Professorial Faculty:

• David G. Hitlin
• Emlyn W. Hughes (left for Columbia)
• Harvey B. Newman
• Frank C. Porter
• Alan J. Weinstein
• Barry C. Barish (Emeritus, Director of the International Linear Collider Global Design Effort)
• Charles W. Peck (Emeritus)

Research Fellows, Associates, and Senior Staff:

• Justin Albert (left to become Asst. Prof. of Physics at Univ. of Victoria in Canada)
• Adi Bornheim
• Julian Bunn
• Fang Fang
• Caius Howcroft
• Phillipe Galvez
• Vladimir Litvin
• Ilya Narsky
• Richard Wilkinson
• Liyuan Zhang
• Hai Zheng
• Ren-Yuan Zhu

The experimental high energy physics (HEP) group studies the ultimate constituents of matter, and the forces governing their interaction. The research is primarily carried out through experiments at high energy particle accelerators, but non-accelerator experiments have also assumed importance. Experiments are currently underway or are being prepared at the European High Energy Physics Laboratory (CERN), the Stanford Linear Accelerator Center (SLAC), the Fermi National Accelerator Laboratory (Fermilab), Lawrence Berkeley National Laboratory (LBL), on campus, and elsewhere. Advanced research and development projects for experiments at the next generation of particle accelerators (as well as in space, underground, etc.), are also being pursued.

Over the last 25 years, an elegantly simple, but incomplete, picture of nature has emerged, in which the elementary constituents of matter consist of quarks and leptons. High energy physicists have established that there are at least three quark generations, each with two different "flavors" (up and down, charm and strange, top and bottom). Each lepton generation consists of a particle with electric charge (electron, muon and tau), accompanied by a neutrino which carries the same "lepton flavor."

The first quark and lepton generations make up ordinary matter --- protons, neutrons and electrons. The heavier strange, charm, and bottom quarks appear in more exotic forms of matter. The very heavy top quark decays too quickly to bind to any other particles.

The forces of nature are carried by "gauge particles." The electromagnetic interaction is carried by the photon, the weak interaction (which governs radioactivity) is carried by the W and Z particles, and photon-like "gluons" carry the strong interaction (responsible for nuclear binding). The unified electroweak theory has been confirmed by a host of experiments, culminating in the discovery and precision measurement of the properties of the heavy W and Z particles over the last 15 years.

Quantum Chromodynamics (QCD) is a successful candidate theory of the strong interaction, but many important phenomena associated with that interaction cannot (yet) be predicted from the theory.

More information about the Standard Model of particle physics can be found at, for example http://www.particleadventure.org (at a layperson level) and the Particle Data Group webpage (at a research-oriented level).

In spite of its impressive successes in describing and predicting the fundamental particles and processes we see in nature, the Standard Model of particle physics has some impressively large holes in its fabric, such as:

• For the Standard Model to be consistent, a "Higgs" particle responsible for particle masses must exist. It has never been seen, but there are hints that it might possibly be just beyond the present lower limits on its mass (primarily set by LEP at CERN in the 1990s). Will we be able to find it at the LHC...?
  
  
• A mysterious "dark matter" must be responsible for most of the matter in the universe (as seen from, for example, galaxy dynamics and gravitational lensing in space), but we do not know what it is (in fact we know that it cannot be made of any of the above particles), nor have we ever seen it in experiments on Earth (although there are hints that it too might be close to discovery). What is it, and how can we find it...?
  
  
• Perhaps even more dramatically, all the matter in the universe (including the above dark matter) appears to only be responsible for approximately 1/3 of the total energy content of the universe (from measurements of supernovae, galaxy clusters, and the cosmic microwave background), assuming general relativity is a correct description of gravity. What could the remaining "dark energy" that comprises 2/3 of the universe be...? (Or could general relativity require modification...?)
  
  
• We produce antimatter (positrons, antiprotons, antineutrons, etc) constantly in the laboratory (even colliding antiparticles with normal particles at PEP-II and the Tevatron, etc). The Big Bang was thought to be pure energy. Energy can divide into matter and antimatter, but it typically tends to divide into equal amounts. So why does our universe appear to be made almost entirely of matter...? What happened to the antimatter...?
  
