Particle production in DIS and photoproduction from ep ... - Zeus

XXXVII International Symposium on Multiparticle Dynamics, Berkeley, CA, USA 

SMD 2007: ”Particle production in DIS and photoproduction from ep collisions” by Anna Galas 1/20 

Particle production in DIS and photoproduction 

from ep collisions 

Anna Galas 

on behalf of ZEUS+H1 

Institute of Nuclear Physics Polish Academy of Sciences 

ul. Radzikowskiego 152, 31-342 Kraków, Poland 

6 August 2007

Presented results 


Measurement of K 0 S , Lambda, Antilambda Production at HERA 

hep-ex/0612023, European Physical Journal C 51 (2007) 1-23 

Bose-Einstein Correlations of Charged and Neutral Kaons in Deep Inelatsic Scattering 

at HERA 

hep-ex/0706.2538, published online in Physics Letters B 

Measurement of (anti)deuteron and (anti)proton production in DIS at HERA 

hep-ex/0705.3770, published online in Nuclear Physics B 

Charged Particle Production in High Q 2 Deep Inelastic Scattering at HERA 

hep-ex/0706.2456, submitted to Physics Letters B 

Scaled Momentum Spectra in the Current Region of the Breit Frame 

ZEUS Preliminary

ep collisions at HERA 


ep → e ′ X collisions at HERA give 

informations about soft and hard 

processes. 

@ HERA: 

e ± (27.6 GeV ) + p (820/920 GeV ) 

√ s = 

√ 

4Ee E p = 300/318 GeV 

where E e , E p are energies of e and p 

beams. 

Tha data presented here were taken 

during: 1996-2000 (L = 121 pb −1 ) 

using ZEUS and in 2000 

(L = 44 pb −1 ) using H1 detector. 

Soft QCD processes @HERA can be measured using: 

◮ hadronization (DIS) Q 2 > 1 GeV 2 

◮ photoproduction (γp) Q 2 ∼ 0 GeV 2

DIS kinematic variables for ep → e ′ X 

P/k the initial-state four 

momenta of the proton and 

electron/positron 

s = (P + k) 2 the cms energy 

squared of the ep system 

DIS processes: 

◮ ep → e ′ X (Neutral Current) 

- exchange γ ∗ , Z 0 

◮ e + (e − )p → ν(¯ν)X (Charged 

Current) - exchange W + , 

W − 

where X - hadronic final state 

W = (P + q) 2 the cms 

energy of the γ ∗ 

virtual-photon-proton 

system 

The photon virtuality Q 2 and 

Bjorken variables are defined as: 

Q 2 = −q 2 = −(k − k ′ ) 2 

x BJ 

= Q2 

2P·q 

y BJ = P·q 

P·k 

Q 2 = s · x BJ y BJ 

SMD 2007: ”Particle production in DIS and photoproduction from ep collisions” by Anna Galas 4/20

Photoproduction processes 


γp processes are characterized by quasi-real exchange photon, γ, 

and very small four-momentum-transfer: Q 2 ∼ 0 GeV 2 

Examples of leading order diagrams 

in γp: (a) direct γp and (b) resolved 

γp. 

◮ direct γp - photon interacts 

directly with a parton in the 

proton 

◮ resolved γp - photon 

behaves as a source of 

partons, one of which takes 

part in the interaction with 

the proton 

Pjets 

xγ OBS = 

E jet 

T e−ηjet 

2y JB E e 

◮ xγ 

OBS 

◮ xγ 

OBS 

= 1 direct γp 

< 1 resolved γp

K 0 S and Λ + ¯Λ signals 

ISMD 2007: ”Particle production in DIS and photoproduction from ep collisions” by Anna Galas 6/20 

◮ Λ + ¯Λ background: ∼ 6% 

◮ K 0 s background: ∼ 4% 

◮ clear signals, large statistics, low 

backgrounds 

Event requirements: 

◮ primary and secondary vertices well 

separated 

◮ two secondary vertex tracks with 

opposite charges

K ± and d, ¯d identification 


An example of K + selection method 

◮ The tracks were accepted if 

f < dE 

dx 

< F , where f , F are 

functions motivated by 

Bethe-Bloch equations: } 

f = 0.08 

p 

+ 1 2 

F = 0.17 for K 

p 

+ 1.03 

+ 

2 

where p is the total track 

momentum. 

