Direct Search for Dirac Magnetic Monopoles in p-pbar Collisi

a r X i v :h e p -e x /0509015v 1 13 S e p 2005Direct Search for Dirac Magnetic Monopoles in p ˉp Collisions at √

2 M.Morello,44P.Movilla Fernandez,28J.M¨u lmenst¨a dt,28A.Mukherjee,16M.Mulhearn,31Th.Muller,25 R.Mumford,24P.Murat,16J.Nachtman,16S.Nahn,58I.Nakano,39A.Napier,54D.Naumov,36V.Necula,17 C.Neu,43M.S.Neubauer,9J.Nielsen,28T.Nigmanov,45L.Nodulman,2O.Norniella,3T.Ogawa,55S.H.Oh,15 Y.D.Oh,27T.Okusawa,40R.Oldeman,29R.Orava,22K.Osterberg,22C.Pagliarone,44E.Palencia,11R.Paoletti,44 V.Papadimitriou,16A.Papikonomou,25A.A.Paramonov,13B.Parks,38S.Pashapour,32J.Patrick,16G.Pauletta,52 M.Paulini,12C.Paus,31D.E.Pellett,7A.Penzo,52T.J.Phillips,15G.Piacentino,44J.Piedra,11K.Pitts,23

C.Plager,8L.Pondrom,57G.Pope,45X.Portell,3O.Poukhov,14N.Pounder,41F.Prakoshyn,14A.Pronko,16 J.Proudfoot,2F.Ptohos,18G.Punzi,44J.Pursley,24J.Rademacker,41A.Rahaman,45A.Rakitin,31S.Rappoccio,21

F.Ratnikov,50B.Reisert,16V.Rekovic,36N.van Remortel,22P.Renton,41M.Rescigno,49S.Richter,25

F.Rimondi,5K.Rinnert,25L.Ristori,44W.J.Robertson,15A.Robson,20T.Rodrigo,11E.Rogers,23S.Rolli,54

R.Roser,16M.Rossi,52R.Rossin,17C.Rott,46A.Ruiz,11J.Russ,12V.Rusu,13D.Ryan,54H.Saarikko,22 S.Sabik,32A.Safonov,7W.K.Sakumoto,47G.Salamanna,49O.Salto,3D.Saltzberg,8C.Sanchez,3L.Santi,52 S.Sarkar,49K.Sato,53P.Savard,32A.Savoy-Navarro,16T.Scheidle,25P.Schieferdecker,31P.Schlabach,16

E.E.Schmidt,16M.P.Schmidt,58M.Schmitt,37T.Schwarz,33L.Scodellaro,11A.L.Scott,10A.Scribano,44

F.Scuri,44A.Sedov,46S.Seidel,36Y.Seiya,40A.Semenov,14F.Semeria,5L.Sexton-Kennedy,16I.S?ligoi,18

M.D.Shapiro,28T.Shears,29P.F.Shepard,45D.Sherman,21M.Shimojima,53M.Shochet,13Y.Shon,57

I.Shreyber,35A.Sidoti,44A.Sill,16P.Sinervo,32A.Sisakyan,14J.Sjolin,41A.Skiba,25A.J.Slaughter,16K.Sliwa,54

D.Smirnov,36J.R.Smith,7F.D.Snider,16R.Snihur,32M.Soderberg,33A.Soha,7S.Somalwar,50V.Sorin,34 J.Spalding,16F.Spinella,44P.Squillacioti,44M.Stanitzki,58A.Staveris-Polykalas,44R.St.Denis,20B.Stelzer,8 O.Stelzer-Chilton,32D.Stentz,37J.Strologas,36D.Stuart,10J.S.Suh,27A.Sukhanov,17K.Sumorok,31

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E.Vataga,36G.Velev,16G.Veramendi,23V.Veszpremi,46T.Vickey,23R.Vidal,16I.Vila,11R.Vilar,11

