1. Electromagnetic Physics Working Group discussion
GSI 10 March 2014

2. Topics of the discussion
Which electromagnetic channels can be studied in the initial phase of PANDA?
But what is the initial phase ?
reduced luminosity (L=1/10* L max= cm-2 s-1)
4 months  0.2 fb-1
reduced detector acceptance and performance, missing subdetectors to fulfill money constraints
not yet clear how such a detector will look like
One possible (maybe the least bad) scenario:
6/16 EMC slices (opposite in radial direction), fully equipped with DIRC, no FW spectrometer

3. Goal of the discussion
Start to identify what are the consequences for each channel of different scenarios
Alternatively list the minimum requirements for some channels to be measured in the initial phase
Summary to be presented on Friday
By no means, we can give at this stage recommandations or draw conclusions
Just present facts and identify which kind of simulations are needed

4. Simple facts, to start the discussion (I)
Main challenge for electromagnetic channels
Rates are very low and decrease very fast with energy
e.g. ppe+e- =0.6 nb at p=1.7 GeV/c, fb at p=13.4 GeV/c
pp   (~50% larger than e+e-), pp ° (150x larger)
pp °e+e- T=1 GeV = 80 pb for q2=2±0.25 (GeV/c)2
pp ° J/Ψ = 150 pb
pp  l+l- X = 0.8 nb 1.5 GeV/c2 < Mll< 2.5 GeV/c2
detection of  , e+e- or µ+µ- in a huge (typically 106 larger ) hadronic background : problems with
misidentification of +- as e+e- or µ+µ-
Misidentification of ° (2  ) as 1  - - - - - 4

5. Simple facts, to start the discussion (II)
Reduction of luminosity and acceptance limits the energy range where sensitive measurements can be done : see examples in Thierry’s talk
Reduced PANDA sensitivity
= 2 pb
100 evts/month at 100% efficiency
(nb)
PANDA sensitivity
=0.2 pb
100 evts/month at 100% efficiency

6. Rejection of pionic background
e+/e- case
DIRC(Cerenkov)
ElectroMagneticCalorimeter
Straw Tube Tracker
- A  °A* 2
Edep/p ~ 1
Non-gaussian tails of truncated
dE/dx distribution
High precision needed for
dE/dx
DIRC Cerenkov angle
(for p<0.8 GeV/c)
EMC energy resolution
4-momentum resolution
Use complementarity of e/ discrimination
capability of the different detectors
To achieve such high rejection factors, the complementarity of the different detectors with respect to PID is exploited. The DIRC detector, which measures a Cerenkov angle in a quartz rod provides an efficient e/pi discrimination at low momentum, but the discrimination power vanishes above GeV/c. The measurements of DE/dx in the central traker are rather powerful over a large range of momenta, while for the electromagnetic calorimeter, the discrimination power obtained by the ratio of E/p is much better at higher momenta. The electrons deposit all their energy in the calorimeter, so they have E/p close to 1. The pion line here is due to the energy loss by ionisation, but a large fraction of the pions undergo hadronic interactions. This is seen clearly on this spectrum which shows the energy deposit of a pion at a momentum p=1.5 GeV/c. The peak is due to energy loss by ionization and a broad bump due to the hadronic interactions. The highest energy deposit are due to the charge exchange process. Indeed, a high energy pi0 is created and the energy of the 2 gammas have a high probability to leave all their energy in the calorimeter. In order to make realistic estimates for e/pi discrimination, we therefore need a model that do not underestimate this charge exchange reactions. As you can see here, different models have been tested and the most pessimistic model is chosen for the simulations. This model is the Bertini cascade and which is also known to be one of the most realistic models to describe hadronic showers, for example, it has been tested recently with the CMS calorimeter and gives good results.
Bayesian PID method R. Kunne
Single pion rejection >3000
Global electron efficiency >60%
Here, we do not want
to loose anything
Use kinematic fit for the given exclusive channel 6 6 6

7. More remarks about detectors
for µ/ discrimination: STT(tracking )+ EMC (acts just as a pion absorber) + Muon detector (tracking)
For /° discrimination, EMC granularity and resolution are essential
For all channels: role of FW Spectrometer to be studied (not present in simulations up to now) essential for form factors at T>10 GeV only
Which dynamics?
For form factors, pe or pµ>500 MeV/c
low energy /electron resolution probably not essential for us

8. What kind of simulations are needed?
fast simulations with acceptance and efficiency matrices, and smearing of particle 4-momenta with parametrized resolution ?
GEANT needs to be run only to calculate the matrices
For which channels and which detector scenarios ?

9. physics channels for the initial phase?
Electromagnetic channels will be fully exploited with full luminosity and detector coverage
We want to achieve these conditions in a not too far future
High luminosity will not come in the initial phase: the measurement at the largest energies will anyway be delayed
For 2-body channels, the luminosity reduction has more effect than the 6/16 option acceptance reduction
For 3 and more body channels, incomplete detector adds further limitation -

10. physics channels for the initial phase?
Focus on two_body channels at the lowest energies ?
form factors: pp e+e- can still be measured for q2< 9 GeV/c in 6/16 scenario.
Problem of competition with BES (e+e-  pp), but PANDA can measure in addition pp µ+µ-
  : same case (but severe background issue) pp   can be measured at BELLE,BES
All others channels: °, °e+e- can only be measured by PANDA and are very poorly known.
No factorization in the low energy regime, but these channels are still very interesting for « soft » reaction mechanisms and to investigate nucleon form factors below threshold
Measurement of background hadronic reactions +- ,+- °,…..will be very important for the preparation of electromagnetic channel measurements. - - - - -