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?

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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

~~

~~~~~~~~

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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)

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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 )(

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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

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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

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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

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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

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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)!

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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

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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:

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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

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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

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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 …

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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!

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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.

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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

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– Theoretical predictions: factorisation!

TOP: SUMMARY TEVATRON

Comparisons: cross-sectionmeasurements Tevatron.

pbban

anpanSjet fbf /

,,,/

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– 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!