Dijets in DIS and PHP at ZEUS - wiki.bnl.gov fileDIS2011,NewportNews,USA,April11-15,2011...
Transcript of Dijets in DIS and PHP at ZEUS - wiki.bnl.gov fileDIS2011,NewportNews,USA,April11-15,2011...
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 1 / 22
Dijets in DIS and PHP at ZEUS
Oleg KuprashDESY / Taras Shevchenko National University of Kyiv
DIS 2011, Newport News
DIS: Eur.Phys.J.C70:965-982,2010PHP: ZEUS-prel-10-014
On behalf of the ZEUS collaboration
HERA collider
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 2 / 22
pP X
e′e
q
l l′
Electrons: 27.5 GeVProtons: 920 GeV√s = 318 GeV
Kinematics:Q2 = −q2 = −(l − l′)2
y = PqP l
xBj =Q2
2Pq
Q2 = xBjys
Q2 ≤ 1 GeV2: photoproduction (PHP)
Q2 ≥ 1 GeV2: DIS
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 3 / 22
Dijets in DIS
Dijet processes in DIS
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 4 / 22
αSq(p1)
g(p2)
e(l) e(l′)
p(P )
X
γ/Z0(q)jet 1
jet 2
q(p1)
q(p2)
e(l) e(l′)
p(P )
X
jet 1
jet 2αS
γ/Z0(q)
• Processes of leading order in αS :• QCD Compton scattering (quark-induced) • Boson-gluon fusion (gluon-induced)
• Jets in Neutral Current DIS:- two hard scales: virtuality of the exchanged boson and transverse energy of the jets- General tests of pQCD (hard matrix element, factorisation, perturbative expansion, PDFuniversality)- High-precision measurements of strong coupling αS
- Sensitive to parton distribution functions in proton σ =∑
n αnS
∑
a=q,q,g fa/p ⊗ σ(n)a
Dijet processes in DIS: observables
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 5 / 22
)2 (GeV2Q125 6750 13375 20000
Eve
nts
1
10
210
310
410
(GeV)jetT,BE
10 20 30 40
Eve
nts
210
310
410 )−1ZEUS (374 pb
LEPTO
ARIADNE
(GeV)jjM20 40 60 80
Eve
nts
210
310
410
Bjx0 0.05 0.1
Eve
nts
210
310
410
)ξ(10
log−2 −1.5 −1 −0.5 0
Eve
nts
1
10
210
310
410
*η0 0.5 1 1.5 2
Eve
nts
10
210
310
ZEUS• Q2 - virtuality of the exchanged boson• xBj - for the partonmodel process is the fraction of the protonmomentum carried by the struck parton
• ¯E
jetT,B = 1
2
(
E1T + E2
T
)
- mean transverse energy• Mjj =
√
(p1 + p2)2 - invariant dijet mass(in LO Mjj is identical to the centre-of-massenergy of the parton-boson system)
• ξ = xBj
(
1 +M2
jj
Q2
)
- in LO is fraction of the protonmomentum carried by the initial parton•η′ = 1
2 |η1 − η2| - difference in pseudorapidityis invariant under longitudinal boosts
Dijets in DIS: samples and selections
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 6 / 22
)2 (GeV2Q125 6750 13375 20000
Eve
nts
1
10
210
310
410
(GeV)jetT,BE
10 20 30 40
Eve
nts
210
310
410 )−1ZEUS (374 pb
LEPTO
ARIADNE
(GeV)jjM20 40 60 80
Eve
nts
210
310
410
Bjx0 0.05 0.1
Eve
nts
210
310
410
)ξ(10
log−2 −1.5 −1 −0.5 0
Eve
nts
1
10
210
310
410
*η0 0.5 1 1.5 2
Eve
nts
10
210
310
ZEUS• 1998-200 and 2004-2007: 203 pb−1
of electron data + 171 pb−1 of positron data• Event phase space:125 < Q2 < 20 000 GeV2
0.2 < y < 0.6• Jets phase space:−1 < η
jetLAB < 2.5
EjetT,B(1,2) > 8 GeV, Mjj > 20 GeV
• Jet selections:- kT clusteralgorithm in the longitudinally invariantinclusive mode running on calorimeter cells- Jetsearch in the η − φ plane of the Breit frame
Dijets in DIS: data corrections and uncertainties
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 7 / 22
• Correction to the hadron level• QED corrections
