Beyond LHC without a machine (new physics from cosmic rays)

[marcus]

Bee Hossenfelder has an interesting post I'd like to get some comment on
http://backreaction.blogspot.com/2013/07/more-mysteries-in-cosmic-rays-and.html
More mysteries in cosmic rays, and a proposed solution

"...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..."
http://backreaction.blogspot.com/2012/04/cosmic-ray-composition-problem.html
http://arxiv.org/abs/1204.1488

"...Now here’s an interesting new paper on the arXiv that adds another mystery. Pierre Auger sees too many muons
A new physical phenomenon in ultra-high energy collisions
Glennys R. Farrar, Jeffrey D. Allen
http://arxiv.org/abs/1307.2322 [hep-ph]
In the paper the authors go through possible explanations for this mismatch between data and our understanding of particle physics..."

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

 

[mfb]

New physics or a bad model?
A bad model (with >90% probability).

In particular a larger cross section than the one extrapolated from low energies can explain the shorter penetration depth.

As far as I know, the proton-proton cross-section at the LHC is a bit above the expectations, too. It agrees with fits, but without a theoretical description extrapolations based on fits are problematic.

TOTEM publication before there was LHC data:

The current large uncertainty on the extrapolation of the proton-proton total cross section at the LHC energy will be resolved by the precise measurement by the TOTEM experiment. Its accurate studies on the basic properties of proton-proton collisions at the maximum accelerator energy could provide a significant contribution to the understanding of cosmic ray physics.

Another thing I noted:

This can be seen very well from the figure below (Fig 2 from Shaham and Piran's paper) which shows the data from the Pierre Auger Collaboration, and the expectation for a composition of protons and Fe nuclei. You can see that adding a second component does have the desired effect of moving the average value to a shorter depth. But it also increases the width. (And, if the individual peaks can be resolved, produces a double-peak structure.)

That is a really weird idea in my opinion. A mixture of protons and various heavier elements is a natural way to resolve those issues.

 

[marcus]

Thanks for commenting!

"...Now here’s an interesting new paper on the arXiv that adds another mystery. Pierre Auger sees too many muons
A new physical phenomenon in ultra-high energy collisions
Glennys R. Farrar, Jeffrey D. Allen
http://arxiv.org/abs/1307.2322 [hep-ph]
In the paper the authors go through possible explanations for this mismatch between data and our understanding of particle physics..."

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

This paper puts Farrar on the map for me, for the first time actually. They are proposing what they describe as a preliminary phenomenological "toy model" involving Chiral Symmetry Restoration (CSR) above 10¹⁷ eV to explain excess muon production in UHECR. By their account the CSR "toy model" is remarkably successful:

PAGE 5 (section 2 A toy model of the New Physics)
"This first, simplistic version of a CSR model gives a remarkably good accounting of all the published shower observations discussed above. At 10¹⁹ eV, where the statistics are good and the energy is well above the postulated energy threshold for CSR, the ground signal and Xmax distributions are essentially perfectly described in every respect: ...[here they refer to several figures comparing predicted/observed distributions]...
Our toy Chiral Symmetry Restoration model is the only model known to date which can simultaneously fit the CIC curve of ground signal versus zenith angle, the Xmax distribution and its energy dependence, and the absolute calibration between SD and FD signals in hybrid events..."

ABSTRACT
"...A 'toy model' of UHE proton-air interactions is presented which provides the first fully consistent description of air shower observations..."

Given Farrar's track record and reputation this makes one sit up and take notice

I got interested in how they arrived at their model, and so explored a bit. Time permitting, I'll quote some excerpts. Hoping for further comment from anyone who has looked at the Farrar paper.

 

[marcus]

Here's a longer excerpt from the Farrar Allen paper, starting on page 4, with section 2:
====quote=====
A toy model of the New Physics
We now turn to identifying possible UHE phenomena with the potential to reduce the electromagnetic energy fraction in the UHECR air shower to account for observations. Two important general observations help restrict the search. First, while the ground muon signal varies from event to event, the variation is not dramatic, so the new phenomenon must affect almost all showers as opposed to being a rare occurrence.

This does not mean that every interaction at UHE needs to manifest new physics, since there are quite a few interactions in the VHE-UHE energy range: on average a 10¹⁹ eV primary proton shower has 2, 20 and 200 secondary collisions at energies above 10¹⁸, 10¹⁷ and 10¹⁶ eV ... Thus as long as the majority of events manifest whatever new physics is responsible for the higher muon content, and the threshold of the new physics is of order 10¹⁷ eV (14 TeV CM energy), the condition that almost all showers have increased muon content can be met.

Second, to produce a 50-100% increase in the muon ground signal, the new physics must impact
a very substantial fraction of the total energy in UHE final states. Production of possible new particles such as those predicted by Supersymmetry cannot account for the UHECR shower observations:... Many types of exotic particle production would actually reduce the muon signal because their decay products include missing energy which would thus be lost to the hadronic shower which yields the muons...
==endquote==