NYU Experimental Particle Physics GroupA jargon-free introduction to particle physics
This page provides an overview to particle physics and our research with minimal jargon. The ATLAS and MILAGRO provide a more detailed description of our activities.
- What particle physics studies.
- The experimental method.
- LHC and ATLAS.
- The ATLAS Collaboration.
- ATLAS Physics Potential.
- The ATLAS Trigger.
- An LHC Upgrade.
What particle physics studies:
- From many materials to just a few particles:
First known to have been discussed by the ancient Greeks, the idea of atoms, fundamental building blocks of matter, was reintroduced about 200 years ago. Today we know that fewer than 100 types of atoms are enough to account for the vast variety of materials we usually interact with in our data lives. In the late 19th and early 20th century it was learned that the atom consists of a small central, positively charged nucleus that contains most of its mass, and negatively charged electrons surrounding it that account for most of its size. In the first few decades of the 20th century it was discovered that the the many different nuclei are all made up of the same two building blocks, one positively charged (called the proton) and the other electrically neutral (the neutron).
- A proliferation of particles:
In the ensuing decades, through the early 1960's, dozens of particles were discovered that were not built of protons and neutrons and hence seemed just as fundamental. The simple picture of a Universe built of just a few basic building blocks seemed to be fading away.
- Symmetry to the rescue:
Colloquially we describe an object as being symmetric when it appears regular or balanced in some way. Mathematically, we can describe symmetry by lack of change under a transformation. For example, if we rotate a circle around it's center by any amount, it looks exactly the same; you can't tell whether it's standing up or lying down (in this sense a circle is more symmetric than a person). It was found that the properties of the many newly discovered particles fit a regular pattern that could be described mathematically in terms of symmetries. This in turn led to the idea of a new set of 3 fundamental particles (for which the name quarks wound up being widely adopted) out of which the proton, neutron, and their many cousins were made.
- An aside about forces:
In the middle decades of the 20th century, it was learned that combining relativity (which describes very fast moving objects) and quantum mechanics (which describes very small objects) leads to a description of forces between particles as being due to the exchange of particles (the mediators of the forces) between them. So the description of particles also contains a description of forces. The observed behavior of forces led to their description in terms of mathematical structures which were also symmetric, in the sense discussed above.
- Too many particles and forces?
In the past few decades, particles have continued to be discovered. Today we know of 6 quarks, each coming in three different types (differing in a property therefore dubbed color). The electron, which is not made of quarks, has two heavier cousins with properties very similar to it, called the muon and tau. There are also three additional particles, called neutrinos, whose properties are similar to those of the electron except that they are electrically neutral and therefore don't interact much. For each of the above particles there is also an anti-particle (a particle with the same properties except for opposite charge). Then there are the force mediating particles: the photon responsible for the electro-magnetic force, 2 W's and a Z responsible for the weak nuclear force, and 8 gluons responsible for the strong nuclear force.
- The Standard Model:
The theoretical framework that incorporates the known particles and describes their interactions is based on symmetry properties and has been so successful that it is now called the "Standard Model". This model has been tested in various ways for about thirty years and has yet to be proven wrong in any of its predictions. Only one major prediction remains to be confirmed, the existence of a new massive particle (about 100 to 1000 times as heavy as the proton) called the "Higgs Particle".
- Beyond the Standard Model:
Though the Standard Model has been very successful, for a number of reasons it appears to be incomplete. The value of the electric charge of the proton and electron are (except for sign) the same to a very high precision. As no Standard Model symmetry relates these particles, there is no reason within that framework that this should be the case. Another issue is the mass of the Higgs particle. Because of its many possible interactions, the effective mass of the Higgs particle would in general wind up being much greater than the value needed by the standard model. Only very precise accidental cancellations could prevent this. A third issue is the astronomical evidence for Dark Matter, which implies the existence of new particles not included in the standard model. Finally, a complete theory of the elementary particles and their interactions must include a description of the gravitational force, something not addressed at all by the Standard Model.
Theories Beyond the Standard Model (BSM), most notably Supersymmetry, provide answers to many or all of these problems. Most BSM theories predict the existence of new particles that, because of their large mass, would not have been observed in previous experiments.
