1. The CMS and TOTEM diffractive and forward physics program
K. Österberg, High Energy Physics Division, Department of Physical Sciences, University of Helsinki & Helsinki Institute of Physics
Low x meeting, Helsinki 
CERN/LHCC 2006039/G124
CMS Note2007/002
TOTEM Note 065

2. Diffraction: a window to QCD and proton structure
Double Pomeron Exchange or Central Diffraction:
Single Diffraction:
Proton det. X
Central & forward det.
Gap
Central & forward det.
Gap X
Gap
Proton det.
p p  p X
Proton det.
p p  p X p
characterized by 2 gluon exchange with vacuum quantum numbers (“Pomeron”)
X = anything : dominated by soft physics
Measurement of inclusive cross sections + their t & MX dependence  fundamental measurements of nonperturbative QCD at LHC!
X = jets, W, Z, Higgs: hard processes calculable in perturbative QCD
Proton structure (dPDFs & GPDs), high parton density QCD, rapidity gap survival probability, new physics in exclusive central diffraction

3. Experimental apparatus at IP5@LHC
Tracking Calorimetry
T1: 3.1 <  < 4.7
T2: 5.3 <  < 6.5
(same as Castor)
10.5 m
IP5 HF
CASTOR
14 m
ZDC
FP420
147 m
IP5
220 m
Roman Pots
Roman Pots

4. Forward proton measurement: principle
Diffractive protons : hit RP220m
high L (low *) low L (high *)
y(mm)
y(mm)
y ~ yscatt ~ | ty |
10
x ~   p/p
10
x(mm)
x(mm)
Detect the proton via:
its momentum loss (low *) its transverse momentum (high *)

5. CMS/TOTEM combined acceptance
Unique coverage makes a wide range of physics studies possible  from diffraction & proton low-x dynamics to production of SM/MSSM Higgs
Acceptance for RP 220 m
TOTEM CMS TOTEM
Log(),  = p/p
Charged particle flow
low *
Charged particle flow
420 m proton low *
Energy flow
* = 1540
* = 90 
Log(t [GeV2])
Wide coverage in pseudorapidity & proton detection At high L proton m enlarge acceptance to  ~ 2  10 For details see B. Cox talk on FP420

6. Running scenario
pp->pX pp->pjjX pp->pjj (bosons, heavy pp->pXp pp->pjjXp pp->pjjp quarks,Higgs...)
soft diffraction (semi)-hard diffraction hard diffraction
cross section luminosity
 mb b nb
* (m)
L (cm2 s1)
TOTEM runs Standard runs
Accessible physics depends on: luminosity * (i.e. proton acceptance)

7. Triggering on soft & semi-hard diffraction
 Estimated Rates (Hz) [acceptance corrected]
L =1030 cm2s1 L =1032 cm2s * = 90 m * = 2 m
14 mb 105
1 mb 103
1b
(pTjet>20 GeV) 0.2
30nb
(pTjet>50 GeV)
60nb
(pTjet>20 GeV) 7103
1.5 nb
(pTjet>50 GeV) 1p
T1/T2 2p
jet(s)

8. Diffraction at low luminosity: soft central diffraction
Study of mass distribution via the 2 protons
 measured directly (() ~ 1.6  103 @  = 90 m) or
 with rapidity gap
 with calorimeters
MX =12s
 = ln ()/ ~ 80 %
  i ETi ei / s ()/ ~ 100 %
Mass resolution,  = 90 m, m
Differential mass distribution, acceptance corrected
asymmetric events
Acceptance  1/N  dN/dMX [1/10 GeV]
symmetric events
low 
MX [GeV]

9. Diffraction at high luminosity: central exclusive production
New physics searches e.g.  (pp  p H(120 GeV)[bb] p) ~ 3 fb (KMR)
JPC selection (0++...)  qq background heavily suppressed Selection: 2 protons + 2 b-jets with consistent mass values Experimental issues: trigger efficiency & pileup background
 in some MSSM scenarios much larger, H  WW if H heavier
(MX)/MX
Acceptance
NB! Assumes proton 420 m
MX [GeV]
MX [GeV]

10. Diffraction at high luminosity: triggering on diffraction
dedicated diffractive trigger stream foreseen at L1 & HLT (1 kHz & 1 Hz)
standard trigger thresholds too high for diffraction at nominal optics  use information from fwd detectors to lower particular trigger thresholds
L1:
jet ET > 40 GeV ( standard 2jet threshold: ~100 L = 2  1033 cm2s1)
1 (or 2) arm m
diffractive topology (fwd rap
gaps, jetproton hemisphere
correlation …)
HLT (more process dependent):
e.g. 420 m proton detectors,
mass constraint, b-tagging..
bench mark process: central
exclusive H(120 GeV)  bb
L1: ~ 12 %
HLT: ~ 7 %
additional ~ 10 % L1 with
a 1 jet + 1  (40 GeV, 3 GeV) trigger
Efficiency
420m
220m m m
L1 trigger threshold [GeV]

