# Presentation: 303032

## Experimental Techniques in Particle Physics

*Eric Prebys*

- Physics 312, April 24th 2000

## Some Definitions...	

**Particle** – the propagation of momentum, energy, and other “information” through space-time.
**Force -  **something which changes a particle in some way (sometimes to a different particle).

- Physics 312, April 24th 2000

## The Standard Model – The Fundamental Particles

- Physics 312, April 24th 2000

## Relativistic Quantum Mechanical Perturbation Theory

- Non-relativistic perturbation theory:

- Potential

- Free particle

- Discrete interaction

- Relativistic perturbation theory “Feynman Diagram”:

- Free particle

- Discrete absorption of intermediate “virtual” particle.

- Physics 312, April 24th 2000

## The Intermediate Vector Bosons (Mediators of Force)

- Physics 312, April 24th 2000

## Some Basic QED Interactions

- e+e- scattering

- e+e- annihilation

- Physics 312, April 24th 2000

## Some Basic Weak Interactions

- e scattering

-  decay

- Physics 312, April 24th 2000

## Some Basic Strong Interaction (QCD)

- Physics 312, April 24th 2000

## The Fundamental Forces

- Physics 312, April 24th 2000

## How Quarks Combine

- Quarks come in three “colors”: red, green, blue
- Combine to form “colorless” (white) particles:
- Three quarks (or antiquarks), one of each color “Baryons”
- A quark of one color and an anti-quark of the associated anti-color ”Mesons”

- Physics 312, April 24th 2000

## The Big Questions in Particle Physics

- What is the origin of mass?
- The standard model diverges if we just “plug-in” a mass for all the particles.
- An *effective* mass comes in through the interaction with a pervasive field with a non-zero vacuum expectation value.
- Perturbations about this vacuum give us a “Higgs Particle”, which probably has a mass 100 GeV< m < 1TeV
- What is the nature of CP violation?
- The physics of matter in a right-handed universe is *almost* the same as that for anti-matter in a left-handed universe.
- This small difference is *accomodated* in the standard model by complex terms in the quark mixing matrix.
- This must be firmly established, and if true, the associated parameters must be measured.

- Physics 312, April 24th 2000

## Big Questions (cont’d...)

- Do neutrinos have mass/do they mix?
- In the Standard Model, all neutrino masses are zero *by definition.*
- There is growing evidence that neutrinos do have mass.
- Solar neutrino deficit.
- Atmospheric neutrino “problem”.
- LSND result.
- If true this could explain the “dark matter” in the universe, at least partially.
- Must be verified, and if true, the details must be studied.

- Physics 312, April 24th 2000

## Big Questions (cont’d...)

- What lies beyond?
- The standard model eventually diverse
- There is a philosophical (aesthetic? religious?) impulse to unify the quark and the lepton sectors, as well as include gravity.
- Supersymmetry (SUSY):
- Every fermion is associated with a boson.
- Predicts a veritable zoo of new particles, the lightest of which should have m<2TeV.
- String theory
- All particles are states of fundamental objects (strings)
- Supersymmetry is a consequence.
- As yet, absolutely no experimental evidence for either of these theories.  Must keep looking.

- Physics 312, April 24th 2000

## What we Actually Study

- incident particle

- incident particle

- big mess!!!

- interaction

- Physics 312, April 24th 2000

## What We Actually Detect

- Almost all of the particles of most interest to us are very unstable; we must detect them *indirectly *through their decay products.
- Everything in the universe ultimately decays to
- In addition, the following particles live long enough (*c****>1*m)* *to be detected *directly*:

- CANNOT be individually detected

- Physics 312, April 24th 2000

## Classes of Particle Detection

- Charged Particle Tracking
- Precision: decay position determination.
- Spectroscopy: measure momentum in conjunction with magnetic field.
- Projection: match information from different detectors.
- Calorimetry
- Electromagnetic: measure energy of photons, identify electrons.
- Hadronic: measure energy of neutral hadrons, identify types of charged particles.
- Particle Identification
- Indirect: based on interaction characteristics
- Direct: determine mass by measuring velocity
- dE/dX
- Time-of-flight
- Cerenkov Radiation

- Physics 312, April 24th 2000

## Charged Particle Tracking

- As charged particles traverse matter, they deposit energy according to the Bethe-Bloch equation:

- Physics 312, April 24th 2000

## The Detection of Charge

- Ultimately almost all types of detectors work through the detection of *ionized charges*, which *induce* electrical signals as they *move*.

