General Search for New Phenomena in ep Scattering at HERA

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3 Gnral Sarch for Nw Phnomna in p Scattring at HERA Von dr Fakultät für Mathmatik, Informatik und Naturwissnschaftn dr Rhinisch-Wstfälischn Tchnischn Hochschul Aachn zur Erlangung ds akadmischn Grads ins Doktors dr Naturwissnschaftn hmigt Dissrtation vorglgt von Diplom-Physikr Martin Wssls aus Hannovr Brichtr: Univrsitätsprofssor Dr. Ch. Brgr apl. Profssor Dr. W. Braunschwig Tag dr mündlichn Prüfung: 9. Juli 4 Dis Dissrtation ist auf dn Intrntsitn dr Hochschulbibliothk onlin vrfügbar.

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5 Abstract A modl-indpndnt sarch for dviations from th Standard Modl prdiction is prformd in + p and p collisions at HERA using H data corrsponding to an intgratd luminosity of 7 pb. For th first tim all vnt topologis involving isolatd lctrons, photons, muons, nutrinos and jts with high transvrs momnta ar invstigatd in a singl analysis. Evnts ar dividd into xclusiv vnt classs according to thir final stat. A novl statistical algorithm is usd to sarch for dviations from th Standard Modl in th distributions of th scalar sum of transvrs momnta and invariant mass of final stat particls and to uantify thir significanc. A good agrmnt with th Standard Modl prdiction is obsrvd in most of th vnt classs and on intrsting vnt is masurd with four jts and an lctron. Th most significant dviation is found in a topology containing an isolatd muon, missing transvrs momntum and a jt, whr a dviation has bn prviously rportd. Kurzfassung In disr Arbit wird in modllunabhängig Such nach Abwichun von dr Vorhrsag ds Standardmodlls in + p und p Kollisionn bi HERA durchgführt. Di analysirtn H-Datn ntsphn inr intgrirtn Luminosität von 7 pb. Erstmals wrdn all Erignistopologin, di isolirt Elktronn, Photonn, Myonn, Nutrinos und Jts mit hohn Transvrsalimpulsn nthaltn, in inr inzi Analys untrsucht. Entsphnd ihrs Endzustands wrdn all Erigniss in xklusiv Erignisklassn untrtilt. Ein nuartigr statistischr Algorithmus wird bnutzt, dr in dn Vrtilun dr skalarn Summ dr Transvrsalimpuls und dr invariantn Mass nach Abwichun vom Standardmodll sucht und drn Signifikanzn uantifizirt. In dn mistn Erignisklassn wird in gut Übrinstimmung mit dr Standardmodllvorhrsag bobachtt, und in intrssants Erignis mit vir Jts und inm Elktron wird gmssn. Di signifikantst Abwichung wird in inr Erignistopologi mit inm isolirtn Myon, fhlndm Transvrsalimpuls und inm Jt fstgstllt, inr Topologi, in dr schon in frührn Analysn Abwichun bobachtt wurdn.

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7 Contnts Introduction Standard Modl Physics at HERA 5. Inclusiv Elctron-Proton Scattring Kinmatics Cross Sctions Virtual Photon-Proton Scattring Structur Functions in QPM and QCD Exclusiv Final Stats Jt Production Photon Production Lpton Pair Production W Production Th H-Exprimnt at HERA. Th Elctron-Proton Acclrator HERA Th H Dtctor Gnral Layout Innr Tracking Systm Calorimtry Cntral Muon Dtctor Luminosity Systm Triggr and Data Acuisition Rconstruction of th Kinmatics Mont Carlo Simulation Mont Carlo Gnration Simulation Data Slction and Classification 4 4. Analysis Stratgy Evnt Slction Particl Idntification i

8 ii Contnts 4.3. Elctrons Photons Muons Jts Nutrinos Evnt Classification Exprimntal Prformanc Enrgy Masurmnt and Calibration Elctromagntic Enrgy Scal Hadronic Enrgy Scal Triggr Efficincis Limitations of th Masurmnt Th -ν Evnt Class Th µ-ν Evnt Class Rsolutions Puritis and Efficincis Systmatic Uncrtaintis Multi-Jt Topologis Exprimntal Rsults 9 7 Sarch for Dviations 7. Sarch Algorithm Mont Carlo Exprimnts and Global Significanc Snsitivity to Nw Physics Signals Sarch Rsults Summary A Rsolutions 3 B Puritis and Efficincis 7 C Systmatic Uncrtaintis 3 D Evnt Display and Tabls of th Rsults 33 List of Figurs 37 List of Tabls 4 Bibliography 43

9 Introduction Today, th knowldg gaind by scintists about th structur of mattr is summarisd in th Standard Modl of particl physics, which has bn dvlopd in th scond half of th last cntury and has provd vry succssfully in dscribing all xprimntally rsults in th fild of high nrgy physics. Th basic assumptions of th Standard Modl ar simpl and can b writtn down in a fw lins. Mattr is composd of lmntary frmions with spin, uarks and lptons, ach occurring in thr familis consisting of two particls. Each of th particls has a corrsponding anti-particl with opposit charg but othrwis idntical proprtis. ( ) ( ) ( ) ( ) ( ) ( ) ν νµ ντ u c t lptons: uarks:. µ τ d s b Ths lmntary particls ar subjct to thr fundamntal forcs, which ar th strong, th lctromagntic and th wak forc. Gravity, th forth known fundamntal intraction, has not yt bn includd in th Standard Modl. Th intractions btwn th particls composing mattr ar mdiatd by bosons carrying spin. Ths xchang particls ar th masslss photon for th lctromagntic forc, th massiv Z and W bosons for th wak forc and ight masslss gluons mdiating th strong intraction. Th third kind of particl in th Standard Modl is th Higgs boson, which is rsponsibl for th cration of particl masss. Towards a grand unification of th fundamntal forcs, th lctromagntic and wak forc ar combind in th Standard Modl into th lctrowak intraction. Th gaug thory dscribing th strong intraction is th Quantum Chromodynamics. Th complx xprimnts nowadays arrangd within th fild of high nrgy physics to vrify th prdictions of th Standard Modl ar basd on th sam principl, which has alrady bn utilisd by Ruthrford and his assistants Gigr and Marsdn in thir xprimnts rvolutionising physics at th start of th th cntury. Aiming alpha particls on a thin gold foil, Ruthrford dducd from th rat at which th alpha particls hav bn scattrd at spcific angls, that gold atoms must b largly mad up of mpty spac containing a small havy cor, which carris narly all of th mass and th total positiv charg of th gold atom. Th atomic nuclus had finally bn discovrd. Sinc thn our undrstanding of natur has improvd, finally rsulting in th concption of th Standard Modl, and xprimntal tchnics hav grown mor complx. Howvr, th basic principl of using point-lik particls as a prob to invstigat anothr particl

