Sunday, July 22, 2007

GZK cutoff confirmed

In an earlier post, Bee explained the physics behind the GZK (Greisen, Zatsepin and Kuzmin) cutoff: protons traveling through outer space will - when their energy crosses a certain threshold - no longer experience the universe as transparent. If their energy is high enough, the protons can scatter with the omnipresent photons of the Cosmic Microwave Background, and create pions. As a result, their mean free paths drops considerably and only very little of them are expected to reach earth. This threshold for photopion production for ultra high energetic protons is known as the GZK cutoff.

The presence of this cutoff had been observed by the HiRes cosmic ray array (Observation of the GZK Cutoff by the HiRes Experiment, arXiv:astro-ph/0703099), but had been disputed by the results from the Japanese detector AGASA (Akeno Giant Air Shower Array) which caused excitement when it failed to see the cut-off in data obtained up to 2004. A third experiment, the Pierre Auger Observatory on the plains of the Pampa Amarilla in western Argentina, which started taking data last year, now settled the question:

"If the AGASA had been correct, then we should have seen 30 events [at or above 1020 eV], and we see two," says Alan Watson, a physicist from the University of Leeds, U.K., and spokesperson for the Auger collaboration [source]. According to Watson, the data also suggests that these highest energy rays comprise protons and heavier nuclei, the latter of which don't feel the GZK drag.

The results were announced on the 30th International Cosmic Ray Conference in Merida, Yucatan, Mexico, and had a brief mentioning in Nature. The Nature article also points out that there is prospect of identifying the regions of the sources of the highest energetic particles, but these data are preliminary. "Unless I talk in my sleep, even my wife doesn't know what these regions are", as Watson was quoted in Nature.

And of course, now that there is new data, somebody is around to claim one needs an even larger experiment to understand it: "Now we understand that above the GZK cutoff there are ten times less cosmic rays than we thought 10 years ago, so we may need a detector ten times as big as Auger," says Masahiro Teshima of the Max Planck Institute for Physics in Munich, Germany, who worked on AGASA and is working on the Telescope Array [source].

The recent paper by the Pierre Auger collaboration with more details was on the arxiv last week:
    The UHECR spectrum measured at the Pierre Auger Observatory and its astrophysical implications
    T.Yamamoto, for the Pierre Auger Collaboration, arXiv:0707.2638

    Abstract: The Southern part of the Pierre Auger Observatory is nearing completion, and has been in stable operation since January 2004 while it has grown in size. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detize. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detector data, and the other of the hybrid data, where the precision of the fluorescence measurements is enhanced by additional information from the surface array. The complementarity of the two approaches is emphasized and results are compared. Possible astrophysical implications of our measurements, and in particular the presence of spectral features, are discussed.


The upper end of the cosmic ray energy spectrum as measured by the Pierre Auger Observatory: The black dots represent data points, the blue and red curves are expectations derived from different models for the composition and energy distribution of the cosmic ray particles, all based on well-established physics including the GZK cutoff mechanism. Two events cannot be understood as stemming from protons, but may well be explained by heavier nuclei. (Figure from T. Yamamoto, The UHECR spectrum measured at the Pierre Auger Observatory and its astrophysical implications, ICRC'07; Credits: Auger Collaboration, technical information)

More plots and data can be found on the websites of the Pierre Auger Observatory.


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12 comments:

CapitalistImperialistPig said...

They should forward their data to the WSJ editorial page. I'm sure those guys would have a really interesting explanation for the huge jump above 20 GeV.

CapitalistImperialistPig said...

In a slightly more serious vein, is there any understanding of acceleration mechanisms that would produce such large energies without disruption of multi-baryon nuclei?

Anonymous said...

The GZK cutoff is strange. Naively one would expect that the higher the energy, the larger the mean free path...

changcho

paul valletta said...

Hi Stephan

Could it be that future experiments would be produced, that incorporate a spacebourne accelerator, at a specific altitude, and then protons could be fired at a detector at a lower altitude, maybe aboard a satallite in low orbit? and then collect the paerticle shower data?

Why not have a particle accelerator "above" the earth, surely this would be a better representation of what is actually occuring?

Do you see this as a future feasible option?

m said...

