Highlights from the NA60 Experiment

06/10/2008 Alessandro De Falco (NA60) SQM08

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Highlights from the NA60 Experiment

Alessandro De Falco

University and INFN Cagliari

on behalf of the NA60 collaboration

SQM08, Beijing, China


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Outline

• The NA60 Experiment– Detector Concept

• Phi Meson Production in In-In Collisions– Analysis details

– pT, y and decay angular distributions

– yield– Raw spectra from KK analysis

• Eta meson production in In-In collisions• Highlights from IMR

– Origin of excess: prompt or charm?

– pT spectra of excess

– T vs mass


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The NA60 detector layout

Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them (in both angles and momentum) to the tracks measured in the spectrometer

Origin of muons can be accurately determinedImproved dimuon mass resolution (~20 MeV/c2 at instead of 80 MeV/c2)Additional bend by the dipole field extends thedimuon coverage down to low pt

High luminosity experiment: possible with radiation tolerant detectors and high speed DAQ

2.5 T dipole magnet

hadron absorber

targets

beam tracker vertex trackermuon trigger and tracking (NA50)

mag

netic field

>10m<1m


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Data sample• In-In collisions at 158 AGeV Incident beam

energy– 5 weeks in Oct.-Nov. 2003– ~ 4 ∙ 1012 ions delivered– ~ 230 million dimuon triggers

• Data analysis– Select events with

only one reconstructed vertex in target region (avoid re-interactions)

– Match muon tracks from Muon Spectrometer with charged tracks from Vertex Tracker (candidates selected using weighted distance

squared matching 2)– Subtract Background


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The puzzle

Historic facts on NA49 KK vs NA50

Yields in hadronic channel lower than in leptonic channel by factor >2 in central collisions

Inverse slopes in central collisions Hadronic (low pT) ~ 300 MeV Leptonic (high pT) ~ 230 MeV

puzzle: in-medium effects on and kaons + kaon absorption and rescattering leading to reduced yield and hardened pT spectrum in hadronic channel?

More recently:– CERES hadronic yield and inverse slope similar to NA49.

Within large errors leptonic yield also compatible with NA49 No puzzle?– New NA50 analysis confirms previous results within 8%


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We select the events on the peak and use two side mass windows to estimate the pT,y and decay

angle distribution of the continuum under the peak

Systematic error: variation of analysis cuts and parameters

5 centrality bins

4000 A data set only

Extraction of differential spectra

Acceptance: Overlay Monte Carlo tuned to data with an iterative process


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All centralities

= 0.1 ± 0.1 ± 0.1

cos1cos

d

dN Fitted with

= 0, independent of centrality

All centralities

= 1.13 ± 0.06 ± 0.05

reflected

Rapidity and cos distributions

constant vs centralityAgreement with previous measurementsin other colliding systems at the same energy


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transverse mass distributions

Box: stat+syst. error

Tm

TT

Tekdm

dN

m/1

Spectra fitted with the function:

Depends on the fit range in presence of radial flowEffective temperature (larger T at low pT)


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T slopes in In-In and Pb-Pb collisions

NA60 In-In (pT < 1.6 GeV)NA49 Pb-Pb (pT < 1.6 GeV)NA50 Pb-Pb (pT > 1.1 GeV)

NA60 fits at low pT (NA49 range)


NA60 In-In (pT > 1.1 GeV)NA49 Pb-Pb (pT < 1.6 GeV)NA50 Pb-Pb (pT > 1.1 GeV)


NA60 low vs high pT: maximal difference in T slopes only ~ 15 MeV

presumably related to radial flow

well below difference between NA50 and NA49 (~ 70 MeV) in the most central bin significant extra hardening of hadronic channel beyond radial flow?

NA60 fits at high pT (NA50 range)

Ceres Pb-Pb (KK, pT > 0.75 GeV) Ceres Pb-Pb (ee, pT < 1.5 GeV)


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cross section and yield

DAQtrigevselrec

obs

effAions Acc

N

LN

A

N

1

Beam Counters

Effective Target Length• Beam Absorption• Transverse Size

Monte Carlo

Dedicated Runs

Beam Counters(DAQ-vetoed)

Total Systematic Uncertainties ~ 10%

InelInIn

multiplicity

b32.4)(23/13/13/13/12 BABARInel

InIn

Eur. Phys. J. C13 69 (2000)


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Yield: Alternative Method

multiplicity extracted from the measured ratio

Alternative method with independent systematics! Total systematic error ~ 13%

Total number of binary collisions in a given centrality bin

J/ cross section x BR in a nucleon-nucleon collision

Inelastic nucleon-nucleon cross section

collNN

JNN

CORRNJ

/

/

CORRJ

/

J/ multiplicity is corrected for nuclear and anomalous suppression


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Yield: Results

b7.005.001.7 16.001.062.1 2.004.048.1

Yield integrated in centrality:• Direct method: • J/ Calibration:

Results in full phase space and corrected for BR = 2.86 · 10-4

Centrality Dependence (average of the 2 methods)

Box: stat+syst. errorBox: stat+syst. error

scales faster than Npart


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yields In-In - comparison to other systems

