- Email: [email protected]
Diffraction at CDF
Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34 www.elsevierphysics.com
Di raction at CDF Christina Mesropian for the CDF Collaborationa a The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA The results of the di ractive measurements at CDF experiment at the Tevatron pp collider are presented. Di ractive structure functions are studied in single di ractive dijet production. The exclusive dijet and low-mass state production in double pomeron exchange are discussed in context of the exclusive Higgs production at LHC.
1. Introduction The hadronic diractive process can be dened as a reaction in which no quantum numbers are exchanged between the colliding particles and/or a large, non exponentially suppressed, rapidity gap (region devoid of particles) is present. In the framework of Regge theory diractive reactions are characterized by an exchange of the pomeron, a hypothetical object with vacuum quantum numbers. Diractive reactions that incorporate hard processes, such as production of jets in pp collisions, allow one to study diraction in a perturbative QCD framework, thus providing an opportunity to study the nature of the pomeron. CDF collaboration at Fermilab pp collider investigated various diractive reactions at three ps=1800 center of mass energies, GeV (Run I), ps=630 GeV (Run IC), and ps=1960 GeV (Run II). The CDF Run I detector is described in detail in Ref. 1]. The detector components relevant to diractive studies are calorimeters, central tracking chambers, and the Roman pot spectrometer (RPS) installed during Run IC. Upgrade for the Run II included a new plug calorimeter, gas Cherenkov counters for luminosity measurements, miniplug calorimeters 2] providing forward pseudorapidity coverage (3:5
events at CDF is either a large rapidity gap, or a leading hadron detected in the RPS. In this talk we will discuss hard diraction measurements at CDF.
2. Diractive Structure Function One of the central questions in hard diraction is whether these processes obey QCD factorization, in other words whether the pomeron has a universal, process independent \diractive" structure function. Experimental results from the DESY ep collider HERA show that QCD factorization holds in diractive deep inelastic scattering (DDIS). In Run I CDF experiment measured single diractive (SD) rates of W-boson 4], dijet 5], b-quark 6], and J= 7] production relative to the non-diractive (ND) rates. It was observed that the rates were a factor of 10 lower than expectations based on the corresponding structure functions determined at HERA. Fig. 1 shows the diractive structure function, FjjD , measured by CDF from Run I data sample with the leading antiproton detected in Roman pot spectrometer. The FjjD is presented as a function of the momentum fraction of the pomeron carried by the struck parton, = xp = , where xp is the x-Bjorken of the parton in the antiproton and is the momentum loss of the antiproton. The curved lines correspond to the expectations based on diractive parton densities extracted by H1 experiment.
32
C. Mesropian / Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34
D
H1 fit-2
Q
ET
Q
Q
CDF data jet1,2
H1 fit-3
100
Preliminary results also show that there is no signicant dependence of the ratio on 2 h 2 i in the 2 range from 100 GeV2 to 1600 GeV2 , which indicates that the pomeron 2 evolution is similar to that of the proton.
ET
2
2
≥ 7 GeV
Ratio [(SD/ ∆ ξ)/ND]
Fjj (β)
A large discrepancy in normalization is observed. This result implies that there is a breakdown of QCD factorization in hard diraction between Tevatron and HERA.
0.035 ≤ ξ ≤ 0.095
( Q = 75 GeV )
| t | ≤ 1.0 GeV
2
10
10
CDF Run II Preliminary ξ (0.02 to 0.10) (x 1) ξ (0.05 to 0.10) (x 10) ξ (0.02 to 0.05) (x 0.1) CDF Run I χ2 / ndf Prob Constant Slope
1
10
10
4.616 / 12 0.9696 -4.55 ± 0.09523 -0.9466± 0.04685
-1
-2
1 10
± 25% norm. uncertainty
-3
jet
= 14 GeV
0.1
H1 2002 σrD QCD Fit (prel.)
