# Presentation: 320213

## Dimuon Measurements by NA60

**NA60 overview **
**Charmonium production: J/******
**Intermediate mass dimuons and open charm**
**m****T**** distribution from dimuons**
**Summary**

**Axel Drees, Stony Brook University**
**RHIC AGS Users Meeting, June 2007, BNL**

## Lepton-Pair (NA60) Physics Topics

**Many medium effects expected in dilepton spectra**

**known sources of lepton pairs **

- Axel Drees

## Discoveries at the SPS

**Discoveries at the SPS**

**Charm enhancement or thermal radiation?**

**medium modified**
**spectral functions?**

**NA60 designed to resolve **
**these issues**

- Axel Drees

## NA60 Detector Setup

** ****Improved dimuon mass resolution, reduced background,**
**and open heavy flavor identifiction**

**targets**

- beam tracker

- vertex tracker

**Matching of muon tracks in**
**position and momentum space**

**Muon Spectrometer (NA50)**

**Silicon Pixel Vertex Telescope**

**NA60 Detector Setup**

**ZDC**

- Axel Drees

## A New Era: NA60 Data from In-In Collisions

**2003 In-In run at 158 AGeV**
**     ****5x10****12**** ions on target **
**     ****2.3x10****8**** dimuons recorded**
**Raw unlike-sign dimuon mass distributions **

**A New Era: NA60 Data from In-In Collisions**

- Vertex selection and
- muon track matching

- M()  80 MeV

- M(J/)  100 MeV

**Narrow vector mesons clearly resolved**
**Significant background rejection**
**But still remaining unphysical background**

- Axel Drees

## Combinatorial background from decays of  and K mesons 
Use event mixing method
Accuracy estimated by comparing to measured like-sign spectra N++ and N

**Use event mixing method**
**Accuracy estimated by comparing to measured like-sign spectra N****++ ****and N******

**Combinatorial Background from Event Mixing**

**Accuracy ~****1% independent of mass**

- Axel Drees

## Deconvolve  spectrum by sequentially fitting different contribution 
Mass shape of signal processes from MC (PYTHIA+GRV94LO PDF)
First fit Drell-Yan for mass > 2 GeV
Fix combinatorial background and charm in intermediate mass region 2.2<M<2.5 GeV 
Last extract charmonia yields 
Results from both data sets consistent  use average in the following
Systematic checks of the J/ / DY ratio:
Change of input distributions in MC calculation  0.3% (cos), 1% (rapidity) 
Tuning of quality cut for muon spectrometer tracks  < 3%

**Deconvolve  spectrum by sequentially fitting different contribution **
**Mass shape of signal processes from MC (PYTHIA+GRV94LO PDF)**
**First fit Drell-Yan for mass > 2 GeV**
**Fix combinatorial background and charm in intermediate mass region 2.2<M<2.5 GeV **
**Last extract charmonia yields **
**Results from both data sets consistent ******** use average in the following**
**Systematic checks of the J/******** / DY ratio:**
**Change of input distributions in MC calculation ******** 0.3% (cos********), 1% (rapidity) **
**Tuning of quality cut for muon spectrometer tracks ******** < 3%**

**J/******** / DY Analysis**

- Axel Drees

## J/ / DY vs Centrality

**3 centrality bins defined by E****ZDC**
**bin1 ******** ********Npart******** = 63  **
**bin2 ******** ********Npart******** = 123 **
**bin3 ******** ********Npart******** = 175 **
**Data points have been normalized to the expected J/******** normal nuclear   absorption, calculated with****	****	****	****	**
**	****with as measured with p-A NA50 data at 400 and 450 GeV**
**Quantitative agreement with  NA50 results as a function of N****part**

**Anomalous suppression present in Indium-Indium**

- J/abs = 4.18  0.35 mb

- B. Alessandro et al., Eur. Phys. J. C39(2005) 335

- Axel Drees

## J/ yield extracted from fit to data 
J/ expectation (NuclearAbsorption) calculated as:
pp yield scaled by number of binary collisions
Suppressed by nuclear absorption (abs =4.18 mb) 
Both taken from measurements by NA50  
Relative normalization adjusted to min. bias value of (J/)/DY = 0.87 ± 0.05

**J/******** expectation (N****uclear****A****bsorption****) calculated as:**
**pp yield scaled by number of binary collisions**
**Suppressed by nuclear absorption (********abs**** =4.18 mb) **
**Both taken from measurements by NA50  **
**Relative normalization adjusted to min. bias value of (J/********)/DY = 0.87 ± 0.05**

**J/******** Yield vs Nuclear Absorption (NA)**

- Axel Drees

## Discussion of Uncertainties (in terms of NA)

**Statistical errors substantially reduced by avoiding Drell-Yan normalization**
**At the price of slightly increased systematic errors:**
**statistical error on normalization ****	****	****~  6%**
**Glauber model parameters****	****	****	****~ 12% (only central collisions)**
**E****ZDC**** to N****part**** ****	****	****	****	****~ 9% (mostly for central coll.)**
**Sytematic error common to both analysis:**
**nuclear absorption parameters**
**	****   ********abs****(J/********)  ****	****	****	****	****~  4%**
**	****   ********pp****(J/********)  ****	****	****	****	****	****~  8%**

**Systematic error on scale (analysis 2)****	****	****11%**
**Systematic error comparing analysis 1&2****	****6%**
**Centrality dependent sys. Errors ****	****	****<15%**

- Axel Drees

## Annomalous J/ Suppression in In-In

- Axel Drees

## Na50: Npart estimated through ET (left), or EZDC (right, as in NA60)
Good agreement with PbPb
S-U data may show a different behavior?

