Jet Physics at the LHC

1
Jet Physics at the LHC

• Ivan Vitev, Nuclear Theory, T-16 , LANL

Thanks to Camelia Mironov and Simon Wicks
P-25 Nuclear Physics seminar, LANL, Los Alamos, NM
2
What this seminar is all about

• The opportunity that exists for a group of
• theorists and experimentalists at LANL to
• make a major contribution to fundamental
• high energy and high energy nuclear physics
• within the Standard model and beyond

• Use the CMS experiment at the LHC as
• opposed to more traditional heavy ion
• experiments

3
Outline of the Talk

• The physics case for the LHC
• Discovery of the Higgs, supersymmetry, search
  for extra dimensions
• New states of matter, quark-gluon plasma, jets
  in nuclear collisions
• Definition of jets and jet observables
• Historical overview, jet algorithms and jet
  variables
• Calculating jet cross sections in perturbative
  QCD
• Calculating jet shapes and comparison to
  Tevatron data
• Jets in nuclear collisions
• The contribution of medium induced
  bremsstrahlung
• Strategy for combining the vacuum and the
  medium-induced radiation
• Defining a sensitive observable for the jet
  interactions in matter (QGP tomography)
• Leading single inclusive particles as proxies to
  jets
• Light particle quenching and limitations in
  sensitivity
• Dissociation new approach to D- and B-mesons
  suppression in the QGP
• Discriminating power of charm and beauty jets -
  including AdS/CFT conjectures

4
What is LHC?
pp _at_ pA _at_ AA _at_
LHC is what SSC should have been
CMS
LHC
And LHCb
ALICE
ATLAS
5
LHC capabilities hard probes
Hard probes high pT and ET, particles and jets,
heavy quarks and quarkonia, vector mesons and the
Higgs
CMS estimates for 106s provided by Camelia Mironov
I.V. hep-ph/0212109
For reference in pp s1/2 14 TeV, L 1034
cm-2s-1
6
Planned discovery of the Higgs

• Higgs
• The remaining scalar field after the
• spontaneous
• The only missing SM particle

M.Spira, Fortsch. Phys. 46 (1998)
J. Hogan, Nature 445 (2007)
7
Golden channels (Higgs)

• Branching ratios

• Detected via

Jet physics as the basis for Higgs searches
8
Planned discovery of Supersymmetry

• Theoretical appeal
• Based on tried and true symmetry
• principles
• No new gauge couplings
• Unification of the coupling constants
• Excellent candidate for cold dark
• matter

I would argue that the first discovery at the
LHC will not be the Higgs but supersymmetry
J. Ellis, CERN
colloquium
P. Mercadante, Braz. J. Phys. 46 (2004)
9
Discovery channels (supersymetry)

• Rich spectroscopy

• Detected via high jet multiplicity missing
  energy ( since there is lightest
• supersymmetric particle - stable neutralino
  
  )

Really high (gt10) jet multiplicities Understandin
g jets, jet energy flow, and QCD backgrounds is
critical for discovering physics beyond
the standard model

• The decay chain of the gluino

10
Jet definitions

• Jet variables

• Transverse energy
• (Pseudo) Rapidity
• Angle

R
Jets are collimated showers of energetic
particles that carry a large fraction of the
energy available in the collisions
Jet algorithms

• Kt algorithm preferred (collinear and
• infrared safe to all orders in PQCD)
• Cone algorithm

11
Jets from factorized PQCD

• Single and double inclusive hard production
• in PQCD - applicable from photons to heavy
• quarks

Pd
X
Pd / zd
Pc / zc
X
Pc
For jets
Caveat parton-hadron duality

• Jet cross sections are more inclusive and
  therefore more robust PQCD observables

CDF studies
12
The difficult path to jet physics

• Fixed target experiment

• Exponential spectrum

• Energy spectrum consistent with exponential soft
  string breaking mechanism. No indication of hard
  scattering (power law)

• Most of the HEP community doubts the model
• of hard scattering constituents in the proton
• (partons) around 1980

C. DeMarzo et al., NA5 Phys.Lett.B 112 (1982)
13
The discovery of jets
Paris 1982 the beginning of modern jet physics

• The highest energy jets measured at the
• Tevatron are of ET gt 600 GeV

M. Banner et al., UA2 Phys.Lett.B 118 (1982)
14
Jet cross sections comparison to LO and NLO PQCD
W. Horowitz, I.V., S. Wicks, in preparation

• Good comparison to the shape at LO.
• Meaningful K-factor

• Even better comparison at NLO.

