the large hadron collider machine, experiments, physics summary lecture part 1 johannes haller...
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The Large Hadron ColliderMachine, Experiments, PhysicsSummary Lecture Part 1
Johannes HallerThomas Schörner-Sadenius
Hamburg UniversitySummer Term 2009
JH/TSS UHH SS09: LHC 2
The Standard Model (SM)- Renormalisable gauge theory
SU(3)C×SU(2)L×U(1)Y.
- Incorporates GSW model and QCD.Particle content:- 6 quarks, 3 charged/neutral leptons
(plus antiquarks)- 8 gluons, W±, Z0, γ;
what about the Higgs boson?
Particle interactions- by the gauge bosons exchange,- depend on charges (EM, weak, color).- Calculable using perturbative methods (for QCD: lattice!).
All current data in agreement with SM !- Data from LEP, HERA, TEVATRON etc.
STANDARD MODEL: SUCCESSES AND PROBLEMS
SM Problems: – large number of free parameters; explanation in fundamental theory?– Gauge structure? Number of generations? Connect fermions-bosons? leptons=quarks?– Without Higgs: SM diverges at 1 TeV! Only scalar boson can avoid divergencies.– SM decoupled from gravitation!– In SM, no unification of couplings, masses.– Hierarchy and fine-tuning problems?– Question of dark matter?
JH/TSS 3
POSSIBLE SM EXTENSIONS: SUSY
Supersymmetry: Symmetry between fermions and bosons
proposed by Wess and Zumino in 1973.Introduce SUSY partner for all SM particles.SUSY must be broken symmetry!
Arguments for SUSY:– Why not - compatible with all experimental
data! – Last extension of Poincare group of SRT.– Explanation of spin?– removes fermion-boson asymmetry!– Leads to unification of interactions (GUT).– Solves hierarchy problem.– Introduces dark matter candidate (LSP).– Predicts MH < 130 GeV.
– Allows introduction of gravity.
Leptonen (e, Leptonen (e, ee, …), …)Quarks (u, d, …)Quarks (u, d, …)GluonenGluonenWW
ZZ00
Photon (Photon ())HiggsHiggsGravitonGraviton
1/21/2
11
0022
SpinSpin SM particlesSM particles SpinSpin00
1/21/2
1/21/23/23/2
Sleptonen (e, Sleptonen (e, ee, …), …)Squarks (u, d, …)Squarks (u, d, …)
SUSY partnersSUSY partners
GluinosGluinosWinoWinoZinoZinoPhotino ( Photino ( ))HiggsinoHiggsinoGravitinoGravitino
~~
~~~~~~~~
UHH SS09: LHC
JH/TSS 4
Higgs, SUSY, ED, CI, …“new” physics at high energy scales and high particle masses
Other view: We are interested in the smallest possible structures of matter
high energy
Problem in ring accelerators: synchrotron radiation! Choose heavy particles
in large radius!
MOTIVATION FOR THE LHC
History of particle physics: Discoveries mostly at hadron machines (cf. Livingston plot):
2mcE ~px
41
m
E
RE
– proton-proton collider, – √s = 14 TeV = 14·1012 eV– frequeny: 40 MHz (25 ns, +overlays) – max. luminosity: 1034 cm-2s-1, 100fb-1/a. – Start: Spring 2008 (√s = 900 GeV?) – Experiments ATLAS, CMS (LHCb, ALICE)
UHH SS09: LHC
JH/TSS 5
Description/understanding of the different stages:The rate of proton-proton interactions is connected to the (proton-proton) cross-section σpp via the luminosity:
Luminosity L is machine parameter; related to the bunch-crossing frequency f, the number of particles per bunch, and the cross-sections of the bunches:
The proton-proton cross-section σpp is connected to the parton-parton cross-section σij (for partons i,j) via the parton distribution functions fi/p (probability to find parton of type i in proton p):
The parton-parton cross-sections σij can (in principle) be calculated using perturbative methods.
Need to disentangle multi-proton (overlay) and multi-parton collisions experimentally.
pp reactions: … the global picture:
OVERVIEW OF pp REACTIONS
ij
pjijpipp ff //
2835×2835 bunchesin the LHC ring.
109 protons / bunch
≤30 pp collisions per bunch crossing
N parton-partoncollisions / pp collision
Complex final-statesin every parton-partoncollision.
LN pp
yx
NNfL
4
21
n
nS
nijij c )(
UHH SS09: LHC
JH/TSS 6
Remember the formula for the pp cross-section:
– The “hard scattering matrix element” σij: perturbation theory with Feynman diagrams: large energy scales.– The PDFs fi must be determined experimentally from data (HERA!). They resum soft / long-range contributions to the cross-section.
Quarks only exist in color-neutral hadrons (“confinement”); only at very small distances / high energies relevant coupling becomes small enough to probe quarks (“asymptotic freedom”, NP 2004).
