Diffractive Higgs searches:
The Pomeron as little helper in
tracking down the Higgs ? The FP420 project
Monika Grothe
U Turin/ U Wisconsin
Johns-Hopkins workshop
Heidelberg August 2007
Why ?
How in principle ?
What’s available already ?
Specific challenges ?
Current status ?
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Why bother with diffraction
at the LHC ?
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Suppose you want to detect a light
SM Higgs (say MH=120 GeV) at the LHC...
shields color charge of
other two gluons
Vacuum quantum numbers
“Double Pomeron exchange”
SM Higgs with ~120 GeV:
gg  H, H  b bbar highest BR
But signal swamped by gg  jet jet
Best bet with CMS: H  
Central exclusive production
pp  pXp
Suppression of gg  jet jet
because of selection rules forcing
central system to be
(to good approx) JPC = 0++
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Diffraction as tool for discovery physics:
Central exclusive production pp  pXp
Experimental assets of central exclusive production:
 Selection rules: central system is JPC = 0++ (to good
approx) I.e. a particle produced with proton tags has known
quantum #
 Excellent mass resolution achievable from protons,
independent of decay products of X in central detector:
“CEP as superior lineshape analyser”
 CP quantum numbers and CP violation in Higgs sector
directly measurable from azimuthal asymmetry of the
protons: “CEP as spin-parity analyzer”
 Proton tagging improves S/B for SM Higgs dramatically
Case in point: pp  pHp with H(120 GeV)  b bbar
In non-diffractive production hopeless, signal swamped
by QCD di-jet background
 CEP may be discovery channel in certain regions in
MSSM where the Xsection can be much larger than in SM
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Central exclusive production:
Standard Model light Higgs
Standard Model Higgs
Generator studies with detector cuts
b jets :
MH = 120 GeV;  = 2 fb (uncertainty factor ~ 2.5)
MH = 140 GeV;  = 0.7 fb
H
MH = 120 GeV : 11 signal / O(10) background in 30 fb-1
with detector cuts
Note: This H decay channel is impossible in
non-CEP production !
WW* :
MH = 120 GeV;  = 0.4 fb
MH = 140 GeV;  = 1 fb
MH = 140 GeV : 8 signal / O(3) background in 30 fb-1
with detector cuts
Note: Use semi-leptonic decays for measurement
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Central exclusive production:
Observation at Fermilab
Search for exclusive 
 3 candidate events found
 1 (+2/-1) predicted
from ExHuME MC*
hep-ex/0707237
Same type of diagrams as for Higgs  validation of KMR model !
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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How go about measuring
central exclusive production ?
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Measuring central exclusive production:
Experimental signature
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Measuring central exclusive production:
Principle of measurement
Diffractively scattered protons survive interaction intact and lose
only a small fraction of their initial momentum in the process
Needed: Proton spectrometer using the LHC beam magnets
Detect protons that are very slightly off-momentum wrt beam protons,
i.e. detection needed inside of beam pipe
beam
dipole
dipole
p’
roman pots
p’
roman pots
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Measuring central exclusive production:
Where to put the detectors
beam
dipole
dipole
p’
roman pots
p’
roman pots
With nominal LHC optics:
x1 x2 s = M2
x=0.015
x=0.002
With √s=14TeV, M=120GeV
on average:
x=0 (beam)
x  0.009  1%
x fractional momentum loss of the proton
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Measuring central exclusive production:
Where to put the detectors (II)
x1 x2 s = M2
With √s=14TeV, M=120GeV on average:
x  0.009  1%
Nominal LHC beam optics
Low * (0.5m): Lumi 1033-1034cm-2s-1
@220m: 0.02 < x < 0.2
@420m: 0.002 < x < 0.02
Detectors at 420m
•complement acceptance of 220m detectors
•needed to extend acceptance down to low x values, i.e. low MHiggs
Detectors closer to IP, e.g. ~220m
• optimize acceptance (tails of x distr.)
• can be used in L1 trigger, while 420m too far away for detector signals to reach L1 trigger
within latency
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Current experimental situation at the
ATLAS and CMS IP’s:
ALFA and TOTEM
Possible extension of the ATLAS/CMS
baseline detectors: FP420
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d(epeXp)/dxL [nb]
Existing proton tagging detectors
TOTEM
det
@420
data points from ZEUS
xL=P’/Pbeam= 1-x
CMS IP: TOTEM
ATLAS IP: ALFA
 Approved experiment for tot, elastic meas.
