The CMS collaboration is presenting its latest results this week at the 2011 Europhysics Conference on High-Energy Physics, held in Grenoble, France. These results are based on about 1 inverse femtobarn of data (100 trillion proton-proton collisions) from LHC running at an energy of 7 TeV, which were collected in 2010 and 2011. They include a wide range of searches for new physics and precise measurements of Standard Model processes.
The Higgs boson is the only particle predicted by the Standard Model that has not yet been seen by experiments. It helps explain how elementary particles acquire mass. If the Higgs boson exists it will be produced in proton-proton collisions at the LHC and then swiftly decay into several known and well-studied particles. The CMS search for the Higgs boson is being carried out using a range of decay products: two photons; two tau leptons; two W bosons; and two Z bosons. The W is observed through its decay to an electron plus a neutrino, or a muon plus a neutrino. The Z is observed through its decay to a pair of electrons, or a pair of muons, (see Figure 1) or a pair of jets of hadronic particles. Analysing all these channels ensures that the search is sensitive to observing the Higgs irrespective of its own mass.
CMS observes no convincing excesses of events in the explored mass range of 120-600 GeV (for full details see [HIG-11-011]). The analysis excludes, with a confidence level of 95%, the existence of a Higgs boson in two broad Higgs mass ranges, 149-206 GeV and 300-440 GeV, as well as several narrower intervals in between (Figure 2). At a lower confidence level of 90%, the existence of a Higgs boson is excluded for the range 145-480 GeV. Re-interpreting the results in the context of the Standard Model with a fourth generation of fermions in addition to the known three generations (SM4), allows us to exclude the SM4 Higgs boson with a mass in the range 120-600 GeV with a confidence level of 95%.
It should be noted that a modest excess of events is observed for Higgs boson masses below 145 GeV. With the data we will collect in the next few months we will be able to distinguish between the possible interpretations: the production of a Higgs boson or a statistical fluctuation of the backgrounds. During the ongoing LHC proton-proton run CMS will record substantially more data that should be sensitive to observing a Higgs boson, if it exists, over the full range of possible masses.
Search for rare decays of Bs→μμ
Bs mesons, comprising a 'bottom' and a 'strange' quark, are produced copiously at the LHC. The fraction that subsequently decays (known as the "branching fraction") to a pair of easily-detected muons is highly suppressed in the Standard Model — only about three such decays are expected per billion Bs particles produced. Several extensions of the SM, for instance Supersymmetric models, predict significant enhancements in the number of decays to pairs of muons, thanks to new particles that would contribute to the decay through "virtual loops". Therefore, any enhancement of this Bs decay rate could indicate the existence of new physics.
CMS has searched for the decays of Bs (and B0 particles, comprising a 'bottom' and a 'down' quark) to muon pairs using proton-proton collision data collected up to June 2011 (Figure 3). A challenging aspect of this search is reducing the large backgrounds from other B-hadron decays or particles misidentified as muons.
The number of candidate decays observed in the available data sample is consistent with the Standard Model expectations for signal and background (see Figure 4). Given the absence of a significant excess, CMS has excluded (at 95% confidence level) branching fractions larger than 1.9x10-8 and 4.6x10-9 for the decay of Bs and B0 particles, respectively.
This result constitutes one of the most stringent exclusion limits achieved until now. The data CMS will collect in the remainder of 2011 and in 2012 will be sensitive to smaller branching fractions, at the level of SM expectations, and may lead to the observation of an enhanced decay rate which could be indicative of a non-Standard-Model physics process.
CMS has searched for many other possible signatures of exotic new physics. No significant signals for new physics have yet been seen and therefore new constraints, at 95% confidence level, have been placed on many extensions to the Standard Model, including the following:
- The Supersymmetry (SUSY) theory provides a symmetry between matter and forces, and predicts that for each known particle there is a 'supersymmetric' partner. SUSY quarks, or squarks, are ruled out by CMS up to 1.2 TeV for a large range of SUSY parameters in the models tested [SUS-11-003].
- High-mass W′ and Z′ particles with Standard Model couplings are ruled out up to masses of 2.27 TeV [EXO-11-024] and 1.94 TeV [EXO-11-019] respectively. Constraints are placed on other models producing high-mass di-lepton resonances. Analysis of top-anti-top final states rules out the "Kaluza Klein gluon" (in Randal Sundrum models) with mass below 1.5 TeV [EXO-11-006].
- A study of di-jet resonances excludes excited quarks in the mass range of 1.0 TeV to 2.5 TeV and similarly constrains models involving E6 di-quarks, string resonances, the W′, axigluons and colorons [EXO-11-015].
- A search for the pair production of a fourth-generation t′ quark and its antiparticle ruled them out up to a t′ mass of 450GeV [EXO-11-051].
- Microscopic black-hole production is ruled out for black-hole masses up to between 4 and 5 TeV, depending on the model in question [EXO-11-071].
CMS also has a rich programme that confronts the predictions of the Standard Model with precise experimental measurements at the high-energy frontier of the LHC. These include top-quark production that has been studied in detail, with recent measurements made of the cross-section for top quark-top antiquark pairs decaying fully hadronically [TOP-11-007], the cross-section measurement using events with a tau lepton in the final state [TOP-11-006] and the top charge asymmetry [TOP-11-014]. Many aspects of the Electroweak and Strong forces as well as b-quarks are being studied. These analyses have already resulted in more than thirty publications in refereed journals, with many more results in the pipeline. A good understanding of Standard Model processes is also important as they contribute to the backgrounds to new physics processes.
The LHC machine is running extremely well and has already delivered more proton-collision data than was expected in the entire 2011 run. The high-precision CMS experiment is recording good quality data with high efficiency. By the end of 2012 we expect to increase the size of our data sample by a factor of about ten. We are just starting to explore a vast territory where new physics could manifest itself and the CMS experiment is ready for whatever Nature has in store for us.
More information, including images and animations of CMS collision events, may be found on the CMS web site: http://cms.cern.ch.
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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