New particle discovery at LHCand first properties measurements

Estelle ScifoLaboratoire de l’Accélérateur Linéaire

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OUTLINE

1. Theory2. Higgs Status before LHC3. LHC and its detectors4. Data taking and results

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1. THEORY

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THE ATOM

3 elementary particles: e, u, d21/01/2013

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NEUTRINOS• An exemple of

complementarity between theory and experiment;

• Postulated in 1930 by Wolfang Pauli (and called neutrino in 1933 by Enrico Fermi) to explain an observed missing energy in beta decay;

• Weakly interacting particle;• Discovered in 1956 by Frederick

Reines and Clyde Cowan (Nobel prize 1995)

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INTERACTIONS

• Interaction = exchange of mediator particles

• Electromagnetic: between electrically charged particles (photons γ)

• Strong: needed to explain nucleus coherence (gluons g)

• Weak: reponsible for some radioactive decays (W± & Z0)

• Gravity: (graviton ?)21/01/2013

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STANDARD MODEL OF PARTICLE PHYSICS

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Fermions:

Bosons:

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MASS HIERARCHY

• Photons (m<10-18 eV) and gluons massless • Neutrinos: massless in minimal SM but some hints (such as neutrinos

oscillations) indicate they should have a very small mass21/01/2013

Order of magnitudeme = 0.5 GeV ≈ 10-30 kg

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HIGGS MECHANISM

• Problem:– In the theory so far (described by quantum field

theory), W and Z bosons have no mass, – but experiments found masses ~ 80 GeV

• Solution:– Nice suggestion by (Brout-Englert) Higgs in the 60’s :

• Vacuum is filled with a ‘’Higgs’’ field that interact with particles and give mass to elementary particles.

• The mediator associated to this interaction: the Higgs boson

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SPONTANEOUS SYMMETRY BREAKING• Higgs potential:

– Mexican hat– Stable solutions symmetric– The system has to chose one

of them spontaneous symmetry breaking(= the stable solution do not have the same symmetry than the physics)

• Other SSB in physics:

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Phase transition Column buckling

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ANALOGY WITH SUPERCONDUCTIVITY

• For T<Tc, magnetic field locked outside the material → photons have a mass ≠ 0

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• Critical temperature for SSB related to the BEH mechanism corresponds to ~100 GeV and is therefore equal to ~ 1015 K. • It happened at a time of ~ 10-10 s after the big bang

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‘’QCD MASS’’• Without Higgs, you would also have a mass!• Ex: a proton is made of 3 quarks

– Mproton = 938 MeV

– ∑mquarks = 9.4 MeV

• Explanation: more realistic view of protons

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DU

U

DU

U

g

gg

DU

U

g

g

DU

U

g

g

DU

U

• A proton at time t:

• The proton mass is mainly due to energy: m=E/c²

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TOWARDS EXPERIMENTS

• Need to observe this ‘’(BE)Higgs boson’’ to validate the theory;

• All properties predicted by the theory except its mass;

• In particular, we know its lifetime (≈10-24s !!) and its decay modes;

• The (BE)Higgs boson will therefore be observed through its decay modes and not directly. → several channels can be analyzed

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2. HIGGS STATUS BEFORE LHC

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LABORATORIES

• Several laboratories have contributed to the construction and evolution of particle physics

• Accelerators : LHC, TeVatron• Detectors (experiments): Atlas, CMS, D0, CDF21/01/2013

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CERN• European organisation for Nuclear Research• Created in 1954• Member states: (at the beginning) (joined)

• Contributions from outside Europe Country (Japan, USA…)• Budget in 2012: 887 million CHF 16 ≈ 707 million €21/01/2013

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FIRST HIGGS SEARCHES: LEP

• LEP– Electron-positron collider– 27km circumference– Center of mass energy up to 200 GeV– 4 detectors

• Results about Higgs:– No discovery– But allow to exclude some mass range…

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LIMIT ON THE HIGGS MASS– LEP – TeVatron (1 TeV proton-antiproton collider in Chicago)

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3. LHC AND ITS DETECTORS

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LARGE HADRON COLLIDER (LHC)

• Proton-proton collider• In the same tunnel than LEP

(27 km)• Each proton energy: 7 TeV

(nominal value)• Previous CERN facilities still

used to prepare the beam before injection in the LHC

• Collisions each 25ns (nominal value), 50 ns (today) → each experiment needs a very effective trigger system to select interesting signature (low energy events eg are skipped)