The experimental method - speed particles up and smash them together
The nucleus was first discovered when a gold target was exposed to radioactive
material. Measuring the directions the radiation bounced off the target
showed that there had to be something heavy and small inside the atom. Smashing
particles into each other is the main method we have to study the building blocks
of matter, and simultaneously teaches us about the particles and the forces between
them. Also, as embodied in the equation E=mc2, the theory of relativity states that matter and energy are different forms of the same stuff. In collisions between particles the energy of the particles can be transformed to mass and new particles can thus be created. The main requirement is that the energy in the collision be at least as large as the mass equivalent energy of the new particles one wants to create. For very massive particles, such as the Higgs or those predicted by supersymmetry, one needs very high energy in the colliding particles.
Accelerators are machines that use a mixture of electric fields to speed particles up to very high energies, and magnetic fields to steer them in their path. These accelerated particles can then be smashed into a target, in a manner very much like that of the original experiment that discovered the nucleus, in what are known as "fixed target experiments". As an alternative, to make more energy available to produce new particles, particles can be smashed in head-on collisions with particles going the other way. This is done in a type of particle accelerators called a collider.
Because sub-atomic particles are so small, there is no way to aim a single one at another. Instead, two counter rotating bunches of particles continuously pass through each other, and occasionally when the bunches cross, a particle from one bunch will hit one in a bunch coming at it. Despite the large number of particles in a bunch, colliders require very precise steering and focusing of these particle "beams" in order to work. There are usually actually also several bunches going in each direction. The number of collisions per unit time depends on how many bunches cross each other in that time, the number of particles per bunch, how small a space the bunch has been squeezed in, and how well the counter-rotating bunches overlap when they pass through each other. Taken together, these are called "luminosity". The number of collisions will also depend on the effective size of the individual particles, called the "cross-section".
The more energetic a particle is, the harder it gets to make its path deviate from a straight line. For colliders that usually have counter rotating particles moving in a circular path, the necessary circle size thus grows as beam energy is increased.
Once the beam particles collide, detectors are needed to measure the properties of the produced particles. The many different types of particles created and measurements that need to be made result in detectors which often consist of nested cylinders within cylinders. At the center is the beam pipe, evacuated as well as possible so that the beam particles don't collide with air molecules instead of hitting each other. Surrounding the beam pipe and as close to it as possible are tracking chambers, used to measure the paths of the produced particles. Outside these there are often another set of tracking detectors and a large magnet. The curved path of charged particles, caused by the magnet and measured by the tracking system, allows measurement of the the produced particle momentum. Then come various detectors that measure the energy of the produced particles, called "ionization calorimeters". These measurements are destructive - particle energy is measured by having it interact in the detector. Upon collision it creates many new particles, the number of which allows one to determine the original energy of the particle that entered the calorimeter. Muons, which interact much less often than electrons, usually survive intact in their path through the calorimeter, and are detected by a final outside layer of detectors.
Some particles interact so infrequently that they usually don't interact in the detector at all. The production of these particles must be inferred, from the imbalance of momentum in the detected particles. In order for this work, the detector must surround the interaction region as completely as possible, with no gaps through which a particle can escape undetected.
If you want to learn more, and excellent introduction to the theory and experimental methods of particle physics can be found at:
A more ATLAS specific introduction can be found at: The Large Hadron Collider and the ATLAS experiment
At the Large Hadron Collider
(
http://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/ , see Figures 1,2 and 3 )
at CERN ( http://public.web.cern.ch/public/),
the European particle physics laboratory that straddles the French-Swiss border,
counter rotating protons are accelerated to 0.99999999 times the speed of light and smashed together in a
ring with a circumference of 27 kilometers. Upon completion, scheduled for some time in 2008,
the LHC will be the most energetic collider on Earth. With 2808 filled bunches in each direction,
1011 protons per bunch, and a bunch transverse radius at collision of 8 microns,
the machine also provides very high luminosity.
If they exist and if their masses are as predicted, the
LHC should be able to produce the Higgs particle, supersymmetric particles, and/or evidence for many
other Beyond the Standard Model theories.
