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Basics of dilepton decay distributions. Examples: quarkonium and vector bosons A general demonstration of an old and surprising “ perturbative -QCD” relation, using only rotation invariance Model-independent spin characterization of the Higgs-like di -photon resonance.

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Basics of dilepton decay distributions examples quarkonium and vector bosons

Basics of dilepton decay distributions. Examples: quarkonium and vector bosons

A general demonstration of an old and surprising “perturbative-QCD” relation,using only rotation invariance

Model-independent spin characterization of the Higgs-like di-photon resonance

Angular momentum and decay distributionsin high energy physics: an introduction and use cases for the LHC

PietroFaccioli, CERN, April 23th, 2013


Why should we study particle polarizations

Why should we study particle polarizations?

  • testofperturbative QCD [ZandWdecaydistributions]

  • constrain universal quantities [sinθWand/orproton PDFsfromZ/W/ * decays]

  • acceleratediscoveryofnewparticlesorcharacterizethem[Higgs, Z’, anomalousZ+, graviton, ...]

  • understand the formation of hadrons (non-perturbative QCD)


Example formation of and

3/34

Example: formationofψ and

  • We want to know the relative contributions of the following processes,differing for how/when the observed Q-Qbar bound state acquires its quantum numbers

colour-singlet state

J = 1

  • Colour-singlet processes:quarkonia produceddirectly as observablecolour-neutralQ-Qbar pairs

red

antired

purely perturbative

+ analogous colour combinations

red

antired

  • Colour-octet processes: quarkonia are produced through colouredQ-Qbar pairs ofany possible quantum numbers

J = 1

colour-octet state

J = 0, 1, 2, …

green

antiblue

Transition to the

observable state.

Quantum numbers change!

J can change!  polarization!

perturbative

non-perturbative


Polarization of vector particles

Polarization of vector particles

J = 1 → three Jzeigenstates1, +1 ,1, 0 ,1, -1  wrt a certain z

Measure polarization = measure (average) angular momentum composition

Method: study the angular distribution of the particle decay in its rest frame

The decay into a fermion-antifermion pair is an especially clean case to be studied

The shape of the observable angular distribution is determined by a few basic principles:

1) “helicity conservation”

=

2) rotational covarianceof angular momentum eigenstates

f

* , Z, g, ...

NO

YES

z'

1, +1 

3) parity properties

1

1

1

1, +1  + 1, 1  1, 0 

2

2

√2

z

?


1 helicity conservation

1: helicity conservation

EW and strong forces preserve the chirality (L/R) of fermions.

In the relativistic (massless) limit, chirality = helicity = spin-momentum alignment

→ the fermion spin never flips in the coupling to gauge bosons:

f

* , Z, g, ...

YES

NO

YES

NO


Example dilepton decay of j

example: dilepton decay of J/ψ

z'

1, M’ℓ+ℓ

(–1/2)

+1/2

c

( )

ℓ+

ℓ+

( )

J/ψ rest frame:

c

*

z

J/ψ

1, MJ/ψ

( )

(–1/2)

ℓ

( )

+1/2

ℓ

J/ψangular momentum component alongthepolarizationaxisz:

MJ/ψ = -1, 0, +1

(determinedbyproductionmechanism)

Thetwoleptonscan onlyhave total angular momentumcomponent

M’ℓ+ℓ = +1 or -1along their common direction z’

0 is forbidden


2 rotation of angular momentum eigenstates

2: rotation of angular momentum eigenstates

z'

change of quantization frame:

R(θ,φ):z → z’

y → y’

x → x’

Jz’ eigenstates

J, M’

θ,φ

z

+ J

J, M 

Σ

J, M’ =DMM’(θ,φ)J, M 

J

M = - J

Jz eigenstates

Wigner D-matrices

z'

1, +1 

Example:

90°

z

1

1

1

1, +1  + 1, 1  1, 0 

Classically, we wouldexpect1, 0 

2

2

√2


Example m 0

example: M = 0

z'

J/ψ (MJ/ψ= 0) →ℓ+ℓ(M’ℓ+ℓ = +1)

ℓ+

1, +1 

J/ψ rest frame

θ

z

1,0

ℓ

→ the Jz’eigenstate1, +1  “contains” the Jzeigenstate1, 0

with component amplitude D0,+1(θ,φ)

1

→ the decay distribution is

1

 |D0,+1(θ,φ)|2 = ( 1  cos2θ)

|1, +1 |O1, 0|2

1*

1, +1 =D1,+1(θ,φ)1, 1 + D0,+1(θ,φ) 1, 0 + D+1,+1(θ,φ) 1, +1

1

1

1

2

z

ℓ+ℓ← J/ψ


3 parity

ℓ+

ℓ+

3: parity

θ

θ

z

z

1,+1

1,1

1,1 and 1,+1 distributions

are mirror reflections

of one another

ℓ

ℓ

z

z

  •  |D+1,+1(θ,φ)|2

  •  1 + cos2θ+ 2cos θ

  •  1 + cos2θ 2cos θ

  •  |D1,+1(θ,φ)|2

1*

1*

Decay distribution of 1,0 stateis always parity-symmetric:

Are they equally probable?

