Measurement of Magnetic Moments of Charmed Baryons Stephan

Measurement of Magnetic Moments of Charmed Baryons Stephan Paul TU-München

Overview CERN workshop - New Opportunities 2008 • Magnetic moments and measurements • Challenges for charmed baryons • Experimental approach – Crystal channeling – Requirements • Count rate estimates 2

Status and Predictions of Magnetic Moments • Basic quantity characterising fermion • The case of strange baryons CERN workshop - New Opportunities 2008 Good description possible but problems with XPT (loops) • The case of heavy baryons: 3 – Predictions: m. Lc=0. 3 -0. 5 m. N m. Xc=0. 3 -0. 8 m. N – Mostly : m. Lc ~ 0. 37 m. N and m. Lc ~ 0. 42 m. N

Measurement Technique • ‘Standard’ method for short lived particles (hyperons) – Produce polarized hyperons with high energy proton beams CERN workshop - New Opportunities 2008 • High x. F and high p. T required • Polarization observed transverse to production plane • Mechanism theoretically not understood but many models – Hyperon extraction via magnetic channel • Spin precession in guiding B-field – Spin analysis • Use parity violating decays (interference of S- and P-waves in decay amplitudes) • Use decays with high analyzing power - L p p- (a = 0. 642) S+ p p 0 (a = -0. 980) – Vary B-field to vary spin precession angle 4

Challenge for Charmed Particles • Lifetime of charmed baryons small (200 -400 fs) – Flight path (ctgb) of L+c ~ 0. 8 cm @300 Ge. V; 8 cm @3 Te. V – Flight path (ctgb) of X+c ~ 1. 6 cm @300 Ge. V; 16 cm @3 Te. V CERN workshop - New Opportunities 2008 • Production cross section very small - s(Lc) ~40 mb/nucleon (10 -3 stot) at 300 Ge. V - s(Lc) ~400 mb/nucleon (10 -2 stot) at 8 Te. V) crude extrapolation • Polarisation unknown (except for R 608 experiment) • Branching ratio for individual decays small 5 BR Lc Lp+ ~ 1± 0. 3% Analyzing power large (a. Lp = − 0. 91± 0. 15) BR Lc Λ l+ν ~ 2± 0. 6% Analyzing power large (a Λ l+ν = − 0. 86± 0. 04) BR Lc S+p 0 ~ 1± 0. 34% Analyzing power large (a. Sp = − 0. 45± 0. 31) • Effect expected to be small : mc~ 0. 4 m. N ~ m. Lc

Experimental realization I • • Channeling for particles with ET magnetic < Ecri Use channeling to produce high fields • Channeling in bent crystal: - need positively charged particle (scattering off nuclei for -) straight crystal centrifugal gives NO netpotential E (B) field --superimpose CERN workshop - New Opportunities 2008 need bent --Lowers Ecritcrystal -bending and pparticle gives effective B-field seen 6

Spin Rotation for Channeling Particles • Measurement for charged particles in a static B-field CERN workshop - New Opportunities 2008 – Sensitivity to g-2 - Dw = w. Lamor – wcyclotron - Typical B-fields : m. X-=-0. 65 m. N X- • For channeling in crystal: • E-fields: 1010 V/cm, B-fields 103 T • for l ~ 1 cm m p 7 E

Experimental Realization CERN workshop - New Opportunities 2008 • Assume FNAL realization (S+): Dftrajectory = 1. 6 mr FNAL 8 mc 0. 2 0. 3 0. 4 0. 6 [m. N] 4 Te. V DFLc -1500 -784 -73 1350 [mr] 6 Te. V DFLc -2240 -1176 -110 2020 [mr] • 5% measurement on m. Lc 200 mrad in DFspin • Wide range of momenta accepted

Count Rates Estimates I • Assume CERN workshop - New Opportunities 2008 – - 9 8 Te. V ( s ~ 120 Ge. V) s(Lc) ~400 mb/nucleon x. F > 0. 4 and p. T > 0. 6 Ge. V/c (as in hyperon polarization) 108 p/s (100 days) assume R 608 cross section and p spectrum N Lc = 4 Lc /s / %l. I ( Lp) 107 s assume L = 8 cm for silicon and crystal (decay losses 63%) Channeling efficiency: - Crystal height: 1 cm Crystal length 4 -5 cm Crystal width 1 mm vert: no limits hori: < cri t~ 10 mrad echanneling ~ 0. 01

Count Rates Estimates II – Dechanneling (interactions with e- ) : no problem – Bending dechanneling (distortions due to jig) CERN workshop - New Opportunities 2008 • Rcurv = 30 m with p ~6 Te. V: fdechannel ~ 0. 5 – Sacc : Surface acceptance: (channeling/total area: 0. 5) • Total acceptance: DW fdechannel Sacc ~ 2. 5 10 -3 (~ 5 10 -4) – B = 45 Tm (2. 5 cm length, D beam = 1. 6 mr) at FNAL – B ~ 300 Tm (mechanically tried D beam = 12 mr) at FNAL • Dx = 3 -5% l. I • Ntotal ~ 50000 Lc ( Lp) 10

Set-up and ‘But’s: • What is the polarization of Lc ? • Which final state to take ? CERN workshop - New Opportunities 2008 - 11 Lc Lp+ : flight path of L (ct = 7. 8 cm) ~ 160 m (low detection efficiency) Lc S+p 0 : flight path of S (ct = 2. 4 cm) ~ 45 m Lc p K-p+: unknown analyzing power (but good sec. vertex) …… - Reconstruction efficiency (typical) 5 -10%

Measurement • How to identify channeled particles CERN workshop - New Opportunities 2008 – Deflection angle (short silicon telescope DL = 2 -3 cm) – Specific (reduced) energy loss in crystal (instrumented silicon crystal) • Effective number of events: 12 - P(Lc ) = 0. 6 (e. g. Bis-2) - a. Lc a. L = 0. 9 0. 64 = 0. 57 - s for DFSpin= 200 mr DP/P = 20% (m. Lc = 0. 4 m. N)

Conclusion • Charged weakly decaying members of charm/beauty baryons can be measured via crystal channeling CERN workshop - New Opportunities 2008 – Charm: – Beauty • Effect: D : [-500, 500]mr • Events needed: few hundred • Need: very high energy beam (Te. V) – Short lifetime – High effective field (spin precession) 13 • • Cross section and polarization uncertain Analyzing power of most interesting channel unknown Seems feasible based on present knowledge Helps to understand heavy baryons

Estimates of mc from radiative D*-decays CERN workshop - New Opportunities 2008 • What do we know about mc ? • Lets look at radiative decays – Experiment: – Predictions: (see e. g. Lepage et al. ) • Agreement: but large uncertainties 14

References CERN workshop - New Opportunities 2008 • S. Paul (1993) Talk given at an LHB meeting (spokesperson G. Carboni) • V. M. Samsonov NIM B 119 (1996) 15

16 CERN workshop - New Opportunities 2008