We report the first evidence for the decay Σ+→pμ+μ- from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(Σ+→pμ+μ-)=[8.6-5.4+6.6(stat)±5.5(syst)]×10-8. The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, Σ+→pP0,P0→μ+μ- with a P0 mass of 214.3±0.5  MeV/c2 and branching ratio B(Σ+→pP0,P0→μ+μ-)=[3.1-1.9+2.4(stat)±1.5(syst)]×10-8

Holmstrom, Timothy

HyperCP Collaboration

Park, H. K

English

Longwood University

Longwood UniversityDigital Commons @ Longwood UniversityChemistry and Physics Faculty Publications Chemistry and Physics1-18-2005Evidence for the Decay Σ+→pμ+μ-H. K. ParkUniversity 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 CitationPark, H. K.; Holmstrom, Timothy; and HyperCP Collaboration, "Evidence for the Decay Σ+→pμ+μ-" (2005). Chemistry and PhysicsFaculty Publications. Paper 13.http://digitalcommons.longwood.edu/chemphys_facpubs/13arXiv:hep-ex/0501014v2  24 Jan 2005Evidence for the Decay Σ+ → pµ+µ−H.K. Park,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,10M. Huang,10 C. James,3 C. M. Jenkins,9 T. Jones,7 D. M. Kaplan,5 L. M. Lederman,5 N.Leros,6 M. J. Longo,8, ∗ F. Lopez,8 L. C. Lu,10 W. Luebke,5 K.B. Luk,2, 7 K. S. Nelson,10 J.-P.Perroud,6 D. Rajaram,5 H. A. Rubin,5 J. Volk,3 C. G. White,5 S. L. White,5 and P. Zyla7(HyperCP Collaboration)1Institute of Physics, Academia Sinica, Taipei 11529, Taiwan, Republic of China2University of California, Berkeley, California 94720, USA3Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA4Universidad de Guanajuato, 37000 Leo´n, Mexico5Illinois Institute of Technology, Chicago, Illinois 60616, USA6Universite´ de Lausanne, CH-1015 Lausanne, Switzerland7Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA8University of Michigan, Ann Arbor, Michigan 48109, USA9University of South Alabama, Mobile, Alabama 36688, USA10University of Virginia, Charlottesville, Virginia 22904, USA(Dated: February 7, 2008)We report the first evidence for the decay Σ+ → pµ+µ− from data taken by the HyperCP (E871)experiment at Fermilab. Based on three observed events, the branching ratio is B(Σ+ → pµ+µ−) =[8.6+6.6−5.4(stat)± 5.5(syst)]× 10−8. The narrow range of dimuon masses may indicate that the decayproceeds via a neutral intermediate state, Σ+ → pP 0, P 0 → µ+µ− with a P 0 mass of 214.3 ± 0.5MeV/c2 and branching ratio B(Σ+ → pP 0, P 0 → µ+µ−) = [3.1+2.4−1.9(stat)± 1.5(syst)]× 10−8.PACS numbers: 13.30.Ce, 14.20.Jn, 14.80.MzIn the standard model (SM), the decay Σ+ →pl+l− (Σ+pll, l = e, µ) can be described as proceedingthrough a flavor-changing neutral-current (FCNC) inter-action and by internal conversion, as shown in Fig. 1(a)–(c). Bergstro¨m et al. [1] argue that in the SM the FCNCcontribution for the decay Σ+pll is not dominant. The de-cay Σ+pll is of interest since it also allows a direct searchfor a new scalar or vector particle, which could contributean s→ d transition at the tree level [2] (Fig. 1(d)).Current literature reports only an upper limit B(Σ+ →pe+e−) < 7 × 10−6 [3]. The decay rate for the processΣ+ → pl+l− was studied in [1, 4] in the context of theSM. Using the measured partial decay width and the αγparameter for Σ+ → pγ decay, Bergstro¨m et al. [1] find:Γ(Σ+ → pe+e−)/Γ(Σ+ → pγ) ≥ 7.2× 10−3 and 1/120 >∼Γ(Σ+ → pµ+µ−)/Γ(Σ+ → pe+e−) >∼ 1/1210. A largeviolation of these limits would signal new physics.In this Letter we report the first evidence for theΣ+ → pµ+µ− decay, a measurement of the branchingratio for this decay, and possible evidence for a new stateP 0 observed via Σ+ → pP 0, P 0 → µ+µ−.The HyperCP experiment was located in the MesonCenter beamline at Fermilab. The spectrometer, shownin Fig. 2, is described in detail elsewhere [5]. Charged sec-ondary