Particle Physics History

“I cannot do physics alone, I have to have someone to talk to.”- George Stranahan

It started small: a few dozen physicists making their way to Aspen in the summer, with plenty of time to think and discuss but no formal program. It wasn’t a conference or a school; it was a new way for theoretical physicists from around the world to get together and interact. As described in Loyal (Randy) Durand’s companion memoir “The Early Years”, the work spaces were shared and spartan, and the main gathering places were a couple of picnic tables on the lawn and (eventually) the outdoor blackboard behind Stranahan Hall. Fifty years later, despite the addition of laptops and wireless internet, the blackboards and the picnic tables are still where the action is.

During the first years of the ACP, particle theorists were struggling to make sense of a zoo of dozens of new particles discovered during the previous decade. These strongly interacting unstable particles, collectively known as hadrons, made nuclear forces look like a mess compared to the elegant relativistic quantum field theory description of electromagnetism. A large fraction of particle theorists were pushing the notion that quantum field theory was not even the correct way to think about hadrons and their interactions, while a few visionaries led by Murray Gell-Mann pursued the idea that hadrons were made out of more fundamental particles, called quarks. By the mid 1970’s, a series of theoretical breakthroughs and key experiments left quantum field theory triumphant, the quark model vindicated, and ushered in a new era of explaining and predicting hadronic interactions from an elegant fundamental force called QCD.

Stephen L. Adler, photo by Bernice Durand

Many of the key players in this revolution used Aspen as a forum to work out their new ideas. While QCD was not invented in Aspen, Gell-Mann’s presence at the ACP was the catalyst for a series of insights showing that QCD was in fact both a viable and predictive theory of strong interactions. In their groundbreaking paper “Advantages of the color octet gluon picture”, Gell-Mann and Harald Fritzch thanked the ACP for making possible their discussions with Aspen contributors Steve Adler, Bill Bardeen, and Heinz Pagels. A few years earlier Adler spent the summer of 1968 at the ACP, writing (in long hand!) his famous paper on quantum anomalies. As Adler recalls:

“In early June I went to Aspen where I spent the summer working on a manuscript on the properties of the axial anomaly, which became the body of the final published version. Towards the end of my Aspen stay, Sidney Coleman arrived and told me about the related work of Bell and Jackiw. This conversation led to my writing the appendix to my anomaly paper, containing the application of the anomaly to explaining the rate of pi zero to two gamma decay. This calculation showed that a single triple of fractionally charged quarks leads to too small a decay rate, giving, as later interpreted by Bardeen, Fritzsch, and Gell-Mann, one of the first pieces of evidence for the ‘color’ hypothesis.”

In the summer of 1974, two young theorists named Tom Appelquist and David Politzer used their time at Aspen make the first detailed prediction from a perturbative calculation in QCD; the resulting Physical Review Letter thanked both the ACP and Aspen attendees Leonard Susskind and Ken Wilson. Wilson’s first Aspen appearance was in 1966, when he wrote an “exit report” at the end of his stay with the prophetic comment that he had spent his time “working on a better understanding of quantum field theory.” Wilson also acknowledged the ACP in famous papers written in the 1970s.

Heinz Pagels was an early convert to QCD and one of the most interactive of all Aspen visitors. Aspen workshops inspired his seminal papers on chiral perturbation theory (1975) and nonperturbative aspects of QCD (1976). Aspen workshops were the genesis of many insights about the confining nature of QCD, with Syd Meshkov, Bob Jaffe, Ken Johnson and others showing how QCD predicts not only confined states of quarks and gluons but also confined states of gluons alone, known as glueballs. Aspen has continued to be a hotbed for ideas related to QCD over the decades; for example in 1996 Zvi Bern and Lance Dixon spent their summer at the ACP building the foundations for a revolution in our ability to make predictions for perturbative QCD processes with complicated final states.

In parallel with the development of QCD, Aspen was also a hotbed for a completely different approach to hadronic physics known as string theory. Pioneers including Pierre Ramond and John Schwarz used their summers in Aspen as a crucible for foundational ideas that led to the development of superstrings and supersymmetry, as detailed in the companion memoirs on this site “Aspen and Supersymmetry” and “Aspen and String Theory”. Important string theory papers with Aspen origins continued through the 70’s, including work by Corrigan and Fairlie (1974), Bardeen, Bars, Hanson, Peccei (1975), and Sasha Polyakov (1979). By the end of the decade it was clear that the greater promise of string theory was not as a competitor to QCD, but rather, as suggested by John Schwarz and Joel Scherk, as the ultimate unified description of all known forces and particles.

