Appleby RB

Azhgirey I

Baishev I

Kurochkin I

Potter K

Roncarolo F

Ruspa M

Sellers G

Talanov V

English

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MACHINE INDUCED BACKGROUNDS FOR FP420∗R. B. Appleby† , K. Potter, F. Roncarolo, G. Sellers, Cockcroft Institute/University of Manchester, UKI. Azhgirey, I. Baishev, I. Kurochkin, V. Talanov, IHEP Protvino, Protvino, RussiaMarta Ruspa, Universit del Piemonte Orientale and INFN, Torino, ItalyAbstractThe LHC FP420 R&D project is assessing the feasibil-ity of installing forward proton detectors at 420m fromthe ATLAS and/or CMS interaction points. Such detec-tors aim at measuring diffracted protons, which have lostless than 2% of their longitudinal momentum. The successof this measurement requires a very good understandingof the charged and neutral particle environment in the de-tector region in order to avoid the signal being swampedas well as for detector survivability. This background re-ceives contributions from beam-gas interactions, halo par-ticles surviving from the betatron and momentum cleaningsystems and secondary showers produced by particles fromthe 14 TeV collision region striking the beampipe upstreamof the FP420 detectors. In this paper, such backgroundsources are reviewed, and the expected background ratescalculated.INTRODUCTION TO FP420The FP420 R&D project [1] aims at exploring the im-mense amount of LHC physics potential by instrumentingwith detectors the forward regions starting at about 420 mfrom IP1 and IP5. The tag on one or both of the forwardprotons can provide information on the mass of the cen-tral system in the detector and allowing measurements ofquantities like the mass and quantum numbers of the HiggsBoson. This experimental programme would add consider-able depth and potential to the main detector experimentalprogrammes. The proposed detectors consist of trackingand timing stations, mounted on movable beam pipes, ap-proaching the beam as close as allowed by LHC protection/beam stability and as required by the experiment accep-tance studies. The present proposed setup is at a horizontaldistance of 5mm from the beam.CLASSIFICATIONS OF BACKGROUNDThe charged and neutral particle backgrounds at 420mcan be classified according to the origin of the particleflux:1) At the interaction point, the colliding beams producemany final state protons dominantly in the forward direc-tion with a large coverage of phase space. These particlesmay be transported down the beamline and irradiate the420m region. This contribution to the background isreferred to as interaction point (IP) particles.2) Elastic and inelastic proton-nucleus collisions between∗Work supported by STFC, UK† robert.appleby@manchester.ac.ukthe beam and residual gas molecules resulting in protonswith large scattering angles represent a direct backgroundwhen the collisions occur close to the FP420 detector sta-tions. This is referred to as the near beam-gas background.3) Beam halo consists of protons in the distribution tailsthat can circulate for many turns and constitute the beamhalo background. The primary halo is populated by variousbeam instabilities including distant beam-gas interactionsresulting in small scattering angle protons. The consequentbeam losses on both the betatron and momentum cleaningcollimation systems produce secondary and tertiary halos.4) Particles transported down the beam line arisingfrom proton-proton interactions at the IP, near beam-gascollisions or any other beam-halo, can interact, and subse-quently shower, with elements of the machine or detector.These processes create secondary showers which canirradiate the detector region at 420m with large chargedand neutral particle fluxes. This background is known assecondary interactions.In this paper, the present status of each backgroundsource at 420m is reviewed and the possible impact on thedetector assessed.IMPACT OF BACKGROUND SOURCESNear Beam-gas InteractionsThe near beam-gas contribution arises from the interac-tion of beam particles with residual gas in the beam piperegion immediately before 420 m, and study of this back-ground requires a detailed model of the beam line, coupledwith gas pressure profiles and computation of proton/gasinteractions. However, an estimate of the number of beam-gas interactions per bunch in the 420 m detector region canbe extracted by scaling results obtained for the straight sec-tion [2]. These simulations give a charged hadron rate forparticles lost after scattering with the gas nuclei and sec-ondary particles as np240 m = 2.4 s−1, at a location 240mfrom IP1. The Hydrogen equivalent pressure is taken tobe [2, 3] ρ240 m = 3.4 · 1011 molecules ·m−3. The syn-chrotron radiation at 420m implies that the dynamic pres-sure is higher than the straight sections and, as a con-servative upper limit, we consider a hydrogen density ofρ420 m = 1 · 1015 molecules · m−3 This corresponds to abeam-gas lifetime of 100 