Visualizing Data from the LHC with the Atlantis Event Display Program Joshua Auriemma Advisor: Andy Haas

Part I: Everything You Always Wanted to Know About ATLAS, Only Wrong A general overview of detector components.

A Toroidal LHC Apparatus

Inner Detector Is mainly a “tracking detector”, required in order to determine the momentum, position, and impact parameter (λ INT ) The inner detector consists of three main sections: the pixel detector, the semi- conductor tracker, and the transition radiation tracker.

The Pixel Detector Provides tracking information for pattern recognition near the collision point and largely determines the Inner Detector’s ability to find secondary vertices. Three measurements over the full acceptance are performed in order to find the impact parameter resolution, and the detector’s ability to find short-lived particles such as B hardrons and t leptons.

Semi-Conductor Tracker A charged particle passing through this detector liberates charge carriers, electrons, and "holes". When separated by the E-field, the "holes" drift to the back-plane, and the electrons to the readout strip. This creates a potential difference which can be measured.

Transition Radiation Detector Why Have a TRD? Many high-energy particles “look” very similar to each other due to similar momentum. The TRD measures E/m – a value which differs greatly depending on the particle. How Does it Work? As high-energy electrons transverse the detector, X-Rays are produced which ionize a gasseous Xe and CO 2 compound. All particles will create a signal!

Calorimeter Detectors The calorimeter detectors are designed to stop basically every particle except neurtinos and muons. The inner-most calorimeter detector is known as the electromagnetic detector. The outer-most calorimeter detector is the hadronic calorimeter detector.

Electromagnetic Calorimeter Responsible for stopping electrons, positrons, and photons. Detector material causes bremsstrahlung. The energy lost is found by: Insufficient technique for photons (photoelectric, Rayleigh, Compton, pair production…).

Hadronic Calorimeter Hadrons have most likely already interacted with the EM Calorimeter. Generally in the first interaction jets of particles are produced. Charged particles will interact with scintillator.

Muon Chambers Because muons are so massive, they generally pass straight through the inner detector, and both calorimeters without losing very much energy. Extremely massive muon detectors must therefore be constructed in order to detect these particles. The detection system is based on the deflection of muons by the large barrel toroid. The muons are measured in three layers of chambers around the beam axis using Monitored Drift Tubes and a Cathode Strip Chambers. Detection will occur inside the Resistive Plate Chambers and the Thin Gap Chambers.

Monitored Drift Tubes Aluminum tubes pictured above act as beam pipes for the muons. Each tube has a wire within it with a potential difference. Changes in the muon’s course due to the E-field provided by the toroid will cause induction in the wire and can be measured. Knowledge of the drift-speed allows for tracking.

Cathode Strip Chambers The CSCs are multi-wire proportional chambers with cathode strip read-outs. The precision coordinate is obtained by measuring the charge induced on the segmented cathode by the avalanche formed by the anode wire. Relatively good special resolutions can be obtained through charge interpolations (resolution of ~60 mm). Absence of hydrogen in the gas mixture, combined with a small gap width, allows for a low sensitivity to neutron background. Less sensitive to variations in gas parameters than the MDTs

Resistive Plate Chambers The RPS is a gaseous detector (C2H2$F4). There is a narrow gap formed by two parallel plates made from bakelite of 2 mm thickness which is separated by insulating spacers. Avelanches are generated by high field of 4.5 kV/mm. The signal is measured through a capacitive coupling by metal strips on both sides of the detector.

Thin Gap Chambers In the TGCs, however, the wire distances are small enough to guarantee short drift times, and therefore, good timing resolution. The anode plane is sandwiched between two cathode planes made of 1.6 mm G-10 plates on which the graphite cathode is deposited. On the backside of the cathode plates facing the center plane of the chamber, etched copper strips provide the readout of the azimuthal coordinate. On the outside, the gas pressure is sustained by 5 mm thick paper honeycomb panels.

Part II: Neat Physics with Atlantis

About Atlantis The Atlantis project is a java-based event display program that represents over 40 years of work. The Atlantis collaboration currently consists of 14 members at 8 different institutions. Atlantis subscribes to the following 3 principles: 1.Atlantis is fast 2.Atlantis is used intuitively. 3.Atlas is used for complete ATLAS events.

Reasons for Atlantis The Atlantis program has two main objectives. 1.To serve as a means of examining and verifying pre-flagged “interesting physics”. 2.To obtain a physics understanding of algorithms related to the detector.

Confirming New Physics While Atlantis will likely never be the first program to find new physics, it certainly serves as a useful verification of physics. For example, one may note an obvious B hadron due to the track reconstruction near the point of interaction. That same person may note that two very prominent electrons were absorbed in the calorimeter detector. Theoretically, it would seem logical that the missing ET of this event would be low, since it is likely that a top quark decayed into a W-boson and a b-quark, and that b-quark then decayed into an electron and an electron neutrino.

Algorithm Construction While algorithm construction is loosely tired to the first goal, many of Atlantis' contributors actually consider this reason the number one reason for the existence of a graphical display program. One may be programming something which uses vertices in order to b-tag. While out of context, the algorithm may make very little sense, applying it in the context of an event display such as Atlantis, the code can become much more intuitive.

Understanding the Coordinate System The ATLAS coordinate system (x,y,z) is defined as: z = beam axis = cylinder axis x = horizontal axis y = vertical axis Additionally, the coordinate system ( f,h,r) is defined as: r = sqrt(x 2 + y 2 ) f = arctan(y/x) h = arctan( r/Z)

Revisiting the Detector The resolution of the detector is fine enough such that it is possible to make out the structure of the detector through hits.

The Pixel Detector

Inner Detector

Impact Parameter

Examining Simulated Events Having simulated data for specific types of events, Atlantis can be used in order to verify that these algorithms are working properly.

Electron (XY)

Electron ( r Z)

Electron (Lego plot)

Muon (XY)

Muon ( r Z)

Jet (XY)

Jet ( r Z)

Jet (Lego Plot)

In Conclusion Although it is not meant to replace triggering systems, Atlantis is a great way of visualizing events. Extremely complicated events can be very easily examined through the use of a graphical display program. Atlantis is great!