Search for exclusive events at the
Tevatron and the RP220 project in
ATLAS
Christophe Royon
DAPNIA-SPP, CEA Saclay
Workshop on diffraction at the LHC,
October 18-19 2007, Cracow
Contents:
• Inclusive diffraction at the LHC
• Search for exclusive events at the Tevatron (CDF)
• Diffractive Higgs production at the LHC: exclusive and
inclusive production
• Background studies: How to measure the high β gluon?
• RP220 project at the LHC in ATLAS
Diffraction at Tevatron/LHC
(a)
φ
(c)
(b)
Gap
Jet+Jet
η
φ
Jet
Gap Jet
φ Gap Jet+Jet Gap
η
η
Kinematic variables
• t: 4-momentum transfer squared
• ξ1 , ξ2 : proton fractional momentum loss (momentum
fraction of the proton carried by the pomeron)
• β1,2 = xBj,1,2 /ξ1,2 : Bjorken-x of parton inside the
pomeron
• M 2 = sξ1 ξ2 : diffractive mass produced
• ∆y1,2 ∼ ∆η ∼ log 1/ξ1,2 : rapidity gap
“Inclusive” models
• “Inclusive” models: Take the hadron-hadron “usual”
cross section convoluted with the parton distributions in
the pomeron
• Take shape of H1 measurement of gluon density
• Normalisation coming from CDF run I cross section
measurement
• Inclusive cross sections need to be known in detail since
it is a direct background to search for exclusive events
x1 , v1
p
P
g
x1
g
x2
k
1
k
2
P
p
x2 , v 2
H , QQ, gg
Inclusive Higgs mass production
Large cross section, but mass poorly reconstructed since part
of the √
energy lost in pomeron remnants
(M = ξ1 ξ2 S ∼Higgs + remnant mass)
“Exclusive models”
p
P
p
g
x1
g
x2
k
k
1
g
k1
g
k2
x1
H , QQ , γγ
x2
H , QQ , γγ
2
P
p
p
"Standard"
"Exclusive "
p
P
g
k
1
g
k
2
x1
x2
H , QQ , γγ
P
p
"Inclusive"
All the energy is used to produce the Higgs (or the dijets),
namely xG ∼ δ (See: R. Peschanski, M. Boonekamp, J.
Cammin, C. R. and Durham group: A. Martin, V. Khoze, M.
Ryskin,
see the talks by A. Martin, M. Tasevsky, V. Juranek
Advantage of exclusive Higgs production?
• Good Higgs mass reconstruction: fully constrained
system, Higgs mass reconstructed using both tagged
protons in the final state (pp → pHp)
• MH =
q
ξp ξp̄ S
• No energy loss in pomeron “remnants”
Entries
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22777
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Mx
Entries
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Mx
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DPEMC Monte Carlo
• DPEMC (Double Pomeron Exchange Monte Carlo): New
generator to produce events with double pomeron
exchange (both “Durham” and “Saclay” models)
http://boonekam.home.cern.ch/boonekam /dpemc.htm,
hep-ph/0312273
• Interface with Herwig: for hadronisation
• Exclusive and inclusive processes included: Higgs, dijets,
diphotons, dileptons, SUSY, QED, Z, W ...
• DPEMC generator interfaced with a fast simulation of
LHC detector (as an example CMS, same for ATLAS),
and a detailled simulation of roman pot acceptance
• Gap survival probability of 0.03 put for the LHC: is it
possible to check this at the Tevatron?
• Another available MC: “Exhume” for Durham model
Signal over background: standard model Higgs
S/B
For a Higgs mass of 120 GeV and for different mass windows
as a function of the Higgs mass resolution
10
9
8
7
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2
1
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Mass resolution
Diffractive SUSY Higgs production
S/B
At high tan β, possibility to get a S/B over 50 (resp. 5.) for
100 (resp.10) fb−1 !
10 2
10
1
0
1
2
3
4
5
Mass resolution
Uncertainty on high β gluon
• Important to know the high β gluon since it is a
contamination to exclusive events
• Experimentally, quasi-exclusive events indistinguishable
from purely exclusive ones
zG
• Uncertainty on gluon density at high β: multiply the
gluon density by (1 − β)ν (fit: ν = 0.0 ± 0.6)
2
1.5
1
0.5
0
2
1.5
1
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1
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0
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-2
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-1
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-3
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-2
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-1
10
-3
10
-2
10
-1
z
Look for exclusive events at the Tevatron
• “exclusive” events: events without pomeron remnant,
search for exclusive events in dijet, diphoton, χC channels
• The full available energy is used in the hard interaction
• Interesting for LHC... (diffractive W , Higgs, photon
anomalous couling...
p
P
p
g
x1
g
x2
k
k
1
g
k1
g
k2
x1
H , QQ , γγ
x2
2
P
p
p
"Standard"
"Exclusive "
p
P
g
k
1
g
k
2
x1
x2
P
p
"Inclusive"
H , QQ , γγ
H , QQ , γγ
χC exclusive production at the Tevatron?
