# Presentation: 831230

## Probing resonance matter with low-mass electron pairs

Joachim Stroth, Univ. Frankfurt/GSI

- Joachim Stroth, Univ. Frankfurt/GSI

- School of collective dynamics in high-energy collisionsBerkeley, May 2007

## The DiLepton Spectrometer at LBL 

- The                                 puzzle !

- Theory by C. Fuchs et al.

**1. Introduction**

**10****th**** ****anniversary**

## Plan of the lecture

- Introduction
- The HADES experiment
- Results
- Comparison to DLS
- Comparison to transport
- The future at FAIR

## Exploring the phase diagram of matter

- Chemical freeze-out puts landmarks in the experimentally unknown regions.
- Universal conditions for freeze-out?
- Limiting temperature Tmax?

- QCD inspired models demonstrate the melting of the condesates.

**P. Braun-Munzinger, K. Redlich, J. Stachel (e.g. nucl-th/0304013)**
**J. Cleymans, K. Redlich (e.g. PRC 60 054908) **

- LQCD explores unknown regions from solid grounds at B=0.
- Tc = Tmax?
- Tc = 170  MeV or TC = 190 MeV?

**<qqbar>**
**Schäfer, Wambach**
**priv. communication**

**<gg>**
**Schäfer, Wambach**
**priv. communication**

**<(qqbar)****2****>**
**Leupold**
**J.Phys.G32:2199,2006**

**1. Introduction**

## A simplified (non-relativistic) microscopic picture

- From medium-modification to deconfinement: **What is the role of:**
- Temperature ?
- Baryon density ?
- QCD condensates ?

**1. Introduction**

## The  meson in nuclear matter

- Additional contributions to the self-energy in the medium.

- Meson nucleon molecules?

**1. Introduction**

## -symmetry restoration: historical slide 

- -symmetry restoration: historical slide

- Gerry's and Mannque's conjecture  transformed into a sketch used to motivate students and to justify funds.
-  (With some support by Wolfram Weise and Mathias Lutz)

**1. Introduction**

## Probing resonance matter with dileptons

- The dilepton signal contains **contributions from throughout the collision**, ..
- ...  i.e. also **direct radiation** from the **early phase.**
- It probes the **electromagnetic structure of dense/****hot**** nuclear****(or partonic) matter.**

- [Bratkovskaya et al., PRC 69 (2004) 054907]

**1. Introduction**

## Electromagnetic structure of dense/hot matter

- lepton pairs couple through time-like photons and test the **spectral properties** of hadronic matter.
- time reversed processes of reactions in a e+e- collider.

- Decays of neutral mesons

- Annihlation,
- Resonance decay

- Bremsstrahlung
- Drell-Yang

**1. Introduction**

## Observable

- Thermal dilepton rate ...

- isentropic expansion

**1. Introduction**

## Overview

- time (advance in technology)

**LHC**

**RHIC**

**SPS**

**SIS300**

**SIS18**
**Bevalac**

**SIS 100****AGS**

- HFT
- HBD

- upgrade

- TPC

**1. Introduction**

- The HADES experiment

## The HADES experiment @ GSI

## Spectrometer concept

- Geometry
- Full azimuth ,  polar angles 18o - 85o
- Pair acceptance  0.35
- About 80.000 detector channels 
- Fast particle identification
- RICH CsI solid photo cathode, No 80, C4F10 radiator
- TOF (Scintillator rods)
- TOFino (Scintillator paddles) temporary solution, RPC in future
- Pre-Shower 18 pad chambers & lead converters
- Momentum measurement
- ILSE, super conducting toroid B = 0.36 Tm
- MDC Multi-wire drift chamber, single cell resolution  100 m

- 1 m

**2. The HADES experiment**

**Notes:**
You heard already a lot about the RICH. Has to cope with a rather short radiator. Solid VUV photo cathode (CsI)