beams, with mean momenta of about 160 GeV/c,were produced by 800 GeV/c protons incident on coppertargets and momentum selected by a curved collimatorsituated in a dipole magnet (hyperon magnet). The signof the charged secondary beam was periodically changedby reversing the field of the hyperon magnet. We an-alyzed 2.14 × 109 triggers from the positive-secondary-beam data set and 0.37× 109 from the negative.The signature of the Σ+pµµ decay is two unlike-signmuon tracks and a proton track originating from a com-mon vertex. The transverse-momentum kick of the an-alyzing magnets was such that muons from this decaywere always deflected to opposite sides and outside ofthe intense secondary beam at the rear of the spectrom-eter. Hence the signal trigger used to select candidateevents required hits in the same-sign and opposite-signhodoscopes (Left-Right trigger), in coincidence with hitsin the vertical and horizontal hodoscopes in each of themuon stations situated on either side of the secondarybeam.l-l+suuduu{ }pΣ+(d)P0suuduu{ }pWΣ+(a)l-l+γsuuuud{ }pWΣ+(b)l-l+γuusuud{ }pWΣ+l-l+γ(c)FIG. 1: Feynman diagrams for Σ+pll decays in the SM (a)–(c)and via new physics (d). The SM processes are referred to asFCNC (a) and internal conversion (b)–(c).2FIG. 2: Plan view of the HyperCP spectrometer.The basic selection cuts discussed below were appliedto the Σ+pµµ event candidates. Each event was requiredto have a track in each muon station and a higher-momentum track (the proton candidate) with the samecharge sign as the secondary beam. The total momen-tum of the three tracks had to be within the range 120to 240 GeV/c. The Σ+ trajectory had to extrapolate towithin 3.5 mm (≈ 3 σ) of the center of the target, wherethe nominal target position was determined using well-reconstructed K+ → pi+pi+pi− decays and the extrapola-tion resolution was determined from a Monte Carlo (MC)simulation of Σ+pµµ decays. The decay vertex of the threetracks was calculated by the method of distance of closestapproach (DCA), and the z-coordinate of the vertex zvwas required to be more than 68 cm downstream of theentrance of the vacuum decay region and more than 32cm upstream of its exit. The average distance betweenpairs of tracks in the x–y plane at zv was required to beless than 0.25 mm. The hits in the multiwire propor-tional chambers upstream of the analyzing magnets wererefit with a constraint that they share a common vertex.The resulting χ2/ndf was required to be less than 1.5.After imposition of the above cuts, three candidateΣ+pµµ events were observed in the positive-secondary-beam data with masses (Mpµµ) within 1 σ (1 MeV/c2)of the Σ+ mass, as shown in Fig. 3(a) and (b). No otherevents were found within ≈ 20 MeV/c2 of the Σ+ mass.The signal events were verified by applying two addi-tional cuts that removed almost all of the higher-massbackground events without affecting the three signalevents. The first cut required that the ratio of the pro-ton momentum to the summed three-track momentumbe larger than 0.68, as MC simulations showed that thiscut preserved 100% of the signal while removing mostof the K+ decay background. The second cut removedevents whose mass was within ±10 MeV/c2 (3 σ) of theK+ mass when calculated using the pi+µ+µ− decay hy-pothesis to reject K+ → pi+µ+µ− decays. The resultingpµ+µ− invariant mass distribution is shown in Fig 3(c).Possible background sources were extensively studied,using both MC and data. Typically 100 times as manyFIG. 