We have searched for θ+(1.54)→K0p decays using data from the 1999 run of the HyperCP experiment at Fermilab. We see no evidence for a narrow peak in the KS0p mass distribution near 1.54  GeV/c among 106 000 KS0p candidates, and obtain an upper limit for the fraction of θ+(1.54) to KS0p candidates o

Holmstrom, Timothy

HyperCP Collaboration

Longo, M. J

English

Longwood University

Longwood UniversityDigital Commons @ Longwood UniversityChemistry and Physics Faculty Publications Chemistry and Physics12-1-2004High Statistics Search for the θ+ (1.54) PentaquarkStateM. J. LongoUniversity of Michigan - Ann ArborTimothy HolmstromLongwood University, holmstromtk@longwood.eduHyperCP CollaborationFollow this and additional works at: http://digitalcommons.longwood.edu/chemphys_facpubsPart of the Physics CommonsThis Article is brought to you for free and open access by the Chemistry and Physics at Digital Commons @ Longwood University. It has been acceptedfor inclusion in Chemistry and Physics Faculty Publications by an authorized administrator of Digital Commons @ Longwood University. For moreinformation, please contact hinestm@longwood.edu.Recommended CitationLongo, M. J.; Holmstrom, Timothy; and HyperCP Collaboration, "High Statistics Search for the θ+ (1.54) Pentaquark State" (2004).Chemistry and Physics Faculty Publications. Paper 15.http://digitalcommons.longwood.edu/chemphys_facpubs/15arXiv:hep-ex/0410027v2  3 Nov 2004High Statistics Search for the Θ+(1.54) Pentaquark StateM.J. Longo∗,8 R.A. Burnstein,5 A. Chakravorty,5 Y.C. Chen,1 W.S. Choong,2, 7 K. Clark,9 E.C. Dukes,10 C.Durandet,10 J. Felix,4 Y. Fu,7 G. Gidal,7 H.R. Gustafson,8 T. Holmstrom,10 M. Huang,10 C. James,3 C.M. Jenkins,9T. Jones,7 D.M. Kaplan,5 L.M. Lederman,5 N. Leros,6 F. Lopez,8 L.C. Lu,10 W. Luebke,5 K.B. Luk,2, 7 K.S.Nelson,10 H.K. Park,8 J.-P. Perroud,6 D. Rajaram,5 H.A. Rubin,5 J. Volk,3 C.G. White,5 S. White,5 and P. Zyla7(HyperCP Collaboration)1Institute of Physics, Academia Sinica, Taipei 11529, Taiwan, Republic of China2University of California, Berkeley, California 947203Fermi National Accelerator Laboratory, Batavia, Illinois 605104Universidad de Guanajuato, 37000 Leo´n, Mexico5Illinois Institute of Technology, Chicago, Illinois 606166Universite´ de Lausanne, CH-1015 Lausanne, Switzerland7Lawrence Berkeley National Laboratory, Berkeley, California 947208University of Michigan, Ann Arbor, Michigan 481099University of South Alabama, Mobile, Alabama 3668810University of Virginia, Charlottesville, Virginia 22904(Dated: February 7, 2008)We have searched for Θ+(1.54) → K0p decays using data from the 1999 run of the HyperCPexperiment at Fermilab. We see no evidence for a narrow peak in the K0Sp mass distribution near1.54 GeV/c among 106 000 K0Sp candidates, and obtain an upper limit for the fraction of Θ+(1.54)to K0Sp candidates of <0.3% at 90% confidence.PACS numbers: 12.39.MK, 13.75.-n, 13.85.Rm, 14.20.-cI. INTRODUCTIONA number of experiments have recently reported a nar-row peak in the K+n and K0Sp mass spectra near 1.54GeV/c2 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Negative searcheshave also been reported [12, 13, 14, 15, 16, 17, 18, 19, 20,21, 22]. A resonance in the K+n system with strangeness+1 and baryon number +1 would require five quarks asconstituents. The K0Sp system, on the other hand, isambiguous since the strangeness could be either +1 re-quiring the system to be “exotic”, or −1 like the normalhyperons. The presumed pentaquark state is referredto as the Θ+. The width of the peak seems to be nar-rower than the experimental mass resolutions, which varybetween about 9 and 27 MeV/c2. However, Cahn andTrilling [23] argue that the intrinsic width would have tobe <1 MeV/c2 based on the data of Barmin et al. [4] andK+d cross section data. This is far narrower than anypreviously discovered hadronic resonance, and thereforethe dynamics of the Θ+ decay would have to be “exotic”,as well as its quantum numbers. On the other hand, theproduction of Θ+, which involves the rearrangement ofquarks, just as in any other hadronic production pro-cess, should not be exotic. Thus, for example, a virtualN* produced diffractively from an incident proton coulddecay into a Θ+ and K, and a K+ scattering off a neu-tron could produce Θ+X0. The general expectation isthat the inclusive Θ+ production cross sections should be∗Corresponding author, email: mlongo@umich.edufairly large, especially for K+ beams [24], and approxi-mately energy independent well above threshold, just asfor any other hadronic state.II. THE HYPERCP EXPERIMENTThe HyperCP detector (Fig. 1) is described