15
Jet shape variables

• Much more stringent tests of the underlying PQCD
  theory

• Differential jet shape

r

• Example at CDF

16
Calculating the jet shape (I)

• Infrared safety theorem

• LO splitting functions

Kinoshita, T., 1962, J. Math. Phys. 3, 650
Lee, T.D., and M. Nuenberg, 1964, Phys. Rev.
B133, 1549

• Definition of a distribution

• QCD kernel

Note Kinoshita, Lee, Nauenberg does not
guarantee collinear safety
Obtain correct at r gtgt 0
17
Calculating the jet shape (II)

• Sudakov resummation (the way to deal with the
  collinear divergence)

M.H. Seymour, Nucl. Phys. B 531 (1998)
R
r
Obtain
correct at r -gt 0 but not r gtgt
0

• Matching correct behavior at both small and
  large

• Including power corrections

• Final expression resummed, matched, and power
  corrected

18
Comparison to Tevatron data (I)
CDF collaboration ( s1/2 1.96 TeV, improved
cone algorithm, R 0.7 )

• Sensitivity to fq (quark jet fraction)

• Sensitivity to Rsep (cone separation variable)

Rsep - min. separation to form 2 jets
Fq Fg 1 (Theoretically calculable)
Calculations done by Simon Wicks
W.Horowitz, I.V., S.Wicks, in preparation
19
Comparison to Tevatron data (II)
CDF collaboration ( s1/2 1.96 TeV, improved
cone algorithm, R 0.7 )

• Sensitivity to Q0
• (non-perturbative effects and
• power corrections)

• Good description of the differential Tevatron jet
  shape data
• Reasonable Rsep
• Reasonable Q0

Calculations done by Simon Wicks
W.Horowitz, I.V., S.Wicks, in preparation
20
Medium-induced bremsstrahlung
2








...


Organizing principle!
M.Gyulassy, P.Levai, I.V., Nucl.Phys.B594 (2001)
Phys.Rev.Lett.85 (2000)
21
Angular distribution to Ist order in opacity
Naive picture
Solution to first order in the mean of
scatterings
2Re
I.V., Phys.Lett.B 630 (2005)
22
Gluon energy and angular distributions

• Gluon energies range
• from

• Angles range from
• to

• Conditions central PbPb at the LHC

First order in opacity
23
Goal study of jet shapes in nuclear collisions
Vacuum
Consistent approach to calculating jet shapes in
nuclear collisions

• Clearly large effects - significant
• redistribution of the energy
• Non-Gaussian shapes, actual depletion
• near the center of the jet cone. As
• opposed to assuming
• Expect small contribution to Sudakov
• form factors

Medium - Induced
I.Lokhtin, A.Snigerev, Euro Phys. J. C 46 (2006)
24
Light hadrons as proxies to jets

• Very special jets atypical, sensitive
• to hadronization effects

Significantly different values are indicative of
theoretical inconsistency
I.V., Phys.Lett.B 639 (2006)
25

Single inclusive pion suppression at the LHC
Running
Fixed

• High pT suppression at the LHC can be
• comparable and smaller than at RHIC
• LHC quenching follows the steepness
• of the partonic spectra
• Reduced sensitivity to medium properties

S.Wicks et al., in progress
26
Direct photon at the LHC

• Direct photons also have limited sensitivity
  isospin effects,
• cold nuclear matter e-loss

Prompt photons

Fragmentation photons

• Better jet interaction
• measures?

