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The importance of knowing the primary mass – and how little we really know
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Transcript of The importance of knowing the primary mass – and how little we really know
The importance of knowing the primary
mass – and how little we really know
Alan Watson
University of Leeds
Pylos: 7 September 2004
Key Questions about UHECR
• Energy Spectrum above 1019 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!
“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 protonic nature of the primaries at the highest energies does not exist. This is a very important problem. Experimentally it is quite a difficult problem.”
G Cocconi: Fifth International Cosmic Ray Conference, Guanajuato, Mexico, 1955
Question of Mass Composition
“Fere libenter homines id, quod volunt, credunt!”
“Men wish to believe only what they prefer” Thanks to Francesco Ronga
~ 5%
Song et al Astroparticle Physics 2000
The energy estimates are HIGHER if Fe is assumed
Corrections necessary to determine energy from fluorescence
From Takeda et al Astroparticle Physics 2003
1.04
1.13
1.09
1.13
For S(600), the energy estimates are LOWER if iron is assumed
S0 = 50 vem
Mass Composition (i): Xmax with energy
Elongation Rate (Linsley 1977, Linsley and Watson 1981)
dXmax/ dlog E < 2.3X0 g cm-2/decade
from Heitler model Xmax = ln (Eo/c)/ ln 2 extended to baryonic primaries:
dXmax/ dlog E = 2.3X0 (1 - Bn - B)
where Bn = d ln(n)/ d ln E
and B = (-N/X0)(d ln N/d ln E)
Composition from depth of maximum (i)
Model dependent AND < 1019.25 eV
Abbasi et al: astro-ph/0407622
Some personal comments on the recent HiRes Composition PaperAbbasi et al (astro-ph/0407622)
Selection of events:
χ2 per dof < 20
2 measures of Xmax within 500 g cm-2
Measurements within 400 g cm-2 for global fit to 2 eyes
But resolution of Xmax 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 Xmax fluctuations (ii)
p
Fe
Solid lines: data
Models are Sibyll and QGSjet
BUT diurnal and seasonal atmospheric changeslikely to be very important
Astroparticle Physics in press; also data shown at ICRC2003
M. Risse et al ICRC03
“Standard” Atmospheres can bias composition inferences
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 ~ A0.14
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
Claim: Consistent with proton dominant component
19 19.5 20 20.5
Log(Energy [eV])
−2
−1
0
1
Log(
Muo
n de
nsity
@10
00m
[m–2
])
Results from the AGASA array
Kenji Shinosaki: 129 events > 1019 eV
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 ~ 1017 eV
From Ralph Engel’spresentation in Leeds,July 2004
Plots by Maria Marchesini
AGASA data: a second look
QGSjet(i)
(i)(ii)
(ii)
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 distributionHaverah Park data: Ave et al. 2003
Estimate of Mass Composition
The fraction of protons (F
p
) as a function of energy for two QGSjet models (’98, dotted line and ’01, solid line). The three low energy points correspond to a range in which there is a well-understood trigger bias that favours steep showers [24].
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 K-H Kampert’s talk
- protons or iron? HARDER: will useXmax , LDF, FADC traces,Radius of curvature…
Mass information from study of Inclined Showers
M. Ave: 80°, proton at 1019 eVDetails in Ave, Vazquez and Zas, Astroparticle Physics
Ave et al. PRL 85 2244 2000
Haverah Park:Photon limit at 1019 eV < 40% (@95% CL)
AGASA: muon poor events
Gamma-ray fraction upper limits (@90%CL)
34% (>1019eV) (/p<0.45)
56% (>1019.5eV) (/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 1017 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 1019 eV at your own risk!