“During the summer of 1984, Michael Green gave an outdoor seminar announcing his discovery, together with John Schwarz, of the cancellation of anomalies in string theory. I was returning to Princeton in the next day or so. Upon my return, I was asked over lunch “What’s the news from Aspen?” and I related what I had just heard. At the time it seemed to me to be technically clever but rather far removed from any of the ‘real’ physics I was thinking about. But Ed Witten grasped its significance immediately, and instantly started focusing on string theory.”- Laurence Yaffe

“In September of 1984 I had lunch with Andy Strominger, a friend from our student days at MIT. He had just returned from the Aspen Center for Physics, a place I had never visited. ‘So’, I said, ‘did anything interesting happen there?’ ‘Yes’, said Andy. ‘Let me put it this way: whatever it is that you are working on, drop it immediately and start learning string theory.’ I did, and the next summer was my first at Aspen.”- Joe Lykken

The Aspen Center for Physics has been a focal point for research and workshops on string theory ever since its invention by ACP contributors Pierre Ramond, John Schwarz, and other visionaries in the 1970s. While string theory was originally formulated as a competitor to QCD, discussions in Aspen crossed over to another strong intellectual theme of the 1970s and early 80s: the desire to formulate a unified theory of forces and matter. Gell-Mann co-organized a pioneering ACP workshop in 1974 on Unified Field Theory of Hadrons and Leptons, which in turn engendered the influential 1976 review by Gell-Mann, Ramond, and Dick Slansky that introduced a systematic framework for thinking about unified gauge theories and their experimental consequences. Slansky extended this work during his next several summers in Aspen, producing the classic 1980 article “Is the Proton Stable?” in Science, co-authored with Maurice Goldhaber and ACP contributor Paul Langacker. In 1982 Ramond co-organized another entire workshop devoted to Grand Unification, which included the by then burgeoning number of experimental results.

The superstring revolution of 1984 grew out of discussions at Aspen in the summers of 1983 and 84. The key anomaly cancelation mechanism of Green and Schwarz was discovered at the ACP, while Steve Shenker and Dan Friedan showed how to use conformal field theory first to understand critical pheneomena and then to understand quantum gravity. In short order, string theory became the dominant topic in particle theory, as many became convinced that strings provided the best hope for unifying gravity with the other forces. Nearly every summer a significant fraction of leading string theorists gathered at the ACP to extend the frontiers of string theory in new directions.

During the summer of 1989 Steve Shenker and Michael Douglas crystallized the idea of the matrix model to approach to strings, which for the next several years stimulated many of the most important advances. Stephen Hawking first visited Aspen in 1989, with three more visits to the ACP during the 90s. Hawking’s visits coincided with a growing push towards understanding nonperturbative aspects of string theory and its relation to black holes. Andy Strominger and Cumrun Vafa participated in ACP string workshops in both 1995 and 1996, and in between published their breakthrough paper deriving the black hole entropy formula from string theory.

The 1995 ACP summer workshop Nonperturbative Supersymmetric String and Gauge Theories was a major factor in the emerging “second string revolution” and discoveries revolving around dualities and branes. Science writer Gary Taubes documented the importance of this workshop in a news article for Science, in which ACP contributors Jeff Harvey, John Schwarz, Andy Strominger, Lenny Susskind, and Cumrun Vafa are quoted. Aspen played a major role in subsequent evolution of the subject, including the 1996 workshop Duality in Physics and Mathematics, where Ken Intriligator finished his paper with Nathan Seiberg on gauge theory dualities and mirror symmetry, while discussing related issues with Tom Banks, Brian Greene, Shamit Kachru, and Eva Silverstein.

The 1997 workshop Supersymmetry and Non-Perturbative Quantum Geometry featured Steve Shenker discussing his new matrix theory with Shamit Kachru and Greg Moore, while Juan Maldacena, Andy Strominger, and Edward Witten developed the ideas leading to their joint paper “Black hole entropy in M theory”. Maldacena has noted that several discussions that summer at Aspen—with Gary Horowitz on anti-de Sitter space, with Strominger on supersymmetry enhancement at black hole horizons, and with Ofer Aharony on conformal field theory—played important roles in his synthesis of ideas to discover AdS/CFT duality.

As the validity and importance of AdS/CFT duality was realized, Aspen played a key role in the challenging program of extending this duality to systems with less supersymmetry, as is more relevant to Nature. The 2000 ACP summer workshop on String Dualities and their Implications was especially lively and productive, with a large and diverse group of leading researchers that included Steve Giddings, Steve Gubser, Shamit Kachru, Igor Klebanov, Clifford Johnson, Juan Maldacena, David Morrison, Amanda Peet, Joe Polchinski, and Eva Silverstein.

