AnomalousAnomalous SoftSoft Photon Photon ProductionProduction

inin MultipleMultiple HadronHadron ProcessesProcesses

Vassili PerepelitsaVassili PerepelitsaITEP, Moscow/IFIC, ValenciaITEP, Moscow/IFIC, Valencia

for for WA83,WA83, WA91,WA91, WA102WA102 and and DELPHIDELPHI Collaborations Collaborations

ContentContent

• Introduction: The puzzle of anomalous Introduction: The puzzle of anomalous soft photonssoft photons

• Experiments with hadronic beamsExperiments with hadronic beams

• LEP, DELPHI observations: LEP, DELPHI observations: experimental technique; experimental technique;

the signal;the signal; cross-checks;cross-checks;

• Striking behaviour of the signalStriking behaviour of the signal

• DiscussionDiscussion

ContentContent• Introduction: The puzzle of anomalous Introduction: The puzzle of anomalous

soft photonssoft photons

• Experiments with hadronic beamsExperiments with hadronic beams

• LEP, DELPHI observations: LEP, DELPHI observations: experimental technique; experimental technique;

the signal;the signal; cross-checks;cross-checks;

• Striking behaviour of the signalStriking behaviour of the signal

• DiscussionDiscussion

Theory:Theory: BremsstrahlungBremsstrahlung fromfrom externalexternal lineslines shouldshould dominatedominate

• SoftSoft Photons:Photons: having transverse momenta having transverse momenta ppTT <<<< p pT T of of typical transverse momenta of hadrons in HE typical transverse momenta of hadrons in HE interactions=interactions=300-400300-400 MeV/cMeV/c

• LowLow theorem/Gribovtheorem/Gribov extensionextension

M ~ 1/[(p – k) – m ]

2

= 1/2pk2

andand⇝⇝

⇝⇝ ⇝⇝ISRISR FSRFSR RadiationRadiation fromfrom centralcentral blobblob

BremsstrahlungBremsstrahlung calculationscalculations

where K and k denote photon four- and three-momenta,where K and k denote photon four- and three-momenta,P are the four-momenta of all the charged particles P are the four-momenta of all the charged particles

participatingparticipatingin the reaction. in the reaction. ƞ = +1 for negative incoming and for positive ƞ = +1 for negative incoming and for positive outgoing particles, ƞ = -1 for positive incoming and negative outgoing particles, ƞ = -1 for positive incoming and negative outgoing outgoing particles, and the sum is extended over all the N + 2particles, and the sum is extended over all the N + 2charged particles involved. The last factor in the integrand is acharged particles involved. The last factor in the integrand is adifferential hadron production ratio. differential hadron production ratio.

CERN WA27: beginning the puzzle (1983-1984)

• K p hadrons + gamma at 70 GeV/cK p hadrons + gamma at 70 GeV/c BEBCBEBC Photons: -0.001<X <0.008, p <60 MeV/cPhotons: -0.001<X <0.008, p <60 MeV/c After subtraction of photons coming from all known hadronic decaysAfter subtraction of photons coming from all known hadronic decays the residual signal was found to be similar in shape to the brems-the residual signal was found to be similar in shape to the brems- strahlung, but bigger in size by a factor of about fourstrahlung, but bigger in size by a factor of about four

= 4.0 = 4.0 ±± 0.8 0.8

In such a way the effect of anomalous soft In such a way the effect of anomalous soft photons hasphotons has

TF

++

CERNCERN NA22NA22 andand WA83:WA83: confirmationconfirmation ofof thethe signalsignal

• CERNCERN NA22NA22 (1990-1991), EHS Spectrometer (1990-1991), EHS Spectrometer

ππ p hadrons + gamma at 250 GeV/cp hadrons + gamma at 250 GeV/c

K K p hadrons + gamma at 250 GeV/cp hadrons + gamma at 250 GeV/c p < 40 MeV/cp < 40 MeV/c = 6.9 = 6.9 ±± 1.3 (pion beam) 1.3 (pion beam) = 6.3 = 6.3 ±± 1.6 (kaon beam) 1.6 (kaon beam)• CERNCERN WA83WA83 (1985-1992), (1985-1992), ΩΩ

Spectrometer+El.Mag.Cal.Spectrometer+El.Mag.Cal.