in detailelsewhere [25]. It was designed principally to investigateCP violation in Ξ−/Ξ+decays. A charged secondarybeam was produced at 0◦ by 800 GeV protons interact-ing in a copper target. A collimator channel was em-bedded in a 6.0-meter-long dipole magnet with a field of1.667 T that deflected the secondary beam upward at anangle of 19.5 mrad. The exit aperture of the collimatorwas 2 cm wide by 1 cm high made with machined tung-sten blocks. The collimator selected a broad momentumband ranging from about 120 to 220 GeV/c. When a pos-itively charged secondary beam was selected, the beamwas a mixture of mainly protons and pi+, with about5% K+, and a smaller fraction of positively charged hy-perons. Most of the long-lived particles produced in thetarget passed through an evacuated pipe (Vacuum De-cay Region) and encountered only the thin windows ofthe Vacuum Decay Region and wire chambers that werein the beam, while most hyperons decayed in the Vac-uum Decay Region or upstream of it. However, over halfof the triggers recorded were from events produced in thetungsten near the exit of the defining collimator. Eventsproduced there were used in the pentaquark search.The HyperCP spectrometer employed two dipole mag-nets (Analyzing Magnets) with a total bending power2of 4.72 T m. The Analyzing Magnets were preceded byfour wire chambers and followed by five wire chambers.Each chamber had two planes of vertical wires and planeswith wires inclined at angles of +26.6◦ and −26.6◦ tothe vertical. The wire pitches ranged from 1 mm for theupstream chambers to 2 mm for the larger downstreamchambers. The momentum resolution of the spectrome-ter was 0.44% at 100 GeV/c [25]. The spectrometer anddata acquisition [26] were designed for high rates, andin the course of the experiment, 2×1011 triggers wererecorded on magnetic tape. The majority of the run-ning was done using a positive-polarity secondary beamin order to obtain a sufficiently large sample of Ξ+ de-cays for the CP violation test. The trigger used for thepentaquark search required a coincidence between hitsin scintillation counter hodoscopes on either side of thebeam. This trigger was prescaled by a factor of 100, sothat only 1% of the potential candidates were recorded.The experiment did not have particle identification.However, K0S→ pi+pi− decays could be cleanly identifiedby reconstructing the two-pion mass and requiring thatit be consistent with the K0 mass. Charged tracks with> 50% of the total momentum had a very high proba-bility of being protons. Thus the pentaquark search wasrestricted to the decay Θ+ → K0Sp.FIG. 1: Elevation (top) and plan (bottom) views of the Hy-perCP spectrometer with blowups of the collimator.III. DATA ANALYSISFor the pentaquark search, events were selected withvertices originating from the lower lip of the tungstencollimator. Cuts on the vertex position along the beamdirection were applied to select events originating in thelast 20 cm of the collimator. The vertical position of thevertex was required to be within 4.0 mm of the top edgeof the lower lip of the collimator. Events were rejectedif their total momentum vector traced back to the pro-duction target. Events with three or more tracks werechosen with the total momentum of all tracks between120 and 250 GeV/c. Events containing Λ → ppi− orK+ → pi+pi+pi− decays were rejected. Cuts were alsomade to reject “ghost” tracks which are near-duplicatetracks generated by the track-fitting program. (Eventswith ghost tracks from Λ→ ppi− decays that also fit theK0Sp hypothesis were found to give a narrow peak at 1.54GeV/c2 in the K0Sp mass [27]).Note that the high beam momentum and broad mo-mentum spread in the beam are advantages in thepentaquark search because the mechanism proposed byDzierba et al. [28] by which narrow enhancements in theKN mass distribution can be produced through kine-matic reflections of mesons, such as f2(1.275), a2(1.320)and ρ3(1.690), would not be effective. Also, the kine-matic reflection mechanism for producing false masspeaks involving K0 and Λ decay overlap, as suggestedby Zavertyaev [29], would not work.FIG. 2: (a) K∗(0.892) signal obtained by subtracting theK0sidebandpi data from the K0pi data. (b) Monte Carlo sim-ulation of the K∗(0.892) produced in the last 20 cm of thecollimator. The simulation assumes a Breit-Wigner shapewith a full width Γ = 51 MeV.3FIG. 3: Reconstructed pi+pi− mass for the events in the Θ+search sample. The cuts to select K0S → pi+pi− are shown.Events with K0S’s were selected by requiring that thetwo lower momentum particles have opposite sign andreconstructing their mass