Marina Cobal

Università di Udine

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Physics at Hadron Colliders Part 2

QCD Approach: Quarks & Gluons

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Parton Distribution Functions

Q2 dependence predicted from QCD

Quark & Gluon Fragmentation Functions

Q2 dependence predicted from QCD

Quark & Gluon Cross-Sections

Calculated from QCD

Proton-Proton Collisions at the LHC

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Elastic Scattering

Single Diffraction

M

σtot = σEL + σSD + σDD + σND

Double Diffraction

M1

M2

Proton Proton

“Soft” non-diff. (no hard scattering)

Proton Proton

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event

Initial-State Radiation

Final-State Radiation

“Hard” non-diff. (hard scattering)

Underlying

Event

Hard Core

1.8 TeV: 78mb = 18mb + 9mb + (4-7)mb + (47-44)mb

The “Min-Bias” trigger picks up most of the “hard core” cross-section plus a small amount of single & double

diffraction.

The “Non diffractive” component contains

both “hard” and “soft” collisions.

Beam-Beam Counters

σtot = σEL + σIN

The structure of an event

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One incoming parton from each of the protons enters the hard process, where then a number of outgoing particles are produced. It is the nature

of this process that determines the main characteristics of the event.

Hard subprocess: described by matrix elements

An event: resonances

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The hard process may produce a set of short-lived resonances, like the Z0/W± gauge bosons.

Resonances

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•In this range the momentum scale is known at the permill level. • it is a cross-check of the detector performance in particular for the lepton energy measurements

The structure of an event: ISR

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One shower initiator parton from each beam may start off a sequence of branchings, such as q → qg, which build up

an initial-state shower.

Initial state radiation: spacelike parton shower

The structure of an event: FSR

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The outgoing partons may branch, just like the incoming did, to build up final-state showers.

Final state radiation: timelike parton showers

An event: Underlying events

•  Proton remnants ( in most cases coloured! ) interact: Underlying event,consist of low pT objects.

• There are events without a hard collision ( dependent on pT cutoff)

Underlying Event

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•  Studying underlying event is crucial for understanding high pT SM events at LHC.

•  ingredient for many analyses. In fact they affect:

the jet reconstructions and lepton isolation, jet tagging etc..

•  One can look at charged track multiplicities Nch in transverse regions which are little affected by the high pT objects. •  Reasonably described by models

TransverseRegion

TransverseRegion

Toward Region

Away Region

3/ = q6

3/2 = q6

3/- = q6

3/-2 = q6

Leading Charged-Particle Jet = 0q

> [G

eV]

Tp<

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8R=0.2 Transverse region

ATLAS

[GeV]jetTp

10 20 30 40 50 60 70 80 90 100

MC

/DAT

A

0.8

1

1.2

QCD Monte-Carlo Models: High PT Jets

•  Start with the perturbative 2-to-2 (or sometimes 2-to-3) parton-parton scattering and add initial and final-state gluon radiation (in the leading log approximation or modified leading log approximation).

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•  The “underlying event” consists of the “beam-beam remnants” and from particles arising from soft or semi-soft multiple parton interactions (MPI).

•  Of course the outgoing colored partons fragment into hadron “jet” and inevitably “underlying event” observables receive contributions from initial and final-state radiation.

The “underlying event” is an unavoidable background to most collider observables and having good understand of it leads to

more precise collider measurements!

The structure of an event: Pile up

In addition to the hard process considered above, further semi-hard interactions may occur between the partons of two other incoming hadrons.

