How does one measure the mass of a neutrino, using cosmology?

I'm going to tell you how, soon, humanity might measure the masses of neutrinos just by observing past events in the universe. I like this topic because it is one of the few situations in fundamental physics where a measurement of the greater universe might detect something about fundamental particles and/or their interactions, before we manage to measure it in a lab. Another example is the existence of dark matter; however the mass of dark matter will almost certainly be first measured in a lab. Perhaps with neutrinos it will go in the other direction?

What is a neutrino?

I guess that before telling you how to measure a neutrino's mass, it might be pertinent to tell you what a neutrino is and how we can know it has mass before we've measured that mass. Well...

When an atomic nucleus decays, the decay products we see are other nuclei, electrons and/or positrons. These visible products always carry less energy and momentum than the amount that the initial nucleus had. This suggests strongly that some unknown other particle is also being created in the decay and that we just can't see it. This hypothetical particle was dubbed the neutrino and when theories were developed for the force responsible for nuclear decays, the neutrino became an important part of them. And, eventually, neutrinos were detected directly. It took a while because neutrinos interact incredibly weakly, which means you need either a lot of neutrinos or a lot of transparent stuff for the neutrino to interact with (or both) before you will see them.

Initially, it was assumed that neutrinos are massless. They don't need to be massless, but for a long time there was no evidence that they did have mass, so the simplest assumption was that they didn't. There are three types of neutrinos: those emitted in interactions with electrons, those emitted in interactions with muons and those emitted in interactions with tau particles. If neutrinos were massless, then a neutrino emitted as an electron neutrino would always remain an electron neutrino. Similarly, a muon neutrino would always remain a muon neutrino. However, if neutrinos do have mass, then a neutrino emitted in an interaction with an electron will actually travel as a superposition of an electron neutrino, muon neutrino and tau neutrino. The net result being that this neutrino could be detected as a different type of neutrino. Therefore, a smoking gun thing to look for when determining whether neutrinos have mass is this characteristic signal whereby one type of neutrino appears to oscillate into another type of neutrino.

This effect was then seen and seen and seen again. Neutrinos appear to have mass. From the perspective of particle physics this is a bit weird. Neutrinos must have really small masses and it is unclear why these masses are so small. Unfortunately, this mechanism of neutrino oscillations doesn't directly give the masses of the neutrinos. Although, it can be used to measure the differences of the masses of the neutrinos, thus setting lower bounds on the possible masses of the neutrinos.

What has this got to do with cosmology?

The universe as seen by Planck - Day Two

The cosmic microwave background (CMB) is the best probe we've yet found to study the early universe. The CMB's temperature is very nearly uniform. However this temperature does have very small anisotropies that can be used to study sound waves that existed in the primordial universe. The Planck satellite (an ESA funded experiment) has mapped these temperature anisotropies over the entire sky with the best resolution to date. Last month, Planck released its data and it immediately became the new benchmark for the testing of cosmological models and the measurement of cosmological parameters.

This week ESA is hosting the first conference since Planck released its data. The conference is at ESTEC in the Dutch town of Noordwijk. I am attending this conference and will be doing my best to write updates about what was discussed during the week.You can read my introductory post where I give my motivation for doing this, here.

The CMB is not just useful for studying the primordial universe. As soon as the CMB forms, everywhere in the universe, it travels freely, in every direction, at the speed of light. This means that, in every direction, the CMB we measure here on Earth today has travelled to us from a point billions of light years away. In principle, this makes the CMB not just a really good probe of the state of the universe where and when it was emitted, but also of everything it passed on its way to us.

This secondary use for the CMB turns out to be very useful and many of the highlights from Planck relate to the way in which the CMB interacts on its way to us. The existence of matter in the universe affects the CMB gravitationally. This causes the CMB to bend towards regions of over-density and away from regions of under-density. It also causes the CMB's temperature to shift as it falls into and out of over and under-dense regions. This first effect is known as lensing and one of Planck's most impressive results is a map of the locations of matter in the universe through this lensing effect. The second effect is known as the Sachs-Wolfe effect, something I've written about in some detail.

