aa r X i v : . [ nu c l - e x ] A ug K-Long Facility for JLab and its Scientific Potential
Igor I. Strakovsky ,⋆ InstituteforNuclear Studies, Department ofPhysics, TheGeorgeWashington University, Washington, D.C.20052, U.S.A.
Abstract.
Our main interest in creating a secondary high-quality KL-beam is to in-vestigate hyperon spectroscopy through both formation and production processes. Wepropose to study two-body reactions induced by the KL-beam on the proton target. Theexperiment should measure both di ff erential cross sections and self-analyzed polariza-tions of the produced Λ -, Σ -, and Ξ -hyperons using the GlueX detector at the Je ff ersonLab Hall D. New data will greatly constrain partial-wave analysis and reduce model-dependent uncertainties in the extraction of strange resonance properties, providing anew benchmark for comparisons with QCD-inspired models and LQCD calculations.The measurements will span c.m. cos θ from -0.95 to 0.95 in c.m. range above W = At the beginning of February of 2016, Je ff erson Lab hosted a Workshop Physics with Neutral Kaonbeam at JLab . It was dedicated to the physics of hyperons produced by the neutral Kaon beam onboth unpolarized and polarized targets [1]. The workshop follows our LoI–12–15–001 [2] (JLab KLFProject) to help to address the comments made by the JLab PAC43 and to prepare the full proposal forPAC45. The emphasis is on the hyperon spectroscopy. Mini-Proceedings of the KL2016 are availableat arXiv [3].The
Excited Hyperons in QCD Thermodynamics at Freeze-Out (YSTAR2016) Workshop [4] isa successor to the recent KL2016. This workshop will discuss the influence of possible "missing"hyperon resonances on QCD thermodynamics, on freeze-out in heavy ion collisions and in the earlyuniverse, and in spectroscopy. Recent studies that compare LQCD calculations of thermodynamiccalculations, statistical hadron resonance gas models, and ratios between measured yields of di ff erenthadron species in heavy ion collisions provide indirect evidence for the presence of "missing" reso-nances in all of these contexts. The aim of the workshop is to sharpen these comparisons, advance ourunderstanding of the formation of baryons from quarks and gluons microseconds after the Big Bangand in today’s experiments, and to connect these developments to experimental searches for direct,spectroscopic, evidence for these resonances.The JLab12 energy upgrade, with the new Hall D, is an ideal tool for extensive studies of non-strange and, specifically, strange baryon resonances [5]. Our plan is evolving to take advantage ofthe existing high quality photon beam line and experimental area in the Hall D complex at Je ff erson ⋆ e-mail: [email protected] ab to deliver a beam of K L particles onto a liquid hydrogen cryotarget within the GlueX detector.The recently constructed GlueX detector in Hall D is a large acceptance spectrometer with goodcoverage for both charged and neutral particles that can be adapted to this purpose. Obviously, Kaonbeam facility (KLF) with good momentum resolution is crucial to provide the data needed to identifyand characterize the properties of hyperon resonances. The masses and widths of the lowest Λ and Σ baryons were determined mainly with Kaon-beam experiments in the 1970s [6]. Pole position incomplex energy plane for hyperons has began to be studied only recently, first of all for Λ (1520) − [7]. A comparison of recent coupled-channel analyses [8–11] comes to the conclusion that, for most cases,it is only the first excited state in each partial wave whose detailed properties [branching reaction(BRs), helicity amplitudes] are known. Di ff erent analyses may agree on the existence of the secondstate (in each partial wave) but not on their decay properties, while there is no agreement even on theexistence of a third state in a particular partial wave. Given the arduous nature of the task involvedin obtaining high-quality data that enter these multichannel analyses it is reasonable to address thequestion as to the final scope of this e ff ort. In other words: How many resonances do we need toidentify in order to convince ourselves that we have achieved a solid understanding of the baryonspectrum from QCD? As examples, we examine this question from the viewpoint of lattice gauge andconstituent quark model (QM) calculations.Our current "experimental" knowledge of Λ ∗ , Σ ∗ , Ξ ∗ , and Ω ∗ resonances is far worse than ourknowledge of N ∗ and ∆ ∗ resonances; though they are equally fundamental. Specifically, the propertiesof multi-strange baryons ( Ξ ∗ and Ω ∗ states) are poorly known. For instance the Review of ParticlePhysics lists only two states with BR to K Ξ , namely, Λ (2100) − (BR < Σ (2030) + (BR < ∗ and 3 ∗ . Many more states have been predicted by QMs. For example in case of S U (6) × O (3), it would be required 434 resonances, if all revealed multiplets were completed (three70 − and four 56 − ).Three light quarks can be arranged in 6 baryonic families, N ∗ , ∆ ∗ , Λ ∗ , Σ ∗ , Ξ ∗ , and Ω ∗ . Number ofmembers in a family that can exist is not arbitrary [12]. If SU(3) F symmetry of QCD is controlling,then for the octet: N ∗ , Λ ∗ , and Σ ∗ , and for the decuplet: ∆ ∗ , Σ ∗ , Ξ ∗ , and Ω ∗ . Number of experimentallyidentified resonances of each baryon family in PDG2014 summary tables is 17 N ∗ , 24 ∆ ∗ , 14 Λ ∗ , 12 Σ ∗ , 7 Ξ ∗ , and 2 Ω ∗ . Constituent QMs, for instance, predict existence of no less than 64 N ∗ and 22 ∆ ∗ states with mass less than 3 GeV. Seriousness of "missing-states" problem [13] is obvious from thesenumbers. To complete SU(3) F multiplets, one needs no less than 17 Λ ∗ , 41 Σ ∗ , 41 Ξ ∗ , and 24 Ω ∗ . There are two particles in the reactions K L p → π Y and KY that can carry polarization: the target andrecoil nucleon / hyperon. Hence, there are two possible double-polarization experiments: target / recoil.While a formally complete experiment requires the measurement, at each energy and angle, of at leastthree independent observables, the current database for K L p → π Y and KY is populated mainly byunpolarized cross sections. Figure 1 illustrates this quite clearly. igure 1. Experimental data available for K L p → K + n , K L p → K S p , K L p → π + Λ , and K L p → π + Σ as a function of c.m. energy W [14]. The number of data points (dp) isgiven in the upper righthand side of each subplot [blue (red) shows amount of unpolarized(polarized) observables]. Total cross sections are plotted at zero degrees. The experiments using unpolarized LD (to get "neutron" data) and polarized target (aka FROST)for both hydrogen and deuteriun components, we will leave for the following proposals. Obviosly, itwill open up a new avenue to the complete experiment. Note that the "neutron" data are critical todetermine parameters of neutral Λ ∗ s and Σ ∗ s hyperons which were considered recently [15]. Following Höhler [16], the di ff erential cross section and polarization for K L p → π Y and KY are givenby d σ d Ω = Ż ( | f ( W , θ ) | + | g ( W , θ ) | ) , P d σ d Ω = Ż Im( f ( W , θ ) g ( W , θ ) ∗ ) , (1)where Ż = ~ / k , with k the magnitude of c.m. momentum for the incoming meson. Here f ( W , θ ) and g ( W , θ ) are the usual spin-nonflip and spin-flip amplitudes at c.m. energy W and meson c.m. scatteringangle θ . In terms of partial waves, f ( W , θ ) and g ( W , θ ) can be expanded as f ( W , θ ) = ∞ X l = [( l + T l + + lT l − ] P l (cos θ ) , g ( W , θ ) = ∞ X l = [ T l + − T l − ] P l (cos θ ) , (2)where l is the initial orbital angular momentum, P l (cos θ ) is a Legendre polynomial, and P l (cos θ ) isan associated Legendre function. The total angular momentum for the amplitude T l + is J = l + ,while that for the amplitude T l − is J = l − . For hadronic scattering reactions, we may ignore smallP-violating terms and write K L = √ K − K ) , K S = √ K + K ) . (3)We may generally have both I = I = KN and KN scattering, so that theamplitudes T l ± can be expanded in terms of isospin amplitudes as T l ± = C T l ± + C T l ± , (4)where T Il ± are partial-wave amplitudes with isospin I and total angular momentum J = l ± , with theappropriate isospin Clebsch-Gordan coe ffi cients C I .We plan to do a coupled-channel PWA with new GlueX KLF data in combination with availableand new J-PARC K − p measurements when they will be available. Then the best fit will allow to deter-mine data driven (model independent) partial-wave amplitudes and associated resonance parametersas the SAID group does, for instance, for analysis of π N-elastic, charge-exchange, and π − p → η n data [17]. With the new GlueX KLF data, the quantitative significance of resonance signals can be de-termined. Additionally, new PWA with new GlueX data will allow to look for "missing" hyperons vialooking for new poles in complex plane positions. It will provide a new benchmark for comparisonswith QCD-inspired models and LQCD calculations. Figure 2.
