Narrow correlated positron-electron peaks discovered in superheavy nuclear collisions may be signatures for previously undetected neutral particle-like objects having masses of 1--2 MeV/c{sup 2}. We have designed an experiment to definitively test this hypothesis by searching for resonant states formed directly in the scattering of monoenergetic electrons incident on a gas of cold positrons confined magnetically in a Malmberg trap. This technique will provide a hundred-fold improvement in sensitivity to e{sup +}e{sup {minus}} resonances compared to previous positron-beam, thin-foil scattering experiments. Together with a recoil-shadow technique, this experiment will explore a five decade range in neutral-particle lifetimes (10{sup {minus}13}s to 10{sup {minus}8}s) which cannot be probed directly to other methods. 14 refs., 2 figs., 1 tab

Cowan, T. E.

Fajans, J. (California Univ., Berkeley, CA (USA). Dept. of Physics)

Howell, R. H.

Rohatgi, R. R. (Lawrence Livermore National Lab., CA (USA))

UNT Digital Library

UCRL-JC--1053791DEgl 005062SEARCH FOR RESONANT STATES IN POSITRON-ELECTRON SCATTERING USING A POSITRONGAS TARGETpTHOMAS E. COWAN, RICHARD H. HOWELL, and RAJEEV R. ROt{ATC;IDepartment of Physics, Lawrence Livermore National Laboratory,Livermore, California 94550, USAJOE1, FAJ ANSDepartment of Physics, University of California,Berkeley, California 94720, USAAbstractNarrow correlated positron-electron peaks discovered in superheavynuclear collisions may be signatures for previously undetected neutralparticle-like objects having masses of i-2 MeV/c 2 We have designed anexperiment to definitively test this hypothesis by searching for resonantstates formed directly in the scattering of monoenergetic electrons incidenton a gas of cold positrons confined magnetically in a Malmberg trap. Thistechnique will provide a hundred-fold improvement in sensitivity to e+e -resonances compared to previous positron-beam, thin-foil scatteringexperiments. Together with a recoil-shadow technique, this experiment willexplore a five decade range in neutral-particle lifetimes (lO-13s to lO-Ss)which cannot be probed directly by other methods.Work preformed under the auspices of the U.S. Department of Energy by theLawrence Livermore National Laboratory under contract No. W-7405-ENG-48.OleTrmt_l/TIC)lM E3F THIS DOCUMENT I$ UNLIMI ! l=L"i. IntroductionSince their discovery at GSI Darmstadr_ in 1985 I)v the EPOS Collaboration e[I], the origin of the narrow correlated e.e - peaks emitted from heavy-ioncollisions has remained a mystery. At least three sets of tortetared e_/e -lines in each of two collision systems (U+Th and U+Ta) have been firmlyestablished [2], which aannot be attributed to conventional nuclear or atomicprocesses. The peaks appear to arise from the two-body decays of previouslyundetected neutral objects of mass 1.63, 1.77, and 1.83 MeV/c 2, havinglifetimes between-10 -19 s to -10 -9 s. Moreover, the existence of narrowpeaks has been confirmed by an independent group at GS1 [3], and most recentlyby ourselves and co-workers with a new spectrometer at the LBL SuperHILAC [4].The simplest interpretation of these data in terms of a family of new point-like elementary particles (e.g., axion-like bosons) is unlikely in light ofnegative results from a variety of particle searcb experiments [2,5]. lt hasbeen pointed out, however, that a spatially extended object (e.g. with aradius _> i00 fm) would have evaded detection so far, and might in a naturalway account for both the existence of the several e+/e - lines (irl terms of aspectrum of internal excitations) and the low velocity (_<0.2c) with which theobjects are emitted in the HI collisions [6]. Many theoretical modelsproposed to date, ranging from non-perturbative effects in QED to new types offermions and/or fundamental interactions, but none has yet been proven toquantitatively reproduce the heavy-ion observations [5].2. Previous ExperimentsSeveral experiments have investigated the neutral particle hypothesis byattempting to create these states directly in the time-reversed