Latest Results of NA60 - UCLA Physics & Astronomyhome.physics.ucla.edu/calendar/conferences/sqm2006/... · A. Förster,CERNPH 2 Introduction - NA60 is a second generation experiment,

A. Förster,CERNPH 1

Andreas Förster

CERN-PH

on behalf of the NA60 Collaboration

Latest Results of NA60


Introduction

- NA60 is a second generation experiment, designed to answer specific questions left open, in the leptonic sector, by the previous round of SPS experiments, finished in 2000

- It has been designed in order to reach unprecedented accuracy in the measurement of muon pair production in HI collisions

- After its approval in 2000, NA60 has taken data in 2002 (p+A), 2003 (In+In) and 2004 (p+A), now being analyzed


Experimental Setup

MUON FILTERBEAM

TRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

BEAM

IC

not to scale

Matching in coordinate and in momentum space: - improved dimuon mass resolution - origin of muons can be accurately determined

!or

beam

~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal MagnetIronwallHadron absorber

ZDC

Target area µ

µ


Vertex and Offset Resolution

Beam Trackersensors

Vacuum boxwindows

Vertex reconstruction

J/

- Vertex resolution: ~200 m in z ~10-20 m in x,y- Good target ID even for most peripheral collisions (> 3 tracks)

- Offset resolution of the dimuon vertex: 40 - 50 m


Centrality Estimation

Target

Projectile

Target

Projectile

EZDC (GeV)

- Collision centrality estimated from the energy released in the Zero Degree Calorimeter (ZDC)

Number of participants

- Number of participants (Npart

) and

other centrality variables estimated using a Glauber calculation


In+In 158 AGeV

LMR IMR HMR

Analysis Topics:

1) Dimuons: a) Low Mass Dimuons (LMR) - vector mesons in medium b) Intermediate Mass Dimuons (IMR) - open charm/thermal radiation c) High Mass Dimuons (HMR) - anomalous J/suppression

2) Charged Particles a) → KK b) Elliptic Flow

Indium beam of 158 GeV/nucleon ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape


Dimuons:Comb. Background Subtraction

dimuon mass [GeV]

- Most pairs are comb. backgr.- Subtraction by event mixing- Quality controlled by comparing mixed event like sign dimuon spectra to measured like sign dimuon spectra

- and peaks visible with a mass resolution of 20 - 23 MeV

signal statistics:360 000 events


Low Mass Dimuons

CERES/NA45 - CERES: well established excess in Pb+Au above the hadronic decay cocktail (that describes p+Be data)

- Different theoretical explanations: mass shift, broadening, ...

- Good resolution and statistics needed


Peripheral Collisions

- 4 centrality bins, defined by Nch

- Fit independently in 3 bins in pt

- Free parameters: DD, overall normalization

- The peripheral data are well reproduced by the hadronic cocktail - Good fit quality down to low mass and low p

t

(low acceptance region well under control)


Excess in Central Collisions

• data sum of the cocktail sources including the ρ

- Comparison to a “conservative” cocktail, definition: see next slide

- ρ/ω fixed to 1.2

- Clear excess of data above cocktail, rising with centrality


Isolating the Excess

- Conservative approach: Use particle ratios such as to set a lower limit to a possible excess: - fix yields such as to get a smooth underlying continuum after subtraction

- set upper limit, defined by “saturating” the measured yield around 0.2 GeV → leads to lower limit for the excess at low masses

- not subtracted


Comparison to Model Predictions

- Predictions for In+In ( ⟨dNch

/d⟩=140 )

by Rapp et al. (2003) for different scenarios

- Theoretical yields folded with NA60 acceptance and normalized to data for M < 0.9 GeV

- Excess shape consistent with broadening of the (Rapp,Wambach RW)

- Models predicting a mass shift clearly disfavoured (Brown,Rho BR)


Low Mass Dimuons: Summary

Lepton pair excess at SPS energies confirmed

Mass shift of the intermediate clearly disfavoured

Broadening of the intermediate describes the data


High Mass Dimuons: J/ Suppression

- Anomalous J/ suppression discovered by NA50 in Pb+Pb collsions

- Generally considered one of the most direct signatures of deconfinement

- NA60: is anomalous J/ suppression as well present in the lighter system In+In?


J/ Suppression in In+In

J/ψ

ψ’

DYBackground

Charm

- NA50: Normalization to Drell-Yan → scales with number of NN-collisions → no sizeable final state effects

- NA60: very limited Drell-Yan statistics (~ 300 for M

> 4.0 GeV)

→ only three bins


Direct J/ Sample

Nuclearabsorption

- directly compare measured J/ yield with the theoretical distribution as expected in the case of pure nuclear absorption


Comparison to Model Predictions

Satz, Digal, FortunatoRapp, Grandchamp, BrownCapella, Ferreiro

J/ψ absorption by produced hadrons (comovers) - Capella and Ferreiro, Eur.Phys.J. C42 (2005) 419

J/ψ suppression in the QGP and hadronic phases - including thermal regeneration and in- medium properties of open charm and charmonium states Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; J.Phys.G 30 (2004) S1355

χc suppression by deconfined partons when geometrical percolation sets in - Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.