)2 (GeV2Q1000 10000
rela
tive
unce
rtai
nty
−0.1
0
0.1
)2 (GeV2Q1000 10000
rela
tive
unce
rtai
nty
−0.1
0
0.1
21
)2syst
δ+2ES
δtotal systematic uncertainty: (
ESδcorrelated systematic uncertainty:
statδstatistical uncertainty:
ZEUS
Uncertainties:• on theabsolute energy scaleof the jets: ±1% forE
jetT,LAB > 10 GeV and
±3% for lower EjetT,LAB
•on the absolute energyscale of the electroncandidate: ±2%• very high precision of measurements
Dijets in DIS: NLO calculations
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 8 / 22
rela
tive
diffe
renc
e
−0.1
0
0.1
rela
tive
diffe
renc
e
−0.1
0
0.1
−0.1
0
0.1
−0.1
0
0.1
−2 −1.5 −1 −0.5 0
−0.1
0
0.1
−0.1
0
0.1
)ξ(10
log−2 −1.5 −1 −0.5 0
)ξ(10
log
2 < 250 GeV2125 < Q 2 < 500 GeV2250 < Q
2 < 1000 GeV2500 < Q 2 < 2000 GeV21000 < Q
2 < 5000 GeV22000 < Q 2 < 20000 GeV25000 < Q
scaleR
µPDF uncertainty (CTEQ6.6)MSTW 2008ZEUS−JETSZEUS−S
• NLO (O(α2S)) calculations were
obtained using NLOJET++ / DISENT
• CTEQ6.6parameterisations of the proton PDFs
• predictions corrected to the hadronlevel (corrections differ from unity by 5%)
• Sources of uncertainty
in the theoretical predictions:
- due to terms beyond NLO:±6% at low Q2 and ±3% at high Q2
- dueto uncertainty on αS : mostly below ±3%- uncertainty of the modellingof the parton shower: less than 2%- due to proton PDFs:±4% at low Q2 and ±2% ath high Q2
Dijets in DIS: results - Single-differential cross sections
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 9 / 22
)2 (
pb/G
eV2
/dQ
σd
−410
−310
−210
−110
)−1ZEUS (374 pb
0Z C⊗hadr C⊗NLO
2jetT,BE+2=Q
R2µ
2=Q2R
µ2jet
T,BE=2R
µ
)2 (GeV2Q1000 10000
rel.
diff.
to N
LO
−0.2
0
0.2 jet energy scale uncertainty
NLO uncertainty
(pb
)B
j/d
xσd
210
310
Bjx0.02 0.04 0.06 0.08 0.1re
l. di
ff. to
NLO
−0.2
0
0.2
(pb
/GeV
)je
tT
,BE
/dσd
−110
1
10
)−1ZEUS (374 pb
0Z C⊗hadr C⊗NLO
2jetT,BE+2=Q
R2µ
2=Q2R
µ2jet
T,BE=2R
µ
(GeV)jetT,BE
10 20 30 40 50 60rel.
diff.
to N
LO
−0.2
0
0.2 jet energy scale uncertainty
NLO uncertainty
• Kinematic region: - 125 < Q2 < 20 000GeV2, 0.2 < y < 0.6−1 < ηjetLAB < 2.5, Ejet
T,B(1,2) > 8 GeV, Mjj > 20 GeV
• Good description of data by NLO QCD in the whole measured range
Dijets in DIS: results - Single-differential cross sections
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 10 / 22
(pb
/GeV
)jj
/dM
σd
−110
1
(GeV)jjM20 40 60 80 100 120re
l. di
ff. to
NLO
−0.2
0
0.2
* (p
b)η
/dσd10
210
)−1ZEUS (374 pb
0Z C⊗hadr C⊗NLO
2jetT,BE+2=Q
R2µ
2=Q2R
µ2jet
T,BE=2R
µ
*η0 0.5 1 1.5 2re
l. di
ff. to
NLO
−0.2
0
0.2 jet energy scale uncertainty
NLO uncertainty
) (p
b)ξ(
10/d
log
σd
0.05
0.1
0.15
310×
)ξ(10
log−2 −1.5 −1 −0.5 0re
l. di
ff. to
NLO
−0.2
0
0.2
η∗ = 12|η1 − η2|, ξ = xBj
(
1 +M2
jj
Q2
)
• Good description of data by NLO QCD in the whole measured range
Dijets in DIS: gluon induced events
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 11 / 22
gluo
n fr
actio
n (C
TE
Q6.
6)
0
0.2
0.4
0.6
0.8
gluo
n fr
actio
n (C
TE
Q6.
6)
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
−2 −1.5 −1 −0.5 00
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
)ξ(10
log−2 −1.5 −1 −0.5 0
)ξ(10
log
2 < 250 GeV2125 < Q 2 < 500 GeV2250 < Q
2 < 1000 GeV2500 < Q 2 < 2000 GeV21000 < Q
2 < 5000 GeV22000 < Q 2 < 20000 GeV25000 < Q
gluo
n fr
actio
n (C
TE
Q6.
6)
0
0.2
0.4
0.6
0.8
gluo
n fr
actio
n (C
TE
Q6.
