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NSTAR2011, JLab USA, May 18, 2011Slide 3Tamer Tolba, IKP, FZ Jülich Double pion production in pp collisions is dominated by resonance production. Relative strengths of the production mechanisms strongly momentum dependent. pp → ppππ high energiesnear threshold pp→pN*(1440) p π π ∆ (1232) π p π pp→∆ ∆(1232) p π * L. Alvarez-Ruso, E. Oset, and E. Hernández, Nucl. Phys. A633, 519 (1998). (Valencia model) Double pion production in pp collision
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NSTAR2011, Jefferson Lab, USA
May 17-20, 2011
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Tamer Tolba for the WASA-at-COSY collaboration
Institut für KernphysikForschungszentrum Jülich
Double-Pion Production in Proton-Proton Interactions at Tp = 1.4 GeV
NSTAR2011, JLab USA, May 18, 2011 Slide 2Tamer Tolba, IKP, FZ Jülich
Outline
- Physics Motivation
- Previous Work
- WASA Detector at COSY
- Data Analysis
- Experimental Results
* Total Cross Section
* Differential Cross Sections
- Summary and Outlook
NSTAR2011, JLab USA, May 18, 2011 Slide 3Tamer Tolba, IKP, FZ Jülich
Double pion production in pp collisions is dominated by resonance production.
Relative strengths of the production mechanisms strongly momentum dependent.
pp → ppππ
high energiesnear threshold
pp→pN*(1440)
p π π ∆ (1232) π
p π
pp→∆ ∆(1232)
p π p π
* L. Alvarez-Ruso, E. Oset, and E. Hernández, Nucl. Phys. A633, 519 (1998). (Valencia model)
Double pion production in pp collision
NSTAR2011, JLab USA, May 18, 2011 Slide 4Tamer Tolba, IKP, FZ Jülich
Goal: Study 2π0 σtotal and production mechanism at Tp
=1.4 GeV (√s ~ 2.48 GeV)
pp→ppπ0π0
Absence of differential cross sections at high energies
(i.e. Tp > 1.3 GeV)Exclusive studies from
threshold up to Tp = 1.3 GeV
Previous work
NSTAR2011, JLab USA, May 18, 2011 Slide 5Tamer Tolba, IKP, FZ Jülich
WASA Detector at COSY
Proton selectionInv. mass γ1γ2 vs. Inv. mass γ3γ4
Peak at π0- π0 mass
p
p
pp
π0
π0
Central Detector (CD):-Covers scattering angle θ = 20° – 169°.- Energy, momentum reconstruction and PID for charged particles and photons.
Forward Detector (FD):- Covers scattering angle θ = 3° – 18°.- Energy, momentum reconstruction and PID for charged hadrons.- Used to tag mesons (via missing-mass technique).
COSY
NSTAR2011, JLab USA, May 18, 2011 Slide 6Tamer Tolba, IKP, FZ Jülich
Main selection criteria: Demanding 1 or 2 protons in the forward detector and 4γ in the central detector.
The full phase space is covered
Data Analysis
Geometrical acceptance of protons in the forward detector and photons in the central detector.
Central Detector
Forward Detector
θγ vs. Eγ θp vs. Ep
MC simulations MC simulations
Background free
NSTAR2011, JLab USA, May 18, 2011 Slide 7Tamer Tolba, IKP, FZ Jülich
Total cross section pp → ppπ0π0
@ Tp = 1.4 GeV
Experimental Results
σtot = (324±61) μb
Preliminary
Total error:
- Statistical error is negligible.
- Systematical error dominated
by absolute normalization ~
18%
NSTAR2011, JLab USA, May 18, 2011 Slide 8Tamer Tolba, IKP, FZ Jülich
Differential Cross Sections @ Tp = 1.4 GeV
MΔ(1232)
Modified Valencia model *
Preliminary
* T. Skorodko, et al., Phys. Lett. B 695 (2011) 115
NSTAR2011, JLab USA, May 18, 2011 Slide 9Tamer Tolba, IKP, FZ Jülich
No strong N*(1440) contribution(pole position = 1360 MeV/c2)
Indication for ΔΔ->pπ0 pπ0
MΔ
MΔ
MN*(1440) = 1.36 GeV/c2
Preliminary
NSTAR2011, JLab USA, May 18, 2011 Slide 10Tamer Tolba, IKP, FZ Jülich
Summary and Outlook
Double pion production in pp→ppπ0π0 at Tp = 1.4 GeV.
