Detection techniques for high energy physics experiments

gianluigi cibinetto

Issue 1 Lecture 1

Outline

•  What’s behind a physics measurement: a miscellanea of examples, problems and solutions.

•  Much more details will be addressed during the following lectures.

high energy physics lab detection techniques 2

Making the puzzle

high energy physics lab detection techniques 3

Detector

Simulation

Data analysis

Accelerator

Let’s start with an example:���the discovery of CP violation

high energy physics lab detection techniques 4

The Fitch and Cronin experiment

•  Un upper limit of an eventual CP violation had been established before the experiment to 1/300 for the fraction of KL that would decay into 2π instead of 3π.

•  Fitch and Cronin wanted to ameliorate this upper limit by analyzing more events of KL decay.

high energy physics lab detection techniques 5

The experiment

The experiment was done at Brookhaven AGS. It was a fixed target experiment, in which the beam was made of protons of 30 GeV energy and the target was made of Be.

high energy physics lab 6 detection techniques

The detector itself is made up of two arms of spectrometer, each one made up of 2 spark chambers, with a magnetic field of 178 kG in between them. They are followed by a Cerenkov counter and a scintillation chamber as a trigger.

Analysis strategy

•  Let’s consider the vector sum of 2 of the decay products and θ the angle between this and the initial direction of the KL beam.

•  There are 2 cases that are essential here in discriminating between the 2π decay and the 3π decay of the KL .

–  In a 2 body decay, this θ is rigorously null (0).

–  In a 3 body decay, it can generally take a wide range of values.

high energy physics lab detection techniques 7

Calibration

•  The apparatus had to be calibrated before being used.

•  As they wanted to see KL → π+π− , as KL and KS have very similar masses, they could calibrate the detector using the regular KS→ π+π− .

•  But the beam contains usually only KL and not KS. That’s why they make the beam pass through tungsten.

high energy physics lab detection techniques 8

Analysis and results They saw that the mass distribution peaks around 500 MeV, which is coherent with the already mass of the KS of 498 MeV. If CP is conserved, than the masses of KS and KL should be exactly equal, if CP is violated, their masses should be different, but the less it is violated, the smaller the mass difference. The already known upper limit of 1/300 for the branching fraction of the CP violation was also the upper limit of the mass difference. So, their relative mass difference would be less that 1/300 (as order of magnitude). They kept the events with masses between 490 and 510 MeV as the events that have the KL decay. But there are 2 ways that KL can decay: the CP conserving 3π mode and the CP violating 2π mode. The only way to separate between the 2 of them is using the angle θ. θ would take values over a large range for the 3π mode but only values of zero (0) and cosine 1 for the 2π mode. They plotted again the distribution of events as a function of cos θ for cosθ > 0.9995 and they did that for 3 regions of masses, one with masses smaller than the expected value for the KL , another one centered around this value and another one after this value. In the figure we can see these plots, as published in their paper.

high energy physics lab detection techniques 9

Possible sources of errors

high energy physics lab detection techniques 10

Possible sources of errors

•  KS regeneration in He bag

•  Particle ID

•  Other KL decay modes

high energy physics lab detection techniques 11

Another example���the charmonium discovery

high energy physics lab detection techniques 12

Charmonium discovery (I)

•  The BNL group investigated –  p+Be→e+e- + X

–  interpretation: new bound state: cc→ e+e-

high energy physics lab detection techniques 13

Charmonium discovery (II)

high energy physics lab detection techniques 14

•  At SLAC, cross-sections for –  e+e- → hadrons

–  e+e- →e+e-

–  e+e- →μ+μ-

Exercise

•  Imagine an experiment to study narrow hadronic states that might be produced in ppbar annihilation. Antiprotons stored inside a ring would collide with a gas jet of hydrogen injected into the ring perpendicular to the beam. By adjusting the momentum of the beam in the storage ring the dependence of the ppbar cross section on the center-of-mass energy can be studied. A resonance would show up as a peak in the cross section to some final state.

•  Assume that there exists a hadron that can be produced in this channel with a mass of 3 GeV and a total width of 100 keV:

(a) What beam momentum should be used to produce this state?