◮ 

dE 

dx 

> 1.25 to minimize background 

from π 

◮ p < 0.9 GeV to compromise 

between purity and momentum 

range (trouble in particle 

separation for large p)

K 0 s , Λ and ¯Λ cross-sections 


The cross-section were calculated in the following kinamatic regions: 

⎫ 

◮ Q 2 > 25 GeV 2 and 0.02 < y BJ < 0.95⎪⎬ 

◮ 5 GeV 2 < Q 2 < 25 GeV 2 and ⎪⎭ + |ηK 0 ,Λ,¯Λ| s < 1.2 

0.6 

T 

< 2.5 GeV 

0.02 < y BJ < 0.95 

◮ Q 2 < 1 GeV 2 and 0.2 < y BJ < 0.85 

(γp) 

} 

+ |ηjet | < 2.4 

E jet + 2 jets 

> 5 GeV 

T 

dσ Ks 0 ,Λ,¯Λ 

dY = 

N 

A·α·B·∆Y 

where: 

N - number of K 0 s , Λ, ¯Λ in a bin of 

width ∆Y , α - luminosity, A - 

acceptance and B - branching ratio 

The differential cross-sections 

were measured as functions of: 

◮ DIS: 

p K s 0 ,Λ,¯Λ 

T 

, η K s 0,Λ,¯Λ , x BJ , Q 2 

◮ γp: 

p K s 0 ,Λ,¯Λ 

T 

, η K s 0,Λ,¯Λ , xγ 

OBS


Differential K 0 s 

cross-sections 

ZEUS 

ZEUS 

dσ/dP LAB 

T (pb/GeV) 

ZEUS 121 pb -1 

LEPTO (0.3) 

ARIADNE (0.3) 

ARIADNE (0.22) 

10 4 

10 3 

1 1.5 2 2.5 

dσ/dP LAB 

T (pb/GeV) 

K S 

0 

LAB 

dσ/dP T (pb / GeV) 

dσ/dη LAB (pb) 

14000 

12000 

10000 

8000 

6000 

4000 


8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 

10 3 

5 < Q 2 < 25 GeV 2 Q 2 > 25 GeV 2 

1 1.5 2 2.5 

P LAB 

T (GeV) 

P LAB 

T (GeV) 


3000 

2500 

2000 

1500 

1000 

500 

0 

-1 -0.5 0 0.5 1 

η LAB 

Q 2 > 25 GeV 2 

-1 -0.5 0 0.5 1 

η LAB 

OBS 

dσ/dx γ (pb) 

10 4 1 1.5 2 2.5 

40000 

35000 

30000 

25000 

20000 

15000 

10000 

5000 

0 

LAB 

P T (GeV) 

0 0.2 0.4 0.6 0.8 1 

OBS 

x γ 

2000 

0 

-1 -0.5 0 0.5 1 

● ZEUS 121 pb -1 

η LAB 

Jet energy scale uncertainty 

– PYTHIA 

K 0 s 

Photoproduction 

◮ ARIADNE (λ s = 0.3) describes 

data reasonably well 

◮ ARIADNE (λ s = 0.22) and 

LEPTO describe the data less 

satisfactory 

◮ reasonable description by PYTHIA

Differential Λ + ¯Λ cross-sections 


dσ/dP LAB 

T (pb/GeV) 


ZEUS 

10 4 10 3 

5 < Q 2 < 25 GeV 2 


LEPTO (0.3) 

ARIADNE (0.3) 


Λ+Λ – 

10 3 

Q 2 > 25 GeV 2 

1 1.5 1 1.5 2 2.5 

2 2.5 

P LAB 

T (GeV) 

P LAB 

T (GeV) 

4000 

3500 

3000 

2500 

2000 

1500 

1000 

500 

0 

dσ/dP LAB 

T (pb/GeV) 


1000 

800 

600 

400 

200 

0 

-1 -0.5 0 0.5 1 

η LAB 

Q 2 > 25 GeV 2 

-1 -0.5 0 0.5 1 

η LAB 

LAB 

dσ/dP T (pb / GeV) 

OBS 

dσ/dx γ (pb) 

10 4 

10 3 

1 1.5 2 2.5 

25000 

20000 

15000 

10000 

5000 

0 

LAB 

P T (GeV) 

