FORWARD CALORIMETRY AT ILC. I. Božović-Jelisavčić. Vinča Institute of Nuclear Sciences, M. Petrovića Alasa 12-14, Belgrade, Serbia

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1 Anall Univrsităţii d Vst din Timişoara Vol. L, 2007 Sria Fizică FORWARD CALORIMETRY AT ILC I. Božović-Jlisavčić Vinča Institut of Nuclar Scincs, M. Ptrovića Alasa 12-14, Blgrad, Srbia Abstract Th vry forward rgion of a dtctor at th Intrnational Linar Collidr (ILC) is particularly challnging ara for instrumntation. Forward calorimtrs should provid luminosity masurmnt with th high prcision, tuning of th bam paramtrs, dtctor hrmticity at lowst polar angls and shilding of th cntral dtctors from backscattrd particls. High-nrgy lctron idntification is particularly important to vto Standard Modl background to nw particl sarchs. In addition, xtrm radiation hardnss in rquird for th innrmost calorimtr du to svral tns of TV dpositions of bamstrahlung rmnants pr bunch crossing. Kywords: ILC, luminosity masurmnt, bam diagnostics 1. Introduction Intrnational Linar Collidr is a proposd futur intrnational particl acclrator. It will crat high-nrgy particl collisions btwn lctrons and positrons, and provid a tool for scintists to addrss many of th most complling qustions lik ons rlatd to lctrowak symmtry braking, unification of forcs, dark mattr, spac-tim with additional dimnsions and many othrs. It is dsignd [1] to achiv an initial cntr-of-mass nrgy up to 500 GV with th ability to upgrad to 1 TV, and th nominal luminosity of cm -2 s -1. ILC will work in complmnt with th LHC (Larg Hadron Collidr) at CERN ddicatd primarily to discovr Higgs bosons and SUSY particls, whil ILC will provid prcis masurmnts of charactristics of ths particls (masss, quantum numbrs, couplings) sinc a collision btwn an lctron and a positron is much simplr than a collision btwn many quarks, antiquarks and gluons confind in a proton. In 2007 th Rfrnc Dsign Rport of ILC has bn dlivrd with an Enginring Dsign Rport xpctd in Th vry forward rgion of ILC (blow 150 mrad) will accommodat thr sub-systms in dcrasing ordr of polar angl: th luminosity calorimtr (LumiCal) for prcision luminosity masurmnt, th bam calorimtr (BamCal) for bamdiagnostics and highnrgy lctron idntification clos to th bam pip and th bamstrahlung photons monitor (GamCal) also for bamdiagnostics and instantanous luminosity masurmnt. 23

2 Instrumntation of th vry forward rgion is facing particular challngs lik high prcision in polar angl rconstruction, fficint particl idntification, radiation hardnss, high occupancis and a fast rad-out. Th layout of th forward rgion of th LDC dtctor, as considrd in th prsnt study, is givn in Figur 1 [2]. Figur 1. Layout of th vry forward rgion of th LDC dtctor. BCal, LCal, HCAL, ECAL, TPC, QUAD and LHCAL ar dnoting Bam Calorimtr, Luminosity Calorimtr, Hadronic Calorimtr, EM Calorimtr, Tim Projction Chambr, final quadrupol and a possibl lowangl hadronic calorimtr. 2. Luminosity Calorimtr A cylindrical lctromagntic calorimtr has bn dsignd for masurmnt of th total luminosity. It is a compact highly granulatd sandwich calorimtr (Figur 2 [2]) consisting of 30 longitudinal layrs of silicon snsor followd by tungstn absorbr and th intrconnction structur. Each dtctor layr corrsponds to about on radiation lngth. It is locatd at z = 2270 mm from th IP, covring th polar angl rang btwn 44 and 155 mrad, for th 14 mrad crossing-angl btwn th bams. In addition, dtctor is placd around th outgoing bam in ordr to avoid phi-dpndncy of th luminosity rlativ rror. Figur 2. Silicon snsor layout for th LumiCal. 24

3 Prcision of luminosity masurmnt is drivn by physics rquirmnts for th crosssction masurmnts (i.. th total hadronic cross-sction at Z 0 rsonanc, 2-frmion production at high nrgy) and prcision EW masurmnts (EWSB - anomalous gaug boson couplings) rquiring L / L bing of ordr of 10-4 to Intgratd luminosity at ILC will b dtrmind from th total numbr of Bhabha vnts producd in th accptanc rgion of th luminosity calorimtr and th corrsponding thortical cross-sction (1). L N th int = (1) σ B Bhabha scattring at small angl is prcisly calculabl in QED ( σ th/ σ th 10-4 ) and has a sufficintly larg cross-sction to dlivr high statistics for luminosity masurmnt of th rquird prcision. With th cross-sction of approximatly 4 nb in th luminosity calorimtr angular rang, at 500 GV cntr-of-mass nrgy and th nominal ILC luminosity of cm -2 s -1, about 10 9 vnts will b collctd pr yar, corrsponding to th statistical rror of ordr of Howvr, on has to provid prcision particl rconstruction and, in addition, to control numrous systmatic ffcts. Th cross-sction is falling stply with incrasing polar angl as 1/θ 3, causing luminosity masurmnt to b snsitiv to th dtctor aprtur θ min (or corrsponding innr radius of th luminosity calorimtr) (2). L θ 2 L θ min Two paramtrs ar dscribing quality of th position rconstruction: bias θ and rsolution of th polar angl masurmnt σ(θ). Suggstd dsign of th LumiCal snsors has bn provn through simulation to provid bias and polar angl rsolution bttr thn 10-3 and 10-2 mrad rspctivly. (2) Figur 3. Rlativ rror on luminosity as a function of polar angl rsolution. 25

