What is Particle Physics --and why I like doing it (Horst Wahl, October 2001)

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What is Particle Physics --and why I like doing
it(Horst Wahl, October 2001)

• Particle physics
• Goals and issues -- Why do it?
• How to do a particle physics experiment
• Accelerator, detector
• DØ detector as example
• Overview of the Standard Model
• Symmetry, constituents, interactions
• Problems of standard model -- look beyond
• The Holy Grail of (present) particle physics
• Going beyond the SM new experiments
• Upgraded DØ detector
• Triggering
• The Silicon Track Trigger
• Webpages of interest
• http//www.hep.fsu.edu/wahl/Quarknet (has links
  to many particle physics sites)
• http//www.fnal.gov (Fermilab homepage)
• http//www.fnal.gov/pub/tour.html (Fermilab
  particle physics tour)
• http//ParticleAdventure.org/ (Lawrence
  Berkeley Lab.)
• http//www.cern.ch (CERN -- European Laboratory
  for Particle Physics)

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Goals of particle physics

• particle physics or high energy physics
• is looking for the smallest constituents of
  matter (the ultimate building blocks) and for
  the fundamental forces between them
• aim is to find description in terms of the
  smallest number of particles and forces
  (interactions)
• at given length scale, it is useful to describe
  matter in terms of specific set of constituents
  which can be treated as fundamental
• at shorter length scale, these fundamental
  constituents may turn out to consist of smaller
  parts (be composite).
• Smallest constituents
• in 19th century, atoms were considered smallest
  building blocks,
• early 20th century research electrons, protons,
  neutrons
• now evidence that nucleons have substructure -
  quarks
• going down the size ladder atoms -- nuclei --
  nucleons -- quarks preons, toohoos, voohoos,
  ???... ???

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Issues of High Energy Physics

• Basic questions
• Are there irreducible building blocks?
• Are there few or infinitely many?
• What are they?
• What are their properties?
• What is mass? charge? flavor?
• How do the building blocks interact?
• Are there 3 forces?
• gravity, electroweak, strong
• (or are there more?) or fewer??
• Why bother, why do we care?
• Curiosity
• Understanding constituents may help in
  understanding composites
• Implications for origin and destiny of Universe

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About Units

• Energy - electron-volt
• 1 electron-volt kinetic energy of an electron
  when moving through potential difference of 1
  Volt
• 1 eV 1.6 10-19 Joules 2.1 10-6 Ws
• 1 kWhr 3.6 106 Joules 2.25 1025 eV
• mass - eV/c2
• 1 eV/c2 1.78 10-36 kg
• electron mass 0.511 MeV/c2
• proton mass 938 MeV/c2 0.938 GeV/ c2
• professors mass (80 kg) ? 4.5 1037 eV/c2
• momentum - eV/c
• 1 eV/c 5.3 10-28 kg m/s
• momentum of baseball at 80 mi/hr ? 5.29 kgm/s ?
  9.9 1027 eV/c
• Most of the time, use units where c h 1
  (natural units)

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Luminosity and cross section

• Luminosity is a measure of the beam intensity
  (particles per
  area per second) ( L1031/(cm2s) )
• integrated luminosity is a measure of the
  amount of data collected (e.g. 100 pb-1)
• cross section s is measure of effective
  interaction area, proportional to the probability
  that a given process will occur.
• 1 barn 10-24 cm2
• 1 pb 10-12 b 10-36 cm2 10-40 m2
• interaction rate

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WHY CAN'T WE SEE ATOMS, .. QUARKS?