Beautiful ideas developed in one area of physics often have unexpected applications in others. The AdS/CFT correspondence was initially discovered to understand quantum states of black holes in string theory, but more recently it has found potential application to a variety of systems, such as the quark-gluon plasma, hadron physics, quantum-phase transitions, and quantum fluids in condensed matter physics. Aspen is a natural venue for this kind of cross-disciplinary challenge, bringing together critical masses of experts from different fields in an informal environment, where a string theorist is likely to find herself sharing an office with a condensed matter physicist. As Eric D’Hoker notes “One aspect of the Center that I came to value tremendously was sitting in on seminars and discussions in fields different from my own, especially in condensed matter physics.” As further stimulus, the ACP hosted the 2009 summer workshop, String Duals of Finite Temperature Low-Dimensional Systems scheduling it to overlap with an AMO/condensed matter workshop, Quantum Simulation/Computation with Cold Atoms and Molecules, a subject where the AdS/CFT correspondence has potential applications. Several AMO/condensed matter physicists and string theorists began important dialogues during the workshop that continue to this day. For example, Allan Adams and Lincoln Carr, who met at the workshop for the first time, produced a special issue of the New Journal of Physics on gauge/gravity applications. In a similar vein, the workshop entitled Strong Dynamics beyond the Standard Model in 2010 covered applications of the AdS/CFT correspondence to QCD and unresolved issues in hadronic physics.

See Pierre Ramond’s essay, “Aspen on the Road to Supersymmetry”.

See Michael Green’s and John Schwarz’s essay, “The Early Years of String Theory at the ACP”.

The role of Aspen in fundamental advances of neutrino physics goes back at least to the summer of 1972, when Manny Paschos and Lincoln Wolfenstein developed the Paschos-Wolfenstein formula, summarizing their insights into using neutrino-nucleon scattering as a precision test of the new electroweak theory. Wolfenstein, an Aspen participant almost every summer since 1962, was even more productive in 1977 when he discovered the possibility of matter effects in neutrino oscillations, and in the summer of 1979 used his time at the ACP to clarify the role of neutrino oscillations in stellar collapse. During the same summer of 1979, Gell-Mann, Ramond and Slansky came to the ACP, revealing to the larger community the idea of the see-saw mechanism for neutrino masses (also developed independently by Minkowski and by Yanagida). These ideas are the foundation for almost every investigation of neutrino physics during the past three decades.

More recently Aspen continues to be an important venue for progress in neutrino physics. After the SuperKamiokande experiment confirmed the existence of neutrino oscillations and tiny neutrino masses, a 2000 ACP workshop Neutrinos with Mass assembled a working group of leading neutrino theorists and experimentalists to lay out the most promising future directions for neutrino physics. After rapid progress in the field, a similar Aspen workshop in 2007 entitled Neutrino Physics: Looking Forward brought together physicists working at both low and high energies to advance all aspects of neutrino physics, including interactions, oscillations, and masses. The overlaps between neutrinos and other cosmic frontier physics continue to be of great interest to the astrophysics community, and the Aspen summer workshops Neutrino Physics on Earth, In the Stars, and in the Cosmos (2009), and Fingerprints of the Early Universe (2009), provided a valuable bridge between the communities of particle physicists and astrophysics.

CP Violation and Flavor physics has a long and illustrious connection to the ACP. After CP violation was discovered in 1964 in the kaon system, theorists attempted to provide an explanation for the origin of CP violation. Some particle physicists advocated that CP violation was unique to the kaon system and would not be detected anywhere else. But all attempts to describe CP violation were only phenomenological in nature. Indeed, a realistic framework for CP-violating phenomena was not possible until the modern theory of electroweak interactions was established. The discovery of the b quark in 1977 was one of the turning points as it showed the way to the six quark model and the attendant Cabbibo-Kobayashi-Maskawa (CKM) mixing in the quark sector.

1983 was a banner year for Flavor physics at the ACP: Lincoln Wolfenstein proposed his parameterization of the mysterious hierarchies built into the CKM matrix, a parameterization which now bears his name. It was a modest proposal at the time, one ACP participant recalls, that was quickly published by Physical Review Letters, and has since amassed over 2000 citations and is recognized as one of the most famous papers in particle physics. In the same 1983 workshop, Michael Gronau began a long-term collaboration with Jonathan Rosner. Much of their joint work in subsequent years on the physics of B and D mesons was stimulated by discussions held at the ACP in six subsequent summer workshops between 1988 and 2011.

Gronau recollects 1990 as another critical year for heavy flavor physics at the ACP. With the B-factories on the horizon, theorists were looking for reliable techniques for extracting the angles of the unitarity triangle (called alpha, beta and gamma) which describe the CKM matrix. That summer marked the beginning of a famous collaboration with David London, which produced two of the most important techniques in the analysis of B and D decays (which were later employed so successfully at the B factories)–the isospin analysis in B decay to two pions and a method to measure gamma using the decay of B to DK. These techniques turned out to be major new developments in flavor physics. The isospin analysis provided the first demonstration of how to employ flavor symmetries as an alternative to direct calculations of QCD matrix elements. It provides for the most accurate experimental determination of alpha, and also finds applications in studies of various B and D decays at the B-factories. The Gronau-London analysis of B to DK decay yielded a very clean way to measure gamma based on tree level decays, and provides the most precise measurement of a flavor parameter. The ultimate benefits of this analysis are only now being realized by experimental analyses, as we move into the era of LHCb and the super-flavor factories.

