We present the recent advances in the analysis of the machine-induced background generation and formation at the LHC. Different aspects of the study of the machine background problem at the LHC are reviewed, including the background production at the different stages of the machine operation, the role and influence on the background from the collimators in the experimental insertions and the background shielding. The potential use of the machine background for the purposes of detector testing and alignment is also discussed

Azhgirey, L S

Baishev, I S

Macina, D

Potter, KM

Talanov, V

Tsesmelis, E

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics   Administrative Secretariat LHC Division CERN CH - 1211 Geneva 23 Switzerland  Geneva, 08 August 2006   Large Hadron Collider Project   LHC Project Report 952 THE STUDY OF THE MACHINE-INDUCED BACKGROUND AND ITS APPLICATIONS AT THE LHC I.Azhgirey 1, I.Baishev 1, D.Macina 2, K.M.Potter 2 , V.Talanov 3 and E.Tsesmelis 2  Abstract  We present the recent advances in the analysis of the machine-induced background generation and formation at the LHC. Different aspects of the study of the machine background problem at the LHC are reviewed, including the background production at the different stages of the machine operation, the role and influence on the background from the collimators in the experimental insertions and the background shielding. The potential use of the machine background for the purposes of detector testing and alignment is also discussed. 1 IHEP, Protvino, Russia 2 TS Division, CERN, Geneva, Switzerland 3 TS Division, CERN, Geneva, Switzerland (on leave from IHEP, Protvino, Russia)  Presented at the Tenth European Particle Accelerator Conference (EPAC 2006) 26-30 June 2006, Edinburgh, United Kingdom  THE STUDY OF THE MACHINE-INDUCED BACKGROUND AND ITSAPPLICATIONS AT THE LHCV. Talanov , I. Azhgirey, I. Baishev, IHEP, Protvino, Russia,D. Macina, K.M. Potter, E. Tsesmelis, CERN, Geneva, SwitzerlandAbstractWe present the recent advances in the analysis of themachine-induced background generation and formation atthe LHC. Different aspects of the study of the machinebackground problem at the LHC are reviewed, includingthe background production at the different stages of the ma-chine operation, the role and influence on the backgroundfrom the collimators in the experimental insertions and thebackground shielding. The potential use of the machinebackground for the purposes of detector testing and align-ment is also discussed.INTRODUCTIONThe formation of the machine induced background in thestructure of the machine depends on the combination of thedifferent parameters of the operation such as machine op-tics, beam intensity, residual gas pressure, cleaning ineffi-ciency, and for any experimental insertion is defined by thedistribution of the sources of background with respect toa particular interaction point. In addition the backgroundrates at the entrance to the experimental zones strongly de-pend on the mechanical layout of the straight section partsclose to the interaction point.The high luminosity insertion regions IR1 and IR5 ofthe Large Hadron Collider at CERN [1] house ATLAS andCMS experiments that will operate at the nominal lumi-nosity of L = 10  cm s . The heavy shielding at thetunnel entrance to the zones of these experiments well pro-tects the detectors from the secondary radiation from thetunnel down to a distance close to the beam. At the sametime, at the machine start-up with the reduced luminosityin the IP, the use of the high energy muon component ofthe machine induced background is considered as one ofthe options for the commissioning of the detectors in bothATLAS and CMS [2].In the LHC experimental insertions IR2 and IR8 twodedicated ion and B-physics experiments, ALICE andLHCb, are located. They will operate at the moderate lu-minosities of L = 3 10  cm s  and 2 10  cm s respectively. At the nominal LHC beam intensity the prob-lem of the machine induced background can be consideredmore relevant for these experiments. The betatron cleaninginsertion IR7 is located upstream from IR8 in the directionopposite to the LHC beam 1 and another cleaning insertionIR3 — upstream from the IR2. Vadim.Talanov@ihep.ruFigure 1: Charged hadron and muon flux (left) and fluxdensity (right), at the UX15 entrance due to the beam-gaslosses in SS1L (blue) and sectors 78-81 of the LHC (red)with two tertiary collimators in the IR1 for the case of ma-chine start-up with the beam of 0.01 A.MACHINE BACKGROUND AT THE LHCCOMMISSIONINGThe first estimates of the machine background for thepurpose of the detector alignment in IR1 and IR5 were ob-tained by scaling the background rates, calculated for theconditions of the LHC nominal operation, to the cases ofthe lower beam intensity. Since then, the calculations of theresidual gas pressures in the LHC experimental insertionswere completed for different cases of the machine condi-tioning with a beam of reduced intensity [3]. Correspond-ing estimates of the machine background from the beam-gas losses in the IR1 insertion were done for the case of themachine commissioning with the beam current of 0.01 A(start-up with 43 bunches) [4].Calculated radial distributions of charged hadron andmuon flux are given in Figure 1. These distributions clearlyreflect the structure of the inner shielding of the machinetunnel entrance to the UX15 cavern as shown in [5]. Thetotal flux is estimated to be 2.2 kHz for all hadrons and-200-1000100200300-300 -200 -100 0 100 20010-210-111010 2x, cmy, cm-200-1000100200300-300 -200 -100 0 100 20010-210-111010 2x, cmy, cmFigure 2: Hadron flux density [particles/cm/s] at the entrance to the UX85 cavern without (left) and with (right) theshielding, for the halo losses on the vertical collimator at the IR7 side of IR8 and assumed beam lifetime of 30 h.66.1 Hz for muons that is a factor of  10  lower than thepreviously estimated levels for the nominal operation [6].These estimates were obtained with two tertiary collima-tors TCTV/H which will be installed on both sides of theLHC interaction points [1].On the same Figure 1 are given the distributions of thebackground fluxes due to the beam-gas losses in the twosectors 78-81 upstream from the IR insertion. The as-sumed pressure level in these cold sectors of the LHC weretaken 30 times higher than the corresponding value for theQ6 magnet in IR1 from [3], following the somewhat pes-simistic estimate given in [7], based on the conservative as-sumptions on the beam screen pumping speed and photoncritical energy, flux rate and desorption. The observed con-tribution to the background fluxes in IR1 from the lossesin the cold sectors was estimated to 1.13 kHz (55 %) forhadrons and 183 Hz (374 %) for muons.The comparison of those new estimations for the parti-cle fluxes with the earlier simulations [6] showed that up to90 % of the protons out-scattered in the sectors 78-81 fromthe beam which were previously lost on the limiting aper-tures of the IR1 are now intercepted by tertiary collimators.The resulting cascades that are initiated in the material ofthe collimator jaws become an additional source of   and flux which then decay to muons in the downstreamdrift space. The maximum of the observed difference be-tween the muon fluxes from the two studied sources is ob-tained at the distance from the beam line corresponding tothe outer radius of the UX15 inner shielding fixed tube FTwhere the secondary particles are attenuated only by theconcrete layer. The resulting muon flux constitutes up to80 % of the total muon machine background and becomesthe dominant background component in the insertion.BACKGROUND SHIELDING IN THE LOWLUMINOSITY IR’SThe shielding at the tunnel entrance to the experimen-tal cavern is one of the ways required to reduce the ma-chine background rates in the LHC experiments. In theLHC high luminosity insertions this role is performed byan inner/forward shielding located inside the experimentalzones around the TAS absorber and the first inner tripletmagnet, to protect the outer detectors of the experimentsfrom the backsplash from these machine elements. Inthe low luminosity insertions, due to the mechanical con-straints of the installation such shielding has to be posi-tioned in the LHC tunnel.The model of such shielding for the IR8 experimental in-sertion was proposed in [8], and now the final configurationof this shielding whose design was done compatible withthe overall structure of the machine elements and equip-ment is being installed. This realistic design was intro-duced in the model of the machine background simulationsfor IR8, and the efficiency of the shielding was evaluatedversus the dominant source of the background in IR8 in thecase of a reduced beam lifetime [2] — the background fromthe tertiary losses on the collimators at the IR7 side of IR8.Simulated hadron flux density distributions at the en-trance from the IR7 side to the UX85 experimental cavernof the interaction point IP8 are given in Figure 2, with andwithout the background shielding in IR8. The total rate ofhadrons without the shielding was estimated to 9.2 MHz,for the assumed collimation beam lifetime of 30 h and cor-responding rate of primary losses in the collimation sys-tem of 2.8 10 protons/s. The installation of the shieldingdecreases the hadron background rate to 0.27 MHz, effec-tively removing charged hadron component of the back-ground and leaving only the background fluxes close to theposition of the beam unaffected.MACHINE BACKGROUND IN THE LSS’SBesides the case of the machine background formationalong the LHC beam, incoming to the interaction point inthe experimental insertion, we consider a few cases of thebackground, generated in the LHC long straight sections(LSS’s) along the beam, outcoming from the IP. We esti-mate the beam-gas background in the sensitive areas of theRoman Pot detectors that will be located in the machinetunnel in the IR5 experimental insertion. We also studythe