process,namely by searching for their resonant contribution to elastic positron-electron (Bhabha) scattering at center-of-mass energies equal to the invariantAmass of the anomalous e+e - pairs [7,8]. This channel is insensitive to thesize or internal structure of such a state, and the resonance cross-section is#simply described by Breit-Wigner theory. Over the relevant lifetime range,the decay widths Fe+ e_ are narrow compared the experimental resolution, so theexpected signal equals the energy-integrated BW cross-section divided by theCMS energy spreadaex p = (_/2) OBW Fx / 6Ecm s,whereaBW - (2J+l) 4_ h 2 (Mx2c 4 - 4m2c4) -I (Fe+e_/Fx)2is the Breit-Wigner cross-section at Ecm s = Mx c2. lt is known that two photondecay is small compared the e+e - channel (F2_/Fe+ e_ _ 2,10 -3 [9]), andassuming that the e+e - branch is the dominant decay mode (Fe+e_-rx) , aBW2100 barns for a spin J-O object of mass M x - 1.83 MeV/c 2.Previous searches for resonant e+e - scattering have all involvedpositrons incident on thin foils (usually Be). The momentum of the boundtarget electrons, when transformed to the CM system, leads to a spread in theCMS collision energy (6Ecm s _ 7.8 keV for Be [8]), which limits the size ofthe expected signal, and hence the sensitivity to resonances above theunderlying elastic scattering continuum (doe+e_/d_ = 20 mb/sr at 6cm s = 90o).The present limit [7] is a,F _ 6.3 b.eV, or re+ e_ > 3.5,10 -13 s, beyond whichlittle progress can be expected using atomic targets.Several groups have attempted to extend the excluded parameter range tolonger lifetimes by exploiting the large velocity of the CMS (Vcm = 0.8c) withthe use of recoil shadow or beam-dump techniques. (A Grenoble experimentclaims to exclude 4.5,10 -12 s < re+ e _ < 7.5.10 -12 s [I0].) These resultshowever are high±y sensitive to the detailed interaction of the extendedobjects in that the), require that the object penetrate the rather thick target '_(Grenoble' 4.6 mg/cm 2 Be) or beam-dump material. A re-examination of theheavy-ion data [ii] indicates that the peak positrons and electrons are likelyemitted in the vicinity (-103 fm) of the colliding heavy ions, implying thatthe neutral state may be created and quickly destroyed in the nuclear Coulombfield. Assuming a polarization mechanism for its dissociation, proportionalto the square of the electric field, the calculated absorption length in Be isonly -I mg/cm 2. Substantially thinner targets are therefore required for ameaningful, essentially model-independent, search for long-lived states.Dissociation of the object in the nuclear field also invalidates the 10 -9 supper lifetime limit derived from the fiducial volume of the EPOSspectrometer, raising the possibility of a much narrower production widthFe+ e_ (longer decay time re+e_ ) in vacuum.3. Experimental DesignWe have designed a new experiment [12] which both achieves a greatersensitivity to the direct production of resonance states, and uses a thinnertarget to provide model-independent sensitivity to long-lived states. This isaccomplished by using a "gas" target of magnetically confined leptons tocircumvent the resolution limitations imposed by the Fermi momentum of boundelectrons in solid targets. To obtain useful e+e - scattering rates (-i Hz for_cms _ 90°) in light of the availability of only weak (-pA) e+ beams, we usethe combination of an intense e- beam (-lO _A) incident on a cold gas ofmagnetically confined positrons (-I0 I0 e+/cm3), as illustrated in Fig. I. Anequilibrium e+ temperature of _I00 K, and unambiguous lepton identificationmade possible by a novel detection geometry, will provide a factor of i00greater sensitivity to resonant e+e - scattering (i.e., for re+ e_ <3.5.10 -II s)than previous positron-beam, thin-foil experiments.3.1 Positron TrapThe positrons will be stored in a magnetic trap, which consists of acoaxialarray of cylindrical gold-plated copper electrodes mounted along theaxis of a 1 meter long 6 Tesla solenoid magnet. Radial confinement isprovided by the strong magnetic field, and the positrons are confined