The Search for the Higgs Boson, and the CMS Project

Transcription

1 The Search for the Higgs Boson, and the CMS Project John Palsmeier Benedictine College Advisor: Stan Durkin The Ohio State University

2 Introduction For the past summer I have been working with Professor Stan Durkin and Postdoc Jason Gilmore on the CMS project. My involvement with this project has been primarily in the area of computer programming. The CMS project was started over ten years ago and will not be finished for another two years, however the few months that I have been here have provided me with a glimpse into the world of High Energy Physics as we search for the Higgs Boson. The Higgs So why do we want to find the Higgs Boson, and what is it? We want to find the Higgs because it is the answer to a major problem with the Standard Model. The Standard Model has been the guide to our basic understanding of matter for years. It consists of two groups, fermions and bosons, the fermions can be further divided into the six types of quarks and six types of leptons. The bosons are force carrying particles, they tell other particles about the forces that are acting between them. A corresponding boson has been found for each force, except for gravity. The graviton is believed to exist, it simply has not yet been found. This is part of the other major problem with the Standard Model, gravity does not fit in with the other forces. However, our interest lies with the Higgs Boson, and the problem it would solve. The Standard Model has no explanation for the cause of what we perceive to be

3 mass. Simply put, according to the Standard Model all particles should have zero mass. To complicate matters further, the force carriers for the weak interaction are extremely massive, while the rest of the bosons have little or no mass. The best theory for why this happens is that particles acquire their mass through their interaction with a field; this field was named the Higgs Field, and the force carrier, the Higgs Boson. This field is believed to permeate all of space and it is the same everywhere. If this theory is correct, then our concept of mass as an intrinsic property of matter is incorrect, mass would depend on the strength of a particle's interaction with the Higgs Field. CMS No current particle accelerator is capable of producing a Higgs Boson. However, the Large Hadron Collider is currently under construction, large because of the type of particle it accelerates, not its 27 km circumference. When it comes online in 2007 it will be able to accelerate a proton beam to 7 TeV, so that when two beams collide the total energy will be 14 TeV. This is well above the expected mass range for the Higgs of 80 GeV to 1 TeV. If the Higgs Boson exists, the LHC will be quite capable of creating one. Creating a Higgs is not enough, evidence must be found for its existence, and that is where the Compact Muon Solenoid comes in. The CMS project involves about 2000 physicists from 159 institutions in 36 countries. They are working to complete a monstrous 12,500 ton muon detector that contains the largest solenoid ever built. The reason that it is called a muon detector and not a Higgs detector is that the Higgs particle is too short lived to be detected directly, but muons live long enough to be detected and

4 they are part of one of the possible Higgs decay chains. So if the right combination of muons at the correct velocity is found then they must have come from a Higgs decay, and so the particle must exist. The reason that the detector has a huge solenoid in it is because charged particles move in a curved path when traveling through a magnetic field, so the stronger the magnet the tighter the curve. This allows very precise calculations of the particles velocity, which in turn is used to calculate its energy. The CMS muon system contains three types of detectors: Resistive Plate Chambers, used for triggering because of their fast response time; Drift Tubes, which are used to determine momentum and position in low data rate areas; and Cathode Strip Chambers, which perform the same function as Drift Tubes, only they are better and therefore more expensive. Thus CSCs are only used in high data rate areas, which would be the endcaps. A CSC is a gas filled, multiwire proportional chamber in which one cathode plane is segmented into strips running across wires. When a muon travels through the chamber it strips electrons off gas atoms. When these drift electrons are within 1 2 radius of the wire they accelerate toward the wire, causing the electric field to become so large that an electron photon cascade begins, creating about one thousand electron ion pairs for each initial drift electron. These electrons register as a hit as they accelerate to the anode wire; while the ions drifting away from the wire induce an electric charge on the hit wire and on the adjacent cathode

