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The CMS and TOTEM diffractive and forward physics programK. Österberg, High
Energy Physics Division, Department of Physical Sciences, University of Helsinki & Helsinki Institute of
PhysicsLow x meeting, Helsinki 29.8. 1.9.2007
CERN/LHCC 2006039/G124CMS Note2007/002TOTEM Note 065
Diffraction: a window to QCD and proton structure
Double Pomeron Exchange or Central Diffraction:
X
Single Diffraction:
p p p XProton det.
Gap
Central & forward det.
X
p p p X p Proton det.
Proton det.
Gap
Gap
Central & forward det.
characterized by 2 gluon exchange with vacuum quantum numbers (“Pomeron”)
X = anything : dominated by soft physicsMeasurement of inclusive cross sections + their t & MX dependence fundamental measurements of nonperturbative QCD at LHC! X = jets, W, Z, Higgs: hard processes calculable in perturbative QCDProton structure (dPDFs & GPDs), high parton density QCD, rapidity gap survival probability, new physics in exclusive central diffraction
FP420
HF
ZDC
(same as Castor)
Tracking Calorimetry
Roman Pots
Roman Pots
T1: 3.1 < < 4.7
T2: 5.3 < < 6.5
Experimental apparatus at IP5@LHC
CASTOR
10.5 m
14 m
IP5
IP5220 m
147 m
Detect the proton via:its momentum loss (low *) its transverse momentum (high *)
Forward proton measurement: principle
Diffractive protons : hit distribution @ RP220m high L (low *) low L (high *)
10 10 x ~ p/p
y(m
m)
x(mm) x(mm)y(m
m)
y ~ yscatt ~ | ty |
CMS/TOTEM combined acceptanceUnique coverage makes a wide range of physics studies possible from diffraction & proton low-x dynamics to production of SM/MSSM Higgs
Charg
ed p
art
icle
flow
TOTEM CMS TOTEM
En
erg
y fl
ow
Ch
arg
ed
part
icle
flow
Log(
),
=
p/p
Log(t [GeV2])
Acceptance for RP 220 m
* = 1540 * = 90
low *
420 m proton detectors @ low *
Wide coverage in pseudorapidity & proton detection At high L proton detectors @420 m enlarge acceptance to ~ 2 103 For details see B. Cox talk on FP420
pp->pX pp->pjjX pp->pjj (bosons, heavy pp->pXp pp->pjjXp pp->pjjp quarks,Higgs...)
soft diffraction (semi)-hard diffraction hard diffractioncross section luminosity
Accessible physics depends on: luminosity * (i.e. proton acceptance)
mb b nb
* (m)1540 90 2 0.5 L (cm2 s1) 1028 1030 1032 1034
TOTEM runs Standard runs
Running scenario
1p T1/T2
2pT1/T2
1pT1/T2jet(s)
2pT1/T2jet(s)
Estimated Rates (Hz) [acceptance corrected]
L =1030 cm2s1 L =1032 cm2s1
* = 90 m * = 2 m -----------------------------------------------------------------
14 mb 6000 1.4105
1 mb 200 3.5103
1b (pT
jet>20 GeV) 0.2 30nb (pT
jet>50 GeV) 0.01 0.5
60nb (pT
jet>20 GeV) 7103 1.5 nb (pT
jet>50 GeV) 0.03
Triggering on soft & semi-
hard diffraction
Diffraction at low luminosity: soft central diffraction
Differential mass distribution, acceptance corrected
Acc
epta
nce
1
/N
dN
/dM
X [
1/1
0
GeV
]
MX [GeV]
Mass resolution, = 90 m, RP@220 m
low
MX =12s
= ln ()/ ~ 80 % i ET
i ei / s ()/ ~ 100 %
symmetric events
asymmetric events
Study of mass distribution via the 2 protons
measured directly (() ~ 1.6 103 @ = 90 m) or with rapidity gap with calorimeters
Diffraction at high luminosity: central exclusive production
MX [GeV]
(M
X)/
MX
MX [GeV]
New physics searches e.g. (pp p H(120 GeV)[bb] p) ~ 3 fb (KMR)
Acc
ep
tan
ce
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
NB! Assu
mes proton detectors @
420 m
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 trigger threshold [GeV]
Effi
ciency
420m
220m
420+420m
420+220m
L1:● jet ET > 40 GeV ( standard 2jet threshold: ~100 GeV @ L = 2 1033 cm2s1)● 1 (or 2) arm RP@220 m● diffractive topology (fwd rap gaps, jetproton hemisphere correlation …)
HLT (more process dependent):e.g. 420 m proton detectors, mass constraint, b-tagging..