Comparison of selected di ff erential cross section data for K − p → π Λ and K L p → π + Λ at W = = K − p → π Λ data [8]. The K − p → π Λ and K L p → π + Λ amplitudes imply that observables for these reactions measuredat the same energy should be the same except for small di ff erences due to the isospin-violating massdi ff erences in the hadrons. No di ff erential cross section data for K − p → π Λ are available at c.m.nergies W < K L p → π + Λ are available at such energies due to longer K L life time. At 1540 MeV and higher energies, di ff erential cross section and polarization data for bothreactions are in fair agreement, as shown in Fig. 2. Meanwhile, the quality of avilable P measurementsdo not have a sensitivity to the fit. We propose to use a Hall D Facility with the GlueX spectrometer, to perform precision measurementsof two-body reactions induced by the K L -beam on the liquid hydrogen cryotarget in the resonanceregion, W = θ from -0.95 to 0.95. This ability of the GlueX providesan ideal environment for these experiment. Figure 3.
Schematic view of Hall D beamline on the way e → γ → K L . There is an advantage factor for K L p vs. K − p experiment. The mean lifetime of the K L is 51.16 ns( c τ = . K − is 12.38 ns ( c τ = . K L p scattering at low beam energies compared with K − p scattering [18].The recently constructed GlueX detector in Hall-D is a large acceptance spectrometer with goodcoverage for both charged and neutral particles that can be adapted to this purpose [5]. Schematic viewof the Hall D beamline is presented in Fig. 3. At the first stage, E e =
12 GeV electrons produced at theCEBAF will scatter in a radiator in the target vault, generating intensive beam of bremsstrahlunghotons (we will not need in the Hall D Broadband Tagging Hodoscope). At the second stage,bremsstrahlung photons, created by electrons, hit the Be-target and produce K L -mesons along withneutron and charged particles. Finaly, K L will reach the LH cryogenic target within GlueX settings.We estimated the flux of K L beam on the GlueX LH target is about 10 K L / s , to be compared toabout 10 K L / s used at NINA [19] and SLAC [20], almost comparable to charged Kaon rates obtainedat AGS and elsewhere in the past and expected for J-PARC [18]. Momenta of neutral Kaons at JLabwill be measured applying the time-of-flight technique using a time structure of 60 ns. The count rateestimates carried out assuming 100 days of data taking are presented in Fig. 4. ) CM θ cos( -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 [ m b / s r ] Ω / d σ d Expected cross sections + uncertainties in 100 days ) CM θ cos( -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 [ m b / s r ] Ω / d σ d Expected cross sections + uncertainties in 100 days
Figure 4.
The cross section uncertainty estimates (statistics only) for K L p → pK S (left) and for K L p → π + Λ (right). Precise new data (both di ff erential cross section and recoil polarization of hyperons) for K L p scatteringwith good kinematic coverage could significantly improve our knowledge on Λ ∗ and Σ ∗ resonances.Clearly, complete understanding of three-quark bound states requires to learn more about baryonresonances in "strange sector". Polarization data are very important to be measured in addition todi ff erential cross sections to help remove ambiguities in PWAs.Unfortunately, the current database for K L p scattering includes very few polarization data. Asnoted here, several K L p reactions are isospin (I =
1) selective, which would provide a useful constraintfor a combined PWA of K L p and K − p reactions. Finally, the long lifetime of the K L compared withthe K − would allow K L p measurements to be made easier at lower energies compared with K − beams.It would be advantageous to combine all K L p data in a new coupled-channel PWA with available andnew J-PARC K − p data when they will be available. The proposed KL facility potentially may unravelmany "missing" hyperons. To complete SU(3) F multiplets, one needs no less than 17 Λ ∗ , 41 Σ ∗ , 41 Ξ ∗ , and 24 Ω ∗ .Measurements of "missing" hyperon states with their spin-parity assignments along with the"missing" non-strange baryons will provide very important ingredients to test QM and LQCDpredictions thereby improving our understanding of QCD in a non-perturbative regime. I thank Moskov Amaryan, Yakov Azimov, William Briscoe, Eugene Chudakov, Ilya Larin, Mark Manley, JamesRitman, and, Simon Taylor for comments on the feasibility of future measurements. This work is supported,in part, by the U.S. Department of Energy, O ffi ce of Science, O ffi ce of Nuclear Physics, under Award Numberde–sc0014133. eferences [1] Workshop on Physics with Neutral Kaon Beam at JLab , JLab, VA, USA, Feb. 2016.[2]
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