- Email: [email protected]
Recent new phenomena searches and prospects for Higgs searches at DØ
SUPPLEMENTS Nuclear
ELSEVIER
Physics
B (Proc.
Suppl.)
126 (2004)
350-353
wwu;.clscviel-phySics.colll
Recent New Phenomena Searches and Prospects for Higgs searches at DO Neeti Parashar (for the DO Collaboration) Center for Applied Physics Studies, Louisiana Tech University Arizona Avenue, Ruston, Louisiana 71272, U.S.A. The upgraded DO detector at the Fermi National Accelerator Laboratory was installed for data taking in March 2091. One of the primary physics goals of the current run, called Run II includes Higgs and SUSY searches. This note outlines the recent New Phenomena searches and the prospects for searching the Higgs boson at the Fermilab Tevatron in Run II.
1. INTRODUCTION The DO detector took its first data run from 1992-1996, called Run I. Ever since, the detector has undergone a major upgrade to participate in the current data taking, called Run II. The discovery of the top quark at the Tevatron in 1995, using the Run I data [l] was a major achievement. With the upgraded detector we would now like to pursue a detailed study of the top quark and search for the Higgs boson and SUSY, which remain the central physics interests for Run II among other physics analyses and precision tests of the Standard Model. The DO detector is designed to study collisions from protons and antiprotons circulating in the Fermilab Tevatron. The Tevatron complex has also been upgraded. The design luminosity for the Run II upgrade is to achieve 5~10~~ cme2sb1, a factor of 30 more than in Run I. The bunch spacing is reduced from 3.5 ps to 396 ns. There is an increase in the center-of-mass energy from 1.8 TeV to 1.96 TeV. The challenges faced by the detector in this environment are large occupancies, event pile-up and radiation damage. The major elements of the upgrade include a new silicon detector placed closest to the interaction point. This is crucial for identifying the bquarks and hence plays a significant role in Higgs searches and new physics. We also have a new scintillating fiber tracker detector. These detectors lie inside a 2 Tesla solenoid, which is a new 0920-5632/$ - see front matter doi:10.1016!j.nuclp1~ysbps.2003.11.059
0 2004 Published
by Elsevier
B.V.
addition for Run II. The forward muon system has been completely replaced in order to accommodate higher event rates. There are now three levels of triggering and a new data acquisition system. 2. New
Phenomena
Searches
at DO
Searching for New Phenomena at the Tevatron can take several different forms, such as Discovery of particles predicted by the Standard Model, such as the Higgs boson Discovery of particles beyond the domain of Standard Model - SUSY, Leptoquarks Identification Topcolor
of new interactions
such as
Confronting complexities beyond the Standard Model such as Compositeness Changes to Fundamental dimensions
physics via Extra
The current DO searches include Supersymmetry, New Physics and Large Extra Dimensions. 2.1. Supersymmetry Cascade decays of squarks and gluinos lead to final states with quarks, gluons and Lightest Supersymmetric Particles (LSP). So jets and missing ET is a generic signature for production of
N. Pamshar/Nucleur
Physics B (Proc. Suppl.) 126 (2004) 350-353
35;
Table 1 The expected and observed number of events for the dielectron data. Sequential cuts
Backgrounds
Data
pT(el) > 15 GeV, pT(ez) > 10 GeV 10 GeV < i&, < 70 GeV MT > 15 GeV Additional Isolated Track, pT > 5 GeV
3216 rt43.2 660.2 rt19.1 96.4 3~8.1
3132 721 123
3.2 f2.3 0.0 rt2.0
3 0
Missing ET > 15 GeV
squarks and or gluinos in the gravity inspired Supersymmetric Model, SUGRA [2]. This signature is dominated by the background from the QCD jet production. Using 4pb-r of Run II data a 95% confidence level limit has been set on the jets and missing ET production cross-sections. There is no sign of new physics yet. For details refer to
DO Run II Preliminary
lo2
[31. The cascade decays of the electroweak gauginos often lead to multilepton signatures. The transeverse momentum pi of the final state leptons is required to be very soft depending on the mass diiference betweent he gauginos and the LSP. This is essential for achieving good sensitivity to this signal. It is also crucial to have the ability to identify hadronic tau decays since the branching fractions for charginos and neutralinos are tau dominated. In the dilepton channel with an electron and muon the analysis is performed in a model independent way due to low backgrounds. The fake rates are estimated using 30 pb-l data and physics backgrounds from the simulation. The data agree well with the background and an upper limit has been set for new physics with an electron and a muon as a function of missing ET. This limit is 400-100 f b for missing ET ranging from O-35 GeV. In the trilepton channel, with two electrons and a third lepton we begin analysis with the dielectron invariant mass spectrum, shown in Figure 1. Using the dielectron sample we understand the trigger, reconstruction and simulation. The QCD fake background is determined from the data. Using 40 pb-’ of data, we notice that the agreement between the expected and observed number
10
1
0
60
80
100
120
1 0
invariant (e,e) mass [GeV]
Figure 1. Dielectron invariant mass spectrum. The points are the data and the histogram is the expectation from simulation.
