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Charm with ZEUS HERA-II data
Falk Karstens on behalf of the ZEUS collaboration
University of Freiburg/ Germany
DIS 2006 – 21th April 2006 – HF-2
Outline:
•D∗± mesons in deep inelastic scattering•D0 decay length significance•D± mesons in deep inelastic scattering
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
1
Charm search with D∗±(2010) → D0(K∓ + π±) + πs
p
e+
g
uu
d
c
cg
D*-
sW-
D0
K+
p-
ps-
u c td s b
2/3-1/3
d s bu c t
1/3-2/3
u u
u
d
dd
• Boson Gluon Fusion is the dominantprocess in DIS – gluon sensitivity
• open charm pair production
• fragmentation F(c, c̄ → D∗±) =0.197± 0.012
• charm deexcitation: Zweig rule -strong interaction, short lifetimeB(D∗± → D0π±s ) = 0.677± 0.005
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
2
Charm search with D∗±(2010) → D0(K∓ + π±) + πs
p
e+
g
uu
d
c
cg
D*-
sW-
D0
K+
p-
ps-
u c td s b
2/3-1/3
d s bu c t
1/3-2/3
u u
u
d
dd
• charm ground state to strangestate: weak interaction, longerlifetime B(D∗± → D0π±s )×B(D0 →K∓π±) = 0.0380± 0.0009
• F × B = (0.51± 0.13) %
• all elementary forces relevant forparticle physics covered
• small phase space of about 6 MeV forslow π±s limited by mass difference
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
3
HERA II
• HERA I 1996 - 2000,HERA II 2002 - 2007
• new elements are micro vertexdetector (MVD) and straw tubetracker (STT) in HERA II
• gated ZEUS data L(e+p) =41 pb−1 and L(e−p) ≈ 160 pb−1in HERA II
HERA delivered
0
50
100
150
200
250
300
0 200 400 600 800 1000 1200 1400
days of running
Inte
gra
ted
Lu
min
osit
y (
pb
-1)
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
4
Selection criteria for
D∗±(2010) → D0(K∓ + π±) + π±smass calculations done with all track combinations and mass assignment
primary vertex tracksmass cuts:
1.80 GeV < M(D0) < 1.92 GeV0.143 GeV < ∆M < 0.148 GeV
∆M = M(K∓ π± π±s )−M(K∓ π±)
DIS cuts:5 GeV2 < Q2 < 1000 GeV2
0.02 < y < 0.7D∗± kinematic cuts:
1.5 GeV < pT(D∗±) < 15 GeVpT(K) > 0.4 GeVpT(π) > 0.4 GeVpT(πs) > 0.12 GeV−1.5 < ηD∗± < 1.5
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
Event display high pt, Q2 from 2-06-2004 5
XY View ZR View
K+ π −K+ −sπ
−sπ
π −
Q2 = 918 GeV2 y = 0.39 pT (D∗−) = 10.3 GeV η(D∗−) = 0.06pT (K+) = 1.5 GeV pT (π−) = 7.9 GeV pT (π−s ) = 1.1 GeV
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
6
Positron scattering: e+ + p → D∗± + X
0.135 0.14 0.145 0.15 0.155 0.16 0.165
100
200
300
400
500
600
700
800
M (GeV)∆
ZEUS
Co
mb
inat
ion
s
0.135 0.14 0.145 0.15 0.155 0.16 0.165
100
200
300
400
500
600
700
800)
-1p (40 pb
+ZEUS (prel.) 03-04 e
Wrong Charge
• subtraction of correlatederrors gives a narrow signal
• invariant mass differencepeaks around the rest massof πs (139 MeV) plus itskinematic energy
• background subtractionmethod to get thenumber of candidatesNrec = Nsig −Nback
• luminosity L = 40pb−1
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
7
Electron scattering: e− + p → D∗± + X
0.135 0.14 0.145 0.15 0.155 0.16 0.165
100
200
300
400
500
600
700
M (GeV)∆
Co
mb
inat
ion
s
ZEUS
0.135 0.14 0.145 0.15 0.155 0.16 0.165
100
200
300
400
500
600
700)
-1p (33 pb
-ZEUS (prel.) 04-05 e
Wrong Charge
• luminosity L = 33 pb−1 →only a fraction of the currentdata
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
8
Ratio between σ(e−p) and σ(e+p) cross-sections
1 10 210 310-1
-0.5
0
0.5
1
1.5
2
2.5
3
)2 (GeV2Q
p)
+(eσ
p)/
- (
eσ
ZEUS
ZEUS 98-00
ZEUS (prel.) 03-05
1 10 210 310-1
-0.5
0
0.5
1
1.5
2
2.5
3 • lower limit in Q2 shifted from1.5 GeV2 to 5 GeV2
• HERA I data inPhys. Rev. D69: 012004,2004 (hep-ex/0308068)given in blue L(e−p) =17 pb−1; L(e+p) = 65 pb−1
• HERA II data confirms thatthe excess in HERA I data wasa statistical fluctuation
⇒ charm is produced equally in e+p and e−p collisions
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
9
Micro Vertex Detector
• two/ three barrel layers andfour forward wheels
• 20 µm intrinsic hit resolution
• impact parameter resolutionof about 100 µm is requiredfor efficient charm tagging ⇒alignment accuracy of 20 µmneeded
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
10
MVD alignment
0
50
100
150
200
25090
8070
6050
40
30
20
10
0
350
340
330
320
310300
290280
270260
250240
230
220
210
200
190
180
170
160
150
140
130120
110100
• impact parameter w.r.p.beam-spot in microns vs φfor high momentum trackspT > 3 GeV in central region|η| < 1 of good quality min.4 MVD hits ⇒ natural spreadof tracks in interaction regioncovered
• cosmic alignment in yellow• ep collision tracks used for
alignment in blue• good agreement with MC
given in red
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
D0 decay length significance 11
Decay length significance
secondary vertex
decay length
primary vertexbeam-spot
x
y
s
P
L
pK
+
sp-
p- • decay length ~L = ~s− ~P
• projection onto momentum in 2DLxy =
(~s−~P )·~p(D0)pT (D
0)
• significance sL = LσL
• ~P = beam-spot gives higher precisionthan ~P = primary vertex
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
12
More data ...