(b) One of the motivations for this experiment is to search for charmonium states that cannot be seen directly as resonance in e+e− annihilation. Which spin-parity states of charmonium would you expect to be visible as resonance in this experiment but not in e+e− annihilation?

high energy physics lab detection techniques 15

More on charmonium

•  E835 was a small experiment for precise charmonium measurements.

•  No magnetic field, apparatus optimized for electrons and photon detection.

high energy physics lab detection techniques 16

Analysis strategy

•  Use the very narrow anti-proton beam to perform resonances scan.

high energy physics lab detection techniques 17

Question

•  How would one measure the mean lifetime of the following particles?

–  238U : τ = 4.5×109 years,

– muon: τ = 2.19 × 10−6 sec,

– B meson: τ = 1.64 × 10−12 sec

– ρ0 meson : τ ≈ 10−22 sec.

high energy physics lab detection techniques 18

CP violation in the B system •  Method

–  Boost: Δt measured via space length measurement Δz between Btag and Brec –  Coherent evolution: at ttag the flavors of Brec and Btag are opposite –  flavor of the Btag determined by its decay product (charge of leptons, K, π) –  flavor of the Brec determined from the flavor of Btag (and Δt)

high energy physics lab detection techniques 19

PEP II and BaBar

high energy physics lab detection techniques 20

Features of the BaBar experiment

high energy physics lab detection techniques 21

" e+e- CM energy ~10.58 GeV - Y(4S)

"   boost βγ ~ 0.56

"   Peak luminosity 1.12x1034cm-2sec-1

Background evaluation -  Off peak data -  Monte Carlo simulation

Background rejection -  Event shape (e.g. B vs continuum) -  Particle identification -  Event reconstruction -  Kinematics constraints

Event reconstruction

high energy physics lab detection techniques 22

The LHC

high energy physics lab detection techniques 23

The LHCb experiment

high energy physics lab detection techniques 24

pp collision Point

Vertex Locator VELO

Tracking System

Muon System RICH Detectors

Calorimeters

~ 1 cm

B

Specific features of LHCb

•  Particle detection in the forward region (down to the beam-pipe) •  Excellent resolution for localization of B decay vertices (Vertex

Locator) •  Excellent particle identification to distinguish p, k, π (RICH

detectors)

high energy physics lab detection techniques 25

CMS @ CERN

high energy physics lab detection techniques 26

Higgs physics

high energy physics lab detection techniques 27

Higgs production

high energy physics lab detection techniques 28

Particles interact differently with matter

high energy physics lab detection techniques 29

Different detectors for different particles

high energy physics lab detection techniques 30

Different detectors for different particles

high energy physics lab detection techniques 31

Some kind-of-important steps to plan your HEP experiment

•  Think about the measurement you want to do •  Plan the experiment •  Simulate the experiment •  Detector research and development •  Construction and commissioning •  Data taking – Data acquisition – Calibrations –  Event reconstruction (tracking, particle ID, etc…)

•  Data analysis

high energy physics lab detection techniques 32

What’s next

•  Brief reminder of the passage of particles through matter

•  Tracking

•  Particle identification techniques

•  Calorimetry

•  Advanced detection techniques

high energy physics lab detection techniques 33

Bibliography

•  CP violation: Physical Review Letters, Volume 13, Number 4, 27 July 1964

•  Charmonium discovery: PhysRevLett.33.1404 and PhysRevLett.33.1406

•  E835 detector: Nucl. Instrum. Meth. A 519, 558 (2004).

•  BaBar detector arXiv:hep-ex/0105044v1

•  LHCb detector arXiv:0910.1740v1 [hep-ex]

•  CMS detector http://doc.cern.ch//archive/electronic/cern/preprints/lhcc/public/lhcc-2006-001.pdf

•  Other information:

–  Introduction to High Energy Physics, Donald H. Perkins (Addison Wesley)

–  Review of Particle Physics Journal of Physics G, Vol 37, No7A, 2010

–  PDG live @ http://pdglive.lbl.gov/listings1.brl?exp=Y

high energy physics lab detection techniques 34