6)
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
20 40 600
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
(GeV)jetT,BE
20 40 60
(GeV)jetT,BE
2 < 250 GeV2125 < Q 2 < 500 GeV2250 < Q
2 < 1000 GeV2500 < Q 2 < 2000 GeV21000 < Q
2 < 5000 GeV22000 < Q 2 < 20000 GeV25000 < Q
• gluon fraction ranges from about 75% at 125 < Q2 < 250 GeV2 and small ξ to about5% at the highest Q2 above 5000 GeV2
• lower Q2-region is not statistically limited → precise input for the PDF fits can beexpected
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 12 / 22
Dijets in PHP
Dijet processes in PHP
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 13 / 22
αS
q(p1)
q(p2)
e(l) e(l′)
p(P )
Xp
jet 1
jet 2
αS
Xγ
• Processesof leading order in αS are the same as in DIS• Processes of higher orders in αS :- direct (point-like exchanged boson, same as in DIS)- resolved(represents ’hadronic structure of the photon’)• Jets in PHP:- one hard scale: jet transverse energy- Generaltests of pQCD (hard matrix element, factorisation,perturbative expansion, PDF universality)- High-precision measurements of strong coupling αS
- dijet cross sections sensitive to both proton and photon parton distribution functionsCross section for resolved photoproduction in LO:
dσγpres =
∑
i,j
∫
xγ
∫
xpdxγdxpfi/γfj/pdσij
xγ =Ejet1
Te−ηjet1
+Ejet2
Te−ηjet2
2yEexp =
Ejet1
Teη
jet1+Ejet2
Teη
jet2
Ep
Dijet processes in PHP: observables
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 14 / 22
αS
q(p1)
q(p2)
e(l) e(l′)
p(P )
Xp
jet 1
jet 2
αS
Xγ
Direct Resolvedγq → qg qg → qg
γq → qq qq → qq
qq → gg
gg → qq (on the figure)gg → gg
qq → qq
qq → q′q′
qq′ → qq′
EjetT - average jet transverse energy
ηjet - average jet pseudorapidityxobsγ - fraction of the incoming photon
momentum taken by the dijet systemMjj - invariant dijet mass| cos θ∗| - scatteringangle in the dijet centre-of-mass system
Dijets in PHP: samples and selections
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 15 / 22
• data collected during 2005-2006: 189 pb−1 of electron data
• Event phase space: - Q2 < 1GeV2
- γp centre-of-mass energy 142 < Wγp < 293 GeV, or equally 0.2 < y < 0.85
• Jets phase space:
−1 < ηjetLAB < 2.5 E
jetT,LAB 1,2 > 17 GeV
Ejet
T,LAB 1(2) > 21 (17) GeV or Mjj > 60 GeV
| cos θ∗| < 0.8• Jet selections:
- kT cluster algorythm in the longitudinally invariant inclusive mode running on calorimeter cells
- Jet search was performed in the η − φ plane of the laboratory frame
Data corrections:
• Correction to the hadronic level
• No need in polarisation corrections
Uncertainties:
• the absolute energy scale of the calorimetric jets in simulated events was varied by its
uncertainty of ±1%• on the CAL energy uncertainty on Wγp was estimated by varying yJB by ±1% in simulated
events
• total systematic uncertainty in the cross sections was typically below ±5%
Dijets in PHP: NLO calculations
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 16 / 22
ZEUS
-0.20
0.2 terms beyond NLO
theo
reti
cal r
elat
ive
unce
rtai
nty
-0.20
0.2 αs(MZ)
-0.20
0.2 hadronisation
-0.20
0.2 pPDFs
-0.20
0.2
20 30 40 50 60 70 80
γPDFs
E––
jet
T (GeV)
• NLO (O(α2S))
calculations were obtained using theprogram by Klasen, Kleinwort and Kramer• µR = µF = (Ejet
T )max
• ZEUS-Sparameterisations of the proton PDFs• GRV-HOparameterisations of the photon PDFs• predictionscorrected to the hadron level (correctionsdiffer from unity by less then 5%)• Sources of uncertainty
in the theoretical predictions:
- due to terms beyond NLO: up to 20%- due to uncertainty on αS : below ±5%- uncertainty of the modellingof the parton shower: less than 2%- due to proton PDFs: less than 5%- due to photon PDFs estimated by using an alternative set of parameterisations, AFG04:less than 5%
Dijets in PHP: results - Single-differential cross sections
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 17 / 22
ZEUS102
10
1
10-1
10-2
10-3
ZEUS (prel.) 189 pb-1
NLO hadr: p/γ PDFs⊗(Klasen et al.)
