The future linear collider

Rob Appleby

ASTeC

Daresbury Laboratory

Overview of talk

Introduction to the linear collider - a physics-driven lepton-colliding precision machine.

What we hope to see at the LC and how we'll see it

The available technology and the choice

ASTeC AP/ID group activities

Broadly speaking - physics then technology

The standard model of particle physics

Has been developed over the last 4 decades - guided by nature and is like the periodic table with interactions.

Is a quantum field theory, built using gauge symmetries

Electroweak sector leptons and neutrinos, which interact through the Z,W, gauge bosons

Strong sector quarks, which interact through the gluons

Theory develops by postulating the existence of matter, whose interactions are governed by the gauge symmetry.Very intricate theory - rich mathematical structure

(~19 free parameters)

Tested to a high precision - has amazing agreement with data(QED is the most tested theory in all of theoretical physics)

Beyond the standard model… (at least minimally)

The theory has many theoretical blemishes!

For example, matter/antimatter asymmetry or the fact that WW scattering is predicted to violate unitarity at large s,

requiring an unseen scalar particle to prevent this

Electroweak gauge invariance forbids the existence of fermion and gauge boson

masses....but we observe particle mass!Solution is the Higgs mechanism, which predicts the scalar Higgs boson which mediates the EW symmetry breaking

new physics at < 1 TeV

EW data suggest mh in range 114-250 GeV

W,Z,f all gain mass (but ,g stays massless)by interaction with the Higgs condensate

The hierarchy problem and SUSY

Hierarchy problem: why is mw << mp? (mp ~ 1019 GeV is scale of gravity)

Or, why is Vcoulomb >> Vnewton? e2 >> G m2

Set by hand? loop corrections? mh = O (/) 2

New physics is needed Cancel boson loops fermions Need | mb

2 – mf2| < 1 TeV2

Strong case for new physics at TeV scale

"fine-tuning problem"(Higgs mass unstable to radiative corrections and grows arbitrarily big)

"problem of numbers"

+

What the Higgs will look like at the LHC…

h0-->2 jets

E-cal deposition

(CMS)

Physics discovery and the LHC

Well defined case of physics at 1 TeV, mainly Higgs and SUSY

The large hadron collider (LHC) is a discovery machine now being built at CERN - commissioning in 2007

Collides p/pbar at s=14 TeV- it should see the new physics.

interaction s not known and high backgrounds LHC is not

a high precision machine

The need for precision measurement and the LC

The LC adds "value-for-money" to the LHC The goal of the LC is to make precision measurements of

new physics, by linearly colliding leptons (electrons) Energy loss per turn stops us making a circular collider. Physics community agrees that a precision linear machine

should be the next big particle physics project. Ideally, overlap LC with LHC and so need to start building at

the end of this decade. Physics benefit for such synergy is well-documented…a

~600 page report is about to be published by the LC/LHC working group.

What precision does for you!

(Wilkinson Microwave Anisotropy Probe) (Cosmic background explorer)

These are maps of the oldest light (379000 years) in the universe - the microwave background. Red shows "warmer"

regions and blue shows "cooler" regions.

The higher resolution resolves tiny fluctuations (1:106 degrees), supporting and strengthening inflation theories

The baseline physics program

This is set by the first stage - the Higgs searches

Main production channels

Higgstrahlung WW fusion

Higgstrahlung peaks at s=220-340 GeV

Need to study h->ttbar and WWh coupling, so baseline machine needs to be around 500 GeV

The luminosity should produce enough Higgs…set by Higgstrahlung and comes out to be 500 fb-1 for base program

(300 fb at mh=115 GeV to 70 at mh=70 GeV)

LC upgrades

Electroweak measurements indicate new physics in the energy range 500-1000 GeV

(Mainly SUSY and possibly extra dimensions)

Also would like polarised beams, as this allows:Study of parity violation in the electroweak sectorpreferential production of scalar selectrons (enhanced by two beams of opposite polarity)

Can also study rare Higgs decays, as WW fusion increases with s, and can studydecays like h0-->+-

INTERNATIONAL SCOPE DOCUMENT

BASELINE MACHINE ECM of operation 200-500 GeV Luminosity and reliability for 500 fb-1 in 4 years Energy scan capability with <10% downtime Beam energy precision and stability below about 0.1% Electron polarization of > 80% Two IRs with detectors ECM down to 90Gev for calibration

UPGRADES ECM about 1 TeV Allow for ~1 ab-1 in about 3-4 years

OPTIONS Extend to 1 ab-1 at 500 GeV in ~ 2 years e-e-, , e-, posi-pol Giga-Z, WW threshold

http://www.fnal.gov/directorate/ icfa/LC_parameters.pdf

Beam size and beam-beam physics

Luminosity requirements dictate a beam size of O(10-9 m)

Beam-beam phenomena at LCbeam-beam disruptionluminosity pinch enhancementphoton emissione+e- pair production

Need to compute a whole range of inter-beam effects

This animation was produced using the beam-beam

simulator GUINEA-PIG, and illustrates the high angular

divergence of a collision beam

A generic linear collider

The technology options

The "cold" technology

Superconducting (or "cold") cavities operating at 1.3 GHz (L-band) have been built with

gradients of 35 MV/m

These cavities produce a time structure with a long time between

pulses (long damping rings)

