“The "Split Supersymmetry" framework emerges as a most economical and compelling option from an analysis of the constraints set by the two evidences above on the physics across the energy frontier.”

Understanding the fundamental laws of nature at energy scales across the present frontier of accelerator experiments is one of the most intriguing problems in particle physics. Our paper uses a new approach to the theoretical investigation of this issue. While the previous investigations used the Higgs mass hierarchy problem as the main (theoretical) guideline, we concentrate instead on established experimental evidences. In our approach, the hierarchy problem is assumed not to be directly relevant to the physics across the energy frontier.

As mentioned above, our method consists in using two important experimental evidences as guidelines for the theoretical investigation of the physics across the energy frontier. The first one is the increasing evidence that almost 1/3 of our universe is made of "dark matter," a mysterious and probably new form of matter that cannot be directly observed because it is not luminous. The second evidence we use is that three out of the four known fundamental forces unify. “Split supersymmetry” emerges from an analysis of the constraints set by the two evidences above.

 The results of our approach reinforce the previous results by Nima Arkani-Hamed and Savas Dimopoulos in hep-th/0405159 [1]. Our approach assumes that the Higgs mass hierarchy problem is solved by a principle that has no direct consequences on the physics probed by accelerator experiments. This is analogous to what appears to be the case for another hierarchy problem, the one associated to the cosmological constant. In fact, the two problems might even have a common "anthropic" solution, which motivated the approach described in [1]. Such a possibility leads to another drastic change of perspective and methodology in our approach to fundamental problems, with the emphasis shifting from dynamical to statistical principles. It also makes a connection with recent developments in string theory.

Split supersymmetry is a motivated guess on what we will find in a world yet to be discovered. Particle physics aims at understanding the laws of physics underlying the structure of our universe at a more and more fundamental level. Our deepest understanding is at present summarized by an extremely successful synthesis called the "standard model," which appears to hold, even down to the smallest subatomic world we have ever probed. However, there are compelling reasons to believe that we are missing an even deeper level of understanding, associated with physics hiding at even smaller scales. Theoretical speculations on the new world waiting for us at those scales have been going on for decades. However, the ultimate answer in science comes from the experiments to be undertaken—in our case from those ones that will be performed at the Large Hadron Collider (LHC) at CERN. Like a giant, incredibly powerful microscope, this impressive machine will be able, in a few years, to explore the world of smaller scales. As mentioned, split supersymmetry is a theoretical guess on what is waiting for us in that world. The guess is based on some hints that nature has offered us. Such hints, unlike the ones pursued by previous speculations, are entirely based on the results of experiments. More specifically, our guess is that the particles living in that world play a major role in two of the most fascinating puzzles of fundamental physics. The first one is the nature of "dark matter," a mysterious form of non-luminous matter that appears to constitute most of the matter in the universe. The second one is the existence of a unifying principle accounting for at least three of the four basic forces of nature.

Previous speculations had mostly been based on another important, purely theoretical hint: the need for a solution of the so called "hierarchy problem." This problem is related to the fact that the ultimate understanding of gravity, the force arising between masses, seems to be associated at scales that are by far (hierarchically) smaller than the scales at which the known particles get a mass. This seems unnatural from a theoretical point of view. In the context of split supersymmetry, this puzzle is not directly relevant to the world that will be explored by the LHC. On the contrary, its solution might require a drastic change of perspective, in which the hierarchy is as accidental as the fact that we live on a planet which satisfies the highly non-trivial conditions necessary for life to develop.

At the beginning of 2004, together with my colleague Gian Giudice, we considered the possibility of putting aside the paradigm that had dominated the theoretical speculations on the nature of the physics beyond the "standard model" in the last couple of decades. The idea was to study the constraints on new physics following uniquely from the requirement of a successful gauge coupling unification. We immediately started investigating this possibility and came up with a few interesting results. However, it was only after the seminal paper of Arkani-Hamed and Dimopoulos in May, 2005, that we started systematically pursuing our approach and obtained the main results of our paper, which turned out to reinforce and provide independent motivations for their work. We then joined forces to perform a detailed study of some theoretical and phenomenological aspects of the split-supersymmetry scenario.

The past has taught us that it is always difficult to imagine the consequences of scientific research on the laws of nature at their most fundamental level. Surely, the effort that will lead to the experimental investigation of the new world on which the theoretical community is speculating is having a crucial impact on the development of new technologies, with clear consequences to the economy and everyday life. It suffices to note the progress in computing infrastructure which, after the clamorous World-Wide Web revolution, now promises to lead to important advances in the "grid" technology. Or to consider the progress made in superconducting magnets and criogenics, which will lead to the creation of the coldest massive object in the universe (the LHC).

Andrea Romanino
Associate Professor
SISSA/ISAS (International School for Advanced Studies)
Trieste, Italy

Related Links:
•	High Energy Physics - Theory, abstract hep-th/0405159
•	http://www.sissa.it
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