 Detectors to determine absolute luminosity
by way of measuring elastic scattering in
Coulomb interference region
 Uses same IP as CMS
 Roman-pot housed Silicon tracking detectors
at 180m and 220m from IP
 TOTEM’s trigger/DAQ system will be
integrated with those of CMS , i.e. common
data taking CMS + TOTEM possible
 However, operation at highest LHC lumi
would require rad hard upgrade of Totem Si
 Approved part of ATLAS experiment
 Roman-pot housed scintillating fiber
detectors at 240m from IP
 Operation at nominal LHC lumi requires
rad-hard upgrade - option subject of an
R&D effort by several ATLAS groups
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The FP420 R&D project
The aim of FP420 is to install high precision silicon tracking and fast timing
detectors close to the beams at 420m from ATLAS and / or CMS
Proposal to the LHCC in June 2005: CERN-LHCC-2005-025
“FP420: An R&D Proposal to Investigate the Feasibility of Installing
Proton Tagging Detectors in the 220m Region at LHC”
Signed by 29 institutes from 11 countries
“The LHCC acknowledges the scientific merit of the FP420 physics program
and the interest in its exploring its feasibility.” - LHCC
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FP420 project:
How to integrate detectors into the
cold section of the LHC
scattered protons emerge here
420m from the IP is in the cold section of the LHC
Modify LHC Arc Termination Modules
for cold-to-warm transition such that detectors can
be operated at ~ room temperature
Turin / Cockcroft Institute / CERN
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FP420:
How to move detectors close to the beam
Movable beam-pipe
with detector stations attached
Move detectors toward beam
envelope once beam is stable
Beam position
monitor
Silicon
detector box
Gastof or Quartic
Turin / Louvain / Helsinki
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FP420:
Which technology for the detectors
3D edgeless Silicon detectors:
Edgeless, i.e. distance to beam envelope can be minimized
Radiation hard, can withstand 5 years at 1035 cm-2 s-1
Use ATLAS pixel chip (rad hard) for readout
Active edges:
the edge is itself
an electrode, so
dead volume at
the edge < 5m.
 Prototype in CERN testbeams
2006 and 2007
 Technology is candidate for
ATLAS tracker SLHC upgrade
Electrodes are
processed inside
the detector bulk
instead of
being implanted
on the wafer’s
surface.
Manchester / Stanford
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FP420 project:
Silicon Detector Stations
Manchester / Mullard Space Science Lab
3 detector stations with 8 layers each
7.2 mm x 24mm
8 mm
8 mm
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FP420 project:
Fast timing detectors
Micro channel plate photo-multiplier tubes (MCP-PMT) were successfully employed in
building Cherenkov-light based TOF detector with resolution of ~10ps (NIM A 528(2004) 763)
Would translate in z-vertex resolution of better than 3mm
 Needed to veto protons from pile-up events
Two technologies; both in FERMILAB test beams 2006 and 2007
Ejection of
gas
Injection of gas
(~ atmospheric
pressure)
pump
Aluminium
proton
Protons
Cherenkov light
~ 5 cm
Cerenkov
medium
(ethane)
Mirror
(Flat or
Spherical?)
~ 15 cm
Lens?
(focusing)
~ 10 cm
PMT
QUARTIC (U Texas-Arlington):
Cherenkov medium is fused Silica
GASTOF (UC Louvain)
Cherenkov medium is a gas
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Putting it all together
FP420 project:
ATM
BPM
Line X
Bus Bar Cryostat
Vacuum Space
BPM
Fixed Beampipe
ATM
Pockets
Vacuum Space
Benoît Florins, Krzysztof Piotrzkowski, Guido Ryckewaert
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FP420:
What resolution does one achieve ?
