f Meson Production in In-In Collisions and Highlights from NA60

Meson Production in In-In Collisions and

Highlights from NA60Michele Floris1

for the NA60 CollaborationStrangeness in Quark Matter 2007

1University and INFN, Cagliari, Italy

June 28, 2007 Strangeness in Quark Matter 2

OutlineThe NA60 Experiment Detector Concept

Phi Meson Production in In-In Collisions Analysis details pT, y and decay angular distributions ratio

Highlights from NA60 In medium modification of the Intermediate mass range excess: prompt or

charm? Centrality dependence of J/ suppression


The NA60 Experiment• Fixed target dimuon experiment at the CERN SPS• Apparatus composed of 4 main detectors

17m

The vertex region (2 detectors):

Zero degree calorimeter(centrality measurements)

Muon Spectrometer

Origin of muons can be accurately determinedImproved dimuon mass resolution (~20 MeV/c2 at instead of 80 MeV/c2)

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

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)

magnetic field

>10m<1m


Data Sample InIn 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 Two data samples

Different current settings in the Muon Spectrometer magnet Different acceptances High current setting suppresses LMR


Meson Production: Motivation Strangeness production in Heavy Ion Collisions Mass and Width changes of the in the medium

Differerences between →KK and → K meson modification in the medium pT dependence of K suppression

Two channels have been studied in 158 AGeV PbPb :

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

puzzles: Absence of in-matter modifications of width in KK Discrepancy between absolute yields Discrepancy between T slope: radial flow (NA49) or no radial flow (NA50)?

Measurements from NA60 in In-In collisions NA60 measures the channel with good pT coverage (0-2.6 GeV) Rapidity, decay angular distribution, pT and ratio

Spectra: Analysis Procedure

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

background

total

Acceptance would require correction with 2D matrices: pT vs y and decay angle vs pT

After tuning MC to data (iterative procedure) 1D acceptance correction

Systematic error: variation of analysis cuts and parameters

5 centrality bins

4000 A data set only


Rapidity Distribution

Width estimated with a Gaussian fitConstant within errorsAgreement with previous measurements in other colliding systems at the same energy

gau

s

All centralities

= 1.13 ± 0.06 ± 0.05

reflected

gau

ss


Decay Angular Distributions

pprojectile ptarget

z axisCS

pµ+

y

x

Viewed from rest frame

Collins-Soper

pprojectile ptarget

z axisGJ

pµ+

y

Viewed from rest frame

Gottfried-Jackson

pprojectile ptarget

z axisHel

pµ+

x

Helicity

y

The angular distribution of the positive muon can be measured with respect to 3 different quantization axes

1. Collins – Soper 2. Gottfried – Jackson3. Helicity

Angular distributions fitted with the function:

2cos1cos

d

dN

polarization

Provides information on the production mechanismPrevious measurements: ACCMOR (h-Be) and Sixel et al. (K- - p / - - p)

Non negligible (GJ Frame)Heavy Ion

Global polarization?

The 3 frames are identical for pT 0First measurement in HI collisions at the SPS

We studied centrality and pT dependence of in the 3 framesFurther developments: azimuthal distributions, study wrt the reaction plane


Helicity Distribution

All centralities

= 0.1 ± 0.1 ± 0.1

= 0, independent of centralityAnalysis repeated at pT < 1 GeV and pT > 1GeV.

No evidence for ≠ 0.

pT > 0.2 GeV/c


Gottfried – Jackson Distribution

All centralities

= 0. ± 0.1 ± 0.04

= 0, independent of centralityAnalysis repeated at pT < 1 GeV and pT > 1GeV.

No evidence for ≠ 0.

pT > 0.2 GeV/c


Collins – Soper Distribution

All centralities

= -0.2 ± 0.2 ± 0.2

Hint for < 0 in peripheral events? Not significant (2).Acceptance limits fit range Large errors on

Analysis repeated at pT < 1 GeV and pT > 1GeV. No evidence for ≠ 0.

pT > 0.2 GeV/c


Tm

TT

Tekdm

dN

m/1

Spectra fitted with the function:

mT Distribution

Depends on the fit range in presence of radial flow Effective temperatureCentrality dependence stronger at low pT

Linear mass dependence at low pT


Tslope: Fit Range Dependence

NA60 In-In (pT < 1.6 GeV/c)NA49 Pb-PbNA50 Pb-Pb

NA60 In-In (pT > 1.1 GeV/c)NA49 Pb-PbNA50 Pb-Pb

Low pT (NA49): Agreement with NA49 when the fit is performed in the same range High pT (NA50): Lower T absolute values, flatter rise with centrality.