hours. The resulting total num-ber of expected hadrons per bunch, due to near beam-gasevents, is aroundnp420 m =np240 mNbs· ρ420 mρ240 m= 1.8 · 10−4, (1)Proceedings of EPAC08, Genoa, Italy MOPP00604 Hadron Accelerators A04 Circular Accelerators559where Nbs = 4 · 107 is the number of bunches per second.Such a background rate is very low.Beam Halo BackgroundsThe LHC cleaning system consists of a multi-stage pro-cess that aims at intercepting the primary beam halo bymeans of primary collimators located in IR3 (momentumcleaning) and IR7 (betatron cleaning). A large amountof the impacting protons will be scattered by the collima-tors and will circulate for many turns before being lost.Concerning on-momentum beam halo protons, the FP420detectors (nominal position at 15 σx) will be well in theshadow of the betatron cleaning system. On the contrary,the off-momentum protons can potentially perturb the op-eration of the FP420 detectors, located in the high disper-sion region. For this reason, a series of simulations usingSTRUCT [4] has been carried out in order to characterizethe beam halo populated by such protons and the effect ofthe cleaning system settings on the FP420 background. Thesimulations have as input N0 = 2 · 106 protons belongingto the primary halo hitting the momentum cleaning primarycollimators in IR3. A multi-turn tracking routine followsthe protons emerging from the collimator surface until theyare absorbed by the cleaning system or lost in other aper-ture limitations of the machine (not including the FP420detectors). At each turn, the proton distribution is recordedat the 420 m locations. In order to estimate the absolutebackground level the distributions must be normalized forthe number of protons that will interact with the momen-tum cleaning collimators during normal LHC operation.If we assume: the nominal LHC beam intensity for highluminosity runs I0 = 3.2 · 1014 protons, an exponential de-cay of the beam current due to off-momentum proton lossesI(t) = I0 ·e−t/τ , and a beam lifetime accounting for lossesof off-momentum particles τ = 150 hours, then the corre-sponding maximum proton loss rate is:r(t = 0) = −dIdt∣∣∣∣t=0=I0τ≈ 5.9 · 108 [p · s−1]. (2)Hence, the loss rate at FP420 as a function of transverseposition can be calculated by normalizing the simulationresults of protons per bin (Np) according to:r(t,Δx) =NpΔx· r(t0)N0[p · s−1mm−1] (3)where Δx is the bin width.Another component contributing to the build up of abeam halo is the elastic and inelastic scattering between7 TeV protons with residual gas nuclei, known as distancebeam-gas. When the resulting proton momentum loss andscattering angle are small, the protons remain within themachine momentum and transverse acceptance and circu-late for several turns. Therefore elastic or inelastic scatter-ing can contribute to the halo.STRUCT [4] has been used to simulate 1 · 107 protons,which were generated at the location of the collimator la-belled as TCP.6L3 (177 m upstream of IP3), with momen-tum equal to 7 TeV and distributed according to the nom-inal transverse phase space. These protons were trackedaround the LHC ring, to simulate the proton phase spacedistribution at 420 m by assuming a uniform gas density inthe LHC arcs and dispersion suppressor regions, and are ei-ther lost or rescattered from a collimator after a proton-gasinteraction. These simulation results must be normalizedfor the expected beam gas life time, and in the results pre-sented here, we considered 500 h as a reasonable value forthe LHC high luminosity runs.The horizontal distribution of the beam halo protons atFP420, recorded for all protons in the simulation with hor-izontal position |x| > 7σx and vertical position |y| > 7σy,is shown in figure 1 which is normalised for the number ofprotons per millimeter, and per bunch crossing.Figure 2 shows the total number of beam halo protonsthat will enter the 420 m regions, for different horizontalpositioning of the detectors (i.e. the number of protons in-tegrated from the outer beam halo edge to the detectors in-ner edge).x [mm]-20 -15 -10 -5 0 5 / mm / bunch crossingpN-510-410-310-210-110110210 Mom. Cleaning Beam 1Mom. Cleaning Beam 2Dist. Beam-Gas Beam 1Dist. Beam-Gas Beam 2FP420 IP1Figure 1: Horizontal beam halo distribution per bunchcrossing at FP420 around IP1. Similar calculations havebeen done for IP5 [1]. [mm]0x-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 / bunch crossingpN-510-410-310-210-110110210310 Mom. Cleaning Beam 1Mom. Cleaning Beam 2Dist. Beam-Gas Beam 1Dist. Beam-Gas Beam 20 integrated for x<xpFP420 IP1, NFigure 2: Total amount of beam halo protons predicted atthe 420 m regions around IP1 for different FP420 detectorhorizontal positions. Similar calculations have been donefor IP5 [1].The peak of the beam-halo distribution is determinedby the LHC momentum cleaning collimator settings. Fornominal collimator settings, positioning the FP420 detec-tors