10
-4
10
0.1
-3
10
2.1. Diractive Dijet Production in Run II
In Run II CDF has continued studies of the single diractive dijet production. The data sample is collected by triggering on a leading antiproton in combination with at least one calorimeter tower with 5 GeV. The is measured by using all calorimeter information. The control ND dijet sample is triggered on the same calorimeter tower requirement. The ratio of SD to ND dijet production rates (see Fig. 2) is studied as a function of , where is evaluated for each event from the andP of the jets according to ; . The results the formula: = p1 =1 are in good agreement with the Run I measurement and do not have appreciable dependence in the ratio. Measurement of dependence at lower values ( 0 02) is one of the Run II goals. ET >
xp
xp
s
n {
{ ET e
{
<
:
-1
1
Figure 1. Diractive structure function measured by CDF compared with expectations from DDIS based on the H1 t.
ET
x Bj 10
β
Figure 2. Ratio of SD to ND dijet event rates per unit as a function of -Bjorken.
xp
-2
x
3. Exclusive Production Observation of the Higgs boson is one of the main goals of the Large Hadron Collider (LHC) in CERN. Recently, considerable interest has been drawn to the subject of \exclusive" Higgs boson production ! + + , where central heavy object is produced alone, separated from outgoing hadrons by rapidity gaps. The exclusive double diractive Higgs production has unique experimental advantages such as clean nal state which contains only the Higgs boson and very forward leading nucleons, improved mass resolution, and small background from the direct production, due to the several suppression mechanisms. Although the cross section for the exclusive Higgs production is too small to be observed at the Tevatron, several processes mediated by the same mechanism but with higher production rates can be studied to calibrate theoretical prepp
p
H
p
bb
33
C. Mesropian / Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34
dictions. Below, we present preliminary results on exclusive dijet and c 0 production at CDF.
3.1. Exclusive Dijet Production by Double Pomeron Exchange
In Run I CDF reported rst observation of dijet production via double pomeron exchange (DPE) 8]. The events are characterized by a leading antiproton, two jets in the central pseudorapidity region, and a large rapidity gap on the outgoing proton side. In Run II our sam-
are calculated from all calorimeter towers. If dijets were produced exclusively, would be equal to one by denition. However, taking into account resolution eects, the exclusive region is dened as 0 8. Fig. 3 shows the distributions for DPE dijets, compared with those for single diractive sample. No signicant excess is observed for 0 8 over a smooth distribution. The upper limit for exclusive dijet production as a cross section for DPE events with 0 8 is 970 65( ) 272( )pb and 34 5( ) 10( )pb for the leading jet 10 and 25 GeV, respectively. These experimental numbers are consistent with recent theoretical predictions 9].
Rjj
Rjj >
:
Rjj
Rjj
Rjj
>
>
:
:
stat
stat
syst
syst
ET >
SDp (Prescale 280)
10 10 10 10 10
DPE = SDp + GAPp(5.5<ηgap <7.5) (Prescale 5)
6
DPE = SDp + GAPp(3.6<ηgap <7.5) (Prescale 5) 5
Energy Scale Uncertainty
L = 26 pb
4
CDF Run 2 Preliminary 3
2
Dijet Mass Fraction
SD p : 0.03 < ξ p < 0.1
10 1
-1
Events/bin
#Events × Prescale/0.05
CDF Run II Preliminary
jet2
ET
0
0.2
data
FakeEvt
4 3.5 3
> 10 GeV
2.5 0.4
0.6
0.8
1
R jj = M jj/M X
2 1.5 1
Figure 3. Dijet mass fraction distributions for DPE events: 5 5 7 5 (open circles) and 36 7 5 (lled circles), and for SD events (triangles). :
:
< gap <
< gap <
0.5
:
0 2.8 3 3.2 3.4 3.6 3.8 4 Di-muon + photon invariant mass (GeV)
:
ple is collected with a dedicated trigger requiring BSC gap on the proton side, a leading antiproton in the Roman pot spectrometer, and a single calorimeter tower with 5 GeV. Additional oine requirement of a gap in the miniplug on the proton side enhances initial sample. For resulting DPE candidate events the \dijet mass fraction", , is dened as the ratio of the invariant mass of the two leading jets, , to the mass of the entire system, excluding leading and , . The dijet mass is measured from the energies of the calorimeter towers inside the jet cones, and mass of p the system is calculated according to = , where values of
Figure 4. Invariant mass of the dimuon and a photon system compared to the Monte Carlo predictions (histogram).
