karin daumbeach 2004, chicago, june 29, 2004 1 searches for exotic baryons at hera karin daum –...
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Karin Daum
BEACH 2004, Chicago, June 29, 2004 1
Searches for Exotic Baryons at HERAKarin Daum – Wuppertal
Outline:
• HERA & kinematics at HERA• Strange Pentaquarks
+ (Hermes, Zeus) -- (Zeus)• Exotic Anticharmed Baryon (H1, Zeus)• Conclusions
6th International Workshop on Hyperons, Charm and Beauty HadronsChicago, June 27-July 3, 2004
on behalf of theH1, HERMES and
ZEUS collaborations
Karin Daum
BEACH 2004, Chicago, June 29, 2004 2
Protons 920 GeV
Electrons 27.6 GeV
The Hera accelerator
DESYHamburgGermany
ep- collisions
eN-fixed target
Karin Daum
BEACH 2004, Chicago, June 29, 2004 3
Hera kinematics in ep collisions
E = 27.6 GeVe E = 920 GeVp
√s = 300-318 GeV (energy c.m.)
W=m(p) (hadronic energy)
Q2=-q2 (photon virtuality)
2 kinematic regimes :Q 0 GeV : PhotoproductionQ > 1 GeV : Electroproduction (DIS)
D*p event in DIS
sKp
e
Karin Daum
BEACH 2004, Chicago, June 29, 2004 4
Fixed target kinematics at Hera
²H
²H
√s = 10 GeV (energy c.m.)
W=m(p) (hadronic energy)
Q2=-q2 (photon virtuality)
E = 27.6 GeVe
2 kinematic regimes :Q 0 GeV : PhotoproductionQ > 1 GeV : Electroproduction (DIS)
Karin Daum
BEACH 2004, Chicago, June 29, 2004 5
The search at HERMES+
Kinematic Range: 0.02 < x < 0.8 at Q2 > 1 GeV2 and W > 2 GeV
Dual radiator RICH for PID
Very good proton ID
Aerogel
104FC
Karin Daum
BEACH 2004, Chicago, June 29, 2004 6
The search at HERMES+
Hadron identification using RICH
Suppress contamination from p
Define appropriate event topology
Unambiguous invariant mass reconstruction due to PID
O(1000) K0s
Peak at 15282.6 MeV=82 MeV; 3.7 Peak at 15272.3 MeV=9.22 MeV; 4.3
bumps
7818 events
e+ + ²H + + X K0s p + X
Karin Daum
BEACH 2004, Chicago, June 29, 2004 7
The search in ZEUS+
DIS (Q2>1 GeV2):96-00 data 121 pb-1
Identify K0s ,
• Remove p and converted ’s
O(900,000) K0s DIS (Q2>1 GeV2):96-00
data 121 pb-1
Identify K0s
• Remove p and converted ’s
• Limited particle identification via
dE/dx
• select proton candidates
p(p)<1.5 GeV, dE/dx>1.15 mips
e + p + + X K0s p +
X
Karin Daum
BEACH 2004, Chicago, June 29, 2004 8
The search in ZEUS+
Clear signal visible only at Q²> 20 GeV²
MC normalised to background,No -bumps included
Fit a 2nd Gaussian to accountfor some of the differences
Peak at 1521.51.5 MeV=6.11.5 MeV; 4.6
221 48 events
O(100,000) K0s
Signal seen for both
charges: K0s p = 9634 !-
Karin Daum
BEACH 2004, Chicago, June 29, 2004 9
Is there a ++ at HERA
HERMES ZEUS
Clear Signal for (1520) pK-
No Signal for pK+ + isoscalar
Karin Daum
BEACH 2004, Chicago, June 29, 2004 10
The search in eN collisions at HERA
+
Experiment Kinematics K0s p/ K0
s
HERMES √s, W, Q² small 7 %
ZEUS
√s, W, Q² largeSignal atHigh Q²small W
0.2 %
Effect of phase space/acceptance ?
Factor: 35 !
Karin Daum
BEACH 2004, Chicago, June 29, 2004 11
Search for at ZEUS--
NA49
ZEUS analysis for DISsimilar to NA49
p-
0-
-- --
Λ Λ
ΞΞ- +
All charge combinations
• Clean Ξ(1530) P13
• No signal at (1860)
preliminary
Karin Daum
BEACH 2004, Chicago, June 29, 2004 12
Evidence for anti-charmed baryon by H1
If a strange pentaquarkWhy not a charmed one also
Experimentally best suitedD* p
Theoretically not favoured
-
96-00 data 75 pb-1 DIS: Q²>1 GeV²
Good Signal/Background
3400 D*’s in DIS to start with
Golden channelD*+ D0
s (K)s
D* signal regionsubsequently used
{
Karin Daum
BEACH 2004, Chicago, June 29, 2004 13
Particle identification
Reasonable PID via dE/dx
-all data calibrated- parameterisation accurate to 3-5%- MIP resolution 8%
Most probable dE/dx
Normalized likelihood based on:measured dE/dx & expectationsfor , K, p and resolution:L()+L(K)+L(p) = 1
Use dE/dx for background suppression
Karin Daum
BEACH 2004, Chicago, June 29, 2004 14
Background well described by D* MC and “wrong charge D” from data
Apply mass difference technique
M(D*p)=m(K p)-m(K)+MPDG(D*)
no enhancement in D* Monte Carlo
no enhancement in wrong charge D
• equally significant signal visible in separate D*+p and D*-p
• signal visible in different data taking periods
_narrow resonance at M=3099 3(stat.) 5 (syst.) MeV
Opposite sign D*p mass distribution
Karin Daum
BEACH 2004, Chicago, June 29, 2004 15
All events scanned:
No anomalies observede.g. split tracks, wrongreconstruction
K
-s
+-p
HERA-II
Typical D*p candidates
p
+
+D
0
D*
K -
HERA-I
s
Karin Daum
BEACH 2004, Chicago, June 29, 2004 16
M(D*p) [GeV]
M(D
*) [
GeV
]
Look at the correlationof M(D*) vs. M(D*p)
Side band scaled to the widthof the signal window in M(D*p)
the (D*p) signal region is richer in D*
D*p signal region
(D*p) side bands
D*
win
dow
Does the resonance come from D*’s?
