CMS Masterclass WZH Measurement
You will study events with multiple signatures to:
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determine which are the best candidates for W, Z, and Higgs bosons,
distinguish W+ from W- candidates,
distinguish decays into electrons from decays into muons,
determine key ratios,
and build a mass plot of the Z and Higgs candidates.
Background
When a W or a Z boson decays, it produces two particles called leptons. Electrons, muons, and
neutrinos are all leptons. So are their antiparticles.
Since the Z boson has zero charge, it can decay into two leptons of opposite charge (for example,
e+ and e-). W bosons have +1 or -1 charge, so a W can decay into a single electron or muon plus
a neutrino, which has no charge. This story goes deeper...but this is enough for now.
A Higgs boson has several possible decays. The two you might see are:
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Higgs boson decays to two Z bosons.
Higgs boson decays to two photons.
There are only a very few of either of these in your data.
W boson candidate events
When a W+ or W- is produced in a proton-proton collision inside CMS, the W decays promptly. The
two leptons (electron or muon plus neutrino) leave from the point of decay out from the beamline
and into the detector itself. Electrons and muons are revealed as tracks in the inner detector. They
curve in the strong magnetic field in CMS: clockwise for positive charge or anticlockwise for
negative, when seen in an x-y view in the event display. Neutrinos are undetected; however,
missing momentum in the system is shown as a yellow arrow and generally attributed to the neutrino.
Choose the image to the left to get a larger version for close
inspection. Can you locate the "missing momentum" vector in
each of these events? Look closely at the curvature of the
"other" track in each. Which is most likely the decay of a
W+? a W-? The short yellow track in Event A indicates an
electron. The longer red track in Event B which penetrates
out to the red boxes indicates a muon.
Z boson candidate events
When a Z boson is made in CMS, it too decays right away. Here, the two leptons are either a
muon pair (μ-μ+) or an electron pair (e-e+). No neutrino is produced, so there should be no
missing momentum. In practice, there is a lot going on in any one event and the detector might
miss something, so some missing momentum is possible.
Choose the image to the left to get a larger
version for close inspection. Which of these is
most likely to be a Z decaying into muons? a Z
to electrons? Which could be a W event? How
can you distinguish a W candidate from a Z
candidate that might have some missing energy?
Higgs boson candidate events
There are several ways a Higgs boson can decay. We are concerned with two cases:
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The Higgs decays to two Z bosons. The Z particles decay to leptons, as above,
so we get four lepton tracks: two muon pairs (2 x μ-μ+), two electron pairs
(2 x e-e+), or one muon and one electron pair (μ-μ+e-e+).
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The Higgs decays to two photons. Photons are uncharged and so are not seen
in the tracker. They instead leave large energy deposits in ECal.
Choose the image to the left to get a larger version
for close inspection. Both of these events are Higgs
candidates. One of them shows two photons (two
green energy towers from ECal with no corresponding
tracks). The other shows a possible Higgs decay
into two Z bosons (in this case, each Z promptly
decayed to two electrons. Which do you think is
which?
Elements of Events in iSpy-online
For each event, the beamline is along the common axis of the ECAL and HCAL wire-frame cylinders.
Which is the better W candidate? the better Z candidate? In each event, where do the collision
and particle decay occur? Learn more about CMS events.
Before You Start
If you are not familiar with iSpy Online, go to the Introduction or ask your mentor or teacher.
Analyze the Events
You will have the following to work with:
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Event Display (iSpy Online)
Spreadsheet
Partner
You and your partner examine the event display, interpret it, and use the speadsheet to record
your observations. The spreadsheet will help you find key ratios and calculate masses of Z boson
candidates. The mass data are combined for the whole Masterclass Institute and go to the
histogram package. Your mentor helps the whole group understand the results before they are
sent to the video conference moderators.
You and your partner will have 100 events to examine. When you observe an event, ask:
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Are the observable lepton tracks muons or electrons?
Is this a W candidate or a Z candidate or a Higgs candidate? Or is it a "zoo" event?
If it is more likely a W, is it W+ or W-?
If it is more likely a Z or a Higgs, what is the invariant mass in GeV (from the spreadsheet)?
There will be places to record all of these in the spreadsheet.
Results
Your mentor will help you combine the results from all students (up to 2100 events) to get the
following for the Masterclass Institute as a whole:
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the W/Z ratio (not as easy as it seems).
the W+/W- ratio.
the e/μ ratio.
the masses of the Z and other particles with similar decays from the statistical combination
in a mass plot. Can you find a Higgs signal as well?
Your CMS Masterclass Institute will combine with others in a video conference. The video
conference activities are:
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Each Masterclass Institute will briefly present its results.
The moderators will combine data from all Institutes and show a combined mass plot.
A Q&A session in which you can ask questions about anything from how the LHC
works to what a particle physicist does for fun.
Follow-up
Ask your mentor or your teacher to help you go further in your study of particle physics at
CMS. Here are some projects:
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Visit the CMS website to see more events and live updates from CMS.
The spreadsheet will have a wealth of data. Pick variables and make new plots. You can:
o make scatter plots (e.g. momentum of lepton 1 vs momentum of lepton 2)
to see what you can discover.
make new histograms to learn the distributions of different quantities
Go to the CMS e-Lab for deeper explorations of CMS and long-term investigations
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with CMS data.