History of High Energy Physics Experimental Research at Caltech


The experimental high energy physics program at Caltech began with the construction of the Caltech Electron Synchrotron, built and used by R. Bacher, M. Sands, R. Walker and A. Tollestrup. Now Caltech, like other university groups, performs its research through participation in large experiments on major facilities away from the campus.

The Caltech administration has traditionally given strong support to our high energy program, and within physics, activities in astrophysics and fundamental particle physics have had the highest priority. Institutional support has been given in the form of faculty appointments, special grants for developing or purchasing equipment, start-up funds for new faculty, funds for visitors, flexible teaching arrangements, special support for postdoctoral fellows, substantial support for computing, contributions to upgrades of our computing facilities, forward financing for equipment projects, etc.

Our HEP experimental program is made up of an active group of faculty, a strong post-doctoral research staff and very bright graduate students, in addition to a number of outstanding undergraduates engaged in research. Our experimental activities consist of a mix of productive on-going physics, plus significant effort toward future upgrades or new projects.

History of the Caltech Experimental Group

The development of experimental high energy physics at Caltech closely parallels that of the whole field of experimental particle physics. At the beginning, more than forty years ago, research at Caltech was concentrated primarily on the Caltech Electron Synchrotron. As our field developed, the need for ever higher energies required larger accelerators whose cost necessitated their placement in national laboratories and their use to be shared between many different university and laboratory groups. The experimental effort at Caltech, like that at other Universities, evolved into "user group activities" at the national facilities at Brookhaven, Fermilab, and SLAC. The Caltech synchrotron was turned off in the mid-60's and the remaining infrastructure, building, etc., have served us well as a technical center for staging experiments conducted elsewhere.

The Caltech (and indeed the entire U.S. high energy physics) effort has been extraordinarily successful during the subsequent period. The Caltech experimental group has played major roles in many important experiments since that time. In the 1970’s at Fermilab, the Caltech group led experiments on multiparticle production (Fox, Gomez and Pine) using the Fermilab Multiparticle Spectrometer which made the first observation of quark-quark scattering in hadronic processes. Charge exchange processes at high energy (an experiment by Tollestrup and Walker) showed clearly how the quark substructure of the proton becomes visible as you probe at shorter and shorter distances. And finally, the neutrino experiments of Barish and Sciulli made a particularly important impact on the other major theoretical development, i.e., the unified theory of the weak and electromagnetic interactions introduced by Glashow, Weinberg and Salam that has led to the Standard Model of particle physics. In the late 1970’s and 1980’s our efforts shifted to SLAC where results from the Crystal Ball (Peck and Porter) were extremely important in understanding radiative transition of states involving charmed quarks. DELCO (Barish) at SLAC and the Mark J (Newman) at DESY made important contributions to our understanding of production of heavy quarks. Newman at the Mark J also was a main contributor to the discovery of gluons in 1979, along with the observation of electroweak interference and the search for the top and Higgs particles at then-highest energy electron-positron collider PETRA. Finally, the Mark III at SLAC provided impressive quantitative results on both and D-physics in the rich SPEAR energy region.

During this time Newman also made major advances in computing, including the first three dimensional simulations for detectors at colliders, the first modern event generators, and the start of international networking for high energy physics.

From the mid-1980’s to the early 1990’s the Caltech program encompassed a wide range of major programs, both in the U.S. and overseas. The upgraded Mark II and SLD experiments at the SLAC Linear Collider (SLC), the CLEO experiment at Cornell’s CESR storage ring, the L3 program at the Large Electron Positron Collider at CERN, the MACRO detector at the Gran Sasso, and the Beijing Spectrometer (BES) at the Beijing Elecron Positron e+e- Collider (BEPC). We also led the plans for one of the major detectors planned for the SSC (GEM).

