Trigger & Lumi challenge
Last updated on 2024-07-18 | Edit this page
Estimated time: 30 minutes
Overview
Questions
- Which triggers are likely the most useful for selecting top quark events?
- What are the prescale values of some possible triggers?
- What is the luminosity collected by my chosen trigger?
Objectives
- Learn to make trigger choices based on analysis goals, expected physics objects, and available trigger paths.
- Practice using brilcalc to extract trigger prescale information
- Practice using brilcalc to compute integrated luminosity
Workshop analysis example: \(Z' \rightarrow t\bar{t} \rightarrow (bjj)(b\ell\nu)\)
Later in the workshop we will search for a Z’ boson that decays to a top quark-antiquark pair. The signal for this measurement is one top quark that decays hadronically, and one top quark that decays leptonically, to either a muon or an electron. For Z’ bosons with mass in the 1 - 5 TeV range, the top quarks should both be produced with high momentum.
Choosing a trigger path
Using what you have learned about CMS physics objects and triggers, let’s sketch out some ideas for potential trigger selections. To begin with:
Which final state particles would you expect to observe in the detector from your “signal” process?
Based on these particles, consider:
- Which physics objects could appear in a useful trigger path? At a hadron collider, which physics objects might be best for triggering?
- What kinematic conditions do you expect for the trigger physics objects?
- We saw trigger “trade-offs”, like requiring isolated particles in exchange for lower momenta – would that trade-off be beneficial in this case?
Physics exercise: choose trigger paths
Visit the NanoAOD HLT branch listing for our signal sample and choose 3-4 trigger options that might be effective for selecting \(Z' \rightarrow t\bar{t}\) events
For the \(t\bar{t}\) final state we will consider, we expect one electron or muon, MET from the accompanying neutrino, and several jets, some of which could be tagged as b quark jets. There might even be “fat jets” tagged as entire top quark decays. This analysis is perfect for the common “single lepton” triggers. Since the top quarks will be produced with high momentum, the lepton might not be well separated from a b quark jet, so it would be wisest to include a non-isolated trigger. Since the signal will likely produce high-pT leptons, we can afford to use triggers that require higher momentum in order to relax isolation requirements.
The triggers used in the CMS analysis that we are mimicking were:
- HLT_Mu50
- HLT_Ele50_CaloIdVT_GsfTrkIdT_PFJet165
- HLT_Ele115_CaloIdVT_GsfTrkIdT
For any of these triggers, the leptons (and jet, if applicable) should ideally be selected using criteria that put them in the “efficiency plateau” of the trigger. For example, the HLT_Mu50 trigger is designed to select muons with at least 50 GeV of transverse momentum, but if you plotted the number of muons selected as a function of pT, you would certainly not see a vertical line step function with 0 muons selected below 50 GeV and 100% of muons selected above 50 GeV. Right in the region of the threshold there will be a “turn-on” effect where the efficiency rises quickly, but not exactly following a step function. This turn-on can be difficult to model well in simulation, so many analysts choose selection criteria for physics objects that will not be subject to the turn-on effects of the trigger. For example, in an analysis using HLT_Mu50, the muon selection criteria would typically be pT > 55 GeV.
Brilcalc exercises
Exercise 1: test brilcalc
As a test, let’s see what the integrated luminosity was for run 284044 by running the command:
Note: It is important that you use the -c web option
when running brilcalc. This specifies that you use indirect access to
BRIL servers via a web cache. For users of CMS open data outside CERN
and CMS this is the only option that will work.
You should see the following output:
OUTPUT
#Data tag : 24v1 , Norm tag: onlineresult
+-------------+-------------------+-----+------+-------------------+-------------------+
| run:fill | time | nls | ncms | delivered(/ub) | recorded(/ub) |
+-------------+-------------------+-----+------+-------------------+-------------------+
| 284044:5451 | 10/26/16 20:46:05 | 594 | 40 | 5450634.956154574 | 4912825.742705812 |
+-------------+-------------------+-----+------+-------------------+-------------------+
#Summary:
+-------+------+-----+------+-------------------+-------------------+
| nfill | nrun | nls | ncms | totdelivered(/ub) | totrecorded(/ub) |
+-------+------+-----+------+-------------------+-------------------+
| 1 | 1 | 594 | 40 | 5450634.956154574 | 4912825.742705812 |
+-------+------+-----+------+-------------------+-------------------+Exercise 2: 2016 validated runs & luminosity sections
All CMS data is studied by data quality monitoring groups for various
subdetectors to determine whether it is suitable for physics analysis.