ET >
Rjj
Mjj
p
MX
Mjj
MX
p
p
s
p
3.2. Exclusive
c
0
Production
Experimental studies of the exclusive 0 production at CDF can serve as a benchmark process for the exclusive Higgs production predictions at LHC. The exclusive 0 production in DPE was studied at CDF using Run II data. The events were selected by requiring dimuon trigger with 1 5 GeV/c, j j 0 6, and the invariant c
c
pT
>
:
<
:
34
C. Mesropian / Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34
mass of the muon pair in the J= mass window. The large rapidity gaps were required after rejecting cosmic background. Starting with data sample of 93 pb;1 , 10 candidate events are found for the exclusive c 0 ! J= + . The mass of the dimuon- system is shown in Fig. 4. The upper limit of the exclusive c0 ! J= + production cross section is set based on these 10 events and is equal to 49 18(stat) 39(syst) pb. The measured cross section is consistent with the theoretical prediction of the exclusive rates 10].
4. Conclusions CDF collaboration continues extensive program of diractive studies. Our Run II goals include measurements of Q2 and dependence of the diractive structure functions with dedicated triggers to extend range down to 0:001 and 2 Q range up to 104 GeV2 . Study of exclusive production processes in DPE is another goal for Run II. We are developing new triggers for b b, c0 , and exclusive processes, which would be useful for calibration of predictions for the exclusive Higgs production at the LHC.
REFERENCES
1. F. Abe et al., Nucl.Instrum.Methods A 271, 387 (1988). 2. K. Goulianos et al., Nucl.Instrum.Methods A 518, 42 (2003). 3. M. Gallinaro et al., Proceedings of the \Diraction at the LHC", Rio de Janeiro, Brazil, 2004, hep-ph/0407255. 4. F. Abe et al., Phys.Rev.Lett., 78, 2698 (1997). 5. F. Abe et al., Phys.Rev.Lett., 79, 2636 (1997). 6. F. Abe et al., Phys.Rev.Lett., 84, 232 (2000). 7. F. Abe et al., Phys.Rev.Lett., 87, 241802 (2001). 8. T. Aolder et al., Phys.Rev.Lett., 85, 4215 (2000). 9. V.A. Khoze et al., Eur.Phys.J C23, 311 (2002).
10. V.A. Khoze et al., Eur.Phys.J C19, 477 (2001).
Di raction at CDF Christina Mesropian for the CDF Collaborationa a The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA The results of the di ractive measurements at CDF experiment at the Tevatron pp collider are presented. Di ractive structure functions are studied in single di ractive dijet production. The exclusive dijet and low-mass state production in double pomeron exchange are discussed in context of the exclusive Higgs production at LHC.
1. Introduction The hadronic diractive process can be dened as a reaction in which no quantum numbers are exchanged between the colliding particles and/or a large, non exponentially suppressed, rapidity gap (region devoid of particles) is present. In the framework of Regge theory diractive reactions are characterized by an exchange of the pomeron, a hypothetical object with vacuum quantum numbers. Diractive reactions that incorporate hard processes, such as production of jets in pp collisions, allow one to study diraction in a perturbative QCD framework, thus providing an opportunity to study the nature of the pomeron. CDF collaboration at Fermilab pp collider investigated various diractive reactions at three ps=1800 center of mass energies, GeV (Run I), ps=630 GeV (Run IC), and ps=1960 GeV (Run II). The CDF Run I detector is described in detail in Ref. 1]. The detector components relevant to diractive studies are calorimeters, central tracking chambers, and the Roman pot spectrometer (RPS) installed during Run IC. Upgrade for the Run II included a new plug calorimeter, gas Cherenkov counters for luminosity measurements, miniplug calorimeters 2] providing forward pseudorapidity coverage (3:5
events at CDF is either a large rapidity gap, or a leading hadron detected in the RPS. In this talk we will discuss hard diraction measurements at CDF.
2. Diractive Structure Function One of the central questions in hard diraction is whether these processes obey QCD factorization, in other words whether the pomeron has a universal, process independent \diractive" structure function. Experimental results from the DESY ep collider HERA show that QCD factorization holds in diractive deep inelastic scattering (DDIS). In Run I CDF experiment measured single diractive (SD) rates of W-boson 4], dijet 5], b-quark 6], and J= 7] production relative to the non-diractive (ND) rates. It was observed that the rates were a factor of 10 lower than expectations based on the corresponding structure functions determined at HERA. Fig. 1 shows the diractive structure function, FjjD , measured by CDF from Run I data sample with the leading antiproton detected in Roman pot spectrometer. The FjjD is presented as a function of the momentum fraction of the pomeron carried by the struck parton, = xp = , where xp is the x-Bjorken of the parton in the antiproton and is the momentum loss of the antiproton. The curved lines correspond to the expectations based on diractive parton densities extracted by H1 experiment.
