Studying Timelike electromagnetic Baryonic Transitions with HADES Batrice
Studying Time-like electromagnetic Baryonic Transitions with HADES Béatrice Ramstein, IPN Orsay, France for the HADES collaboration Nucleon and Resonance structure in hard exclusive processes Orsay, 29 May 2017 N R Orsay workshop 2017 B. Ramstein e+ e 1
Outline q. Introduction: Motivations of measuring e+e- emission with HADES at GSI q Results from pp reactions Sensitivity of e+e- emission to the electromagnetic structure of baryonic resonances q Results obtained with the GSI pion beam π+π- , e+e- production q Conclusions Orsay workshop 2017 B. Ramstein 2
HADES experimental program: e+e- production hadronic matter studies: • A+A (E/A<2 Ge. V), p+A (E<3. 5 Ge. V), π+A (0. 5<E<1. 6 Ge. V) (SIS 18) • Study modifications of in-medium vector meson spectral functions • depends on NN* coupling - main players: N(1520), (1620), N(1720) Elementary reactions: p+p, d(n)+p, π+p Constrain interpretation of in-medium dilepton spectra by elementary processes Inclusive e+e- production: reference spectra Dilepton exclusive channels e. g. pp ppe+e-, π-p ne+e • Dalitz decay of baryonic resonances R Ne+e- • Sensitivity to Time-like electromagnetic structure • Role of Vector Meson Dominance Orsay workshop 2017 B. Ramstein 3
Electromagnetic baryonic transitions in Time-Like and Space-Like region: towards a global picture ? Time-Like electromagnetic form factors No data - R F(q 2) n Space-Like electromagnetic form factors Precise data from JLab/CLAS up to -q 2=4 Ge. V 2 Inverse pion electroproduction e- * e+ p e- e- * p q 2 =(M ee )2> 0 q 2 <0 fixed R F(q 2) p - Workshop « Space-Like and Time-like electromagnetic baryonic transitions » 8 -12 May 2017, Trento P. Cole, B. R. , A. Sarantsev Orsay workshop 2017 B. Ramstein 4
Theoretical description of Dalitz decay process Space Like: q 2<0 real GSL(q 2) Models constrained by data Analytic continuation Time Like: q 2>0 complex GTL(q 2) For N- : In the Dalitz decay process: • q 2 stays small , q 2 < (MR -Mp)2 e. g. at M =1232 Me. V/c 2 , q 2 < 0. 09 Ge. V/c 2 • influence of vector meson poles for large MR 2 options: take constant form factors (photon point value) + vector meson contribution added incoherently use models form factors GE(q 2), GM(q 2), GC(q 2) Béatrice Ramstein 5
Time-like form factor models Models: fitted to space-like (q 2 0) form factors data analytically continued to time-like region (q 2>0) • Two-component model (direct coupling+VDM) q 2 Zetenyi and Wolf, Phys. Rev. C 86 (2012) 065209 • e-VDM model ( , ’, ’’, …) M. Krivoruchenko, Ann. Phys. 296 (2002) 299 Currently being extended (work in progress) to recent data - SL transition FF (CLAS) - ρ/ωNN* couplings (PWA) • constituent quark-core+ meson cloud existing for N-Δ(1232) and N-N(1520) G. Ramalho, T. Pena Phys. Rev. D 85 (2012), 113014 G. Ramalho, T. Pena Phys. Rev. D 95 (2017), 014003 Space Like Time Like 6 Orsay workshop 2017 B. Ramstein
Acceptance: Full azimuth, polar angles 18 o - 85 o identification: e+/e-, +/ - , K+/K-, p Particle 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 in 2018 Orsay workshop 2017 B. Ramstein 7
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 Orsay workshop 2017 Coimbra (Univ. ), LIP, Portugal B. Ramstein 8