  •  1 + cos2θ+ 2[P(+1)P (1)] cos θ

  •  1  cos2θ

z

z

z

dN

dN

dN

dN

z

P (1) > P(+1)

P (1) = P(+1)

P (1) < P(+1)

  •  |D0,+1(θ,φ)|2

1*


Transverse and longitudinal

“Transverse” and “longitudinal”

z

“Transverse” polarization,

like for real photons.

The word refers to the

alignment of the field vector,

not to the spin alignment!

J/ψ = 1,+1

or 1,1

 1 + cos2θ

x

(parity-conserving case)

y

z

“Longitudinal” polarization

J/ψ = 1,0

 1 – cos2θ

x

dN

dN

y


Why photon like polarizations are common

Why “photon-like” polarizations are common

We can apply helicity conservation at the production vertex to predict that

all vector states produced in fermion-antifermion annihilations (q-q or e+e–) at Born level have transverse polarization

q-q rest frame

= V rest frame

q

( )

(–1/2)

+1/2

( )

( )

q

z

V

  • V = 1,+1

  • (1,1)

V = *, Z, W

q

q

( )

The “natural” polarization axis in this case is

the relative direction of the colliding fermions

(Collins-Soper axis)

1

.

5

Drell-Yan

1

.

0

Drell-Yan is a paradigmatic case

But not the only one

(2S+3S)

0

.

5

E866, Collins-Soper frame

dN

0

.

0

 1 +λcos2θ

-

0

.

5

0

1

2

pT [GeV/c]


The most general distribution

The most general distribution

z

z

chosenpolarization axis

θ

ℓ +

production plane

φ

y

x

particle

rest frame

average

polar anisotropy

average

azimuthal anisotropy

correlation

polar - azimuthal

parity violating


Polarization frames

Polarization frames

zHX

zHX

zHX

zGJ

zGJ

Helicity axis (HX): quarkonium momentum direction

Gottfried-Jacksonaxis (GJ):direction of one or the other beam

zCS

Collins-Soperaxis (CS): average of the two beam directions

h1

h2

Perpendicular helicity axis (PX): perpendicular to CS

hadron

collision

centre

of mass

frame

h2

h2

h2

h1

h1

h1

production plane

p

zPX

particle

rest

frame

particle

rest

frame

particle

rest

frame


F rame dependence

Framedependence

For |pL| << pT , the CS and HX frames differ by a rotation of 90º

z

z′

helicity

Collins-Soper

90º

x

x′

y

y′

longitudinal

“transverse”

(pure state)

(mixedstate)


All reference frames are equal but some are more equal than others

Allreference frames are equal…but some are more equalthanothers

What do different detectors measure with arbitrary frame choices?

  • Gedankenscenario:

  • dileptons are fully transversely polarized in the CS frame

  • the decay distribution is measured at the (1S) massby 6 detectors with different dilepton acceptances:


The lucky frame choice

The lucky frame choice

(CS in this case)

dN

 1 +cos2θ


Less lucky choice

Less lucky choice

(HX in this case)

+1/3

λθ = +0.65

λθ = 0.10

1/3

artificial (experiment-dependent!)

kinematic behaviour

 measure in more than one frame!


Frames for drell yan z and w polarizations

Frames for Drell-Yan, Z and W polarizations

  • polarization is always fully transverse...

V = *, Z, W

_

Due to helicity conservation at the q-q-V(q-q*-V)vertex,

Jz = ± 1 along the q-q (q-q*) scattering direction z

_

z

  • ...but with respect to a subprocess-dependent quantization axis

_

_

q

z = relative dir. of incoming q and qbar

( Collins-Soper frame)

q

q

V

important only up to pT = O(partonkT)

V

q

z = dir. of one incoming quark

( Gottfried-Jackson frame)

q

V

q*

q*

g

QCD

corrections

q

V

z = dir. of outgoing q

(= parton-cms-helicity lab-cms-helicity)

q*

g

q


Optimal frames for drell yan z and w polarizations

“Optimal” frames for Drell-Yan, Z and W polarizations

Different subprocesses have different “natural” quantization axes

q

For s-channel processes the natural axis is

the direction of the outgoing quark

(= direction of dilepton momentum)

V

q*

g

q

  • (neglecting parton-parton-cms

  • vs proton-proton-cms difference!)