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LHC TUNNEL

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LHC CONTROL ROOM

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LUMINOSITY• Caracteristic of the accelerator• Related to the number of collisions delivered• The higher luminosity, the higher the number of

recorded event, the better for search for rare process

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Number of collisions per second

Luminosity Probability of interaction btw 2 protons

• Dimension: L-2T-1; Current unit: inverse barn per second

• Integrated luminosity

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HIGGS PRODUCTION AT LHC

• Modes:

– Important because involve different Higgs couplings (to W, Z or quarks)

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SENSIBLE HIGGS DECAY MODES

• H→bb:– Highest branching ratio– But: very high background– Study possible in very specific cases

lower number of events and smaller mass resolution

• H→ZZ: the ‘’golden channel’’;• H→γγ: clear signature, another important channel at LHC;• Other (less important because poor mass resolution):

– H →WW ,– H→τ τ

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WHAT TO DO WITH DECAY PRODUCTS ?

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ΨAMass=mA

Part. 1

Part. 2

Signal:

Background:

Ψ

Part. 1

Part. 2

A

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WHAT TO DO WITH DECAY PRODUCTS ?

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Total:

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H→ZZ→4l: THE GOLDEN CHANNEL

• 3 possibilities: ZZ→eeee; ZZ→µµµµ; ZZ→eeµµ• Signal over Background ratio very high;• But very few expected events

(due to the BR(Z→ll)=3%);• Expected m4l distribution:

– mass resolution ≈1.5 GeV

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H

l

l

Z

l

l

Z

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H→gg: THE DIPHOTON CHANNEL

• Vertex Hgg impossible in the SM because photons are massless (= no coupling to Higgs boson)– Process allowed through loops involving massive particles:

• Clear signature• Expected mgg distribution

– mass resolution ≈1.5 GeV

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H

γ

γ

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INTERPRETATION:STATISTICAL TOOLS

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• Important variable: p-value (p0): estimate

‘’the probability for the background to fluctuate at least as much as the observed data’’

– If p-value is high: observed data are consistent with the hypothesis of background only

– If p-value is small (<10-7): there are more events in data than the expected background, even taking into account fluctuations

→ there might be a signal !

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EXPERIMENTS ON THE LHC

• 4 detectors:– ATLAS: general purpose– CMS: general purpose– LHCb: asymmetry matter/antimatter (CP-violation)– ALICE: heavy ions studies (Quarks and gluons plasma)

• ATLAS and CMS designed mainly for Higgs (and SUSY) searches

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A GENERAL DETECTOR DESCRIPTION• Need to measure:

– Trajectories– Energies: calorimeters

(need to destroy incident particle)

• Ex: CMS

• And need to identify particles21/01/2013

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THE ATLAS DETECTOR

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ATLAS DURING ASSEMBLING

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CMS Silicon Tracker

The Silicon tracker (200m2) has 10M channels

Limoges 14-1-2013

Limoges 14-1-2013 37

(E)/E = 3%/EGeV 0.7 %

CMS EM calorimeter more than 75000 cristals of PbW04

IMPORTANCE OF GRANULARITY

• Ex. of ATLAS calorimeter: 3 longitudinal layers• 1st sampling very fine transverse granularity

– Allow to distinguish between photons and their main background: two close photons coming from the decay of energetic π°'s

• Photon π°

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π°γγ

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4. DATA TAKING AND RESULTS

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LHC SCHEDULE

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2008

2009

2011

10th september 2008 : first beams around19th september 2008 : incident

20th november 2009 : first beams around (again) december 2009 : collisions at 2.36 TeV

14 months of major repairs and consolidationNew Protection system

30th march 2010 : first collisions at 7 TeV august 2010 : luminosity of 1031 cm-2 s-1

2010

november 2011 : integrated luminosity ~ 5 fb-1

13th december 2011 : first ‘signal’ around 126 GeV

2012 march 2012 : start again at 8 TeV

4th July 2012 : evidence for a new boson ( total 8 TeV integrated luminosity for discovery : 6 fb-1) luminosity = 8 1033 cm-2s-1

now at 8 TeV : 13 fb-1 published 20 fb-1 taken

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DATA TAKING CONDITIONS• Btw 2010 and 2012, LHC

increases its performance: more and more events collected per unit of time

• Good point for search for rare phenomenon

• Drawback: more and more pile-up: lot of particles to identify in a single event

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2010 2011 2012

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FIRST ATLAS RESULTS(2010 DATA)

• All known particles were rediscovered at the right mass → give confidence in the new results

