production in proton-nucleus and indium-indium collisions

Michele FlorisUniversity and INFN, Cagliari, Italy.

on behalf of the NA60 Collaboration

ISMD 2005, Kromeriz, Czech Republic, August, 14th 2005.

Outline Motivation Apparatus Collected data Results in p-A Results in In-In Ongoing work for KK

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Motivation The study of meson production in heavy ion collisions

carries information about strangeness production Two channels have been studied:

Muons not influenced by the medium Previous SPS measurements: NA50

• Acceptance limited to high pT

KK Better mass resolution No physical BG Previous SPS measurements: NA49

• Broad pT coverage, but dominated by low pT

The discrepancy between the values of the inverse slope T measured by these experiments led to the so-called

puzzle New measurements from NA60 in In-In collisions

NA60 measures the channel with very good pT coverage Has access to the KK channel

17m

The vertex region:

The NA60 experiment

• Fixed target dimuon experiment at the CERN SPS• Apparatus composed of 4 main detectors

Zero degree calorimeter(centrality measurements)

Muon Spectrometer

Analysis strategy• Create sample of matched muons• Subtract combinatorial BG via event

mixing• Estimate fakes via overlay MC• Fit experimental spectra with

expected sources • Hadronic cocktail (Genesis)• IMR continuum (open charm and

Drell-Yan or empirical)

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Data Taking

Two samples discussed in this talk p-A Collisions (400 GeV protons)

Six targets, three nuclei (Be, In, Pb) Microstrips silicon vertex detector 4 days in 2002, 600 000 dimuons collected (at “low”

beam intensity: 1–3 x 108 protons/burst) New high statistics sample collected in 2004, still to be

analyzed In-In collisions (158 GeV ions)

Seven indium targets Silicon pixels vertex detector 5-week-long run in 2003, very good statistics (~ 230

million dimuon triggers on tape) ~ 50% statistics discussed in this talk

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Detector performance

Dimuon mass resolution at M ~ 1 GeV:In-In: ~ 23 MeV (independent of centrality)p-A: ~ 30 MeVPrevious dimuon experiments: ~ 80 MeV

p beam

400 GeV

In Pb

3 x BeBe

Clear separation of all targets (Z vertex resolution ~ 600–900 m in p-A, better than 200 m in In-In)

target boxwindows

7 In targets

z-vertex (cm)

Transverse vertexingwith 20 µm accuracy

Beam tracker station

Indium beam

158 A GeV

The acceptance of NA60 extends, all the way down to small M and pT

A (

%)

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BG+charm

production in proton-nucleus collisions

• Mass spectra described as a superposition of the low mass resonances decays into muons + charm + DY

• Fit to the mass spectra to extract the ratio

p-Be 0.062

0.04

p-In 0.083 0.07

p-Pb 0.081 0.06

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production in indium-indium collisions4 Centrality bins:• Npart estimated from a Glauber fit to

the EZDC spectrum

Fit input:• Hadron cocktail (genesis)• Low-level (empirical) continuum

source with exponential fall-off (to mimic continuum under the vector mesons)

• Parameters allowed to vary: , , and the continuum

• Arbitrarily normalize to the

Total sample: • 570 000 events after BG subtraction • 50% of the full statistics• S/B = 1/4

peripheralall pT

Signal

Cocktail

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Peripheral Bin

Peripheral bin studied in three pT bins The normalizations of the hadron decay cocktail and of the

continuum are independently fit in each pT bin and ratios are

nearly pT independent The peripheral

bin is well described in terms of expected sources but:

“Too many” low pT mesons Peripheral In-In is

not quite pp, it’s more like CC or OO

Effect of pion annhilation cannot be neglected

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/ cross section ratio – Vacuum

Vacuum contribution ( annihilation) important at low pT even in peripheral collisions

Effect becomes dramatic in more central collisions complicated

continuum below the

However, the excellent mass resolution of NA60 allows us to extract a

robust yield

Vacuum Cocktail Cocktail

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/ cross section ratio – Centrality Dependence

As a function of centrality:

We restrict analysis topT > 1 GeV

Increase of a factor ~2 from peripheral to central collisions

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/ comparison to NA50

A direct comparison is impossible, due to the contribution from pion annihilation, which must be even higher in Pb-Pb

collisions, and which NA50 cannot isolate

NA50 points converted to the windowpT>1.1 GeV/c assuming T=228 MeV

used

(lower limit for NA50 ratio)

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/ comparison to NA49 /

Same trend as a function of Npart constant

If we set the ratio to 0.07–0.08, as suggested by statistical models, then

the NA60 yield is a factor 1.5–2 higher than

the NA49 value

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transverse momentum spectrum

