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Electroweak PhysicsLecture 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

fragmentation

partondistribution

partondistribution

Jet

Underlyingevent

Photon, W, Z, t, H etc.

ISRFSR

Hard scattering

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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 m

15cm

BUNCH : 1011 P: BUNCH

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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:

• 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

parton

hadron

px

p

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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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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

Knowledge of PDFs is Vital!PDFs = Particle Density Functions

How many quarks and gluons are in proton and how much of each

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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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• Current integrated luminosity: 1500 pb−1

• Current Analysis: up to 400 pb−1

• Analysis with 800 pb−1 underway

Run II Luminosity

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When protons & (anti-)protons

collide• 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

Collision Energy

( anything)pp

( )pp W X

( )pp tt

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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

Needle in a haystack moving at 186,000 miles per second ...

75 HzTape Robot ~ few Tb / daydisks...

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

L1 : hardware

5 kHz

375 Hz

L2 : firmware

L3 : software

7.5 MHz

Triggering at the Tevatron

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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

Polar Angle: θ

φ

:

:

ln tan 2

Detector Coordinates

psudeorapidity

axial angle

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CDF in Real Life

Central trackingη 1.0

Muon Chambersη 1.5

Central+Plug Calorimetery

η 3.6

Silicon trackingη 2.0

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Transverse Quantities• Colliding partons have small momentum transverse to beam• We detect all interactions transverse to the beam

Missing ET direction

0 0x ypart part

p p • Any “missing momentum”

in x,y plane is attributed to the neutrino– Or other non-interacting

particles eg neutralinos

– Transverse momentum:

2 2T x yp p p

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An easy example: reconstructing Z→ℓ+ℓ−

• pT(μ+) = 54.8 GeV/c

• pT(μ−) = 39.2 GeV/c

• M(μ+μ−) = 93.4 GeV/c²

• Select events with– 2 leptons, – Opposite charge– momentum transverse to

beam, pT>20 GeV/c

• 66 < M(ℓ+ℓ−)/GeVc−2 < 116

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )x x y y z zM E E p p p p p p

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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

( ) ( ) ( ) ( ) ( ) ( ) ( )T x x y yM E E p p p p

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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 identifySmall cross sectionMET 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±νℓ

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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