Monday, July 15, 2013

More mysteries in cosmic rays, and a proposed solution

The highest energetic particle collisions that we observe on our planet are created by particles from outer space that hit atomic nuclei in Earth’s upper atmosphere. The initial particle produces a large number of secondary particles which decay or scatter again, creating what is called a cosmic ray shower. The shower rains down on the surface where it is measured in large arrays of detectors. The challenge for the theoretical physicist is to reconstruct the cosmic ray shower so that it is compatible with all data. In practice this is done with numerical simulations in which enters our knowledge about particle physics that we have from collider experiments.

Cosmic ray shower, artist's impression. Source: ASPERA

One of the detectors is the Pierre Auger Observatory whose recent data has presented some mysteries.

One mystery we already discussed previously. The “penetration depth” of the shower, ie the location where the maximal number of secondary particles are generated, doesn’t match expectation. It doesn’t match when one assumes that the primary particle is a proton, and Shaham and Piran argued that it can’t be matched either by assuming that the primary is some nuclei or a composite of protons and nuclei. The problem is that using heavier nuclei as primaries would change the penetration depth to fit the data, but on the expenses that the width of the distribution would no longer fit the data. Back then, I asked the authors of the paper if they can give me a confidence level so I’d know how seriously to take this discrepancy between data and simulation. They never came back to me with a number though.

Now here’s an interesting new paper on the arXiv that adds another mystery. Pierre Auger sees too many muons


In the paper the authors go through possible explanations for this mismatch between data and our understanding of particle physics. They discuss the influence of several parameters on the shower simulation and eventually identify one that has the potential to influence both, the penetration depth and the number of muons. This parameter is the total energy in neutral pions.

Pions are the lightest mesons, that is particles composed of a quark and an anti-quark. They get produced abundantly in highly energetic particle collisions. Neutral pions have a very short lifetime and decay almost immediately into photons. This means essentially all energy that goes into neutral pions is lost for the production of muons. Besides the neutral pions there are two charged pions and the more energy is left for these and other hadrons, the more muons are produced in the end. Reducing the fraction of energy in neutral pions also changes the rate at which secondary particles are produced and with it the penetration depth.

This begs the question of course why the total energy in neutral pions should be smaller than present shower simulations predict. In their paper, the authors suggest that a possible explanation might be chiral symmetry restoration.

The breaking of chiral symmetry is what accounts for the biggest part of the masses of nucleons. The pions are the (pseudo) Goldstone bosons of that broken symmetry, which is why they are so light and ultimately why they are produced so abundantly. Pions are not exactly massless, and thus “pseudo”, because chiral symmetry is only approximate. The chiral phase transition is believed to be close by the confinement transition, that being the transition from a medium of quarks and gluons to color-neutral hadrons. For all we know, it takes place at a temperature of approximately 150 MeV. Above that temperature chiral symmetry is “restored”.

In their paper, the authors assume that the cosmic ray shower produces a phase with chiral symmetry restoration which suppresses the production of pions relative to baryons. They demonstrate that this can be used to fit the existing data, and it fits well. They also make a prediction that could be used to test this model, which is a correlation between the number of muons and the penetration depth in individual events.

They make it very clear that they have constructed a “toy model” that is quite ad-hoc and mainly meant to demonstrate that the energy fraction in neutral pions is a promising parameter to focus on. Their model raises some immediate questions. For example it isn’t clear to me in which sense a cosmic ray shower produces a “phase” in any meaningful sense, and they also don’t discuss to what extent their assumption about the chirally restored phase is compatible with data we have from heavy ion physics.

But be that as it may, it seems that they’re onto something and that cosmic rays are about to teach us new lessons about the structure of elementary matter.

28 comments:

Juan F. said...

Wonderful post, Bee! Thank you for the paper link! It had scaped to my attention. Personally, I find stunning these new results.
Cosmic rays mysteries, neutrino mysteries, AMS02 mysteries, ... And yet, no clear signal from "light" new particle species in LHC or direct DM experiments (and I mean evidence close to the critical 5sigma barrier). We have a hard job ahead of us...

