1 Methods of Experimental Particle Physics Alexei Safonov Lecture #13

Presentations Two presentations today: Chris Davis – about the design of the CMS Drift Tubes (DT) muon detector cells Jeff Roe – on RICH detectors 2

Chris Davis

 Filled with an Ar/CO 2 gas mixture  Charged particles (muons) cause a cascade of ionized electrons  electrons detected at the anode  Question: Why are there electrodes?

 Track inclination affects the resolution  electrons with the shortest drift time not produced in the middle of the cell  Magnetic field affects the moving electrons  Simulation with 0.5 T magnetic field

 Drift Tubes Trigger System of the CMS Experiment at LHC: Commissioning and Performances. Carlo Battilana, PhD Thesis University of Bologna.

Jeffrey Roe 3/18/2013 Cherenkov Radiation and RICH Detectors

Cherenkov Radiation Cherenkov radiation occurs when a charge particle traverses a material faster than light in that material Predicted by Oliver Heaviside in 1888 Discovered by Pavel Alekseyevich Cherenkov (Nobel Prize 1958) Polarization and de-excitation of molecules in the medium emits photons In the slower than light case, photons would interfere destructively When the photons are produced faster than the light travels, the emitted photons interfere constructively, resulting in visible photon emission at a fixed angle

Cherenkov Radiation for Particle ID

Ring Imaging Cherenkov (RICH) Detectors Design considerations Angular Resolution Angular dispersion for different frequencies Flaws in optical systems (mirrors, lenses, etc.) Position resolution for detector – Number of collected photons Depth of radiating material Photon transmission trough the material and optical system Efficiency (quantum) of photon detectors Design components – Radiating material – Optical system (optional) – Photodetector Example RICH detectors with (left, “focusing RICH”) and without (right, “proximity RICH”) optical systems

RICH Detectors An Example RICH Design Example from the HADES experiment RICH at the HADES experiment for e + e - pairs – Relatively low particle momentum Velocity of electrons/positrons and hadrons much different (easy to make it “hadron blind”) – Radiator: CaF 2 n = , χ 0 = g/cm 2 High UV emittance – Photodetector Segmented photocathode (CsI) Multi-wire proportional chamber (MWPC)

RICH Detectors More Examples: LHCb RICH-1: Lower momentum tracks (10-65 GeV/c) – Radiator: C 4 F 10 RICH-2: Higher momentum tracks ( GeV/c) – Radiator: CF 4 Hybrid Photon Detectors (HPDs) – Combination of photocathodes and silicon based photodiodes RICH 1 RICH 2

MAIN LECTURE: QCD 16

Quantum ChromoDynamics (QCD) Another quantum field theory Much like QED or the electroweak part of the Standard Model QCD describes interactions of quark and gluons, which are constituents of all hadrons Hadrons are bound states of quarks (“matter fields”) kept together by gluons (“force carriers”) Not too different from the hydrogen atom where charged fermions (protons and electrons, the “matter fields”) are kept together by the electromagentic field (photons, the “force carriers”) Also based on a gauge symmetry SU(3) in this case, which is like SU(2) but a higher dimension group 17

QCD Charge The QCD Charge is “color” In QED, the charge is electric charge All QED interactions preserve conservation of the electric charge (can’t convert an electron into a positron by interacting with the force carriers) In EWK theory, the electroweak hypercharge preserves the leptons and quarks from converting into each other (a muon can convert into a neutrino via interaction with W, which is the generator of the group, but it doesn’t allow it to convert into a quark as those interactions are explicitly not allowed) QCD preserves quark flavor (a top quark cannot convert into a charm or up quark) Quarks interact with each other by exchanging gluons but gluons do not change the flavor of the quark, it remains what it was In some sense QCD is like QED, electrons can emit photons to interact with other electrons or positrons but you can’t kill an electron or convert it into a positron Each quark can have three colors (that’s why it is SU(3)) Quarks can change colors by exchanging gluons, =8 gluons as they are the generators of SU(3) 18

Asymptotic Freedom and Confinement QCD is an attractive force that grows with distance That’s why you can’t have a free quark, they always appear in “colorless” combinations Any free hanging color will cause a huge force This is “confinement” in QCD, it takes infinite amount of energy to separate two quarks At very small distances (high energies), QCD force all but disappears – “asymptotic freedom”: Quarks inside a proton are much like three pool balls inside a shell 19

Energy Behavior QCD does not have the usual QFT troubles at high energy As the force drops at small distances (large energies), there are no ultraviolet divergences and so no worries about renormalizability Running coupling: Where k is the usual momentum transfer (same as q 2 we used before) and  is “Lambda QCD” There are some details to it but  is about 250 MeV But it has troubles at low energies: Alpha becomes too large to do perturbative calculations at already a few GeV scale Any bound states (hadrons) have typical energy transfers much smaller than several GeV’s (most of them have their entire mass smaller than a GeV) 20

Bound States Despite being a nice theory, for a long time QCD has been only partially usable What’s good in a theory that can’t predict the masses of mesons and baryons? The issue is that we aren’t good at dealing with regimes where perturbative methods don’t work QCD is one of them “Lattice QCD” is an area of calculational particle physics where they calculate these using non-perturbative methods 21

Path Integral This is another way to do quantum field theory Solution functions are those that minimize action Much like in regular mechanics In QFT you will write something like: An “integral over all possible functions over all possible points in space” 22

Path Integral in Discrete Case It is a little difficult to grasp what is an “integral over all possible functions over all possible points in space” It makes more sense if instead of all space, you do it on a discrete lattice, so that you sum over values at specific x i and so you are trying to find a function that gives such values  (x i ) that minimize action: Lattice QCD is effectively calculating path integrals for “x to x” transitions which are dominated by the ground state as time (T) is infinite And so they can find masses of the ground states (mesons) 23

QCD Interactions For high energy collisions, there are two main implications: You can produce quarks in colliding beams and something needs to happen with them as they can’t live by themselves If your collider is a hadron machine, you need to know how to calculate cross-sections as what interacts in QCD is quarks and gluons and not protons 24

QCD Jets Let’s separate the problems: first lets make quarks/gluons with a e+e- machine What will happen with it? Turns out you get “jets” Gluon discovery at Petra 25

Number of Quark Colors Stay away from QCD yet, look at EW production of quarks at an e+e- machine Note we can check how many colors are there: 26

Jet Fragmentation As quarks can’t live by themselves, they need to bleach their color They do it by “showering” emitting gluons that can produce quark pairs One would have the right color to form a color neutral hadron (meson), the other one will have to get another quark from somewhere to bleach itself As in the beginning you had a colorless system, you can always find a pair for each quark/gluon to end up with a set of colorless hadrons 27

Jet Production Two things to note: Each quark showers a lot as the QCD coupling is strong Means we would have to do calculations to some potentially very high orders Even though the two initial quarks give the direction and energy to two jets, they still “talk” As they need to cancel each other’s color the color string is getting stretched producing some particles flying far from the directions of the parent quarks 28

Factorization Doing any calculable predictions would be impossible without simplifications: Break into two stages with very different energy scales: Produce two energetic quarks Hard process as energies are high Very perturbative regime, calculations should work fine Let each quark independently shower (called fragmentation) – difficult: This is much softer process, one would wonder if QCD would even work there At the end of it the groups of quarks/gluons form hadrons (very soft process, perturbative methods can’t work – need some modeling) Then apply correction to account for broken strings (add particles between main quarks that are there to cancel the color flow) 29

Next time QCD fragmentation QCD in higher orders: LO, NLO, LL, NLL Parton model and parton distribution functions 30