axiallywith.in a given section by biasing the adjacent electrodes on either side of/the plasma column to a positive voltage. This so-called Ma lmberg trap wasdeveloped and has been used extensively by J.H. Malmberg and co-workers atU.C. San Diego to investigate non-neutral single-component pure-electronplasmas [13]. Their cryogenic high-field version has demonstrated thefeasibility of a high electron density and low temperature re_'uired for thisexperiment. An e-plasma forms as a column of nearly uniform density whichspins as a rigid rod about its symmetry axis (due to ExB forces from theradial space charge electric field) with a constant total angular momentum (inan ideal trap). Azimuthal asymmetries in, or misalignments of the trapelectrodes and/or magnetic field appear as RF sources by the rotating plasmacolumn, and exert torques which tend to radially deconfine the plasma. Wehave fabricated an confinement structure in which the electrodes are mountedin a precision ceramic V-block to provide a mechanical tolerance of 2.5 #mover a 60 cm length. Absolute alignment with the field axis is achieved bysuperconducting magnetic shim coils, and we expect an azimuthal uniformity ofp• _Bi/B . 10 -4 Confinement times of up to 300 s may be achievable•One important feature of a high-field trap is that the thermal energy ofthe leptons, exhibited as cyclotron motion superimposed on the ExB rotation,is dissipated by synchrotron radiation in the strong magnetic field. Thisprovides a natural cooling mechanism for the positrons which will be exploitedto offset the Coulombic heating of the plasma by the i_jected electron beam.The choice of the positron plasma and electron beam parameters: presented aboveand in the following discussion represents a reasonable optimization ofvarious competing effects, such as the plasma filling vs. contairmlent times,electron beam heating vs. radiative cooling rates, possible beam-plasmainstabilities, and the need for a reasonable e+e - scattering rate. Themagnitudes of the relevant physical processes are s_mnarized in Table 1 forour nominal conditions, together with their functional dependence on variousparameters such as positron density, plasma radius and length, and beamcurrent (ne+ , re+, Le+ , and Ie_ , respectively).The positrons will be injected along the solenoid axis into the trapfrom the LLNL intense low-energy pulsed positron source (driven by a i00 MeVe- Linac) [14]. Each e+ beam pulse will be e].ectrostatically accelerated to-5 keV in order to penetrate the strong magnetic field, during which it willbe compressed to -10 -2 cm and have attained significant transverse energy.The positrons must be re-moderated in the high field region to typically a -2eV energy spread, and will be trapped by time-of-flight utilizing the pulsedtime structure of the beam (_t - 1 ns, 1440 Hz repetition rate). Onceconfined, the positrons will radiatively cool on a 0.i s time scale to theambient 4.2 K temperature of the l_qe cooled electrodes. After a -25 s longinjection phase, the trap will contain a 30 cm long by 1 mm diameter thread-like plasma of 2.5oi09 positrons, having an average density of I0 I0 cm -3.q3.2 Electron BeamOnce the trap is filled with positrons, the electron beam from aNational Electrostatics Corporation model 9SDH-2e 3 MV Pelletron accelerator%will be injected along the axis of the solenoid. TI_e.-i0 #A beam current,chosen to achieve an equilibrium positron temperature of _I00 K (-8 meV), willproduce a beam-target luminosity of -2oi025 e+e-/cm2/s, providing a -0.5 Hzevent rate for e+e - scattering with Ocm s = 90 ° , c'omparable to previous beam-foil experiments. Including the e- beam energy resolution (<500 eV peak-to-peak), we will achieve a collision energy spread in the CM system of 6Ecm s _<200 eV (compared to 7.8 kev for Be target experiments). The electron beamenergy will be fine-tuned at any given Pelletron setting by floating theentire electrode structure to high voltage. In this way, we plan to reducesystematic errors in normalizing adjacent points of the excitation function