variation of the beam-gas background in the LSS’s forthe case of an accidental leak or local pressure bump in theLHC vacuum chamber, for the purpose of vacuum diagnos-tics with the radiation monitors.A set of Roman Pot detectors will be installed in IR5 forthe forward physics programme of the TOTEM experimentat the LHC [9]. To estimate the background rates in thesedetectors, the correct identification of machine backgroundsources in the region of detector installation is required.The analysis of the limiting apertures in IR5 showed thatthe p-p interactions in IP5 and the beam-gas losses on themachine sector between IP5 and Roman Pot stations hadto be considered as the sources of the background. Esti-mates for the background from the p-p interactions duringthe TOTEM runs are obtained by scaling the numbers from[10] to the luminosity of 2.4 10cm /s.For the beam-gas background during TOTEM operationdetailed simulations were performed [11]. Backgroundrates were estimated for the specially developed high- optics of the machine, most recent profiles of the resid-ual gas density in IR5, calculated in [3] for the period ofmachine operation with the reduced beam intensity andthe latest layout of the experimental insertion. Estimationsshowed that at the luminosity given above, the backgroundrates from the two sources considered are on the same levelof a few kHz. Full information on the background tracks,recorded in these simulations, allowed a thorough analysisof the background and the development of methods for thereduction of the background trigger rates [12].For the purposes of vacuum diagnostics at the LHC weconsider the possibility to evaluate the gas density profilein the room temperature part of the LSS via the variationin the radiation levels, caused by the beam-gas backgroundin the insertion. We introduce a local pressure bump inthe model of the LHC vacuum chamber and compare theestimates of the resulting background produced in protoninteractions with the residual gas nuclei with the particlerates from the other sources of the secondary radiation inthe LSS [13].The estimates showed that the background producedalong the beam, outcoming from the interaction point, fromthe local pressure bump in the region of the separationdipoles is a factor  100 higher than the background levelfor the predicted gas pressures during stable machine oper-ation, in a few locations of IR5. Positioning of the radiationmonitors in these locations would allow to detect this localincrease of gas pressure in a reasonable time from a few toseveral minutes, during injection, ramp and squeeze.REFERENCES[1] O. Bru¨ning, P. Collier, P. Lebrun et al. (editors), “LHC De-sign Report. Volume I The LHC Main Ring”, CERN 2004-003, Geneva, 2004.[2] G. Corti and V. Talanov, “Aspects of Machine InducedBackground in the LHC Experiments”, In: Proc. of the LHCProject Workshop — Chamonix XV, 2006, p.179–185.[3] A. Rossi, “Residual Gas Density Estimations in the LHCInsertion Regions IR1 and IR5 and the Experimental Re-gions of ATLAS and CMS for Different Beam Operations”,CERN LHC Project Report 783, Geneva, 2004.[4] V. Talanov, “Estimation of the Machine Induced Back-ground for the Commissioning Period with Tertiary Colli-mators in the IR1 of the LHC”, CERN LHC Project Note371, CERN, Geneva, 2005.[5] I. Azhgirey, I. Baishev, K.M. Potter et al, “Machine InducedBackground in the High Luminosity Experimental Inser-tion of the LHC Project”, CERN LHC Project Report 722,Geneva, 2004.[6] I. Azhgirey, I. Baishev, K.M. Potter et al., “Cascade Simula-tions for the Machine Induced Background Study in the IR1of the LHC”, CERN LHC Project Note 324, Geneva, 2003.[7] A. Rossi, “Expected Vacuum Performance in the ColdArcs”, LHC Machine Induced Background Work-ing Group meeting, CERN, Geneva, June 2005, 	  	[8] I. Azhgirey, I. Baishev, K.M. Potter et al., “Evaluation ofSome Options for Shielding from Machine Induced Back-ground in the IR8”, LHC Project Note 307, CERN, Geneva,2002.[9] The TOTEM Collaboration, “TOTEM Technical Design Re-port: Total Cross Section, Elastic Scattering and DiffractionDissociation at the Large Hadron Collider at CERN”, CERNLHCC 2004-002, Geneva, 2004.[10] A. Drozhdin, D. Macina, N. Mokhov et al., “AcceleratorRelated Backgrounds in the LHC Forward Detectors”, In:Proc. of the PAC’2003, Portland, OR, May 12–16 2003,p.1742–1744.[11] V. Avati, M. Deile, D. Macina et al., “First Results of theMachine Induced Background Estimation for the ForwardPhysics Detectors in the IR5 of the LHC”, CERN LHCProject Note 360, Geneva, 2004.[12] M. Deile, “Beam–Gas Background Studies for theTOTEM Roman Pots”, LHC Machine Induced BackgroundWorking Group meeting, CERN, Geneva, June 2005, 	  	[13] V. Baglin, V. Talanov, T. Wijnands, “Radiation Monitors asa Vacuum Diagnostics for the Room Temperature Part of theLSS”, CERN LHC Project Note 378, Geneva, 2006.

The study of the machine-induced background and its applications at the LHC

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