3: Mpµµ distribution for the positive-secondary-beamdata, (a) after the standard cuts, (b) within ±15 MeV/c2 ofthe Σ+ mass, and (c) after the additional cuts. The arrowrepresents the Σ+ mass.MC events as would be expected for each potential back-ground source were generated.Other positively-charged hyperon decays, Ξ+ →Λpi+ → ppi+pi+ and Ω+ → ΛK+ → ppi+K+, were notsignificant backgrounds as decays in flight of the daugh-ter pi+ or K+ would produce muons of only one po-larity, and the probability of misidentifying the sign ofthe charge of any of the decay daughters was negligiblysmall. Contributions from charged kaon decays such asK+ → pi+pi+pi−, K+ → pi+pi−µ+νµ, K+ → pi+µ+µ−,and K+ → µ+µ+µ−νµ were estimated using MC simu-lations allowing in-flight decay of pions, and found to benegligible. Muon pair production by photon conversionin material inside the vacuum decay region was stud-ied using MC simulation for K+ → pi+pi0, pi0 → γγ,K+ → pi+γγ, Σ+ → pγ, and Σ+ → ppi0, pi0 → γγ de-cays. Such sources of background were also found to benegligible.In addition, we used real data to investigate possi-ble backgrounds otherwise missed, including the unlike-sign dimuon sample from the negative-secondary-beamdata, as well as a sample of events (single-muon sam-ple) for which only one muon track was required in ei-ther the left or right muon station. For the single-muonsample both the positive- and negative-secondary-beamdata were included, and the non-muon track was re-quired to be within the fiducial volume of the appropriatemuon station. For the positive-secondary-beam data, thesingle-muon sample was one order of magnitude largerthan the unlike-sign dimuon sample. These backgroundstudies showed that after cuts none of these sources con-tributed in the pµ+µ− invariant-mass region below 1200MeV/c2. Finally, we relaxed each cut to increase thebackground level shown in Fig. 3(c) by an order of mag-3nitude. However, there still were no background eventswithin 8 MeV/c2 of the Σ+ mass.Figure 4(a) compares the dimuon mass distribution ofthe three signal candidates with that expected in theSM with the form-factors described below. The recon-structed dimuon masses for the three candidates, 214.7,214.3, and 213.7 MeV/c2, all lie within the expecteddimuon mass resolution of ≈ 0.5 MeV/c2. The dimuonmass distribution for Σ+pµµ decays is expected to be broadunless the form factor has a pole in the kinematically al-lowed range of dimuon mass.The expected SM distribution was used to estimate theprobability that the dimuon masses of the three signalcandidates be within 1 MeV/c2 of each other anywherewithin the kinematically allowed range. The probabil-ity is 0.8% for the form-factor decay model and 0.7% forthe uniform phase-space decay model. The unexpectedlynarrow dimuon mass distribution suggests a two-bodydecay, Σ+ → pP 0, P 0 → µ+µ− (Σ+pPµµ), where P0 is anunknown particle with mass 214.3 ± 0.5 MeV/c2. Thedimuon mass distribution for the three signal candidatesis compared with MC Σ+pPµµ decays in Fig. 4(b), andgood agreement is found. Distributions of hit positionsand momenta of the proton, µ+, and µ− of the three can-didate events were compared with MC distributions, andwere found to be consistent with both decay hypotheses.To extract the Σ+pµµ branching ratio, the Σ+ →ppi0, pi0 → e+e−γ (Σ+peeγ) decay was used as the normal-ization mode, where the γ was not detected. (HyperCPhad no γ detectors.) The trigger for the Σ+peeγ events wasthe Left-Right trigger prescaled by 100. The proton andtwo unlike-sign electrons were required to come from asingle vertex, as were the three tracks of the signal mode.The proton was selected to be the positively-chargedtrack with the greatest momentum, and the event wasdiscarded if the proton candidate did not have at least66% of the total three-track momentum, as determinedby a MC simulation of Σ+peeγ decays. The reconstructedmass