1. Prospects for diffractive and forward physics at the LHC
Monika Grothe
U Turin/ U Wisconsin
CALTECH, March 2007 
Brief introduction to diffraction and the Pomeron
Its interest at the LHC
Experimental situation around the CMS interaction point
The CMS/TOTEM/FP420 program
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

2. Diffraction ?! Pomeron ?! Esoterica ?!
Diffraction ?! Pomeron ?! Esoterica ?!
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

3. Brief introduction to diffraction and the Pomeron: Diffraction
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

4. o) Forward peak for q=0 (diffraction peak)
Brief introduction to diffraction and the Pomeron: Diffraction in optics
o) Forward peak for q=0 (diffraction peak)
o) Diffraction pattern related to size of target and wavelength of beam
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

5. Brief introduction to diffraction and the Pomeron: Diffraction in hadron scattering
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

6. Diffraction is a feature of hadron-hadron interactions (30% of stot):
Brief introduction to diffraction and the Pomeron: Diffraction in hadron scattering (II) a a a X b
Single Dissociation a X
vacuum
quantum
numbers IP
LRG b b b Y
Elastic
Double Dissociation
Diffraction is a feature of hadron-hadron interactions (30% of stot):
o) Beam particles emerge intact or dissociated into low-mass states.
Energy  beam energy (within a few %)
o) Final-state particles separated by large polar angle
(or pseudorapidity, ln tan(q/2)): Large Rapidity Gap (LRG)
o) Interaction mediated by t-channel exchange of object with vacuum
quantum numbers (no colour): the Pomeron (IP)
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

7. LO realisation of vacuum
Brief introduction to diffraction and the Pomeron: Pomeron !? - A new way to probe the proton
Pomeron goes back to the ‘60s: Regge trajectory, ie a moving pole in complex
angular momentum plane (huh ?!)
Thanks to HERA and the Tevatron, we now understand diffraction in
terms of the theory of strong interactions, QCD, and of the constituents
of the proton – quarks and gluons (need a hard process)
In diffractive events look at the proton constituents through a lens that filters out all parton combinations except those with the vacuum quantum numbers p IP p
2-gluon exchange:
LO realisation of vacuum
quantum numbers in QCD
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

8. Great. And why bother at the LHC ?
Great. And why bother at the LHC ?
The Pomeron as little helper in tracking down the Higgs ?!
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

9. gg  H, H  b bbar highest BR But signal swamped by gg  jet jet
Why bother with diffraction at the LHC ? Suppose you want to detect a light SM Higgs (say MH=120 GeV) at the LHC...
Vacuum quantum numbers
“Double Pomeron exchange”
shields color charge of
other two gluons
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 JPC = 0++
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

10. In addition: Rich QCD program
Why bother with diffraction at the LHC - Detecting CEP of a light Higgs: Necessary ingredients
Central exclusive production pp pXp: Discover a light (~120 GeV) Higgs
In non-diffractive production hopeless, signal swamped
with QCD dijet background
Selection rule in CEP (central system is JPC = 0++ to
good approx) improves S/B for SM Higgs dramatically
In certain MSSM scenarios the signal cross section is
orders of magnitude higher than for the SM case
CP violation in the Higgs sector can be measured via
azimuthal asymmetry of the protons
In addition: Rich QCD program
In addition: Rich program of gamma-gamma mediated processes
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

11. CEP of a light Higgs: Necessary ingredients (II)
beam p’
roman pots
dipole
Proton spectrometer using the LHC beam magnets:
Detect diffractively scattered protons inside of beam pipe
x=0 (beam)
x=0.002
x=0.015
With nominal LHC optics:
1 2 s = M2
With √s=14TeV, M=120GeV
on average:
   1%
 fractional momentum loss of the proton
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

12. CEP of a light Higgs: Necessary ingredients (III)
1 2 s = M2
With √s=14TeV, M=120GeV on average:
   1%
Nominal LHC beam optics
Low * (0.5m): Lumi cm-2s-1
@220m: <  < 0.2
@420m: <  < 0.02
Detectors at 420m
complement acceptance of 220m detectors
needed to extend acceptance down to low  values, i.e. low M(Higgs)
Detectors at ~220m
needed in addition to optimize acceptance (tails of  distr.)
needed because 420m too far away for detector signals to reach L1 trigger within latency
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

