High Energy Physics (HEP) experiments located at CERN are breaking new ground in the understanding of the unification of forces, the origin and stability of matter, and structures and symmetries that govern the nature of matter in our universe. To improve our understanding of the fundamental constituents of matter, and the nature of space-time itself, researchers must isolate and measure rare events. The Higgs particles, thought to be responsible for mass in the Universe, will typically only be produced in one interaction among 1013. A new generation of particles at the upper end of the LHC’s energy reach may only be produced at the rate of a few events per year, or one event in 1015 to 1016. Experimentation at increasing energy scales and luminosities, and the increasing sensitivity and complexity of measurements, have necessitated a growth in the scale and cost of detectors as well as particle accelerators, and a corresponding increase in the size and geographic dispersion of scientific collaborations. The largest collaborations today, ATLAS and CMS – each encompass around 2000 physicists from 150 institutions in more than 35 countries.

The key to discovery is the ability to detect a signal with high efficiency, and in many cases (such as in the Higgs searches) to measure the energies and topology of the signal events precisely while suppressing large, and potentially overwhelming, backgrounds. Physicists must scan through the data repeatedly, devising new or improved methods of reconstructing, calibrating and isolating the “new physics” events with increasing selectivity as they progressively learn how to suppress the backgrounds. The initial strategies for separating out the Higgs and other signals will initially rely on massive simulations performed using distributed Grid facilities. These strategies will evolve as the LHC data accumulates, and a more precise picture of the backgrounds that need to be suppressed is obtained, by studying the real data themselves.

HEP data volumes to be processed, analyzed and shared are expected to rise from the multi-Petabyte (1015 Byte) to the Exabyte (1018 Byte) range within the next 10-15 years, and the corresponding network speed requirements on each of the major links used in this field are expected to rise from the 10 Gigabit/sec (Gbps) to the Terabit/sec (Tbps) range during this period.

Several of the challenges in physics analysis are the sheer volume of data that needs to be analyzed (possible multiple times) distributed in multiple storage elements and the limited amount of resources available for a large physics community. Important is the multi user scenario in which many individuals need to get access to limited compute, storage and network resources without congesting the system, where end-2-end monitoring plays an important role. The challenge for (many) single physics user(s), is to have transparent access to (software) resources, to perform physics analysis, possibly in an iterative manner (submit, analyze, submit again….).