• CDF observation: Upper limit of χC exclusive production
at the Tevatron in the J/Ψγ channel σ ∼ 49 pb ±18 ±
39 pb for y < 0.6 (result not corrected for cosmics, χ2
contamination)
• Exclusive prediction: 59 pb
• Quasi-exclusive contamination:
mass fraction ν = 0 ν = −1 ν = −0.5 ν = +0.5
≥ 0.8
5.4
119.1
27.2
0.9
≥ 0.85
2.0
62.0
11.2
0.2
≥ 0.9
0.3
19.6
2.9
0.0
≥ 0.95
0.8
1.7
0.8
0.0
ν = +1.0
0.2
0.0
0.0
0.0
• Contamination of quasi-exclusive events strongly
dependent on assumption on high-β gluon density in
pomeron (completely unknown...), and also on precision
and smearing of dijet mass distribution (a cut at 0.9 at
generator level corresponds to ??? at reconstructed
level), true also for jet studies...
Dijet mass fraction measurement in CDF
• Look for exclusive events (events where there is no
pomeron remnants or when the full energy available is
used to produce diffractively the high mass object)
• Select events with two jets only, one proton tagged in
roman pot detector and a rapidity gap on the other side
Events
• Predictions from inclusive diffraction models for Jet
pT > 10 GeV
700
FM INC
DPE data (stat.)
preliminary
ν=0
ν=0.5
ν=-0.5
ν=1
ν=-1
600
500
400
300
200
100
0
0
0.2
0.4
0.6
0.8
1
Rjj=Mjj/Mx
Prediction from inclusive and exclusive diffraction
• Add the exclusive contribution (free relative normalisation
between inclusive and exclusive contribution)
• Good agreement between measurement and predictions
• As an example: exclusive and inclusive models for
pT > 10 GeV and for pT > 25 GeV
700
FM INC + KMR EXC
DPE data (stat.)
preliminary
600
ν=0
Events
Events
• See O. Kepka, C. Royon, arXiv:0704.19956 accepted by
Phys. Rev. D, arXiv0706.1798
35
30
exclusive contribution
25
400
20
300
15
200
10
100
5
0.2
0.4
0.6
0.8
preliminary
ν=0
500
0
0
BL INC modified
+ KMR EXC
DPE data (stat.)
1
Rjj=Mjj/Mx
0
0
exclusive contribution
0.2
0.4
0.6
0.8
1
Rjj=Mjj/Mx
Soft Colour Interaction models
• A completely different model to explain diffractive
events: Soft Colour Interaction (R.Enberg, G.Ingelman,
N.Timneanu, hep-ph/0106246)
• Principle: Variation of colour string topologies, giving a
unified description of final states for diffractive and
non-diffractive events
• No survival probability for SCI models
p
qq
q
qq
q
H
p
q
qq
qq
q
H
q
qq
H
q
qq
What about SCI?
• SCI models give correct normalisation for single
diffraction at Tevatron and diffraction at HERA without
any additional parameter
700
DPE data (stat.only)
preliminary
600
Soft color interaction
500
Events
Events
• Exclusive events and SCI: Contribution of exclusive
events needed much lower compared to Pomeron-like
models, even vanishes for jet pT > 25 GeV...
40
DPE data (stat.only)
preliminary
35
Soft color interaction
30
25
400
20
300
15
200
10
100
0
0
5
0.2
0.4
0.6
0.8
1
Rjj=Mjj/Mx
0
0
0.2
0.4
0.6
0.8
1
Rjj=Mjj/Mx
Comments about SCI
• Contribution of exclusive events much smaller for SCI
• “DPE” exchange in SCI models dominated by the
following configuration for CDF events: 1 antiproton
tagged in the final state, a bunch of particles going
through the beam pipe on the other side (dominated by
pions), no proton in the final state, due to the fact that
only a rapidity gap is requested
0.05
dN
N
dN
N
• Jet rapidity boosted towards high rapidity: SCI model
worth to be studied in more detail, but needs further
improvement
0.04
0.12
0.1
0.08
0.03
0.06
0.02
0.04
0.01
0.02
0
-10
-5
0
5
10
η
0
-3
-2
-1
0
1
2
3
ηJet1
LHC: Exclusive and inclusive events
• Study of exclusive and inclusive production to be made
at the LHC: study cross section of both components as a
function of jet pT and perform DGLAP QCD fits
• Important to understand background and signal for
exclusive production of rare events: Higgs, SUSY...
