Doug Berry's blog

Over the last couple days I put together a presentation on the test beam analysis, an analysis which is still in progress. The talk looks at how radiation damage affects the energy resolution of the ECAL endcap Lead Tungstate Crystals. During the course of the LHC, the ECAL endcap will be exposed to a large amount of hadronic radiation. This is mostly due to protons scattering of each other at low angle. This process has a very large cross section, so overtime, this radiation damages the crystals in the endcap. You can find the talk here.

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I have been working for the last couple months on analyzing the data from the test beam from the summer. As per Tom’s request, here are the slides of a talk I gave last Wednesday summing up the work that I have done on the test beam.

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Kaons are tricky mesons. Kaons, just like pions, can be charged (K+,K-) or neutral (K0). Charged kaons leave tracks in the tracker and deposits in the HCAL. Neutral kaons don’t make tracks and leave deposits in the HCAL. Since kaons are mesons they are composed of a quark and an anti quark. In the case of kaons, one of the quarks has to be a strange quark. A K+ has an anti-strange quark and an up quark and a K- has an anti-up quark and a strange quark. The K0 is a down and anti-strange quark or an anti-down and strange quark. The neutral kaon is a tricky particle because it is a superposition of K-short and K-long. K-shorts and K-longs are both neutral kaons that decay to different particles.

In terms of test beam physics kaons can be created by colliding protons on a fixed target. Charged kaons mean lifetime is 12.38 nanoseconds and they decay to muons or pions. Since their lifetime is so short charged kaons are generally used to produced muons. Neutral kaons are studied extensively because neutral kaon oscillation is an example of CP-violation (Charge Conjugation and Parity), which will be discussed later. K-shorts decay in .08953 nanoseconds where K-longs decay in 51.16 ns and they have different decay products. K-shorts decay to charged and neutral pions and K-longs decay to pions, leptons, and neutrinos. Kaons are generally not identified or reconstructed in CMS because the collision environment is too busy.

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Pions are light mesons. Mesons are composite particles that are made of a quark and an antiquark. There are charged and neutral pionst. A positive pion is composed of one up quark and one antidown quark and a negative pion is composed of one down quark and one antiup quark. A neutral pion is a superposition of a down and antidown quark and up and antiup quark. Neutral pions decay to two photons 98.8% of the time. Charged pions decay to a muon and a muon neutrino 99.99% of the time, but since the mean lifetime of a charged pion is 26 nanoseconds, it doesn’t decay inside the detector. Pions are produced in copious amounts in the LHC environment. This is because the strong force, Quantum Cromodynamics or QCD for short, produces many light quarks that then form pions.

Pions are fairly easy to produce in a test beam. When protons collide with a dense target, pions are produced from QCD interactions. The pions are then selected with a series magnets and collimators and aimed down a beam pipe.

Charged pions leave tracks in the tracker and energy deposits in the HCAL. Neutral pions only have a mean lifetime of 84 attoseconds (.000000000000000084 seconds), they decay to two photons before they touch any part of the detector. When a charged pion impacts the HCAL is deposits energy in the brass absorber plates and creates a shower of particles. These particles then encounter a scintillator that produces light, the light then travels to a hybrid photo diode, which produces an electronic signal proportional to the light output of the scintillator.

Pions are generally not identified in CMS because there are so many of them. If there is a group of high energy tracks and a deposit in the HCAL it is labeled as a jet. The CMS environment so busy that it is imposable to identify all the particles in a collision. Below is a particularly busy Z->ee event, some of the green tracks are definitely pions.

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Muons are very similar to electrons. They are both spin 1/2 fermions with one unit of charge; however, muons are 207 times more massive than electrons. This has several important consequences for muons. One thing that muons do that electrons do not is decay. Muons are a second generation particles where electrons are a first generation particles. Muons decay to their first generation counterpart via the weak force. Below is a Feynman diagram of muon decaying to an electron, an antielectron neutrino, and a muon neutrino.

Even though muons naturally decay they rarely decay inside our detector. This is because muons have a ‘long’ mean lifetime, and by a ‘long’ lifetime I mean 2.2 microseconds. This may not seem like a long time, but muons in our detector are produced with GeV of energy so they travel very close to the speed of light. It only takes 75 nanoseconds for muons to leave our detector, so they never decay in CMS.

Muons in test beams are produced by creating particles that decay to muons. Protons are accelerated and collided with a dense material. This produces kaons and pions. The charged kaons then decay to muons and pions. The charged pions also decay to muons. This creates a stream of muons, which can be used to calibrate any detector components.

Muon are neat particles because they travel through most materials. Cosmic rays collide with the upper atmosphere and this produces muons. These muons can travel through hundreds of feet of earth. The muons that are produced in CMS do the same thing. The muons travel through the tracker, the ECAL, the HCAL, the magnet, and then finally the muon chambers. The muon chambers perform position measurements of any charged particle that makes it through the detector. Pions, which are not stopped by the detector, can fake muons because they leave hits in the muon chambers.

Muons are very important for physics because they are easy to detect and are used for triggering and calibration. Zs, Ws, bottom and top quarks, and the higgs, if it’s the right mass, have muons in their decay products. Initial calibration of CMS uses cosmic muons, which are created when high energy particles impact the upper atmosphere. By measuring the impact of cosmic muons on CMS, physicist can measure the performance of every component of the detector and their alignment. Below is an image of one such event in CMS.

