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High Energy Physics
 


Particle Physics Research at Penn Overview

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

The answers to these questions not only affect the understanding of elementary particle physics; they can also have important implications for cosmology and 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:

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 will be in Philadelphia July 30th - August 5th 2008

Experimental Particle Physics seminars at PENN

Theoretical Particle Physics seminars at PENN

Emeritus Faculty: Walter Selove, Al Mann, Sherman Frankel, William Frati, Paul Langacker, Sidney Bludman, Gino Segre.

EXPERIMENTAL PARTICLE PHYSICS

CDF Experiment at Fermilab

More information on CDF at PENN
Faculty: Joe Kroll, Nigel Lockyer, Evelyn Thomson, Brig Willliams
Staff Scientist: Joel Heinrich
Postdoctoral Researchers: Anadi Canepa, Aart Heijboer, Christopher Neu
Graduate students: Yan-jun Tu, Tatiana Rodriguez, Elisabetta Pianori

The Collider Detector at Fermilab

The Collider Detector at Fermilab (CDF).

The CDF experiment at the Fermilab Tevatron is one of the premier high energy physics facilities in the world. The versatility of the detector and proton-antiproton collisions allows the study of a broad range of fundamental issues in particle physics, including the search for new phenomena, tests of the strong and electroweak theory, and the physics of heavy quarks (bottom and top).

The Penn group has played a significant role in the discovery of the top quark in 1995 and in more recent studies of top quark properties.  Both Brig Williams (1994-1996) and Evelyn Thomson (2004-2006) have taken important leadership roles in the Top Quark Physics Group, one of five physics groups in the 600-person international collaboration.  Recent papers with significant contributions from the Penn group include measurements of the top quark mass, measurements of the pair production rate of top quarks, and studies of the decay properties of top quarks.

The startling news in 2006 was the precise observation by CDF of the mixing frequency of the strange B meson into its anti-particle. Joe Kroll played a significant leadership role in the data analysis, and in the design and construction of a crucial particle identification system for kaons. At 3 trillion oscillations per second, this frequency had been beyond the sensitivity of all previous experiments in the world for almost thirty years. 

Nigel Lockyer was co-spokesman of the CDF collaboration from 2002 to 2004 and has broad interests in B physics, top quark physics, and searches for new physics. He received the Panofsky Prize in 2006, along with Bill Ford (Colorado) and John Jaros (SLAC) for pioneering work in the measurement of the unexpectedly large lifetime of the B meson at SLAC in the 1980's. In May 2007, Nigel Lockyer will leave Penn to become director of TRIUMF in Canada.

ATLAS Experiment at CERN

More information on ATLAS at PENN
Faculty: Brig Willliams,Evelyn Thomson, Joe Kroll, Elliot Lipeles
Staff Scientists: Rick Van Berg, Mitch Newcomer, Paul Keener, Godwin Meyers, Ben LeGeyt, Mike Reilly, Nandor Dressnandt
Postdoctoral Researchers: Franck Martin, James Degenhardt, Sasa Fratina, Peter Wagner, Mauro Donega
Graduate students: Mike Hance, Ryan Reece, John Alison, Dominick Olivito, Liz Hines, Josh Kunkle, Brett Jackson, Chris Lester and Jon Stahlman

The exploration of the high energy frontier will take a leap forward when the Large Hadron Collider (LHC) at commences operation in 2008. The LHC and the ATLAS detector are currently in the final stages of construction at CERN near Geneva, Switzerland. The LHC will collide intense beams of 7 TeV protons with 7 TeV protons every 25 ns. The higher energy and higher collision rate will allow the study of more massive particles and rarer interactions than reachable at the Tevatron. The ATLAS experiment will explore the most fundamental questions being pursued in particle physics: the origin of mass, searches for supersymmetry and new particles predicted by some string theories, and the unexpected.

Since 1994, the University of Pennsylvania group has played a large role in the design and construction of ATLAS, one of two large "general purpose" experiments being constructed at the LHC. The ATLAS detector, shown below, stands nearly five stories tall and includes precision tracking systems for observing the trajectories and thus measuring the momemta of particles produced in the interactions, calorimetry for measuring the total energies of all observable particles produced, and muon chambers for observing muons which escape the calorimeter.

Schematic of the ATLAS Detector
Schematic of the ATLAS Detector.

