The

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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

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22

NA60