PowerPointPrsentation

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Exotic States Challenges for QCD Bad Honnef,
January 17-21, 2005
Evidence for a narrow anti-charmed baryon state
Karin Daum Wuppertal
on behalf of

• Outline
• HERA Deep inelastic scattering
• Charm production at HERA
• Search for an anti-charmed baryon with H1
• Signal checks and signal assessment
• Summary of ?c searches
• Conclusions

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The HERA accelerator
DESY Hamburg Germany
?s 300 - 320 GeV
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HERA kinematics in ep collisions
vs 300-318 GeV (energy c.m.) DIS
kinematics Photon virtuality Q2-q2 Electron
inelasticity y Scaling variable x Hadronic
mass W
Kinematic regimes
Scattered e detected Q² gt 1 GeV
Electroproduction (DIS) Scattered e not
detected Q² 0 GeV Photoproduction
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In the focus of HERA Strong interactions
HERA is the machine for precision
measurements/tests of strong interactions (QCD)
RGE has negative ?
Gluon self coupling ? Asymptotic freedom
Gluon emission ? Scaling violation
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Physics at HERA (I)
Strong coupling constant ?s at Hera
Asymptotic freedom
Scaling violation
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Physics at HERA (II)
hep-ph/0407067 B.Allanach P.Zerwas
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Charm physics at HERA (I)
Heavy quark mass Charm is not a constituent of
the proton
copious production from gluonsin the proton
?
charm production is dominated By Boson Gluon
Fusion (BGF)
?g ? cc (bb)
LO QCD
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Charm physics at HERA (II)
Charm contribution to The total cross section
HERA is a charm factory
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Charmed pentaquark search
Inspired by the evidences for the strange
pentaquark ?? from Kn and K0sp analyses
Why not a charmed pentaquark ?

• ? formation may be due to features of the QCD
  vacuum
• (fragmentation process)
• Universality of QCD vacuum e.g. flavour blind
• ?
• ? Features of ?c similar to those of ?
• (i.e. Q-value, width)
• Similarities in fragmentation process of
• charmed hadrons
• ?
• Search for ?c in D-p (c.c.)

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The H1 detector
p
e
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D Signal
Golden channel D? D0 ? s ?(K?)?s (low BR but
clean signal) M(D)-M(D0) 145.4 MeV Q-value
only 6 MeV
96-00 data 75 pb-1 DIS Q²gt1 GeV²
Mass difference technique ?MDM(K??s) - M (K?)
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Proton selection
Most probable dE/dx

• Particle identification
• via dE/dx
• 3-5 accuracy
• 8 MIP resolution

Use dE/dx for background suppression
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Opposite sign Dp mass distribution
Apply mass difference technique M(Dp)m(K??
p)-m(K??)MPDG(D)
Background well described by D MC and wrong
charge D from data
narrow resonance at M3099? 3(stat.) ? 5 (syst.)
MeV

• signal visible in different data taking periods

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Signal in both D-p and in Dp
M(Dp) m(Kppp)-m(Kpp)m(D)
M(Dp)3.102 0.003 GeV
M(Dp)3.096 0.006 GeV
Signal of similar strength observed for both
charge combinations at compatible M(Dp)
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Signal in like sign Dp combinations?
No significant peak in like sign Dp
Reasonably described by D MC and wrong charge D
from data
No
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Typical Dp candidates
HERA-I
All events scanned No anomalies observed e.g.
split tracks, wrong reconstruction Signal not due
to kinematic reflections
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Does the resonance come from Ds?
Look at the correlation of ?M(D) vs. M(Dp)
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Is the D-p1) signal due to protons?
Use proton in this region
Well identified protons
Yes
1) Charge conjugate always implied
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Physics changes on-resonance ?

• ? Single particle momentum spectra
• are steeply falling
• This feature is preserved in the
• combinatorial background of
• invariant mass analyses

Harder spectrum for particles from decay due to
mass release Harder spectrum for particles from
decay of charmed hadrons due to hard charm
fragmentation
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Physics changes on-resonance ?
Fit slope with ??exp -?p(p)
Look at momentum distribution of proton
candidates w/o dE/dx
No dE/dx cuts !
Yes
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Physics changes on-resonance ?
No dE/dx cuts !
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Kinematic tests
Monte Carlo expectation
wrong mass assignment
CPQ MC
Mass M independent of decay angle ? only for
correct mass assignment
correct mass assignment
M(Dp) GeV
CPQ MC
CPQ MC
M(Dp) GeV
M(Dp) GeV
M(Dp) GeV
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Kinematic tests
Monte Carlo expectation
wrong mass assignment
CPQ MC
Mass M independent of decay angle ? only for
correct mass assignment
correct mass assignment
M(Dp) GeV
CPQ MC
CPQ MC
M(Dp) GeV
Band like structure visible in the M(Dp)-M(Dx)
plane in data?
M(Dp) GeV
M(Dp) GeV
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Kinematic test Dp vs. Dp
p-mass hypothesis excluded from the shape and
range of Dp mass distribution !
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Dp in photo-production