**Good agreement with PbPb**
**S-U data may show a different behavior?**

**Comparison with Previous Results (vs Npart)**

- Axel Drees

## Suppression vs energy density and fireball’s transverse size
 Anomalous suppression sets in at  ~ 1.5 GeV/fm3 (0=1 fm/c)
 What is the best scaling variable for the onset ?
 Clear answer requires more accurate Pb-Pb suppression pattern

-  **Anomalous suppression sets in at ******** ****~ 1.5 GeV/fm3 (********0=1 fm/c)**
** ****What is the best scaling variable for the onset ?**
****** ****Clear answer requires more accurate Pb-Pb suppression pattern**

**Various Centrality Estimators (********,l)**

- Axel Drees

## Comparison with Theoretical Predictions

- Suppression by hadronic
- comovers (co = 0.65 mb,
- tuned for Pb-Pb collisions)

- Dissociation and
- regeneration in QGP
- and hadron gas

- Percolation, with
- onset of suppression
- at Npart~140

** ****Size of the anomalous suppression reasonably reproduced**
** ****Quantitative description not satisfactory**

- centrality dependent 0

- fixed termalization time 0

- A. Capella, E. Ferreiro
- EPJ C42(2005) 419

- R.Rapp,
- EPJ C43(2005) 91

- S. Digal, S. Fortunato, H. Satz,
- EPJ C32(2004) 547

- Axel Drees

## Smooth effect or Sharp Drop ?

- Step folded with EZDC resolution (~10%):
-      Step position: Npart = 86 ± 8
- 		( Bj ~  1.6 GeV/fm3 )
-       A1= 0.98 ± 0.02
-       A2= 0.84 ± 0.01
-       2/dof = 0.7

**data are compatible with a sharp drop**
**onset smoother than our resolution on N****part ****(****~20) is disfavored**

- Axel Drees

## Comparison between SPS and RHIC

**Comparison between SPS and RHIC**

** ****Plot  J/******** yield vs N****part ****,**** ****normalized to collision scaling expectations**

**Challenge for theorists:**
**Consistent interpretation **
**of SPS and RHIC data**

**Surprising scaling of **
**suppression from **
**SPS to RHIC!**

- Axel Drees

## Dimuons in the Intermediate Mass Region (IMR)

**Method used by NA50:**
**Describe mass distribution by two components: DY and charm **
**Shape from PYTHIA**
**Fit Drell-Yan above J/******
**Fit Charm in range 1.2 to 2.6 GeV, keeping DY fixed**

**Apparent increase of **
**charm by factor ~2**

- Axel Drees

## Disentangling the Signal Sources in the IMR

**NA60 vertex telescope**
**Excellent resolution (****~20-30 μm, in the transverse plane)**
**Sufficient to identify the typical offset of  D-meson decay (****~100 μm)**

**The dileptons from charm decay can be identified by ****tagging their  production point**** with respect to the primary interaction vertex**

**Signal requires enhanced**
**prompt component and **
**not enhanced charm!**

- Axel Drees

## Charm and Prompt Contributions

**Fit offset data with both components: prompt and charm**
**Prompt component factor ~2.4 larger than DY **
**Charm contribution ~70% of yield extrapolated from NA50 p-A**

**Large prompt component**
**Possibly reduced charm yield**

- Axel Drees

## Decomposition of Mass Spectrum (1.16<M<2.56 GeV/c2)

**Sum 4000 and 6500 A data samples **

**Decomposition of Mass Spectrum (1.16<M<2.56 GeV/c****2****)**

**Define the ****Excess**** as ****Signal  –  [ Drell-Yan (1********± 0.1)  +  Open Charm (0.7±0.15) ]**

- Axel Drees

## Centrality & pT Dependence of IMR Excess

**Excess increases faster than proportional to N****part**** but also faster than hard processes (N****coll****) **

**Excess has significantly softer p****T**** distribution than Drell-Yan **

- Axel Drees

## More Detailed Look at pT Dependence

**More Detailed Look at p****T**** Dependence**

**Present Excess in different mass regions as function of m****T**
**Fit exponential function**
**Extract T****eff**** slope parameter**

