What have we seen so far ?

Position and momentum measurement - little or no loss of energyTime measurement - little or no loss of energyEnergy measurement - loss of energy

Today ‘s goals :

Identification• general principles• muon/electron/pion/jets • b or c quark• Cherenkov radiation • Transition radiation

Literature:-paper linked in ISIS page by the slides -D.Green pg 55-87

What do we need to measure the mass, henceidentify a particle uniquely ?

E p

Ideally: measure E and p very precisely , then calculate m(for particles decaying, sum up all E_i and p_i)

So we do not need any particle identification

How can we identify a particle ?

Energy(1)

Energy(2)

Momentum(2)

Momentum(1)

Example : ALEPH experiment at LEP

The goal of the ALEPH experiment is to proof thate+e- → Z0 → ff exists, and ff is the decay of a particle, Z0 boson

Ok, I clearly see 2 objects, so I should do (E(1)+E(2))^2 - (p(1)+p(2)) ^2 = M^2

But :

E(i) << p(i) , so ???

I need to understand a bit more what I see

So if I doE(1)=sqrt( p(1)^2 + m_muon ^2) and I then reconstruct M, what doI get ?

if 1, 2 are muons then I know they are M.I.P.and E in the calorimeter is not full E of muon , and I should trusttracker for full P.This interpretation is also confirmed by two muonsleaving signal in the outer chambers

E(1)<<p(1), E(2)<<p(2) !

What should I make of this ?

Resonance shape ! It’s a Z0 we observed !

They also help with identification, not all detectors have an ad-hoc particle identificationdetector or rely only on that one for identification

ALEPH didn’t have one ad-hoc detector for identification only

• Energy(1)/momentum(1) < 1.

• Energy(1)/momentum(1) ~ 1.

Energy(1)

Momentum(1)

Energy(1)

Momentum(1)

• Stops in electromagnetic calorimeter

• Does not stop entirely in electromagnetic (EM) calorimeter• There is signal around in EM and has no track attached

electron

pion

photons

τ -> e νντ -> π π0 ν

(what do we expect the total energy to be ?)

electron

positron

20 50 …

Calorimeter helps:

E.g. e/π-Lateral Shape-E/p

Tracking helps:

Identification is not simply done “by eye”, but combines severalinformations from several parts of the detector

• We have seen the case of the muon (E not fully collected)• electron, pion (again, we use ALEPH example) ?

σ(pT)/pT = 0.6 10-3 pT

calorimeter Aleph tracking

Try eg. p=p_T= 45 GeV

Using measured E and p is actually worse than using best of the twoand identification (-> mass well known) -> energy flow

Identification methods :

Use the fact that dE/dx depends on velocity, hence particle mass (fora given momentum)

Use the fact that in 20 cm of iron the probability for a electron to stop is much higher than for a pion (brehmstrahlung goes like 1/mass 2)so energy collected and size of shower is quite different

Exercise :

Try to identify the following events from ALEPH detector

Remember: physics process happening is

e+ e- → Z0 / gamma → f f with any f

3 minutes for each figure, discuss 2 and 2 andwrite down your identification

Discussion over evet displays.

My identification is :1) electron positron2) tau tau3) muon muon4) q q5) tau tau6) Mu mu gamma

For Alpeh performance, see paper linked from ISIS page for this week’s lectures

How to find a secondary vertex (http://www.phys.ufl.edu/~avery/fitting.html)

B and c flavour tagging: identifying long lived quarks

2mm

SLD detector

ee→Z0→bb

SLD : e+ and e-linear collider, c.m.s. 91 GeVZ boson production, similar program to LEP experiments

Best vertex resolution so far achieved -> best tagging of b and c quarkoriginated jets.

First, construct fi(r) = probability tube in 3-D for the i-th track trajectory

In ee collisions at 91 GeV , b and c quark from Z0 live relatively long (b ~1 mm)

Track Functions Vertex Functiontracks tracks

IPB

3-D Spatially resolved clusters of V(r) maxima form candidate vertices.

No combinatorics, you just see the right pattern

Third, tracks are attached to these resolved regions to form a set oftopological vertices

Need carefull tuning ofthese T and L/D cutsbefore using them for agiven vertex detectordesign

All non-primary vertices foundmust be n-prong with n ≥ 2

BUT

For IP_B_D decay chain canhave 1-prong B or D decay

Can consider n-prong ZVTOPvertex as a seed vertex

A track not directlyassociated with the Primary

or Seed vertex, but withT<Tmax and L/D>L/Dmin, is

likely to come from B decaychain

Finally variables coming out for tagging are

Apply a kinematiccorrection to MVTX to

partially recover effectof missing neutral

particles:

PTmiss

MPT

flavour tagging informations available are :

Vertex MassVertex MomentumDecay LengthDecay Length significanceN tracks associated to sec. Vtxs …..

Finally, Vertex Mass reconstructed

SLD Ghost track Algorithm

Angle between true B flight and jet axisAngle between true B flight and ghost track

Will improve these mostly

D

B

• • • •

In e+e- →Z0/γ→qq events, at 91 GeV c.m.s. energy (simulated data)

SLD-c

SLD-b

Efficiency of determing flavour “b” for a jet

Efficiency of determing flavour “c” for a jet

Results from SLD CCD-based vertex detector (stars) and from a new designfor a CCD-based vertex detector with parallel column readout

50 µs (8 msNLC)

216 msreadout time (1 layer)

0.96 (3 hits)0.90 (2hits)

cos(θ) max0.10.4layer thickness (%Xo)1528inner layer radius(mm)53n. of layers799307n. of pixels (106)27.512.8CCD active area (cm2)12096CCDsFuture LCSLDDetector