32
C. Mesropian / Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34
D
H1 fit-2
Q
ET
Q
Q
CDF data jet1,2
H1 fit-3
100
Preliminary results also show that there is no signicant dependence of the ratio on 2 h 2 i in the 2 range from 100 GeV2 to 1600 GeV2 , which indicates that the pomeron 2 evolution is similar to that of the proton.
ET
2
2
≥ 7 GeV
Ratio [(SD/ ∆ ξ)/ND]
Fjj (β)
A large discrepancy in normalization is observed. This result implies that there is a breakdown of QCD factorization in hard diraction between Tevatron and HERA.
0.035 ≤ ξ ≤ 0.095
( Q = 75 GeV )
| t | ≤ 1.0 GeV
2
10
10
CDF Run II Preliminary ξ (0.02 to 0.10) (x 1) ξ (0.05 to 0.10) (x 10) ξ (0.02 to 0.05) (x 0.1) CDF Run I χ2 / ndf Prob Constant Slope
1
10
10
4.616 / 12 0.9696 -4.55 ± 0.09523 -0.9466± 0.04685
-1
-2
1 10
± 25% norm. uncertainty
-3
jet
0.1
H1 2002 σrD QCD Fit (prel.)
10
-4
10
0.1
-3
10
2.1. Diractive Dijet Production in Run II
In Run II CDF has continued studies of the single diractive dijet production. The data sample is collected by triggering on a leading antiproton in combination with at least one calorimeter tower with 5 GeV. The is measured by using all calorimeter information. The control ND dijet sample is triggered on the same calorimeter tower requirement. The ratio of SD to ND dijet production rates (see Fig. 2) is studied as a function of , where is evaluated for each event from the andP of the jets according to ; . The results the formula: = p1 =1 are in good agreement with the Run I measurement and do not have appreciable dependence in the ratio. Measurement of dependence at lower values ( 0 02) is one of the Run II goals. ET >
xp
xp
s
n {
{ ET e
{
<
:
-1
1
Figure 1. Diractive structure function measured by CDF compared with expectations from DDIS based on the H1 t.
ET
x Bj 10
β
Figure 2. Ratio of SD to ND dijet event rates per unit as a function of -Bjorken.
xp
-2
x
3. Exclusive Production Observation of the Higgs boson is one of the main goals of the Large Hadron Collider (LHC) in CERN. Recently, considerable interest has been drawn to the subject of \exclusive" Higgs boson production ! + + , where central heavy object is produced alone, separated from outgoing hadrons by rapidity gaps. The exclusive double diractive Higgs production has unique experimental advantages such as clean nal state which contains only the Higgs boson and very forward leading nucleons, improved mass resolution, and small background from the direct production, due to the several suppression mechanisms. Although the cross section for the exclusive Higgs production is too small to be observed at the Tevatron, several processes mediated by the same mechanism but with higher production rates can be studied to calibrate theoretical prepp
p
H
p
bb
33
C. Mesropian / Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34
dictions. Below, we present preliminary results on exclusive dijet and c 0 production at CDF.
3.1. Exclusive Dijet Production by Double Pomeron Exchange
In Run I CDF reported rst observation of dijet production via double pomeron exchange (DPE) 8]. The events are characterized by a leading antiproton, two jets in the central pseudorapidity region, and a large rapidity gap on the outgoing proton side. In Run II our sam-
are calculated from all calorimeter towers. If dijets were produced exclusively, would be equal to one by denition. However, taking into account resolution eects, the exclusive region is dened as 0 8. Fig. 3 shows the distributions for DPE dijets, compared with those for single diractive sample. No signicant excess is observed for 0 8 over a smooth distribution. The upper limit for exclusive dijet production as a cross section for DPE events with 0 8 is 970 65( ) 272( )pb and 34 5( ) 10( )pb for the leading jet 10 and 25 GeV, respectively. These experimental numbers are consistent with recent theoretical predictions 9].