• seeing an object
• detecting light that has been emitted
  (scattered, reflected,..) from the object's
  surface
• light electromagnetic wave
• visible light those electromagnetic waves that
  our eyes can detect
• wavelength of e.m. wave (distance between two
  successive crests) determines color of light
• no sharp image if size of object is smaller than
  wavelength
• wavelength of visible light between 4?10-7 m
  (violet) and 7? 10-7 m (red)
• diameter of atoms 10-10 m, nuclei 10-14 m,
  proton 10-14 m,
  quark lt 10-19 m
• generalize meaning of seeing
• seeing is to detect effect due to the presence of
  an object, and the interpretation of these
  effects
• quantum theory ? particle waves, with
  wavelength ?1/(m v)
• use accelerated (charged) particles as probe, can
  tune wavelength by choosing mass m and
  changing velocity v
• this method is used in electron microscope, as
  well as in scattering experiments in nuclear
  and particle physics

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Particle physics experiments

• Particle physics experiments
• collide particles to
• produce new particles
• reveal their internal structure and laws of
  their interactions by observing regularities,
  measuring cross sections,...
• colliding particles need to have high energy
• to make objects of large mass
• to resolve structure at small distances
• to study structure of small objects
• need probe with short wavelength use particles
  with high momentum to get short wavelength
• remember de Broglie wavelength of a particle ?
  h/p
• in particle physics, mass-energy equivalence
  plays an important role in collisions, kinetic
  energy converted into mass energy
• relation between kinetic energy K, total energy E
  and momentum p E K mc2 ?(pc)2 (mc2)c2

___________
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How to do a particle physics experiment

• Outline of experiment
• get particles (e.g. protons, antiprotons,)
• accelerate them
• throw them against each other
• observe and record what happens
• analyze and interpret the data
• ingredients needed
• particle source
• accelerator and aiming device
• detector
• trigger (decide what to record)
• recording device
• many people to
• design, build, test, operate accelerator
• design, build, test, calibrate, operate, and
  understand detector
• analyze data
• lots of money to pay for all of this

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Collisions at the Tevatron

• p-antip Collisions ? qq(g) Interactions

Fermilab
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Fermi National Accelerator Laboratory(near
Batavia, Illinois)
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Old Fermilab accelerator complex
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Detectors

• use characteristic effects from interaction of
  particle with matter to detect, identify and/or
  measure properties of particle
  has transducer to translate direct
  effect into observable/recordable (e.g.
  electrical) signal
• example our eye is a photon detector
• seeing is performing a photon scattering
  experiment
• light source provides photons
• photons hit object of our interest -- absorbed,
  some reemitted (scattered, reflected)
• some of scattered/reflected photons make it into
  eye focused onto retina
• photons detected by sensors in retina
  (photoreceptors -- rods and cones)
• transduced into electrical signal (nerve pulse)
• amplified when needed
• transmitted to brain for processing and
  interpretation

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Typical particle physics detector system
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The DØ Collaboration
500 scientists and engineers 60 institutions 16
countries 110 Ph.D. dissertations 80 papers
?
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The old DØ detector
Muon System 1.9T magnetized Fe, Prop. drift
tubes 40,000 channels
Central Tracking Drift chambers, TRD
Calorimeter Uranium-liquid Argon 60,000 channels
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Typical DØ Event
MJJ 1.18 TeV Q2 2.2x105
ET,1 475 GeV, h1 -0.69, x10.66 ET,2 472
GeV, h2 0.69, x20.66
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Typical DØ Event
MJJ 1.18 TeV Q2 2.2x105
ET,1 475 GeV, h1 -0.69, x10.66 ET,2 472
GeV, h2 0.69, x20.66
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The Standard Model

• A theoretical model of interactions of elementary
  particles
• Symmetry
• SU(3) x SU(2) x U(1)
• Matter particles
• Quarks in six flavors
• up, down, charm,strange, top bottom
• leptons
• electron, muon, tau, neutrinos
• Force particles
• Gauge Bosons
• ? (electromagnetic force)
• W?, Z (weak, electromagnetic)
• g gluons (strong force)
• Higgs boson
• spontaneous symmetry breaking of SU(2)
• Mass

