A search for antiproton decay has been made at the Fermilab Antiproton
Accumulator. Limits are placed on fifteen antiproton decay modes. The results
are used to place limits on the characteristic mass scale Mx that could be
associated with CPT-violation accompanied by baryon number violation.Comment: 11 pages, 1 figure. Submitted to Phys. Rev. Let

APEX Collaboration

Geer, S.

English

arXiv

A search for antiproton decay has been made at the Fermilab Antiproton Accumulator. Limits are placed on fifteen antiproton decay modes. The results are used to place limits on the characteristic mass scale Mx that could be associated with CPT-violation accompanied by baryon number violation

Collaboration, APEX

Geer, S

UCL Discovery

VOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000590New Limit on CPT ViolationS. Geer, J. Marriner, M. Martens, R. E. Ray, J. Streets, and W. WesterFermi National Accelerator Laboratory, Batavia, Illinois 60510M. Hu and G. R. SnowUniversity of Nebraska, Lincoln, Nebraska 68588T. ArmstrongPennsylvania State University, University Park, Pennsylvania 16802C. Buchanan, B. Corbin, M. Lindgren, and T. Muller*University of California at Los Angeles, Los Angeles, California 90024R. GustafsonUniversity of Michigan, Ann Arbor, Michigan 48109(The APEX Collaboration)(Received 7 October 1999)A search for antiproton decay has been made at the Fermilab Antiproton Accumulator. Limits areplaced on fifteen antiproton decay modes. The results are used to place limits on the characteristic massscale mX that could be associated with CPT violation accompanied by baryon number violation.PACS numbers: 11.30.Er, 11.30.Fs, 13.30.Ce, 14.20.DhThe CPT theorem requires that the proton (p) andantiproton (p ) lifetimes are equal. Searches for pdecay have yielded lower limits on the p lifetimetp . O1032 yr [1]. A search for p decay with a shortlifetime (tp ø tp) tests the CPT theorem. In this paperwe summarize results from a search for p decay at theFermilab Antiproton Accumulator, and discuss limits onthe characteristic mass scale mX associated with CPTviolation. For a mass dimension-5 CPT violating opera-tor these limits approach the Planck scale.CPT invariance is one of the most fundamental sym-metries of modern physics. There have been a variety ofsearches [1–4] for CPT violation based upon comparingparticle and antiparticle masses, lifetimes, and magneticmoments. For example, the p and p masses have beenshown to be equal with a precision of a few parts in 108[3], while the particle and antiparticle masses in the neu-tral kaon system have been shown to be equal to about onepart in 1018 [1,4]. A search for p decay complements theseCPT tests by providing a search for CPT violation ac-companied by a violation of baryon number. Indeed, sincethe p is the only very long-lived antiparticle that could inprinciple decay into other known particles without violat-ing charge conservation, a search for p decay provides aunique test of the CPT theorem, and a unique test of theintrinsic stability of antimatter.The sensitivity of a p decay search to the presenceof a CPT violating interaction has been characterized byconsidering a dimension-n CPT violating quantum fieldoperator (n . 4) with characteristic mass scale mX . Di-mensional analysis then provides the estimate [5] mptp 0031-90070084(4)590(4)$15.00mpmX2n28, yieldingmXmp  4.5 3 1038 tp10 Myr12n28. (1)For a given lower limit on tp the implied lower limit onmXis most stringent for n  5. Note that ifmX is at the Planckscale 1.2 3 1019 GeVc2 and n  5, the expected tpwould be 10 Myr.The most stringent lower limit on tp has been obtained[6] from a comparison of recent measurements of the cos-mic ray p flux [7] with predictions based on expecta-tions for the secondary production of antiprotons in theinterstellar medium. The agreement between the observedand predicted rates implies that tp is not small comparedto Tg, where T is the p confinement time within thegalaxy (107 yr) and g is the Lorentz factor for the ob-served antiprotons. After taking into account the uncer-tainties on the relationship between the interstellar p fluxand the flux observed at the Earth, at 90% C.L. the limittp . 8 3 105 yr has been reported [6]. This indirect limitis not valid if current models of p production, propagation,and interaction in the interstellar medium are seriouslyflawed. For example, within the minimal supersymmetricextension to the standard model, a significant cosmic rayp flux can be produced by cold dark matter neutralino an-nihilation. In this scenario, if the depletion of the spectrumdue to p decay is compensated by additional contributionsfrom neutralino annihilation, it has been claimed [8] thatthe current cosmic ray data can accommodate p lifetimesas low as #105 yr.Laboratory searches for p decay have, to date, providedless stringent limits on tp . However, these limits do not© 2000 The American Physical SocietyVOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000suffer from large model dependent uncertainties. Themost stringent published laboratory limit on inclusivep decay has been obtained from a measurement of thecontainment lifetime of 1000 antiprotons stored inan ion trap, yielding tp . 