Zephir said...

As usually, this effect has been predicted with some completely forgotten russian physicists before thirty years. We already observed it at colliders as so-called di-muon anomaly.

Arun said...

Question - assuming that cosmic rays are protons, and we're mispredicting what happens as these slam into N, O, H nuclei; is the energy scale different enough that we still trust our cosmological and supernova nucleosynthesis calculations?

Uncle Al said...

"The breaking of chiral symmetry is what accounts for the biggest part of the masses of nucleons" Physics observes vacuum is mirror-symmetric toward massless photons. Physics assumes vacuum is mirror-symmetric toward matter. Theory then hemorrhages parity violations and chiral anomalies. Unending inserted symmetry breakings are excuses.

Vacuum is a trace left foot toward matter, hence parity violations and chiral anomalies. A quartz’ unit cell has 0.113 nm^3 volume. A 20 gram single crystal test mass holds 6.68×10^22 3-D gaplessly packed unit cells - shoes - within 7.55 cm^3 volume. Measure differential vacuum chirality toward opposite shoes. One geometric Eötvös experiment outweighs 56 years of Yang and Lee dry labs (second definition).

Phillip Helbig said...

What? You mention "chirality" and Uncle Al hasn't chimed in yet?

Xerxes said...

I'm puzzled what the authors think the difference is between a chiral-symmetry restored phase and a quark-gluon plasma. The transition temperatures for CSR and deconfinement are are basically the same. They dismiss QGP but somehow expect to get a CSR phase without QGP?

Sabine Hossenfelder said...

Phillip,

Uncle submitted a comment, but all of his comments get caught in Blogger's spam filter. I don't know why. They only show up when I go through the list and free the not-spam from the queue. Some of Plato's comments also tend to go into spam. This comment though has a link in it, so I think I'll leave it in the spam folder. Best,

B.

Sabine Hossenfelder said...

Arun,

I don't know much about supernova. Nucleosynthesis is at much lower energies though. Best,

B.

Sabine Hossenfelder said...

Xerxes,

I'm not sure why you say they "dismiss the QGP". I think they basically say that the confinement transition is an unlikely source for the discrepancy. See, it's the chiral symmetry breaking that creates the pions as pseudo-Goldstone modes, so it seems plausible that it has an impact on the ratio at which they are produced. Best,

B.

Plato Hagel said...

Maybe google is deciding some things for you Bee?:)

Systemically the experimental process and energy valuations come it to question about the relationship that leads to QGP and the transition that is taking place.

Sean Carroll talks about past historic evidences displayed in the cosmos. This has to have this basis and consideration for information that had always existed and never dies, as it transmits information about where the particle decay comes from? Other universes, or even regions within the cosmos itself?

You know I have been interested in this subject for a long time now.

Best,

Plato Hagel said...

Under label of QGP

First image of Jet quenching

"We want to measure when the quark-gluon plasma behaves like a perfect fluid with zero viscosity, and when it doesn't," says Lauret. "When it doesn't match our calculations, what parameters do we have to change? If we can put everything together, we might have a model that reproduces everything we see in our detector." See:Probing the Perfect Liquid with the STAR Grid

Plato Hagel said...

oops...Jet Quenching

Plato Hagel said...

Lastly, a layman at heart wants to continue to know......,

At RHIC large elliptic flow has been observed and is one of the key experimental discoveries [2{6]. Theoretical models, based on ideal relativistic hydrodynamics with a QGP equation of state and zero shear viscosity, fail to describe elliptic flow measurements at lower energies but
describe RHIC data reasonably well [7]. Theoretical arguments,
based on the AdS/CFT conjecture [8], suggest a universal lower bound of 1=4 [9] for the ratio of shear viscosity to entropy density. Recent model studies incorporating viscous corrections indicate that the shear viscosity
at RHIC is within a factor of 5 of this bound [10{
13]. See: Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV

Plato Hagel said...

