Studies of Pion Induced Reactions with the HADES Spectrometer Izabela Ciepał for the HADES Collaboration

Transcription

1 Studies of Pion Induced Reactions with the HADES Spectrometer Izabela Ciepał for the HADES Collaboration Institute of Nuclear Physics PAS 1

2 Outline 1) Motivation of the HADES experiment, 2) Electromagnetic structure of baryons, 3) The HADES detector, 4) Sample results, 5) Pion GSI, - first measurement and results, 6) Outlook. 2

3 Motivations two main pillars of HADES experiment Heavy ion physics (A+A): in medium hadron properties nuclear matter at high B, T proton+a Elementary collisions (proton proton, pion proton): vector meson baryon couplings cross sections.. 3

4 Primary Motivations short lived mesons in medium Klimt, Lutz, Weise Phys. Lett. B249 (1990) 386 p/ / + A SPS RHIC e+ e p - beams SIS 18 LHC SIS T [MeV] G.E. Brown / M. Rho: Scaling of masses with quark condensate - order parameter of Chiral Symmetry restoration (PRL 1989, 1991) * m m q q * qq dileptons radiation rare probes (e+e BR~10 5) do not interact strongly with nuclear matter u 4

5 In Medium Mas Thermal emission: dn ee em2 B f (q0, T )Im D / / d xd q M in medium spectral function depends on NN* coupling (N(1520), D(1620), N(1720),...) (1232) N-1 + N -1 R(, N*) +... Rapp, Wambach, Adv. Nucl. Phys. A25 (2000)1 coupling of to baryonic resonances can be directly studied in NN and N 1 2 GeV via N* ( ) Ne+e decays 5

6 Electromagnetic Structure of Baryons N* Ne+ee+ N q2 >0 N e- Transition Form Factors F(Q2 ) N Time Like electromagnetic form factors N (N*) N* N Space Like electromagnetic form factors Dalitz decays, appearance of intermediate vector mesons! / / JPC = 1-- (! ) 6

7 Models for Dalitz Decays constant etff experimental cross section for / used no off shell coupling to vector mesons QED: point like R * vertex M. Zetenyi/M. I. Krivoruchenko 2 d Γ (N e+e )=QED point like F (Q )QCD 2 F (Q )QCD < r > q 1 q /Λ off shell coupling to vector mesons VMD: em. transition FF (Mee) M. Zetenyi/M. I. Krivoruchenko 7

8 Electromagnetic Structure of Baryons Role of meson cloud in baryons G. Ramalho, T. Peña, Phys. Rev. D 93, (2016) Example: 33 (J=3/2) N (J=1/2) * time like bare quark core lattice QCD Resonance Nucleon transitions : em. Transition Form Factors : GM (q2), GE (q2), GC (q2) Important role of pion cloud at small q2 quark core + pion cloud 8

9 High Acceptance Di Electron Spectrometer Side View Beams from SIS18: protons (1 4 GeV), nuclei (1 2 AGeV), pions (0.4 2 GeV/c) secondary beam Spectrometer with DM/M 2% at / /p/k separation with TOF/tracking electrons : RICH (hadron blind), TOF/Pre Shower Trigger: particle mult in TOF DAQ: ~20 KHz with Au+Au collisions FT START RPC (from 2010) 1m Geometry full azimuthal, polar angles 18o - 85o e+e- pair acceptance ~

10 GeV Resonance Production p p Initial : SLJ Final: S L Partial Wave Analysis (PWA) Coherent sum of partial waves Energy dependent solutions: many experimental sets treated together by max. log likehood method event by event Detector acceptances taken into account maximum log-likelihood event-by-event initial NN system transition amplitude A. V. Anisovich et al. Eur. Phys. J. A34 (2007) 129 system of two final particles final state amplitude (resonant, non resonant) 10 10

11 GeV Resonance Production G. Agakishiev et al. Eur. Phys. J. A (2015) FINAL STATES S, P, D waves in pp or pn state P33(1232) and P11(1440) in N state red Δ(1232)P33 blue N(1440)P11 11