11. Diffraction at high luminosity: reducing pileup background
At high luminosity, non-diffractive events overlaid by soft diffractive events mimic hard diffractive events (“pileup background”)
Pileup event probability with single proton in (420m) ~ 6 % (2 %)  probability for fake central diffractive signal caused by pileup protons
Pileup background reduction:
correlation  & mass measured with central detector & proton detectors
fast timing detectors to distinguish proton origin & hard scattering vertex from each other (R&D project)
pileup background depends on LHC diffractive proton spectrum
 rapidity gap survival probability

12. Lowx physics
high LHC allows access to quark & gluon densities for
small Bjorkenx values: fwd Drell-Yan (jets) sensitive to quarks (gluons)
 > 5 & M < 10 GeV
 x ~ 106107
T2 + CASTOR !!
M2 of Drell Yan pairs in CASTOR
x of Drell Yan pairs in CASTOR

13. Forward physics air shower models, possible new phenomena…
 Forward particle & energy flow: validation of hadronic
air shower models, possible new phenomena…
 Study of underlying event
  & p mediated processes: e.g. exclusive dilepton production
()  (true)
known
rms ~ 10-4
Calibration process for luminosity and energy scale of proton detectors
Allows proton  value reconstruction with a 104 resolution < beam energy spread
Striking signature: acoplanarity angle between leptons

14. CMS 2008 diffractive & forward program
First analysis in 2008 most likely based on large rapidity gap selection Even if m available need time to understand the data …
Concentrate first analysis on data samples where pileup negligible
Developing rapidity gap trigger with forward calorimeters (HF, CASTOR…)
Program:
“Rediscover” hard diffraction a la Tevatron, for example:
measure fraction of W, Z, dijet and heavy flavour production with large rapidity gap
observe jet-gap-jet and multi-gap topologies
Forward physics with CASTOR
measure forward energy flow, multiplicity, jets & Drell-Yan electrons
 & p interactions, for example
observe exclusive di-lepton production

15. TOTEM Detectors: Roman Pots
2 RP220 m stations installed into LHC in May-June, remaining by end of summer
Si edgeless detector
VFAT hybrid with detector
First edgeless Si detectors mounted with final VFAT hybrid successfully working with source & in test beam  all detector assemblies ready for mounting by April 2008.

16. TOTEM detectors: T1 & T2 T1 Telescope T2 Telescope testbeam setup
Mechanical frames and CSC detectors in production; tests in progress. During 2007 complete CSC production. Plan to have two half-telescopes ready for mounting in April 2008.
All necessary GEM’s produced (> half tested upto gains of 80000). Energy resolution %. The 4 half-telescopes should be ready for mounting in April 2008.

17. Optics & beam parameters
(standard step in LHC start-up)
= 90 m
(early TOTEM optics)
= 1540 m
(final TOTEM optics)
Crossing angle
0.0
N of bunches
156 43
N of part./bunch
(4 – 9)  1010
3  1010
Emittance [m ∙ rad]
3.75 1
10  vertical beam width at RP220 [mm]
~ 3
6.25
1.3
Luminosity [cm-2 s-1]
(2 – 11)  1031
(5 – 25)  1029
1.6  1028
* = 90 m ideal for early running:
fits well into the LHC start-up running scenario;
uses standard injection  easier to commission than 1540 m optics
wide beam  ideal for RP operation training (less sensitive to alignment)
* = 90 m optics proposal submitted to LHCC & well received.

18.  = 90 m optics Optics optimized for elastic & diffractive scattering
Proton coordinates w.r.t. beam in the RP at 220 m:
(x*,y*):vertex position
(x*,y*):emission angle
 = p/p
Ly = 265 m (large)
vy = 0
Lx = 0
vx = – 2
D = 23 mm
vertical parallel-to-point focusing
 optimum sensitivity to y*
i.e. to t (azimuthal symmetry)
elimination of x* dependence
enhanced sensitivity to x in diffractive events,
horizontal vertex measurement in elastic events
x RP220 m
x RP220 m for elastic events  measurement of horizontal vertex IP
Luminosity estimate together with beam parameters if horizontal-vertical beam symmetry assumed
Beam position measurement to ~1 m every minute

19. TOTEM 2008 physics program
For early runs: optics with * = 90 m requested from LHCC
Optics commissioning fits well into LHC startup planning
Typical running time: several periods of a few days
Total pp cross section within ± 5%
Luminosity within ± 7%
Soft diffraction with independent proton acceptance (~ 65%)
proton resolution limited by
LHC energy spread
via rapidity gap (T1, T2)
via rapidity gap (CMS)
s(x)/x = 100%
via proton (b*=90m)

20. CMS and TOTEM diffractive and forward physics program
Summary
CMS and TOTEM diffractive and forward physics program
CMS+TOTEM provides unique pseudo-rapidity coverage
Integral part of normal data taking both at nominal & special optics
Low Luminosity (< 1032 cm-2s-1):
nominal & special optics
Single & central diffractive cross sections (inclusive & with jets, rapidity gap studies)
Lowx physics
Forward Drell-Yan & forward inclusive jets
Validation of Hadronic Air Shower Models
Increasing Luminosity (> 1032 cm-2s-1):
nominal optics
Single & central diffraction with hard scale (dijets, W, Z, heavy quarks)  dPDF’s,
GPD’s, rapidity gap survival probability
 & p physics
High Luminosity (> 1033 cm-2s-1) : nominal optics
Characterise Higgs/new physics in central diffraction?
Running with TOTEM optics: large proton acceptance
No pileup
Pileup not negligible:
important background source
Need additional proton detectors

