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Standard Model's next theory, typically predict new phenomena to show up at high energies
:: 25 October, 2007
A famous character once said, "Life is like a box of chocolates… You never know what you're gonna get." In particle physics, we do not follow this principle often. Experiments are designed and data analyses are developed with the help of theoretical predictions and powerful simulation tools, to make sure that the experiments operate optimally. If new effects are discovered, they can be quickly compared to theories and identified. The Standard Model, a well-established theory of particles and interactions, has been very successful in predicting the outcomes of the experiments over the last 25 years. New theories, developed on the basis of these measurements and building on the success of the Standard Model, typically predict new phenomena to show up at high energies. According to conventional wisdom, new discoveries would be made at the energy frontier: at the most energetic colliders such as Fermilab's Tevatron, or the Large Hadron Collider (LHC) at CERN, the new machine to be commissioned next year. Yet, BaBar physicists are attempting to bend this conventional wisdom ever so slightly.
The SLAC PEP-II storage ring and the BaBar detector are collectively known as the "B-Factory," which implies the main purpose of the experiment: precision study of charge-parity (CP) violation in the decays of B mesons (Bs). The Bs are heavy equivalents of pions, particles commonly produced in cosmic rays, and contain a heavy b-quark. The accelerator produces about 20 Bs each second, and the BaBar researches look for subtle differences in the time evolution and decay of the Bs and their anti-particles (B-bars). PEP-II has produced nearly 500 million B/B-bar pairs, and the BaBar Collaboration has written over 300 scientific papers since the start of the experiment in 1999.
But just like a GM factory may churn out Chevy, GMC, or Isuzu cars, and even an occasional Saab, electron-positron annihilations at the SLAC B-Factory produce more than just B mesons. Lighter quark flavors, from up/down to charm, are produced in even larger quantities, as well as muons, tau-leptons, and even undetectable neutrinos. BaBar physicists look at those processes too, and in fact some of the more unexpected discoveries have come out from these studies: the discoveries of several new charm mesons, the discovery of the D0-D0bar mixing, to name a few.
Then, there are truly exotic possibilities. What if the Higgs boson, expected within the Standard Model to have the mass above 120 GeV/c2, is instead unconventionally light? Such a particle could be produced in e+e- annihilations, in association with energetic photons, but the process might be too rare to have been observed at other colliders that more energetic but less prolific in data rates. The experiments at the LHC, designed to discover the Higgs bosons, would probably miss a very light Higgs (or a Higgs-like) particle too. What if the dark matter particles, which carry about a quarter of the mass of the Universe and believed to relatively slow and heavy (several hundred GeV), are instead light and weakly interacting? There are a number of (decidedly unconventional) theoretical models that predict such light states, motivated by astrophysical observations, or simply exploring the possibilities. Very light new particles might be hard to identify at high-energy colliders, and that is where BaBar comes in.
BaBar physicists have started to explore the opportunities to look for such exotic events. Most models agree on one thing: even if collisions at the B-Factory have enough energy to produce new particles, the probability to create such a state in a given e+e- annihilation is very low. Researches need to look through a lot of data, in quantities matched only by the BaBar's sister experiment, Belle, and be able to reject copious "background" events, to have a chance for discovery. In addition, the signature for such events sometimes includes a single particle—a high-energy photon—and nothing else detectable. Such events used to be ignored by the event selection algorithms, but this fall BaBar has enabled a new "trigger": an electronic marker to record events that look interesting for further analysis. This trigger will enable new searches next year, and—if Nature proves to be sufficiently exotic—potential discoveries.
BaBar collaboration is organizing a special workshop on October 29 to jump-start these efforts, and to generate new ideas. Theorists and BaBar experimentalists alike are invited, and interesting ideas, even wild ones, are welcome. Who is to say we won't find a little exotic nugget?
Note for Particle physics
Particle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called "high energy physics", because many elementary particles do not occur under normal circumstances in nature, but can be created and detected during energetic collisions of other particles, as is done in particle accelerators.
Note for Standard Model
The Standard Model of particle physics is a theory which describes three of the four known fundamental interactions between the elementary particles that make up all matter. It is a quantum field theory developed between 1970 and 1973 which is consistent with both quantum mechanics and special relativity. To date, almost all experimental tests of the three forces described by the Standard Model have agreed with its predictions. However, the Standard Model falls short of being a complete theory of fundamental interactions, primarily because of its lack of inclusion of gravity, the fourth known fundamental interaction, but also because of the large number of numerical parameters (such as masses and coupling constants) that must be put "by hand" into the theory (rather than being derived from first principles).
About European Organization for Nuclear Research
The European Organization for Nuclear Research (French: Organisation européenne pour la recherche nucléaire), commonly known as CERN (see Naming), pronounced [sɝn] (or [sɛʀn] in French), is the world's largest particle physics laboratory, situated just northwest of Geneva on the border between France and Switzerland. The convention establishing CERN was signed on 29 September 1954. From the original 12 signatories of the CERN convention, membership has grown to the present 20 member states. Its main function is to provide the particle accelerators and other infrastructure needed for high-energy physics research. Numerous experiments have been constructed at CERN by international collaborations to make use of them.
The main site at Meyrin also has a large computer centre containing very powerful data processing facilities primarily for experimental data analysis, and because of the need to make them available to researchers elsewhere, has historically been (and continues to be) a major wide area networking hub.
CERN currently has approximately 2600 full-time employees. Some 7931 scientists and engineers (representing 500 universities and 80 nationalities), about half of the world's particle physics community, work on experiments conducted at CERN.
As an international facility, the CERN sites are not officially under Swiss or French jurisdiction, and some company vehicles have diplomatic number plates.
Release link: http://public.web.cern.ch/Public/Welcome.html
Tags: particle physics , data analyses , theoretical predictions , Standard Model , conventional wisdom , Fermilab's Tevatron , Large Hadron Collider (LHC) , CERN , PEP-II , B-Factory , b-quark. ,
