The Large Hadron Collider LHC Operation III: pPb, PbPb collisions: operation, luminosity limits Luminosity in a hadron collider: Van der Meer scan Absolute luminosity, High beta physics, Luminosity leveling [R. Alemany] [CERN BE/OP] [Engineer In Charge of LHC] Lectures at NIKHEF ( )

pPb collisions 1.LHC main dipoles are connected in series  both rings experience the same magnetic field at any time  (B ρ) Pb = (Bρ) p  momentum is fixed 2.The revolution period (T) of a particle is a function of its charge (Z) and mass (A)  T p(Z=1,A=1) ≠T Pb(Z=82,A=208) IP1 IP2 IP8 IP5 How do you make them collide?

RF frequency is the key Courtesy of J. Jowett Those are the frequencies that keep protons and ions on a stable CENTRAL ORBIT of length Cref=CPb=Cp B1(p) B2(Pb) H V H V Beam orbits during ramp But this is not a problem since (RF system) B1 is independent of (RF system) B2

RF frequency is the key  distort the closed orbit fixed! T p(Z=1,A=1) ≠ T Pb(Z=82,A=208) We need T p(Z=1,A=1) = T Pb(Z=82,A=208) IP1 IP2 IP8 IP5 C ref ≠ C Pb ≠ C p

RF frequency is the key  distort the closed orbit B1(p) B2(Pb) H(mm) V(mm) H(mm) V(mm) Beam orbits at top energy with RF frequencies locked to B1 Horizontal offset given by the dispersion:

Ions are difficult particles A qualitative introduction to luminosity limits when working with ions: 1.Intra Beam Scattering (IBS) 2.Bound Free Electron-Positron Pair-Production (BFPP) 3.Electromagnetic Dissociation (EMD) … the last two only relevant for PbPb collisions

Intra Beam Scattering (IBS) in one slide Multiple small-angle elastic scattering processes leading to an increase in emittance  limits the beam lifetime  luminosity lifetime. B1(p) B2(Pb) Why trail bunches have more intensity than head bunches? IBS in SPS at 450 GeV!! IBS at high energy can be compensated by radiation damping, so the effect is mostly impressive at injection. Now a question concerning beam-beam, could you explain why those trains have more intensity than the others?

Intra Beam Scattering (IBS) in one two slides Bunch intensity Bunch number SPS 2 bunches from PS into SPS up to 12 injections, then the 2x12 bunches are transferred to LHC Time line

Ultra-peripheral interactions Two types of (inelastic) UPC: Photon-photon interactions: in PbPb events 75% of the   interactions are accompanied by nucleus excitation, 40% leading to GDR Photon-nucleus interactions f A A A* A One inelastic vertex f A A A* Two inelastic vertex A A A* A Single excitation A A A* Mutual excitation: 1  exchange A A A* Mutual excitation: 2  exchange GDR: Harmonic vibration of protons against neutrons. GDR decays via 1 (2 less likely) neutron emission or dissociation without neutrons. A* 1.Excitation of discrete nuclear states 2.Giant Dipole Resonances (GDR)* 3.Quasideuteron absorption 4.Nucleon resonance excitation Change of Z/A Change of A A A(Z=81) A BFPP e+e+ e

BFPP & EMD  high cross section  beam losses  faster decay in luminosity Courtesy of J. Jowett During the ultra-peripheral interaction the pT of the recoil ion is very small, so the modified ions merge at small angles to the main beam  D x (s) makes the rest

BFPP & EMD  high cross section  beam losses  faster decay in luminosity Courtesy of J. Jowett

Luminosity in a Hadron Collider Luminosity optimization during a regular fill Luminosity calibration during special fills  Van der Meer scans Cross-section measurement Luminosity leveling

Luminosity optimization during a regular fill N = number of collisions / second L = luminosity (cm-2s-1) σ = cross section of a given process x y The detectors record the LHC delivered luminosity and the recorded (logged) luminosity. Ideally delivered == recorded, but sometimes the detector is unable to take data because being busy with a previous event, or a sub detector tripped. HF (CMS) Both beams are moved against each other by ~1-2σ in steps of 1/2 σ  this allows us to find the maximum luminosity