I.Vollrath,32I.Volobouev,28F.W¨u rthwein,9P.Wagner,51R.G.Wagner,2R.L.Wagner,16W.Wagner,25

R.Wallny,8T.Walter,25Z.Wan,50M.J.Wang,1S.M.Wang,17A.Warburton,32B.Ward,20S.Waschke,20

D.Waters,30T.Watts,50M.Weber,28W.C.Wester III,16B.Whitehouse,54D.Whiteson,43A.B.Wicklund,2

E.Wicklund,16H.H.Williams,43P.Wilson,16B.L.Winer,38P.Wittich,43S.Wolbers,16C.Wolfe,13S.Worm,50

T.Wright,33X.Wu,19S.M.Wynne,29A.Yagil,16K.Yamamoto,40J.Yamaoka,50Y.Yamashita.,39

C.Yang,58U.K.Yang,13W.M.Yao,28G.P.Yeh,16J.Yoh,16K.Yorita,13T.Yoshida,40I.Yu,27S.S.Yu,43

J.C.Yun,16L.Zanello,49A.Zanetti,52I.Zaw,21F.Zetti,44X.Zhang,23J.Zhou,50and S.Zucchelli5

(CDF Collaboration)

1Institute of Physics,Academia Sinica,Taipei,Taiwan11529,Republic of China

2Argonne National Laboratory,Argonne,Illinois60439

3Institut de Fisica d’Altes Energies,Universitat Autonoma de Barcelona,E-08193,Bellaterra(Barcelona),Spain

4Baylor University,Waco,Texas76798

5Istituto Nazionale di Fisica Nucleare,University of Bologna,I-40127Bologna,Italy

6Brandeis University,Waltham,Massachusetts02254

7University of California,Davis,Davis,California95616

8University of California,Los Angeles,Los Angeles,California90024

9University of California,San Diego,La Jolla,California92093

10University of California,Santa Barbara,Santa Barbara,California93106

11Instituto de Fisica de Cantabria,CSIC-University of Cantabria,39005Santander,Spain

12Carnegie Mellon University,Pittsburgh,PA15213

13Enrico Fermi Institute,University of Chicago,Chicago,Illinois60637

14Joint Institute for Nuclear Research,RU-141980Dubna,Russia

15Duke University,Durham,North Carolina27708

16Fermi National Accelerator Laboratory,Batavia,Illinois60510

17University of Florida,Gainesville,Florida32611

18Laboratori Nazionali di Frascati,Istituto Nazionale di Fisica Nucleare,I-00044Frascati,Italy

19University of Geneva,CH-1211Geneva4,Switzerland

20Glasgow University,Glasgow G128QQ,United Kingdom

21Harvard University,Cambridge,Massachusetts02138

3 22Division of High Energy Physics,Department of Physics,

University of Helsinki and Helsinki Institute of Physics,FIN-00014,Helsinki,Finland

23University of Illinois,Urbana,Illinois61801

24The Johns Hopkins University,Baltimore,Maryland21218

25Institut f¨u r Experimentelle Kernphysik,Universit¨a t Karlsruhe,76128Karlsruhe,Germany 26High Energy Accelerator Research Organization(KEK),Tsukuba,Ibaraki305,Japan

27Center for High Energy Physics:Kyungpook National University,Taegu702-701;Seoul National University, Seoul151-742;and SungKyunKwan University,Suwon440-746;Korea