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The Standard Model

• Fundamental constituent particles
• leptons q 1, 0 e ? ? ?e ?? ??
• Fundamental forces (mediated by force
  particles)
• strong interaction between quarks, mediated by
  gluons (which themselves feel the force)
• leads to all sorts of interesting behavior, like
  the existence of hadrons (proton, neutron) and
  the failure to find free quarks
• Electroweak interaction between quarks and
  leptons, mediated by photons (electromagnetism)
  and W and Z bosons (weak force)
• Role of symmetry
• Symmetry (invariance under certain
  transformations) governs
  behavior of physical systems
• Invariance ? conservation laws (Noether)
• Invariance under local gauge transformations ?
  interactions (forces)
• SM has been thoroughly tested in many experiments
  -- embarrassingly good description of data

quarks q 2/3 . 1/3
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Inclusive Jets - DØ
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anomalous couplingsDØ and LEP Combined
-0.16 lt ?? lt 0.10 _at_95 CL
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W Boson Mass
mass of W
World average MW 80.394 ? 0.042 GeV
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W Boson Width
SM W ? ??, qq If additional non-SM particles
exist which are lighter than and couple to the W
boson ? additional contribution to the W boson
width
Indirect measurements from the ratio of W and
Z cross sections DØ ?(W) 2.107 ? 0.054
GeV CDF?(W) 2.179 ? 0.040 GeV
Upper limit on non-SM decays of W ??(W) ? 132
MeV
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Constraints on Higgs Mass
From combined analysis of all available data,
obtain constraints on Higgs mass Present SM Higgs
Mass limits (95 CL) 107.7 lt MH lt 188 (GeV)
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The SM works great ! Why change it ?

• SM, developed in the 1970s, has been thoroughly
  tested in many experiments -- embarrassingly
  good description of data
• Why are we not happy with it?
• has 18 arbitrary parameters(e.g. quark, lepton
  masses) ? Where do they come from ?
• does not include gravity
• E.M. symmetry breaking mechanism via Higgs Boson
  is put in by hand
• Is the Higgs really what we think it should be ?
• Higgs mass calculation within SM is not stable
  quadratic divergences
• SM at very high energies inconsistent (violates
  unitarity)
• Looking for the Theory of Everything (TOE)
  that contains SM as approximation many
  extensions proposed and considered
• GUTs, technicolor, SUSY, superstring theory,
• Need guidance from experiment
• Frantically looking for deviations from SM

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Electroweak Symmetry Breaking

• One of the big unanswered question in high
  energy physics
• the couplings of the photon and the W/Z to matter
  are the same (except for mixing angles) and all
  agree with the Standard Model
• but
• In the SM, this occurs because the W and Z
  interact with a new, fundamental scalar particle,
  the Higgs boson
• SM predicts relation between masses of W, top,
  Higgs

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Looking beyond the SM

• Strategies
• look harder -- do more precise tests of SM
• get a bigger hammer - more energy, and look for
  new phenomena not compatible with SM
• Tools needed for this
• Accelerator with higher energy
• to make massive particles predicted by some of
  the SM extensions
• to look closer into structure of proton and
  antiproton
• Higher beam intensity
• new phenomena are rare
• to improve precision, need lots of data
• better detectors
• cope with higher collision rates
• provide more end more precise information
• be more selective in what is recorded
• Fermilab upgrade program
• Accelerator energy from 1.8 to 2.0TeV, raise
  luminosity by factor gt 5
• upgraded detectors DØ and CDF

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• TeVatron collider at Fermilab
• Peak Luminosity 1032 cm-1s-1 (5X1032 cm-1s-1 )
• Energy in c.m.s. 2 TeV
• Integrated Luminosity 2fb-1 ( 830?fb-1 )

• Turn-on March 1, 2001
• First collisions April 3, 2001
• Bunch crossing time 396 ns (132ns)

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DØ upgrade detector
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The DØ detector in the collision hall (March 2001)
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The DØ detector in the collision hall (March 2001)
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DÆ Upgrade Tracking