bench mark process: central exclusive H(120 GeV) bbL1: ~ 12 %HLT: ~ 7 %additional ~ 10 % efficiency @ L1 with a 1 jet + 1 (40 GeV, 3 GeV) trigger
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 RP@220m (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
Lowx physicshigh s @ LHC allows access to quark & gluon densities for small Bjorkenx values: fwd Drell-Yan (jets) sensitive to quarks (gluons)
> 5 & M < 10 GeV x ~ 106107
T2 + CASTOR !!
x of Drell Yan pairs in CASTOR
M2 of Drell Yan pairs in CASTOR
Forward particle & energy flow: validation of hadronicair shower models, possible new phenomena… Study of underlying event & p mediated processes: e.g. exclusive dilepton production
Forward physics
() (true)
rms ~ 10-4
Calibration process for luminosity and energy scale of proton detectors
Allows proton value reconstruction with a 104 resolution < beam energy spreadStriking signature: acoplanarity angle between leptons
known
CMS 2008 diffractive & forward program
First analysis in 2008 most likely based on large rapidity gap selection Even if RP@220 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
2 RP220 m stations installed into LHC in May-June, remaining by end of summer
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.
Si edgeless detector VFAT hybrid with detector
TOTEM Detectors: Roman Pots
T1 Telescope
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.
T2 Telescope
All necessary GEM’s produced (> half tested upto gains of 80000). Energy resolution 20-30 %. The 4 half-telescopes should be ready for mounting in April 2008.
testbeam setup
TOTEM detectors: T1 & T2
Parameters = 2 m(standard step in LHC start-up)
= 90 m(early TOTEM optics)
= 1540 m(final TOTEM optics)
Crossing angle 0.0 0.0 0.0
N of bunches 156 156 43
N of part./bunch (4 – 9) 1010 (4 – 9) 1010 3 1010
Emittance [m ∙ rad] 3.75 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.
Optics & beam parameters
(x*,y*):vertex position(x
*,y*):emission angle
= p/p
Optics optimized for elastic & diffractive scatteringProton coordinates w.r.t. beam in the RP at 220 m:
* *y y yy L v y
Ly = 265 m (large) vy = 0
* *x x xx L v x D
Lx = 0 vx = – 2
vertical parallel-to-point focusing optimum sensitivity to y
*
i.e. to t (azimuthal symmetry)
D = 23 mm
elimination of x* dependence
enhanced sensitivity to x in diffractive events, horizontal vertex measurement in elastic events
x distribution @ RP220 m x distribution @ RP220 m for elastic
events measurement of horizontal vertex distribution @ IP
Luminosity estimate together with beam parameters if horizontal-vertical beam symmetry assumed
Beam position measurement to ~1 m every minute
= 90 m optics
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%)
TOTEM 2008 physics program
proton resolution limited by LHC energy spread
via rapidity gap (T1, T2)
via rapidity gap (CMS)
via rapidity gap (T1, T2)
=
10
0%
via
prot
on (* =
90m
)
SummaryCMS and TOTEM diffractive and forward physics program
CMS+TOTEM provides unique pseudo-rapidity coverageCMS+TOTEM provides unique pseudo-rapidity coverageIntegral part of normal data taking both at nominal & special opticsIntegral part of normal data taking both at nominal & special optics
Low Luminosity (< 10Low Luminosity (< 103232 cm cm-2-2ss-1-1): ): nominal & special opticsnominal & special optics Single & central diffractive cross sections
(inclusive & with jets, rapidity gap studies) Lowx physics Forward Drell-Yan & forward inclusive jets Validation of Hadronic Air Shower Models
Increasing Luminosity (> 10Increasing Luminosity (> 103232 cm cm-2-2ss-1-1): ): nominal opticsnominal optics Single & central diffraction with hard scale
(dijets, W, Z, heavy quarks) dPDF’s, GPD’s, rapidity gap survival probability & p physics
High Luminosity (> 10High Luminosity (> 103333 cm cm-2-2ss-1-1) : nominal optics) : nominal optics Characterise Higgs/new physics in central diffraction?