ATLAS underground cavern   barrel TRT in the center of ATLAS
Left: ATLAS underground cavern (November 2005) with all of the muon toroidal magnets.
Right: Installation of the barrel TRT in the center of ATLAS (August 2006).

One of the key charged particle tracking systems is the Transition Radiator Tracker (TRT), based on a novel design which enables excellent tracking of the individual particles while also improving significantly the idenfication of electrons via transition radiation. The TRT is constructed from 400,000 "straw" proportional tubes which detect the passage of charged particles. Penn is one of the primary institutions developing the electronics for the ATLAS Transition Radiation Tracker (TRT). Our efforts at Penn include development of two integrated circuits: the first incorporates an amplifier/shaper/discriminator with baseline restoration (ASDBLR). The challenge in developing this chip is that it must provide signal detection, amplification, and detecion cleanly at a rate of more than 20MHz. Penn is also involved in the development of a second integrated circuit, the DTMROC chip, which measures the time at which the individual pulses occur. We also provide significant expertise in the area of instrumentation, grounding, shielding, and overall system design.

Brig Williams was co-coordinator for the Front End Electronics for ATLAS for several years and is a member of the US ATLAS executive board. The group's first graduate student on ATLAS, Mike Hance, is already based at CERN and making significant contributions to the commissioning of the TRT with cosmic rays. With the recent addition of the research groups of Evelyn Thomson and Joe Kroll, the Penn group is expanding its efforts on ATLAS as commissioning activity heats up in 2007.

Babar Experiment at Stanford

Faculty:Larry Gladney

The BaBar experiment at SLAC is focused on the observation of CP violation in the B quark system and the long term study of the bottom and charmed-quark systems. This program will be a major emphasis of the U.S. and world program for the years between 1999 and the turn on of the LHC. The goal is to look for physics beyond the Standard Model through precision measurements of CP-violating asymmetries and observation of extremely rare decays which might be enhanced through the presence of new fundamental physics.

Penn has played major roles in developing the object-oriented (OO) framework for the BaBar trigger and for the reconstruction software. Given the significant competition for discoveries in CP-violation around the year 2000, rapid analysis of data soon after physics turn-on is essential. The turn towards modern software design tools and methods is a crucial part of the plan to ensure that the BaBar reconstruction and analysis system will be robust, flexible, and timely. Penn currently leads the efforts for the reconstruction software and for the software trigger of the experiment. Some of the crucial aspects for these efforts which we have provided include:

Professor Larry Gladney and graduate student Qinghua Gao

Professor Larry Gladney and graduate student Qinghua Gao discussing analysis of B-hadron data.

International Linear Collider: accelerator design

More information on ILC at PENN
Faculty: Nigel Lockyer
Staff Scientists: Walter Kononenko, Mitch Newcomer
Graduate students: Justin Keung, Anna Grassellino

A new effort has begun with our involvement in the International Linear Collider (ILC) project. The International Linear Collider is a proposed new electron-positron collider. Together with the Large Hadron Collider (LHC) at CERN , it would allow physicists to explore energy regions beyond the reach of today's accelerators. At these energies, researchers anticipate significant discoveries that will lead to a radically new understanding of what the universe is made of and how it works. The group is currently involed in the design of Superconducting Radio Frequency cavities for the accelerator.

Sudbury Neutrino Observatory, SNO+, DEAP/CLEAN

More information on SNO at PENN
Faculty: Gene Beier, Josh Klein (from summer 2008)
Postdoctoral researchers: Jeff Secrest, Huaizhang Deng, Gabriel Orebi Gann
Graduate Students: Tim Shokair, Richie Bonventre
Recent graduate students: Monica Dunford, Chris Kyba, Mark Neubauer, Vadim Rusu, Peter Wittich

The Sudbury Neutrino Observatory (SNO detector). Shown in the figure are Penn graduate student Doug McDonald, Professor Josh Klein and graduate student Peter Wittich (from left) and others with the acrylic vessel underground in Sudbury.

The Sudbury Neutrino Observatory project (SNO) has solved one of the great puzzles of twentieth-century physics and astrophysics---the anomalously low flux of neutrinos coming from the sun. Since the late 1960's when Ray Davis first announced that he detected about one-third the number of neutrinos predicted by models of stellar evolution, scientists were in a quandary regarding the source of the discrepancy. Was his experiment wrong? Was our understanding of stars wrong? Or was there something else, perhaps an inadequate understanding of the properties of neutrinos?

