The Large Hadron Collider

LHC Operation III: pPb, PbPb collisions: operation,

luminosity limits Luminosity in a hadron collider: Van der

Meer scanAbsolute luminosity, High beta physics, Luminosity leveling

[R. Alemany][CERN BE/OP]

[Engineer In Charge of LHC]Lectures at NIKHEF (22.03.2013)

pPb collisions1. LHC main dipoles are connected in series

both rings experience the same magnetic field at any time

(Bρ)Pb = (Bρ)p momentum is fixed2. The revolution period (T) of a particle is a

function of its charge (Z) and mass (A) Tp(Z=1,A=1) ≠TPb(Z=82,A=208)

IP1

IP2 IP8

IP5How 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!

Tp(Z=1,A=1) ≠ TPb(Z=82,A=208) We need Tp(Z=1,A=1) = TPb(Z=82,A=208)

IP1

IP2 IP8

IP5

Cref ≠ CPb ≠ Cp

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 particlesA 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

Bun

ch in

tens

ity

Bunch number

SPS measurement @17GeV 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*

AOne inelastic vertex

f

A

A

A*

A*Two inelastic vertex

A

A

A*

ASingle excitation

A

A

A*

A*Mutual excitation:

1 exchange

A

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 states2. Giant Dipole Resonances (GDR)*3. Quasideuteron absorption4. Nucleon resonance excitation

Change of Z/A∆ 𝑝/𝑝

Change of

A

A

A(Z=81)

ABFPP

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

Dx(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=𝐿𝜎N = number of collisions / secondL = 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 currentB2 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

yOne beam is fixed the other scans the effective cross section area, first one plane, then the other.

Beam separation

dN/dt

6σ

Aeff6σ

Describes the overlap profile𝐹 (𝛿𝑥 ,𝛿 𝑦 )𝐹 (𝛿𝑥 ,𝛿 𝑦 )=𝐹 (𝛿𝑥 ) 𝐹 (𝛿𝑦 )

The double Gaussian fit gives A & σS. 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?

fel : 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)=180o & 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 luminosityOptimal detector operation and physics performance with TPC = no pile-up

1029 cm-2s-1

For 3 1030 cm-2s-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?

revb fNiL

Why luminosity leveling is needed in IP2/8?

High luminosity & high σ(bƃ) ~ 500 µbarn (@ 14 TeV)

B mesonsExperiment Design L

(cm-2s-1)interesting events (107s)

ATLAS/CMS 1034 103 Higgses/IP to be found in 1016 minimum bias events

LHCb 1-5 1032 1012 bƃ events*

LHCb doesn’t need to push the luminosity; it has important advantages to run at low lumi:

1. Less radiation slow aging2. Less pile-up: events are easy to analyze, less detector

occupancyDetector limitations:1. The signal/background ratio of most analyses does not improve with a

pileup>2.52. The output bandwidth of the Readout Boards (event size per event

fragment) max pileup ~2.53. 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 currentB2 current

~ 20 hours of STABLE BEAMSCourtesy F. Alessio

Designed value

Integrated luminosity pictures

2012:23 fb-1

at 8 TeV

20115.6 fb-1

at 7 TeV

20100.05 fb-1

at 7 TeV

4th July seminar

Total 2010-2012: 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 8 145 2-5 pb-1

pPb 5.02 24 > 30 nb-1

ALICE