3.4 months [3]. The sensi-tivity of laboratory p decay searches can be improvedby looking for explicit p decay modes at a p storagering. Angular momentum conservation requires that adecaying p would produce a fermion (electron, muon,or neutrino) in the final state. A search for explicitp decay modes with an electron in the final state wasmade by the T861 experiment at the Fermilab Antipro-ton Accumulator. The T861 search yielded the 95%C.L. limits [9]: tpBp ! e2g . 1848 yr, tpBp !e2p0 . 554 yr, tpBp ! e2h . 171 yr, tpBp ! e2K0S . 29 yr, and tpBp ! e2K0L . 9 yr.Following the T861 results, the APEX experiment [10]was designed to enable a more sensitive search for pdecay. The APEX detector, located in a straight sectionof the 474 m circumference Fermilab Antiproton Accu-mulator ring, was designed to identify p decays withina 3.7 m long evacuated decay tank. Particles exiting thetank at large angles to the circulating p beam traversed a96 cm diameter 1.2 mm thick stainless steel vacuum win-dow. The experiment was optimized to detect a single en-ergetic charged track (electron or muon), originating fromthe beam, and accompanied by one or more neutral pi-ons or photons. A detailed description of the detectorcan be found in Ref. [10]. In brief, the detector con-sists of (i) an upstream system of scintillation countersarranged around the 10 cm diameter beam pipe, locatedupstream of the tank, and used to reject upstream inter-actions. (ii) Three planes of horizontal and three planesof vertical scintillation counters downstream of the tank.Each plane consisted of two 50 3 100 3 1.27 cm3 coun-ters. The last counter planes were downstream of a 2.3radiation length lead wall, providing a preradiator to aidin identifying electrons and photons. The first and secondcounter planes, upstream of the lead, provided pulse heightinformation used to count the traversing charged particles.(iii) A lead-scintillator sampling electromagnetic calorime-ter [11] constructed from 144 rectangular 10 3 10 cm2modules that are 14.7 radiation lengths deep, arranged ina 13 3 13 array with six modules removed from eachof the four corners, and the central module removed toallow passage of the beam pipe. (iv) A tail catcher (TC)downstream of the calorimeter consisting of a 20 cm deeplead wall followed by two planes of scintillation counters.(v) A muon telescope (MT) downstream of the TC, 10 nu-clear interaction lengths deep, and aligned to point towardsthe center of the decay tank. The MT consists of a sand-wich of five iron plates and five 30 3 30 cm2 scintilla-tion counters, and was used to identify penetrating chargedparticles (muon candidates). (vi) A tracking system con-sisting of three planes of horizontal and three planes ofvertical 2 mm diameter scintillating fibers downstream ofthe tank and upstream of the preradiator lead. These detec-tors provided three space points along the track trajectorywith typical residuals of 620 mm in the directions trans-verse to the beam direction, enabling the origin of tracksemerging from the decay tank to be reconstructed with anrms precision given by sz  12 cm.The APEX experiment took data at times whenthere were typically 1012 antiprotons circulating inthe Accumulator ring with a central p momentum of8.90 6 0.01 GeVc (g  9.54 6 0.01). A measure ofthe sensitivity of the APEX data sample is given byS 1gZNp t dt  3.31 6 0.03 3 109 yr, (2)where Np t is the number of circulating antiprotons attime t, the integral is over the live time of the experi-ment, and the uncertainty arises from the precision withwhich the time dependent beam current was recorded.Hence, if tp  3.31 3 109 yr then on average 0.63 p de-cays would have occurred somewhere within the Accu-mulator ring during the live time of the experiment and0.63 3 3.7474  5 3 1023 decays would have occurredwithin the APEX vacuum tank.Energetic particles passing through the detector dur-ing the Accumulator operation predominantly arise frominteractions of the p beam with the residual gas in thedecay tank or with material surrounding the beam. To sup-press these backgrounds, and select candidate p ! m2Xand p ! e2X decays, signals from the calorimeter andthe scintillation counters were used to form an MT triggerwhich recorded 1.2 3 106 events, and calorimeter triggerswhich recorded 37.8 3 106 events. Off-line, events wereselected for analysis if they had (i) a single charged trackreconstructed in