physics at the lhc experiments - desy
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Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 1
Physics at the LHC experimentsPhysics at the LHC experiments
Summer School Lectures Summer School Lectures DESY Zeuthen, August 25DESY Zeuthen, August 25thth-26-26thth
20082008
Marcello BarisonziMarcello BarisonziSlides by Christiane RislerSlides by Christiane Risler
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 2
Physics at the LHC experiments
Outline of the lecture
1. Introduction to hadron colliders and pp physics Overview of collider physics, LHC 2. standard model physics at LHC (and Tevatron) QCD Jets, top and W/Z physics and precision measurements 3. searches for the Higgs boson and new physics beyond the standard model 4. ATLAS and CMS how detectors work
Physics at the LHC experimentsPhysics at the LHC experiments
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 3
Physics at the LHC experiments
Outline of part1● 1. The standard model of particle physics● 2. Hadron Colliders : LHC● 3. some More details on pp physics and what to expect at LHC● 4. LHC experiments ATLAS and CMS
Physics at the LHC experimentsPhysics at the LHC experiments
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1. The standard model of particle physics: short introduction, its success and its limitations
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The Standard Model of Particle Physics
Building blocks of matter
quarks and leptonsin three generations
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The Standard Model of Particle Physics
building blocks of matter
quarks and leptonsin three generations
and forces
electromagnetic, weak strong forcemediated by exchange of gauge bosons
photon m=0gluon g m=0 W,Z-boson m
W=80.4, m
Z= 91 GeV
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Origin of mass in the standard model so far particles massless
● electron and top massm
e=0.5 MeV m
t=171.2 GeV
● gauge Boson masses,g m=0 W,Z m
W=80.4, m
Z= 91 GeV
Higgs mechanism: (Peter Higgs, 1964) masses of W and Z boson quark and lepton masses via coupling to the Higgs boson
mass of Higgs boson = free parameter constraints 114.4 GeV (exp.) < m
H < ~1 TeV (theory) Does the Higgs
particle exist?
The Standard Model of Particle Physics
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Success of the Standard Model
electro weak theory Glashow, Salam, Weinberg , 1974 SU(2) x U(1) Higgs mechanism prediction of neutral weak currents and massive weak gauge bosons
experimental confirmation: Gargamelle and UA1, UA2
charm quark, top (heavier and found much later...) theory of strong interactions QCD SU(3)
C
also formulated in 1974 describes data over large energy range
el.weak precision measurements
e+e- colliders (LEP, SLC), Tevatron (pp), HERA(ep),...energy range up to ~100 GeV with high precisionSM consistent with all experimental data!However, ...
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 9
The Standard Model: Open Questions
● Origin of mass in the standard model ? does the Higgs boson exist, what is it mass ?
● Unification of forces ?
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Unification of Forces ?
● unification of electric and magnetic forces : Maxwell
● standard model unifies electro-magnetic and weak forces
can electroweak and strong force (and even gravity) be unifiedat some higher scale = grand unification scale GUT ?
All forces described by one unified theory with SM as low energyapproximationmore symmetry (broken), more fundamental theory...
possible in supersymmetric theories:
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 11
The Standard Model: Open Questions
● Origin of mass in the standard model ? does the Higgs boson exist, what is it mass ?
● Unification of forces ? More fundamental theory where forces are unified at higher scale?
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 12
The Standard Model: Open Questions
● Origin of mass in the standard model ? does the Higgs boson exist, what is it mass ?
● Unification of forces ? More fundamental theory where forces are unified at higher scale?
● Hierarchy Problem ? Why is the electro weak scale ~100 GeV << Planck scale 1019 ? In other words: Why is gravity so weak?
● Finetuning Problem radiative corrections to the Higgs mass quadratically diverge cancellation possible only with extreme fine tuning of parameters
● Dark Matter ?
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Dark Matter
Does not emit or reflect electro magnetic radiation, undetected
Where do we know it exists?
● motion of galaxies ● gravitational lensing● cluster formation in cosmic microwave background
e.g. SUSYcould provide candidates fordark matter
65%
30%
0.5% 4%0.03%0.03%
Dark energyDark matterFree H and HeStarsNeutrinosHeavy elements
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The Standard Model: Open Questions
● Origin of mass in the standard model ? does the Higgs boson exist, what is it mass ?
● Unification of forces ? more fundamental theory where forces are unified at higher scale?
● Hierarchy Problem ? why is the electro weak scale ~100 GeV << Planck scale 1019 ? In other words: Why is gravity so weak?
● Finetuning Problem radiative corrections to the Higgs mass quadratically diverge cancellation possible only with extreme fine tuning of parameters
● Dark Matter ? what is it ?