10 Introduction that scattrs th first has not changd for th nt xprimnts. Bnath ths scattring xprimnts th invstigation of lctron-proton intractions has alrady playd an important rol in rvaling th structur of th proton and stablishing Quantum Chromodynamics as th thory of strong intractions. This succssful story is continud at th H and ZEUS xprimnts at th Hadron-Elktron-Ring-Anlag HERA, which is th only acclrator colliding lctrons on high-nrgtic protons worldwid. At HERA lctrons and protons collid with a cntr-of-mass nrgy of up to 39 GV. Ths high-nrgy lctron-proton collisions provid a uniu tsting ground for th Standard Modl. Although no dviation from th Standard Modl has bn stablishd, it is widly blivd that th Standard Modl is incomplt and that nw physics signals may appar blow nrgis of TV. A larg varity of xtnsions to th Standard Modl has bn constructd during th last dcads, prdicting various phnomna apparing at high nrgis or larg transvrs momnta. HERA data hav bn usd to tst som of ths modls of nw procsss by analysing thir anticipatd xprimntal signaturs, and limits on thir paramtrs hav bn drivd []. Contrary to ths ddicatd analyss, th approach dscribd in this thsis consists of a comprhnsiv and ric sarch for dviations from th Standard Modl prdiction at larg transvrs momnta P T in all final stat topologis with at last two objcts. For th first tim all high-p T final stat configurations involving lctrons, muons, jts, photons or nutrinos ar systmatically invstigatd in a cohrnt analysis. Th prsntd analysis is basd on th complt HERA-I data sampl ordd at th H xprimnt btwn 994 and. All slctd vnts ar classifid into xclusiv vnt classs according to th numbr and typs of objcts dtctd in th final stat. For all vnt classs th invariant mass and th scalar sum of transvrs momnta distributions of th high P T -final stat objcts ar systmatically rsarchd for th apparanc of nw physics signals. Th analysis covrs phas spac rgions whr th Standard Modl prdiction is sufficintly pis to dtct anomalis and dos not rly on assumptions concrning th charactristics of any Standard Modl xtnsion. An in this spirit calld modl-indpndnt sarch might thrfor b abl to discovr unxpctd manifstations of nw physics. It addrsss th important ustion of whthr vidnc of nw physics might still b hiddn in th data ordd at collidr xprimnts. Th prliminary rsults of this analysis ar rportd in [, 3]. Th prpard publication contains th rsult plots prsntd in this thsis. In th first chaptr of this thsis a thortical ovrviw of th scattring procss of lctrons on protons is givn. In particular th production mchanism of th various final stat topologis of th p intraction ar discussd. In th scond chaptr th HERA collidr and th H xprimnt will b dscribd. Th third chaptr introducs th Mont Carlo rators usd in this analysis. Sinc a ral sarch for dviations from th Standard Modl ruirs a pis stimat of all

11 Introduction 3 rlvant p scattring procsss, multipl vnt rators ar usd. Th vnt slction and th uality critria of th considrd objcts ar dscribd in chaptr 4. Spcial mphasis will b placd on th lctron and photon idntification. Morovr th classification of vnts into xclusiv vnt classs is introducd. Chaptr 5 is concrnd with th prformanc of th xprimntal masurmnt. Both th lctromagntic and hadronic nrgy scal uncrtainty ar vrifid and th rsolutions of th latr on rsarchd uantitis ar drivd. Puritis and fficincis for ach vnt class ar dtrmind and th systmatic uncrtaintis considrd ar discussd. Finally, th fasibility of a masurmnt of multi-jt final stats is invstigatd. In chaptr 6 th xprimntal rsults of th data masurmnt ar introducd. Th global vnt yilds of th vnt classs ar compard to th Standard Modl xpctation and th invariant mass and th scalar sum of transvrs momnta distributions of th high-p T final stat objcts ar discussd. Finally in th svnth chaptr, ths distributions ar systmatically invstigatd within th framwork of an statistical analysis using a novl algorithm, which locats th rgions with th largst dviation btwn th data and th Standard Modl prdiction. In ordr to uantitativly dtrmin th lvl of agrmnt btwn th data and th Standard Modl xpctation, th probability of occurrnc of such a dviation is drivd.

12 4 Introduction

13 Chaptr Standard Modl Physics at HERA At th Hadron-Elktron-Ring-Anlag HERA lctrons and protons collid with a cntrof-mass nrgy of up to 39 GV. Ths high-nrgtic lctron-proton intractions provid an uniu tsting ground for th Standard Modl and allow for th invstigation of complmntary aspcts compard to lctron-positron annihilation and proton-antiproton scattring procsss. In th first part of this chaptr th thortical foundations on th inclusiv scattring procss of lctrons and protons ar introducd. Subsuntly th production procsss of xclusiv final stats ar dscribd. Most of th argumnts prsntd in th following ar drivd from th discussions on lctron-proton scattring as givn in [4, 5, 6, 7, 8, 9].. Inclusiv Elctron-Proton Scattring Th invstigation of lctron-proton scattring procsss has alrady playd an important rol in rvaling th structur of th proton and in stablishing Quantum Chromodynamics as th thory of th strong intraction btwn lmntary particls. At modrat nrgis th scattring of lctrons on protons is ssntially an lastic procss, whr th proton oils from th impact but is actually rsumd intact aftr th intraction. Th diffrntial cross sction for th lctron-proton lastic scattring has bn calculatd for th first tim in 95 and is known as Rosnbluth formula. It is dominatd by so-calld form factors, which dtrmin how th scattring procss is rducd from its valu for a pointlik proton. In 954 physicists supplid th xprimntal vidnc that protons hav a masurabl diamtr. Raising th momntum transfr btwn th lctron and proton and thus incrasing th spatial rsolution, th structur of th proton is rsolvd. Within this high-nrgtic probing procss th proton is gtting likly to brak up, rfrrd to as inlastic lctron-proton scattring, whr no rstriction is applid on th typ of th hadronic final stat. In 964 Gll-Mann and Zwig postulatd that all hadrons ar composd of pointlik particls, which Gll-Mann calld uarks. From that point of viw th lastic lctron-uark scat- In this thsis lctron rfrs to both lctrons and positrons, if othrwis not statd. 5

