Programas de pregrado

the physics goals and significant performance improvements of the LHC machine during the first run.

5.1 L1 menus

From 2010 to 2012, several L1 menus (and corresponding prescale columns) were developed to meet the experiment’s physics goals and to cope with the evolution of the LHC operational conditions, i.e., the change of the center-of-mass energy between 2011 and 2012, the varying number of colliding bunches for LHC fills, and the growth of luminosity per bunch. While designing new L1 menus, improved algorithms and thresholds were utilized to continuously maintain the L1 trigger output rate within the 100 kHz bandwidth limit. When the luminosity ramp-up phase stabilized in 2011 and 2012, the strategy focused on reducing the number of L1 menus being developed to a few per year, and adapting for different machine operational conditions by using multiple prescale columns rather than different L1 menus. At the end of 2012, during a twelve-hour-long fill, the instantaneous luminosity delivered by LHC varied significantly spanning from≈7×1033cm−2s−1 to≈2.5×1033cm−2s−1. The average number of pileup events per interaction ranged from≈30 at the beginning to≈12 at the end of the run.

To aid the L1 menu development using data, a special reduced-content event data format (containing only GCT, GMT and GT readout payloads) was defined and used to record events in a special data set. These events were collected on the basis of BPTX and L1 trigger GT decision only. Hence, with such recorded zero bias and L1 bias data sets, it was possible to properly account for rate overlaps of the algorithms operated in parallel in the GT (section2.4) while designing new menus. Additionally, since the event size was significantly smaller than the standard event sizes [3], it was possible to collect a much higher trigger rate of these events than the standard event-data payload, enabling frequent offline analysis and cross-checks of the L1 trigger decision.

5.1.1 Menu development

The L1 menu development for the first LHC run was to a large extent based on data. Data recorded during standard collision runs and from special LHC setups including high pileup runs. To better understand the features of the LHC machine, different magnet and collimator settings were used. In addition, some data were taken with very few proton bunches. Large number of protons per bunch lead to significantly more collisions per bunch crossing, resulting in high-pileup events. These events were used to project trigger rates at improved LHC performance. Simulated data samples were also used to evaluate the impact of the 7 TeV to 8 TeV LHC energy increase in 2012.

For the L1 menu development, as well as for the development of the L1 trigger algorithms, we followed the following principles and strategy:

• use single-object triggers as baseline algorithms and adjust thresholds to be sensitive to the electroweak physics as well as new physics, e.g., heavy particles, multi-object final states, events with large missing transverse energy;

• in case the thresholds of the single-object triggers are too high with respect to the given physics goals (or if the acceptance for a given signal can be largely increased), use multi- object triggers, e.g., two muons or one muon plus two jets;

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Table 15. Machine operational conditions, target instantaneous luminosity used for rate estimation, and

approximate overall L1 rate for three sample L1 menus, representative of the end of the year data-taking conditions for 2010, 2011, and 2012.

Year √s[TeV] Ref. L[cm−2s−1] hpileupi hL1 ratei[kHz]

2010 7 0.15×1033 ≈2.5 56.9 2011 7 3.00×1033 ≈14 80.9 2012 8 5.00×1033 ≈23 56.5 Rate [kHz] 0 2 4 6 8 10 12 14 16 18 20 22 24

2010 Single EG 8 2011 Single EG 15 2012 Single EG 20 2010 Double EG 5 2011 Double EG 12 5 2012 Double EG 13 7 2011 Triple EG 7 2012 Triple EG 7

= 7 or 8 TeV s CMS b] µ Cross section [ 1 10 2 10

2010 Single EG 8 2011 Single EG 15 2012 Single EG 20 2010 Double EG 5 2011 Double EG 12 5 2012 Double EG 13 7 2011 Triple EG 7 2012 Triple EG 7

= 7 or 8 TeV s

CMS

Figure 71. Rates (left) and cross sections (right) for a significant sample of L1 e/γtriggers from 2010, 2011, and 2012 sample menus.

• prefer algorithms which are insensitive to changing LHC run conditions, e.g., prefer algo- rithms that are less sensitive to pileup events; and

• the algorithms and thresholds in a new L1 menu developed, e.g., for a different instantaneous luminosity, should result, if possible, in a similar sharing of rates for the same type of triggers: i.e., the muon triggers, e/γ and jet/sum triggers should have the same rate at a different instantaneous luminosity compared to the existing L1 menu.

Table15gives an overview of typical output rates of the L1 trigger system in 2010, 2011, and 2012, and table16shows details for a typical 2012 menu. The examples are chosen for LHC run periods where the measured instantaneous luminosities were close to the ones the different menus were designed for. The overall L1 trigger output rate was significantly higher than 50 kHz and well below the 100 kHz limit, as intended. The differences of observed and predicted total trigger rates largely depended on how the L1 trigger was operated, i.e., if a prescale column was changed at instantaneous luminosities different from the desired operating instantaneous luminosity of a specific L1 menu it followed that the total trigger output rate changed significantly (O(10 kHz)).

The average L1 total trigger output rate varied from year to year due to adaptations to the changing LHC conditions. Figures 71, 72, and 73 show trigger rates and cross sections of the

2017 JINST 12 P01020