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LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons HUGS 2015 Bolek Wyslouch

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Page 1: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons

HUGS 2015

Bolek Wyslouch

Page 2: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

Techniques to study the plasma

2

Radiation of hadrons Azimuthal asymmetry and radial expansion

Energy loss by quarks, gluons and other particles Suppression of quarkonia

Page 3: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

Energy loss by quarks, gluons and other particles

•  Equivalent of x-ray of the plasma, the loss of energy can tell us about the density, composition and the microscopic structure of the plasma

•  We use probes created during the elementary collisions between the initial quarks and gluons –  Large transverse momentum quarks or gluons

appearing as jets –  Particles that do not interact strongly can be used as

a reference: Z, W, photon

3

Page 4: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

4

Probe the medium

External source

Material

l  Goal: Understand the property of QGP

l  Problem: the lifetime of QGP is so short (O(fm/c)) such that it is not feasible to probe it with an external source.

l  Solution: Take the advantage of the large cross-sections of high pT jets, γ/W/Z, quarkonia at the LHC energy, use hard probes produced with the collision.

Page 5: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

5

Three types of hard probes

Electroweak probes W/Z bosons, high pT γ

Quarks and gluons Jets

Quarkonium J/ψ, Υ family

QGP QGP γ Jet QGP

C C

Probe the initial state Probe the opacity of QGP Sensitive to the temperature of QGP

Page 6: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

6

Factorization

proton

proton

Page 7: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

7

Factorization

proton

proton

Parton Distribution Function (PDF)

high-pt processes result from interaction between parton constituents [quarks and gluons] of the incoming hadrons •parton content of hadrons described by parton distribution functions

Page 8: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

8

Factorization

Gluon

proton

proton

Cross-section of 2à2 process

Quark Quark

Parton Distribution Function (PDF)

Page 9: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

9

Factorization

Gluon

Cross-section of 2à2 process

Quark Quark

Nuclear Parton Distribution Function (nPDF)

Page 10: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

10

How do we extract the medium effect in PbPb collisions?

pp reference PbPb measurements

One typical way is to compare PbPb data to pp reference measurement

Page 11: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

11

How do we extract the medium effect in PbPb collisions?

pp reference PbPb measurements

One typical way is to compare PbPb data to pp reference measurement

Ncoll à Number of binary scatterings

Npart à Number of participating nucleons

Npart = 2 Ncoll = 1

Npart = 5 Ncoll = 6

Example:

Page 12: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

12

How do we extract the medium effect in PbPb collisions?

RAA < 1 (suppression) dηdpσddηdpNd

Nσ

=RTpp

TAA

coll

inelpp

AA //

2

2 “QCD Medium”

“QCD Vacuum”

RAA > 1 (enhancement)

RAA = 1 (no medium effect) ~

pp reference PbPb measurements

inelpp

collAA σ

N=T ''NN equivalent integrated luminosity per AA collision'‘ Reduces the uncertainty from pp inclusive cross-section

One typical way is to compare PbPb data to pp reference measurement

‘Nuclear modification factors’

Can also be written as 1/TAA

Ncollà Averaged number of binary scattering

Page 13: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

13

How do we extract the medium effect in PbPb collisions?

RAA < 1 (suppression)

“QCD Medium”

“QCD Vacuum”

RAA > 1 (enhancement)

RAA = 1 (no medium effect) ~

pp reference PbPb measurements

One typical way is to compare PbPb data to pp reference measurement

‘Nuclear modification factors’

Questions: How do we know the Glauber model calculation of Ncoll is correct?

