delmastro esipap2014 1marco delmastro! experimental particle physics! 3! quantity! hep units! si...
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Marco Delmastro Experimental Particle Physics 1
particle physics Experimental
1.
units, quantities, kinematics, measurements
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Order of magnitudes
Marco Delmastro Experimental Particle Physics 2
with p = transferred momentum
�r ⇠ 1
sin ✓
Optical microscope resolution
with θ = angular aperture of the light beam
De Broglie wave length
�r ⇠ h
p
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HEP, SI and “natural” units
Marco Delmastro Experimental Particle Physics 3
Quantity HEP units SI units
length 1 fm 10-15 m
charge e 1.602⋅10-19 C
energy 1 GeV 1.602 x 10-10 J
mass 1 GeV/c2 1.78 x 10-27 kg
ħ = h/2 6.588 x 10-25 GeV s 1.055 x 10-34 Js
c 2.988 x 1023 fm/s 2.988 x 108 m/s
ħc 197 MeV fm …
“natural” units (ħ = c = 1)
mass 1 GeV
length 1 GeV-1 = 0.1973 fm
time 1 GeV-1 = 6.59 x 10-25 s
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How much energy to probe these distances?
Marco Delmastro Experimental Particle Physics 4
What? Dimension [m]
p [GeV/c]
Atom 10-10 0.00001
Nucleus 10-14 0.
Nucleon 10-15 1
Quark 10-18 100
� =h
p=
2⇡~cpc
=2⇡ ⇥ 197MeV fm
pc
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What do we want to measure?
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H!Higgs boson!
2012: CERN!
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Baryons and Mesons
Marco Delmastro Experimental Particle Physics 6
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Measuring particles • Particles are characterized by
! Mass [Unit: eV/c2 or eV]
! Charge [Unit: e]
! Energy [Unit: eV] ! Momentum [Unit: eV/c or eV]
! (+ spin, lifetime, …)
• … and move at relativistic speed
Marco Delmastro Experimental Particle Physics 7
� =1p
1� �2
length contraption
time dilatation
Particle identification via measurement of: e.g. (E, p, Q) or (p, β, Q) (p, m, Q) ...
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Relativistic kinematics in a nutshell
Marco Delmastro Experimental Particle Physics 8
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Center of mass energy • In the center of mass frame the total momentum is 0 • In laboratory frame center of mass energy can be computed as:
Marco Delmastro Experimental Particle Physics 9
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2-bodies decay
Marco Delmastro Experimental Particle Physics 10
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Invariant mass
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3-bodies decay
Marco Delmastro Experimental Particle Physics 12
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3-bodies decay
Marco Delmastro Experimental Particle Physics 13
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3-bodies decay: pion decays
Marco Delmastro Experimental Particle Physics 14
in most cases muon decays at rest
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3-bodies decay: Dalitz plot
Marco Delmastro Experimental Particle Physics 15
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Multi-bodies decay
Marco Delmastro Experimental Particle Physics 16
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Reaction threshold
Marco Delmastro Experimental Particle Physics 17
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Fixed target vs. collider
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How much energy should a fixed target experiment have to equal the center of mass energy of two colliding beam?
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Collider experiment coordinates
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y
z
x
r θ Φ
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Rapidity
• Particle physicists prefer to use modified rapidity along beam axis
Marco Delmastro Experimental Particle Physics 20
Lorentz factor Hyperbolic cosine of “rapidity”
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Pseudorapidity
Marco Delmastro Experimental Particle Physics 21
if E >> m
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Transverse variables • At hadron colliders, a significant and unknown fraction of the beam
energy in each event escapes down the beam pipe.
• Net momentum can only be constrained in the plane transverse to the beam z-axis!
Marco Delmastro Experimental Particle Physics 22
XpT (i) = 0
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Missing transverse energy and tranaverse mass • If invisible particle are created, only their transverse momentum can be
constrained: missing transverse energy
• If a heavy particle is produced and decays in two particles one of which is invisible, the mass of the parent particle can constrained with the transverse mass quantity
Marco Delmastro Experimental Particle Physics 23
if
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W " e ν discovery
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Marco Delmastro Experimental Particle Physics 25
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W " e ν
) (GeV)i(eTm60 70 80 90 100
even
ts /
0.5
GeV
0
5000
10000
) MeVstat 19± = (80408 WM
/dof = 52 / 482r
-1 2.2 fb5 L dt 0CDF II preliminary
Marco Delmastro Experimental Particle Physics 26
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Interaction cross section
Marco Delmastro Experimental Particle Physics 27
Flux [L-2 t-1]
Reactions per unit of time
[t-1]
[L-2 t-1] [L-1] [L] [?]
Reaction rate per target particle
[t-1]
area obscured by target particle
Cross section per target particle
[L2] = reaction rate per unit of flux
1b = 10-28 m2 (roughly the area of a nucleus with A = 100)
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Fermi Golden Rule
Marco Delmastro Experimental Particle Physics 28
From non-relativistic perturbation theory…
[t-1] [E-1] [E]
matrix element energy density of final states transition probability
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Cross section: magnitude and units
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Proton-proton scattering cross-section
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Cross-sections at LHC
Marco Delmastro Experimental Particle Physics 31
108 events/s
10-2 events/s ~ 10 events/min
~1010
[mH ~ 120 GeV]
0.2% H " γγ 1.5% H " ZZ