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«STUDENT RESEARCH PAPERS SUMMER 2011 VOLUME 22 REU DIRECTOR UMESH GARG, PH.D. REU Student Research Papers – Summer 2011 University of Notre Dame – ...»

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Conclusion For this experiment a 60Fe sample was placed inside the lead castle to obtain a measurement for 60Co. The decay peaks at the energies 1099 keV and 1291 keV of 59Fe was analyzed. The data displayed in figures 2-9 was measured and calculated for the activity of 60Co. This activity will be plugged into equation 4. The number of 60Fe nuclei, N, is currently being worked on too by graduate students in the process of AMS. When both λ and N is calculated and implemented into equation 4 the decay constant of 60Fe will be measured. From this we can find the half-life of 60Fe.

The gamma energies for 60Co have not come up yet because the 59Fe hasn’t decayed completely. Run 7 data was just analyzed and shows that 59Fe is now background radiation. Since the 59Fe has went to background the gamma energies of 60Co will be analyzed as it starts to appear.

–  –  –

[1] Rugel, G. et al., “New Measurement of the 60Fe Half-Life.” Physical Review Letters.

(2009): 1-4.

[2] Knoll, Glenn. Radiation Detection and Measurement. 2rd ed. New York, NY: John Wiley & Sons, Inc., 1989, 65-96, 387-397 [3] Kutschera, Walter et al., “Half-Life of 60Fe.” Elsevier Science Publishers B.V..

(1984): 430-435.

[4] Krane, Kenneth. Introductory Nuclear Physics. John Wiley & Sons, Inc., 1988

–  –  –

The research performed revolves around creating tracking algorithms for the proposed ten-year upgrade to the silicon tracker for the Compact Muon Solenoid (CMS), one of two main detectors for the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. The proposed upgrade to the silicon tracker for CMS will use high-speed electronics to trace particle trajectories so that they can be used immediately in a trigger system. The additional information will be combined with other sub-detectors in CMS to distinguish interesting events from background, enabling the good events to be read-out by the detector. The algorithms would be implemented directly into the Level-1 trigger, i.e. the first trigger in a two-trigger system, to be used in real time.

Specifically, by analyzing computer generated stable particles over various ranges of transverse momentum and the various tracks they produce, we created and tested various simulated trigger algorithms that would be hopefully used in hardware. As one algorithm has proved very effective, the next step is to this algorithm against simulated events with an environment equivalent to SLHC luminosities.

To Table of Contents

1 Introduction: The LHC Machine In Geneva, Switzerland, home to the European Council for Nuclear Research (CERN), is the world’s most powerful man-made particle accelerator. Located an average 100 meters below the surface, the Large Hadron Collider (LHC) accelerates protons and heavy ions in a 27-kilometer ring for collision. The LHC looks to answer some of the most fundamental questions concerning the nature of the universe. The Standard Model of Particle Physics is currently acknowledged as the most complete description of particle interactions between the electromagnetic, weak, and strong forces. However, under certain symmetries, the electromagnetic and weak forces can be unified under one electroweak force, but only if the particles mediating the weak force have no mass. Yet, the W and Z bosons are observed to have mass, and so something must be added to the theory. One explanation is the Higgs boson, which continues to remain elusive. Finding the Higgs boson is one of the main reasons behind the construction of the LHC and its detectors. In addition to solving the electroweak symmetry breaking, the LHC looks to answer questions concerning the nature of dark matter, dark energy, supersymmetry, and extra dimensions. All these questions seek to test physics beyond the Standard Model.

At design specifications, the LHC reaches a maximum of 14 TeV center of mass energy for proton-proton collisions. Each proton beam has an average energy of 7 TeV, which corresponds to approximately 7500 times the rest mass energy of a proton. Due to initial setbacks, the LHC is currently operating at only half of this energy. The current peak luminosity, or the number of particles per unit area per unit time, of the proton beam collisions is on the order of 1.25 ∗ 1033 cm−2 /s [2]. Design specifications have bunch crossings occur every 25 ns, or 40 MHz, resulting in 2808 bunches in the entire LHC [1].