● Flavor or generation problem why three generations? origin of CP violation? why no strong CP violation neutrino masses and mixing
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Possible Answers from LHC
How could LHC give answers to these questions?
● search for the Higgs Boson● Test of standard model at highest energies – find deviations?● search for supersymmetric particles? elegant, unifies forces, helps with fine tuning and hierarchy problem, provides dark matter candidate● ..and of course any other physics beyond the standard model
new machine to find new physics:LHC - answer on new physics at few TeV scale!
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2. Hadron colliders LHC @ CERN
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Why hadrons ?
Collide compound objects, hadrons in initial state: hadron collisions more complex!
7 TeV 7 TeV
e+e-
pp
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LEP event (higgs candidate 4 jet event)e+e- -> ZH -> bb qq
● no hadrons in initial state● “clean” environment● √s = 2E
beam
simulated Higgs -> event in CMS
● hadron collisions more complex...● √s of hard interaction of constituents ≠ 2 E
beam
● overlap with soft hadronic interactions
Why hadrons ?
e+e- pp
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Advantage of hadron beams
wanted : highest energiesbut losses due to synchrotron radiation electrodynamics: accelerated charges radiate ring accelerators: x-rays via bremsstrahlung
Energy loss per term :
how to reduce E? reduce E ? no! increase R? done! increase m!
Ratio of e/p :
future ● pp ring accelerator LHC● e+e- linear accelerator ILC (R&D, planning phase)
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energy of electron and hadron colliders
History of √s
higher center ofmass energies athadron colliders
constituent cms!
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Hadron Colliders and Detectors
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CERN
Conseil Européen pour la Recherche Nucléaire
European Organization for Nuclear Research
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LHC
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LHC
proton-proton accelerator in the LEP tunnel = 27 km beam energies 7 TeV
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LHC
four experiments: ATLAS, CMS (pp physics) LHC-B (physics of b quarks in pp) ALICE (Pb-Pb collisions)
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LHC
Superconducting dipole magnets1232 magnets, 15 m longmagnetic field 8.3 Teslatemperature 1.9 K
Accelerator units 8 superconducting structuresgradient 5 MV/m
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LHC - details and status
some LHC parameters
SC magnets 1235, 15m , 8.33 Teslabeam energy 7 TeV
bunches/beam 2835particles/bunch 1.15 · 1011
bunch spacing 25 ns
Lumi initial phase (1-2yr) 10 33 cm-2 s-1
Lumi design 10 34 cm-2 s-1
1 year ~ 107 s runningintegrated Lumi 10 -100 fb-1/ year
for comparison:design Tevatron Run II (2001 – now): L = 2 · 1032 cm-2 s-1
LHC design 50 x larger !
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LHC - details and status
Start up scenario
now : beam commissioning in part of the ring, seen by LHCb!
Sep 2008: Beams in whole ring
Oct 2008: first interactions at 10 TeV used for calibration and commissioning of experiments October 21st is the official inauguration
Dec 2008: End of first run
2009 : Commissioning for 14 TeV April 2007lowering last LHC dipole
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Beam halo at LHCb
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Tevatron at Fermilab
Proton antiproton colliderexp.: CDF and D0
1992 – 1996 RunI √s=1.8 TeV L =125 pb-1
1996 – 2001 upgrade of accelerator and detectors
since 2001 RunII √s=1.96 TeV
2001 - 2006 Run IIa L =1.2 fb-1
2006 – 2009 Run IIb L =5-8 fb-1
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3. physics at hadron colliders
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 32
Definition of variables: pT and
● transverse momentum pT: momentum perpendicular to the beam axis
● pseudorapidity
rapidity
massless particles (mass unknown) use pseudorapidity instead:
°
°
°
=11 unit in pseudorapiditysame number of chargedparticles<n> ~7 (for min bias events)
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 33
pp collisions
collide complex objects
interaction of partons in the protonquarks and gluons (a,b)
hard subprocess a+b -> c+d
final state:hadrons and jetsproton remnants X
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pp colliders: QCD machines
Quarks and gluons interact stronglywhat was special about QCD ?