With significant advances in flavor physics already initiated at the ACP, and two high-luminosity B factories (at SLAC and KEK) preparing to turn on in 1999, an ambitious ACP workshop in 1997 on lattice, heavy quarks and perturbative QCD focused on the impact of the strong interactions for the experimental and theoretical study of heavy quark phenomena. In particular, if one wishes to explore the fundamental underlying parameters of the theory (quark masses and mixing angles), one must be able to control both short-distance perturbative QCD effects and long-distance strong-interaction effects (best studied by lattice techniques). Only then, can one translate between observed phenomena (which involve mesons and baryons) and the fundamental constituents (quarks and gluons).

During the first decade of the 2000s, there was a flood of experimental data on the properties of charm, bottom and top quarks. A number of ACP summer workshops dedicated to flavor physics were held on a regular basis, which brought together experts on effective field theories, lattice gauge theory, rare decays, CP violation and model building to discuss the interpretation of these data and their meaning for physics beyond the Standard Model. Lattice gauge theory computations and further development of soft collinear effective field theory improved significantly the theoretical predictions of heavy quark systems and can be used to confront the high-precision samples of B-decays produced at the B-factories. Indeed, numerous important experimental results on B and D decays were announced for the first time at Aspen winter conferences in 2005, 2006, 2008 and 2009. The informal interactive format of Aspen provided a valuable venue for the discussions of experimental anomalies among theorists and experimentalists.

By the end of the 1970s, the structure of the neutral current predicted by the SU(2)xU(1) electroweak theory of Glashow, Weinberg and Salam was strongly supported by experiment. The discovery of the W and Z bosons at CERN a few years later cemented this theory, which together with QCD was dubbed the Standard Model of particle physics. Nevertheless, one aspect of the theory remained clouded in mystery: the origin of electroweak symmetry breaking. In Weinberg’s classic 1967 paper, a single complex doublet of scalar fields was employed to break the electroweak theory via the scalar self-interactions. A consequence of this hypothesis was the existence of a new elementary scalar state: the Higgs boson. Although the theory predicted the strength of the Higgs couplings to the particles of the Standard Model, the Higgs mass was a free parameter of the model. The experimental data provided very weak constraints on the size of the Higgs mass.

Meanwhile, a number of theorists began to consider other mechanisms for electroweak symmetry breaking. In contrast to the weakly-coupled Higgs sector, a new strongly-coupled sector of fermions and gauge bosons was posited. The condensation of fermion pairs could also be used to break the electroweak symmetry and reproduce many of the known features of the Standard Model. Technicolor was one of a number of examples of strong electroweak symmetry breaking. One of the important early papers on isospin breaking in technicolor by Pierre Sikivie, Lenny Susskind, Mikhail Voloshin and Valentin Zakharov was developed in part at Aspen in the summer of 1979. This paper presaged the challenges to technicolor models that would eventually result from precision electroweak data a decade later. So, the question was posed: what is the fundamental mechanism that breaks the electroweak symmetry?

This issue took on added urgency in 1989 with the start of the Stanford Linear Collider (SLC) at SLAC and the LEP collider at CERN. The first dedicated Aspen summer workshop on these matters took place in the same year, and in the 1990s workshops on electroweak symmetry breaking, often coupled with discussions of physics beyond the Standard Model, became regular occurrences on the ACP summer program schedule. The debate on the origins of electroweak symmetry pitted two camps against each other: those favoring weakly-coupled electroweak symmetry breaking (EWSB) dynamics and elementary Higgs fields vs. those supporting models of strongly-coupled EWSB dynamics.

Sekhar Chivukula and Liz Simmons recall the 1996 workshop on flavor and gauge hierarchy problems as being especially influential in communicating the importance of experimental searches for strong EWSB dynamics at the Tevatron (and eventually the LHC). The 1996 workshop attracted a significant number of particle experimental colleagues and the interactions at Aspen inspired a number of dedicated searches at the Tevatron for phenomena predicted by various theories of dynamical electroweak symmetry breaking. Such strongly-interacting gauge theories present a number of theoretical challenges, as suitable approximation schemes for computations are difficult to achieve. In this regard, lattice methods developed for QCD can provide a reliable framework for analyzing other strongly -coupled systems, and the 2010 ACP workshop on Strong Dynamics Beyond the Standard Model succeeded in bringing these two disparate communities together.

