Beamgas background in LHCb p A mostly H
Beam-gas background in LHCb p A mostly H, C, O from H 2, CO 2, CH 4 , H 2 O • 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 Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP
The issue l l down-streaming 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 zp-A * relative to particles coming from an IP event 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 zdetector) Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP
The issue l (continued) 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 ? Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP
Distributions for p-H (very preliminary) Paolo Bartalini 104 normalized entries d. Nch/d PYTHIA 7 Te. V proton beam on fixed proton target 103 102 101 p-p charged & 1 Gev p. T cut 100 p-p charged p-H gammas p-H other neutrals 10 -1 10 -2 10 -3 p-H charged & 1 Gev p. T cut 10 -1 10 -2 10 -3 10 -4 10 -5 10 -4 -20 -12 -4 4 12 20 pseudo-rapidity Lausanne, Generator Miniworkshop, March 2001 10 -6 0 2 4 6 8 10 transverse momentum (Ge. V/c) 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-Te. V proton beam. . . (LHC: s of few 100 Ge. V) p-air with plab 0. 1. . . 2 1018 e. V/c - extrapolate on A from pp (pp) data - “interpolate”on s (cosmic, FNAL, SPS) Lausanne, Generator Miniworkshop, March 2001 LHC p-H Lab beam momentum (Ge. V/c) Massimiliano Ferro-Luzzi, CERN/EP
Very gross estimate for crosssections of p-A scattering l Absorption cross section: expect roughly p. A pp A with pp 50 mb (7 Te. V/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 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 tmax 1 1016 H 2/cm 2 ( = 27 km 3. 8 109 molecules/cm 3 ). In VELO: density of 10 -7 mbar 2 m (H 2 at 300 K) corresponds to 4. 8 1011 H 2/cm 2 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 (H 2 300 K) 1. 5 % of LHCb nominal luminosity 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 Unbaked VELO tank hi as for unbaked LHCb cone (NEG saturated, i. e. not pumping) hi as for baked surface Note: There is no stringent request on vacuum performance from the LHCb experiment Pressure p (mbar) ion desorption yield (incident ion energy Eion ~ 300 e. V) Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP
Expected dynamic pressure profile: comments l l l 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 L (beam-gas) < 5% of L (beam-beam) at LHCb nominal luminosity 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 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 Yellow. Book 95) 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 ? Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP
Beam losses and halo cleaning l l l CMS IP 5 2. 4 109 Total loss p/s (2 beams at nominal luminosity in collision mode) Most of beam losses happen at high luminosity IP’s Hence, in IP 1 (ATLAS), 1/2 of which goes to LHCb before reaching the BCS, but ä fortunately, it comes from the “good” direction, and ä apertures should always be multiplied with their local Dump BCS ATLAS IP 1 Lausanne, Generator Miniworkshop, March 2001 Supposed to be a “ 1 -turn” beam cleaning system IP 8 LHCb Massimiliano Ferro-Luzzi, CERN/EP
Conclusions l l 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 ! 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 IP 8) Beam halo scattering from VELO materials should be considered Lausanne, Generator Miniworkshop, March 2001 Massimiliano Ferro-Luzzi, CERN/EP
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