Energy Frontier: Probing the Standard Model at ATLAS and

Physik-Institut

PHY213 Kern- und Teilchenphysik II (FS 2020)

Energy Frontier: Probing the Standard Model at ATLAS and

CMS

Lea Caminada

lea.caminada@physik.uzh.ch

Overview

2

• LHC

• ATLAS and CMS

• Detector, particle identification and event reconstruction

• Testing the Standard Model

– Physics with jets

– Physics with W and Z bosons

The Large Hadron Collider (LHC)

27 km circumference proton-proton collider at CERN Largest and highest energy accelerator ever built Started operation in 2010

CERN accelerator complex

27 km circumference proton-proton collider at CERN Largest and highest energy accelerator ever built Started operation in 2010

LHC

5

Dipole cross section magnetic field configuration

• 9300 magnets

• 1232 dipole magnets

• B_max = 8.3T

• ”2 in 1” concept:

both beams share same mechanical structure and cryostat (1.9K)

• E = 7 TeV

• E_inj = 450 GeV

13 TeV Beam Energy 10³⁴cm^-2s^-1 Design luminosity 2808 Bunches/beam 10¹¹ Protons/bunch

Bunch crossing 4Ÿ10⁷Hz (40 MHz) Proton proton collisions 10⁹Hz

Parton collisions

Higgs production 10^-5 Hz (Hà4l)

LHC parameters

6

LHC Fill

• An LHC fill usually lasts a few hours

• After injection and

acceleration, the beams are

– first, focused

– then, brought into collision

• LHC then “declares stable beams” à experiments start data-taking

• Luminosity decreases over the course of the run

• Divide run into “luminosity sections” (also called

“luminosity blocks”)

7

LHC luminosity

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1 Hz/nb = 10³³cm^-2s^-1

à Maximum instantaneous luminosity at the LHC 50 times higher than the Tevatron

• Average number of proton-proton collisions per bunch crossing

• s: inelastic proton-proton cross section

• L: instantaneous luminosity

• f: revolution frequency

• n_b: number of bunches

Pileup

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à Major challenge for physics at the LHC experiments

Pileup in CMS

Pileup in ATLAS

p_T > 0.5 GeV

Pileup in ATLAS

p_T > 2 GeV

Pileup in ATLAS

p_T > 10 GeV

Event reconstruction

14

• Every event is complicated!

• “Underlying event”:

– Initial state radiation

– Interactions of other partons in proton

• Additional pp interactions

– up to an average of 40 pileup events

• Many forward particles escape detection

– Transverse momentum ~0 – Longitudinal momentum >>0

H ®ZZ®µ⁺µ^-µ⁺µ^-)

p_T p_z pq

ATLAS and CMS

… and the heaviest particle detectors ever built

• Two general-purpose experiments at LHC:

The largest…

44 m x 22 m 7000 t

21 m x 15 m 14000 t

ATLAS and CMS

• General-purpose detectors à a wide physics program

Main interest

s_inel

ATLAS Detector

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(2 T)

22 m

Total weight:

7'000 tonnes

x z

y

CMS Detector

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Tracker (Pixels+Strips)

Electromagnetic

calorimeter (PbWO4 crystals) Hadronic sampling calorimeter

Muon chambers

3.8 T magnet

15 m

Total weight:

14'000 tonnes

• Track reconstruction is not just about the reconstruction of charged particles

• Tracks are used in almost every element of the event reconstruction

– Leptons

– Particle-flow jets – Primary vertices

– Pileup removal for jets and missing energy

– Jet flavor tagging

Tracking at the LHC

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Primary vertex reconstruction

Pileup removal

Jet flavor tagging

Tracking at CMS

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• Tracking system made of layers of silicon pixel and strip detectors

• 3 key characteristics of a good tracking detector

– High efficiency – Low fake rate – Good resolution

• Advantage of CMS

– High magnetic field (3.8T) à good momentum resolution and separation of charged and

neutral particles

now 4 pixe

l laye rs

Perfect detector Poor resolution Poor resolution and low efficiency

CMS pixel detector

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Pixel detector closest to the collision point

à Built from four cylindrical layers and three endcap disks on each side à Each layer segmented in pixels (3D

space points)

Installed@CERN

CMS pixel detector

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CMS pixel detector

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CMS Pixel Detector is built from more than 1000 sensor modules

2cm 160 pixels

6.6cm 416 pixels

Sensor made from silicon

Segmented in 66’560 active pixels Pixel size is 100x150 μm²

(Thickness of a hair 50-100 μm)

CMS pixel detector module

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à Supply voltages, programming signals, readout signals

à Electronics

à Particle detection

à Electronics

à Mechanical part

Hybrid pixel detector

1 cm

500 um 300 um20 um180 um

• Pixelated sensor is connected to pixelated readout electronics

• Charged particles passing through silicon sensor generates e/h pairs through ionization

Electronics chip Single pixel cell

Sensor pixel cell

Bump bond

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Charge deposited in sensor:

Q = E/E_i e

E: deposited energy E_i: ionization energy e = 1.602 10^-19 C

Example: MIP in 300um Si E = 100 keV (Bethe Bloch) E_i (Si) = 3.6 eV

à Q = 4fC. Tiny current!