0 0.2 0.4 0.6 0.8 1 

ZEUS 

OBS 

x γ 


9000 

8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 

-1 -0.5 0 0.5 1 

η LAB 

● ZEUS 121 pb -1 


– PYTHIA 

Λ+Λ – 


◮ ARIADNE (λ s = 0.3) describes 

the data reasonably well 

◮ ARIADNE (λ s = 0.22) and 

LEPTO do not describe the data 

well 

◮ reasonable description by PYTHIA

Baryon to meson ratio 


- (N Λ +N Λ ) / NK 0 

S 

0.6 

0.5 

0.4 

0.3 

0.2 


LEPTO (0.3) 

ARIADNE (0.3) 


ZEUS 

- (N Λ +N Λ ) / NK 0 

S 

0.6 

0.5 

0.4 

0.3 

0.2 

(N Λ + N Λ 

– ) / NKs 

0 

1 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

ZEUS 

(N Λ + N Λ 

– ) / NKs 

0 

1 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.1 

5 < Q 2 < 25 GeV 2 0.1 

0 

Q 2 > 25 GeV 2 

0.2 

0.1 

0.2 

0.1 

- (N Λ +N Λ ) / NK 0 

S 

0 

0.6 

0.5 

0.4 

0.3 

1 1.5 2 2.5 

P LAB 

T (GeV) 

- (N Λ +N Λ ) / NK 0 

S 

0.6 

0.5 

0.4 

0.3 

1 1.5 2 2.5 

P LAB 

T (GeV) 

(N Λ + N Λ 

– ) / NKs 

0 

0 

1 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

1 1.5 2 2.5 

LAB 

P T (GeV) 

0 

-1 -0.5 0 0.5 1 

● ZEUS 121 pb -1 

η LAB 


– PYTHIA 

0.2 

0.1 

0 

5 < Q 2 < 25 GeV 2 0.2 

0.1 

0 

-1 -0.5 0 0.5 1 

η LAB 

Q 2 > 25 GeV 2 

-1 -0.5 0 0.5 1 

η LAB 

0.3 

0.2 

0.1 

0 

0 0.2 0.4 0.6 0.8 1 

OBS 

x γ 


◮ ARIADNE (λ s = 0.3) follows the 

shape of the data 

◮ Baryon to meson ratio increases at 

low xγ 

OBS up to 0.7, not predicted 

by PYTHIA 

◮ For xγ 

OBS = 1, same baryon to 

meson ratio as in DIS and e + e −

BEC between K 0 s K 0 s and K ± K ± 


Bose-Einstein effect is an enhancement 

in the production of identical 

bosons with similar momenta. 

BE effect is related to the timespace 

characteristic of the particle 

emission source. 

Two bosons with momenta ⃗p 1 , 

⃗p 1 produced at points r 1 and r 2 

x 2 

r 2 π 1 

r 1 

π 1 (p → 1 ) 

π 2 (p → 2 ) π 2 

x 1 

R(Q 12 ) = P(Q12) 

In experiment: 

P ref (Q 12) 

where Q 12 = p q 

−(p 1 − p 2 ) 2 = M 2 − 4m 

boson 

2 

π 1 , π 2 - bosons 

Standard parametrisation of R(Q 12 ) is Goldhaber-like parametrisation: 

R(Q 12 ) = α(1 + λe −Q2 12 r2 ) 

The correlation function R(Q 12 ) is measured using double ratio method: 

R(Q 12 ) = 

P(Q 12) data 

P mix (Q 12 ) data 

/ P(Q 12) MCnoBEC 

P mix (Q 12 ) MCnoBEC

BEC between K 0 s K 0 s and between K ± K ± 


R(Q 12 ) 

2.2 

ZEUS 

KS 0 KS 

0 

R(Q 12 ) 

2 

1.8 

1.6 

1.4 

1.2 

1 

0.8 

1.8 

1.6 

1.4 

< Q 2 > = 35 GeV 2 

ZEUS (121 pb -1 ) 

Fit 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

Q 12 (KS 0 KS) 0 GeV 

ZEUS 

K ± K ± 

< Q 2 > = 35 GeV 2 

ZEUS (121 pb -1 ) 

