Post on 17-Jan-2016
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 1
Leading Baryonsat HERA
• Introduction• Diffractive structure function measured in events with a leading proton (See also M. Kapishin’s talk)• D* photoproduction with a leading neutron• Dijet with a leading neutron• Summary
32nd International Conference on High Energy PhysicsBeijing, China, Aug. 16-22, 2004
Heuijin Lim(DESY) on behalf of the H1 and ZEUS Collaborations
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 2
IntroductionIntroduction
, IP
Standard fragmentation :• p or n appears in fragmentation of target jet
p, n
p, n
• A significant fraction of ep scattering events contains a leading baryon produced at small t (soft process).
In exchange models :• p from exchange of neutral iso-scalar or iso-vector (, IR, IP) • n from exchange of charged iso-vector (+, +, …)
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 3
DDiffractive structure function measured with a leading proton
22 )kk(Q 2)qp(W
22X )ppkk(M
22
2X
2
IPWQ
MQx
IP22
X
2
x
x
QM
Q
2)pp(t
Kinematics of ep eXp
IPpZL x1Epx
)t,x,Q,(F)R1(2
yy1
Q
4
dtdxdQd
dIP
2)4(D2D
2
4
2
IP2
eXpep
Diffractive structure function of proton
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 4
t dependence from LPS t dependence from LPS
• Fit t distribution to
dd|t| shows steep fall-off with t as in elastic hadron-hadron scattering.
)tbexp(tdd 29.0
5.0 GeV.)syst(.)stat(5.09.7b
• H1 b-value considerably below ZEUS result. • Regge phenomenology predicts “shrinkage” of the diffractive peak:
Additional dependence expected in models.Data not yet precise enough to show presence/absence of IP’ term.
IPIP02
X
2
0x
1ln2b
M
Wln2bb
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 5
QQ22 dependence of cross section with LPS dependence of cross section with LPS
• At high Q2, ddMX falls with Q2.• As Q2 0, d/dMX depends weakly on Q2.
• Data are compared with the color dipole model (BEKW parametrization).
• At high Q2 in high MX bins,, Data above BEKW. Expect this region (xIP>0.01) to be dominated by Reggeon exchange.
(Bartels, Ellis, Kowalski and Wüsthoff)
)1(FTqq
)1(FTgqq
For medium ,
For small ,
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 6
Diffractive structure function of the proton Diffractive structure function of the proton xxIPIPFF22
D(3)D(3) (x (xIPIP,,,Q,Q22) with LPS) with LPS
• Regge fit (xIP < 0.01) with common Pomeron flux factor
)Q,(F)x(fF 2IP2IPIP
)3(D2
dtx
e)x(f
1)t(2IP
tb
IPIPIP
IP0
t)0()t( IPIPIP
with
.)syst(02.0.)stat(02.016.1)0(IP
• xIP>0.01 xIPF2
D(3) increases as xIP1 Reggeon contributions• xIP<0.01 xIPF2
D(3) increases as xIP0 Parton evolution as xIP0
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 7
Diffractive hard scattering factorization Diffractive hard scattering factorization
)Q,z()t,x,Q,z(f)Xpp( 2
iIP2D
pi*D
*
fi/pD(z,Q2,xIP,t) : Probability to find in a proton, with a probe of resolution Q2, parton i
with momentum fraction z, under the condition that the proton remains intact and emerges with small energy loss, xIP, and momentum transfer, t.
[Collins (1998); Trentadue, Veneziano (1994); Berera, Soper (1996)…]
Universal partonic cross sectionDiffractive parton distribution function
NLO QCD fits to the ZEUS dataNLO QCD fits to the ZEUS data • Perform fit to the results from LPS and from diffractive charm in DIS.xIP < 0.01 and Q2>2 GeV2
• Regge factorisation assumption, • Donnachie and Landshoff Pomeron flux• Parametrise PDFs(quark flavour singlet and gluon) using zf(z) = a1+a2z+a3z2)(1-z)a4
For light quark distribution, assuming Charm quarks were treated in Thorne-Roberts variable flavor number (TRVFN) scheme with mc=1.45 GeV.
