# Desert (particle physics)

In particle physics, the desert is a hypothetical gap in fundamentally new physics. The gap extends, in theory, from the energy scale of weak interactions to the energy scale of grand unified theories, or, perhaps, the Planck scale.

## Quotes

* Since 1974, the guiding principle for building extensions of the standard model has been the so-called desert hypothesis. ... Its premise is that there is no new physics between the energy scales of electroweak unification (10$^9$ GeV or 1 TeV) and the vicinity of the Planck mass M$_{Pl}$ (10$^{19}$ GeV). That implies an enormous "desert," 16 orders of magnitude wide, where one would expect to encounter nothing new. M$_{Pl}$ is the mass at which a particle's Compton wavelength becomes equal to its Schwarzschild radius—a realm where one can't do without a quantum theory of gravity. ... The leading contender for the elaboration of the desert picture has been the supersymmetric extension of the standard model ...
** Nima Arkani-Hamed, Savas Dimopoulos, and Georgi Dvali, (2002). "Large extra dimensions: A new arena for particle physics". Physics Today 55 (2): 35-41. (quote from p. 35)
* Let me come finally to the question that is of more direct interest to the audience here: ‘what are the scales of string unification?’ or put more provocatively, ‘will we see strings and extra dimensions at the LHC?’ (this is also discussed in Peskin’s talk ...) The conventional (and conservative) hypothesis is that the string, compactification and Planck scales lie all to within two or three orders of magnitude from each other, and are hence far beyond direct experimental reach. The non-gravitational physics at lower energies is thus described by a 4d supersymmetric quantum field theory (SQFT), which must at least include in it the MSSM. This conventional hypothesis is supported by the following three solid facts : (i) Softly broken SQFTs can indeed be extrapolated consistently to near-Planckian energies without destabilizing the electroweak scale ; (ii) the hypothesis is (almost) automatic in the weakly-coupled heterotic string theory, and (iii) the minimal (or ‘desert’) string-unification assumption is in remarkable agreement with some of the measured low-energy parameters of our world.
** Constantin Petros Bachas, (2000). "String/M theory". arXiv:hep-ph/0003259v1. (quote from pp. 6–7)
* The most natural expectation away from asymptotic limits in moduli space of supergravity theories is the desert scenario, where there are few states between massless fields and the quantum gravity cutoff. In this paper we initiate a systematic study of these regions deep in the moduli space, and use it to place a bound on the number of massless modes by relating it to the black hole species problem. There exists a consistent sub-Planckian UV cutoff (the species scale) which resolves the black hole species problem without bounding the number of light modes. We reevaluate this in the context of supersymmetric string vacua in the desert region and show that even though heuristically the species scale is compatible with expectations, the BPS states of the actual string vacua lead to a stronger dependence of the cutoff scale on the number of massless modes. We propose that this discrepancy, which can be captured by the “BPS desert conjecture”, resurrects the idea of a uniform bound on the number of light modes as a way to avoid the black hole species problem. This conjecture also implies a stronger form of the Tadpole Conjecture, which leads to an obstruction in stabilizing all moduli semi-classically for large number of moduli in flux compactifications.
** Cody Long, Miguel Montero, Cumrun Vafa, and Irene Valenzuela, (2021). "The Desert and the Swampland". arXiv:2112.11467. (quote from abstract)

## External links

* Encyclopedic article on Desert (particle physics) on Wikipedia