bycontinuously sweeping the beam energy by !lO keV, and recording theinstantaneous voltage offset with each scattering event trigger. ThePelletron beam energy operating _ange of 0.6 to 2.7 MeV will allowinvestigations over an e+e - pair invariant mass range of 1.29 to 1.95 MeV/c 2.3.3 Lepton DetectionThe scattered positrons and electrons will be detected with >_90%collection efficiency and <i0 kev energy resolution in an array of Sidetectors at the downstream end of the trap. A novel collection geometry hasbeen developed which exploits the adiabatic elongation of the lepton orbits asthey are transported out of the solenoid along the axis to a low field region(-I kG). Here the positrons and electrons are deflected by a localizedftransverse dipole field (formed by SmCo permanent magnets) and spiral (withopposite helicity) in the fringe field of the solenoid into the separatedetector arrays. Positrons are further unambiguously identified by thecoincident detection of their 511 keV annihilation radiation in an array ofNal crystals surrounding the detector.A prototype of this spectrometer, using a 2.7 T solenoid, has beensuccessfully tested at the LBL SuperHILAC in which we m_,asured coincidentpositrons and electrons emitted from a U beam injected along the axis to a Tatarget located in the center of the solenoid 14]. Irl our first shakedown run,we achieved a 15-fold improvement over the EPOS Spectrometer e+e - detectionefficiency, and have at least qualitatively confirmed the existence of narrow+ -e e peaks in U+Ta collisions. Minor improvements presently underway willyield a 25-fold increase over the EPOS efficiency for additional runs plannedfor the near future.The measured laboratory energies of the leptons and the e+/e -discrimination will allow complete reconstruction of the CM collisionkinematics. This is a substantial improvement over present beam-foilexperiments which fail to distinguish positron from electron. We willconcentrate our search for resonance states at backward CMS scattering angles(e.g., 90 ° - 180°), whereas other experiments measure only the totalsymmetrized cross-section around 8cm s = 90 ° [7] where the elastic scatteringcontinuum is three tim,es more intense. Combined with the above improvement inCMS energy resolution, we expect a factor of i00 increase in sensitivity.Systematic uncerltainties will be minimized by sweeping the _eam energyas well as by redundant normalizations to the forward-angle elastic scatteringcontinuum, the integrated beam current, and the total e+ charge and beam-plasma overlap which are :_easured destructively at the termination of eachfill-scatter-dump cycle. A limit on the existence of peaks at a level of 0.3%above the elastic scattering continuum, comparable to the Grenoble result,translates into a a,F _< 0.2 b,eV limit on the integrated cross-section,extending the explored region of parameter space to lifetimes of <3.5.10 -11 s.3.4 Recoil Shadow. The existence of neutral objects with long lifetimes will bei_llvestigated with a recoil-shadow geometry as indicated schematically inFig. 2. An annular filter, thick enough to stop or substantially degrade theenergy of prompt elastically scattered positrons and electrons, will be placeddownstream of a short segment (-5 cm) of e+ plasma. The e- beam, and anylong-lived neutral objects, will pass through the hole into a relativelybackground-free region where infrequent e+e- decay events will be detectable.An optional conversion foil (which plays the role of the thick Be target or Utarget in heavy-ion collisions) m_y be used to stimulate the decay of the verylong lived objects which would otherwise exit the magnetic trap. Thistechnique will cover the lifetime range of -5,10 -12 s < _e+e- < 10-8 s, andhas the significant advantage over previous experiments [i0] in that thetarget has el06 times fewer charge centers per cm -2 than -I mg/cm 2 thick Betargets (i,e_, an effective thickness of -10 -9 g/cm2). The polarization-Ldissociation cross-section is approximately 1012 times smaller in the e+plasma target. Whereas solid-target