Hades « strategy » 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 (cf KEK, Jlab, CBELSA/TAPS) Elementary collisions pp, dp and -p • reference to heavy-ion spectra • understand dilepton production mechanism (exclusive channels) • 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) Béatrice 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- Shyam and Mosel, , PRC 82 (2010)062201 Deviation from « point-like » transition HADES coll. , ar. Xiv: 1703. 07840 [nucl-ex](PRC, accepted) Orsay workshop 2017 B. Ramstein 10
(1232) Dalitz decay: angular distributions and branching ratio pp ppe+e- E=1. 25 Ge. V Non-resonant Bremsstrahlung subtracted Acceptance corrected Production angle Helicity distributions * e+ee * e+ N scaled contribution from pp pp 0 (PWA) Dalitz decay simulation (Ramalho&Pena form factor) Only transverse : d /dΩe~ 1+cos 2 Fit result : d /dcos ~ 1 + B cos 2 B=1. 17 0. 34 first measurement of (1232) Dalitz decay branching ratio BR( Ne+e-) = 4. 19 0. 42 (model) 0. 46 (syst. ) 0. 34 (stat. ) 10 -5. Orsay workshop 2017 B. Ramstein 11
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) Orsay workshop 2017 B. Ramstein 12
Higher lying resonances: electromagnetic channels p+p pp e+e - 3. 5 Ge. V Effect of VDM like em 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 Inclusive e+e- production reproduced by resonnace model with ρNN* couplings ppe+e- R ω G. Agakishiev et al. Eur. Phys. J. A 50 (2014) 8 J. Weil, et al. , EPJA 48, 111 (2012) Orsay workshop 2017 B. Ramstein 13
Interest of N reaction study 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) Orsay workshop 2017 B. Ramstein 14
Pion beam at GSI Pion beam tracker Diamond detectors HADES GSI pion beam momentum 0. 6 < p <1. 5 Ge. V/c pion flux ~ 106/s at 1 Ge. V/c 2 Double-sided Silicon sensors 100 x 100 mm 2, 300μm thick 2 x 128 channels η Orsay workshop 2017 ω Pion momentum acceptance =2% Resolution Δp/p < 0. 3% B. Ramstein 15
Pion beam experiment with HADES 2014 experiment: Limited beam time +intensity Use of Polyethylene (CH 2)n and Carbon targets Motivations: 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 (Link to time-like transition electromagnetic structure) No existing data N R e+ e- Subtraction of C contribution and Normalisation using π-+p elastic scattering Orsay workshop 2017 B. Ramstein 16
Bonn-Gatchina PWA: HADES data collaboration with A. Sarantsev 4 data samples from HADES (π-p nπ+π- and π-p pπ0π- ) + photon (CB-ELSA, MAMI) and pion (Crystal Ball) data base One example: HADES data at p=683. 5 Me. V/c
Bonn-Gatchina Partial Wave Analysis in 2 production channels -p + - n -p - 0 p Preliminary results • production: 1. 3 mb at 0. 69 Ge. V/c • N(1520): N coupling =12 2 % PDG 2015 : 15 -25% PDG 2016: no info n - total 3/2 - (D 13+. . ) 1/2 - (S 11+. . ) 3/2+ (P 33+. . • Only N(1520) and N(1440) play a significant role around s=1. 5 Ge. V • 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) Orsay workshop 2017 B. Ramstein 18
Analysis of e+e- channels Raw e+e- invariant mass spectrum Signal = Ne+e- -CB CB: same-event like-sign pairs CB rejection cuts: Tracking optimized to reject conversion e+e- opening angle > 9° Signal (M<140 Me. V/c 2) = 13138 Signal (M>140 Me. V/c 2) = 3300 Efficiency corrections based on GEANT simulations Measurement on CH 2 target: (d /d. M)H+0. 5 (d /d. M)C C 4 H Orsay workshop 2017 B. Ramstein 19
