# Presentation: 122346

## NA60 results on the  spectral function            in Indium-Indium collisions

- Sanja Damjanovic NA60 Collaboration

- Asilomar, 14 June 2006

## Outline

- Isolation of excess dimuons above hadron decays
- Mass spectra (published in PRL)
- Shape analysis of mass spectra (new)
-           Comparison to theory
- Acceptance-corrected pT spectra (new)
-           Comparison to theory

##  Event sample: Indium-Indium

-   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

## Main steps of the data analysis 

-   reconstruction of the event vertex within the      segmented target
-    matching of tracks from muon spectrometer      and silicon vertex telescope
-   assessment of combinatorial background by     event mixing
-    assessment of fake matches by overlay MC       and/or event mixing

-  **talk of Andre David**

## Subtraction of combinatorial background and fakes

- For the first time,  and  peaks clearly visible in dilepton channel ; even μμ seen

- Net data sample: 360 000 events

- Mass resolution:23 MeV at the  position

- Fakes / CB < 10 %

****

****

****

- Progress over CERES:  statistics: factor >1000resolution: factor 2-3

## Associated track multiplicity distribution

- Track multiplicity from VT tracks for triggered dimuons for

| Centrality bin | multiplicity | 〈dNch/dη〉3.8 |
| --- | --- | --- |
| Peripheral |      4–28 |        17 |
| Semi-Peripheral   |    28–92 |        63 |
| Semi-Central |   92–160 |      133 |
| Central |      > 160 |      193 |

**4 multiplicity windows:**

**opposite-sign pairs** **combinatorial background** **signal pairs**

- new: some part of the analysis also in 12 multiplicity windows

## Understanding the Peripheral data

- Fit hadron decay cocktail and DD to the data
-     5 free parameters to be fit:
- DD, overall normalization
-     (0.12fixed)
-     do the fits for all pT and three bins in pT

- Extrapolate fit parameters to full phase space
-      (using particle generator  “Genesis”)

## Comparison of hadron decay cocktail to data

## Full-phase-space particle ratios from the cocktail fits

-   and nearly
-   independent of pT;
-   10% variation due to
-   the 
-   enhanced  mostly
-   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

## Goal

-     Find excess above hadron decays     without fits
- Conservative approach    Use particle yields so as to set a     lower limit to a possible excess

## Comparison of data to “conservative” cocktail

- --  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 subtracting

- Isolate possible excess by subtracting
- cocktail (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 on the 10% level, since the consequences of such mistakes are 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

- Phys.Rev.Lett. 96 (2006) 162302

## Systematics                

- Systematic errors of continuum 0.4<M<0.6 and 0.8<M<1GeV   25%

- Illustration of sensitivity        to correct subtraction of      combinatorial background     and fake matches;   to variation of the yield

- Structure in  region  completely robust

##      Shape analysis of

-    excess mass spectra

## Excess mass spectra in 12 centrality windows

## Shape vs. centrality

-  3/2(L+U)  “continuum”
-   R=C-1/2(L+U)  “peak”

-  RR  peak/continuum

- nontrivial changes of all three variables at dNch/dy>100 ?

****

## RMS of total excess 

- RMS of total excess

- Consistency with shape analysis
- Further rise    starting at    dNch/dy =100    significant!
- (bad fit (2=3) for linear rise above dNch/dy=30)

****

##   Comparison of data 

-   to model predictions

## Two alternatives how to compare data to predictions 

-     use predictions in the form
-      decay the virtual photons * into +- pairs, propagate these     through the NA60 acceptance filter and compare  results to       uncorrected data at the output (presently done for mass    spectra in selected pT regions)
-     correct data for acceptance in 3-dim. space M-pT-y and     compare directly to predictions at the input (presently done   for pT spectra in selected mass regions)

-    conclusions as to agreement or disagreement of         data and predictions are independent of whether       comparison is done at input or output

## understanding the spectral shape at the output

- output:
- white spectrum !

- By pure chance, for the M-pT characteristics of direct radiation, without pT selection,the NA60 acceptance roughly compensates for the phase-space factors and directly “measures” the <spectral function>

- input:
- thermal radiation based on white spectral function

- all pT

- Acceptance filtering of theoretical prediction:

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

- Predictions for In-In by Rapp et al. (2003) for 〈dNch/d〉 = 140, covering all scenarios

- Theoretical yields 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 

- Data and predictions as shown, after acceptance filtering, roughly mirror the respective spectral functions, averaged over space-time and momenta.

