# Presentation: 919892

## Leading Neutron Energy and pT Distributions from ZEUS

- A. Solano
- Univ. of Torino and INFN
- On behalf of the ZEUS Collaboration

- Outline:
-  Introduction and models
-  Data-sets
-  Leading neutrons in DIS
-  Leading neutrons in photoproduction
-  Leading neutrons & leading protons
-  Summary

- Low-x, 29/6/2006, A.Solano

## Leading neutron production

- Neutrons often carry a large fraction of the proton beam energy  *x*L* **= E**n**/E**p*

- Test of production models:

-  `Standard` fragmentation:
-    compare to usual MC generators

-   Particle exchange:
-    compare to One Pion Exchange (OPE) models
-    probe the structure function of the exchange particle
-    test vertex factorization: many models predict
-       factorization violation due to rescattering on the
-       photon (neutron absorption)

**e’**

**e’**

- Low-x, 29/6/2006, A.Solano

## Absorption

- p

-    d’Alesio and Pirner, EPJ A7 (2000) 109
-  the larger the photon, the fewer the n’s
-   detected (more absorption in PHP than DIS)
-  the smaller the nπ system, the fewer the n’s
-   detected (more absorption at high pT in PHP
-   vs. DIS)

- Absorption from additional pomeron exchange: recently Kaidalov, Khoze, Martin, Ryskin
- (Nikolaev, Speth, Zakharov, hep-ph/9708290)                 hep-ph/0602215;
-                                                                                            Khoze, Martin, Ryskin  hep-ph/0606213

-  evaluate correction due to enhanced
-   absorptive diagrams **B** (~ 15%)
-  show importance of migration due to
-   rescattering for *x*L< 0.8
-  include effects of  ρ, a2 exchange
-  estimate the gap survival factor (important
-   for LHC!) which takes into account that
-   rescattering may populate the rapidity gap
-   with secondary particles carrying away
-   energy from the leading neutron

**A**

**B**

- Low-x, 29/6/2006, A.Solano

## Leading baryon detectors

- y

**Forward Neutron Calorimeter (FNC):**
-  10 λ*l** *Pb-scintillator sandwich
-  σ/E = 0.65/√E,  energy scale accuracy ±2%
**Forward Neutron Tracker (FNT):**
** **scintillator hodoscope at 1 λ*l* (position detector)
-  σX,Y = 0,23 cm             σθ = 22 μrad

- (*x**o *, *y**o*):  neutron 0o point

**Leading Proton Spectrometer (LPS):**
-  6 stations of silicon μstrips detectors (S1-S6)
-  *σ**XL* < 1%

*p*T resolution is dominated by *p**T ** *spread of the proton beam (50-100 MeV)

- Low-x, 29/6/2006, A.Solano

## Data samples

- For all samples:    neutron:  0.2 < *x*L < 1
-                                              θn < 0.75 mrad          *p*T2  < 0.476 *x*L2 GeV2
-                                                                     (due to magnet apertures)

- Deep Inelastic Scattering (DIS):   40 pb-1,  Q2 > 2 GeV2
- Photoproduction (PHP):    6 pb-1,  Q2 < 0.02 GeV2
- Dijets in photoproduction:    40 pb-1,  Q2 < 1 GeV2, 130 < W < 280 GeV
-                                                              *E*Tjet1 > 7.5 GeV, *E*Tjet2 > 6.5 GeV
-                                                              -1.5 <  ηjet1,2 <  2.5

- DIS and PHP have very different inclusive cross sections  σinc
-         for sensible comparisons look at  σLN/σinc
- Additional benefit: systematic uncertainties of central ZEUS cancel

- Low-x, 29/6/2006, A.Solano

## xL distribution in DIS

- Below *x*L ~ 0.7 yield drops due to decreasing *p*T2 range

- Systematic uncertainties from:
-  0o point
-  FNC energy scale
-  dead material before FNC

- Low-x, 29/6/2006, A.Solano

## pT2 distributions in DIS

-  Note varying *p*T2 ranges in different *x*L bins
-  Data are well described by exponential distributions:  get intercepts and slopes

- Low-x, 29/6/2006, A.Solano

## pT2 distributions in DIS: intercepts & slopes

**Intercepts a(*****x*****L****)**

**Slopes b(*****x*****L****)**

- Low-x, 29/6/2006, A.Solano

## b(xL) in DIS: comparison with OPE models

- Numerous parametrizations of the pion flux  *f*π/p(*x**L**,p**T**2*)  in literature

- Best agreeing models shown here
-           More refinement needed:
-           absorption, migration, more exchanges

- A. Martin, 2*nd* HERA-LHC Workshop

- Low-x, 29/6/2006, A.Solano

## LN in DIS: comparison with non-OPE MCs

- Compare with several popular MC models w/o OPE, all with default settings:
-   LEPTO ~ ok in shape and magnitude for the *x*L distribution
-   All models fail in reproducing the slopes

- Low-x, 29/6/2006, A.Solano

## xL distribution in PHP vs. DIS

- Curves from model by
- Kaidalov, Khoze, Martin, Ryskin

-   Data in agreement with absorption hypothesis
-   Gap survival factor from *x*L distribution:
-     (A. Martin, 2*nd* HERA-LHC Workshop)

- ρ

- S2 ~ 0.5  (~ 0.4 including ρ, a2)

- Low-x, 29/6/2006, A.Solano

## pT2 distributions in PHP vs. DIS

- normalized to 1 at *p*T2 = 0

- π

- π,ρ,a2

-  Clear difference:
-    b(PHP) > b(DIS)  for 0.6 < *x*L < 0.9
-  Qualitatively consistent with absorption:
-    more abs. at small rnπ , i.e. large *p*T, in PHP

- Comparison with model by
- A. Martin, 2*nd* HERA-LHC Workshop

- Low-x, 29/6/2006, A.Solano

## LN in dijet photoproduction vs. DIS

- Neutron energy spectra suggest phase space limitation:
- with energetic dijets in the final state
- little room is left for leading neutron

- Slopes have similar magnitude, statistics limits any conclusion on possible differences

- Hard to draw any conclusion on absorption

- Low-x, 29/6/2006, A.Solano

## Leading neutrons & leading protons in DIS

-   Clear different trends
-   LN: main contribution is π exchange
-   LP: contribution of other trajectories
-   Similar magnitude for *x*L~ 0.7-0.8
-            π exchange LN ~ LP

- Low-x, 29/6/2006, A.Solano

## Summary

-   Precise measurements of leading neutron *x*L and *p*T2 distributions were presented
-   Pure OPE models do not describe the data
-   MCs with `standard` fragmentation do not describe the data, LEPTO is promising
-   The b-slopes in DIS are better reproduced including ρ, a2 exchanges
-   Comparing PHP with DIS shows that leading neutron production is suppressed
-     in photoproduction at low *x*L, high *p*T, in agreement with absorption hypothesis
-   The neutron energy spectrum in photoproduction is compatible with effects of
-     absorption and migration as calculated by Kaidalov, Khoze, Martin and Ryskin,
-     by which a gap survival factor S2 ~ 0.4 has been evaluated
-   Leading neutrons in dijet photoproduction have similar slopes but a different
-     energy spectrum than in DIS
-   The b-slopes of protons and neutrons have different behaviours but agree at
-     *x*L ~ 0.7-0.8  where π exchange is dominant in both cases

- Low-x, 29/6/2006, A.Solano