• NLO evolution : QCDNUM
sdusdu
)Q,(F)x(f)x,Q,(F 2IP2IPIPIP
2)3(D2
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 8
NLO QCD fit on LPS+charm data NLO QCD fit on LPS+charm data
• QCD fit describes data with 2/ndf = 37.8/36
• Fraction of the t-channel momentum carried by gluons
ZEUS at Q2=2 GeV2
H1(2002) at Q2=10 GeV2.
consistent with H1.
.)%syst(.)stat(882 516
%1575
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 9
Comparison of LPS and H1-FPS Comparison of LPS and H1-FPS
• Good agreement between the two experiments.
• H1 QCD fit (from large rapidity gap data, see M. Kapishin’s talk) describes the results of ZEUS(LPS) and H1(FPS).
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 10
Forward neutron calorimeter Forward neutron calorimeter
Z = 106 m
Leading Proton SpectrometerLeading Proton Spectrometer
FNCFNC P beam hole
Window for n-acceptance n < 0.8 mrad
ZEUS
H1
• ZEUS FNC consists of 10 I Pb-scintillator sandwich calorimeter.•
nnn E65.0E)E(
107
• H1 FNC consists of Pb-scintillator fibers.
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 11
Factorization hypothesis Factorization hypothesis
)s()t,x(fdtdx
dLp/
L
Xnp
pion form-factor
with s’=s(1-xL)
n
Independentvertices
parton distribution in pion
• Study relationship between soft and hard interactions : Hard scale : mc
2 for charm production ET for dijet events Q2 for DIS. Soft scale : pT of the neutron
• Study vertex factorization hypothesis.
• Ratios (d/dY neutron tagged)/(d/dY inclusive) are flat with Y kinematic variables of the event.
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 12
DD** photoproduction with a leading neutron photoproduction with a leading neutron
n
X s
0* DD s)K(
nXeDeXnep *
nb.)R.B(05.0.)syst(.)stat(22.008.2 12.018.0
DLN
*
tagged in FNC : 0.2 < xL < 1 n < 0.8 mrad
Q2 < 1 GeV2
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 13
Differential cross sections of D* production Differential cross sections of D* production with a leading neutronwith a leading neutron
• Standard fragmentation (HERWIG, PYTHIA, RAPGAP(inc)) and RAPGAP(OPE) describe W, pT and dist. Only OPE describes xL dist.
• Data is not sensitive to the different variants of the pion structure function.• Light-cone(f1) and exponential(f3) pion form factors are compatible with data in shape.
Different pion PDFs
Different pion form factors
Q2 < 1 GeV2
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 14
Ratio if D* yield with and without a neutronRatio if D* yield with and without a neutron
• Ratio is flat with respect to W, pT(D*) and (D*).
.)%syst(.)stat(93.085.8 48.061.0
)/(r *Dinc
*DLN
*D
Agreement with factorization hypothesis.
Q2 < 1 GeV2
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 15
Dijet with a leading neutronDijet with a leading neutron
• fLN
flat with Etjet
Support factorization hypothesis rising with x
jet
Jet
Jet Dijet cross sections compared to pion exchange MC (POMPYT) with different pionPDFs. Not sensitive.
)/(r jjinc
jjLN
jj
Heuijin Lim ICHEP04, Beijing, 16-22 Aug. 16
SummarySummary
• Diffractive structure function measured with a leading proton in DIS Recent data from ZEUS with improved precision and extended kinematic range. Slope of d/d|t| is compatible with soft interaction at the proton vertex. Data can be described by color dipole model (e.g. BEKW parameterisation). Data described by a NLO QCD fit
• H1 Very Forward Proton Spectrometer (VFPS) Tag the scattered proton at HERA II for high rate studies of diffraction Large acceptance for low xIP and 0<|t|<0.5 GeV2
Expect physics data soon.
• D* and dijet with a leading neutron Data described by one-pion-exchange model. Data agree with the factorization hypothesis.
Acceptance :Red : 90-100 %, Blue:0-10%
FPS-HVFPSFPS-V