is called the GRID.
Since the start of the LHC and data taking at the end of 2009, CP3 researchers are involved in various measurements ranging from top quark physics, the search for supersymmetric particles and of course the search for the Higgs boson. In particular, the Group specializes in the study of events with two leptons and two b quarks in the final state. This final state is particularly important to confirm and understand the nature of the Higgs particle. It is maybe the only channel that would allow to check if the Higgs boson interacts with both bosons (W, Z and photon) and quarks (b quarks here). The challenges behind this analysis require, however, a very good understanding of other processes with the same signature.
In this context, CP3 theorists develop original models that could explain in a more coherent way the symmetry breaking mechanism. These models are then faced with data from the CMS experiment by the experimenters from the Centre. The detection of the Higgs boson determines clearly the future of particle physics. Predicted in the context of a unification of the electromagnetic and weak interactions, the boson may be the last in elementary particle to be discovered, more than a century after the discovery of the electron. However, it differs from the quanta of matter and interaction with very special intrinsic properties. So, missing link or first element of a class of particles so far escaping any observation? Axion, dilaton, inflaton, quintessence, ... are indeed much like the Higgs boson and regularly invoked by theorists at CP3 in attempts to resolve major problems in the context of strong and gravitational interactions, as well as cosmology.
Reference url: http://cms.web.cern.ch/news/cms-search-standard-model-higgs-boson-lhc-data-2010-and-2011