The goal of particle physics is to understand what are the most fundamental constituents of matter and how these elementary particles interact. The next few years hold great promise for major advances in our understanding of this field of physics, both in theory and in experiment. Several new experimental facilities have just begun operation or will begin operation soon. These facilities will address fundamental questions such as

• What is the origin of electroweak symmetry breaking and mass (the Higgs sector)?
• Are there additional fundamental particles (e.g. supersymmetric partners of the known particles)?
• What is the origin of the matter anti-matter asymmetry in the Universe?

The answers to these questions not only affect the understanding of elementary particle physics; they can also have important implications for cosmology and our understanding of the large-scale structure of the Universe. Theoretical particle physics is focused on understanding whether there is a unified theory that explains all elementary particles and their interactions, including gravity. The most promising approaches such as string theory and membrane theory also involve modern mathematics. One of the biggest challenges is to extract unique predictions from these theories that can be verified by experiment.
Experimentalists in the Penn faculty are working on the following projects:

• The CDF experiment at the Tevatron collider at Fermi National Accelerator Laboratory, near Chicago. The Tevatron collides protons on their anti-matter counterparts, anti-protons, at a center of mass energy of 1.96 TeV, the highest energy in the world.  The top quark, the most massive fundamental particle,  was discovered here from data collected during 1989-1995. In the current operational period, from 2001 to 2009 , the Tevatron is accumulating over 50 times more data,  providing the best opportunites until 2009 to study the top quark and to discover new fundamental particles.
• The ATLAS experiment at the Large Hadron Collider (LHC) at CERN on the French-Swiss border. This facility will commence operation in late 2009 (after repairs on the collider are complete) and continue through 2020. The LHC will collide protons with protons at a center of mass energy of 14 TeV, that is seven times the Tevatron energy. Furthermore, the design luminosity of the LHC will be 100 times higher. This will be the premier facility of the era and will address the origin of electroweak symmetry breaking and other outstanding fundamental issues in particle physics.

• SNO+ will take advantage of the SNO cavity, vessel, water system, electronics and phototubes but will replace the heavy water with scintillator to lower the energy threshold. This lower threshold will allow study of the solar pep spectrum but, perhaps more interestingly, the addition of a small amount of 150Nd, a double beta decay isotope. Neutrinoless double beta decay is the primary goal of the Penn SNO+ group and the SNO+ experiment.
• The DEAP/CLEAN experiment is a planned liquid noble gas detector sensitive to Weakly Interacting Massive Particles (WIMPs), which will be capable of using both liquid argon and liquid neon as the target and detection medium. What makes DEAP/CLEAN unique is that, unlike other liquid noble gas experiments that require both liquid and gas phases to detect scintillation light and ionization, DEAP/CLEAN is a purely liquid phase detector.
• DUSEL, the Deep Underground Science and Engineering Laboratory being constructed by the NSF at the former Homestake mine in South Dakota (site of Ray Davis’s Nobel winning measurement of solar neutrino flux) will house a very large long baseline neutrino oscillation (and nucleon decay) experiment. The planning for this experiment is just beginning but much of the argument for the project is based upon work done over the past several decades by Penn faculty (Davis, Lande, Mann).
• DES (the Dark Energy Survey) is building a new wide field camera for the existing 4 meter Blanco Telescope in Chile and execute an imaging survey in four filters of _ 5000 square degrees. The survey will provide four complementary probes of dark energy: galaxy clusters, baryon oscillations, supernovae and weak lensing. It will be carried out between 2011-2016. The current activities of most relevance to the Penn effort are: the development of the software pipeline; the lensing science requirements on image quality and therefore camera and telescope design; interface of lensing modules with the data management system; and the development and testing of the lensing and Supernovae science codes.
• LSST (the Large Synoptic Survey Telescope) is a proposed 8.4m class wide field telescope and camera with a 9.6 square degree field-of-view to image half the sky in six filter bands and at significantly greater depth than DES and other precursor surveys. It will provide an enormous dataset for dark energy measurements via gravitational lensing, Supernovae and other methods. Penn is leading the LSST scientific team in weak lensing and is contributing heavily to the design of the electronics for the 3 Gpixel camera.
• SNAP / JDEM (Super Nova Acceleration Probe / Joint Dark Energy Mission) is a planned space based DOE/NASA observatory designed to measure the expansion of the Universe and to determine the nature of the mysterious Dark Energy that is accelerating this expansion.

• The Penn instrumentation group provides an invaluable resource to the field of experimental particle physics, with expertise in detector design and electronics. The group is heavily involved in ATLAS upgrades for Super LHC, in the LSST camera electronics, in data acquisition upgrades for SNO+ and in planning for a large DUSEL detector. The group also collaborates with the Penn Medical School on Positron Emission Tomography work, Arjun Yodh’s group in Physics & Astronomy on diffuse optical sensing, and with the GluEx group at Thomas Jefferson National Accelerator Lab on wire chamber chip design.

Two experiments with major Penn involvement have just finished data taking:

• The BaBar experiment at the PEP-II B meson factory at Stanford Linear Accelerator Center in California. Here 9 GeV electrons collide with 3 GeV positrons for a center of mass energy in the region of the Upsilon(4S) resonance for enhanced production of B meson pairs. This facility was constructed to study Charge Parity violation in the B meson system and to make precision measurements of the weak interaction decay of B mesons. The B factory began operation in 1999 and finished its last run in 2008.
• The Sudbury Neutrino Observatory in Ontario.  The results from this facility have provided revolutionary insight into the properties of neutrinos and the core of the sun.  The solar neutrinos produced by fusion interactions in the core of the sun are electron neutrinos.  En route from the sun to the earth, these electron neutrinos change into other types of neutrinos. The direct evidence for solar neutrino transformation also indicates that neutrinos have mass. The facility began operating in 1999 and data taking was completed in 2006.

Penn has a very active and strong elementary particle physics theory group. The central thread is the unification of all interactions. This includes theoretical efforts in string and membrane theory, phenomenological studies of the electroweak interaction, and attempts to connect the fundamental theory with experiment.
Both experimentalists and theorists collaborate closely with the astrophysics group at Penn. Recent evidence that the expansion of the universe is accelerating implies that more than two-thirds of the energy density in the universe is in the form of a mysterious dark energy.  We participate in several future experiments, including the Supernovae Acceleration Probe and the ground-based Dark Energy Survey.   Theoretical research in particle astrophysics includes inflationary cosmology, studies of the microwave anisotropies, and theoretical studies of solar and supernova neutrinos.

ICHEP 2008 was held in Philadelphia last year - July 30th - August 5th 2008
Experimental Particle Physics seminars at PENN
Theoretical Particle Physics seminars at PENN

Emeritus Faculty: Sherman Frankel, Paul Langacker, Sidney Bludman, Gino Segre.