Studies of the Lambda(1405) in Proton-Proton Collisions with ANKE at COSY-Juelich
aa r X i v : . [ nu c l - e x ] N ov Studies of the
Λ(1405) in Proton-ProtonCollisions with ANKE at COSY-J¨ulich
MENU 200711th International Conferenceon Meson-Nucleon Physics andthe Structure of the NucleonSeptember10-14, 2007IKP, Forschungzentrum Jülich, Germany
I. Zychor
The Andrzej So ltan Institute for Nuclear Studies05-400 ´Swierk, Poland
Abstract
The lineshape of the Λ(1405) was studied in the pp → pK + Y reactionat a beam momentum of 3.65 GeV/c at COSY-J¨ulich. The ANKE spectrom-eter was used to identify two protons, one positively charged kaon, and onenegatively charged pion in the final state. Invariant–mass and missing–masstechniques were applied to separate two neighbouring neutral excited hyperonresonances, the Σ (1385) and Λ(1405). Both the shape and the position ofthe Λ(1405) distribution are similar to those measured in other reactions andthis information contributes to the ongoing debate regarding the structureof this resonance. The Λ(1405) is a well established four–star resonance [1] but it is still notwell understood as a baryonic state; it does not fit in easily within the simplequark picture [2]. The Λ(1405) might be the spin-multiplet partner of the J P = − Λ(1520), a meson–baryon resonance, a KN quasibound state [3], ora q q pentaquark state [4]. Recent theoretical investigations based on chiraldynamics predict the existence of two poles in the vicinity of the Λ(1405) [5–7] with a decay spectrum that depends upon the production process. Inany event, the Λ(1405) does not have a Breit–Wigner shape because of theopening at 1432 MeV/c of the decay mode KN [8–10]. Independent of themodel, if the Λ(1405) were a single quantum state, its lineshape should beindependent of the method of production.The Σ (1385) and Λ(1405) resonances overlap significantly because theirwidths of 36 MeV/c and 50 MeV/c , respectively, are much larger than themass difference of ∼
20 MeV/c . This is the main experimental difficulty ininvestigating the Λ(1405) nature via the Σ + π − and Σ − π + decay modes sincethese are also possible final states for the Σ (1385) disintegration. However,1. Zychor Studies of Λ(1405) with ANKE@COSYthe Λ(1405) → Σ π decay can be used to identify this resonance unambigu-ously because isospin forbids this mode for the Σ (1385).In Fig. 1 the simplified decay scheme of excited neutral resonances withmasses below 1432 MeV/c demonstrates the differences between Σ (1385)and Λ(1405) utilised in the present analysis. p(938)(1405) L (1116) L (1385) S (1193) S p p - p g (1193) S Figure 1: Simplified decay scheme for the Λ(1405) and Σ (1385) hyperonresonances The experiment was performed at the Cooler Synchrotron COSY, a mediumenergy accelerator and storage ring for protons and deuterons, which is oper-ated at the Research Center J¨ulich (Germany) [11]. COSY supplied a storedproton beam with a momentum of 3.65 GeV/c at a revolution frequency of ∼ s − . Using a hydrogen cluster–jet target, the average luminosity duringthe measurements was L = (55 ±
8) pb − .The ANKE spectrometer [12] used in the experiments consists of threedipole magnets that guide the circulating COSY beam through a chicane.The central C–shaped spectrometer dipole D2, placed downstream of thetarget, separates the reaction products from the beam. The ANKE detec-tion system, comprising range telescopes, scintillation counters and multi–wire proportional chambers, registers simultaneously positively and nega-tively charged particles and measures their momenta [13].2. Zychor Studies of Λ(1405) with ANKE@COSYThe following configuration of detectors was used to measure particlesover a particular momentum range:1. forward (Fd) and side–wall (Sd) counters for protons between 0.75 GeV/cand the kinematic limit,2. telescopes and side–wall scintillators for K + between 0.2 and 0.9 GeV/c,3. scintillators for π − between 0.2 and 1.0 GeV/c.The angular acceptance of the spectrometer dipole D2 is | ϑ H | . ◦ hori-zontally and | ϑ V | . ◦ vertically. Momenta, reconstructed from tracks inmulti–wire proportional chambers, allow the masses of particles to be deter-mined to within ∼
10 MeV/c .A multiparticle final state, containing two protons, a positively chargedkaon, a negatively charged pion and an unidentified residue X selected the pp → pK + pπ − X reaction. In the Σ (1385) → Λ π decay the X residue isa π while, for the Λ(1405) → Σ π decay, X = π γ (see Fig. 1). ), MeV/c - p Sd M(p E N T R I ES / M e V / c PRELIMINARY ), MeV/c - p p + MM(p K50 100 150 200 250 300 350 400 ) , M e V / c + K Fd MM ( p PRELIMINARY