5 strips. In a CSC two coordinates per plane are available by the simultaneous and independent detection of the signal induced by the same track on both the wires and the strips. The wires give the radial coordinate while the strips measure φ. Between all these detectors an incredible amount of information is collected and must be processed, to record all of it would require an equivalent of ten thousand Encyclopedia Britannicas per second. About forty million times per second the proton beams will cross in the detector, with about twenty five collisions per crossing, this results in one billion events per second, which the system then reduces to one hundred of the most interesting events. Ohio State Involvement The endcaps are where Ohio State is making its contribution. Most of the electronics used in the endcaps to process the insane amount of data produced by the detectors were designed at OSU, along with the software to operate them. The five hundred forty CSCs each contains five Cathode Front End Boards. Each CFEB receives data from a section of the chamber sixteen strips wide by six layers deep. CFEBs do short term analog storage of the data and inform the chamber's Trigger Motherboard when a hit occurs, (the TMB was not designed at OSU.) The TMB then compares the data from multiple CFEBs to

6 determine if there was a track through multiple layers. If so the CFEBs digitize the data and send it to the chamber's Data Acquisition Motherboard. The DAQMB collects all the data from the CSC that is to be kept and sends it out to the Device Dependant Unit, which collects data from fifteen DAQMBs. The DDU is a very important piece of equipment because it is the first point where data is in a radiation free environment, so it is here that errors in the data can be checked. The DAQMB sends a key created with a special algorithm as part of the data packet to the DDU, if the key does not match the data then then the data, or the key, has been corrupted and the event is thrown away. The other very important function served by the DDU is that it is the main interface point between the CSC and the rest of the CMS. My Projects Because most of the boards that OSU is responsible for have already been designed, built, and tested, software was the main project. Most of my involvement was based on writing programs to analyze data, so my first project was to learn C and C++, and to familiarize myself with the Linux operating system. Electronics generate internal noise that can corrupt data, the purpose of my first program was to generate autocorrelated noise for the Monte Carlo simulation. A Monte Carlo simulation is basically a statistical simulation that utilized sequences of random numbers. Next I helped update a muon track fitting program that had been written three years ago and needed modifications to work properly. This program calculates a floating zero point voltage for each hit, since the zero point is not constant; it finds tracks using the least

7 squares method; it calculates noisecoefficients and cross talk and then eliminates them; and finally it plots a track as shown in the figure to the right. My last project was to write a program that determines the resolution of the muon tracks. It does this by calculating a least squares fit for each muon track six times without using one of the layers each time, it then finds the difference between the fit line and the excluded point, this is the resolution. This distribution was then compared to the distance the point was from the center of the strip. An interesting pattern exists, the standard deviation of the distribution when the charge had been found in the center of the strip was approximately twice the

8 standard deviation of the distribution when the charge had passed through toward the edge of the strip. This effect was found to be due to the way the position of the particles path through the strip is determined; when a particle passes through the center of a strip only that strip receives a large charge, when it passes through the edged of a strip then two strips receive a large charge and a more accurate location can be determined. The result of this is that the center of the strips in the data sample used have a resolution of about four hundredths of a strip while the edges have a resolution better than two hundredths of a strip. Most strips are one centimeter wide so this gives an actual resolution of four hundred microns for the center of strips and better than two hundred microns for the edge of strips. Conclusion This has been a great experience, I learned a great deal about High Energy Physics and how actual research is done. I have learned how the electronics of the detectors and data processing computer boards works. I have seen how the data collected is translated into useful information by various computer programs, some of which I worked on, or even wrote. I have had the opportunity to help with actual research work, and I have seen how much of a group effort physics research is.

9 References CMS Collaboration. CMS The Muon Project: Technical Design Report. CERN, CMS Outreach: Lederman, L. & Teresi, D. The God Particle: If the universe is the answer, what is the question? Boston: Houghton Mifflin Co, Discussions with Stan Durkin Discussions with Jason Gilmore