Baryon transitions in the timelike region at HADES

Baryon transitions in the timelike region at HADES Béatrice Ramstein, IPN Orsay, France for the HADES collaboration Hadronic physics with lepton and hadron beams, JLab , Sept. 5 -8, 2017 N R Jlab Sep. 2017 e+ e- B. Ramstein 1

Outline q Introduction Accessing time-like electromagnetic structure of baryonic transitions through Dalitz decays N R q Results from pp reactions with HADES at GSI e+ e- q Preliminary results with the GSI pion beam q Conclusions Jlab Sep. 2017 B. Ramstein 2

Baryon electromagnetic structure: from space like to time like regimes Most information on electromagnetic baryon transitions comes from electron scattering, probing the space like (q 2<0) region Time like (q 2> 0 ) region : analytically connected to space like Annihilation q 2 > (MB+MB’)2 Scattering q 2<0 Dalitz decay 0< q 2 <(MB’-MB)2 q 2= M ee 2 Jlab Sep. 2017 B. Ramstein 3

Time like form factors of baryons: experimental situation Annihilation q 2 > (MB+MB’)2 Focus on threshold and asymptotic (large q 2) behavior Nucleon: data for elastic form factors: e+e- (FENICE, LEAR, Fermilab, pp, e+e- nn, Cleo, BABAR, BES) on-going studies at BESIII, plans for PANDA Hyperons: a few points for , elastic form factors from BABAR, BELLE and CLEO On-going studies at BESIII Dalitz decay : 0<q 2 <(MB’-MB)2 Data only at q 2=0 (radiative decay/ photoproduction) Finite q 2: on-going studies at HADES ( , N*), plans for HADES and PANDA (hyperons: Y’ Ye+e-) Jlab Sep. 2017 B. Ramstein 4

Interest of Dalitz decay studies Electromagnetic structure at small positive q 2 (0 < q 2 < (MB’-MB)2) - Dalitz decays (B’ Be+e-) branching ratios Timelike electromagnetic form factors as a function of q 2 Continuity with space like region via photon point ( q 2=0 ) Vector meson dominance (VDM) Input for studies of e+e- production in medium (main HADES program) modification of the meson spectral function Rapp, Chanfray and Wambach NPA 617, (1997) 472 Rapp and Wambach EPJA 6 (1999) 415 Jlab Sep. 2017 B. Ramstein 5

Theoretical description of Dalitz decay process Differential decay width as a function of q 2 e. g. for N- : Additional information from lepton angular distributions 2 options: take constant « point-like » form factors (photon point value) use models form factors GE(q 2), GM(q 2), GC(q 2) Space Like: q 2<0 real GSL(q 2) Models constrained by data Jlab Sep. 2017 Analytic continuation B. Ramstein Time Like: q 2>0 complex GTL(q 2) 6

Time-like form factor models • e-VDM model ( , ’, ’’, …) M. Krivoruchenko, Ann. Phys. 296 (2002) 299 Currently being updated (work in progress) to recent data - SL transition FF (CLAS) - ρ/ωNN* couplings (PWA) • Two-component model (real photon +vector meson couplings) Zetenyi and Wolf, Phys. Rev. C 86 (2012) 065209 constituent quark model existing for N-Δ(1232) Space Like constituent quark-core+ meson cloud existing for N-Δ(1232) and N-N(1520) Time Like • N- (1232) core Wan and Iachello, Int. J. Mod. Physics A 20 (2005) 1846, I. Fröhlich et al. , Eur. Phys. J. A 45 (2010) 401 -411 • G. Ramalho, T. Pena Phys. Rev. D 85 (2012), 113014 G. Ramalho et al. , Phys. Rev. D 93, 033004 (2016) G. Ramalho, T. Pena Phys. Rev. D 95 (2017), 014003 core 7 Jlab Sep. 2017 B. Ramstein

