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Forward PAG
One of the 10 PAGs of CMS
We are a genuine cross-borders PAG:
Physics subjects range from QCD and EWK to Higgs and Exotica
We are a across-political-boundaries PAG:
(Attempt at) collaboration with TOTEM experiment
FP420 R&D joint R&D project of ATLAS, CMS, LHC
In CMS, becoming a PAG has brought us into the mainstream:
We have already two PAS to our name:
“Exclusive dilepton and Upsilon photoproduction” PAS DIF-07-001
“Single diffractive W production” PAS DIF-07-002
We contribute to the iCSA08 exercise (Dimuon spectrum)
We have a (unfortunately not very up-to-date) TWIKI page:
https://twiki.cern.ch/twiki/bin/view/CMS/DiffractionAndFwdPhysics
We have a hypernews forum
https://hypernews.cern.ch/HyperNews/CMS/get/diffraction.html
We meet every other week on Friday afternoon
And we btw get more requests for conference talks than we can possibly fill
Monika Grothe, UW meeting May 29 2008
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Forward physics
Experimental definition:
All processes for which particles produced
At small angles and hence large
Pseudorapidities provide a defining feature
 = 90o
 =0
 = 10o    2.4
 = 170o    -2.4
 = 1o    5.0
edge of coverage of central
CMS/ATLAS detectors
Monika Grothe, UW meeting May 29 2008
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Forward detectors
CMS
Hadronic
Forward (HF)
Hadronic
Forward (HF)
(3.0 < || < 5.0)
(3.0 < || < 5.0)
T2
Services routing:
From Castor to Racks
CASTOR
(|| > 8.1)
Patch Panels
TOTEM T2
(5.2 < || < 6.6)
TOTEM T2
(5.2 < || < 6.6)
(|| > 8.1)
Not present in start-up
TOTEM T1
TOTEM T1
TOTEM: Total Cross Section, Elastic Scattering and
Diffraction Dissociation at the LHC
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Monika Grothe, UW meeting May 29 2008
Forward detectors around CMS IP
CMS has unprecedented calorimetric coverage, down to 6.5 for charged
Particles and even more for neutral ones
ATLAS cannot do this !
With FP420/Totem, CMS would also have unprecedented coverage for
Detecting leading protons with momentum loss xi between 20% and 0.2%
At nominal LHC optics, *=0.5m
Points are ZEUS data
TOTEM
diffr
peak
FP420
xL=P’/Pbeam= 1-x
Monika Grothe, UW meeting May 29 2008
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Physics with forward calorimeters
and tracking detectors
Proton PDF at very low x, parton evolution, saturation
Particle and energy flow measurements
Monika Grothe, UW meeting May 29 2008
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Saturation
(Colour glass
condensate)
1/x
Non perturbative
region
Structure function F2
Low-x QCD - Saturation
P momentum fraction carried by parton
Qs2(x)
pQCD
Q2 [GeV2]
• Steep rise in the gluon density at small x observed at HERA
• Growth cannot continue indefinitely, would eventually violate unitarity
• Growth tamed by gluon fusion: saturation of parton densities
• So far not observed in pp interactions
Monika Grothe, UW meeting May 29 2008
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Proton-proton collisions at low x
Parton evolution
The proton is a dynamic object !
Exact nature of dynamics not fully
Understood: DGLAP, BFKL, saturation
CMS
CASTOR
p
rapidity
p
•
•
Partons from each proton “decelerate”
and meet to produce the hard scattering
subsystem (ME)
Low x ↔ long parton showers
→ at LHC (for Q ≳ 10 GeV and η = 6):
xBjorken ≳ 10-6
→ xBjorken decreases by factor ~ 10
for each 2 units in rapidity
Monika Grothe, UW meeting May 29 2008
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Proton pdf at low-x from Drell-Yan
Forward lepton pairs
Kinematic coverage of CMS Castor
calorimeter reaches down to <10-6
x1 >> x2

qq → γ* → e+e−


PYTHIA DY
PYTHIA DY, η3 or η4 CASTOR
PYTHIA DY, η3 and η4 ∈ CASTOR
EHKQS: “saturated” pdf with nonlinear terms in
gluon evolution
CASTOR
acceptance
window
5.2 < ηe+,ηe− < 6.6
CTEQ5L
Cross section
reduced by factor 2!
EHKQS
First time observation of saturation
in pp possible !
Monika Grothe, UW meeting May 29 2008
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Forward jets from QCD evolution




