Transcript Electroweak Physics Lecture 4

Electroweak Physics
Lecture 4
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Physics Menu for Today
• Top quark and
W boson
properties at
the Tevatron
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Hadron-Hadron Collisions
Photon, W, Z, t, H etc.
parton
distribution
Hard scattering
Underlying
event
FSR
parton
distribution
ISR
fragmentation
Jet
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Physics at Hadron Colliders
• Since hadron colliders collide composite objects – the
extraction of the physics is often ''messy'' and not
straight-forward.
• Need to understand:
– underlying event, multiple interactions
– proliferation of QCD radiation
– high event rates
• Places a premium on:
– real-time triggering (selection of interesting events)
– accurate detectors with some redundancy
– understanding QCD
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Life at a Hadron Collider
•
What happens when two hadrons collide:
1. ~ 25% ELASTIC collisions – hadrons change direction/momenta but there is
no energy loss : dull !
2. ~ 75% INELASTIC collisions – one or both of the hadrons have a change in
energy and direction : rate ~ 1/Q4 : Q is energy transfer – mostly dull !
•
In a collider we have bunches of hadrons circulating the accelerator
–
•
each bunch contains ~ 1011 protons (anti-protons ~109)
We can have more than one collision as the bunches pass through each
other at the interaction region : ''Multiple Interaction''
30 mm
: BUNCH
BUNCH : 1011 P
15cm
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A typical (interesting) event
For EWK physics: Try to extract the information about the subprocess
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Hard Subprocesses
• The hadrons (protons and anti-protons) are made of quarks
and gluons
• The momentum distribution of the quarks and gluons as a
function of Feynman x:
x
p parton
phadron
• Effective energy of the collision: Ecmx1x2
– Not known on an event by event basis
• To make predictions (to compare with the Lagrangian) we
need to know about the x distribution of the quarks and
gluons. Parton Density Functions
– This is known (to some precision) from lepton-nucleon experiments
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Hard Subprocesses
• Three possible hard scattering processes:
– qq: quark-quark, quark-antiquark, antiquark-antiquark
– qg: quark-gluon, antiquark-gluon
– gg: gluon-gluon
• at the Tevatron (2 TeV) quark-antiquark is dominant
• at the LHC (14 TeV) gluon-gluon is dominant … the LHC is
really a gluon-gluon collider !
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Knowledge of PDFs is Vital!
PDFs = Particle Density Functions
How many quarks and gluons are in proton and how
much of each
To relate what we want to know to what we want to
measure define ''luminosity functions'' to determine
what the important partonic sub-processes will be.
- this is where HERA measurements are vital
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Remember the TeVATRON!
• At Fermilab
• Proton anti-proton collider
– Run 1 from 1987 to 1995: √s=1.8 TeV
– Run 2 from 2000 to 2009: √s=1.96 TeV
• Two experiments: CDF and DØ
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Run II Luminosity
• Current integrated luminosity: 1500 pb−1
• Current Analysis: up to 400 pb−1
• Analysis with 800 pb−1 underway
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When protons &
(anti-)protons collide
 ( pp  anything)
• Physics at proton collider is
like…
• Drinking from a firehose
– At TeVATRON: 1 collision
every 396ns
– 1 to 2 interactions per
collision
• Panning for gold
– W, Z, top are rare events!
– Need high luminosity
– Use high momentum muons
and electrons to select
interesting events
 ( pp  W  X )
 ( pp  tt )
Collision Energy
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Triggering at the Tevatron
Vital at hadron collider eg:
- b quark was discovered with one b event per 1010 collisions
- top quark was discovered with one top per 1012 collisions!
by comparison, this is trivial at a lepton collider
7.5 MHz
L1 : hardware
5 kHz
L2 : firmware
375 Hz
L3 : software
Needle in a haystack moving at 186,000 miles per second ...
CHALLENGES
- ensuring high trigger efficiency & retaining purity
- knowing what the trigger efficiency is
(use pass-through triggers and rely on pre-scaled
triggers with lower thresholds)
Rejection factor of 1:20,000 after level-2
75 Hz
Tape Robot
disks...
~ few Tb / day
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Electroweak Lagrangian
Important for MW
Important for mtop
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Electroweak Lagrangian
Higgs couples to all
fermions in proportion
to their mass
Important for mtop
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Electroweak Lagrangian
Higgs couples to W and Z
WWH vertex
ZZH vertex
Important for MW
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Electroweak Lagrangian
Higgs quartic coupling to
W and Z
WWHH vertex
ZZHH vertex
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Putting it all Together
• W mass predicated in EWK Lagrangian
– Corrections from interactions with Higgs boson and top quark
• Top corrections important for many processes
– including those from LEP
– Need accurate measurement of top quark mass to make comparisons
between theory and experiment
• Top is by far the heaviest fundamental particle known (~175 GeV/c²)
– Same scale as W & Z: it may offer insights into the nature of
electroweak symmetry breaking (Higgs mechanism)
– doesn’t have time to hadronise
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Theory <−> Experiment
• Now we know what physics to expect, let’s make some
measurements
• For that we need…
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Detector Coordinates
Detector Coordinates
 psudeorapidity:
 