• Direct photon tagging
• can be compromised by
• fragmentation photons

27
Z0 / Virtual Photon - tagged jets

• A better way of tagging

Although the rates are smaller, they
are sufficient to pinpoint the jet interactions
In matter
Calculations done by Camelia Mironov
28
Non-Photonic Electron / Heavy Flavor Quenching

• Langevin simulation of heavy quark
• diffusion

• Radiative and collisional energy loss

S. Wicks et al., (2005)
N. Armesto et al., (2006)

• Ratio
• Opacity of the QGP

H. van Hees, R. Rapp, (2005)
G. Moore, D.Teaney (2005)

• Diffusion coefficient D and eventually
• Existence of heavy heavy
• resonances near Tc in the QGP

29
Conceptually Different Approach to D / B

• Problem treated in the same way as light quarks

• Fragmentation and dissociation of hadrons from
  heavy quarks inside the QGP

Lowest order lightcone Fock component
30
Heavy Meson Dissociation at RHIC and LHC
Coupled rate equations

• The asymptotic solution in the QGP -
• sensitive to t00.6 fm and expansion
• dynamics
• Features of energy loss
• B-mesons as suppressed as D-mesons
• at pT 15-20 GeV at the LHC

Unique feature
31
Quenching of Non-Photonic Electrons

• Full semi-leptonic decays of C- and B-
• mesons and baryons included. PDG
• branching fractions and kinematics.
• PYTHIA event generator

• Similar to light , however, different
• physics mechanism

• B-mesons are included. They give a
• major contribution to (ee-)

Note on applicability
D-, B-mesons to
(ee-) to 25 GeV
32
The Path Forward

• An interesting idea valid physics
  explanation

• To understand heavy flavor modification in the
  QGP we need direct and
• separate measurements of D- and B-mesons,
  excellent statistics

Measurable at RHIC
Measurable at the LHC
W.Horowitz, M. Gyulassy, (2007)
A.Adil, I.Vitev, Phys. Lett. B (2006)
Meson dissociation
PQCD, Transport
String theory AdS/CFT
50-100
10-15
PT GeV
Never
33
Conclusions

• The physics case for major involvement at LHC via
  CMS
• Outstanding jet physics capabilities
• Modern new physics experiment
• The first measurement of jets in nuclear
  collisions
• Bridging the gap between high energy and nuclear
  physics
• First real measurement of the QGP-induced
  modification of jet properties
• Rich phenomenology of topological jet
  observables
• Significantly larger discriminating power for
  theoretical models
• Solution to heavy quark puzzles from RHIC
• Jets in pp collisions
• Higgs discovery
• Supersymmetry discovery
• Precision perturbative QCD
• Opportunity for LANL leadership

34
Heavy Flavor Elliptic Flow and Suppression

• Understand the structure of mesons
• light cone wave functionsc

Test coalescence model fits to the v2 of
light hadrons via heavy flavor
Sensitive to the opacity of the QGP and its
formation time
D. Molnar (2004)
A. Adil, I. Vitev, Phys.Lett.B (2007)
35
Jet cross sections
Calculations done by Simon Wicks
36
Jet cross sections
37
(No Transcript)
38
Medium properties
RHIC
LHC
m
a
2
2
2
E

(
1
)
s
L
g
.
.
.
,

• Small probability not to radiate

l
m
2
(
L
)
L
g
-














S
t
a
t
i
c

m
e
d
i
u
m
3
p
a
g
9
C
1
d
N
2
E
D
?

