title: Indications of proton-dominated cosmic-ray composition above 1.6 EeV.
abstract: We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d<X(max)>/d[log(E)] of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4x10(18) eV.
authors.affiliation: Department of Physics, University of Utah, Salt Lake City, Utah, USA.
authors.forename: R U
authors.initials: RU
authors.lastname: Abbasi
authors.forename: T
authors.initials: T
authors.lastname: Abu-Zayyad
authors.forename: M
authors.initials: M
authors.lastname: Al-Seady
authors.forename: M
authors.initials: M
authors.lastname: Allen
authors.forename: J F
authors.initials: JF
authors.lastname: Amman
authors.forename: R J
authors.initials: RJ
authors.lastname: Anderson
authors.forename: G
authors.initials: G
authors.lastname: Archbold
authors.forename: K
authors.initials: K
authors.lastname: Belov
authors.forename: J W
authors.initials: JW
authors.lastname: Belz
authors.forename: D R
authors.initials: DR
authors.lastname: Bergman
authors.forename: S A
authors.initials: SA
authors.lastname: Blake
authors.forename: O A
authors.initials: OA
authors.lastname: Brusova
authors.forename: G W
authors.initials: GW
authors.lastname: Burt
authors.forename: C
authors.initials: C
authors.lastname: Cannon
authors.forename: Z
authors.initials: Z
authors.lastname: Cao
authors.forename: W
authors.initials: W
authors.lastname: Deng
authors.forename: Y
authors.initials: Y
authors.lastname: Fedorova
authors.forename: C B
authors.initials: CB
authors.lastname: Finley
authors.forename: R C
authors.initials: RC
authors.lastname: Gray
authors.forename: W F
authors.initials: WF
authors.lastname: Hanlon
authors.forename: C M
authors.initials: CM
authors.lastname: Hoffman
authors.forename: M H
authors.initials: MH
authors.lastname: Holzscheiter
authors.forename: D
authors.initials: D
authors.lastname: Ivanov
authors.forename: G
authors.initials: G
authors.lastname: Hughes
authors.forename: P
authors.initials: P
authors.lastname: Hüntemeyer
authors.forename: D
authors.initials: D
authors.lastname: Ivanov
authors.forename: B F
authors.initials: BF
authors.lastname: Jones
authors.forename: C C H
authors.initials: CC
authors.lastname: Jui
authors.forename: K
authors.initials: K
authors.lastname: Kim
authors.forename: M A
authors.initials: MA
authors.lastname: Kirn
authors.forename: E C
authors.initials: EC
authors.lastname: Loh
authors.forename: J
authors.initials: J
authors.lastname: Liu
authors.forename: J P
authors.initials: JP
authors.lastname: Lundquist
authors.forename: M M
authors.initials: MM
authors.lastname: Maestas
authors.forename: N
authors.initials: N
authors.lastname: Manago
authors.forename: L J
authors.initials: LJ
authors.lastname: Marek
authors.forename: K
authors.initials: K
authors.lastname: Martens
authors.forename: J A J
authors.initials: JA
authors.lastname: Matthews
authors.forename: J N
authors.initials: JN
authors.lastname: Matthews
authors.forename: S A
authors.initials: SA
authors.lastname: Moore
authors.forename: A
authors.initials: A
authors.lastname: O'Neill
authors.forename: C A
authors.initials: CA
authors.lastname: Painter
authors.forename: L
authors.initials: L
authors.lastname: Perera
authors.forename: K
authors.initials: K
authors.lastname: Reil
authors.forename: R
authors.initials: R
authors.lastname: Riehle
authors.forename: M
authors.initials: M
authors.lastname: Roberts
authors.forename: D
authors.initials: D
authors.lastname: Rodriguez
authors.forename: N
authors.initials: N
authors.lastname: Sasaki
authors.forename: S R
authors.initials: SR
authors.lastname: Schnetzer
authors.forename: L M
authors.initials: LM
authors.lastname: Scott
authors.forename: G
authors.initials: G
authors.lastname: Sinnis
authors.forename: J D
authors.initials: JD
authors.lastname: Smith
authors.forename: P
authors.initials: P
authors.lastname: Sokolsky
authors.forename: C
authors.initials: C
authors.lastname: Song
authors.forename: R W
authors.initials: RW
authors.lastname: Springer
authors.forename: B T
authors.initials: BT
authors.lastname: Stokes
authors.forename: S
authors.initials: S
authors.lastname: Stratton
authors.forename: S B
authors.initials: SB
authors.lastname: Thomas
authors.forename: J R
authors.initials: JR
authors.lastname: Thomas
authors.forename: G B
authors.initials: GB
authors.lastname: Thomson
authors.forename: D
authors.initials: D
authors.lastname: Tupa
authors.forename: A
authors.initials: A
authors.lastname: Zech
authors.forename: X
authors.initials: X
authors.lastname: Zhang