  •  optimal frame (= maximizing polar anisotropy): HX

example: Z

y = +0.5

HX

CS

PX

GJ1

GJ2

(negative beam)

(positive beam)

1/3


Optimal frames for drell yan z and w polarizations1

“Optimal” frames for Drell-Yan, Z and W polarizations

Different subprocesses have different “natural” quantization axes

V

q

q

V

For t- and u-channel processes the natural axis is

the direction of either one or the other incoming parton ( “Gottfried-Jackson” axes)

q*

q*

g

  •  optimal frame: geometrical average of GJ1 and GJ2 axes = CS (pT < M) and PX (pT > M)

_

q

example: Z

y = +0.5

HX

CS

PX

GJ1 = GJ2

MZ

1/3


A complementary approach frame independent polarization

A complementaryapproach:frame-independent polarization

The shape of the distribution is (obviously) frame-invariant (= invariant by rotation)

→ it can be characterized by frame-independent parameters:

z

λθ = –1

λφ = 0

λθ = +1

λφ = 0

λθ = –1/3

λφ = –1/3

λθ = +1/5

λφ = +1/5

λθ = +1

λφ = –1

λθ = –1/3

λφ = +1/3

rotations in the production plane


Reduces acceptance dependence

Reducesacceptance dependence

Gedankenscenario: vector state produced in this subprocess admixture:

 60%processes with natural transverse polarization in the CS frame

 40%processes with natural transverse polarization in the HX frame

assumed indep.

of kinematics,

for simplicity

CS

HX

M = 10 GeV/c2

polar

azimuthal

  • Immune to “extrinsic” kinematic dependencies

  • less acceptance-dependent

  • facilitates comparisons

  • useful as closure test

rotation-

invariant


Physical meaning drell yan z and w polarizations

Physical meaning: Drell-Yan, Z and W polarizations

  • polarization is always fully transverse...

V = *, Z, W

_

Due to helicity conservation at the q-q-V(q-q*-V)vertex,

Jz = ± 1 along the q-q (q-q*) scattering direction z

_

z

  • ...but with respect to a subprocess-dependent quantization axis

q

_

_

“natural” z = relative dir. of q and qbar

λθ(“CS”)= +1

q

q

V

~

~

~

λ = +1

wrtanyaxis:λ = +1

λ = +1

~

V

q

z = dir. of one incoming quark

λθ(“GJ”)= +1

q

V

λ = +1

q*

q*

any frame

g

(LO) QCD

corrections

z = dir. of outgoing q

λθ(“HX”)= +1

q

V

q*

~

N.B.: λ = +1 in both

pp-HX and qg-HX frames!

g

q

In all these cases the q-q-V lines are in the production plane (planar processes);

The CS, GJ, pp-HX and qg-HX axes only differ by a rotation in the production plane


Basics of dilepton decay distributions examples quarkonium and vector bosons

~

λθvsλ

Example: Z/*/W polarization (CS frame) as a function of contribution of LO QCD corrections:

Case 2: dominating q-g QCD corrections

Case 1: dominating q-qbar QCD corrections

pT = 50 GeV/c

y = 2

pT = 50 GeV/c

pT = 50 GeV/c

pT = 50 GeV/c

(indep. of y)

pT = 200 GeV/c

y = 2

pT = 200 GeV/c

1

pT = 200 GeV/c

pT = 200 GeV/c

(indep. of y)

y = 0

y = 0

y = 2

y = 2

0

.

5

y = 0

y = 0

~

~

~

~

~

M = 150 GeV/c2

M = 80 GeV/c2

M = 150 GeV/c2

M = 80 GeV/c2

λ = +1

λ = +1

λ = +1

λ = +1

λ = +1 !

mass dependent!

0

fQCD

fQCD

fQCD

fQCD

  • depends on pT , y and mass

    • by integrating we lose significance

  • is far from being maximal

  • depends on process admixture

    • need pQCD and PDFs

0

1

0

2

0

3

0

4

0

5

0

6

0

7

0

8

0

λθCS

W by CDF&D0

λθ

“unpolarized”?

