Leading Baryons at HERAwith the ZEUS

Forward Detectors.

Alberto Garfagnini.Universita’ della Calabria and INFN

on behalf of the ZEUS Collaboration

Brussels, DIS 98, 4-8 April 1998

DIS98 4-8 Apr 1998 A.Garfagnini 2

Outline

• Physics motivations;

• ZEUS forward detectors: LPS and FNC;

• General properties of Leading Baryons (LB) in DIS: energy spectra, uncorrected ratios, rapidity gaps, ...;

• dN/dxL for LP;

• |t| slopes for LP and LN;

• Conclusions

DIS98 4-8 Apr 1998 A.Garfagnini 3

Leading Baryon Kinematics

222

2 1)'(

;'

pL

L

L

t

L

mx

x

x

pppt

p

px

Fraction of beam momentum carried by the LB

Four-momentum squared transferred at the proton vertex

The LB kinematics can be described by:• pt’ transverse momentum of the LB;• pz’ longitudinal momentum of the LB.

Scattered electron measured inCAL (Q2 > 4 GeV2) orBPC (Q2 ~ 0.1-0.8 GeV2)

Leading Baryon (LB) detected in LPS (p) or FNC (n)

Additional variables:

DIS98 4-8 Apr 1998 A.Garfagnini 4

LB Production Pictures

• ‘REGGE’ approach:

• ‘Fragmentation’ approach:

neutral exchange: LPS• diffraction (Pomeron) • exchange

charged exchange: FNC• O.P.E. • structure function ?

LB from recombination effect of the remnant jet fragmentation

p p , n

ee’

DIS98 4-8 Apr 1998 A.Garfagnini 5

ZEUS Forward Detectors

6 plans of detectors per station: • three different strip orientation 0o; +45o; -45o; (strip pitch 115 m (0o), 81 m (45o).• detector cut-out to follow the 10 beam profile.

Leading Proton Spectrometer:6 stations of strip detectorsalong beam line.

Forward Neutron Calorimeter:lead-scintillator calorimeter (10 interaction lengths)

Towers of 5cm vertical dim.

DIS98 4-8 Apr 1998 A.Garfagnini 6

Forward Detectors AcceptanceFNC acceptance: determinedby the beam-pipe apertures

LPS acceptance(S4-S6 - 1994 configuration): determined by the beam-pipe apertures and magnet strength.

Energy scale ~ 1%

Energy resolution: (En) ~ 0.65 En

Measure:- p

t

- xL = |p’|/|pbeam|

Resolution:- pz ~ 0.3%

- pt ~ 3.0%

Measure:- En;

DIS98 4-8 Apr 1998 A.Garfagnini 7

LP Momentum Spectrum (xL vs Q2)

LP xL spectrum measured for low and high photon virtualities.

Arbitrary normalization to the same area for xL < 0.91.

No strong dependence on Q2 for LP momentum spectrum for 0.6 < xL < 0.9

DIS98 4-8 Apr 1998 A.Garfagnini 8

LP Momentum Spectrum (xL vs W2)

DIS98 4-8 Apr 1998 A.Garfagnini 9

LN Energy Spectrum

No dependence of the xL spectrum of LP and LN on W and Q2 for 0.6 < xL < 0.9

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Properties of Events with a LN

Rate of DIS events with a neutron in bins of Q2 and xBJ .

The plot is uncorrected for FNC acceptance and integrated

over neutron energy 0.2 < xL < 1.0

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Properties of Events with a LN

No strong dependence of the ratio on Q2, in xBJ slices, for different neutron energy.

DIS98 4-8 Apr 1998 A.Garfagnini 12

Some xBJ dependence for xL > 0.8 neutrons.The exchange model better than Pomeron parametrisation Not just a kinematical effect; we are sensitive to F2

.

Properties of Events with a LN

22

22

,

,1

QxF

Qx

xFxfx

rBJ

pL

BJLL

FNC

unc

Results are compared with a numerical PDF integration.

02 HPGRVF

08.0

2 1

L

BJ

x

xF

DCTEQF p 42

or

DIS98 4-8 Apr 1998 A.Garfagnini 13

Properties of Events with a LP

Same ratio for LPs with 0.6 < xL < 0.9.

Within the limit of our statistics, the LP rate is independent of xBJ and Q2.

DIS98 4-8 Apr 1998 A.Garfagnini 14

dN/dxL for LPs

The acceptance corrected 1/N dN/dxL distribution has been

measured for LP at low and high Q2. The integral of the distribution for 0.60 < xL < 0.91 gives:

• LP/NTOT (Q2 > 4 GeV2) = 12.7 ± 0.3 (stat.) ± 0.9 (sys.)%;• LP/NTOT (0.1 < Q2 < 0.8 GeV2) = 13.0 ± 0.5 (stat.)+0.7 -0.8 (sys.)%;

DIS98 4-8 Apr 1998 A.Garfagnini 15

dN/dxL MC Comparison

The measured dN/dxL spectrum for LP production is compared with the predictions of Lepto 6.5 and Ariadne MCs.

The soft-colour interaction mechanism of Lepto gives a qualitative description of the LP spectra and is able to predict the strong rise of the diffractive peak.

The integral of the distribution for 0.60 < xL < 0.91 gives:

• LEPTO SCI = 8.2 %;• ARIADNE = 4 %;

DIS98 4-8 Apr 1998 A.Garfagnini 16

GAPCUT selection

• double dissociation1.5 units

• single dissociation

2.5 units

Measured activity

NO activity

Possible activity

pseudorapidity

+4 0 -2+2 -4

Leading Baryon

An event is accepted if:

ZEUS

2tanln

DIS98 4-8 Apr 1998 A.Garfagnini 17

LB and GAPCUT

At xL ~ 1 almost all the LP tagged events show a rapidity gap, while for 0.6 < xL < 0.9 only a small fraction does.

The LN have a almost flat behaviour over the whole xL range, and in the 0.2 < xL < 0.9 interval the majority of LB is generated by a mechanism that is not diffractive.

Lepto SCI reproduces, partly, only the LP spectrum.

Neither Lepto SCI nor Ariadne reproduce the LN behaviour.

DIS98 4-8 Apr 1998 A.Garfagnini 18

Good agreement for LPs and LNs with xL < 0.9 Similar production mechanism.

LB Slopes Parameter ‘b’ vs xL

Systematic uncertainty: LPS ~ 10-20 % on ‘b’ slopes, FNC overall error of 2 GeV-2.

DIS98 4-8 Apr 1998 A.Garfagnini 19

Conclusions

• The xL spectrum of LBs is independent of Q2 and W.

• The rate of LNs has been measured in the xBJ and Q2

plane and found to be largely independent of these variables. At high xL , high xBJ, there is a depletion of

neutrons, consistent with the expectations of a OPE calculation.

• The dN/dxL distribution for LP has been measured. The fraction of LP in the 0.60 < xL < 0.91 interval is: 12.7 ± 0.3 (stat.) ± 0.9 (sys.)% for Q2 > 4 GeV2 and 13.0 ± 0.5 (stat.)+0.7 -0.8 (syst.)% for 0.1 < Q2 < 0.8 GeV2.

• Standard fragmentation model in DIS (ARIADNE) does not describe LB production. Additional soft mechanisms (as in LEPTO) describe some features of the data (e.g. diffractive peak for LPs) but do not fully reproduce LB data.