As precision electroweak data poured forth from SLC and LEP, along with the discovery of the top quark and a precise measurement of the W mass at the Tevatron collider, it soon became apparent that models of strongly-coupled EWSB dynamics were being significantly stressed. But, with no discovery of the Higgs boson forthcoming, creative theorists employed their model-building skills to create clever new models of electroweak symmetry breaking that could withstand the onslaught of precision electroweak data. Inspired by new extra-dimensional models of particle theory (and the mechanism of “deconstruction”), a new class of four-dimensional theories for electroweak symmetry breaking was proposed by Arkani-Hamed, Cohen and Georgi at Harvard. This immediately became a hot topic for discussion at the 2001 ACP workshop on Electroweak Symmetry breaking and TeV Scale Physics after LEP. These new approaches inspired the “little Higgs” model in 2002, and as a result of an initial collaboration by David E. Kaplan and Martin Schmaltz later that summer at the ACP, an elegant little Higgs model based on a simple group was constructed. Nevertheless, the precision electroweak data presented a challenge for this class of models. Hsin-Chia Cheng and Ian Low met the challenge by inventing T-parity as a way of enforcing the necessary amount of custodial symmetry to account for the observed electroweak rho-parameter. The final stages of their work were completed at an ACP workshop Theory and Phenomenology of Physics at the TeV Scale in the summer of 2003. Extra-dimensional models served also as an inspiration for the so-called Higgsless models of electroweak symmetry breaking, in which no Higgs-like state is present in the low-energy spectrum. One of the seminal papers on these models by Csaba Ssaki, Christophe Grojean, Luigi Pilo and John Terning is also a consequence of the 2003 ACP summer workshop.

Electroweak symmetry breaking still inspires new ideas and approaches, and was the central theme of an ACP summer workshop in 2005. But as the LHC era approached, attention returned to the weakly-coupled fundamental Higgs scalar and the LHC Standard Model Higgs search. At the 2011 ACP summer workshop entitled Year One of the LHC, excitement was running high as the first significant data on the LHC Higgs search was released. That initial data set provided intriguing hints that a Higgs signal would soon be revealed, and updates in 2011 from both LHC and Tevatron experiments provided even stronger hints of a Higgs boson with a mass around 125 GeV. The highly anticipated first analyses of the 2012 LHC data sample will be reported this July, and will be a central focus of the upcoming 2012 ACP summer program.

Even if the Higgs boson is discovered at the LHC with its properties as predicted by the Standard Model, there are compelling reasons to expect that new physics beyond the Standard Model must be present. Indeed, the gravitational force is not directly incorporated into the Standard Model of particle physics. This is not particularly troubling since the gravitational force is many orders of magnitude weaker than the strong and electroweak forces. However, the interaction strength depends on the energy scale at which it is probed. Thus, at the so-called Planck energy scale (thought to be sixteen orders of magnitude larger than the energies probed at the LHC), the gravitational interaction becomes comparable in strength to the strong and electroweak interactions, and would have to be incorporated into a more fundamental theory of particle interactions.

Additional arguments have been presented that suggest that the Standard Model must break down at energy scales much closer to the so-called Terascale (about a TeV in energy) probed at the LHC. For example, the existence of dark matter (which is inferred from astrophysical data) has no explanation within the Standard Model. One elegant explanation of the dark matter is a weakly-coupled thermal relic that arises in a theory of new physics beyond the Standard Model (BSM) with a mass in the range of about a hundred GeV to a few TeV. The three gauge couplings of the Standard Model when extrapolated to very high energies converge, but fail to meet as precisely as might be expected if the strong and electroweak forces have a unified origin. However, in certain theories of BSM physics with a new spectrum of particles in the mass range of order one TeV, the modification of the evolution of gauge couplings due to the new particles is just sufficient to provide for a unification of forces at high energies.

Finally, the existence of fundamental scalars in the Standard Model has been long viewed as a serious flaw, as the theory provides no natural mechanism for understanding why the scale of EWSB is seventeen orders of magnitude below the Planck scale that governs the gravitational interactions. For the past 35 years, theorists have endeavored to construct “natural” models of electroweak symmetry breaking, many invoking a new symmetry principle called supersymmetry. Quadratic sensitivity Higgs sector to the Planck scale could now be removed due to the cancelation of contributions of fermions and bosons that is exact if supersymmetry is unbroken. If supersymmetry is broken by mass terms no larger than a few TeV, then naturalness of the Higgs sector can be approximately preserved. A supersymmetric extension of the Standard Model (MSSM) predicts a rich phenomenology of new particles which again must be present in Terascale physics and observable at colliders.

With the motivation for new BSM physics at the Terascale described above, there was a big push by theorists to develop new models of BSM physics to address the deficiencies of the Standard Model. Numerous workshops focusing on different aspects of BSM physics became a regular feature of ACP summer programs starting in 1989. The 1994 ACP workshop on Supersymmetry from the TeV to the Planck Scale provided an opportunity for phenomenological and more formal theorists to interact and provide insights into their studies of BSM physics. This workshop contributed to a better appreciation among model builders of the latest developments in duality and non-perturbative techniques for supersymmetric gauge theories championed by Nathan Seiberg and his collaborators. The work by Nathan Seiberg and Ken Intrilligator on the phases of N=1 supersymmetric gauge theories was one of the notable achievements at this workshop.

