# Presentation: 879594

## Dilepton Analysis in the HADES Spectrometer for C+C at  2 AGeV

- Jaroslav Bielcik
- GSI Darmstadt/Yale University-BNL

-   The Physics Case
-   Description of the HADES spectrometer
-   Simulation of the spectrum of dilepton sources
-   Dilepton analysis of measured data
-   Summary

***             *****for the**** **
**HADES Collaboration**

- Jaroslav Bielcik, Hot Quarks

- July 18-24, 2004

**Notes:**
Good afternoon ladies and gentleman. My name is Jaroslav Bielcik and I would like to present my work on Dilepton spectroscopy with HADES.

## The Physics Case

**spontaneous  breaking of the chiral symmetry** (QCD):

-     **interactions of hadrons in nuclear matter**:
-    couples to N* N-1 => changes of the mass and width

-  **HOW TO MEASURE ?**

****** ** generation of the constituent quark masses

-  reduction in dense/hot medium
-  changes of vector meson masses

- Probe hadron properties (masses, widths, formfactors ...) in hot and
- dense nuclear matter by using dielectrons!

**Notes:**
	There are 2 groups of the theoretical predictions that vector meson changes its mass or width in dense and hot matter. One is related
to the phenomenon of spontaneous breaking of the chiral symmetry, a global symmetry of the Quantum Chromodynamics.  SB is caused
by nonzero value of the quark-antiquark condensate of the QCD vacuum. Interaction of the constituent quarks with quark antiquark
condensate generates their mass. Reduction of the value of the condensate is expected in hot and dense matter and models predicts
that this cause changes of vector mesons masses.
	Another group is related  to rho baryon interactions, in particular the strong rho n-nstar particle hole coupling, that is found to be most
dominating origin of a considerable change of the rho meson mass.	

## Measurement with penetrating probe

** ******** **decays in dense phase to e+e- pair  (BR=4.4x10-5)
-  dileptons are penetrating probe

- detection e+ and e-
- measurement:
-                _momentum_

- dense phase

- only one pair in several millions collisions

- experimental challenge

-  reconstruction of vector meson mass as it was in the dense phase

**Notes:**
Vector meson rho is good candidate for inmedium changes because it has short lifetime and it decays during dense and hot fireball.  It has
also  decay branche to e+e- pair. however this decays is rare. Detecting both leptons from decay, measuring their momentum and opening
angle of the pair we can reconstruct the mass of the rho as it was at a moment of decay. It is an experimental challenge to reconstruct
such decay because it happened only once in several millions collisions.

## HADES spectrometer

**Shower detector**
**    ****+TOFINO**

**Tracking (4 drift chambers)**

**Time of flight**

**Magnet**

- Target inside

**Cherenkov    detector       ****           **

**beam**

**4.5m**

-   **e,**** ********, K** and p** **identification
-   intelligent 3 levels lepton trigger
-   m/m = 1-2% @ / mass
-   up to 35% pair geom. acceptance

-   **SiS at GSI Darmstadt**:
-  **AA, pA, ********A **
-  **pp, ********p**

- High acceptance dielectron spectrometer

**Project launched in late 1994****6 years R&D and construction****.**

**Notes:**
HADES High acceptance dielectron spectrometer  has been build in GSI Darmstadt and was under commisioning during last years. It is dedicated
to measure e+e- pairs from heavy ion collisions with very high invariant mass resolution of 1-2 % in rho-omega mass reagion. It has special
3 levels lepton trigger. For the tracking of the particles and momentum reconstruction a set of 4 multiwire drift chambers is employed with 
magnetic field between them. A lepton identification is done by ring imaging cherenkov detector, that is hadron blind. Aditional electron
identification uses measuring the time of flight of the particles and selection of the  fast electrons. Also shower detector is employed for
electron identification while electromagnetic shower is produced when electron pass through it.  

## Simulation of the dilepton sources

-       SIMULATIONS
-  _no inmedium changes in_
-  realistic detector geometry
-  only 3 drift chamber setup
-  statistics: **2 * 10****9** collisions

**e+**
**e-**

- Dalitz decays

- dilepton decays

- Combinatorial
-   background

**Notes:**
Monte Carlo simulations have been performed to reconstruct the expected dilepton spectrum for CC collisions at 2 AGeV. There are no in medium effectsinside the simulations. The statistics corresponds to 2 miliards collisions. There are several different sources of the dileptons. The most dominant 
are Dalitz decays of pi0, eta mesons. The vector mesons decays dominate spectrum only at higher masses. During the analysis we always combine
all e+ with all e- in the events while we do not know a priori which leptons belong together. Correct combinations contributes to the
peaks and random combination build combinatorial background. In the real data presented later in my talk due small statistics I could
study only left part of spectrum.  