Long-range angular correlations of pi, K and p in p--Pb collisions at sqrt(s_NN) = 5.02 TeV

Ummmm.........

John Merryman said...

File this in the stupid questions category, but if you break the atom, it creates pressure. Think Hiroshima. So wouldn't fusion create a vacuum? We can't find any dark matter, but there is a lot of excess cosmic rays and all that other stuff going on in the interstellar medium, as well as all the heavy metals being created within stars, etc. What if gravity is not so much a property of mass, as it is an effect of energy forming into mass?

Sabine Hossenfelder said...

John,

Not sure I understand your question. As Einstein taught us, mass is just some form of energy. Pressure is also just some form of energy. Fusion doesn't create a vacuum, or maybe I misunderstand what you mean with vacuum. Best,

B.

John Merryman said...

Sabine,
What really is the relationship between light as a wave or field and light as a quantized photon? In last years FQXI contest, Eric Reiter (http://fqxi.org/community/forum/topic/1344) raised the issue of whether the quantization of light is fundamental to light, or is a function of its absorption within atomic structure.
So just for a moment consider the consequences of light traveling as a field/wave and just being absorbed as point-like quantities. We know when we release that energy of which mass consists, it expands. When the light passes through the slits, it does so as a wave. What internal property does light have, that would hold it to a point-like particle in space? So say light in space, where it isn't being tested or measured, is naturally just one entangled element, but that magnetic polarization draws it to that point.
(Here is an interesting interview with Carver Mead on the subject:
http://freespace.virgin.net/ch.thompson1/People/CarverMead.htm.
"Now it's true that if you take a proton and you put it together with an electron, you get something that we call a hydrogen atom. But what that is, in fact, is a self-consistent solution of the two waves interacting with each other. They want to be close together because one's positive and the other is negative, and when they get closer that makes the energy lower. But if they get too close they wiggle too much and that makes the energy higher. So there's a place where they are just right, and that's what determines the size of the hydrogen atom. And that optimum is a self-consistent solution of the Schrodinger equation.")
So my very simple question is that since releasing energy creates pressure; E=mc2. Would the opposite; M=e/c2, create a vacuum? So gravity would emerge from that relation between the particle state and the wave state.

Uncle Al said...

Failure, as demonstrated, is good when acknowledged. Failure fuels the upward surge of mankind. Rederive failed theory from qualified observation. Curve-fitting failure to observation is anti-understanding.

Elegant theory or obtuse experiment can disagree with solid observation. Summed less than a dark matter asteroid locally should be 5.47:1 dark/baryonic matter. This is failure. Macroeconomics is a $(US)10^15 failure. Situationally ethical and Big Pharma-corrupt (DSM-5) psychiatry’s rationalizing sibling psychology oozes hornswoggle. Planaria ooze slime trails, rodents ooze urine trails. If your maze is not cleaned after every run…you get... published.

Ignore Einstein. Ernest O. Lawrence invents the cyclotron and pushes beyond 500 keV electrons. Curve-fitting discovers the beta-factor, then endlessly parameterizes Newton.

Sabine Hossenfelder said...

John,

m=E/c^2 is not "the opposite" of E=mc^2, it's the same. Fusion doesn't create vacuum because there's stuff there, the stuff that you're fusing together, and gravity doesn't "emerge" in nuclear fusion either. I honestly have no clue what you're trying to say. Best,

B.

John Merryman said...

B,
I know there is stuff there. It just ends up in a smaller package.
To me, the whole spacetime curvature thingy is like the giant cosmic gear wheels explaining why epicycles were so mathematically elegant.
We experience time as a sequence of events and physics treats it as a measure of duration, but the actual physics is change. So it the process by which future becomes past and that perception of sequence is only our subjective perception, like the sun moving across the sky. To wit, the earth isn't traveling some fourth dimension from yesterday to tomorrow. Rather tomorrow becomes yesterday because the earth rotates.
This makes time more like temperature than space. You could say time is to temperature what frequency is to amplitude.
So spacetime is correlation of duration and distance, not causation of gravitational effects. Duration doesn't actually transcend the present, but is the state of what is present between the occurrence of events.
I realize much of this is not going to compute for you and I'm used to being ignored and deleted, but I'm finding more and more people, across many professions, who find the issues bedeviling physics to be more amenable to non-linear dynamics, complexity theory, information theory, etc. and not requiring multiverses, multiworlds, expanding universes and many of the other grandiose patches of the last century.
For example, how can you say "space expands," yet assume a constant speed of light against which to compare it? Which is the denominator? I wouldn't want to take that math to the IRS.
Thank you for your time and patience.
Regards,
John

