Coherent and incoherent DVCS and π 0 production on 4 He

Transcription

1 Coheret ad icoheret DVCS ad π 0 productio o 4 He S. Stepaya (JLAB) Nucleo ad Resoace Structure with Hard Exclusive Processes IPN-ORSAY, May 29 31, 2017

2 Outlie 2 DVCS ad p 0 productio o spi ad isospi zero target CLAS/EG6 experimet at JLAB Radial Time Projectio Chamber BSA i coheret ad icoheret DVCS BSA i coheret ad icoheret p 0 productio Future measuremets with CLAS12 i Hall-B Summary

3 Beam Spi Asymmetry i DVCS o uclei 3 ü Coheret DVCS, e A e A γ: Study the partoic structure of the ucleus. For spiless uclei ( 4 He, 12 C, 12 O ), i the forward limit oly oe chiral-eve GPD (H A (x,ξ,t)) is eeded to parametrize the ucleus structure ad hece the BSA - α 7 (φ) ; H => A 34 = α? φ + α A φ ; H BC + α D φ ; H A A BC + H => H α G (φ) Compto Form-factor Fuctios of agle betwee lepto ad hadro scatterig plaes ü Icoheret DVCS, e A e ( N γx: Study the partoic structure of the boud ucleo. Four ciral-eve GPDs (H x, ξ, t, H0 x, ξ, t, E x, ξ, t, E2 x, ξ, t ) A 34 α φ IF? H + ξ(f? + F A )H0 + κf A E}

4 Beam Spi Asymmetry i π 0 productio 4 d 2 σ dϕdt = 1 2π " # $ dσ T dt +ε dσ L dt + 2ε(ε +1) dσ LT dt cosϕ +ε dσ TT dt cos(2ϕ)+ h 2ε(ε 1) dσ LT ' dt % si(ϕ) & ' ü Coheret π 0 productio, ea e A π 0 : At Q 2 ~few GeV 2, trasverse virtual photo cotributio domiates the productio o the ucleo. Cotributio of a logitudial virtual photo icreases i the productio o a spiless uclei, cross sectio of the photo-productio o a spiless uclei ( 4 He, 12 C, 12 O ) vaishes with θ cm 0: 1 dσ O F A (t) dω ~ si2 θ Y> A 34 ~σ 3O (~H0 H0 O ü For the proto target, the flavor structure for π 0 productio process is 2*u+d. For a isospi zero target, e.g. 4 He, the flavor structure is u+d A H0, H0 O A

5 CLAS/Eg6 experimet, Nov-Dec Meso Spectroscopy ad DVCS (coheret ad icoheret) o 4 He Both experimets make use of zero spi ad isospi of the target to restrict productio mechaisms Both experimets require detectio ad idetificatio of recoil a-particles q 2 d Geeratio Radial Time Projectio Chamber with 20 cm log, 6 atm, 4 He gaseous target located iside of the Hall-B supercoductig soleoid maget 20-cm-log, 15 cm i diameter cylidrical detector positioed aroud the 6 atm 4 He gaseous target. The target cell is a 25- cm-log ad 6-mm-diameter Kapto tube with 27-µm-thick walls. Radial Time Projectio Chamber for 4 He Productio data takig ~200mC of beam o target (20 PAC days) with 6.06 GeV polarized electros RTPC calibratio rus at 1.2 GeV

6 RTPC Calibratio Beam eergy 1.2 GeV, the same detector cofiguratio as for productio data takig Both exclusive ad iclusive elastic scatterig q q i.e. with ad without 4 He detectio ratio is the RTPC trackig efficiecy Example Fits Iclusive 6 RTPC Efficiecy Exclusive Z-Vertex (cm) W (GeV)

7 Coheret DVCS aalysis e 4 He e ( 4 He γ M. Hattawy (ORSAY/ANL) 7 Oe electro i CLAS, oe photo with E>2 GeV i the ier calorimeter The recoil α-particles ( 4 He) has bee detected ad idetified i RTPC Set of cuts o kiematical variables defie the exclusivity of the reactio: the coplaarity agle φ betwee the (γ,γ ) ad (γ, 4 He ) plaes, the missig eergy, mass, ad trasverse mometum of the (e 4 He γ) system, the missig mass squared of the e 4 He system, ad the agle θ betwee the measured photo ad the missig mometum of the e 4 He system. Black - all evets, shaded regio after all other exclusivity cuts

