Beam Losses and Background Loads on Collider Detectors Due to Beam-Gas Interactions in the LHC Page: 2 of 4
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BEAM LOSSES AND BACKGROUND LOADS ON COLLIDER
DETECTORS DUE TO BEAM-GAS INTERACTIONS IN THE LHC*
A.I. Drozhdin, N.V. Mokhov#, S.I. Striganov, FNAL, Batavia, IL 60510, U.S.A.
With a fully-operational high-efficient collimation system
in the LHC, nuclear interactions of circulating protons
with residual gas in the machine beam pipe can be a major
source of beam losses in the vicinity of the collider
detectors, responsible for the machine-induced
backgrounds. Realistic modeling of Coulomb scattering,
elastic and inelastic interactions of 7-TeV protons with
nuclei in the vacuum chamber of the cold and warm
sections of the LHC ring - with an appropriate pressure
profile - is performed with the STRUCT and MARS15
codes. Multi-turn tracking of the primary beams,
propagation of secondaries through the lattice, their
interception by the tertiary collimators TCT as well as
properties of corresponding particle distributions at the
CMS and ATLAS detectors are studied in great detail and
results presented in this paper.
Beam loss in the vicinity of interaction points (IP) at the
LHC is an outstanding source of background rates in the
collider detectors, called machine-induced backgrounds
(MIB) [1, 2]. As shown in , the relative importance of
this component can be comparable to that from the pp-
collisions at early operation of the LHC because MIB is
mostly related to beam intensity and not luminosity, and
tuning the machine will require substantial time and
In this paper we consider the design LHC beam with
2808 bunches of 7-TeV protons in the scrubbed machine.
At nominal operation, there are three sources of MIB for
the experiments :
* Collimator tails: protons escaping the betatron and
momentum cleaning insertions (IP7 and IP3,
respectively) and being intercepted by the tertiary
collimators TCT. This term, related to the
inefficiency of the main collimation system, is called
"tails from collimators" or "tertiary beam halo". The
TCTs are situated between the neutral beam absorber
(TAN) and D2 separation dipole at about 148m on
each side of IPI and IP5. They are set to 8.3- to fully
protect the triplet magnets. For the betatron cleaning
in IP7 at the rate of 8.3x109 p/s, a 10-hr beam
lifetime and nominal intensity, the loss rates on the
TCTs are 2.61x106 p/s and 4.28x106 p/s for Beam-2
approaching IP5 and Beam-I approaching IP1,
*Work supported by Fermi Research Alliance, LLC, under contract No.
DE-AC02-07CH11359 with the U.S. Department of Energy.
* Inelastic beam-gas: nuclear inelastic (including
quasi-elastic and diffractive) interactions of the
incoming beam with the residual gas. Products of
such interactions in straight sections and arcs
upstream of the experiments have a good chance to
be lost on limiting apertures in front of the collider
detectors. The rate of beam-gas interactions is
proportional to the beam intensity and residual gas
pressure in the beam pipe. Detailed studies since the
first papers on MIB in LHC [1, 3] have shown that
relatively large-angle inelastic nuclear interactions in
the 550-m regions upstream of IPI and IP5 are
mostly responsible for the beam-gas component of
MIB. The total number of inelastic and quasi-elastic
nuclear interactions in these regions for each of the
beams coming to IPI and IP5 is 3.07x106 p/s .
* Elastic beam-gas: nuclear elastic - coherent and
incoherent (quasi-elastic) - interactions as well as
Coulomb scattering on residual gas around the ring.
First two sources are studied in great detail in thorough
MARS15  calculations. Modeling approach and results
of calculations of energy-dependent particle fluxes in the
machine components, tunnel, and ATLAS and CMS
experimental halls as described in . Third component is
described in this paper.
ELASTIC BEAM-GAS INTERACTIONS
As described in [5, 6], the main process of beam-gas
interaction, Coulomb scattering, results in slow diffusion
of protons from the beam core causing emittance growth.
These particles increase their betatron amplitudes
gradually during many turns and are intercepted by the
main collimators before they reach other limiting
apertures. Similar behavior takes place for small-angle
elastic nuclear scattering. For certain beam parameters,
large-angle nuclear elastic and Coulomb scattering can
behave quite differently . Such processes can result in a
substantial increase of the betatron amplitude and, if not
intercepted by the main collimators, the scattered protons
can be lost in the vicinity of the experimental insertions,
predominantly giving rise to the "scraping" rate on the
TCTs and adding to MIB in the detectors.
A differential cross section of nuclear elastic scattering
can be parameterized as a sum of two exponential
distributions. The slope of coherent elastic scattering has
weak energy dependence and can be taken from . The
slope of quasi-elastic scattering is close to the slope of pp
elastic scattering at high energies. An approximation 
of experimental data on the slope of elastic pp-scattering
at 2-2000 GeV/c is extrapolated to 7 TeV. Total cross
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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/m1/2/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.