There is a third way that the CMB is significantly affected by the intervening universe. Within clusters of galaxies there is a lot of hot gas. If the CMB passes through a cluster it can scatter off electrons in this hot gas. The effect of this scattering on the CMB is known as the Sunyaev-Zeldovich (SZ) effect. Therefore, we should be able to use the CMB to detect the lines of sight along which the most massive clusters lie.

We can. And Planck has.

Highlights from Beijing: COSMO 2012

The obligatory conference photo. The photographer spoke to us in Mandarin. I think what he was trying to say was "more intensity".

Just over a week ago I was at the annual COSMO conference. This year's host was Beijing. I had originally intended to live blog this event, but the Great Wall of China (alternative link) managed to prevent that entirely.

What follows are some reflections on the scientific bits and pieces people presented at the conference that I happened to find interesting. It might be a bit technical, but please ask questions if I use jargon you don't understand. Also, if you're an expert and I write something you want to comment on, please do (especially if something I write is misleading or just plain wrong).

The topics I've chosen below just happen to be what I found memorable. I made no attempt to choose these topics by any sort of theme. I apologise if I've missed anything particularly interesting. Perhaps if you were there and think I missed out something interesting you can either mention it in the comments or write a guest post for us.

Neutrinos and precision cosmology

One of the first images captured by the Dark Energy Survey. The more interesting images it will take will be of very distant galaxies and won't look anywhere near as nice. This one is just for people to put in their blogs.

Jan Hamann gave a talk on the future constraints that cosmology will provide for neutrino physics. I was pleasantly surprised by the power of large scale structure probes, such as Euclid.

We know from particle physics experiments that the difference between the masses of two of the neutrinos is more than 0.06 electron volts. This means that the heaviest neutrino must be heavier than 0.06 electron volts.

A Higgs Hunter's story...

[Note from Shaun: Here is Higgs hunter Mikko Voutilainen's account of the recent search for the Higgs. You can find the teaser to this post here. And my own, partially cynical, but ultimately upbeat, account of Higgs-things, here.]

Here it is, finally

[I assume the readers of this blog are somewhat familiar with the Higgs boson; if not, there's a nice summary on the CMS pages here]

So, this is the follow-up to the teaser I wrote a week ago. Now that everybody knows we found a Higgs boson at \(125.3\pm 0.6\) GeV, I'm free to talk about our finding, what it means and how we got there. Note the intentional use of 'a' Higgs there: although we, beyond reasonable doubt (less than one in a million chance of an error, to be precise), found a new particle, it's not 100% sure yet if it's *the* Higgs boson predicted by the standard model, or one of its many twins predicted by the hundreds of theories out there. There's even a tiny chance of it being an altogether different particle yet.

We actually already know a fair deal about this new particle besides the rather impressively precise estimate of its mass: it seems to be produced at a rate that matches the standard model prediction within about 20% uncertainty, it decays into bosons (W, Z and photon) and fermions (b-quarks and tau-leptons) roughly in the ratios predicted by the standard model, and in particular it decays into W and Z bosons in the ratio predicted by the standard model. The last point is rather important, because the Higgs mechanism, and the Higgs boson along with it, was invented to give mass to the W and Z bosons, and leave the photon massless. This also fixes the ratio of the decay rates to W and Z. If the new particle didn't decay into Z's and W's in just the right ratio, it couldn't be the Higgs boson we predicted.

We've also had a stab at determining the more abstract properties of the particle such as a quantum number called parity, but the statistics are low and the results still inconclusive. Predictions say we should be able to tell by the end of the current run, when we've collected 2--3 times the amount of data we have now. At this point we should also have more precise determination of the particle's decay rates in all the different channels, in order to gain more confidence in calling the particle a Higgs boson or something else.

So, is this the end, or the beginning of something new? I'm really hoping for the latter. If the new particle turns out to be 'just' the standard model Higgs boson and there's nothing new to be found, that would be fairly boring. If instead it's a Higgs twin, we may have just opened a window into a new landscape of particles.