of events is fairly good. This is shown in Table 1. The typical selection efficiency for SUGRA events is 2-4% in parameter space close to the current exclusion limit. Two searches at DO are seeing evidence for 2 --+ ~7, which is difficult to reconstruct. One analysis searches for electrons and hadronic tau decays while the second one searches for the muon and the hadronic tau decays. Taus in the multilepton final states are crucial for improving the sensitivity of the SUGRA events selection, which is still a factor of 7 away from extending the ex-
352
N. Parashur/Nuclear
Physics
eluded area. We use 50 pb-l of data and select events with an electron with pT > 12 GeV and a narrow jet of pT > 7 GeV with only a single track of pT > 1.5 GeV. Neural Networks is used to further discriminate between QCD and tau jets. The di-tau invariant mass is reconstructed using the assumption that the tau direction is the same as the visible tau daughter direction. Finally we subtract the same-sign er events from the opposite sign events. Figure 2 shows the reconstructed di-tau invariant mass calculated using the above described strategy.
B (Proc.
Suppl.)
126 (2004)
2.3. Large Extra Dimensions At D0 the search for Large Extra Dimensions hunts for an excess of high-mass dielectron, diphoton or di-muon events over Standard Model expectations. In this model [4], we combine the dilelectrons and diphotons and no missing ET is predicted. These analyses are based on the assumption that the Standard Model particles are confined to a 3-brane, and gravity propagates in extra dimensions. Limits have been set and are close to Run I for the dielectron and diphoton channel while the dimuon analysis is a new channel at the Tevatron. For details refer to [3]. There is no sign of new physics yet. 3. Prospects
I 20
40
60
80
100
120
140
160
180
200
inv. TT mass [GeV]
Figure 2. Invariant mass of the 2 + TT events. The points are the data and the histogram is the expectation from simulation.
2.2. New Physics The search for leptoquarks is one of the exotic signals for New Physics. In the search for second generation leptoquarks, we assume no crossgenerational couplings and there is a 100% decay to muons. The signature is 2 muons and 2 jets. We require two opposite sign muons with PT > 15 GeV, two jets with pT > 20 GeV and Mp, > 110 GeV. With these requirements a limit on the mass of the leptoquark has been set to 157 GeV, which is approaching Run I sensitivity.
350-353
for Higgs
Search
at DO
One of the major goals of the Tevatron Run II physics program is to search for the Higgs boson. Direct searches at the LEP have set at limit on the Higgs Boson mass, mH > 114.4 GeV 153. Indirect searches with precise measurements of the W and 2 bosons have set an upper limit on the mass of the Higgs boson, mH < 195 GeV [5,6]. The dominant Higgs production at the Tevatron is via the gluon fusion process. This process has a high cross-section but is swamped by the direct production of gluons, making it difficult to filter the signal. The best mode of preference is the “associated production” mode where a Higgs boson is produced in association with a W or a Z boson. When the latter decay leptonically we have a better handle over background rejection. The dominant decay mode at low Higgs masses is H + b& while at higher masses it is H -+ W W* . However, we do not expect a significant result from a single channel. The Higgs discovery potential at the Tevatron for Run II has been evaluated by the Higgs Working Group [7], using a parameterized detector simulation. The expected sensitivity is shown in Figure 3. The result of this study is that a discovery at the 50 can be made only if we combine all the channels and all the data from both the collider detectors, CDF and DO. Currently, we are improving our understanding of the signal as well as background processes. The essential tools to look for the Higgs boson are b-tagging and dijet
N Parashur/Nuclear
Physics
mass resolution. Advanced analysis techniques are also very vital for this search. The crucial factor however is to achieve the largest luminosity.