‘Pile-up’ is distinct from ‘underlying events’ in that it describes events coming from additional proton-proton interactions, rather than additional

interactions originating from the same proton collision. ���

Pile up

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2012 ATLAS event; Z in µµ with 25 primary vertices

Z in µµ event with 25 vertices

•  Multiple interactions between partons in other protons in the same bunch crossing –  Consequence of high rate

(luminosity) and high proton-proton total cross-section (~75 mb)

•  Statistically independent of hard scattering –  Similar models used for soft

physics as in underlying event

Et ~ 58 GeV

Et ~ 81 GeV without pile-up

Prog.Part.Nucl.Phys.60:484-551,2008

Pile up

Et ~ 58 GeV

Et ~ 81 GeV with design luminosity pile-up

Prog.Part.Nucl.Phys.60:484-551,2008

Pile up

•  Multiple interactions between partons in other protons in the same bunch crossing –  Consequence of high rate

(luminosity) and high proton-proton total cross-section (~75 mb)

•  Statistically independent of hard scattering –  Similar models used for soft

physics as in underlying event

Challenge Pile up: example ETmiss

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•  Strict requirements on track vertexing

•  Number of reconstructed vertices proportional to the pile-up

•  Measure pile-up density event by event: Use it to subtract from the jets energy a pile-up term. do the same with isolation cones.

•  For evaluation of the pile-up given the mismodelling of min bias, have to use data!

without PU suppression

with PU suppression

Important for quantities, affected by soft hadrons, for example; ET

miss = -| Σ pT |

σtot=σ

EL+σ

SD+σ

DD+σ

ND

•  Inelastic hadron-hadron events selected with an experiment’s “minimum bias trigger”.

•  Usually associated with inelastic non-single-diffractive events (e.g. UA5, E735, CDF … ATLAS?)

Minimum bias events

¡  Need minimum bias data if want to: 1)  Study general characteristics of

proton-proton interactions 2)  Investigate multi-parton

interactions and the structure of the proton etc.

3)  Understand the underlying event: impact on physics analyses?

¡  In parton-parton scattering, the UE is usually defined to be everything except the two outgoing hard scattered jets: Beam-beam remnants.

1)  Additional parton-parton interactions.

2)  ISR + FSR ¡  Can we use “minimum bias” data to

model the “underlying event”? Ø  At least for the beam-beam

remnant and multiple interactions?

The underlying event

¡  The “soft part” associated with hard scatters

Minimum bias

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•  Non head-on collisions, with only low pT objects. Those are the majority of the events in which there is a small momentum transfer

Δp ~ h/Δx

•  Distributed uniformly in η: dN/dη = 6 •  On average the charged particles in the final

state have a pT~500 MeV

Not well described by models! Shape is sort of OK Normalisation is off

Minimum bias

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•  It is interesting by its own to study such events. Also an ingredient for many analyses you will see.

•  A necessary first step for precision measurements (such as top-quark mass)

•  A key ingredient to modelling pile-up •  As can be seen most of the events do have

quite low pT

•  Anyhow those events constitue a noise of few GeV per bunch crossing

A Monte Carlo Event

Initial and Final State parton showers resum the large QCD logs.

Hard Perturbative scattering:

Usually calculated at leading order in QCD, electroweak theory or some BSM model.

Perturbative Decays calculated in QCD, EW or some BSM theory.

Multiple perturbative scattering.

Non-perturbative modelling of the hadronization process.

Modelling of the soft underlying event

Finally the unstable hadrons are decayed.

SM processes

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•  No hope to observe light objects ( W,Z,H) in the fully hadronic final state! •  We need to rely on the presence of an isolated lepton!

•  Fully hadronic final states can be extracted from the backgrounds only with hardO(100 GeV) pT cuts-> works for heavy objects!

QCD/Electroweak sector

Where do Jets come from at LHC?   Fragmentation of gluons and (light)

quarks in QCD scattering   Most often observed interaction

at LHC

  Decay of heavy Standard Model (SM) particles   Prominent example:

  Associated with particle production in Vector Boson Fusion (VBF)   E.g., Higgs

  Decay of Beyond Standard Model (BSM) particles   E.g., SUSY

t→ bW → jjjt→ bW → lν jj

1.8 TeVs =

14 TeVs =

pT

(TeV)

inclusive jet cross-section

q q→ "q "qWW → Hjj

dσ 2

dηdpT η=0

nb

TeV

Where do Jets come from at LHC?

