Hadron Production in Photon-Photon Processes at the International Linear Collider

Swathi Sasikumar 29th Feb, DPG 2016 Introduction

> The International Linear Collider (ILC) - a proposed e+e- collider Tunable at centre-of-mass energies between 200 GeV to above 500 GeV (will be upgraded to 1 TeV) Luminosity of about 500 fb-1 in first four years at 500 GeV

> background an important issue Every e+e- event is accompanied by one or two photon collisions Overlay of low Pt hadrons over the important physics events

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 2 Impact of Hadron Overlay

> Overlay of soft hadrons affects a few 0.1 ZHH (llbbbb) γγ processes specific but important cases e.g. : M(H1)= 125GeV not included 0.08 included s=500GeV

Measurement of Higgs self coupling from normalised 0.06 double-Higgs production - rare process 0.04 Overlay degrades the mass resolution 0.02

Signals of new particles with small mass 0 differences (dark matter candidates) 50 100 150 200 M(H1) [GeV] Ref:Claude Duerig Visible decay products very soft and thus similar to hadron overlay

Need more differential methods to remove gamma-gamma background • Identify gamma-gamma collision products by explicit reconstruction Very important to have detailed simulation

hep-ph/1307.3566 (2013)

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 3 e+ e- beams are accompanied by : > Luminosity as a function of ps

Real photons fr(x): 10 > real-real real-virtual

‣ Beamstrahlung - emission of real ] virtual-virtual -2 1

photons in high electrical field of cm 33 oncoming bunch 0.1 /GeV) [10

CM 0.01 /d log(E

γγ 0.001

> Virtual photons fv(x): dL

‣ Weizsaecker-Williams process - emission 0.0001 0.01 0.1 1 10 100 of virtual photons which can interact with p s E CM [GeV] an oncoming photon or an electron Ref:hep-ph/0406010v1(2004)

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 4 Vector Meson Dominance

> Photon is a hadron 1/400 of the time > Vector meson dominance the most dominating subprocess in photon-photon processes > A photon fluctuates into a vector meson (⍴,⍵,ϕ, j/⍦, ⌥ ) ( same quantum properties) > The highest probability for the photon to fluctuate is into a rho meson. > Production of number of low momentum soft hadrons

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 5 Cross sections at different ps

> Comparison of cross section ×10-3 predictions from Pythia 6,PDG and 0.7 MD-1 PLUTO

Amaldi et al (hep-ph/9305247) with (mb) OPAL σ 0.6 L3 data from LEP,PETRA and VEPP Pythia standard function(PDG) p Amaldi Parametrization > s > 10 GeV: Good description of 0.5 LEP data with Pythia-6 0.4 p > s < 10 GeV: Measurements have

large uncertainties and widespread Cross sections 0.3

> Cross section from Pythia 6 possibly 0.2 too low? 10 102 103 sγγ (GeV)

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 6 A Dedicated Event generator for processes

> For p s > 2 GeV Pythia 6 could be 10 real-real used to simulate low Pt hadron real-virtual

! ] virtual-virtual

processes -2 1 cm 33

> Below 2 ⇡ threshold, pure QED beam- 0.1 beam interaction is modeled by /GeV) [10

CM Pythia-6 dedicated programs, e.g. Guinea Pig 0.01 Guinea Pig ? /d log(E

> Need to evaluate impact of uncovered γγ 0.001 region - how can it be modeled? dL

0.0001 > Dedicated generator have been 0.01 0.1 1 10 100 p E [GeV] developed in ILC community to study s CM the low energy areas by Tim Barklow

> To study the events :

p s > 2 GeV Pythia-6 is used

p s < 2 GeV Barklow generator is used

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 7 Event Properties of Barklow generator

> Barklow generator takes cross section from Amaldi parametrization thus folding it into the luminosity spectrum giving all combinations of hadrons at given p s

> The Barklow generator produces ⇡⇡ - at pion threshold energy (300 MeV) ⇢⇡ - at rho threshold energy (1.5 GeV) 0 ⇢± ⇡±⇡ ! 0 + ⇢ ⇡ ⇡ ! 0 > The cross sections for producing ⇢ is greater than ⇢±

> Until recently a much simpler version of Barklow generator was used which produced 0 ⇢ ± without width and produced no ⇢ s

> A better version of the generator was thus developed correcting the flaws in older version - big progress

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 8 Investigating Pion threshold energy range

> To study impact on detector, simulated collisions of two real photons in the range 2 ⇡ < p s < 0.5 GeV