Fit 

◮ 96 − 00 e ± p ZEUS data sample ⇒ L = 121 pb −1 

◮ DIS events sample: 2 < Q 2 < 15000 GeV 2 

◮ BE effect clearly visible for both K 0 s K 0 s and K ± K ± 

r = 0.61 ± 0.08(stat) +0.07 

−0.08 (sys)fm K 0 s K 0 s (ZEUS) 

r = 0.57 ± 0.09(stat) +0.15 

−0.08 (sys)fm K ± K ± (ZEUS) 

r = 0.666 ± 0.009(stat) +0.022 

−0.036 (sys)fm π± π ± (ZEUS) a 

◮ good agreement between Ks 0 Ks 

0 and K ± K ± and 

π ± π ± 

◮ the f 0 (980) resonance is expected in the same low Q 12 

region where BEC are measured 

◮ 4% contribution of f 0 (980) in ZEUS data significantly 

change λ value 

1.2 

a Phys. Lett. B583 (2004) 231 

1 

0.8 

0 0.2 0.4 0.6 0.8 1 1.2 

Q 12 (K ± K ± ) GeV

Summary results for BEC 


K 0 s K 0 s 

λ = 1.16 ± 0.29(stat) +0.28 

−0.08 (sys) 

r = 0.61 ± 0.08(stat) +0.07 

−0.08 (sys)fm 

Ks 0 K s 

0 - with 4% of f 0 correction 

λ = 0.70 ± 0.19(stat) +0.28 +0.38 

−0.08 −0.52 (sys) 

r = 0.63 ± 0.09(stat) +0.07 +0.09 

−0.08 −0.02 (sys)fm 

K ± K ± 

λ = 0.37 ± 0.07(stat) +0.09 

−0.08 (sys) 

r = 0.57 ± 0.09(stat) +0.15 

−0.08 (sys)fm 

◮ good agreement with LEP for r 

◮ after f 0 (980) resonance corrections 

of Ks 0 Ks 

0 data good agreement 

with LEP for λ 

◮ different fragmentation processes in 

e ± p (ZEUS) and e + e − (ALEPH, 

DELPHI, OPAL) annihilations 

◮ ZEUS data populate mostly proton 

fragmentation region - we expect 

proton influence

¯d, and ¯p production ratios in DIS 


Ratio 

Ratio 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

1.5 

0.5 

(a) 

ZEUS 

2 

) 

2 

d/p (ZEUS, Q >1GeV 

2 

d/p (ZEUS, Q >1GeV ) 

d/p (H1, γ p) 

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 

p /M 

T 

2 

1 

(b) 

0 

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 

p /M 

T 

2 

2 

) 

p/p (ZEUS, Q >1GeV 

2 

2 

) 

d/d (ZEUS, Q >1GeV 

2 

◮ 96 − 00 e ± p ZEUS data 

sample ⇒ L = 120 pb −1 

◮ DIS range: Q 2 > 1 GeV 2 

◮ the first observation of 

antideuterons in DIS 

◮ ¯d/¯p ratio in good agreement 

with H1 for γp, with pp and 

similar to hadronic Υ(1S), 

Υ(2S) (ARGUS) decays 

◮ the production rate of d is 

higher than that for ¯d 

◮ ¯p/p ratio is consistent with 

unity as expected from 

hadronisation of quark and 

gluon jets


Summary results for B 2 

The parametisation of differential 

cross-section for deuterons 

E d d 3 σ d 

σ dpd 

3 

= B 2 ( Ep 

σ 

d 3 σ p 

d( p d 

2 ) 3 ) 2 

B 2 ∼ r( p n ) 

- the coalescence parameter 

R 

where: 

3 

r( n ) - ratio of neutrons to protons 

p 

R - source radius 

◮ B 2 for DIS is consistent with γp 

◮ B 2 in ep (H1, ZEUS) is 

significantly larger than in 

heavy-ion collisions and e + e − 

annihilations ⇒ smaller sorce 

radius

Charged particles studies in Breit Frame 


scaled momentum distribution 

D(x p , Q) = 1 N 

where: 

dn 

dx p 

N - total number of selected events 

dn - total number of charged tracks 

with x p in the interval dx p 

◮ separetes struck quark and proton 

remnant 

◮ e ± p current region is similar to 

one hemisphere of e + e − 

annihilation 

◮ Q 2 

the scale of the xp in the 

current Breit Frame 

◮ scaled momentum x p = 2·p h 

Q 

e ± p current region of Breit 

Frame (QPM model) 

where p h - momentum of charged tracks 

◮ in e + e − annihilation ⇒ x p = 2·p h 

E ∗ 

in

Scaled momentum distribution D(x p , Q) 