14 6 Chaptr. Standard Modl Physics at HERA!#"$ Fig..: Th lowst-ordr diagrams of lctron-proton scattring for nutral currnt procsss (lft) and chargd currnt procsss (right). Th four-momnta of th particls or particl systms ar givn in parnthss. tring is gtting accssibl within th inlastic scattring procss of lctrons on protons. Finally, th arly 97 s saw th discovry of th uarks in xprimnts prformd at th Stanford Linar Acclrator SLAC... Kinmatics Within th Standard Modl of particl physics th thortical dscription of th scattring procss of lctrons on protons is basd on th thory of lctrowak intractions and th Quantum Chromodynamics. In th most ral cas th amount of possibl ractions is subdividd into two kinds of procsss according to th typ of th intrmdiat bosons as dpictd in figur.. Th incoming lctron with th four-momntum k intracts with th proton carrying th four-momntum p via th xchang of an lctrowak gaug boson transfrring th fourmomntum. Sinc th lpton numbr has to b consrvd, th final stat consist of a scattrd lpton with th four-momntum k, whil th struck proton might fragmnt into th hadronic systm X carrying th four-momntum p. In cas of nutral currnt (NC) procsss p X th xchangd boson is a nutral particl (γ or Z ), whil chargd particls (W + or W ) mdiat th intraction in chargd currnt (CC) procsss p νx rsulting in th outgoing lpton bing a nutrino. Basd on th four-momnta of th initial lctron, th final stat lpton and th incoming proton th kinmatics of th lctron-proton scattring ar dtrmind at th lowst-ordr by th following thr Lorntz-invariant variabls: Q = = (k k ), (.) Q x =, (.) (p ) y = p. (.3) p k

15 .. Inclusiv Elctron-Proton Scattring 7 Q dfind as th ngativ suar of th four-momntum transfr is a masur of th virtuality of th intrmdiat boson and dtrmins th hardnss or rsolving powr of th intraction. Th non-dimnsional variabls x and y ar ranging btwn zro and on and ar bst undrstood in ddicatd frams of rfrnc. x is calld th Bjørkn scaling variabl. Its valu valuatd in th infinit momntum fram of th proton can b intrprtd in th framwork of th Quark Parton Modl as th proton momntum fraction carrid b th struck uark (cf. sction..4). In th rst fram of th proton y dtrmins th rlativ nrgy loss of th scattrd lctron and givs thus a masur of th inlasticity of th intraction. Th suar of th cntr-of-mass nrgy s and th suar of th invariant mass W of th hadronic systm X oiling against th scattrd lpton ar givn by s = (k + p) and W = (p + ) = p. (.4) Basd on th invariant mass W of th hadronic systm th mntiond transition from th lastic to th inlastic rgim of lctron-proton scattring can b formulatd mor uantitativly. In cas of lastic intractions whr th proton dos not brak up th valu of W apparntly amounts to th mass of th proton. For incrasing nrgy transfrs th proton might b xcitd into a rsonant stat, such as th +. In this cas of th socalld uasi-lastic scattring th invariant mass W rachs valus in th ordr of th mass of th according rsonanc. Entring th rgim of inlastic lctron-proton scattring, th complicatd multi-particl final stats with larg invariant masss rsult in a smooth distribution of W. Nglcting th lctron and proton masss th suar of th cntr-of-mass nrgy is connctd with th nrgis of th incoming lctron E k and proton E p via th rlation s = 4 E k E p. (.5) Undr th sam assumption th following uations ar obtaind, rlating th introducd kinmatic variabls Q, x, y, W and s: Q = s x y W = Q x x. (.6) Thus in cas of fixd cntr-of-mass nrgis s, as it is tru for th HERA acclrator, only two of thos four Lorntz-invariant variabls Q, x, y, W ar indpndnt and sufficint to dtrmin th kinmatics of th lctron-proton scattring... Cross Sctions As discussd in th prvious sction th inclusiv diffrntial cross sction, intgratd ovr all possibl hadronic final stats of th lctron-proton scattring procss, is a function In th proton rst fram y is givn by y = E k E k, whr E k and E k incoming lctron and scattrd lpton in this fram of rfrnc. dnot th nrgis of th

16 8 Chaptr. Standard Modl Physics at HERA of two indpndnt variabls uniuly dtrmining th kinmatics of th vnts. In anticipation of th structur of th proton th variabls x and Q ar typically chosn for paramtrisation. Basd on th diagrams prsntd in figur. th doubl diffrntial cross sction for inlastic lctron-proton scattring is givn at lowst ordr in lctrowak intractions in th most ral formulation by th convolution of a lptonic tnsor L µν and a hadronic tnsor W µν, which dscrib th currnts at th corrsponding vrtics [5]: d σ dx dq ɛ V L V µν(k, ) W µν V (p, ). (.7) Th subscript V labls th xchangd vctor bosons γ and Z for nutral currnt procsss and W in cas of chargd currnt intractions. Th according couplings of th intrmdiat bosons to th particls involvd in th scattring procss ar givn by ɛ V. Whras th lptonic part of th diagrams is calculabl within th lctrowak thory th tnsor W µν V srvs to paramtris th ignoranc of th form of th currnts at th hadronic vrtx. Exploiting th Lorntz structur of th hadronic tnsor and th rstriction of currnt consrvation th doubl diffrntial cross sction for unpolarisd lctron-proton scattring can finally b xprssd as [4] [ ] d σ p y dx dq = A x F (x, Q ) + ( y) F (x, Q ) (y y ) x F 3(x, Q ) Th factor A varis for nutral and chargd currnt procsss and is givn by. (.8) A NC = ( ) 4π α and A CC = G F M W, (.9) x Q 4 π x Q + MW whr M W dnots th mass of th W boson and G F is th Frmi constant. Th unknown intraction at th proton sid is absorbd in th dfinition of th ralisd proton structur functions F i (x, Q ), which ar snsitiv to th structur of th proton and ar in ral functions of th kinmatics of th scattring procss []. Th ralisd structur functions ar procss dpndnt, sinc thy ncod th contributions arising from th xchang of th various lctrowak bosons. In such a way th xpctd Z propagator is compltly absorbd in cas of nutral currnt intractions. Th structur function F 3 is non-zro only for wak intractions as it is ratd by th parity violating intractions. Typically, th longitudinal structur function F L is introducd as F L (x, Q ) = F (x, Q ) x F (x, Q ), (.) and th doubl diffrntial cross sction for th inlastic lctron-proton scattring is xprssd in trms of th structur functions F, F 3 and F L.

