Goodbye to Large G? Marek Karliner. Cambridge University and Tel Aviv University. with John Ellis, hep-ph/ ph/

Transcription

1 Goodbye to Large G? Marek Karliner Cambridge University and Tel Aviv University with John Ellis, hep-ph/ ph/ LC2005, Cairns, 7/2005

2 nucleon spin quark helicities: q = Z 1 0 h q (x) q (x) i + h q (x) q (x) i sources of information about q-s: (a) weak decays g A = u d ; g 8 = u + d 2 s (b) polarized DIS on nucleon g p 1 (x) =1 2 X q and n p e 2 q [q (x) q (x)+ q (x) q (x)] = 1 2 +QCD&higher twist corrections X q e 2 q q(x) 2

3 global fit to p and d polarized DIS data: and u = 0.82 ± 0.03 ±... d = 0.44 ± 0.03 ±... s = 0.11 ± 0.03 ±... Σ = u + d + s = 0.27 ± 0.04 ±... = Spin Crisis Angular momentum sum rule: 1 2 = 1 2 Σ + G + L q + L z 3

4 Why Should the Nucleon be Strange? the vacuum is strange: chiral symmetry for π, K mesons h0 ss 0i =(0.8 ± 0.1)h0 qq 0i cannot be expected to disappear when one inserts qqq test charge into the vacuum generated in pqcd: q g ss also by non-perturbative effect: instantons, chiral soliton models 4

5 axial anomaly: the last hope of the naïve quark model: q g q q α s 2π G 2nd term on rhs due to gluon axial current. observables: g q-s, rather than q-s, so in principle can resurrect the NRQM by g s = s α s G 6= 0 : s =0, G 6= 0 2π = large G ' 2 at Q 2 ' 3GeV 2. 5

6 How to measure G/G Photon-Gluon Fusion (PGF) A PGF µ N = a LL G G Two tagging methods: -Highp T hadron pairs - Open charm 6

7 Photon-Gluon Fusion (PGF) COMPASS cuts for jet asymmetry: each hadron p T > 0.7 GeV/c, ³ p T p T 2 2 > 2.5 (GeV/c) 2 current fragmentation: x F > 0.1, z>0.1 7

8 G/G from open charm D 0 K - π + (BR 4%) D *+ D 0 π + + c.c. A cc γn = a LL G G 8

9 Compilation of G G measurements Indirect, using polarized structure function data: BBA: G = ± BBB: G = ± AAC: G = ± Direct, using hadron production asymmetries: HERMES : G/G = 0.41 ± 0.18 ± <x G < 0.28, SMC : G/G = 0.20 ± 0.28 ± 0.10 hx G i =0.07, COMPASS : G/G = 0.06 ± 0.31 ± 0.06 hx G i =0.13, Q 2 > 1GeV 2 COMPASS : G/G = ± ± hx G i = Q 2 < 1GeV 2 9

10 results from open charm no physics background but much less data than hadron asymmetry G/G = 1.08 ± 0.73 x G =0.15 ±

11 G/G from hadron polarization asymmetries 11

12 fits to asymmetry data data not precise enough (yet?) for a full-fledged fit but good enough to estimate G magnitude assume the three trial forms BB3, BB4, AAC for G(x, Q 2 ); normalization A ijk left as a free parameter BB3 : G(x, Q 2 ) = A BB3 f BB3 (x, Q 2 ), BB4 : G(x, Q 2 ) = A BB4 f BB4 (x, Q 2 ), AAC : G(x, Q 2 ) = A AAC f AAC (x, Q 2 ). 12

13 first, fit forcing G=2 a very bad fit 13

14 fit with normalization allowed to float χ 2 =2.5 χ 2 =1.1 χ 2 =1.5 significantly better fit 14

15 BB4 BB3 AAC Q 2 χ 2 χ 2 χ 2 χ 2 χ 2 χ 2 GeV 2 G best for G best for G best for fit G=2 fit G=2 fit G= ± ± ± ± ± ± ± ± ± ± ± ± Fits to the HERMES, SMC and COMPASS G/G data, for Q 2 =1.5, 2, 5 and 10 GeV 2. For each parametrization we list the best-fit value of G and its χ 2,aswellas the χ 2 value corresponding to G =2. 15

16 Goodbye to Large G. Long Live s 0 0! recent exp. evidence on strange magnetic moment of the nucleon from JLab lattice 16