SNO has shown that Davis's experiment was correct, and that the model of the sun is also correct. The puzzle was solved when SNO showed that some of the Boron-8 electron-neutrinos that are produced in nuclear fusion reactions that power the sun transform to another type of neutrino which does not produce a signal in Davis's detector.

Unlike previous solar neutrino experiments, the SNO detector is sensitive to three different neutrino reactions. One of the reactions is, like Davis's experiment, only sensitive to the electron-neutrinos that the sun produces. The other two reactions are sensitive to electron-neutrinos, and, in different proportions, to mu-neutrinos and tau-neutrinos --- types that are not produced in the solar fusion reactions. The three reaction types can be separated using the position, angle, and energy information of the events observed.

By comparing measurements of the flux of solar Boron-8 electron neutrinos (nu_e) to the total flux of all neutrino types (nu_x) coming from the Sun, SNO has shown that Davis's original measurement was correct---the nu_e flux is suppressed---but that the flux of all types of neutrinos is in agreement with the predictions of the model. The conclusion is that some of the electron-neutrinos that were produced in the sun transform into the other types of neutrinos before they are detected on earth. The most likely mechanism for producing this transformation requires that neutrinos have small, but non-zero mass. This is an indication of exciting new physics beyond the Standard Model of elementary particle physics. Although the mass of the neutrinos is tiny, the total mass of all the neutrinos in the universe is comparable to that of all the visible stars.

The unique feature of SNO is the use of a kiloton of heavy water, D2O, as a neutrino target. The valuable D2O is securely contained in a spherical acrylic vessel which is twelve meters in diameter. The vessel is surrounded by light water, H2O, and is viewed by 9500 photomultiplier tubes. To limit backgrounds introduced by cosmic radiation at the earth's surface, the entire laboratory and detector are located two kilometers underground in a cavity in one of the world's most productive nickel mines. The SNO cavity, which is the size of a ten story apartment building, is maintained as a "clean room" to exclude trace contamination from mine dust.

The SNO experiment began taking calibration and neutrino data in May 1999. The program of calibrations determines the optical parameters, the spatial, angular, and energy responses of the detector, the response to signals from neutrinos and processes that produce background, and systematic effects which might bias interpretations. The calibrations are taken routinely to track the time dependence of the detector response.

Neutrino data will be acquired in at least three configurations of the detector. The initial configuration was the simplest, with only heavy water inside the acrylic vessel. In June, 2001, the detector configuration was altered to the first of two configurations that will enhance the detection capability for nu_x. Measurements in these two configurations will produce independent measures of the flux of nu_x and serve as checks on each other and on the result from the initial phase. The additional data will also permit accurate measurements of possible distortions in the electron energy spectrum and day-night spectral differences for nu_e induced events. These measurements will lead to precise evaluation of the physics parameters responsible for neutrino flavor transformation in the solar sector. Additional topics of study include atmospheric neutrinos and a search for anti-neutrino interactions. SNO's neutron detection capability is a unique asset for this work. A program of data acquisition and analysis lasting at least through 2005 is envisioned, in order to obtain the highest precision results possible.

The University of Pennsylvania group constructed and is responsible for maintaining all the front-end signal processing electronics for the detector. This includes PMT signal detection and digitization, triggering, and GPS timing electronics. The effort has required three custom designed integrated circuits and fourteen custom designed printed circuit boards. Graduate students contributed to or were solely responsible for nine of the circuit boards. This represents one of the many substantial and crucial contributions to the SNO experiment by students.

Penn researchers were deeply involved in commissioning the detector and are now active in operations and data analysis at all levels. The opportunities for learning a wide range of physics and experimental techniques---from hardware design to data acquisition software to data analysis---are great; Penn graduate students working on SNO get a broad exposure to both the hardware and analysis skills required to do effective research and an opportunity to work on one of the most exciting experiments in the particle physics.