## New theoretical developments since QM05  

- Brown and Rho, comments on BR scaling,      nucl-th/0509001Brown and Rho, formal aspects of BR scaling, nucl-th/0509002

- Rapp and van Hees,  parameter variations for 2, hep-ph/0604269Rapp and van Hees, 4, 6... processes, hep-ph/0603084

- Renk and Ruppert, finite T broadening, Phys. Rev. C71 (2005)  Renk and Ruppert, finite T broadening and NA60, hep-ph/0603110  Renk, Ruppert, Müller, BR scaling and QCD Sum Rules, hep-ph/0509134 Renk and Ruppert, What the NA60 dilepton data can tell, hep-ph/0605330

- Skokov and Toneev,  BR scaling and NA60, Phys. Rev. C73  (2006)

- Dusling and Zahed,  Chiral virial approach and NA60, nucl-th/0604071

##     Modification of  BR bychange of the fireball parameters  

- Parameter variations for Brown/Rho scaling

- even switching out all temperature effects  does not lead to agreement between  BR and the data

- Van Hees and Rapp,  hep-ph/0604269

## Comparison of data to RH(2+4+QGP)

- Vector-Axialvector Mixing: interaction with real ’s (Goldstone bosons). Use only 4 and higher parts of the correlator V  in addition to 2

- Use         4, 6 ...                 and          3, 5... (+1)    processes from ALEPH data, mix them, time-reverse them and get  +- yields

- van Hees and Rapp, hep-ph/0603084 **                               **

## Comparison of data to RH(2+4+QGP)

- In this model, the yield above 0.9 GeV is sensitive to the degree of vector-axialvector mixing and therefore to chiral symmetry restoration!

- whole spectrum reasonably well described, now even in absolute terms
- direct connection to IMR results >1 GeV from NA60

- Van Hees and Rapp, hep-ph/0603084
**                               **

## Comparison of data to RW(2+4+QGP)

- Comparison of data to RW(2+4+QGP)

- theoretical results plotted in absolute terms

## Comparison of data to RR

- Spectral function only based onhot pions (Dyson-Schwinger) , no baryon interactions included

- Continuum contribution from partons, dominating the region >1GeV

-  broadening described, except for low-mass tail

- Theoretical results obtained in absolute terms

-  **talk of Joerg Ruppert**

- Renk and Ruppert, hep-ph/0603110
**                               **

## Comparison of data to RR

- pT dependences

-  **talk of Joerg Ruppert**

- theoretical results plotted in absolute terms

## Acceptance-corrected    excess pT spectra 

- Acceptance-corrected    excess pT spectra

- Preliminary

##      reduce 3-dimensional acceptance correction in M-pT-y      to 2-dimensional correction in M-pT, using measured       y distribution as an input

-      use slices of m = 0.1 GeV       and pT = 0.2 GeV
-      resum to three extended       mass windows
-           0.4<M<0.6 GeV          0.6<M<0.9 GeV              1.0<M<1.4 GeV

- Present strategy of acceptance correction

- subtract charm from the data (based on NA60 IMR results)before acceptance correction

##  Experimental results on the y distribution of the excess 

- use measured mass and pT spectrum as input to the acceptance correction in y (iteration procedure)

- agreement betweenthe three pT bins

- results close torapidity distribution of pions (from NA49) for the same √s, as expected (RR)

##       Excess pT spectra for three centrality bins          

- (spectra arbitrarily normalized)

##  Centrality-integrated excess pT spectra           

-  Centrality-integrated excess pT spectra

- significant mass dependence (also vs. mT, see below )
- possible origin:
-  different physics     sources
-  radial flow
-  p-dependence of          in-medium spectral     function

- (arbitrarily normalized at pT=1GeV)

##     at high pT, rho like region hardest, high-mass region softest !  

-     at high pT, rho like region hardest, high-mass region softest !