17 .. Inclusiv Elctron-Proton Scattring 9 Th dpndnc of th inlastic cross sctions on Q is principally formd by th propagator trms, which ar givn by /Q 4 for th nutral currnt procss and /(Q + MW ) for chargd currnt scattring. For nutral currnt intractions th contribution arising from th xchang of Z bosons bcoms important at Q > MZ with M Z dnoting th mass of th Z boson. At low valus of Q th lctron-proton intraction is dominatd by th nutral currnt scattring procss, whos cross sction is about thr ordrs of magnitud largr than that of chargd currnt intractions in this kinmatic rgion. But whil th nutral currnt cross sction dcrass vry rapidly with incrasing valus of Q th chargd currnt cross sction falls much lss stply up to valus of Q in th ordr of MW. In this kinmatic rgim, which is in fact th rgim of lctrowak unification, th chargd currnt cross sction masurs up to th cross sction of nutral currnt scattring and th charactristics of both cross sctions bcom similar. For rasons bcoming apparnt in sction..4 th lctron-proton scattring at HERA is rfrrd to as dp-inlastic in th kinmatic rgion of Q > 4 GV. Th lctron-proton scattring is dominatd by procsss with vry small momntum transfrs. In this kinmatic domain th wak intractions mdiatd by Z and W xchang can b nglctd compard to th dominant cross sction of th lctromagntic scattring procss. In th limit Q GV th xchangd photon adopts th charactr of ral photons and th lctron-proton intraction can b considrd as photon-proton scattring. Th lctron-proton scattring is thn rfrrd to as photoproduction, in analogy to th dnotation of th procss of hadron production in th scattring of ral photons on nuclons γ + N X. Th following sction dscribs th approach of virtual photon-proton scattring which allows for th transition from th rgim of dp-inlastic lctron-proton scattring to th rgion of photoproduction...3 Virtual Photon-Proton Scattring Apart from th approach prsntd in sction.. th scattring procss of lctrons on protons can also b dscribd as th convolution of a flux of virtual bosons radiatd from th lctron with th absorption cross sction of a virtual boson by th proton. In th following discussion only th dominant lctromagntic procss, th scattring of virtual photons on protons, is considrd: γ () + P (p) X(p ). Th virtual photon is tratd as a massiv spin particl. Thus th cross sction of th lctron-proton scattring is split into two indpndnt parts, on for th absorption of transvrsly polarisd photons σ T and on for longitudinally polarisd photons σ L [6]: d σ NC dy dq = Γ T σ T (Q, W ) + Γ L σ L (Q, W ) (.) = Γ T [ σt (Q, W ) + ɛ σ L (Q, W ) ] = Γ T σ ff γ p(y, Q, W ). (.)

18 Chaptr. Standard Modl Physics at HERA Th functions Γ T and Γ L dnot th according fluxs of transvrsly and longitudinally polarisd photons and ar givn by Γ T (y, Q ) = ɛ(y) Γ L(y, Q ) = with ɛ(y) = α π Q y ( + ( y) ) (.3) ( y) + ( y). (.4) Th photon polarisation ɛ = Γ T /Γ L is givn by th ratio of th longitudinal to th transvrs photon flux and σ ff γ p = σ T + ɛ σ L is known as th ffctiv γ p cross sction. Th structur functions of th proton F, F and F 3 ar connctd with th absorption cross sctions σ T and σ L by th following rlations: x F = Q 4π α σ T F = Q 4π α (σ T + σ L ) F L = Q 4π α σ L. (.5) Th total virtual photon-proton cross sction σ tot γ p is dfind by th sum of th transvrs and longitudinal absorption cross sctions and is givn by σ tot γ p = σ T (Q, W ) + σ L (Q, W ) = 4π α Q F (Q, W ). (.6) Contrary to th ffctiv γ p cross sction σ ff γ p th total cross sction as drivd abov dpnds only on th variabls Q and W. Within th approach of virtual photon-proton scattring th cross sction for th lctronproton inlastic scattring in th kinmatic rgim of photoproduction at Q GV can b drivd. In th limit Q th longitudinal componnt of th lctron-proton cross sction vanishs, sinc th mdiatd photon adopts th charactr of ral photons, which ar only transvrsly polarisd. Th transvrs absorption cross sction σ T (Q, W ) convrgs to th photoproduction cross sction σ γp (W ). Th diffrntial lctron-proton scattring cross sction as a function of y is givn within this approach by dσ dy = f γ/ σ γp. (.7) Th flux of th photons f γ/ as drivd in th Wizsäckr-Williams approximation [, ] is obtaind by intgrating (.) ovr Q : f γ/ (y, Q max) = α [ ( ) + ( y) ln ( y) ] Q max ( y), (.8) πy m y whr m dnots th mass of th lctron and th valu of Q max is dtrmind by th maximal angl allowd for th scattrd lctron. Compard to th intgral of rlation (.3) th additional trm ( y) within th obtaind xprssion for th photon flux ariss from th considration of trms in th ordr of m.

19 .. Inclusiv Elctron-Proton Scattring..4 Structur Functions in QPM and QCD Alrady in th 96 s th proton was assumd to b composd of a small numbr of pointlik constitunts, th so-calld partons, which hav latr on b idntifid with uarks and gluons. Sinc th structur functions F i paramtris th proton structur, thy ar suitd to xplor th proprtis of th partons. Th physical intrprtation of th structur functions within th Quark Parton Modl (QPM) and th impact of th Quantum Chromodynamics (QCD) ar prsntd in th following. In th lat 96 s Bjørkn prdictd that in th rgim of high nrgis th proton structur functions ar indpndnt of th transfrd momntum Q and functions of th kinmatic variabl x alon 3 : F i (x, Q ) F i (x) Bjørkn scaling. (.9) In 969 Callan and Gross proposd that in cas of scaling structur functions th longitudinal structur function F L would vanish, rsulting from th following rlation btwn F (x) and F (x): F L = x F (x) = F (x) Callan-Gross rlation. (.) Both prdictions hav imprssivly bn confirmd by xprimnts at th SLAC in th arly 97 s and rsult ditly from th intrprtation of th lctron-proton scattring within th Quark Parton Modl, which was proposd by Fynman in 969. Quark Parton Modl In a rfrnc fram whr th proton momntum is larg (infinit momntum fram) th partons composing th proton hav only a small transvrs momntum. Thus th proton is dscribd by a bam of collinar partons, ach carrying a crtain fraction of th total proton momntum. In th Quark Parton Modl th proton is viwd to b composd of thr pointlik partons with spin, idntifid with two up-uarks and on down-uark. Th lctron-proton intraction is xprssd as th incohrnt scattring of th lctron on fr uarks insid th proton, ach carrying th momntum fraction x. Introducing th uark momntum distributions (x) th proton structur functions F i ar givn within this approach by th following rlations [4]: F = f (x) (vf + a f), (.) F = f f x f (x) (v f + a f), (.) F 3 = f f (x) v f a f, (.3) 3 Th so-calld Bjørkn-Limit dnots th transition Q and p with fixd valus for x = Q /( p ). In this kinmatic rgim th lctron-proton scattring is rfrrd to as dp-inlastic.

