Friday, January 11, 2013

Looming dark matter announcements

The LHC is enjoying its well-deserved two years' vacation. When it re-emerges in late 2014 or early 2015, it will be stronger than ever. The collisions will boast the center-of-mass energy of $$13\TeV$$, well above $$8\TeV$$ of 2012, so certain discoveries that were virtually impossible in 2012 could become trivial in 2015. Meanwhile, we're waiting for lots of papers evaluating the 2012 collisions. Most of the articles should be out by March 2013.

Will there be some interesting experiments running in 2013? You bet. The Nova neutrino experiment will look for CP-violation in the neutrino sectors. But most importantly, we shouldn't forget about dark matter experiments. Recall that two months ago, I mentioned that some important insights about dark matter could very well be around the corner.

To make the story short and to include some suggestive rumors I have received, AMS-02 – a cutting-edge spectrometer attached to the International Space Station that may check the previous pro-dark-matter claims by PAMELA in a more reliable way – is likely to publish its results by the end of March 2013 and something newsworthy (but not necessarily the first thing you may guess) should explode out of these results.

Hess is a gamma-ray telescope in Namibia that has entered stage II and may express its opinions about the $$130/135\GeV$$ gamma-ray line identified by Christoph Weniger in the Fermi data and repeatedly discussed on this blog. The rumors available to me say that there won't be any signal at the right place.

Meanwhile, LUX in South Dakota could announce its results on the dark matter direct searches in Spring 2013 – if not Winter 2012/2013. The speed of that experiment seems impressive, too.

Two days ago, astronomer William Dawson and others presented some evidence that there exists a new force (I suppose they mean a long-range force) that only affects dark matter. A claimed new long-range force is pretty extraordinary, I would say, but I can't kill all versions of this claim.

I want to mention two today's preprints on potential experimental anomalies and their explanations. D0 of the Tevatron claims that the ratio of cross sections$\frac{\sigma(p\bar p \to Z+b\text{ jet})}{\sigma(p\bar p \to Z+\text{jet})}$ (i.e. the percentage of b-tagged events with a jet and a Z-boson as a function of angles and rapidities) they measured in their whole dataset seems to disagree with all Monte Carlo event generators they have tried. It's up to Monte Carlo experts to decide whether it's a serious discrepancy and I am not such an expert.

Two Years' Vacation, theme song (CZ).

Also, Kang, Liu, Ning, three Chinese authors, discuss supersymmetric models with an extra Higgs triplet that contain doubly-charged scalars and doubly-charged fermions (the charginos, their superpartners). They speak about the masses of the new particles near and/or below $$100\GeV$$ a lot. It's hard for me to get rid of the suspicion that the content of their paper was shaped by the Christmas rumor on ATLAS dimuons. Note that their doubly-charged charginos may decay two two like-sign W-bosons and the neutralino and the W-bosons may decay to muons – so their doubly-charged chargino decays could potentially agree with the rumored bump at $$105\GeV$$. Of course, one should check many more things than the type of the decay products and the overall energy. ;-)

1. How confident are physicists that a massive dark matter particle produced plentifully in the big bang soup should have weak interactions?

2. Well, there's no objective way to quantify this conviction. There are models that make lots of sense, especially supersymmetric models where the dark matter particles often have this property, but there are also models in which the dark matter is interacting only by even more hidden interactions etc., including supersymmetric ones (e.g. if dark matter is gravitinos, they only interact gravitationally).

We don't want the dark matter to interact via electromagnetism, if it did, it wouldn't be dark.

Everything interacts gravitationally and we surely want this from dark matter as well, that's why it was introduced to physics.

An alternative possibility is that dark matter is actually not dark but radiates much less because it's made of heavy proton atoms with the nucleus replaced by a much heavier particle than proton.

There are many possibilities. There's no way to "calculate" which one is more likely. It depends on the priors for the hypotheses.

3. The expression is "around the corner".

4. Hi Lubos,

How can you have a rumor about what HESS-2 will see with regards to the line, given that they haven't started taking data at the galactic center yet? The declination of Sgr A* is ~ -29 degrees. The HESS latitude is -23 degrees, and so on in early January, there should be about a 30 degree zenith angle for Sgr A* for the HESS instrument. From HESS's website (old configuration of course), the low energy effective area falls off precipitously for higher zenith angles. ( http://bit.ly/VQiQrS ) - so this would be a terrible time to observe the GC -- especially at the very lowest energies where the machine needs to be sensitive to the 130 GeV Line.

If you look at all HESS observations of the galactic center (plotted in Figure 4 here), they always take place between about April 20 and August 20.