1. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Studies on the High Level Trigger System of the ATLAS Detector Stefanidis E. University College London

2. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Outline  ATLAS Trigger System: The challenge. Overview.  Level-2 Trigger: The Region of Interest mechanism.  Reduced Region of Interest: Motivation. Methodology. Implementation.  Results.  Conclusions.

3. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef The ATLAS Trigger System: The challenge Proton bunch crossing rate of 40 MHz Design (high) luminosity of 10 34 cm -2 s -1 s pp-inel = 80 mb @  s = 14 TeV ~25 p-p interactions per crossing ! ~10 9 interactions per second ! Signal events are very rare: H  gg rate is ~10 -13 of the LHC interaction rate. (m H =120 GeV) Limited storage capability + High Granularity of the sub-systems event size ~1.6 MB just before storage. storage rate of O (200) Hz. Trigger System must be: Fast. Efficient: reject background and keep signals. Flexible: Cooperate for different signal signatures. Robust: successful operation during runs.

4. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef The ATLAS Trigger System: Overview 1.LVL1: 40 MHz  75 kHz < 2.5 μs Hardware trigger Coarse granularity Calorimeter and Muon Trigger stations only 2.LVL2: 75 kHz  2 kHz 10 ms Full granularity information from both ID and Calorimeter RoI mechanism 3.Event Filter (EF): 2 kHz  100 Hz  2s Sophisticated algorithms Alignment data available High Level Trigger (HLT)

5. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Level-2 Trigger: The RoI mechanism Region of Interest defines the geometrical region where HLT will start from. It is based on the LVL1 information. Z r Z r Z r Zvertex Z r Middle of the 1 st EM sampling Middle of the 2 nd EM sampling Z r Zvertex Middle of the 1 st EM sampling Middle of the 2 nd EM sampling How the RoI is currently constructed… With the RoI mechanism we manage to minimise the: Processing time. Network traffic. Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f ±0.1 Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f ±0.1 Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f ±0.1 Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f ±0.1 Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f ±0.1 Z r Zvertex 1 st sampling 2 nd sampling Level-1: h, f ±0.1 ±168 mm

6. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Reduced Region of Interest: motivation The size of the RoI in (eta, phi) space has been determined almost arbitrarily. The RoI currently takes into account the full range of the beam spread. Better algorithm design can reduce the size. Faster decisions (and more efficient?), especially in a demanding environment. Never before has been a detailed study for optimising this size. Main tool: the 1st and 2nd samplings of the EM Calorimeter. High granularity information at Level-2.

7. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Reduced Region of Interest: methodology Z r Zvertex 1 st sampling 2 nd sampling 1. Determine the position of the middle of the 1st and 2nd sampling. 2. Extrapolate back to the beam. 3. Open 3sigma around the Z found by the Calorimeter Z r Zvertex 1 st sampling 2 nd sampling Z r Zvertex 1 st sampling 2 nd sampling Z r Zvertex 1 st sampling 2 nd sampling Z r Zvertex 1 st sampling 2 nd sampling Zcalo Z r Zvertex 1 st sampling 2 nd sampling Zcalo Z r Zvertex 1 st sampling 2 nd sampling Zcalo The RoI is reduced by ~30%!

8. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Reduced Region of Interest: implementation 1. Parameterize the middle of the 1st and the 2nd EM samplings 2. Parameterize the width to open.

9. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Results Using our Tracking Algorithm we can test the method: 1.…in terms of efficiency: 2.…in terms of timing:

10. HEP 2005 WorkShop, Thessaloniki April, 21 st – 24 th 2005 Efstathios (Stathis) Stefanidis sstef@hep.ucl.ac.uk www.hep.ucl.ac.uk/~sstef Conclusions A new method has been applied in order to reduce the size of the Region of Interest. Important improvement in terms of efficiency and timing even at this preliminary stage. Further/high order corrections and studies are being carried out in order to optimize the size. Contacts have started to integrate the new method into the official ATLAS Trigger Software.