The experimentalist's task

TTdF – Seminario - DFO Milano-Bicocca 1Nov 20th, 2013


This talk is on the experimental evidence of this fundamental (?) particle (theory in the previous talk)

TTdF – Seminario - DFO Milano-Bicocca 3

The experimentalist's task

Theory: A small number of general

equations with some parameters (unknown or poorly known)

Observables: Cross-sections (probability of

interaction), branching ratios, lifetimes

Experiments: Inquire about what nature does Imperfect measurement of a

(set of) particle interactions in a (set of) detector(s).

Nov 20th, 2013

Compare measurements to observables


Theory… e.g. the standard model

Has parameters Coupling constants

(electric charge and weak charge)

Masses

Predicts: Cross-sections Branching ratios Lifetimes …

Nov 20th, 2013

Adapted from G.Dissertori


… and experimentNov 20th, 2013

Raw data

2×107 GB / year at the LHC(over 1 million DVDs)




Raw data Interactions Electric signals Digitization

Hits Track

p = (px,py,pz)

Event (a unique happening): List of (stable) particles with: E, p, charge and other information Higher level reconstruction: e, μ, γ, hadrons (jets)

Address: where the detector element took the reading

Values: what the electronics wrote out

✚✚

✚✚ pT=eBR




Interactions Electric signals Digitization

Hits Track

p = (px,py,pz)

Event (a unique happening): List of (stable) particles with: E, p, q and other information Higher level reconstruction: e, μ, γ, hadrons (jets)

✚✚

✚✚

A measurement: data and theory prediction (convolved with detector response)


Any definite prediction?

The standard model (SM) of EWK interactions: Non-abelian gauge theory: SU(2)L x U(1)Y With spontaneous symmetry breaking

New unequivocal predictions:1. A neutral massive boson (Z-boson) and its couplings2. A massive scalar field (Higgs boson) of unpredicted mass

Nov 20th, 2013

• 1954 - Yang & Mills non-abelian gauge theory• 1961 - Sh. Glashow, SU(2)xU(1) with mass-less bosons • 1964 - P. Higgs + Brout & Englert – Symmetry breaking • 1967/68 - S.Weinberg / A.Salam :


However… 1973 – Discovery of neutral currents 1982 – Discovery of W/Z bosons 1989-2000 – Over 100 tests of the

standard model with no sign of inconsistency …

… and yet no (direct) sign of a Higgs boson

Nov 20th, 2013

mH > 114.4 GeV (95% CL)

LEP/SLC/Tevatron


More about the Higgs boson Gauge bosons :

MW2 = ¼ g2 v

MZ2 = ¼ (g2+g’2) v

Theory constraint: (MZ/MW)2 = g2/(g2+g’2)

Strong indication of a ‘Higgs mechanism’ through the independent measurement of boson masses and EWK coupling constants

Fermions : m = λ v / √2 Unconstrained by theory

Higgs mass unknown, but: mH > 114 GeV LEP direct bound mH < 152 GeV Indirect bound from SM precision tests mH < 1 TeV Theory bound (e.g. WW scattering)

Nov 20th, 2013

gHVV = mf/v

gHVV = 2MV2/v


The Large Hadron Collider Primary objectives:

Search for the SM Higgs boson up to 1 TeV Characterize it, if found

Search for phenomena beyond the standard model New gauge bosons, ‘new physics’ at the 1 TeV scale Dark matter candidate

Centre of mass energy: 7-8 TeV (2010-2012) 14 TeV (design)

Nov 20th, 2013

18 km/27 km of superconducting dipolesB = 8.3 TT = 1.9 K (120 ton of L-He)80 MW (+ 30 MW for the experiments)

Approved in 1994First collisions in 2009


Detectors at the LHCNov 20th, 2013


The Compact Muon SolenoidNov 20th, 2013


The Compact Muon SolenoidNov 20th, 2013

Pixel / Tacker detector Electromagnetic Calorimeter Hadron Calorimeter Solenoid Muon detector

muon

photons/electrons

hadrons (π,K,p,n,…)made by quarks

Jets (originated by partons)

charged tracks


Electromagnetic calorimeter:cooling system, commissioning, calibration and monitoring, project coordination (2011-2012)

Hγγ candidate

Pixel detector: construction commissioning, data monitoring

Data analysis: Hγγ, HWW, Hττ, Search for heavy gauge and scalar bosons, Heavy flavour physics

Milano-Bicocca in CMS

γ1

γ2

Computing: INFN coordination

The Compact Muon Solenoid


Higgs boson production at pp colliders

Higgs couples to quarks, and bosons; initial and final states include composite hadrons Not an easy problem: (leading theory group in the field at Milano-Bicocca)

