charm with zeus hera-ii data · 3 hera ii • hera i 1996 - 2000, hera ii 2002 - 2007 • new...

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


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


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)


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


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


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


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


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


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


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

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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


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


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


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


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


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


(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


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


ZEUS• big improvement

• background reduced by a factorof 30


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


charm with zeus hera-ii data · 3 hera ii • hera i 1996 - 2000, hera ii 2002 - 2007 • new...

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