> The particles produced here have very low Pt

> Significance of particles produced in these range with respect to ILC condition

> The polar angles of all reconstructed Theta of reconstructed particles (0.1-0.5GeV) particles normalized to particles per 0.5

photon-photon event is plotted 0.4

> Reconstruction of charged particles as normalized 0.3 neutral above 7 degrees due to tracker acceptance 0.2

> Particles at pion threshold can be seen in 0.1 the detector 0 0 50 100 150 θ[°] Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 9 Conclusions

> Although ILC environment is much cleaner than LHC, the overlay from backgrounds is still an issue

> The impact of this overlay is observed on a very few but important physics events

> The existing generators can cover the impact partially (data in insufficient) , we need better methods to cover the impact completely

> Recently significant progress was made in modeling the low energy region (Barklow generator)

> Currently evaluating impact of these processes on Physics studies and detector design for ILC

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 10 Back up

Swathi Sasikumar | Hadron production in gamma-gamma processes | 05-11-2015 | Page 11 Hadron Interactions in VMD

A photon is a hadron a fraction 1/400 of the time Rise in gamma-gamma cross sections much similar to hadronic cross sections All event classes known for ordinary hadron-hadron interactions are found to occur here Behaves more like a Hadron collider than a lepton collider

Reference : Particle Data Group 2014

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 12 Current => low pt hadron simulation

The => low pt hadron processes simulated by methods provided by T.Barklow Based on a blend of

Pythia events at p s > 10 GeV Paper by Chen, Barklow and Peskin for 0.3 GeV< p s <10GeV Integrated into Whizard which provides the => Hadron processes

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 13 Outline

‣ Introduction

‣ Total Cross Sections

‣ Event Properties

‣ Towards an improved description

‣ Summary and Outlook

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 14 Total Cross Sections from Chen, Barklow and Peskin

Determination of photon-photon hadronic cross sections essential for computing hadronic backgrounds

Photon-photon total cross section proportional to ⍴-⍴ total cross section The parametrization of Amaldi et al. give cross sections as

3 2.1 0.37 ( >hadrons)= (1 + (630 10 )[ln(s) +(1.96)s ) 0 ⇤

(Ref: Peskin et al, PRD, 49(3209) 1994)

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 15 - Processes in Pythia 6.4

Direct Interactions(DIR) - Real photons interacts directly

Vector Meson Dominance(VMD) - Photon fluctuates into a vector meson

Anomalous Interactions(GVMD) - Photon fluctuates into a q q¯ pair of larger virtuality

Deep inelastic Scattering(DIS) - A process of probing the Hadrons with very high energy leptons.

Subprocesses Cross-sections (nb) VMD * VMD 239.2 DIR * VMD 87.52 GVMD * DIR 9.77 GVMD * GVMD 12.05

Usage of Gamma - gamma beams with varying energy is preferred - more realistic results Missing of Deep Inelastic scattering process - trying to include by changing some parameters

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 16 Update: Total cross section from PDG

The standard theory for calculating hadronic cross sections as per PDG:

2 2 S S ⌘1 = [H ln ( )+P ]+R1 ( ) SM SM

(Ref: Particle data group 2014) R: Regge term defines the cross section Cross sections for different energies at low energies where the interactions are ×10-3 0.7 (mb)

explained using meson exchange σ 0.6 P : Pomeronchuk term defines the cross section at higher energies where the 0.5 Cross section interactions are explained using pomeron 0.4 exchange. 2 ~c 0.3 H: H = ⇡ ( ) The Heisenberg term M 2 defines the rise in cross section with 0.2 2 3 energy 10 10 Energy (GeV)10

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 17 Total Cross Sections

Cross sections for different energies ×10-3 0.7 The results from Pythia very MD-1

(mb) PLUTO much in accordance with the σ OPAL L3 Pythia standard function and the 0.6 standard function(PDG) measured data. Amaldi Parametrization

0.5

Data for gamma-gamma cross cross section sections at very high energies not available 0.4 Pythia seems to be quiet okay for evaluating gamma- 0.3 gamma backgrounds 0.2 2 3 10 10 Energy(GeV)10

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 18 Outline

‣ Introduction

‣ Total Cross Sections

‣ Event Properties

‣ Towards an improved description

‣ Summary and Outlook

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 19 Event Properties

Taking a look at the individual events from => low pT hadrons

m ⇢ = 770 MeV and ⇢= 145 MeV Barklow generator produces Rho mesons of same mass and no width at all. Most of the events having two rho 0 mesons are ⇢ ± and no ⇢ Events formulated in a way to have three following possibilities + ⇡a ⇡