Feynman diagrams for qq, qg,gg interactions:
for the discovery of
asymptotic freedomin the theory of strong interactions
high lowmomentum transfer Q2
short longdistance interactions
low s high s
perturbative QCD =expansion in orders in S
asympt. confinementfreedom
running coupling
S
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pp collisions
long distance interactions
S large
- inside the proton- formation of hadrons from partons- pert. QCD not applicable
short distance interactionshigh momentum transferand small
S
- hard subprocess
outgoing (hard) partonsparton shower --> jets of partons
partons form hadrons--> jets of hadrons
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 36
pp collisions
long distance interactions
S large
- inside the proton- formation of hadrons from partons- pert. QCD not applicable
short distance interactionshigh momentum transferand small
S
- hard subprocess
outgoing (hard) partonsparton shower --> jets of partons
partons form hadrons--> jets of hadrons
pdfs
fragmentation
Marcello Barisonzi [email protected] LHC experiments August 25-26 2008 37
Hard subprocess
Proton momenta m ometum of partons in proton
x= longitudinal momentum fraction
center of mass energy
of pp collision:of hard subprocess:
which x needed to produce masses M ? M <x> LHC Tevatron100 GeV ~0.007 ~0.055 TeV ~0.36
what is probability to find parton withmomentum fraction x in the proton?
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What is probability to find parton with momentum fraction x in the proton ?
Parton density functions
Probe structure of the protondeep inelastic ep scattering
HERA ep collider DESY, HH e 27.5 GeV p 920 GeV
QCD: structure depends on x, Q2
probability is given by pdfs
HERA tunnel
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Parton density functions
measure structure functions F2 (x,Q2)...
...extract pdfs for valence (up,down),sea quarks and gluons
total ep cross section --> F2
fit of parameterised pdfsto F2 data= test of QCD, extract pdfs
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Cross section calculation
sum over initial partons a,b● parton density functions
non-pert, universal
● partonic hard scatteringcross section
calculable in pert. QCD
Long distance part(pdfs) and short distance interaction(hard subprocess)factorise!
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from partons to jets: fragmentation
at large momentum transfer: quarks act as free (asymptotic freedom)however: no free quarks observed , but jets of color less hadrons before QCD : puzzling ...
string-model
hard parton from subprocesscreates parton shower = gluon radiation and quark-antiquark pairs (pert)!At each splitting: E smaller -->
S increased
non perturbative process: hadronisatione.g. Lund String modeljets of hadrons
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string-model
formation of jets
non-perturbative process, model-dependent
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Minimum-bias events
Inelastic pp scattering cross section (70 mb = very large)dominated by
long distance interaction between pp with low momentum transfer● final state very little pT, very large pL● pt of charged tracks ~ 500 MeV● # charged particles dN/d ~ 7
so-called minimum-bias events why this name ? Trigger on... almost nothing = minimum bias
why interesting?Underlying event = everything but what I'm interested in
e.g. everything except the hard subprocess
min.-bias events = part of underlying event, Lumi dependent pile-up
all interesting events come along with underlying min-bias events!
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Pile up of min-bias events
2835x2835 proton bunchesseparation 7.5 m (25 ns)1011 protons/bunch
bunch crossing rate: 40 MHzLumi (design): 10 34 cm-2 s-1
~109 pp collisions / s and109 / 40 · 106
~ 25 pp interactions/bc = pile up!
Simulated event in CMS
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Pile up of min-bias events
2835x2835 proton bunchesseparation 7.5 m (25 ns)1011 protons/bunch
bunch crossing rate: 40 MHzLumi (design): 10 34 cm-2 s-1
~109 pp collisionen / s and109 / 40 · 106
~ 25 pp interactions/bc = pile up!
Simulated event in CMS
with min-bias pile up events
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Cross sections and rates
Rates for design L=1034 cm-2 s-1
inelastic pp interactions : 109 Hz(total cross section)
bb pairs 5 · 106 Hztt pairs 8 Hz W -> e n 150 HzZ -> ee 15 Hz
Higgs (150 GeV) 0.2 Hz gluinos, squarks(1TeV) 0.03 Hz
most cross sections 7-9 or more orders smaller than total cross section!
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Typical signatures
The signatures we look for are characterised by...
● Leptons and photons at high pT
initial state pp : no leptons , no pT
hight pT leptons in final state : decay of heavy particles signature of interesting physics
● missing Energy --> ETmiss
Higgs, W decays involve neutrinos many SUSY and other BSM scenarios missing Energy measure missing transverse E why not E
Lmiss?
● b quarks, tau leptons from decays long lived particles, decay vertex reconstruction
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Summary
● Hadron colliders play an important role in particle physics discory but also precision measurements
● LHC will open up TeV energy range new particles with 3-5 TeV mass could be produced and hopefully detected
● typical sigantures include high pT objects, leptons and photons and often missing (transverse) energy
● challenges at LHChuge interaction rates and large QCD backgroundpile up
requires detectors and electronics fast, high granularity, radiation hard ... lets have a look at them!