First measurement of the spectral function in high-energy nuclear collisions

S. Damjanovic, QM2005, 4-9 August, Budapest 1

First measurement of the spectral function in high-energy nuclear collisions

Sanja Damjanovic on behalf of the NA60 Collaboration

Quark Matter 2005August 4–9, Budapest, Hungary


Outline

Event sample

Data analysis event selection combinatorial background fake matches

Understanding the peripheral data

Isolation of an excess in the more central data

Comparison of the excess to model predictions


2.5 T dipole magnet

hadron absorber

• Origin of muons can be accurately determined• Improved dimuon mass resolution

Matching in coordinate

and momentum space

targets

beam tracker

vertex trackermuon trigger and tracking

magnetic field

or!

Measuring dimuons in NA60: concept


5-week long run in Oct.–Nov. 2003

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

present analysis: ~1/2 of total data

Event sample: Indium-Indium


Data Analysis


Selection of primary vertex

Beam Trackersensors

windows

The interaction vertex is identified with better than 20 m accuracy in the transverse plane and 200 m along the beam axis.

(note the log scale)

Present analysis (very conservative):

Select events with only one vertex in the target region,

i.e. eliminate all events with secondary interactions


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)

Muon track matching 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.


Determination of Combinatorial Background

Basic method:

Event mixing

takes account of charge asymmetry correlations between the two muons, induced by magnetic field sextant subdivision trigger conditions

talk by Ruben

Shahoyan, 5b


Combinatorial Background from ,K→ decays

Agreement of data and mixed CB over several orders of magnitude Accuracy of agreement ~1%


Fake Matches Fake matches of the combinatorial background are automatically subtracted as part of the mixed-events technique for the combinatorial background

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

Overlay MC (used for LMR): 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 (used for IMR): More complicated, but vital for offset analysis


Example of overlay MC: the

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

= 23 MeV fake = 110 MeV


Subtraction of combinatorial background and fake matches

For the first time, and peaks clearly visible in dilepton channel

Net data sample: 360 000 events

Mass resolution:23 MeV at the position

μμ channel also seen

Fakes / CB < 10 %

Real data !


Track multiplicity from VT tracks for triggered dimuons, shown separately for opposite-sign pairs, combinatorial background and signal pairs after subtraction of total background (including fakes).

Four multiplicity windows used in the further analysis:

Centrality bin multiplicity ⟨dNch/dη⟩3.8

Peripheral 4–28 17

Semi-Peripheral

28–92 63

Semi-Central 92–160 133

Central > 160 193

Associated track multiplicity distribution


Signal and background in 4 multiplicity windows

S/B

2 1/3

1/8 1/11

Decrease of S/B with centrality, as expected


Phase space coverage in mass-pT plane Final data after subtraction of combinatorial background and fake matches

The acceptance of NA60 extends (in contrast to NA38/50) all the way down to small mass and small pT

MC


Results


Understanding the Peripheral dataFit hadron decay cocktail and DD to the data

5 free parameters to be fit:

DD, overall normalization

(0.12fixed)

Fit range: up to 1.4 GeV


Comparison of hadron decay cocktail to data

all pT

Very good fit quality

log


The region (small M, small pT)

is remarkably well described

Comparison of hadron decay cocktail to data

→ the (lower) acceptance of NA60

in this region is well under control

pT < 0.5 GeV


Particle ratios from the cocktail fits

and nearly independent of pT; 10% variation due to the

increase of at low pT (due to ππ annihilation, see later)General conclusion:

peripheral bin very well described in terms of known sources low M and low pT acceptance of NA60 under control


Isolation of an excess in the more central data


Understanding the cocktailfor the more central data

Need to fix the contributions from the hadron decay cocktail Cocktail parameters from peripheral data? How to fit in the presence of an unknown source?

Nearly understood from high pT data, but not yet used

Goal of the present analysis: Find excess above cocktail (if it exists) without fits


Conservative approach

Use particle yields so as to set a lower limit to a possible excess


● data-- sum of cocktail sources

including the

Cocktail definition: see next slide

all pT

Comparison of data to “conservative” cocktail

Clear excess of data above cocktail, rising with centrality

fixed to 1.2

But: how to recognize the spectral shape of the excess?


Isolate possible excess by subtractingcocktail (without ) from the data

set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV

leads to a lower limit for the excess at very low mass

and : fix yields such as to get, after subtraction, a smooth underlying continuum

difference spectrum robust to mistakes even at the 10% level;consequences highly localized


Excess spectra from difference: data - cocktail

all pT

Clear excess above the cocktail , centered at the nominal pole and rising with centrality

Similar behaviour in the other pT bins

No cocktail and no DD subtracted


Systematics

Level of underlying continuum more sensitive

Illustration of sensitivity to correct subtraction of combinatorial background and fake matches; to variation of the yield

Structure in region completely robust


Enhancement relative to cocktail use mass range 0.2–0.9 GeV to normalize to

Total data,no DD subtracted

faster than linear rise with centrality, steeper for low pT

Errors are systematic, statistical errors are negligible


Comparison of excess to model predictions


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



pT dependence same conclusions


Understanding the spectral shape Dilepton rate Example:

thermal radiation based on white spectral function

propagate this through NA60 acceptance:no structure ! recover white spectrum !

By pure chance, for all pT and the slope of the pT spectra of the direct radiation, the NA60 acceptance compensates for the phase space factors and “extracts” the<spectral function>

integrate over space-time and momenta



Data and model predictions as shown (propagated through the NA60 detector) roughly represent the respective spectral functions, averaged over space-time and momenta.


Conclusions

• pion annihilation is a major contribution to the lepton pair excess in heavy-ion collisions

• no mass shift of the intermediate contrary to Brown / Rho scaling

• broadening of the intermediate , consistent with Rapp / Wambach

Documents

First measurement of the spectral function in high-energy nuclear collisions