고에너지 물리 특강

고에너지 물리 특강Experiments of High-Energy Experiments of High-Energy

PhysicsPhysicsLecture 1: Experimental Tools for HEP

- Accelerators & Detectors - Observation of fundamental particles

Lecture 2: Some recent/future HEP experiments- Belle for heavy-flavor physics and CP violation- COREA for UHECR

High-Energy Physics / Experimental Tools Mar.15, 2005 Youngjoon Kwon (Yonsei Univ.) 2

theory

physicalobservables

calculate

experimentwith any necessary

approximations

Theory vs. Experiment

3High-Energy Physics / Experimental Tools Mar.15, 2005 Youngjoon Kwon (Yonsei Univ.)

Experimental tools Particle Accelerators

Particle interactions inside matter

Particle detectors

Particle Accelerators

“precision instruments constructed on a gigantic scale” particles are traversing ~106 km for a few seconds

while maintaining the path within ~m “modern accelerators are like great Gothic cathedrals

of mediaeval Europe…” (R. Wilson) Why accelerate?

the more energy, the deeper structure we can prober p ħ/2

Why not use high-E particles

in the cosmic ray? low flux ; energies cannot be controlled

T = qVlimited to ~ 1 MeV voltage breakdown & discharge

Electrostatic Accelerators

potential difference b/w the ends of drift tubes the fields oscillate, but the particles are protected (from decelerating

phase) by the metallic drift tube the distance b/w gap increases but soon saturates

an everyday proof of special relativity!

Linear Accelerator

An example: Stanford Linear

Accelerator Center

SLAC linear acc.

cyclotron

Circular Accelerators

Cyclotron

Cyclotron[Ex] a cyclotron, with extraction radius R = 0.4 m & B = 1.5T fAC = ? Tmax = ? (for p)

fAC = fc = qB/2m = 22.9 MHz

Tmax = (qBR)2/2m = 17 MeV

As we increase the energy, relativity must be considered. fixed freq. cyclotron would not work for very high E synchronous acceleration is needed!

Synchrotron

Consider we already attained the desired energy (= constant)and the particle goes through a circular orbit under B

f or B (or both) should be changed synchronously with the particle velocity; hence it is called a “synchrotron”

a “magic formula” for charged particles

Then, for v c,

and we obtain a very useful formula

[Ex] p = 3 GeV/c, B = 2T; R = ?

SynchrotronIf we build a cyclotron-style machine, too much steel (and cost!) is needed…hence, a new design!

The beam particles take many turns to achieve the design energy.Q: is it possible to maintain the beam size (within the vac. chamber) for so many turns? beam stability ??

Focusing of beams

Phase stability edge focusing

Strong focusing - FODO lattice

F O D O

focusing with quadrupole magnet

flux return steel

Collider vs. fixed target

How to derive ?

Livingston Plot

Colliders

Particle Detectors

Detector system: an overview Particle interaction inside matter

– dE/dx

– Multiple Coulomb scattering

– photon interaction inside matter Charged particle detection : Neutral particle detection : Detector system for real experiment

ppxx ,

EExx ,

On experimental resolution

Detector System What do we want to measure in a detector system?

positionposition ; event topology, intermediate particle state momentummomentum ; need “tracking” energyenergy ; deposited in a localized place ; “calorimetry” massmass ; i.e. particle identification (PID) chargecharge ; from the curve orientation in the tracking chamber

Constructing (E, ) 4-vector for each particle: charged : tracking & PID => , m => E=(p2+m2) neutral : (E, , ) => is deduced by assuming m and origin

How a detector system works

For colliding beam experiments

For fixed-target experiments

Pt=0.3BR

Particle interaction inside matter

Energy loss of charged particle– Before a particle can be detected,

it must first undergo some sort of interaction in the material of a detector.

– EM interaction is the most important Energy loss as a function of travel distance

10.000

12.000

14.000

0.000 2.000 4.000 6.000 8.000 10.000 12.000

dxdE vs./ dxdE

dE/dx (brief derivation) Coulomb interaction b/w incident charged particle &

another charged particle in the detector material by transverse p

ΔpΔE m = target mass

Then, in the Lab. frame, (t=0 @ r=b)

dtEqp T

(Jackson)

[Ex] energy loss due to bound electrons vs. nuclei

dE/dx (brief derivation) For dE/dx,

count the number of interacting particles in the target!

dxdbbnN e 2

min 22

221 ln

ZZdxdE

bmin and bmax ?

NZnZn A

bmax for dE/dx consider interaction w/ free electron

only if motion orbitalcollision Tt

bmin for dE/dx E cannot exceed the max. allowed energy transfer

for a head-on collision

(let Z2 = 1)

421 ln

eZndxdE e

Bethe-Bloch formula dE/dx calculation with quantum correction

eZndxdE e

I = ionization potential

For small v,

For large v,“relativistic rise”

minimum ionization

Particle ID by dE/dx

Additional tools for charged particle ID

• Time of flightvxt /

p from trackingmass

• Cherenkov radiation

Mass (GeV)

Multiple Coulomb scattering

energy loss in Coulomb collision with nuclei is small

Note: Rutherford scattering formula

as / dd

ping-pong ball bowling ball

In any given layer of material,the net scattering is the result ofa large # of small-angle deviations (indep. of one another)

=> “ Multiple Scattering ”

a Gaussian distribution

for details, see “Intro. to Exp. P.P.” by R. Fernow, Sec.2-7

In a layer of thickness

deflection angle

Ec = “critical energy”

In practice, multiple scattering limits the precision of p

[ex] determine inside of a solenoidp

if no scattering, pc = BeR (Gaussian unit)

or p = BeR (SI unit)