ππ p hadrons + gamma at 280 GeV/cp hadrons + gamma at 280 GeV/c p < 10 MeV/cp < 10 MeV/c = 7.9 = 7.9 ±± 1.6 1.6

++

++

TT

__

TT

WA91WA91 experimentexperiment,, ππ-p exposure, 280 GeV/c-p exposure, 280 GeV/c

WA91WA91 eventevent examplesexamples

WA91WA91 raw signal raw signal

WA91WA91 energy energy dependencedependence

SignalSignal energyenergy dependencedependence agreesagrees withwith thatthat ofof thethe bremsstrahlungbremsstrahlung

TheThe spectraspectra werewere fittedfitted byby aa form:form:

Signal:Signal:

A = (6940A = (6940±540±1910) 1/GeV±540±1910) 1/GeV

αα = ―1.11±0.09±0.04 = ―1.11±0.09±0.04

Bremsstrahlung:Bremsstrahlung:

A = (1460A = (1460±44) 1/GeV±44) 1/GeV

αα = = ――0.93±0.040.93±0.04

dNɣdNɣ——dEdE = A = A ((——), ), EE

EE00

E =1 GeVE =1 GeV00

αα

ObservedObserved photonphoton raterate::

((9292±4±15±4±15) ˣ ) ˣ 1010 ɣ/evtɣ/evt

PredictedPredicted hadronichadronic bremsstrahlung:bremsstrahlung:

(17.4±0.3±1.2) ˣ 10 ɣ/evt(17.4±0.3±1.2) ˣ 10 ɣ/evt

= 5.3±0.2±0.9= 5.3±0.2±0.9

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WA102WA102 experiment,experiment, pp exposure, 450 GeV/c pp exposure, 450 GeV/c

WA102WA102 soft photon events soft photon events

WA102WA102 p pT T and and angular angular distributionsdistributions

ObservedObserved photonphoton raterate::

((47.347.3±1.8±9.1±1.8±9.1) ˣ ) ˣ 1010 ɣ/evtɣ/evt

PredictedPredicted hadronichadronic bremsstrahlung:bremsstrahlung:

(11.6±0.2±0.7) ˣ 10 ɣ/evt(11.6±0.2±0.7) ˣ 10 ɣ/evt

=4.1=4.1±0.2±0.8±0.2±0.8

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--33

ExperimentExperiment,,

Reaction, Beam Reaction, Beam momentummomentum

PhotonPhoton

kinematic rangekinematic rangeSignal/Signal/BremsBrems

ratioratio

SLAC,SLAC, BCBC

ππ++ p p —›—› hadrons+ hadrons+ɣɣ, 10.5 , 10.5 GeV/cGeV/c

0 0 << XXF F << 0.010.01

EEɣɣ > 30 > 30 MeVMeV, , PPT T < 20 < 20 MeV/cMeV/c

1.25 1.25 ±± 0.25 0.25

CERNCERN WA27,WA27, BEBCBEBC

K p K p —›—› hadrons+hadrons+ɣɣ, 70 , 70 GeV/cGeV/c

-0.001 -0.001 << XXF F << 0.008 0.008

EEɣɣ > 70 > 70 MeVMeV, , PPT T < 60 < 60 MeV/cMeV/c

4.0 4.0 ±± 0.8 0.8

CERNCERN NA22,NA22, EHSEHSK p K p —›—› hadronshadrons + + ɣ,ɣ, 250 250 GeV/cGeV/c