12 GeV + (1232) { pe+e } Dalitz Decay First detailed study of a timelike em. baryon transition in p+p: BR( pe+e ) = 4.2x10 5 HADES Collaboration Phys. Rev. C 95, (2017) 0 e+e- pe+e- bare quark core 12

13 3.5 GeV Higher Resonances pp ppe+e HADES Coll., EPJ A50 (2014) 82 Vector Meson Dominance QED point like decay Effect of electromagnetic transition FF coupling to ρ meson of light baryonic resonances R several contributing resonances R: N*(1520), N*(1720), D(1620), D(1905),... excess above QED cocktail can be explained by VDM 13

14 p+p vs 3.5 GeV HADES Coll., PLB 715 (2012) 304 slow sources pe+e < 0.8 GeV/c pp data scaled by Apart scaling nuclear modification factor p+p cockail : based on known sources fixed to data 0 / / /, remarkable difference between p+p, p+a : reduction of, broadening of 14

15 Pion GSI Unique possibility to investigate em. resonance decays via combined Partial Wave Analysis of hadronic and electromagnetic final states pion beam tracker diamond detector polyethylen (CH2)n and C Eur. Phys. J. A (2017) 53:

16 HADES Physics Programe 2014 with Pion Beams Main advantages of pion beams: 1) selectivity: resonances can be excited at given mass by choosing the beam (pion) momentum, HADES starts with s =( ) GeV N*(1520) resonance region data obtained at 4 momenta: 0.656, 0.69, 0.748, 0.8 GeV/c Most of + - data 1.3 < s <2 come from Manley et al., PRD30, (1984) 904 based on events (differential distributions not available) [mb] + - production: off shell coupling of to resonance, + - (~100%) golden channel, N 2 N: Bonn/Gatchina PWA N(1440,1710) + (1910) N(1535) + (1620) N(1720) + (1600) N(1520) + (1700) N(1675) + (1925) N(1680) + (1905) 4`d 3`d 2`d 3) dilepton channel R e+e-, e+e never measured in pion induced reactions. s 16

17 , 0.69, 0.748, 0.8 GeV/c (CH2)n polyethylene target, PE and carbon(c) target, elastic scattering identification: p p, two pion identification in channel: n + -, p - 0 (exclusive channels via missing mass), partial wave analysis focused on N(1520) and production, dilepton identification in channel: ne+e (quasi exclusive channel) baryon resonance Dalitz decays and two body decay. 17

18 , 0.69, 0.748, 0.8 GeV/c red: 35 42% C backgr. (quasi free πp) peak shift due to energy loss in a target statistics of existing database increased by more than 2 orders of magnitude (> 4*107 events for each s) 18

19 PWA Results (n + -) by Bonn Gatchina Group pr el i mi na r y GOAL: extraction of N(1520) BR to D, N, σn N input for dilepton analysis 19

20 PWA Results (n + -) pr el i mi na r y GOAL: extraction of N(1520) BR to D, N, σn N input for dilepton analysis 20

21 Exclusive - p e+e s~1.5 GeV preliminary QED: point-like R- * vertex M. Zetenyi/M. I. Krivoruchenko VMD: em. transition FF (Mee) Models coctail simulations: baryon (?) QED calculated as only D13(1520) Dalitz (with σ of - p nγ), very strong contribution from ρ (using strict VMD), ρ meson contribution derived from PWA of 2 pion channels (n + -, p - 0) measured in the same experiment! 21