B fProc.
Suppl.)
3. 4. 5.
combined
CDF/W ,
6.
thresholds 4 I fb-’ fb-’
-.E
r
80
.
-
$4
95% CL limit 3a evidence 5a discoverv I I .zI
l
I
100
120
140
Higgs
mass
160
180
.I
200
(GeV/c2)
Figure 3. Higgs Reach for Run II.
4. Summary The upgraded DO detector is taking data and many interesting physics analyses are going on. We are also working on improving the sensitivity and performance of the detector. A very exciting time is ahead of us. 5. Acknowledgments I would like to thank the organizers of the Photon 2003 Conference, especially Giulia Pancheri who invited me to give a talk and provided all the necessary documents in time for me to attend the Conference. I would also like to thank Fabio Anulli and Saverio Braccini for their help with the computing facilities and other logistics. REFERENCES 1. S. Abachi et al., Observation of the Top Quark, Phys. Rev. Lett. (1995), 2632. 2. Report of the SUGRA Working Group for Run II of the Tevatron, hep-ph/0003154.
7.
126 (2004)
350-353
353
G. Brooijmans, Searches for new physics at DO, hep-ex/0305009, (2003). N. Arkani-Hamed, S. Dimopoulos and G. Dvali, Phys. Lett. B429 (1998) 263. The LEP Working group for Higgs Boson Searches, LHWG Nate/2002-01. The LEP Electroweak Working Group , http://hepewwg.web.cern.ch/LEPEWWG/ M. Carena et al., hep-ph/0010338 (2000).
ELSEVIER
Physics
B (Proc.
Suppl.)
126 (2004)
350-353
wwu;.clscviel-phySics.colll
Recent New Phenomena Searches and Prospects for Higgs searches at DO Neeti Parashar (for the DO Collaboration) Center for Applied Physics Studies, Louisiana Tech University Arizona Avenue, Ruston, Louisiana 71272, U.S.A. The upgraded DO detector at the Fermi National Accelerator Laboratory was installed for data taking in March 2091. One of the primary physics goals of the current run, called Run II includes Higgs and SUSY searches. This note outlines the recent New Phenomena searches and the prospects for searching the Higgs boson at the Fermilab Tevatron in Run II.
1. INTRODUCTION The DO detector took its first data run from 1992-1996, called Run I. Ever since, the detector has undergone a major upgrade to participate in the current data taking, called Run II. The discovery of the top quark at the Tevatron in 1995, using the Run I data [l] was a major achievement. With the upgraded detector we would now like to pursue a detailed study of the top quark and search for the Higgs boson and SUSY, which remain the central physics interests for Run II among other physics analyses and precision tests of the Standard Model. The DO detector is designed to study collisions from protons and antiprotons circulating in the Fermilab Tevatron. The Tevatron complex has also been upgraded. The design luminosity for the Run II upgrade is to achieve 5~10~~ cme2sb1, a factor of 30 more than in Run I. The bunch spacing is reduced from 3.5 ps to 396 ns. There is an increase in the center-of-mass energy from 1.8 TeV to 1.96 TeV. The challenges faced by the detector in this environment are large occupancies, event pile-up and radiation damage. The major elements of the upgrade include a new silicon detector placed closest to the interaction point. This is crucial for identifying the bquarks and hence plays a significant role in Higgs searches and new physics. We also have a new scintillating fiber tracker detector. These detectors lie inside a 2 Tesla solenoid, which is a new 0920-5632/$ - see front matter doi:10.1016!j.nuclp1~ysbps.2003.11.059
0 2004 Published
by Elsevier
B.V.