- Illustration of mass dependence of pT spectra

- differential fits to pT spectra, assuming locally 1-parameter mT scaling and using gliding windows of pT=0.8 GeV  local slope Teff

- very low Teff at low pT, enormous dynamic range, hardly compatible with flow alone (for only one component)

- systematic errors <15 MeV

##        Systematics of low-pT data: acceptance    

-        Systematics of low-pT data: acceptance

- pT spectrum of  at low pT much flatter (higher Teff)

- acceptance of  inbetweenthat of the twomass windows

- enhanced yield at low pT not due to incorrect acceptance

## Systematics of low-pT data: combinatorial background     

- enhanced yield at low-pT seen at all centralities, including the peripheral bin

- estimate of errors at low pT, due to subtraction of combinatorial background: peripheral           1%semiperipheral  10% semicentral       20%central               25%

- enhanced yield at low pT not due to incorrect subtraction of combinatorial background

##   Comparison of data 

-   to model predictions

##     Comparison to theory: mass window 0.6<M<0.9 GeV

- 1-parameter differential mT fits

-  at low pT: better description by RH  at higher pT much better description by RR (freeze-out ρ incl.)

- (arbitrarily normalized at pT=1 GeV)

- in detail, data different from any theoretical prediction

##     Comparison to theory: mass window 1.0<M<1.4 GeV

- 1-parameter differential mT fits

- (arbitrarily normalized at pT=1 GeV)

-  RH: dominantly hadronic processes (4);  lower T; role of flow? RR: dominantly partonic processes (qq);      high T;  low flow

- comparison to data inconclusive

## Conclusions (I) :   data

- pion annihilation seems to be a major contribution to the lepton pair excess in heavy-ion collisions at SPS energies
- no significant mass shift of the intermediate 
- only broadening of the intermediate 
- strong mass dependence of pT spectra

## Conclusions (II) :   interpretation

-    all models predicting strong mass shifts of the      intermediate including Brown/Rho scaling, are      not confirmed by the data

-    theoretical investigation on an explicit  connection   between broadening and the  chiral condensate clearly required

-    pT spectra not yet fully described by any theory, rich in detail, promising handle on emission  sources and fireball dynamics

## The NA60 experiment

- ~ 60 people
- 13 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. Damjanović, 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, 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

## BACKUP

##    Peripheral Data

## Slide 47

##       Excess pT spectra for four centrality bins          

-       Excess pT spectra for four centrality bins

- peripheral bin at high-pT slightly softer

## Comparison of hadron decay cocktail to data

- is remarkably well described

- → the (lower) acceptance of  NA60 in this region is well under control

- pT < 0.5 GeV

## Comparison of hadron decay cocktail to data

- Again good agreement
- between cocktail and data

- 0.5 < pT < 1 GeV
-     pT > 1 GeV

##   Pt spectra

##     at high pT, rho like region hardest, high-mass region softest ! 

-     at high pT, rho like region hardest, high-mass region softest !

- Illustration of mass dependence of pT spectra

- integral fits to pT spectra, assuming 1-parameter mT scaling and varying the upper cut in pT (bad χ2 for full pT range)

##     Comparison to theory: mass window 0.4<M<0.6 GeV

- 1-parameter differential mT fits

- (arbitrarily normalized at pT=0.7 GeV)

-  at low pT: better description by RH/DZ  at higher pT much better description by RR

- in detail, data different from any theoretical prediction

## Teff from differential fits to pT spectra 

- Teff from differential fits to pT spectra

##   Shape analysis         

## RMS of continuum 

- RMS of continuum

- If observed peak subtracted, the remaining continuum described by RMS of a flat distribution, independent of dNch/dy

- Subtraction of cocktailcreates dip in the middle  RMS>RMSflat

##  of peak (from Gaussian fit)

-  of peak (from Gaussian fit)

- Sigma of peak consistent with sigma of cocktail/vacuum independent of dNch/dy

##   Further theoretical results         

## Comparison of data to RW, BR and Vacuum 

- Comparison of data to RW, BR and Vacuum 

- pT dependence

- same conclusions

## “fusion” of the two scenarios  

- Modification of  DM by
- “fusion” of the two scenarios

- Dropping Mass (DM) vs Rapp/Wambach

- Results of Rapp (8/2005):(not propagated through NA60 acceptance filter)

- Neither fusion nor parameter change able to make DM scenario unobservable

- change of the fireball parameters

## Skokov and Toneev on BR scaling    

- Skokov and Toneev on BR scaling

- full dynamical model including deconfinement transition

- n=0.3

- n=0

- standard scaling of pole mass, no broadening

- same modification to the vector dominance coupling

- theoretical results normalized to data

- even for n=0, no agreement with the data (like Rapp)