(
1
R
s
E
L
o
g
.
.
.
,
L
m
2
4
A
d
y
(
L
)
L

-















1

1
D

B
j
o
r
k
e
n
Fundamental
Derivative
39
Hard Probes from Factorized PQCD
Pd

• Single and double inclusive hard production
• in PQCD - applicable from photons to heavy
• quarks

X
Pd / zd
Pc / zc

• Single and double inclusive hard production
• in PQCD - applicable from photons to heavy
• quarks

X
Pc
Power laws
Quenching factor
40
Cold Nuclear Matter Effects

• Initial-state E-loss

Energy scale

• Effect of cold nuclear matter energy
• loss is equal to the doubling of the
• parton rapidity density

41
Direct Photon Production at RHIC and the LHC

• Direct photon production calculated in
• the QCD factorization approach

Prompt photons
Fragmentation photons
Suppressed

• Fragmentation functions Owens

Determines the final-state interactions

• Same ratio in NLO, Wogelsang

42
Cold Nuclear Matter Effects for ?0 and Direct ?

• Where it starts from

• Dynamical shadowing (coherent final
• state scattering)
• Cronin effect (initial state transverse
• momentum diffusion)
• Initial state energy loss (final state at
• these energies - negligible)

43
Direct Photon Quenching at the LHC
Reminder isospin and Initial-state energy loss
effects
Nuclear modification RAA(?) follows the ratio
Correction to the energy in photon-tagged jets
44
Heavy Quark Production and Correlations

• Fast convergence of the perturbative
• series

• Possibility for novel studies of heavy
• quark-triggered (D and B) jets hadron
• composition of associated yields

45
Conceptually Different Approach to D / B

• Problem treated in the same way as light quarks

• Fragmentation and dissociation of hadrons from
  heavy quarks inside the QGP

Lowest order lightcone Fock component
46
Heavy Meson Dissociation at RHIC and LHC
Coupled rate equations

• The asymptotic solution in the QGP -
• sensitive to t00.6 fm and expansion
• dynamics
• Features of energy loss
• B-mesons as suppressed as D-mesons
• at pT 15-20 GeV at the LHC

Unique feature
47
Quenching of Non-Photonic Electrons

• Full semi-leptonic decays of C- and B-
• mesons and baryons included. PDG
• branching fractions and kinematics.
• PYTHIA event generator

• Similar to light , however, different
• physics mechanism

• B-mesons are included. They give a
• major contribution to (ee-)

Note on applicability
D-, B-mesons to
(ee-) to 25 GeV
48
References to the Four Contributions, Collabs.
Energy loss
Light hadrons
Direct photons
Heavy flavor
49
Conclusions
Looking forward to jet physics at the LHC
50
Scales in Thermalized QGP (GP)

• Experimental Bjorken expansion

• Theoretical Gluon dominated plasma

• Energy density

• Transport coefficients (not a good measure for
  expanding medium)

• Define the average for Bjorken

51
E-loss in Back-to-Back Di-jets and Correlations

• Multi-particle modification

• Angular gluon distribution

Tag
Two particle suppression / enhancement in AA
reactions
See talk by M. Brooks
I.Vitev, Phys.Lett.B630 (2005)
52
When One ? One

• Theoretical results cancellation between factor
  of 4 Cronin enhancement and 2- to 3-fold
  quenching

• Experimental findings

Ncoll,PbPb 807 81
S.Bathe., LANL seminar
I.V., Phys.Lett.B 632 (2005)

• With any multiple scattering effect there is
  no reason to expect

• If one understands this in AA collisions one
  should also accept this is
• pA collisions

53
Langevin Simulation of Heavy Quark Diffusion
Input in a Langevin simulation of heavy quark
diffusion
H. van Hees, I.V., R. Rapp, in preparation

• Drag coefficient

• Diffusion coefficient

Equilibration is imposed by Einsteins
fluctuation-dissipation relation
Radiative energy loss is dominant except for
b-quarks and very small systems
54
Transport Quenching Approach
Numerical results for heavy quark diffusion
Results are preliminary
H. van Hees, I.V., R. Rapp, in preparation

• The suppression and v2 are large when e-loss and
  q-resonance interactions are
• combined
• Normal hierarchy c quarks are significantly
  more suppressed than b-quarks

55
Wang
Wicks et al.
How do you build from T
400 MeV
LHC from T 1 GeV