## HADES production runs

- November 2002: C+C 2 AGeV, commissioning and physics runs
- target= 2 x 2.5%,  56% LVL1 trigger + 44% LVL2 trigger
- 6 outer drift chambers (MDC) in 4 sectors
- February 2004: p+p 2.2 GeV
- target 5 cm l-H2
- August 2004: C+C 1 AGeV
- 3x1.5 % target, 56% LVL1 trigger + 44% LVL2 trigger
- October 2005: Ar+KCl 1.75 AGeV
- 3x1.5 % target, 56% LVL1 trigger + 44% LVL2 trigger
- May 2006: p+p 1.25 GeV
- target 5 cm LH2
- 2007: p,d+p 1.25A, 3.5 GeV
- 2008: p+A (approved)
- From 2009 on: heavy systems

**2. The HADES experiment**

## Trigger and Data taking capability (C+C 2AGeV)

**2. The HADES experiment**

## Hadron identification in HADES

- K+

- p

- π-

- π+

- 1.75 AGeV Ar+KCl

**dE/dx in MDC**

**PhD thesis: A. Schmah**

**2. The HADES experiment**

## Large phase-space coverage for pions ...

- Pion dN/cosθCM

- C+C
- 2 AGeV

**2. The HADES experiment**

## Strange hadrons from Ar+KCl @ 1.75 AGeV

- High statistics run: below spectra from 40 % of total statistics available.
- 118 000 K+
- 3300 K-
- 1290 K0
- 2400 
-  , ...

**PhD thesis: A. Schmah**

**2. The HADES experiment**

**Notes:**
In 2005 HADES took high statistics data for the collision system Ar+KCl. Besides the electron pair observable, we also look into strange hadron production. In the analysis we make use of energy loss information in the drfit chambers, obtained from a time-above-threshold information of the drift chamber timing signal. As is seen we reconstruct strange hadrons with very good signal to noise.  The statistic shown amounts to about 40% of the totally available statistics. 

## Lepton candidate selection

- Events with lepton multiplicity  1 in LVL1 triggered events  1.2 %
- Require META and RICH signal (LVL2)
- + matching with fully reconstructed tracks

- cut on 

**2. The HADES experiment**

## Background rejection

- "Conversion rejection":
- C1, C2 = "close pair candidates" !
- Remove the track if a non-fitted track candidate is within 10o

- "0-Dalitz rejection":
- C3:  opening angle < 9o
- Remove both tracks from the sample

- RICH

- MDC I-II

- TOF/Shower

- B-field

**C1                       C2**

**2. The HADES experiment**

- The HADES experiment

## C+C 2AGeV:   e+e- invariant mass spectrum

- signal **< **140 MeV/c2:  20971 counts
- signal **>** 140 MeV/c2:    1937 counts

**3. Results from HADES**

## C+C  2AGeV, compared to hadronic cocktail

_**Event generator**__ __**PLUTO**_ :
- thermal source (T=80MeV)
-  polar angle distribution from charged  analysis
- η (TAPS data)
- ρ, ω :   m**** -scaling
- Δ scales with 

**3. Results from HADES**

**PhD thesis: M. Sudol**

- PRL 98, 052302 (2007)

## p┴ & y distributions compared to cocktail

- p┴ & y distributions compared to cocktail

**3. Results from HADES**

- Comparison to DLS

## Phase space coverage:   HADES  vs.  DLS 

- A comparison of HADES and DLS results is not  straightforward!!!

- mid-rapidity

- mid-rapidity

- HADES

- DLS

**4. Comparison to DLS**

## Comparison of DLS to HADES

- Enhancement factor F over η for 150<Mee<500 MeV/c2 :

- R. J. Porter et al.,  Phys. Rev. Lett. 79 1229 (1997)

**4. Comparison to DLS**

## A closer look to the excess yield

- _Excess yield scales like ____0 _ !

- At SIS energies effectively all pions come from resonance decay (regeneration).

