# Presentation: 267890

## PARTICLE PHYSICS

*Exploring the world of the small*
-                                 Dr Emily Nurse

## The structure of atoms

- proton/neutron
-      (10-15 m)

- atom (10-10 m)

- nucleus (10-14 m)

- electron

- ‘up’ and ‘down’
-         quarks

## Matter Particles:

- Matter Particles:

- interact via forces, carried by more particles:

**increasing mass**

## electron  positron

***antimatter*** particle with the same mass
- But opposite charge.

- electron  positron
- quark      antiquark
- proton     antiproton

##  Quarks and jets

-  Quarks carry a ***colour*** ***charge***** **{cf. electric
-  charge} that the ***strong force*** acts on.

-  The ***further apart*** two colour charges are the
*** ******greater ***the attraction so that quarks are
-  confined into colourless objects known as
*** ******hadrons***.

-  Attempting to separate quarks  creates a ***jet***
-  of hadrons.

## A mathematical theory called the 

-                        Standard Model
- describes these particles and their interactions.
- The Standard Model is very successful. It has
- predicted particles *before* they are discovered!
- The top quark was predicted in 1977 but not
- discovered until 1995! This late discovery is due
- to its HUGE mass...
-        ...50,000 times heavier than the up quark!
- The hunt is now on for the remaining particle in
- the Standard Model... The Higgs boson.

## How do we research particle physics?

- The particles are detected via their interactions with
- matter in a ***particle detector***.
- The aim is to search for ***new ***particles and measure the
- properties (e.g. masses) of existing particles to test
***Standard Model*** predictions.

- Particles are accelerated and smashed together at
- huge energies in ***particle accelerator*** machines.

- Many different particles are
- created. The higher the
***energy*** the more ***massive***
- the particles can be.

## Home to the Tevatron collider

**Situated near Chicago, **
**USA**

**Home to the Tevatron collider**

## The Tevatron(the world’s highest energy collider)  

- The Tevatron(the world’s highest energy collider)

- collides protons with antiprotons
-    (accelerates them through
-      1, 000,000,000,000 volts!)

**p**

**p**

- 6 km circumference

## Slide 10

## Slide 11

## Many different processes occur when the 

- proton and antiproton collide             surround
- the interaction point with a *particle detector*.

## Slide 13

## The DØ detector

- The DØ detector

** **
**Calorimeter**

**South **
**Endcap**

**Magnet**

**Tracking**
**Volume**

## The DØ Detector in Collision Hall, January 2001

## Z

- with each other and produce more particles.

- e.g.

- Heavy particles tend to be unstable ...
- they decay to lighter particles.

- e.g.

**e****+**

**e****-**

- P

- P

## This process can be represented in a Feynman diagram:

## Unstable particles cannot be directly detected if their lifetime is much less 
   than the time it takes to traverse the detector.

Their existence is inferred by detecting their decay products.

The mass of an unstable particle is 
    found from the energy and momenta 
    of its decay products.

-    than the time it takes to traverse the detector.
- Their existence is inferred by detecting their decay products.
- The mass of an unstable particle is
-     found from the energy and momenta
-     of its decay products.

## Relativity

- The velocity of an object can only be measured *relative** *to another object (e.g.
-     a car travels at 30 mph *relative* to the Earth).
- A particles momentum and energy depends on its *frame of reference**.*
- A property called the *invariant mass* is the same whatever the *frame of reference**:* * *
*              **M **2** = ( *∑ *E )**2** - ( ∑ *_*p*_* )**2*
- In the particles *rest **frame*: *M ≡ mass*

- Note: units of c=1

##    You will be looking at the production and decay of:
       (1) the Z boson
       (2) the W boson
       (3) the top quark

   You will measure the mass of these particles using the energy and momenta of their decay products.

-        (1) the Z boson
-        (2) the W boson
-        (3) the top quark
-    You will measure the mass of these particles using the energy and momenta of their decay products.

- Your Task

## Z production and decay:

-      The Z can also decay to other pairs of particles:
-                        **e****+****e****-****         ****μ****+****μ****-****       ****τ****+****τ****-    **** **
**                  ****ν****e****ν****e****       ****ν****μ****ν****μ****        ****ν****τ****ν****τ**
**               ****uu     dd     cc     ss     bb**

##  Z  +- candidate

## Z  e+e- candidates

**DØ**

## W± bosons are also produced:

-      The W+ can also decay to other pairs of particles:
-                           **e****+****ν****e****      ****μ****+****ν****μ****    ****τ****+****ν****τ**
**                     ****ud        us       ub   **
**                     ****cd        cs        cb**

##   Added complication .... 
  neutrinos are not detected!

*  **neutrinos are not detected!*

- before collision

- after collision

- beam into wall

-   After a collision the sum of the momentum in the
-    direction transverse to the beam must add up to zero.

## transverse energy! 

- of the neutrino by measuring the total missing
- transverse energy!

- Define the *transverse invariant mass**:*
*   **M**T**2** = ( *∑ *E**T** )**2** - ( ∑ *_*p*__*T*_* )**2*

*(M**T ** *is less than or equal to *M)*

## W e candidates

**EM cluster**
**with track**

**DØ**

## Top quarks are also produced:

-      The W+ and W- can also decay to other pairs of particles.

## from the e+, ν and the bottom (b) jet. 

- The mass of the top (t) quark can be found
- from the e+, ν and the bottom (b) jet.

- The mass of the antitop (t) quark can be
- found from the antibottom (b) jet and the
- two remaining jets.

##    You will be looking at the production and decay of:
       (1) the Z boson
       (2) the W boson
       (3) the top quark.

   You will measure the mass of these particles using the energy and momenta of their decay products.

-        (1) the Z boson
-        (2) the W boson
-        (3) the top quark.
-    You will measure the mass of these particles using the energy and momenta of their decay products.

- Task recap

## Physics beyond the Standard Model...

- _Physics beyond the Standard Model..._

- Supersymmetry?

- The answer to the Universe’s missing matter?

## YOU         can help to find out !

- Is the World 11 dimensional ??

- More dimensions than meet the