28Ernest Orlando Lawrence Berkeley National Laboratory,Berkeley,California94720

29University of Liverpool,Liverpool L697ZE,United Kingdom

30University College London,London WC1E6BT,United Kingdom

31Massachusetts Institute of Technology,Cambridge,Massachusetts02139

32Institute of Particle Physics:McGill University,Montr′e al,

Canada H3A2T8;and University of Toronto,Toronto,Canada M5S1A7

33University of Michigan,Ann Arbor,Michigan48109

34Michigan State University,East Lansing,Michigan48824

35Institution for Theoretical and Experimental Physics,ITEP,Moscow117259,Russia

36University of New Mexico,Albuquerque,New Mexico87131

37Northwestern University,Evanston,Illinois60208

38The Ohio State University,Columbus,Ohio43210

39Okayama University,Okayama700-8530,Japan

40Osaka City University,Osaka588,Japan

41University of Oxford,Oxford OX13RH,United Kingdom

42University of Padova,Istituto Nazionale di Fisica Nucleare,

Sezione di Padova-Trento,I-35131Padova,Italy

43University of Pennsylvania,Philadelphia,Pennsylvania19104

44Istituto Nazionale di Fisica Nucleare Pisa,Universities of Pisa,

Siena and Scuola Normale Superiore,I-56127Pisa,Italy

45University of Pittsburgh,Pittsburgh,Pennsylvania15260

46Purdue University,West Lafayette,Indiana47907

47University of Rochester,Rochester,New York14627

48The Rockefeller University,New York,New York10021

49Istituto Nazionale di Fisica Nucleare,Sezione di Roma1,

University of Rome“La Sapienza,”I-00185Roma,Italy

50Rutgers University,Piscataway,New Jersey08855

51Texas A&M University,College Station,Texas77843

52Istituto Nazionale di Fisica Nucleare,University of Trieste/Udine,Italy

53University of Tsukuba,Tsukuba,Ibaraki305,Japan

54Tufts University,Medford,Massachusetts02155

55Waseda University,Tokyo169,Japan

56Wayne State University,Detroit,Michigan48201

57University of Wisconsin,Madison,Wisconsin53706

58Yale University,New Haven,Connecticut06520

We search for pair-produced Dirac magnetic monopoles in35.7pb?1of proton-antiproton col-

√

lisions at

4 the Dirac quantization condition:

ge

2??g

2α≈68.5·n

where n is an integer andαis the?ne structure constant. In this search,we consider an n=1monopole with mass less than1TeV/c2,spin1

p collisions at √

5

likely overestimates the uncertainty;varying the energy-loss model between a naive model where e→gβand the full treatment of Ref.[17]has a negligible e?ect.

The TOF acceptance depends on the monopole pro-duction kinematics.To quantify this dependence,we consider separately the Drell-Yan mechanism without the additional velocity dependence and with monopole pro-duction uniform in the cosine of the polar angle in the center of mass frame.The total variation in the accep-tance is10%.We therefore present results for our bench-mark mechanism only,with the understanding that mass limits for other production mechanisms can be inferred from the cross-section limit with reasonable accuracy. During each event,the TOF electronics makes a sin-gle measurement for each PMT.Light from other par-ticles,called spoilers,can reach a PMT before the light from monopoles,starting the charge integration.If the monopole light does not reach the PMT within the20ns charge integration window,the monopole’s light will not be integrated and trigger will not?re.Our studies show that pure Monte Carlo underestimates the e?ect of spoil-ers seen in data.We therefore estimate the spoiler frac-tion by embedding Monte Carlo produced monopoles in real Z→e+e?data.Because these are high-mass cen-tral events produced by a Drell-Yan mechanism,we ex-pect the distribution of other particles in the event to be similar to that of a monopole-pair production event. We exclude the bars with signals from the electrons and count the number of spoiler events,which have real pulses arriving more than20ns before the simulated pulse from a magnetic monopole.

The systematic uncertainty is dominated by the un-certainty in the time needed to integrate enough of the monopole’s charge to cause a trigger.To quantify this e?ect,we note that rise times for TOF pulses are typi-cally less than1ns and redo the calculation with a15ns integration window.We take one-half the di?erence as a systematic uncertainty.Other e?ects,such as the de-pendence on luminosity,are much smaller for our sam-ple.For a400GeV/c2monopole,the spoiler fraction is 2%±1%with a3%systematic uncertainty.