  
• We have learned in the past 10 years the relatively surprising fact that neutrinos have mass. But why are they so light compared with the other particles...?
  
  
• Our Standard Model of particle physics does not incorporate gravity (and, in fact, one cannot incorporate gravity into a "typical" quantum field theory). How would we incorporate gravity into a combined theory of nature, such as string theory (and, most importantly, test such a theory)...?
  
  

Caltech scientists in experimental high energy physics have made key contributions to the rapid advance of the field. Experiments at SLAC, CESR, and Fermilab in the U.S., and CERN and DESY in Europe, have been instrumental in revealing the existence of quarks and gluons, confirming electroweak unification, studying the nature of mesons and baryons containing Charm or Bottom, measuring the fundamental coupling constant of the strong interaction, and searching for phenomena beyond the Standard Model. They also have pivotal roles in the next generation of particle physics experiments, currently in preparation.

The research program being carried out by the Caltech group for ongoing and future experiments is described below.

at PEP-II: The SLAC Asymmetric B Factory

Four decades after the discovery of the CP violation in K_S(0,L) meson decay, the phenomenon defies detailed explanation. The key to an understanding of CP violation is the measurement of a variety of CP-violating asymmetries in the hadronic decays of neutral B mesons. This is best done using B_0 mesons produced as particle-antiparticle pairs at an asymmetric energy e+e- storage ring, PEP-II at SLAC (and KEK-B in Japan). Caltech HEP faculty David Hitlin and Frank Porter have been motivators of this project from its inception and now lead a group performing physics measurements using the large dataset collected by BaBar since 1999 (and presently accumulating) as well as leading the effort to increase the luminosity of the e+e- beams. This effort has resulted in the discovery of CP violation in the B meson system in 2001, and and is providing uniquely stringent tests of the Standard Model, by facilitating a series of precision measurements in b, charm and tau physics.

Caltech has performed, and is performing, numerous measurements at Babar to search for deviations from the Standard Model of particle physics that could potentially provide indications of a new source of matter-antimatter asymmetry in the universe. Caltech also is responsible for several parts of the Babar and PEP-II hardware, including the electromagnetic calorimeter calibration system, the computing system, and the PEP-II beam size measurement instrumentation.

Caltech Babar members: Justin Albert, Ed Chen (grad student), David Doll (grad student), Alexei Dvoretski (grad student), Fang Fang, David Hitlin, Tim Piatenko (grad student), Frank Porter.

The Compact Muon Solenoid at the LHC

We may have the opportunity for a very limited number of new graduate students to work on CMS.
Please see this Powerpoint presentation and/or contact Prof. Harvey Newman (newman@hep.caltech.edu) for details.

The CMS experiment (old public webpage, webpage for collaboration members) will use a large high precision tracker and Lead Tungstate (PbWO4) crystal calorimeter housed in a 4 Tesla magnet, together with hadron calorimeters and a very large muon detection system to search for the Higgs, supersymmetric particles and other signs of the breakdown of the Standard Model up to the TeV mass scale. The LHC will be by far the highest-energy collider to date: 7 times the center-of-mass energy of the Tevatron --- and far higher intensity as well. We have only vague notions about what may lurk at these newly accessible energy scales, and thus the start of the LHC promises to be a very exciting time. The CMS experiment and the LHC are now being developed, with the start of physics running scheduled for 2007. Caltech leads the US CMS Collaboration,  and has major roles in the physics program, in the development of the experiment's computing, software and network systems, as well as its crystal calorimeter.

The Caltech work on CMS capitalizes on the group's expertise on the development and calibration of crystal scintillators, and of computing systems for large experiments, over the last 16 years. This includes the development of BGO for L3 (1983-7), and of fast rad-hard Barium Fluoride scintillators for the SSC (1988-1992), as well as the design, implementation and operation of the L3 computing systems (1982-1997). The construction and testing of the CMS PbWO4 calorimeter is presently ongoing. We are also developing new concepts in wide area networks and distributed computing systems.