20 Chaptr. Standard Modl Physics at HERA whr th sum runs ovr all uark and antiuark flavours f which participat in th intraction. Dpnding on th undrlying procss th according vctor and axial-vctor couplings of th lctrowak gaug bosons on th uarks ar givn by v f and a f, rspctivly. For nutral currnt procsss mdiatd by photons only, th couplings v f amount to th chargs of th uarks f whil th axial-vctor couplings a f ual to zro. In cas of chargd currnt intractions th vctor and axial-vctor couplings ar givn by v f = a f = for uarks and v f = a f = for antiuarks. Th couplings hav a mor complicatd structur for nutral currnt intractions involving th Z xchang and its intrfrnc with th photon contribution. Th xplicit xprssions for th structur functions in thos cass ar givn for instanc in [3]. Apparntly, th Bjørkn scaling is ralisd within th Quark Parton Modl, sinc no dpndnc of th structur functions on th momntum transfr Q is drivd. This bhaviour rsults from th assumption, that th proton is composd of pointlik but non-intracting constitunts. Th compltion of th Callan-Gross rlation, which is also ditly dducibl from th givn rlations for th structur functions, is a dit consunc of th considrations of chargd partons with spin. Th Impact of Quantum Chromodynamics Both th Bjørkn scaling and Callan-Gross rlation ar violatd whn laving th naiv approach of th Quark Parton Modl and taking into account th strong intractions btwn th uarks as dscribd by th Quantum Chromodynamics. Within th dscription of QCD th uarks ar bound in th proton du to intractions mdiatd by gluons, involving procsss such as gluon radiation ( g) and th convrsion of gluons into uark-antiuark pairs (g ). Th mission and rabsorption of gluons by th uarks modifis th transvrs uark momnta and allows th uarks to coupl on longitudinally polarisd bosons. Thus th contribution of th longitudinal absorption cross sction σ L (cf. sction..3) can not b nglctd any longr, rsulting in th violation of th Callan-Gross rlation. In addition th radiation of gluons rsults in a logarithmic dpndnc of th structur functions on Q, rprsnting th violation of Bjørkn scaling. As a consunc of th convrsion of gluons into uark-antiuark pairs, which thmslvs may radiat gluons crating furthr uark-antiuark pairs and so on, th proton contnt is modifid. Contrary to th approach of th QPM th proton xists no longr only of thr valnc uarks, but contains also a fraction of gluons and sa-uarks, which carry small momntum fractions x. Dpnding on th momntum of th probing gaug boson this partonic structur of th proton is rsolvd.

21 .. Exclusiv Final Stats 3. Exclusiv Final Stats Th prvious chaptr has concntratd on th inlastic scattring procss of lctrons on protons abstracting away from th composition of th ratd final stat. Th masurmnts of th inclusiv cross sctions giv ditly accss to th proton structur functions, which rval th natur of th partons and allow in particular for th invstigation of Quantum Chromodynamics. Anyhow, th pattrn of prturbativ QCD is mayb vn bttr accssibl in th xploration of th hadronic final stat. Although colourd partons cannot b obsrvd ditly, thir fragmntation producs jts of hadrons collimatd around th primal dition of th partons. In such a way th gluon has bn ditly obsrvd for th first tim in th masurmnt of thr-jt vnts at th PETRA acclrator at DESY in 979. Th invstigation of xclusiv final stats allows for th sarch for signals of nw physics which would manifst thmslvs in ddicatd vnt topologis. Morovr, th varity of final stat topologis rflcts th physics of lmntary particls at larg and thir xploration allows thrwith for a ral vrification of th Standard Modl. Th following paragraphs introduc th thortical foundations on th formation of xclusiv final stats in lctron-proton scattring at HERA... Jt Production In cas of th lctron-proton scattring bing inlastic, th proton is braking up within th probing procss and th final stat is xpctd to includ jts of hadronic particls arising from th partons composing th proton. At th lowst ordr of th strong coupling constant α s, which is actually O(α s), th final stat of a typical dp-inlastic lctron-proton intraction consist of a jt of hadrons originating from th struck uark, calld th currnt or lading jt, which balancs th transvrs momntum of th scattrd lpton. Th rmnant of th proton also fragmnts into hadrons, which rmain in th cntr-of-mass fram collimatd around th primal dition of th incoming proton. Thus th hadronic final stat of vnts of this kind is rfrrd to as bing composd of (+) jts. Th corrsponding diagram is shown in figur. (a) for th dominant lctromagntic p scattring. Th diagrams for procsss involving th xchang of Z or W bosons look similar. At th first ordr of α s (+) jt final stats ar producd. A gluon may split into a pair of uarks with larg rlativ transvrs momnta, bfor on of th uarks absorbs th virtual boson. This procsss is known as Boson-Gluon fusion and shown in figur. (b). Anothr procss rsulting in two high transvrs momntum jts is th so-calld QCD Compton scattring, in which th uark radiats a hard gluon (bfor) absorbing th virtual boson as shown in figur. (c). Evn highr jt multiplicitis ar producd taking into account furthr QCD radiation ffcts. Th production of jts in th inlastic lctron-proton scattring is dominatd by photoproduction procsss. As discussd in sction..3 th incoming lctron can b rgardd

22 γ γ l l P 4 Chaptr. Standard Modl Physics at HERA l l P γ γ γ g γ γ g g γ g P P P P P P (a) (b) (c) Fig..: Diagrams for jt production at lowst (a) and first ordr (b-c) of th strong coupling constant α s for thinlastic lctromagntic p scattring. Th procsss displayd in figur (b) and (c) ar known as Boson-Gluon fusion and QCD Compton scattring, rspctivly. For γ both γ procsss only th t-channl contribution is shown. γ γ g g asp a sourc P of photons, which ar favourd to hav uasi-ral P P charactr du to th charactristics of th photon propagator. Rsulting from th consrvation of four-momntum th production of (+) jt final stats as displayd in figur. (a) is not ralisd in th rgim of photoproduction for photons on th mass shll. As a furthr consunc of its low virtuality th photon may fluctuat bfor th intraction into uark pairs, rsulting in a partonic structur of th photon. Consuntly th photoproduction of jts is dscribd ithr by th dit scattring of a ral photon on a parton of th proton (dit photoproduction) or th intraction btwn on of th partons insid th photon with on of th partons of th proton (rsolvd photoproduction). Apart from th virtuality of th photon, th dit photoproduction of jts is in th sam way as for th dp-inlastic rgim in lading ordr of α s dscribd by th diagrams prsntd in figur. (b) and (c). Although th hard subprocss of th rsolvd photoproduction of (+) jts is in th ordr of αs th impact of th parton dnsity function of th photon provoks both procsss to contribut roughly in th sam ordr of magnitud in th rgim of high transvrs momnta. Th lading-ordr diagrams for th rsolvd photoproduction of jts ar displayd in figur.3. g g.. Photon Production Photons in th final stat of th lctron-proton intraction aris ithr from dit production procsss or from th fragmntation procss of th partons. In th lattr cas th photons ar typically includd in hadronic jts and thus in ral not idntifiabl as isolatd particls. Th following paragraphs dscrib th production of hard photons in lctron-proton scattring.