at 5 mm from the beam would locate them well awayfrom this peak. To operate closer than 5 mm an adjust-MOPP006 Proceedings of EPAC08, Genoa, Italy04 Hadron Accelerators560A04 Circular Acceleratorsment of the collimator positions would likely be required.Furthermore, for detector distances greater than 5 mm, thisbackground is dominated by distant beam gas and the back-ground rate is low.Secondary nteraction ackgroundsThe proton-proton interactions at the IP result in finalstate proton distributions with a long momentum tail. Thesubsequent transport of this phase space distribution downthe beamline will result in the interaction of the protonswith the material of the machine and the detector, due tolimited aperture and the high dispersion. The protons willbecome lost from the beam and results in electromagneticand hadronic showers, causing deposited energy and theproduction of background particle species. The calcula-tion of the resulting shower particle distribution requirescoupling of a beam line and detector model with an initialparticle phase space distribution.To obtain a full simulation of secondary productionalong the beam line the simulation tool STRUCT has beenused [2] to calculate the proton distribution on the dipoleimmediately preceding FP420. The showers in this dipoleare then computed using MARS. Preliminary results areshown in Fig. 3.x [mm]-25 -20 -15 -10 -5 0 / mm / bunch crossingpN-310-210-110Protons (from p-p at the IP)Hadrons (from showers) (from showers)-,e+eFigure 3: Secondary particles flux at the entrance of the420 m region downstream IP5 (preliminary results). Theshower source is IP protons, generated with loss on the lastdipole magnet before FP420.BDSIM [5] (Beam Delivery System Simulation) hasbeen used to crosscheck the STRUCT-MARS results andto estimate the neutrons flux level. The code com-bines vacuum tracking of particles in the beam pipe withGEANT4 [6], which models the interaction of beam par-ticles with matter and is used whenever particles leave thebeampipe and enter solid parts of the machine. The BDSIMcalculations were done using DPMJET [7] calculation ofthe protons at the interaction point, for which the LHC totalproton-proton cross section gives about 35 proton-protoncollisions per bunch crossing. Approximately 1/3 of thesegive forward protons. The loss map calculated with BD-SIM has been crosschecked with the LHC machine track-ing code MADX [8], as shown in Fig. 4. The BDSIMestimation of the neutron rate is 0.11 neutrons per bunchs [m]300 320 340 360 380 400 420 440]-1Np [m110210310MADXBDSIMFigure 4: Loss maps produced for the IP5 beamline with aDPMJET phase space sample using MADX and BDSIM.crossing at 420 m. This is equivalent to an integrated rateof 44.4 · 103 neutrons per cm2·s. Showers from near beamgas and beam halo are expected to be negligible comparedto showers arising from IP protons.The detector region has also been simulated withGEANT4 [6]. The results of these studies are describedin [1], where their impact on the design of the layout of thedetector region is discussed. The aim is integration into thebeamline models and a complete start-to-end simulation.CONCLUSIONS AND OUTLOOKThe machine induced background in the LHC forwardexperiments at 420 m from IP1 and IP5 will be dominatedby IP proton losses in the beam line, distant beam-gas andmomentum cleaning collimators beam halo. The last twoof these contributions give a proton background, which isdescribed by a peak determined by the momentum clean-ing collimator settings, and a tail dominated by far beam-gas halo protons. At detector transverse distance of 5 mmor greater, the expected integrated number of protons frombeam halo is expected to be less than 1 per bunch cross-ing. All particle losses occurring immediately upstreamof 420m produce showers of charged and neutral particlesthat have been (or are in the process of being quantified.All background contributions need to be combined with de-tailed detector and signal studies to understand the impacton the FP420 experiment.REFERENCES[1] M. Albrow et al., ”The FP420 R&D Project ”, hep-ex/0806.0302[2] Minutes of the CERN Machine Induced Background Work-ing Group, 14-12-07, http://lhc-background.web.cern.ch/[3] A.Rossi, CERN-LHC-Project-Report-783 (2004)[4] I.Baishev, CERN LHC Project note 407, 2007[5] G. A. Blair et al., EuroTeV report 2006-035 (2006)[6] S. Agostinelli et al., “Geant4: A simulation toolkit”, Nucl.Instrum.Meth. A, vol. 506, no. 3, pp. 250–303, 2003.[7] S. Roesler, R. Engel and J. Ranft, hep-ph/0012252 (2000).[8] MADX homepage, http://mad.web.cern.ch/mad/I BProceedings of EPAC08, Genoa, Italy MOPP00604 Hadron Accelerators A04 Circular Accelerators561

Geant4: A simulation toolkit”,

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LHC Project note 407,

Machine induced backgrounds for FP420

Machine induced backgrounds for FP420

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