the tracking system that pointed back tothe beam line within the fiducial volume of the decay tank,and (ii) a pattern of hits in the scintillation counters andcalorimeter cells consistent with a two-body or three-bodyp decay topology. After these requirements, 416 muoncandidate events and 2.2 3 104 electron candidates sur-vive for further analysis. Events were then required to beTABLE I. Summary of results: 90% C.L. limits on tB for15 antiproton decay modes.tB Limit tB LimitDecay mode (years) Decay mode (years)m2g 5 3 104 e2g 7 3 105m2p0 5 3 104 e2p0 4 3 105m2h 8 3 103 e2h 2 3 104m2gg 2 3 104 e2gg 2 3 104m2K0L 7 3 103 e2K0L 9 3 103m2K0S 4 3 103 e2K0S 9 3 102e2r 2 3 102e2v 2 3 102e2K01 3 103591VOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000FIG. 1. Upper limits on the antiproton lifetime. APEX limits(circles), T861 limits (boxes), and limits [3,16,17] prior to theT861 experiment (triangles) are shown as a function of the pub-lication year.kinematically consistent with the particular decay chan-nel being considered (see Table I). No statistically signifi-cant p decay signal was observed. A detailed descriptionof the triggers, data taking, and analysis can be found inRefs. [10,12–14]. For a specific decay mode with branch-ing ratio B this null result can be used to place a limit ontB which is given in years bytpBp ! m2X . 2 1ln1 2 NmaxeS , (3)where e is the calculated fraction of decays taking placeuniformly around the accumulator ring that pass the triggerand event selection requirements. The upper limit on thenumber of signal events Nmax is given by the prescriptionof Ref. [15]:Nmax  mmax 3 1 1 mmaxs2r2 , (4)where sr  see, se is the systematic uncertainty on e,and mmax is the 90% C.L. upper limit corresponding to theobservation of N events that pass the trigger and selectionrequirements. Note that N  0 for all of the p ! m2Xdecay modes and for p ! e2g and p ! e2p0 modes.For the other modes, N is between two and four events,where the surviving events are consistent with arising fromthe tails of the background distributions. A detailed MonteCarlo simulation of the detector geometry and response hasbeen used to calculate e and sr .Table I lists the resulting limits on tB for 15 decaymodes in which there is an electron or muon in the finalstate. Our results are shown in Fig. 1 to be significantlymore stringent than previous limits [2]. In particular, weplace the first explicit limits on the muonic decay modesof the p, and the first limits on the decay modes e2gg,e2r, e2v, and e2K0 . Our most stringent limit is on thedecay p ! e2g for which we find tB . 7 3 105 yr592TABLE II. Summary of limits on the CPT violating scale mX(GeVc2) shown as a function of the mass dimension n of theassociated quantum field operator.Decay nmode 5 6 7 8 9m2g 1 3 1018 1 3 109 1 3 106 3 3 104 4 3 103m2p0 1 3 1018 1 3 109 1 3 106 3 3 104 4 3 103m2h 6 3 1017 7 3 108 8 3 105 3 3 104 3 3 103m2gg 9 3 1017 9 3 108 9 3 105 3 3 104 4 3 103m2K0L 5 3 1017 7 3 108 8 3 105 3 3 104 3 3 103m2K0S 4 3 1017 6 3 108 7 3 105 2 3 104 3 3 103e2g 5 3 1018 2 3 109 2 3 106 5 3 104 5 3 103e2p0 4 3 1018 2 3 109 2 3 106 4 3 104 5 3 103e2h 9 3 1017 9 3 108 9 3 105 3 3 104 4 3 103e2gg 9 3 1017 9 3 108 9 3 105 3 3 104 4 3 103e2K0L 6 3 1017 7 3 108 8 3 105 3 3 104 3 3 103e2K0S 2 3 1017 4 3 108 5 3 105 2 3 104 3 3 103e2r 9 3 1016 3 3 108 4 3 105 2 3 104 2 3 103e2v 9 3 1016 3 3 108 4 3 105 2 3 104 2 3 103e2K02 3 1017 4 3 108 6 3 105 2 3 104 3 3 103(90% C.L.). Table II summarizes the limits on the CPTviolating mass scale derived from Eq. (1), listed as a func-tion of the mass dimension n for 5 # n # 9. Note thatfor the most sensitive decay modes the lower limit on mXis O1018 GeVc2 for n  5. For n  6 the limits are inthe range 108 109 GeVc2 for all 15 decay modes. Evenfor n  8 the limits on mX are still at the 10 TeVc2 scale.These are unique limits on the presence of CPT violationaccompanied by a violation of baryon number.The APEX experiment was performed at the FermiNational Accelerator Laboratory, which is operatedby Universities Research Association, under ContractNo. DE-AC02-76CH03000 with the U.S. Department ofEnergy. We are grateful to Dallas Kennedy for commentson the paper.*Present address: Universität Karlsruhe, 76128 Karlsruhe,Germany.[1] C. Caso et al., Eur. Phys. J. C 3, 1 (1998).[2] R. Bluhm, V. A. Kostelecký, and N. Russell, Phys. Rev.Lett. 79, 1432 (1997).[3] G. Gabrielse et al., Phys. Rev. Lett. 65, 1317 (1990).[4] R. Carosi et al., Phys. Lett. B 237, 303 (1990); CPLEARCollaboration, R. Adler et al., and J. Ellis et al., Phys. Lett.B 364, 239 (1995).[5] D. C. 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A New Limit on CPT Violation

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A New Limit on CPT Violation

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