LHC: Exclusive and inclusive events
• Number of dijet events as a function of jet pT :
dominated by uncertainty on gluon density
• Dijet mass fraction (average value as an example):
sensitive to exclusive production, quite easy to measure
0.7
10 6
0.65
0.6
10 5
0.55
10 4
0.5
10 3
0.45
0.4
10 2
0.35
10
0.3
1
100
150
200
250
300
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400
pT
0.25
100
150
200
250
300
350
400
pT
Concept of survival probability
• Survival probability: Probability that there is no soft
additional interaction, that the diffractive event is kept
• Important to measure the survival probability in data:
estimated to be of the order of 0.1 at the Tevatron
π
1
0
0
1
1
0
0
1
+
LRG
LRG
+
Y
a)
1
0
0
1
b)
1
0
0
1
φ
0
Roman pot projects
• TOTEM project accepted, close to CMS
• FP420: Project of installing roman pot detectors at 420
m both in ATLAS, CMS; collaboration being built
• Roman pot detectors at 220 m in ATLAS:
– Natural follow-up of the ATLAS luminosity project at
240 m to measure total cross section
– Complete nicely the FP420 m project
– Collaboration between Saclay. Prague, Cracow, Stony
Brook, Pars 6, Michigan State University, Heidelberg,
Giessen and also University of Chicago and Argonne for
timing detectors (so far) being pursued
– Collaboration with the FP420 m project concerning
detectors, triggers, simulation...
• For more information, see the web pages of FP420, CMS,
TOTEM, ATLAS
Scheme of roman pot detectors
Roman pots at ATLAS
IP
4m
220m
As close as possible
to the beam:
10 s = 1mm
8s
3cm
Silicon
Detectors
Assume roman pots located at 216 and 224 m
Acceptance for 220 m pots
• Steps in ξ: 0.02 (left), 0.005 (right), |t|=0 or 0.05 GeV2
• Detector of 2 cm × 2 cm will have an acceptance up to
ξ ∼ 0.16, down to 0.008 at 10 σ, 0.016 at 20 σ
∆Y [mm]
∆Y [mm]
• As an example Higgs mass acceptance using 220 m pots
down to 135 GeV and upper limit due to cross section
and not kinematics
0
S = 216 m
-5
1
ξ = 0, 0.005, 0.010, 0.015, 0.020
S = 216 m
t = 0, -0.1 GeV2
0.5
0
-10
-0.5
-15
S = 224 m
-1
-20
10σ: ξ~0.008
-1.5
-25
20σ: ξ~0.016
-2
0
5
10
15
20
25
∆X [mm]
-0.5
0
0.5
1
1.5
2
2.5
∆X [mm]
acceptance [%]
Roman pot projects
100
90
80
RP220 full simulation
combined acceptance
RP 220+220
RP 220+420
70
RP 420+420
60
50
40
30
20
10
0
100 200 300 400 500 600 700 800
missing mass [GeV]
Both FP420 and RP220 needed to have a good coverage of
acceptance (NB: acceptance slightly smaller in CMS than in
ATLAS)
Roman pots / movable beam pipe at 220 m
Schematic view of 220 m pots: horizontal pots to detect
diffractive events, vertical pots used for alignment
Which kind of detectors?
• Requirement: good resolution in position (good
measurement of mass, kinematical propwerties), and in
timing
• Position detectors:
– Size of Si detectors (either 3D or strips) to be put in
roman pots: 2cm × 2cm
– Spatial resolution of the order of 10-15 µm: Si strip
detectors of 50 µm: 5 layers, 2 vertical, 1 horizontal, 1
U, 1 V (45 degrees)
– Edgeless detectors: Between 30 to 60 µm
– First prototype of strip detector made by CANBERRA,
3D detectors made for FP420: test-stand (laser and
radioactive source) being installed in Saclay, tests also
possible at CERN/Prague
– Trigger: Strip detectors of 100-200 µm, Si or pixel chip
to be modified to include trigger possibility
Which kind of detectors?
