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Where do the protons go II? Mike Lamont LBOC 2 nd February 2016 Acknowledgements TOTEM in the first few slides
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Total cross section 2
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Cross sections 7 to 14 TeV 3 √selasticinelastictotalCOMPETE 725.4 ± 1.573.1± 1.398.6 ± 2.898 ± 5 827.1 ± 1.474.7 ± 1.7101.7 ± 2.9101 ± 5 1329.780.0109.7n/a 1430.280.9 ± 1.7 ± 2.2111.1111.5 ± 1.2 + 4.1 -2.1 7 and 8 TeV: TOTEM luminosity independent 14: extrapolation by ATLAS and CMS (inel)/TOTEM(el) 13: LPC scaling (inel) /TOTEM extrapolation (el) But good enough for government work
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Inelastic includes diffractive 4 1 proton survives with momentum loss ξ Some of these will either stay within the beam or get lost in the DS or IR3 Cross-sections for different momentum lost ranges can be evaluated Assume here that surviving protons get lost locally Non-diffractive processes: ~60 mb at 7-8 TeV
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Elastic scattering 5
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7 s: square of the center-of-mass energy √s = 13 TeV in 2015 Elastic Scattering A and B given by e.g TOTEM ~Elastic cross section
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Elastic scattering mean scattering angle 9 Slope parameter B (TOTEM 8 TeV value)~19.9 GeV -2 √s13000 GeV 0.05 GeV 2 rms scattering angle34.5 urad rms scattering angle – one plane24.4 urad Angular divergence at IP (80 cm, 3.5 um)25.1 urad What is the fate of the elastically scattered protons? Question 1: through what angle are protons scattered at the IP?
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Angle through which protons are scattered at the IP Differential cross-section well described by Ae -Bt For the sake of this analysis cut off at t=0.53 – 4 orders of magnitude in differential cross-section – Integral of Ae -Bt out to t=0.53 very close to el. xsec Very few particles get scattered by these high angles 10 80 cm: minimum t accessible by RP at 5.5 sigma (say) in 2015: ~0.7 GeV 2 (~90 urad)
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So… We have a distribution of angles at the IP characterized by the rms divergence Have the elastic cross section – and thus for a given luminosity the number of elastic events per second Have the differential cross-section for elastic scattering i.e. probability for scattering with a given angle Form a 2D Gaussian probability density function (pdf) for distribution of (x,x’) at IP (truncate at 6 sigma) Form a pdf for elastic scattering – truncate at t = 0.53 11
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Toy Monte Carlo 1 In 1 second ~66,000 elastic collisions per 1.1e11 bunch with a luminosity of 5e33 cm -2 s -1 Take a 66,000 (x,x’) sample in 2-D Gaussian phase space For each particle, sample pdf, convert t to θ Randomize projection to get θ x Add scattered angle to x’ New distribution of angles/phase space 12 Vanishing small chance of multiple collisions Mean = 0.05 GeV^2 Mean = 24.2 urad
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Phase space at IP 13 Remember - only looking at scattered particles, there’s another 1e11 out there
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Phase space at IP 14
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Toy Monte Carlo 2 Transport distributions to collimator (e.g vert) Number of particles outside n sigma? Apply one turn matrix a few times, remove and sum any lost particles (e.g. > 4 sigma) 15 “Lose” 400 – 500 particle/s above 4 sigma – Gaussian beam remember Gently populating tails ES will also gently clean non-Gaussian tails (to be quantified)
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90 m is a different story 16 10 minutes sample IP Roman Pot
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2.5 km! 17 An hour’s worth of ES
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Emittance growth 18 Peak normalized emittance growth from elastic scattering in 2015 ~ 0.015 mm.mrad/hour See D.A. Edwards and M.J Syphers or A.W. Chao and M. Tigner
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Loss rates during Stable Beams Luminosity – calculate losses based on inelastic cross- section at 6.5 TeV – Assume 80 mb for inelastic cross-section – Sum luminosity from ATLAS, CMS, LHCb Use SVD/losses at D,C,B, IR3 B1 & B2 to establish losses in collimation regions (see Belen) – (Or scaled 2012 calibration) Ignore for the moment: – residual gas; diffractive component Sum loss rates to get overall dN/dt Calculate lifetimes etc. 19
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Beam losses from SVD analysis 20 Thanks to Belen, Mirko and Michal Wyszynski Sum components to get total losses Raw data SVD breakdown
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Loss breakdown 2012 21 Before OCP After OCP Not SVD… Settings make a difference!
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Loss breakdown 2015 22 Green: inelastic luminosity losses (ATLAS+CMS+LHCb) Pink: total number of elastically scattered protons – possibly not lost! Note high loss rate during 1 st hour… presumably tails – DA less than 5.5 sigma?
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Cross-check losses versus BCT 23 After 8 hours Work in progress on the matrix Certainly some issues with B2
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Single beam lifetimes 24 Fit to BCT data – sliding 10 minute window Lifetime from loss contribution from collimation and inelastic luminosity Note 1 st hour
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Single beam lifetime breakdown 25
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Luminous region (ATLAS) 26 OFFLINE data
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Emittance from luminous region 27 Standard picture Emittances similar at t=0 as per 2012 Naively calculate corresponding emittances
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Luminosity 28
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Emittance from luminosity 29 -0.008 um/hour at t=0 -0.002 um/hour at t=22hr
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Comparison of lumi and lumireg 30
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Luminosity lifetime 31 Rolling 15 minute window Bit ratty because of the drifts
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Luminosity lifetime breakdown 32 More-or-less makes sense Dominated by losses rather than emittance Emittance and F are both net positive bringing L(t) up
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First hour v. BBTS 2012 33
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First hour v. BBTS 2015 34 Hum…
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Conclusions The majority of elastically scattered protons stay within the beam. Elastic scattering (ES) contributes to the emittance growth. Relatively high loss rate during 1 st hour of SB – Presumably tails – Cleaning effect of ES to be quantified – DA < 5.5 sigma? Losses later in the fill considerably down on 2012 – But still significant – If not ES then octupoles, Q’, e-cloud, beam-beam, WP? – If latter then diligent optimization in SB should be able to reduce these losses. The luminosity lifetime is brilliant! 35
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