2 The CMS Detector The Compact Muon Solenoid is the detector for one of two major particle physics experiments being performed at the LHC, the other being ATLAS. CMS gets its name from being small relative to ATLAS, from the solenoid inside of it which produces a 3.8T magnetic field, and from the muon detectors which sit outside the solenoid and make up a large percentage of the overall volume. CMS is designed for both flexibility in studying various results of particle collisions and especially for efficiency and accuracy in detecting muons. CMS has 4 layers, each consisting of a barrel and an endcap. See Figure 2 below for schematic layout. The layers are the silicon tracking layer, the electromagnetic calorimeter, the hadron calorimeter, and the muon detector when they are listed from innermost to outermost. The

To Table of Contents

solenoid is directly between the hadron calorimetry layer and the muon system. The barrel of each layer is optimized to detect events which are close to perpendicular from the beam line (low η, or psuedorapidity), while the endcap tends to be oriented perpendicular to the barrel and is optimized for higher η events. η is a measure of how close a particle is to being parallel with the beam. η=0 corresponds with a perpendicular, while η=∞ describes a line parallel with the beam. η=1.2, the cutoff for most of the barrel detectors, is an angle 33.5o off the beam line.





–  –  –

2.1 Detectors in CMS 2.1.1 The Muon Detector The outer shell of CMS is the muon detector. It is a detector array consisting of 3 different types of muon detectors: Drift Tubes, Cathode Strip Chambers, and Resistive Plate Chambers. For the details of the workings of these detectors, please see the section

3.1 of the CMS TDR [1].

To Table of Contents

2.1.2 The Solenoid

Directly inside of the muon layer is the super conducting solenoid. This produces a 3.8T magnetic field inside of it for the purpose of bending particles in order to investigate their momenta. The uniform magnetic field inside of this detector provides a method of converting a curvature of a particle into a momentum, which can then be translated to energy once the identity of the particle is known. The inner radius of this solenoid is about 3m, so the HCAL, ECAL, and the silicon tracking make up only about 15% by volume of CMS.

2.1.3 The Hadron Calorimeter

The HCAL lies inside of the solenoid and is used for finding the energies of hadrons resulting from different particle events. The HCAL barrel portion achieves this with layers of metal (mostly brass) plates sandwiching scintillating crystals. The HCAL also includes an outer hadron calorimeter; it is a thin layer of scintillating crystals placed just outside the solenoid. This outer layer is used to measure how much of the hadron shower is missed [1].

The hadron endcap contains similar detectors oriented in a direction consistent with capping the cylinder. The hadron detector layer is unique in its inclusion of a forward detector to cover angles even less perpendicular to the beam line than the typical endcaps (3.0 η

5.0 instead of 1.2 η 3.0). muons will deposit very little energy in this layer and can therefore be tracked and measured in the outer muon layer.

2.1.4 The Electromagnetic Calorimeter

Below the HCAL is the electromagnetic calorimeter (ECAL), which is used to measure the energies of photons and electron resulting from events. The ECAL consists of many lead tungstate crystals that emit light with a short distance of interaction and in a short amount of time, but fail to produce much light per energy of incident particle. The light is therefore amplified and readout with 15 bits of precision (over a dynamic range) [1]. The endcaps of the ECAL are made of the same material. Both heavy hadrons and muons will deposit minimal energy in this layer, so that accurate readings can be taken as they move to the outer layers.

2.1.5 The Silicon Tracker

Closest to the beam line is the silicon tracking layer, which is a series of cylinders made from silicon strip detectors and, in the case of the innermost layers, silicon pixel detectors.