Massive monopoles can have low velocities causing them to arrive at the TOF too late to cause a trigger.The timing acceptance is calculated with a Monte Carlo sim-ulation by requiring pulses to arrive within the54ns tim-ing window.Only heavy monopoles move slowly enough to be a?ected:a900GeV/c2monopole is out of time in10%of events.This is a negligible e?ect on lighter monopoles.

Monopoles curve in the rz plane,in sharp contrast to electrically charged particles,which curve in the rφplane.

A specialized reconstruction program isolates monopole candidates using data from the COT.Candidates consist of coincident track segments composed entirely of hits with large ionization,consistent with a straight line in the plane perpendicular to the magnetic?eld.

The COT electronics encodes the integrated charge as the width of a hit,which is the ionization measurement used for monopole candidate selection.A typical MIP produces hit widths of about20ns.An extrapolation of the non-linear COT response for ordinary particles pre-dicts that monopoles would produce hit widths of about 230ns(1000MIPs),still within the dynamic range of the COT.We do not use this extrapolation.Instead we cut in the tail of the width distribution from ordinary tracks, found to be at140ns(50MIPs)in minimum-bias data collected with an open trigger highly e?cient for inelastic pˉp collisions.Hits with charge below this amount are not considered by the monopole reconstruction.As magnetic monopoles have much greater ionization than the tracks used to determine this cut,it has a negligible e?ect on the e?ciency.

The default COT tracking algorithm?rst reconstructs track segments in each of eight superlayers.It checks for hits loosely consistent with a straight-line,using a tol-erance of20ns.The identi?ed hits in each segment are then?t to a circular trajectory.In the monopole algo-rithm,the segments are required to be composed entirely of high-ionization hits.Also,because a monopole can be as slow asβ～0.1with changing transverse velocity,the usual timing assumption(t?ight=r/c)cannot be used. Instead,the time of?ight to each superlayer is varied between r/c and10r/c in5ns increments.

A monopole candidate consists of severalφ-coincident, low-curvature segments.From Monte Carlo simulation, we choose a loose cut on the segment curvatureρ< 0.001cm?1,which for an electron would correspond to p T>4GeV/c.Likewise,theφtolerance is a loose 0.2radians.The remaining cuts are on the minimum number of hits needed in a segment and on the total num-ber ofφ-coincident segments required for a monopole can-didate.By ignoring the width cut,the segment-?nding algorithm e?ciency is measured in an independent data sample using high-p T tracks.In this manner,we choose a highly e?cient cut requiring seven coincident superlayers with at least eight hits in each segment.This has a94% e?ciency with a1%statistical uncertainty.For these cuts,the e?ciency for?nding high-mass monopole pairs calculated with the Monte Carlo simulation is nearly 100%.The e?ciency for high-p T electrons in simulation, after removing the width cut,is also nearly100%.There are real detector e?ects contributing a small ine?ciency. As an ionizing particle passes through matter,the most energetic electrons form delta rays.For highly relativis-tic low-mass monopoles,the large number of delta rays confuses the segment?nding algorithm,lowering the ef-?ciency.We check that GEANT is properly producing delta-rays by comparing the e?ciency of monopoles to kinematically equivalent heavy-ions simulated in the ab-sence of a magnetic?eld.We scale the e?ciency deter-mined from Monte Carlo simulation to make the high-mass monopole e?ciency agree with the high-p T track

6

FIG.2:Number of monopole candidates as a function of

COT width cut in o?ine reconstruction,in100k minimum-

bias events and the entire130k event trigger sample.A100

ns width cut corresponds to15MIPs,we expect monopoles

to ionize more than1000MIPs or232ns.

E?ect

TOF geometric(MC)

100%

TOF spoilers

99%±1%±1%

100%

COT segment?nding

7

search Foundation;the Particle Physics and Astronomy Research Council and the Royal Society,UK;the Russian Foundation for Basic Research;the Comisi′o n Interminis-terial de Ciencia y Tecnolog′?a,Spain;in part by the Eu-ropean Community’s Human Potential Programme un-der contract HPRN-CT-2002-00292;and the Academy of Finland.

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