
Hydrogen made with muons reveals proton size conundrum - iProject
http://arstechnica.com/science/2013/01/hydrogen-made-with-muons-reveals-proton-size-conundrum/
======
richardjordan
This is tremendously interesting. In particular their suggestion that one
solution to the conundrum is the existence of additional very light force
carriers.

Perhaps a more up to date particle physicist than I can chip in bu such
particles must presumably be outside of the Standard Model (as iirc from my
particle physics, which is admittedly getting on for 20 years ago, there ain't
any more slots left). I know there are some interesting new theories which are
very widely disputed and which have a lot of other problems, such as the
Exceptionally Simple Theory, which predict a whole host of additional
particles - but there are reasons such things are on the fringe right now.

With all the progress in the last couple of decades, and all these new results
pointing to new Physics in all kinds of areas, it's a fascinating time for the
field - and makes one wonder where we'd be if the entire faculty hadn't
dedicated decades down the String theory rabbit hole.

~~~
incision
>...decades down the String theory rabbit hole.

Where does string theory stand these days?

As a fascinated layman, strings always reminded me quite a bit of epicycles
and I've had an impression that string theory had been falling out of favor
for some time now.

~~~
Steuard
The "popular fad" aspect of string theory has certainly lost much of its
luster, no question, particularly as far as the general public is concerned.
(The early excitement about the theory in the 1980's was apparently rather
over the top.) The fact that folks like Peter Woit and Lee Smolin were able to
publish popular books about their distaste for string theory reflects that
(and those books in turn did a lot to establish the "string theory is
overrated" counter-meme).

But within actual physics departments, as far as I've been able to tell string
theory holds much the same position that is has for many years: it's the
direction that most people interested in pursuing a "theory of everything"
seem to find most promising. (Plenty of physicists have other interests, of
course, even within particle physics: the phenomenology involved in
interpreting LHC data and predicting new phenomena to look for at that scale
is a big deal these days, and string theory really isn't relevant to those
questions.) As far as I can tell from the inside, that's primarily a genuine
process of ongoing scientific judgement rather than pure groupthink.

As a string theorist, I can see to some degree why you'd say it reminds you of
epicycles: I think most of us would agree that there's got to be some deeper
underlying truth that would make the structures of string theory seem more
unified and elegant. The difference between that and epicycles, at least to a
string theorist, is that we're optimistic that string theory as we know it is
a valid, well-defined approximation to the underlying theory, whatever form
that may take. Epicycles, on the other hand, were (as I understand it) sort of
a hack, without any intrinsic connection to the true heliocentric model.

~~~
sanxiyn
"The phenomenology involved in interpreting LHC data and predicting new
phenomena to look for at that scale is a big deal these days, and string
theory really isn't relevant to those questions."

I think "string theory is overrated" meme is mainly fueled by the fact that it
"really isn't relevant to those questions". I got the impression that many
string theorists claimed string theory to be relevant to those questions, and
only changed position once it became clear that won't be the case.

------
shardling
This has been a pretty big deal for a while now -- its a huge, unexplained
effect that ever theoretical physicist in a remotely related field will have
poked at. (And the expected result is actually pretty easy to calculate, as
far as anything in QED is.)

A lot of folk were betting on some sort of experimental error (just as ended
up happening with the superluminal neutrinos), so if they've managed to
replicate the result with more in depth experiments, that's kind of important.

------
ChuckMcM
I find the physics fascinating, but that muonic hydrogen stuff, that has
marketing gold all over it. SmartWater, now made with Muonic Hydrogen, get it
before it decays!

I expect its going to take several days to digest the paper though, it is
quite dense.

~~~
SideburnsOfDoom
> get it before it decays

A muon has a mean lifetime of 2.2 microseconds (2.2 * 10^−6 seconds). Hurry,
if you blink you'll miss it two hundred thousand times over.

~~~
sliverstorm
You don't have to tell your customers that.

Or maybe you can combine it with homeopathy while you're at it. The regular
water will _remember_ the properties of the muon water.

------
goodcanadian
Obligatory: <http://xkcd.com/955/>

Humour aside, I love these sorts of things. Even if there is no new physics
involved, if it turns out to be some sort of error in the experiment, we are
bound to learn something.

~~~
gus_massa
I read the preprint in the arxive, but I'm not an expert in this area.

This doesn't look like the usual mixture of crackpotology and
misinterpretation. It's better skip this article and to keep your $200 for the
next one.