x2 ≃ x1 → X can be (di-)jets in central
CMS detector
In BFKL-like QCD-evolution forward jets
can have large pT
Also possible:
jet-gap-jet events
Mueller-Navelet jets
PYTHIA jets (MSEL=1)
central dijet with pT > 60 GeV,
|η| < 3
BFKL: large yield of
high E forward jets
ARIADNE
(CDM)
jets
PYTHIA
(DGLAP)
“Jet energy” in CASTOR
Sensitivity to BFKL dynamics
Monika Grothe, UW meeting May 29 2008
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Measuring fwd particle and energy flow:
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, UW meeting May 29 2008
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Physics with !(forward detectors)
Hard diffraction
Photon-photon physics
Monika Grothe, UW meeting May 29 2008
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Forward physics:
Escaping protons
Elastic scattering
Monika Grothe, UW meeting May 29 2008
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Diffraction in optics –
Diffraction in hadron scattering
p
p
p
p
o) Forward peak for =0
(diffraction peak)
o) Diffraction pattern
related to size of target
and wavelength of beam
Monika Grothe, UW meeting May 29 2008
| t | ( p )2
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Forward physics:
Escaping protons
Hard diffraction
Monika Grothe, UW meeting May 29 2008
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Mind the gap:
Selecting
Gaps in the diffractive
hadronic finalevent
state
Generated particles – energy weighted
No color flow between
proton and X
Traditionally called
“Pomeron exchange”
X
Diffractive sample generated with gap in
–plus side
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Monika Grothe, UW meeting May 29 2008
dPDF
Diffraction with a hard scale: W prod
 pp → p W X, W sensitive to quark component of dPDFs
 Rap gap based selection, i.e. no pile-up
 Require absence of activity in the forward
calorimeters (HF 3< || < 5, Castor 5.2 < || < 6.6 ) of CMS
 For rap gap survival factor of S2 = 5%, arrive at
O(100) evts/100pb-1 in the [n(Castor), n (HF)] = [0,0] bin
 S/B  20 with Castor veto
 Signal enhancement by ~30% due to diffractive dissociation
2
S
…
dPDF
100pb-1
Other hard diffraction processes possible: Dijets, heavy quarks
Monika Grothe, UW meeting May 29 2008
PAS DIF-07-002
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 physics: Exclusive dilepton production
Monika Grothe, UW meeting May 29 2008
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 physics: Exclusive dilepton production
Monika Grothe, UW meeting May 29 2008
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 physics: Exclusive dilepton production
Monika Grothe, UW meeting May 29 2008
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 physics: Exclusive dilepton production
Nearly pure QED process
→ Absolute luminosity determination with
precision of 4% is feasible
→ Selection via exclusivity condition in central
detector + veto on CASTOR & ZDC activity
→ Calibration/alignment of proton taggers
→ rejection of 2/3 of p dissociative background
with CMS fwd calorimeters
pp → pp l+l−
PAS DIF-07-001
~700  events in 100 pb-1 with
dominant background from
p dissociative events (~200)
ATLAS cannot do this !
Monika Grothe, UW meeting May 29 2008
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Vectormeson photoproduction
ATLAS cannot do this !
pp  pp, → 
PAS DIF-07-001
Possibility to measure pT2 slope:
Monika Grothe, UW meeting May 29 2008
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Physics with near-beam
proton taggers
Monika Grothe, UW meeting May 29 2008
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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  ,
where in 30 fb-1 S/√B  4.4
Central exclusive production
pp  pXp
Suppression of gg  jet jet
because of selection rules forcing
central system to be
(to good approx) JPC = 0++
Monika Grothe, UW meeting May 29 2008
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Central exclusive Higgs production & FP420
shields color charge of
other two gluons
pp pHp with H (120GeV) -> bb
 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 particular beneficial for SUSY Higgs:
Production cross section considerably higher than for SM
Vacuum quantum numbers
“Double Pomeron exchange”
dipole
beam
p’
detector
May allow discovery of heavy SUSY Higgs bosons in
LHC wedge region
 CP quantum numbers & CP violation in Higgs sector
directly measurable from azimuthal asymmetry of protons
dipole
p’
detector
Proton spectrometer using the LHC beam magnets:
Detect diffractively scattered protons inside of beam pipe
Monika Grothe, UW meeting May 29 2008
