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The future linear collider Rob Appleby ASTe. C Daresbury Laboratory
Overview of talk l Introduction to the linear collider - a physics-driven lepton-colliding precision machine. l What we hope to see at the LC and how we'll see it l The available technology and the choice l ASTe. C AP/ID group activities Broadly speaking - physics then technology
The standard model of particle physics l l 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 (~19 free parameters) 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 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 W, Z, f all gain mass (but , g stays massless) by interaction with the Higgs condensate EW data suggest mh in range 114 -250 Ge. V new physics at < 1 Te. V
The hierarchy problem and SUSY l l Hierarchy problem: why is mw << mp? (mp ~ 1019 Ge. V is scale of gravity) Or, why is "problem of numbers" Vcoulomb >> Vnewton? e 2 >> G m 2 "fine-tuning problem" Set by hand? loop corrections? (Higgs mass unstable to radiative corrections and mh = O ( / ) 2 grows arbitrarily big) New physics is needed Cancel boson loops fermions + Need | mb 2 – mf 2| < 1 Te. V 2 Strong case for new physics at Te. V scale
What the Higgs will look like at the LHC… h 0 -->2 jets E-cal deposition (CMS)
Physics discovery and the LHC Well defined case of physics at 1 Te. V, 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 Te. V- 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 l l l 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! (Cosmic background explorer) (Wilkinson Microwave Anisotropy Probe) 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 Ge. V Need to study h->ttbar and WWh coupling, so baseline machine needs to be around 500 Ge. V 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 Ge. V to 70 at mh=70 Ge. V)
LC upgrades Electroweak measurements indicate new physics in the energy range 500 -1000 Ge. V (Mainly SUSY and possibly extra dimensions) Can also study rare Higgs decays, as WW fusion increases with s, and can study decays like h 0 --> + Also would like polarised beams, as this allows: ØStudy of parity violation in the electroweak sector Øpreferential production of scalar selectrons (enhanced by two beams of opposite polarity)
INTERNATIONAL SCOPE DOCUMENT l BASELINE MACHINE l l l l UPGRADES l l l ECM of operation 200 -500 Ge. V 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 90 Gev for calibration ECM about 1 Te. V Allow for ~1 ab-1 in about 3 -4 years OPTIONS l l l Extend to 1 ab-1 at 500 Ge. V 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) Need to compute a whole range of inter-beam effects Beam-beam phenomena at LC Øbeam-beam disruption Øluminosity pinch enhancement Øphoton emission Øe+e- pair production 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 Ge. V 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 Ge. V machine Both warm and cold technologies are limited to ~1 Te. V
CLIC (Compact Linear Collider) 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 Layout for s=3 Te. V
The technology decision and the ITRP l l 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-Te. V 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 ASTe. C l l 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 ASTe. C as part of the larger community.
TESLA crossing angle schemes (Rob and Deepa) l l l 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 ASTe. C, 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) l l l 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).
BDS Collimation (Frank Jackson) l l 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 beam halo simulation in TESLA BDS collimation section detector masking apertures desired SR fan
Ground Motion & Emittance Tuning (James Jones) l l Stability of linear collider, from damping ring through to IP, has a direct consequence on the beam size and so the luminosity. l Motion in damping ring leads to wakefield effects and higher order magnetic field effects – direct emittance increase. l Motion in linac leads to wakefield and HOMs – direct emittance increase. l 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. l Effects of Feedback on Luminosity ATL Motion in the NLC damping ring The real machine will need to be ‘tuned’ by using: l A static steering algorithm using corrector magnets (introduce spurious dispersion) and quadrupole movers to align the magnetic components l A fast feedback scheme (inter-train) to correct the orbit in the DR, linac and LET l A fast IP based feedback system at few train or intra- train speeds (NLC or TESLA)
Undulator for Polarised Positron Production (Duncan Scott) l l Circularly polarised radiation (20 Me. V) will produce polarised positrons in a thin target (by pair production) 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 Super-Conducting Bifilar helix l permanent magnet undulator
Summary l l l I've talked about the need for a Te. V 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 ASTe. C LC work is now fully underway and we hope to make significant contributions in the coming years.