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Photons are neutral, spin one vector bosons and are the messenger particle for the electromagnetic force. Photons are important to new physics because, if the Higgs boson is light, it can decay to two photons. All electromagnetic radiation is made of photons. Visible light is composed of photons between 1 and 3 electron volts. The photons that are observed in CMS can be up to 100 billion times more energetic than visible light. Below is an image of a Higgs boson decaying to two photons.

Photons can be produced in a variety of ways for a test beam. In high energy physics, photons can be produced by colliding protons on a very dense material, like tungsten, which produces neutral pions. The neutral pions then decay to two photons. Another way to produce photons is to use synchrotron radiation from electrons. Synchrotron radiation is produced by bending high energy electrons in a magnetic field. Using synchrotron radiation as a photon source has become popular in the fields of chemistry and biology, because it can used for many different scientific experiments, such as high speed imaging and atomic spectroscopy.

The ECAL measures photon energy the same way as it measures the energy of an electron. The photon impacts the ECAL and produces an electron-positron pair. The positron and electron then bremsstrahlung and the bremsstrahlung photons produces electron positron pairs, which causes a shower of light that is measured by a photo detector at the back of the ECAL.

The reconstruction of a photon is very simple in CMS, since the photon is neutral it doesn’t have a track so it just looks like a deposit of energy in the ECAL; however, photons can be mistaken for other things. If there is a charged particle that happens to impact the ECAL in the same location as the photon, it can look like and electron. If there happens to be a deposit in the HCAL near the ECAL deposit, it can be labeled as a jet. While photons are easily reconstructed, it can be difficult in an environment with as many particles as the LHC.

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The electrons that we use in test beams are the same electrons that every student learns about in fourth grade science class. Electrons are charged, light fermions, which leave hits in the tracker and deposit all of their energy in the ECAL. Electrons are important because they are the decay products of electroweak interactions. W and Z bosons both decay to electrons 10.8% and 3.4% of the time respectively. Depending on its mass, the Higgs boson can decay to two W or two Z bosons. Below is an image from Wikipedia of an electron beam in a magnetic field.

Elections are very easy to produce and isolate. For example, by simply heating a filament in a vacuum, free electrons are produced. However, this is not how we produce electrons in our test beam. This is because electron are difficult to accelerate. The reason electrons are difficult to accelerate is because if an electron has a curved path it looses energy through a process called synchrotron radiation. If the electron looses energy through synchrotron radiation it looses kinetic energy. The only good option to accelerate electrons is to build a linear accelerator instead of a circular one. For our test beam we accelerated protons in the Super Proton Synchrotron, and then used the protons to make electrons.

The ECAL for CMS is an absorption calorimeter. That means it measures the energy of the incoming electrons by absorbing the kinetic energy of the electron. Electrons loose energy in the ECAL through two methods, bremsstrahlung and pair production. Bremsstrahlung radiation occurs when the electron impacts the high Z material of the ECAL and rapidly decelerates. The photons that are released from bremsstrahlung radiation then split into an electron-positron pair, which then bremsstrahlung again and the process repeats.

Reconstruction of electrons in CMS is a little tricky because as the electron curves in the magnetic field it produces bremsstrahlung radiation. An electron in CMS has a track connected to a deposit in the ECAL. The deposit in the ECAL is a large, spread out cluster because both the electron and bremsstrahlung radiation leaves deposits in the ECAL. Below is an image of a Z->ee Monte Carlo event with the electrons in light blue.

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This is the first post that relates to the milestones listed on I2U2 test beam elab. The point of a test beam is to characterize the detector. Physicists do this by bombarding each detector components with know particles. This allows physicists to obverse how individual parts of CMS behave before they are integrated into the whole experiment. Each detector component is designed to detect and measure different types of particles. The ECAL measures photons and electrons. The HCAL measures hadronic particles, which are mostly pions, kaons, protons, and neutrons. The muon chambers measure any particle that makes it through the HCAL. Most of the time those particles are muons, but sometimes a pion can punch through the HCAL and fake a muon. The tracker measures the position of charged particles. These position measurements are then connected creating a track. Tracks can only be made from charged particles. Over the next few posts, I will be talking about the different particles used in test beams and how they behave in the detector. Below is a picture of different types of particles passing through CMS. There is an animated version of the image below on the CMS web page.

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The building where the ECAL test beam took place is building 887. Building 887 is an enormous building. It is about 950 feet long, 4 stories high, and 160 feet wide, and it is packed full of physics equipment.

On the left is a picture of the interior of building 887. It’s so large that you can barely make out the 130 foot wide crane in the back. On the right is the actual beam line that contains the electrons. It stretches all the way back to the end of building 887. The beam pipe is under vacuum; however, there are physical gaps in the beam pipe where the electrons travel through air for a brief moment. Below is a picture of a physics thingamajig. It looks like it is used for coolant, but it is not used on my experiment. I think it looks neat, plus there is a sign on it that says, “danger” which is pretty cool.

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My friend and fellow Notre Dame graduate student, Nil, got some pictures of the crystal expert exchanging the regular crystal with a molybdenum doped crystal. The hope is that adding molybdenum to the crystal with prevent radiation damage and not significantly affect performance.

You can see the 5×5 crystal tower with brackets and mounting in front of them. There are also water cooling pipes on top of the crystals and photo multiplier tubes (PMTs) mounted on back connected to fiber optic, high voltage, and signal cables.

On the left is a Mo doped lead tungstate crystal being held by Sasha the crystal expert. On the right is an image of a photo multiplier tube. The red cables are high voltage cables for the PMTs, the black cables are the signal out for the PMTs, and the fine white wires are fiber optical cables that carry an LED signal, which is used to calibrate the PMTs.

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