1. Detector building Notes of our discussion
Stagiere group
15 Feb 2017

2. Outline
We considered a timescale of the next 10 years, looking at the project to upgrade the CMS Tracker
We talked mainly about prototyping phase (going on now in 2017 and for next 2-3 years)
Some particular questions we discussed
How do we decide what to build (e.g. how to make aTracker)
Why such light material used?
How do we meaasure 3D spacepoints with 2D microstrip detectors
Why are optical fibres used?
Why are we using QR codes?
Engineering and Physics - How do these different fields connect at CERN in detector building?
Why we need to collaborate?
We talked briefly about what follows prototyping
Production and testing (2019 – 23)
Why do we need to do the production of parts in batches?
Assembly and testing ( ) and how do we test the detector at CERN Meyrin at the same time as getting the CMS cavern ready to install the new detector…
All to get ready for final installation and testing (2025) then 10+ more years of measurements

3. The beginning, what and why?
How to decide what to build?
First, what is the objective of the Tracker?
We try to measure precisely the point of origin of the particle, when and where the particle was produced, which direction it is going, and how fast, and what charge sign the particle has.
Combining Tracking information with other information from CMS (calorimeters and muon detectors) allows to identify more precisely the type of particles made in the collision
Some key points:
The tracker detectors each need to make a precise measurement of where the particles cross
Means we need to use very fine detectors (the lines or strips on the piece of silicon)
With a collection of points we can put all the information together to reconstruct the tracks
The electronics has to be fast and well synchronized to be able to measure signals over modules (72000 electronic chips and fibres) from collisions that occur every 25 nanoseconds (40 million times per second!)
The Tracker has to be as lightweight

4. Why is the Tracker lightweight??
Particles can bump into any piece of material in the Tracker that is in their path
If this happens the particles are deflected in a different direction
which can spoil the tracking performance
So, we use thin silicon (just thick enough for a good signal) and light mechanics (carbon-fibre, aluminium, etc., thin materials)
We make the electronics to use as little power as possible
to avoid needing too large and too many copper cables to bring the electricity
and to reduce the amount of cooling needed to take away the heat from the silicon sensors, electronics and power cables

5. First steps in detector building
in parallel:
Test small prototypes silicon sensors, early electronics, and basic mechanical modules
Understand how to use latest technology
Figure out how to improve existing detector designs
Physics and tracking performance simulations
Physics theory (modeled in the simulation programmes) predicts what we might be able to create with LHC collisions
Detector physics simulation predicts how well we can measure particles from the collisions
Allows us to figure out how many sensors (and how many different types) we need and where to put them.

6. Prototyping
Prototyping = Designing and testing the ingredients that we will use to build the detector
First we make and test very small prototype detectors
For example, checking different silicon sensor designs in the case of the new Tracker
We didn’t talk much about this, but why silicon detectrs?
we use Silicon to profit from all the advances in the last decades in the electronics industry, where silicon chips have been getting cheaper and cheaper, and more advanced, with smaller features.
Also, test small versions of the electronics chips
Simplified versions collecting signals from only a few microstrips.
Small versions of prototype sensors and chips are more affordable and we can then test more versions before deciding on the final designs
In parallel, we also design the first prototype detector module mechanics (carbon fibre, aluminium, ceramic, kapton cables, thermal glue, etc..)
To figure out how to attach sensor to the electronics, cables and cooling
How to do this solidly and reliably with the lightest materials

7. Are the prototypes good enough?
Make “beam-tests”, putting one or more modules into a particle beam
can be done at CERN, or other accelerator facilities
Checks the detector performance
Optimise the mechanics to be as stiff and light as possible for holding many modules in place throughout the Tracker
Make other tests e.g. radiation damage
Every part of the tracker is hit by about a 100 million million particles per square centimetre during the 10 year lifetime.
Each particle can make a tiny bit of damage, which builds up and can kill the sensor or the electronics, or the fibre optics components.
Estimate the expected costs to make industrial production based on the prototypes
Can we afford to build the detector we would like to have???
Have to make these first steps over and over (usually it takes several years work of prototyping!) until we are in a good enough situation to continue to production

8. Collaboration
There are at least 3000 people involved in the CMS experiment, and about 500 involved in the Tracker alone.
Why do we collaborate?
We have so much development, testing, production and assembly work to do
Not all of it can be done by automatic machines
So, we must share the work over many groups and people
E.g. if we need to make modules in no more than 2 years time period to fit the planning overall, we better have several (5+) groups able to make at least several thousand modules each in 1-2 years of time.
A big challenge:
to organise that every group gets all the ingredients it needs on time,
has all the lab space and test equipment it needs,
Has the people and training needed for all the assembly and testing
This organisation has to include how to manage the purchasing from industry to get on time all the raw ingredients (sensors, chips, hybrids, fibres, connectors, etc..)
Money has to be available and collected from the different countries
sometimes millions of CHF

9. Other questions
How do we measure 3D spacepoints with 2D microstrip detectors
We put detectors back to back with a small tilt angle on one of the layers
Why are optical fibres used?
Lightweight way of sending huge amounts of data at low power from a very small laser chip
Why are we using QR codes?
We need to keep track of what is connected to what using a cabling map. The codes on the QR label are unique and allow us to make a map of what is connected to what.
Why is production done in batches?
Production then done in batches of a certain size to manage the risk of faults in production
and allow time for us to test each batch before making the next batch.
Also we do not then have to pay all the money at once for all the material!

10. Conclusion With a collection of:
good ideas of how to use the latest technology,
a good detector design, that can be built and has the performance to measure with the right precision and speed,
a great team in each partner institute ready to dedicate to years of work,
a good organisation over many groups across the world,
enough money when we need it to buy parts,
high quality materials from all of our industrial partners,
and lots and lots of testing at each stage
These are some of the ingredients for successful detector building!