Kuc, H.

Ramstein, B.

Salabura, P.

GSI Repository

Dipion and dielectron production in quasi-free np reactions with HADES ∗H. Kuc1, P. Salabura1, and B. Ramstein†21Jagiellonian University, Crakow, Poland; 2IPNO, Orsay, FranceThe origin of the yield excess in the inclusive dielec-tron production in np over the pp collisions measured byHADES [1] still needs to be clarified to provide a globalreliable interpretation of dielectron spectra measured in nu-clear collisions by HADES. Several attempts to solve thisissue by including a contribution from off-shell ρ mesonwere recently proposed [2,3,4]. Since the main decay ofthe ρ is the channel with two charged pions, an analysis ofthis channel provides an important constraint for dielectronspectra [5]. Furthermore, the results on dipion productionprovide valuable verification of the di-baryon formation,recently reported by the WASA collaboration [6,7], in acomplementary phase space region as covered by HADES.We report on results on exclusive dipion and dielectron pro-duction in np collisions in the deuterium formation chan-nel.Signal selectionThe quasi-free np reaction was studied using measure-ments of the dp reaction with a deuterium beam of 2.5GeV. The quasi-free np reaction was tagged by the pro-ton spectator detection at small angles (0.3-7◦) in the For-ward Wall with a velocity close to the beam velocity. Sincefor both np → de+e− and np → dpi+pi− reactions, thedeuteron angular distribution is very forward peaked, veryfew deuterons reach the HADES detector, which covers po-lar angles from 18◦ to 85◦, while the detection in the For-ward Wall (FW) is more suited. Due to the large statisticsin the np → dpi+pi− channel, both cases (deuteron de-tected in FW or in HADES) were investigated, while for themore rare np → de+e− channel, only the detection of thedeuteron in the FW was investigated. Deuterons are identi-fied in HADES with a very high purity using the correlationbetween time-of-flight and momentum. For the deuteronidentfication in the FW a coplanarity constraint on all par-ticles in the final state was used, in addition to cuts on thetime-of-flight to suppress events corresponding to the un-bound processes (np→ nppi+pi− or np→ npe+e−) [5].Dipion productionAfter a subtraction in each bin of the background con-tribution using the coplanarity condition, the experimentaldistributions of the pn → dpi+pi− have been compared tothe predictions of a model provided by M. Bashkanov [6],∗Work supported by CNRS/IN2P3 (France) and by grant2013/10/M/ST2/00042 (Poland).† ramstein@ipno.in2p3.fr]2 [GeV/cs2.2 2.3 2.4 2.5 2.6 [arbitrary units]σ051015-pi+pi d→npPRELIMINARY Figure 1: Cross-sections (full symbols) for the quasi-freepn→ dpi+pi− reaction deduced from measurements with adeuteron beam at 2.5 GeV, where the two pions are detectedin HADES and an additional charged particle is detected inthe Forward Wall [5]. The blue histogram shows the re-sult of a simulation taking into account double ∆(1232),N(1440) and N(1520) excitation, as well as the contribu-tion of a dibaryon with a mass of 2.38 GeV. The data havebeen normalized using quasi-free pp elastic scattering andare compared to the model predictions on the same absolutescale. The yellow area shows the distribution expected fora phase space simulation of the reaction with an arbitrarynormalization.which includes conventional t-channel processes (double∆(1232),N(1440) andN(1520) excitation), as well as thecontribution of a dibaryon (d⋆) with a mass 2.38 GeV anda width Γ ∼ 70MeV , as suggested by the results of theWASA collaboration in the pn → dpi0pi0, pn → dpi+pi−and pp → dpi+pi0 reactions [2,3]. The model underesti-mates the yields measured in the case of the deuteron de-tected in HADES, which might be due to a contaminationof non quasi-free processes. However, when the deuteronis detected in the FW, a reasonable description of the yieldsand differential distributions is achieved, while the conven-tional sources alone underestimate the measured yields [5].A procedure to unfold the neutron momentum distributionand extract the np → nppi+pi− cross section as a func-tion of the center-of-mass energy (√s) of the np pair wasapplied to the experimental data, as well as to simulatedevents generated either with the model [6] or with a phase-space model. The results are shown in Fig.1. The datahave been normalized using quasi-free pp elastic scatter-GSI SCIENTIFIC REPORT 2014 MU-NQM-HADES-03DOI:10.15120/GR-2015-1-MU-NQM-HADES-03 3ing events and are compared to the model predictions onthe same absolute scale, which is displayed here in arbi-trary units. The results obtained with the phase space dis-tributions have an arbitrary normalization. A peak is ob-served in our experimental data. Even though located at aslightly larger√s, it is very similar to the one predictedby the model, where it is attributed to the d⋆ contribution.Though systematic effects still need to be investigated, thisresult seems to confirm the observations of the WASA col-laboration.Dielectron production]2 [GeV/ce+e-Minv0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 [arbitrary units]e+e-/dMσd-410-310-210-1101)-e+ de→exp.(loose cut for np)-e+ de→simulation (np)-e+ de→exp (upper limit for np X-e+ e→npE=1.25 GeVPRELIMINARY Figure 2: np → Xe+e− reaction as a function of thee+e− invariant mass [5]. The empty squares and full blueline show experimental results corresponding respectivelyto a loose cut based on time-of-flights to enhance the con-tribution of the np → de+e− channel and to the upperlimit for this reaction. The results of a simulation of thenp→ de+e− process are shown as a dot-dashed blue line.The data have been normalized using quasi-free pp elasticscattering and are compared to the model predictions on thesame absolute scale.In the dielectron channel, no signal for np→ de+e− re-action is visible. However, the Feldman & Cousins methodwas applied to extract an upper limit for the differentialcross sections of this reaction using constraints from thecoplanarity condition. To show the power of this method,this upper limit is displayed in Fig. 2, in comparison tothe experimental differential cross sections obtained afterappplying only time-of-flight cuts on the FW hits. This up-per limit is in agreement with the predictions of [3], wherethe np → dγ differential cross sections are deduced fromthe known photo disintegration (np → dγ) cross sectionsand the extension to virtual photon emission is based on theVector Dominance Model. However, it is overestimated by60% by the model [4], which uses an off-shell ρ productiondue to the ∆∆ Final State Interaction.References[1] G. Agakichiev et al., Phys. Lett. B690 (2010) 118.[2] R. Shyam and U. Mosel Phys. Rev. C82 (2010) 062201.[3] B. Martemyanov et al. Phys. Rev. C84 (2011) 047601[4] M. Bashkanov and H. Clement Eur. Phys. J. A50 (2014) 107.[5] H. Kuc, PhD thesis, Paris-Sud University, Dec. 2014.[6] P. Alardson et al., Phys. Rev. Lett. 106 (2011) 242302.[7] P. Alardson et al., Phys. Lett. 721 (2013) 229.MU-NQM-HADES-03 GSI SCIENTIFIC REPORT 20144 DOI:10.15120/GR-2015-1-MU-NQM-HADES-03

Dipion and dielectron production in quasi-free np reactions with HADES

http://repository.gsi.de/record/183779/files/MU-NQM-HADES-03.pdf

Dipion and dielectron production in quasi-free np reactions with HADES

Abstract

Similar works

Full text

Available Versions

GSI Repository