13. 
Have 220 m detectors, want 420 m ones: Current experimental situation at the CMS IP
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

14. +TOTEM +FP420 pp interactions at 14TeV
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

15. An approved experiment at LHC for measuring
Experimental situation at the CMS IP CMS + TOTEM (+ FP420): Coverage in 
CMS IP
T1/T2, Castor ZDC
possibly
Castor
ZDC
TOTEM:
An approved experiment at LHC for measuring
tot and elastic, uses same IP as CMS
TOTEM’s trigger and DAQ system will be
integrated with those of CMS , i.e.
common data taking CMS + TOTEM possible
TOTEM aims at start-up at the same timescale
as CMS (second half of 2007)
Possible addition FP420
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

16. Experimental situation at the CMS IP CMS + TOTEM (+ FP420): Coverage in 
At nominal LHC optics, *=0.5m
diffractive
peak
TOTEM
Points are ZEUS data
FP420
xL=P’/Pbeam= 1-x
Note: Totem RP’s optimized for special optics runs at high *
β* is measure for transverse beam size at vertex
TOTEM coverage in  improves with increasing *
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

17. Proposal to ATLAS and CMS in first half of 2007
Experimental situation at the CMS IP Additional option: 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
FP420 R&D fully funded (~1000K CHF)
Proposal to ATLAS and CMS in first half of 2007
Detector installation could take place during first long LHC break (~2009)
Technological challenge:
420m is in the cryogenic region of the LHC
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

18. 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 !
Need to modify LHC cryostat:
Use Arc Termination Modules
for cold-to-warm transition such that
detectors can be operated at
~ room temperature
Movable beam-pipe (pipelets)
with detector stations attached
Move detectors toward beam envelope
once beam is stable
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

19. FP420 project: 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
Protons
PMT
Lens?
(focusing)
Mirror
Cerenkov medium (ethane)
~ 15 cm
~ 5 cm
(Flat or Spherical?)
Aluminium
pump
Injection of gas
(~ atmospheric
pressure)
Ejection of gas
~ 10 cm
Time-of-Flight detectors:
Time resolution of ~10ps would translate
in z-vertex resolution of better than 3mm
GASTOF (UC Louvain)
Cherenkov medium is a gas
QUARTIC (U Texas Arlington)
Cherenkov medium is fused Silica
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

20. Integration of the moving beampipe and detectors
ATM
BPM
Line X
Bus Bar Cryostat
Vacuum Space
BPM
QRL
Fixed Beampipe
ATM
Pockets
Transport side
Vacuum Space
Benoît Florins, Krzysztof Piotrzkowski, Guido Ryckewaert
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

21. The goal: Carry out a program of diffractive and forward physics
The goal:
Carry out a program of diffractive and forward physics
as integral part of the routine data taking at CMS,
i.e. at nominal beam optics and up to the highest
available luminosities.
This program spans the full lifetime of the LHC.
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

22. Joint CMS/TOTEM program
A step on the way ...
Joint CMS/TOTEM program
Central experimental issues in measuring
fwd and diffractive physics at the LHC
addressed for the first time - by way of
a number of exemplary processes.
Included already as option:
Near-beam detectors @420m
submitted to the LHCC in Dec ‘06
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

23. Prospects for diffractive and forward physics at the LHC
Central experimental issues for selecting diffraction at the LHC
Trigger is a major limiting factor for selecting diffractive events
Background from non-diffractive events that mimic diffractive
events because of protons from pile-up events
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

24. Double Pomeron exchange (DPE):
“Prospects for diffractive and forward physics at the LHC” Part I: “Diffractive” part The processes we are concerned with
Double Pomeron exchange (DPE): X
Single diffraction (SD):
Near-beam
detectors
central CMS
apparatus X IP IP
central CMS
apparatus
rap gap IP
Near-beam
detectors
Near-beam
detectors
o) If X = anything:
Measure fundamental quantities of soft QCD
Contributes significantly to pile-up
Inclusive SD  15 mb, inclusive DPE  1 mb, where 1 mb = cm-2 s-1
o) If X includes jets, W’s, Z’s, Higgs (!):
Hard processes, calculable in perturbative QCD.
Measure proton structure, QCD at high parton densities, discovery physics
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

25. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Trigger (I)
2 x 1033 cm-2 s-1 L1 jet trigger rates
no fwd
detectors
condition
pp  p jj X
2-jet trigger
Efficiency
Events per pb-1
single-arm
220m
L1 output
bandwidth:
100 kHz
single-arm
420m
L1 trigger threshold [GeV]
Attention: Gap survival probability not taken into
account; normalized to number of events with
0.001 <  < 0.2 and with jets with pT>10GeV
At 2x 1033 cm-1 s-1 without any additional condition on fwd detectors:
L1 1-jet trigger threshold O(150 GeV)
L1 2-jet trigger threshold O(100 GeV)

26. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Trigger (II)
→ CMS 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 diffractive 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 
Achievable total reduction: 10 (single-sided 220m) x 2 (jet iso) x 2 (2 jets same hemisphere as p) = 40
Adding L1 conditions on the near-beam detectors provides a rate reduction sufficient to lower the 2-jet threshold to 40 GeV per jet while still meeting the CMS L1 bandwidth limits for luminosities up to 2x 1033 cm-1 s-1
Much less of a problem is triggering with muons, where L1 threshold for 2-muons is 3 GeV
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

27. Central exclusive production pp pHp with H (120GeV)  bb
“Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Trigger (III)
Central exclusive production
pp pHp with H (120GeV)  bb
Trigger is a major limiting factor !
Efficiency
H(120 GeV) → b bbar
L1 trigger threshold [GeV]
Level-1:
2-jets (ET>40GeV, ||<3) & single-sided 220m condition results in efficiency ~12% (L1)
HLT:
Efficiency of above 2-jet condition ~7%
To stay within 1 Hz output rate, needs to either prescale or add 420 m detectors in trigger
 Can add another ~10% efficiency by introducing a 1 jet & 1 (40GeV, 3GeV) trigger condition
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

28. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Pile-up background
TOTEM
xL=P’/Pbeam= 1-x
d(epeXp)/dxL [nb]
Number of PU events with protons
within acceptance of near-beam
detectors on either side:
~2 % with 420m
~6 % with 220m
Translates into a probability of obtaining a
fake DPE signature caused by protons from PU:
Eg at 2x 1033 cm-2s-1 10% probability for a fake DPE signature.
This is independent of the type of signal.
Depends critically on the leading proton spectrum at the LHC which in turn depends on size of soft rescattering effects (rapidity gap survival factor) !
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

29. Side remark: Rapidity gap survival probability
Proton and anti-proton are large objects, unlike pointlike virtual photon
In addition to hard diffractive scattering, there may be soft interactions among spectator partons.
 Fill rapidity gap & slow down outgoing protons  Hence reduce the rate of diffractive events.
Quantified by rapidity gap survival probability.
jet b
hard
scattering IP
dPDF
Diffractive PDFs:
probability to find a parton of given x under
condition that proton stays intact –
sensitive to low-x partons in proton
dPDF
Closely related to the
underlying event at the LHC
F2D
without
and with
rescattering
effects
CDF data b
Predictions based on HERA diffractive PDFs
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

30. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Pile-up background (II)
Can be reduced by:
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 (planned in FP420)
; 1 2 s = M2
CEP H(120) bb
incl QCD di-jets + PU
(jets)
(220m)
CEP of H(120 GeV) → b bbar:
Possible to retain O(10%) of signal up to
2 x 1033 cm-2 s-1 in a special
forward detectors trigger stream
S/B in excess of unity for a SM Higgs
might be in reach
M(2-jets)/M(p’s)
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

31. Program on diffractive and forward physics 1. Diffraction
Low Luminosity (≤1032 cm-2s-1): low & high b*
Measure inclusive SD and DPE
cross sections and t, MX dependence
Rapidity Gap selection possible
High Luminosity (> 1032 cm-2s-1) : low b*
Measure SD and DPE in presence of hard scale
(dijets, vector bosons, heavy quarks)
gg and gp phyics
Highest Luminosity (> 1033 cm-2s-1) : low b*
Discovery physics in central exclusive production
(SM or MSSM Higgs, other exotic processes)
Running with TOTEM optics: large proton acceptance
No pile-up
Pile-up not negligible:
main source of background
Need additional forward proton
detector
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