• Silicon Tracker
• Four layer barrels (double/single sided)
• Interspersed double sided disks
• 793,000 channels
• Fiber Tracker
• Eight layers sci-fi ribbon doublets (z-u-v, or z)
• 74,000 830 mm fibers w/ VLPC readout

1.1
cryostat

• Preshower detectors
• Central
• Scintillator strips
• 6,000 channels
• Forward
• Scintillator strips
• 16,000 channels
• Solenoid
• 2T superconducting

1.7
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Silicon Tracker

1/2 of detector
50 cm
3
12 Disks F
7 barrels
8 DisksH
1/7 of the detector (large-z disks
not shown)
387k ch in 4-layer double sided Si barrel
(stereo)
405k ch in interspersed disks (double sided
stereo) and large-z disks
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Central Fiber Tracker

• 16.000 channels
• Read-out SVX-II chips
• Fast enough for L1
• 2.6 m scintillation fibers, VLPC readout 10m
  waveguides
• Mounted on 8 cylinders 20 lt r lt 50 cm
• 8 alternating axial and stereo doublets (2o
  pitch)

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Silicon Microstrip Tracker

• Provides very high resolution measurements of
  particle tracks near the beam pipe
• a) measurement of charged particle momenta
• b) measurement of secondary vertices for
  identification of b-jets from top, Higgs, and for
  b-physics
• Track reconstruction to h 3
• Track impact parameter trigger (STT)
• Point resolution of 10 mm
• Radiation hard to gt 1 Mrad
• Maximum silicon temperature lt 10o C

240 cm
6 barrel sections
12 Disks F
8 Disks H
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Tracking with the SMT
Readout
pqBR
Charged Particle
VB
p
50 mm

-
300 mm

-
n Si

-
n
Reverse-Biased Diode
Si Detector

• charge collected in sensors ? Points for
  Track Fit
• Precise Localization of Charge ? accurate
  particle trajectories
• SMT precision 10 mm

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Trigger

• Trigger device making decision on whether to
  record an event
• why not record all of them?
• we want to observe rare events
• for rare events to happen sufficiently often,
  need high beam intensities ? many collisions take
  place
• e.g. in Tevatron collider, proton and antiproton
  bunches will encounter each other every 132ns
• at high bunch intensities, every beam crossing
  gives rise to collision ?
  about 7 million collisions per second
• we can record about 20 to (maybe) 50 per second
• why not pick 50 events randomly?
• We would miss those rare events that we are
  really after
  e.g. top production ?
  1 in 1010 collisions Higgs
  production ? 1 in 1012 collisions
• ? would have to record 50 events/second for 634
  years to get one Higgs event!
• Storage needed for these events ? 3 ? 1011
  Gbytes
• Trigger has to decide fast which events not to
  record, without rejecting the goodies

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Our Enemy High Rates

• too much is happening, most of which we dont
  want to know about
• Collision Rate 7 MHz
• Data to Tape 20 to 50 Hz
• Trigger
• Try to reject uninteresting events as quickly
  as possible, without missing the interesting
  ones
• Strategy
• 3 Level System L1, L2, L3 with successively more
  refined information and more time for decision
  available

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DØ Trigger System
L3

• L3

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Run II Trigger Scheme

• Bandwidth Allocations
• L1 in 7MHz, out 5-10kHz time 4.2 ms
• L2 in 10kHz, out 1kHz time 100 ms
• L3 in 1kHz, out 20Hz time 100 ms/ 100
  CPUs
• Trigger configuration
• L1 Uses Calorimeter, Fiber tracker (CFT),
  Muon and Preshower objects trigger on
• Cal ET (em and jets),
• muon pT (use CFT),
• track pT,
• track-preshower stubs
• L2 preprocessors for detectors, global L2
  combines L1 objects into electrons, muons, jets,
  makes decision
• L2STT use of SMT information in trigger
• refine momentum measurement
• determine impact parameter

