The problem of background generation in the experimental insertion IR5 of the LHC during machine operation in the dedicated TOTEM mode with low intensity beams and the specially designed Î²* = 1540 m optics is discussed. The sources of the machine-induced background in the IR5 forward physics areas are identified and their relative importance is evaluated. The results of the background simulation in the IR5 are presented, based on the most recent estimates of the residual gas density for TOTEM beam conditions. The methods for background analysis and rejection are explained

Avati, V

Deile, M

Macina, D

Talanov, V

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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 954 ESTIMATION AND ANALYSIS OF THE MACHINE-INDUCED BACKGROUND AT THE TOTEM ROMAN POT DETECTORS IN THE IR5 OF THE LHC V.Avati 1, D.Macina 2, M.Deile 3 and V.Talanov 4  Abstract  The problem of background generation in the experimental insertion IR5 of the LHC during machine operation in the dedicated TOTEM mode with low intensity beams and the specially designed β* = 1540 m optics is discussed. The sources of the machine-induced background in the IR5 forward physics areas are identified and their relative importance is evaluated. The results of the background simulation in the IR5 are presented, based on the most recent estimates of the residual gas density for TOTEM beam conditions. The methods for background analysis and rejection are explained. 1 Case Western Reserve University, Cleveland, OH, USA 2 TS Division, CERN, Geneva, Switzerland 3 PH Division, CERN, Geneva, Switzerland 4 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  ESTIMATION AND ANALYSIS OF THE MACHINE-INDUCEDBACKGROUND AT THE TOTEM ROMAN POT DETECTORSIN THE IR5 OF THE LHCV. Talanov , IHEP, Protvino, Russia, M. Deile, D. Macina, CERN, Geneva, Switzerland,V. Avati, Case Western Reserve University, Cleveland, OH, USAAbstractThe problem of background generation in the experi-mental insertion IR5 of the LHC during machine operationin the dedicated TOTEM mode with low intensity beamsand the specially designed    = 1540 m optics is discussed.The sources of the machine-induced background in the IR5forward physics areas are identified and their relative im-portance is evaluated. The results of the background sim-ulation in the IR5 are presented, based on the most recentestimates of the residual gas density for TOTEM beam con-ditions. The methods for background analysis and rejectionare explained.SIMULATION APPROACHThe physics programme of the TOTEM experiment atLHC requires the measurement of elastic p-p scatteringdown to a four-momentum transfer of  10  GeV [1].A set of Roman Pot detectors will be located in the inser-tion region IR5 along the beam inside the LHC tunnel sym-metrically on both sides of the interaction point IP5. Eachside of IP5 is featured by strong fields in the inner triplet,heavy shielding and the TAS absorber with a small me-chanical aperture [2]. For the background in the detectorson Beam 1 only those interactions of beam particles withthe residual gas that occur on the machine sector betweenupstream TAS and the Roman Pot station need be taken intoaccount. Simulation of beam-gas collisions and subsequentsecondary cascades was performed using the methodicalapproach developed for the solution of the radiation prob-lems in the LHC [3]. Technical details of simulations forRoman Pot detectors can be found in [4].ROMAN POT DETECTORS IN IR5The mechanical layout of the right part of IR5 longstraight section whose model was used in simulations isshown in Figure 1. Three locations marked as XRP1–3 areforeseen for Roman Pot stations with their positions deter-mined in an interplay with the development of the high-   optics and by the requirements of integration with otherLHC components. The estimations of machine inducedbackground are presented for a plane at 220 m from theIP5, in the region of the Roman Pot station XRP3.Each Roman Pot station is composed of two units, sep-arated by a distance of 4 m, with each unit consisting of Vadim.Talanov@ihep.ruFigure 1: Mechanical layout of IR5 right straight sectionpart with the locations of Roman Pot stations XRP1-3.two pots that move vertically and one that moves horizon-tally. The model of a Roman Pot detector insertion and asketch of the detector positions in one unit are given in Fig-ure 2 where  10+0.5cm indicates the nominal distancebetween the Roman Pot detectors and the beam.Figure 2: 3D model of Roman Pot (right) and a sketch (left)showing the overlap of two vertical and one horizontal pot.MACHINE CONDITIONS AT TOTEMOPERATIONOne of the requirements for the study of elastic anddiffractive proton scattering at the LHC is the possibilityto measure scattering angles down to a few rads. For thispurpose a specific option of parallel-to-point optics was de-signed for the TOTEM operation in IR5 of the LHC [1].Optics functions for this solution are given in Figure 3 com-pared to the nominal optics of IR5. This optics provides thephase advance of /2 at the location of the detectors, allow-ing the position of the scattered proton in the detector to beobtained independently of its transverse position at the in-teraction point. The required phase advance is achieved ata distance of 220 m from the IP5 in both planes.The minimal detectable angle is proportional to   so for a nominal emittance a value of     greater than1000 m is required. With the increase of the upstream Q6quadrupole gradient to 3.4 % above the nominal setting avalue of    = 1540 m can be obtained. This optics op-tion was introduced in the model of simulations. Sincethis high-   optics differs significantly from the nominalFigure 3: Functions for IR5 nominal optics (    = 0.55 m)(left) and special high-   optics (right) of LHC beam 1.optics for high luminosity insertions, the formation of themachine induced background in the IR5 during TOTEMoperation is different from the nominal machine runningscenario.The density of gas components defines the rate of inter-actions between beam protons and the nuclei of residualgas. Gas density distributions in LHC experimental inser-tions for the latest design of the LHC vacuum chambersand pumping system, different machine filling schemes andperiods of operation are given in [5]. The estimations