High Energy Physics forthe 21st Century

Step one: into the unknown

Christopher Lester

Where are we now?The Standard Model

• Higgs not yet found• Quark mixing not over-

constrained yet• Quark masses poorly

measured• Top-quark charge

undetermined!

Standard Model BadStandard Model Good

• No conflict with experiment (yet)

• Parts (QED) in extremely good agreement with experiment – even with atomic physics! (Lamb Shift, magnetic moments)

• Elementary particle content “reasonably” small …

Based on 17 events. [Markus Klute]

Preliminarily excludes exotic top-quark charge of -4/3 at 94% confidence. (365 pb-1) Spring 2006.

What is the charge of the top-quark?

Dark corners of the Standard Model

• Higgs not yet found• Quark mixing not over-

constrained yet• Top-quark charge

undetermined!• Quark masses poorly

measured• Fine-tuning / “hierarchy

problem” (technical) – Why are particles light?

• Does not explain Dark Matter

• No gauge coupling unification

Standard Model BadStandard Model Good

• No conflict with experiment (yet)

• Parts (QED) in extremely good agreement with experiment – even with atomic physics! (Lamb Shift, magnetic moments)

• Elementary particle content “reasonably” small …

New Physics, e.g. Supersymmetry, can help.

(1) What might the new physics be?

(2) What sort of experiment will help us?

(3) How will we go about extracting answers from the data?

(4) Can we trust the answers?

Four Questions:

Will describe some later.

Coming next!

Very much the work of people in The Cavendish.

… if time allows …

Simple experimental aim:

Collide protons and see what happens.

Large Hadron Collider (LHC)

Inside the LHC

“ATLAS” Experiment

7 TeV

7 TeV

The Semiconductor Tracker

LHC protons

protons

Note concerning units

eV = electron-volt = 1.6 x 10-19 J

GeV = 10 9 eV = 1.6 x 10-10 JTeV = 1012 eV = 1.6 x 10-7 J

(= K.E. of 1.3mg mosquito at 0.5 m/s)

Express most particle energies and masses in GeV …

… but LHC proton beams are 7 TeV each (14 mosquitos in total)

Anatomy of the detector

Layered like Onion

Different layers for different types of particles

NeutrinoNeutrino

MuonMuon

So main things we can do• Distinguish the following

from each other– Hadronic Jets,

• B-jets (sometimes)– Electrons, Positrons,

Muons, Anti-Muons• Tau leptons (sometimes)

– Photons

• Measure Directions and Momenta of the above.

• Infer total transverse momentum of invisible particles. (eg neutrinos)

Hadronic Jet

electron

photon

Average direction of

things which were

invisible

Muon Detector

Man for scale

MAGNETIC FIELD

MAGNETIC FIELD

Muons bend away from us.

Anti-muons bend toward us.

Calorimetersand Central Solenoid

Right Honourable and Most Reverend Dr Rowan Douglas Williams, the 104th Lord Archbishop of Canterbury and Primate of All England

Transition Radiation Tracker (TRT) – tracks charged particles and distinguishes electrons from pions

The SemiConductor Tracker (SCT)

Many components designed and built in

The Cavendish

Records tracks of charged particles

Most of the data-acquisition and

calibration/monitoring software designed and written in Cambridge

SCT contains 4088 “Modules”768 sensitive-strip diodes per side. (200 V)

3 infra-red communication channels.

Collisions recorded @ 40MHz (every 25 ns)

Neutron bombardment will degrade silicon over time.

Individual strips will need recalibration.

Optical properties need adjustment.

May need to use redundant links.

10cm

SCT Data Acquisition Software

• Present size:– 350,000 lines of code– ~6 developers

• Much still to be done:– Have managed to control 500 modules at once

• only 1/8th of final number• “multi-crate” development - parallelisation

– Needs to become usable by non-experts– Needs to recover from anomalies automatically

Evidence that it will work:

Data from morning of 18th May 2006

First cosmic rays seen in SCT and TRT!

PRELIMINARYPRELIMINARY

Back to the new physics• Fine-tuning / “hierarchy

problem” (technical) – Why are particles light?