## Chiral Virial Approach Dusling/Zahed

- First attempt to describe the centrality dependence of the excess data.
- Reasonable description, but increasing overestimate of central  peak

## DZ Integrated Dimuon Rates

##         Acceptance

***      ******NA60 vs CERES***

## Comparison of lepton pair acceptance

**CERES**

- reduction of acceptance at low M and low pT similar

**CERES**

## Acceptance comparison of CERES and NA60 

-  acceptance variation of NA60 between <RW> and <BR> factor of 3
-  acceptance variation between 0.4 and 0.8 GeV different by     a factor of 2  between CERES and  NA60

- Source:Rapp/Wambachlow pT dominated

## Phase space coverage in pT-mass plane 

- CERES

- NA60

- Phase space coverage in pT-mass plane

##  Comparison of   NA60 to CERES

- Suppression of low mass part of RW similar in CERES and in NA60

## Phase space coverage in y-pT plane 

- Comparison to CERES
- MC simulation with RW (low-mass and low-pt dominated):
- both acceptances shifted relative to midrapidity, but difference only δy=0.3

## Phase space coverage in y-pT plane 

- Examples from MC simulations

- Optimal acceptance:
- at high mass, high pT
-         <y> = 3.5
- at low mass,  low pT
-            <y> = 3.8

- Shift of acceptance away from midrapidity not much different from CERES

## Experimental set-up

## Measuring dimuons in NA60: concept

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

- Matching in coordinate _and_ momentum space

- targets

- beam tracker

- vertex tracker

##    Background

##  Analysis Topics

- Low Masses:vector mesons + continuum rad. (LMR)
- Intermediate Masses: charm + continuum rad. (IMR)
- High Masses:J/, ’ and DY (HMR)

## Selection of primary vertex

**Beam Tracker****sensors**

**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

## Muon track matching 

- 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

## Combinatorial Background from ,K→ decays

- 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)
-      are 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.
-  Event mixing :
-       More complicated, but less sensitive to systematics

## Fake-match background

- example from overlay MC: the  fake-match contribution localized in mass (and pT) space:  = 23 MeV,              fake = 110 MeV; fake prob. 22%

- complete fake-match mass spectrum agreement between overlay MC and event mixing, in absolute level and in  shape, to within <5%

## Signal and background in 4 multiplicity windows 

- S/B

|    2 |  1/3 |
| --- | --- |
|  1/8 |  1/11 |

- Decrease of S/B with centrality, as expected

## Mixing events

- The event mixing requires 12 “pools” of single muons
- Two muon charges, times
- Six spectrometer trigger sextants
- And we can only mix single muons from
- The same target
- The same centrality class
- The same field polarities, running conditions, etc
- (~6000 event samples)
- For the mixing we only use single muons from the like-sign dimuon triggers
- The mixed event technique gives both shape and absolute scale

## Combinatorial background: normalization

- Assume that the probability to accept the pair ij is Pij = Pi Pj,
- Pi and Pj are the single muon probabilities for sextants *i* and *j *(as if NA60 would have collected single muon triggers)
- The observed number of pairs ij is Nij
- Then the single muon underlying probabilities are
- And the **absolute **probability to have a given muon is

- Muon spectrometertrigger system sextants

**Notes:**
Can go out

##       Sensitivity of                 

***  ******difference procedure***

## Sensitivity of the difference procedure 

- Sensitivity of the difference procedure

- Change yields of ,  and  by +10%:
-  enormous sensitivity, on the level of  1-2%, to mistakes in the particle yields.

- The *difference spectrum* is robust to mistakes even on the 10% level, since the consequences of such mistakes are highly localized.

## Excess spectra from difference data-cocktail

- No cocktail   and no DD subtracted

- pT < 0.5 GeV

- Clear excess above the cocktail , centered at the nominal  pole and rising with centrality
- Similar behaviour in the other pT bins

##   medium modification                             of  

## Slide 88

## Masking of  by pion annihilation

- Masking of  by pion annihilation

**Central Bin**

****

- The chances are better at high pT but no effect is visible there!

## Medium modification of 

- Flattening of the pt distributions develops very fast with centrality.
- Compatible with radial flow?
- Or indication of  broadening in the medium??

##   results from KEK-E325

## Slide 92

## Alternative interpretation: / interference

- Alternative interpretation: / interference

- Rob Veenhof, Winter Workshop on Nucl.Dynamics, Breckenridge 2005