The model tries to explain this problem by postulating that our universe, with its four dimensions (three spatial ones plus time), exists on a membrane in a higher dimensional space.
It is then suggested that the other forces of nature (the electromagnetic force, strong interaction, and weak interaction) operate within this membrane and its four dimensions, while the hypothetical gravity-bearing particle, the graviton, can propagate across the extra dimensions.
[clarification needed][1] The size of the dimensions in ADD is around the order of the TeV scale, which results in it being experimentally probeable by current colliders, unlike many exotic extra dimensional hypotheses that have the relevant size around the Planck scale.
[2] The model was proposed by Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali in 1998.
If a graviton were to be formed in the collision, it could propagate into the extra dimensions, resulting in an imbalance of transverse momentum.
In models of large extra dimensions, the fundamental scale is much lower than the Planck.
In the years after the publication of ADD, much of the work of the beyond the Standard Model physics community went to explore how these problems could be solved with a low scale of quantum gravity.
[22][23] Using extra dimensions as a new source of small numbers allowed for new mechanisms for understanding the masses and mixings of the neutrinos.
Physicists quickly realized that there were novel mechanisms for getting small numbers necessary for explaining these very rare processes.
[26][27][28][29][30] In the traditional view, the enormous gap in energy between the mass scales of ordinary particles and the Planck mass is reflected in the fact that virtual processes involving black holes or gravity are strongly suppressed.
The suppression of these terms is the principle of renormalizability – in order to see an interaction at low energy, it must have the property that its coupling only changes logarithmically as a function of the Planck scale.
A different way to suppress these interactions in the context of extra-dimensional models is the "split fermion scenario" proposed by Arkani-Hamed and Schmaltz in their paper "Hierarchies without Symmetries from Extra Dimensions".
[31] In this scenario, the wavefunctions of particles that are bound to the brane have a finite width significantly smaller than the extra-dimension, but the center (e.g. of a Gaussian wave packet) can be dislocated along the direction of the extra dimension in what is known as a "fat brane".
Integrating out the additional dimension(s) to obtain the effective coupling of higher-dimensional operators on the brane, the result is suppressed with the exponential of the square of the distance between the centers of the wave functions, a factor that generates a suppression by many orders of magnitude already by a dislocation of only a few times the typical width of the wave function.
In electromagnetism, the electron magnetic moment is described by perturbative processes derived in the QED Lagrangian: which is calculated and measured to one part in a trillion.
But it is also possible to include a Pauli term in the Lagrangian: and the magnetic moment would change by
QED is not the full theory, and the Standard Model does not have many possible Pauli terms.
This should start contributing to the electron magnetic moment at the sixth decimal place.
A similar term should contribute to the muon magnetic moment at the third or fourth decimal place.
The only reason to introduce a right-handed partner is to produce neutrino masses in a renormalizable GUT.
If the Planck scale is small so that renormalizability is no longer an issue, there are many neutrino mass terms which do not require extra particles.
Even with a relatively low energy pion scale, this type of interaction could conceivably give a mass to the neutrino of size
[citation needed] The popularity, or at least prominence, of these models may have been enhanced because they allow the possibility of black hole production at the LHC, which has attracted significant attention.
[5][6][7][8][9][10] In 2012, the Fermi/LAT collaboration published limits on the ADD model of Large Extra Dimensions from astrophysical observations of neutron stars.
, the results presented here imply that the compactification topology is more complicated than a torus, i.e., all large extra dimensions (LED) having the same size.
For flat LED of the same size, the lower limits on the unification scale results are consistent with n ≥ 4.
[32] The details of the analysis is as follows: A sample of 6 gamma-ray faint NS sources not reported in the first Fermi gamma-ray source catalog that are good candidates are selected for this analysis, based on age, surface magnetic field, distance, and galactic latitude.
Based on 11 months of data from Fermi-LAT, 95% CL upper limits on the size of extra dimensions
In addition, the limits from all of the analyzed NSs have been combined statistically using two likelihood-based methods.
The results indicate more stringent limits on LED than quoted previously from individual neutron star sources in gamma-rays.