Quantum Diaries

Hi there QD readers!

Instead of any further intro about me than my bio for the moment, I’m starting my contribution on this group blog right off with a first post about a movie that I think is going to become a fairly major deal culturally, in the coming weeks and months, and isn’t that partly what science blogging is kind of about — that is, identifying those scientific things that could be important especially to, and have an impact on, the future of our society..?

So, to jump right to it —  I’d been seeing bits of previews of this movie:

(Full description: Interstellar at IMDb )

here and there online, and then there was a post to the KIPAC Facebook page about its scientific accuracy for the black hole depiction, with well-known physicist Kip Thorne’s involvement:

https://www.facebook.com/KIPAC/posts/839371609436219

I then saw the preview fully through a couple of days ago, and read more about the background and story arc and reactions to the film — and am impressed, and inspired already by what I’ve seen.

I can see this is a film in the spiritual tradition of Carl Sagan and Contact, and it may be the first movie I go see in a theater in a long while.

It’ll be out on Nov 7, and I’m looking forward to it.

Of course — I can tell I will have some problems with it too already — like with the ideas of indulging in escapism and finding another world vs. doing the hard work of fixing the only one we have — and are going to have, for a long, long time yet, as Carl himself used to point out, which can be read in e.g. the penultimate paragraph of the famous Pale Blue Dot talk, or seen as part of a short 6min clip put into more context here:

But with all that in mind, I am stoked about the positive aspects of what this one is about: inspiring us to be adventurers, showing strong familial bonds, and getting us to act in coordination as a species because of its apocalyptic vision of what the Earth will become if we do not.

So — anyone else also getting enthused about it, just yet..? 🙂

I travel a lot for my work in particle physics, but it’s usually the same places over and over again — Fermilab, CERN, sometimes Washington to meet with our gracious supporters from the funding agencies.  It’s much more interesting to go someplace new, and especially somewhere that has some science going on that isn’t particle physics.  I always find myself trying to make connections between other people’s work and mine.

This week I went to a meeting of the Council of the Open Science Grid that was hosted by the Oklahoma University Supercomputing Center for Education and Research in Norman, OK.  It was already interesting that I got to visit Oklahoma, where I had never been before.  (I think I’m up to 37 states now.)  But we held our meeting in the building that hosts the National Weather Center, which gave me an opportunity to take a tour of the center and learn a bit more about how research in meteorology and weather forecasting is done.

OU is the home of the largest meteorology department in the country, and the center hosts a forecast office of the National Weather Service (which produces forecasts for central and western Oklahoma and northern Texas, at the granularity of one hour and one kilometer) and the National Severe Storms Laboratory (which generates storm watches and warnings for the entire country — I saw the actual desk where the decisions get made!).  So how is the science of the weather like and not like the science that we do at the LHC?

(In what follows, I offer my sincere apologies to meteorologists in case I misinterpreted what I learned on my tour!)

Both are fields that can generate significant amounts of data that need to be interpreted to obtain a scientific result.  As has been discussed many times on the blog, each LHC experiment records petabytes of data each year.  Meteorology research is performed by much smaller teams of observers, which makes it hard to estimate their total data volume, but the graduate student who led our tour told us that he is studying a mere three weather events, but he has more than a terabyte of data to contend with — small compared to what a student on the LHC might have to handle, but still significant.

But where the two fields differ is what limits the rate at which the data can be understood.  At the LHC, it’s all about the processing power needed to reconstruct the raw data by performing the algorithms that turn the voltages read out from millions of amplifiers into the energies and momenta of individual elementary particles.  We know what the algorithms for this are, we know how to code them; we just have to run them a lot.  In meteorology, the challenge is getting to the point where you can even make the data interpretable in a scientific sense.  Things like radar readings still need to be massaged by humans to become sensible.  It is a very labor-intensive process, akin to the work done by the “scanner girls” of the particle physics days of yore, who carefully studied film emulsions by eye to identify particle tracks.  I do wonder what the prospects are in meteorology for automating this process so that it can be handed off to humans instead.  (Clearly this has to apply more towards forefront research in the field about how tornadoes form and the like, rather than to the daily weather predictions that just tell you the likelihood of tornado-forming conditions.)