• Does not explain Dark Matter

• No gauge coupling unification

Remember the aim was to fix some of these problems with

the Standard Model

Possibilities:

• Supersymmetry– Minimal– Non-minimal– R-parity violating or conserving

• Extra Dimensional Models– Large (SM trapped on brane)– Universal (SM everywhere)– With/without small black holes

• “Littlest” Higgs ?• ….

We will look at supersymmetry (SUSY)

Supersymmetry!CAUTION!

• It may exist• It may not• First look for

deviations from Standard Model!

Gamble: IF DEVIATIONS ARE SEEN:• Old techniques won’t work• New physics not simple• Can new techniques in SUSY

but can apply them elsewhere.

Experiment must lead theory.

What is Supersymmetry?

Electron Higgs Anti-ElectronHiggs

Selectron Higgsino Anti-selectronHiggsino

Matter Antimatter

Supersymmetric Matter

ReverseReverse the the chargescharges, ,

retain the spins.retain the spins.

Retain the charges,Retain the charges,reversereverse the the spinsspins..((exchange boson exchange boson with fermionswith fermions).).

For technical reasons each sparticle can be heavier than its partner by no more than a TeV or so.

Great!

• Fix Hierarchy Problem

• The Lightest Neutralino (LSP) is a prime candidate for neutral stable cold Dark Matter

• Can have gauge coupling unification

Neutralinos : The collective name of the supersymmetric partners of the photon, the Z-boson and the higgs boson.

LSP :Lightest Supersymetric Particle. Often the lightest neutralino.

ΩCDMh2 = 0.103 ± 0.009

(WMAP 3-year data)

Unfortunately

• Doubling of particle content

• Conservation of “R-parity”– LSPs generated in pairs– LSPs invisible to ATLAS

• Large number of tuneable parameters– Assume just five of them exist for the

moment – unification arguments

What might events look like?

What we can see

What we can see

Here Be Monsters! (again)

This is the high energy physics of the 21st Century!

(What they really look like)

An example of an event where a higgs boson decayed to a pair of b-quarks/

b

b

soft gluon radiation?

• Lots of missing energy • Lots of leptons• Lots of jets

• ATLAS Trigger: ETmiss > 70 GeV, 1 jet>80 GeV. (or 4 lower energy jets). Gives 20Hz at low luminosity.

So main EASY signatures are:

Just

Cou

nt E

vent

s!

Indicates deviation from The Standard Model.

Squark/gluon mass scale

(GeV)effM

even

ts

Signal

S.M. Background

Peak of Meff distribution correlates well with SUSY scale “as defined above” for mSUGRA and GMSB models. (Tovey)

What you measure:

The real test comes when you want to measure individual masses etc.

Technique 1: Kinematic Edges

Plot distributions of the invariant masses of what you can see

Technique 1: Kinematic Edgesll

lq high

llq Xq

lq low

llq

Xqllq

lq lowlq high

llqll

Technique 1: Kinematic Edges

Account for all ambiguities:

Both look the same

to the detector

Det

erm

ine

how

edg

e po

sitio

ns d

epen

d on

sp

artic

le m

asse

s

Technique 1: Kinematic EdgesUse custom Markov-Chain algorithms to sample efficiently from the high dimensional parameter spaces of the model according to the Bayesian posterior probability.

Shape of typical set is often something quite

horrible.

Technique 1: Kinematic Endpoints

Finally, project onto space of interest:

Slepton massCorrelation between slepton mass measurement and neutralino mass

measurement.

01

~

01

~Rl~

Rl~

l

l

Other Techniques:• Look at the shapes of the distributions

– Systematic errors harder to control

• Create new variables– “Cambridge MT2 Variable”

now international used methodfor sparticle mass measurementin pair production

• Incorporate cross sections and branching ratio measurements– again, Cambridge “leading the way” as home

to the most developed samplers for H.E.P.