) (GeV)π) - M(KsππM(K0.135 0.14 0.145 0.15 0.155 0.16 0.165
Can
dida
tes
0
100
200
300
400
500
600
700 (+ c.c.)+sπ 0 D→ +D*
+π - K→ 0D-1ZEUS (prel) 120 pb
FitWrong-charge background
) (GeV)π) - M(KsππM(K0.135 0.14 0.145 0.15 0.155 0.16 0.165
Can
dida
tes
0
100
200
300
400
500
600
700
ZEUS • current 2005 data added forthe signal
• L(e−p) = 120 pb−1
• signal width improved from0.93 MeV to 0.76 MeVbecause of improved track fits
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
D0 decay length significance 13
Decay length significance II
)xyL
σ/xy = LLDecay Length Significance (S-10 -8 -6 -4 -2 0 2 4 6 8 10
Can
dida
tes
0
100
200
300
400
500 (+ c.c.)+sπ
0 D→ +D*+π - K→ 0D
L from D* S
0D
-1ZEUS (prel) 120 pbWrong-charge background(x 1.09)
ZEUS • D0s taken from D∗±s
• wrong-charge backgroundnormalized to signal
• shift seen because of decaylength cτ = 123µm
• exponential decay lengthdistribution hidden byresolution effects, what is nota surprise
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
D± → K∓ + π± + π± selection criteria 14
)(GeV)ππM(K1.75 1.8 1.85 1.9 1.95 2
Co
mb
inat
ion
s
0
500
1000
1500
2000
2500
3000
+ (c.c.)+π+π- K→+D
> 2.5 GeV±D
Tp
-1ZEUS(prel.) 120 pb
ZEUS • same DIS cuts as for D∗±spT (D±) > 2.5 GeVpT (K∓) > 0.65 GeVpT (π±) > 0.45 GeV
• additional quality cut min. 4MVD hits
• hardly any signal visible
• lifetime longer: cτ = 314µm
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
D± → K∓ + π± + π± decay length significance 15
(L)σLSignificance
-10 -8 -6 -4 -2 0 2 4 6 8 10
En
trie
s
1
10
210
310
410
+ (c.c.)+π+π- K→+D(L)σL positive →
(L)σLMirror negative
-1ZEUS(prel.) 120 pb
(L)σLSignificance
-10 -8 -6 -4 -2 0 2 4 6 8 10
En
trie
s
1
10
210
310
410
ZEUS • negative side mirrored to the left⇒ enhancement
• signed decay lengthL = |~L|sign(~L · ~pD±)
• cut Lσ(L) > 3 is used to enrich thesignal
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
D± → K∓ + π± + π± signal 16
)(GeV)ππM(K1.75 1.8 1.85 1.9 1.95 2
Co
mb
inat
ion
s
0
20
40
60
80
100
120
140
160
180
> 3(L)σL
43±) = 231 ±N(D
+ (c.c.)+π+π- K→+D
Fit (Gaussian + p2)
> 2.5 GeV±D
Tp
-1ZEUS(prel.) 120 pb
ZEUS• big improvement
• background reduced by a factorof 30
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06
17
Summary and outlook
• D∗± cross-section ratio: HERA II dataconfirms that the excess in HERA I data was astatistical fluctuation
• decay length of D0s from D∗±s visible in decaylength significance plot and could be used toclean up the signal
• further use of decay length significance foruntagged D0s
• decay length significance cut useful to reduces√b
for D± production
Falk Karstens Charm with ZEUS HERA-II data @ DIS 2006 21|04|06