Pentaquark Searches in H1

0.17 MB·English

by  J. E. Olsson

February7,2008 8:39 WSPC/TrimSize: 10inx7inforProceedings ichep06
PENTAQUARK SEARCHES IN H1
∗
J.E. OLSSON
DESY, Hamburg, Germany
7 ∗E-mail: [email protected]
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Talk given on behalf of the H1 Collaboration at ICHEP06, Moscow, Russia
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n We report on searches in deep inelastic ep scattering for narrow baryonic states decaying into
a Ξ−π−,Ξ−π+,KS0p and their charge conjugates, at centre-of-mass energies of 300 and 318 GeV.
J Nosignalforanewnarrowbaryonicstateisobservedinthemassranges1600-2100MeV(Ξπ)and
from threshold up to 1700 MeV (K0p). The standard baryon Ξ(1530)0 is observed inthe decay
9 modeΞ−π+,andmassdependentuSpperlimitsontheratioofthehypotheticalpentaquarkstates
Ξ−− andΞ0 totheΞ(1530)0 signalaregiven. Alsoforthehypothetical strangepentaquark Θ+
1 5q 5q
v massdependent upperlimitsonσ(ep→eΘ+X)×BR(Θ+→K0p)areobtained.
1 Finallymeasurements oftheacceptance corrected ratiosσ(D∗p(3100))/σ(D∗)fortheelectropro-
ductionoftheanti-charmedbaryonstate D∗p(3100)decayingintoD∗ andparepresented.
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Keywords:pentaquark; HERA.
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0 1. Introduction by any other experiment.
/ In this report the current results of
x Pentaquarksareexotictothestandardquark
e searches by the H1 experiment for the Θ+
model of mesons and baryons, although not
- and Σ−− states are briefly presented. The
p excludedinQCD.Combinationofthemeson
e searcheshave been performed in the HERA-
and baryon octets leads to the anti-decuplet
h
Iepdata(1996-2000),whichencompass75−
: qqqqq¯, where q stands for the light quarks
v 100 pb−1. In the last section some details of
u,d and s. These 5-quark states are non-
Xi the characteristics of the D∗p(3100)produc-
minimal, colour neutral combinations. The
r apex states uudds¯,ssddu¯ and uussd¯are pre- tion are presented.
a
dicted to have masses in the range 1.5−2.1
GeV and to be very narrow1. For two of 2. The strange pentaquark Θ+
these states decay modes are experimentally
The Θ+ was first seen by the LEPS
relatively easily accessible,and consequently
collaboration5 in the decay mode K+n, at
many searches have been performed, both
a mass of 1.52 GeV. The H1 collaboration
in fixed target and colliding beam environ-
searched6foranarrowstatedecayingtoK0p
ments. For the uudds¯ state, called Θ+, a S
inthemassintervalfromthreshold1.48GeV
large number of observations in the mass
to 1.7 GeV, using DIS data with 5 < Q2 <
range 1.52 − 1.54 GeV is matched by an
100 GeV2 and 0.1 < y < 0.6. The K0 was
equally large number of non-observations2. S
identified through the decay K0 → π+π−
The doubly strange ssddu¯ state, Σ−−, has S
and events were accepted if they contained
only been observed by one experiment so
atleastoneK0 andatleastoneprotonacan-
far3. Also here many non-observations have S
didate. Charged tracks had p > 0.15 GeV
been reported2. t
and pseudorapidity |η|<1.75, while the K◦
Pentaquarks which contain a charm S
candidate had p > 0.3 GeV. Backgrounds
t
quark have also been searched for. One
observation4, named D∗p(3100) by the H1
aChargeconjugationisalwaysimplied,unlessother-
collaboration, has so far not been confirmed
wisestated.
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from Λ and converted photons were rejected atic errors,are shown below the mass distri-
with the restrictions M > 1.125 GeV and butions in Fig. 1.
pπ
M >0.05 GeV on the ππ system. Protons Thesearchwasrepeatedwithseparation
ee
wereidentifiedthroughthespecificionization of the K0p and K0p¯ distributions, with no
S S
loss dE/dx in the inner drift chambers,with significant peak as result. The obtained up-
efficiencies between 65 and 100%. per limits vary between 30 and 90 pb.
The ZEUS collaboration has reported
MeV128000 a) H1 MeV116800 b) H1 evidence8 for a 1.52 GeV signal in the KS0p
Entries / 5 11118024600000 H1 data Entries / 5 11180240000 H1 data m1σ.5a<2ssGd1ei0Vs0tripisbbσut(i9<o5n%.72CT.pLhb.e,)wHwh1hiecunhpetprxeatrnraslpliamotlieatstetadot
60 bgr fit 60 bgr fit to the ZEUS y-range. This value is barely
40 40
20 5 < Q2 < 10 GeV2 20 10 < Q2 < 20 GeV2 compatible withthe ZEUSpreliminarycross
pb]800 pb]800 section9, σ =125±27+36 pb.