8 BSA: coheret DVCS 1 < Q 2 < 2.3 GeV < -t < 0.2 GeV < x B < 0.25 A 34 = dσ\ dσ ] dσ \ + dσ ] = 1 N \ N ] P _ N \ + N ] 8

9 Im ad Re parts of the Compto Amplitude 9 Curves from S. Liuti ad K. Taeja, Phys. Rev. C 72, (2005) HERMES Curves: Covolutio-Dual V. Guzey, Phys. Rev. C 78, (2008) Covolutio-VGG M. Guidal, M. V. Polyakov, A. V. Radyushki ad M. Vaderhaeghe, Phys. Rev. D 72, (2005). OFf-shell model J. O. Gozalez-Heradez, S. Liuti, G.R. Goldstei ad K. Kathuria, Phys. Rev. C 88, o. 6, , (2013). α 7 (φ) ; H => A 34 = α? φ + α A φ ; H BC + α D φ ; H A A BC + H => PRL draft is i collaboratio review

10 Icoheret DVCS e 4 He e ( pγx 10 Detected particles: scattered electro ad recoil proto i CLAS, DVCS photo with E>2 GeV i the ier calorimeter Exclusivity cuts o kiematical variables, similar to hydroge-dvcs aalysis. Boud ucleo GPDs

11 DVCS BSA o boud protos 11 A 34 = dσ\ dσ ] dσ \ + dσ ] = 1 N \ N ] P _ N \ + N ] A 34 α φ IF? H + ξ(f? + F A )H0 + κf A E} bis i t=(p-p ) 2 - smeared due to Fermi motio bis i t =(q-q ) 2 - smeared due to radiative effects Both effects are small i out kiematics PRELIMINARY PRELIMINARY PRELIMINARY

12 BSA ad Geeralized EMC Ratio 12 EMC effect: ucleo structure is modified. Compare BSA measured i icoheret DVCS with the oe measured o the hydroge target. Icoheret BSA is domiated by DVCS off of MF protos (o taggig) R = F A fg i h F A A He LU A p 90 A He LU /A p 90 PRELIMINARY PRELIMINARY

13 Coheret p 0 Productio B. Torayev (ODU) Detected particles: scattered electro i CLAS, two photos i the ier calorimeter, the recoil 4 He is idetified i RTPC Exclusivity cuts o kiematical variables, similar to coheret-dvcs aalysis. Yellow shaded regio - all evets Blue shaded regio = after all other exclusivity cuts e 4 He e ( 4 He π 7 13

14 Coheret p 0 Productio 14 Total of ~800 coheret π 0 evets. si(ϕ A ) LU = σ + σ σ + +σ = 1 P B " $ # N + N N + + N % ' & ΔA LU # si(ϕ ) = 2 % N ΔN + P B % N + + N $ ( ) 2 + ( N + ΔN ) 2 ( ) 2 & ( ( ' PRELIMINARY χ 2 ad u-bied maximum log-likelihood fits to BSA φdepedece si(ϕ A ) LU (χ 2 ) = 0.09 ± 0.05(stat.) si(ϕ A ) LU (LL) = 0.10 ± 0.05(stat.)

15 Icoheret exclusive p 0 Productio 15 Detected particles: scattered electro ad the recoil proto i CLAS, two photos i the ier calorimeter. Kiematic cuts to optimize sigal over backgroud from the fits to the MM-distributios. e 4 He e ( pπ 7 X

16 Icoheret semi-exclusive p 0 Productio e 4 He e ( Nπ 7 X Detected particles: scattered electro i CLAS, two photos i the ier calorimeter. Number of evets i each beam helicity ad φ-bi is extracted from the fit to the MM distributio of (e ( π 7 ) asuumig e N e ( π 7 X 16

17 BSA i coheret ad i-coheret p 0 productio 17 e 4 He e ( 4 He π 7 e 4 He e ( Nπ 7 X PRELIMINARY PRELIMINARY Note the asymmetry sig chage betwee spi/isospi zero target ad the ucleo

18 Hadroic exchages i p 0 productio e 4 He e ( Nπ 7 X p(γ *,π 0 )p V cuts oly NO Poles A LU Productio o 4 He selects exchages with spi ad isospi =0. The 2-π exchage has the strogest J=T=0 cotributio e 4 He e (4 Heπ J.M. Laget / Physics Letters B 695 (2011) Q 2 = 1.50 GeV 2 W = GeV X = 0.18 φ = 90 0 ε = t (GeV 2 ) NO cuts Full