Weather forecasting data is generally public information, accessible by anyone.  The National Weather Service publishes it in a form that has already had some processing done on it so that it can be straightforwardly ingested by others.  Indeed, there is a significant private weather-forecasting industry that makes use of this, and sells products with value added to the NWS data.  (For instance, you could buy a forecast much more granular than that provided by the NWS, e.g. for the weather at your house in ten-minute intervals.)  Many of these companies rent space in buildings within a block of the National Weather Center.  The field of particle physics is still struggling with how to make our data publicly available (which puts us well behind many astronomy projects which make all of their data public within a few years of the original observations).  There are concerns about how to provide the data in a form that will allow people who are not experts to learn something from the data without making mistakes.  But there has been quite a lot of progress in this in recent years, especially as it has been recognized that each particle physics experiment creates a unique dataset that will probably never be replicated in the future.  We can expect an increasing number of public data releases in the next few years.  (On that note, let me point out the NSF-funded Data and Software Preservation for Open Science (DASPOS) project that I am associated with on its very outer edges, which is working on some aspects of the problem.)  However, I’d be surprised if anyone starts up a company that will sell new interpretations of LHC data!

Finally, here’s one thing that the weather and the LHC has in common — they’re both always on!  Or, at least we try to run the LHC for every minute possible when the accelerator is operational.  (Remember, we are currently down for upgrades and will start up again this coming spring.)  The LHC experiments have physicists on on duty 24 hours a day, monitoring data quality and ready to make repairs to the detectors should they be needed.  Weather forecasters are also on shift at the forecasting center and the severe-storm center around the clock.  They are busy looking at data being gathered by their own instruments, but also from other sources.  For instance, when there are reports of tornadoes near Oklahoma City, the local TV news stations often send helicopters out to go take a look.  The forecasters watch the TV news to get additional perspectives on the storm.

Now, if only the weather forecasters on shift could make repairs to the weather just like our shifters can fix the detector!

An interesting headline piqued my interest when browsing the social networking and news website Reddit the other day. It simply said:

“The first direct detection of dark matter particles may have been achieved.”


Well, that was news to me! 
Obviously, the key word here is “may”. Nonetheless, I was intrigued, not being aware of any direct detection experiments publishing such results around this time. As a member of LUX, there are usually collaboration-wide emails sent out when a big paper is published by a rival group, most recently the DarkSide-50 results . Often an email like this is followed by a chain of comments, both good and bad, from the senior members of our group. I can’t imagine there being a day where I think I could read a paper and instantly have intelligent criticisms to share like those guys – but maybe when I’ve been in the dark matter business for 20+ years I will!

It is useful to look at other work similar to our own. We can learn from the mistakes and successes of the other groups within our community, and most of the time rivalry is friendly and professional. 
So obviously I took a look at this claimed direct detection. Note that there are three methods to dark matter detection, see figure. To summarise quickly,

The three routes to dark matter detection

• Direct detection is the observation of an interaction of a dark matter particle with a standard model one
.
• Indirect detection is the observation of annihilation products that have no apparent standard model source and so are assumed to be the products of dark matter annihilation.
• Production is the measurement of missing energy and momentum in a particle interaction (generally a collider experiment) that could signify the creation of dark matter (this method must be very careful, as this is how the neutrinos are measured in collider experiments).

So I was rather surprised to find the article linked was about a space telescope – the XMM-Newton observatory. These sort of experiments are usually for indirect detection. The replies on the Reddit link reflected my own doubt – aside from the personification of x-rays, this comment was also my first thought:

“If they detected x-rays who are produced by dark matter axions then it’s not direct detection.”