Figures 1 through 3:
The ATLAS (A Toroidal LHC ApparatuS)
experiment
(http://atlas.ch/ , Figure 4 through 6), surrounds one
collision area at the LHC.
It is a general purpose detector designed to measure as many characteristics as
possible of the particles produced in LHC collisions.
The innermost ATLAS detector can measure the location of a particle in two
dimensions to 14 microns by 115 microns,
better than the thickness of a human hair (about 100 microns), whereas the outermost muon detector
is about the height of a 5 story building. ATLAS is located in a cavern who's floor is 92 meters below
ground level.
Figures 4 through 6:
The ATLAS Collaboration:
The 35 ATLAS experiment collaborating nations are shown in Figure 7. An up-to-date
list of collaborating institutions can be found at
http://atlas.web.cern.ch/Atlas/Management/Institutions.html .
Figure 7:
ATLAS has an enormous potential to discover new physics and to teach us something very profound about nature. It may unviel the origin of mass for elementry particles and new symmetries of nature. More on our group's physics interests can be found here.
END OF JARGON FREE ZONE
The ATLAS Trigger
The time between bunch crossings at the LHC is 25 nano-seconds, and at LHC design luminosity there will be about 25 proton-proton interactions per beam crossing. The average detector information for each beam crossing is about 2 Megabytes, so one second of running produces about 80 tera-bytes of data, or enough to fill 1000 80 Giga-byte disks (each of which is about the size disk probably connected to the computer you are viewing this web page with). Since there is no way of storing so much information, or analyzing it even of it could be stored, ATLAS uses a system of triggers to quickly decide which events are interesting to keep and stores about 100 of these per second.
ATLAS has a three-level trigger system. The first level (LVL 1) trigger is hardware based and keeps about 1 in 500 events, passing a maximum of 75,000 events per second. As the full information must be stored for each event until it is rejected, that there are a total of over 107 detector channels (not counting pixel detectors), and that cost and reliability make it desirable to keep the pipeline as short as possible, the LVL1 trigger is required to make its decision in less than 2.5 micro-seconds. When an event selected by LVL1, "Derandomizers" match the instantaneous pipeline output rate with the available input rate of Read Out Drivers (RODs), that transfer the event via point-to point Read Out Links (ROLs), each with a maximum rate of 160 Mbyte/second, to ~1600 Read Out Buffers (ROBs).
This results in a rate of ~160 GBytes/sec input to the subsequent High Level Trigger (HLT), a two tier system. Events passing LVL1 first go to the level 2 (LVL2) trigger, which passes about 1% of the events sent to it by LVL1, resulting in rate of about 1000 events per second. LVL2 has a latency of ~1 to 10 milli-seconds. Events passing LVL2 go through the "Event Building" process, where data from the different ROBs are combined to form individual events. These in turn are analyzed by the Event Filter (EF), which passes about 1 in 10 of the LVL2 triggers, giving a final rate of about one hundred ~1.5 Megabyte events per second stored for later analysis. Many, though not all, of the off-line algorithms can be used at the EF stage.
The NYU group is working on the Missing Transverse Energy (MET) part of the HLT. MET, which measures an imbalance of momentum in the plane transverse to the particle collision, is sensitive to production of particles that don't interact in the detector. It is therefore a vital part of the search for new physics, and is a natural trigger component for us to study given our group's interest in the search for supersymmetry. Some of the possible sources of fake MET include mismeasurement of jet energies in QCD jet events, detector cracks, and beam-halo interactions. As these can be many orders of magnitude more frequent than SUSY events, they put severe constraints on the MET trigger.
Detector performance during the experiment may change due to problems with the detector components, software, or changes in beam condition (such as luminosity or focus). The NYU group is also active in design and implementation of data quality monitoring, which will allow early detection of any problems and tuning the trigger for best possible performance.
An LHC Upgrade
Finally, there are possible plans for an upgrade to the LHC, the super LHC (SLHC) that would increase machine luminosity by a factor of 10 some time around 2015. This would make it necessary to upgrade several parts of the ATLAS detector. NYU is participating in R&D for a new outside portion of the inner detector.