BASICS OF QCD, FACTORISATION
ij
pjijpipp ff //
2
22
22)1(
ln12
2331
ZZS
f
ZSloopS
M
QM
nM
Q
2242
2
,4
QxFxQdxdQ
d
PDFs from F2 structure functiondata from HERA
UHH SS09: LHC
JH/TSS 7
Connection between structure function F2 and PDFs
– “DGLAP” evolution with Q2:
Sufficient information to extract PDFs from behaviour of F2 with x and Q2!
PDFs, CROSS-SECTIONS, PARTON LUMIS
1222
2
2
2
22
),()(2),()(
2
)(
ln
),(
x iiqgqq
S
QQzgz
xzPQzF
z
xP
z
x
z
dz
Q
Q
QxF
i
ii QxxqeQxFF ),(),( 22222
Cross-sections at LHC (TEVATRON):
ijij
ij
ijijpjpi
sd
dL
s
d
xfxfdxdxs
1
2/1/21
0
1
,,
ssxxs 21ˆ
1
0 212/21/121 21,,1
1xxxfxxfxdxdx
d
dLpjpi
ij
ij
UHH SS09: LHC
JH/TSS 8
OVERVIEW OF HEP ACCELERATORS, LHC
Collider
Start/end Beams Max. energy [GeV]
Circumference /
length [km]
PETRA)
1978-86 e+e– 23.4 2.304
SLC 1989-99 e+e– 50 1.45 + 1.47
LEP 1989-2000
e+e– 104 26.7
ILC (?) 20??-?? e+e– 500? 15+15 (?)
KEKB 1999-?? e+e– 8 x 3.5 3.0
PEP-II 1999-?? e+e– 9 x 3.1 2.2
HERA 1991-2007
ep 27.5 x 820/920
6.3
SppS 1981-1990
ppbar 315 6.9
TEVATRON
1987-2009/10
ppbar 1000 6.28
LHC 2008-?? pp 7000 26.7
UHH SS09: LHC
JH/TSS 9
LHC IMPRESSIONS, DETAILS
Comparison:– Energy of A380 at 700 km/h corresponds to energy stored in the LHC magnet system!– Sufficient to heat up and melt 12 tons of copper!Energy in the beams:– corresponds to 90 kg of TNT– 8 litres of gasoline– 15 kg of chocolate
Increase wrt existing accelerators :
•A factor 2 in magnetic field•A factor 7 in beam energy•A factor 200 in stored energy
UHH SS09: LHC
JH/TSS 10
COMPARISON OF SIZES
ATLAS
CMS
– Electrons, photons, jets very important(<1%, 3% energy scale knowlegde):
– Aim: Determine energy of charge/neutral particles– Measurement is destructive! - loss of total energy requires high material density. - Different energy loss mechanism for hadrons, leptons separation into EM + HA parts.– Calorimeters are needed high-energy particles:
– Homogenous versus sampling calorimeters:
pp
pEE
E driftcalo 1
– Response ε: linear and same for all!– MIP: minimum-ionising particle.– Compensation if e/h = 1! Normally: - h < e: invisible E in HA component! e/π > 1! And non constant! Problem! Try to achieve compensation (later)!
UHH SS09: LHC
JH/TSS 11
Electrons: Bremsstrahlung and Ionisation – Bremsstrahlung: radiation length X0:
– Ionsiation: Bethe-Bloch!
Photons: Mostly pair production
Simple model (Heitler): Assume interaction after one X0 (and symmetric energy sharing).
ENERGY LOSS, EM SHOWERS
Z
C
I
cm
cm
nZez
dx
dE
2
2ln
4 2222
22
42
0X
E
dx
dE
3
1220
183ln4 ZrZN
AX
eN
- After T X0: 2T particles with energy E0/2T. Continue particle production until E<Ecrit. Then only ionisation left.- Note: cascade stops after tmax generations:
- Moliere radius (trans- verse profile):
- Energy resolution: 1/sqrt(E) term from statistics:
2ln
ln 0max
critEEt
a
btbtbE
dt
dE a
exp1
0
aEEt crit
peak 2lnln 0
critM E
cmXR
210
E
cb
E
a
E
E
UHH SS09: LHC
JH/TSS 12
For some recent HEP experiments:
COMPARISON; HA SHOWERS
Showers initiated by hadrons are different:– nuclear reactions (strong interactions)! Multitude of processes with probabilities to be determined in experiment.– invisible contribution worse resolution! – About 7λ needed to contain shower better use heavy absorbers:– Problem: different efficiencies/resolution for
measurement of EM / hadronic part of showers.
1.0 ,ln1 kEkE
Eh
eheehhh EES
%8.1%42
%35.0%10
EE
EEHA
EM
%5.6%127
%45.0%4
EE
EEHA
EM
ATLAS:
CMS:
UHH SS09: LHC
JH/TSS 13
Cross-section overview:
– Small cross-sections, branching ratios for BSM!
– Large rates for SM, <=30 overlay events!– SM processes as backgrounds to new physics– Use SM calibration processes Zl+l-, W jj,
… – Luckily new physics at high
energy/momentum scales! hint for triggering!