Npart in central In-In collisions in the muon channel exceeds the corresponding values in Pb-Pb collisions in the KK channel Unambiguous comparison to NA50 in full phase space not feasible due to the observed differences in the T valueAn extrapolation with the extreme hypoteses T=220 MeV and T=300 MeV leadsto values with respect to In-In that are larger than a factor of 2

CERES Pb-Pb KK

CERES Pb-Pb ee


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KK in In-In collisions• No PID Huge combinatorial background• Event mixing technique• Residual background present in spectra• Data taken mainly with dimuon trigger: high multiplicity more populated

0.6 < pT < 1.8 GeV/c

510)3.06.6( NS/B~1/400

510)1.07.1( NS/B~1/300

410)4.00.3( NS/B~1/100

<Npart> = 39 <Npart> = 75 <Npart> = 132


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KK: mass spectra vs pT

Despite of the residual backgrounda pT distribution can be extractedin most central bins starting from pT>0.9 GeV/cEnough statistics up to ~2.5 GeV/c

Still ongoing analysis

<Npart> = 132


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Raw pT spectrum in the KK channel

Not corrected for the acceptance

<Npart> = 132


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production in In-In collisionsStrangeness content of = 40% (for it is 100%)

enhancement from peripheral to central In-In collisions

What about enhancement?

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/

/

InIn

Periphpart

InIn

Centralpart

N

N

increases by a factor of 1.34 from most peripheral to most central In-In collisions

InIn

Periphpart

InInpart

partCPN

NNR

/

/)(


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RCP: vs

increases by a factor of 3, while increases by a factor of 1.34 from most peripheral to most central In-In collisions


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To describe the strange particle yield, an additional parameter S is introduced in the canonical partition functionS suppresses the phase space of particles composed of valence s or s quarks

Statistical model Becattini et al.:

For central In-InS~0.8

Taking from the SM that production is primary

S vs Npart

Phys.Rev.C73:044905,2006


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Strangeness content of = 40%. One expects

6.0)(40.0 2 PartSPart

NN

In case of feed-down from other resonances:

kk

imary

jj jkBr )(Pr

The largest contribution to secondary yield owes to 1(1400) and a0 which are NOT S suppressed

Knowing the centrality dependence of S we can calculate what one should expect for the RCP of production if there would not be any feed down from higher resonances and compare to the measured values.

/Npart calculated from S


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The measured values of the eta RCP (red points) agree within errors with the expectations for the primary production of the

RCP data vs expectation for primary production


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IMR: previous measurements NA38/NA50 was able to describe the IMR dimuon spectra in p-A (Al, Cu, Ag, W) collisions at

450 GeV as the sum of Drell-Yan and Open Charm contributions.

The yield observed by NA50 in heavy-ion collisions (S-U, Pb-Pb) exceeds the sum of DY

and Open Charm decays, extrapolated from the p-A data (factor ~2 excess for central Pb-Pb)

What is the origin of this excess?

NA38/NA50 proton-nucleus data

centralcollisions

M (GeV/c2)


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Offset and mass distributions

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measurement of muon offsets :distance between interaction vertex and track impact point

charm not enhanced; excess prompt; 2.4 × DY

excess similar to open charmsteeper than Drell-Yan

isolation of excess by subtraction of measured open charm and expected Drell-Yan


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Excess/DY vs pT

The process responsible for the production of excess dimuons is significantly softer than Drell-Yan dimuons


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IMR excess pT and mT spectraThe pT spectra in 3 different masswindows are clearly differentwhile Drell-Yan pT spectra and massspectra factorize, the primordial kT=0.8 GeV/c being independent of mass

T=199 ± 21 ± 3 MeVT=193 ± 16 ± 2 MeVT=171 ± 21 ± 3 MeV

mT spectra fitted by the function

Tm

TT

Tedm

dN

m/1

for pT>0.5 GeV/c


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Centrality integrated mT spectra

effTTT

Tmdm

dN

mexp~

1

steepening at low mT; not observed for hadrons (like

fit mT spectra for pT>0.4 GeV with

monotonic flattening of spectra with mass up to M=1 GeV, followed by a steepening above

signs for mass-dependent radial flow?

Phys. Rev. Lett. 100 (2008) 022302


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Evolution of inverse slope Teff with mass

Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV

Rise consistent with radial flow of a hadronic source (here →→) , taking the freeze-out ρ as the reference

Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→)

Phys. Rev. Lett. 100 (2008) 022302


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Summary puzzle:

– Yield in leptonic channel in In-In larger than in hadronic channel in Pb-Pb

– Dependence of T slope on pT suggests NA49 vs NA50 T difference larger than expected from radial flow

– Physical origin of effect?

– New NA60 data on KK to come soon!

– PbPb measurements in a NA60-like experiment?

production: only primary

Intermediate mass region

– Prompt excess

– Drell-Yan process does not reproduce mass spectrum

– Excess softer than Drell-Yan

– T slope dependence on mass suggests a source of partonic origin

Documents

Highlights from the NA60 Experiment