addition for Run II. The forward muon system has been completely replaced in order to accommodate higher event rates. There are now three levels of triggering and a new data acquisition system. 2. New
Phenomena
Searches
at DO
Searching for New Phenomena at the Tevatron can take several different forms, such as Discovery of particles predicted by the Standard Model, such as the Higgs boson Discovery of particles beyond the domain of Standard Model - SUSY, Leptoquarks Identification Topcolor
of new interactions
such as
Confronting complexities beyond the Standard Model such as Compositeness Changes to Fundamental dimensions
physics via Extra
The current DO searches include Supersymmetry, New Physics and Large Extra Dimensions. 2.1. Supersymmetry Cascade decays of squarks and gluinos lead to final states with quarks, gluons and Lightest Supersymmetric Particles (LSP). So jets and missing ET is a generic signature for production of
N. Pamshar/Nucleur
Physics B (Proc. Suppl.) 126 (2004) 350-353
35;
Table 1 The expected and observed number of events for the dielectron data. Sequential cuts
Backgrounds
Data
pT(el) > 15 GeV, pT(ez) > 10 GeV 10 GeV < i&, < 70 GeV MT > 15 GeV Additional Isolated Track, pT > 5 GeV
3216 rt43.2 660.2 rt19.1 96.4 3~8.1
3132 721 123
3.2 f2.3 0.0 rt2.0
3 0
Missing ET > 15 GeV
squarks and or gluinos in the gravity inspired Supersymmetric Model, SUGRA [2]. This signature is dominated by the background from the QCD jet production. Using 4pb-r of Run II data a 95% confidence level limit has been set on the jets and missing ET production cross-sections. There is no sign of new physics yet. For details refer to
DO Run II Preliminary
lo2
[31. The cascade decays of the electroweak gauginos often lead to multilepton signatures. The transeverse momentum pi of the final state leptons is required to be very soft depending on the mass diiference betweent he gauginos and the LSP. This is essential for achieving good sensitivity to this signal. It is also crucial to have the ability to identify hadronic tau decays since the branching fractions for charginos and neutralinos are tau dominated. In the dilepton channel with an electron and muon the analysis is performed in a model independent way due to low backgrounds. The fake rates are estimated using 30 pb-l data and physics backgrounds from the simulation. The data agree well with the background and an upper limit has been set for new physics with an electron and a muon as a function of missing ET. This limit is 400-100 f b for missing ET ranging from O-35 GeV. In the trilepton channel, with two electrons and a third lepton we begin analysis with the dielectron invariant mass spectrum, shown in Figure 1. Using the dielectron sample we understand the trigger, reconstruction and simulation. The QCD fake background is determined from the data. Using 40 pb-’ of data, we notice that the agreement between the expected and observed number
10
1
0
60
80
100
120
1 0
invariant (e,e) mass [GeV]
Figure 1. Dielectron invariant mass spectrum. The points are the data and the histogram is the expectation from simulation.
of events is fairly good. This is shown in Table 1. The typical selection efficiency for SUGRA events is 2-4% in parameter space close to the current exclusion limit. Two searches at DO are seeing evidence for 2 --+ ~7, which is difficult to reconstruct. One analysis searches for electrons and hadronic tau decays while the second one searches for the muon and the hadronic tau decays. Taus in the multilepton final states are crucial for improving the sensitivity of the SUGRA events selection, which is still a factor of 7 away from extending the ex-
352
N. Parashur/Nuclear
Physics
eluded area. We use 50 pb-l of data and select events with an electron with pT > 12 GeV and a narrow jet of pT > 7 GeV with only a single track of pT > 1.5 GeV. Neural Networks is used to further discriminate between QCD and tau jets. The di-tau invariant mass is reconstructed using the assumption that the tau direction is the same as the visible tau daughter direction. Finally we subtract the same-sign er events from the opposite sign events. Figure 2 shows the reconstructed di-tau invariant mass calculated using the above described strategy.
B (Proc.
Suppl.)