CEP of Higgs:
Si pitch 40-50 mm
x and y orientation
(x) ~ (y) ~15 mm
CMS IP
ATLAS IP
S/B for 120GeV Higgs  b bbar depends critically on mass window width
around signal peak
Glasgow / Manchester
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Central problems to solve
in the analysis of diffractive events
at the LHC
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Experimental challenge: Trigger
The difficulty of triggering on a 120GeV Higgs
Trigger at ATLAS/CMS based on high pT/ET jet and lepton candidates in event
In order to keep output rate at acceptable level, for example at 2x 1033 cm-1 s-1:
 L1 2-jet trigger threshold O(100 GeV) per jet
But: 120 GeV Higgs decays preferably into 2 b-jets with ~60 GeV each
Possible strategies:
 Rely on muon trigger only, where 2-muon trigger thresholds are 3 GeV
 Take hit in statistics
 Allow lower jet thresholds by assigning bigger chunk of available bandwidth
 Could be considered once Higgs has been found and one knows where to look
 Allow lower jet thresholds without increase in assigned bandwidth by
combining central detector jet condition with condition on forward proton taggers
Note: 220m proton taggers usable in L1 trigger, 420m taggers only on HLT because 420m too
far away from IP for signal to arrive within L1 latency of 3.2 ms
Experimental challenge: Trigger
A dedicated forward detectors
L1 trigger stream
→ Trigger thresholds for nominal LHC running too high for diffractive events
→ Use information of forward detectors to lower in particular CMS jet trigger thresholds
→ The CMS trigger menus now foresee a dedicated forward detectors trigger
stream with 1% of the total bandwidth on L1 and HLT (1 kHz and 1 Hz)
!
single-sided
220m condition
without and with
cut on x
Achievable total reduction: 10 (single-sided 220m) x 2 (jet iso) x 2 (2 jets same hemisphere as p) = 40
Demonstrated that for luminosities up to 2x 1033 cm-1 s-1 including 220m detectors
into the L1 trigger provides a rate reduction sufficient to lower the 2-jet threshold
substantially, to 40GeV, while requiring only 1% of L1 bandwidth
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Experimental challenge: Trigger
Trigger Efficiency for
central exclusive Higgs production
Central exclusive production pp  pHp with H (120GeV)  bb:
420m
Assuming 1% of total bandwidth available:
220m
Di-jet trigger threshold of 40GeV &
single-sided 220m condition possible,
would retain 10% of the events
420+420m
420+220m
H(120 GeV) → b bbar
This would double the efficiency provided
by the CMS muon trigger (no fwd detectors
condition)
L1 trigger threshold [GeV]
Central exclusive production pp  pHp with H (140GeV)  WW:
Same efficiency as non-CEP production, no improvement from fwd detectors jet trigger condition
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
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Experimental challenge:
Pile-up background !
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Experimental challenge: Pile-up background
Pile-up background (II)
Number of PU events with protons
within acceptance of near-beam
detectors on either side:
~2 % with p @ 420m
~6 % with p @ 220m
d(epeXp)/dxL [nb]
Diff events characterized by low fractional proton momentum loss
diffractive
peak
TOTEM
det@420
xL=P’/Pbeam= 1-x
Coincidence of non-diffractive event with protons from pile-up events in the near-beam
detectors:  fake double-Pomeron exchange signature
Non-diffractive event with signature in the central CMS detector identical to some DPE
signal event: At 2x 1033 cm-2s-1 10% of these non-diffractive events will be mis-identified
as DPE event. This is independent of the specific signal.
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Experimental challenge: Pile-up background
Handles against pile-up background
Can be reduced on the High Level trigger:
Requiring correlation between
ξ, M measured in the central detector and
ξ, M measured by the near-beam detectors
Fast timing detectors that can determine
whether the protons seen in the near-beam
detector came from the same vertex as the
hard scatter within 3mm
;
CEP H(120) bb
x1 x2 s = M2
incl QCD di-jets + PU
Further offline cuts possible:
Condition that no second vertex be
found within 3mm vertex window
left open by fast timing detectors
x(p tagger)
Exploiting difference in
multiplicity between diff signal
and non-diff background
CEP of H(120 GeV) → b bbar and
H(140 GeV) → WW:
S/B of unity for a SM Higgs
M(2-jets)/M(p’s)
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Side remark:
CMS + Totem (+ FP420) program
Experimental issues of detecting diffractive processes at the LHC discussed in:
Prospects for diffractive and forward physics at the LHC,
CERN/LHC 2006-039/G-124
Written by CMS and TOTEM to express interest in carrying out a joint program of
diffractive and forward physics as part of the routine data taking at the CMS IP, i.e.
up to the highest available luminosities and spanning the full lifetime of the LHC.