FP420 is an R&D project
that proved the feasibility of
putting proton detectors
at 420m from the IP
Decision whether FP420
will be built in CMS
expected by this summer
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Proton taggers @ 220m and 420m from IP
Beampipe
s
TOTEM uses Roman pot technique to approach the beam with their Si detectors
FP420, because of location in cryogenic
region of LHC, uses movable beampipe
Extremly rad hard novel Si technology:
3-d Silicon
Cherenkov timing detectors with
t ~ 10 ps to filter out events with
protons from pile-up
Monika Grothe, UW meeting May 29 2008
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Discovery potential of CEP of Higgs
H
b, W, 
b, W, 
CEP may be the discovery channel for MSSM Higgs:
Heavy Higgs states decouple from gauge bosons, hence
preferred search channels at LHC not available
But large enhancement of couplings to bb,  at high tan
Detailed mapping of
discovery potential
for pp→p + H,h + p
CEP Higgs may also open door to
discovery of an NMSSM Higgs in
channel
h  aa  4
which would be unique at the LHC
Monika Grothe, UW meeting May 29 2008
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Summa Summarum
Monika Grothe, Woekshop on high energy photon processes, CERN, April 2008
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Topics in the fwd PAG cover QCD – EWK – Higgs – Exotica
Technically, our analyses replicate in part what other groups do
(e.g. W selection from EWK, jets from QCD, etc)
We look at corners of the phase space that the mainstream usually
doesn’t look at: for example low pT end of the DY spectrum, quiet detector, etc
Most of our topics are prime start-up topics
For example J. Hollar is now leading the iCSA08 dimuon analysis
In the next years the only area in CMS where additional detector construction
may happen (FP420)
Finally, already before FP420 upgrade, CMS has a fwd physics program that
ATLAS cannot match
Monika Grothe, Woekshop on high energy photon processes, CERN, April 2008
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Extra
Monika Grothe, UW meeting May 29 2008
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Cross sections and production rates
Rates for L = 1034 cm-2 s-1: (LHC)
• Inelastic pp reactions:
• bb pairs
• tt pairs
109 / s
5 106 / s
8
/s
• W  e n150 / s
• Z ee
15 / s
• Higgs (150 GeV)
• Gluino, Squarks (1 TeV)
• Diffraction
• Elastic scattering
0.2 / s
0.03 / s
2.5 108 / s
2.2 108 / s
~50% of the total cross section !
Monika Grothe, UW meeting May 29 2008
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A new way to probe the proton
Double Pomeron exchange (DPE):
Single diffraction (SD):
X
Near-beam
detectors
central CMS
apparatus
IP
IP
rap gap
IP
Near-beam
detectors
X
central CMS
apparatus
Near-beam
detectors
In diffractive events look at the proton constituents through a lens that filters
out all parton combinations except those with the vacuum quantum numbers
If X = anything:
Measure fundamental
quantities of soft QC
IP
p
p
If X includes jets, W’s, Z’s, Higgs (!):
p
p
Hard processes, calculable in perturbative QCD.
2-gluon exchange:
Measure proton structure, QCD at high parton densities,
discoveryofphysics
LO realisation
vacuum
quantum numbers in QCD
Monika Grothe, UW meeting May 29 2008
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Forward physics at
nominal LHC optics
The CMS+TOTEM (+FP420) joint program:
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.
CERN/LHC 2006-039/G-124
Monika Grothe, UW meeting May 29 2008
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Photon-mediated processes:
Exclusive μμ production
x(di-muons) - x(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 x values with resolution of 10-4, i.e. smaller than beam dispersion
 Expect ~300 events/100 pb-1 after CMS muon trigger
Monika Grothe, UW meeting May 29 2008
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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~10-7
 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, UW meeting May 29 2008
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Summary and Outlook (II)
Still an opportunity for an original contribution to the LHC detectors !
possible upgrade
RP220 with Si detectors
possible
addition
SLHC
Monika Grothe, UW meeting May 29 2008
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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.
•Diffractive PDFs:
probability to find a parton of given x under
condition that proton stays intact –
sensitive to low-x partons in proton
F2D
hard
scattering
without
and with
rescattering
effects
jet

