First measurement of the rho spectral function in high-energy nuclear collisions.
We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360,000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi- -->rho -->mu+mu- annihilation. The associated space-time averaged spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.
Arnaldi
R
R
Università di Torino and INFN, Turin, Italy.
Averbeck
R
R
Banicz
K
K
Castor
J
J
Chaurand
B
B
Cicalò
C
C
Colla
A
A
Cortese
P
P
Damjanovic
S
S
David
A
A
De Falco
A
A
Devaux
A
A
Drees
A
A
Ducroux
L
L
En'yo
H
H
Fargeix
J
J
Ferretti
A
A
Floris
M
M
Förster
A
A
Force
P
P
Guettet
N
N
Guichard
A
A
Gulkanian
H
H
Heuser
J M
JM
Keil
M
M
Kluberg
L
L
Lourenço
C
C
Lozano
J
J
Manso
F
F
Masoni
A
A
Martins
P
P
Neves
A
A
Ohnishi
H
H
Oppedisano
C
C
Parracho
P
P
Pillot
Ph
P
Puddu
G
G
Radermacher
E
E
Ramalhete
P
P
Rosinsky
P
P
Scomparin
E
E
Seixas
J
J
Serci
S
S
Shahoyan
R
R
Sonderegger
P
P
Specht
H J
HJ
Tieulent
R
R
Usai
G
G
Veenhof
R
R
Wöhri
H K
HK