At the 1996 ACP workshop on Flavor and Gauge Hierarchy Problems, the “more minimal” supersymmetric model of Andy Cohen, David Kaplan and Ann Nelson was developed and inspired numerous subsequent approaches in supersymmetry model building in which two of the squark generations were much heavier than the third. Sixteen years later, the absence of squark signals in the 7 TeV LHC data could be a consequence of such a structure. The 1998 workshop on Theoretical and Experimental Issues in Electroweak Dynamics revisited theories of electroweak symmetry breaking based on strong dynamics of new particles and interactions. Constraints on such approaches from precision electroweak measurements were considered and the prospects for discovery at future colliders were examined. One of the important results to emerge from this workshop was the discovery of a new constraint on strongly-coupled field theories by Tom Appelquist, Andy Cohen and Martin Schmaltz consisting of an inequality limiting the number of degrees of freedom in the infrared description of a theory relative to the number of underlying, ultraviolet degrees of freedom.

The 1999 workshop on the Phenomenology of Superparticles and Superbranes and the 2000 workshop on New Physics at the Weak Scale and Beyond focused in part on new theoretical approaches for TeV-scale physics inspired by recent developments of superstring theory. Anomaly-mediated supersymmetry breaking and extra-dimensional theories were all the rage and dominated the discussions among theorists. The puzzle of anomaly mediation in supergravity theories was resolved during this period, thanks in part to work by ACP contributors Jon Bagger, Takeo Moroi and Erich Poppitz. Extra-dimensional theories and brane worlds emerged as the primary theoretical focal point for the 2000 workshop, leading to important papers such as the work by Tony Gherghetta and Alex Pomarol on a warped supersymmetric standard model.

Models of warped extra dimensions were pioneered by Lisa Randall and Raman Sundrum in two seminal papers published in 1999. At a 1999 ACP workshop, Lisa Randall and Joe Lykken extended Randall’s new ideas to formulate a model in which the fifth dimension is infinite in extent, but gravity nevertheless appears four-dimensional in our universe, and the hierarchy of the TeV scale relative to the Planck scale is naturally generated. Sundrum describes eloquently the role that the ACP played in further developments of these ideas. He writes: “in the early 2000’s, soon after the original “Randall-Sundrum” (RS) model came out as a solution to the Hierarchy Problem, I was engaged in trying to turn it into a comprehensive framework for experimentally accessible particle physics. This was a very challenging program, trying to integrate our understanding of electroweak precision tests, grand unification, flavor physics, neutrinos, and so on. I was fortunate to have participated in an ACP summer program at this time, bringing me in contact with several of the world experts also engaged in aspects of this program. The Center gave us the opportunity to really test out, criticize and flesh out half-formed ideas, as well as to brain-storm on new possibilities and review developments in related topics. In particular, I would attribute three important benefits to me of this stay: (i) It greatly improved the technical substance of the paper that my collaborators and I later wrote on “RS1, Custodial Isospin and precision tests” (ii) Even more importantly, the deep interactions I had at Aspen sharpened for me the central issues that defined the next decade of my research in this direction. (iii) At Aspen, I had the first intense discussions of a recent ‘crazy idea’ by Gherghetta and Pomarol on the phenomenological viability and attractiveness of the strong-coupling mechanism of ‘accidental’ supersymmetry, rendered in RS form via AdS/CFT duality. While our discussions led to finding some technical objections, the idea resonated greatly with me, and I took home from Aspen the goal of working through the technical issues and adding substantively to the original idea.” Sundrum cites his 2009 paper, “SUSY Splits, but then Returns,” as the outcome of these discussions.

As the decade of the 2000s came to a close, the BSM activity went into high gear. Theorists were well aware that LHC data would soon provide serious experimental constraints for new theoretical extensions of the Standard Model. Here are a few of the notable accomplishments of that time period. The 2006 ACP summer workshop Particle Theory in Anticipation of the LHC brought together Csaba Csaki, Yuri Shirman and John Terning, whose paper on a simple model of low-scale direct gauge mediation attracted considerable attention. The same workshop gave rise to a productive collaboration of Kaustubh Agashe and Gilad Perez who then co-authored with other theorists a well-cited paper on LHC signals from warped extra dimensions. The same collaboration met again the following summer in Aspen and added Tao Han, producing an influential follow-up study of warped electroweak neutral gauge boson signals at the LHC. In 2008, Roni Harnik and Graham Kribs wrote an important paper on the theory of Dirac dark matter that was initiated at the ACP summer workshop LHC: Beyond the Standard Model Signals in a QCD Environment.