We select the events on the peak and use two side mass windows to estimate the pT distribution of the continuum

under the peak

Then we correct for the acceptance, calculated (by Monte Carlo) as a 2-dim matrix: pT and y

background

total

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There is no significant variation of the extracted inverse slope parameter, T, with rapidity

There is a clear increase from peripheral to central collisions

With full statistics, extension up to pT > 3 GeV/c should be feasible

pT spectrum versus y and centrality

Comparison to NA49/NA50

Pb-Pb

In-In

Si-Si

C-C

pp

The In-In measurement of NA60follows the NA49 systematics,

The disagreement between NA49 and NA50 is not due to the different decay channel

Average NA60 value

1. All pT: 253 ± 2 MeV

2. NA50 range (mT > 1.65 GeV/c): 244 ± 5 MeV

3. NA49 range (pT < 1.5 GeV/c): 260 ± 5 MeV Only small variations…

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pT (GeV/c)

MC

KK, Analysis strategy KK can be studied using charged tracks

reconstructed in vertex telescope Brute force method (no PID):

Assume all tracks are kaons Make invariant mass from all track pairs

Huge combinatorial BG Subtracted by event mixing

technique Kinematical cuts

Single tracks avoid phase space boundaries pT and p improve Signal/BG

Pair Opening angle improve Signal/BG

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Signal

Very clean signal!

MC

KK, Status

Measured spectrumCombinatorial BG

DATA

Something yet to be tuned (BG?)

Subtraction

• Detailed Monte Carlo (Venus) studies (full detector description)

• First attempts to get a signal out of the data

MC

Semi-peripheral In-In collisions

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Summary

NA60 is well suited to help understanding the “ puzzle” New In-In measurements Better pT coverage than previous experiments Capability to measure KK

ratio: Rise with Npart consistent with NA49 and NA50 Absolute values between NA49 and NA50

Inverse slope T of the pT-distribution: Agreement between NA49 and NA60 The difference between NA49 and NA50

is not due to the different channels probed KK

Full MC simulation shows the feasibility of the study Final tuning still needed for background subtraction in real data

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Lisbon

CERN

Bern

Torino

Yerevan

CagliariLyon

Clermont

BNL Riken

Stony Brook

Palaiseau

Heidelberg

BNL

56 people13 institutes 8 countries

R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen,

B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. de Falco, N. de Marco,

A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord,

N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço,

J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho,

G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,

P. Sonderegger, H.J. Specht, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof, D. Walker and H. Wöhri

The NA60 Collaborationhttp://na60.cern.ch/

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BACKUP

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Measuring DimuonsMWPC &

trigger hodosMWPC &

trigger hodosToroidalmagnet

Iron WallHadron absorber

TargetLast trigger

station

Limiting Factor:• Enegy loss• Multiple Scattering

Tracking before the hadron absorber

Vertex Detector

High multiplicity+

High luminosity

Rad-hard silicon pixels

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hadron absorber

and trackingmuon trigger

magnetic field

iron w

all

muonother

targets

Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorberand match them to muon measured in the spectrometer

Improved kinematics (~20 MeV/c2 at instead of 80 MeV/c2 in NA50)Origin of muons can be accurately determined

2.5 T dipole magnet

beam tracker vertex tracker

Concept of NA60

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Peripheral bin

0.5 < pT < 1.0 GeV/c pT > 1.0 GeV/cpT < 0.5 GeV/c

SignalCocktail

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Clear increase of the extracted slope parameter T with multiplicity

Purely statistical errors

pT spectra vs multiplicity

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Fit of the pT distribution in different pT ranges Differential” fits

Fix fit interval at pT=0.8 GeV and move the extremes

Dynamic range of 40 MeV in Teff (all centralities)

Flat trend for

peripheral

collisions Indication for flow

in In-In collisions

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Agreement of data and mixed CB over several orders of magnitude

Accuracy of agreement ~1%

Combinatorial Background from ,K→ decays

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A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with 2 < 3 are selected.

Fake matches are subtracted by a mixed-events technique (CB) and an overlay MC method (only for signal pairs, see below)

Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Telescope (VT) is done using the weighted distance (2) in slopes and inverse momenta. For each candidate a global fit through the MS and VT is performed, to improve kinematics.

Muon track matching

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Fake-match contribution localized in mass (and pT) space

= 23 MeV fake = 110 MeV

Example of overlay MC: the

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Comparison of data to RW, BR and Vacuum

Vacuum Cocktail Cocktail

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Mass spectrum in semi-central In-In collisionsComplicated continuum under the in more central collisionsHowever, the excellent mass resolution of NA60 allows us to extract a robust yield

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Predictions for In-In by Rapp et al (2003) for <dNch/d > = 140, covering all scenarios Theoretical yields, folded with acceptance of NA60 and normalized to data in mass interval < 0.9 GeV

Only broadening of (RW) observed, no mass shift (BR)

Comparison of data to RW, BR and Vacuum