Results:
- Total and differential cross section.
Reaction Dynamics:
- The production mechanism is dominated by double Δ(1232).
- No significant contribution from N*(1440).
Outlook:
- Extension to higher energies: contribution of higher resonances.
- Study pp→ppπ+π- : contribution of ρ(770).
- Investigations on pn and pd reactions: include mixed final states (π-π0,π+π0).
NSTAR2011, JLab USA, May 18, 2011 Slide 11Tamer Tolba, IKP, FZ Jülich
Spares
NSTAR2011, JLab USA, May 18, 2011 Slide 12Tamer Tolba, IKP, FZ Jülich
- Toy-model:
Tune the MC simulations to match the data.
GIN phase space generator. * Provides 4-vector momentum for each particle in the final state at the interaction point. * Provides individual weight for each event. * Availability to simulate resonances production by changing the generated according to
the expected production mechanism.
Correction terms added to the generated weight: * 2Δ propagators, only Breit-Wigner width. (Risser, T. and M.D. Shuster, Phys. Lett. B 43(1973) 68).
* Angular distributions for p and π in CM frame. * Inv. mass 2π0. * Inv. mass2 pπ0.
Monte Carlo Models
NSTAR2011, JLab USA, May 18, 2011 Slide 13Tamer Tolba, IKP, FZ Jülich
- Valencia-model (Alvarez-Ruso et. al., Nucl. Phys. A 633(1998) 519).:
Model describing NN→ππNN reaction. Studies both resonance and prompt (non-resonance) terms from near threshold up to 1400
MeV nucleon incident energy. Chiral Lagrangian terms involving nucleons and pions. Plus terms involving the excitation of the Δ(1232) and N*(1440) resonances.
NSTAR2011, JLab USA, May 18, 2011 Slide 14Tamer Tolba, IKP, FZ Jülich
- Kinematical Fit (KFit) with constraints:
total (E,P) conservation = 4 2 x Inv. mass π0 = 2 Number of degrees of freedom (NDF) = 3 1 particle in final state is treated as unmeasured. KFit used as combinatorial selection tool for the best γ-pair forming π0. The γ-pair combination of minimum χ2
KFit was chosen as best γ-pair forming π0.
- data- MC simulation
NSTAR2011, JLab USA, May 18, 2011 Slide 15Tamer Tolba, IKP, FZ Jülich
Data – MC Comparison After KFit
NSTAR2011, JLab USA, May 18, 2011 Slide 16Tamer Tolba, IKP, FZ Jülich
Scale from luminosity determination Use the pp→ppη(→γγ and →3π0) cross section as reference channel. σpp→ppη (Tp = 1400 MeV) = (9.8 ± 1) μb. (Chiavassa, PL 322B, (1994) 270).
Errors generated from acceptance correction to different models Different models (Toy-model, Valencia-model and Phase space) lead to different acceptance results.
Edge Effects Selection on the scattering angles of protons and photons in the FD and CD.
Errors generated from confidence level of the KFit Different confidence level selections.
- Systematic Errors
Error
~ 5%
~ 1%
~ 4%
~ 18%
Total 19%
- Statistical Errors ~ 10-3%
Total Error
NSTAR2011, JLab USA, May 18, 2011 Slide 17Tamer Tolba, IKP, FZ Jülich
Differential cross sections
NSTAR2011, JLab USA, May 18, 2011 Slide 18Tamer Tolba, IKP, FZ Jülich
NSTAR2011, JLab USA, May 18, 2011 Slide 19Tamer Tolba, IKP, FZ Jülich
NSTAR2011, JLab USA, May 18, 2011 Slide 20Tamer Tolba, IKP, FZ Jülich
NSTAR2011, JLab USA, May 18, 2011 Slide 21Tamer Tolba, IKP, FZ Jülich
MC-Toy
NSTAR2011, JLab USA, May 18, 2011 Slide 22Tamer Tolba, IKP, FZ Jülich