DIS2006

1
LHC Forward Physics
Experiments ALICE ATLAS CMS FP420 (RD
project) LHCf TOTEM
Jim Whitmore Penn State University
2
LHC Forward Physics

• Total cross-section (and luminosity) with a
  precision of 1
• Elastic pp scattering
• in the range 10-3 lt t (p ?)2 lt 10 GeV2
• Forward Physics
• Low-x dynamics
• Diffractive phenomena
• Soft and Hard
• Inclusive and exclusive Double Pomeron Exchange
  (DPE)
• Leading particle and energy flow in the forward
  direction
• pA, AA, gg and gp processes (sorry, I will not
  cover these topics)

Many of these topics can be studied best at
startup luminosities
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We are not studying a possibility of forward
physics with LHCb at the moment
LHCf
FP420
TOTEM
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Forward Detectors

• General philosophy
• Additional detectors near the IP
• Proton (Roman Pot) detectors
• want to detect small scattering angles (few
  mrad)
• and the beam divergence
• so want large values of b. However, luminosity
• want small b
• So expect a selection of b values (0.5-1540 m)
• RP detectors at 140-220 m from IP
• Need to go to 420 m ? the cold region

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Roman Pot acceptance
M2x1x2s
- 240 m
? proton momentum loss Dp/p Reconstruct ?
with roman pots ? lt 0.1 ? O(1) TeV Pomeron
beams
TOTEM (ATLAS)
FP420
Low ? (0.5m) Lumi 1033-1034cm-2s-1
220m 0.02 lt ? lt 0.2 300/400m
0.002 lt ? lt 0.02 Detectors in the 420 m
region are needed to access the low ?
values
(A. deRoeck)
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TOTEM CMS
Experimental Apparatus
T1 3.1 lt h lt 4.7 T2 5.3 lt h lt 6.5
CMS Castor 5.25lt? lt6.5
IP5
10.5 m
T1
T2
14 m
CASTOR (CMS)
IP5
RP1 (147 m)
RP2 (180 m) (later option)
RP3 (220 m)
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T1 Telescope
3.1lt h lt4.7

• 5 planes with measurement of three coordinates
  per plane.
• 3 degrees rotation and overlap between adjacent
  planes
• Primary vertex reconstruction
• Trigger with CSC wires

3 m
T2 Telescope
Digital r/o pads
5.3lt lhl lt 6.5
GEM (Gas Electron Multiplier) Telescope 10
½-planes 13.5 m from IP
Analog r/o circular strips
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Roman Pots
Test beam data
reconstructed tracks in y
u,v info
RP in SPS beam and the detector is measuring the
halo

• Roman Pot unit
• - Vertical and horizontal pots mounted as close
  as possible
• - TOTEM at the RP sbeam 80 mm
• - Leading proton detection at distances down to
  10sbeam d
• Need edgeless detectors that are efficient up
  to the physical edge to minimize d
• Currently two tech. (5-10 mm and 40-50 mm dead
  areas)

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Forward Detectors in ATLAS
Roman Pots at 240 m Cerenkov Counter (LUCID) a
lumi monitor at 5.4 lt?lt 6.1 neutral energy at
zero degrees
(I. Efthymiopoulos)
IP1
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Running Scenarios
Scenario Physics 1 low t elastic, stot , min. bias, soft diffraction 2 diffraction 3 large t elastic 4 hard diffractionlarge t elastic (under study)
b m 1540 1540 18 90
N of bunches 43 156 2808 156
N of part. per bunch (x1011) 0.3 0.6 - 1.15 1.15 1.15
Half crossing angle mrad 0 0 160 0
Transv. norm. emitt. mm rad 1 1 - 3.75 3.75 3.75
RMS beam size at IP mm 454 454 - 880 95 200
RMS beam diverg. mrad 0.29 0.29 - 0.57 5.28 2.4
Peak luminosity cm-2 s-1 1.6 x 1028 2.4 x 1029 3.6 x 1032 2 x 1030
TOTEM
(V. Avati, M. Deile)
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pp total cross sectionand luminosity monitor
TOTEM-CMS ATLAS
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pp total cross section
pp total Cross-Section
Luminosity-independent measurement using the
Optical Theorem
(M. Deile)
Measure the total rate (NelNinel) , sdiff 18
mb and min. bias 65 mb, with an
expected precision of 0.8 (running for 1
day at L 1.6 x 1028cm-2s-1). Extrapolate the
elastic cross-section to t 0 systematics
dominated
0.5 (statistical error after 1 day
0.07 ) ? Re f(0)/Im f(0) unknown using
COMPETE pred. 0.2

