23rd October 2006 Alan Barr 1

Discovering and exploring the new world

• Why we’re looking for new particles• Making dark matter in the lab• My work: discovering and understanding

new particles• ATLAS semiconductor tracker upgrade• Grand objectives

Alan Barr

23rd October 2006 Alan Barr 2

Why high energies? (1)• See deeper

– High energy high momentum small wavelength

cells: 50 μm

DNA: 2 nm

atom nucleus: 2 fm

quarks:< 0.001 fm

x 25,000

x 1,000,000

x up to2,000

ph

wavelength

Planck’s constant

Momentum

Scatters offbulk

Scatters offconstituent

λ

23rd October 2006 Alan Barr 3

2.. cmE Energy

Why high energies? (2)• Create new particles

– Energy matter interchangeable

– High energies can make heavy particles

Heavy particle production is the main purpose of the new highest energy colliders

Heavy particle production is the main purpose of the new highest energy colliders

2.. cmE anti-quarkquark

annihilate

Produce new heavy-weight

speed / c

Velocity factor 21

221

cv

Speed of light

Mass

Velocity factor

23rd October 2006 Alan Barr 4

The new periodic table

• Building blocks and mathematical theory understood• Mostly extremely well tested

– Higgs (h) and Graviton (G) not directly observed yet

Electron, e

d quark

Photon, γ

Gluon, gDiagrammatic only

Commonplace particles

u quark

Matter Particles

u

Quarks

d

c

s

t

b

Leptons

νe νμ ντ

e μ τ

Known force carriers

W Z γg

Others

h G

23rd October 2006 Alan Barr 5

The whole story?

• Stuff we understand ~ 4%• Evidence for Dark Matter from:

– Rotation curves of galaxies– Microwave background radiation– Galaxy cluster collision

astro-ph/0608407

Invisible mass

Visible mass

• Colliding galactic clusters• Normal matter mostly in

interstellar gas– X-ray detection– Hits and slows down

• But the bulk of the mass has not interacted– From gravitational lensing

Large Dark Matter component

Particle physicists should hunt: Weakly Interacting, Stable, Massive Particles

Particle physicists should hunt: Weakly Interacting, Stable, Massive Particles

23rd October 2006 Alan Barr 6

Candidates?• Particles related to

“normal” matter by a symmetry:– Supersymmetry

• Relationship between particles with spins differing by ½h

– Spatial symmetry• With extra dimensions

– “Gauge” symmetry• Extra force

interactions (and often matter particles)

electron

quarks

exoticpartners?

Force-carriers

Related bysymmetry

Related bysymmetry

neutrino

x3x2

…?

…?

…?

Alreadyobserved

_

23rd October 2006 Alan Barr 7

Producing exotics?

Time

standard

exotic

Time

standard

exotic

Time

standard

exotics

Time

standardexotics

• If exotics can be produced singly they can decay– No good for

Dark Matter candidate

• If they can only be pair-produced they are stable– Only

disappear on collision (rare)Require an even number of exotic legs to/from blobs

(Conserved multiplicative quantum number)

Require an even number of exotic legs to/from blobs(Conserved multiplicative quantum number)

23rd October 2006 Alan Barr 8

How do they then behave?

• Events build from blobs with 2 “exotic legs”

• A pair of cascade decays results

• Complicated end result

• Events build from blobs with 2 “exotic legs”

• A pair of cascade decays results

• Complicated end result

Time

standard

2 exotics

Production part

Time

standard

heavyexotic lighter

exotic

Decay part Time

Complete “event”

= exotic= standard

23rd October 2006 Alan Barr 9

How to discover at colliders?

• Can’t see dark matter particles themselves– Weakly interacting– Pass through

detector– “Invisible”

• Observe some visible decay products

• Plus apparent non-conservation of momentum(Perpendicular to beams)

• “Missing” momentum is sum of momenta of invisible particles

• Can’t see dark matter particles themselves– Weakly interacting– Pass through

detector– “Invisible”

• Observe some visible decay products

• Plus apparent non-conservation of momentum(Perpendicular to beams)

• “Missing” momentum is sum of momenta of invisible particles

proton proton

short-livedexotic

short-livedexotic

Invisibleexotic

Invisibleexotic

Visibleparticle

Visible particle

missing

z

x

23rd October 2006 Alan Barr 10

The “real thing”(a simulation of…)

