Higgs boson couples to mass …. Can be produced (or decay to) and pair of particles with mass

It can also be produced (or decay to) photon or gluon pairs via (eg top) loops… In fact, due to large gluon PDF, gg is by far the dominant channel

It can also be radiated by heavy particles

What does Standard Model Higgs decay to?

For detection, ideally want clean decays to high pt e or µ

•  For low MH, tricky! – Higgs decay products are at low pt - b bbar has biggest branching ratio, but high background decays to jets!  - γγ more promising, but  low rate & still backgrounds!

•  For high MH, dream scenario is to use WW or ZZ, decays to leptons

Standard Model Higgs decay branching fractions depend only on its mass …

LEP, Tevatron suggest its lighter than this…

Dream scenario - `heavy` Higgs (CMS)

If Higgs is heavy enough, detect H→ZZ →µ+µ-µ+µ-

e.g. if MH=200 GeV, muons could have pt=50GeV each …. fast discovery, MH could be measured to 1GeV or better

… even then a full simulation looks scary!

ATLAS

HZZ4 leptons

Another example: Hγγ

Higgs Discovery Significance at LHC

•  5σ `Discovery‘: prob. of statistical fluctuation < 0.0001 % •  With 100fb-1, all masses covered by γγ and ZZ channels .... but that could take a few years!

Expected signal significance (see later lectures) …

… “number of standard deviations by which signal lies above the background”

c.f. 2010-2011 run expected to yield 1 fb-1

More LHC Physics

Massive topic! – both for new physics searches and precision data - early examples of jet and Z0 measurements - June 2010, with 1 nb-1 , 16 nb-1 … now we have 50 pb-1 !

Jet pairs at the LHC

High  cross  sec+on  process  accessible  with  data  already  taken…  already  beyond  reach  of  

Tevatron  

pT  (j1)=420  GeV  pT  (j2)=320  GeV  

Highest-­‐mass  di-­‐jet  event  observed  so  far:  Mjj  =  2.55  TeV  

… looking increasingly likely that the Tevatron will run until 2014, by which point it will have ~15 pb-1 per experiment

Next Part of Course: How detectors work

Annihilation of an antiproton in the 80 cm Saclay liquid hydrogen bubble chamber. A negative kaon (K- meson) and a neutral kaon (K0 meson) are produced in this process as well as a positive pion (π+ meson).

… starting with tracking of charged particles

… an old subject with applications way beyond particle physics!

Today: Gaseous tracking detectors

•  To detect a particle, we have to make it interact with our detector! •  Tracking detectors detect CHARGED PARTICLES … want them to interact as little as possible to precisely track the particle trajectory in conditions as close to vacuum as possible. •  In calorimeters, we want CHARGED AND NEUTRAL PARTICLES to interact as much as possible until they stop completely … then measure all of the energy they leave behind.

•  Trackers are therefore located inside calorimeters!

•  Multiple Coulomb (electromagnetic) Scattering •  Ionisation •  Bremsstrahlung (especially electrons) – see calo lecture •  Nuclear Interactions (hadrons) – see calo lecture

•  Charged particle detectors rely on IONISATION. •  Measurement accuracy usually limited by Multiple Coulomb Scattering •  The measured quantity is the MOMENTUM (see next time)

•  be made from low density material •  have minimal dead material between them and the interaction

Ideally, tracking detectors should ….

A Hundred years of Particle Tracking

1900:Cloud Chamber Supersaturated vapour condenses to tiny liquid droplets around ions

2000:Time Projection Chamber: >1000 tracks!

Different centuries, same principle!… persuade the particle to ionise and collect the resulting charge

Energy Loss by Ionisation in Gas •  Primary energy loss by ionisation of atoms leads to

creation of ion-electron pairs in gas volume. •  Number of primary ionisation pairs is small •  Primary ionisation depends on particle type and medium

•  Discussed in terms of the `specific ionisation energy loss’ or `stopping power’:

- dE/dx is the energy loss of the particle per distance travelled. ... depends on the projectile, its velocity and the medium.

•  The Bethe Bloch formula describes dE/dx over much of the interesting kinematic range (derived from Rutherford scattering, with some quantum and relativistic corrections)

see eg Fernow, Perkins or Particle Data Booklet.

The Bethe-Bloch Formula

z is projectile charge Z, A are atomic number and atomic mass of medium β, γ are the usual Lorentz variables I is the ionisation potential of the medium Tmax is the maximum kinemtic energy gained by ionised electron δ  is a relativistic correction which depends on βγ K = 4π NA α2/ me c2 is a constant independent of the medium:

me is the electron mass, α is the fine structure constant NA is Avagadro’s number

Bethe-Bloch Formula

•  Depends quadratically on charge and velocity of particle, but not its mass.  Particles with same momentum (as measured from curvature in B field) but different mass can be distinguished! Shape is always the same … ~1/β2 at low momentum, ~lnγ at high mom, minimum in between

(β=v/c, γ=1/sqrt(1-β2)

The complicated formula is well approximated by:

~1/β2

~lnγ

Particle Identification with dE/dx

Relativistic rise

~log γ

Ionisation deposit per unit distance

Example from H1 experiment … dE/dx used to distinguish kaons, pions, protons at low momentum

Ionisation Signal Collection with Electric Field

•  Close to the anode the field is strong: the electrons are accelerated such that they produce secondary electrons, which make more electrons via Bremsstrahlung etc ......................an AVALANCHE resulting in a large GAIN!

•  Use dense inert gases (e.g. Argon) with various admixtures

Ground V=0

Ground V=0

Anode wires at high voltage

… most common modern method of collecting the ionisation … Ionisation chamber consists of gas to be ionised in `drift space’, enclosed in uniform electric field.

Liberated electrons drift to anode wires

Gas Detector Operation Modes  Ionisation mode:No gas multiplication, signal from primary electrons only (used in dosimetry)

 Proportional mode: Signal proportional to energy loss of particle (dE/dx). Gain of order 10^5 (used in most detectors).

 Limited proportionality:Space charge effects of ions alter effective electric field.

 Geiger Mode: Electric breakdown of gas. Recombination of ions result in photo-emission. Avalanches merge -> spark emission (damages detector!)

Response curves for α (highly ionising) and β (~ minimum ionising) particles

Limited Streamer Tubes •  H1 muon detectors operate in limited proportionality mode

•  Iron return yoke of magnet instrumented with gas chambers

•  Thick wires, highly quenched gas

•  Signal no longer proportional to primary ionisation ( no particle ID, but who cares, we know they are muons!)

•  Easy to build, large & efficient signal, simple electronics

Multi-Wire Proportional Chambers

G. Charpak et al. 1968, Nobel Prize 1992

Typically d=2mm,w=8mm

- Large number of electrically isolated cells

- Proportional mode. – 1D info, multi-layers for precision and 3D signal

- Digital readout of anode pulses

-  Fast response, ideal for triggering

-  Poor spatial resolution ~500µm, determined by (but less than) cell size.

d w

Field map: Low field between anodes

E