• Timing detectors
– Why do we need timing detectors? At the LHC, up to
30 interactions by bunch crossing, and we need to
identify from which vertex the protons are coming, same
problem for FP420
– Timing detector in movable beam pipe: resolution
needed: of the order of 5 picoseconds (space resolution
slightly more than 1 mm)
– Radiation hardness
– Detector space resolution: few mm, the total width of
the detectors being 2.5 cm (4.5 cm available in roman
pot)
– Reference clock: either the LHC clock (resolution of 7-8
ps), or atomic clock (they need to be calibrated on each
side)
– Trigger information: at L1 (rough compatibility between
both sides of ATLAS) and specially at L2 (compatibility
with vertex position)
– Development: new timing detectors in collaboration
with the Universities of Chicago, Stony Brook, and
Argonne, and with Photonis
Trigger: principle
Horizontal roman pots
(a la TOTEM)
- 224 m
xA
T
L
- 216 m
T
L
Diffractive Higgs Trigger
xB
jet
Front end
PA
SH
Chip ABC next
Pipeline
buffer
(> 3.6 µsec)
xA xB
T
Left
Pretrigger
jet
Right
Pretrigger
ATLAS detector
T
R
xD - xC =0
xA - xB =0
LR Trigger Logic
+850 ns (air cable)
7 plans Si strips /Roman Pot x,y
σ position 5 microns
σ time < 10 psec
T
R +730 ns
T
1,0 µsec
• LP AND RP
•TR - TL < 200 psec
L1
L1CTP
CTP
2 Jets with
Pt > 20 Gev/c
Max 75KHz
ATLAS standard
RR
OO
DD
US15
HLT
HLTTrigger
Trigger
(ROB)
(ROB)
ATLAS Standard
2,0 µsec
2,5µsec
Refined Jet Pt cut
Vertice within millimeter
∆ time < 5 to 10 psec
Trigger: strategy
• L1 trigger when two protons tagged at 220 m
• L1 trigger when only one proton is tagged at 220 m: in
that case, cut on acceptance at 220 m corresponding to
the possibility of a tag at 420 m
• Cuts used:
– 2 jets in central detector with pT > 40 GeV
– Exclusiveness of the process (2 jets carrying 90% of the
energy) (ET1 + ET2 )/HT > 0.9
– Kinematics requirement (η1 + η2 ) × η220 > 0
– At least one proton tagged at 220 m with ξ < 0.05
(compatible with the eventual presence of a proton at
420 m on the other side) or one proton tagged at 220
m on each side
• With those cuts, possibility to get a L1 rate less than 1
kHz for a luminosity less than 3.1033 cm−2 s−1
Collaboration between FP420 and RP220
• Development for RP220 project:
– Roman pots: 3 arms (horizontal, up and down for
diffractive events and elastics, do we need a magnet
between pots? (avoid low energy particles coming from
the first pot)
– Pixel detectors: modify size of pixel detector to 2 cm ×
2 cm
– Trigger capability: trigger performed by reading lines of
pixels to be able to compute ξ at L1
– Final timing detectors: 5 ps resolution, aim to develop
by 2012, MCP technology, (to be used also by FP420)
• FP420 and RP220 together:
– Pixel detector technology and readout
– Movable beam pipe technique: used for timing
detectors in RP220
– Timing detectors for 2010: temporary MCP (resolution
of 30-40 ps?), gastof developped by Louvain (15 ps?)
– RF pickup: to be computed by Manchester team
• FP420 and RP220 backup: Si strip detectors
Conclusion
• Study of inclusive events: determination of gluon at high
β, QCD studies, search for SUSY events (or any
resonance) when dijet background is known
• Exclusive events: promising results in dijet channel at the
Tevatron
• Exclusive Higgs: Signal over background: ∼ 1 if one gets
a very good resolution using roman pots (better than 1.5
GeV), enhanced by a factor up to 50 for SUSY Higgs at
high tanβ
• QED WW pair production: cross section known precisely,
allow to calibrate prescisely the roman pot detectors,
study of photon anomalous coupling
• Project of installing roman pot detectors in ATLAS well
started: collaboration between Prague, Cracow, Saclay,
Stony Brook, Heidelberg, Giessen, Paris 6, Michigan
State University
• Collaboration about timing detectors (and medical
applications): University of Chicago, Argonne, Saclay
• TDR to be presented in ATLAS together with FP420 in
the beginning of next year