Is the nuclear PDF modified with respect to nucleon PDF? Motivates the studies of electroweak probes

dηdpσddηdpNd

Nσ

=RTpp

TAA

coll

inelpp

AA //

2

2

Ncollà Averaged number of binary scattering

Page 14: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

14

Electroweak probes •  High pT Photons, W and Z bosons:

• Colorless à Not affected by the QGP • Good theoretical control • Check the validity of Ncoll calculation (ex. from Glauber Model) • Constraint the nuclear parton distribution function (nPDF)

ArXiv:1103.1471 PbPb 5.5 TeV

ArXiv:1010.5392

nucl-ex/0701025v1

Photons Z bosons

Page 15: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

15

Photons

•  Ideally: LO photons from hard scattering •  Real world: huge background from the decay and fragmentation photons •  Need a consistent definition between measurements and theoretical calculations

LO NLO

Page 16: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

16

Isolated high pT photons l  Solution: measurement of the isolated photons l  Decay photons from hadrons in jets such as π0, ηà γ γ are largely suppressed l  UE subtracted isolation variables are developed

γ

Isolated Isolated Non-isolated

LO NLO

γγ

same object to the detector

Isolated Non-isolated

Page 17: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

17

Isolated photon RAA

l  No modification of the photons as expected!

0-10% PbPb compared to pp

Theory

Page 18: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

18

Zàµ+µ- Zàe+e-

Z boson production in PbPb collisions

Page 19: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

19

Z boson production in PbPb collisions

l  No modification is found with respect to the pp reference

l  Normalized yield is not varying as a function of centrality

2010 data

2011 data

Page 20: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

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W boson

µ

Wàµυ

Wàµυ Single high pT µ + Missing pT

µ

υ

W φ

Transverse mass

Page 21: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

21

W boson RAA

RAA(W) = 1.04 ± 0.07 ± 0.12

l  Normalized yield is not varying as a function of centrality

Page 22: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

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W boson RAA

RAA(W) = 1.04 ± 0.07 ± 0.12 RAA(W+) = 0.82 ± 0.07 ± 0.09 RAA(W–) = 1.46 ± 0.14 ± 0.16

l  Isospin effect is seen if we differentiate W+ and W-

Page 23: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

23

Summary of electroweak probes •  Electroweak probes are unmodified •  Confirmed Ncoll scaling of hard scattering •  Constraint nuclear Parton

Distribution Function

Ncoll scaling

pp

PbPb

nPDF

PDF

Page 24: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

24

How about quarks and gluons?

Gluon

Quark

•  Quarks and gluons in pp collisions

Page 25: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

25

How about quarks and gluons? •  Want to measure quarks and gluons which carry

color charge and see how they interact with QGP

Gluon

Quark

Page 26: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

26

Quarks and gluons

Color confinement:

Quarks and gluons à groups of hadrons

Page 27: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

27

How about quarks and gluons? •  Want to measure quarks and gluons which carry

color charge and see how they interact with QGP

Gluon

Quark

Page 28: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

28

How about out going quarks and gluons?

Hadrons

Hadrons

“Fragmentation”

“Fragmentation”

Jet

Jet

•  Want to measure quarks and gluons which carry color charge and see how they interact with QGP

•  à Practically: measure hadrons and jets

Gluon

Quark

Page 29: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

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An easier measurement: charged particle RAA

Provide constraints on the parton energy loss models

R AA=σppinel

⟨N coll⟩d2N AA/dpT dηd2σ pp/dpT dη

“QC

D M

ediu

m”

“QC

D V

acuu

m”

~

Ncoll validate by photons W/Z bosons

If PbPb = superposition of pp ...

Page 30: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

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Charged particle spectra

Absorption? Energy loss?