The exploration of the Terascale began with the Tevatron and the LEP and SLC colliders in the 1990s. An upgraded Tevatron came online in 2001 and ran for ten years. However a comprehensive exploration of the Terascale would require the services of an even more powerful machine: the Large Hadron Collider. During the 1980s, particle theory at the ACP was dominated by string theory and more formal particle theorists. That began to change in the 1990s, as phenomenologically-oriented particle physicists began detailed studies of the implications of the various BSM theories for present and future colliders. During the 1980s, experimental and theoretical particle physicists collaborated in large workshops held at Snowmass every two years to plan for a supercollider that would explore the Terascale. The Snowmass workshops, which differ greatly in style from the ACP summer workshops, attract hundreds of particle, detector and accelerator physicists who collaborate on a wide range of issues associated with future colliders. When the US supercollider was canceled in 1993, momentum began to build for the LHC at CERN. Meanwhile, with fewer Snowmass meetings, phenomenologically-oriented particle physicists began to take more of an interest in ACP summer workshops, which provided unique opportunities for collaborations between experts in phenomenology and more formal areas like string theory and BSM model-building.

In 1994, a ground-breaking ACP summer workshop entitled Supersymmetry from TeV to the Planck Scale was held, which attracted both formal and phenomenological theorists along with a number of experimental particle physicists from the Tevatron. This workshop led to numerous collaborations across particle theory subfields. Phenomenologist Howard Baer and Tevatron experimentalist Andrew White co-led an effort to study BSM signals in future Tevatron searches. Their work helped to jump start the D0 Collaboration supersymmetry search team. In 1996, the Division of Particles and Fields (DPF) organized another Snowmass meeting. This time, the ACP was prepared and the 1996 ACP workshop on Flavor and Gauge Hierarchy Problems was scheduled to overlap with the Snowmass meeting. Many of the ACP workshop participants were able to join the Snowmass effort with contributed talks and Snowmass working group projects.

The 1997 workshop on New Physics at LEP2 and the Tevatron centered on the results of the completed Run 1 at the Tevatron collider and speculated on possible phenomena that could be discovered at LEP2 at its maximal energy. Here, the main focus was on Terascale physics that could be probed at these two facilities. This workshop provided an opportunity for theorists to interact with a number of experimental colleagues and gain insights into experimental methods for searching for new physics phenomena at colliders. The participation of Tevatron experimentalists Andrew White, John Womersley and Henry Frisch was particularly useful in this regard. One notable project initiated at this workshop by Marcela Carena, Stephen Mrenna and Carlos Wagner explored the discovery reach for Higgs bosons of the minimal supersymmetric model at the upgraded Tevatron collider.

The DPF-sponsored Snowmass meeting returned in 2001, and the ACP summer workshop on Electroweak Symmetry Breaking and TeV Scale Physics after LEP was organized to ensure a large overlap of interest between the two workshops. Participants traveled back and forth between Snowmass and Aspen. The ACP particle physics workshop enlarged the scope of the Snowmass activities and added physicists from related disciplines. This was repeated again in 2005, when a DPF-sponsored Snowmass meeting overlapped with the ACP summer workshop on New Approaches to Electroweak Symmetry Breaking.

During the past few years, the ACP summer particle theory workshop has focused on various aspects of LHC physics. At the 2009 ACP summer meeting Beyond the Standard Model: Physics at the Threshold, a collaboration of Martin Schmaltz and Jesse Thaler led to an influential paper with three other authors entitled “Supermodels for Early LHC.” This work helped spark a significant effort among particle theorists to examine LHC discovery scenarios that could be relevant for the first two years of LHC running, and led many particle theorists to educate themselves about the intricate and difficult issues faced by the particle experimentalists in their search for new BSM physics. The most recent ACP summer workshops on LHC physics in 2010 and 2011 have featured more participation from our experimental colleagues than ever before, and the back-and-forth discussions between theorists and experimentalists are having a noticeable impact on the rapid progress of collider physics at the energy frontier.

When planning for the new National Accelerator Laboratory (now known as Fermilab) began in 1968, founding director Robert Wilson saw Aspen as the ideal retreat to attract physicists from around the country for summer studies planning the physics program of the new laboratory. Wilson even convinced the Atomic Energy Commission to foot the bill. This led to the construction of a new building – Hilbert Hall – tripling the then existing office space for physicists. George Stranahan backed the construction; costs were repaid by use fees from NAL and other groups over several years. After successful summer studies in 1968 and 69 led to more than a hundred research papers and ninety experimental proposals from prospective NAL users, the first Program Advisory Committee (later renamed Physics Advisory Committee, or PAC) met in Aspen in the summer of 1970. The PAC met at the Aspen Center in 1971-1974, and has continued to meet in Aspen on a quasi-regular basis to the present day. The crossover between particle theorists on the PAC who were also ACP contributors (beginning with Murray Gell-Mann in 1970) gave more physics focus and scope to all of the Fermilab planning exercises. The Aspen-Fermilab connection also benefited the governance of the ACP, with Robert Wilson serving as a Trustee of the Center for the period 1969-75, and as Chair of the Board for 1986-89, succeeded by Leon Lederman, the second director of Fermilab and Nobel laureate, for 1989-92.