**<T****eff****> ~ 190 MeV/c****2**
**Is this related to temperature?**

- Axel Drees

## Interpretation of Teff 

**Interpretation of T****eff ****from fitting to exp(-m****T****/T****eff****) **
**Static source: T****eff**** interpreted as the source temperature **
**Radially expanding source: **
**T****eff**** reflects temperature and flow velosity**
**T****eff ****dependens on the m****T**** range **
**Large p****T**** limit:**
**Low p****T**** limit: **
**F****inal p****T**** spectra from space-time history T****i**** → T****fo**** and emmision time**
**hadrons: ****	****interact strongly**
**	****	****	****freeze out at different times depending on cross section with pions**
**	****	****	****T****eff**** ********  temperature and flow velocity at thermal freeze out **
**dileptons: ****	****do not interact strongly **
**	****	****	****decouple from medium after emission**
**	****	****	****T****eff**** ******** temperature and velocity evolution averaged over emission time**

- Axel Drees

- (specific for In-In – Dusling et al.)

**Example of Hydrodynamic Evolution**

**hadron**
**phase**

**parton**
**phase**

**Monotonic decrease of T from: **
**early times to late times**
**medium center to edge**
**early times to late times**
**medium center to edge**

**Dileptons may allow decoupling or deconvolving of emission times:**
**Early emmision times (from parton phase):  ****high T****, small v****T**
**Late emission times (from hadronic phase):  low T, ****high v****T**

- Axel Drees

## NA60 analysis of mT spectra in In-In 

**Decompose low mass region**
**Contributions of mesons (********,********,********)**
**Continuum plus ******** meson**
**Extract vacuum ******
**Hadron m****T**** spectra for **
******,********,******** **
**Vacuum ******
**Dilepton mT spectra for **
**Low mass excess **
**Intermediate mass excess**

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

- Axel Drees

## Examples of mT Distributions

- Axel Drees

**Hadrons (********) **
**T****eff**** depends on mass **
**T****eff**** smaller for ********, decouples early**
**T****eff**** large for ********, decouples late**
**Low mass excess**
**Clear flow effect visible**
**Follows trend set by hadrons**
**Possible late emission**
**Intermediate mass excess**
**No mass dependence of T****eff**
**Indication for early emission**

- Axel Drees

## Summary

**Anomalous J/******** suppression in In-In collisions**
**Consistent with NA50 data**
**Scaling with N****part****, ********, L can not be distinguished**
**Theoretical predictions only qualitatively describe data**
**Unexpected similarity of suppression pattern at SPS and RHIC**
**Large prompt component in IMR**
**Excess in mass range between ******** and J/******** **
**Not from open heavy flavor, it is a prompt component**
**Excess increases faster than Drell-Yan (N****coll****) with centrality **
**Excess exponential in m****T ****independent of mass with T****eff ****~ 190 MeV/c****2**
**Dilepton m****T**** spectra promise to separate time scales**
**Low mass dileptons shows clear flow contribution indicating late emission **
**Intermediate mass show no flow contribution hinting toward early emission**

- Axel Drees

## 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

- Axel Drees

## Backup

- Axel Drees

## NA60 Pixel Vertex Detector

**DIPOLE MAGNET**
**2.5 T**

**HADRON ABSORBER**

**TARGETS**

**~40 cm**

**1 cm**

**NA60 Pixel Vertex Detector**

**12 tracking points**** with good acceptance**
**8 “small” 4-chip planes, plus**
**8 “big” 8-chip planes (4 tracking stations)**
**~ 3% X0**** per plane**
**750 μm Si read-out chip**
**300 μm Si sensor**
**ceramic hybrid**
**800’000 R/O channels**** - 96 pixel assemblies**

- Axel Drees

## Outlook: the NA60 2004 proton-nucleus data

- NA60 collected p-nucleus data with 7 nuclear targets and two beam energies:
- _400 GeV:_ ~ 300 000 J/ events, before matching            study c, open charm, low and intermediate mass dimuons
- _158 GeV:_ ~ 11 000 J/ events, after matching            determine the normal nuclear absorption curve at the energy of the heavy-ion data                by measuring 0(J/) and abs(J/)

- Matched J/ z-vertex

- Al

- 3 × Be

- In

- Cu

- W

- U

- Pb

- p-A 158 GeV

- OS

- LS

**J/******

******’**

- Axel Drees

## ’ suppression in In-In collisions

-  Use selection 2 (matching of muon spectrometer tracks)
-  Study limited by statistics (N’ ~ 300)
-  Normalized to Drell-Yan yields

-  Most peripheral point
-   (Npart ~ 60) does not show
-   an anomalous suppression

-  Good agreement with
-    Pb-Pb results

- Preliminary

- 450, 400 and 200 GeV points
- rescaled to 158 GeV

- Axel Drees

## Extension of  pT Spectra to 3 GeV  

**nearly exponential shape, ****no hardening**** towards high p****T**

** ****large ****difference**** between ******** and ******** (same mass) to within**** 9******

- Axel Drees