L3 at LEP (H. Newman, R. Mount, R. Zhu, G. Gratta, C. Tully)

L3 carried out precision electroweak tests, QCD tests and searches the Higgs, supersymmetry, excited leptons among many new particle searches. After accumulating 4 million events at the Z0 resonance during LEP Phase I in 1989 - 1995, L3 moved on to study W-pair production and to extend its Higgs and other searches to center of mass energies of 170 to 209 GeV. At the upper end of the energy range in 2000, L3 and the other LEP experiments were able to exclude a Standard Model Higgs up to 114 GeV. Caltech led one third of L3’s 300 publications and the same fraction of the major analysis groups throughout the L3 program including New Particles, Supersymmetry, tau-Decays, Hadron Physics and the W mass measurement, and was a major contributor to the group measuring the Z0 lineshape. Caltech also led the development and implementation of off-line computing, wide area networking, and the production of L3 simulated events on computers at L3 Collaboration sites throughout the U.S. and Europe. Caltech implemented the RFQ calibration system, which achieved sub-percent resolution in the BGO crystal calorimeter during LEP Phase II.

The work on photon signatures, radiation hard crystals, electromagnetic calorimeter calibration, Higgs searches, computing and networking during this period was the foundation of the many leading roles the group has in the CMS experiment as of today.

BES at Beijing (D. Hitlin, F. Porter)

Carrying on a tradition of involvement in e+e- experiments near tau and charm threshold (Mark II, Crystal Ball, Mark III), Hitlin and Porter are members of the joint US-China BES collaboration. A precise measurement of the mass of the tau lepton has been made, and now verified and improved further by studying additional modes. We expect to report preliminary results in charm on the pseudoscalar decay constant and absolute branching fractions in the near future. Data has been accumulated which will permit the study of a variety of topics in charmonium decays. Caltech is constructing a luminosity monitor as part of a machine/detector upgrade.

B Factory at SLAC (D. Hitlin, F. Porter, A. Weinstein)

Since 1988, this group has worked to bring the opportunity to study CP violation and rare processes in B decays to reality. The asymmetric e+e- PEP-II collider has been approved and is under construction, with physics scheduled to begin in 1999. Our attention is now devoted principally to the detector and physics aspects, with an active R&D program and an active role in the leadership of the newly-formed collaboration.

MACRO at Gran Sasso (B. Barish, D. Michael, C. Peck)

The MACRO experiment is the largest underground detector in the world, designed to search for rare particles in cosmic rays (monopoles, nuclearites, etc.), study downgoing muons of very high initial energy and therefore the interactions and composition of the primary cosmic rays that produced them, study upgoing muons induced by neutrino interactions and search for bursts of antineutrinos from gravitational collapse within the galaxy. During the last year, we have completed the construction of the detector and it is now in acquisition with the full scintillator system for searching for fast monopoles, muon and neutrino physics. We expect that searches for slow monopoles will begin near the end of the summer following installation of the final waveform digitizer system. During the next months, MACRO will make a transition to search for rare particles in cosmic rays (slow GUT monopoles, nuclearites, etc.), study downgoing muons of very high initial energy and therefore the interactions and composition of the primary cosmic rays that produced them, study upgoing muons induced by neutrino interactions and search for bursts of antineutrinos from gravitational collapse within the galaxy. A highlight of accomplishments in the past year, in addition to continued studies of cosmic ray penetrating muons has been our studies of upward muons from neutrino interactions. The se have provided limits for WIMP's and Astrophysical point sources and most importantly information on the atmospheric neutrinos anomaly.

CLEO (B. Barish, A. Weinstein)

Our CLEO activity has a special emphasis on the physics of tau-leptons. We have published results on tau neutrino mass, tau mass, multi-pi0 decays, five-prong decays (5pi(pi0)nu_tau), and rare decay modes (tau -> mu gamma). A precision measurement of the largest tau decay mode, to h- pi0 nu_tau, has been submitted for publication; this result drives the so-called "one-prong problem" in tau decays to insignificance. Results on tau decays containing K0_S mesons were presented at the Washington APS meeting; further results are in preparation. Results on the Lorentz structure of leptonic tau decays, aiming at the most precise measurements of the Michel Parameters, are in preparation.

We are also focusing on the semileptonic and rare decays of B mesons. Results on the semileptonic decay branching fractions using a novel method for background estimation are in preparation for publication, and a measurement of the vector form factor in the decay B -> D* l nu is in progress.

Our group is also leading the effort in the development and construction of a data acquisition system for the new silicon vertex detector, Permitting far greater precision in the study of primary charm and charm from B decays. We are getting more involved in the upgrade to CLEO III for higher luminosities. The CLEO experiment affords us a rich and productive physics program for years to come.