• total 4900 D to start
• Dp peak at the same mass in ?p
• no enhancement in non-charm bg
• 95 bg due to non-charm

Background well described by wrong charge D from
data
Photo-production more difficult due to large
non-charm background
but
independent confirmation of the signal
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Significance estimate
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Results of qc searches
H1 observation in ep ? cc
X R(?c?Dp/D) 1.46?0.32 (uncorrected)
Negative results for qc from ALEPH ee- ?
Z0 ? cc FOCUS ?N ? cc X CDF pp ? cc
X BELLE ee- ? ?(4s) ? B0B0 B(B0 ?
?cp?)?B(?c ?Dp)/ B(?c ?Dpp?)lt11 _at_ 90
C.L. ZEUS ep ? cc X
prelim.
Not contradicting H1
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Search for charmed PQ, qc? Dp, in ZEUS
1995-2000 data, 127 pb-1 Selection of D, p close
to H1 cuts
DIS (Q2gt1 GeV) 5920?90 Ds ?p (Q2lt1
GeV)11670?140 Ds
No signal seen in Dp

• Limits on ?c/D for DIS
• R(?c?Dp/D)lt0.51 _at_95 C.L.
• Includes some systematic uncertainties
• But selection different from H1
• Assumes production mechanism
• of ?c to be the same as for D

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H1 vs. ZEUS observation What does it mean?

• H1 R(?c?Dp/D) 1.46?0.32 in DIS
  observed
• ZEUS R(?c?Dp/D) lt 0.51 _at_ 95 C.L. in DIS with
  corrections
• ?Numbers are not consistent, but
• different selection
• different triggers
• ZEUS DIS events only 40 are from the DIS
  trigger
• H1 DIS events 100 are from the DIS
  trigger
• ? different phase space explored by H1 and ZEUS
• ?c and D production mechanism may be different
  at HERA
• (as suggested by the ?/K0s yields observed by
  ZEUS)
• ? We have to understand more about ?c production

prelim.
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Conclusions

• Evidence for a neutral anti-charmed baryon state
• decaying to Dp in deep inelastic scattering
  from H1
• Signal due to D mesons and protons
• Harder proton spectrum in the signal region as
  expected
• for secondaries from the decay of charmed
  hadrons
• Kinematic tests agree only with the Dp
  hypothesis
• Independent confirmation from photo-production
• Poissonian background fluctuation probability
  lt410-8
• Searches from ZEUS for Dp yield negative
  results
• ?
• Situation unclear more understanding of Dp
• production dynamics needed

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Backup Slides
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Remarks on ALEPH
D signals for different xE
Rb ?22, Rc ?17
D _at_ LEP are produced predominantly by beauty
ltxEgtcc ?0.488
In case of ?c?Dp ltxEgtcc ?0.32
D selection may not be appropriate for ?c Likely
that possible ?c is cut out by D selection No
?c Monte Carlo used for ?c ?Dp/D yields
Similar to selection For Dp analysis
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Remarks on FOCUS
Fixed target experiment 180 GeV photons on
9Be ?hadronic mass W18 GeV
Hera 60ltWlt280 GeV
Hera
D
?c threshold
M(DD)
D
nucleon
Large phase space suppression for ?c in FOCUS No
Monte Carlo used by FOCUS
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Remarks on CDF
Charm production via gluon gluon fusion Similar
to BGF at HERA Depends quadratically on the
gluon density No details on the analysis
obtainable e.g. effect of trigger D
selection CDF used a Monte Carlo for ?c signal
estimation but model completely wrong elastic
J/? production decaying to Dp
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Remarks on ee- data
?Production baryons and light nuclei in high
energy processes not understood e.g.
anti-deuteron production H1 ?p d/p
(5.0?1.0?0.5)10-4 RHIC Au-Au d/p 2
10-3 LEP ee- d/p lt 1.6 10-4 Anti-deuteron
production (6 quarks) strongly process
dependent Could be similar for pentaquarks
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Possible signature of the charmed pentaquark

• Common belief Q D p (pseudo-scalar D
  meson)