TRIGGER
bunch crossing rate: 40 MHz
total interaction rate: ~1 GHz
event size: ~ 1.5 MB
affordable: ~ 300 MB/s
storage rate: ~ 200 Hz online rejection:
99.9995%Considered signatures:– inclusive and di-leptons
(electrons, muons)– Photons: Hγγ– High-pT hadronic jets (SUSY,
LQ, resonances, …)– missing transverse energy
(MET): SUSY, t, W.
Selection 2·1033 cm-2s-1
1034
cm-2s-1
MU6(20?) (20) 23 (3?) 4.0
2MU6 --- (1?) 1.0
EM25i (30) 11 22.0
2EM15i (20) 2 5.0
J200 (290) 0.2 0.2
3J90 (130) 0.2 0.2
4J65 (90) 0.2 0.2
J60+xE60 (100) 0.4 0.5
TAU25+xE30 2.0 1.0
MU10+EM15i --- 0.4
others 5.0 5.0
total ~ 44 (25?)
~ 40
UHH SS09: LHC
JH/TSS 14
ATLAS TRIGGER
Multiplicities
Regions-of-
InterestEvent decisionfor L1
Interface tofront-end
Muoncandidatesabove pT
thresholds
Interface to highertrigger levels/DAQ:objects with pT,,
Candidates forelectrons/photons,taus/hadrons,jetsabove pT thres-holds.
Energy sumsabove thresholds
UHH SS09: LHC
JH/TSS 15
QCD
pbban
anpanSjet fbf /
,,,/
Jet production: QCD background to everything!
CDF data
Learn about PDFs from jets? About αS?
Huge pT reachable
Test of pQCD ! Large BSM sensitivity!
Understand minimumbias, underlying events!
Need to understandPDFs, αS, to get SUSY, resonances …
UHH SS09: LHC
JH/TSS 16
ELECTROWEAK
W and Z production: high-rate, good for lumi deter-mination, PDFs, couplings.
All results compatible with SM!
GeV025.0398.80 WM
Aim LHC: Errors < 15 MeV !
coscos1
2cos2
2
BAsd
d
p p
l+
l-
θ
Rosner, J.L.: Phys. Rev. D 54, 1078 (1996) Rosner, J.L.: Phys. Rev. D 54, 1078 (1996)
Mee [GeV/c2]
AFB
uuee
Measurement of AFB will reveal structure beyond SM!
UHH SS09: LHC
JH/TSS 17
SM: triple and quartic gauge boson couplings (con-sequence of non-abelian structure of underlyinggauge group SU(2)LxU(1)Y):
… but no γZZ vertex!
SM: g, κ, λ=1. measurement of κγ, λ,gZ is powerful test of SM.\
EW PHYSICS: MULTI-BOSONS VERTICES
All
WWZig
ZWWWWig
WWAig
AWWWWigL
W
W
W
Wgauge
ˆcos
ˆˆcos
ˆsin
ˆˆsin
WWW
Example: Zγ final states with Zll. Question: Do we observe a non-SM ZZγ contribution? Answer: No!
So far:
– TGC observed and measured. – Limits on non-SM couplings derived. – Cross-sections for diboson production measured with good accuracy.
UHH SS09: LHC
JH/TSS 18
TOP PHYSICS Current status topmass from Tevatron:
Learn about:– ttbar cross-section, spin correlations, charge asymmetries.– Top mass, charge.– Cross-section for single top production ( FCNC?).– Top decays (W helicity, FCNC, tH+b, …)
This makes the top quark unique in many respects:– Mass: BSM might couple to high masses. – Decays as quasi-free quark no hadronisation chance to study free quark + properties (spin!).– Top might decay to yet undiscovered particles.– Top quark allows precise SM tests via connection to W and Higgs masses.
€
mt =173.1±1.3GeV
9991.0039.0009.0
040.09747.0221.0
003.0221.09753.0
tbtstd
cbcscd
ubusud
VVV
VVV
VVV
bWVt
igtb
5122
UHH SS09: LHC
JH/TSS 19
– Theoretical predictions: factorisation!
TOP: SUMMARY TEVATRON
Comparisons: cross-sectionmeasurements Tevatron.
pbban
anpanSjet fbf /
,,,/
UHH SS09: LHC
JH/TSS 20
– Possibility to produce single top quarks in pp:
Latest result from CDF:
SINGLE TOP PRODUCTION
SM: 1.98±0.25 pb
SM: 0.88±0.11 pb
– Search for flavour changing neutral currents.– Search for fourth-generation quarks:
Additional generations might effectively reduce Vtb (which is only indirectly known!) and thus the single-top cross-section. Do we observe that? Also: Are the unitarity relations fulfilled?– Measurement of tt+jets events test of SM couplings.– Measurement of tt+photon events measurement of top quark electric charge.– …
Lots to do for LHC!
UHH SS09: LHC
discovery at 5σ!