------
habosa
I'm no physics expert but I did find the article very interesting. Can someone
who knows more than I do possibly explain, simply, what may be going on here
and what its implications would be?

~~~
davrosthedalek
In my PhD, I measured the proton form factor and, from that, calculated the
radius. So let me explain my view :)

There are essentially three ways to measure the radius:

1) Electron-proton scattering. This gives you the form factors (related to the
charge distribution by the Fourier transform), and from that you can calculate
the proton radius. 2) Measurements of the electron energy levels of "normal"
Hydrogen 3) Measurements of the muon-proton energy levels.

Re 1) The Mainz proton form factor experiment is to date the most precise
proton radius determination from electron scattering and is compatible with
the larger value. Our results are along the line of earlier measurements using
similar techniques. The first measurements of this kind where done in the 50's
and 60's (but produced quite a wrong radius).

Re 2) These measurements are very hard, the proton size effect is very small.
Nevertheless, the results are of similar precision as those from 1) and give a
compatible radius

Re 3) In muonic hydrogen, the muon is "much closer" to the proton and
therefore the proton size has a much larger influence. Because of this, the
method produces by far the best precision.

The 7 standard deviations are calculated with the precision of 1) and 2), the
error of 3) is negligble!

So, what can go wrong?

In my opinion, the muon experiment is very clean, so I don't believe in an
experimental error. The fact that 1) and 2) agree make it unlikely that they
are wrong, as they are complete separate methods. However, it is possible.

It could be that we are missing an important part in our understanding of the
radiative corrections, i.e. the theory needed to calculate the levels. This
could mean a simple error in one of the calculations, but most if not all of
them have been checked by different groups. It could also mean a flaw in one
of the solving techniques. Or maybe something which was overlooked.

It is also possible that there is another particle at work here. A possible
candidate is a dark photon. This solution has some benefits, especially since
such a new particle might also solve the muon g-2 puzzle. But it is not easy
to construct a theory of such a particle without violating other experiments.
A lot of fine tuning.

It could also be that the muon just behaves differently from the electron.
That would shatter a rather basic and widely accepted believe.

I attended a very interesting workshop recently which focused on this puzzle.
Unfortunately, we didn't find a solution. However, there are a lot of
experiments in the pipeline which might clarify the situation: \- There are
several experiments to measure the proton radius using eletron scattering,
with specialized instruments and new methods. -Muse is an experiment which
will scatter a combined electron and muon beam from protons. This will test
certain aspects of the radiative corrections and allow a direct comparison of
the exctracted radii. -There will be measurements of other muonic atoms.

All in all, this is a very interesting topic right now, with the added benefit
that it brings together the often separate communities of nuclear and atomic
physics.

Nice tidbit: Our result and the first muonic result was presented at the same
conference... and both speakers didn't know what the other would say.

Other nice tidbit: The first try at the muonic measurement didn't work out.
When they got it working, they scanned the energy region around the suspected
radius value. But they didn't find a resonance. After improving the experiment
and double checking everything, they still didn't find anything, until they
started looking further away, when suddenly, the resonance appeared. They just
didn't scan wide enough the first time!

~~~
pdonis
_Muse is an experiment which will scatter a combined electron and muon beam
from protons._

I'm a bit surprised that muon-proton scattering experiments haven't already
been done. It it just that there have been other priorities up to now, or is
there something particularly difficult about doing such experiments, compared
to doing them with electrons?

~~~
davrosthedalek
The main difference is that electrons are readily available while muons have
to be created. I think there are some experiments with high energy muons, but
for the proton radius, you want low energy. PSI creates them accelerating
protons and smashing them into nuclei. This produces pions, which then decay
into muons. These beams are then rather wider and harder to handle, as muons
decay quite quickly. With higher energies, it is possible to store them long
enough in rings, relativistic time dilation helps then.

------
drucken
The article makes this seem like a relatively straightfoward experiment
concept and implementation plus high justification. Why have measurements of
this sort not been made already?

Is it because of the difficulty of forming muonic hydrogen or making
measurements before the decay?

~~~
nagrom
None of it is easy. I haven't read this particular paper, but I have worked on
similar experiments. First, you have to create muons and then isolate them in
the particle beam and this costs quite some money (typically fire accelerated
protons at a target and allow the pions and other decay products to be dumped
in some dumb absorber that the muons pass through). Then you have to capture
the muons into orbits around protons that come from hydrogen that needs to be
disassociated and ionised. Then you have to pulse the hydrogen with a laser
and excite the muon into a new energy state and accurately measure the energy
of the photons that come off in the decay. You almost certainly need
subnanosecond accuracy in your timing at this point. Of course, you can never
be absolutely certain what you're measuring in any particular event so you sum
over millions of events and average. And you have to simulate the whole thing
(typically using a C++ framework called GEANT running on massive processor
farms analysed with a horror-show of an analysis toolkit called ROOT) to see
what could go wrong and determine the possible sources of error and
uncertainty in your measurement.

To get the money to do this, you have to apply for government grants that may
take up to a year to get approved. And no government wants to fund it so you
need an 'in' with the lab, because they'll be the ones that sponsor your
application for it to have a chance of success, and you'll need to know people
from a whole load of other countries to collaborate with and prove your _bona
fides_. And the people writing all these grant applications and doing most of
the middle management are distracted by teaching literally hundreds of
students every week during term time, so they can't even work full-time on it.

There are other experimental ways of doing this (e.g. measure correlated form
factors and parton distribution functions from elastic scattering and
extrapolate to zero momentum transfer), but they're all harder. They'll
probably be done within the next 30 years. Probably. This particular effort is
likely the work of 30-50 PhDs working for five or more years (at 50% of the
salary they'd earn working for commercial companies), discounting any of the
commercially available technology like electronics developed by CAEN or
detector components developed by the likes of ElJen or Hamamatsu. And I would
be surprised if they didn't mint another 10-15 PhDs during the effort. And I
haven't covered the highly trained technicians and electronic engineers that
are usually needed to build a lot of the custom experimental equipment.

~~~
deltasquared
What do you have against ROOT ?