1
(r 0.13610.00150.0058-0.0025)
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pp total Cross-Section

• Current models predictions 90-130 mb
• Aim of TOTEM 1 accuracy (1 mb)

PRL 89 201801 (2002) Cudell et al.
COMPETE Collaboration fits all available hadronic
data and predicts
LHC
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ATLASs Plans

• ATLAS submitted a Letter of Intent to complement
  the experiment with a set of forward detectors
  for luminosity measurement and monitoring as part
  of a two stage scenario
• Short time scale
• Roman Pots at 240 m from IP1
• Probe the elastic scattering in the Coulomb
  interference region
• Dedicated detector for luminosity monitoring
  LUCID
• Used also to transfer the calibration from 1027?
  1034
• Goal Determine absolute luminosity at IP1
• (2-3 precision)
• 2. Longer time scale
• Study opportunities for diffractive physics with
  ATLAS
• Propose a diffractive physics program using
  additional detectors

(I. Efthymiopoulos)
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Physics interest -- ATLAS

• Luminosity Measurement Why?
• Important for (precision)
  comparison with theory
• e.g. ?bb, ?tt, ?W/Z, ?n-jet,
  cross-section deviations
  from SM could be a signal
  for new physics

Systematic error dominated by the luminosity
measurement (ATLAS-TDR-15, May 1999)
(I. Efthymiopoulos)
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pp elastic scattering
TOTEM
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Elastic scattering from ISR to Tevatron
1.5 GeV2
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pp elastic scattering cross-section
104 per bin of 10-3 GeV2
B(s) B0 2aP ln (s/s0) 20 GeV-2 at LHC
1/t8
BSW Bourrely, Soffer and Wu
b 1540 m L 1.6 x 1028 cm-2 s-1 (1)
b18 m L 3.6 x 1032 cm-2 s-1 (3)
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Elastic Scattering Models (eg. Islam et al)

• Observations
• fwd diffraction cross section increases
• diffractive peak shrinks
• interference dip moves to smaller t
• at t ? 1 GeV2
• ds/dt ? 1/t8
• (3-gluon exchange)
• little ?s dependence

? 1/t8
Islam et al
BSW
Desgrolard et al
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Elastic Scattering- ?el/?tot
Rdiffsel(s) sSD(s) sDD(s)/stot(s)
Rel sel(s)/stot(s)
0.3
?0.30
0.4
0.375
0.2
0.3
0.1
0.2
4
5
6
3
3
4
6
5
log(s/s0)

• sel ? 30 of stot at the LHC ?
• sSD sDD ? 10 of stot ( 100-150mb) at the
  LHC ?

(M. Deile)
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Low-x at the LHC

• LHC due to the high energy can reach small
  values of Bjorken-x
• in structure of the proton F(x,Q2)
• Processes
• ? Drell-Yan
• ? Prompt photon production
• ? Jet production
• ? W production
• If rapidities below 5 and
• masses below 10 GeV can be
• covered ? x down to 10-6-10-7
• Possible with T2 upgrade in TOTEM
• (calorimeter, tracker) 5lt?lt 6.7 !
• Proton structure at low-x !!
• Parton saturation effects?

(A. deRoeck)
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Diffractive physics
ALICE TOTEM CMS F420 project
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2 gluon exchange with vacuum quantum numbers
Pomeron
p p ? p X
p p ? p X p
Double diffraction
Y
p p ? X Y
The accessible physics is a function of the
integrated luminosity
(M. Ruspa)
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CMS TOTEM Acceptance
largest acceptance detector ever built at a
hadron collider
90 (65) of all diffractive protons are detected
for b 1540 (90) m
107 min bias events, incl. all diffractive
processes, in 1 day with b 1540 m
Total TOTEM/CMS acceptance
Charged particles
b90m
dNch/dh
ZDC
RPs
CMS central
T1 HCal
T2 CASTOR
Energy flux
dE/dh
b1540m
Pseudorapidity ? ln tg ?/2
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Soft Diffractive Event rates
ALICE is studying the possibility of implementing
a trigger requiring a rapidity gap on both sides
of a central region of 1.5 units of rapidity. The
selection can include EM energy deposition in the
PHOS, protons in the HMPID (RICH), or electrons
identified with the TRD, opening the possibility
to study heavy flavour production in double
diffractive events.
DPE pp ? pXp Acc 27.8 for detecting both
protons (b 90 m)
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DPE
Dh2 ln x2
Dh1 ln x1
Events/GeV-day
Exchange of color singlets (Pomerons) ?
rapidity gaps Dh
Measure gt 90 (65) of leading protons with RPs at
b 1540 (90) m and diffractive system X with
T1, T2 and CMS.
Scenario (2) (4) b (m) 1540 90
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Hard Diffractive Events
Diffractive events with high pT particles produced