23 .. Exclusiv Final Stats 5 g g g g g g g g g g g g g g g g g g g g g g Fig..3: Lading-ordr diagrams for rsolvd photoproduction of jts, corrsponding to th parton scattring procsss,, gg and gg gg. Prompt Photon Production In th rgim of photoproduction th production 5 of photons in hard intractions is rfrrd to as prompt photon production. Both dit and rsolvd photoproduction procsss contribut. Th procss of prompt photon production is snsitiv to th partonic structur of th proton and du to th rsolvd componnt also to th parton contnt of th photon. Exampl diagrams of prompt photoproduction at lading ordr ar displayd in figur.4. Radiativ Procsss Apart from th production in hard intractions of photoproduction procsss, hard photons may aris from th nxt-to-lading ordr QED cortions on th lctron-proton scattring procss involving th radiation of ral photons. Figur.5 shows th four diagrams contributing to th nutral currnt scattring, corrsponding to th initial and final stat radiation from both th lctron and uark lins.

24 γ γ γ γ 6 Chaptr. Standard Modl Physics at HERA γ γ γ γ γg γg g γ g γ γ γγ γγ γ/z γ/z (a) (b) (c) γ/z (d) g g γ γ Fig..4: Exampl diagrams for dit (a-b) and rsolvd (c-d) prompt photon production procsss at lading ordr. γ γ/z γ γ γ γ γ γ γ/z γ/z γ/z γ/z γ γ/z γ/z γ γ/z γ γ/z γ γ (a) (b) (c) (d) 6 6 γ/z Fig..5: Th diagrams for ral photon radiation in nutral currnt p scattring. Figur γ γ γ γ (a) and (b) rprsnt th initial and final stat photon radiation from th lctron lin, diagrams (c) and (d) th corrsponding procsss for th uark lin. γ/z γ/z γ γ/z γ γ γ 6 γ g γγ 6 g γγ lins ar supprssd du to th rlativ larg ffctiv uark masss. Th radiation of ral photons procds in both lastic and inlastic scattring procsss. Dpnding on th kinmatics of th intraction th radiatd photon ivs larg trans- vrs g momntum and isg thus ditly obsrvabl g g in th masurmnt. g Th diagrams for chargd currnt intractions look similar, xcpt for th fact that photon radiation from th lctron lin can only procd in th initial stat. Sinc th cross sction for th mission of ral photons dpnds invrsly proportional at last on th logarithm of th mass of th radiating particl, th radiation procsss from th uark Figur.6 shows th diagrams for th initial and final stat radiation procsss in lctromagntic p scattring, now including th assignmnt of th four-momnta to th participating particls. With th following dfinitions γ γ γ of th four-momnta transfrd by th intrmdiat photon and th virtual lctron stats g γ g g γ ˆQ = ˆ = (p p) (.4) g g Q Q = = (k g) (.5) g g = = (k + g) (.6) th doubl diffrntial cross sctions for th initial and final stat radiation procsss in lctromagntic p scattring ar dtrmind by d σ d ˆQ dq ˆQ (Q + m ) and d σ d ˆQ dq ˆQ (Q + m ). (.7)

25 .. Exclusiv Final Stats 7 (k) γ(g) (k) γ(g) (k ) ˆ (k ) ˆ P (p) X(p ) P (p) X(p ) Fig..6: Th diagrams for th initial and final stat photon radiation in lctromagntic p scattring. Th cross sctions ar gtting larg clos to th pols of th virtual photon and virtual lctron stats. Morovr, th siz of th transfrd momnta dtrmins th rlativ transvrs momnta of th scattrd lctron and radiatd photon and thus th vnt signatur in th masurmnt. Basd on th rlativ valus of th transfrd momnta Q, Q and ˆQ th following procsss ar distinguishd: Small Angl Brmsstrahlung (Q, Q 7; ˆQ ) As a rsult of th small transfrd momnta both th scattrd lctron and radiatd photon aris at vry small angls. In th lastic cas p γp th procss is known as Bth-Hitlr procss and is accuratly computabl within th QED. Sinc furthr th vnt rat is larg, Bth- Hitlr vnts ar usd at th H xprimnt to xprimntally masur th luminosity (cf. sction..5). Th according inlastic brmsstrahlung procsss rprsnt radiativ photoproduction procsss. QED Compton Scattring (Q, Q ; ˆQ ) Brmsstrahlung procsss, whr th transfrd momntum ˆQ is small ( < 4 GV ) but th scattrd lctron and radiatd photon mrg at larg angls, ar rfrrd to as QED Compton scattring. Consuntly ths procsss ar also known as wid angl brmsstrahlung. Th QED Compton scattring is dominatd by lastic lctron-proton intractions whr th mdiatd photon faturs uasi-ral charactr. Ths procsss xhibit a typical vnt signatur, sinc thy includ two lctromagntic particls which ar balancd in thir transvrs momnta. Radiativ Dp-Inlastic Scattring (Q, Q > ; ˆQ ) In this kinmatic domain th diagrams shown in figur.6 rprsnt th ral QED cortions at th lctron lin to th dp-inlastic lctromagntic scattring procss. Du to th charactristics of th propagator trms in (.7) th photon mission procds for both th initial and final stat radiation procsss prfrably collinar to th dition of th incoming and scattrd lctron, rspctivly (Q, Q ). If th photon is radiatd in th initial stat th lctron momntum is dcrasd, rsulting in a rduction of th ffctiv cntr-of-