This set of detectors extends to a radius of approximately 1.2m. Silicon detectors are strips of metal placed on top of two layers of alternately doped silicon making what is essentially a

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very large diode this allows them to detect the current of charged particles passing through them, and this can be read out directly. The spatial resolution is limited by how small the metal plates can be created, as a voltage change only allows for the information that the particle hit somewhere on the plate. This can be improved somewhat if the particle goes in between two plates and leaves some charge on each, as then the charges can be shared to determine where in between the plates the particle passed. The silicon pixel detectors have three layers with radii 4.4cm, 7.3cm, and 10.2cm away from the beam line, and provide accurate 3 dimensional location data for charged particles passing through them. Between 20cm and 55cm are silicon strip detectors, which sacrifice one dimension of resolution in space for a large savings in readout electronics, and out wide of this range are still larger silicon strip detectors, for a total of ten layers. The problem of spatial resolution in strip detectors can be somewhat resolved if 2 layers are positioned closely together with the strips in an slightly off parallel and one event fired relatively few strips; this is performed by using the data in both layers to pinpoint the exact 3D location where the particle must have been if it went through both strips.

Despite being thick, which can cause a particle to change trajectory, silicon detectors can accurately measure positions of particles that pass through them. This high precision in position leads to another well-defined measurement: transverse momentum (pT ). Due to the solenoid’s magnetic field being parallel to the beam, from electromagnetism, a charged particle will bend while moving in the transverse, or perpendicular, direction of the beam.

A particle’s radius of curvature can be determined based only on 2 points, and pT can be directly found from magnetic strength and radius of curvature. Precisely, if CMS is in a cylindrical coordinate system with the beam line as the z axis, φ being the angle, and r the radius, then it is in the r direction (φ cross z). Since the transverse momentum is always oriented away from the middle of the detector, the sign of the momentum is used to indicate the charge of the particle (and therefore which direction it will curve in the magnetic field).

It is possible to measure the total momentum, given the angle between the transverse plane and the particle. By measuring a component of the momentum, it is possible to obtain the full vector quantity of the momentum. These determinations can only be made if the particle’s initial position is directly on the beam line, as the most energetic particles will be.

2.2 CMS: Triggering At design specifications events occur in the center of the detector at a rate of 40MHz.

Each event produces approximately 1 MB of detector data, which corresponds to a data rate of 40 TB/s. It is not feasible to write data out at this rate, much less store it. Therefore there

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is a system of triggers which quickly identifies which events are interesting, and thus which events to store. These triggers are adjusted based on what kind of physics is being studied at a given time, but they eventually reduce this data to write rate to 100 Hz. As a quick note, currently write rate at CMS is 300 Hz. This reduction is done in two stages: the Level-1 (L1) trigger and the Higher Level Trigger (HLT). The L1 trigger brings the data rate down to 100kHz, but can only do so much, as it must make decisions at the full detector rate and is limited by processing time. In order to react this fast this trigger must be implemented in custom hardware, which limits the complexity of the algorithm, as complexity scales price and size of the component quickly. Triggered events are then passed to the HLT which is implemented in software, and so can run with more complexity at the slower rate. As an example, the L1 muon trigger takes data from all of the detectors in the muon layer and decides if a muon is likely, if so, the event is passed to the HLT, which then uses the data in the muon layer and the silicon tracking layer to reconstruct the muon track and determine its momentum and energy [1]. The HLT can implement different types of tracking algorithms on the data; many start first from the calorimetry layers and muon systems to reconstruct objects, creating partial tracks. If these hits are not rejected by the trigger, this information can then be combined with information from the tracking layer in order to get the exact path of the particle. This is a process known as tracking. Currently, the L1 trigger does not implement any form of tracking but leaves that process to the HLT. The reasons for not including a track trigger before the HLT are historical. When CMS was first envisioned back in the late 1980’s, lower level triggers with track reconstruction were seen as ineffective.

CMS also made a large gamble in only using two triggers, whereas almost all detectors use three. CMS projected the developments in computing would be ready to handle the large computational demands present for processing the high rates of events to the HLT by the time the LHC turned on. With an already robust calorimetry and muon system, and low amounts of interactions per bunch crossing, there was no immediate need to include another trigger or any sort of lower level tracking.

2.3 CMS: Tracking By using all of the information on all of the layers together, CMS can identify tracks.



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