• Double pomeron Ex pp ? pjjXp
• 1 mb
• pT gt 10 GeV
• Acc 29.3 (for b 90 m, prel.)

(V. Avati)
Single diffraction pp ? p 3j
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Exclusive Double Pomeron Exchange
TOTEM-CMS FP420 (with ATLAS/CMS)
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Exclusive Double Pomeron Exchange
Quantum numbers are defined for exclusive
particle production Gluonic states ?c , ?b ,
Higgs, supersymmetric Higgs,..
MX2 x1 x2 s

• Motivation from KMR calculations (e.g. hep-ph
  0111078)
• Selection rules mean that central system is (to
  a good approx) 0
• H?b-bbar QCD b-bbar bkgd suppressed by Jz0
  selection rule
• If you see a new particle produced exclusively
  with proton tags you know its quantum numbers
• Tagging the protons means excellent mass
  resolution ( GeV) irrespective of the decay
  products of the central system
• Proton tagging may be the discovery channel in
  certain regions of the MSSM

Trigger studies were discussed by M. Ruspa
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SUSY Higgs h, H, A, (H, H--)
Tasevsky et al
Diffractive H? bb Yuk. coupling, DMH, 0
5s
Inclusive H,A? tt wide bump
L60 fb-1
mH140
mH160
From A. Martins parallel session talk
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From A. Martins parallel session talk
Alans Conclusions
There is a very strong case for installing proton
taggers at the LHC, far from the IP ---- it is
crucial to get the missing mass DM of the Higgs
as small as possible The diffractive Higgs
signals beautifully complement the conventional
signals. Indeed there are significant SUSY Higgs
regions where the diffractive signals are
advantageous ---determining DMH, Yukawa H?bb
coupling, 0 determinn ---searching for
CP-violation in the Higgs sector
s(pp ? p H p) 3 fb at LHC for SM 120
GeV Higgs

• L(LHC)60 fb-1 10 observable events after cuts
  efficiency

• Higgs needs L 1033 cm-2 s-1, i.e. a running
  scenario for b 0.5 m
• trigger problems in the presence of overlapping
  events (see M. Ruspas talk)
• install additional Roman Pots in cold LHC region
  (420 m) at a later stage

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FP420 Project
The aim of FP420 is to install high precision
silicon tracking and fast timing detectors close
to the beams at 420 m from ATLAS and/or CMS.
FP420 turns the LHC into a glue-glue collider
where you know the beam energy of the gluons to
2 GeV.

• With nominal LHC beam optics
• _at_ 1033-34 cm-2s-1
• 220 m 0.02 lt ? lt 0.2
• 420 m 0.002 lt ? lt 0.02

?1 ?2 s M2 With vs 14TeV, MH 120
GeV on average ? ? 0.009 ? 1 Hence the
need for FP420
(See B. Coxs talk in the diffractive parallel
session)
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Forward physicsconnection to cosmic rays
ALICE TOTEM LHCf
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Issues in UHE cosmic rays
1. Spectrum / GZK Cutoff
29th ICRC Pune
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Issues in UHE cosmic rays
2. Composition
p
Measurements of the very forward energy flux
(including diffraction) and of the total cross
section are essential for the understanding of
cosmic ray events At LHC pp energy 104
cosmic events km-2 year-1 gt 107 events at the LHC
in one day
Fe
(O. Adriani)
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UHE Cosmic Rays
g
p
Fe
Cosmic ray showers Dynamics of the high energy
particle spectrum is crucial
Interpreting cosmic ray data depends on hadronic
simulation programs Forward region poorly
known/constrained Models differ by factor 2 or
more Need forward particle/energy measurements
e.g. dE/d?
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Model Predictions pp at the LHC
Predictions in the forward region within the
CMS/TOTEM acceptance
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LHCf