experiments may be sensitive only toobjects _i00 fm, we will be sensitive to objects of up to >107 fm radius.4. Summary and StatusCombining the increased sensitivity to neutral resonance states due tothe low temperature of the target positrons, with the model-independentosensitivity for long-lived objects of the recoil-shadow technique, thisexperiment will constitute a definitive search for new neutral particle-likeobjects throughout the five decade range in lifetimes (10-13s < Tc+ e_ < lO-8s)which cannot be reliably and unambiguously probed by other methods.I0fFabrication of the positron trap electrode structure is essentiallycomplete, as is the PC-based computer control system and #VAX-based plasmadiagnostics system_ We presently await the delivery of the high azimuthaltuniformity 6 T solenoid from Nalorac Cryogenics Corporation, and the 3 MVelectron Pelletron from National Electrostatics Corporation. Scatteringexperiments may begin in late 1991.iiREFERENCESq[I] T• Cowan et al., Phys. Rev. Lett. 56 (1986) 444.[2] T.E. Cowan and J.S. Greenberg, in • Physics of Strong Fields, cd. W.Greiner (PlenL_, New York, 1987) pp• 111-194;T.E. Cowan, Ph.D. dissertation (Yale University, New Haven, 1988);P. Salabura et al., Phys. Letr• B 245 (1990) 253.[3] W. Koenig et al., Phys. Lett. B 218 (1989) 12.[4] T.E. Cowan, C.L. Bennett, M. Clark, R.H. Howell, D.A. Knapp, J. McDonald,R.R. Rohatgi, D.H. Schneider, J. Schweppe, and J. Fajans, to bepublished.[5] See B. M_ller, in" Atomic Physics of Highly Ionized Atoms, cd. R. Marrus(Plenum Press, New York, in press), and refs. therein.[6] A. Sch_fer, Phys. Lett_ B 211 (1988) 207.[7] H. Tsertos et al., Phys. Rev. D 40 (1989) 1397, and refs. therein.[8] A. Scherdin et al., "Low Energy e+e - Scattering," Univ. Frankfurtpreprint UFTP 248/1990, and refs. therein.See also J. Reinhardt et al., Z. Phys. A 327 (1987) 367.[9] K. Danzmann et al., Phys. Rev. Lett_ 62 (1989) 2353•[I0] S.M. Judge et al., Phys. Rev. Lett. 65 (1990) 972.[ii] T.E. Cowan, to be published.[12] T.E. Cowan et al., Bull. APS 34 (1989) 1812;R.R. Rohatgi et al., Bull. APS (1989) 1933.w[13] See, e.g., J.H. Malmberg et al., in • Proc• Symp. on Non-Neutral Plasma4Physics (1988). /• . 73.[14] R.H. Howell et al., Nucl. Inst and Meth. B i0/ii (1985)J12FIGURE CAPTIONSFigure I. Schematic diagram of e+e - scattering experiment. A positron plasmais formed by capturing positrons from the LLNL pulsed e_ source. Theelectron beam is then injected along the axis, and scattered positronsand electrons are transported out of the trap region to a Si detectorarray. Positrons are identified by coincident_ detection of 511 keVphoton in Nai crystal.Figure 2. Recoil-Shadow arrangement for investigating long-lived neutralobjects. The annular filter stops elastically scattered positrons and _electrons. Long-lived states decay downstream in a reduced-backgroundregion. An optional converter foil stimulates the decay of very long-lived objects.TABLE CAPTIONTable i. Design values of important experimental parameters and theirdependence on the magnetic field (B), electron beam current (le_), andpositron plasma density (ne+), radius (re+) and length (Le+) . Nominalvalues for these parameters are B = 6 T Ie_ = i0 #A ne+ = i0I0 cm-3and re+ - 0.5 n_. A 30 cm long e+ plasma column is ass_ned to bedivided into 15 segments each Le+ = 2 cm long.,, QTABLE 1wExpeiqmental Parameter Functional Devendence Design ValueConfinemertt Time tl/2 o_ B2 Le+ "2 nc -2 = 300s2 Lc = 25 sFill Time trill = ne+ r e+ +Cyclotron Cooling Time tra d oc B"2 -- 0._[1 sBeam Heating Rate dEe+/dt cx le. re+ "2 = 0.(38 eV s"1-2 B-2Equilibrium e + Temperature kTe+ ,_ Ie_re+ = 0.008 eVe + e" Scattering Rate < R > _ Ie. ne+ Le+ -- 0.:5s'lr__ ___ _____.._ , _- ........._ tr ........................................b b

Search for resonant states in positron-electron scattering using a positron gas target

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Search for resonant states in positron-electron scattering using a positron gas target

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