- Equipotential surfaces

- Physics 312, April 24th 2000

## Proportional Wire Chambers

- - - - - - - - -- - - --

- A charged particle ionizes gas molecules as it passes.
- This ionized charge drifts toward a wire which is held a relatively positive potential.

- As the ionized charge gets close to the wire, the rapidly increasing field results in an *avalanche* of multiple ionization.
- The motion of the resulting *ions* away from the wire *induces* a signal.
- The total signal is *proportional* to the total ionized charge.
- The *time* of the signal can accurately measure the *position* of the track.

- Track

- Physics 312, April 24th 2000

## Silicon Detectors

- -

- -

- -

- -

- -

- -

- -

- -

- o

- o

- o

- o

- o

- o

- o

- o

- p-type

- n-type

- -70V

- Fully depleted

- Readout strips

- Electron-hole pairs

- Physics 312, April 24th 2000

## Typical Charged Tracking Resolution

- Physics 312, April 24th 2000

## Electromagnetic Calorimetry

- Energetic electrons in material loose energy through *bremsstrahlung**. *

- Energetic photons in material can convert to e+e- pairs through Bethe-Heitler pair production

- These processes continue, ultimately depositing all the energy of the incident particle (e+, e-, or ) in a well characterized *shower*.

- Physics 312, April 24th 2000

## Electromagnetic Calorimetry (cont’d)	

- Electromagnetic Calorimetry (cont’d)

- There are basically two types of EM calorimeters...

- shower

- Scintillation light

- shower

- Absorption layer (shower develops)

- Detection layer (scintillator, PWC, liq. Ar, etc)

- Total Absorption

- Sampling

- Physics 312, April 24th 2000

## Electromagnetic Calorimeter Resolution

- Energy in GeV

- Physics 312, April 24th 2000

## Hadronic Calorimetry

- Hadronic Shower

- Nuclear collision

- Escaping neutron

- EM Component

- Based on nuclear interactions.
- Longer and messier than EM showers.
- Always use sampling calorimeters (e.g. steel+scintillator)
- Very good resolution would be

- Physics 312, April 24th 2000

## Particle Identification

- We can broadly distinguish particles by how they interact (we’ll discuss this in a minute).
- But particles of the same time class (eg charged hadrons) must be distinguished by their different *masses*.
- We determine the mass by independently measuring the *momentum* and *velocity*.
- One way to do this is to directly measure the time of flight
- Can usually measure time to better than 100 ps
- In a central detector, this can separate  and K up to about 1 GeV
- In addition, there are common *indirect* ways to measure velocity.

- Physics 312, April 24th 2000

## dE/dx

- Recall that as particles traverse matter, the energy they deposit is dependent only on the *velocity*.
- particles of the *same momentum* will deposit *different amounts of energy* if their masses are different.
- This can be easily measured with proportional wire chambers.

- Physics 312, April 24th 2000

## Cerenkov Radiation

- A charged particle which is traveling faster than the speed of light in a particular medium will radiate its energy in the form of photons in a cone whose angle is

- The existence of such light can be used to *discriminate two particles* of different masses for a range of momenta (threshold Cerenkov detector).
- OR the angle can be *directly measured* (more accurate but more difficult).

- Physics 312, April 24th 2000

## General Detector Layout and Classes of Particles

- Hadronic Calorimetry

- Charged Tracking

- Electromagnetic Calorimetry

- Particle ID (sometimes)

- Precision Tracking

- Physics 312, April 24th 2000

## Example – the BELLE Detector

- Physics 312, April 24th 2000

## DAQ Overview

- Physics 312, April 24th 2000

## Pictures (Tracking)

- SVD

- Central Drift Chamber

- Physics 312, April 24th 2000

## Pictures (Electromagnetic Calorimeter)

- Physics 312, April 24th 2000

## Pictures (Particle ID)

- Module Assembly

- Barrel Detector

- Physics 312, April 24th 2000

## Pictures (K-long Catcher/Muon Tracker)

- Barrel Module

- Endcap Module

- Physics 312, April 24th 2000

## Pictures (DAQ/Control)

- Readout Electronics

- Custom

- LeCroy 1877

- Event Builder

- Control Room

- Champaign Bottles

- Physics 312, April 24th 2000

## All Finished!!

- Me

- Physics 312, April 24th 2000

## What an Event Looks Like

-  J/
-  M() = 3.1 GeV

- Precision Tracking
- Charged Tracking
- Particle ID (Cerenkov)
- Electromagnetic Calorimetry
- Solenoidal Magnet
- Muon ID/Hadronic Calorimetry

- Physics 312, April 24th 2000