Q [UL4600 Q [UL4600 The ZEUS collabo−ra2t8ionalso found that
s 20 95 % C.L. s 20 95 % C.L.
01.451.51.551.61.651.71.751.8 01.451.51.551.61.651.71.751.8 the observed resonance is most prominent
M(KS0p(p)) [GeV] M(KS0p(p)) [GeV] with the cuts Q2 > 20 GeV and proton mo-
MeV250 c) H1 MeV100 d) H1 mentumpp <1.5GeV.Alsointhiskinematic
ntries / 5 125000 ntries / 5 6800 rseaengFeigt.he1dH.1collaborationobservesnopeak,
E H1 data E 40 H1 data
100
bgr fit bgr fit
sQ [pb]UL24685000000 95 % 2C0. L< .Q2 < 100 GeV2 sQ [pb]UL24682000000 9 5 % 2C0. L< .Q2 < 100 GeV2 3Tre.hsoeTnNhanAec4ep9esnctotrlualacqtbuuoarreraktiasotnΞ1−5o8qb−6s2ear±vnedd2ΞaM05nqeaVrroiwn
01.451.51.551.61.651.71.751.8 01.451.51.551.61.651.71.751.8 Ξ(1321)π mass spectra3. Both Ξ−− and Ξ0
M(K0p(p)) [GeV] M(K0p(p)) [GeV]
S S
Fig. 1. K0p(p¯) invariant mass. a-c) peaks were seen, leading to the interpreta-
S
in bins of Q2, d) in the highest Q2 bin tion of these states being the neutral and
doubly charged members of the Ξ (1862)
andwith p <1.5GeV.The fullcurves 3/2
p
pentaquark multiplet. Other experiments
showthe backgroundfunction fits. Up-
could not yet confirm this observation.
perlimitsσ at95%C.L.onthecross
U.L.
The H1 search for this state10 uses the
section are shown below the mass dis-
decay chain Ξ → Ξ(1321)π, Ξ(1321) →
tributions.
Λπ, Λ →pπ. DIS events were selected with
In the resonance search, K0p combina- 2 < Q2 < 120 GeV2 and 0.05 < y < 0.7. In
S
tionswereselectedundertherestrictionp > amasswindowof±8MeV∼ 158000Λcan-
t
0.5 GeV and |η| < 1.5 for the K0p system. didates were identified with a 3-dimensional
S
MassdistributionsK0pinthreeQ2 intervals vertex fit, using cuts on the momentum
S
are shown in Fig. 1a-c. No significant re- p > 0.3 GeV and decay length > 0.75
t,pπ
sonance signal is seen, and the distributions cm. Λπ combinations were also subjected to
are well described by a smooth background a3-dimensionalvertexfit, withafurthercut
function. Mass dependent upper limits for onthedistanceofclosestapproachtothepri-
the cross section σ(ep → eΘX)×BR(Θ → maryvertex. ThesignificanceoftheΞ(1321)
K0p), obtained using a modified frequentist signal is increased by a restriction to < 0.6
approach7basedonlikelihoodratiosandtak- radon the angle between secondaryand ter-
ing into accountboth statisticalandsystem- tiary vertex vectors. In the resulting mass
February7,2008 8:39 WSPC/TrimSize: 10inx7inforProceedings ichep06
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distribution of Λπ,∼ 1650 Ξ(1321) form a plots of Fig. 2 and lie in the range 0.15−
clear narrow peak. 0.6, with the value R (1860) ∼ 0.5(0.2)
U.L.
FinallytheΞ(1321),inamasswindowof for the neutral (doubly charged) combina-
±15MeV,wascombinedwithachargedpion tion. Summing all combinations, the limit
fromtheprimaryvertex. Acutp >1.0GeV R (1860)∼ 0.5isobtained,whichisfully
t U.L.
was imposed on the Ξ(1321)π combinations. compatible with the upper limit value 0.29,
Fig. 2 shows the final mass distributions. obtained by the ZEUS experiment in a simi-
In the neutral Ξ(1321)π combinations lar analysis11.
