Richard E. Hughes Atlas and CMS; p.1 What Questions Remain in Particle Physics? Why are there three types of quarks and leptons of each charge? Why

Atlas and CMS; p.1Richard E. Hughes

What Questions Remain in Particle Physics?

Why are there three types of quarks and leptons of each charge?

Why do the particles haves the masses they do? Are there more types of particles and forces to be

discovered at yet higher-energy accelerators? Are the quarks and leptons really fundamental, or do

they, too, have substructure? How can the gravitational interactions be included in

the standard model?


The Higgs The “Rule Book” that particle physicist use to describe the

universe includes the observed 6 quarks and 6 leptons This rule book also has 3 forces, each of which has associated

particles 1) Electromagnetic: for electricty ad magnetism (photons) 2) Strong force: (hold the nucleus together (gluons) 3) Weak force: beta decay, powers the sun (W and Z)

Gravity (4th force) is not included (more on this later) But there is a 5th force, which explains why particles – any particle

has mass. This force has a field associated with it, just like the gravitional force has the gravitational field associated with it.

The Standard Model proposes that there is another field not yet observed, a field that is almost indistinguishable from empty space. We call this the Higgs field. We think that all of space is filled with this field, and that by interacting with this field, particles acquire their masses. Particles that interact strongly with the Higgs field are heavy, while those that interact weakly are light.

The Higgs field has at least one new particle associated with it, the Higgs particle (or Higgs boson).


The Higgs Mechanism See http://www.hep.ucl.ac.uk/~djm/higgsa.html To understand the Higgs mechanism, imagine that a room full of

physicists chattering quietly is like space filled with the Higgs field


a well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step


this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field


if a rumor crosses the room


it creates the same kind of clustering, but this time among the scientists themselves.

In this analogy, these clusters are the Higgs particles.


Grand Unified Theories Physicists hope that a Grand Unified Theory will unify the

strong, weak, and electromagnetic interactions. There have been several proposed Unified Theories, but we need data to pick which, if any, of these theories describes nature.

If a Grand Unification of all the interactions is possible, then all the interactions we observe are all different aspects of the same, unified interaction.


Super Sysmmetry! Many physicists have developed theories of supersymmetry,

particularly in the context of Grand Unified Theories. The supersymmetric theories postulate that every particle we observe has a massive "shadow" particle partner. For example, for every quark there may be a so-called "squark" tagging along.


String Theory Modern physics has good theories for quantum mechanics, relativity, and

gravity. But these theories do not quite work with each other. There are problems caused by our living in three spatial dimensions. If we lived in more than three dimensions, these problems would naturally resolve themselves.

String Theory, one of the recent proposals of modern physics, suggests that in a world with three ordinary dimensions and some additional very "small" dimensions, particles are strings and membranes. Yes, membranes in extra dimensions are weird and hard to visualize. And what are "small dimensions?"


Extra Dimensions Extra Dimensions String theory and other new proposals require more than three space

dimensions. These extra dimensions could be very small, which is why we don't see them. How can there be extra, smaller dimensions? Think about an acrobat and a flea on a tight rope. The acrobat can move forward and

backward along the rope. But the flea can move forward and backward as well as side to side. If the flea keeps walking to one side, it goes around the rope and winds up where it started. So the acrobat has one dimension, and the flea has two dimensions, but one of these dimensions is a small closed loop.

So the acrobat cannot detect any more than the one dimension of the rope, just as we can only see the world in three dimensions, even though it might well have many more. This is impossible to visualize, precisely because we can only visualize things in three dimensions!


The Theory of Everything

The long range goal of physics is to unify all the forces, so that gravity would be combined with the future version of the Grand Unified Theory. Then the gravitational interaction would be thought of as quantized, like the other forces, so that the gravitational force is transmitted by particles called gravitons.

This poses a formidable problem. Einstein showed us that the gravitational force arises due to curvature in the fabric of spacetime. Thus, the task is to quantize spacetime to produce the desired gravitons. Achieving this type of quantum field theory is quite a challenge both conceptually and mathematically.


How do we find Higgs and SUSY?