Data can be accepted or rejected in units of “luminosity sections”.
These are periods of time covering \(2^{18}\) LHC revolutions, or about 23
seconds. The list of validated runs and luminosity sections is stored in
a json file that can be downloaded from the Open Data
Portal.
First, obtain the file with the list of validated runs and luminosity sections for 2016 data:
BASH
wget https://opendata.cern.ch/record/14220/files/Cert_271036-284044_13TeV_Legacy2016_Collisions16_JSON.txtUse brilcalc to display the luminosity for each valid
run in the 2016 Open Data. Currently, we have released 2016G and 2016H
data, which contain runs 278820 through 284044.
BASH
brilcalc lumi -c web -b "STABLE BEAMS" --begin 278820 --end 284044 -i Cert_271036-284044_13TeV_Legacy2016_Collisions16_JSON.txt > 2016GHlumi.txt- Can you identify the run in this range that had the most luminosity sections?
- What is the total recorded luminosity for this run range?
You can investigate the output file best using head:
The longest run is 279931!
OUTPUT
#Data tag : 24v1 , Norm tag: onlineresult
+-------------+-------------------+------+------+---------------------+---------------------+
| run:fill | time | nls | ncms | delivered(/ub) | recorded(/ub) |
+-------------+-------------------+------+------+---------------------+---------------------+
| 278820:5199 | 08/14/16 11:00:52 | 1524 | 1524 | 300457014.643834889 | 287580217.903829336 |
| 278822:5199 | 08/14/16 21:52:48 | 1627 | 1627 | 195829703.718308955 | 189724216.323258847 |
| 278873:5205 | 08/15/16 23:25:46 | 67 | 60 | 12468667.004980152 | 2012170.733444575 |
| 278874:5205 | 08/15/16 23:51:54 | 484 | 484 | 88533994.547763854 | 82142606.203014016 |
| 278875:5205 | 08/16/16 03:01:21 | 834 | 834 | 120598272.705723718 | 115092509.056569114 |
| 278923:5206 | 08/16/16 12:16:33 | 413 | 413 | 103255455.458636224 | 98881486.964829206 |
...
| 279887:5270 | 09/01/16 23:20:53 | 319 | 319 | 76790312.907530844 | 72932440.002667427 |
| 279931:5274 | 09/02/16 11:37:40 | 2936 | 2936 | 485408960.517024338 | 470459175.415192366 |
| 279966:5275 | 09/03/16 09:20:25 | 363 | 363 | 95862775.891636893 | 91937407.130460218 |
| 279975:5276 | 09/03/16 14:45:09 | 1024 | 1024 | 244951346.260912448 | 235979324.421879977 |The units are inverse microbarns as default and can be changed to
inverse femtobarns with the option -u /fb.
The total recorded luminosity for the run range is listed in the summary below the individual runs:
OUTPUT
#Summary:
+-------+------+-------+-------+-----------------------+-----------------------+
| nfill | nrun | nls | ncms | totdelivered(/ub) | totrecorded(/ub) |
+-------+------+-------+-------+-----------------------+-----------------------+
| 64 | 156 | 90807 | 90526 | 17062540322.934297562 | 16290713419.734096527 |
+-------+------+-------+-------+-----------------------+-----------------------+CMS recorded 16.3 inverse femtobarns of proton-proton collisions in 2016 G and H!
Exercise 3: selecting the best luminometer
Update to
For Run-2 data, the luminometer giving the best value for each luminosity section is recorded in a ‘normtag’ file, provided in the 2016 luminosity information record (‘normtag_PHYSICS_pp_2016.json’). Use it with the corresponding list of validated runs from 2016 to find the best estimate of the 2016G-H luminosity.
BASH
wget https://opendata.cern.ch/record/1059/files/normtag_PHYSICS_pp_2016.json
brilcalc lumi -c web -b "STABLE BEAMS" --begin 278820 --end 284044 -i Cert_271036-284044_13TeV_Legacy2016_Collisions16_JSON.txt -u /fb --normtag normtag_PHYSICS_pp_2016.json > 2016GHlumi_normtag.txtDoes it change compared to the previous exercise?