Figure 2:
Left:
Invariant mass M ( p Sd π − ) measured in the 3.65 GeV/c pp → pK + Y reaction. The yellow horizontal box shows the band usedto select the Λ. Right:
Missing mass
M M ( p F d K + ) versus the missing mass M M ( pK + π − p ). The left (red) vertical box covers the π region and the right(blue) one has M M ( pK + π − p ) >
190 MeV /c ≫ m ( π ).The following method was used to separate the Λ(1405) from the Σ (1385):1. identify four particles: p F d , p Sd , K + and π − ,2. analyse events with the invariant mass of the p Sd π − pair equal to themass of the Λ, 3. Zychor Studies of Λ(1405) with ANKE@COSY3. select events with the missing mass of ( p F d , p Sd , K + , π − ) equal to a π mass to isolate the Σ (1385) and much higher than the π mass toidentify the Λ(1405).In the left part of Fig. 2 the invariant mass M ( p Sd π − ) of the p Sd π − pairsis shown, where the protons were registered in the side-detector counters. Inthe mass region around 1116 MeV/c a peak with a FWHM of ∼ is visible on a background that is mostly combinatorial in nature. The ver-tical box marks invariant–masses between 1112 and 1120 MeV/c . Eventswithin this box are plotted in the right panel of Fig. 2 in a distribution of M M ( p F d K + ) versus M M ( pK + π − p ). The two vertical bands show the four–particle missing–mass M M ( pK + π − p ) criteria used to separate the Σ (1385)candidates from those of the Λ(1405). The left band is optimised to identifya π whereas the right one selects masses significantly greater than m ( π ).The deviation of ∼ from the nominal pion mass of 135 MeV/c isnot unexpected in a mass reconstruction involving four particles. ), MeV/c + K Fd MM(p1300 1350 1400 1450 1500 1550 1600 E N T R I ES / M e V / c PRELIMINARY ), MeV/c + K Fd MM(p1300 1350 1400 1450 1500 1550 1600 E N T R I ES / M e V / c PRELIMINARY
Figure 3: Missing–mass
M M ( p F d K + ) distribution for the pp → pK + pπ − X reaction for events with M ( p Sd π − ) ≈ m (Λ). The distribution obtainedfor M M ( pK + π − p ) ≈ m ( π ) is presented in the left panel and for M M ( pK + π − p ) >
190 MeV/c in the right one.In the left part of Fig. 3 the missing–mass M M ( p F d K + ) distribution isshown for M M ( pK + π − p ) ≈ m ( π ). A peak around a mass of 1385 MeV/c and a width of ∼
50 MeV/c is seen on a rather small background. In the rightpart of Fig. 3 the distribution, obtained for M M ( pK + π − p ) >
190 MeV/c ,has a peak near 1400 MeV/c and a tail on the high missing–mass side.In order to explain the measured spectra, Monte Carlo simulations wereperformed to estimate backgrounds from non–resonant and resonant reac-tions. The following non–resonant processes have been included:4. Zychor Studies of Λ(1405) with ANKE@COSY ), MeV/c + K Fd MM(p1300 1350 1400 1450 1500 1550 1600 E N T R I ES / M e V / c experimentsimulationsimulation wo 1385simulation 1385 p = X NK p S PRELIMINARY
Figure 4: Missing–mass
M M ( p F d K + ) distribution for the pp → pK + pπ − X reaction for events with M ( p Sd π − ) ≈ m (Λ) and M M ( pK + π − p ) ≈ m ( π ).Experimental points with statistical errors are compared to the red his-togram of the fitted overall Monte Carlo simulations. The simulation includesresonant contributions (yellow) and non–resonant phase–space production(green). Arrows indicate the Σ π and KN thresholds.1. pp → N K + πX ( γ )2. pp → N K + ππX ( γ )with X representing any allowed Λ or Σ hyperon. The second group consistsof the following exclusive hyperon production reactions:1. pp → pK + Σ (1385)2. pp → pK + Λ(1405)3. pp → pK + Λ(1520)The simulations, based on the GEANT3 package, were performed in a similarmanner to those in Ref. [14].In the study of Σ (1385) production and its backgrounds, events weregenerated according to phase space using a relativistic Breit–Wigner param-eterisations for the known hyperon resonance [1]. The relative contributionsof the resonant and non–resonant reactions were deduced by fitting the ex-perimental data to the simulated spectra. In Fig. 4 the histograms show theresonant contribution from the pp → pK + Σ (1385) reaction (solid-yellow)and the sum of non–resonant contributions (solid-green). The result of theoverall simulations is shown as a red histogram.5. Zychor Studies of Λ(1405) with ANKE@COSYTurning now to the Λ(1405), simulations show that the Σ (1385) does notcontaminate the missing–mass M M ( pK + π − p ) range above 190 MeV/c (seeFig. 5). This point is crucial since it allows us to obtain a clean separationof the Σ (1385) from the Λ(1405). ), MeV/c - p p + MM(p K0 50 100 150 200 250 300 350 400 ) , M e V / c + K Fd MM ( p PRELIMINARY