19 Quark degrees of freedom i p 0 productio o 4 He 19 The BSA for this process is: A 34 ~σ 3O (~H0 H0 O These GPDs are proportioal (ormalized) to: Δq ; Δq O where Δq is the polarized PDF ad Δq O is the tesor aomalous momet. For the proto target, the flavor structure for this process is: 2u + d For 4 He, the flavor structure is u + d. Sice Δd ad Δu are opposite i sig, the ehacemet of the d-quark cotributio for 4 He should decrease the resultig beam-spi asymmetry relative to the proto case. (Note clear if this is eough to flip the sig) V. Guzay, private commuicatio

20 Low eergy recoil taggig with CLAS12 New proposal to JLAB PAC45 very ambitious program tued for studies of partoic structure of light uclei ad boud ucleos usig low eergy recoil taggig techique. 20

21 q q q q q q Summary CLAS experimet for coheret photo- ad electro-productio of the photo ad mesos o a spi ad isospi zero target, 4 He, ru i 2009 Scatterig of a 6.06 GeV polarized electros o a 6 atm., 20 cm log 4 He gas target was used to collect data for DVCS ad π 0 electroproproductio A low eergy recoil detector, a cylidrical RTPC, has bee deployed for detectio ad idetificatio of the recoilig α particles BSA i the fully exclusive fial state, for the first time, have bee measured i coheret DVCS ad π 0 productio o 4 He. Coheret DVCS allows modelidepedet extractio of the Re ad Im parts of Compto amplitude. The BSA of π 0 productio has differet sig of asymmetry tha BSA o the ucleo. BSA i icoheret DVCS opes up a ew opportuities for studyig the partoic structure of boud ucleos through spectator taggig A ew measuremets are plaed for CLAS12 usig up to 11 GeV logitudially polarized electro beams, a high pressure gaseous target, ad a ew low eergy recoil detector, ALERT, with much improved PID that is very importat for spectator taggig (proposal have bee re-submitted to PAC45) 21

22 22 Thaks you

23 CLAS12 Desig Parameters 23 Cetral Detector Forward Detector L =10 35 cm 2 s 1 Forward Cetral Detector Detector Agular rage Tracks Photos Resolutio dp/p (%) < 5 GeV/c < 1.5 GeV/c dq (mr) < 1 < Df (mr) < 3 < 5 Photo detectio Eergy (MeV) > dq (mr) 1 GeV --- Neutro detectio N eff < 0.7 (EC+PCAL).a. Particle ID e/p Full rage --- p/p < 5 GeV/c < 1.25 GeV/c p/k < 2.6 GeV/c < 0.65 GeV/c K/p < 4 GeV/c < 1.0 GeV/c p(h)ggg Full rage ---

24 24 Selectio of elastic evets W-distributio of all evets 2 2 W = M He + 2νM He Q2 f- vs. Z-RTPC distributio of elastic electro i CLAS ad a track i RTPC ν = Ee Ee' Q 2 = 2Ee Ee' (1 cos ϑ e' ) Differece betwee measured ad calculated from elastic kiematics polar agle of recoil 4He Differece betwee measured ad calculated from elastic kiematics polar agle of recoil 4He

25 RTPC Efficiecy: Detect Elastic 4 He 25 With electro selected, look for RTPC track with matchig vertex ad elastic agles q q Each Δ-quatity below has a cut o the others peaks Resolutios are roughly 8mm, 2,3 o z,φ,θ Pass Fail Elastic Cuts Iclusive Test cut sesitivity, particularly Δθ Result: ~400K Exclusive Elastic Evets W (GeV) ΔZ CLAS-RTPC (cm) Δθ CLAS-RTPC ( ) Δϕ CLAS-RTPC ( )

26 Target & RTPC assembly for eg6 26 Workig gas, 1 atm 2 alumiized mylar foils, 2 ad 6 µm thick 1 atm New target cell, 6 mm ID & with 27 µm wall thickess, target gas 6 atm. (Bous ru with 50 µm thick target cell) Ope geometry (has ot bee utilized effectively) Improved backed readout Total material thickess o the way of particles at 90o is ~39 µm

27 RTPC calibratio 27 Elastic scatterig o 4 He at 1.2 GeV D, N*