The importance of knowing the primary mass – and how little we really know

Alan Watson University of Leeds [email protected]

Pylos: 7 September 2004

Key Questions about UHECR

•

Energy Spectrum above 10 19 eV?

•

Arrival Direction distribution?

•

Mass Composition?

•

Aim of talk is to show where I think that we have got to in trying to answer the fundamental question of what is the mass at the highest energies.

Life may be less simple than some theorists seem to think!

Question of Mass Composition

“We remain with the dilemma: protons versus heavy

nuclei. A clear cut decision cannot be reached yet. I believe that up to the highest energies the protons are the most abundant in the primary cosmic rays. However, I must confess that a leak proof test of the problem. Experimentally it is quite a difficult

problem.” G Cocconi: Fifth International Cosmic Ray Conference, Guanajuato, Mexico, 1955

Corrections necessary to determine energy from fluorescence ~ 5% The energy estimates are

HIGHER

assumed if Fe is Song et al Astroparticle Physics 2000

For S(600), the energy estimates are

LOWER

if iron is assumed S 0 = 50 vem

1.04

1.13

1.09

1.13

From Takeda et al Astroparticle Physics 2003

Mass Composition (i): X max with energy Elongation Rate (Linsley 1977, Linsley and Watson 1981) dX max / dlog E < 2.3X

0 g cm -2 /decade from Heitler model X max =



ln (E o /



c )/ ln 2 extended to baryonic primaries: dX max / dlog E = 2.3X

0 (1 - B n - B



) where B n = d ln(n)/ d ln E and B



= (-



N /X 0 )(d ln



N /d ln E)

Composition from depth of maximum (i)

Model dependent AND < 10 19.25

eV Abbasi et al: astro-ph/0407622

Some personal comments on the recent HiRes Composition Paper Abbasi et al (astro-ph/0407622)

Selection of events: χ 2 per dof < 20 2 measures of X max within 500 g cm -2 Measurements within 400 g cm -2 for global fit to 2 eyes But resolution of X max claimed as 30 g cm -2 from Monte Carlo

-

BUT surely the resolution will depend on the distance from the Eyes (apparently not considered) Periods of calibrated and uncalibrated atmosphere (419 and 134 events) put together

-

would have been interesting to have seen these groups apart

HiRes Composition from X max fluctuations (ii) p

BUT diurnal and seasonal atmospheric changes

Solid lines: data

likely to be very important

Fe Models are Sibyll and QGSjet

Astroparticle Physics in press; also data shown at ICRC2003

“Standard” Atmospheres can bias composition inferences

M. Risse et al ICRC03

From L Perrone (Auger group): Catania CRIS meeting

Mass Composition (iii): muons

Muon Content of Showers: N



(>1 GeV) = AB(E/A

 

) p ( depends on mass/nucleon ) N



(>1 GeV) = 2.8A(E/A

 

) 0.86 ~ A 0.14

So, more muons in Fe showers Muons are about 10% of total number of particles Used successfully at lower energies (KASCADE) VERY expensive - especially at high energies - conclusions derived are rather model dependent

Results from the AGASA array Claim: Consistent with proton dominant component

Kenji Shinosaki: 129 events > 10 19 eV 1 0 − 1 − 2 19 19.5

20 Log(Energy [eV]) 20.5

Model dependence of muon signals Sibyll 1.7: Sibyll 2.1: QGSjet98 1: 1.17:1:45 Important to recall that we do not know the correct model to use.

LHC CMS energy corresponds to ~ 10 17 eV

From Ralph Engel’s presentation in Leeds, July 2004

(i) QGSjet (ii) AGASA data: a second look (ii) (i) Plots by Maria Marchesini

Mass Composition (iv): Using the lateral distribution



(r)~ r –(



+ r/4000)

circa 1978: Feynman Scaling Primary Uranium?!

Sample LDF compared with new model: QGSjet’98

Distribution of lateral distribution

Haverah Park data: Ave et al. 2003

T h

Estimate of Mass Composition

QGSjet models (’98, dotted line and ’01, solid line).

First 3 points: trigger bias

Lateral distribution data from Volcano Ranch interpreted by Dova et al (2004) Astropart Phys (in press)

Comparisons from Dova et al (2004) Astropart Phys

Are results consistent between different methods applied by same experimental group? An extreme situation HiRes/MIA data: Abu-Zayyad et al: PRL 84 4276 2000

Ideas to explain the Enigma

•

Decay of super heavy relics from early Universe (or top down mechanisms) Wimpzillas/Cryptons/Vortons

•

New properties of old particles?

•

Breakdown of Lorentz Invariance?

or is it ‘simple’?

•

Are the UHE cosmic rays iron nuclei?

•

Are magnetic field strengths really well known?

Potential of the Auger Observatory

•

Directions

 •

Energy

 •

Mass

-

photons

 -

neutrinos



protons or iron?

K-H Kampert’s talk

HARDER:

will use X max , LDF, FADC traces, Radius of curvature…

Mass information from study of Inclined Showers

M. Ave: 80°, proton at 10 19 eV Details in Ave, Vazquez and Zas, Astroparticle Physics

Ave et al. PRL 85 2244 2000

Haverah Park: Photon limit at 10 19 eV < 40% (@95% CL) AGASA: muon poor events Gamma-ray fraction upper limits (@90%CL) 34% (>10 19 eV)

( g /p<0.45)

56% (>10 19.5

eV

) ( g /p<1.27) 60 ° < θ < 80° Ave, Hinton, Vazquez, aaw, and Zas PRL 85 244 2000

An Elegant Mass Determination Method

•

Zatsepin Effect

Zatsepin 1951 Zatsepin and Gerasimova 1960 Solar Magnetic Field Important Medina Tanco and Watson (1998) “..events from this very beautiful idea are too infrequent to be of use in any real experiment…”

Typical scale is ~ 1000 km

Conclusions

Beware

: the experimentalists are still some way from AGREED statements about the mass composition above 10 17 eV - after one studies the differences between different experiments - and even

the different conclusions

from within the same experiment.

From Auger,

we will get neutrino and photon limits (signals?) more readily than baryonic masses - but we have many tools in our armoury and should succeed in getting the latter, when we fully understand the showers and our hybrid detector. (

Recall: ground breaking was only 5 years ago)

.

Personal view

: assume 100% protons above 10 19 eV at your own risk!