for the 3pi hypothesis was required to be outside±10 MeV/c2 of theK+ mass. The cuts on χ2/ndf , DCA,FIG. 4: Real (points) and MC (histogram) dimuon mass dis-tributions for (a) Σ+pµµ MC events (arbitrary normalization)with a form-factor decay (solid histogram) and uniform phase-space decay (dashed histogram) model, and (b) Σ+pPµµ MCevents normalized to match the data.and the total momentum were the same as for the signalmode. However, the decay vertex had to be more than168 cm downstream of the entrance of the vacuum decayregion and more than 32 cm upstream of its exit. Sincethe γ momentum was not measured, the x and y positionsof the Σ+ trajectory at the target were determined usingonly the three charged tracks, and those positions had tobe consistent with that expected from a MC simulationof Σ+peeγ decays.To significantly reduce contamination from photon-conversion events, the dielectron mass was required tobe between 50 and 100 MeV/c2. After application of theabove selection criteria, a total of 211 events remained,as shown in Fig. 5. We performed a binned maximum-likelihood fit for the mass distributions for data and threeMC samples: Σ+peeγ decays, K+ → pi+pi0, pi0 → e+e−γ(K+pieeγ) decays, and uniform background. From thefit, the number of observed Σ+peeγ decays was Nobsnor =189.7± 27.4 events, where the uncertainty is statistical.To extract the total number of normalization events, val-ues of (51.57±0.30)% and (1.198±0.032)% were used re-spectively for the Σ+ → ppi0 and pi0 → e+e−γ branchingratios [6].The kinematic parameters for Σ+ production at thetarget were tuned to match the data and MC Σ+peeγ mo-mentum distributions. The MC Σ+peeγ decays were gen-erated using the decay model in Ref. [7] for pi0 → e+e−γ(pi0eeγ) decays, and the pi0 electromagnetic form-factor pa-rameter a = 0.032± 0.004 was taken from Ref. [6]. Aftertuning of the parameters, comparisons of the distribu-tions of the MC events with the data for Σ+peeγ decays,the decay vertex positions, momentum spectra, recon-structed mass, hit positions of each charged particle, etc.,showed good agreement.In the simulation of the Σ+pµµ decays, we used the form-factor model of Bergstro¨m et al. [1], although we foundlittle difference between results using it and a uniformFIG. 5: The reconstructed pe+e− mass distribution for thenormalization mode after all cuts. The histogram is the sumof MC samples of Σ+peeγ , K+pieeγ decays and a uniform back-ground, where the relative amounts of each were determinedby a fit, and the number of MC events was normalized tomatch the number of data events. The hatched area showsthe main background source (uniform background).4TABLE I: Acceptances (A) and efficiencies (ǫ) for the signaland normalization modes.Mode A (%) ǫ (%)Σ+pµµ decays 0.259 71.2Σ+pPµµ decays 0.731 69.4Normalization Σ+peeγ 0.255 5.6phase-space decay model. The form-factor model usesthe SM processes (FCNC and internal conversion) shownin Fig. 1(a)–(c). In the model B(Σ+ → pl+l−) dependson three parameters: (b2/b1), (c1/b1) and (c2/b1). Us-ing the Particle Data Group values of the partial decaywidth and the αγ decay parameter of the Σ+ → pγ de-cay, we determined (b2/b1) = −0.46 ± 0.07. The othertwo parameters were chosen to make the Σ+pee branchingratio as small as possible so that it is consistent with theexperimental upper limit, < 7 × 10−6 [3, 6]. This gives(c1/b1) = 0.7 and (c2/b1) = −13.0, and a branching ratioB(Σ+ → pe+e−) = 8.9× 10−6.In the simulation of the Σ+pPµµ decay, we assumedthat the P 0 mass was 214.3 MeV/c2 with negligible de-cay width, and that the P 0 decayed immediately to thedimuon pair. The matrix element for the P 0 → µ+µ−decay was taken to be uniform.MC