ofgas densities for the machine start-up period were usedin the simulations, with 156 bunches and 1.1510 pro-tons/bunch which gives the value of 35 mA for the current.Corresponding profiles of gas density are given in Fig-ure 4. The vacuum chamber inside the cold mass of themagnets will be cooled down to 1.9 or 4.5 K while in theinterconnections the cold bore will operate at 10 K. Therea high pressure of hydrogen is predicted, represented bythe spikes of the Hcurve in Figure 4, since this gas com-ponent will not be pumped onto the surface of the coldbore in these regions. The relatively high density of othergas components there is determined by higher desorptionFigure 4: Profile of residual gas density components in IR5for the period of machine start-up with nominal bunch in-tensity and 156 bunches [5].yields from the unbaked surfaces. The methane is the dom-inant component in the room temperature regions sinceNEG coating there does not pump CH. Other peaks showthe locations of uncoated and unbaked elements of vacuumchamber like stainless steel bellows and pumping ports.PARTICLE FLUXES AND SPECTRAThe results of the simulations were particle distributionsrecorded at the scoring plane at 220 m with a 100 keVthreshold on kinetic energy, including only those producedin the interactions with the residual gas or in the subsequentcascades. Concerning the proton flux, this excludes the un-perturbed primary beam particles. Figure 5 gives particleflux maps on the cross-section of the Q5–Q6 warm vac-uum chamber. Charged hadrons are clearly focused anddefocused by quadrupoles of the insertion in vertical andhorizontal planes, while neutrons are centered around thebeam position.Radial distributions of hadron fluxes and spectra for allparticle types considered in these simulations are given inFigure 6. The peak flux density at the center of the beam isa few kHz/cm for all components while pions start to dom-inate in the background flux at high radii. At a distance of3–4 cm from the beam the density of  flux exceeds theflux density of protons and neutrons by about an order ofmagnitude. The energy distribution of protons is featuredby the diffractive peak at an energy close to 7 TeV. Thepion spectra are clearly dominated by the first generationparticles which are produced directly in the primary protoninteraction on gas nuclei and have a resulting energy of 1TeV. Since  also dominate in the radial distribution ofparticle flux, this high energy component is the most im-portant part of the machine induced hadron background atthe location of the Roman Pot station at 220 m.BACKGROUND REJECTIONTo evaluate the contamination of the Roman Pot triggerswith the background, several rejection criteria were appliedon the particle distributions, shown on the Figures 5 and 6:-4-3-2-101234-4 -2 0 2 4x, cmy, cm-4-3-2-101234-4 -2 0 2 411010 210 3x, cmy, cmFigure 5: Particle flux density [particles/cm/s] for chargedhadrons (left) and neutrons (right) inside the warm vacuumchamber at the location of the Roman Pot station at 220 m.Figure 6: Particle flux density (left) and particle spectra (right) calculated for the region of the Roman Pot station at 220 m.(1) Simple pot cut Particle must be seen in the sensitiveareas of the Roman Pot — this means application ofthe “three pots” mask from Figure 2 to the flux maps.(2) Simple coincidence 216220 m Tracks of the parti-cles are extrapolated from 220 m to 216 m — particlesseen in the pot of one unit of the station must be seenalso in the same pot of the other unit.(3) Coincidence with roads Each detector is divided into16 groups of 32 strips of 66 m width — particleseen in a 2 mm wide group in one pot must also beseen in the other unit within a road consisting of thesame group and the 2 neighboring groups (this roadwidth is determined by the angular spread of the elas-tic/diffractive signal).Background trigger rates for different types of particles ob-tained by this rejection procedure are given in Table 1. Theobserved trigger rate from  is further reduced by the prob-ability of  to interact in the Roman Pot material. From thespectrum on Figure 6 this “efficiency” correction brings theof  triggers down to 235 Hz. Similar reasoning reduces theexpected neutron trigger rate to 0.5 Hz.Beam-gas background levels from Table 1 can be com-pared with the estimates of the background from the p-p in-teractions in the IP, given in [6] for the nominal optics with  = 0.55 m in IP5 and L = 10   cm/s. Although as seen	  1 344 174 616 406 4630 3361 94.722 307 131 479 289 75 122 10.173 303 129 385 220 21 14 3.9Table 1: Particle rate integrated over the detector area ofthe Roman Pot station at 220 m with background rejectioncriteria 1–3 applied sequentially. Units are Hz, except for where flux is given in kHz.from Figure 3, nominal optics of IR5 differs dramaticallyfrom the optics considered for TOTEM operation, scalingthese estimates to the IP5 luminosity of 2.410 cm/sduring TOTEM runs can be used as a first approximation.This gives a total rate of 1.62 kHz integrated over the to-tal detector area, for charged hadron background at 220 mfrom p-p interactions in IP5. The corresponding numbersfrom Table 1 give a rate of 1.37 kHz for charged hadrons,indicating that at L = 2.4 10 cm/s secondary chargedhadrons from the beam-gas losses and from the p-p interac-tions will be at the same level. The last holds for the otherparticle types as well.REFERENCES[1] 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.[2] 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.[3] I. Azhgirey, I. Baishev, K.M. Potter et al., “Methodical Studyof the Machine Induced Background Formation in the IR8 ofLHC”, LHC Project Note 258, CERN, Geneva, 2001.[4] 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.[5] A. Rossi, “Residual Gas Density Estimations in the LHC In-sertion Regions IR1 and IR5 and the Experimental Regionsof ATLAS and CMS for Different Beam Operations”, CERNLHC Project Report 783, Geneva, 2004.[6] A. Drozhdin, D. Macina, N. Mokhov et al., “Accelerator Re-lated Backgrounds in the LHC Forward Detectors”, In: Proc.of the PAC’2003, Portland, OR, May 12–16 2003, p.1742–1744.

Estimation and analysis of the machine-induced background at the TOTEM roman pot detectors in the IR5 of the LHC

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