No,

~

λ is constant, maximal and

independent of process admixture

pT [GeV/c]


Basics of dilepton decay distributions examples quarkonium and vector bosons

~

λθvsλ

Example: Z/*/W polarization (CS frame) as a function of contribution of LO QCD corrections:

Case 2: dominating q-g QCD corrections

Case 1: dominating q-qbar QCD corrections

pT = 50 GeV/c

y = 2

pT = 50 GeV/c

pT = 50 GeV/c

pT = 50 GeV/c

(indep. of y)

pT = 200 GeV/c

y = 2

pT = 200 GeV/c

pT = 200 GeV/c

pT = 200 GeV/c

(indep. of y)

y = 0

y = 0

y = 2

y = 2

y = 0

y = 0

~

~

~

~

M = 150 GeV/c2

M = 80 GeV/c2

M = 150 GeV/c2

M = 80 GeV/c2

λ = +1

λ = +1

λ = +1

λ = +1

mass dependent!

fQCD

fQCD

fQCD

fQCD

~

On the other hand, λ forgets about the direction of the quantization axis.

This information is crucial if we want to disentangle the qg contribution,

the only one resulting in a rapidity-dependentλθ

Measuring λθ(CS) as a function of rapidity gives information on the gluon content

of the proton


The lam tung relation

The Lam-Tung relation

A fundamental result of the theory of vector-boson polarizations (Drell-Yan, directly produced Z and W) is that, at leading order in perturbative QCD,

independently of the polarization frame

Lam-Tung relation, PRD 18, 2447 (1978)

This identity was considered as a surprising result of cancellations in the calculations

Today we know that it is only a special case of general frame-independent polarization relations, corresponding to a transverse intrinsic polarization:

It is, therefore, simply a consequence of

1) rotational invariance

2) properties of the quark-photon/Z/W coupling

Experimental tests of the LT relation are not tests of QCD!


Beyond the lam tung relation

Beyond the Lam-Tung relation

Even when the Lam-Tung relation is violated,

can always be defined and is always frame-independent

→ Lam-Tung. New interpretation: only vector boson – quark – quarkcouplings (in planar processes)  automatically verified in DY at QED & LO QCD levels and in several higher-order QCD contributions

→ vector-boson – quark – quarkcouplings in

non-planar processes (higher-order contributions)

→ contribution of different/new couplings or processes

(e.g.: Z from Higgs, W from top, triple ZZ coupling,higher-twist effects in DY production, etc…)


Spin characterization of the higgs like di photon resonance

Spin characterizationoftheHiggs-likedi-photonresonance

z'

±1

g



θ

T restframe

T

z



g

±1

Usual approach to “determine” the J of T:

comparison between J=0 hypothesis and ONE alternative hypothesis. Example:

graviton with minimal-couplings to SM bosons ( “boson helicity conservation”)

SM Higgs boson

J=2

J=0

+

+

OR

OR

OR

OR

Mgg = 0

Mgg = 0

Mgg = 0, ±1, ±2

Mgg = 0, ±1, ±2

Decay distribution calculated case-by-case


Spin characterization of the higgs like di photon resonance1

Spin characterizationoftheHiggs-likedi-photonresonance

z'

±1

g



θ

T restframe

T

z



g

±1

Usual approach to “determine” the J of T:

comparison between J=0 hypothesis and ONE alternative hypothesis. Example:

graviton with minimal-couplings to SM bosons ( “boson helicity conservation”)

SM Higgs boson

J=2

J=0

Mgg = ±2

Mgg = ±2


Likelihood ratio approach

Likelihood Ratio Approach

  • Method:

    • measure distribution of the likelihood ratio between hypothesis A and hypothesis B

    • here A = SM Higgs (JA = 0), B = a new-physics hypothesis (JB)

L  decay angular distribution

L [B] / L [A]

  • Ingredients (for each set of A and B hypotheses):

    • the angular momentum quantum numbers JA and JB

    • the coupling properties of A and B to initial and final particles (gluons and photons)

    • calculations of the helicity amplitudes for the production and decay processes

  • Question addressed:

    • is the observed resonance more likely to be particle A or particle B?