1/N dn/dx p 

1/N dn/dx p 

1/N dn/dx p 

200 

150 

100 

50 

0 

10 

< 0.02 

0 < x p 

H1 Data 

DELPHI 

TASSO 

MARKII 

AMY 

2 

10 

Q,E* (GeV) 

0.1 < x p < 0.2 

12 

10 

8 

0.8 

0.6 

0.4 

6 

10 

Q,E* (GeV) 

0.4 < x p < 0.5 

1 

10 

2 

10 

2 

10 

Q,E* (GeV) 

1/N dn/dx p 

1/N dn/dx p 

1/N dn/dx p 

0.02 < x p < 0.05 

100 

80 

60 

40 

20 

0 

10 

4 

3 

2 

0.4 

0.3 

0.2 

1 

10 

2 

10 

Q,E* (GeV) 

0.2 < x p < 0.3 

5 

10 

2 

10 

Q,E* (GeV) 

< 0.7 

0.5 < x p 

2 

10 

Q,E* (GeV) 

1/N dn/dx p 

1/N dn/dx p 

1/N dn/dx p 

0.05 < x p < 0.1 

35 

30 

25 

20 

15 

10 

10 

2 

1.5 

0.06 

0.04 

0.02 

1 

10 

10 

2 

10 

Q,E* (GeV) 

< 0.4 

0.3 < x p 

2 

10 

Q,E* (GeV) 

< 1.0 

0.7 < x p 

2 

10 

Q,E* (GeV) 

◮ quite good agreement between 

e + p and e + e − 

◮ at low x p significant increase in 

the number of hadrons from low 

to high Q 

◮ at high x p the number of hadrons 

decreases with Q ⇒ this 

corresponds with less tracks 

observed in e + p data than in 

e + e − 

◮ the middle x p and low Q region 

affected by high order QCD 

processes (BGF, ICQCD) which 

occur as a part of hard 

interactions in e + p but not in 

e + e − 

SMD 2007: ”Particle production in DIS and photoproduction from ep collisions” by Anna Galas 18/20

Scaled momentum distribution D(x p , Q) @ ZEUS 


1/σ dσ / d x p 

0< x p < 0.02 

300 

ZEUS (prel.) 0.5 fb -1 

200 

100 

ZEUS 38 pb -1 

0 

10 2 10 3 10 4 

0.1< x p < 0.2 

15 

10 

5 

0 

10 2 10 3 10 4 

0.4< x p < 0.5 

1 

0.75 

0.5 

0.25 

0 

10 2 10 3 10 4 

Q 2 (GeV 2 ) 

80 

60 

40 

20 

ZEUS 

0.02< x p < 0.05 

100 

Kretzer 0.5 < µ R < 2 

AKK 

KKP 

0 

10 2 10 3 10 4 

0.2< x p < 0.3 

5 

4 

3 

2 

1 

0 

10 2 10 3 10 4 

0.5< x p < 0.7 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

10 2 10 3 10 4 

Q 2 (GeV 2 ) 

0.05< x p < 0.1 

40 

30 

20 

10 

0 

10 2 10 3 10 4 

0.3< x p < 0.4 

3 

2 

1 

0 

10 2 10 3 10 4 

0.7< x p < 1 

0.15 

0.1 

0.05 

0 

10 2 10 3 10 4 

Q 2 (GeV 2 ) 

◮ HERA I+II ZEUS data 

⇒ L = 0.5 fb −1 

◮ similar measurements as 

in H1 

◮ small differents between 

different fragmentation 

function parametrisations 

(KKP, KRETZER, AKK) 

◮ similar to H1, NLO 

predictions do not 

describe the D(x p, Q)

Summary 


◮ Ks 

0 and Λ + ¯Λ cross-sections generally well described by ARIADNE 

(λ s = 0.3). The baryon to meson ratio increases up to 0.7 at low xγ OBS , 

not predicted by PYTHIA 

◮ The first measurements of BEC for Ks 0 Ks 

0 and K ± K ± . Good agreement 

for radius between Ks 0 Ks 0 , K ± K ± , π ± π ± and LEP results 

◮ The first observation of antideuterons in DIS. The production rate of d is 

higher than that for ¯d 

◮ The scaled momentum distribution D(x p, Q) broadly supports quark 

fragmentation universality between e + p and e + e − . NLO fails to describe 

the scaling violations seen in H1 and ZEUS data

Particle production in DIS and photoproduction from ep ... - Zeus

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