These x-rays supposedly come from a particle called an axion – a dark matter candidate. But to address the comment, I considered LUX, a direct dark matter detector, where what we are actually detecting is photons. These are produced by the recoil of a xenon nuclei that interacted with a dark matter particle, and yet we call it direct – because the dark matter has interacted with a standard model particle, the xenon. 
So to determine whether this possible axion detection is direct, we need to understand the effect producing the x-rays. And for that, we need to know about axions.

I haven’t personally studied axions much at all. At the beginning of my PhD, I read a paper called “Expected Sensitivity to Galactic/Solar Axions and Bosonic Super-WIMPs based on the Axio-electric Effect in Liquid Xenon Dark Matter Detectors” – but I couldn’t tell you a single thing from that paper now, without re-reading it. After some research I have a bit more understanding under my belt, and for those of you that are physicists, I can summarise the idea:

• The axion is a light boson, proposed by Roberto Peccei and Helen Quinn in 1977 to solve the strong CP problem (why does QCD not break CP-symmetry when there is no theoretical reason it shouldn’t?).
• The introduction of the particle causes the strong CP violation to go to zero (by some fancy maths that I can’t pretend to understand!).
• 
It has been considered as a cold dark matter candidate because it is neutral and very weakly interacting, and could have been produced with the right abundance.

Conversion of an axion to a photon within a magnetic field (Yamanaka, Masato et al)


For non-physicists, the key thing to understand is that the axion is a particle predicted by a separate theory (nothing to do with dark matter) that solves another problem in physics. It just so happens that its properties make it a suitable candidate for dark matter. Sounds good so far – the axion kills two birds with one stone. We could detect a dark matter axion via an effect that converts an axion to an x-ray photon within a magnetic field. The XMM-Newton observatory orbits the Earth and looks for x-rays produced by the conversion of an axion within the Earth’s magnetic field. Although there is no particular interaction with a standard model particle (one is produced), the axion is not annihilating to produce the photons, so I think it is fair to call this direct detection.

What about the actual results? What has actually been detected is a seasonal variation in the cosmic x-ray background. The conversion signal is expected to be greater in summer due to the changing visibility of the magnetic field region facing the sun, and that’s exactly what was observed. In the paper’s conclusion the authors state:

“On the basis of our results from XMM-Newton, it appears plausible that axions – dark matter particle candidates – are indeed produced in the core of the Sun and do indeed convert to soft X-rays in the magnetic field of the Earth, giving rise to a significant, seasonally-variable component of the 2-6 keV CXB”

Conversion of solar axions into photons within the Earth’s magnetic field (University of Leicester)

Note the language used – “it appears plausible”. This attitude of physicists to always be cautious and hold back from bold claims is a wise one – look what happened to BICEP2. It is something I am personally becoming familiar with, last week having come across a lovely LUX event that passed my initial cuts and looked very much like it could have been a WIMP. My project partner from my masters degree at the University of Warwick is now a new PhD student at UCL – and he takes great joy in embarrassing me in whatever way he can. So after I shared my findings with him, he told everyone we came across that I had found WIMPs. Even upon running into my supervisor, he asked “Have you seen Sally’s WIMP?”. I was not pleased – that is not a claim I want to make as a mere second year PhD student. Sadly, but not unexpectedly, my “WIMP” has now been cut away. But not for one second did I truly believe it could have been one – surely there’s no way I‘m going to be the one that discovers dark matter! (Universe, feel free to prove me wrong.)

These XMM-Newton results are nice, but tentative – they need confirming by more experiments. I can’t help but wonder how many big discoveries end up delayed or even discarded due to the cautiousness of physicists, who can scarcely believe they have found something so great. I look forward to the time when someone actually comes out and says ‘We did it – we found it.” with certainty. It would be extra nice if it were LUX. But realistically, to really convince anyone that dark matter has been found, detection via several different methods and in several different places is needed. There is a lot of work to do yet.

It’s an exciting time to be in this field, and papers like the XMM-Newton one keep us on our toes! LUX will be starting up again soon for what we hope will be a 300 day run, and an increase in sensitivity to WIMPs of around 5x. Maybe it’s time for me to re-read that paper on the axio-electric effect in liquid xenon detectors!