SM

param

s

Can even bring these techniques to bear on the data we have today

• m0

• M1/2

• A0

• Tan beta• Sgn mu• mb

• mt

• αs(Mz)

• Know • Don’t know

Quantity Measured value

ΩDMh2 (WMAP) 0.1126 +0.0081 -0.0091

muon (g-2)/2 (19.0 ± 9.4) * 10-10

BR(b->s γ) (3.52 ± 0.42)*10-4

mb 4.2 ± 0.2 GeV

mt 172.7 ± 2.9 GeV

αs(Mz) 0.1187 ± 0.002

SU

SY

pa

ram

s

2D Slices of 5D SUSY parameter space tell you very little …

Roszkowski et.al.

Even worse news:

Standard Model errors are very important!

Standard Model uncertainty:

mtop = 170 GeV mtop = 180 GeV

Experiment: mtop = 178 ± 4.3 GeV in 2006 (was 174.3 ± 3.2 GeV in 2004)

Top Quark Mass

Standard Model uncertainty:

mbot = 4.0 GeV mbot = 4.5 GeV

Experiment: mbot = 4.1 to 4.4 GeV in MS scheme

Bottom Quark Mass

h0 poleregion

Slepton-neutralinoco-annihilation

region

Pseudoscalar higgs A0 s-channel

annihilation region

First analysis able to fold everything together was from Cambridge:

“Multi-Dimensional mSUGRA Likelihood Maps”, B.C. Allanach & C.G. Lester (Phys.Rev. D73 (2006) 015013)

The parts of Supersymmetric Parameter Space are consistent with Today’s data:

What if the Dark Matter isn’t all SUSY?Dark matter is just made of SUSY neutralinos:

Other sources of Dark Matter allowed in addition to SUSY:

Favoured regions of SUSY model don’t change an awful lot!Prediction fairly robust.

Future plans• The whole programme is about the future.• If we knew what the experiment will tell us we wouldn’t

need to build it. Experiment must lead.

• In short term, must continue to integrate further with CERN physics analysis teams.– Analysis will be in collaboration

• In 10 years the SCT will have been radiation damaged beyond repair, and the LHC may be upgraded.– Need to start work on “SCT version 2” long before 10 year

lifetime of “version 1” is reached– LHC luminosity upgrade will place more demands on tracking

systems

• Cavendish HEP group in ideal position to play leading role in that endeavour.

• Must strive to draw maximum inference from LHC data!

R.I.P.S.C.T.

2017

Conclusions• Expect new particles, new physics and other discoveries at the LHC• May include a Dark Matter candidate ?

• Many competing physical theories:– Supersymmetry is one possibility – There are many others:

• (UED, Large Extra Dimensions, Littlest Higgs …)

• An example experimental technique was presented in the context of Supersymmetry– Kinematic endpoints and other measurements + care + efficient

sampling from Posterior Distribution on parameter space – Supersymmetry may not be what nature has chosen!

• Techniques will be applicable to any theory with large particle content and Dark Matter candidate – and to others too

• Many more things I would like to have shown you:– How to measure particle spins and distinguish SUSY from UED etc ….

Cambridge Office

The End, and the ATLAS Collaboration

Christopher Lester

2006

Spare slides

Pos

terio

r m

aps

• 19th Century– 1897: Electron (Thomson)

• 20th Century:– 1911: Nucleus (Rutherford)– 1930: Neutrino postulated (Pauli, beta decay)– 1936: Muon (Anderson, cosmic rays)– 1956: Neutrino observed (Cowan, Reines, et al)– 1960s and 1970s: Growing support for light quarks– 1960s: Higgs boson postulated– 1970s: Tau discovery– 1996: Top quark discovered (Tevatron)

• 21st Century– Something’s coming, something good, (West Side Story)

Progress in the last Century

25 year waitfor neutrino

20-30 year waitfor top quark

45 year wait for Higgs ??

Anatomy of a Detector

ATLAS blind data challenge

• Didn’t discover anything that wasn’t there.

What do events look like?

(Baryon number violating)

RPV RPV

(Lepton number violating)

RPC RPC

The SCT Software

Overall SCT Controller

VME Crate Controller

ModuleModule

Module

VME Crate Controller

ModuleModule

Module

VME Crate Controller

ModuleModule

Module

Various

etc.GUIs

andusers

SctApi