21.2.3. Simulation: The Monte Carlo¶

We need to be able to compare data from our detector to the expectation we have. In very rare cases this might not be necessary, for example the discovery of the J/ψ was so clear a signal that we didn’t need any comparison to understand that it was something new. But most of the time we need to make sure what we see is not some artefact of our very very complex experiment.

To do this we create simulated events which should behave as closely as possible to the real detector events. This is done using sampling of random numbers repeatedly and thus called the Monte Carlo method. In HEP we usually just call the whole process Monte Carlo or MC for short.

Now there are two parts of this procedure we need to distinguish: Generation of an event and simulation of the event.

Event Generation¶

This is the physics part: the interaction we want to simulate. Given the initial conditions of the electron and positron colliding we generate a number of particles according to the physics model we want to study. This could be any advanced physics model (SUSY, dark matter) or basic standard model physics.

It depends on the analysis: Usually we have specific samples for the decay we analyse, the “signal MC”. And we compare these to simulation of basic standard model processes, the “generic MC”. There might be additional simulations needed for specific processes which we want to exclude in our analysis, the “background MC”.

For all these different samples the principle is the same: We generate positions and four-vectors of particles according to a physics model. In Belle II this is usually a very fast process and takes of the order of milliseconds per event to generate.

There is a large variety of different generators for different use cases: EvtGen, KKMC, Tauola, Madgraph, CRY, AAFH, babayaganlo, PHOKARA, … . All simulate specific physic processes and will be used for different use cases from performance studies to different analysis types. There is an internal Belle II Note with more details if you’re interested.

Simulation¶

After we generated the four-vectors of our event we need to now make it look like output from the real detector. And the real detector measures the interaction of these particles with the material of our detector: ionisation, scintillation, bremsstrahlung, pair production, Cherenkov radiation and so forth.

All these processes are well known and can in be simulated. There has been a lot of effort put into this by many experiments to create simulation software capable of all of these processes. The most well known one is Geant4 and we also use it in Belle II.

Geant4 takes the four-vectors and simulates their interaction with a virtual Belle II detector. In the end we get deposited energy and particles produced by the interactions in each sub detector.

On top of that we have custom software to convert the result from Geant4 into signals as we see from the detector. For example the pixel detector software will convert the energy deposited into information which pixels were fired.

Simulating the full detector is an expensive process and takes of the order of a second for Belle II. For other experiments like ATLAS and CMS it can also get close to minutes per event due to the much higher energy.

Question

Assuming it takes one second per event, how long would it take to simulate all the 770 million $B\bar{B}$ events collected at Belle on one CPU?

How long would it take to simulate all the $B\bar{B}$ events we intend to collect for Belle II?

Hint

You already know the cross section for $B\bar{B}$ events now you only need the planned total luminosity for Belle II.

Solution

Multiplying 770 million by one gives us 770 million seconds which is around 8912 days or roughly 24 years.

For Belle II we intend to collect $50\ \textrm{ab}^{-1}$ and the cross section is 1.1 nb. So we expect 55 billion $B\bar{B}$ events. Equivalent to 636574 days or 1744 years.

Question

Computing time doesn’t come for free. Real numbers are hard to determine, especially for university operated computing centers. But in 2020 one hour of CPU time can be bought for around $0.025 on demand so lets take for a very quick estimate.

How many CPUs do we need to buy in the cloud and how much would it cost to simulate the equivalent of $50\ \textrm{ab}^{-1}$ $B\bar{B}$ events in six months?

Solution

We need 55 billion seconds of CPU time, equivalent to 15.3 million hours. It would cost us roughly $382,000.

Six months have roughly $30 \times 6 \times 24 = 4320$ hours so we need 3540 CPUs.

Now bear in mind: this is only the simulation part, there is still more work to do during reconstruction as will be explained in the next section.

Differences between MC and real data¶

Now after this simulation we have data which looks like what we might get from the real detector and we can use it to compare our expectations to measurements. But Geant4 uses an ideal detector description we put in. In reality the detector itself consists of thousands of tons of hardware, some of it trying to measure positions in micrometer precision. We don’t know it perfectly and we cannot put every little thing correctly in the simulation: We simply don’t know the exact material composition and place of every single screw precisely enough. And even if we did this would slow down Geant4 massively because the system would become much too complex to simulate.

There will thus always be simplifications we will have to live with but we need to strive to make the differences as small as technically possible.

But especially in the early phases of the experiment we’re still in the process of understanding the real detector so we cannot have everything correct in the MC yet. This is a long and tedious process where small detail in the detector response need to be understood and modelled accordingly in the MC.

This is an ongoing work in the Performance group which tries to understand the differences between MC and data by looking at specific samples and studies.

Generating MC samples¶

As you saw above, generating sufficient MC is a tedious process which requires large amount of CPU time. It also is prone to errors where something might not be setup exactly correct. These mistakes would be costly for larger productions.

So we have the Data Production group to organize and manage the production of large MC samples. They make sure that the requests of the physicists are met and that the computing resources we have are not wasted.