22 -p e+e n Formula for Lepton Production 4 2 (2 ) e μν 3 d σ= ( H μ ν 4 L ) d Φ ne+e, 4 k s Q Hμν (hadronic tensor) hadron production and decay to * (dependence on spin and parity of the transition FF): input either from theory or from data (PWA),.. Lμν (lepton tensor) transition * e+e (known from QED!) dφ phase space factor E. Speranza et al., Phys.Lett.B764 (2017) d σ A = 2 pol λ,λ ' had ' λ,λ lep ' λ,λ Spin Density Matrix Elements (SDME): had =ϵμ (k, λ) Hμ ν ϵ ν (k, λ ' ) hadron decay to * λ,λ ' μ μν ν ' lep =ϵ (k, λ) L ϵ (k, λ ) * decay to e+e (QED) λ,λ ' virtual photon polarization (in the helicity basis): 1 ( 0,1, i, 0) 2 longitudinal ϵμ ( k,0)=(0,0,0,1) 1 μ transverse ϵ (k,+ 1)= (0,1,i, 0) 2 transverse ϵμ (k, 1)= 22

23 -p e+e n Angular Distributions dσ had lep λ, λ λ,λ dmdcosθ dcos θe λ, λ ' known from QED ' ' invariant mass shows deviation from point like baryon transitions additional information on the electromagnetic transitions can be provided by the angular distribution SDME depend on m, z=cos θcm baryo n general formula for * e+e angular distribution: 4 parameters: θ e+, e, e+,e, m CM, z=cos θ SDME can be extracted from fit to the angular distributions 23

24 Separation of Resonance Contributions microscopic model including N(1440) and N(1520) excitations in s and u channels and VDM electromagnetic form factors E. Speranza et al., Phys.Lett.B764 (2017) anisotropy coefficient info on the virtual photon polarization angular distributions depends on spin and parity of resonance state (λθ from the model ): s=1.49gev, M ee=0.5 GeV 24

25 HADES Physics Program with Pion Beams Near Future (?) High statistics beam energy scan : continuation and extension to third resonance region Hadronic final states, one pion, 2 pion, hyperon production to control resonance excitation (HADES upgrade with el. calorimeter! neutral final states: / / ) Dielectron measurements : R couplings S31(1620), D33(1700), P13(1720),.. 25

26 Outlook Phase 0/FAIR : joined effort with PANDA/CBM +p / +A Tracking stations (FTS1, FTS2) based PANDA Straw technology p+p, max SIS18 (4.5 GeV) (Krakow)(IFJ, UJ, AGH) A+A Forward TOF based on RPC New ECAL (lead glass) New RICH photon detector Schedule for construction of STS2 I mplementation of the RPC response. Concepts f rom current TOF-RPC. Modules for STS2 at test stand Digitizer responsibility of JU-Krakow 6 long modules have been built HADES acceptance: next 6 will be finished till May 2017 Response of a real detector 18 <θ<85 4 short modules will be built in summer 2017 o o # of strips with signal Vs Charge & position one support frame is ready 2 double layers mounted on support frame (1/2 of STS2) will be ready in Sept FDetector second half of STS2 will be built in Krakow in autumn 2017/ winter 2018 acceptance: 0.5 <θ<6.5 funds for materials of the second half of STS2 will be provided by the Institute de Physique Nucleaire d Orsay o Obtained f rom previous prototype built in 2012 for the NeuLand upgrade. Since then, operative in the Coimbra Mechanical Workshop working as a cosmic ray telescope. o 26

27 Electromagnetic Decays of Hyperons Em. Decays are important for baryon structure Possible with upgraded HADES detector (DAQ/ECAL/Forward Detector) at SIS18,SIS100 Complementary to PANDA programme of Hyperon Spectroscopy in p pbar (1405) = 50 Predictions for K-p Y* Ye+e- I=1 J = 1/20.85% 0.15% (1115) J =1/2+ VMD:R. Williams et. al. PRC48(1993) (1520) = 15.6 J= 3/2- I=0 (1385) = 36 J = 3/2+ 1.2% 0.13% 100 % Only very few Y transitions are known! No Y e+e- ever measured Very strong VM contribution 27

28 Summary HADES & pion beam is an unique tool to understand in details baryon couplings and em. decays of baryon resonances, Strong contribution to e+e production from resonance decays along the VDM but more detail investigations will follow (microscopic models, PWA with e+e..), Large impact on interpretation of e+e production in pa and AA, Exciting possibility to continue this physics with upgraded HADES at FAIR. 28

29 Thank You for Your Attention 29

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