⇡a± ⇢± + ⇢a ⇢ Two groups of rhos: Should have neutral Rhos and more • One with correct width -Pythia complex events too • One with zero width-not pythia

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 20 Shortcomings with Pythia

Pythia can create very nice 10 real-real events with all variety of rhos real-virtual and complex events ] virtual-virtual -2 1 cm

Hadron productions initialized 33 at 300MeV 0.1

Crucial to understand /GeV) [10 processes at these energies CM 0.01 Guinea Pig ? Pythia Pythia cannot simulate for /d log(E energies below 2.5GeV γγ 0.001 dL Trying for various solutions 0.0001 By changing few 0.01 0.1 1 10 100 p parameters Ref:hep-ph/0406010v1(2004) E CM [GeV] s Looking at Barklow's methods

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 21 Outline

‣ Introduction

‣ Total Cross Sections

‣ Event Properties

‣ Towards an improved description

‣ Summary and Outlook

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 22 Status of events in Current Simulation

Energy of Barklow Events > Whizard events a blend of events from Energy of Barklow events >>h2 Entries 6887 pythia and Barklow generator. 400

Events Mean 0.9349 Events upto 10GeV from Barklow RMS 0.5652 > 300 generator

> Events above 10GeV from Pythia 200

> Can Use Pythia to generate events 100 from 2.5GeV to 10 GeV

0 0 0.5 1 1.5 2 2.5 3 Energy(GeV) Energy of Whizard Events Energy of Whizard Events (real-real) Energy of Whizard Events (real-virtual)

Pythia Pythia 3 3 10 Barklow 10 3 Events Barklow 10 Pythia Barklow

102 2 10 102

10 10 10

1 1 1 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Energy (GeV)

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 23 Status of events in Current Simulation

Energy of Barklow Events Whizard events a blend of events from Energy of Barklow events >>h2 > Entries 6887 400

pythia and Barklow generator. Events Mean 0.9349

RMS 0.5652 > Events upto 10GeV from Barklow 300 generator

> Events above 10GeV from Pythia 200 > Can Use Pythia to generate events from 2.5GeV to 10 GeV 100

0 0 0.5 1 1.5 2 2.5 3 Energy(GeV) Energy of Whizard Events Energy of Whizard Events (real-real) Energy of Whizard Events (real-virtual)

Pythia 3 3 Pythia Pythia Events 10 10 103 Barklow Barklow Barklow

102 2 10 102

10 10 10

1 1 1 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Energy (GeV) 0 5 10 15 20 25 30 cm

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 24 Summary and Outlook

Summary: Through the detailed study of the events show that reconsidering the events in Whizard is important Very important to have better simulation ways to remove backgrounds Outlook: Try to change the cut of Pythia from 10 GeV to 2.5 GeV in Whizard Try to bring Pythia for lower energies by changing parameters How much important is the 0.3-2.5 GeV range and solutions Check alternative programs Improve Barklow generator

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 25 Back up slides

Swathi Sasikumar | Hadron production in gamma-gamma processes | 05-11-2015 | Page 26 Pythia as a tool

Using Pythia to evaluate the properties of particles in the Hadronic interactions Plotting for Energy, Transverse Momentum and Cosine of Polar angle at 30 GeV centre of mass energy we have

mcene {mcgst==1} sqrt(mcmox*mcmox+mcmoy*mcmoy) {mcgst==1}

104 104

103 103

102

102

10

10

1

0 5 10 15 0 1 2 3 4 5 Energy(GeV) Transverse Momentum (GeV)

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 27 Improvements in Barklow Generator

> Improvements were implemented in Invariant Mass Distribution of rho mesons Invariant Mass Distribution for rho meson>>h2 Barklow generator. 14000 Entries 140953 Mean 0.7395

RMS 0.1303 > Rho meson produced with full width 12000 10000 > Neutral Rho mesons at lower energies produced with higher cross section 8000

6000 > Events formulated to have possibilities as in table 4000

> A big progress towards an improved 2000 version of event generator 0 0 0.5 1 1.5 2 GeV

Before + + Improvements ⇡ ⇡ ⇡±⇢± ⇢ ⇢

+ 0 0 0 0 0 0 After + ⇡±⇢± ⇢ ⇢ ⇢ ⇢ ⇢ ⇡± ⇢ ! Improvements ⇡ ⇡ ⇡ ⇡

Swathi Sasikumar | Hadron Production in photon-photon processes | 29-02-16 | Page 28