Prospettive di fisica a LHC

1
Prospettive di fisica a LHC
Parte II) La fisica dello Standard Model
(escluso l Higgs, trattato successivamente) -mi
sure di precisione di Mw e Mtop. - b- physics
2
Lo Standard Model a LHC
LHC una fabbrica di top quarks
s(bb) 500mb Possibili studi di processi
rari (es. Bs-gt2m, studio di violazione di CP con
alta statistica Caveat capacità di Trigger)
misure di alta precisione di MW (goal dMW ?10
MeV, limitato dalla sistematica)
108 tt/anno (ad alta luminosità)
3
Le misure di precisione a LHC motivazioni
La predizione SM di MHiggs
DM t 5.2 GeV
incertezze attuali
DMW 39 MeV
Misure dirette
consistenza?!
80.385
Fit agli osservabili elettrodeboli
Dipendenza da mt, mH nello SM
4
La fisica del top
Produzione di coppie di top
stt(th)825150 pb
NNLO-NNNLL Kidonakis, Vogt, PRD 68 (03) 114014
This means 8 millions tt pairs/year (1
pair/second) at low luminosity!
1033cm-2s-11nb-1s-1
gg-gttt 87
qq-gttt 13
quark annihilation
gluon fusion
5
La fisica del top

• easiest channel
• tt ? bb qq l? (semi-leptonic)

BR 43

• 3.5 million semileptonic events
• corresponding to 10 fb-1
• CMS analysis with hard cuts
• 0.14 of the events kept (!!!)
• Goal
• Error on mt ? ? 1 GeV
• statistical error 250 MeV
• largest sys. errors
• pT spectrum 400 MeV
• b-jet energy scale ?

• Measurements at 1 fb-1
• initial mass determination
• total diff. cross sections

6
La fisica del top
tt ? bb qq ?? event simulated in CMS
7
Top quark capacita di trigger
Efficienza di trigger
0.8
0.6
0.4
Low Lumi HLT threshold
0.2
10 fb-1
Full L1 simulation HLT reconstruction
Off-line reconstruction
gt 70 efficiency in fiducial region, for
typical HLT threshold
8
La fisica del top eventi di-leptonici

• Both top quarks decay
• semi-leptonically
• - BR ? 5
• - low background
• - Mass determination free from
• Jet-scale uncertainties
• but two neutrinos in final state
• Exploit M(ll)-Mtop correlation
• to determine Mtop
• ?mt ? ? 1.7 GeV

9
La fisica del top
Metodo alternativo per la misura di Mtop
Lepton J/y
1000 events/year _at_ L1034
J/y-gtmm
J/y-gtee
J/y-gtmm is a very clean signal. MlJ/y has a
dependence on Mt.

• Independent from jet scale
• Among the main sistematics
• b fragmentation

10
Mtop da leptone J/y
Per confronto di-leptoni
11
La fisica del top
tt spin correlation

• Very short lifetime,
• no top bound states
• Spin info not diluted
• by hadron formation

• Distinguishes between
• quark annihilation
• A -0.469
• and gluon fusion
• A 0.431

Use double leptonic decays tt ? bb l? l?
A 0.311 ? 0.035 ? 0.028 (using 30 fb-1)
12
Produzione di top singolo
t-channel
s-channel
Wt-channel
s10 pb
s247 pb
s56 pb
Main bkgs ttbar, Wbb

• Tevatron puts only an upper limit on s
• Directly related to Vtb
• Sensitivity to new physics FCNC (t-ch.), new
  gauge bosons (s-ch.), H-gttb
• Background to tt and several searches (ttH,
  WH-gtlnbb, )
• Possibility to study top properties (mass,
  polarization, charge) with very little
  reconstruction ambiguities

13
Produzione di top singolo
Production mechanisms and cross sections
? 245 pb
? 60 pb
? 10 pb
Experimental determination of Vtb to percent
level (with 30 fb-1)
14
Produzione di W e pdf

• Fundamental processes at LHC are
• the scattering of
• Quark Antiquark
• Quark Gluon
• Gluon Gluon

Examples
gg ? H

• need precise of parton density
• Functions pdf(x,Q2)
• QCD corrections (scale)