Charm fragmentation fractions
c
Vector mesons not suppressed
But what is experimentally feasible ?
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Details of fit
Charges MMeV sMeV
NS D-p Dp 3099 3 12 3 50.6
11.2 D-p 3102 3 9 3
25.8 7.1 Dp 3096 6 13 6
23.4 8.6
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All Checks (I)

• check events
• signal events scanned visually no anomalies
• double entries ?
• 1.) Within - 24 MeV around peak 1 double
  entry
• 2.) All M(Dp) lt 3.6 GeV 1.12 entries /
  event
• signal from D,p?
• backward D analysis signal region D rich
• well identified protons (plt1.2, hard dE/dx)
  signal there
• average norm. likelihood in signal region
  ltLpgt0.92
• physics in signal and bgr region?
• physics on/off resonance proton spectrum harder
  on resonance
• peak stable?
• signal present in subsamples (in Q2, x, y, h,
  pt, data taking period)
• variations of binning and selection mass, width
  stable
• signal present in photoproduction

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All Checks (II)

• signal from bgr or from D, protons?
• wrong charge D bgr instead of real D no peak
• D sidebands instead of DM(D) signal window no
  peak
• K, p selected (via dE/dx) instead of protons
  (p-mass assigned) no peak
• Kp combinations with masses above region where
  charm contributes no peak
• check reflections
• protons assigned K, p mass no peak
• Invariant masses m(pK), m(pp), m(pps) and all
  other possible
• 2-particle masses no res. structures
• reflections from D10, D20 expected
  contribution (MC)
• 4 events (24MeV)
• Signal due to D0? D0 g ? D0 e e- ? no
• (electrons misidentified as ps and proton)

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D signal in DIS and photoproduction

• DIS cleaner signal
• photoproduction supporting evidence

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Acceptance effects?
Pion survival probability
Proton efficiency
Smooth variation with M(Dp) Shape reflects
opening of phase space
M(Dp) GeV
M(Dp) m(Kppp)-m(Kpp)MPDG(D)
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Reflections from decays to Dp ?
D10, D20 ? Dp
loose D cuts p selection

D cuts of Dp p selection
D cuts of Dp proton selection
Expect 3.5 decays (D10, D20 ? Dp) in Dp signal
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Could signal be due to decay D0 ? D0 g ?

• D0 ? D0 g? D0 ee-
• electrons from g-conversion
• asymmetric in energy
• misidentified as proton and ps ?

No accumulation at small mee in Dp signal region
or elsewhere
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Non observation at ZEUS
D decay channels D? D0 ps ? K- p ps
( c.c.) D? D0 ps ? K- ppp- ps
( c.c.) gt 60000 D
DIS (Q2gt1GeV2) and photoproduction
(Q2lt1GeV2) 1995-2000 data, 127 pb-1
No peak observed results not compatible with H1
Upper limit on R(q0c ? Dp/D) 0.35 (both
channles, Q2gt1GeV2)
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Lots of further kinematic test

• Reflections from a possible signal in DK mass
  distribution ruled out
• Possible contributions from D ?D ? with
  ?-conversion ruled out
• Possible contributions from D /D ?D K
  ruled out
• Possible peak structures in all possible mass
  correlations with all possible mass hypotheses of
  the particles making the D and the Dp system to
  search for real or fake resonances, e.g ?, ? , ?
  , K ,?, f no enhancements found
• Possible peak structures in all possible mass
  correlations among the proton candidate the
  remaining charged particles of the event with all
  possible mass assignments to search for real or
  fake peaks, no enhancements found

0
0
0
S1 S2
0
0 S
2
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Remarks on Dp searches at LEP
D signals for different xE
Rb ?22, Rc ?17
D _at_ LEP are produced predominantly by beauty
D acceptance vs. xE?
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Remarks on Dp searches at LEP
Rb ?22, Rc ?17
D _at_ LEP are produced predominantly by beauty
ltxEgtcc ?0.488
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Remarks on Dp searches at LEP
beauty
charm
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D from Dp and direct D at HERA
MC
MC
ltxEgtcc ?0.56
ltxEgtcc ?0.43
Ds from Dp significantly softer than normal
Ds
Should also hold for LEP !
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Physics at Hera
Strong coupling constant ?s at Hera
Asymptotic freedom
Scaling violation
Different processes give consistent picture of QCD
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Search for charmed PQ, qc? Dp, in ZEUS
Photoproduction
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Significance estimate
5.4 s
Background only hypothesis Nb51.72.7
Background fluctuation probability 4x10-8
(Poisson)5.4 s (Gauss)