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Our Friend the b-Quark

• Many of the phenomena that we would like to study
  have b-quarks associated with them
• Tag Top Decays
• t?bW 100
• Tag Higgs (H?bb)
• ?(H?ff) ? mf2
• new Particles (e.g. SUSY) ? bs
• new Physics couples to mass
• CP Violation
• Matter / Antimatter Asymmetry
• Should be Large in B systems

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Silicon Track Trigger

• Idea use SMT information at L2, to improve
  background rejection
• Goals
• Sharpen PT Measurement
• Identify b-events
• B Event Properties
• Impact Parameter / Vertex Triggers

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Silicon Track Trigger

• STT Preprocessor, prepares information for
  decision by L2GLB
• Include SMT hits on CFT Track in L2 trigger

50 ms Time Budget
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STT concept and design goals

• Refit Tracks ? PT, ?o, b
• Use CFT A,H SMT 4(3) Layers
• Use only r-? information
• stereo strips are clustered
• Use only PTgt1.5 GeV, blt2 mm
• L1CTT efficiency
• 30o sectors in SMT independent
• system relies on this geometry
• loss in efficiency 2
• L1CTT roads ? search in SMT
• CFT geometry remapped in L1CTT
• use SMT hits closest to road center
• fixed road width 2 mm
• t t(select) t(fit) t(bus) 16 ?s
• budget 50 ?s
• t(bus) lt 5.8 ?s
• t(select) t(fit)
• t(select) ?N(hits in road)
• Queuing Simulation

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STT Functionally
L1CTT Tracks
FRC
broadcast Trig/Road data
SMT Data (2 HDI / fiber)
Trigger (SCL)
at input rate (no buffering)
1 / input
roadslt46 / 60o
clusters
coord transf
compare clst / rd
compare clst / rd
1 / road
clusters in roads
Averages Averages
ltN(trk)gt 2 / 30o
ltN(clst)gt 14 / 30o
ltclst/trkgt 3.7
STC
fit matrix LUT
TFC
coord transf
fit
fit
8 DSP/30o
L2CTT
fitted tracks
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STT Architecture
Numbers Numbers Numbers
Crates 6
FRC 1 / cr 60o
trig in 1 scl
road in 1 fiber?vtm
max rds 46
STC 9 / cr
smt in 4 fiber?vtm
HDI/fiber 2
TFC 2 / cr 30o
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STT history and status

• Project started in 1996 (first feasibility
  studies, assess physics merit)
• 1997 to 1999 proposals to DØ, Fermilab PAC, NSF
• Summer 1999 consortium of 4 universities
  (Boston U., ColumbiaU., FSU, Stony Brook obtains
  funding (1.8M from NSF and DOE)
• Dec. 1999 Reginald Perry joins
• He and his students (Shweta Lolage, Vindi Lalam,
  Sean Roper,.) developed the VHDL code for the
  cluster finder and hitfilter part, probably the
  most challenging part of the project
• Sept. Nov 2001 system tests with first
  prototypes, first at BU, now at Fermilab in the
  DØ environment
• Second (final?) prototype tests Nov. Dec.
• Production Jan March 2002
• March 2002 Installation in DØ

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A WH event in the DØ detector
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Mtop vs MW in Run 2

• Run 2 scenario
• DM t 3 GeV
• DM W 40 MeV

• Within SM, Mtop and MW constrain MHiggs to an
  accuracy of 80
• The relation between these 3 masses provides a
  good consistency check of the SM

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Summary

• DØ has a new detector which promises to be up to
  the task of incisive testing of the SM, and
  capable of discovering new physics phenomena
• New trigger, in particular the STT, greatly
  enhances potential
• We are looking forward to finally seeing
  something which clearly disagrees with the SM!
• Many thanks to Reginald Perry and ECE Dept.
• Hope for continued collaboration