No agreement with NA50 Difference between NA50 and NA49 was not due to different decay channel Hint for the presence of radial flow → Blast Wave analysis


ratio: Analysis Procedure

ratio extracted from a fit of the mass distribution with expected sources:

MCData

• 2-body and Dalitz decays of low mass mesons• Open charm continuum (low level)

Parameters allowed to vary: and the continuum

region in central bins parameterised to reproduce the NA60 excess data. Little dependence on the parameterisation (~ 5%)

pT > 1 GeV/c


ratio

pT > 1 GeV/c

Full pT and y

yield increases from peripheral to central collision by a factor ~ 3(Consistent with previous measurement)Absolute yield measurement in progress


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.

However, 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).

The study of this excess was one of the main objectives of the NA60 experiment at SPS.

NA38/NA50 proton-nucleus data

centralcollisions

M (GeV/c2)

IMR Excess: Previous Measurements


NA60 Measurement of the IMR excessNA60 can separate the prompt and open charm contribution on a statistical basis by measuring the dimuon offset with respect to the primary vertex

2/)2( 11212

xyyyxx VyxVyVx

2/)( 22

21

Single muon weighted offset

Dimuon weighted offset

To eliminate the momentum dependence of the offset resolution, we use the muon offset weighted by the error matrix of the fit:

J/ muons

Offset resolution ≈ ~40 m, < c(D+ : 312 m, Do : 123 m)


Sources (open charm and Drell Yan) simulated using PythiaMonte Carlo dimuons reconstructed on top of a real event

IMR: Expected Sources

Analysis of the mass spectra in the range 1.16 GeV/c2 < M < 2.56 GeV/c2 Coverage in Collins-Sopper angle: | cos CS | < 0.5

Analysis repeated for the 2 samples (4000 A and 6500 A) and for different cuts on the matching 2 (2

match < 1.5 and 2match < 3.0)

Relative normalization:Drell Yan: Reproduce high mass cross section measured by NA3 and NA50Open Charm: Cross section which reproduces the NA50 p-A dimuon data Yield of expected sources in units of expected cross section in the following

Fit range

4000 A, 2match <1.5

Fit of the mass spectra with prompts fixed to Drell-Yan (within 10%) shows that the dimuon yield in IMR is higher than expected

4000 A,2match <1.5

6500 A, 2match

<1.5

Fit range

6500 A, 2match <1.5

Data integrated in collision centralities and in pT

and the fit to the offset spectra shows that the excess is prompt.


~2.4 times more prompts are required than what Drell-Yan provides. The two data sets, with different systematics, are consistent with each other Obtained Charm contribution is lower than extrapolation from NA50 p-A data. Statistics is not enough

for its study vs centrality and pT, it will be fixed to 0.7 0.15 (average of 4 and 6.5 kA data)

4000 A, 2match < 3

6500 A, 2match < 3

4000 A, 2match <1.5

6500 A, 2match <1.5

Offset fits with free prompt and charm


Our statistics is not enough to study the differentially in centrality.

But bulk of existing measurements is in agreement with expectation of its scaling with number of

binary collisions, characteristic for hard process: = 1 in

In further analysis the normalization factor for will be fixed to 0.7 0.15 (wrt extrapolation from

NA50 pA data) leading to 9.5±2 b/nucleon

cc

AccpA 0

H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180

cc

cc Cross Section


NA60

H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180

Effect of nuclear modification of PDFs

CC: comparison with other measurements


The excess acceptance correction is done differentially in M and pT

Assuming: flat cos CS distribution for decay angle and rapidity distribution similar to Drell-Yan (y~1)

Once the excess is corrected for acceptance, the two data sets can be summed up

Systematic errors account for uncertainty in Drell-Yan and

Charm normalization factors

Excess corrected for acceptanceDefine the excess as Signal – [ Drell-Yan (1± 0.1) + Open Charm (0.7±0.15) ]


Centrality and pT dependence of excess

Excess/Nparticipants(arb. scale) Excess already present in peripheral collisions, scales faster than NPart

Excess stronger at low pT


Fit in 0.5<PT<2 GeV/cFit in PT<2.5 GeV/c

pT Spectra of the excess

TEFF is rather low compared both to the Drell-Yan and to the Low Mass Region (T ~ 250 MeV)

Drell Yan


Summary Meson Production T slope increases with centrality and depends

on fit range → hint for radial flow Compatible with NA49

Absolute yields measurement in progress

IMR excess Excess is prompt Open charm yield agrees with NA50 p-A Excess is qualitatively different from Drell-Yan


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/

BACKUP


ratio

pT > 1 GeV/c

Full phase space

Compared to NA50 : NA50 data have a common mT > 1.5 GeV/c2 cut

Extrapolated to pT > 1 GeV/c using Tslope measured by NA50Ambiguity: need to assume to extract

(Arbitrary rescaling of NA50 data)

NA60NA50 (Arb. Rescaled)


pT Distribution

peripheral

central

Tm

TT

Tekdp

dN

p/1 Spectra fitted with the function:

to extract the Tslope Depends on the fit range!