Rjj
Rjj >
:
Rjj
Rjj
Rjj
>
>
:
:
stat
stat
syst
syst
ET >
SDp (Prescale 280)
10 10 10 10 10
DPE = SDp + GAPp(5.5<ηgap <7.5) (Prescale 5)
6
DPE = SDp + GAPp(3.6<ηgap <7.5) (Prescale 5) 5
Energy Scale Uncertainty
L = 26 pb
4
CDF Run 2 Preliminary 3
2
Dijet Mass Fraction
SD p : 0.03 < ξ p < 0.1
10 1
-1
Events/bin
#Events × Prescale/0.05
CDF Run II Preliminary
jet2
ET
0
0.2
data
FakeEvt
4 3.5 3
> 10 GeV
2.5 0.4
0.6
0.8
1
R jj = M jj/M X
2 1.5 1
Figure 3. Dijet mass fraction distributions for DPE events: 5 5 7 5 (open circles) and 36 7 5 (lled circles), and for SD events (triangles). :
:
< gap <
< gap <
0.5
:
0 2.8 3 3.2 3.4 3.6 3.8 4 Di-muon + photon invariant mass (GeV)
:
ple is collected with a dedicated trigger requiring BSC gap on the proton side, a leading antiproton in the Roman pot spectrometer, and a single calorimeter tower with 5 GeV. Additional oine requirement of a gap in the miniplug on the proton side enhances initial sample. For resulting DPE candidate events the \dijet mass fraction", , is dened as the ratio of the invariant mass of the two leading jets, , to the mass of the entire system, excluding leading and , . The dijet mass is measured from the energies of the calorimeter towers inside the jet cones, and mass of p the system is calculated according to = , where values of
Figure 4. Invariant mass of the dimuon and a photon system compared to the Monte Carlo predictions (histogram).
ET >
Rjj
Mjj
p
MX
Mjj
MX
p
p
s
p
3.2. Exclusive
c
0
Production
Experimental studies of the exclusive 0 production at CDF can serve as a benchmark process for the exclusive Higgs production predictions at LHC. The exclusive 0 production in DPE was studied at CDF using Run II data. The events were selected by requiring dimuon trigger with 1 5 GeV/c, j j 0 6, and the invariant c
c
pT
>
:
<
:
34
C. Mesropian / Nuclear Physics B (Proc. Suppl.) 146 (2005) 31–34
mass of the muon pair in the J= mass window. The large rapidity gaps were required after rejecting cosmic background. Starting with data sample of 93 pb;1 , 10 candidate events are found for the exclusive c 0 ! J= + . The mass of the dimuon- system is shown in Fig. 4. The upper limit of the exclusive c0 ! J= + production cross section is set based on these 10 events and is equal to 49 18(stat) 39(syst) pb. The measured cross section is consistent with the theoretical prediction of the exclusive rates 10].
4. Conclusions CDF collaboration continues extensive program of diractive studies. Our Run II goals include measurements of Q2 and dependence of the diractive structure functions with dedicated triggers to extend range down to 0:001 and 2 Q range up to 104 GeV2 . Study of exclusive production processes in DPE is another goal for Run II. We are developing new triggers for b b, c0 , and exclusive processes, which would be useful for calibration of predictions for the exclusive Higgs production at the LHC.
REFERENCES
1. F. Abe et al., Nucl.Instrum.Methods A 271, 387 (1988). 2. K. Goulianos et al., Nucl.Instrum.Methods A 518, 42 (2003). 3. M. Gallinaro et al., Proceedings of the \Diraction at the LHC", Rio de Janeiro, Brazil, 2004, hep-ph/0407255. 4. F. Abe et al., Phys.Rev.Lett., 78, 2698 (1997). 5. F. Abe et al., Phys.Rev.Lett., 79, 2636 (1997). 6. F. Abe et al., Phys.Rev.Lett., 84, 232 (2000). 7. F. Abe et al., Phys.Rev.Lett., 87, 241802 (2001). 8. T. Aolder et al., Phys.Rev.Lett., 85, 4215 (2000). 9. V.A. Khoze et al., Eur.Phys.J C23, 311 (2002).
10. V.A. Khoze et al., Eur.Phys.J C19, 477 (2001).