Luminosity optimization during a regular fill --- ATLAS LUMINOSITY --- CMS LUMINOSITY --- LHCb LUMINOSITY Beam energy B1 current B2 current ~ 20 hours of STABLE BEAMS

Luminosity calibration  Van der Meer scans Van der Meer, CERN’s Intersecting Storage Ring accelerator in the 1960s x y One beam is fixed the other scans the effective cross section area, first one plane, then the other. Beam separation dN/dt 6σ6σ  A eff 6σ6σ  Describes the overlap profile The double Gaussian fit gives A & σ S. White, R. Alemany, H. Burkhardt, M. Lamont First Luminosity Scans in the LHC IPAC10

Total pp cross section determination  The Optical Theorem Discovered independently by Sellmeier and Lord Rayleigh in 1871 (optics study). Later extended to quantum scattering theory by several individuals, and referred to as the Optical Theorem in 1955 by Hans Bethe and Frederic de Hoffmann. In physics, the optical theorem is a general law of wave scattering theory, which relates the forward scattering amplitude to the total cross section of the scatterer. The determination of the total pp cross section and absolute luminosity requires measurements of small scattering angles of ~ µrad. Special detectors are needed  Roman Pots (already used in the 70’s at the ISR) How can we enhance low scattering angles in the accelerator? f el : scattering amplitude, k: momentum transfer, θ: forward scattering angle

Total pp cross section determination  The Optical Theorem Courtesy of S. White IP5 Small beam divergence

High β * physics  draw backs Large tune changes Aperture limitations at very high β * Operation of some insertions quadrupoles at the limits 1) β -function in a drift space around a symmetry point (IP): 2) phase advance over the distance s: A low β * insertion with β *<<drift space length  ψ (s)=180 o &  Q=~0.1 A high β * insertion with β *>>drift space length  ψ(s) & Q contributions  0   Q=~0.3 The problem?  Those tune changes are too big to be fully compensated locally Courtesy of H. Burkhardt

Why luminosity leveling is needed in IP2/8? Simulated ALICE TCP pile-up event composed of 25 single pp events. The black tracks are the triggered event. ALICE Luminosity limitations for pp collisions:  TPC and Silicon detectors pile-up  µ-Trigger RPCs trips with high luminosity Optimal detector operation and physics performance with TPC = no pile-up  cm -2 s -1 For cm -2 s -1  interaction rate ~200 kHz  20 overlapping events in the TPC  95% of the data volume corresponds to unusable partial events 1.To limit the PILE-UP!!! 2.To limit the LUMINOSITY (Nb >>) What is the PILE-UP?

Why luminosity leveling is needed in IP2/8? High luminosity & high σ (b ƃ ) ~ 500 µbarn 14 TeV) B mesons ExperimentDesign L (cm -2 s -1 ) interesting events (10 7 s) ATLAS/CMS Higgses/IP to be found in minimum bias events LHCb b ƃ events* LHCb doesn’t need to push the luminosity; it has important advantages to run at low lumi: 1.Less radiation  slow aging 2.Less pile-up: events are easy to analyze, less detector occupancy Detector limitations: 1.The signal/background ratio of most analyses does not improve with a pileup>2.5 2.The output bandwidth of the Readout Boards (event size per event fragment)  max pileup ~2.5 3.Total bandwidth across readout network 65 Gigabyte/s (!) 4.Complete event readout rate to High Level Trigger farm 1.1 MHz (!)

How do we reduce luminosity ? Currently we separate the beams at the IP: But other methods are possible: β * leveling Crossing angle leveling Crab cavities  no crossing angle

LHCb pile-up = f(beam separation) 1-1.5σ >1.5σ

Luminosity leveling during LHC operation --- ATLAS LUMINOSITY --- CMS LUMINOSITY --- LHCb LUMINOSITY Beam energy B1 current B2 current ~ 20 hours of STABLE BEAMS Courtesy F. Alessio Designed value

Integrated luminosity pictures 2012: 23 fb -1 at 8 TeV fb -1 at 7 TeV fb -1 at 7 TeV 4 th July seminar Total :  28.8 fb -1 delivered to ATLAS  27 fb -1 recorded  ~26 fb -1 good-for-physics (~90% of delivered), in spite of challenging conditions System√s (TeV)Days of running Integrated luminosity pp pb -1 pPb5.0224> 30 nb -1 ALICE