HLAB meeting paper 2011/1/18 T.Gogami CLAS ( CEBAF Large Acceptance Spectrometer ) Clam shell is open

Paper

Abstract

Introduction

In this article CLAS g11a data set (5/7/2004 – 7/29/2004) – γp → K + Λ differential cross section – Λ recoil polarization (P Λ )

Experimental setup

Tagger ΔE/E ≈ MAGNET photon electron [GeV] gravity 6.06 [m] E-plane (384) T-plane (61) 4mm thickness 20mm thickness σ time ≈ 110 [ps] ( intrinsic resolution 2×10 -4 ) Radiator Gold foil (10 -4 rad. length ) The bremsstrahlung tagged photon beam in Hall B at JLab, D.I. Sober, Nuclear Instruments and Methods in Physics Research A 440 (2000) 263}284

CLAS CLAS ( CEBAF Large Acceptance Spectrometer )

Toroidal Magnet Superconducting toroidal magnet – Temperature : 4.4 [K] (Liquid He) – 1920 [A], 1.8 [T] ( Max : 3861[A], 3.5 [T] ) B B People

Toroidal Magnet Superconducting toroidal magnet – Temperature : 4.4 [K] (Liquid He) – 1920 [A], 1.8 [T] ( Max : 3861[A], 3.5 [T] ) B B Magnetic field in midplane between two coils

CLAS (CEBAF Large Acceptance Spectrometer) Target : Liquid H 2 (cylindrical Kapton chamber 40cm L × 4cm D ) Momentum resolution ~ 5×10 -3 Trigger – tagger × 2/6 ( Start × TOF ) DAQ rate ~ 5 [kHz] Tracking ~300 [μm] Timing (~80 – 160 [ps]) γp → K + Λ

Spec The CEBAF large acceptance spectrometer (CLAS), B.A. Mecking, Nuclear Instruments and Methods in Physics Research A 503 (2003) 513–553

DATA and EVENT selection Want to see “γp → K + Λ” – γp → K + pπ - ・・・ Tree track analysis – γp → K + p(π - ) ・・・ Two track analysis +28% of full set 1.Momentum, Energy correction 2.Loose cut – Missing mass, Invariant mass 3.Kinematic fit  confidence level cut

Confidence level cut Λ Λ σ ~ 8 [MeV/c 2 ] Confidence level The effect of the confidence level cut is dramatic ! Missing mass

Background reduction Cut misidentified particles

Background reduction (2) 1.Given event “i” neighbors 3.Missing mass 4.Maximum likelihood fit – Gaussian signal (s) – linear background (b) 5.Quality factor weighted by (1-Q) All events Missing Mass √s = GeV bin

Other consideration Detector acceptance Extracting P Λ Normalization

Systematic Uncertainties

Result

Mandelstam variables proton photon Kaon Lambda

Cross section Count Corrected Number of photons Target Binning width Acceptance

Cross section( イ ) Red : two track analysis Blue : three track analysis Error Statistic Acceptance

Cross section( ロ ) Red : two track analysis Blue : three track analysis Error Statistic Acceptance

Cross section( ハ ) Red : two track analysis Blue : three track analysis Error Statistic Acceptance

Agreement between the two analysis Agreement between the two analysis, Very little systematic offset Consistent within statistical and acceptance uncertainties

Weighted mean differential cross sections Weighted mean differential cross sections : i = 2,3 j = 2,3 ρ = 0.28 For kinematic points where only a two-track measurement exists, Cross section : its value Uncertainty : scaling by 1 + |μ| = 1.136

Cross section γp → K + Λ Red : CLAS Blue : previous CLAS Green : SAPHIR Measurement of p->K+ Λ and p->K+ Σ 0 at photon energies up to 2.6 GeV, K.H. Glander, arXiv:nucl-ex/ v1 26 Aug 2003 The γ p → K+ and γ p → K+0 reactions at forward angles with photon energies from 1.5 to 2.4 GeV, M. Sumihama, PHYSICAL REVIEW C 73, (2006) Measurement of the γ p → K+ reaction at backward angles, K. Hicks, PHYSICAL REVIEW C 76, (R) (2007) LEPS SAPHIR

Model comparison ( イ ) K-MAID (fit with SAPHIR) – K+,K*(892),K 1 (1270) RPR (fit with p CLAS) – K+ Regge trajectory, K* Regge trajectory – N(1650)S 11,N(1710)P 11, N(1720)P 13,N(1900)P 13, missing D 13 – Forward angle Bonn-Gatchina (fit with p CLAS) – Partial wave analysis

Model comparison ( ロ ) K-MAID (fit with SAPHIR) – K+,K*(892),K 1 (1270) RPR (fit with p CLAS) – K+ Regge trajectory, K* Regge trajectory – N(1650)S 11,N(1710)P 11, N(1720)P 13,N(1900)P 13, missing D 13 – Forward angle Bonn-Gatchina (fit with p CLAS) – Partial wave analysis

Model comparison ( ハ ) K-MAID (fit with SAPHIR) – K+,K*(892),K 1 (1270) RPR (fit with p CLAS) – K+ Regge trajectory, K* Regge trajectory – N(1650)S 11,N(1710)P 11, N(1720)P 13,N(1900)P 13, missing D 13 – Forward angle Bonn-Gatchina (fit with p CLAS) – Partial wave analysis

Conclusion

END

Start counters

Introduction (γp → K + Λ) γp → K + Λ – Polarization Pseudoscalar nature of the K+ Self-analyzing decay of the Λ – Isospin structure of the final state Allows coupling only to I=1/2 N* (not the I=3/2 Δ*)

PΛPΛ CLAS : Blue SAPHIR2004 : Green GRALL2007 : Black

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