Measurement of Double-Polarization

Transcription

1 Measurement of Double-Polarization Observables in γ p p π + π V. Credé 1, M. Bellis 2, S. Strauch 3, and the CLAS Collaboration 1 Florida State University, Tallahassee, Florida 2 Carnegie Mellon University, Pittsburgh, PA 3 University of South Carolina, Columbia, SC Meeting of PAC 29, 1/12/2005

2 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

3 Outline Introduction Baryon Spectroscopy 1 Introduction Baryon Spectroscopy 2 3 4

4 Spectroscopy Introduction Baryon Spectroscopy Atomic spectra allow access to QED Discrete spectrum of absorption and emission lines Does excitation spectrum of nucleon provide access to QCD?

5 Baryon Spectroscopy The Challenges in Baryon Spectroscopy Unfortunately, N spectral lines look more like Baryons are broad and overlapping Rescattering Effects Require Coupled-Channel Analysis (need to measure as many final states as possible) Polarization (need complete experiments)

6 Baryon Spectroscopy E γ [GeV] Reaction Thresholds p ηη p π 0 ω p π 0 η η p πππ ππ p π 2,5 2,0 1,5 1,0 W [GeV/c 2 ] Existing Facilities 0, ,0 BONN CLAS MAMI-C GRAAL LEGS

7 Baryon Spectroscopy W [GeV/c 2 ] 2,5 Existing Facilities 2,0 1,5 Why have the missing resonances not been discovered, yet? 1,0 Investigated mass region was mainly below 1800 MeV/c 2 0,5 0,0 BONN CLAS MAMI C GRAAL LEGS

8 Baryon Spectroscopy W [GeV/c 2 ] 2,5 2,0 1,5 ELSA Pol. Program FROST Existing Facilities Why have the missing resonances not been discovered, yet? 1,0 Investigated mass region was mainly below 1800 MeV/c 2 Polarization 0,5 (essentially) only available at low energies 0,0 BONN CLAS MAMI C GRAAL LEGS

9 Baryon Spectroscopy W [GeV/c 2 ] 2,5 2,0 1,5 ELSA Pol. Program FROST Existing Facilities Why have the missing resonances not been discovered, yet? 1,0 Investigated mass region was mainly below 1800 MeV/c 2 Polarization 0,5 (essentially) only available at low energies Channels with more than one meson still not explored 0,0 BONN CLAS MAMI C GRAAL LEGS

10 Great Chance... Introduction Baryon Spectroscopy The Double-Polarization Program (FROST) at JLab: E Photoproduction of Hyperons E π Photoproduction E E η Photoproduction PR π + π Photoproduction (same exp. setup) The Double-Polarization Program at ELSA (Crystal Barrel Experiment): (among many other proposals) ELSA 6/2005 π 0 π 0 Photoproduction ELSA 7/2005 π 0 η Photoproduction

11 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

12 Motivation: Low-Energy Regime P 11 (1440) (Roper Resonance) too low in mass? dynamically-generated resonance effect state with a strong gluonic component small (qqq)-component with a substantial contribution from the meson cloud Parameters depend strongly on data and analysis Contribution of D 13 (1520) to γ p p π + π cross section Different interpretations of γ p p π + π total cross section data Oset et al.: D 13 (1520) π dominant contribution Laget et al.: P 11 (1440) pσ dominant D 13 (1520) π in D-wave (PDG: %) and S-wave (5 12 %)? P 33 (1600) (Roper Resonance of system) too low in mass?

13 Motivation: Medium-Energy Regime 3 rd resonance region F 15 (1680) D 13 (1700) D 33 (1700) P 13 (1720) How to disentangle? Discrepancy of CLAS P 13 (1720) with PDG: two close-by P 13 states? This would be in contradiction with quark models!

14 Motivation: High-Energy Regime N Spectrum ** 2600 *** Mass [MeV] J π L 2T 2J **** **** 2090 ** * 2000 * **** 1990 ** 1986 S ** 1900 ** ** S S *** **** 1650 **** *** **** **** **** 939 **** **** **** Missing Resonances U. Löhring et al., EPJ A10, 395 (2001) 1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 1/2-3/2-5/2-7/2-9/2-11/2-13/2- P11 P13 F15 F17 H19 H1 11 K1 13 S11 D13 D15 G17 G19 I1 11 I1 13

15 Motivation: High-Energy Regime Spectrum ** 2750 ** * 2300 ** 2420 **** 2150 * 2350 * 2200 * 2400 ** Mass [MeV] **** 1750 * 1920 *** 2000 ** 1905 **** 1950 **** 1900 ** * *** *** **** **** **** Missing Resonances Negative-Parity Resonances at 1900 MeV/c 2 J π 1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 15/2+ 1/2-3/2-5/2-7/2-9/2-11/2-13/2-15/2- L 2T 2J P31 P 33 F 35 F 37 H 39 H 3 11 K 3 13 K 3 15 S 31 D 33 D 35 G 37 G 39 I 3 11 I 3 13

16 Motivation: High-Energy Regime Reactions with two or more mesons in the final state account for most of the cross section at W 2 GeV/c 2 Large efforts at ELSA for neutral decay modes: π 0 π 0, π 0 η, etc. π + π at CLAS! There is certainly resonance production above 2 GeV/c 2 But: resonances broad and overlapping Big Challenge Ultimate goal: coupled-channel analysis including pol. constraints to nail down these resonances