32. Prospects for diffractive and forward physics at the LHC
Castor
ZDC
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

33. Program on diffractive and forward physics 2. Forward physics
Cosmic ray physics
→ Models for showers caused by primary cosmic rays (PeV = 1015 eV range) differ substantially
→ Fixed target collision in air with 100 PeV center-of-mass E corresponds to pp interaction at LHC
→ Hence can tune cosmic ray shower models at the LHC
Study of the underlying event at the LHC:
→ Multiple parton-parton interactions and rescattering effects accompanying a hard scatter
→ Closely related to gap survival and factorization breaking in hard diffraction
Heavy-ion and high parton density physics:
Proton structure at low xBj → saturation → Color glass condensates
Photon-photon and photon-proton physics:
Also there protons emerge from collision intact and with very low momentum loss
Multiple connection points to other areas in High-Energy-Physics !
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

34. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Photon-mediated processes: Exclusive μμ production
(di-muons) - (true)
known
rms ~ 10-4
Calibration process both for luminosity and energy scales of near-beam detectors
 Striking signature: acoplanarity angle between leptons
Allows reco of proton  values with resolution of 10-4, i.e. smaller than beam dispersion
 Expect ~300 events/100 pb-1 after CMS muon trigger
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

35. pQCD 1/x So far not observed in pp interactions Q2 [GeV2]
“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Low-x QCD
pQCD
Non perturbative
region
Saturation
(Colour glass
condensate)
Qs2(x)
1/x
Q2 [GeV2]
When x  0 at Q2 > a few GeV2
DGLAP predicts steep rise of
parton densities
At small enough x, this violates
unitarity
Growth is tamed by gluon fusion:
saturation of parton densities at
Q2=Qs2(x)
So far not observed in pp
interactions
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

36. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Low-x QCD: Forward jets
Inclusive forward (3<|| < 5) low-ET (20-100GeV) jet production 
Inclusive single jet cross section
would be reduced by saturation
Di-jets sensitive to gluons with:
x2 ~ 10-4, x1 ~ 10-1
PYTHIA ~ NLO
1 pb-1
Large expected yields (~107 at ~20 GeV) !
At the LHC, accessible xBJ(min) decreases by ~10 every 2 units of rapidity
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

37. Sensitivity to saturation:
“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Low-x QCD: Forward Drell-Yan
Sensitivity to saturation:
saturated PDF
Gives access to low-xBJ quarks in proton
in case of large imbalance of fractional
momenta x1,2 of leptons, which are then
boosted to large rapidities
CASTOR with 5.3 ≤ || ≤ 6.6 gives
access to xBJ~10-6 to 10-7
Measure angle of electrons with T2
Pdf’s known at large xBJ, hence can
extract pdf’s at low xBJ
DY pairs suppressed in saturated PDF
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

38. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Validation of hadronic shower models in cosmic ray physics
→ Models for showers caused by primary cosmic rays (PeV = 1015 eV range) differ substantially
→ Fixed target collision in air with 100 PeV center-of-mass E corresponds to pp interaction at LHC
→ Hence can tune shower models by comparing to measurements with T1/T2, CASTOR, ZDC
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

39. Summary CMS aims at carrying out a program of diffractive and forward
physics as integral part of its scientific program during routine
data taking at the LHC and up to the highest available luminosities
The combination of CMS (CASTOR, ZDC) with TOTEM (T1, T2,
near-beam detectors at 220m) and with FP420 offers ideal
conditions for carrying out a rich program of diffractive and forward
physics as integral part of the routine data taking at the LHC
and up to the highest achievable luminosities.
CMS aims at a formal agreement with TOTEM on this collaboration.
Feasibility of the detectors suggested by FP420 essentially proven.
Decision on building detectors at 420m, as suggested by FP420,
as part of CMS to be addressed soon.
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

40. Last words...
“QCD at a hadron collider is always a ghetto, and there the
worst neighborhood is diffraction.” - W.Smith
Recently the diffraction and forward physics group
was promoted to one of the principal physics analysis groups of CMS.
We are on the way to respectability ! 
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007