Figure 5: Simulated distribution of events with missing mass
M M ( p F d K + ) versus M M ( pK + π − p ). The left shaded vertical box covers the π regionand the right one has M M ( pK + π − p ) >
190 MeV/c ≫ m ( π ). Notice anabsence of events in the right box.In order to extract the Λ(1405) distribution from the measured Σ π de-cay, the non–resonant contributions have first been fitted to the experimentaldata. The resulting non–resonant background is indicated by the shaded his-togram in the left panel of Fig. 6. When this is subtracted from the data,we obtain the distribution shown as experimental points in the right panelof Fig. 6. In Table 1 the information that is relevant for the evaluation of the totalcross section is given. For both the hyperons measured this is of the order ofa few µ b.The (Σ π ) invariant–mass distributions have been previously studied intwo hydrogen bubble chamber experiments. Thomas et al. [10] found ∼ + π − or Σ − π + events corresponding to the π − p → K Λ(1405) → K (Σ π ) reaction at a beam momentum of 1.69 GeV/c. Hemingway [8] used a 4.2 GeV/ckaon beam to investigate K − p → Σ + (1660) π − → Λ(1405) π + π − → (Σ + π − ) π + π − and measured 1106 events [8].In Fig. 7 our experimental points are compared to the results of Thomasand Hemingway, which have been normalised by scaling their values down by6. Zychor Studies of Λ(1405) with ANKE@COSY ), MeV/c + K Fd MM(p1300 1350 1400 1450 1500 1550 1600 E N T R I ES / M e V / c g p = X NK p S experimentsimulation PRELIMINARY ), MeV/c + K Fd MM(p1300 1350 1400 1450 1500 1550 1600 E N T R I ES / M e V / c NK p S PRELIMINARY
Figure 6:
Left:
Experimental missing–mass
M M ( p F d K + ) distribution forthe pp → pK + pπ − X reaction for events with M ( p Sd π − ) ≈ m (Λ) and M M ( pK + π − p ) >
190 MeV/c compared to the shaded histogram of thefitted non–resonant Monte Carlo simulation. Right:
The background–subtracted lineshape of the Λ(1405) decaying into Σ π .factors of ∼ ∼
7, respectively. The effect of the KN threshold is quiteobvious in these data, with the Λ(1405) mass distribution being strongly dis-torted by the opening of this channel. Despite the very different productionmechanisms, the three distributions have consistent shapes.This might suggest that, if there are two states present in this region, thenthe reaction mechanisms in the three cases are preferentially populating thesame one. It should, however, be noted that by identifying a particularreaction mechanism, the proponents of the two–state solution can describethe shape of the distribution that we have found [7].Table 1: Total cross section for the production of the Σ (1385) and Λ(1405)resonances in the 3.65 GeV/c pp → pK + Y reactionΣ (1385) Λ(1405)number of events 170 ±
26 156 ± . × − . × − combined BR (%) 56 21luminosity (pb − ) 55 ± ± ±
11 55 ± µ b) 5 . ± . stat ± . syst . ± . stat ± . syst
7. Zychor Studies of Λ(1405) with ANKE@COSY ), MeV/c + K Fd MM(p1300 1350 1400 1450 1500 1550 1600 E N T R I ES / M e V / c experimentThomasHemingway PRELIMINARY
Figure 7: The background–subtracted lineshape of the Λ(1405) decayinginto Σ π (points) compared to π − p → K (Σ π ) [10] (black–solid line) and K − p → π + π − Σ + π − [8] (red–dotted line) data. The decay of excited hyperons Y ∗ via Λ π and Σ π → Λ γπ can be detecteddirectly in electromagnetic calorimeters by registering neutral particles, i.e. γ and/or π . Measurements of such channels are underway in γp reactions(CB/TAPS at ELSA [15], SPring − pp collisions with WASA at COSY [17]. Acknowledgments
References [1] W.-M. Yao et al. , J. Phys. G , 1 (2006), but see also the minireviewin D.E. Groom et al. , Eur. Phys. J. C , 1 (2000).[2] N. Isgur and G. Karl, Phys. Rev. D , 4187 (1978).[3] R.H. Dalitz and S.F. Tuan, Ann. Phys. (N.Y.) , 307 (1960).8. Zychor Studies of Λ(1405) with ANKE@COSY[4] T. Inoue, arXiv:0708.2339 [hep-ph].[5] D. Jido et al. , Nucl. Phys. A , 181 (2003).[6] V.K. Magas, E. Oset and A. Ramos, Phys. Rev. Lett. , 052301 (2005).[7] L.S. Geng and E. Oset, arXiv:0707.3343 [hep-ph].[8] R.J. Hemingway, Nucl. Phys. B , 742 (1984).[9] R.H. Dalitz and A. Deloff, J. Phys. G , 289 (1991).[10] D.W. Thomas, et al. , Nucl. Phys. B , 15 (1973).[11] R. Maier, Nucl. Instr. Meth. A , 1 (1997)[12] S. Barsov et al. , Nucl. Instr. Meth. A , 364 (2001).[13] M. B¨uscher et al. , Nucl. Instr. Methods A , 378 (2002[14] I. Zychor et al. , Phys. Rev. Lett. , 012002 (2006).[15] H. Schmieden, ELSA Letter of Intent ELSA/4-2003.[16] H. Fujimura, AIP Conf. Proc. , 737 (2007).[17] H.-H. Adam et al. , nucl-ex/0411038.[18] I. Zychor et al.et al.