The importance of knowing the primary

mass – and how little we really know

Alan Watson

University of Leeds

a.a.watson@leeds.ac.uk

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!