Karin Daum
BEACH 2004, Chicago, June 29, 2004 17
Physics changes on-resonance ?
Look at momentum distribution ofproton candidates w/o dE/dx
No dE/dx cuts !
The momentum spectrum of the particlesin the signal region is harder than in the M(D*p) side bands
At large p(p) (>2 GeV)At large p(p) (>2 GeV)Signal more pronounced
Karin Daum
BEACH 2004, Chicago, June 29, 2004 18
Photo-production more difficult due to large non-charm background but
independent confirmation of the signal
D*p in photo-production
• total: 4900 D* to start• D*p peak at the same mass in p• no enhancement in non-charm bg• 95 % bg due to non-charm Background well described
bywrong charge D from data
Karin Daum
BEACH 2004, Chicago, June 29, 2004 19
Background only hypothesis: Nb=51.7±2.7
Background fluctuation probability: 4x10-8 (Poisson)=5.4 (Gauss)
5.4
Significance estimate
Karin Daum
BEACH 2004, Chicago, June 29, 2004 20
Search for charmed PQ, c D*p, in ZEUS
D.I.S. Selection differs from H1 analysis
Karin Daum
BEACH 2004, Chicago, June 29, 2004 21
Search for charmed PQ, c D*p, in ZEUSPhotoproduction + DIS
Karin Daum
BEACH 2004, Chicago, June 29, 2004 22
Conclusions
Positive evidence for the production of + at HERA
No doubly charged partner + is an isoscalar large differences in the +/ K0
s yields from HERMES and ZEUS
Negative results on the search for -- by ZEUS
Evidence for a neutral anti-charmed baryon state decaying to D*p from H1
- not confirmed by ZEUS
prelim.
prelim.
Karin Daum
BEACH 2004, Chicago, June 29, 2004 24
Like-sign D*p mass distribution
no significant peak in like-sign D*p
no enhancement in D* Monte Carlo
no enhancement in wrong charge D
Background well described by D* MC and “wrong charge D” from data
Karin Daum
BEACH 2004, Chicago, June 29, 2004 25
M(D*p)=3.104 0.003 GeV
Use this regionwith L(p)>0.5
Does the resonance come from protons ?
P(p)<1.2 GeV
dE/dx >1.15
Signal is there for well identified protons
<L(p)> = 0.92
Karin Daum
BEACH 2004, Chicago, June 29, 2004 26
Is the physics different in the signal region?
Single particle momentum spectraare steeply fallingThis feature is preserved in thecombinatorial background of invariant mass analyses
In decays particles are also emitted in the direction of flight Particles from a decay should have a harder spectrum than thecombinatorial background
Check the proton momentum !
H1
Karin Daum
BEACH 2004, Chicago, June 29, 2004 27
Fit slope with exp {-p(p)}
The momentum spectrum of the particlesin the signal region is harder than in the M(D*p) side bands
Signal region=1.270.09
D*side band=1.860.13
D*p side bands=1.740.06
M(D*p) [GeV]
M(D
*) [
GeV
]Is the physics different in the signal region?
Karin Daum
BEACH 2004, Chicago, June 29, 2004 28
D*p rest framewrong mass assignment
correct mass assignment M(D*) [GeV]
M(D*p) [GeV] M(D*p) [GeV]
M(D
*)
[GeV
]
221
2 )( PPM
)22( **22
* XDXDXD ppEEmm
2-Body Decay
independent of decay angle cos* only for correct mass assignment
2M
CPQ MC
CPQ MC
CPQ MC
Basics of kinematic tests
Karin Daum
BEACH 2004, Chicago, June 29, 2004 29
wrong mass assignment
correct mass assignment M(D*) [GeV]
M(D*p) [GeV]M(D*p) [GeV]
M(D
*)
[GeV
]
221
2 )( PPM
)22( **22
* XDXDXD ppEEmm
2-Body Decay
Do we see a band like structurein the M(D*p)-M(D*x) plane indata? Let’s have a look
D*p rest frame
CPQ MC
CPQ MC
CPQ MC
Integrated in cos*
Basics of kinematic tests
Karin Daum
BEACH 2004, Chicago, June 29, 2004 30
*2 DD
*1 DD
Sign for X D*p: available phase space in D*completely used
Go to the D*p signal region
Signal due to D* ? NO!
Karin Daum
BEACH 2004, Chicago, June 29, 2004 31
Lots of further kinematic test
• Reflections from a possible signal in D*K 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 D*p 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
S1 S2
0 ++ 0S
2
0 0
0
Karin Daum
BEACH 2004, Chicago, June 29, 2004 32
Remarks on D*p searches at LEP
D* signals for different xE
D* @ LEP are producedpredominantly by beauty
Rb 22%, Rc 17%
D* acceptance vs. xE?
Karin Daum
BEACH 2004, Chicago, June 29, 2004 33
Remarks on D*p searches at LEP
D* @ LEP are producedpredominantly by beauty
Rb 22%, Rc 17%
<xE>cc 0.488