At the moment it's too early to tell for sure, but there are a few interesting features to the way the new particle decays. It seems to decay into photons more often than expected, and to tau-leptons less often than expected. Taking all the decays to fermions together, they only seem to add up to about half of the rate predicted by the standard model, albeit with an error of about 50% as well. That coincidence is causing a bit of excitement nevertheless.

It might not be too bad for the standard model, though, it could just indicate that it's 'non-minimal'. While the Higgs coupling to W and Z is pretty tightly constrained, all the other particle masses are more of an ad-hoc addition to the theory, and there's some freedom to adjust how these particles couple to the Higgs boson without breaking everything else. Another good example of something that would require a 'non-minimal' standard model are the neutrino masses, which in the simplest expectation are exactly zero. We now know they are not zero, although we've still to nail down exactly how much they weigh (it's very very little in any case).

What for me was most interesting in this was to see first-hand how things have evolved towards a big discovery. Things started rolling about six months ago, when the first results from LHC Higgs boson searches were presented last December. Back then both ATLAS and CMS saw a hint of a Higgs at 125 GeV, with about 2-2.5 sigma statistical confidence. If you were a Higgs-believer, you could have given the signal more than 95% chance of being true.

After December it was decided that we wouldn't look at the 2012 data in the signal region before we had enough to confirm or refute the hint seen in 2011. This process is called blinding, and its important for making sure the analyzers are not unconciously affected by their prior expectations. Blinding is also one of the reasons we've tried to keep a lid on the results until today's seminar so that the experiments would not affect each other's findings between opening their signal box opening and presenting the final results. I think we were fairly successful in the end, although rumors started circulating on the blogs within days, and by yesterday almost every major newspaper (including Nature) had run a story on Higgs.

Between opening the signal box and seeing the first evidence of a new particle there was a whole lot of work going on for 2--3 weeks to prepare for ICHEP. The analyses added around 50% more data, the particle properties were studied in more detail, the CMS management had regular meetings with both ATLAS and CERN directors, people were working day and night to scrutinize the results, prepare documentation, etc. The final days were spent polishing plots, rehearsing presentations and fine-tuning press releases. Although I didn't happen to be at CERN during that period (I did attend the signal box opening in the beginning, though), I could at least participate through the almost daily video meetings and by keeping my own small piece of CMS running (I'm responsible for a team calibrating jets).

Just two days prior to the seminar there was also a presentation of the Tevatron Higgs results at Fermilab. The Tevatron people had done a superb job in squeezing every last bit of sensitivity out of their data and fell just a hair's width short of claiming evidence for the Higgs (they got 2.94 sigma by the most optimistic count, and needed 3.0). The Tevatron experiments collected data for ten years before shutting down last summer, and have the same amount of data (10 fb-1) available for analysis as the LHC experiments now. The lower collision energy of the Tevatron, 2 TeV versus 8 TeV at LHC, means roughly ten times less Higgs bosons are produced, but they still have better sensitivity in one single channel, the Higgs decaying into two b-quarks. I was watching that live on video, too, cheering for my old colleagues (I did my PhD on D0, one of the two experiments at the Tevatron).

And then, finally, today we had a chance to see how our colleagues and rivals at ATLAS were doing with their Higgs search. According to blog rumors, newspaper leaks and sensitivity estimate just a tad behind CMS, but never far. As it turned out, both CMS and ATLAS came up with the same significance in the end, within 0.1 sigma precision. Both experiments have now just made it to the 5-sigma milestone, and it's pretty clear that the signal has been effectively confirmed by at least three experiments (counting D0 and CDF together as a single Tevatron experiment).

P.S. I wrote a lengthy story about the box opening the same evening when I was at CERN, and stored it on a time capsule on my e-mail account. I'm not sure if it's interesting anymore, but at least I shouldn't be breaking any confidentiality rules by releasing it. [Shaun speaking: I now have this item in my possession, so if anyone wants to see it please let me know and I will upload it in a few days.]

At the neutrino OPERA, the fat lady is warming up.

If you weren't living under a rock last year, you will remember the neutrinos that seemed to be going faster than the speed of light. Everybody and their cat wrote about it, including me (though my cat only had editorial supervision of the final manuscript).

