## Background & Online Resources

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While physicists often use concepts like relativity in their work, teachers rarely discuss them simply because they are difficult to convey in a "hands-on" manner. Physics teachers can use this Website to help their students build an understanding of relativity based on analysis of data.

This project uses data recorded as part of an experiment at Fermi National Accelerator Laboratory in Batavia, Illinois. Experiments 687 and 831 investigate several topics in the physics of the charm quark, a fundamental particle. (E831 is an upgraded version of E687.) Experimenters at Fermilab focus a beam of protons on a target of beryllium (E687) or liquid deuterium (E831). A beam of photons emerges from this target and encounters a target of beryllium. Within this target, several particles are produced, including D mesons which are also called charmed mesons. These exotic particles consist of a charm quark and either an up or a down quark. Detectors at the Lab determine which particles are produced, how long they live before they decay, and how much energy they have.

These particles are extremely energetic; their speeds are nearly that of light. Thus they suffer huge relativistic "shifts" in simple measurements. Any physical measurement made in the frame of the particles, even one as simple as time, is different than the same measurements made in the detector. This physical effect is well understood and arises from the nature of space and time as described in Einstein's Theory of Special Relativity. There is a rather complex correction factor that one must apply to measurements in the detector to determine what the measurements would be had the particle recorded them. This correction depends on the velocity; the larger the relative velocity between those making the measurement, the larger the correction factor. We simply do not see this effect at everyday speeds because this correction factor is very close to 1 at these speeds. The data in this experiment have speeds so large that the correction factor is at least an order of magnitude.