Traversing a distance x, the angular deflection is

[ p(MeV/c), x(m), B(T) ]

10106.1)GeV(

)m/s(100.3)m()C(106.1)(

Tracking error due to multiple scattering

Compare with scat B

rmsscat

why 1/2 ? consider only projection onto the plane of the trajectory

Energy loss via radiation

Photon interactions in matter

Pair production

In the high energy limit (E>>2me),

the mean distance a photon will travel before pair producing is

Note: Why are XP and X0 similar?=> because bremsstrahlung and pair production are simply time and space rearrangements of the same process

0)7/9(~ XX P“conversion length”

Photon interactions in matter

Calorimetry Most calorimeters measure the ionization energy

deposited by all the charged particles in the "showers" produced as the particle is absorbed. – EM shower– Hadronic shower

The scale length– EM: radiation length, X0 (~2cm in Fe);

relevant for both bremsstrahlung & pair production

– Hadronic: mean hadronic interaction length, I (0.2m in Fe).

Scintillation detectors

PhotoMultiplier Tube

Belle EM Calorimeter

Tower structure projected to the vicinity of IP. 30 cm long (16.2 X0),

8736 CsI(Tl) crystals (6624 in barrel). 12< < 155 (lab frame)

Inner radius – 1250 mm

How to detect a charged track?

E ~ 10 kV/cmcharge amplification ~ 105

Ionization and detectionsensitive medium +V

Multi-Wire Proportional Chamber

Charpak, 1992 Nobel physics prize

x ~ O(wire spacing)

Drift Chamber

in each cell, the combination of (+) and () HV wiresprovides a relatively uniform E (= V/x)in a direction to normal incdidence

drift distance measurement

Drift Chamber

Field lines inside Drift Chamber

Precision Vertex Detection

Particles with c (charm) or b (bottom) quarks have lifetimes ~ 10-12 s

[Ex] In the Belle experiment, e+ (3.5GeV) and e (8 GeV) beams are collided and a pair of B mesons (mB = 5.28 GeV/c2) are produced.

(1) Find for each B meson?

(2) How far will it travel before it decays?

(3) spatial resolution to measure B meson decay position?

Precision Vertex Detection

To detect heavy-flavor particles (c or b), we need a position measurement of ~ 100 m resolution or better

Near the collision point, the density of particle tracks is very high

Surround the collision point with a high-resolution silicon sensors

Why silicon?

Silicon Vertex Detector To create a electron-hole pair in a semiconductor (e.g.

Si or Ge), only ~ 3 eV is needed large signals with very little energy deposition

Very thin wafer (~ 0.3 mm) is enough to achieve good signals

Conducting electrodes are implanted in separated stripes, orthogonally for p and n sides HV

Track finding & fitting

Detector System What do we want to measure in a detector system?

positionposition ; event topology, intermediate particle state momentummomentum ; need “tracking” energyenergy ; deposited in a localized place ; “calorimetry” massmass ; i.e. particle identification (PID) chargecharge ; from the curve orientation in the tracking chamber

Constructing (E, ) 4-vector for each particle: charged : tracking & PID => , m => E=(p2+m2) neutral : (E, , ) => is deduced by assuming m and origin

putting things together…

quiz time!

The Belle Detector

Particle Flavor Q/|e| e –1

leptons e 0

u c t +2/3 quarks

d s b –1/3

The fundamental fermions

1st 2nd 3rdgenerations

Heavy quarks ( c, b, t )

As of 1974…

GIM mechanism predicted the 4-th quark

which was named “charm”

because it did some good things…

November, 1974!

SLACB. Richter

MITS. Ting

ffee Xeep Be

J/ - the quantum #?

Interference with a photon

same quantum # as photon

interference

After heated discussion among theorists, it was concluded that J/ is a bound state.cc

The 4-th quark was discovered!

' : 1st radial excitation of J/

)1(/)2( SJS

One amazing feature about J/ was its long lifetime

(J/875 keV

(150 MeV

But the partial decay width to e+e was similar to other vector (JP = 1) mesons

Lifetime of J/

keV )32.077.6()(keV )37.026.5()/(

Something special about J/ hadronic decays?– OZI suppression!

OZI suppression in J/ decays

4 quarks & 4 leptons - are we happy?

Reines & Cowan (1950’s)

Leptons and their mystery

so similar?

But, then why do we not see ewhile seeing plenty of ? ee

The 3rd charged lepton

)missing( eeeWhat could this be?

The 3rd charged lepton

)missing( eee What could this be?

Martin Perl (1975)

So, in less than a year...

Upsilon resonances

anythingPt Cu, Be, pLederman (1977)

Upsilon resonanceswith better resolutions

BBS )4(

OZI suppression -> narrow resonances

What about the 6th quark?

Indirect evidences of “top” quark

Vtd ARGUS (1987)

(1) B mixing

(2) Kane-Peskin limit (1982)

Double pendulum as a mechanical analog of flavor-

mixing

-1.500

-1.000

-0.500

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0

Pendulum 1

Pendulum 2

The 6th quarkCDF & D0 (both at Fermilab) in 1995

How to see the top quark?

How to see “top” quarks?

CDF DetectorCDF Detector

“top” quark event : one lepton + 4 jets

“top” quark event : di-lepton + 2 jets

Mass of top quark

CDF, PRL (1998)

Each top candidate is fit to obtain Mrec

kinematical constraints tob-jets and light-q-jets

missing transverse energy for neutrino max. likelihood fit to obtain mass

from a sample of candidate events

1.53.174

(GeV) 3.58.49.175)(

world average

How about gauge bosons?

WWee 0Zee

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