The summary should now read:
OUTPUT
#Summary:
+-------+------+-------+-------+-------------------+------------------+
| nfill | nrun | nls | ncms | totdelivered(/fb) | totrecorded(/fb) |
+-------+------+-------+-------+-------------------+------------------+
| 64 | 156 | 90807 | 90526 | 17.168970230 | 16.393380532 |
+-------+------+-------+-------+-------------------+------------------+To three significant figures, the luminosity is better reported as 16.4 inverse femtobarns.
Exercise 4: Luminosity and prescale values for an HLT trigger path
One of the triggers that is a good candidate for the Z’ search
example is HLT_Mu50. Let’s calculate the integrated
luminosity covered by this trigger for the long run 279931:
Compare this luminosity to the similar trigger HLT_Mu27
– what difference do you observe? What is happening to Mu27 that is not
occurring for Mu50?
In run 279931, the luminosity collected with Mu50* is:
OUTPUT
+-------------+-------------------+------+-----------------------------+---------------------+---------------------+
| run:fill | time | ncms | hltpath | delivered(/ub) | recorded(/ub) |
+-------------+-------------------+------+-----------------------------+---------------------+---------------------+
| 279931:5274 | 09/02/16 11:36:53 | 2945 | HLT_Mu50_IsoVVVL_PFHT400_v3 | 486636050.343178093 | 471451252.657074332 |
| 279931:5274 | 09/02/16 11:36:53 | 2945 | HLT_Mu50_v4 | 486636050.343178093 | 471451252.657074332 |
+-------------+-------------------+------+-----------------------------+---------------------+---------------------+The luminosity collected with Mu27* is:
OUTPUT
+-------------+-------------------+------+--------------------------------------+---------------------+---------------------+
| run:fill | time | ncms | hltpath | delivered(/ub) | recorded(/ub) |
+-------------+-------------------+------+--------------------------------------+---------------------+---------------------+
| 279931:5274 | 09/02/16 11:36:53 | 2945 | HLT_Mu27_Ele37_CaloIdL_GsfTrkIdVL_v4 | 486636050.343178093 | 471451252.657074332 |
| 279931:5274 | 09/02/16 11:36:53 | 2945 | HLT_Mu27_TkMu8_v3 | 334796076.249607444 | 325826000.870547831 |
| 279931:5274 | 09/02/16 11:36:53 | 2945 | HLT_Mu27_v4 | 3373601.765050163 | 3273957.230715709 |
+-------------+-------------------+------+--------------------------------------+---------------------+---------------------+The Mu27 trigger is significantly prescaled!
Note: you could combine the example in this exercise with the
brilcalc options --begin, --end,
-i, and --normtag to confirm the total
integrated luminosity for a given trigger path. This command can take a
while! If you wish to use wildcards to get results for many trigger
paths, you can consider adding --output-style csv and
redirecting the results to a .csv file for easier analysis.
OUTPUT
+--------+-------+----------+-------------+--------------------+-------+---------------------------------------+
| run | cmsls | prescidx | totprescval | hltpath/prescval | logic | l1bit/prescval |
+--------+-------+----------+-------------+--------------------+-------+---------------------------------------+
| 279931 | 1 | 2 | 260.00 | HLT_Mu27_v4/260 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 71 | 6 | 140.00 | HLT_Mu27_v4/140 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 74 | 2 | 260.00 | HLT_Mu27_v4/260 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 262 | 3 | 230.00 | HLT_Mu27_v4/230 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 521 | 4 | 200.00 | HLT_Mu27_v4/200 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 752 | 5 | 160.00 | HLT_Mu27_v4/160 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 1088 | 6 | 140.00 | HLT_Mu27_v4/140 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |
| 279931 | 1946 | 7 | 100.00 | HLT_Mu27_v4/100 | OR | L1_SingleMu22/1.00 L1_SingleMu25/1.00 |The total prescale value is the product of any prescale applied at L1
with any prescale applied at HLT. As this long run proceeded, the
prescale values changed several times. During a portion of the run this
trigger was not allowed to collect any luminosity, and in other portions
it was scaled down by at least a factor of 100. In contrast, you can
confirm that Mu50 always has a prescale of 1. Any wildcard can be given
in the --hltpath option to compare many triggers for a
certain physics object and find unprescaled options.
Key Points
- Triggers need to be chosen with the event topology of the analysis in mind! They place constraints on your choices for selecting physics objects in an analysis.
- The brilcalc tools allows you to calculate luminosity for a run, a range of runs, or a trigger path.
- The brilcalc tool can also share information about trigger prescales throughout a run.