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