The high speed and radiation resistance of a calorimeter based on these crystals would have unique ability to detect the Higgs in the "intermediate mass" range favored by precision electroweak measurements (115 - 140 GeV, just beyond the reach of LEP), through measurements of the rare Higgs decay H⁰ -> gamma gamma. The Caltech CMS group is currently carrying out detailed simulation studies of Higgs detection in CMS using this channel, taking advantage of the very large HP "Exemplar" system at Caltech's Centre for Advanced Computing Research.

Caltech CMS members: Justin Albert, Adi Bornheim, Julian Bunn, Phillipe Galvez, Marat Gataullin (grad student) Heather Gray (grad student), Emlyn Hughes, Iosef Legrand, Vladimir Litvin, David Lopez-Mateos (grad student), Zachary Marshall (grad student), Harvey Newman, Conrad Steenburg, Vladlen Timciuc (grad student), Jan Veverka (grad student), Alan Weinstein, Rick Wilkinson, Yong Yang (grad student), Liyuan Zhang, Ren-Yuan Zhu.

MINOS at Fermilab and Soudan

We have the opportunity for a very limited number of new graduate students to work on Minos (and Nova).
Please see this Powerpoint presentation and/or contact Prof. Harvey Newman (newman@hep.caltech.edu) for details.

The Main Injector Neutrino Oscillation Search (MINOS) experiment utilizes protons from the Fermilab Main Injector to produce a high-energy neutrino beam (the NuMI beamline) aimed in the direction of the Soudan underground laboratory located 730 km away in northern Minnesota. The goal of the experiment is to search for neutrino oscillations suggested by anomalies in the observed numbers of atmospheric neutrinos in several underground detectors. If observed, the MINOS experiment will be equipped to study the oscillations in detail and determine the participation in the oscillations of different neutrino flavors. MINOS uses an 8.1 kT sampling calorimeter with magnetized iron absorbers and planes of plastic scintillator at the Soudan site to observe neutrino interactions from the Fermilab beam and also atmospheric neutrino interactions. A similar, but smaller, detector is taking data at Fermilab for comparison of neutrino interactions before and after the 730 km "flight" to Soudan. The experiment began data acquisition in early 2005.

Caltech MINOS members: Barry Barish, Caius Howcroft, Harvey Newman, Juan Ochoa (grad student), Charlie Peck, Hai Zheng.

New Ventures

The International Linear Collider

The International Linear Collider (ILC) will be a 30 km long electron-positron collider, initially colliding at 500 GeV center-of-mass energy and upgradeable to 1 TeV, and slated to begin taking data in approximately 2015. The ILC is a worldwide global effort involving over 3000 physicists in over 30 countries on 5 continents, with Caltech faculty member Barry Barish as the leader of the Global Design Effort. There will be one or two high-energy e+e- interaction points with state-of-the-art detectors. Caltech is involved with beam instrumentation (beam size monitors), the ILC damping rings, and the structure of an ILC silicon tracking detector.

Caltech people involved with the ILC are: Barry Barish, Justin Albert, David Hitlin, Frank Porter, Ren-Yuan Zhu.

: A Next-Generation B-Factory

BaBar is scheduled to end data taking in 2008. It is likely, however, that many of the questions in B-physics (including the ultimate question of the matter-antimatter asymmetry) will still remain unanswered at that point. Most of the measurements at BaBar are still limited by statistics (rather than by systematic uncertainties) and thus a higher-luminosity B Factory is well-motivated. Many of the designs for a next-generation B-Factory have much in common with the ILC project above (and perhaps even the possibility of being a part of it). Caltech works closely with groups at INFN Frascati in Italy, SLAC, Pisa, and elsewhere on this project.

Caltech people involved with SuperB are: Justin Albert, David Hitlin, Frank Porter, Ren-Yuan Zhu

Determining the Nature of Dark Energy at and

In 1999, two groups, the Supernova Cosmology Project and the High-z Supernova Search Team, reported the very surprising discovery that the universe is not just expanding (as discovered by Hubble in 1936) but that its expansion is in fact accelerating. If general relativity is an accurate description of nature, then this could only be caused by a form of energy permeating the universe. One possibility for this is a so-called "cosmological constant" first suggested by Einstein. However, such a cosmological constant would appear to have implications for the quantum nature of gravity, that are not clearly suggested by present quantum theories of gravity. Thus, we need to determine if the dark energy is a cosmological constant or something else.

