Low-mass dimuons in Indium collisions
1
The NA60 Experiment at CERN http://cern.ch/na60 Monte-Carlo simulation The QDC Phase Diagram Low-mass dimuons Low-mass dimuons in Indium in Indium collisions collisions Physics motivation Strongly interacting matter under extreme conditions: Restructuring of QCD vacuum towards chiral symmetry restoration Disappearance of <qq> → change of spectral properties of light hadrons like masses and widths Degenerate parity doublets Most sensitive hadron: ρ (lifetime only 1.3 fm/c) The dilepton spectrum may provide a signal for the existence of a chirally restored phase created in heavy ion collision Energy dependence may elucidate the relative importance of T and µ B Needs good mass resolution, high statistics and associated multiplicity First hints for the ‘melting’ of the ρ: Chiral symmetry restoration? Much more statistics, better signal to background ratio and better mass resolution required for a convincing case Combined 95/96 data Effective number of electron pairs for m ee > 0.2 GeV/c 2 : 215±15 Mass resolution at the ω: ~ 6% Expectation for 2000 data About the same effective number of pairs Mass resolution at the ω: ~ 4% ω The dipole field in the target region leads to much better p T coverage than previous dimuon experiments after muon track matching S/B ~ 1/1.3 No centrality selection opposite-sign signal combinatorial background <1 % of total statistics From a very preliminary analysis of a very small event sample… With respect to CERES: • Higher statistics by factor ~200 • Signal/background improved by factor ~10 • Higher effective statistics by factor 2000 • Mass resolution ~2% better by a factor 2 • Full information on associated track multiplicity • Completely different systematic uncertainties The combinatorial background from π and K decays is estimated through a mixed-event technique using like-sign muon pairs. The normalization is preliminary and fake matches are not yet included. M (GeV) N ch < 90 90 < N ch < 180 180 < N ch < 320 The analysis of the dimuon mass distributions can be done as a function of the collision centrality ω ϕ dN/dM no centrality selection after applying a first order acceptance correction raw data ϕ→µµ events NA60 will solve the long standing ϕ → µµ puzzle between NA49 and NA50 • 100 000 ϕ → µµ decays in the full data sample • ϕ → K + K - decays also under analysis Good p T coverage down to the lowest dimuon masses Critical behaviour of the Chiral Condensate q , H Chiral susceptibility H reflects critical behaviour Directly related to hadronic spectral function Restoration of Chiral Symmetry Previous results on low-mass electron pairs from CERES m ee (GeV/c 2 ) CERES Pb-Au 158 GeV Phase-space coverage Yields and mass resolution Charged track multiplicity dependence p T spectra NA60 ϕ No dipole field Dimuons now competitive with respect to dielectrons! A(%) With 2.5 T field A(%) by a factor 50 for M ~ 500 MeV and p T ~ 500 MeV/c Acceptance improves in all M and p T windows Mass resolution 20–25 MeV at M µµ ~ 1 GeV ω ϕ ω ϕ central peripheral 1 2 3 Charged particle multiplicity for reconstructed dimuon events 1 2 3 ω and ϕ peaks still visible in central Indium-Indium collisions Chiral symmetry is spontaneously broken in hadronic matter… …and is restored in the deconfined phase d ) q , ( lim VT ) q q ( ) q q ( T V s q , m 0 0 2 2 NA60
description
Low-mass dimuons in Indium collisions. central. CERES Pb-Au 158 GeV. peripheral. m ee (GeV/c 2 ). Physics motivation. Phase-space coverage. The QDC Phase Diagram. ϕ. ω. The dipole field in the target region leads to much better p T coverage than previous dimuon experiments. - PowerPoint PPT Presentation
Transcript of Low-mass dimuons in Indium collisions
The
NA
60
Exp
erim
ent
at C
ER
Nht
tp:/
/cer
n.ch
/na6
0
Monte-Carlo simulation
The QDC Phase Diagram
Low-mass dimuonsLow-mass dimuonsin Indium collisionsin Indium collisions
Physics motivation
Strongly interacting matter under extreme conditions:
Restructuring of QCD vacuum towards chiral symmetry restoration
Disappearance of <qq> → change of spectral properties of light hadrons like masses and widths
Degenerate parity doublets
Most sensitive hadron: ρ(lifetime only 1.3 fm/c)
The dilepton spectrum may provide a signal for the existence of a chirally restored phase created in heavy ion collision
Energy dependence may elucidate the relative importance of T and µB
Needs good mass resolution,high statistics andassociated multiplicity
First hints for the ‘melting’ of the ρ:
Chiral symmetry restoration?
Much more statistics, better signal to background ratio and better mass resolution required for a convincing case
Combined 95/96 data
Effective number of electron pairsfor mee > 0.2 GeV/c2 : 215±15
Mass resolution at the ω: ~ 6%
Expectation for 2000 data
About the same effective number of pairs
Mass resolution at the ω: ~ 4%
ω The dipole field in the target region leads to much better pT coverage than previous dimuon experiments
after muontrack matching
S/B ~ 1/1.3
No centrality selection
opposite-sign
signal
combinatorial background
<1 % of total statistics From a very preliminary analysis of a very small event sample…
With respect to CERES:
• Higher statistics by factor ~200• Signal/background improved by factor ~10
• Higher effective statistics
by factor 2000• Mass resolution ~2%better by a factor 2• Full information on associated track multiplicity• Completely different systematic uncertainties
The combinatorial background from π and K decays is estimated
through a mixed-event technique using like-sign muon pairs. The
normalization is preliminary and fake matches are not yet included.
M (GeV)
Nch < 90
90 < Nch < 180
180 < Nch < 320The analysis of the dimuon mass distributions can be done as a function of the collision centrality
ω
ϕ
dN/d
M
no centrality selection after applying a first orderacceptance
correctionraw data
ϕ→µµ events
NA60 will solve the long standing ϕ → µµ
puzzle between NA49 and NA50• 100 000 ϕ → µµ decays in the full data sample• ϕ → K+K- decays also under analysis
Good pT coverage down to the lowest
dimuon masses
Critical behaviour of the Chiral Condensate
q,H
Chiral susceptibility H reflects critical behaviour
Directly related to hadronic spectral function
Restoration of Chiral Symmetry
Previous results on low-mass electron pairs from CERES
mee (GeV/c2)
CERESPb-Au 158 GeV
Phase-space coverage
Yields and mass resolution
Charged track multiplicity dependence
pT spectra
NA60
ϕ
No dipole field
Dimuons now competitive with respect to dielectrons!
A(%
)
With 2.5 T fieldA(%
)
by a factor 50 for
M ~ 500 MeV and
pT ~ 500 MeV/c
Acceptance improvesin all M and pT windows
Mass resolution20–25 MeV at Mµµ ~ 1 GeV
ωϕ
ωϕ
central
peripheral
1 2 3Charged particle multiplicity for reconstructed dimuon events
1
2
3
ω and ϕ peaks still visible in central Indium-Indium collisions
Chiral symmetry is spontaneously broken in hadronic matter…
…and is restored in the deconfined phase
d )q,(limVT
)qq()qq(T
V
sq,
m
00
22
NA60
