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LE RICERCHE SPERIMENTALI

SUL BOSONE DI HIGGS

LEP: collider e+e- con 4 rivelatori:

ALEPH, DELPHI, L3, OPAL;

TEVATRON: collider protone-antiprotone

con 2 rivelatori: CDF e D0;

LHC: collider protone-protone con 4

rivelatori: ATLAS, ALICE, CMS e LHCB;

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IL MODELLO STANDARD

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IL MODELLO STANDARD

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IL MODELLO STANDARD

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W, Z production

gluon-to-Higgs fusion

squarks, gluinos (m ~ 1 TeV)

,W Zq

q

g

g

t0H

g

g

q

q

High-pT QCD jets g

g

Quark-flavour

production g

g

q

q

Cross Sections at Hadron Colliders

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Higgs Cross Sections at Hadron Colliders

Tevatron LHC

qq W/Z + H cross sections ~10 × larger at the LHC

gg H ~70–80 × larger at the LHC

Higgs production cross sections versus MH at the Tevatron and the LHC

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Searching for the Higgs at Hadron Collliders

The dependence of the branching fractions on MH drive search strategy

For heavy Higgs:

• Lepton final states via WW, ZZ

For light Higgs:

• Lepton final states via WW*, ZZ*

• Di-photon final state

• Di-tau final state

The dominant H bb mode is only

exploitable in association with W/Z or tt

(associated leptons help to reduce overwhelming

QCD background)

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Searching for the Higgs at Hadron Collliders

Detector requirements for Higgs searches and measurements

Excellent identification and measurement

of high-pT muons, electrons and photons

Excellent measurement of missing

transverse energy (for W and t final states)

requiring energy measurement up to very

forward region (|h | ~ 5)

Jet tagging in forward direction

(for weak-boson-fusion process)

Efficient and pure b-tagging* and t

identification

*requiring silicon pixel detectors close to beam pipe

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The Tevatron Collider at Fermilab

CDF D0

Anti-protons Protons

Tevatron

Main injector & recycler

Proton source

Booster

Proton–antiproton collider:

•6.5 km circumference

•0.98 TeV beam E (ECM = 1.96 TeV)

•36 bunches at 396 ns separation

•main challenge: antiproton production

and storage luminosity & operational

stability

Two general purpose experiments:

CDF and D0

Tevatron is running in Run II:

•Run I (1990–1996, 0.13 fb–1 lumin.):

top discovery

•Run IIa (2001–2006, 1.2 fb–1):

Bs oscillation discovery

•Run IIb (2006–2011, 10–12 fb–1) :

Higgs exclusion at ~ 160 GeV

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Tevatron versus LHC

Tevatron (achieved)

LHC (design)

Centre-of-mass energy 1.96 TeV 14 TeV

Number of bunches 36 2808

Bunch spacing 396 ns 25 ns

Protons / bunch [×1011] 0.9(anti-p) / 2.8(p) 1.2

Energy stored / beam 1 MJ 360 MJ

Peak luminosity 4 × 1032 cm–2s–1 1033–1034 cm–2s–1

Integrated luminosity / year ~3 fb–1 10–100 fb–1

Inelastic interactions / crossing 10 ~24

• 7 times more centre-of-mass energy at LHC

• 3–30 times higher peak luminosity

• 10–100 times larger physics cross sections for hard scattering reactions

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Higgs Searches at the Tevatron

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Higgs Search Strategy

Separate analyses into low-mass and high-mass searches 135 GeV

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Low Mass Search: MH < 135 GeV

Largest VH cross section –

however strongly background contaminated

WH l bb : lepton + E

T

miss + bb

Smaller cross section than WH –

but less background

ZH l l bb : 2 leptons + bb

3 × more signal than charged-lepton mode –

however large and difficult backgrounds

(includes events from WH mode when lepton missed)

ZH bb : E

T

miss + bb

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Low mass searches: CDF/D0

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Sensitivity of Low-Mass Search

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High-Mass Searches: MH > 135 GeV

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High mass searches: D0 & CDF

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Results of Tevatron Combination

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Higgs Searches at the LHC

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: the price to pay for high luminosity

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Searching for the Higgs at Hadron Collliders

The dependence of the branching fractions on MH drive search strategy

For heavy Higgs:

• Lepton final states via WW, ZZ

For light Higgs:

• Lepton final states via WW*, ZZ*

• Di-photon final state

• Di-tau final state

The dominant H bb mode is only

exploitable in association with W/Z or tt

(associated leptons help to reduce overwhelming

QCD background)

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H gg

H gg: Significance of the excess

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H ZZ 4l (l = e, m)

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H tt

Observed: 1.3s

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VH with Hbb

2,2s excess

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Is this excess “The” Higgs boson?

The parameter µ is measured, i.e. the ratio between the s X BR for each

channel and the SM prediction for a fixed Higgs mass (here 125 or 125.8 GeV).

µ = 0,88 ± 0,21 (𝑠𝑡𝑎𝑡. +𝑠𝑦𝑠𝑡. )

Il 4 Luglio ha segnato una data storica per la fisica: dopo quasi 20 anni

dalla scoperta del quark top, una nuova particella è stata scoperta.

Al momento tutto fa pensare che sia il Bosone di Higgs previsto dal

Modello Standard. E che sia quindi il “responsabile” della massa di tutte

le particelle fondamentali conosciute.

Ma sarà davvero così? E se sì, è il solo o ce ne sono altri? Per motivi di

tempo non vi ho parlato di altri scenari legati al bosone di Higgs:

Supersimmetria? Higgs non elementare? Che spin/parità ha?

Per avere una risposta definitiva a tutti questi quesiti servirà attendere

l’analisi di tutti i 27 fb-1 raccolti a 7 e 8 TeV dagli esperimenti fino a lunedì

scorso (cosa che avverrà entro il 2013) ma soprattutto dei dati a 13-14

TeV che si prenderanno a partire dal 2015, alla ripartenza di LHC.

Come vedete, quindi, di lavoro da fare ce n’è tantissimo e c’è anche

tutto il tempo per farlo…..

Per concludere….

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… o forse no ??