Acceptance: Full azimuth, polar angles 18 o - 85 o e+/e-, +/ - , K+/K-, Particle identification: p RICH, Time Of Flight, Pre-Shower (pad chambers & lead converter) ( also MDC (K )) HADES High Acceptance Di. Electron Spectrometer at GSI, Darmstadt Trigger: ~ 50 k. Hz Momentum measurement Magnet: ∫Bdl = 0. 1 - 0. 34 Tm MDC: 24 Mini Drift Chambers Leptons: x~ 140 per cell, p/p ~ 1 -2 % M/M ~ 2% at peak Electromagnetic Calorimeter Forward Detector (0. 5 -7°) to be ready end of 2018 Jlab Sep. 2017 B. Ramstein 8

The Collaboration Cracow (Univ. ), Poland Darmstadt (GSI), Germany Dresden (FZD), Germany Dubna (JINR), Russia Frankfurt (Univ. ), Germany Giessen (Univ. ), Germany Jülich (FZJ), Germany Milano (INFN, Univ. ), Italy München (TUM), Germany Moscow (ITEP, MEPh. I, RAS), Russia Nicosia (Univ. ), Cyprus GSI Orsay (IPN), France Rez (CAS, NPI), Czech Rep. SIS Sant. de Compostela (Univ. ), Spain Wuppertal (BUG), Germany Jlab Sep. 2017 Coimbra (Univ. ), LIP, Portugal B. Ramstein 9

Hades experimental program Study dilepton emission in dense and hot matter A+A reactions in the 1 -2 AGe. V energy range C+C 1 and 2 Ge. V/nucleon, Ar+KCl 1. 75 Ge. V/nucleon, Au+Au 1. 25 Ge. V/nucleon cold matter at normal nuclear density p+Nb 3. 5 Ge. V , -+W/C 1. 7 Ge. V/c Elementary collisions pp, dp and -p • reference to heavy-ion spectra • dilepton emission is probing time-like electromagnetic structure of hadronic transitions! Simultaneous measurements of hadronic channels (inclusive and exclusive meson production 1 , 2 , , …) • • • Production mechanism Baryon spectroscopy (baryonic resonance couplings, . . ) Partial Wave Analysis: pp pp 0, pp pn + , -p n + -, -p p 0 - …. strangeness measurement program: K- , K 0, , (1385), (1405), p correlations Jlab Sep. 2017 B. Ramstein

(1232) Dalitz decay: form factors pp ppe+e- E=1. 25 Ge. V and 0 production cross section deduced from PWA. of one pion production channels HADES coll. , Eur. Phys. J. A 51 (2015) 137 π° Dalitz: π° e+e- pp Bremsstrahlung Shyam and Mosel, , PRC 82 (2010)062201 Helicity distributions * e+e- e * e+ N Fit result : d /dcos ~ 1 + B cos 2 B=1. 17 0. 34 Similar to transition at q 2=0 (real ) HADES coll. , Phys. Rev. C 95 (2017) 065205 first measurement of (1232) Dalitz decay branching ratio BR( Ne+e-) = 4. 19 0. 42 (model) 0. 46 (syst. ) 0. 34 (stat. ) 10 -5. Jlab Sep. 2017 B. Ramstein 11

Higher lying resonances: electromagnetic channels p+p pp e+e - 3. 5 Ge. V Effect of electromagnetic transition FF for light baryonic resonances (N(1520), …) ppe+e- ω R ρ Dalitz decays of point-like baryonic resonances constrained by ppπ0 and pnπ+ channels + “direct” ρ and ω ppe+e- R Inclusive e+e- production reproduced by resonnace model with ρNN* couplings ω G. Agakishiev et al. Eur. Phys. J. A 50 (2014) 8 J. Weil, et al. , EPJA 48, 111 (2012) Jlab Sep. 2017 B. Ramstein 12