One of the early ACP contributors with a connection to Fermilab was Ben Lee, who became head of the Fermilab Theory Group in 1973 and a trustee of the ACP in 1976. During these years his research output was extraordinary. Having already completed his foundational work on spontaneously broken gauge theories, he developed with Chris Quigg and Hank Thacker the “no-lose” theorem for the Higgs boson that later provided the most compelling argument for building the Large Hadron Collider. In this same work they noted in passing the possibility of a “golden mode” for Higgs boson decays – the same four-lepton signature being used now at ATLAS and CMS for Higgs discovery. With Mary K. Gaillard and Jon Rosner, Lee produced a comprehensive guide to experimental charm physics shortly before the discovery of the first charmed meson in 1974, and in 1977 Lee and Steven Weinberg wrote what we now recognize as the foundational paper on the relic abundance of WIMP dark matter.

For the summer of 1977 Ben Lee planned to combine the PAC meeting with an eight-week stay at the ACP. Driving from Chicago to Aspen with his family, he was killed in a car crash on June 16, 1977. Today a plaque in his memory has a central place in the green space next to Stranahan Hall, where physicists gather to discuss and debate the newest ideas.

Heinz Pagels, one of the scientific pillars of the ACP, died in a tragic climbing accident near Aspen in July, 1988. Jeremy Bernstein’s memoir describes how the Aspen community of physicists responded:

“At the Center, we debated as to the best way to honor his memory. We finally decided that given his interests in the public understanding of science the most appropriate thing would be to name a lecture series in his honor. The first lecturer in the series, which was on July 5, 1989, was Stephen Hawking. To say it was a success grossly understates the case. We could not fit the people into Paepcke Auditorium. It was filmed and made available on television. Hawking’s lecture set a standard of public interest that was impossible to maintain. But we tried. The lectures played another role. Compared to the Institute and the music people, we kept a pretty low profile. All anyone saw were a few physics professors riding on bicycles or hiking in the mountains. Public lectures gave us visibility.” – Jeremy Bernstein

In our companion history of ACP astrophysics, Craig Wheeler notes that astrophysics was entangled in the DNA of the ACP from the beginning. In the same way, the DNA of astrophysics is entangled with that of particle physics, in ways that we still only dimly understand despite decades of research. No wonder then that particle physicists like Hans Bethe, Curt Callan, Dan Freedman, Joel Primack, and Leonard Susskind appear in the highlights of early astrophysics and cosmology in Aspen. By the middle years of the ACP, the rate of intellectual intermarriage had increased to the point that the distinction between particle physicists and astrophysicists began to blur, starting with innovators like David Schramm, Michael Turner, and Rocky Kolb, and extending far and wide to include leading Aspen contributors like Andy Albrecht, Katie Freese, Josh Frieman, Marc Kamionkowski, Lawrence Krauss, Pierre Sikivie, and Neil Turok.

As a result, the ACP can boast one of the longest and proudest histories of two-way interactions between these fields. Today the younger generation of string theorists use their time in Aspen to learn about inflation, and budding theorists in physics beyond the Standard Model come to the ACP to find out the latest on dark matter searches.

How we do particle physics at the ACP has changed over this half century. Some changes are clearly for the better: more women and participants from under-represented groups, more young people, more diversity of topics and cross-disciplinary overlaps. Some changes are a mixed blessing: shorter stays, a faster pace, laptops. What remains unchanged is the basic dynamic that makes the ACP a unique experience for researchers from all over the world.

The future? Part of the fascination of particle physics is that it is a discipline that continually expands its intellectual horizons, reinventing itself as we gain more insight into Nature’s fundamental underpinnings. From particles to fields to symmetries, strings, branes, extra dimensions, condensates, dark sectors and multiverses, our theoretical imaginations will be continually tested by experimental discoveries on many fronts.

The ACP summer workshops provide a great opportunity for particle theorists to discuss the hottest topics of our field and catch up on some of the latest developments. But the particle (theorist) interactions are not solely relegated to the ACP campus. In the early days of the ACP, there was a long standing tradition of carrying on some of the lively physics discussions at the Pine Creek Cookhouse, which is situated 13 miles from the ACP up Castle Creek Road. The preferred method of transportation was the bicycle. That tradition eventually faded away, as workshops began to dominate the ACP summer programs.

However, the number of enthusiastic bicyclists among the particle physics community began to grow significantly in the 1990s and eventually a new tradition was established. Every summer, one of the particle theory workshops sponsors a lunch at the Pine Creek Cookhouse (although any ACP participant and their families are welcome to join). Even the bicycle-impaired are not left out, and a number of ACP physicists feel no shame in using a car to reach the destination.