DALI , the Aleph event display

You look for a new particle, long expected but never observed before, and you know exactly what its mass, charge, spin etc.. should be , so you know how to look for it among the large data sample collected by your experiment. Once you finally find a few events passing successfully your selection, wouldn't you want to “take a look” at them?

You are developing some software to reconstruct the path of a muon passing throughyour detector, and to test this software you have simulated many muons. Some muons are not found by your reconstruction, wouldn't you like to “look at them”, to better understand why they fail your algorithm?

For these, and many other reasons, all high energy physics experiments develop a software tool to display the data they collect.DALI is the event display of Aleph, one of the 4 experiments which operated at the electron-positron high energy collider LEP at CERN from 1989 to 2000. In this exercise you will use this event display to measure how many times the Z0 boson (oneof the particles mediating the weak force) decays into an electron and a positron, or a muon and an anti-muon, or a tau and an anti-tau, or finally a quark and an anti-quark.You will then compare your result to the world average measurement of this, and draw conclusions about your result.In the first few pages you are given some examples of event displays for the events you areasked to classify. Try to go through them carefully, to get familiar with which characteristicsyou need then later on to look for. Ask questions, if something is not clear!

Z0

µ-

µ+

High energy particle physics Typical detector layout

Trackingchamber

ElectroMagneticcalorimeter

Hadroniccalorimeter

Muonchamber

Innermost Layer.... ... Outermost Layer

photons

electrons

muons

pions,protons

neutrons

ee →Z0 → ee

Efl=Total energy, reconstructed using tracking or calorimeter information from all the particles observed.If there are neutrinos in the event, Efl and ECM will be different

= Polar angle

Hadronic calorimeter

Electromagnetic calorimeter

Tracking detector

Nch will be large for Z0→ q-antiq events, wrt e.g. ee,mumu

ee →Z0 → ee e+

e-

e+

e-

Z0,large polar angle

e+

e-

e+

e-

small polar angleBhabha scattering

You will observealso ...

ee →Z0 →µµ e+

e-

mu+

mu-

Z0,

ee →Z0 → ττ e+

e-

tau+

tau-

Z0,

τ have shorterlifetime thane,mu, so theydecay in τ andpions

ee →Z0 → q anti-q e+

e-

q

anti-q

Z0,

Some instructions:

login: aleph, passwd: aleph

then go to xterm and type:dalinewAnd then:filename=data2.edirrun=20922SC=15, AC=16,AC=17

then go to display and clickread displayand then start: display next

Use attached spreadsheets to write down which category you think the events you observe belong too. Try to scan at least 100 of them, and then calculate the fraction of times you think you have observed ee,mumu,tautau,... final states. Then compare toPDG table and draw your conclusions.

Now you are ready to get started !

from http://pdg.lbl.gov

ee mumu tautau qq qqg ee BhabhaEv #

Run :

ThT

ee mumu tautau qq qqg ee BhabhaEv #

Run :

ThT

ee mumu tautau qq qqg ee BhabhaEv #

Run :

ThT

ee mumu tautau qq qqg ee BhabhaEv #

Run :

ThT

ee mumu tautau qq qqg ee BhabhaEv #

Run :

ThT