15
Esempio di early physics a LHC produzione di W

• pT and rapidity distributions are very sensitive
  to pdf
• particularly sensitive variable
• ratio of W/W? cross section measures u(x)/d(x)

Example study for 0.1 fb-1, i.e. 2106 W???
produced
Sensitive to small differences in sea quark
distribution
16
MW
2004 mW 80 412 ? 42 MeV
2007 mW ? 80 ... ? 20 MeV
(2.5 10-4)
LEP Tevatron Run I
from Tevatron Run II
Improvement at the LHC to ? 10 MeV
envisaged requires control of systematic error to
10-4 level
Traditional method mW from transverse mass
distribution
17
MW
18
MW

• General idea at LHC
• take Z ? ?? events
• remove one ? to fake Z ? ? ?
• mZ 91 187.5 ? 2.1 MeV known

19
MW

• Example
• use Z ? ? ? data to create
• transverse mass distribution for
• arbitrary MX
• compare MXT and compare to W data
• statistical error from 1 fb-1
• ?mW ? 20 MeV
• Systematic errors to be investigated
• small differences in W/ Z production
• (pT ? distributions)
• final state radiation
• (? doesnt radiate!)
• different backgrounds

20
MW
21
W/Z jet(s) physics

• An ideal physics final state to connect data to
  theoretical predications, and improve our
  understanding of event generation which is
  critical to many physics analysis
• Test perturbative QCD at large momentum transfer
• Indirect measurement of PDF including heavy
  flavor
• Very large cross section to reduce the
  statistical uncertainty and compare to NLO and
  NNLO calculations
• Serious background process of new physics
• Improve Reconstruction technique
• Precise Luminosity measurement
• Reliable Background normalization

Results of W jets in Tevatron Phys. Rev. Lett.
79, 4760 (1997)
22
W/Z jet(s) physics

• Minimum Jet Pt 25 GeV (because of large amount
  of jets from pileup and underlying events below
  25 GeV, see plots in later slide)
• Minimum Jet-Jet distance 0.5
• Jet Eta from 5.0 to 5.0
• W/Z semileptonic decay, fragmentation and
  hadronization, multiple parton scattering by
  Pythia

Alpgen prediction _at_ LHC
sensitive to , e.g. gt
Z1 jet
W1 jet
Z2 jet
W2 jet
Z3 jet
W3 jet
Z4 jet
Z4 jet
23
Triple Gauge Boson (W,Z,?) Couplings

• Leading Order Feynman diagrams
• Only s-channel has three boson vertex
• Charged Couplings
• Allowed in the Standard Model
• WWW, WWZ, WW?
• Neutral Couplings
• Forbidden in the Standard Model
• ZZZ, ZZ?, Z??
• What we might expect to see
• Cross section enhancement
• Enhancement at high pT of V 1,2
• Enhancement at high MT
• Production Angle

24
Triple Gauge Boson Couplings
Test CP conserving anomalous couplings at the
WW? vertex ?? and ?
Sensitivity pT spectrum SM couplings vs current
limits at 1.5 TeV

• Method
• W? final states
• W ? e? and ??
• pT spectrum of photon

25
B physics

• b physics topics at CMS
• inclusive b production
• at 14 TeV and in ?? collsions
• b decays
• e.g. b ? J/? X
• rare b decays
• e.g. FCNC in B0s ? ???
• CP violation in B0s ? J/? ?
• B0s oscillations
• Bc studies

Example reconstruction of B0s ? J/? ? ? ???
KK?

• Main difficulties
• Trigger (low pT thresholds needed)
• background rejection

26
B physics example B0s decays
s(bb) 500 mb 103 Bs/s _at_ L1031
Rare decay Bs ? mm
SM BR O(10-9 ) from FCNC b ? s reachable in 1
year
L 1031
isorate trigger curves
Non-rare channel
Bs ? J/psi f ? mmKK
20 Hz
30 Hz
Trigger efficiencies for Bs ? mm
8 ?104 decays/10 fb-1 feasable for CP
violation studies
27
B physics example Sensitivity to B0s
Oscillations
mass resolution 18.5 MeV
TRIGGER (not easy !)
critically dependent on background rejection
capability
28
B0s Oscillations
Current status
29
B0s Oscillations
Tevatron will certainly improve this result