   ln  tan  2 


 axial angle: 
Polar Angle: θ
φ
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CDF in Real Life
Central+Plug
Calorimetery
η  3.6
Muon Chambers
η  1.5
Central tracking
η  1.0
Silicon tracking
η  2.0
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Transverse Quantities
• Colliding partons have small momentum transverse to beam
• We detect all interactions transverse to the beam
p
x
p
0
part
y
0
part
• Any “missing momentum” in
x,y plane is attributed to the
neutrino
– Or other non-interacting
particles eg neutralinos
–
Transverse momentum:
pT 
p p
2
x
2
y
Missing ET direction
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An easy example: reconstructing Z→ℓ+ℓ−
• Select events with
– 2 leptons,
– Opposite charge
– momentum transverse to beam,
pT>20 GeV/c
• 66 < M(ℓ+ℓ−)/GeVc−2 < 116
• pT(μ+) = 54.8 GeV/c
• pT(μ−) = 39.2 GeV/c
• M(μ+μ−) = 93.4 GeV/c²
M(
 
)  E (  )  E (  )  px (  )  px (  )  p y (  )  p y (  )  pz (  )  pz (  )
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W Mass
• Current best single measurement of MW is ±58MeV
• World Average: (80.425±0.038) GeV/c²
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Extracting W Mass
• Value of MW is sensitive to PT of lepton and Missing-ET
• The combination of both quantities in the Transverse Mass (MT) has
best sensitivity to MW
• Generate lots of MC samples with different MW
• Fit each one to date to find test MW value
Transverse Mass of muon and neutrino.
Invariant mass only using components of
the momentum transverse to the beam
M T ( m )  E ( m )  E ( )  px ( m )  px ( )  p y ( m )  p y ( )
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Largest W Mass Systematics
• How well do we understand energy scale of calorimeter?
– Use Z→e+e− to calibrate detector
• How well do we understand hadronic recoil
– Effects resolution of missing ET
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Current & Predicted W Mass Measurements
No Run II measurement yet!
CDF Expected error for
200pb−1 is ±76 MeV/c²
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Top Quark Production
• Main mode for top quark production at Tevatron is through two quarks
fusing to form a gluon, which decays into top-antitop
• Gluon-gluon fusion too
– All QCD production, no EWK involved
• Cross section decreases as mtop increases
• Predicted cross section for mtop=175 GeV/c²: (6.23-6.82) nb
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Top Quark Decays
• CKM matrix: top decays 99% of the time into b-quark and
W.
• Two tops: two b-quark jets + 2W
– Two lepton channel
Easy to identify
Small cross section
MET from 2 neutrinos
– Lepton+jets
 30% of cross section
 Only 1 neutrino
– All jet channel
 v. hard to reconstructed masses
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Top Event in the Detector
• 2 jets from W decay
• 2 b-jets
• ℓ± ν ℓ
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Top Event Reconstruction
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Tevatron Summary
• Hadron Colliders are great for discovery of new particles
• Need to use a trigger to select useful events
• Can also be used for precision physics:
– Need to understand PDFs of colliding hadrons
• CDF and DØ have extensive
physics programme
• Aim measure:
– mtop ±2.5 GeV/c2
– MW to ±40 MeV/c2
– Probably can do better
– Other EWK tests possible too!
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Backups:
Other Tevatron EWK Measurements
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