charm and beauty proton structure functions f 2 cc and f 2 bb at hera
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HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 1
Charm and beauty proton structure functions F2
cc and F2bb
at HERA Vladimir Chekelian (Shekelyan)
MPI for Physics, Munich
Measurements of F2cc and F2
bb using the H1 vertex detector
at high Q2: Q2 > 150 GeV2 (scattered positron in Lar calorimeter) Eur. Phys. J. C40 (2005) 349 [hep-ex/0411046]
at low Q2: 6.3 < Q2 < 120 GeV2 (scattered positron in Spacal) DESY-05-110 (July 2005), submitted to Eur. Phys. J. C [hep-ex/0507081] ’ the e+p data 1999-2000, int= 57.4 pb-1
--
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 2
Structure Functions, Heavy Quarks and QCD
22
4 2
22
2 2
2(1 (1 ) )
1 (1 )[ ]
pC
L
eN F
yd yydxd xQ QF
DIS cross section and SFs
2 22 ( , ) ( )
iq i iF x Q x e q q in QPM:
Boson Gluon Fusion (BGF)Q2 ~M2
HQ
QPMQ2 >>M2
HQ
’ exp. checks of HQ calculations in pQCD (calculation techniques, pdfs from QCD fits) direct access to gluon via BGF
Treatment of HQ in QCD mc
2 2.5 GeV2, mb2 25 GeV2
Q2 >>M2HQ massless scheme
Q2 ~ M2HQ massive approach (FFNS)
full Q2 range: Variable FNS (VFNS: ACOT,MRST)
,b
- ,b
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 3
Main methods used so far to study HQ at HERA
Open Charm:
- exclusive D* and D meson production - small branching (-> limited statistics) - large extrapolation to get F2
cc
ptrel
( ) ( )slowm K m K
’ see talk of S. Fourletov
’ see talk of B. Naroska
Open Beauty:
b ’ X make use of large b mass and long lifetime: pt
rel ( transverse to b-jet)
(impact parameter)
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 4
The H1 vertex detector
used in this analysis:
CST(30o < < 150o)
- 2 cylindical layers of double-sided silicon strip detectors surrounding the beam pipe
- radii 5.7 cm and 9.7 cm
- hit resolution: 12 μm in r-φ 25 μm in z
- impact parameter resolution (for central tracks with CST hits in both layers):
9033 [ ]
T
mm GeV
P
FST CST BST Forward, Central and Backward Silicon Trackers e p27.6 920 GeV
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 5
Inclusive lifetime tagging of HQ
secondary vertex
- use impact parameter in r-φ plane for
- all tracks with pt> 500 MeV and 2 hits in CST
- sign of impact parameter is defined using φ of the HQ direction:
Goal: be as inclusive as possible
-> the method is cross checked at high Q2 by using explicit reconstruction of the secondary vertex
primary vertex
x
y
r-φ plane
(resolution + secondary vertices) (due to resolution only)
-> high acceptances for long lived c (b) hadrons: 82% (96%) at low Q2; 93% (96%) at high Q2
-> small extrapolation to the full phase space
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 6
Reference axis (HQ direction) Reference axis in r-φ plane ref. axis = jet of the highest transverse momentum jet with pt>2.5 GeV
low Q2 : jet is found for 81% (95%) c(b) events
ref. axis = 180o – scattered elec. if no jet found
Checks of the resolution:
D* - ref. axis : azimuthal difference between reconstructed D* and reference axis
jet no jetfound
Resolution in is 5o for events with jets and 35o if jet is not found
’ data are well described by MC
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 7
Signed impact parameter and SignificanceTracks matched to the reference axis |trref. axis
Significance S = /() of the track (in r-φ plane) - remove ||>1mm (to suppress long lived strange particles, e.g. Ks
0)
/ ( )S
Signed impact parameter Significance
(r-φ plane)
distributions are well described by MC
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 8
Significance distributions at low Q2
S1 – highest absolute significance track
S2 – 2nd highest absolute significance track
with the same sign as S1
S3 – 3d highest absolute significance track
with the same sign as S1 and S2
S1 S2
S3
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 9
Subtracted significance distributions at low Q2
subtract the contents of negative bins from the contents of the corresponding positive bins -> substantially reduce uncertainty due to resolution of and light quark normalisation
-> dominated by c with increasing b fraction at high Si
Simultaneous fit to the subtracted S1,S2,S3 distributions
and total number of inclusive events before track selection using c, b and uds shapes from MC 3 parameters:
- Pc scale factor for charm MC (b decays are excluded)
- Pb scale factor for beauty MC
- Pl scale factor for light quark MC
S1 S2
S3
For complete low Q2 sample: 1.28 0.04, 1.55 0.16, 0.95 0.01c b lP P P
’ individual fit for each x-Q2 interval
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 10
Reduced cross sections and F2cc, F2
bb
2 2( , ) ( , )MCgen
cccBCCMCgen MCgen MCgen
c b l
cc
l
c
bc
Nx Q x Q
N N N
P
P PP
’ F2cc,bb are obtained using FL
cc,bb from the NLO QCD predictions
2 4 2
2 2 2 22
22 (1 (1 )(
),
))
1 (1HQ
HH QQ
QHL
d xQ yF
dx Q
xdQ yF
y
2 2( , ) ( , )MCgen
bbcBCCMCgen MCgen MCgen
cc l
bb
l
b
bb
Nx Q x Q
N N N
P
PP P
2( , )x Q measured inclusive reduced cross section
bin center correction to convert the bin average to x-Q2 point,cc bb
BCC
Pc , Pb , Pl defined for each x-Q2 interval
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 11
Charm structure function F2
cc
lifetime results are consistent with H1 and ZEUS D* measurements
extention of H1 measurements to higher Q2
consistent with gluon from scaling violations
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 12
Beauty reducedcross section
the first measurement of bb (and F2bb)
consistent with pQCD prediction
important for LHC experiments (e.g. bb->H)
bb
MRST04: VFNSCTEQ6HQ: VFNSCCFM: massive (BGF)
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 13
F2cc vs Q2 F2
bb vs Q2
Scaling violations of F2cc, F2
bb are increasing with decreasing of x (similarly to F2)
Compared with NLO and NNLO QCD predictions:
’ the data are more precise than the spread of QCD predictions
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 14
Cross section fractions2 2 2
2 2 2
( , )/
( , )
HQ HQHQ d d x Qf
dxdQ dxdQ x Q
Charm fraction fcc
- roughly constant with Q2
- slightly decreasing with x
- around 24% on average
Beauty fraction fbb - increases rapidly with Q2 from ~0.3% to ~3% (i.e. by a factor of 10)
fcc
fbb
Well described by MRST04 (VFNS)
HEP2005, 21 July 2005 V. Chekelian, Charm and Beauty SF 15
Charm and beauty structure functions F2cc and F2
bb are measured in the Q2 range from 12 GeV2 to 650 GeV2 using lifetime tagging (impact parameter method)
Charm contribution to F2 is around 24% on average
Beauty contribution to F2 changes from 0.3% to 3% F2
bb is measured for the first time
Both F2cc and F2
bb are well described by NLO/NNLO QCD
Summary
Large potential for HERA II:
H1: CST, FST, BST ZEUS: MVD (see talk of M. Corradi)
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