Single hadron spectra itself do not provide details of the underlying mechanism

à Need direct jet reconstruction and correlation studies

Page 31: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

31

Jet events in PbPb collisions at LHC ATLAS

CMS

Page 32: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

32

Jet reconstruction

Need rules to group the hadrons A popular algorithm is anti-kT algorithm

Used in ALICE, ATLAS and CMS analyses

Cacciari, Salam, Soyez, JHEP 0804 (2008) 063

Small radius parameter à jet spliting

Large radius parameter

ΔR = 0.2, 0.3, 0.4, 0.5 are used in LHC analyses

Radius parameter: decide the resolution scale

Page 33: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

33

Jet composition

On average, charged hadrons carry 65% of the jet momentum

Measure the known part Correct the rest by MC simulation

Goal:

•  Make use of the redundancy of measurements from calorimeter and tracker

•  Improve the sensitivity to low pT particles in jet àReduce the dependence on MC

(ex: PYTHIA)

Optimize the use of calorimeter and tracker Example: “Particle Flow” in CMS A typical high pT jet

Page 34: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

34

Underlying event background

Multiple parton interaction

Large underlying event from soft scattering

Jet

Need background subtraction

ATLAS

Page 35: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

35

Summary of jet reconstruction

Remove underlying events contribution

MC Simulation PYTHIA

Raw jet energy Background subtraction

Jet energy correction Jet energy

correction

Page 36: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

36

Three possible scenarios

Soft collinear radiation

Hard radiation Large angle soft radiation

“QGP heating”PYTHIA inspired models Modified splitting functions AdS/CFTGLV + others

To explain the suppression of high pT particles

Page 37: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

37

Inclusive jet RAA, RCP

Track Jet Calorimeter Jet Strong suppression of inclusive high pT jets! A cone of R=0.3, 0.4 doesn’t catch all the radiated energy

Compare PbPb to PYTHIA (pp generator) RCP: Compare to

Page 38: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

38

Correlation study: Di-jet imbalance

Small AJ (Balanced dijet)

Large AJ (Un-balanced dijet)

Page 39: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

39

Correlation study: Di-jet imbalance

Parton energy loss is observed as a pronounced energy

imbalance in central PbPb collisions No apparent modification in the dijet Δφ distribution

(Dijet pairs are still back-to-back in azimuthal angle)

Small AJ (Balanced dijet)

Large AJ (Un-balanced dijet)

Δφ

π π π π

0-10% 10-20% 20-40% 40-100%

Δφ Δφ Δφ Δφ

Page 40: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

40

Low pT jets in PbPb collisions

ΔηpT Δϕ Δϕ ΔϕΔη Δη

0-10% 60-70% pp

2 < pT,trig < 3 1 < pT,assoc < 2

à Motivates jet shape analysis and fragmentation function with low pT particles

à  Look at low pT reconstructed jet

Two particle correlation from ALICE: Jet like near side correlation with background subtraction Strong centrality dependence, widening of the angular correlation

Page 41: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

41

Challenge of jet as a trigger: surface bias

•  Selection on a high pT leading jet (charged particle) may bias the position of the hard scattering in the QGP

High pT leading jet Triggered sample

All hard collisions Can happen in any place in the QGP

Page 42: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

42

How about correlate photons and jets?

Quark

Gluon

Hadrons

Jet

Photon

photon+jet

Surface bias is removed!

pTphoton ~ pTJet

“quark-gluon compton scattering”

Page 43: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

43

Photon jet angular correlation

pp

Azimuthal angle between photon and jet

PbPb

pppp

“QGP Rutherfold experiment”

Jet

Jet

Photon

Photon“Backscattering?”

Page 44: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

44

Compare photon-jet momentum balance

xjg = pTJet/pTphoton in vacuum (pp collision) to the

QGP (PbPb collision)

PbPb

pp

Quarks lose about 15% of their initial

energy

In addition, 20% of photons lose their jet

partner

PbPb PbPb

Photon-jet momentum balance

Page 45: LHC Heavy Ion Physics Lecture 5: Jets, W, Z, photons...T jets, γ/ W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. 5 Three types of hard probes Electroweak

Summary

•  Quarks and gluons lose a lot of energy traversing the hot nuclear medium: huge effect!

•  It is transparent to W, Z, γ •  Detailed theoretical studies of “where the energy

goes” and details of energy loss models are work in progress

45


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