Phillip Helbig said...

@John Merryman: If your ideas come up with testable predictions which differentiate them from conventional science, then publish them. If not, then it is not science.

John Merryman said...

Phillip,
20 years ago it occurred to me redshift was more likely evidence of Einstein's cosmological constant, ie. a balancing effect to gravity. 10 years after that, Perlmutter and Co. measured the redshift as more reflective of some non-zero cosmological constant. Now even though, according to both theory and measurement, expansion is balanced by gravity, presumably leaving no additional expansion for the universe as a whole to grow, it is still being shoehorned into this singular expanding universe model. So my prediction is that eventually, who knows how long, the physics community will wake up and realize those galaxies are not just inert points of measurement across intergalactic space, but are gravity wells and they effectively balance the expansion between such points. It is a convection cycle of expanding radiation and contracting mass. To use the ball on the rubber sheet analogy, it is as though that sheet is on the surface of water and where the ball is not directly pushing down, the water pushes back up in equal measure, thus leaving an overall average flat surface, as COBE and WMAP show.
But no, it's much more fun to talk about the universe springing forth from some immaculate conception, inflating out at multiples of the speed of light, throwing out multiverses, etc. Do you suppose the multiverse is a "testable prediction?"

Uncle Al said...

@John Merryman: Look up "multipoles," their meaning and fitting vs. Planck data. WMAP is lower resolution. A universe without expansion. Peer review finds no fault with the derivation. (Perhaps non-linear Gmm'/r^2 will appear in a later paper.)

Phillip Helbig said...

"Now even though, according to both theory and measurement, expansion is balanced by gravity, presumably leaving no additional expansion for the universe as a whole to grow, it is still being shoehorned into this singular expanding universe model."

This is wrong on so many levels that a blog comment can't even begin to address it.

Phillip Helbig said...

"A universe without expansion. Peer review finds no fault with the derivation."

Probably true, but perhaps misleading. Peer review has found no fault with a host of crackpot ideas---since they haven't been peer-reviewed at all! I'm not saying that the author is a crackpot (he is not) nor that the paper is (I haven't read it), merely that your, presumably true, statement does not imply that the article has in fact passed peer review. The link provides no information as to whether it has been submitted and if so, where. It certainly doesn't mention any peer-reviewed journal.

John Merryman said...

Phillip,
I'm not asking or expecting you to think this through, you asked for a prediction and I gave you one. Life is a billion year trial and error experiment and so long as multiverses pay anyone's bills, someone will believe in it.
I once had a Catholic priest and future in-law quiz me on my piety. I said that since the absolute is basis, not apex, a spiritual absolute would be the essence from which we rise, not an ideal from which we fell. His response was to cross himself and walk away. I get a similar response from cosmologists, in pointing out gravity already balances expansion.
Uncle Al,
Here are a few links:
http://www.fqxi.org/data/forum-attachments/2008CChristov_WaveMotion_45_154_EvolutionWavePackets.pdf

http://freespace.virgin.net/ch.thompson1/People/CarverMead.htm
http://www.americanscientist.org/issues/pub/2007/9/modern-cosmology-science-or-folktale
http://fqxi.org/community/forum/topic/1578

Sabine Hossenfelder said...

John,

Thanks, but that's enough. It's off-topic. If you want to continue this exchange, please do so elsewhere. Best,

B.

John Merryman said...

B,
Thanks for your patience and tolerance.