26 γ l γ l γ l γ 8 Chaptr. Standard Modl Physics at HERA γ γ γ γ l l l l γ γ l l γ γ γ γ l l l l γ γ (a) (b) (c) Fig..7: Diagrams for lpton pair production in inlastic lctromagntic p scattring. Figur (a) shows th t-channl l diagram of th two-photon procss. Th radiation pro- csss displayd in figur (b) and (c) ar known as Cabibbo-Parsi and Drll-Yan procss, γ rspctivly. l γ γ γ γ γ l g mass nrgy availabl in th subsunt scattring procss. Th radiatd γ photon mostly P P rmains undtctd in th primal dition of th lctron. In P cas of final stat radiation l P both lctromagntic particls typically form a common nrgy clustr prvnting th xprimntal discrimination btwn th lctron and photon. For both procsss th photon bcoms obsrvabl in th masurmnt with incrasing valus of Q and Q...3 Lpton γ γ Pair Production γ γ γ Lpton pairs ar producd g in lctrowak lctron-proton g intractions in both g lastic g and inlastic scattring procsss. Th dominant production mchanism is th two-photon raction P P as dpictd in figur.7 P (a). Du to th charactristics P P of th photon propagator P th momntum transfrd to th scattrd particls is rally small. Th cross sction of this procss can b calculatd in good approximation from th convolution of th photon fluxs at th lctron and proton sid with th cross sction of th cntral two-photon subprocss. Whil th photon flux from th lctron has alrady bn prsntd in sction..3 th dscription of th flux of photons radiatd from th proton is morγ complicatd sinc th partonic structur of th proton has γ to b takn into account. Its xplicit calculation can for instanc b found in [4]. g g Anothr lss fficint sourc of lpton pairs ar radiation procsss involving th convrsion of P th radiatd photon or Z boson into lpton pairs. Figur P.5 has alrady introducd th four possibl diagrams of ral photon radiation in nutral currnt intractions, corrsponding to th initial and final stat radiation from th lctron and uark lins. In th ral lctrowak cas all photon propagators in th diagrams can b rplacd by th propagator trm of a Z boson. For both th radiation procsss from th lctron and uark lins th initial stat radiation producing lpton pairs dominats th corrsponding final stat procsss. In cas of th boson bing radiatd from th initial lctron, th procss corrsponds to an + γ γ g

27 .. Exclusiv Final Stats 9 annihilation into lpton pairs, in which on of th incidnt lctrons is mittd from a photon radiatd from th proton. This mchanism is known as Cabibbo-Parsi procss and displayd in figur.7 (b). Th siz of th corrsponding cross sction and thrwith its contribution to th total lpton pair production cross sction dpnds on th flavour of th producd lptons. Whn th boson is radiatd from th initial uark lin th undrlying lpton pair production procss can b rgardd as th annihilation of a uark pair into a lpton pair. This procss is rfrrd to as Drll-Yan procss and th corrsponding diagram is shown in figur.7 (c). Its total contribution is small compard with th two-photon and Cabibbo- Parsi procsss, nvrthlss it gains in importanc at high transvrs momnta of th lptons...4 W Production Ral W bosons ar producd in th inlastic lctron-proton scattring ithr via th procss p W X or th procss p ν W X. Th W production is dominatd by th W procss, which has a cross sction roughly an ordr of magnitud largr than th procss p ν W X [5]. Subsunt to its production th W boson dcays into a frmion pair, ithr a chargd lpton and its corrsponding nutrino or a uark pair. At th parton lvl th procss p W X is dscribd by th diagrams prsntd in figurs.8 (a-c) and figurs.8 (f-g) and th crossing rlatd diagrams with an antiuark in th initial stat. Th procsss displayd in diagrams (a) and (b) can b viwd as nutral currnt intractions with an additional radiation of a W boson from th uark in th initial or final stat, rspctivly. Du to th prsnc of both th photon t-channl and th frmion u-channl in diagram (a) this particular production mchanism involving th initial stat W radiation dominats alrady th total cross sction. Diagram (c) includs a W W γ tripl boson vrtx. Diagrams (f) and (g) fatur a W boson radiatd from th lpton lin, both procsss ar strongly supprssd by th prsnc of a scond W propagator trm. Finally, diagrams (d) and () ar ndd in ordr to prsrv lctromagntic gaug invarianc whn considring th subsunt dcay of th W boson into a frmion pair.

28 Chaptr. Standard Modl Physics at HERA W γ/z l l γ/z W l l W W γ/z l l W γ/z l l W γ/z l l W ν W l l W ν W l l 3 W γ/z l l γ/z W l l W W γ/z l l W γ/z l l W γ/z l l W ν W l l W ν W l l 3 (a) (b) W γ/z l l γ/z W l l W W γ/z l l W γ/z l l W γ/z l l W ν W l l W ν W l l 3 W γ/z l l γ/z W l l W W γ/z l l W γ/z l l W γ/z l l W ν W l l W ν W l l 3 W γ/z l l γ/z W l l W W γ/z l l W γ/z l l W γ/z l l W ν W l l W ν W l l 3 (c) (d) () l l γ/z W l l W W γ/z l l l l W γ/z l l l l W ν W l l 3 W γ/z l l γ/z W W γ/z l l W γ/z l l γ W ν W l l W 3 (f) (g) Fig..8: Diagrams for ral W production via th procss p W X, including th subsunt dcay into frmion pairs.

29 Chaptr Th H-Exprimnt at HERA Th Dutschs Elktronn-Synchrotron DESY was foundd in 959 at Hamburg in Grmany as an autonomous foundation of common law with th objctiv of basic rsarch in natural scinc. Whil initially stablishd as a particl physics facility for Grman univrsitis it has dvlopd into a rsarch cntr of intrnational rnown. Th mphass of th prsnt work at DESY ar th invstigation of fundamntal proprtis of mattr and forcs within th scop of high nrgy physics as wll as th rsarch with synchrotron radiation within various filds of activity, such as solid stat physics, matrial scinc, chmistry, molcular biology and mdical scinc. High nrgy physics xprimnts at DESY wr takn up in 964 at th lctron synchrotron, which has givn its nam to th rsarch cntr until today. Up to th bginning of th ightis th rsarch program continud at th + storag rings DORIS and PETRA. On of th most important discovris at DESY, th dit vidnc of gluons, succdd at PETRA by th obsrvation of thr-jt vnts in 979. Both facilitis ar opratd for th rsarch with synchrotron radiation today. Apart from its oprational mod as synchrotron radiation sourc th PETRA collidr is usd as pr-acclrator for th storag ring HERA, which has bn commissiond in 99. Until this day th rsarch in high nrgy physics at DESY is taking plac at HERA, th first and only acclrator colliding lctrons on high-nrgtic protons worldwid.. Th Elctron-Proton Acclrator HERA At HERA lctrons with an nrgy of 7.6 GV collid with protons at an nrgy of 9 GV. Th rsulting cntr-of-mass nrgy of 39 GV xcds thos rachd in prvious xprimnts colliding lptons on fixd proton targts by mor than a factor of tn and provids thrwith a much dpr insight into th proton. As a rsult of th diffrnt particl masss th lctron and proton bams of HERA ar stord in two sparat magntic rings, which ar placd in a tunnl of 6336 m circumfr- Doppl-Ring-Spichr Positron-Elktron-Tandm-Ring-Anlag