High Mass Dimuons : Summary

Anomalous J/ supression observed in In+In

Centrality dependent, with an onset around Npart

= 90

Theoretical predictions (tuned on Pb+Pb) do not properly describe the In+In data


Charged Particles:Reconstruction of the Reaction Plane

- NA60 acceptance: ~ 0 < ycm

< 1

- Use elliptic flow v2 to estimate the event

plane (v1 = 0 at midrapidity)

- Determination from charged particle tracks as measured in the silicon pixel vertex tracker

- Reaction Plane: Plane defined by the beam direction and the impact parameter vector


Determination of the Event Plane

- Event Plane: estimate of Reaction Plane from experimental data using the flow itself- has to be corrected for the influence of the resolution

- Emission angles of all particles i are related to the event plane angle

via the event flow vector Q

- we use n=2 (v2), since v

1 = 0 at midrapidity

Qn , x = Qn cosnn =∑i

pt , i cosni

Qn , y = Qn sin nn =∑i

pt , i sin nin =

1n arctan Qn , y

Qn , x


The Azimuthal Acceptance of NA60

- The NA60 acceptance is highly assymetric in the azimuthal angle - Beam not perfectly centered in the experimental coordinate system


<cos(2)>

Recentering(Poskanzer,Voloshin PRC58 (1998) 1671 , NA49, PRC 68 (2003) 034903)

- Recentering performed per track in (pt,y)-acceptance bins (like in NA49)

Qx = Q cos2=∑i

pt , icos2i−⟨cos2⟩ pt , y

Qy = Q sin 2=∑i

pt , isin 2i−⟨sin 2⟩ pt , y

- Separately for each of the 7 subtargets- Only (p

t,y)-bins with

correction factors < 0.2 used


Flattening

uncorrected after recentering▴ after flattening

- Recentering by definition only removes distortions in the same Fourier order as used for the event plane determination- Higher orders removed by a flattening procedure (Poskanzer,Voloshin PRC58 (1998) 1671)- This imposes that the event plane distribution has to be flat


Event Plane Resolution- Resulting values for v'

2 have to be corrected for the event plane resolution

- v2 = v'

2 / <cos[2()]>

- Correction factors determined from subevents using the method of Ollitrault


MonteCarloSimulation

- Generation of + and -

with an assumed v2(p

t)

- Tracking through experimental setup using Geant- Using the same data analysis as for real data

- Input is reproduced very well


Azimuthal Distributions

h±

- Fit to the data: dN/d ~ 1 + (2 v'

2 cos[2])

- v2 = v'

2 / <cos[2()]>


pt - Dependence of v

2

- Results are not corrected for non-flow correlations due to HBT effect- Therefore, we only show results for p

t > 0.5 GeV

- ~ 50% of total measured statistics

- v2 saturates for large p

t


pt and Centrality Dependence of v

2


Azimuthal Distributions of J/

J/ J/

- Large statistical errors (~50% of the total measured statistics)


Azimuthal Distributions : Summary

NA60 can measure azimuthal particle emission patterns

Charged particles show a significant v2 , saturating at high p

t

Azimuthal distributions of J/have large statistical errors (50% of total statistics analyzed so far)


Summary

Low Mass Dimuons: - Lepton pair excess at SPS energies confirmed - Mass shift of the intermediate clearly disfavoured - Broadening of the intermediate describes the data

High Mass Dimuons: - Anomalous J/ supression observed in In+In - Centrality dependent, with an onset around N

part = 90

- Theoretical predictions (tuned on Pb+Pb) do not properly describe the In+In data

Charged Particles: - NA60 can measure azimuthal particle emission patterns - Charged particles show a significant v

2 , saturating at high p

t

- Azimuthal distributions of J/have large statistical errors (50% of total statistics analyzed so far)


The NA60 Collaboration

Lisbon

CERN

Bern

Torino

Yerevan

CagliariLyon

Clermont

Riken

Stony Brook

Palaiseau

Heidelberg

BNL

~ 60 people13 institutes8 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. Damjanovi , A. David, A. de Falco, N. de Marco,ć

A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A.Förster,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, P. Pillot, G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin,

J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, H.J. Specht, R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri

Documents

Latest Results of NA60 - UCLA Physics & Astronomyhome.physics.ucla.edu/calendar/conferences/sqm2006/... · A. Förster,CERNPH 2 Introduction - NA60 is a second generation experiment,