  • The answer

    • may be given unhesitatingly, i.e. L [A] >> L [B], even when neither A nor B coincide with the correct hypothesis

    • is never conclusive until the whole set of possible models for A and B is explored.Do we know this set of models in a totally model-independent way?As a matter of fact, a very restricted set of “B” models is currently considered


Mpc approach

MPC approach

MPC = Minimal Physical Constraints

  • Method:

    • measure the angular distribution

dN

 1 + λ2 cos2θ + λ4 cos4θ + λ6 cos6θ + … + λNcosNθ

  • Ingredients:

    • angular momentum conservation

    • initial gluons and final photons are transversely polarized

    • no hypothesis on J nor on couplings, no explicit calculations of helicity amplitudes

[J=1 hypothesis forbidden by Landau-Yang theorem]

The general physical parameter domains of the J=2, 3 and 4 cases are mutually exclusive!

And do not include the origin (J=0)!


Mpc approach1

MPC approach

MPC = Minimal Physical Constraints

  • Method:

    • measure the angular distribution

J = 3

J = 4

J = 0

J = 2

dN

 1 + λ2 cos2θ + λ4 cos4θ + λ6 cos6θ + … + λNcosNθ

  • Ingredients:

    • angular momentum conservation

    • Initial gluons and final photons are transversely polarized

    • no hypothesis on J nor on couplings, no explicit calculations of helicity amplitudes

The cosθ distribution discriminates the spin univocally:


Mpc approach2

MPC approach

MPC = Minimal Physical Constraints

  • Method:

    • measure the angular distribution

dN

 1 + λ2 cos2θ + λ4 cos4θ + λ6 cos6θ + … + λNcosNθ

  • Ingredients:

    • angular momentum conservation

    • Initial gluons and final photons are transversely polarized

    • no hypothesis on J nor on couplings, no explicit calculations of helicity amplitudes

  • This method directly addresses the question:

    • how much is J?

  • The answer

    • is model-independent and can be compared to any theory

    • is always conclusive, if the measurement is sufficiently precise


Lr vs mpc

LR vs MPC

The binary strategy of the LR approach aims at discriminating between two hypotheses:

From this point of view,

this measurementwould correspond to a J=0 characterization

99% C.L.

J = 2 minimally-coupling graviton

J=2

In the MPC approach

this measurementwould represent an unequivocal spin-0 characterization

In the MPC approach it would exclude all models lying outside the ellipse, but it would not exclude J=2, nor J=3!

99% C.L.

J=3

J = 0

(SM Higgs)


Further reading

Further reading

  • P. Faccioli, C. Lourenço, J. Seixas, and H.K. Wöhri,J/psi polarization from fixed-target to collider energies,Phys. Rev. Lett. 102, 151802 (2009)

  • HERA-B Collaboration, Angular distributions of leptons from J/psi's produced in 920-GeV fixed-target proton-nucleus collisions,Eur. Phys. J. C 60, 517 (2009)

  • P. Faccioli, C. Lourenço and J. Seixas,Rotation-invariant relations in vector meson decays into fermion pairs,Phys. Rev. Lett. 105, 061601 (2010)

  • P. Faccioli, C. Lourenço and J. Seixas,New approach to quarkonium polarization studies,Phys. Rev. D 81, 111502(R) (2010)

  • P. Faccioli, C. Lourenço, J. Seixas and H.K. Wöhri,Towards the experimental clarification of quarkonium polarization,Eur. Phys. J. C 69, 657 (2010)

  • P. Faccioli, C. Lourenço, J. Seixas and H. K. Wöhri,Rotation-invariant observables in parity-violating decays of vector particles to fermion pairs,Phys. Rev. D 82, 096002 (2010)

  • P. Faccioli, C. Lourenço, J. Seixas and H. K. Wöhri,Model-independent constraints on the shape parameters of dilepton angular distributions,Phys. Rev. D 83, 056008 (2011)

  • P. Faccioli, C. Lourenço, J. Seixas and H. K. Wöhri,Determination of ­chi_c and chi_­b polarizations from dilepton angular distributions in radiative decays,Phys. Rev. D 83, 096001 (2011)

  • P. Faccioli and J. Seixas, Observation of χc and χb nuclear suppression via dilepton polarization measurements,Phys. Rev. D 85, 074005 (2012)

  • P. Faccioli,Questions and prospects in quarkonium polarization measurements from proton-proton to nucleus-nucleus collisions,invited “brief review”, Mod. Phys. Lett. A Vol. 27 N. 23, 1230022 (2012)

  • P. Faccioli and J. Seixas, Angular characterization of the Z Z → 4ℓ background continuum to improve sensitivity of new physics searches,Phys. Lett. B 716, 326 (2012)

  • P. Faccioli, C. Lourenço, J. Seixas and H. K. Wöhri,Minimal physical constraints on the angular distributions of two-body boson decays,submitted to Phys. Rev. D


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