Running with TOTEM optics: Running with TOTEM optics: large proton acceptancelarge proton acceptance
No pileupNo pileup
Pileup not negligible:Pileup not negligible:important background important background sourcesource
Need additional Need additional proton detectors proton detectors
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
Includes important experimental issues in measuring forward and diffractive physics but not an exhaustive
physics study
Detailed studies of acceptance & resolutionof forward proton detectors
Trigger
Background
Reconstruction of kinematical variables Several processes studied in detail
An important milestone for collaboration between the 2 experiments
Content of common physics document
1.15 (0.6 - 1.15) 1.150.3 N of part. per bunch
(x 1011)
2 x 1030
2.3
200
3.75
0
156
90
Soft &
semi-hard diffraction
2.4 x 1029
0.29 - 0.57
454 - 880
1 - 3.75
0
156
1540
Soft diffraction
3.6 x 10321.6 (7.3) x 1028Peak luminosity
[cm-2 s-1]
5.280.29 (2.3)RMS beam diverg.
[rad]
95454 (200)RMS beam size at IP [m]
3.751 (3.75)Transv. norm. emitt. [m rad]
1600Half crossing angle
[rad]
280843N of bunches
18, 2, 0.51540 (90)
large |t| elasticlow |t| elastic,
tot ,
min bias
Physics:
*[m]
Running scenarios
Diffraction at low luminosity: rapidity gaps
Measure via rapidity gap: = ln
Achieved resolution: ()/ 80 %
diffractive system Xp2 p1
min max
Gap measurement limit due to acceptance of T2
Gap vs ln () T1+T2+Calorimeters
ln = 2103 2102
* = 90 * = 2 same acceptance / limit
Gap
Diffraction at medium luminosity: semi-hard diffractionass
um
ed
cro
ss s
ect
ion
s
pTjet(GeV)
SD:pp->pjjX
CD:pp->pjjXp
Measure cross sections & their t, MX, pT
jet 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 pb1
SD: pT>20 GeV 6x104 CD: “ 2000
* = 2 m ∫L dt = 100 pb1 SD: pT>50 GeV 5x105 CD: “ 3x104
d/dpT
(b)
In case of jet activity also determined from calorimeter info:
i ETi ei / s
()/ 40 %
Forward proton measurement: acceptance (@220 m & 420 m)
RP 220m
FP420
* = 1540
* = 2/0.5
* = 90
Log
(),
=
p
/p
Log(t)
*= 0.5 2 m
420 m
220 m
Proton detectors at 420 m would enlarge acceptance range down to = 0.002 at high luminosity (= low *).
Forward proton measurement: momentum resolution (* = 90 m)
Individual contribution to the resolution: Transverse vertex resolution (30 m)
Beam position resolution (50 m)
Detector resolution (10m)
p/p
() total 220 m
Final state dependent; 30 m for light quark & gluon jets
Forward proton measurement: momentum resolution (* = 2/0.5 m)
Individual contribution to the resolution:
Detector resolution (10m)
Beam position resolution (50 m)
Transverse vertex resolution (10 m)
Relative beam energy spread (1.1104)
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.
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
Cosmic ray connection
Inclusive forward “low-ET” jet
(ET ~20-100 GeV) production:
Sensitive to gluons with: x2 ~ 10-4, x1 ~ 10-1
p + p → jet1 + jet2 + X
Large expected yields (~107 at ~20 GeV) !
PYTHIA ~ NLO
1 pb-1
Low-x physics: forward jets
• 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
GPDjet
jet
jet
jet
hard scattering
IP dPDF
• 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
Diffractive PDF’s and GPD’s