occasional meanderings in physics' brave new world

Name:
Location: New Zealand

Marni D. Sheppeard

Thursday, July 12, 2007

GRG18 Day 4

I am told that the wine was flowing liberally on the (expensive) harbour cruise last night. Three plenary sessions this morning: H. Ringstrom on mathematical results in Cosmic Censorship, Johnathon Feng on Collider Physics for cosmology and D. Shaddock from JPL on the current status of the LISA mission, which will know around September 8 if it is scheduled for first launch under Beyond Einstein.

Let's focus on Feng's talk, which was very, very entertaining and took a relatively balanced point of view on theoretical considerations, including for instance a flowchart of possible outcomes from comparisons between LHC analyses of Dark Matter and cosmological observations. They expect only a single year's worth of LHC data to determine the existence, or not, of standard SUSY wimps based on a single species analysis. However, note that the existence of SUSY may be supergravity, or otherwise, inspired, rather than a typical string scenario.

The talk began with some recent CERN-cam pretty pictures from the LHC, which Feng noted colour-matched the design layout beautifully. The LHC is expected to yield 10^7-10^9 top quarks per year, compared to 10^2-10^4 at the TEVATRON. The current schedule for first collisions is July 2008. Feng listed some very general, but unanswered, questions in particle physics and cosmology. In particular, he asked how $\Omega_{\Lambda}$ could be around 10^{120}, and then how $\Omega_{\Lambda}$ could be zero, and then he pointed out that it was a remarkable situation when one could ask both questions side by side and sort of be making sense. The focus shifted to Dark Matter, once he pointed out that there were no (well known) compelling solutions to the DE problem. In short, DM is gravitationally interacting, not short lived, and not hot: unambiguous evidence for new physics. He gave a long list of candidates (along with a suitable jibe at the theorists) starting with primordial black holes, and then ran through the unnaturalness argument for cancellation in quantum corrections to $m_{H}^{2}$ (1 part in 10^{34}), strongly suggesting new particles, perhaps of SUSY type (again, note, that SUSY is a general term here).

The 'WIMP miracle' is the fact that by assuming a single DM species, initially in thermal equilibrium, which interacts until it freezes out to a DM relic (let's call it $\chi$) such that $\Omega_{DM}$ depends on the annihilation cross section, one can show that $\Omega_{DM}$ goes as the mass $m$ of $\chi$ squared, and miraculously this suggests (from the observed DM density) that the preferred mass for $\chi$ is in the range 100GeV - 1 TeV of the LHC. Moreover, precision constraints from LEP indicate that new particles interact in pairs, as opposed to as single components in SM particle interactions. In short, the LHC REALLY ought to see new particles. The question is, what are they?

The problem with the neutral $\chi$ particle is that it appears as missing momentum from the 2 $\chi$ at the end of the decay chain. This is not an identification of Dark Matter for cosmological purposes, although it would certainly confirm SUSY if it was found. However, a full analysis of all possible processes for $\chi$ will allow a comparison between particle physics abundance and observed DM density. On that note, Feng briefly discussed the ILC proposal for a variable E beam electron-positron collider. With constraints from WMAP, the Planck mission (2010) and the LHC, the ILC would fix the relic density as a function of $m$, to a value around 96 GeV. That is, the ILC would allow a 1% comparison between collider and cosmology DM identification.

But the most interesting part of Feng's talk was a discussion at the end of an alternative SUSY scenario, based on work of Feng et al, in which the neutral $\chi$ (usually a neutralino) is replaced as a DM candidate by the gravitino, which itself appears in a decay chain from new massive charged particles with longish lifetimes, of the order of a month. These would be seen at the LHC in water detectors built outside the other detector chambers. Such DM is only gravitationally interacting. This possibility also satisfies the WIMP miracle phenomenon, and would allow a particle physics measurement of Newton's constant $G$. Needless to say, the cosmology is radically different. The 1 month lifetime modifies Big Bang nucleosynthesis, offering a possible solution to the Li-7 anomaly which is not always discussed in the WMAP literature. It also suggests a suppression of small scale structure, such as fewer galactic halo objects.

In summary, whatever happens (short of us all killing each other) GRG19 should revolutionise cosmology.

a quantum diaries survivor said...

Hi Kea, great report!

One thing, a pedantic detail - I think he was being optimistic with the 10^7-10^9 tops at LHC in a year. With a cross section of 800/pb, getting 10^7 of them tops means collecting 12.5/fb, which is something we will only reach in 2010, probably. 10^9 will never be
managed.

Cheers,
T.

July 12, 2007 10:03 PM
Kea said...

Hi Tommaso, thanks a lot!

July 13, 2007 9:35 AM