22. An important milestone for collaboration between the 2 experiments
Content of common physics document
Includes important experimental issues in measuring forward and diffractive physics but not an exhaustive physics study
Detailed studies of acceptance & resolution
of forward proton detectors
Trigger
Background
Reconstruction of
kinematical variables
Several processes
studied in detail
Ch 1: Introduction
Ch 2: Experimental Set-up
Ch 3: Measurement of Forward Protons
Ch 4: Machine induced background
Ch 5: Diffraction at low and medium luminosity
Ch 6: Triggering on Diffractive Processes
at High Luminosity
Ch 7: Hard diffraction at High Luminosity
Ch 8: Photon-photon and photon-proton physics
Ch 9: Low-x QCD physics
Ch 10: Validation of Hadronic Shower Models
used in cosmic ray physics
An important milestone for collaboration between the 2 experiments

23. Running scenarios 1.15 (0.6 - 1.15) 0.3 N of part. per bunch 2 x 1030
2.3
200
3.75
156 90
Soft &
semi-hard diffraction
2.4 x 1029
1540
Soft diffraction
3.6 x 1032
1.6 (7.3) x 1028
Peak luminosity
[cm-2 s-1]
5.28
0.29 (2.3)
RMS beam diverg.
[rad] 95
454 (200)
RMS beam size at IP [m]
1 (3.75)
Transv. norm. emitt. [m rad]
160
Half crossing angle
2808 43
N of bunches
18, 2, 0.5
1540 (90)
large |t| elastic
low |t| elastic,
tot ,
min bias
Physics:
*[m]

24. Diffraction at low luminosity:
rapidity gaps
diffractive system X p2 p1
Measure  via rapidity gap:  = ln
Achieved resolution: ()/  80 %  
Gap vs ln () T1+T2+Calorimeters
min
max
Gap 
* = 90 * = 2
same acceptance
/ limit
Gap measurement limit due to acceptance of T2
ln
 = 2103 2102

25. Diffraction at medium luminosity: semi-hard diffraction
d/dpT
(b)
Measure cross sections & their t, MX, pTjet dependence
Event topology: exclusive vs inclusive jet production
 access to dPDF’s and rapidity gap survival probability
N event collected
[acceptance included]
* = 90 m ∫L dt = 0.3 pb1
SD: pT>20 GeV x104
CD: “
* = 2 m ∫L dt = 100 pb1
SD: pT>50 GeV x105
CD: “ x104
SD:pp->pjjX
assumed cross sections
CD:pp->pjjXp
pTjet(GeV)
In case of jet activity  also determined from calorimeter info:
  i ETi ei / s
()/  40 %

26. Forward proton measurement: acceptance (@220 m & 420 m)
RP 220m
*= 0.5  2 m
Log(),  = p/p
* = 2/0.5
FP420
420 m
220 m
* = 1540
* = 90
Log(t)
Proton detectors at 420 m would enlarge acceptance range down to  = at high luminosity (= low *).

27. Forward proton measurement: momentum resolution (* = 90 m)
Individual contribution
to the resolution:
Transverse vertex
resolution (30 m)
Beam position
resolution (50 m)
Detector
resolution (10 m)
total
()
220 m
Final state dependent; m for light quark & gluon jets
p/p

28. Forward proton measurement: momentum resolution (* = 2/0.5 m)
Individual contribution
to the resolution:
Detector
resolution (10 m)
Beam position
resolution (50 m)
Transverse vertex
Relative beam energy
spread (1.1104)
()/
total
p/p
* = 90 m vs. * = 2/0.5 m
Better ()/ for * = 2/0.5 m & larger luminosity but limited  acceptance ( > 0.02)
Very little final state dependence since transverse IP size is small.

29. Cosmic ray connection
Interpreting cosmic ray data depends on cosmic shower simulation programs
Forward hadronic scattering poorly known/constrained
Models differ up to a factor 2 or more
Need forward particle/energy measurements at high center-of-mass energy (LHC  Elab=1017 eV)
Achievable at low luminosity using T1/T2/HF/Castor

30. Low-x physics: forward jets
Inclusive forward “low-ET” jet
(ET ~ GeV) production:
PYTHIA ~ NLO
1 pb-1
p + p → jet1 + jet2 + X
Sensitive to gluons with: x2 ~ 10-4, x1 ~ 10-1
Large expected yields (~107 at ~20 GeV) !

31. Diffractive PDF’s and GPD’s
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,
complementary to standard PDFs
GPD
jet
Generalised Parton Distributions (GPD) quantify correlations between parton momenta in the proton
t-dependence sensitive to parton distribution in transverse plane
When x’=x, GPDs are proportional to the square of the usual PDFs