HADES pion beam programme Electromagnetic channels • Exclusive -p n e+e- can be easily selected • Resonance produced in s-channel with fixed mass = s (less overlapping contributions than in pp) • N interaction better controlled than pp • Imputs from pion photo-/electro-production can be used - n R p * e+ e- Hadronic channels: • Very poor data base for pion beam data • New data needed to advance knowledge in baryon and meson spectroscopy (J-PARC, projects of meson beam facilities W. J. Briscoe et al. , Eur. Phys. J. A 51 (2015) 129) Jlab Sep. 2017 B. Ramstein 13

Pion beam experiment at GSI pion beam momentum 0. 6 < p <1. 5 Ge. V/c pion flux ~ 106/s at 1 Ge. V/c Pion beam tracker: Acceptance =2% Resolution Δp/p < 0. 3% Diamond detectors η HADES ω 2014 experiment: commissioning beam time, use of Polyethylene (CH 2)n and Carbon targets investigate the N(1520) region + - and - ° production in an energy scan (4 measurements s =1. 46 -1. 55 Ge. V/c 2) Improve database for baryon spectroscopy: 2 N ( N, N, ) branchings e+e- production s =1. 49 Ge. V/c 2 Resonance Dalitz decays R→Ne+e. No existing data (information on time-like transition electromagnetic structure) Jlab Sep. 2017 B. Ramstein 14

Bonn-Gatchina Partial Wave Analysis in 2 production channels collaboration with A. Sarantsev HADES data (π-p nπ+π- and π-p pπ0π- at 4 energies ) + photon (CB-ELSA, MAMI) and pion (Crystal Ball) data base -p + - n -p - 0 p pre lim n - total 3/2 - (D 13+. . ) 1/2 - (S 11+. . ) 3/2+ (P 33+. . Preliminary results • production in -p 1. 3 mb at 0. 69 Ge. V/c • N(1520): N coupling =12 2 % PDG 2015 : 15 -25% PDG 2016 : no info ina ry • New HADES data are crucial for the determination of the contribution • Still no data on between 1. 54 and 1. 75 Ge. V/c 2 (part of HADES future program) Jlab Sep. 2017 B. Ramstein 15

Inclusive pre lim ina + ee production PLUTO event generator ry Fröhlich et al, POS (2007) 076 π-p: • Meson production: Landolt-Börnstein 0 e+e- e+e • « N(1520) » : Point-like baryonic contribution from n -p e+e- distribution as N(1520)0 ne+eπ-C: • quasi-free process (momentum distr. of nucleons taken into account) scaled to C / H 4 Measurement on CH 2 target: (d /d. M)H+0. 5 (d /d. M)C C 4 H Signal (M<140 Me. V/c 2) = 13138 Signal (M>140 Me. V/c 2) = 3300 Jlab Sep. 2017 Simulations filtered by acceptance • Strong η contribution • Cocktail of point-like sources underestimates the e+e- yield at high invariant mass B. Ramstein 16

Exclusive ne+e- channel with pion beams: 0. 9 <Mmissee < 1. 03 Ge. V/c 2 Minvee > 0. 12 Ge. V/c 2 pre lim π-p ne+e- ina ry pre N N*, 1500 events e+ e- lim ina Total π0 e+e-γ η e+e-γ N(1520) ne+eπ-p n + - ry PWA Off-shell ρ e+e- Vector Dominance Model (d /d. Mee)= (d /d. M )* BR(Mee=M ) * (M /Mee)3 Off-shell ρ π+πI=1 pre lim ina Minvππ ( Ge. V/c 2) Empirical way of taking into account VDM form factors for baryon electromagnetic decays 17 Jlab Sep. 2017 B. Ramstein ry

Information on em structure from angular distributions Spin density matrix formalism: E. Speranza et al. Phys. Lett. B 764 (2017) 282 -288 A. Sarantsev (ECT* meeting, May 2017) 3 independent density matrix elements: 11 , 10, 1 -1 depending on Mee and • contain information on electromagnetic structure e. g. pure transverse (E/M): 11 =1/2, 10 =0 N R • Related to helicity amplitudes and/or form factors e+ Jlab Sep. 2017 B. Ramstein e 18