• Measurement of Photons and Neutral Pions in the
  Very Forward Region of LHC

Simulation of an atmospheric shower due to a 1019
eV proton.
(O. Adriani)

• The dominant contribution to the energy flux is
  in the very forward region
• In this forward region the highest energy
  measurements of p0 cross section were done by UA7
  (E1014 eV, y 57)

The direct measurement of the p production cross
section as function of pT is essential to
correctly estimate the energy of the primary
cosmic rays (LHC 1017 eV)
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LHCf
Experimental Method2 independent detectors on
both sides of IP
INTERACTION POINT
Detector II Tungsten Scintillator Silicon mstrips
Detector I Tungsten Scintillator Scintillating
fibers
IP1 (ATLAS)

• The vacuum tube contains two counter-rotating
  beams. The beams transition from one beam in each
  tube to two beams in the same tube.
• Detectors will be installed in the TAN region,
  140 m away from the Interaction Point, in front
  of luminosity monitors
• Charged particle are swept away by magnets
• LHCf will cover up to y ? 8

(O. Adriani)
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Summary

• There are plans at the LHC for a wide range of
  Forward and Diffractive measurements that can be
  achieved at a variety of different luminosities
• Measure total cross-section stot with a precision
  of 1
• Measure elastic scattering in the range 10-3
  lttlt 8 GeV2
• A study of soft and hard diffractive physics
• semi-hard diffraction (pT gt 10 GeV)
• hard diffraction
• Inclusive DPE
• Studies of Exclusive Double Pomeron Exchange
  events
• Studies of very forward particle production
• Connection with UHE Cosmic ray phenomena
• Special exotics (centauros, DCCs in the forward
  region)

41
Extra slides
42
Elastic Scattering ? Re f(s,0)/Im f(s,0)

TOTEM

• ? Ref(s,0)/Imf(s,0)
• (analyticity of the
• scattering amplitude via dispersion relations)
• constant/lns with s??

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Pile-up numbers!
PHOJET ALL PROCESSES 110 mb
NONDIF.INELASTIC 51 mb ELASTIC
33 mb
DOUBLE POMERON 1.95 mb
SINGLE DIFFR.(1) 7.66 mb
SINGLE DIFFR.(2) 7.52 mb
DOUBLE DIFFRACT. 9.3 mb
1 mb 100 events/s _at_ 10 29 cm-2 s-1

• Number of pileup events per bunch crossing
• Lumi cross section bunch time width total
  lhc bunches / filled bunches
• 1034 cm-2 s-1 104 (cm2/m2) 10-28 (m2 / b)
  110 mb 10-3 (b/mb) 25 (ns) 10-9 (s/ns)
  3564 / 2808 ? 35
• 1x1032 ? 0
• 1x1033 ? 3.5
• 2x1033 ? 7

Number of pileup events per bunch crossing
Lumi cross section bunch time width total
lhc bunches / filled bunches 1034 cm-2 s-1
104 (cm2/m2) 10-28 (m2 / b) 51 mb 10-3
(b/mb) 25 (ns) 10-9 (s/ns) 3564 / 2808 ?
17 This number is valid in the central detector
region, but must be corrected for the elastic and
diffractive cross section in the forward
region!
Selection of diffractive events with rapidity
gap selection only possible at luminosities below
10 33 cm-2s-1, where event pile-up is absent
44
RP
T1 T2
RP
45
FP420 Acceptance and Resolution
3 mm 3 mm
3 mm
25 mm
30 mm
5 mm
7.5 mm
10 mm
22 mm
MB apertures
46
Edgeless silicon detectors for the RP
10 planes/pot
Planar technology Testbeam
40 ?m dead area
66 mm pitch
50 mm dead area
10 mm dead area
active edges (planar/3D)
planar technology CTS (Curr. Termin. Struct.)
47
Diffraction at b 1540 m Acceptance
RP at 220 m
kinematically excluded

• Diffractive protons are observed in a large x-t
  range
• xDp/p t-(pq)2
• 90 are detected
• -t gt 2.5x10-3 GeV2
• 10-8 lt x lt 0.1
• x resolution 5x10-3

acc. lt 10
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Diffraction at b 90 m Acceptance
Resolution in x s(x) 4x10-4 (prel.) Llt2x1031
cm-2s-1
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Diffraction at b 0.5 m