Fragmentation of gluons and (light) quarks in QCD scattering Most often observed interaction at LHC

Decay of heavy Standard Model (SM) particles Prominent example:

  Associated with particle production in Vector Boson Fusion (VBF)   E.g., Higgs

  Decay of Beyond Standard Model (BSM) particles   E.g., SUSY

t → bW → jjj

t → bW → lν jj

qq q q WW Hjjʹ′ ʹ′→ →% %

top mass reconstruction

Where do Jets come from at LHC?

Fragmentation of gluons and (light) quarks in QCD scattering

  Most often observed interaction at LHC

  Decay of heavy Standard Model (SM) particles   Prominent example:

  Associated with particle production in Vector Boson Fusion (VBF)   E.g., Higgs

  Decay of Beyond Standard Model (BSM) particles   E.g., SUSY

t→ bW → jjjt→ bW → lν jj

q q → ′q ′qWW → Hjj

η

Where do Jets come from at LHC?

  Fragmentation of gluons and (light) quarks in QCD scattering   Most often observed interaction

at LHC

  Decay of heavy Standard Model (SM) particles   Prominent example:

  Associated with particle production in Vector Boson Fusion (VBF)   E.g., Higgs

  Decay of Beyond Standard Model

(BSM) particles   E.g., SUSY

t→ bW → jjjt→ bW → lν jj

q q→ "q "qWW → Hjj

electrons or muons jets

missing transverse

energy

,jets

,leptons

Te f Tjf T ppM p= + +∑ ∑ l

Jets •  Definition (experimental point of view):

bunch of particles generated by hadronisation of a common confined source –  Quark-, gluon fragmentation

•  Signature –  energy deposit in EM and HAD calos

Several tracks in the inner detector

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•  Calorimeter energy measurement

- Gets more precise with increasing particle energy

- Gives good energy measure for all particles except µ’s and ν’s

- Does not work well for low energies

Particles have to reach calorimeter, noise in readout

Jet Reconstruction Task

Jet Reconstruction •  How to reconstruct the jet?

–  Group together the particles from hadronization

–  2 main types •  Cone •  kT

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Requirements to jet algorithm choices

•  Infrared safety –  Adding/removing soft particles should

not change the result of jet clustering •  Collinear safety

–  Splitting of large pT particle into two collinear particles should not affect the jet finding

•  Invariance under boost –  Same jets in lab frame of reference as in

collision frame •  Order independence

–  Same jet from partons, particles, detector signals

•  Many jet algorithms don’t fulfill these requirements!

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Jet reconstruction algorithms: cone

What is a jet?

Jet physics: jet energy scale Before looking at jet physics be aware of few issues, first of all when we have steeply falling cross sections-> we have a sensitivity of its measurement from the energy scale -Jet energy determined from calorimeter (+tracking information) -Sophisticated calibration procedure

Different contributions to JES error. (jets reconstructed with the Anti-kT alogrithm cone 0.6 that is used in ATLAS)

Jet physics: JES calibration from data

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Different physics processes can be used to calibrate the JES. - recoil against Z and photons -reconstruction of W’s in ttbar events Such methods are useful for different energy ranges and can be used at different ECM

Jet production

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•  NLO QCD works over ~9 orders of magnitude! •  excellent exp. progress: jet energy scale

uncertainties at the 1-2% level •  for central rapidities: similar exp. and theo.

uncertainties, 5 - 10% •  inclusive jet data : starts to be important tool for

constraining PDFs, eg.also by using ratios at different c.o.m. energies

Searches with Jets

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•  But..How come both ATLAS and CMS are showing search results from 8 TeV data, but jet cross sections from 7 TeV data?”

•  It is linked to the 10% differences in x-

section predictions

– If I want 10% precision on the cross section, I need about 2% precision on the jet energy scale. This is about the state of the art today. – In contrast, there is no requirement for an absolute background prediction in the searches: the only requirement is that the background be smooth.

Jet multiplicity

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•  Another possible test of QCD is obtained by checking the jet multiplicity •  Tests also the modelling of the radiation