Earlier this year, OPERA (the experiment that apparently saw faster than light neutrinos) updated their press release informing us that they had spotted a few mistakes in their experiment. The mistake was an unfortunate one, but wasn't in OPERA's actual analysis or method itself. The mistake was basically a piece of faulty hardware. Normally you don't need to know things at a neutrino detector to within nanoseconds, so this fault hadn't been noticed before.

I don't hold OPERA to be at fault for going public with their initial findings. They had checked everything they could think of and still had this seemingly crazy result. If the result stood up to scrutiny and was repeated by other experiments, then they'd made one of the biggest discoveries for a long time, if not ever. Once they had checked everything they could think of, any lingering mistake is far more likely to be found by a group trying to repeat their experiment, than by someone in their collaboration.

I didn't write about this update, mostly because I was travelling, but also because nothing conclusive had yet occurred. OPERA made a mistake, fine, but it wasn't proven yet that this mistake was responsible (of course everyone, myself included, strongly suspected that it was – but we all strongly expected that some mistake had been made, and said so, from the start!).

Today, something genuinely new can be added to the story, due to ICARUS, another neutrino experiment. Ironically, both ICARUS and OPERA are at the same laboratory, at Gran Sasso, and in fact measure exactly the same neutrino beam from CERN. The new piece of the story is that ICARUS has measured the speed of these neutrinos and found them to be consistent with the speed of light, and crucially, inconsistent with the results from OPERA.

Here is the crucial figure from their article:

The neutrinos' arrival at ICARUS and OPERA. δt=0 corresponds to when light would arrive.

The purple bars show the scatter in time when neutrinos were measured to arrive at ICARUS. δt=0 corresponds to when light would arrive. Clearly the ICARUS neutrinos are arriving at the same time as light would. The bars on the right are what OPERA's previous measurements indicated.

So, given that (a) a lot of other (measured!) things in physics would be very difficult to reconcile with something going faster than light (b) We know OPERA have made a mistake that they need to fix and (c) ICARUS is measuring the exact same neutrino beam at the exactly same place and finding the neutrinos to be travelling at light speed, it seems that, in this opera, "the fat lady" might not just be warming up, but could very well be in the final stages of her aria.

But, as Matt Strassler pointed out in his own blog, there is a silver lining to all of this. 

The experimental particle physics community has learned how to make long-range distance and timing measurements that are more precise and more accurate than were ever possible before. Don’t be surprised if this knowledge turns out to be useful, in some unexpected way, in future experiments.

In fact, I'm sure this precision timing will be extremely useful in increasing the bit-rate of the neutrinos that have very recently been used to send digital messages through the ground, something that is impossible with any other known form of communication.

As a final passing comment I want to add that I am still in awe of the fact that both ICARUS and OPERA could make such a precise measurement of the speed of neutrinos. OPERA's mistake was unfortunate, but it doesn't change how impressive the measurement itself was. It just changes how impressive the result is.

Follow @just_shaun

Yet another voice in the neutrino OPERA

I feel just a little prophetic, given that only a week ago I wrote,

“... it is also useful to simply measure as many new things as possible. History is littered with moments where we didn't bother measuring something because we knew what the result would be, only to get a big surprise whenever someone finally did measure it. So nowadays if something is measurable, then someone, somewhere, is trying to measure it.”

Well, true to the claim, someone out there had the ridiculous idea that measuring the speed of neutrinos was a good idea, go figure. And, lo and behold, it seems they've found something! Maybe? Who knows!

Well, what sort of blog writer, who is a scientist (physicist even), would I be if I didn't join in with the neutrino cacophony? So, what follows is my attempt to say something interesting that hasn't already been said a million times already. I will focus on why this is such a surprising result and why this means nobody believes it.

This disbelief has nothing to do with the quality of the experiment. The measurement was recorded with impeccable accuracy and has been extremely carefully analysed. Neither is the disbelief because the result "proves Einstein wrong". It even has very little to do with the prospect of over-turning special relativity.

The disbelief is due to the fact that this observation brings into question the holy principle of causality – that is, the seemingly incontrovertible fact that cause must come before effect and not after

And, as my former PhD supervisor, said in the Guardian today:

“If we do not have causality, we are buggered."