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This posting is regarding the recent confirmation of the DAMA results that might be due to underlying differences in proton and neutron cross section with the dark matter particles, which reflect on the differences between Xenon and Germanium detectors

in the early days of particle physics, neutrinos were discovered as missing momentum in decay events

my question is the following: How did the current accelerators missed a dark matter product of 7GeV? possible answers i think:

1) does by any chance theory predict that these relatively low energy particles will not be created with normal particle collisions in our current multi-TeV range?

2) does the current complexity of analysis of TeV-magnitude collision products just cannot detect such missing momenta signals?

3) there are no dark matter particles in such low energy range, otherwise we would have already spotted them in particle accelerators


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Just because the particle has a rest mass that is small relative to TeV energies doesn't mean that it has a large cross-section with ordinary matter at TeV energies. – Jerry Schirmer May 9 '11 at 15:34
Nice question, lurscher, and good answer, Jerry - but can you write some numbers how often one creates a photino etc. and what the cross sections would be for LEP for a model that explains it? And isn't a sufficient cross section required for the particle's abundance in the Universe to drop to the 23 percent of the dark energy we have today? – Luboš Motl May 9 '11 at 15:58
up vote 5 down vote accepted

If dark matter interacts only gravitationally, then the cross section for producing it in the e+e- machines is inherently too low to be detected. I am discussing e+e- machines because those are the ones that can give a closed enough system to be able to detect missing mass and energy cleanly.

The cross section at the Y ( about 10GeV in mass)is something like 10^-2 millibarn. Now the coupling constant in front of the calculations (squared) is the electromagnetic one, which is orders of magnitude larger than the gravitational one. This will affect to practically zero both the magnitude and the width of any reaction producing the hypothetical 7 Gev particle, either in some pair production, or associated production.

There was some talk of finding more positrons than electrons associated with the measurements reported. In that case there exists a coupling between electromagnetic fields and these proposed particles, but a specific model would be needed to say at what level the production of these would be excluded by the existing world data from e+e- machines.

There are limits given assuming super symmetry is the valid theory. See this ALEPH thesis which gives limits over 40 GeV .

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Dear Anna, good but if dark matter only interacts gravitationally, it won't have enough time since the Big Bang to dilute its density to the acceptable value we measure today, will it? There must be a large enough annihilation cross section for it, right? – Luboš Motl May 9 '11 at 16:12
Well, unless one has a TOE how can one assume how much existed at the beginning of the big bang with respect to normal mass? Maybe this is just the right density for the dilution due to the expansion after only gravitational interactions? – anna v May 9 '11 at 16:41
indeed, in principle we don't know the actual distribution of these 7 GeV particles in the universe, and we don't know how much of it actually accounts for what we usually refer to as 'dark matter' which is expected to be condensed around galaxy halos – lurscher May 9 '11 at 17:08
Dear Anna, you don't need a "theory of everything" to answer these questions, so you shouldn't be using a "theory of everything" in a derogatory way as an excuse that you haven't answered the key question so far. The density of the dark matter particles at their characteristic era of the Big Bang evolution is just given thermally and the later evolution of the density is dictated by the Big Bang solution and a few cross sections. – Luboš Motl May 10 '11 at 5:27
The baryons have annihilated against antibaryons, so that 1 billion and one of the former and 1 billion of the latter led to 1 baryon only. Similarly dark matter must be able to annihilate at a not-too-vastly-lower rate so that it's reduced to the current density - otherwise it could be predicted billions of times denser than what is observed. This is a nontrivial constraint - a lower bound on some cross sections - which is not based on accelerator experiments but it doesn't mean that it may be ignored. – Luboš Motl May 10 '11 at 5:30

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