Hints of a light stop squark at ATLAS?

Reading some rumors tonight about a 2.4 sigma excess from ATLAS in the 330-500 GeV range. This hints at a possible stop squark (the top quark superpartner) which would, if confirmed, be the first of the long awaited sparticles (long awaited by me in any case). My understanding is that ATLAS will get 4-5X more data by the end of year, so if this tantilizing excess is real, we should have better stats. 3 sigma would be a suggestion, but 5 sigma is the particle physics standard for a discovery.

Supersymmetry, or SUSY, is necessary for String Theory. The Universe can be supersymmetric without String Theory being true, but if String Theory is true, the Universe is supersymmetric. So a lot of people are hoping we find a sparticle soon.

Sparticles are the superpartners of the ‘ordinary’ Standard Model particles; the sparticles of bosons (force carriers) are fermions (matter constituents) and vice versa. The naming convention for sparticles is weird. Quarks and leptions, being fermions, will have bosonic superpartners, squarks and sleptions (the top quark’s superpartner is the stop squark, the electron has the selectron). Photons, being bosonic, will have fermionic photinos as superpartners (gluons’ superpartners are gluions, and so forth). For various reasons, the stop squark is expected to be one of the lightest sparticles, and so one of the first that accelerators can produce.

Dark Matter, Dwarf Galaxies, and the current standard model of cosmology.

The current standard model of cosmology (SMoC) requires The Dual Dwarf Galaxy Theorem to be true according to which two types of dwarf galaxies must exist: primordial dark-matter (DM) dominated (type A) dwarf galaxies, and tidal-dwarf and ram-pressure-dwarf (type B) galaxies void of DM. Type A dwarfs surround the host approximately spherically, while type B dwarfs are typically correlated in phase-space. Type B dwarfs must exist in any cosmological theory in which galaxies interact. Only one type of dwarf galaxy is observed to exist on the baryonic Tully-Fisher plot and in the radius-mass plane.

Pavel Kroupa remains unconvinced that Dark Matter exists. He cites observational evidence from dwarf galaxies. His Dual Dwarf Galaxy Theorem basically says that we should see two kinds of dwarf galaxies: Dark Matter dominated which formed in the early universe, and non-Dark Matter dominated dwarf galaxies that formed from tidal or ram pressure interactions later in time. Since Dark Matter doesn’t respond to electromagnetic forces, galaxies passing through each other will only affect Dark Matter via gravitational force, not via interactions with normal, visible matter. Hence the dwarfs formed from matter interactions–the ram pressure type–will be deficient in Dark Matter. It isn’t clear to me why dwarfs formed by tidal interactions would not include Dark Matter. After all, Dark Matter does respond to gravity, and tidal forces are differential gravitational forces. Anyway, his conclusion is that we don’t see two classes of dwarf galaxies, we know for a fact that ram-pressure and tidally formed dwarf galaxies exist, so there are no Dark Matter dominated primordial dwarfs, hence there is no Dark Matter.

I haven’t thought this through enough to do anything but observe that 1) I don’t understand why tidal dwarfs would be Dark Matter deficient, 2) I don’t understand how he distinguishes between primordial dwarfs and dwarfs formed by later galactic interactions. I’m simply observing that there are objections to the Dark Matter explanations, and that Kroupa has identified a potential for dwarf galaxies to be a Dark Matter probe. I think it is a sound and clever idea, and Dark Matter proponents need to have an explanation for this.

I’ll end by noting that if Kroupa is correct, and dwarf galaxies offer evidence against the existence of Dark Matter, we will need to revise our understanding of General Relativity and spacetime. Something is causing the appearance of excess mass on galactic scales, and that something could well be departures from pure GR at astronomical distances. This proposal isn’t new, but there has been little evidence in favor of it (or proof that it must be wrong) due to the difficulty in doing experiments where the distances involved are galactic scale and up.

Particle physicists haven’t produced any DM candidates to date, but we haven’t looked in the right energy ranges for all that long. Discovery of a DM candidate particle, say the lightest supersymmetric matter partner, would really help resolve the doubts about the existence of DM.

Dark Matter in a galactic cluster detected via gravitational lensing.


The gravity of massive objects such as galaxy clusters acts as a lens to bend and distort the light from more distant objects as it passes. Dietrich’s team observed tens of thousands of galaxies beyond the supercluster. They were able to determine the extent to which the supercluster distorted galaxies, and with that information, they could plot the gravitational field and the mass of the Abell 222 and 223 clusters. Seeing this for the first time was “exhilarating,” Dietrich said.

More evidence, albeit indirect, for Dark Matter. Something is causing ‘excess’ gravity in our Universe, but that something doesn’t seem to respond to the electromagnetic force, so it isn’t directly visible (it doesn’t interact with photons except via gravity) and it emits no light (no emission of photons). I really need to sit down an do a review of Dark Matter evidence to date. Even back in my grad student days, there were puzzling results from studies of galactic rotation. Galaxies rotated as if there was a lot more mass present than what we could see from visible light.