GZK anomaly gone, PVLAS anomaly gone, LSND anomaly gone. Great year for physics

Plato said...

Hi Paul,

The ground would certainly be much more fertile for future energies needed to think about the role string theory might play? We need the higher energies:)

One does not denign that high energy events take place, or that calorimeters used in Glast are design for for a reason.

We just needed a way in which the photon voiced it's nature in the gravitatonal field. It's color?

As if L1 or L2 in lagrangian coordinates, would show these deviatons in the nature of the gravity field?


Some physicists have speculated that such energetic particles could only come from the decay of exotic, heavy subatomic particles formed immediately after the Big Bang. But seeing five high-energy rays from the same point rules that out, says Farrar, because the chance of finding that many exotic decays along the same line of sight is minuscule.

Our window on the Universe is much diffeent now that SNO has given us a deeper understanding of the events in the cosmos.

Take our sun for instance.

Thomas Larsson said...

Hm. If luminosity drops to zero above 10^20 eV, the COM energy for a cosmic-ray experiment is limited to 10^10 eV, right? How can this compete with accelerators?

Thomas Larsson said...

Uh, my brain was evidently not working this early in the morning. If the cosmic particles hit 1 GeV protons, the COM energy is sqrt(10^29) = 3*10^14 eV, of course. Nevertheless, this is not that much more than the LHC energy 10^13 eV, and the luminosity is close to nothing.

Bee said...

Hi Changcho:

The GZK cutoff is strange. Naively one would expect that the higher the energy, the larger the mean free path...

Think about the higher energetic particle moving faster: the faster it travels, the denser the medium will appear. For the proton it looks as if the typical distances between photons become smaller and smaller.

Hi M,

GZK anomaly gone, PVLAS anomaly gone, LSND anomaly gone. Great year for physics

;-) Makes one feel a bit like a doctor who's supposed to be happy if everybody is healthy, but actually is longing for an unknown disease?

Hi Thomas,

If luminosity drops to zero above 10^20 eV, the COM energy for a cosmic-ray experiment is limited to 10^10 eV, right? How can this compete with accelerators?

This is the com for proton photon. Cosmic ray showers are instead produced by collisions of the incoming particles with atoms in the upper atmosphere, usually not by scattering with the CMB.

Best,

B.

Bee said...

Hi Thomas:
Sorry, our comments crossed, you were 5 seconds faster :-) Yes, the com energy is not so much higher, but in addition cosmic rays have other problems to cope with (e.g. the understanding of the shower causes additional uncertainties). The observations are a good source of information - they can complete but not replace collider physics. Best,

B.

stefan said...

Hi Paul,

Could it be that future experiments would be produced, that incorporate a spacebourne accelerator, at a specific altitude, and then protons could be fired at a detector at a lower altitude, maybe aboard a satallite in low orbit? and then collect the particle shower data?

Hm, practical points aside - how do you deploy a sufficiently strong accelerator in space, and provide the electrical power to run it - I could imagine that it may be interesting to have such an experiment. You could create atmospheric showers by well-defined primary particles, and use this to gauge the models which simulate these showers, and which are essential to estimate the energy of the primaries from the Cherenkov light and the shower particles you detect at ground level. However, I do noth think that this alone wold be worth the enormous cost of such an experiment.

I do not see much sense in targetting such a spacebourne accelerator at a detector on a satellite. Putting detectors for cosmic ray primaries on satellites may be interesting in principle, but one has to keep in mind that the luminosity of the highest energy cosmic ray primaries is very low - remember the roughly one event per square kilometer per century at the GZK cutoff energy.

Best, stefan

stefan said...

Hi changcho,

The GZK cutoff is strange. Naively one would expect that the higher the energy, the larger the mean free path...

Yes, but one has to keep in mind also that the cross section for proton-photon reactions gets quite high at sufficiently large energies, when you hit the Delta resonances. This means that the CMB gets quite "opaque" for protons within this energy range. At energies beyond the Delta resonances, the cross section drops again a bit, but then keeps on growing slowly with ever higher energies - see figure 39.17 in this PDG compilation, the upper curve on the lower figure, labelled γp_total.


Best, stefan