~~~
nagrom
I dislike the fact that the library is forever expanding its feature set while
a lot of the core code is badly written or buggy (see the method for
calculating the angle between two T3vectors, for example). This is crazy when
development is not under commercial competitive pressure. I dislike the use of
global pointers to control the behaviour of the library and I find the design
decisions in particular regarding histograms and the lack of separation
between data and presentation bewildering (e.g. a graph is a subtype of a
histogram, which should contain it so that it can be plotted. A plot frame is
a type of histogram object, which makes no sense that I can understand, etc.).
The library makes no use of a lot of core C++ features; it really is C-with-
classes, where the use of exceptions could really improve the code base and
potential usage. I feel quite strongly that a numeric calculation should not
return a number if something went wrong, for example.

I dislike that it has a strong feeling of not-invented-here syndrome where it
could include dependencies on well tested code, e.g. the gnu scientific
library, but instead rewrites everything. And up until about 6 months ago the
default plot styles were damned ugly, although this has gotten a lot better.
There seem to be three different math libraries that each implement some
subset of mathematical functions independently and a lot of the graphics stuff
makes little sense to me (take a look at how to ensure platform-independent
type face size in a plot, for example). The CINT interface almost seemed
designed to ensure that when students were learning, they learned ROOT and not
C or C++, which is great for productivity in the short term, and incredibly
sucky in the medium to long term.

I understand that a lot of this is legacy cruft (and even made sense at the
time!) and that I could contribute patches for it, but I'm busy doing my
actual job. I'm impressed that a lot of my objections are being worked on too
- I was heavily dependent on the framework for my PhD about 5 years ago and a
lot of my frustration stems from then. It often feels like the ROOT team
learn-by-doing; they want to understand something, so they make it in ROOT.
Which is a fine way to learn, but not the best way to develop stable code that
should be used by thousands and thousands of people. In some senses ROOT is an
amazing achievement and I still find myself using it on occasion. But I've now
mostly replaced what I use it for with matplotlib and the GSL and my life is
easier.

 _breathes_

I also dislike the way that it seems partially to have enabled physics to go
in a direction where we pump out PhDs who don't understand code or physics,
but act as worker-drones for the large collaborations. But that's not really
ROOT's fault, and is a whole 'nother topic.

~~~
davrosthedalek
I second that. The aim of root is certainly good. But the implementation of
that idea, especially the overall design, is absolutely awful. It's
understandable, it has grown over a long time and was written by physics
experts, not software design experts. People who where used to Fortran and
paw. The actual code implementation is bad but not hopelessly so. The
interface is broken and because so many people are used to it, it will be hard
to replace with anything new.

For Mainz experiment, we have or own code base. Not pretty, but because it's a
lot more specialist, less confusing. I am working now on OLYMPUS, and we are
using ROOT for that. I created a Framework for the analysis based on ROOT,
trying to hide the most problematic areas and making it easier for use by the
other collaboration members. Also trying to make them write programs, not ROOT
macros. Every time I look up a new feature, I'm surprised that they managed to
find a non standard way of doing it. My pet peeve? TH1D is a 1-d historgram
class. What does TH1D.Clear do?

Wrong! It clears the histogram title and name, not the histogram itself. For
that, you need Reset. It makes kind of sense if you know the class hierarchy,
TNamed and all. But who remembers that? I saw this mistake in the wild a lot.

Protip: Gnuplot. While also a little bit arcane in its command language, it's
for me the best tool to produce paper-ready plots. With the tikz terminal you
can include it in your Latex flow, and with some Makefile trickery you can
have e.g. \cites resolve correctly in plot labels. With numbering correctly
reflecting the position of the plot in the paper!

~~~
davrosthedalek
(Can't reply to the child post)

Lol! It's Jan. The world is small.

------
TerraHertz
Annnnd.... once again when we want to read the actual paper (paid for with
public funds) we hit a damned paywall.

<http://www.sciencemag.org/content/339/6118/417.full>

Can anyone suggest a free source?

~~~
gus_massa
Once again, every preprint (in some areas) is in the arxiv:
<http://arxiv.org/pdf/1208.2637.pdf>

~~~
TerraHertz
I wasn't sure that was the same paper, due to the different title and somewhat
different list of authors. Even though the subject matter is the same.

~~~
gus_massa
My bad. Same team, different (recent) paper.

Edit: There is a discussion about this in reddit:
[http://www.reddit.com/r/Physics/comments/177wfm/hydrogen_mad...](http://www.reddit.com/r/Physics/comments/177wfm/hydrogen_made_with_muons_reveals_proton_size/)

------
TerraHertz
So how is the mystery of observed isotopic decay rate variations (with
Earth/Moon/Sun spin/orbit period correlations) going these days? That's now
two solid results that indicate things we thought were constants, are not.
What's the bet they are related?