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HIGGS DISCOVERY STEPS

• 13th December 2011: observation of an excess

• 4th July 2012: discovery

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RESULTS PER DECAY MODEINVARIANT MASS DISTRIBUTION

H→ZZ→4l H→gg

21/01/2013 NB: plots shown are updated plots from December 2012

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p0 DISTRIBUTIONS

H→ZZ→4l H→gg

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COMBINED RESULTS

• ATLAS

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• CMS

p0 < 10-7 = discoveryFor both ATLAS and CMS There is a new particle

EVOLUTION OF THE EXCESS

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UPDATE DECEMBER 2012

• Update with more data 6→13 fb-1

• Blue line ≈ July value

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EVENT DISPLAY: H→γγ

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EVENT DISPLAY: H→ZZ→4e

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EVENT DISPLAY: H→ZZ→4Μ

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NEXT

• Measure the properties of this new particle – Check it is really the Higgs boson…– Probe the SM

• Properties to measure:– Signal strength

(=number of observed signal wrt expected)– Couplings to other particles– Spin (0 in the SM)

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SOME PRELIMINARY RESULTSSIGNAL STRENGTH

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Small excess in γγ wrt SM ? To be confirmed by more data.

Very good agreement with SM

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SOME PRELIMINARY RESULTS SPIN

• How to measure the spin ? • Idea: spin has an impact on angular

distribution– Spin 0: totally symmetric wrt space

orientation: angular distribution flat– Spin ≠ 0: angular distribution not flat…

• Plus: statistical analysis

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From γγ channel only:Probability for spin 0: 29%Probability for spin 2: 8.6%

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LIMITATIONS OF THE SM

• We already know some phenomenon that can not be explained by the SM– Neutrinos masses and oscillations– Mass hierarchy– Dark matter/energy

• Theories exist to explain this (eg supersymmetry) predict the existence of new heavy particles

(eg. SUSY have at least 5 Higgs bosons)

• That can contribute to the loop processes (eg H→gg) and change the experimental rate of this process by a few permil (otherwise would already have been observed) necessity to measure very precisely the branching ratios

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LHC FUTURE

• 2012 pp run finished• Long shutdown:

2013-2015• Restart in 2015 at 13 TeV

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CONCLUSIONS

• After many years and thanks to many people, a new particle has been observed at the LHC, by both ATLAS and CMS

• Very similar to the predicted (BE)Higgs-boson• Mass:

– ATLAS: mH=125.2 ± 0.3 (stat) ± 0.6 (syst)

– CMS: mH=125.8 ± 0.4 (stat) ± 0.4 (syst)

• Still need to measure very precisely its properties (spin, parity, BR) to probe the SM and perhaps find new physics ?

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BACKUP

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PRELIMINARY RESULTSCOUPLINGS

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CONFINEMENT

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LES JETS

• Confinement un quark libre n’existe pas• Comment est-ce qu’ils se manifestent dans

nos expériences ? Sous la forme d’un « flot » de particules appelé

‘’jet’’

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OBSERVATION DE JETS DANS ATLAS

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Un événement avec jets

Un événement sans jet

Collision

Une particule chargée Une autre particule chargée

Détecteur de traces

1) MESURE DE LA TRAJECTOIRE

Les électrons proches de la trajectoire de la particule chargée sont arrachés de leur atome et sont collectés par un système électronique sous la forme d’un signal électrique.

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2) MESURE DE L’ÉNERGIE• Formation de gerbe électromagnétique : nécessite la destruction de

la particule initiale

• Même principe pour mesurer l’énergie des électrons/positrons et des autres particules.

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Calorimètreélectromagnétique

Électron (ou position)Photon

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ParticuleStable

Détecteur de traces

Calorimètre électro-

magnétique

Calorimètre hadronique

Détecteur de muons

Photon

Électron

Quark/Gluon Jets

Muons

Neutrinos

• Beaucoup de particules• Seules celles qui sont stables peuvent être identifiées

directement• Pour les autres, une analyse plus poussée est nécessaire

3) Identifier les particules

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LHC BUDGET

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54%

12%1%

1%

3%

2%

2%

15%

3%2% 3% 2%

Aimants

Cryogénie

Décharge de faisceau

Radio-frequence

Vide

Convertisseurs de puissance

Instrumentation du faisceau

Génie civil

Refroidissement & ventilation

Distribution d'énergie

Infrastructures & services

Installation & coordination

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LIMITS: BRAZILIAN PLOT

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