30 Chaptr. Th H-Exprimnt at HERA Hall North H p 36m R=779m Hall East HERMES NW p HERA hall wst N NO Hall Wst HERA-B HERA PETRA 36m Volkspark Stadion W SW cryoic hall PETRA magnt tst-hall + -linac PIA DESY II/III - H -linac --linac p proton bypass SO O Hall South ZEUS Fig..: Th lctron proton storag ring HERA (lft) and th pr-acclrators at th DESY trrain (right). Elctrons: linar acclrator LINAC II (45 MV), synchrotron DESY II (8 GV), synchrotron PETRA II ( GV). Protons: linar acclrator LINAC II (5 MV), synchrotron DESY III (7.5 GV), synchrotron PETRA II (4 GV). nc btwn and 5 m blow grad. Bfor bing injctd into HERA th lctron and proton bams ar passd through svral pr-acclrators situatd at th DESY trrain. Exprimntal halls ar locatd at four placs of th HERA tunnl including th dtctors of th xprimnts H (Hall North), ZEUS (Hall South), HERMES (Hall East) and HERA-B (Hall Wst). An ovrviw of th HERA storag ring and its pr-acclrators is prsntd in figur.. Whil th colliding xprimnts H and ZEUS invstigat th high-nrgtic scattring procss of lctrons and protons, ithr th lctron or th proton bam of HERA ar usd by th fixd targt xprimnts HERMES and HERA-B. Th HERMES xprimnt startd oprations in 995 and studis phnomna rlatd to th spin of nuclons in collisions of th longitudinal polarisd lctron bam with polarisd gas targts. Th HERA-B xprimnt finalisd data taking in 3. It studid collisions of high-nrgtic protons with th nucli of targt wirs positiond in th halo of th HERA proton bam. Th xprimnt was optimisd to masur a violation of th combind charg conjugation and parity consrvation (CP) in dcays of B msons. Th achivabl nrgy of th lctron bam is rstrictd by th loss rsulting from synchrotron radiation. Sinc th amount of radiatd nrgy is scaling with th fraction of particl nrgy and mass to th powr of four, no significant incras of th lctron nrgy abov 3 GV can b rachd at HERA. Th proton nrgy is mainly rstrictd by th rachabl strngth of th magntic filds guiding th bam, sinc th nrgy loss du to radiation can b nglctd for havy parti-

31 .. Th Elctron-Proton Acclrator HERA 3 INTEGRATED LUMINOSITY (4.8.) Fig..: Th intgratd luminosity dlivrd by HERA as a function of th running tim during th first data taking priod btwn 99 and (HERA-I). cls 3. Th proton storag ring at HERA is solly uippd with suprconducting magnts in ordr to stor high-nrgtic protons. Th main dipol magnts dlivr a nominal fild of 4.7 T, about thr tims as strong as thos attainabl using normal iron yoks and coppr coils. As a rsult of th acclration by cavity rsonators th lctron and proton bams consist of singl bunchs, ach containing - particls. About 8 bunchs ar stord at th sam tim, rsulting in typical currnts of 4 ma for lctrons and 9 ma for protons. Th dsign bunch spacing at HERA amounts to 96 ns corrsponding to a bunch crossing rat of.4 MHz. Sinc th HERA collidr startd oprations in 99 th dlivrd luminosity has stadily incrasd (cf. figur.). With a spcific luminosity of L =. 3 cm s HERA has xcdd its dsign luminosity in its first data taking priod (HERA-I), which ndd in autumn. In ordr to incras th luminosity by roughly a factor of four and to giv 3 E / E p 3 / assuming ual bam nrgis.

32 4 Chaptr. Th H-Exprimnt at HERA th colliding xprimnts H and ZEUS accss to rar physics procsss, an upgrad of th intraction rgions has bn implmntd providing a strongr focusing of th bams. HERA has succssfully rsumd oprations nd of 3. Th prsntd analysis is basd on th HERA-I data takn btwn 994 and with th H xprimnt.. Th H Dtctor Th basic rsarch objctivs pursud at th H xprimnt ar th masurmnt of th intrnal structur of th proton, th study of fundamntal forcs acting btwn particls as wll as th sarch for nw physics byond th Standard Modl. In ordr to distinguish btwn th wid varity of physics procsss and to obtain accurat masurmnt rsults, th idntification of all raction products must b nsurd. Th univrsal layout of th H dtctor allows for th pis masurmnt of lctrons and photons as wll as muons and hadronic jts. Th particls ar classifid according to th dgr of intraction thy hav with th dtctor matrial. Th analysis of lctron-proton ractions producing wakly intracting particls, such as nutrinos arising from chargd currnt procsss, ruirs a hrmtic dtctor layout in ordr to infr th xistnc of thos particls inditly from th consrvation of nrgy and momntum. As a rsult of th diffrnt bam nrgis th cntr-of-gravity of th lctron-proton systm is boostd in dition of th proton bam, lading to an asymmtric dtctor layout. Figur.3 prsnts an isomtric viw of th H dtctor. Th asymmtric layout is rflctd by th nforcd amount of matrial in th forward dition. Th origin of th H coordinat systm is th nominal lctron-proton intraction point, with th dition of th proton bam dfining th positiv z-axis (forward rgion). Th x-axis and th y-axis point towards th cntr of th HERA ring and in upward dition, rspctivly. Th following sction givs an ovrviw of th layout of th H dtctor. Subsuntly th main dtctor componnts ar dscribd in mor dtail... Gnral Layout Th univrsal dtctor of th H xprimnt [6] is dsignd in ordr to ord th maximum numbr of lctron-proton ractions and to dtct as many of th raction products as possibl. Th intraction point whr th particl bams collid is thus almost ntirly surroundd by layrs of highly snsitiv dtcting uipmnt. Th innrmost layr of th H dtctor is build up by a systm of tracking dvics, which dtcts th intraction point as wll as th dcay positions of short livd particls with high pision and ords th tracks lft bhind by chargd particls. A magntic fild dflcts th flight dition of thos chargd particls and allows thir momntum to b dtrmind. Th innr tracking systm, subdividd into a forward (fig..3, (3)), cntral

33 .. Th H Dtctor 5 Siz: 5 m 3 Wight: 8 t Bam pip and bam magnts Cntral track dtctors 3 Forward track dtctors 4 Elctromagntic LAr calorimtr 5 Hadronic LAr calorimtr 6 Suprconducting coil (.5 T) 7 Compnsating magnt 8 Hlium supply for 7 9 Muon chambrs Instrumntd iron yok Forward muon toroid Backward calorimtr and drift chambr 3 PLUG calorimtr 4 Concrt shilding 5 LAr cryostat Fig..3: Isomtric viw of th H dtctor at HERA.