there is a clear signal of the wellknown
Ξ(1530)0, with ∼ 170 events. The doubly 4. D∗p(3100) production in DIS
chargedΞ(1321)π combinations do not show
any resonant structure, in particular not at Pentaquarkmultipletscontainingtheheavier
1.86GeV,themassoftheNA49observation. c or b quarks have also been considered12. If
This is also true when separating Ξ−π− and the anti-charmed pentaquark Θ0c, with the
Ξ+π+ distributions. The Ξ(1530)0 is well quark content uuddc¯, is heavy enough the
seen in both neutral charge combinations. decay Θ0c → D∗−p would be possible. Ev-
idence for a narrow peak, provisionally la-
Entries / 8 MeV11102480000 a) X2(H 1<5 13Q0 2)[p0G sreiVegn2l]ai <ml: 112.700 Entries / 8 MeV11102480000 b) 2H < 1Q 2[pGreVe2l]i <m 12.0 tgbiieovlnelendinbDyD∗pItSh(3e1a0nH0d1),pcihonollttahobpeorDroad∗tuipocnmti4oa.nssIhdnaisssturbbiebseuen--
60 60 quentsearches,nootherexperimentwasable
40 40 to confirm this observation.
20 20 Additional preliminary information for
–pX+)(Ru.l.000...4681 95 % C.L. –pX–)(Ru.l.000...4681 95 % C.L. tHh1eDco∗lpla(b31o0ra0t)iopnro13d.uctAiocnceisptparnocveidceodrbreyctthede
0.2 0.2
01.41.5 1.61.71.81.9M(X2–p+2) .[1Ge2V.2] 01.41.5 1.61.71.81.9M(X2–p–2) .[1Ge2V.2] yield ratios relative to inclusive D∗ produc-
tion, anddifferential distributions ofthe vis-
Fig. 2. Ξπ invariant mass, summed
ible cross section ratio as function of event
for a) two opposite and b) two equal kinematics and D∗ quantities are presented,
charge combinations. Solid lines show
the latter hinting at some features of the
fits of a background function, in a) in- D∗p(3100) production mechanism.
cludingaGaussian. Upper limits R
U.L.
The acceptance corrected yields ratio
at95%C.L. onthe ratioof the number
R (D∗p(3100)/D∗)is definedinthe visible
ofeventsofahypotheticalΞ(1321)πre- cor
range given by p (D∗p(3100)) > 1.5 GeV,
sonance to the Ξ(1530)0 are shown be- t
−1.5 < η(D∗p(3100)) < 1.0, p (D∗) >
low the mass distributions. t
1.5 GeV, −1.5 < η(D∗) < 1.0 and z(D∗) >
Mass dependent upper limits are de- 0.2 (including the D∗ from the D∗p(3100)
fined in terms of the ratio of a hypothetical decay). η and z are the pseudorapidity and
Ξ(1321)π resonanceto the Ξ(1530)0, using a elasticity, respectively. The acceptance cor-
narrow Gaussian for a possible signal in the rections are calculated using RAPGAP, un-
range 1.6 − 2.1 GeV. The background is a dertheassumptionthatpentaquarksarepro-
smooth function and again the modified fre- duced by the fragmentation (simulated with
quentist approach7 is used. Separate upper the Lundstringmodel). Theobservedyields
limits were obtained for neutral and doubly ratio R(D∗p(3100)/D∗) = 1.46 ± 0.32%
charged combinations, as well as for their becomes, after the acceptance correction,
sum. The ratiolimits areshowninthe lower R (D∗p(3100)/D∗)=1.59±0.33+0.33. The
cor −0.45
February7,2008 8:39 WSPC/TrimSize: 10inx7inforProceedings ichep06
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ZEUScollaborationfoundR <0.59%(at D∗p(3100) fragmentation function, the vari-
cor
95% C.L.), using larger statistics and a dif- able
ferent definition of the visible range14. (E−p )
z charm
x (charm)=
obs
P (E−p )
s(D*)D*p(3100)) / 000...000468 a) HR1A PPGreAlP s(D*)D*p(3100)) / 000...000468 b) HR1A PPGreAlP xicsroobssds(Desfi∗en)cetdiao.nndrathtiTeohdσei(ffDehdr∈∗eephpn(ete3min1aid0le0cn)r)co/esσs(zDoshfe∗c)ttioohnne
s ( s ( dσ(D∗p(3100))/dxobs(D∗p(3100)) are shown
0.02 0.02
in Fig. 4a and 4b, respectively. The com-
-01.5 -1 -0.5 0 0.5 1 01 2 3 4 5 parison with the RAPGAP expectation (in
h (D*) h* (D*) whichD∗ andD∗p(3100)havethesamepro-
Fig. 3. Acceptance corrected ratio ductionmechanism)showsthatthe D∗ from
σ (D∗p(3100))/σ (D∗)asafunction the D∗p(3100) decay is softer than the in-
vis vis
ofthe D∗ pseudorapidity,in the a) lab- clusive D∗, and that the fragmentation of
oratory and b) hadronic c.m. system. D∗p(3100) is harder than the inclusive D∗
Only statistical errors are shown. fragmentation.
Extrapolating to the full phase space,
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