under the pi+pi− assumption.Events with K0S’s were used to search for K∗(0.892) →K0Spi decays that were produced in the collimator. Fig-ure 2 compares the K∗(0.892) signal with a Monte Carlosimulation. The K∗ signal was isolated by subtractingthe pi+pi+pi− mass distribution for events with a pi+pi−mass in the K0Smass sidebands from the K0Spi mass dis-tribution. A similar subtraction was used to isolate theΣ(1.385) resonance peak in Λpi events; this also agreedwith Monte Carlo expectations.The reconstructed pi+pi− mass for the events in thepentaquark sample is shown in Fig. 3. The reconstructedK0Sp mass is shown in Fig. 4, where we have combinedthe positive and negative beam data samples, and bothΘ+ → K0Sp and Θ− → K0Sp candidates are included.With a cut requiring the proton momentum fraction to be>0.50, we estimate the non-proton contamination in theK0Sp sample to be ∼1%. The upper plot in Fig. 4 showsthe mass distribution expected from a Monte Carlo sim-ulation of a narrow Θ+(1.54)→ K0Sp. The expected rmsmass resolution is about 11 MeV/c2 with a non-gaussiandistribution. In the total sample of 106 000K0Sp andK0Sp¯candidates there is no sign of a peak near 1.54 GeV/c2.The smooth curve overlaying the histogram in the1.51–1.57 GeV/c2 range is a fit to the data with the shapein the upper plot centered at 1.53 GeV/c2 plus a linearbackground. This shows the maximum number of eventsin the putative Θ+(1.53) peak to be 317 at 90% confi-dence level. Thus we find that at most 0.3% of the K0Spcandidates could come from Θ+ decays. By comparison,Barmin et al. [4] reported 29 events in their Θ+ peakamong 541 K0Sp events with a similar mass spectrum inthe DIANA bubble chamber experiment. Because of thelarge momentum spread and mixed beam composition,as well as the unknown production spectrum, it was notpossible to estimate limits for Θ+ production cross sec-tions.IV. DISCUSSIONThe main differences between our experiment andthose that reported Θ+(1.54) peaks are the higher beamenergy, a mixed beam containing p, pi+,K+, and chargedhyperons with a broad momentum spread, as well as bet-ter mass resolution and far more events. There is nofundamental reason why the ratio of Θ+ to overall K0pproduction should decrease significantly at higher beamenergies. To produce a pentaquark, a beam proton hasto pick up a ds¯ quark pair from the sea. To produce aΞ+, u¯, s¯, s¯ quarks need to be created. Naively one wouldexpect Θ+ production to be much larger than Ξ+. Yetthe data sample used in the pentaquark search containedover 75 000 Ξ+decays and 70 000 Ξ− decays that orig-inated from the lower lip of the collimator. Thus theabsence of substantial Θ+ production in our data is ex-tremely puzzling.FIG. 4: The K0Sp invariant mass distribution. (a) MonteCarlo events with the Θ(1.54) produced in the last 20 cmof the collimator. (b) Reconstructed K0Sp mass for the totalpentaquark candidate sample. The smooth curve between1.51 and 1.57 GeV/c2 is a fit to the shape in the top plotcentered at 1.53 GeV/c with normalized amplitude P1, plusa linear background, P2 + P3× (Mass).4AcknowledgmentsWe wish to thank Alex Dzierba for useful conversa-tions. The authors are indebted to the staffs of Fermilaband the participating institutions for their vital contribu-tions. This work was supported by the U.S. Departmentof Energy and the National Science Council of Taiwan,R.O.C. E.C.D. and K.S.N. were partially supported bythe Institute for Nuclear and Particle Physics. 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Christian, Quarks & Nuclear Physics Conference,Bloomington, 2004; http://www.qnp2004.org/.[17] C. Blume, www.ikf.physik.uni-frankfurt.de/na49/collab-2001/Collaborations Meeting 12 2003/Blume/Blume coll-dec-03-penta.pdf.[18] T. Wengler, hep-ex/0405080.[19] C. Pinkenburg, J. Phys. G30:S1201 (2004).[20] S. Schael et al., Phys. Lett. B 599, 1 (2004).[21] B. Aubert et al., hep-ex/0408064.[22] S. Salur, nucl-ex/0403009.[23] R. Cahn and G. Trilling, Phys. Rev. D 69, 011501(2004).[24] W. Liu and C. M. Ko, Phys. Rev. C 68, 045203 (2003).[25] R.A. Burnstein et al., hep-ex/0405034 (to be publishedin Nucl. Instrum. Methods).[26] C.G. White et al., Nucl. Instrum. Meth. A 474, 67(2001).[27] M. Longo, Quarks & Nuclear Physics Conference, Bloom-ington, 2004; http://www.qnp2004.org/.[28] A. R. Dzierba et al., Phys. Rev. D 69, 051901 (2004).[29] M. Zavertyaev, hep-ph/0311250.

High Statistics Search for the θ+ (1.54) Pentaquark State

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High Statistics Search for the θ+ (1.54) Pentaquark State

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