simulations were used to estimate the geometricacceptances and the event-selection efficiencies for thesignal and normalization modes. Table I shows a sum-mary of the results. The relative trigger efficiency forthe signal mode with respect to the normalization modeand the unlike-sign dimuon detection efficiency were de-termined from the full data sample, and were found tobe (91.9± 1.4)% and (96.2± 0.3)%, respectively.The systematic errors in the measurement of thebranching ratios are listed in Table II. They were stud-ied by varying the ranges of the kinematic parametersin the modeling of the Σ+ production at the target, theassumed target positions, the magnetic fields, and theparameters describing the Σ+pµµ and pi0eeγ decays in theMC simulations. The systematic errors were dominatedby uncertainties in the modeling of the Σ+ production,which was mainly due to the small normalization sample.The systematic error includes the statistical error fromthe normalization mode.The branching ratio for the Σ+pµµ decay hypothesis is[8.6+6.6−5.4(stat) ± 5.5(syst)] × 10−8, in apparent disagree-ment with the expected branching ratio using the form-factor parameters given above, (1.1± 0.3)× 10−8, wherethe error includes only the uncertainty in the parameter(b2/b1). The branching ratio for the Σ+pPµµ hypothesis isB(Σ+ → pP 0, P 0 → µ+µ−) = [3.1+2.4−1.9(stat)±1.5(syst)]×10−8. The statistical errors for the branching ratio mea-surements were estimated by using the statistical tablein Ref. [8]. If the three signal events are assumed to bebackground events from some unknown source, then us-ing the method in Ref. [9] to take the systematic errorTABLE II: Fractional systematic errors (σB/B) in the branch-ing ratios of Σ+pµµ and of Σ+pPµµ.σB/B (%)SourceΣ+pµµ Σ+pPµµNormalization 14.7 14.7Modeling of Σ+ production 54.3 44.6Beam targeting 11.1 8.7Magnetic field 2.2 3.9Trigger efficiency 1.5 1.5Muon identification 0.3 0.3Σ+pµµ form factor 28.9π0 form factor 1.8 1.8B(Σ+ → pπ0) 0.6 0.6B(π0 → e+e−γ) 2.7 2.7MC statistics 1.3 1.3Total 64.4 48.1into account, we obtain an upper limit at 90% C.L. ofB(Σ+pµµ) < 3.4× 10−7.In summary, we observe three clean Σ+ → pµ+µ− can-didates. This is the first evidence for this decay. Theprobability that the three events have a dimuon masswithin 1 MeV/c2 of each other in the SM is estimatedto be < 1%. The three events are consistent with theΣ+ → pP 0, P 0 → µ+µ− decay hypothesis, with a P 0mass of 214.3± 0.5 MeV/c2.We thank the staffs of Fermilab and the participat-ing institutions for their contributions. We acknowl-edge many useful discussions with D.S. Gorbunov andP. Singer. This work was supported by the U.S. De-partment of Energy and the National Science Council ofTaiwan, R.O.C. E.C.D. and K.S.N. were partially sup-ported by the Institute for Nuclear and Particle Physics.K.B.L. was partially supported by the Miller Institute.∗ Corresponding author.Email: mlongo@umich.edu[1] L. Bergstro¨m, R. Safadi and P. Singer, Z. Phys. C 37, 281(1988).[2] D. S. Gorbunov and V. A. Rubakov, Phys. Rev. D 64,054008 (2001).[3] G. Ang et al., ZPHY 228, 151 (1969).[4] D. Corrigan and N.N. Trofimenkoff, Nucl. Phys. B 40, 98(1972).[5] R. A. Burnstein et al., hep-ex/0405034 (2004).[6] S. Eidelman et al. (Particle Data Group), Phys. Lett. B592, 1 (2004).[7] K. O. Mikaelian and J. Smith, Phys. Rev. D 5, 1763(1972).[8] G. J. Feldman and R. D. Cousins, Phys. Rev. D 57, 3873(1998).[9] R. D. Cousins and V. L. Highland, Nucl. Instrum. Meth-ods Phys. Res., Sec. A 320, 331 (1992).

Evidence for the Decay Σ+→pμ+μ-

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Evidence for the Decay Σ+→pμ+μ-

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