jet
dPDF
CDF data
Closely related
the
IPtodPDF
underlying event at the LHC

Predictions based on HERA diffractive PDFs
Monika Grothe, UW meeting May 29 2008
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Inclusive SM Higgs production with H->WW*
Two most important production channels
Higgs boson likes mass
Decays preferably into the heaviest
particle kinematically possible
Discovery potential already with ~1fb-1
for M(H) ~ 2M(W)
Vector-Boson Fusion
VBF channel could profit
from CASTOR calorimeter
Monika Grothe, UW meeting May 29 2008
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VBF SM Higgs production with H->WW*
May profit from CASTOR which extends
detector coverage to angles smaller than 1o
Higgs
Decay
Tag jets

Contributes significant discovery potential
In addition, may provide evidence for
spin-0 Higgs, leptons prefer small 
1o 10o
Monika Grothe, UW meeting May 29 2008
90o
170o
179o

(non-linear scale)
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Benefits from Castor calorimeter for Higgs
Extends CMS calorimetric coverage
in the very fwd direction,
to below 1o wrt beam axis
Benefits for Higgs searches:
 Provides handle on effect
of underlying events/multiple
parton-parton interactions
 Use in Vector Boson Fusion
channels for detecting
forward tag jets
W
Monika Grothe, UW meeting May 29 2008
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CMS + TOTEM (+ FP420)
CMS IP
T1/T2, Castor
ZDC RPs@150m
RPs@220m
possibly detectors@420m
TOTEM:
 An approved experiment at LHC for measuring tot & elastic, at same IP as CMS
 TOTEM aims at start-up on the same timescale as CMS
 Expression of wish of CMS + TOTEM to carry out a joint physics program, with
joint CMS+TOTEM data taking given to LHCC:
“Prospects for diffraction and forward physics at the LHC”
CERN LHCC 2006-039 G124, CMS note 2007-02, TOTEM note 06-5
Possible addition FP420:
 Proposal to install high precision silicon tracking and fast timing detectors close to
the beams at 420m from the CMS IP
 Proposal currently under scrutiny in CMS
 If approved, could be installed in 2010 after LHC start-up
Monika Grothe, UW meeting May 29 2008
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Forward instrumentation around CMS IP
CMS central detector
Hadronic Forward (HF)
CASTOR
IP 5
T1
Zero Degree Calorimeter (ZDC)
T2
RP 147
RP 220
FP 420
IP 5
CMS: blue
Monika Grothe, UW meeting May 29 2008
TOTEM: green
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The CMS CASTOR calorimeter
•
•
•
•
•
•
•
extends the coverage to 5.2 < η < 6.6
enhances the hermiticity of
CMS!
14.37 m from the interaction point
octogonal cylinder with inner radius 3.7cm,
outer radius 14cm and total depth 10.5 λI
signal collection through Čerenkov photons
transmitted to PMTs through aircore
lightguides
W absorber & quartz plates sandwich, with
45° inclination with respect to the beam
axis
electromagnetic and hadronic sections
16 seg. in φ, 14 seg in z
no segmentation in η
Currently funding available only for CASTOR on one side of IP
If LHC allows, installation of 1/2 CASTOR on one side of IP 07/2008
Monika Grothe, UW meeting May 29 2008
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The CMS Zero Degree Calorimeter
•
•
•
•
•
140 m from interaction point
in TAN absorber
Thungsten/quartz Čerenkov
calorimeter with separate
e.m. (19 X0) and had.(5.6 λI)
sections
em: 5-fold horizontal seg
had: 4-fold seg in z
Acceptance for neutrals (γ,
π0, n) from η > 8.1
(100% for η > 8.4)
Ready for 2008 run
Monika Grothe, UW meeting May 29 2008
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TOTEM T1 & T2 tracking detectors
3m
Test Beam
 Cathode Strip Chambers (CSC)
 Mounted in front of HadronForward
calorimeter of CMS
 3.1 < | < 4.7
 5 planes with 3 coordinates/plane
 6 trapezoidal CSC detectors/plane
 Resolution  ~ 0.8mm
 Gas Electron Multiplier (GEM)
 Mounted in front of CASTOR
 5.3 < | < 6.5
 10 planes formed by 20 GEM
semi-circular modules
 Radial position from strips, , from pads
 Resolution strip~70m
Monika Grothe, UW meeting May 29 2008
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Multiplicity and ETitel
flow measurements
Can differentiate between certain MC tunes
to describe underlying event with CASTOR/T2
Can differentiate between
MC models for cosmic rays
with CASTOR/T2/ZDC
Monika Grothe, UW meeting May 29 2008
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