ZEUS-S/GRV-HO
MSTW08/GRV-HOZEUS-S/AFG04
ZEUS-S/CJK
Ejet1
T > 21 GeV, Ejet2
T > 17 GeV
-1 < η jet < 2.5
Q2 < 1 GeV2
0.2 < y < 0.85
jet energy scale uncertainty
dσ/
dE–– je
t
T
(pb/
GeV
)
-0.5
0
0.5
20 30 40 50 60 70 80
E––
jet
T (GeV)
rel.
diff
. to
NL
OZEUS
0
250
500
750
1000
ZEUS (prel.) 189 pb-1
NLO hadr: p/γ PDFs⊗(Klasen et al.)
ZEUS-S/GRV-HO
MSTW08/GRV-HOZEUS-S/AFG04
ZEUS-S/CJK
Ejet1
T > 21 GeV, Ejet2
T > 17 GeV
-1 < η jet < 2.5
Q2 < 1 GeV2
0.2 < y < 0.85
jet energy scaleuncertainty
dσ/
dxγ
ob
s (p
b)
-0.5
0
0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
xγ obs
rel.
diff
. to
NL
O• Kinematic region: Q2 < 1GeV2, 0.2 < y < 0.85−1 < η
jetLAB < 2.5, Ejet
T,LAB 1(2) > 21 (17) GeV
• Good description in shape and normalisation
xγ =Ejet1
Te−ηjet1
+Ejet2
Te−ηjet2
2yEecross section is expected to be most sensitive to the photon
PDFs, especially at low xγ , where resolved processes are dominant
Dijets in PHP: results - Single-differential cross sections
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 18 / 22
ZEUS10
1
10-1
10-2
10-3
ZEUS (prel.) 189 pb-1
NLO hadr: p/γ PDFs⊗(Klasen et al.)
ZEUS-S/GRV-HO
Ejet
T > 17 GeV -1 < η jet < 2.5 Q2 < 1 GeV2
0.2 < y < 0.85 |cosθ*| < 0.8
jet energy scale uncertainty
dσ/
dMjj (
pb/G
eV)
-0.5
0
0.5
60 80 100 120 140 160
Mjj (GeV)
rel.
diff
. to
NL
OZEUS
0
100
200
300ZEUS (prel.) 189 pb-1
NLO hadr: p/γ PDFs⊗(Klasen et al.)
ZEUS-S/GRV-HO
Ejet
T > 17 GeV -1 < η jet < 2.5
Q2 < 1 GeV2
0.2 < y < 0.85 Mjj > 60 GeV
jet energy scaleuncertainty
dσ/
d|co
sθ* | (
pb)
-0.5
0
0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
|cosθ*|
rel.
diff
. to
NL
O• Kinematic region: Q2 < 1GeV2, 0.2 < y < 0.85−1 < η
jetLAB < 2.5, Ejet
T,LAB 1,2 > 17 GeV, | cos θ∗| < 0.8, Mjj > 60 GeV2
• Good description in shape and normalisation• Demonstates validity of the description of the dynamics of dijet production by pQCD atO(α2
S)
Conclusions
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 19 / 22
• Dijet measurements in both PHP and DIS were performed at HERA using
ZEUS detector
• The measurements of dijets in DIS have very small statistical and
systematical uncertainties
• ... and the description of the data by the predictions of NLO QCD is very
good
• → DIS dijet data will provide useful precision information for the
determination of the strong coupling constant and the exctraction of the
proton PDFs
• Precise measurements of dijets in PHP was done
• Given cross sections are sensitive to the parton densities in the proton and
photon
• ... and the description of the data by the predictions of NLO QCD is very
good
• → Measurements allow for improving the determination of the photon and
proton PDFs in future QCD fits
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 20 / 22
Backup slides
Dijets: Infrared Sensitivity
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 21 / 22
• In some regions of the dijet phase space the cancellation between soft and collinearsingularities in NLO theoretical predictions is incomplite• Real emissions of hard gluons are suppressed in regions in which the azimuthal difference∆φjj of the two jets is close to π, where ∆E
jjT and Mjj is close to threshold
• But if jets slightly decorrelated,soft gluon emission is allowedbut kinematically constrained• ... some of the virtualdivergences are left uncancelledand theory calculations becomesensitive to the soft gluon emission• in order to make theoryinfrared insensitive asymmetric ET
cut can by applied or cut on Mjj
• In DIS dijet analysis cut onMjj > 20 GeV has been applied
• In PHP dijet analyses asymmetric cut on Ejet1(2)T,LAB > 21 (17) GeV has been applied for
cross sections as functions of EjetT , ηjet, xobs
γ and cut on Mjj > 60 GeV for Mjj and| cos θ∗|
Correlation between Mjj and η′
DIS 2011, Newport News, USA, April 11-15, 2011 Oleg Kuprash – 22 / 22
M2jj = 2 · Ejet1
T · Ejet2T · [cosh(η1 − η2)− cos∆φjj ]