Acceptance would require correction with 2D matrices: pT vs y and decay angle vs pT.After tuning MC to data (iterative procedure) 1D acceptance correction

Systematic error: variation of analysis cuts and parameters:

• 2 cut of Matched Dimuon• Fake subtraction method • Side windows Offset• Mass window width

Vertex resolution (in the transverse plane)

The interaction vertex is identified witha resolution of 10–20 m accuracy in the transverse plane

Dispersion between beam track andVT vertex

Vertex resolution (deconvoluting BT=20 m)

10

20

30

0

(

m)

Number of tracks

Beam Tracker measurement vs. vertex reconstructed with Vertex Tracker

BTBT

The BT measurement

( = 20 m at the target) allows us to control the vertexing resolution and systematics


Charm and Drell-Yan contributions are obtained by overlaying real event data on dimuons generated by Pythia 6.326 (CTEQ6L PDFs with EKS98 nuclear modifications. mc=1.5 GeV/c2.

kT=0.8 for Drell-Yan and 1 for Charm)

The fake matches in the MC events are subtracted as in the real data, by event mixing.

Relative normalizations:

Drell-Yan: K-factor of 1.9. Reproduces In-In data at M>4 GeV/c2 and cross sections measured by NA3 and NA50 (J. Badier et al. (NA3 Coll.), Z.Phys.,C26: (1985) 489. M.C. Abreu et al. (NA50 Coll.), Phys. Lett. B410 (1997) 337).

Charm: = 13.6 b/nucleon. Obtained from the cross section describing NA50 p-A dimuon

data at 450 GeV by its rescaling to 158 GeV using Pythia.

Note: this is factor ~2 higher than the extrapolation from the “world average” cross section

(H.Woehri and C.Lourenco, J.Phys. G30 (2004) 315)

Possible explanation: both NA60 and NA50 detect

dimuons only in |cos|<0.5, while

shows very strong rise at large cos Our full phase space acceptance for charm is

very sensitive to the correctness of kinematic

distribution from Pythia

DD

cc

NA60 Signal Analysis: simulated sources


Absolute normalization: the expected Drell-Yan contribution, as a function of the collision centrality, is obtained from the number of observed J/ events and the suppression pattern:

Data is split in 12 bins in collision centrality (number of participants obtained from the measured charged multiplicity in the Vertex Tracker).

In each bin the number of J/ events is extracted and corrected for the anomalous suppression (E.Scomparin, proceedings of Quark Matter 2006, Shanghai)

Expected number of events Drell-Yan events at 2.9<M<4.1 GeV/c2 is extracted from DY accounting for the nuclear absorption of the J/

A 10% systematical error (mostly due to the uncertainty of the J/ nuclear absorption cross section) is assigned to this normalization.

Signal shapes used to fit the dimuon weighted offset distributions are:

prompt : mixture of J/ and data (open charm contamination is < 1%)

charm: Monte Carlo smeared by amount needed for J/ and MC to reproduce data

The fits to mass and weighted offset spectra are reported in terms ofthe DY and Open Charm scaling factors needed to describe the data

DD

NA60 Signal analysis: simulated sources


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

Fake matches are subtracted by a mixed-events technique 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 Tracker (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


Combinatorial BackgroundCB (uncorrelated muon pairs coming from and K decays) is estimated with an Event Mixing technique

Take muons from different events and calculate their invariant mass. Takes account of charge asymmetry, correlations between the two muons (induced by magnetic field sextant subdivision: detector geometry), trigger conditions

Apparatus triggers both opposite sign () and like sign () pairs. Quality of CB is assessed comparing LS spectra. Accuracy ~1% over several orders of magnitude!

Fakes in CB also subtracted!


Fake Matches “Fake Matches” are those tracks

where a muon track from the Muon Spectrometer is matched to the wrong track from the Vertex Tracker

Fake matches of the signal pairs (<10% of CB) can be obtained in two different ways: Overlay MC

Superimpose MC signal dimuons onto real events. Reconstruct and flag fake matches. Choose MC input such as to reproduce the data. Start with hadron decay cocktail + continuum; improve by iteration.

Event mixing More rigorous, but more complicated. Less statistics

hadron absorber

muon trigger and tracking

target

fake

correct


= 23 MeV

fake = 110 MeV

Example of Overlay MC: the

Fakes calculation with Overlay MC and Mixing

method agree in absolute level and shape within 5%!


Fakes/CB < 10 %

For the first time and peaks clearly visible in dilepton channel

(23 MeV mass resolution at the

also visible

Clean Spectrum

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f Meson Production in In-In Collisions and Highlights from NA60