ππ pp —›—› hadrons + hadrons + ɣ,ɣ, 250250 GeV/cGeV/c

- - 0.001 0.001 < < XXF F < < 0.0080.008

EEɣɣ >> 70 70 MeVMeV, , PPT T << 40 40 MeV/c MeV/c

6.46.4 ±± 1.61.6

6.96.9 ±± 1.31.3

CERNCERN WA83,WA83, OMEGAOMEGA

ππ p p —›—› hadrons + hadrons + ɣ, ɣ, 280280 GeV/c GeV/c

2 2 << y yc.m.s. c.m.s. << 55

0.2 0.2 << EEɣɣ << 1 1 GeVGeV, , PPT T << 10 10 MeV/cMeV/c

7.97.9 ±± 1.41.4

CERNCERN WA91,WA91, OMEGAOMEGA

ππ p p —›—› hadrons + hadrons + ɣ, 280ɣ, 280 GeV/cGeV/c

1.4 1.4 << y yc.m.s. c.m.s. << 55

0.20.2 << E Eɣ ɣ < 1 < 1 GeVGeV, , PPT T << 20 20 MeV/cMeV/c

5.3 5.3 ±± 0.9 0.9

BNLBNL

p Be p Be —›—› hadrons + hadrons + ɣ, 18 ɣ, 18 GeV/cGeV/c

-1.4 -1.4 < < yyc.m.s. c.m.s. << 00

1515 << E Eɣ ɣ << 150150 MeV MeV, , PPT T << 10 10 MeV/cMeV/c

<< 2.7 2.7

(at 90% CL)(at 90% CL)

CERNCERN NA34NA34 (HELIOS)(HELIOS)

p Be p Be —›—› hadrons + hadrons + ɣ, 450 ɣ, 450 GeV/cGeV/c

-1.4 -1.4 << y yc.m.s. c.m.s. << 00

15 15 << EEɣɣ << 150 150 MeVMeV, , PPT T << 10 10 MeV/cMeV/c

<< 1.5 – 3 1.5 – 3

(at 90% CL)(at 90% CL)

CERNCERN WA102,WA102, OMEGAOMEGA

p p p p —›—› hadrons + hadrons + ɣ,ɣ, 450450 GeV/cGeV/c

1.2 1.2 << y yc.m.s. c.m.s. << 55

0.20.2 << E Eɣ ɣ < 1 < 1 GeVGeV, , PPT T < 20 < 20 MeV/cMeV/c

4.1 4.1 ± 0.8± 0.8

++

--

--

++ ++

LEP, DELPHI observationsLEP, DELPHI observations

• Signal discoverySignal discovery• Check-upsCheck-ups• Muon inner bremsstrahlung: control Muon inner bremsstrahlung: control

experimentexperiment• Signal dependence on the parent jet Signal dependence on the parent jet

characteristicscharacteristics• Non-trivial behaviour with the jet Non-trivial behaviour with the jet

neutral and total multiplicitiesneutral and total multiplicities

LEP,LEP, DELPHIDELPHI observationsobservations

• Signal discovery Signal discovery EPJEPJ C47C47 (2006)(2006) 273273

• Check-upsCheck-ups• Muon inner bremsstrahlung: control Muon inner bremsstrahlung: control

experiment experiment EPJEPJ C57 C57 ((2008) 4992008) 499

• Signal dependence on the parent jet Signal dependence on the parent jet characteristics characteristics CERN-PH-EP/2009-14CERN-PH-EP/2009-14

• Non-trivial behaviour with the jet neutral Non-trivial behaviour with the jet neutral and total multiplicitiesand total multiplicities

TheThe DELPHIDELPHI detectordetector

Typical hadronic event with soft ɣ

e , p=100MeV/c e , p=100MeV/c —›—›

<—<— e , p=390MeV/c e , p=390MeV/c

--

++

High neutral flow soft ɣ

Neutral je

t energy=40GeV

Neutral je

t energy=40GeV—›—›

EEɣɣ=63

0MeV

—›

=63

0MeV

—›

SignalSignal observationobservation

==3.4±0.2±0.63.4±0.2±0.6 =4.0±0.3=4.0±0.3±±0.80.8

ObservedObserved photonphoton rate:rate:

(69.1±4.5±12.9) ˣ 10 ɣ/jet(69.1±4.5±12.9) ˣ 10 ɣ/jet

PredictedPredicted hadronichadronic bremsstrahlungbremsstrahlung::

(17.1±0.1±1.2) 10 ɣ/jet(17.1±0.1±1.2) 10 ɣ/jet

=4.0±0.3±0.8=4.0±0.3±0.8

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Check-upsCheck-ups

ChangingChanging generatorgenerator TestTest withwith chargedcharged particlesparticles

Difference/Signal = 1:7Difference/Signal = 1:7 Difference/Signal = 1:11Difference/Signal = 1:11