## Conversion pairs

**Conversion in materials**

**60% leptons conv.**

**20% leptons ********0**** Dalitz**

- very close pairs:
-   unresolved in drift chamber
-   if one lepton bends out
-   track identified as single lepton
-   contribution to combinatorial
-                             background

********

**Rejection method:**
-  based on MDC hits properties
**90%** double tracks rejected
-   with  90% single track efficiency

## First data from HADES C+C

**DATA set**

- commissioning run November 2001

- system: C+C 2AGeV

- low resolution: two drift chambers

- multiplicity trigger:  Mch>3  (LVL1)

- statistics:  **45*****10****6**** **collisions

- dilepton spectrum

- production run November 2002

-   + online e+/e- selection (LVL2)

-  10x enhanced pair statististics       @ 92% e+e- pair efficiency

- comp. to LVL1

- statistics:  **200*****10****6**** **collisions

- Feb04: p+p @2 AGeV 420*106 coll.

- + outer drift chambers

## Lepton identification

## Momentum lepton spectra of HADES

** ****data and simulations (UrQMD) consistent  **
**  ****90% purity of lepton selection**

** ****large lepton acceptance**

**preliminary**

##  Dilepton spectra Nov’01C+C@2AGeV

**Pair analysis**:
-  combine e+ with e-
-  reject close pairs
-  combinatorial background:
-  like sign pairs  method:

-   low mass dominated continuum

-   large errors due to low statistics

-   signal: **2165 e****+****e****-**
-   signal/background 2.6

**preliminary**

## Comparison with simulations

-  no absolute normalization yet
-  no acceptance and efficiency
-                                   corrections

-  93% purity of the reconstructed signal

-  normalization to 0–Dalitz
-  shape of the spectra very similar

**preliminary**

## Dilepton spectra Nov’02  C+C@2AGeV

- Dilepton spectra Nov’02  _[C+C@2AGeV](mailto:C+C@2AGeV)_

**preliminary**

***No*** Efficiency / Acceptance ***Correction***

**inv. Mass [MeV/c****2****]**

**dN/dM [1./MeV/coll.]**

**N****e+e-**

**CB **

**N****e+e- ****- CB**

- only statistical errors
**no** outer drift chambers in analysis
- equivalent to                          ~ 604 * 106  LVL1 triggers

**Combinatorial Background**
**CB = N****e+e+**** + N****e-e-**

**Thanks T.Eberl**

**Notes:**
This is a preliminary invariant mass plot with the raw e+e- , the cb and the net signal spectrum.
The used statistics is equivalent to about 600 million LVL1 events.
It is not yet corrected for acceptance and efficiency. 

## Dilepton spectra Nov’02  C+C@2AGeV  

**preliminary**

***No*** Efficiency / Acceptance ***Correction***

- Syst. Errors only due to *cuts* in     data & sim

**True pairs in Simulation:**
**75% (M < 140 MeV/c****2****)**
**50% (M > 140 MeV/c****2****)**

**Thanks T.Eberl**

**Notes:**
Comparison of measured data and the URQMD simulation.
From analysis a systematic uncerertainty of about 25% was extracted and is marked by the red shaded band.
In the pion mass region the systematic uncertainties in simulation were estimated as marked by the blue shaded band.
The error bars indicate purely statistical errors.
In the simulation 75% of the reconstructed pairs are true pairs in the pion region and for higher masses about 50%.
There were no physical e+e- sources in the simulation above 550 MeV/c2.
The spectra are not yet efficiency and accepatance corrected. 

## Summary

-  HADES takes data since 2001
-  large dielectron statistics (Nov01: 2165   Nov02: 16500 signal pairs )
-  leptons have been identified with 90% purity

**Outlook**
- Normalization to + / - yield
- Final spectra after
-    ... improvement of track purity and momentum resolution in sectors with outer tracking
-   ... absolute efficiency calibration (pppp******)**
- Event mixing procedure to calculate combinatorial background (work in progress)
- Next run: ***C+C @ 1 AGeV in September 2004***
- 			(with outer MDCs in the detection system)

## HADES Detector @ GSI

## The Collaboration

- Catania  (INFN - LNS)
- Cracow  (Univ.)
**Darmstadt  (GSI)**
- Dresden (FZR)
- Dubna  (JINR)
- Frankfurt  (Univ.)
- Giessen  (Univ.)
-  Milano  (INFN, Univ.)
-  Munich  (Tech. Univ.)
-  Moscow  (ITEP, MEPhI, RAS)
-  Nicosia  (Univ.)
-  Orsay (IPN)
-  Rez  (CAS, NPI)
-  Sant. de Compostela  (Univ.)
-  Valencia  (Univ.)
-  Coimbra (Univ.)

**The Collaboration**

**GSI**

**Notes:**
The collaboration consists of 17 institutions in 10 countries and is distributed all over Europe.