- _Hadron data_
- R. Averbeck et al., TAPS coll., Z. Phys. A 359, 65 (1997)
- R. Holzmann et al., TAPS coll., Phys. Rev. C 56, R2920 (1997)

**4. Comparison to DLS**

## Efficiency corrected data: C+C 1 AGeV 

**preliminary**

**PhD theses: Y. Pachmayer & T. Christ**

- Enhancement factor       **F(1.0) ≈ 5.8**                           (in 150<Mee<500 MeV/c2 range)
- Enhancement within errors in agreement with DLS

- signal **< **140 MeV/c2:  17800 counts
- signal **>** 140 MeV/c2:    670 counts

**4. Comparison to DLS**

- Comparison to transport

## Comparison with transport

**RQMD: **M. D. Cozma, C. Fuchs, E. Santini, A. Faessler, Phys. Lett. B 640, 150 (2006)
**UrQMD: **D. Schumacher, S. Vogel, M. Bleicher, nucl-th/06080401.
**HSD(v2.5): **W. Cassing and E. L. Bratkovskaya, Phys. Rep. 308, 65 (1999).

**in-medium results**

**vacuum results**

**5. Comparison to transport**

## Spectra from just finished run (April - May 2007)

- pp at 3.5 GeV, full analysis on-line available
- No tuning of cuts, no final calibration and alignment yet – compressed mass scale!
- 1,8 109 recorded events  4.7 109 triggered events (LVL1)

- 54000 signal electron pairs, after 3/4 of the collected statistics

- 4-prong events (+++-) selected

- Yield [arb. units]

- Yield [arb. units]

- pp missing mass [GeV/C2]

- e+e- invariant mass MeV/C2]

- On-line spectra

**Notes:**
Since April 10th, 2007 we are running with p beam on a liquid hydrogene target. We run a full analysis on-line to monitor the quality of the data. The transparancy show on the left a proton proton missing mass spectrum for a four prong final state with one negative and three positive particles. In a kinematic fit, the final state is minimized under the contrained that a neutral pion was not observed. Clearly, eta and omega production are visible. 
On the right, the on-line e+e- invariant mass spectrum is shown. In the last two weeks of the beam time we are running deuteron beam on  lH2 at 1.25 AGeV beam energy. Together with a run in 2006 on p+p at 1.25 GeV we can soon provide cross sections on inclusive e+e- production needed to further constrain the transport models.  

## Spectra from just finished run (April - May 2007)

- Yield [arb. units]

- e+e- invariant mass MeV/C2]

- pp 3.5 GeV

- The future at FAIR

## Somewhere east of Wixhausen

**6. Future at FAIR**

## From HADES to CBM @ FAIR

**HADES**
**2 – 8 AGeV**

**CBM**
**8 – 45 AGeV**** **

***Just five steps ;-)***

**6. Future at FAIR**

## Dielectron reconstruction in CBM

- Fast, high-precision tracking using silicon sensors.
- No electron identification before tracking

**6. Future at FAIR**

## Exploit the track topology

**Track  Fragment**

**Full Track**

**STS 1****z=10cm**

**6. Future at FAIR**

## Background rejection performance

- Au+Au 25 AGeV, central collisions
- Signal mixed into UrQMD events

- accepted

- after cuts applied

****

****

****

**π****0**

****

****

****

- More than two orders suppression

**bg**

**bg**

**6. Future at FAIR**

## The muon option in CBM

- C/Fe absorbers + detector layers

- Simulations Au+Au 25 *A*G*e*V:
- Excellent signal to background ratio in high mass region.
- Low efficiency for small invariant masses and/or low pt (enhancement region).
- Challenging muon detector (high particle densities)

- J/ψ→μ+μ-

- s/b ~ 100

- ρ

- φ

- ω

**6. Future at FAIR**

## Summary

- Lepton pairs are an ideal tool to study the microscopic structure of dense/hot nuclear medium
**HADES** confirms DLS observation (puzzle) in C+C systems
- The excess yield is in general complex. Systematic studies needed to disentangle various effects (excitation function, double differential spectra).
- Excess yield can possibly be used to determine bulk properties of the fireball (trajectory in B-T plane)
- Elementary reactions and measurements at nuclear ground state density (**TAPS/CBELSA, KEK, CLAS, HADES** ) important to settle aspects of the spectral function.
- Dilepton spectroscopy is considered one of the major programs of **CBM**.

## collaboration

**GSI**

**collaboration**