There are two main ways to do this. One is to obtain higher statistics (and better systematic uncertainties) of the "type Ia" supernovae that were originally used to find the dark energy. Another is to use the technique of "weak gravitational lensing": look at correlations in the shapes of galaxies for patterns that indicate the presence of foreground matter and energy that bends, or "lenses", the light from the background galaxies. When this information is combined with the redshifts of the background galaxies, contraints on the amount and the properties of dark energy can be obtained.

Caltech is working on the near-IR detectors for SNAP, and on hardware for redshift calibration ("spectrophotometric calibration") for both SNAP and LSST (see the "STARCaL" project below).

Caltech HEP people involved with SNAP and LSST are Justin Albert and Alan Weinstein. They work closely with the Astronomy Department group working on SNAP: Richard Ellis, Richard Massey, Roger Smith, and Keith Taylor, as well as Jason Rhodes, Michael Seiffert, and others at the nearby NASA/Caltech Jet Propulsion Laboratories (JPL).

The STARCaL Satellite Project

Determinations of the nature of dark energy by the next generation of telescopes including LSST and SNAP will be limited by our knowledge of relative and absolute spectrophotometry (i.e., the precision measurement of light flux as a function of color) of objects (supernovae and galaxies). Present means of spectrophotometric calibration are fundamentally limited by our understanding of standard sources (stars) and by the atmosphere. The best-understood stellar spectra are only known to absolute uncertainties of 2-4 percent. New techniques to calibrate absolute and relative flux to better than 1 percent are needed. One possibility, both cost-effective and without model-dependence, is to put a well-calibrated (~0.1 percent) light source on a satellite, such as the TSAT defense communications satellites scheduled for launch in 2012-2014 (to then be observed by LSST, SNAP, etc.). This project also has benefits for the defense and the atmospheric science communities.

Please contact Justin if you are interested in working on STARCaL. STARCaL is led by Justin and Susana Deustua (AAS), and collaborates closely with groups at Harvard, Hawaii, LBNL, LLNL, NOAO/Arizona, SLAC/Stanford/KIPAC, and UC Davis, as well as Air Force Research Laboratories and other Department of Defense facilities at the Los Angeles AFB and Kirtland AFB, etc.

The Experiment

We have the opportunity for a very limited number of new graduate students to work on Minos/Nova.
Please see this Powerpoint presentation and/or contact Prof. Harvey Newman (newman@hep.caltech.edu) for details.

The NOvA experiment is primarily intended to measure 3 things: the neutrino sector mixing angle theta_13 (which so far only has upper limits on its magnitude), CP violation in neutrino oscillations, and the "mass hierarchy" of the neutrinos. NOvA could also improve on MINOS constraints on the "atmospheric" mixing angle theta_23. NOvA will be a large detector located in rural northern Minnesota about 15 milliradians "off-axis" of the NuMI beamline presently used by MINOS, using liquid scintillator as the active element and wood particleboard as an absorber. The use of the off-axis neutrinos (when combined with a larger, more massive detector) allows better constraints on the mixing angles, and on the potentially non-zero CP-violating phase, than a detector located right in the center of the neutrino beamline. A detection of CP violation in the neutrino system would be a major milestone that would potentially have a large impact on our understanding of the matter-antimatter asymmetry of the universe.

Caltech people involved with NOvA are: Caius Howcroft, Harvey Newman, Jason Trevor, Hai Zheng.

Some Related Ventures

The Caltech LIGO gravitational wave group counts HEP group faculty Alan Weinstein and Barry Barish (the former director of LIGO [until 2005], now the director of the ILC GDE above) amongst its members, and thus works very closely with the HEP group.

The Cryogenic Dark Matter Search (CDMS) experiment is a germanium and silicon crystal detector to search for weakly interacting massive particles (WIMPs) that may form the bulk of the dark matter in the universe. Caltech faculty member Sunil Golwala leads the Caltech CDMS group. CDMS shares the Soudan Mine cavern with the Minos detector (and plans to move to Canada to inhabit the deeper SNO mine after an upgrade later this decade).

We also work very closely with the nuclear, particle theory, and astrophysics groups, etc., at Caltech (and elsewhere).