We already have a proton-antiproton collider at Fermilab There are two detectors studying these collisions: CDF and D0 They *might* see hints of this new physics BUT THEY HAVE NOT SEEN ANYTHING LIKE THESE PARTICLES! YET!

What are the difficulties? Not enough energy Not enough “luminosity”

What to do: Build something even bigger More energy in the Collider Better Detectors


The Large Hadron Collider LHC = Large Hadron Collider; located at CERN

It is large: 27km in circumference It collides “hadrons” in this case: protons are collided with other

protons


What is CERN? The major European International Accelerator Laboratory located

near Geneva, Switzerland. (originally called Centre European pour Rechearche Nucleaire).

The World Wide Web originated in this laboratory in 1989 when its staff proposed a multimedia, hyperlinked system of documents. This laboratory has sometimes been referred to as the "home of the Web." The NCSA staff developed the first graphical browser, Mosaic, for the World Wide Web. Mosaic was released (free) for public use in 1990.


Another View The LHC is 27 km in circumference, about 100m underground To curve the protons in the circle will require ~1300

superconducting magnets Each about 14m long, generating an 8T field


LHC Detectors

B-physicsCP Violation

Heavy IonsQuark-gluon plasma

General-purposeHiggsSUSY??

General-purposeHiggsSUSY

??


Fermilab Vs LHCParameter Fermilab LHC

Type of Collider Proton-antiproton

Proton-proton

C.M. Energy 2 TeV 14 TeV

Size of ring 6.3km circum 27km circum

Collisions every 396nsec 25nsec

Main Detectors CDF & D0 CMS & Atlas

Luminosity 2 x 1032cm-2sec-1 100 x 1032cm-2sec-

1


The LHC Tunnel This is the underground tunnel of the Large Hadron Collider (LHC)

accelerator ring, where the proton beams are steered in a circle by magnets.


How Fast are the Protons moving?

Accelerators raise the energy and therefore speed of particles. However, the particle speed gets to be near that of light before the particle has traveled very far. After that, the energy can continue to increase quickly, but the particle speed very slowly gets closer and closer to the speed of light.

Energy of Proton

Fraction of Speed of Light

1eV 0.00005

1GeV (109eV) 0.875

1 TeV (1012eV) 0.99999956

7 TeV 0.999999991


What is a TeV?Energies are often expressed in units of "electron-volts". An electron-volt (eV) is the

energy acquired by a electron (or any particle with the same charge) when it is accelerated by a potential difference of 1 volt.

Typical energies involved in atomic processes (processes such as chemical reactions or the emission of light) are of order a few eV. That is why batteries typically produce about 1 volt, and have to be connected in series to get much larger potentials.

Energies in nuclear processes (like nuclear fission or radioactive decay) are typically of order one million electron-volts (1 MeV).

The highest energy accelerator now operating (at Fermilab) accelerates protons to 1 million million electron volts (1 trillion electron volts, 1 TeV =1012 eV).

The Large Hadron Collider (LHC) at CERN will accelerate each of two counter-rotating beams of protons to 7 TeV per proton.


A collision at the LHC in action


The two Giants!ATLASA Toroidal LHC ApparatuS

µ

CMSCompact Muon Solenoid

µ



ATLAS Detector


Transverse slice through CMS detector

Click on a particle type to visualise that particle in CMSPress “escape” to exit


An Event at CMS


An Event at ATLAS A simulated collision event viewed

along the beampipe. The event is one in which a mini-blackhole was produced and decayed immediately. The black area in the center with many particle tracks represents the inner detector (pixel detector, semiconductor tracker, and transition radiation tracker), which has been enormously magnified relative to the rest of the detector (in this view) . The colors of the thin tracks have no significance. The thick yellow lines are the two electrons in this event. The green area is the electromagnetic calorimeter, while the red area is the hadronic calorimeter. The green and red histograms show the energy deposits by particles in the electromagnetic and hadronic calorimeters. A muon was added by hand to the event to show how it would look in the detector; it is a thick blue line in the inner detector and orange in the (blue) muon chambers.

http://pdg.lbl.gov/atlas/atlas_photos/fulldetector/fulldetector/atlas_event_cross.html


The Detectors are LARGE!