126 (2004)
2.3. Large Extra Dimensions At D0 the search for Large Extra Dimensions hunts for an excess of high-mass dielectron, diphoton or di-muon events over Standard Model expectations. In this model [4], we combine the dilelectrons and diphotons and no missing ET is predicted. These analyses are based on the assumption that the Standard Model particles are confined to a 3-brane, and gravity propagates in extra dimensions. Limits have been set and are close to Run I for the dielectron and diphoton channel while the dimuon analysis is a new channel at the Tevatron. For details refer to [3]. There is no sign of new physics yet. 3. Prospects
I 20
40
60
80
100
120
140
160
180
200
inv. TT mass [GeV]
Figure 2. Invariant mass of the 2 + TT events. The points are the data and the histogram is the expectation from simulation.
2.2. New Physics The search for leptoquarks is one of the exotic signals for New Physics. In the search for second generation leptoquarks, we assume no crossgenerational couplings and there is a 100% decay to muons. The signature is 2 muons and 2 jets. We require two opposite sign muons with PT > 15 GeV, two jets with pT > 20 GeV and Mp, > 110 GeV. With these requirements a limit on the mass of the leptoquark has been set to 157 GeV, which is approaching Run I sensitivity.
350-353
for Higgs
Search
at DO
One of the major goals of the Tevatron Run II physics program is to search for the Higgs boson. Direct searches at the LEP have set at limit on the Higgs Boson mass, mH > 114.4 GeV 153. Indirect searches with precise measurements of the W and 2 bosons have set an upper limit on the mass of the Higgs boson, mH < 195 GeV [5,6]. The dominant Higgs production at the Tevatron is via the gluon fusion process. This process has a high cross-section but is swamped by the direct production of gluons, making it difficult to filter the signal. The best mode of preference is the “associated production” mode where a Higgs boson is produced in association with a W or a Z boson. When the latter decay leptonically we have a better handle over background rejection. The dominant decay mode at low Higgs masses is H + b& while at higher masses it is H -+ W W* . However, we do not expect a significant result from a single channel. The Higgs discovery potential at the Tevatron for Run II has been evaluated by the Higgs Working Group [7], using a parameterized detector simulation. The expected sensitivity is shown in Figure 3. The result of this study is that a discovery at the 50 can be made only if we combine all the channels and all the data from both the collider detectors, CDF and DO. Currently, we are improving our understanding of the signal as well as background processes. The essential tools to look for the Higgs boson are b-tagging and dijet
N Parashur/Nuclear
Physics
mass resolution. Advanced analysis techniques are also very vital for this search. The crucial factor however is to achieve the largest luminosity.
B fProc.
Suppl.)
3. 4. 5.
combined
CDF/W ,
6.
thresholds 4 I fb-’ fb-’
-.E
r
80
.
-
$4
95% CL limit 3a evidence 5a discoverv I I .zI
l
I
100
120
140
Higgs
mass
160
180
.I
200
(GeV/c2)
Figure 3. Higgs Reach for Run II.
4. Summary The upgraded DO detector is taking data and many interesting physics analyses are going on. We are also working on improving the sensitivity and performance of the detector. A very exciting time is ahead of us. 5. Acknowledgments I would like to thank the organizers of the Photon 2003 Conference, especially Giulia Pancheri who invited me to give a talk and provided all the necessary documents in time for me to attend the Conference. I would also like to thank Fabio Anulli and Saverio Braccini for their help with the computing facilities and other logistics. REFERENCES 1. S. Abachi et al., Observation of the Top Quark, Phys. Rev. Lett. (1995), 2632. 2. Report of the SUGRA Working Group for Run II of the Tevatron, hep-ph/0003154.
7.
126 (2004)
350-353
353
G. Brooijmans, Searches for new physics at DO, hep-ex/0305009, (2003). N. Arkani-Hamed, S. Dimopoulos and G. Dvali, Phys. Lett. B429 (1998) 263. The LEP Working group for Higgs Boson Searches, LHWG Nate/2002-01. The LEP Electroweak Working Group , http://hepewwg.web.cern.ch/LEPEWWG/ M. Carena et al., hep-ph/0010338 (2000).