Study of Baryon Form factor and Collins effect at BESIII

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1 Study of Baryon Form factor and Collins effect at BESIII Wenbiao Yan On behalf of BESIII Collaboration INT Program INT-17-3 Hadron imaging at Jefferson Lab and at a future EIC 1

2 Bird s View of BEPCII & BESIII Storage ring Linac BESIII detector Beijing electron positron collider BEPCII Beam energy GeV Energy spread: Design luminosity /cm 2 (3770) 2

3 BESIII detector Solenoid Magnet: 1 T Super conducting NIM A (2010) MDC small cell & He gas σ xy =130 μm δp/p = 0.5%@1GeV de/dx=6% TOF σ T = 90 ps Barrel 110->80ps Endcap EMCAL: CsI crystal ΔE/E = GeV σ φ,z = 0.5~0.7 cm/ E Data Acquisition: Event rate = 3 khz Throughput ~ 50 MB/s Muon ID: 8~9 layer RPC σ RΦ =1.4 cm~1.7 cm Trigger: Tracks & Showers Pipelined; Latency = 6.4 μs Hermetic spectrometer for neutral and charged particle with excellent resolution, PID, and large coverage 3

4 Baryon form factors at BESIII 4

5 Proton form factor Hadron vertex are described by Dirac FF (F 1 ) & Pauli FF (F 2 ) Sachs FFs: electric G E and magnetic G M G E & G M : spatial distribution of charge and magnetization, charge density distribution 5

6 Proton form factor Energy scan method Initial state radiation (ISR) method 6

7 e + e - pഥp Differential cross section Coulomb correction C: subtle and important near threshold Born cross section e + e - -> pഥp Effective FF, assume G E = G M G eff = 3q 2 s Born 4pa 2 bc(1+1/ 2t ) PLB 630 (2005) 14 7

8 e + e - pഥp Steep rise toward threshold Rapid decrease Ratio R= G E /G M : disagreement between PS170 & Babar/CMD-3 Poor precision Limited energy points 8

9 Proton G E & G energy scan Proton G E & G M with 12 scan points between 2.22 and 3.71GeV PRD 91 (2015) s Born = N obs - N bkg Le(1+d) N obs : observed e + e - -> pഥp event number N bkg : background event number L: luminosity; e: detection efficiency; (1+d): radiative correction G eff = 3q 2 s Born 4pa 2 bc(1+1/ 2t ) Consistent with previous results Compared with Babar results, uncertainty improved by ~30% 9

10 Proton R= G E /G energy scan GeV GeV ( ) GeV PRD 91 (2015)

11 R em ( G E G / G eff M ) Proton G E & G ISR method use e + e - -> g ISR pഥp with tagged photon Effective Form Factor Stat. 5% - 32% E760 PS170 E835 ADONE73 DM1 FENICE DM2 CLEO BESII BABAR (LA) BABAR (SA) BESIII (R-Scan) BESIII ISR (LA) B E S III P relim inary q 2 [(GeV/c) ] PS170 BABAR (LA) BESIII (R Scan) CMD-3 BESIII (ISR LA) B E S III P relim inary Stat. 16% - 34% q 2 [(GeV/c) ] 11

12 Baryon pair production e + e - -> pഥp Cross section around threshold Charged baryon Neutral baryon G E & G M form factor 12

13 e + e - Λ c +ഥΛ c BESIII Preliminary GeV First measurement of Λ c FFs Very close to threshold ~1.6MeV Coulomb correction C? BESIII Preliminary GeV 13

14 e + e - ΛഥΛ Non-zero behavior around threshold Consistent with previous results, improved by 10% 14

15 Prospects: new energy scan 2015 Energy scan between 2 and 3.08GeV (552pb -1 ) 15

16 Collins effect at BESIII 16

17 Spin-dependent Fragmentation PLB396 (1993) 161 D 1 : unpolarized fragmentation function (FF) H 1 : Collins FF Describes fragmentation of a transversely polarized quark into a spinless hadron h Depend on z=2e h / s, and P h Leads to an azimuthal modulation of hadrons around quark momentum 17

18 Collins FF: Global analysis University of Collins FF PRD 87 (2013) Q 2 evolution of TMD FFs BEPCII: Q 2 similar of SIDIS e + e - annihilation process at different energy with respect to B factories PRD 88 (2013)

19 Collins FF at BESIII Jet structure at BESIII is not clear, can not reconstruct thrust axis correctly. e + e - pp + X: q pp >120 O, back-to-back pion Difficult to suppress backgrounds with on-resonance datasets, prefer off-resonance data in continuum region 62 pb 3.65GeV, below open charm threshold PRL 116 (2016)

20 e + e - p p + X Two Collins FFs NP B806 (2009) 23 20

21 e + e - p p + X Normalized ratio R= N(2φ 0) <N 0 > N(2φ 0 ): di-pion yield in each 2φ 0 subdivision < N 0 >: averaged bin content R U : unlike sign (π ± π ) R L : like sign (π ± π ± ) R C : all pion pair Double ratio: reduce acceptance and radiation effect Fit function A UL/UC mainly contains Collins effect B should be consistent with unity 21

22 e + e - p p + X Non-zero A UL/UC asymmetries in data MC data (without Collins FF): A UL/UC are consistent with zero 22

23 Compare with theory A UL/UC prediction is from Z. B. Kang et al., PRD93 (2016) Asymmetry dependence transverse momentum 23

24 Compare with other experiments Results at e+e- collision Babar & Q 2 ~110GeV 2 Q 2 ~13GeV 2 Prediction in Collins paper Larger asymmetry at lower Q 2 Asymmetry increase as z grows 24

25 Summary Rich program on baryon form factor at BESIII Proton FFs are studied by energy scan and ISR methods First measurement of Λ c FFs, very close to threshold Non-zero behavior around threshold for ΛഥΛ Collins 3.65GeV Clear non-zero asymmetry Larger that that of Babar and Belle Comparable to theoretical predictions e + e - K p + X; e + e - p p 0 /h + X; e + e - K K s + X: plan 25

26 Collins FF at BESIII 26