TestTest withwith neutralneutral pionspions

TwoTwo convertedconverted photonsphotons ConvertedConverted + + HPCHPC photons photons

Combined upper limit: RD/MC < 1.015 at 95% CLCombined upper limit: RD/MC < 1.015 at 95% CL

DELPHIDELPHI dimuon event dimuon event

thethe samesame event,event, the photon the photon region region

Muon inner bremsstrahlung in μ μ events of Z decays

00

++ __

Signal corrected for efficiency:

25.9±4.0±1.4

Muon inner bremsstrahlung:

23.30±0.01±0.93

Signal/muon brems:

1.06±0.12±0.07

Upper limit for an excess:

1.29 at 95% CL

DeadDead conecone of the muon of the muon bremsstrahlungbremsstrahlung

Max at Max at √√3/3/ГГ

ГГ = 432 = 432

4 mrad4 mrad

Max at 1/Max at 1/ГГ22

Dependences on jet characteristics

SIMILARLY TO BREMSSTRAHLUNGSIMILARLY TO BREMSSTRAHLUNG

Jet momentumJet momentum Jet charged multiplicityJet charged multiplicity

Dependences on jet mass and hardness

Similarly to bremsstrahlungSimilarly to bremsstrahlung

mmjet jet = = √ √ EEjet ― jet ― ppjetjet22 22 κκj j = = EEjet jet sin sin

θθ/2/2ΘΘ is angle to the closest jet is angle to the closest jet

Dependences on Nneu and Npar

What about explanation?

• No theoretical explanation of the

phenomenon still exists, in spite of the problem being

under active investigation.

StrongStrong dependencedependence onon NNneuneu suggests:suggests:a) either the radiation comes from individuala) either the radiation comes from individual quarks and/or quark-antiquark pairs;quarks and/or quark-antiquark pairs;b) or it comes, due to some collective effects,b) or it comes, due to some collective effects, from a jet as a whole.from a jet as a whole.

Collective models fail experimental tests:Collective models fail experimental tests:no dependence on Mno dependence on Mjetjet, neither on jet net charge, neither on jet net charge(collective behaviour predicts N(collective behaviour predicts Nnet net dependence). dependence).

NoncoherentNoncoherent modelsmodels agree well with linear agree well with lineardependence on total particle multiplicitydependence on total particle multiplicity

(the radiation (the radiation ~ ~ sum of quark charges squared)sum of quark charges squared)

ModificationModification ofof noncoherentnoncoherent approach:approach:consider quark-antiquark pairs as radiating consider quark-antiquark pairs as radiating

(electromagnetic) dipoles:(electromagnetic) dipoles:

d = d = Ʃ q Ʃ q i i r r ii

22

ii=1=1

22—›—›—›—›

String fragmentation model

List of (failed) models• String (Lund) model• Van-Hove/Lichard model (cold quark-gluon

plasma, via processes qq->gɣ, qg->qɣ)• Collective model (Barshay’s pion

condensate)• Armenian model (Unruh-Davies effect)• Nachtmann’s model (quark synchrotron

radiation in the stochastic QCD vacuum)• Shuryak’s model (confinement forces)

__

Models still alive but underdeveloped

• Internal quark loop model (Simonov Yu.A.):

based on nonperturbative QCD methods applied to the large size systems (contains a strong enhancement

mechanism)• Gluon dominance model (Kokoulina E.S.) appeals to a new physics phenomenon: exitation of physical vacuum leading to thermal radiation with T ~ 30 MeV

__

Wong’s model (arXiv:1001.1691)

• Exploits longitudinal dominance and transversalconfinement of fragmentation process in order touse the formalism of 2-dimensional QED (QED2)for calculation of anomalous soft photon yieldassociated with the meson production. • Production of mesons in the model arises from theoscillations of colour charge densities of the quark vacuum inthe flux tube when a quark and antiquark pull away from eachother at high energies. Because a quark carries both a colour charge and an electric charge, the underlying dynamical motion of quarks will also generate electric charge oscillations which will lead to ASP production.

__

Is it a tail of New Is it a tail of New Physics?Physics?

Is it a tail of Is it a tail of NewNew PhysicsPhysics??