NSTAR2011, Jefferson Lab, USA May 17-20, 2011 Mitglied der Helmholtz-Gemeinschaft Tamer Tolba for the WASA-at-COSY collaboration Institut für Kernphysik

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


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


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


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


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


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%


Differential Cross Sections @ Tp = 1.4 GeV

MΔ(1232)

Modified Valencia model *

Preliminary

* T. Skorodko, et al., Phys. Lett. B 695 (2011) 115


No strong N*(1440) contribution(pole position = 1360 MeV/c2)

Indication for ΔΔ->pπ0 pπ0

MΔ

MΔ

MN*(1440) = 1.36 GeV/c2

Preliminary


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).


Spares


- 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


- 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.


- 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


Data – MC Comparison After KFit


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


Differential cross sections





MC-Toy


Documents

NSTAR2011, Jefferson Lab, USA May 17-20, 2011 Mitglied der Helmholtz-Gemeinschaft Tamer Tolba for the WASA-at-COSY collaboration Institut für Kernphysik