This is just one small piece of the CMS detector, called the barrel yoke:


The CMS Cavern


Some Fun Facts about CMS

The total mass of CMS is approximately 12500 tonnes - double that of ATLAS (even though ATLAS is ~8x the volume of CMS)

The CMS tracker comprises ~250 square metres of silicon detectors - about the area of a 25m-long swimming pool The silicon Pixel detector comprises (in its basic

form) more than 23 million detector elements in an area of just over 0.5 square metres

The lead tungstate crystals forming the ECAL are 98% metal (by mass) but are completely transparent

The 80000 crystals in the ECAL have a total mass equivalent to that of ~24 adult African elephants - and are supported by 0.4mm thick structures made from carbon-fibre (in the endcaps) and glass fibre (in the barrel) to a precision of a fraction of a millimetre

The brass used for the endcap HCAL comes from recuperated artillery shells from Russian warships


Some Fun Facts about CMS

The CMS magnet will be the largest solenoid ever built The maximum magnetic field supplied by the solenoid is

4 Tesla - approximately 100000 times the strength of the magnetic field of the earth

The amount of iron used as the magnet return yoke is roughly equivalent to that used to build the Eiffel Tower in Paris

The energy stored in the CMS magnet when running at 4 Tesla could be used to melt 18 tonnes of solid gold

During one second of CMS running, a data volume equivalent to 10,000 Encyclopaedia Britannica is recorded The data rate handled by the CMS event builder (~500

Gbit/s) is equivalent to the amount of data currently exchanged by the world's Telecom networks

The total number of processors in the CMS event filter equals the number of workstations at CERN today (~4000)


What happens when the LHC Starts?

The expected start date for LHC (and thus CMS and Atlas) is around 2008

• If H0 found at the expected Standard Model mass, it will validate the GWS Electroweak Theory and complete the model.

• Measurements of the Higgs couplings and comparison with particle masses will verify mass-generating mechanism.

• A lighter than 130 GeV/c2 mass Higgs boson could support a theory beyond the Standard Model, known as Supersymmetry.

• If a Higgs boson with a mass < 1 TeV is not found, it would indicate that the Electroweak symmetry must be broken by a means other than the Higgs mechanism.


Puzzle

Find 4 straight tracks.

View along beam line of the inner tracking, with a H 4 event superimposed. The are very high energy, so leave straight tracks originating from the centre and travelling to the outside


Puzzle solution

Make a “cut” on theTransverse momentumOf the tracks: pT>2 GeV


Since the two proton beams are traveling in separate beam pipes passing through oppositely directed magnetic fields, how do they ever collide? At certain locations around the ring, called "collision points", there are no magnetic fields, and the protons are moving in straight lines. At those places, the two beams can be brought together into a single vacuum enclosure and allowed to collide head on.

The protons come in roughly

cylindrical bunches a few centimeters long and a few millionths of a meter in radius. The distance from one bunch to the next is 7.5 m. Since it takes light 25 billionths of a second (25 nanoseconds or 25 ns) to travel 7.5 m, and the protons are practically moving at the speed of light, head-on meetings between bunches at every collision point occur every 25 ns, or 40 million times per second.


How Many Collisions? If two bunches of protons meet head on,

the number of collisions between protons of one beam and protons from the other might be ten, one, or even zero. How often are there actually collisions? For a fixed bunch size, this depends on how many protons there are in each bunch, and how large each proton is.

Actually a proton can be roughly thought of

as being about 10-15 meter in radius. If you had bunches 10-6 meters in radius, and only, say, 10 protons in each bunch, the chance of even one proton-proton collision when two bunches met would be extremely small.

On the other hand, if each bunch had a billion-billion (1018) protons so that its entire cross section were just filled with protons, every proton from one bunch would collide with one from the other bunch, and you would have a billion-billion collisions per bunch crossing.

The LHC situation is in between these two extremes, a few collisions (up to 20) per bunch crossing, which requires about a billion protons in each bunch

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Richard E. Hughes Atlas and CMS; p.1 What Questions Remain in Particle Physics? Why are there three types of quarks and leptons of each charge? Why