Very first results on density matrix elements Quasi-free -p ne+e s =1. 49 Ge. V/c 2 Mee> 400 Me. V/c 2 Microscopic model s and u channels: N(1440) and N(1520) with VDM form factors: B. Friman, M. Zetenyi, E. Speranza Phys. Lett. B 764 (2017) 282 -288 pre lim ina ry Transverse • Despite low statistics, significant information on the electromagnetic structure of the transition • Presence of longitudinal contribution • 11 consistent with pure N(1520) in VDM model Cos * CM Federico Scozzi et al. (HADES coll. ) EPJ Web Conf. , 137 (2017) 05023 Exp. proposal at GSI/SIS 18 (2018 -2019) : explore third resonance region ( s~1. 7 Ge. V/c 2) channels: -p ne+e- , -p N, n , K 0 , K , …. +photons (new electromagnetic calorimeter) Possibly after 2019: s~2 Ge. V/c 2 region ( (1405), …) Jlab Sep. 2017 B. Ramstein 19

HADES upgrade New RICH photon detector & read-out (coll. with CBM@FAIR) Gain in e+e- efficiency x 5 Forward detector (0. 5 -6. 5°) Straws+RPC TOF (coll. with PANDA@FAIR) (x)~150 m (TOF)~70 ps To be ready end of 2018 Jlab Sep. 2017 B. Ramstein 20

More plans for baryon em structure measurements in the time like region : hyperon Dalitz decays Hyperons are narrow ( =15 -40 Me. V) : can be studied in pp, p. A with HADES (and later pp with PANDA at FAIR). Radiative decays of hyperons Y • Badly known, e. g. no measurement for (1405), • High sensitivity to internal structure (quark corrélations ) Dalitz decays of hyperons Y e+e- (BR~10 -5) No measurement. Relevance of Vector Dominance in the hyperon sector? • Feasibility study: about 100 events/day in pp at 4. 5 Ge. V (1520) e+e. Doubly strange baryons ( ) (S= - 2) unexpected high yield below threshold Ar. KCl (-640 Me. V) et p. Nb (-70 Me. V) Elementary mechanism to be understood HADES collab PRL 103(2009)132310 HADES collab PRL 114( 2015) 212301 Jlab Sep. 2017 B. Ramstein 21

Conclusion • Pioneering studies for Time-Like electromagnetic baryon transitions Dalitz decay branching ratio (pp reactions), N(1520) region ( pion beam) • 2 channels: baryon spectroscopy 2018: 3 -4 year time slot for HADES experiments with beams (p, , A) at SIS 18 before the start of FAIR Higher acceptance (Forward Detector)+ Electromagnetic Calorimeter Pave the way for future Investigate heavier resonances Δ(1620), N(1720), … meson beam facilities in e+e- channels and many hadronic channels, W. J. Briscoe et al. , Eur. Phys. e. g. -p n , K 0 , K , …. J. A 51 (2015) no. 10, 129 Electromagnetic decays of hyperons in pp reactions : Y , Y e+e After 2025: HADES experiments at FAIR (p and ion beams, possibly pions in future…. ) 22 Jlab Sep. 2017 B. Ramstein

Thank you Jlab Sep. 2017 B. Ramstein 23

Higher lying resonances: meson production Cocktail of baryonic resonances Fitted to distributions in ppπ0 , pnπ+ and pp channels HADES coll. . Eur. Phys. J. A 50 (2014) 8 2 channels under study (A. Belounnas PHD Orsay) Jlab Sep. 2017 B. Ramstein 24

Higher lying resonances: meson production Cocktail of baryonic resonances Fitted to distributions in ppπ0 , pnπ+ and pp channels HADES coll. . Eur. Phys. J. A 50 (2014) 8 2 channels under study (A. Belounnas PHD Orsay) Jlab Sep. 2017 B. Ramstein 25