17 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

18 Helicity Dependence of the Reaction γ p n π 0 π + σ(µb) (p,π + π o ) TAPS DAPHNE DAPHNE σ 3/2 /2 DAPHNE σ 1/2 /2 total (Nacher et al.) σ 3/2 /2 σ 1/2 /2 GDH Collaboration (Mainz): Largest contribution from σ = 3 2 Nacher et al. underestimates σ = E γ (GeV)

19 Beam Asymmetry I in γ p p π + π 0.3 W = 1.40 GeV W = 1.45 GeV W = 1.50 GeV I W = 1.55 GeV W = 1.60 GeV W = 1.65 GeV CLAS Measurements (S. Strauch et al.) I 0 and model calculations: I 0 W = 1.70 GeV W = 1.75 GeV W = 1.80 GeV Mokeev et al. (solid) Fix and Arenhövel (dashed) W = 1.90 GeV W = 2.10 GeV W = 2.30 GeV I φ (deg) φ (deg) φ (deg)

20 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

21 Beam-Target Polarization Observables d σ d Ω = σ 0 { 1 δ l Σ cos 2φ + Λ x ( δ l H sin 2φ + δ F ) Λ y ( T + δ l P cos 2φ) Λ z ( δ l G sin 2φ + δ E)} Single-Meson Final States (7 Observables) Two-Meson Final States (15 Observables) I = I 0 { ( 1 + Λ i P ) + δ (I + Λ i P ) + δ l [ sin 2β ( I s + Λ i P s ) cos 2β ( I c + Λ i P c ) ] }

22 Circular Beam and Longitudinal Target Polarization d σ d x i = σ 0 { ( 1 + Λ z P z ) + δ (I + Λ z P z )} ( ) := ( ) := d σ( ) d σ( ) = 2 σ 0 {δ (I + Λ z P z d x i d x )} i d σ( ) d σ( ) = 2 σ 0 {δ ( I + Λ z P z )} d x i d x i 1) ( ) + ( ) := d σ 3/2 d x i d σ 1/2 d x i = 4 σ 0 δ (Λ z P z ) 2) ( ) ( ) := 4 σ 0 (Λ z P z )

23 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

24 Model Calculations of P z (known as E) by A. Fix D 13 full D 13 no π (s-wave) D 13 no π (d-wave) 0.4 D 13 full D 13 no ρp D 13 no π W = 1700 MeV Asymmetry Asymmetry W = 1520 MeV Invariant Mass M(π - p) (GeV) -0.4 W = 1520 MeV Invariant Mass M(π - p) (GeV) Can clearly distiguish between solutions if A 0.05 Reality will be a mixture of S-/D-wave! Needs very small errors to distinguish between different contributions!

25 Model Calculations of P x by W. Roberts φ = rad (almost 0), φ = 0.56 rad, φ = 2.09 rad, φ = 3.04 rad (almost π) W = 2 GeV Invariant π + π mass [GeV/c 2 ] Circ. Beam Trans. Target Solid Line Full Calculation Dashed Line S 11 (1900) Omitted Dashed-Dotted Line P 31 (1910) Omitted goal: P c z 0.05

26 Model Calculations of P y by W. Roberts φ = rad (almost 0), φ = 0.56 rad, φ = 2.09 rad, φ = 3.04 rad (almost π) W = 2 GeV Circ. Beam Trans. Target Solid Line Full Calculation Dashed Line S 11 (1900) Omitted Dashed-Dotted Line P 31 (1910) Omitted Invariant π + π mass [GeV/c 2 ] goal: P c z 0.05

27 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

28 Background Estimate 10 6 Counts / Channel 10 6 Counts / Channel (a) p π + π - X m X (GeV 2 ) (c) p π + X m X (GeV 2 ) 10 6 Counts / Channel 10 6 Counts / Channel 0.8 (b) π + π - X m X (GeV 2 ) 0.6 (d) p π - X m X (GeV 2 ) g1c Data Set ( γ p) At least two particles detected Distributions essentially background free

29 Estimate of an Effective Dilution Factor Counts (a) p π + π - X m 2 X (GeV 2 ) Counts (b) π + π - X m 2 X (GeV 2 ) Solid Line total number of events Dashed Line polarized hydrogen Dotted Line unpolarized nucleons (c) p π + X (d) p π - X Counts 2000 Counts 2000 a) D = 0.05 b) D = 0.27 c) D = 0.44 d) D = m X 2 (GeV 2 ) m X 2 (GeV 2 ) D eff = 0.38

30 Outline Introduction 1 Introduction Baryon Spectroscopy 2 3 4

31 Statistical Uncertainties of the Measurements The asymmetry is given by A z = and the statistical error by A z ( stat.) 1 D eff δ Λ z N N N + N 1 D eff δ Λ z 1 N + N The total number of counts required to reach a certain precision A z ( stat.): ( 1 N + N D eff δ Λ z A z ( stat.) and thus, the beam time needed to reach a certain statistical accuracy A z : T = 1 Ṅ γ (E) 1 ( A z ) 2 1 σ unpol 1 ρ p target ɛ ) 2 D 1 eff (δ Λ z ) 2 N bins

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33 Setting Energy [ GeV ] σ p tot [ µb ] Ṅ γ [ MHz ] N bins δ / δ l Λ tg A T [ h ] A B E e = circ/trans C E γ = lin/long E γ = E γ = E γ = D circ/long E e = Σ 27 d Required beam time to study γp pπ + π at and above 2 GeV/c 2

34 Setting Energy [ GeV ] σ p tot [ µb ] Ṅ γ [ MHz ] N bins δ / δ l Λ tg A T [ h ] A B circ/trans E e = C E γ = lin/long E γ = D circ/long E γ = E γ = E e = PAC 29 : 4 days for setting B Will prove success of FROST and come back later...