Transcript High Energy Physics for the 21 Century Step one: into the unknown

High Energy Physics for
the 21st Century
Step one: into the unknown
Christopher Lester
Where are we now?
The Standard Model
Standard 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 …
Standard Model Bad
• Higgs not yet found
• Quark mixing not overconstrained yet
• Quark masses poorly
measured
• Top-quark charge
undetermined!
Dark corners of the Standard Model
What is the
charge of the
top-quark?
Based on 17 events.
[Markus Klute]
Preliminarily excludes exotic
top-quark charge of -4/3 at
94% confidence. (365 pb-1)
Spring 2006.
Standard 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 …
Standard Model Bad
• Higgs not yet found
• Quark mixing not overconstrained 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
New Physics, e.g.
Supersymmetry, can help.
Four Questions:
(1) What might the new physics be?
Will describe some later.
(2) What sort of experiment will help us?
Coming next!
(3) How will we go about extracting
answers from the data?
Very much the work of people in The Cavendish.
(4) Can we trust the answers?
… 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
Note concerning units
eV = electron-volt = 1.6 x 10-19 J
GeV = 10 9 eV
= 1.6 x 10-10 J
TeV = 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
Neutrino
Muon
So main things we can do
• Distinguish the following
from each other
– Hadronic Jets,
• B-jets (sometimes)
– Electrons, Positrons,
Muons, Anti-Muons
Average
direction of
things
which were
invisible
electron
• Tau leptons (sometimes)
– Photons
• Measure Directions and
Momenta of the above.
• Infer total transverse
momentum of invisible
particles. (eg neutrinos)
Hadronic
Jet
photon
Muon Detector
MAGNETIC FIELD
MAGNETIC FIELD
Muons bend away from us.
Anti-muons bend toward us.
Man for scale
Calorimeters
and Central Solenoid
Transition Radiation Tracker (TRT) – tracks charged particles and
distinguishes electrons from pions
Right Honourable and Most Reverend Dr Rowan
Douglas Williams, the 104th Lord Archbishop of
Canterbury and Primate of All England
The SemiConductor Tracker (SCT)
Records tracks of charged particles
Many components
designed and built in
The Cavendish
Most of the dataacquisition and
calibration/monitoring
software designed and
written in Cambridge
SCT contains 4088 “Modules”
10cm
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.
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:
First cosmic rays seen in SCT and TRT!
Data from morning of 18th May 2006
Back to the new physics
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 ?
• ….
• Fine-tuning / “hierarchy
problem” (technical) –
Why are particles light?
• Does not explain Dark
Matter
• No gauge coupling
unification
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?
Matter
Reverse the charges,
retain the spins.
Electron
Higgs
Selectron
Higgsino
Antimatter
Higgs
Anti-Electron
Retain the charges,
reverse the spins.
(exchange boson
with fermions).
Higgsino Anti-selectron
Supersymmetric Matter
For technical reasons each sparticle can be heavier than its partner by no more than a TeV or so.
Neutralinos :
Great!
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.
• Fix Hierarchy Problem
• The Lightest Neutralino (LSP) is a prime
candidate for neutral stable cold Dark
Ω h = 0.103 ± 0.009
Matter
2
CDM
(WMAP 3-year data)
• Can have gauge coupling unification
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
Here Be Monsters! (again)
What we can see
This is the high energy physics of the 21st Century!
(What they really look like)
b
soft gluon radiation?
b
An example of an event where a higgs
boson decayed to a pair of b-quarks/
So main EASY signatures are:
Just Count Events!
• 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.
Indicates deviation from The Standard Model.
eff
jet i
T
missing
T
i
events
What you
measure:
Squark/gluon mass scale
M E
 | p |
Signal
S.M.
Background
Peak of Meff distribution correlates well with SUSY scale “as defined
above” for mSUGRA and GMSB models. (Tovey)
M eff (GeV)
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 Edges
ll
llq
ll
llq
lq high
lq low
lq high
lq low
llq
Xq
llq
Xq
Technique 1: Kinematic Edges
Account for all ambiguities:
Both look
the same
to the
detector
Determine how edge
positions depend on
sparticle masses
Technique 1: Kinematic Edges
Use 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 mass
Correlation between slepton mass
measurement and neutralino mass
measurement.
Other Techniques:
• Look at the shapes of the distributions
– Systematic errors harder to control
• Create new variables
– “Cambridge MT2 Variable”
now international used method
for sparticle mass measurement
in pair production
l
~
lR
~10
~
lR
~10
• Incorporate cross sections and branching
ratio measurements
– again, Cambridge “leading the way” as home
to the most developed samplers for H.E.P.
l
Can even bring these techniques to
bear on the data we have today
• Don’t know
• Know
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
mt
αs(Mz)
4.2 ± 0.2 GeV
172.7 ± 2.9 GeV
0.1187 ± 0.002
m0
M1/2
A0
Tan beta
Sgn mu
mb
mt
αs(Mz)
SM params
mb
•
•
•
•
•
•
•
•
SUSY params
Quantity
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:
Experiment: mtop = 178 ± 4.3 GeV
mtop = 170 GeV
Top Quark
Mass
in 2006 (was 174.3 ± 3.2 GeV in 2004)
mtop = 180 GeV
Standard Model uncertainty:
Experiment: mbot = 4.1 to 4.4 GeV
mbot = 4.0 GeV
Bottom
Quark Mass
in MS scheme
mbot = 4.5 GeV
The parts of Supersymmetric Parameter
Space are consistent with Today’s data:
h0 pole
region
Pseudoscalar higgs
A0 s-channel
annihilation region
Slepton-neutralino
co-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)
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 ….
The End,
and the ATLAS Collaboration
Cambridge Office
Christopher Lester
2006
Spare slides
Posterior maps
Progress in the last Century
• 19th Century
– 1897: Electron (Thomson)
• 20th Century:
–
–
–
–
–
–
–
–
1911: Nucleus (Rutherford)
1930: Neutrino postulated (Pauli, beta decay)
25 year wait
1936: Muon (Anderson, cosmic rays)
for neutrino
1956: Neutrino observed (Cowan, Reines, et al)
1960s and 1970s: Growing support for light quarks
1960s: Higgs boson postulated
1970s: Tau discovery
20-30 year wait
1996: Top quark discovered (Tevatron)
for top quark
• 21st Century
– Something’s coming, something good, (West Side Story)
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?
RPV
(Baryon number violating)
RPC
RPV
(Lepton number violating)
RPC
The SCT Software
Various
GUIs
and
users
etc.
SctApi
Overall SCT Controller
VME Crate Controller
VME Crate Controller
VME Crate Controller
Module
Module
Module
Module
Module
Module
Module
Module
Module