1. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Beam-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 pA  mostly H, C, O from H 2, CO, CO 2, CH 4, H 2 O

2. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP The issue down-streaming l p-A reaction from down-streaming beam: ä all particles produced go downstream into LHCb ä arrival time shift* of (relativistic) particles into detectors is always zero (as if produced at IP) independent of z p-A * relative to particles coming from an IP event l p-A reaction from up-streaming beam: ä all particles produced go opposite to “physics” stream ä arrival time shift* of particles into detectors depends on position of detector (  t = 2 c z detector )

3. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP The issue (continued) l Possible nuisances: ä 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,...) ä increased occupancies and number of tracks l Remarks: ä 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... ä Is rest gas the only (or main) source of p-A ? What about beam halo scattering from the VELO materials ?

4. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Distributions for p-H (very preliminary) Paolo Bartalini P YTHIA 7 TeV proton beam on fixed proton target -20 -12 -4 4 12 20 pseudo-rapidity  10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 dN ch /d  p-p charged p-H charged p-H gammas p-H other neutrals p-p charged & 1Gev p T cut 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 p-H charged & 1Gev p T cut normalized entries 0 2 4 6 8 10 transverse momentum (GeV/c)

5. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Cross-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 p lab 0.1...2  10 18 eV/c LHC p-H

6. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Very gross estimate for cross- sections of p-A scattering l 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. p T ) l Multiplicity: does not scale with “disc surface” ! up to a factor 2 higher in p-A than in p-p

7. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Vacuum constraints for VELO: a first glance From LHC: Beam life time limit:   24 h requires LHC integrated density t max  1  10 16 H 2 /cm 2 ( = 27 km  3.8  10 9 molecules/cm 3 ). In VELO: density of 10 -7 mbar  2 m (H 2 at 300K) corresponds to 4.8  10 11 H 2 /cm 2  0.005% of t max.  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 (H 2 300K)  1.5 % of LHCb nominal luminosity

8. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Expected dynamic pressure profile slide presented to LEMIC 23-Jan-01 by Adriana Rossi CERN/LHC-VAC Pressure p (mbar) Unbaked VELO tank  i as for unbaked Note: There is no stringent request on vacuum performance from the LHCb experiment ion desorption yield (incident ion energy  E ion  ~ 300 eV) LHCb cone (NEG saturated, i.e. not pumping)  i as for baked surface

9. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Expected dynamic pressure profile: comments l In this model, it results that  cone  (z) dz   velo  (z) dz l There is a new VELO design: it can be baked out  less outgassing and beam-induced desorption  NEGs will pump ! l most of CO x desorption in Adriana Rossi’s calculation is photon-induced ! (assumed photon flux ~ 10 16 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 )

10. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP 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  10 22 cm -2  1 m  25 bar (!) of Al “gas” at 300 K But 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 ~ 10 9 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 ?

11. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Beam losses and halo cleaning l Total loss 2.4  10 9 p/s (2 beams at nominal luminosity in collision mode) l Most of beam losses happen at high luminosity IP’s l Hence, in IP1 (ATLAS), 1/2 of which goes to LHCb before reaching the BCS, but äfortunately, it comes from the “good” direction, and äapertures should always be multiplied with their local  Supposed to be a “1-turn” beam cleaning system ATLAS IP1 IP8 LHCb BCS CMS IP5 Dump

12. Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP Conclusions l Physics: beam-gas background can hardly be expected to be an issue Still, we should state an official LHCb limit for vacuum pressure profile ! l Monte-Carlo strategy (my modest opinion): ä pre-2006: use current models based on extra/interpolations, with safety factors if desired. This is good enough for design studies. ä 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) l Beam halo scattering from VELO materials should be considered