Program covers in addition to central exclusive production:
• Diffraction in the presence of a hard scale: “Looking
at the proton through a lense that filters out anything
but the vacuum quantum numbers
• Diffractive structure functions
• Soft rescattering effects/underlying event and
rapidity gap survival factor
• Low xBJ structure of the proton
• Saturation, color glass condensates
• Rich program of and p physics
• Validation of cosmic ray air shower MC
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Current status of FP420
and
Summary
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 FP420 is an R&D collaboration with members from ATLAS, CMS and the LHC
 FP420 aims at providing the necessary tools for measuring central exclusive
production at the LHC under nominal LHC running conditions
 FP420 suggests to instrument the location 420m from the ATLAS/CMS IP with
Silicon tracking detectors and fast TOF detectors
FP420 will extend the physics potential of the ATLAS/CMS baseline detectors:
 For the SM Higgs, FP420 makes feasible observing a light SM Higgs
in the bb decay channel
 For the MSSM Higgs, in certain parts of the parameter space FP420
has discovery potential
 FP420 renders possible a direct measurement of the Higgs quantum
numbers
 Both in ATLAS and CMS internal evaluation of FP420 proposal has started
 FP420 is preparing a Technical Design Proposal with the results of R&D studies
 If approved by ATLAS (CMS) as proper ATLAS (CMS) project, independent
Technical Design Proposals for ATLAS-FP420 and CMS-FP420, building on
common R&D
Installation could take place in 2009/2010, i.e. no interference with LHC start-up
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BACKUP
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The physics case for FP420
MSSM: intense coupling regime
Intense-coupling regime of the MSSM:
Mh~MA ~ MH ~ O(100GeV): their coupling to, WW*, ZZ* strongly suppressed
 discovery very challenging at the LHC
Cross section of two scalar (0+) Higgs bosons
enhanced compared to SM Higgs
100 fb
Production of pseudo-scalar (O-) Higgs
suppressed because of JZ selection rule
Superior missing mass resolution
from tagged protons allows to separate h, H
-
see Kaidalov et al,
hep-ph/0307064,
hep-ph/0311023
1 fb
Spin-partity of Higgs can be determined from
the azimuthal angles between the two tagged
protons (recall JZ rule only approximate)
 CEP as discovery channel
120 140
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The physics case for FP420
MSSM: intense coupling regime
Azimuthal angle between outgoing protons sensitive to Higgs spin-parity:
JP=0+ vs JP=0- (recall JZ selection rule only approximate)
100 fb
01 fb
0+
Kaidalov et al.,
hep-ph/0307064
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MSSM Scenario Studies
MA = 130 GeV tan = 50
Hbb
S. Heinemeyer et al
to appear
No-mixing scenario
Contours of ratio of signal events in the MSSM over the SM
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CMS + TOTEM (+ FP420)
Unprecedented kinematic coverage
Castor
Castor
CMS Castor thungsten/quartz
Cherenkov calorimeter
TOTEM T2:
GEM tracking detector
d(epeXp)/dxL [nb]
TOTEM Silicon tracking
det. housed in Roman pots
ZDC
ZDC
TOTEM
det
@420
CMS ZDC thungsten/quartz
Cherenkov calorimeter
xL=P’/Pbeam= 1-x
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ALFA and LUCID
ALFA: Absolute Luminosity for ATLAS
LUCID: Luminosity measurement with
a Cherenkov Integrating Detector
2 stations at 240m
from ATLAS IP
approaching the
beam to within 1.2mm
10+10 planes of
scintillating fibre
detectors
 spatial resolution 30mm
 edge <100mm
Installation of detectors during first
long LHC shutdown (2009 ?)
Aluminium tubes filled with isobutane in
cylinder (length 1.5m, diameter 13.7cm)
around beam pipe 17 m from ATLAS IP
Absolute lumi measurement at ~ 10-27 cm-2 s-1
Extrapolation from there to luminosity at
nominal LHC running via track counting in LUCID
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Forward detectors at ATLAS/CMS IP’s
possible upgrade
RP220 with Si detectors
possible
addition
SLHC
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CDF: exclusive processes at Fermilab (II)
On the way to diffractive Higgs
production:
p
p
cc
c
J/y
cc
p

c
p
•H proceeds via the same diagram
but t-loop instead of c-loop
•Important for calibrating models on
diffractive Higgs
10 candidate events (but unknown background)
<49  18 (stat)  39 (syst) pb
for exclusive cc production for |y|<0.6
Monika Grothe, Diffractive Higgs searches: The FP420 project, August 2007
MJ/y-
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Alignment
Online: Beam-Position Monitors plus a wirepositioning system: aiming for 10 micron
precision on beam-detector separation.
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