Beam Losses and Background Loads on Collider Detectors Due to Beam-Gas Interactions in the LHC Page: 4 of 4
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The contributions to the proton loss rate on the tertiary
collimators from the incoherent (quasi-elastic), coherent
and Coulomb scattering processes in the ring are 46, 52.6
and 1.4 percent, respectively, not very different for Beam
1 and 2 as well as for IPI and IP5. Tagging the gas
components involved in these processes, reveals that H, C
and O contribute 17, 29 and 54 percent, respectively, to
the losses on TCTs, again quite similar for both beams
and both high-luminosity insertions.
LOADS ON ATLAS AND CMS
The full MARS15 shower simulations are performed in
IPI and IP5 with the source terms on their TCTs
generated for Beam 1 and 2 as described in the previous
section. As with two other sources - "collimator tails" and
"inelastic beam-gas" - all the details of geometry,
materials and magnetic fields around IP are included in
the model at 22.6 < z < 150 m and r < 12 m. For the
simulated nominal conditions of Beam 2, the sources in
IP5 (R) are rather similar (rate-wise): 2.61 MHz on
TCT.B2R5 for "collimator tails", 3.07 MHz at 22.6 < z <
550 m for "inelastic beam-gas", and 3.07 MHz on
TCT.B2R5 for "elastic beam-gas". Distributions of all
particle types are calculated and analyzed at the machine-
detector interface plane z=22.6 m as in . As an
example, Fig. 5 shows the lateral distributions of particle
fluxes at the IP5 (R) interface plane.
0 200 400 600 800 1000
Radius in cm
U lU 2
0 200 400 600 800 1000
Radius in cm
Radial distr. at CMS (E > 0.02 GeV): bgas elastic, MARS15
Figure 5: Radial distributions of particle fluxes (E>20
MeV) at z=22.6 m from IP5 (R) for "elastic beam-gas".
Not a surprise that with the same loss rates on TCT (a
"point-like" source at z-148 m with respect to the
interface plane), the distributions for "elastic beam-gas"
are very similar to those for "collimator tails". At the
same time, radial distributions of particle fluxes at the
interface plane for the "inelastic beam-gas" case are very
different . The dominant component is now muons
from decays of pions and kaons generated in large-angle
beam-gas interactions distributed along a few hundred
meters. The highest interaction rate happens in the arcs at
z > 270 m from IP (Fig. 2), with average pion trajectories
tangent to the ring. Therefore, muons and other particles,
which accompany muons, arrive at the IPI and IP5
interface planes at large distances from the beam axis,
resulting in pretty flat lateral distributions in the orbit
plane in the direction outside the ring center (Fig. 6). The
loads to sub-detectors at radii r < 2 m are not so different
for both inelastic and elastic beam-gas sources.
0 200 400 600 goo 1000
Radius in cm
Figure 6: Radial distributions of particle fluxes (E>20
MeV) at z=22.6 m from IP for "inelastic beam-gas" .
 N.V. Mokhov, Nucl. Physics B, 51A (1996) 210.
 N.V. Mokhov, T. Weiler, "Machine-Induced
Backgrounds: Their Origin and Loads on
ATLAS/CMS", Fermilab-Conf-08-147-APC (2008).
 A.I. Drozhdin, M. Huhtinen, N.V. Mokhov, Nucl. Instr.
And Meth. A381 (1996) 531.
 N.V. Mokhov, "The Mars Code System User's
Guide", Fermilab-FN-628 (1995); N.V. Mokhov, S.I.
Striganov, "MARS15 Overview", in Proc. of
Hadronic Shower Simulation Workshop, Fermilab,
September 2006, AIP Conf. Proc. 896, p. 50 (2007);
 N.V. Mokhov, V.I. Balbekov, "Beam and Luminosity
Lifetime", in Handbook of Accelerator Physics and
Engineering, 2"d Printing, Ed. A.W. Chao, M. Tigner,
P. 218, World Scientific (2002).
 A.I. Drozhdin, V.A. Lebedev, N.V. Mokhov et al.,
"Beam Loss and Backgrounds in the CDF and DO
Detectors due to Nuclear Elastic Beam-Gas
Scattering"', PAC'03, Portland, Oregon, 2003; also
Fermilab-FN-734 (2003). http://www.JACoW.org.
 A. Schiz et al., Phys. Rev., D21 (1980) 3010.
 J.P. Burq et al Nucl. Phys. B217, (1983) 285.
 A. Rossi, CERN-LHC-Project-Report 783 (2004).
 I.S. Baishev, A.I. Drozhdin, N.V. Mokhov, X. Yang,
"STRUCT Program User's Reference Manual",
t n , , A,
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Drozhdin, A.I.; Mokhov, N.V.; Striganov, S.I. & /Fermilab. Beam Losses and Background Loads on Collider Detectors Due to Beam-Gas Interactions in the LHC, article, April 1, 2009; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc935212/m1/4/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.