Electronics Instrumentation

More information on electronics at PENN
Staff Scientists: Rick Van Berg(Instrumentation Group Leader), Mitch Newcomer, Paul Keener, Godwin Meyers, Walter Kononenko, Ben LeGeyt, Mike Reilly, Nandor Dressnandt

As is probably evident in the earlier discussions of CDF, ATLAS and SNO, the sophisticated capability which we have built up at Penn to design and test custom integrated circuits has been put to good use. In the last few years we have finished the development of two bipolar integrated circuits for SNO, have completed a custom bipolar circuit for ATLAS in two separate radiation hard technologies, a custom bipolar circuit for ATLAS in two separate radiation hard technologies, have played a critical role assisting Queen's University in the development of the CMOS chip for SNO, have completed a custom bipolar circuit for CDF, and have recently played a major role (along with CERN and Lund University) in completing the design of a CMOS circuit for ATLAS. We have also helped dozens of other institutions around the world utilize the ASD8 chip or one of its variants for high rate wire tracking systems. Penn front end chips are in use at major experiments at Fermilab, Brookhaven, CERN, DESY, TRIUMF, and elsewhere around the world.

With the combination of the IMS Integrated Circuit tester purchased via an NSF University Infrastructure Grant and the automatic wafer probe system, Penn has a very sophisticated capability for testing integrated circuits. Production testing of IC's for SNO processed over 10,000 chips, there were also almost 10,000 ASDQ chips tested for CDF, and we are looking forward to testing almost 70,000 chips for ATLAS. We have also done testing of custom circuits designed by other groups and some detailed characterization of commercial circuits required for some special needs. The integrated circuit design and test capabilities are the outgrowth of a tradition of building novel and elegant electronics systems for particle physics experiments. Clearly the integrated circuits are only one facet, albeit the single most sophisticated portion, of such systems. At the more conventional level of system architecture and printed board design the Penn group has covered a wide gamut of projects recently from the full system of fifteen different printed circuits that make up the SNO electronics to the single boards used in the the CDF 30,000 wire Central Outer Tracker readout and Central Electromagnetic Calorimeter calibration systems and the very high density, high rate, low noise TRT readout card assemblies for ATLAS.

THEORETICAL PARTICLE PHYSICS

More information on theory at PENN and Math/Physics Research Group
Faculty:Vijay Balasubramanian, Mirjam Cvetic, Mark Trodden, Justin Khoury, and Burt Ovrut
Postdocs: Timo Weigand, Inaki Garcia-Etxebarria (from fall 2008), Volker Braun
Graduate students: Klaus Larjo, Bartlomiej Czech, Robert Richter, Mike Ambroso, Tamaz Brelidze, Jim Halverson, Godfrey Miller, Jan Homann

The group has a diverse research program, pursuing topics in supersymmetry, superstring theory and M-theory, model building and phenomenology, mathematical aspects of supergravity, electroweak physics, precision tests, black hole physics, particle cosmology and neutrino astrophysics.

During recent years members of the Penn Theory Group have made substantial contributions in several forefront fields:

Electroweak physics: Especially precision electroweak tests of the standard model, searches for such new physics as supersymmetry, additional gauge bosons, and neutrino masses and mixings.
Model building and phenomenology: Including predictions of heavy Z bosons and other new particles, the consequences of specific string model constructions, and applications of non-perturbative field theory and string theory to particle physics problems such as supersymmetry breaking and fermion masses.
Formal string theory and field theory: Formal problems in areas of supergravity, supersymmetric field theory, superstrings and the parent M-theory from which they arise, including the study of holomorphic instantons, non-perturbative phase transitions, conformal field theories, mathematical aspects of string theories defined on blown-up singularities. Applications to black holes, cosmology, and the construction of realistic models of the world.
Particle physics implications of M-theory: Derivation of M-theory and string theory compactifications that can implement the features of the standard model of electroweak and strong interactions.
Dualities in string theory and field theory: Approaches to the non-perturbative definition of M-theory such as the Matrix model and the correspondence between gravity on spaces with a negative cosmological constant and conformal field theories; Duality transformations relating gauge theories to each other and to higher dimensional theories of gravity.
Black hole physics: Especially constructions of new black hole and brane solutions, microscopic counting of black hole states, string theoretic investigations of the information loss paradox for black holes, and the holographic principle for quantum gravity.
Cosmology and astrophysics: Especially neutrino astrophysics with applications to solar neutrinos and pulsars, and the cosmological implications of superstring and M-theory.

The group encourages collaborations across disciplinary and departmental boundaries. In particular, interactions with particle experimentalists, mathematicians, astrophysicists, condensed matter physicists and biophysicists make Penn a rich environment for cross-disciplinary research.

