lausanne, generator miniworkshop, march 2001 massimiliano ferro-luzzi, cern/ep beam-gas background...
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Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Beam-gas background in LHCbBeam-gas background in LHCb
• What nuisances for LHCb from p-A background ?
• Cross-sections for p-A scattering
• Vacuum: what are the current constraints from LHC and LHCb ?
• What rates can we expect ?
• Summary
p A mostly H, C, O from H2, CO, CO2, CH4 , H2O
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
The issueThe issue down-streaming
p-A reaction from down-streaming beam: all particles produced go downstream into LHCb arrival time shift* of (relativistic) particles into detectors is
always zero (as if produced at IP) independent of zp-A
* relative to particles coming from an IP event
p-A reaction from up-streaming beam: all particles produced go opposite to “physics” stream arrival time shift* of particles into detectors depends on
position of detector (t = 2 c zdetector)
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
The issueThe issue (continued)
Possible nuisances: if p-A in VELO region (down-streaming case): adds a vertex
which however should be identifiable as p-A (track “polarity” not a bunch-bunch primary vertex, energy not a decay vertex, ...)
increased occupancies and number of tracks
Remarks: p-A also produces B mesons (see Hera-B, GAJET, …) but with
BB down by about 500 whereas tot is down by less than 2. for LHCb one can assume p-A makes only noise...
Is rest gas the only (or main) source of p-A ? What about beam halo scattering from the VELO materials ?
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Distributions for p-H (very preliminary)Distributions for p-H (very preliminary)Paolo Bartalini
PYTHIA 7 TeV proton beam on fixed proton target
-20 -12 -4 4 12 20
pseudo-rapidity
104
103
102
101
100
10-1
10-2
10-3
10-4
dNch
/d p-p charged
p-H chargedp-H gammasp-H other neutrals
p-p charged& 1Gev pT cut
10-1
10-2
10-3
10-4
10-5
10-6
p-H charged& 1Gev pT cut
norm
aliz
ed e
ntri
es
0 2 4 6 8 10transverse momentum (GeV/c)
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Cross-sections for p-A scatteringCross-sections for p-A scattering
There is scarse data on p-A scattering with fixed targets and a multi-TeV proton beam...(LHC: s of few 100 GeV)
- extrapolate on A from pp (pp) data- “interpolate”on s (cosmic, FNAL, SPS) Lab beam momentum (GeV/c)
p-air with plab 0.1...2 1018 eV/cp-air with plab 0.1...2 1018 eV/c
LHC p-HLHC p-H
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Very gross estimate for cross-sections of p-A scattering
Very gross estimate for cross-sections of p-A scattering
Absorption cross section: expect roughly pA pp A
with pp 50 mb (7 TeV/c proton beam momentum)
and 0.7
e.g. for C and O : A 6 and 7
(in reality, more complicated: depends a bit on e.g. pT)
Multiplicity: does not scale with “disc surface” ! up to a factor 2 higher in p-A than in p-p
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Vacuum constraints for VELO: a first glance
Vacuum constraints for VELO: a first glance
From LHC:• Beam life time limit: 24 h requires LHC integrated density tmax 1 1016 H2/cm2 ( = 27 km 3.8 109 molecules/cm3 ).
In VELO: density of 10-7 mbar 2 m (H2 at 300K) corresponds to
4.8 1011 H2/cm2 0.005% of tmax. rather “loose” constraint for the pressure in LHCb.• But, dynamic vacuum (beam-induced) effects must be taken into account ! stringent constraints on surface desorption properties!
From LHCb:• 10-7 mbar 1.2 m (H2 300K) 1.5 % of LHCb nominal luminosity
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
1.00E-10
1.00E-09
1.00E-08
1.00E-07
1.00E-06
-1200 1200 3600 6000 8400 10800 13200 15600 18000 20400
distance from IP (mm)
pre
ss
ure
(to
rr)
CH4
CO2
CO
Expected dynamic pressure profileslide presented to LEMIC 23-Jan-01 by Adriana Rossi CERN/LHC-VAC
Expected dynamic pressure profileslide presented to LEMIC 23-Jan-01 by Adriana Rossi CERN/LHC-VAC
Pre
ssur
e p
(m
bar)
Unbaked VELO tanki as for unbaked
Note: There is no stringent request on vacuum performance from the LHCb experiment
ion desorption yield (incident ion energy Eion ~ 300 eV)
LHCb cone (NEG saturated, i.e. not pumping)
i as for baked surface
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Expected dynamic pressure profile: commentsExpected dynamic pressure profile: comments
In this model, it results that cone (z) dz velo (z) dz
There is a new VELO design: it can be baked out
less outgassing and beam-induced desorption
NEGs will pump ! most of COx desorption in Adriana Rossi’s calculation is
photon-induced ! (assumed photon flux ~ 1016 m-1 s -1)
if needed: add shielding upstream against photon flux (?)
The calculated pressure values are to be taken as an upper limit
at LHCb nominal luminosity
L (beam-gas) < 5% of L (beam-beam) L (beam-gas) < 5% of L (beam-beam)
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
What about the beam halo... What about the beam halo... … scattering from the VELO materials ?
First, note that:The VELO shield is very close to the beam axis (~ 6 mm)
1 cm of bulk Al 6 1022 cm-2 1 m 25 bar (!) of Al “gas” at 300 KBut also: a p-Al vertex is very much off axis
Then assume: beam = 85 hours (LHC YellowBook95)
Which implies: 3.3 ppm of stored protons lost per second ~ 109 protons lost/second/beam
So: having 1 % of these “lost” protons pass once through the VELO shield is about equivalent to having ~ 10-7 mbar Al “gas” in the tank...
What halo can we expect at LHCb ?
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
Beam losses and halo cleaningBeam losses and halo cleaning
Total loss 2.4109 p/s (2 beams at nominal luminosity in collision mode)
Most of beam losses happen at high luminosity IP’s
Hence, in IP1 (ATLAS), 1/2 of which goes to LHCb before reaching the BCS, but fortunately, it comes from the
“good” direction, andapertures should always be
multiplied with their local
Supposed to be a “1-turn” beam cleaning system
ATLASIP1
IP8 LHCb
BCS
CMSIP5
Dump
Massimiliano Ferro-Luzzi, CERN/EPLausanne, Generator Miniworkshop, March 2001
ConclusionsConclusions Physics: beam-gas background can hardly be expected to be an issue
Still, we should state an official LHCb limit for vacuum pressure profile !
Monte-Carlo strategy (my modest opinion): pre-2006: use current models based on extra/interpolations, with
safety factors if desired. This is good enough for design studies. post-2006: use beam-gas data from non-colliding bunches to
precisely study and take into account beam-gas effects on physics analysis ( ~6% of all bunches are “non-colliding” at IP8)
Beam halo scattering from VELO materials should be considered
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