These cavities are the basis of the TESLA linear collider proposal for a 500/800 GeV machine

The "warm" technology

Normal conducting (or "warm") RF cavities have been developed with

gradients of 50 MV/m

They operate at either X-band (11.4 GHz) or C-band (5.7 GHz), and

produce very closely spaced pulses

They form the basic of the (very similar) NLC and GLC (formally JLC) designs for a 500/1000 GeV machine

Both warm and cold technologies are limited to ~1 TeV

CLIC (Compact Linear Collider)

Layout for s=3 TeV

CLIC uses a two beam system to achieve

gradients of 150 MV/m

A high-current low-energy drive beam transfers RF power to the main beam

Operates in 30 GHz region, with normal conducting accelerating structures

The technology decision and the ITRP

A global review of the technology choice has been made by the TRC, and the bottom line is that both the warm and the cold technology meet the requirements of the LC and are viable in the short term (multi-TeV CLIC is not).

Currently a recommendation panel (ITRP) of 12 "wise men" (4 from Europe, 4 from Asia and 4 from the US) are assessing both technologies, and should make a recommendation to ICFA/ILCSC before the end of the year.

When this happens, the community should unite behind the chosen technology and form the GDI, with a TDR and detector designs being published in ~2007. Construction should begin around 2010 and commissioning around 2015.

Such issues as the site have yet to be officially approached.

The positioning of the UK and ASTeC

The linear collider community in the UK have formed the LC-UK forum. A collection of institutions has also formed, called LC-ABD, which focuses on the BDS design.

The beam delivery system is the final part of the collider, which takes the accelerated beams from the Linac, focuses them and collides them at the interaction region.

It's important that, until the technology choice is made, that the work done is as technology independent as possible. We need to be ready for the recommendation whatever it is!

The following slides show the current work on elements of the (mostly) TESLA BDS, which is currently being done by the AP/ID group of ASTeC as part of the larger community.

TESLA crossing angle schemes (Rob and Deepa)

The time sequence of the NLC/GLC designs means that the two beams must cross at an angle to avoid extra collisions.

The TESLA design allows a head-on collision. However, problems with head-on collision beam extraction have led to the formulation of "hybrid" crossing angle schemes, where the beams cross with a very small (~1-2 mrad) angle.

The TESLA project has yet to choose between a head-on and a crossing angle collision geometry..

In the AP group of ASTeC, we've developed a vertical crossing angle scheme for TESLA. The

design has a final-focus magnetic quadruplet to aid beam extraction.

Final Focus System (FFS) Optimisation (Deepa Angal-Kalinin)

FFS to focus the beam to the required beam sizes at the Interaction Point (typically x~ 200-500 nm, z~1-5 nm)

Strong final focus quadrupoles required to get demagnification.

Chromatic and geometric aberrations need to be minimised up to fourth order.

Emittance growth due to SR should be minimum.

Luminosity as a function of energy spread is the yardstick.

We have developed an expertise to do these optimisations (Collaboration with CEA, Saclay group).

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Final Focus Optics for L*=5m with quadruplet

Beam Sizes and Luminosity

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Goal is to reduce the beam halo which may emit synchrotron radiation in the final focus and thus generate background in particle physics experiments

System should also provide machine protection in event of beam energy error

TESLA BDS first design shows poorer collimation efficiency than other designs (NLC etc)

Simulations (MERLIN) underway to investigate collimation efficiency in TESLA

detector masking apertures

beam halo simulation in TESLA BDS collimation section

desired SR fan

BDS Collimation (Frank Jackson)

Ground Motion & Emittance Tuning (James Jones)

Stability of linear collider, from damping ring through to IP, has a direct consequence on the beam size and so the luminosity. Motion in damping ring leads to wakefield effects and

higher order magnetic field effects – direct emittance increase.

Motion in linac leads to wakefield and HOMs – direct emittance increase.

Motion in LET leads to vertical dispersion and coupling – direct beam size increase.

This motion can be modelled through the, so called, ATL law for slow motion, and as elastic ground waves for fast motion.

The real machine will need to be ‘tuned’ by using: A static steering algorithm using corrector

magnets (introduce spurious dispersion) and quadrupole movers to align the magnetic components

A fast feedback scheme (inter-train) to correct the orbit in the DR, linac and LET

A fast IP based feedback system at few train or intra- train speeds (NLC or TESLA)

Effects of Feedback on Luminosity

ATL Motion in the NLC damping ring

Undulator for Polarised Positron Production (Duncan Scott)

Circularly polarised radiation (20 MeV) will produce polarised positrons in a thin target (by pair production)

Super-Conducting Bifilar helix permanent magnet undulator

A “Helical Undulator” is used with the main electron beam to create the circularly polarised radiation.

Two technology choices are available – prototypes of each design will be built to test their feasibility

Summary

I've talked about the need for a TeV linear collider, and what we hope to discover from this machine.

The physics case for new physics is very persuasive and gives us a physics-driven accelerator design.

The world LC community is waiting for the technology recommendation to be made later this year, and then hopes to begin the global design process. The goal is to start construction 2010; this will allow concurrent running with the large hadron collider at CERN.

The challenge of colliding nanometre scale, high density charged particle beams is immense and there is a lot of challenges to face before the LC will be taking data.

The ASTeC LC work is now fully underway and we hope to make significant contributions in the coming years.