Nov 20th, 2013

Gluon-gluon fusion ~ 500 kevts in data

Vector boson fusion ~ 40 kevts in data

Two forward jets

LEVEL-1 Trigger Hardwired processors (ASIC, FPGA) Pipelined massive parallel

HIGH LEVEL Triggers Farms of

processors

10-9 10-6 10-3 10-0 103

25ns 3µs hour yearms

Reconstruction&ANALYSIS TIER0/1/2

Centers

ON-line OFF-line

sec

Giga Tera Petabit


Higgs detection at the LHC Higgs boson mass

from decay products:

“Invariant mass” of final state particles originated by the Higgs boson decay

The Higgs Hunter’s vademecum Identify decay products Measure with high resolution the

energy and the momentum Search for a (narrow) peak in the

invariant mass spectrum

Probability of Higgs boson decay to a final state

Discovery channels: H ZZ(*) 4 leptons H γγ

Nov 20th, 2013


Experimental challenges Trigger

Total collision rate 40 MHz Storage capability ~300 Hz

Must trigger efficiently Higgs boson candidates (and other processes)

Crowded environment Pileup events affect ‘particle isolation’

and energy reconstruction

Rejection/mitigation of background events:

E.g. in the Hγγ case: Fake photons from neutral pions in

γ+jet, or dijet events Prompt diphoton production

Fine transverse segmentationHigh invariant mass resolution

Nov 20th, 2013

ATLAS

2010O(2) collisions

per beam crossing

2011O(10) collisions

per beam crossing

2012O(20) collisions

per beam crossing


The Higgs boson signal: HZZ*

Signal significance ATLAS: 6.6 standard deviations

MH = 124.3 +/- 0.5 +/- 0.5 GeV CMS: 6.7 standard deviations

MH = 125.8 +/- 0.5 +/- 0.2 GeV

Nov 20th, 2013

H ZZ* μ+μ-e+e-


The Higgs boson signal: H γγ

Signal significance ATLAS: 7.4 standard deviations

MH = 126.8 +/- 0.2 +/- 0.7 GeV CMS: 3.9 standard deviations

MH = 125.4 +/- 0.5 +/- 0.6 GeV

Nov 20th, 2013

H γγ


Restrospective view Comparison between HZZ* results and projections from the first

LHC Workshop in 1990 (I was a PhD student atin DELPHI though)

Nov 20th, 2013

From Marumi Kado

Simulation for three mass hypotheses

Much better than anticipated


Is it the SM Higgs boson?Nov 20th, 2013

Test, spin, couplings, etc. Example: Measurement of the modifiers of the SM Higgs

coupling to gauge bosons and fermionsAll measurements so far consistent with the SM

κf

kf and κV

κv

This Higgs boson couples to fermions: not a trivial result!


Yet, the standard model is not enough E.g. dark matter:

An experimental evidence that there is more than the standard model

Astrophysical evidence from rotation curves, gravitational lensing, bullet clusters

Six times more abundant than ordinary matter

¼ of the total energy budget of the Universe

Particle candidates proposed in several models

Detection at LHC: Jet of hadrons or photon Missing energy

(Dark matter footprint)

Nov 20th, 2013

photon partpn jet

Dark matterparticles


Monojet eventsNov 20th, 2013

• No anomalies observed compared to known (SM) processes

Adapted from Sh.Rahatlou


Adapted from Sh.Rahatlou


Prospects for the next runs Found a scalar boson: its relatively small mass

leaves theoretical puzzles to be addressed:

EWK scale ~ 100 GeV ??? Mplanck = 1019 GeV

No evidence of new phenomena New gauge bosons excluded up to 3 TeV New fermions excluded up to 0.5 GeV

We have just gone through the first run! LHC running planned for the next ~20 y (through upgrades)

Resume operation in 2015: Centre of mass energy: 13 TeV (a factor ~2 more) Integrated luminosity: 300 fb-1 (10 times more data)

Objectives: Precision measurements of the Higgs boson properties

Couplings to 5-10% to see deviations from SM Higgs phenomenology Search for new particles/phenomena at higher mass scales Rare processes

Nov 20th, 2013


Conclusions and outlook S’io fui del primo dubbio disvestito

per le sorrise parolette brevidentro ad un nuovo più fui irretitoe cominciai: “Già contento requievi,di grande ammirazion, ma ora ammirocom’io trascenda questi corpi lievi”.

Ond’ella, appresso d’un pio sospiro,li occhi drizzò ver me con quel sembiante che madre fa sovra figlio deliroe cominciò: “Le cose tutte quantehan ordine tra loro …

… sì come cader si può vederefuoco di nube, sì l’impeto primos’atterra torto da falso piacere… “

Dante, Paradiso, Canto I

1. There is a scalar boson compatible with the SM

2. and nothing else3. Still puzzled by the way

mass is given to massless particles

4. There is a higher level symmetry

5. Spontaneously broken

Let’s check this part of the theory in the next LHC run(s)

Nov 20th, 2013

The experimentalist's task

Documents

Transcript of The experimentalist's task