+ Inclusive e e production pr elim ina ry PLUTO event generator Fröhlich et al, POS (2007) 076 π-p: • Meson production: Landolt-Börnstein 0 e+e- • « N(1520) » : Point-like baryonic contribution cross section from n -p e+e- distribution as N(1520)0 Ne+e- π-C: • quasi-free process (momentum distr. of nucleons taken into account) scaled to C / H 4 Simulations filtered by acceptance • Cocktail of point-like sources underestimates the e+e- yield at high invariant mass • Strong η contribution Orsay workshop 2017 B. Ramstein 20
Exclusive ne+e- channel with pion beams: 0. 9 <Mmissee < 1. 03 Ge. V/c 2 Minvee > 0. 12 Ge. V/c 2 π-p ne+e- Total π0 e+e-γ η e+e-γ N(1520) ne+e- π-p ne+epre lim ina e+ e- ry PWA ρ e+e- Vector Dominance Model (d /d. Mee)= (d /d. M )* BR(Mee=M ) * (M /Mee)3 ρ π+πMinvππ ( Ge. V/c 2) • Deviation from point-like behaviour consistent with VDM (ρ e+e-) • ρ cross section and mass shape derived from π-p π+π- n measured in the same experiment ! • Empirical way of taking into account VDM form factors for electromagnetic decays 21 Orsay workshop 2017 B. Ramstein
e+e- exclusive production: comparison to models • Lagrangian model : real γ + VDM coupling Large Born term Zetenyi and Wolf Phys. Rev. C 86 (2012) 065209 • Cocktails from resonance models (Gi. BUU) N(1520)0 n ne+e. Overestimates contribution at low q 2 (related to too large radiative decay BR (R N ) in pure VDM models) • s channel model based on ρ/ωNN* couplings Destructive interference too low e+e- yield B. Kaempfer , A Titov , R. Reznik NPA 721(2003)583 M. Lutz , B. Friman, M. Soyeur NPA 713 (2003) 97– 118 Orsay workshop 2017 B. Ramstein 22
Information on em structure from angular distributions • e+e- invariant mass distributions are only sensitive to a linear combination of |FF|2 • Additional information in angular distributions • Similarly to polarization in (e, e’) scattering 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+ Orsay workshop 2017 B. Ramstein e 23
Fit of helicity angle distribution Fitted to HADES data in HADES acceptance : quasi-free -p ne+e- s =1. 49 Ge. V/c 2 Cos CM < 0 Orsay workshop 2017 0<Cos CM < 0. 5 B. Ramstein Cos CM > 0. 5 24
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 Cos CM < 0 0<Cos CM > 0. 5 CM < 0. 5 Orsay workshop 2017 Microscopic model s and t channels: N(1440) and N(1520) with VDM form factors: B. Friman, M. Zetenyi, E. Speranza Phys. Lett. B 764 (2017) 282 -288 • Despite low statistics, significant information on the electromagnetic structure of the transition • Presence of longitudinal contribution • Consistent with pure N(1520) in VDM Model Link to helicity amplitudes to be worked out Federico Scozzi (PHD IPNO/Darmstadt) EPJ Web Conf. , 137 (2017) 05023 B. Ramstein 25
SL and TL transitions: towards a global picture? Still a long way to go in time like region… SL + TL Orsay workshop 2017 B. Ramstein 26
Outlook-Future plans for HADES • Dalitz decay measured for the first time • pioneering promising studies with HADES and the GSI pion beam in the N(1520) region… e+e- channels : Time-like baryonic transitions 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 2022: HADES experiments at FAIR (p and ion beams, possibly pions in future…. ) 27 Orsay workshop 2017 B. Ramstein
Thank you Orsay workshop 2017 B. Ramstein 28
MR=1. 520 Ge. V/c 2 Orsay workshop 2017 B. Ramstein 29
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Separation of channels elastic p - (normalization) PE C p 791 n + - 791 938, 8 Me. V ( = 1. 7%) 939, 4 Me. V ( = 1. 0%) beam tracker p - 0 791 132, 8 133, 2 PE C, C e. b. e p beam tracker SAID ECT 2017
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