Random Thoughts on the Universe

Monday, May 17, 2010

Where is the Antimatter - UPDATE

The Fermilab today released some preliminary findings of a particle reaction that produces slightly more matter than antimatter, possible explaining why there is so much matter in the visible Universe but no antimatter.

In nature there are two types of massive particle, dubbed by scientists as matter and antimatter. When the two meet they destroy each other and convert their masses to pure energy. And in every experiment conducted thus far, they are produced and destroyed in exactly equal amounts - there is no known way to make matter without making antimatter.

Except if that were true in nature, than every star in every galaxy we see would have been destroyed a long time ago. (Unless of course there is an unknown mechanism for separating the two into different regions of the Universe, but this has never been seen either).

So where is all the antimatter?

What Fermilab appears to have observed is a reaction where very slightly more matter is produced than antimatter - it seems to be less than a 1% difference. But if that is true, then the Universe could have used the same process to generate the matter excess.

Of course those are very preliminary results and could very easily be an experimental error or anomaly. It will take a lot more data before we can be sure that they have discovered something interesting...

Saturday, May 08, 2010

More Conflict in the Universe

As I predicted, the debate over dark matter detection has already heated up. (See my last post for the background details)

The three experiments with positive dark matter signals (DAMA, CDMS-II, and CoGent) are standing by their results, and at least two of these groups have raised serious concerns about the XENON experiment. And probably with good reason.

The three prior experiments had long runs before publishing results, while XENON has run for approximately 11 days. They will run for longer in future, but in my opinion it is too early to publish - I would have completed at least one or two more runs before making the bold claim that the well studied and peer reviewed results of other groups are false. I have a feeling that with so much previous data stacked against XENON, their reputation could suffer for this. And that would be sad, because it is a very good experiment in principle.

I am not an experimentalist, and have never claimed to be an expert on these large scale detectors. However my suspicion is that XENON may have made two mistakes in their analysis.
The first is in the background exclusion. For the non-technical readers, all experiments have a background signal which contains false positives caused by anything from radiation in the surrounding soil to cosmic rays, and this must be removed in such a way as to not remove real dark matter signals. XENON is supposed to have a very low background signal, so it is possible that some form of exotic dark matter has been lost in their processing of the data. Although that is pure speculation on my part, and not based on any analysis yet completed.
The second possibility, and perhaps the stronger argument, is that XENON may not be as sensitive to certain types of dark matter. For several years there has been interest in low mass dark matter candidates which fit the data of DAMA and CDMS, and perhaps CoGent as well. As I understand the XENON analysis, they assume that their detector is as sensitive to low mass dark matter as it is to heavier particles - but this is not true of any previous experiment and so is unlikely at this one. In fact, all previous experiments are known to be insensitive to very light particles. The question is where this cutoff is for XENON. My feeling from reviewing the detector proposals is that it can detect light dark matter but at a suppressed rate - and that would explain the lack of a signal in the small data set analyzed.

With such a short experimental run of 11 days, it is even possible that the Earth was travelling through a low dark matter bubble at the time! Contrary to popular belief, dark matter is not necessarily uniform in the galaxy and it could be that the Earth travels through clumps and voids at different times of the year (and even different years through the century). If XENON collected data in a void it could explain the missing signal.

In the end, only time and further reviews will resolve this conflict. I just hope that a good experiment like XENON doesn't lose its reputation by publishing too quickly.

Wednesday, May 05, 2010

The Mysterious Dark Matter Detections

I read an interesting article today on new experimental data from the XENON dark matter search. But first I should give a little background...

For the last few decades there has been growing evidence that the majority of matter in the Universe is in some unknown exotic form. All we know about it is that it has a mass, and that it doesn't interact very much if at all with light. There are literally thousands of proposed models for dark matter, and it seems at times that there are just as many experiments searching for it.

About five years ago, a large experiment called DAMA found that there was a signal of dark matter colliding with the Earth as it orbits the sun.

Another experiment called CDMS went looking, and they found that the dark matter signal does not exist. Theorists immediately point out that this could happen if dark matter is about as heavy as a hydrogen atom.

Last year CDMS-II, the next generation detector, discovered a signal of dark matter. But this week another experiment, called XENON, released their data that shows both DAMA and CDMS-II are wrong and there was no signal.

Except that XENON itself could have missed the signal if the dark matter particle weighs more than 80 times the mass of a hydrogen atom. But if that is true then the original DAMA-CDMS conflict may re-arise.

Assuming that all three experiments are accurate (which is never certain in this business) that means that dark matter must be even more exotic than we thought.

What a great time to be a physicist!

Monday, May 03, 2010

How I Learned to Stop Worrying and Love the LHC

I saw this post in another blog, and just had to re-post it here :)

With the news that the Large Hadron Collider has started colliding particles, there has been a lot of mis-information flowing on the hazards of subatomic black holes and other dangers of high energy physics. This seems to be a combination of chicken-littles who love to panic, members of the media who love a scandal, and scientists who are too enamored of their own theories or experiments to give practical information on the LHC to the public.

Since I am also a trained physicist, let me say clearly the LHC will not destroy the Earth!

And here is the reason why we are so certain. The LHC set a record yesterday for the highest energy man-made collisions. But it is a tiny fraction of the energy the occurs in nature, even in our own atmosphere. The Earth is constantly being bombarded by gamma-rays and cosmic rays from space, and when they hit the atmosphere the reactions are easily a million times higher than anything the LHC will do.

So if tiny black holes can be formed (and as a sideline, most astrophysics evidence says they won't) then they have been forming in our atmosphere for billions of years. Modern physics and the lack of any observed black holes indicate that they will decay very quickly.We are still here, so therefore they are not dangerous.

The real advantage of the LHC isn't the energies it reaches - those are already reached naturally. The advantage is that we can make the reactions happen when and where we want - like inside a very big laboratory where the results can be studied.

So quit worrying, and love the LHC!

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