Details of the research interests of our faculty members can be obtained from their webpages. Briefly, Vijay Balasubramanian is a string theorist whose recent work has focussed on the description of black hole and de Sitter thermodynamics in string theory, the analysis of "holographic" duality symmetries that relate theories of gravity to lower dimensional gauge theories, and the cosmological implications of string theory. Mirjam Cvetic has made broad contributions in areas ranging from basic theory to its phenomenological implications. She is an expert on gravitational physics, and has done pioneering work on domain walls and black holes in supergravity and M-theory. These efforts have recently focussed on the study of consistent compactifications of M-theory, the analysis of extended solitons in M-theory, and their implications for the physics of gauge theories. She has also worked on scenarios in which the universe lives on a domain wall embedded in a large higher dimensional space. Mirjam Cvetic and Paul Langacker have pursued a research program aimed at deriving the particle physics implications of string theory, including the construction of quasi-realisitic models that contain the Standard Model of particle physics. Paul Langacker is a world expert on electroweak precision physics and neutrino physics. He continues to pursue a long-term research program on electroweak signatures in precision experiments, neutrino physics, and the implications of new gauge bosons that generically appear within string models. Burt Ovrut has been studying the low-energy particle physics and cosmological implications of M-theory. Ovrut has been developing the idea that M-theory has a five-dimensional phase in which our observable Universe is trapped on a 3+1 dimensional membrane. This is a rigorous theory of so-called ``brane universes''. His recent efforts involve the construction of brane-world theories which closely resemble the standard model, and the study of their particle physics and cosmological implications. This work has required interdisciplinary collaboration with mathematicians at Penn, and has led to new results in pure mathematics and to the establishment of an active Penn Math/Physics program. Gino Segre continues to work on pulsar phenomenology, and is interested in K decays and the problem of the cosmological constant. Sidney Bludman continues his work on solar and stellar structure, and on cosmological parameters.

Our postdocs include T. Li (particle phenomenology and brane world physics), A. Naqvi (M-theory, string field theory, AdS/CFT correspondence), G. Shiu (particle physics implications of string theory and M-theory), and F. Brito (topological defects in supersymmetric theories). Former postdocs J. Erler (electroweak precision physics) and J. Park (extensive work in string theory and M-theory) have recently taken faculty positions in Mexico City and in Korea, respectively. At present, the group supports six students who are very much involved in our research and activities. An expanded and vibrant visitor program is bringing many researchers in high energy physics to Penn for extended durations.

NUCLEAR PHYSICS

Faculty: D. P. Balamuth, H. T. Fortune
Emeritus Faculty: F. Ajzenberg-Selove, R. D. Amado, Robert W. Zurmuhle

Experiment The goal of nuclear physics is to understand the structure and behavior of many-particle systems in which strong, weak, and electromagnetic interactions all play a significant role. These questions are asked at a wide range of energy scales, ranging from ultra-cold neutrons (far below thermal energies) to systems of hundreds of nucleons excited to relativistic energies, and including nearly everything in between. Nuclear physics has a long and distinguished history at Penn, involving research with both on-campus accelerators (the most recent was a 10 MV tandem Van de Graaff) and national user facilities. Important milestones in this program included what was arguably the most comprehensive program of nuclear structure studies ever undertaken using a tritium beam, the discovery of highly deformed, highly excited 'molecular' resonances in heavy nuclei, an extended series of rigorous assignments of discrete quantum numbers to bound and unbound states using angular correlation techniques, and an important series of measurements investigating nuclear structure and reaction mechanisms with single and double charge exchange reactions involving pi mesons. Today, this program has evolved in two principal directions. At low energies, David Balamuth's research program is focussed on nuclei with unusual ratios of neutrons to protons, thereby lying far from the valley of beta stability. These systems are studied at national accelerator facilities using both the techniques of modern gamma ray spectroscopy and with secondary beams of unstable nuclei produced at a high energy heavy ion accelerator.

At higher energies, Terry Fortune's work is focussed on both experimental and theoretical nuclear physics. The experimental emphasis is on direct and resonance reactions, using a variety of probes, which range from heavy ions to elementary particles. The theoretical interests include calculations of nuclear structure and reaction mechanisms. A component of experiments and calculations is related to questions in nuclear astrophysics. Most recent research has utilized beams of low-energy heavy ions and intermediate-energy pi mesons, neutrons, and electrons from various accelerators in the US and Canada.