Introduction to

World Wide Data Day

https://cern.ch/w2d2

What is the LHC and what happens there?

LHC=Large Hadron Collider

• ~100 m underground near Geneva, collides protons
• Most energetic accelerator in the world (currently > 13 TeV)

What is the LHC and what happens there?

When protons collide...

• Many particles are produced due to E=mc² (13 TeV → particles)
• Most are known processes: background
• What happens when protons collide in LHC:
• Some of the collision energy makes a new particle
• PROMPTLY, new particle → muon* + antimuon*
• The new particle can decay into other things but we will focus on muons
• Muons are easy to ID, easy to measure, and pretty interesting!

*A muon is a more massive cousin of the electron. An antimuon is its antimatter counterpart.

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Proton collision produces dimuon pair

What are the preferred directions of muons, if any?

Images of rho-z (side) view of a CMS event showing measurement of THETA-1 and THETA-2. The long red tracks are muons.

What are the preferred directions of muons, if any?

Images of x-y view of an ATLAS event showing measurement of PHI-1 and PHI-2. The long white tracks are muons.

Your job: calibrate the detector according to the model.

• Over many events, how does the distribution in PHI behave? Is it as we expect?
• Over many events, what do we see in the distribution in THETA? Is it as we expect?
• Were our expectations correct?
• What do the results tell us about particle collisions and about the detector?

Seeing dimuon events in ATLAS and CMS displays:

• 2 long tracks
• Any other things in event are background - ignore
• If not 2 muons in event, entire event is background - ignore

Dimuon in ATLAS

Dimuon in CMS - muons coded red

Dimuon or background? (ATLAS)

Dimuon or background? (CMS)

Measure angles in ATLAS

0 <phi < 360 deg. Measure from +x-axis anti-clockwise only as far as you need to go.

Measure angles in CMS.

0 <phi < 360 deg. Measure from +x-axis anti-clockwise only as far as you need to go.

Record angles in tally sheet

• Find PHI-1 and PHI-2 from event display, e.g. 101 deg and 276 deg (ATLAS example)
• Record with a mark in the first table at the nearest angle, e.g. 110 and 270
• Do the same for your two values of THETA.
• Do this for all dimuon events; when done, make 2 histograms on back and report results

Ready?

Make use of:

• ATLAS or CMS page for W2D2
• Screencasts
• Event display
• Tally sheet

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Form up:

• Partners - 2 to a computer
• Each pair takes one set of 50 events (ATLAS) or one set of 100 events (CMS)
• Fill out tally sheet - turn in
• Make class histograms!
• Ask questions!

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