Beam-gas background in LHCb

1. Beam-gas background in LHCb p A  mostly H, C, O from H2, CO, CO2, CH4 ,H2O • 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 Massimiliano Ferro-Luzzi, CERN/EP

2. The 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 czdetector) Massimiliano Ferro-Luzzi, CERN/EP

3. The 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/EP

4. Distributions for p-H (very preliminary) Paolo Bartalini PYTHIA 7 TeV proton beam on fixed proton target 104 103 102 101 100 10-1 10-2 10-3 10-4 p-p charged p-H charged p-H gammas p-H other neutrals 10-1 10-2 10-3 10-4 10-5 10-6 dNch/d normalized entries p-p charged & 1Gev pT cut p-H charged & 1Gev pT cut 0 2 4 6 8 10 transverse momentum (GeV/c) -20 -12 -4 4 12 20 pseudo-rapidity  Massimiliano Ferro-Luzzi, CERN/EP

5. 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) p-air with plab 0.1...2  1018 eV/c LHC p-H Lab beam momentum (GeV/c) Massimiliano Ferro-Luzzi, CERN/EP

6. 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/EP

7. 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/EP

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

9. Expected 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) Massimiliano Ferro-Luzzi, CERN/EP

10. 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 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 ~ 109protons 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/EP

11. Supposed to be a “1-turn” beam cleaning system CMS IP5 Dump BCS IP8 LHCb ATLAS IP1 Beam losses and halo cleaning • Total loss 2.4109p/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, and • apertures should always be multiplied with their local  Massimiliano Ferro-Luzzi, CERN/EP

12. Conclusions • 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 Massimiliano Ferro-Luzzi, CERN/EP