4 Dpndnc of th rlativ rror of luminosity on polar angl rsolution is givn in Figur 3 [2]. At th sam tim, numrous sourcs of systmatic uncrtaintis hav to b controlld: position and alignmnt of th dtctor, background from physics procsss (4-frmion production via Nutral Currnt), bam-bam intraction ffcts. Tabl 1 [2], summarizs rquirmnts on dtctor prcision and alignmnt paramtrs. Tabl 1. Rquirmnts on luminosity calorimtr position and alignmnt. Dtaild studis [3] showd that bam-bam intraction rsults in ffctiv supprssion of th Bhabha cross-sction of ordr of a prcnt, at 500 GV cntr-of-mass nrgy. It also implis asymmtric slction of Bhabha vnts in ordr to accommodat for th nhancd Bhabha acolinarity du to EM dflction. As can b found in [4], with such slction it is still possibl to kp th rror of luminosity masurmnt originating from uncrtainty of th numbr particls from physics background misidntifid as Bhabhas at th lvl of Bam Calorimtr and Gamma Calorimtr As luminosity calorimtr, bam calorimtr is also compact silicon-tungstn sandwich calorimtr, consisting of 30, on radiation lngth thick, layrs of snsor and absorbr plans. Snsor plans ar sgmntd into pads of Molièr radiuss. Elctrons and positrons, originating from convrsion of bamstrahlung photons, carrying svral 10 TV of nrgy pr bunch crossing, will hit bam calorimtr. Figur 4. Annual dpositions at BamCal snsor plan from pairs convrtd from bamstrahlung. [5] 26

5 On th top of widly sprad dpositions from bamstrahlung (Figur 4), it is important to fficintly idntify a high-nrgtic lctron. Standard Modl procsss lik + γ l l can fak SUSY procss + + χ 0 χ 0 l l du to th loss of spctator lctron along th bam pip. In addition, corrsponding cross-sctions for SM 2-photon procsss and th SUSY signal ar of ordr of nb and fb rspctivly. Figur 5. Elctron idntification fficincy and simulation on 2-photon lctron in th BamCal on th top of bamstrahlung rmnants [2]. Bam calorimtr is also providing masurmnt of th bam paramtrs basd on obsrvabls lik total dpositd nrgy, first radial momnt, lft/right and up/down asymmtris of bamstrahlung dpositions, by mploying (3) O=O nom +T B (3) whr O and O nom ar matrics of obsrvabls for changd and nominal bam paramtrs, B is a chang of bam paramtrs from its nominal valus and T is a Taylor xpansion matrix. Moor-Pnros Mthod is applid to obtain T -1. This information can b usd for a fast luminosity optimization by providing it to th bam dlivry fdback systm. Th Gamma Dtctor GamCal is providing complmntary information to th BamCal on bamdiagnostics. By xploiting ida that th small fraction of bamstrahlung photons will convrt into Bth-Hitlr pairs, with th ratio of BH pairs to bamstrahlung photons proportional to th numbr of lctrons in th bam, GamCal is providing information on instantanous luminosity. In addition, nrgy of Bth-Hitlr pairs and bamstrahlung gammas ar snsitiv to shap and offst of bunchs thus srving for supplmntary bamdiagnostics. Tabl 2 illustrats diagnostics of th basic bunch paramtrs by simulation of th bamstrahlung rmnants in th BamCal [2]. Dpositions of high radiation doss from convrtd pairs rquir radiation hard snsors oprational, in trms of linarity of th rspons, CCD dgradation, tc, aftr absorbing 27

6 svral MGy pr yar. Svral matrials ar undr study, including poly and mono CVD, GaAs and radiation-hard Si. Tabl 2. Prcision of dtrmination of bunch sizs with th BamCal. Prcision Bam paramtr Nominal valu Had-on 2 mrad 20 mrad σ x [nm] σ y [nm] σ z [µm] Conclusions Th FCAL Collaboration dvlops dtctors in th vry forward rgion of th ILC. FCAL dtctors ar going to provid prcision luminosity masurmnt (LumiCal), fficint particl rconstruction rlvant for nw physics sarchs (BamCal), bamdiagnostics (BamGal and GamCal) and luminosity monitoring (GamCal). MC simulations ar providing clar undrstanding of implications of dtctor dsign on physical masurmnts. At th sam tim, systmatic ffcts such as physics background, bam-bam ffcts, tc. hav to b wll undrstood. Acknowldgmnts Prsntd work rsultd from a joind ffort of th FCAL Collaboration ( Rfrncs 1. T. Ab t al., Intrnational Linar Collidr Rfrnc Dsign Rport, Volum 4: Dtctors, August 2007, 161pp, ISBN H. Abramowicz, I. Bozovic-Jlisavcic, M. Pandurovic t al, Rport for th ILC Dtctor R&D Panl: Instrumntation of th Vry Forward Rgion, May 2007, 78 pp. 3. C. Rimbault, Limitation on prcision luminosity masurmnt from bam-bam ffcts, Europan LC Workshop, January 2007, Darsbury, Unitd Kingdom. 4. M. Pandurovic, I. Bozovic-Jlisavcic, Physics background as a systmatic ffct in luminosity masurmnt at ILC, to b publishd in th Procdings of th LCWS07, Hamburg, Grmany, May 30 - Jun 04 (2007). 5. C. Grah, Bamdiagnostics using BamCal, FCAL Workshop LAL Orsay, Novmbr 5,