• Two high-energy jets of particles– Visible decay

products

• Missing momentum– From two

invisible particles

• More complicated than on previous page

Proton beams perpendicular to screenProton beams perpendicular to screen

Invisibleparticles

23rd October 2006 Alan Barr 11

The new machine at CERN

• Large– 27 km circumference

• Hadron – Mostly protons

• Collider~ 7 x higher collision energy~ 100 x increase in collision

rateCompared to current best machine

(Tevatron near Chicago)

• Timeline:– Currently in commissioning– First collisions:

November 2007– First high-energy run:

Spring 2008– Much background work

already done in simulations

Magnets tobend beams

Detectors at collision points

23rd October 2006 Alan Barr 12

My work (1)

• Digging out the exotic stuff:– Want to isolate ~ one event per billion– Lots of less interesting stuff going on– Intelligent choice vastly improves quality of selection

• Relevance:– Major discovery about nature– Motivates further study…

• International Linear Collider

“interesting”“interesting”

“lessinteresting”

“lessinteresting”

hep-ph/0208214 hep-ph/0304226

23rd October 2006 Alan Barr 13

My work (2)

• Measuring particle masses– Interpreting incomplete information– Reconstructing complex decays

• Relevance:– Supersymmetry breaking– Sizes of extra dimensions– Unification of masses at very high energies?– Dark matter relic “predictions”

Mass

hep-ph/0208214 hep-ph/0102173hep-ph/0106304

23rd October 2006 Alan Barr 14

My work (3)

• Measuring the particle angular momenta– How much spin do the exotic particles posses – Previously thought to require a precision machine ($)– I have found good methods for measuring it at ATLAS

• Relevance?– Which of the candidate theories is relevant?– Supersymmetry? Extra dimensions?

Measure spin

hep-ph/0405052 hep-ph/0511115

23rd October 2006 Alan Barr 15

Spin determination

θ*p

S

Lq~ Lq

Rl~

02~

Rl

Measureinvariant mass=> find angle

Measureinvariant mass=> find angle

1

0~ LqP

S

Fermions

Polarisedfermion(Partner of W0)

Chiral couplings

Scalar

Scalar

Scalar: spin-0Fermion: spin-½

Scalar: spin-0Fermion: spin-½

hep-ph/0405052

23rd October 2006 Alan Barr 16

Detecting the debrisDetecting the debris

ATLAS:Diameter ~ 20 m

ModuleLength ~ 12 cm

Tracker:Diameter ~ 2 m

23rd October 2006 Alan Barr 17

Silicon detectors

• Cross-section through a sensor– Charged particle excites

electrons in silicon– Electric field sweeps them

towards electrode– Electrical signal amplified and

digitised– Data sent to off-detector

electronics

• One of 4088 ATLAS semiconductor tracker modules– Complete tracker is

like a 6 MegaPixel digital camera

– Our camera takes 40 million photos per second

12 cm

23rd October 2006 Alan Barr 18

My contributionsR&D Assembly Commissioning

• Irradiations and beam tests at CERN• Performance tests of prototypes

• Functional testing during assembly

• Verification of performance

• Cosmic ray detection at CERN•

• Getting ready for physics

At each of these stages I’ve played major roles in making the system work,reading out the detectors, and understanding the results

At each of these stages I’ve played major roles in making the system work,reading out the detectors, and understanding the results

NIM.A 538:384-407,2005

June 06,CERN

Sep 2004, downstairsJune 2000, CERN

23rd October 2006 Alan Barr 19

Into the future?• Various important

measurements are likely to be statistics-limited

• Motivates study of luminosity upgrade – Need about a factor of 10

increase in collision rate– Redesign detectors

• Various important measurements are likely to be statistics-limited

• Motivates study of luminosity upgrade – Need about a factor of 10

increase in collision rate– Redesign detectors

• International R&D effort started– UK involved in several aspects

of upgraded tracker design• Grant proposal submitted

– My continuing interest is in the off-detector electronics

(and associated readout and calibration systems)

– Compliments optical design and super-module testing already planned in Oxford

• International R&D effort started– UK involved in several aspects

of upgraded tracker design• Grant proposal submitted

– My continuing interest is in the off-detector electronics

(and associated readout and calibration systems)

– Compliments optical design and super-module testing already planned in Oxford

High significance spin-determinationoften requires hundreds of fb-1

hep-ph/0511115

23rd October 2006 Alan Barr 20

Grand Objectives• Discover new particles

– Focus on Dark Matter-motivated signals

• Extract maximum information about them– What type of particles are these?– What can they us about:

• New symmetries of nature?• Dark Matter?• Higher-scale physics? (Unification …)

• Upgrade tracker for high luminosity– Construct UK demonstrator prototype in Oxford