à Amplification and processing in readout electronics

Particle Identification at CMS

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Missing energy

• Particles such as neutrinos are invisible to the detectors à their signature is a missing signature, identified by an

energy imbalance in the plane transverse to the beams

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• Missing transverse

momentum (E_T^miss, MET) is defined as the negative sum of the transverse momenta of all reconstructed particles

Jet reconstruction

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Reconstruction of short-lived particles

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• The presence of particles that decay before reaching the detector is inferred from the reconstruction of their decay products

– e.g. t lepton, heavy mesons and baryons (B, D, L), top quark, W and Z bosons, Higgs boson, …

• Example of t lepton

m(t) = 1.78 GeV m(p) ~ 140 MeV m(K) ~ 500 MeV m(D)~ 1.86 GeV

64% hadronically 36% leptonically

Reconstruction of t leptons

• Start from reconstructed jet

• Decay mode reconstruction (signal cone):

– Leptons

– Charged tracks (p^±) (1 or 3-prong) – ECAL clusters (p⁰)

• t identification

– Isolation – Lifetime – Kinematics

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A reconstructed event

Try it yourself: http://opendata.cern.ch/visualise/events/cms

From collision data to physics analysis

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Event size ~1MB

Full data rate ~ 60TB/s

Accept

Accept

✘

or discard

✘

From collision data to physics analysis

33

Event size ~1MB

Full data rate ~ 60TB/s

Accept

Accept

✘

or discard

✘

From collision data to physics analysis

34

Event size ~1MB

Full data rate ~ 60TB/s

Accept

Accept

✘

or discard

✘

Standard Model cross section measurements as test of QCD

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• Jets

• W and Z bosons

How to measure a cross section?

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• Differential cross section: ds/dW:

– Probability of a scattered particle in a given state per solid angle dW

• Other differential cross sections: ds/dE_T(jet) – Probability of a jet with given E_T

• Integrated cross section – Integral: s =∫ds/dW dW

s=(N_obs-N_bg)/(AeL_int)

Measurement:

N_obs: Number of observed events N_bg: Number of background events A: Detector acceptance

e: Efficiency

L_int: Integrated luminosity

N_tot = s L_int

Jet cross sections

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Inclusive jets: processes qq, qg, gg

§ Highest E_T probes shortest distances

§ LHC: r_q<10^-19 m

§ Could e.g. reveal substructure of quarks

§ Tests perturbative QCD at highest energies

Jet cross section measurements

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20 GeV up to 2 TeV (central region)

NLO perturbative QCD describes

data over 17 orders of magnitude!

Jet cross section measured as a function of p_T in bins of rapidity y

Di-jet mass spectrum

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Phys. Rev. D 96, 052004 (2017)

Excellent agreement between NLO theory and data

over eight orders of magnitude

“Bump hunt”:

Heavy resonances decaying to two jets (predicted by any strongly interacting new physics)

excluded up to few TeV

High-pt di-jet event

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mass_di-jet = 7.7 TeV

W and Z bosons

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• Focus on leptonic decays:

– Hadronic decays close to impossible due to enormous QCD dijet

background

• Selection:

– Z:

– Two leptons (e⁺e^-,µ+µ^-,t+t^-) p_T>20 GeV

– W:

– One lepton (e,µ,t) p_T>20 GeV – MET > 20 GeV

• Excellent calibration signal for many purposes:

– Electron energy scale – Track momentum scale

– Lepton ID and trigger efficiencies – Missing E_T resolution

– Luminosity …

42

• Z mass reconstruction

– Invariant mass of two leptons

– Sets electron energy scale by comparison to LEP measured value

• W mass reconstruction

– Do not know neutrino p_Z

– No full mass reconstruction possible

– Transverse mass:

W and Z boson mass reconstruction

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Di-muon spectrum

• Z events provide excellent tool for measurement of lepton reconstruction efficiency and resolution

• Efficiency in simulation corrected by scale factors to match efficiency measured in data

• Electron and muon resolution measured at <<1% precision

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Performance of lepton reconstruction

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W and Z boson cross section measurement

• Measured W and Z boson cross section compared to theoretical prediction obtained with different PDFs

Phys. Lett. B 759 (2016) 601

Sensitivity to PDFs

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• Measurement of rapidity distribution sensitive to flavor content of proton

• Can be used to get improved Parton Distribution Functions (PDFs) and thus more precise cross section predictions

Theoretical prediction

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• Higher-order calculations in perturbative QCD are crucial for precision tests of the Standard Model at LHC

• NLO and NNLO corrections have strong effects on cross sections and kinematic distribution

• Rapidity distributions are symmetric, but different for W⁺/W^-

C. Anastasiou

Measurement of differential cross sections

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Summary of Standard Model measurements at LHC

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W and Z

References

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• Stefanos Leontsinis, ”The CMS experiment”, KT2 2018

• https://www.physik.uzh.ch/de/lehre/PHY213/FS2018.html

• Annapaola de Cosa, “Higgs discovery”, KT2 2017

• https://www.physik.uzh.ch/de/lehre/PHY213/FS2017.html

• Beate Heinemann, “Hadron Collider physics”, 2010

• https://www.desy.de/~bheine/homepage/publictalk.html

• Babis Anastasiou, “Physics at Large Hadron Collider and challenges for perturbative calculations”,

Coordinate system

Pseudorapidity h = - ln tan(q/2)

Right-handed coordinate system:

• x-axis points toward interaction point

• y-axis points upwards

• z-axis points along the beamline

CMS tracking detector covers up to here